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Electrochemical Hydrogen Compression Efficient PressurizationConcept Derived from an Energetic EvaluationTo cite this article Michel Suermann et al 2017 J Electrochem Soc 164 F1187

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Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1187

Electrochemical Hydrogen Compression Efficient PressurizationConcept Derived from an Energetic EvaluationMichel Suermanna Thomas Kiupela Thomas J Schmidtablowast and Felix N Buchiaz

aElectrochemistry Laboratory Paul Scherrer Institut CH-5232 Villigen PSI SwitzerlandbLaboratory of Physical Chemistry ETH Zurich CH-8093 Zurich Switzerland

Electrochemical hydrogen compression is a potentially high efficient low-maintenance and silent technology to produce highpressure hydrogen A new pressure concept with increased compression efficiency termed intermediate differential pressure polymerelectrolyte water electrolysis is proposed With slightly pressurized oxygen and a much higher hydrogen pressure this pressureconcept profits from the advantages of the lower gas crossover of differential (only hydrogen compressed) pressure water electrolysisand the improved oxygen evolution reaction kinetics with increasing pressure of balanced pressure operation Data for gas pressuresup to 100 MPa is modeled based on experimental results up to 5 MPa of the three pressure concepts and validated with literaturedata up to 70 MPa While differential pressure electrolyzer operation following ideal isothermal compression can be more efficientthan todayrsquos best mechanical compressors up to 40 MPa the intermediate pressure concept shows higher compression efficiency upto 90 MPacopy The Author(s) 2017 Published by ECS This is an open access article distributed under the terms of the Creative CommonsAttribution 40 License (CC BY httpcreativecommonsorglicensesby40) which permits unrestricted reuse of the work in anymedium provided the original work is properly cited [DOI 10114921361712jes] All rights reserved

Manuscript submitted June 13 2017 revised manuscript received August 21 2017 Published August 30 2017

Electricity generation from renewable sources is constantly ex-panding eg the wind and photovoltaic (PV) power capacity installedin the European Union (EU-28) has increased from 172 GW in 2010to 236 GW in 2015 and the concurrent electricity generation to about400 TWh12 A major drawback of these technologies is their inter-mittent generation Balancing production with consumption in theelectric grid becomes a challenge and unused electric energy needsto be curtailed3 At the national level in 2015 the largest curtailmentwas required in Germany with 47 TWh (127 GWh in 2010) causingestimated costs of 480 Meuro4

Polymer electrolyte water electrolysis (PEWE) ie electrochem-ical splitting of water into hydrogen and oxygen is a well suitedtechnology to follow excess power loads and convert it to chemicalenergy in the form of hydrogen5 With all curtailed energy in Ger-many in 2015 converted into hydrogen at 79 kWhkgH2 (compressedto gt 70 MPa and precooled for fast filling)67 the fuel would havebeen sufficient for about 590000 fuel cell electric vehicles (FCEVs1 kgH2100 km and 10000 km annual driving)

In todayrsquos electrolysis plants hydrogen is produced typically at 3MPa89 and is further compressed mechanically to be fed into the nat-ural gas grid or transported to a hydrogen refueling station (HRS)89

where the gas is compressed to pressure levels up to 100 MPa10 inorder to refuel 70 MPa FCEVs

Conventionally hydrogen is compressed mechanically using pis-ton compressors In the best case this still requires energy of approxi-mately a factor of 14 above the thermodynamic requirement ie idealisothermal compression11 Differential pressure PEWE (only hydro-gen is compressed) has been demonstrated up to 70 MPa12 Likewiseelectrochemical hydrogen compressors (EHCs) where hydrogen isconcentrated andor compressed from a lower to a higher (partial)pressure have been demonstrated up to 100 MPa13

In energy related applications the conversion efficiency is cru-cial especially while investment costs for PEWE systems of about1000ndash2000 eurokW1415 are still relatively high In the electrolyzer theconversion efficiency is reduced by the water splitting reaction overpo-tentials and losses by gas crossover An overview on the overpotentialsin PEWE is given by Carmo et al16 and on membrane gas perme-ability by Mittelsteadt and Staser17 The reduced gas drying effortat the plant level when using pressurized PEWE is exemplified byBensmann et al18

In this work we determine the hydrogen compression efficiencyconsidering PEWE cell voltage and gas crossover losses due to pres-

lowastElectrochemical Society MemberzE-mail felixbuechipsich

surization Conventionally PEWE is operated with either a differen-tial or balanced (both gases compressed at same pressure) pressureconcept The effect of gas pressure on cell voltage is experimentallyanalyzed at 1 MPa for the classical PEWE pressure strategies Basedon an energetic evaluation a novel pressure concept with a slightly in-creased oxygen pressure and a much higher hydrogen pressure is thenintroduced as intermediate differential pressure concept and results areshown up to 5 MPa The results from the electrochemical characteri-zation are then combined with the pressure dependent gas crossoverlosses to calculate the total hydrogen compression efficiency whichis extrapolated up to values of 100 MPa and which is discussed forthe cases shown in Figure 1 when hydrogen is compressed i) directlyusing PEWE ii) with downstream mechanical compression or iii) us-ing EHCs As also shown EHCs could be even used to concentrateandor compress hydrogen from a lower to a higher partial pressureout of a gas mixture eg out of the (natural) gas grid

Experimental

Polymer electrolyte water electrolysis test bench and cellmdashAsmall scale electrolysis cell with a square active area of 4 cm2 isused Commercial catalyst coated membranes (CCMs) with Ir- andPt-based catalysts (Nafion 117CS and Nafion 212 Greenerity E400Greenerity DE) are sandwiched between two 10 mm thick poroustransport layers (PTLs) with a porosity of 35 plusmn 2 made from titaniumsintered powder (SIKA T10 GKN Sinter Metal Filters DE)19 Gasand water is distributed from and to the PTLs by flow-fields with fiveparallel channels (2 times 1 mm width x depth) made from gold-coatedstainless steel to eliminate contact resistances and corrosion issues

The cell contains a special membrane electrode assembly (MEA)compression mechanism keeping the contact pressure (here 25 MPa)independent of the clamping pressure needed for the flat gaskets Thetest bench consists of two separate gaswater loops with a forcedwater recirculation at the anode of 30 mL middot minminus1 middot cmminus2 while atthe cathode natural convection takes place Both PEWE cell and testbench are described in detail in Ref 20

PEWE measurement methodsmdashThe overpotentials are charac-terized with galvanostatic currentvoltage characteristics (iE-curves)from 0001 to 4 A middot cmminus2 with a holding time at each current densityof 10 s plus 1 s for a high frequency resistance (HFR) measurementElectric controls are based on a Bio-Logic potentiostat (VSP-300Bio-Logic SAS FR) The relatively short holding time is chosen dueto safety requirements (gas crossover at low current densities and highpressures) and has been validated in previous work21 The HFR is mea-sured at 25 kHz to correct data for the ohmic overpotential Before

F1188 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 1 Power-to-gas paths for storing intermittent elec-tric power from renewable sources by conversion into chem-ical energy via water electrolysis and three different ways ofhydrogen transportation temporary storage (indicated withpressure state p1) and compression of hydrogen to the desiredpressure state p2 From top to bottom the three different pathsare i) direct compression via PEWE ii) partial compressionvia PEWE with downstream temporary storage and mechan-ical andor electrochemical compression to p2 and iii) partialcompression via PEWE with downstream temporary storageor direct transport via the gas grid and a concentration andcompression via EHC to p2

measuring iE-curves the cell is operated at a constant current densityof 2 A middot cmminus2 for at least 10 minutes to achieve thermal equilibriumconditions The cell temperature is set to 50C However at highercurrent densities a temperature gradient of about 1 K per Wcm2 wasteheat production leads to a curvature with negative concavity of the re-ported (iR-free) cell characteristics (iE-curves) as explained in detailin Ref 19

Electrochemical hydrogen compressor experimental setupmdashThe EHC setup is based on a single cell with a rectangular active areaof 200 cm2 (226 times 88 mm length x width) Commercial 7-layer MEAsbased on Nafion XL-100 with Pt-based catalysts microporous layers(MPLs) and gas diffusion layers (GDLs) are used (H500E GreenerityDE) which are actually designed for fuel cell applications The MEAis sandwiched between two flow fields with 50 parallel channels witha channel depth of 03 mm

EHC measurement methodsmdashIn EHCs hydrogen is oxidized atthe anode and then the protons are reduced back to hydrogen withhigher partial pressurecompression at the cathode Two different EHCscenarios are investigated i) compression of pure hydrogen from alower to a higher pressure level and ii) concentration and compressionof hydrogen from a gas mixture of 5 H2 in N2 to 100 H2 repre-senting possible conditions for separating hydrogen from the naturalgas grid Gas is fed in a counter-flow mode to the two MEA sidesand humidified to 80 relative humidity via controlled evaporationand mixing systems (CEMs Bronkhorst CH) Pure hydrogen is al-ways fed on the cathode side The cell temperature is 60C controlledby heatingcooling channels inside the plates A local current densitymeasurement device (Current scan shunt S++ Simulation ServicesDE) with 288 individual segments is implemented

For the EHC characterization iE-curves in the current densityrange from 0005 to 25 A middot cmminus2 with holding times at individualcurrent densities of 40 s are recorded using a power supply (AgilentTechnologies US) For the 5 H2 in N2 experiments the maximumcurrent density is 05 A middot cmminus2 due to limitations with gas humidifi-cation For both cases the anode (low pressure side) volume flow isequal to 15 times the stoichiometric requirement (λa = 15) whilethe cathode volume flow is equal to λc = 05 for case i) and λc = 2 forcase ii) based on the anodic consumption of hydrogen Stoichiomet-ric conditions are reached at current densities greater or equal than05 A middot cmminus2 for case i) and 01 A middot cmminus2 for case ii) Local currentdensities (S++ unit) are measured simultaneously during iE-curvesIn the absence of differences across the channels only the averagevalues and the corresponding standard deviations for each of the 16columns along the channel are shown in the results Before measuringiE-curves at given parameters the EHC is conditioned for 15 minutes

at 025 A middot cmminus2 The mean cell voltage and corresponding standarddeviation reported are based on at least three measurements

Theoretical Background

In this work two aspects are considered which determine theenergetic compression losses due to pressurization The first is referredto the cell voltage the second to gas crossover losses In theoryboth losses should increase with pressure With respect to the cellvoltage from thermodynamics an isothermal compression behavioris expected according to the Nernst equation given in Equation 1

E0cell(p T ) = E0(T ) + R middot T

z middot Fln

(a(H2) middot radic

a(O2)

a(H2O)

)[1]

where E0(T) is the temperature dependent equilibrium potential22 z isthe number of electrons exchanged in the electrochemical reaction (2for hydrogen and 4 for oxygen) and a is the activity of the reactantsFor liquid water an activity of unity and for hydrogen and oxygenideal gas behavior is assumed23ndash25 Thus the thermodynamic cell volt-age decreases with temperature and increases with pressure eg at50C a voltage increase of +32 mV per decade of hydrogen pressure(differential) and +48 mV per decade of both hydrogen and oxygenpressure (balanced) is calculated For the EHC Equation 1 simplifiesto Equation 2

E0cellEHC(p T ) = R middot T

2 middot Fln

(a(H2cathode)

a(H2anode)

)[2]

The measured cell voltage Ecell (pT) ie the sum of thermody-namic cell voltage and all overpotentials increase between pressur-ized and ambient pressure operation Ecell normalized to the expectedthermodynamic cell voltage is considered as the electrochemical com-pression efficiency ηCEC according to Equation 3

ηCEC = E0cell(p T ) minus Ecell

E0cell(p T )

[3]

Since the polymer electrolytes suffer from gas crossover by thesolution-diffusion mechanism the faradaic efficiency is less thanunity According to Fickrsquos law the gas crossover can be expressedas a molar permeation flux where the driving force is given by thepartial pressure difference p for hydrogen and oxygen respectivelyas shown in Equation 4

(H2O2) = minusP(H2O2) middot p(H2O2)

δ[4]

here P is the gas permeability and δ is the wet thickness of the polymerelectrolyte during operation Both hydrogen and oxygen gas crossover

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1189

losses can be expressed as a gas crossover current density equivalentjx(H2O2) with the unit A middot mminus2 middot Paminus1 as formulated in Equation 5

jx(H2O2) = z middot F middot P(H2O2)

δ[5]

Oxygen crossover is important also for the faradaic efficiency ofthe hydrogen produced as permeated oxygen reacts chemically at thecathode electrocatalyst to form water consuming the stoichiometricamount of hydrogen The faradaic efficiency can be expressed withthe ratio of the de facto produced hydrogen gas to the theoreticallyproduced amount as given in Equation 6

ηF = j minus jx(H2) minus jx(O2)

j[6]

where j is the applied current density Additional losses due to leakageare vanishingly small and thus neglected20 Finally the electrochemicalcompression efficiency and the faradaic efficiency are combined to thetotal compression efficiency as given in Equation 7

ηCtotal = ηCEC middot ηF [7]

The total electrochemical compression efficiency is calculatedbased on the thermodynamic cell voltage corresponding to the lowerheating value (LHV) of hydrogen of 240 kJmol Because in most en-ergy applications only the hydrogen produced is of interest and oxygenproduced is considered as a by-product all given electrochemical com-pression efficiencies are calculated based on the thermodynamic cellvoltage increase due to hydrogen pressurization only (see Equation1) If oxygen would be of interest too the electrochemical compres-sion efficiencies for balanced pressure PEWE would be even higherNevertheless in all cases both oxygen and hydrogen gas crossover areconsidered for the faradaic efficiency in Equation 6

When comparing different PEWE operating parameters or mate-rials ultimately not only the compression work is of interest but alllosses Consequently the cell voltage is considered in the cell effi-ciency according to Equation 8

ηcellPEWE = E0cell(p T )

Ecell(p T )middot ηF = E0

cell(p T )

Ecell(ambientT )middot ηCtotal [8]

In this work the cell efficiency is normalized to Ecell = f(pH2T)

according to Equation 1 If oxygen would be of interest too the cellefficiencies for balanced pressure operations would be even higherbecause of an increase in the thermodynamic cell voltage with oxygenpressure

Results

Electrochemical compression losses in differential and balancedpressure PEWEmdashThermodynamics predict an isothermal compres-sion behavior for pressurized electrolysis and EHCs increasing thecell voltage (Equation 1) Experimental results for the cell voltageincreases due to pressurization are discussed in the form of the elec-trochemical compression efficiency (Equation 3)

In all experiments the high frequency resistance (HFR) is mea-sured at 25 kHz Only negligible changes are observed for the HFRwith pressure (plusmn001 Ohm middot cm2 at 2 A middot cmminus2)20 and therefore theohmic overpotential is considered as not relevant for the evaluationof pressurization losses and thus all data is iR-corrected In Figure2A iE-curves at 50C for ambient (01 MPa) and 1 MPa balancedand differential pressure operation are reported When the data is ana-lyzed for the pressure induced cell voltage increase for the differential1 MPa operation as expected a behavior close to isothermal com-pression is observed For balanced pressure operation however thecell voltage coincides only at low current densities (lt 05 A middot cmminus2)with the differential pressure curve at higher current densities (gt ca3 A middot cmminus2) it approaches the cell voltage values for ambient pressureoperation Consequently with oxygen compression beneficial pro-cesses take place reducing the apparent compression effort Theseeffects have been referred to improved oxygen evolution reaction

Figure 2 Effect of pressure strategy in PEWE on compression efficiency(A) iR-free iE-curves for 01 MPa (ambient) 1 MPa differential and balancedpressure PEWE at 50C (B) Absolute differences between the pressurizedand ambient pressure cell voltages and expected thermodynamic cell voltageincreases at 50C (C) ratio of cell voltage differences between pressurizedand ambient pressure operation (Ecell) and the theoretical thermodynamiccell voltage increase (E

cell(pH2T)) considering only hydrogen compres-sion ie 100 represents an isothermal compression behavior according toEquation 1

(OER) kinetics ie an increased apparent exchange current densitywith pressure2126

The different behavior of differential and balanced pressure on thepressure induced voltage increase is more closely analyzed by plottingthe cell voltage difference between the 10 and 01 MPa data versusthe current density in Figure 2B The theoretical cell voltage increaseat 50C is 32 mV per decade of hydrogen pressure and 48 mV perdecade of both hydrogen and oxygen pressure increase For the dif-ferential pressure operation an isothermal compression behavior ofabout 80 of the theoretical value is observed for current densitiesabove 1 A middot cmminus2 This means that the compression via differentialpressure mode is slightly less but close to ideal isothermal compres-sion In contrast for the balanced pressure operation the compressioninduced cell voltage increase drops with increasing current densityand at a current density of 36 A middot cmminus2 no voltage increase at all isobserved At this current density the improvements in OER kineticsfully compensate the pressure induced cell voltage increase expectedfor an ideal isothermal compression according to Equation 1

A detailed analysis of previously reported data in the literature forthe differential and balanced pressure strategies shows that the presentexperimental data are in good agreement with literature as summarizedin Table I Literature values are extracted from the published graphs

The observations of the relative compression losses from this studyand the above literature data are compared in Figure 2C where the

F1190 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Table I Literature overview of balanced and differential pressure PEWE compression behavior

Pressure mode Reference Operating pressure [MPa] Isothermal compression behavior []

differential Ayers et al27 14 76 and 166 asymp 100 plusmn 30Bernt and Gasteiger28 01 to 3 asymp 80ndash110Ishikawa et al12 01 35 and 70 35 MPa asymp 100

70 MPa asymp 150Suermann et al20 01 1 and 5 asymp 80ndash95

balanced Grigoriev et al26 01 and 25 ge07 A middot cmminus2 lt 0Grigoriev et al29 01 to 13 asymp 0Siemens30 01 1 and 10 ge07 A middot cmminus2 asymp minus20Suermann et al21 01 1 and 10 ge15 A middot cmminus2 asymp 0

Figure 3 Electrochemical compression (EC) efficiency for differential andbalanced pressure PEWE at 50C as a function of the operating pressure Dataare based on experimental results up to 1 MPa from Figure 2B Isothermalcompression is calculated according to Equation 1 No gas crossover lossesare considered here

cell voltage differences between pressurized and ambient pressure arenormalized to the corresponding thermodynamic cell voltage increase(again only hydrogen is of interest and oxygen is considered as a by-product) For differential pressure operation a compression behaviorclose to the isothermal values are observed up to 35 MPa The 70 MPadifferential pressure data from Ishikawa et al12 shows a significantlyhigher apparent compression effort of about 150 which might beexplained with an increased non-ideal gas behavior ie a hydrogencompressibility factor of approximately 14 (60C 70 MPa)31 Thebalanced pressure operation data at 10 and 10 MPa of Siemens30

as well as the results of Grigoriev et al2629 show a higher effect ofthe improved OER kinetics than this study with negative apparentcompression effort of up to minus20

Summarizing the results of this study are qualitatively in line withliterature up to 35 MPa differential pressure When extrapolating theseresults to higher pressures the compression work required is underes-timated for the differential pressure case In contrast for the balancedpressure our experimental results are less optimistic than literatureso when extrapolating our data to higher pressures the compressionlosses are rather overestimated Therefore the comparison betweendifferential and balanced pressure based on the present experimentaldata when extrapolating to higher pressures will produce conserva-tive estimates

The electrochemical compression efficiencies extrapolated up to100 MPa are shown in Figure 3 The losses for the differential pressureare based on 80 isothermal compression behavior and independentof the current density For balanced pressure however the electro-chemical compression efficiency increases with current density and

curves are shown for 1 2 and 3 A middot cmminus2 At 3 A middot cmminus2 an electro-chemical compression efficiency at 100 MPa of 99 is calculated

Electrochemical hydrogen compressormdashElectrochemical hydro-gen compressors (EHCs) concentrate andor compress hydrogen fromanode to cathode In this study two different scenarios are investigatedi) compression of pure hydrogen and ii) extraction and compressionof hydrogen out of the natural gas grid For both scenarios hydrogenis compressed from 01 to 02 MPa absolute pressure for case i) purehydrogen is supplied and in case ii) a gas mixture with 5 H2 in N2in order to simulate the dilution in the natural gas grid where the 5are considered an upper limit with respect to todayrsquos regulations32

Thus compression ratios of 2 and 40 are realized in scenarios i) andii) respectively

The two EHC scenarios are compared in Figure 4A When extrap-olating the cell voltage from the stoichiometric range ie above 05A middot cmminus2 for case i) and above 01 A middot cmminus2 for case ii) to equilib-rium conditions offsets of about 50 plusmn 14 mV for case i) and 586plusmn 05 mV for case ii) can be extracted which are close to the calcu-lated theoretical values (10 and 53 mV based on Equation 2 withoutconsidering any water vapor and hydrogen consumption along thechannel) Hence for hydrogen concentration and compression usingEHC an isothermal compression behavior close to the theoreticalvalues is observed similar to the results described for differentialpressure PEWE (see Figure 3)

The slopes of the iE-curves for cases i) and ii) in Figure 4A aresignificantly different In the absence of kinetic overpotential contri-butions due to the highly reversible hydrogen oxidation and evolutionreactions (HOR and HER)33 the slope represents the area specific re-sistance For the case of pure hydrogen a value of 95 m middot cm2 and forthe diluted hydrogen 149 m middot cm2 are observed The pure hydrogencase is in good agreement with literature results of Shi et al34 whoreported a through-plane area specific resistance for the same type ofmembrane with a thickness of about 33 μm in the order of 80ndash90m middot cm2 (70C rh = 80 based on ex-situ HFR measurements) Thedeviation observed for case ii) may originate from the uneven currentdensity distribution between the anode inlet to outlet as outlined inFigure 4B The uneven current density distribution is attributed to aninsufficient membrane humidification especially at the anode outletThis hypothesis was underlined with additional experiments injectingliquid water into the cathode (rhc ge 100) resulting in temporarilydecreased area specific resistances in the order of 90 to 100 m middot cm2similar to case i) and in more homogenous current density distribu-tions as shown in Figure 4B However having partly liquid water inthe cathode no steady-state conditions were obtained with the cur-rent setup used A further possibility is the appearance of transportlosses at the anode outlet Here two-thirds of the diluted hydrogen hasbeen consumed and the hydrogen content in nitrogen drops to 17eventually giving rise to anodic transport overpotential

For EHCs in technical applications the humidification concept isa key challenge in particular when hydrogen is concentrated andorcompressed out of a dry gas mixture eg out of a natural gas pipelineThe water management in EHCs which is strongly influenced by theelectro-osmotic water drag from anode to cathode is also reflected in

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1191

Figure 4 (A) EHC iE-curves at 60C using Nafion XL-100 membrane for compressing hydrogen from 01 to 02MPa (cell pressures) for case i) pure hydrogen (red circles)and case ii) 5 hydrogen in nitrogen (blue squares) therelative humidity of all gases is 80 lines show linear fits ofthe cell voltage above 05 A middot cmminus2 for case i) and above 01A middot cmminus2 for case ii) extrapolated to equilibrium conditionsat 0 A middot cmminus2 Linear fit and y-axis intercept for case i) are95 m middot cm2 and 5 mV for case ii) 149 m middot cm2 and 59 mV(B) EHC local current density distribution at 60C at meancurrent densities indicated The rhc = 80 data correspondsto the 5 H2 in N2 iE-curve shown in (A) For the counter-flow arrangement the cathode inlet is at length 0 and theanode inlet at length 100

the work of Grigoriev et al35 using less diluted hydrogen in nitrogengas mixtures The authors observed a turning point in the EHC iE datawith diluted anode gas at relatively low current densities of only 02 to03 A middot cmminus2 which was attributed to an insufficient humidification ofthe Nafion 117 membranes used35 Nevertheless compression ratiosup to 130 were realized confirming the largely isothermal compressionbehavior of the EHC35 In summary the experimental findings confirman isothermal compression behavior of EHCs which is also in linewith earlier literature data133637 From the energetic point of viewthe compression behavior of EHCs and differential pressure PEWEare similar therefore for energy considerations only differential andbalanced pressure PEWE are compared below However from thesafety point of view EHCrsquos may have advantages at high pressuresas no pure oxygen is present in the vicinity of high pressure hydrogenAlso from the application point of view EHCrsquos may be used whenthe hydrogen is stored at an intermediate pressure andor transportedfrom the electrolyzer to its final destination (Figure 1)

Faradaic losses due to gas crossovermdashIn addition to the abovediscussed electrochemical compression losses faradaic losses occurdue to gas crossover through the polymer electrolyte Assuming thatpressure dependence of the gas permeability of Nafion membranescan be described by a pure solution-diffusion process as experimen-tally demonstrated up to 22 MPa differential pressure by Barbir38 datafrom Schalenbach et al39 are used which have been validated for 10MPa balanced pressure earlier20 Recent research has questioned thisassumption for electrolyzers it was postulated that the current indi-rectly influences the gas crossover especially for conditions close toambient pressure due to increased partial pressures in the catalystlayers with increasing current density40 The fact that oxygen perme-ability is about half that of hydrogen has consistently also measuredby Sakai et al41

Considering that the thickness of water swollen membranes duringoperation is larger than their nominal thickness (209 152 and 63 μmfor the nominally dry 175 125 and 50 μm ie in the often used impe-rial units the 7 5 and 2 mil membranes)39 the gas crossover currentdensity equivalent jx(H2O2) is calculated according to Equation 5and given in Figure 5A Its apparent linear behavior with temperatureis visualized using linear fits allowing for interpolating for differenttemperatures Even though the permeability of oxygen is only abouthalf that of hydrogen the corresponding gas crossover currents aresimilar to hydrogen because the current equivalent of oxygen is twicethat of hydrogen when permeated oxygen reacts at the cathodic cata-lyst to form water Unreinforced thin membranes are discussed hereto illustrate the influence of membrane thickness on gas permeationand overall cell efficiency For long term field applications howevermore advanced materials will be needed1742

From the gas crossover data the faradaic efficiency is calculatedfor different membrane thicknesses and current densities (Equation 6)as a function of the operating pressure and given in Figure 5B for thedifferential pressure at 50C For EHCs similar gas crossover lossesare obtained but for balanced pressure operation the gas crossoverlosses are roughly twice as large Furthermore estimates for (hypo-thetic) new membrane materials with different fractions of the Nafiongas permeability (05 02 and 01) are added For the 110 Nafion 112membrane the faradaic efficiency at 100 MPa increases to above 96from less than 70 for the standard material (Figure 5B) Improve-ments in the membrane permeability seem to be possible by reinforc-ing the membrane27 or by replacing the perfluoroalkylsulfonic acidbased membranes with different chemistry eg hydrocarbons17

Total hydrogen compression efficiencymdashIn order to predict thetotal hydrogen compression efficiency both electrochemical com-pression and faradaic efficiency need to be combined as given inEquation 7 As discussed above because only hydrogen is of interestas a product both pressure strategies are compared to the thermody-namic cell voltage increase of +32 mV per decade of hydrogen pres-sure (at 50C) However both hydrogen and oxygen (recombining atthe cathode to water) crossover losses are included in the calculations

The results for Nafion 117 for differential and balanced pressure forthe electrochemical compression efficiency (Figure 3) and the corre-sponding faradaic efficiency (Figure 5B) are combined and the resultsare given in Figure 6 With respect to the resulting total compressionefficiency several general statements can be formulated

At low operating pressures the total compression efficiency isdominated by isothermal compression behavior (linear curve shape inthe semi-logarithmic plots)

With increasing operating pressure the losses due to gascrossover become more dominant and curve shapes change from linearto exponential

Differential pressure PEWE has better total compression ef-ficiency at low current densities (up to about 1 A middot cmminus2) becauseof similar electrochemical compression behavior but lower oxygenpermeation losses compared to balanced operation

Balanced pressure strategy has lower apparent compressionlosses at high current densities (here 3 A middot cmminus2) up to about 10 MPausing Nafion 112 or up to 50 MPa using Nafion 117 because theoxygen crossover losses are offset by the OER kinetic gains

From an energetic point of view electrochemical compressionis more efficient especially at the high current density up to 10 MPafor Nafion 112 or 50 MPa using Nafion 117 than the (probably) bestmechanical 5-stage ionic liquid hydrogen compressors11 (having aloss factor of about 14 times the thermodynamic requirement)

F1192 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 5 (A) Hydrogen and oxygen gas crossover current density equivalents for wet 7 5 and 2 mil thick Nafion membranes (actual thickness 209 152 and63 μm) Linear fits are added for both hydrogen (solid lines) and oxygen (dashed lines) Data is calculated based on the experimental results of Schalenbachet al39 (B) The corresponding faradaic efficiency is calculated for differential pressure PEWE using wet Nafion 117 or Nafion 112 at 1 and 3 A middot cmminus2 for 50Cas a function of the operating pressure The data is based on Figure 5A Hypothetic new membranes with the thickness of Nafion 112 and different fractions of thepermeability (05 02 01) are added for discussion

Hypothetic new membranes with a permeability of only onetenth of Nafion dramatically reduce the gas crossover losses enhanc-ing compression efficiency with such a membrane material 93 com-pression efficiency would be obtained for a 2-mil membrane at 100MPa (3 A middot cmminus2 and balanced pressure) This is similar to isothermalcompression behavior and would outperform todayrsquos best mechanicalcompressors

Considering the total cell efficiency membrane thickness does notonly influence the gas crossover losses but also the correspondingohmic overpotential Thus a tradeoff between both losses has to befound for the optimal cell efficiency as discussed below

PEWE cell efficiencymdashThe main focus is on the hydrogen com-pression efficiency However when discussing the gas permeabilityproperties of different membrane types or thicknesses it should also

be paid attention on the cell efficiency considering the faradaic losseselectrochemical compression losses and all other overpotentials in-cluding the (ionic) ohmic resistances of the membrane Hence de-pending on the desired hydrogen pressure a tradeoff between themembrane induced ohmic overpotential and gas crossover needs tobe found Therefore the total compression efficiency data in Figure 6are combined with measured PEWE cell performance (according toEquation 8) using Nafion 112 and 117 membranes at 50C and ambi-ent pressure and results shown in Figure 7 The given cell efficienciesare based on the LHV of hydrogen The corresponding cell voltages at1 and 3 A middot cmminus2 are 185 and 211 V for Nafion 112 and 197 and 250V for Nafion 117 respectively Similar iE-curves at 1 MPa balancedpressure are given in Ref 21

The observations concerning the hydrogen compression efficiency(Figure 6) are still valid thus only few new insights are deduced

Figure 6 Total compression efficiencies for differential and balanced pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50C for 1 and 3 A middot cmminus2

as a function of the operating pressure Hypothetic new membranes with 110 of the Nafion permeability are added for comparison the isothermal compressionbehavior at 50C and best mechanical compression technology11 are also shown The PEWE data is based on Figures 3 and 5B

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1193

Figure 7 Cell efficiency (including all electrochemical andfaradaic losses) according to Equation 8 normalized to E0

cell= f(pH2T) or LHV of hydrogen for differential and bal-anced pressure PEWE using Nafion 112 and Nafion 117membranes at 50C as a function of the operating pressureat (A) 1 A middot cmminus2 and (B) 3 A middot cmminus2

Cell efficiencies are almost constant up to 1 MPa at 1 A middot cmminus2

and up to 10 MPa at 3 A middot cmminus2 Further pressure increase reduces cellefficiencies but less at high current densities (here 3 A middot cmminus2) due tothe smaller influence of gas crossover and lower apparent compressioneffort for balanced pressure operation

At low current densities (here 1 A middot cmminus2) Nafion 112 has bettercell efficiencies with about 62 up to several MPa and about 60 athigh current densities up to about 10 MPa compared to Nafion 117with about 60 and 50 respectively However at higher pressuresthe better gas barrier properties of Nafion 117 offset the higher ohmicoverpotential

Nevertheless from an energetic point of view the data show thatlow gas crossover membrane materials will be key for efficient electro-chemical compression For ultimate efficiency these materials wouldbest be combined with the beneficial processes of the balanced pres-sure strategy where OER kinetic gains are (partly) compensating forthe thermodynamic cell voltage increase due to pressurization How-ever high-pressure oxygen (typically above 3 MPa) is a severe safetyconcern and difficult to implement in a technical process Hence itseems sensible to limit oxygen pressure This will also be beneficialfor the oxygen crossover losses be it with conventional or future lowgas permeation materials For this novel pressure strategy we use theterm ldquointermediate differentialrdquo pressure PEWE Its characteristicswith respect to hydrogen compression efficiency are discussed below

Intermediate differential pressure PEWEmdashWith the OER kinet-ics following a logarithmic and the gas crossover following a linearpressure relation there must be an optimum operating point mini-mizing the total compression losses by choosing a slightly increasedanode and a considerably increased cathode pressure Experimentshave therefore been performed to better understand this novel pres-sure concept where the anode is pressurized less than the cathodeIn Figure 8A data with low anode pressures (1 and 2 MPa) and highcathode pressures (3 and 5 MPa) are shown A relatively small anodepressure of 1 MPa is sufficient to retain the voltage gains due to in-creased OER kinetics as no further decrease in cell voltage is observedwith the oxygen pressure increase to 2 MPa as shown in Figure 8B

The experimental (1 5 MPa) data are extrapolated to higher pres-sures with an isothermal compression behavior of 84 at 1 A middot cmminus2

and 24 at 3 A middot cmminus2 and compared to those from differential andbalanced pressure in Figure 9 The extrapolated data show that athigher current densities the intermediate differential pressure conceptis energetically more efficient than differential pressure operation dueto a higher electrochemical compression efficiency which is overcom-pensating the relatively small additional oxygen gas crossover lossesIn particular at higher pressures the limited oxygen gas crossover

losses for intermediate differential pressure are crucial for being moreenergetically efficient than all other pressure strategies and even betterthan mechanical compression up to almost 100 MPa for Nafion 117

Those experimental findings are rather counter-intuitive becauseeven if oxygen is considered as a by-product and only pressurizedhydrogen is of interest a 1 MPa compression of oxygen is energeti-cally beneficial Furthermore operating and system investment costs(opex and capex) should be lower for intermediate differential than forbalanced pressure operation due to lower safety requirements of thelow pressurized oxygen and thus be similar to conventional differen-tial pressure operation where the oxygen pressure is close to ambientpressure

Conclusions

In this study the cell voltage and gas crossover related losses dueto electrochemical pressurization were investigated experimentally upto 5 MPa and based on literature study up to 70 MPa for differentialand balanced pressure PEWE and EHCs As expected from ther-modynamics differential pressure electrolysis and EHCs generallyfollow an isothermal compression behavior In contrast the apparentcompression effort for balanced pressure electrolysis decreases withincreasing current density due to improved OER kinetics Part of thisenergetic advantage is offset by gas crossover losses roughly twicethose of differential pressure operation

The energetic analysis of the total hydrogen compression effi-ciency combining electrochemical and faradaic losses up to 100MPa shows that differential pressure electrolysis is preferable to bal-anced pressure at low current densities (le 1 A middot cmminus2) due to lowergas crossover losses and similar compression behavior However athigher current densities the decreasing apparent compression effort ofthe balanced pressure PEWE compensates for the higher gas crossoverlosses resulting in higher total compression efficiencies

Intermediate differential pressure electrolysis with a slightly pres-surized oxygen side and a higher pressure at the hydrogen side com-bines the advantages of both pressure concepts retaining the low oxy-gen gas crossover losses of the differential pressure and the decreasingcompression effort at higher current densities of the balanced pres-sure operation Based on the extrapolation of experimental data theintermediate differential pressure strategy with cells having a mem-brane electrolyte with the gas permeation characteristics of Nafion117 energetically outperform todayrsquos best mechanical compressorsup to tens of MPa with similar compression losses at 100 MPa

With respect to industrially relevant application the intermedi-ate differential pressure strategy respects the challenges and costsof compressed oxygen which will be limited to close to those forconventional differential pressure PEWE with oxygen production at

F1194 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 8 (A) intermediate differential pressure PEWE iR-free iE-curves at 50C at indicated pressure levels (B) ra-tio of the measured iR-free cell voltage difference betweenpressurized and ambient pressure operation (E(iR-free))and the theoretical thermodynamic cell voltage increase(E0

cell(pH2T)) considering only hydrogen compressionie 100 represents an isothermal compression behavioraccording to Equation 1

Figure 9 Total compression efficiencies for differential balanced and intermediate differential pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50Cfor 1 and 3 A middot cmminus2 as a function of the operating pressure For comparison the isothermal compression behavior at 50C and best mechanical compressiontechnology11 are added The PEWE data is based on Figures 3 5B and 8

atmospheric pressure If the technical risk to have highest pressurehydrogen in the electrolyzer in close vicinity to pure oxygen is consid-ered too high then a strategy with a combination of intermediate dif-ferential PEWE up to intermediate pressures of about 10 MPa and finalcompression to highest pressures (100 MPa) by EHCs seems a promis-ing concept with respect to both energetic and safety properties

Acknowledgments

Funding by the Swiss Federal Office of Energy (SFOE under grantno SI500904-01) Belenos Clean Power Holding Ltd and the En-ergy System Integration (ESI) platform at PSI as well as technicalsupport by Martin Ammann and Thomas Gloor (both PSI) are grate-fully acknowledged TJS thanks the Commission for Technology andInnovation Switzerland and the Swiss Competence Center for EnergyResearch Heat amp Electricity Storage

References

1 httpeceuropaeueurostatwebenergydatadatabase (accessed on 31012017)2 Erneuerbare Energien in Zahlen - Nationale und internationale Entwicklung im Jahr

2015 in Bundesministerium fur Wirtschaft und Energie (BMWi) (2016)3 C J Barnhart M Dale A R Brandt and S M Benson Energy amp Environmental

Science 6 2804 (2013)

4 Monitoringbericht 2016 in G Bundesnetzagentur fur Elektrizitat Telekommu-nikation Post und Eisenbahnen Bundeskartellamt Editor Bundesnetzagentur furElektrizitat GasTelekommunikation Post und Eisenbahnen Bundeskartellamt(2016)

5 T Smolinka E T Ojong and J Garche in Electrochemical Energy Storage forRenewable Sources and Grid Balancing P T Moseley and J Garche Editors p 103(2015)

6 httpsenergygoveerefuelcellsdoe-technical-targets-hydrogen-production-electro-lysis (accessed on 10032017)

7 J Alkire Technology Validation Program Area - Plenary Presentation U S Depart-ment of Energy 2017 Annual Merit Review and Peer Evaluation Meeting (2017)

8 httpwwwenergiepark-mainzde (accessed on 10032017)9 M Kopp D Coleman C Stiller K Scheffer J Aichinger and B Scheppat Inter-

national Journal of Hydrogen Energy 42(19) (2017)10 S Metz Fuel Cells Bulletin 2014 12 (2014)11 M Stefan in 20th World Hydrogen Energy Conference (WHEC) KDJ Convention

Center Gwangju South Korea (2014)12 H Ishikawa E Haryu N Kawasaki and H Daimon Honda RampD Technical Review

28 (2016)13 P J Bouwman J Konink D Semerel L Raymakers M Koeman W Dalhuijsen

E Milacic and M Mulder ECS Transactions 64 1009 (2014)14 D G Bessarabov H Wang H Li and N Zhao PEM Electrolysis for Hydrogen

Production Principles and Applications CRC Press Boca Raton FL (USA) (2015)15 L Bertuccioli A Chan D Hart F Lehner B Madden and E Standen Study on

Development of Water Electrolysis in the EU by E4tech Slsquoarl with Element EnergyLtd for the Fuel Cells and Hydrogen Joint Undertaking in (2014)

16 M Carmo D L Fritz J Mergel and D Stolten International Journal of HydrogenEnergy 38 4901 (2013)

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1189

losses can be expressed as a gas crossover current density equivalentjx(H2O2) with the unit A middot mminus2 middot Paminus1 as formulated in Equation 5

jx(H2O2) = z middot F middot P(H2O2)

δ[5]

Oxygen crossover is important also for the faradaic efficiency ofthe hydrogen produced as permeated oxygen reacts chemically at thecathode electrocatalyst to form water consuming the stoichiometricamount of hydrogen The faradaic efficiency can be expressed withthe ratio of the de facto produced hydrogen gas to the theoreticallyproduced amount as given in Equation 6

ηF = j minus jx(H2) minus jx(O2)

j[6]

where j is the applied current density Additional losses due to leakageare vanishingly small and thus neglected20 Finally the electrochemicalcompression efficiency and the faradaic efficiency are combined to thetotal compression efficiency as given in Equation 7

ηCtotal = ηCEC middot ηF [7]

The total electrochemical compression efficiency is calculatedbased on the thermodynamic cell voltage corresponding to the lowerheating value (LHV) of hydrogen of 240 kJmol Because in most en-ergy applications only the hydrogen produced is of interest and oxygenproduced is considered as a by-product all given electrochemical com-pression efficiencies are calculated based on the thermodynamic cellvoltage increase due to hydrogen pressurization only (see Equation1) If oxygen would be of interest too the electrochemical compres-sion efficiencies for balanced pressure PEWE would be even higherNevertheless in all cases both oxygen and hydrogen gas crossover areconsidered for the faradaic efficiency in Equation 6

When comparing different PEWE operating parameters or mate-rials ultimately not only the compression work is of interest but alllosses Consequently the cell voltage is considered in the cell effi-ciency according to Equation 8

ηcellPEWE = E0cell(p T )

Ecell(p T )middot ηF = E0

cell(p T )

Ecell(ambientT )middot ηCtotal [8]

In this work the cell efficiency is normalized to Ecell = f(pH2T)

according to Equation 1 If oxygen would be of interest too the cellefficiencies for balanced pressure operations would be even higherbecause of an increase in the thermodynamic cell voltage with oxygenpressure

Results

Electrochemical compression losses in differential and balancedpressure PEWEmdashThermodynamics predict an isothermal compres-sion behavior for pressurized electrolysis and EHCs increasing thecell voltage (Equation 1) Experimental results for the cell voltageincreases due to pressurization are discussed in the form of the elec-trochemical compression efficiency (Equation 3)

In all experiments the high frequency resistance (HFR) is mea-sured at 25 kHz Only negligible changes are observed for the HFRwith pressure (plusmn001 Ohm middot cm2 at 2 A middot cmminus2)20 and therefore theohmic overpotential is considered as not relevant for the evaluationof pressurization losses and thus all data is iR-corrected In Figure2A iE-curves at 50C for ambient (01 MPa) and 1 MPa balancedand differential pressure operation are reported When the data is ana-lyzed for the pressure induced cell voltage increase for the differential1 MPa operation as expected a behavior close to isothermal com-pression is observed For balanced pressure operation however thecell voltage coincides only at low current densities (lt 05 A middot cmminus2)with the differential pressure curve at higher current densities (gt ca3 A middot cmminus2) it approaches the cell voltage values for ambient pressureoperation Consequently with oxygen compression beneficial pro-cesses take place reducing the apparent compression effort Theseeffects have been referred to improved oxygen evolution reaction

Figure 2 Effect of pressure strategy in PEWE on compression efficiency(A) iR-free iE-curves for 01 MPa (ambient) 1 MPa differential and balancedpressure PEWE at 50C (B) Absolute differences between the pressurizedand ambient pressure cell voltages and expected thermodynamic cell voltageincreases at 50C (C) ratio of cell voltage differences between pressurizedand ambient pressure operation (Ecell) and the theoretical thermodynamiccell voltage increase (E

cell(pH2T)) considering only hydrogen compres-sion ie 100 represents an isothermal compression behavior according toEquation 1

(OER) kinetics ie an increased apparent exchange current densitywith pressure2126

The different behavior of differential and balanced pressure on thepressure induced voltage increase is more closely analyzed by plottingthe cell voltage difference between the 10 and 01 MPa data versusthe current density in Figure 2B The theoretical cell voltage increaseat 50C is 32 mV per decade of hydrogen pressure and 48 mV perdecade of both hydrogen and oxygen pressure increase For the dif-ferential pressure operation an isothermal compression behavior ofabout 80 of the theoretical value is observed for current densitiesabove 1 A middot cmminus2 This means that the compression via differentialpressure mode is slightly less but close to ideal isothermal compres-sion In contrast for the balanced pressure operation the compressioninduced cell voltage increase drops with increasing current densityand at a current density of 36 A middot cmminus2 no voltage increase at all isobserved At this current density the improvements in OER kineticsfully compensate the pressure induced cell voltage increase expectedfor an ideal isothermal compression according to Equation 1

A detailed analysis of previously reported data in the literature forthe differential and balanced pressure strategies shows that the presentexperimental data are in good agreement with literature as summarizedin Table I Literature values are extracted from the published graphs

The observations of the relative compression losses from this studyand the above literature data are compared in Figure 2C where the

F1190 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Table I Literature overview of balanced and differential pressure PEWE compression behavior

Pressure mode Reference Operating pressure [MPa] Isothermal compression behavior []

differential Ayers et al27 14 76 and 166 asymp 100 plusmn 30Bernt and Gasteiger28 01 to 3 asymp 80ndash110Ishikawa et al12 01 35 and 70 35 MPa asymp 100

70 MPa asymp 150Suermann et al20 01 1 and 5 asymp 80ndash95

balanced Grigoriev et al26 01 and 25 ge07 A middot cmminus2 lt 0Grigoriev et al29 01 to 13 asymp 0Siemens30 01 1 and 10 ge07 A middot cmminus2 asymp minus20Suermann et al21 01 1 and 10 ge15 A middot cmminus2 asymp 0

Figure 3 Electrochemical compression (EC) efficiency for differential andbalanced pressure PEWE at 50C as a function of the operating pressure Dataare based on experimental results up to 1 MPa from Figure 2B Isothermalcompression is calculated according to Equation 1 No gas crossover lossesare considered here

cell voltage differences between pressurized and ambient pressure arenormalized to the corresponding thermodynamic cell voltage increase(again only hydrogen is of interest and oxygen is considered as a by-product) For differential pressure operation a compression behaviorclose to the isothermal values are observed up to 35 MPa The 70 MPadifferential pressure data from Ishikawa et al12 shows a significantlyhigher apparent compression effort of about 150 which might beexplained with an increased non-ideal gas behavior ie a hydrogencompressibility factor of approximately 14 (60C 70 MPa)31 Thebalanced pressure operation data at 10 and 10 MPa of Siemens30

as well as the results of Grigoriev et al2629 show a higher effect ofthe improved OER kinetics than this study with negative apparentcompression effort of up to minus20

Summarizing the results of this study are qualitatively in line withliterature up to 35 MPa differential pressure When extrapolating theseresults to higher pressures the compression work required is underes-timated for the differential pressure case In contrast for the balancedpressure our experimental results are less optimistic than literatureso when extrapolating our data to higher pressures the compressionlosses are rather overestimated Therefore the comparison betweendifferential and balanced pressure based on the present experimentaldata when extrapolating to higher pressures will produce conserva-tive estimates

The electrochemical compression efficiencies extrapolated up to100 MPa are shown in Figure 3 The losses for the differential pressureare based on 80 isothermal compression behavior and independentof the current density For balanced pressure however the electro-chemical compression efficiency increases with current density and

curves are shown for 1 2 and 3 A middot cmminus2 At 3 A middot cmminus2 an electro-chemical compression efficiency at 100 MPa of 99 is calculated

Electrochemical hydrogen compressormdashElectrochemical hydro-gen compressors (EHCs) concentrate andor compress hydrogen fromanode to cathode In this study two different scenarios are investigatedi) compression of pure hydrogen and ii) extraction and compressionof hydrogen out of the natural gas grid For both scenarios hydrogenis compressed from 01 to 02 MPa absolute pressure for case i) purehydrogen is supplied and in case ii) a gas mixture with 5 H2 in N2in order to simulate the dilution in the natural gas grid where the 5are considered an upper limit with respect to todayrsquos regulations32

Thus compression ratios of 2 and 40 are realized in scenarios i) andii) respectively

The two EHC scenarios are compared in Figure 4A When extrap-olating the cell voltage from the stoichiometric range ie above 05A middot cmminus2 for case i) and above 01 A middot cmminus2 for case ii) to equilib-rium conditions offsets of about 50 plusmn 14 mV for case i) and 586plusmn 05 mV for case ii) can be extracted which are close to the calcu-lated theoretical values (10 and 53 mV based on Equation 2 withoutconsidering any water vapor and hydrogen consumption along thechannel) Hence for hydrogen concentration and compression usingEHC an isothermal compression behavior close to the theoreticalvalues is observed similar to the results described for differentialpressure PEWE (see Figure 3)

The slopes of the iE-curves for cases i) and ii) in Figure 4A aresignificantly different In the absence of kinetic overpotential contri-butions due to the highly reversible hydrogen oxidation and evolutionreactions (HOR and HER)33 the slope represents the area specific re-sistance For the case of pure hydrogen a value of 95 m middot cm2 and forthe diluted hydrogen 149 m middot cm2 are observed The pure hydrogencase is in good agreement with literature results of Shi et al34 whoreported a through-plane area specific resistance for the same type ofmembrane with a thickness of about 33 μm in the order of 80ndash90m middot cm2 (70C rh = 80 based on ex-situ HFR measurements) Thedeviation observed for case ii) may originate from the uneven currentdensity distribution between the anode inlet to outlet as outlined inFigure 4B The uneven current density distribution is attributed to aninsufficient membrane humidification especially at the anode outletThis hypothesis was underlined with additional experiments injectingliquid water into the cathode (rhc ge 100) resulting in temporarilydecreased area specific resistances in the order of 90 to 100 m middot cm2similar to case i) and in more homogenous current density distribu-tions as shown in Figure 4B However having partly liquid water inthe cathode no steady-state conditions were obtained with the cur-rent setup used A further possibility is the appearance of transportlosses at the anode outlet Here two-thirds of the diluted hydrogen hasbeen consumed and the hydrogen content in nitrogen drops to 17eventually giving rise to anodic transport overpotential

For EHCs in technical applications the humidification concept isa key challenge in particular when hydrogen is concentrated andorcompressed out of a dry gas mixture eg out of a natural gas pipelineThe water management in EHCs which is strongly influenced by theelectro-osmotic water drag from anode to cathode is also reflected in

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1191

Figure 4 (A) EHC iE-curves at 60C using Nafion XL-100 membrane for compressing hydrogen from 01 to 02MPa (cell pressures) for case i) pure hydrogen (red circles)and case ii) 5 hydrogen in nitrogen (blue squares) therelative humidity of all gases is 80 lines show linear fits ofthe cell voltage above 05 A middot cmminus2 for case i) and above 01A middot cmminus2 for case ii) extrapolated to equilibrium conditionsat 0 A middot cmminus2 Linear fit and y-axis intercept for case i) are95 m middot cm2 and 5 mV for case ii) 149 m middot cm2 and 59 mV(B) EHC local current density distribution at 60C at meancurrent densities indicated The rhc = 80 data correspondsto the 5 H2 in N2 iE-curve shown in (A) For the counter-flow arrangement the cathode inlet is at length 0 and theanode inlet at length 100

the work of Grigoriev et al35 using less diluted hydrogen in nitrogengas mixtures The authors observed a turning point in the EHC iE datawith diluted anode gas at relatively low current densities of only 02 to03 A middot cmminus2 which was attributed to an insufficient humidification ofthe Nafion 117 membranes used35 Nevertheless compression ratiosup to 130 were realized confirming the largely isothermal compressionbehavior of the EHC35 In summary the experimental findings confirman isothermal compression behavior of EHCs which is also in linewith earlier literature data133637 From the energetic point of viewthe compression behavior of EHCs and differential pressure PEWEare similar therefore for energy considerations only differential andbalanced pressure PEWE are compared below However from thesafety point of view EHCrsquos may have advantages at high pressuresas no pure oxygen is present in the vicinity of high pressure hydrogenAlso from the application point of view EHCrsquos may be used whenthe hydrogen is stored at an intermediate pressure andor transportedfrom the electrolyzer to its final destination (Figure 1)

Faradaic losses due to gas crossovermdashIn addition to the abovediscussed electrochemical compression losses faradaic losses occurdue to gas crossover through the polymer electrolyte Assuming thatpressure dependence of the gas permeability of Nafion membranescan be described by a pure solution-diffusion process as experimen-tally demonstrated up to 22 MPa differential pressure by Barbir38 datafrom Schalenbach et al39 are used which have been validated for 10MPa balanced pressure earlier20 Recent research has questioned thisassumption for electrolyzers it was postulated that the current indi-rectly influences the gas crossover especially for conditions close toambient pressure due to increased partial pressures in the catalystlayers with increasing current density40 The fact that oxygen perme-ability is about half that of hydrogen has consistently also measuredby Sakai et al41

Considering that the thickness of water swollen membranes duringoperation is larger than their nominal thickness (209 152 and 63 μmfor the nominally dry 175 125 and 50 μm ie in the often used impe-rial units the 7 5 and 2 mil membranes)39 the gas crossover currentdensity equivalent jx(H2O2) is calculated according to Equation 5and given in Figure 5A Its apparent linear behavior with temperatureis visualized using linear fits allowing for interpolating for differenttemperatures Even though the permeability of oxygen is only abouthalf that of hydrogen the corresponding gas crossover currents aresimilar to hydrogen because the current equivalent of oxygen is twicethat of hydrogen when permeated oxygen reacts at the cathodic cata-lyst to form water Unreinforced thin membranes are discussed hereto illustrate the influence of membrane thickness on gas permeationand overall cell efficiency For long term field applications howevermore advanced materials will be needed1742

From the gas crossover data the faradaic efficiency is calculatedfor different membrane thicknesses and current densities (Equation 6)as a function of the operating pressure and given in Figure 5B for thedifferential pressure at 50C For EHCs similar gas crossover lossesare obtained but for balanced pressure operation the gas crossoverlosses are roughly twice as large Furthermore estimates for (hypo-thetic) new membrane materials with different fractions of the Nafiongas permeability (05 02 and 01) are added For the 110 Nafion 112membrane the faradaic efficiency at 100 MPa increases to above 96from less than 70 for the standard material (Figure 5B) Improve-ments in the membrane permeability seem to be possible by reinforc-ing the membrane27 or by replacing the perfluoroalkylsulfonic acidbased membranes with different chemistry eg hydrocarbons17

Total hydrogen compression efficiencymdashIn order to predict thetotal hydrogen compression efficiency both electrochemical com-pression and faradaic efficiency need to be combined as given inEquation 7 As discussed above because only hydrogen is of interestas a product both pressure strategies are compared to the thermody-namic cell voltage increase of +32 mV per decade of hydrogen pres-sure (at 50C) However both hydrogen and oxygen (recombining atthe cathode to water) crossover losses are included in the calculations

The results for Nafion 117 for differential and balanced pressure forthe electrochemical compression efficiency (Figure 3) and the corre-sponding faradaic efficiency (Figure 5B) are combined and the resultsare given in Figure 6 With respect to the resulting total compressionefficiency several general statements can be formulated

At low operating pressures the total compression efficiency isdominated by isothermal compression behavior (linear curve shape inthe semi-logarithmic plots)

With increasing operating pressure the losses due to gascrossover become more dominant and curve shapes change from linearto exponential

Differential pressure PEWE has better total compression ef-ficiency at low current densities (up to about 1 A middot cmminus2) becauseof similar electrochemical compression behavior but lower oxygenpermeation losses compared to balanced operation

Balanced pressure strategy has lower apparent compressionlosses at high current densities (here 3 A middot cmminus2) up to about 10 MPausing Nafion 112 or up to 50 MPa using Nafion 117 because theoxygen crossover losses are offset by the OER kinetic gains

From an energetic point of view electrochemical compressionis more efficient especially at the high current density up to 10 MPafor Nafion 112 or 50 MPa using Nafion 117 than the (probably) bestmechanical 5-stage ionic liquid hydrogen compressors11 (having aloss factor of about 14 times the thermodynamic requirement)

F1192 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 5 (A) Hydrogen and oxygen gas crossover current density equivalents for wet 7 5 and 2 mil thick Nafion membranes (actual thickness 209 152 and63 μm) Linear fits are added for both hydrogen (solid lines) and oxygen (dashed lines) Data is calculated based on the experimental results of Schalenbachet al39 (B) The corresponding faradaic efficiency is calculated for differential pressure PEWE using wet Nafion 117 or Nafion 112 at 1 and 3 A middot cmminus2 for 50Cas a function of the operating pressure The data is based on Figure 5A Hypothetic new membranes with the thickness of Nafion 112 and different fractions of thepermeability (05 02 01) are added for discussion

Hypothetic new membranes with a permeability of only onetenth of Nafion dramatically reduce the gas crossover losses enhanc-ing compression efficiency with such a membrane material 93 com-pression efficiency would be obtained for a 2-mil membrane at 100MPa (3 A middot cmminus2 and balanced pressure) This is similar to isothermalcompression behavior and would outperform todayrsquos best mechanicalcompressors

Considering the total cell efficiency membrane thickness does notonly influence the gas crossover losses but also the correspondingohmic overpotential Thus a tradeoff between both losses has to befound for the optimal cell efficiency as discussed below

PEWE cell efficiencymdashThe main focus is on the hydrogen com-pression efficiency However when discussing the gas permeabilityproperties of different membrane types or thicknesses it should also

be paid attention on the cell efficiency considering the faradaic losseselectrochemical compression losses and all other overpotentials in-cluding the (ionic) ohmic resistances of the membrane Hence de-pending on the desired hydrogen pressure a tradeoff between themembrane induced ohmic overpotential and gas crossover needs tobe found Therefore the total compression efficiency data in Figure 6are combined with measured PEWE cell performance (according toEquation 8) using Nafion 112 and 117 membranes at 50C and ambi-ent pressure and results shown in Figure 7 The given cell efficienciesare based on the LHV of hydrogen The corresponding cell voltages at1 and 3 A middot cmminus2 are 185 and 211 V for Nafion 112 and 197 and 250V for Nafion 117 respectively Similar iE-curves at 1 MPa balancedpressure are given in Ref 21

The observations concerning the hydrogen compression efficiency(Figure 6) are still valid thus only few new insights are deduced

Figure 6 Total compression efficiencies for differential and balanced pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50C for 1 and 3 A middot cmminus2

as a function of the operating pressure Hypothetic new membranes with 110 of the Nafion permeability are added for comparison the isothermal compressionbehavior at 50C and best mechanical compression technology11 are also shown The PEWE data is based on Figures 3 and 5B

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1193

Figure 7 Cell efficiency (including all electrochemical andfaradaic losses) according to Equation 8 normalized to E0

cell= f(pH2T) or LHV of hydrogen for differential and bal-anced pressure PEWE using Nafion 112 and Nafion 117membranes at 50C as a function of the operating pressureat (A) 1 A middot cmminus2 and (B) 3 A middot cmminus2

Cell efficiencies are almost constant up to 1 MPa at 1 A middot cmminus2

and up to 10 MPa at 3 A middot cmminus2 Further pressure increase reduces cellefficiencies but less at high current densities (here 3 A middot cmminus2) due tothe smaller influence of gas crossover and lower apparent compressioneffort for balanced pressure operation

At low current densities (here 1 A middot cmminus2) Nafion 112 has bettercell efficiencies with about 62 up to several MPa and about 60 athigh current densities up to about 10 MPa compared to Nafion 117with about 60 and 50 respectively However at higher pressuresthe better gas barrier properties of Nafion 117 offset the higher ohmicoverpotential

Nevertheless from an energetic point of view the data show thatlow gas crossover membrane materials will be key for efficient electro-chemical compression For ultimate efficiency these materials wouldbest be combined with the beneficial processes of the balanced pres-sure strategy where OER kinetic gains are (partly) compensating forthe thermodynamic cell voltage increase due to pressurization How-ever high-pressure oxygen (typically above 3 MPa) is a severe safetyconcern and difficult to implement in a technical process Hence itseems sensible to limit oxygen pressure This will also be beneficialfor the oxygen crossover losses be it with conventional or future lowgas permeation materials For this novel pressure strategy we use theterm ldquointermediate differentialrdquo pressure PEWE Its characteristicswith respect to hydrogen compression efficiency are discussed below

Intermediate differential pressure PEWEmdashWith the OER kinet-ics following a logarithmic and the gas crossover following a linearpressure relation there must be an optimum operating point mini-mizing the total compression losses by choosing a slightly increasedanode and a considerably increased cathode pressure Experimentshave therefore been performed to better understand this novel pres-sure concept where the anode is pressurized less than the cathodeIn Figure 8A data with low anode pressures (1 and 2 MPa) and highcathode pressures (3 and 5 MPa) are shown A relatively small anodepressure of 1 MPa is sufficient to retain the voltage gains due to in-creased OER kinetics as no further decrease in cell voltage is observedwith the oxygen pressure increase to 2 MPa as shown in Figure 8B

The experimental (1 5 MPa) data are extrapolated to higher pres-sures with an isothermal compression behavior of 84 at 1 A middot cmminus2

and 24 at 3 A middot cmminus2 and compared to those from differential andbalanced pressure in Figure 9 The extrapolated data show that athigher current densities the intermediate differential pressure conceptis energetically more efficient than differential pressure operation dueto a higher electrochemical compression efficiency which is overcom-pensating the relatively small additional oxygen gas crossover lossesIn particular at higher pressures the limited oxygen gas crossover

losses for intermediate differential pressure are crucial for being moreenergetically efficient than all other pressure strategies and even betterthan mechanical compression up to almost 100 MPa for Nafion 117

Those experimental findings are rather counter-intuitive becauseeven if oxygen is considered as a by-product and only pressurizedhydrogen is of interest a 1 MPa compression of oxygen is energeti-cally beneficial Furthermore operating and system investment costs(opex and capex) should be lower for intermediate differential than forbalanced pressure operation due to lower safety requirements of thelow pressurized oxygen and thus be similar to conventional differen-tial pressure operation where the oxygen pressure is close to ambientpressure

Conclusions

In this study the cell voltage and gas crossover related losses dueto electrochemical pressurization were investigated experimentally upto 5 MPa and based on literature study up to 70 MPa for differentialand balanced pressure PEWE and EHCs As expected from ther-modynamics differential pressure electrolysis and EHCs generallyfollow an isothermal compression behavior In contrast the apparentcompression effort for balanced pressure electrolysis decreases withincreasing current density due to improved OER kinetics Part of thisenergetic advantage is offset by gas crossover losses roughly twicethose of differential pressure operation

The energetic analysis of the total hydrogen compression effi-ciency combining electrochemical and faradaic losses up to 100MPa shows that differential pressure electrolysis is preferable to bal-anced pressure at low current densities (le 1 A middot cmminus2) due to lowergas crossover losses and similar compression behavior However athigher current densities the decreasing apparent compression effort ofthe balanced pressure PEWE compensates for the higher gas crossoverlosses resulting in higher total compression efficiencies

Intermediate differential pressure electrolysis with a slightly pres-surized oxygen side and a higher pressure at the hydrogen side com-bines the advantages of both pressure concepts retaining the low oxy-gen gas crossover losses of the differential pressure and the decreasingcompression effort at higher current densities of the balanced pres-sure operation Based on the extrapolation of experimental data theintermediate differential pressure strategy with cells having a mem-brane electrolyte with the gas permeation characteristics of Nafion117 energetically outperform todayrsquos best mechanical compressorsup to tens of MPa with similar compression losses at 100 MPa

With respect to industrially relevant application the intermedi-ate differential pressure strategy respects the challenges and costsof compressed oxygen which will be limited to close to those forconventional differential pressure PEWE with oxygen production at

F1194 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 8 (A) intermediate differential pressure PEWE iR-free iE-curves at 50C at indicated pressure levels (B) ra-tio of the measured iR-free cell voltage difference betweenpressurized and ambient pressure operation (E(iR-free))and the theoretical thermodynamic cell voltage increase(E0

cell(pH2T)) considering only hydrogen compressionie 100 represents an isothermal compression behavioraccording to Equation 1

Figure 9 Total compression efficiencies for differential balanced and intermediate differential pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50Cfor 1 and 3 A middot cmminus2 as a function of the operating pressure For comparison the isothermal compression behavior at 50C and best mechanical compressiontechnology11 are added The PEWE data is based on Figures 3 5B and 8

atmospheric pressure If the technical risk to have highest pressurehydrogen in the electrolyzer in close vicinity to pure oxygen is consid-ered too high then a strategy with a combination of intermediate dif-ferential PEWE up to intermediate pressures of about 10 MPa and finalcompression to highest pressures (100 MPa) by EHCs seems a promis-ing concept with respect to both energetic and safety properties

Acknowledgments

Funding by the Swiss Federal Office of Energy (SFOE under grantno SI500904-01) Belenos Clean Power Holding Ltd and the En-ergy System Integration (ESI) platform at PSI as well as technicalsupport by Martin Ammann and Thomas Gloor (both PSI) are grate-fully acknowledged TJS thanks the Commission for Technology andInnovation Switzerland and the Swiss Competence Center for EnergyResearch Heat amp Electricity Storage

References

1 httpeceuropaeueurostatwebenergydatadatabase (accessed on 31012017)2 Erneuerbare Energien in Zahlen - Nationale und internationale Entwicklung im Jahr

2015 in Bundesministerium fur Wirtschaft und Energie (BMWi) (2016)3 C J Barnhart M Dale A R Brandt and S M Benson Energy amp Environmental

Science 6 2804 (2013)

4 Monitoringbericht 2016 in G Bundesnetzagentur fur Elektrizitat Telekommu-nikation Post und Eisenbahnen Bundeskartellamt Editor Bundesnetzagentur furElektrizitat GasTelekommunikation Post und Eisenbahnen Bundeskartellamt(2016)

5 T Smolinka E T Ojong and J Garche in Electrochemical Energy Storage forRenewable Sources and Grid Balancing P T Moseley and J Garche Editors p 103(2015)

6 httpsenergygoveerefuelcellsdoe-technical-targets-hydrogen-production-electro-lysis (accessed on 10032017)

7 J Alkire Technology Validation Program Area - Plenary Presentation U S Depart-ment of Energy 2017 Annual Merit Review and Peer Evaluation Meeting (2017)

8 httpwwwenergiepark-mainzde (accessed on 10032017)9 M Kopp D Coleman C Stiller K Scheffer J Aichinger and B Scheppat Inter-

national Journal of Hydrogen Energy 42(19) (2017)10 S Metz Fuel Cells Bulletin 2014 12 (2014)11 M Stefan in 20th World Hydrogen Energy Conference (WHEC) KDJ Convention

Center Gwangju South Korea (2014)12 H Ishikawa E Haryu N Kawasaki and H Daimon Honda RampD Technical Review

28 (2016)13 P J Bouwman J Konink D Semerel L Raymakers M Koeman W Dalhuijsen

E Milacic and M Mulder ECS Transactions 64 1009 (2014)14 D G Bessarabov H Wang H Li and N Zhao PEM Electrolysis for Hydrogen

Production Principles and Applications CRC Press Boca Raton FL (USA) (2015)15 L Bertuccioli A Chan D Hart F Lehner B Madden and E Standen Study on

Development of Water Electrolysis in the EU by E4tech Slsquoarl with Element EnergyLtd for the Fuel Cells and Hydrogen Joint Undertaking in (2014)

16 M Carmo D L Fritz J Mergel and D Stolten International Journal of HydrogenEnergy 38 4901 (2013)

F1190 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Table I Literature overview of balanced and differential pressure PEWE compression behavior

Pressure mode Reference Operating pressure [MPa] Isothermal compression behavior []

differential Ayers et al27 14 76 and 166 asymp 100 plusmn 30Bernt and Gasteiger28 01 to 3 asymp 80ndash110Ishikawa et al12 01 35 and 70 35 MPa asymp 100

70 MPa asymp 150Suermann et al20 01 1 and 5 asymp 80ndash95

balanced Grigoriev et al26 01 and 25 ge07 A middot cmminus2 lt 0Grigoriev et al29 01 to 13 asymp 0Siemens30 01 1 and 10 ge07 A middot cmminus2 asymp minus20Suermann et al21 01 1 and 10 ge15 A middot cmminus2 asymp 0

Figure 3 Electrochemical compression (EC) efficiency for differential andbalanced pressure PEWE at 50C as a function of the operating pressure Dataare based on experimental results up to 1 MPa from Figure 2B Isothermalcompression is calculated according to Equation 1 No gas crossover lossesare considered here

cell voltage differences between pressurized and ambient pressure arenormalized to the corresponding thermodynamic cell voltage increase(again only hydrogen is of interest and oxygen is considered as a by-product) For differential pressure operation a compression behaviorclose to the isothermal values are observed up to 35 MPa The 70 MPadifferential pressure data from Ishikawa et al12 shows a significantlyhigher apparent compression effort of about 150 which might beexplained with an increased non-ideal gas behavior ie a hydrogencompressibility factor of approximately 14 (60C 70 MPa)31 Thebalanced pressure operation data at 10 and 10 MPa of Siemens30

as well as the results of Grigoriev et al2629 show a higher effect ofthe improved OER kinetics than this study with negative apparentcompression effort of up to minus20

Summarizing the results of this study are qualitatively in line withliterature up to 35 MPa differential pressure When extrapolating theseresults to higher pressures the compression work required is underes-timated for the differential pressure case In contrast for the balancedpressure our experimental results are less optimistic than literatureso when extrapolating our data to higher pressures the compressionlosses are rather overestimated Therefore the comparison betweendifferential and balanced pressure based on the present experimentaldata when extrapolating to higher pressures will produce conserva-tive estimates

The electrochemical compression efficiencies extrapolated up to100 MPa are shown in Figure 3 The losses for the differential pressureare based on 80 isothermal compression behavior and independentof the current density For balanced pressure however the electro-chemical compression efficiency increases with current density and

curves are shown for 1 2 and 3 A middot cmminus2 At 3 A middot cmminus2 an electro-chemical compression efficiency at 100 MPa of 99 is calculated

Electrochemical hydrogen compressormdashElectrochemical hydro-gen compressors (EHCs) concentrate andor compress hydrogen fromanode to cathode In this study two different scenarios are investigatedi) compression of pure hydrogen and ii) extraction and compressionof hydrogen out of the natural gas grid For both scenarios hydrogenis compressed from 01 to 02 MPa absolute pressure for case i) purehydrogen is supplied and in case ii) a gas mixture with 5 H2 in N2in order to simulate the dilution in the natural gas grid where the 5are considered an upper limit with respect to todayrsquos regulations32

Thus compression ratios of 2 and 40 are realized in scenarios i) andii) respectively

The two EHC scenarios are compared in Figure 4A When extrap-olating the cell voltage from the stoichiometric range ie above 05A middot cmminus2 for case i) and above 01 A middot cmminus2 for case ii) to equilib-rium conditions offsets of about 50 plusmn 14 mV for case i) and 586plusmn 05 mV for case ii) can be extracted which are close to the calcu-lated theoretical values (10 and 53 mV based on Equation 2 withoutconsidering any water vapor and hydrogen consumption along thechannel) Hence for hydrogen concentration and compression usingEHC an isothermal compression behavior close to the theoreticalvalues is observed similar to the results described for differentialpressure PEWE (see Figure 3)

The slopes of the iE-curves for cases i) and ii) in Figure 4A aresignificantly different In the absence of kinetic overpotential contri-butions due to the highly reversible hydrogen oxidation and evolutionreactions (HOR and HER)33 the slope represents the area specific re-sistance For the case of pure hydrogen a value of 95 m middot cm2 and forthe diluted hydrogen 149 m middot cm2 are observed The pure hydrogencase is in good agreement with literature results of Shi et al34 whoreported a through-plane area specific resistance for the same type ofmembrane with a thickness of about 33 μm in the order of 80ndash90m middot cm2 (70C rh = 80 based on ex-situ HFR measurements) Thedeviation observed for case ii) may originate from the uneven currentdensity distribution between the anode inlet to outlet as outlined inFigure 4B The uneven current density distribution is attributed to aninsufficient membrane humidification especially at the anode outletThis hypothesis was underlined with additional experiments injectingliquid water into the cathode (rhc ge 100) resulting in temporarilydecreased area specific resistances in the order of 90 to 100 m middot cm2similar to case i) and in more homogenous current density distribu-tions as shown in Figure 4B However having partly liquid water inthe cathode no steady-state conditions were obtained with the cur-rent setup used A further possibility is the appearance of transportlosses at the anode outlet Here two-thirds of the diluted hydrogen hasbeen consumed and the hydrogen content in nitrogen drops to 17eventually giving rise to anodic transport overpotential

For EHCs in technical applications the humidification concept isa key challenge in particular when hydrogen is concentrated andorcompressed out of a dry gas mixture eg out of a natural gas pipelineThe water management in EHCs which is strongly influenced by theelectro-osmotic water drag from anode to cathode is also reflected in

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1191

Figure 4 (A) EHC iE-curves at 60C using Nafion XL-100 membrane for compressing hydrogen from 01 to 02MPa (cell pressures) for case i) pure hydrogen (red circles)and case ii) 5 hydrogen in nitrogen (blue squares) therelative humidity of all gases is 80 lines show linear fits ofthe cell voltage above 05 A middot cmminus2 for case i) and above 01A middot cmminus2 for case ii) extrapolated to equilibrium conditionsat 0 A middot cmminus2 Linear fit and y-axis intercept for case i) are95 m middot cm2 and 5 mV for case ii) 149 m middot cm2 and 59 mV(B) EHC local current density distribution at 60C at meancurrent densities indicated The rhc = 80 data correspondsto the 5 H2 in N2 iE-curve shown in (A) For the counter-flow arrangement the cathode inlet is at length 0 and theanode inlet at length 100

the work of Grigoriev et al35 using less diluted hydrogen in nitrogengas mixtures The authors observed a turning point in the EHC iE datawith diluted anode gas at relatively low current densities of only 02 to03 A middot cmminus2 which was attributed to an insufficient humidification ofthe Nafion 117 membranes used35 Nevertheless compression ratiosup to 130 were realized confirming the largely isothermal compressionbehavior of the EHC35 In summary the experimental findings confirman isothermal compression behavior of EHCs which is also in linewith earlier literature data133637 From the energetic point of viewthe compression behavior of EHCs and differential pressure PEWEare similar therefore for energy considerations only differential andbalanced pressure PEWE are compared below However from thesafety point of view EHCrsquos may have advantages at high pressuresas no pure oxygen is present in the vicinity of high pressure hydrogenAlso from the application point of view EHCrsquos may be used whenthe hydrogen is stored at an intermediate pressure andor transportedfrom the electrolyzer to its final destination (Figure 1)

Faradaic losses due to gas crossovermdashIn addition to the abovediscussed electrochemical compression losses faradaic losses occurdue to gas crossover through the polymer electrolyte Assuming thatpressure dependence of the gas permeability of Nafion membranescan be described by a pure solution-diffusion process as experimen-tally demonstrated up to 22 MPa differential pressure by Barbir38 datafrom Schalenbach et al39 are used which have been validated for 10MPa balanced pressure earlier20 Recent research has questioned thisassumption for electrolyzers it was postulated that the current indi-rectly influences the gas crossover especially for conditions close toambient pressure due to increased partial pressures in the catalystlayers with increasing current density40 The fact that oxygen perme-ability is about half that of hydrogen has consistently also measuredby Sakai et al41

Considering that the thickness of water swollen membranes duringoperation is larger than their nominal thickness (209 152 and 63 μmfor the nominally dry 175 125 and 50 μm ie in the often used impe-rial units the 7 5 and 2 mil membranes)39 the gas crossover currentdensity equivalent jx(H2O2) is calculated according to Equation 5and given in Figure 5A Its apparent linear behavior with temperatureis visualized using linear fits allowing for interpolating for differenttemperatures Even though the permeability of oxygen is only abouthalf that of hydrogen the corresponding gas crossover currents aresimilar to hydrogen because the current equivalent of oxygen is twicethat of hydrogen when permeated oxygen reacts at the cathodic cata-lyst to form water Unreinforced thin membranes are discussed hereto illustrate the influence of membrane thickness on gas permeationand overall cell efficiency For long term field applications howevermore advanced materials will be needed1742

From the gas crossover data the faradaic efficiency is calculatedfor different membrane thicknesses and current densities (Equation 6)as a function of the operating pressure and given in Figure 5B for thedifferential pressure at 50C For EHCs similar gas crossover lossesare obtained but for balanced pressure operation the gas crossoverlosses are roughly twice as large Furthermore estimates for (hypo-thetic) new membrane materials with different fractions of the Nafiongas permeability (05 02 and 01) are added For the 110 Nafion 112membrane the faradaic efficiency at 100 MPa increases to above 96from less than 70 for the standard material (Figure 5B) Improve-ments in the membrane permeability seem to be possible by reinforc-ing the membrane27 or by replacing the perfluoroalkylsulfonic acidbased membranes with different chemistry eg hydrocarbons17

Total hydrogen compression efficiencymdashIn order to predict thetotal hydrogen compression efficiency both electrochemical com-pression and faradaic efficiency need to be combined as given inEquation 7 As discussed above because only hydrogen is of interestas a product both pressure strategies are compared to the thermody-namic cell voltage increase of +32 mV per decade of hydrogen pres-sure (at 50C) However both hydrogen and oxygen (recombining atthe cathode to water) crossover losses are included in the calculations

The results for Nafion 117 for differential and balanced pressure forthe electrochemical compression efficiency (Figure 3) and the corre-sponding faradaic efficiency (Figure 5B) are combined and the resultsare given in Figure 6 With respect to the resulting total compressionefficiency several general statements can be formulated

At low operating pressures the total compression efficiency isdominated by isothermal compression behavior (linear curve shape inthe semi-logarithmic plots)

With increasing operating pressure the losses due to gascrossover become more dominant and curve shapes change from linearto exponential

Differential pressure PEWE has better total compression ef-ficiency at low current densities (up to about 1 A middot cmminus2) becauseof similar electrochemical compression behavior but lower oxygenpermeation losses compared to balanced operation

Balanced pressure strategy has lower apparent compressionlosses at high current densities (here 3 A middot cmminus2) up to about 10 MPausing Nafion 112 or up to 50 MPa using Nafion 117 because theoxygen crossover losses are offset by the OER kinetic gains

From an energetic point of view electrochemical compressionis more efficient especially at the high current density up to 10 MPafor Nafion 112 or 50 MPa using Nafion 117 than the (probably) bestmechanical 5-stage ionic liquid hydrogen compressors11 (having aloss factor of about 14 times the thermodynamic requirement)

F1192 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 5 (A) Hydrogen and oxygen gas crossover current density equivalents for wet 7 5 and 2 mil thick Nafion membranes (actual thickness 209 152 and63 μm) Linear fits are added for both hydrogen (solid lines) and oxygen (dashed lines) Data is calculated based on the experimental results of Schalenbachet al39 (B) The corresponding faradaic efficiency is calculated for differential pressure PEWE using wet Nafion 117 or Nafion 112 at 1 and 3 A middot cmminus2 for 50Cas a function of the operating pressure The data is based on Figure 5A Hypothetic new membranes with the thickness of Nafion 112 and different fractions of thepermeability (05 02 01) are added for discussion

Hypothetic new membranes with a permeability of only onetenth of Nafion dramatically reduce the gas crossover losses enhanc-ing compression efficiency with such a membrane material 93 com-pression efficiency would be obtained for a 2-mil membrane at 100MPa (3 A middot cmminus2 and balanced pressure) This is similar to isothermalcompression behavior and would outperform todayrsquos best mechanicalcompressors

Considering the total cell efficiency membrane thickness does notonly influence the gas crossover losses but also the correspondingohmic overpotential Thus a tradeoff between both losses has to befound for the optimal cell efficiency as discussed below

PEWE cell efficiencymdashThe main focus is on the hydrogen com-pression efficiency However when discussing the gas permeabilityproperties of different membrane types or thicknesses it should also

be paid attention on the cell efficiency considering the faradaic losseselectrochemical compression losses and all other overpotentials in-cluding the (ionic) ohmic resistances of the membrane Hence de-pending on the desired hydrogen pressure a tradeoff between themembrane induced ohmic overpotential and gas crossover needs tobe found Therefore the total compression efficiency data in Figure 6are combined with measured PEWE cell performance (according toEquation 8) using Nafion 112 and 117 membranes at 50C and ambi-ent pressure and results shown in Figure 7 The given cell efficienciesare based on the LHV of hydrogen The corresponding cell voltages at1 and 3 A middot cmminus2 are 185 and 211 V for Nafion 112 and 197 and 250V for Nafion 117 respectively Similar iE-curves at 1 MPa balancedpressure are given in Ref 21

The observations concerning the hydrogen compression efficiency(Figure 6) are still valid thus only few new insights are deduced

Figure 6 Total compression efficiencies for differential and balanced pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50C for 1 and 3 A middot cmminus2

as a function of the operating pressure Hypothetic new membranes with 110 of the Nafion permeability are added for comparison the isothermal compressionbehavior at 50C and best mechanical compression technology11 are also shown The PEWE data is based on Figures 3 and 5B

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1193

Figure 7 Cell efficiency (including all electrochemical andfaradaic losses) according to Equation 8 normalized to E0

cell= f(pH2T) or LHV of hydrogen for differential and bal-anced pressure PEWE using Nafion 112 and Nafion 117membranes at 50C as a function of the operating pressureat (A) 1 A middot cmminus2 and (B) 3 A middot cmminus2

Cell efficiencies are almost constant up to 1 MPa at 1 A middot cmminus2

and up to 10 MPa at 3 A middot cmminus2 Further pressure increase reduces cellefficiencies but less at high current densities (here 3 A middot cmminus2) due tothe smaller influence of gas crossover and lower apparent compressioneffort for balanced pressure operation

At low current densities (here 1 A middot cmminus2) Nafion 112 has bettercell efficiencies with about 62 up to several MPa and about 60 athigh current densities up to about 10 MPa compared to Nafion 117with about 60 and 50 respectively However at higher pressuresthe better gas barrier properties of Nafion 117 offset the higher ohmicoverpotential

Nevertheless from an energetic point of view the data show thatlow gas crossover membrane materials will be key for efficient electro-chemical compression For ultimate efficiency these materials wouldbest be combined with the beneficial processes of the balanced pres-sure strategy where OER kinetic gains are (partly) compensating forthe thermodynamic cell voltage increase due to pressurization How-ever high-pressure oxygen (typically above 3 MPa) is a severe safetyconcern and difficult to implement in a technical process Hence itseems sensible to limit oxygen pressure This will also be beneficialfor the oxygen crossover losses be it with conventional or future lowgas permeation materials For this novel pressure strategy we use theterm ldquointermediate differentialrdquo pressure PEWE Its characteristicswith respect to hydrogen compression efficiency are discussed below

Intermediate differential pressure PEWEmdashWith the OER kinet-ics following a logarithmic and the gas crossover following a linearpressure relation there must be an optimum operating point mini-mizing the total compression losses by choosing a slightly increasedanode and a considerably increased cathode pressure Experimentshave therefore been performed to better understand this novel pres-sure concept where the anode is pressurized less than the cathodeIn Figure 8A data with low anode pressures (1 and 2 MPa) and highcathode pressures (3 and 5 MPa) are shown A relatively small anodepressure of 1 MPa is sufficient to retain the voltage gains due to in-creased OER kinetics as no further decrease in cell voltage is observedwith the oxygen pressure increase to 2 MPa as shown in Figure 8B

The experimental (1 5 MPa) data are extrapolated to higher pres-sures with an isothermal compression behavior of 84 at 1 A middot cmminus2

and 24 at 3 A middot cmminus2 and compared to those from differential andbalanced pressure in Figure 9 The extrapolated data show that athigher current densities the intermediate differential pressure conceptis energetically more efficient than differential pressure operation dueto a higher electrochemical compression efficiency which is overcom-pensating the relatively small additional oxygen gas crossover lossesIn particular at higher pressures the limited oxygen gas crossover

losses for intermediate differential pressure are crucial for being moreenergetically efficient than all other pressure strategies and even betterthan mechanical compression up to almost 100 MPa for Nafion 117

Those experimental findings are rather counter-intuitive becauseeven if oxygen is considered as a by-product and only pressurizedhydrogen is of interest a 1 MPa compression of oxygen is energeti-cally beneficial Furthermore operating and system investment costs(opex and capex) should be lower for intermediate differential than forbalanced pressure operation due to lower safety requirements of thelow pressurized oxygen and thus be similar to conventional differen-tial pressure operation where the oxygen pressure is close to ambientpressure

Conclusions

In this study the cell voltage and gas crossover related losses dueto electrochemical pressurization were investigated experimentally upto 5 MPa and based on literature study up to 70 MPa for differentialand balanced pressure PEWE and EHCs As expected from ther-modynamics differential pressure electrolysis and EHCs generallyfollow an isothermal compression behavior In contrast the apparentcompression effort for balanced pressure electrolysis decreases withincreasing current density due to improved OER kinetics Part of thisenergetic advantage is offset by gas crossover losses roughly twicethose of differential pressure operation

The energetic analysis of the total hydrogen compression effi-ciency combining electrochemical and faradaic losses up to 100MPa shows that differential pressure electrolysis is preferable to bal-anced pressure at low current densities (le 1 A middot cmminus2) due to lowergas crossover losses and similar compression behavior However athigher current densities the decreasing apparent compression effort ofthe balanced pressure PEWE compensates for the higher gas crossoverlosses resulting in higher total compression efficiencies

Intermediate differential pressure electrolysis with a slightly pres-surized oxygen side and a higher pressure at the hydrogen side com-bines the advantages of both pressure concepts retaining the low oxy-gen gas crossover losses of the differential pressure and the decreasingcompression effort at higher current densities of the balanced pres-sure operation Based on the extrapolation of experimental data theintermediate differential pressure strategy with cells having a mem-brane electrolyte with the gas permeation characteristics of Nafion117 energetically outperform todayrsquos best mechanical compressorsup to tens of MPa with similar compression losses at 100 MPa

With respect to industrially relevant application the intermedi-ate differential pressure strategy respects the challenges and costsof compressed oxygen which will be limited to close to those forconventional differential pressure PEWE with oxygen production at

F1194 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 8 (A) intermediate differential pressure PEWE iR-free iE-curves at 50C at indicated pressure levels (B) ra-tio of the measured iR-free cell voltage difference betweenpressurized and ambient pressure operation (E(iR-free))and the theoretical thermodynamic cell voltage increase(E0

cell(pH2T)) considering only hydrogen compressionie 100 represents an isothermal compression behavioraccording to Equation 1

Figure 9 Total compression efficiencies for differential balanced and intermediate differential pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50Cfor 1 and 3 A middot cmminus2 as a function of the operating pressure For comparison the isothermal compression behavior at 50C and best mechanical compressiontechnology11 are added The PEWE data is based on Figures 3 5B and 8

atmospheric pressure If the technical risk to have highest pressurehydrogen in the electrolyzer in close vicinity to pure oxygen is consid-ered too high then a strategy with a combination of intermediate dif-ferential PEWE up to intermediate pressures of about 10 MPa and finalcompression to highest pressures (100 MPa) by EHCs seems a promis-ing concept with respect to both energetic and safety properties

Acknowledgments

Funding by the Swiss Federal Office of Energy (SFOE under grantno SI500904-01) Belenos Clean Power Holding Ltd and the En-ergy System Integration (ESI) platform at PSI as well as technicalsupport by Martin Ammann and Thomas Gloor (both PSI) are grate-fully acknowledged TJS thanks the Commission for Technology andInnovation Switzerland and the Swiss Competence Center for EnergyResearch Heat amp Electricity Storage

References

1 httpeceuropaeueurostatwebenergydatadatabase (accessed on 31012017)2 Erneuerbare Energien in Zahlen - Nationale und internationale Entwicklung im Jahr

2015 in Bundesministerium fur Wirtschaft und Energie (BMWi) (2016)3 C J Barnhart M Dale A R Brandt and S M Benson Energy amp Environmental

Science 6 2804 (2013)

4 Monitoringbericht 2016 in G Bundesnetzagentur fur Elektrizitat Telekommu-nikation Post und Eisenbahnen Bundeskartellamt Editor Bundesnetzagentur furElektrizitat GasTelekommunikation Post und Eisenbahnen Bundeskartellamt(2016)

5 T Smolinka E T Ojong and J Garche in Electrochemical Energy Storage forRenewable Sources and Grid Balancing P T Moseley and J Garche Editors p 103(2015)

6 httpsenergygoveerefuelcellsdoe-technical-targets-hydrogen-production-electro-lysis (accessed on 10032017)

7 J Alkire Technology Validation Program Area - Plenary Presentation U S Depart-ment of Energy 2017 Annual Merit Review and Peer Evaluation Meeting (2017)

8 httpwwwenergiepark-mainzde (accessed on 10032017)9 M Kopp D Coleman C Stiller K Scheffer J Aichinger and B Scheppat Inter-

national Journal of Hydrogen Energy 42(19) (2017)10 S Metz Fuel Cells Bulletin 2014 12 (2014)11 M Stefan in 20th World Hydrogen Energy Conference (WHEC) KDJ Convention

Center Gwangju South Korea (2014)12 H Ishikawa E Haryu N Kawasaki and H Daimon Honda RampD Technical Review

28 (2016)13 P J Bouwman J Konink D Semerel L Raymakers M Koeman W Dalhuijsen

E Milacic and M Mulder ECS Transactions 64 1009 (2014)14 D G Bessarabov H Wang H Li and N Zhao PEM Electrolysis for Hydrogen

Production Principles and Applications CRC Press Boca Raton FL (USA) (2015)15 L Bertuccioli A Chan D Hart F Lehner B Madden and E Standen Study on

Development of Water Electrolysis in the EU by E4tech Slsquoarl with Element EnergyLtd for the Fuel Cells and Hydrogen Joint Undertaking in (2014)

16 M Carmo D L Fritz J Mergel and D Stolten International Journal of HydrogenEnergy 38 4901 (2013)

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1191

Figure 4 (A) EHC iE-curves at 60C using Nafion XL-100 membrane for compressing hydrogen from 01 to 02MPa (cell pressures) for case i) pure hydrogen (red circles)and case ii) 5 hydrogen in nitrogen (blue squares) therelative humidity of all gases is 80 lines show linear fits ofthe cell voltage above 05 A middot cmminus2 for case i) and above 01A middot cmminus2 for case ii) extrapolated to equilibrium conditionsat 0 A middot cmminus2 Linear fit and y-axis intercept for case i) are95 m middot cm2 and 5 mV for case ii) 149 m middot cm2 and 59 mV(B) EHC local current density distribution at 60C at meancurrent densities indicated The rhc = 80 data correspondsto the 5 H2 in N2 iE-curve shown in (A) For the counter-flow arrangement the cathode inlet is at length 0 and theanode inlet at length 100

the work of Grigoriev et al35 using less diluted hydrogen in nitrogengas mixtures The authors observed a turning point in the EHC iE datawith diluted anode gas at relatively low current densities of only 02 to03 A middot cmminus2 which was attributed to an insufficient humidification ofthe Nafion 117 membranes used35 Nevertheless compression ratiosup to 130 were realized confirming the largely isothermal compressionbehavior of the EHC35 In summary the experimental findings confirman isothermal compression behavior of EHCs which is also in linewith earlier literature data133637 From the energetic point of viewthe compression behavior of EHCs and differential pressure PEWEare similar therefore for energy considerations only differential andbalanced pressure PEWE are compared below However from thesafety point of view EHCrsquos may have advantages at high pressuresas no pure oxygen is present in the vicinity of high pressure hydrogenAlso from the application point of view EHCrsquos may be used whenthe hydrogen is stored at an intermediate pressure andor transportedfrom the electrolyzer to its final destination (Figure 1)

Faradaic losses due to gas crossovermdashIn addition to the abovediscussed electrochemical compression losses faradaic losses occurdue to gas crossover through the polymer electrolyte Assuming thatpressure dependence of the gas permeability of Nafion membranescan be described by a pure solution-diffusion process as experimen-tally demonstrated up to 22 MPa differential pressure by Barbir38 datafrom Schalenbach et al39 are used which have been validated for 10MPa balanced pressure earlier20 Recent research has questioned thisassumption for electrolyzers it was postulated that the current indi-rectly influences the gas crossover especially for conditions close toambient pressure due to increased partial pressures in the catalystlayers with increasing current density40 The fact that oxygen perme-ability is about half that of hydrogen has consistently also measuredby Sakai et al41

Considering that the thickness of water swollen membranes duringoperation is larger than their nominal thickness (209 152 and 63 μmfor the nominally dry 175 125 and 50 μm ie in the often used impe-rial units the 7 5 and 2 mil membranes)39 the gas crossover currentdensity equivalent jx(H2O2) is calculated according to Equation 5and given in Figure 5A Its apparent linear behavior with temperatureis visualized using linear fits allowing for interpolating for differenttemperatures Even though the permeability of oxygen is only abouthalf that of hydrogen the corresponding gas crossover currents aresimilar to hydrogen because the current equivalent of oxygen is twicethat of hydrogen when permeated oxygen reacts at the cathodic cata-lyst to form water Unreinforced thin membranes are discussed hereto illustrate the influence of membrane thickness on gas permeationand overall cell efficiency For long term field applications howevermore advanced materials will be needed1742

From the gas crossover data the faradaic efficiency is calculatedfor different membrane thicknesses and current densities (Equation 6)as a function of the operating pressure and given in Figure 5B for thedifferential pressure at 50C For EHCs similar gas crossover lossesare obtained but for balanced pressure operation the gas crossoverlosses are roughly twice as large Furthermore estimates for (hypo-thetic) new membrane materials with different fractions of the Nafiongas permeability (05 02 and 01) are added For the 110 Nafion 112membrane the faradaic efficiency at 100 MPa increases to above 96from less than 70 for the standard material (Figure 5B) Improve-ments in the membrane permeability seem to be possible by reinforc-ing the membrane27 or by replacing the perfluoroalkylsulfonic acidbased membranes with different chemistry eg hydrocarbons17

Total hydrogen compression efficiencymdashIn order to predict thetotal hydrogen compression efficiency both electrochemical com-pression and faradaic efficiency need to be combined as given inEquation 7 As discussed above because only hydrogen is of interestas a product both pressure strategies are compared to the thermody-namic cell voltage increase of +32 mV per decade of hydrogen pres-sure (at 50C) However both hydrogen and oxygen (recombining atthe cathode to water) crossover losses are included in the calculations

The results for Nafion 117 for differential and balanced pressure forthe electrochemical compression efficiency (Figure 3) and the corre-sponding faradaic efficiency (Figure 5B) are combined and the resultsare given in Figure 6 With respect to the resulting total compressionefficiency several general statements can be formulated

At low operating pressures the total compression efficiency isdominated by isothermal compression behavior (linear curve shape inthe semi-logarithmic plots)

With increasing operating pressure the losses due to gascrossover become more dominant and curve shapes change from linearto exponential

Differential pressure PEWE has better total compression ef-ficiency at low current densities (up to about 1 A middot cmminus2) becauseof similar electrochemical compression behavior but lower oxygenpermeation losses compared to balanced operation

Balanced pressure strategy has lower apparent compressionlosses at high current densities (here 3 A middot cmminus2) up to about 10 MPausing Nafion 112 or up to 50 MPa using Nafion 117 because theoxygen crossover losses are offset by the OER kinetic gains

From an energetic point of view electrochemical compressionis more efficient especially at the high current density up to 10 MPafor Nafion 112 or 50 MPa using Nafion 117 than the (probably) bestmechanical 5-stage ionic liquid hydrogen compressors11 (having aloss factor of about 14 times the thermodynamic requirement)

F1192 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 5 (A) Hydrogen and oxygen gas crossover current density equivalents for wet 7 5 and 2 mil thick Nafion membranes (actual thickness 209 152 and63 μm) Linear fits are added for both hydrogen (solid lines) and oxygen (dashed lines) Data is calculated based on the experimental results of Schalenbachet al39 (B) The corresponding faradaic efficiency is calculated for differential pressure PEWE using wet Nafion 117 or Nafion 112 at 1 and 3 A middot cmminus2 for 50Cas a function of the operating pressure The data is based on Figure 5A Hypothetic new membranes with the thickness of Nafion 112 and different fractions of thepermeability (05 02 01) are added for discussion

Hypothetic new membranes with a permeability of only onetenth of Nafion dramatically reduce the gas crossover losses enhanc-ing compression efficiency with such a membrane material 93 com-pression efficiency would be obtained for a 2-mil membrane at 100MPa (3 A middot cmminus2 and balanced pressure) This is similar to isothermalcompression behavior and would outperform todayrsquos best mechanicalcompressors

Considering the total cell efficiency membrane thickness does notonly influence the gas crossover losses but also the correspondingohmic overpotential Thus a tradeoff between both losses has to befound for the optimal cell efficiency as discussed below

PEWE cell efficiencymdashThe main focus is on the hydrogen com-pression efficiency However when discussing the gas permeabilityproperties of different membrane types or thicknesses it should also

be paid attention on the cell efficiency considering the faradaic losseselectrochemical compression losses and all other overpotentials in-cluding the (ionic) ohmic resistances of the membrane Hence de-pending on the desired hydrogen pressure a tradeoff between themembrane induced ohmic overpotential and gas crossover needs tobe found Therefore the total compression efficiency data in Figure 6are combined with measured PEWE cell performance (according toEquation 8) using Nafion 112 and 117 membranes at 50C and ambi-ent pressure and results shown in Figure 7 The given cell efficienciesare based on the LHV of hydrogen The corresponding cell voltages at1 and 3 A middot cmminus2 are 185 and 211 V for Nafion 112 and 197 and 250V for Nafion 117 respectively Similar iE-curves at 1 MPa balancedpressure are given in Ref 21

The observations concerning the hydrogen compression efficiency(Figure 6) are still valid thus only few new insights are deduced

Figure 6 Total compression efficiencies for differential and balanced pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50C for 1 and 3 A middot cmminus2

as a function of the operating pressure Hypothetic new membranes with 110 of the Nafion permeability are added for comparison the isothermal compressionbehavior at 50C and best mechanical compression technology11 are also shown The PEWE data is based on Figures 3 and 5B

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1193

Figure 7 Cell efficiency (including all electrochemical andfaradaic losses) according to Equation 8 normalized to E0

cell= f(pH2T) or LHV of hydrogen for differential and bal-anced pressure PEWE using Nafion 112 and Nafion 117membranes at 50C as a function of the operating pressureat (A) 1 A middot cmminus2 and (B) 3 A middot cmminus2

Cell efficiencies are almost constant up to 1 MPa at 1 A middot cmminus2

and up to 10 MPa at 3 A middot cmminus2 Further pressure increase reduces cellefficiencies but less at high current densities (here 3 A middot cmminus2) due tothe smaller influence of gas crossover and lower apparent compressioneffort for balanced pressure operation

At low current densities (here 1 A middot cmminus2) Nafion 112 has bettercell efficiencies with about 62 up to several MPa and about 60 athigh current densities up to about 10 MPa compared to Nafion 117with about 60 and 50 respectively However at higher pressuresthe better gas barrier properties of Nafion 117 offset the higher ohmicoverpotential

Nevertheless from an energetic point of view the data show thatlow gas crossover membrane materials will be key for efficient electro-chemical compression For ultimate efficiency these materials wouldbest be combined with the beneficial processes of the balanced pres-sure strategy where OER kinetic gains are (partly) compensating forthe thermodynamic cell voltage increase due to pressurization How-ever high-pressure oxygen (typically above 3 MPa) is a severe safetyconcern and difficult to implement in a technical process Hence itseems sensible to limit oxygen pressure This will also be beneficialfor the oxygen crossover losses be it with conventional or future lowgas permeation materials For this novel pressure strategy we use theterm ldquointermediate differentialrdquo pressure PEWE Its characteristicswith respect to hydrogen compression efficiency are discussed below

Intermediate differential pressure PEWEmdashWith the OER kinet-ics following a logarithmic and the gas crossover following a linearpressure relation there must be an optimum operating point mini-mizing the total compression losses by choosing a slightly increasedanode and a considerably increased cathode pressure Experimentshave therefore been performed to better understand this novel pres-sure concept where the anode is pressurized less than the cathodeIn Figure 8A data with low anode pressures (1 and 2 MPa) and highcathode pressures (3 and 5 MPa) are shown A relatively small anodepressure of 1 MPa is sufficient to retain the voltage gains due to in-creased OER kinetics as no further decrease in cell voltage is observedwith the oxygen pressure increase to 2 MPa as shown in Figure 8B

The experimental (1 5 MPa) data are extrapolated to higher pres-sures with an isothermal compression behavior of 84 at 1 A middot cmminus2

and 24 at 3 A middot cmminus2 and compared to those from differential andbalanced pressure in Figure 9 The extrapolated data show that athigher current densities the intermediate differential pressure conceptis energetically more efficient than differential pressure operation dueto a higher electrochemical compression efficiency which is overcom-pensating the relatively small additional oxygen gas crossover lossesIn particular at higher pressures the limited oxygen gas crossover

losses for intermediate differential pressure are crucial for being moreenergetically efficient than all other pressure strategies and even betterthan mechanical compression up to almost 100 MPa for Nafion 117

Those experimental findings are rather counter-intuitive becauseeven if oxygen is considered as a by-product and only pressurizedhydrogen is of interest a 1 MPa compression of oxygen is energeti-cally beneficial Furthermore operating and system investment costs(opex and capex) should be lower for intermediate differential than forbalanced pressure operation due to lower safety requirements of thelow pressurized oxygen and thus be similar to conventional differen-tial pressure operation where the oxygen pressure is close to ambientpressure

Conclusions

In this study the cell voltage and gas crossover related losses dueto electrochemical pressurization were investigated experimentally upto 5 MPa and based on literature study up to 70 MPa for differentialand balanced pressure PEWE and EHCs As expected from ther-modynamics differential pressure electrolysis and EHCs generallyfollow an isothermal compression behavior In contrast the apparentcompression effort for balanced pressure electrolysis decreases withincreasing current density due to improved OER kinetics Part of thisenergetic advantage is offset by gas crossover losses roughly twicethose of differential pressure operation

The energetic analysis of the total hydrogen compression effi-ciency combining electrochemical and faradaic losses up to 100MPa shows that differential pressure electrolysis is preferable to bal-anced pressure at low current densities (le 1 A middot cmminus2) due to lowergas crossover losses and similar compression behavior However athigher current densities the decreasing apparent compression effort ofthe balanced pressure PEWE compensates for the higher gas crossoverlosses resulting in higher total compression efficiencies

Intermediate differential pressure electrolysis with a slightly pres-surized oxygen side and a higher pressure at the hydrogen side com-bines the advantages of both pressure concepts retaining the low oxy-gen gas crossover losses of the differential pressure and the decreasingcompression effort at higher current densities of the balanced pres-sure operation Based on the extrapolation of experimental data theintermediate differential pressure strategy with cells having a mem-brane electrolyte with the gas permeation characteristics of Nafion117 energetically outperform todayrsquos best mechanical compressorsup to tens of MPa with similar compression losses at 100 MPa

With respect to industrially relevant application the intermedi-ate differential pressure strategy respects the challenges and costsof compressed oxygen which will be limited to close to those forconventional differential pressure PEWE with oxygen production at

F1194 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 8 (A) intermediate differential pressure PEWE iR-free iE-curves at 50C at indicated pressure levels (B) ra-tio of the measured iR-free cell voltage difference betweenpressurized and ambient pressure operation (E(iR-free))and the theoretical thermodynamic cell voltage increase(E0

cell(pH2T)) considering only hydrogen compressionie 100 represents an isothermal compression behavioraccording to Equation 1

Figure 9 Total compression efficiencies for differential balanced and intermediate differential pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50Cfor 1 and 3 A middot cmminus2 as a function of the operating pressure For comparison the isothermal compression behavior at 50C and best mechanical compressiontechnology11 are added The PEWE data is based on Figures 3 5B and 8

atmospheric pressure If the technical risk to have highest pressurehydrogen in the electrolyzer in close vicinity to pure oxygen is consid-ered too high then a strategy with a combination of intermediate dif-ferential PEWE up to intermediate pressures of about 10 MPa and finalcompression to highest pressures (100 MPa) by EHCs seems a promis-ing concept with respect to both energetic and safety properties

Acknowledgments

Funding by the Swiss Federal Office of Energy (SFOE under grantno SI500904-01) Belenos Clean Power Holding Ltd and the En-ergy System Integration (ESI) platform at PSI as well as technicalsupport by Martin Ammann and Thomas Gloor (both PSI) are grate-fully acknowledged TJS thanks the Commission for Technology andInnovation Switzerland and the Swiss Competence Center for EnergyResearch Heat amp Electricity Storage

References

1 httpeceuropaeueurostatwebenergydatadatabase (accessed on 31012017)2 Erneuerbare Energien in Zahlen - Nationale und internationale Entwicklung im Jahr

2015 in Bundesministerium fur Wirtschaft und Energie (BMWi) (2016)3 C J Barnhart M Dale A R Brandt and S M Benson Energy amp Environmental

Science 6 2804 (2013)

4 Monitoringbericht 2016 in G Bundesnetzagentur fur Elektrizitat Telekommu-nikation Post und Eisenbahnen Bundeskartellamt Editor Bundesnetzagentur furElektrizitat GasTelekommunikation Post und Eisenbahnen Bundeskartellamt(2016)

5 T Smolinka E T Ojong and J Garche in Electrochemical Energy Storage forRenewable Sources and Grid Balancing P T Moseley and J Garche Editors p 103(2015)

6 httpsenergygoveerefuelcellsdoe-technical-targets-hydrogen-production-electro-lysis (accessed on 10032017)

7 J Alkire Technology Validation Program Area - Plenary Presentation U S Depart-ment of Energy 2017 Annual Merit Review and Peer Evaluation Meeting (2017)

8 httpwwwenergiepark-mainzde (accessed on 10032017)9 M Kopp D Coleman C Stiller K Scheffer J Aichinger and B Scheppat Inter-

national Journal of Hydrogen Energy 42(19) (2017)10 S Metz Fuel Cells Bulletin 2014 12 (2014)11 M Stefan in 20th World Hydrogen Energy Conference (WHEC) KDJ Convention

Center Gwangju South Korea (2014)12 H Ishikawa E Haryu N Kawasaki and H Daimon Honda RampD Technical Review

28 (2016)13 P J Bouwman J Konink D Semerel L Raymakers M Koeman W Dalhuijsen

E Milacic and M Mulder ECS Transactions 64 1009 (2014)14 D G Bessarabov H Wang H Li and N Zhao PEM Electrolysis for Hydrogen

Production Principles and Applications CRC Press Boca Raton FL (USA) (2015)15 L Bertuccioli A Chan D Hart F Lehner B Madden and E Standen Study on

Development of Water Electrolysis in the EU by E4tech Slsquoarl with Element EnergyLtd for the Fuel Cells and Hydrogen Joint Undertaking in (2014)

16 M Carmo D L Fritz J Mergel and D Stolten International Journal of HydrogenEnergy 38 4901 (2013)

F1192 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 5 (A) Hydrogen and oxygen gas crossover current density equivalents for wet 7 5 and 2 mil thick Nafion membranes (actual thickness 209 152 and63 μm) Linear fits are added for both hydrogen (solid lines) and oxygen (dashed lines) Data is calculated based on the experimental results of Schalenbachet al39 (B) The corresponding faradaic efficiency is calculated for differential pressure PEWE using wet Nafion 117 or Nafion 112 at 1 and 3 A middot cmminus2 for 50Cas a function of the operating pressure The data is based on Figure 5A Hypothetic new membranes with the thickness of Nafion 112 and different fractions of thepermeability (05 02 01) are added for discussion

Hypothetic new membranes with a permeability of only onetenth of Nafion dramatically reduce the gas crossover losses enhanc-ing compression efficiency with such a membrane material 93 com-pression efficiency would be obtained for a 2-mil membrane at 100MPa (3 A middot cmminus2 and balanced pressure) This is similar to isothermalcompression behavior and would outperform todayrsquos best mechanicalcompressors

Considering the total cell efficiency membrane thickness does notonly influence the gas crossover losses but also the correspondingohmic overpotential Thus a tradeoff between both losses has to befound for the optimal cell efficiency as discussed below

PEWE cell efficiencymdashThe main focus is on the hydrogen com-pression efficiency However when discussing the gas permeabilityproperties of different membrane types or thicknesses it should also

be paid attention on the cell efficiency considering the faradaic losseselectrochemical compression losses and all other overpotentials in-cluding the (ionic) ohmic resistances of the membrane Hence de-pending on the desired hydrogen pressure a tradeoff between themembrane induced ohmic overpotential and gas crossover needs tobe found Therefore the total compression efficiency data in Figure 6are combined with measured PEWE cell performance (according toEquation 8) using Nafion 112 and 117 membranes at 50C and ambi-ent pressure and results shown in Figure 7 The given cell efficienciesare based on the LHV of hydrogen The corresponding cell voltages at1 and 3 A middot cmminus2 are 185 and 211 V for Nafion 112 and 197 and 250V for Nafion 117 respectively Similar iE-curves at 1 MPa balancedpressure are given in Ref 21

The observations concerning the hydrogen compression efficiency(Figure 6) are still valid thus only few new insights are deduced

Figure 6 Total compression efficiencies for differential and balanced pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50C for 1 and 3 A middot cmminus2

as a function of the operating pressure Hypothetic new membranes with 110 of the Nafion permeability are added for comparison the isothermal compressionbehavior at 50C and best mechanical compression technology11 are also shown The PEWE data is based on Figures 3 and 5B

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1193

Figure 7 Cell efficiency (including all electrochemical andfaradaic losses) according to Equation 8 normalized to E0

cell= f(pH2T) or LHV of hydrogen for differential and bal-anced pressure PEWE using Nafion 112 and Nafion 117membranes at 50C as a function of the operating pressureat (A) 1 A middot cmminus2 and (B) 3 A middot cmminus2

Cell efficiencies are almost constant up to 1 MPa at 1 A middot cmminus2

and up to 10 MPa at 3 A middot cmminus2 Further pressure increase reduces cellefficiencies but less at high current densities (here 3 A middot cmminus2) due tothe smaller influence of gas crossover and lower apparent compressioneffort for balanced pressure operation

At low current densities (here 1 A middot cmminus2) Nafion 112 has bettercell efficiencies with about 62 up to several MPa and about 60 athigh current densities up to about 10 MPa compared to Nafion 117with about 60 and 50 respectively However at higher pressuresthe better gas barrier properties of Nafion 117 offset the higher ohmicoverpotential

Nevertheless from an energetic point of view the data show thatlow gas crossover membrane materials will be key for efficient electro-chemical compression For ultimate efficiency these materials wouldbest be combined with the beneficial processes of the balanced pres-sure strategy where OER kinetic gains are (partly) compensating forthe thermodynamic cell voltage increase due to pressurization How-ever high-pressure oxygen (typically above 3 MPa) is a severe safetyconcern and difficult to implement in a technical process Hence itseems sensible to limit oxygen pressure This will also be beneficialfor the oxygen crossover losses be it with conventional or future lowgas permeation materials For this novel pressure strategy we use theterm ldquointermediate differentialrdquo pressure PEWE Its characteristicswith respect to hydrogen compression efficiency are discussed below

Intermediate differential pressure PEWEmdashWith the OER kinet-ics following a logarithmic and the gas crossover following a linearpressure relation there must be an optimum operating point mini-mizing the total compression losses by choosing a slightly increasedanode and a considerably increased cathode pressure Experimentshave therefore been performed to better understand this novel pres-sure concept where the anode is pressurized less than the cathodeIn Figure 8A data with low anode pressures (1 and 2 MPa) and highcathode pressures (3 and 5 MPa) are shown A relatively small anodepressure of 1 MPa is sufficient to retain the voltage gains due to in-creased OER kinetics as no further decrease in cell voltage is observedwith the oxygen pressure increase to 2 MPa as shown in Figure 8B

The experimental (1 5 MPa) data are extrapolated to higher pres-sures with an isothermal compression behavior of 84 at 1 A middot cmminus2

and 24 at 3 A middot cmminus2 and compared to those from differential andbalanced pressure in Figure 9 The extrapolated data show that athigher current densities the intermediate differential pressure conceptis energetically more efficient than differential pressure operation dueto a higher electrochemical compression efficiency which is overcom-pensating the relatively small additional oxygen gas crossover lossesIn particular at higher pressures the limited oxygen gas crossover

losses for intermediate differential pressure are crucial for being moreenergetically efficient than all other pressure strategies and even betterthan mechanical compression up to almost 100 MPa for Nafion 117

Those experimental findings are rather counter-intuitive becauseeven if oxygen is considered as a by-product and only pressurizedhydrogen is of interest a 1 MPa compression of oxygen is energeti-cally beneficial Furthermore operating and system investment costs(opex and capex) should be lower for intermediate differential than forbalanced pressure operation due to lower safety requirements of thelow pressurized oxygen and thus be similar to conventional differen-tial pressure operation where the oxygen pressure is close to ambientpressure

Conclusions

In this study the cell voltage and gas crossover related losses dueto electrochemical pressurization were investigated experimentally upto 5 MPa and based on literature study up to 70 MPa for differentialand balanced pressure PEWE and EHCs As expected from ther-modynamics differential pressure electrolysis and EHCs generallyfollow an isothermal compression behavior In contrast the apparentcompression effort for balanced pressure electrolysis decreases withincreasing current density due to improved OER kinetics Part of thisenergetic advantage is offset by gas crossover losses roughly twicethose of differential pressure operation

The energetic analysis of the total hydrogen compression effi-ciency combining electrochemical and faradaic losses up to 100MPa shows that differential pressure electrolysis is preferable to bal-anced pressure at low current densities (le 1 A middot cmminus2) due to lowergas crossover losses and similar compression behavior However athigher current densities the decreasing apparent compression effort ofthe balanced pressure PEWE compensates for the higher gas crossoverlosses resulting in higher total compression efficiencies

Intermediate differential pressure electrolysis with a slightly pres-surized oxygen side and a higher pressure at the hydrogen side com-bines the advantages of both pressure concepts retaining the low oxy-gen gas crossover losses of the differential pressure and the decreasingcompression effort at higher current densities of the balanced pres-sure operation Based on the extrapolation of experimental data theintermediate differential pressure strategy with cells having a mem-brane electrolyte with the gas permeation characteristics of Nafion117 energetically outperform todayrsquos best mechanical compressorsup to tens of MPa with similar compression losses at 100 MPa

With respect to industrially relevant application the intermedi-ate differential pressure strategy respects the challenges and costsof compressed oxygen which will be limited to close to those forconventional differential pressure PEWE with oxygen production at

F1194 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 8 (A) intermediate differential pressure PEWE iR-free iE-curves at 50C at indicated pressure levels (B) ra-tio of the measured iR-free cell voltage difference betweenpressurized and ambient pressure operation (E(iR-free))and the theoretical thermodynamic cell voltage increase(E0

cell(pH2T)) considering only hydrogen compressionie 100 represents an isothermal compression behavioraccording to Equation 1

Figure 9 Total compression efficiencies for differential balanced and intermediate differential pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50Cfor 1 and 3 A middot cmminus2 as a function of the operating pressure For comparison the isothermal compression behavior at 50C and best mechanical compressiontechnology11 are added The PEWE data is based on Figures 3 5B and 8

atmospheric pressure If the technical risk to have highest pressurehydrogen in the electrolyzer in close vicinity to pure oxygen is consid-ered too high then a strategy with a combination of intermediate dif-ferential PEWE up to intermediate pressures of about 10 MPa and finalcompression to highest pressures (100 MPa) by EHCs seems a promis-ing concept with respect to both energetic and safety properties

Acknowledgments

Funding by the Swiss Federal Office of Energy (SFOE under grantno SI500904-01) Belenos Clean Power Holding Ltd and the En-ergy System Integration (ESI) platform at PSI as well as technicalsupport by Martin Ammann and Thomas Gloor (both PSI) are grate-fully acknowledged TJS thanks the Commission for Technology andInnovation Switzerland and the Swiss Competence Center for EnergyResearch Heat amp Electricity Storage

References

1 httpeceuropaeueurostatwebenergydatadatabase (accessed on 31012017)2 Erneuerbare Energien in Zahlen - Nationale und internationale Entwicklung im Jahr

2015 in Bundesministerium fur Wirtschaft und Energie (BMWi) (2016)3 C J Barnhart M Dale A R Brandt and S M Benson Energy amp Environmental

Science 6 2804 (2013)

4 Monitoringbericht 2016 in G Bundesnetzagentur fur Elektrizitat Telekommu-nikation Post und Eisenbahnen Bundeskartellamt Editor Bundesnetzagentur furElektrizitat GasTelekommunikation Post und Eisenbahnen Bundeskartellamt(2016)

5 T Smolinka E T Ojong and J Garche in Electrochemical Energy Storage forRenewable Sources and Grid Balancing P T Moseley and J Garche Editors p 103(2015)

6 httpsenergygoveerefuelcellsdoe-technical-targets-hydrogen-production-electro-lysis (accessed on 10032017)

7 J Alkire Technology Validation Program Area - Plenary Presentation U S Depart-ment of Energy 2017 Annual Merit Review and Peer Evaluation Meeting (2017)

8 httpwwwenergiepark-mainzde (accessed on 10032017)9 M Kopp D Coleman C Stiller K Scheffer J Aichinger and B Scheppat Inter-

national Journal of Hydrogen Energy 42(19) (2017)10 S Metz Fuel Cells Bulletin 2014 12 (2014)11 M Stefan in 20th World Hydrogen Energy Conference (WHEC) KDJ Convention

Center Gwangju South Korea (2014)12 H Ishikawa E Haryu N Kawasaki and H Daimon Honda RampD Technical Review

28 (2016)13 P J Bouwman J Konink D Semerel L Raymakers M Koeman W Dalhuijsen

E Milacic and M Mulder ECS Transactions 64 1009 (2014)14 D G Bessarabov H Wang H Li and N Zhao PEM Electrolysis for Hydrogen

Production Principles and Applications CRC Press Boca Raton FL (USA) (2015)15 L Bertuccioli A Chan D Hart F Lehner B Madden and E Standen Study on

Development of Water Electrolysis in the EU by E4tech Slsquoarl with Element EnergyLtd for the Fuel Cells and Hydrogen Joint Undertaking in (2014)

16 M Carmo D L Fritz J Mergel and D Stolten International Journal of HydrogenEnergy 38 4901 (2013)

Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017) F1193

Figure 7 Cell efficiency (including all electrochemical andfaradaic losses) according to Equation 8 normalized to E0

cell= f(pH2T) or LHV of hydrogen for differential and bal-anced pressure PEWE using Nafion 112 and Nafion 117membranes at 50C as a function of the operating pressureat (A) 1 A middot cmminus2 and (B) 3 A middot cmminus2

Cell efficiencies are almost constant up to 1 MPa at 1 A middot cmminus2

and up to 10 MPa at 3 A middot cmminus2 Further pressure increase reduces cellefficiencies but less at high current densities (here 3 A middot cmminus2) due tothe smaller influence of gas crossover and lower apparent compressioneffort for balanced pressure operation

At low current densities (here 1 A middot cmminus2) Nafion 112 has bettercell efficiencies with about 62 up to several MPa and about 60 athigh current densities up to about 10 MPa compared to Nafion 117with about 60 and 50 respectively However at higher pressuresthe better gas barrier properties of Nafion 117 offset the higher ohmicoverpotential

Nevertheless from an energetic point of view the data show thatlow gas crossover membrane materials will be key for efficient electro-chemical compression For ultimate efficiency these materials wouldbest be combined with the beneficial processes of the balanced pres-sure strategy where OER kinetic gains are (partly) compensating forthe thermodynamic cell voltage increase due to pressurization How-ever high-pressure oxygen (typically above 3 MPa) is a severe safetyconcern and difficult to implement in a technical process Hence itseems sensible to limit oxygen pressure This will also be beneficialfor the oxygen crossover losses be it with conventional or future lowgas permeation materials For this novel pressure strategy we use theterm ldquointermediate differentialrdquo pressure PEWE Its characteristicswith respect to hydrogen compression efficiency are discussed below

Intermediate differential pressure PEWEmdashWith the OER kinet-ics following a logarithmic and the gas crossover following a linearpressure relation there must be an optimum operating point mini-mizing the total compression losses by choosing a slightly increasedanode and a considerably increased cathode pressure Experimentshave therefore been performed to better understand this novel pres-sure concept where the anode is pressurized less than the cathodeIn Figure 8A data with low anode pressures (1 and 2 MPa) and highcathode pressures (3 and 5 MPa) are shown A relatively small anodepressure of 1 MPa is sufficient to retain the voltage gains due to in-creased OER kinetics as no further decrease in cell voltage is observedwith the oxygen pressure increase to 2 MPa as shown in Figure 8B

The experimental (1 5 MPa) data are extrapolated to higher pres-sures with an isothermal compression behavior of 84 at 1 A middot cmminus2

and 24 at 3 A middot cmminus2 and compared to those from differential andbalanced pressure in Figure 9 The extrapolated data show that athigher current densities the intermediate differential pressure conceptis energetically more efficient than differential pressure operation dueto a higher electrochemical compression efficiency which is overcom-pensating the relatively small additional oxygen gas crossover lossesIn particular at higher pressures the limited oxygen gas crossover

losses for intermediate differential pressure are crucial for being moreenergetically efficient than all other pressure strategies and even betterthan mechanical compression up to almost 100 MPa for Nafion 117

Those experimental findings are rather counter-intuitive becauseeven if oxygen is considered as a by-product and only pressurizedhydrogen is of interest a 1 MPa compression of oxygen is energeti-cally beneficial Furthermore operating and system investment costs(opex and capex) should be lower for intermediate differential than forbalanced pressure operation due to lower safety requirements of thelow pressurized oxygen and thus be similar to conventional differen-tial pressure operation where the oxygen pressure is close to ambientpressure

Conclusions

In this study the cell voltage and gas crossover related losses dueto electrochemical pressurization were investigated experimentally upto 5 MPa and based on literature study up to 70 MPa for differentialand balanced pressure PEWE and EHCs As expected from ther-modynamics differential pressure electrolysis and EHCs generallyfollow an isothermal compression behavior In contrast the apparentcompression effort for balanced pressure electrolysis decreases withincreasing current density due to improved OER kinetics Part of thisenergetic advantage is offset by gas crossover losses roughly twicethose of differential pressure operation

The energetic analysis of the total hydrogen compression effi-ciency combining electrochemical and faradaic losses up to 100MPa shows that differential pressure electrolysis is preferable to bal-anced pressure at low current densities (le 1 A middot cmminus2) due to lowergas crossover losses and similar compression behavior However athigher current densities the decreasing apparent compression effort ofthe balanced pressure PEWE compensates for the higher gas crossoverlosses resulting in higher total compression efficiencies

Intermediate differential pressure electrolysis with a slightly pres-surized oxygen side and a higher pressure at the hydrogen side com-bines the advantages of both pressure concepts retaining the low oxy-gen gas crossover losses of the differential pressure and the decreasingcompression effort at higher current densities of the balanced pres-sure operation Based on the extrapolation of experimental data theintermediate differential pressure strategy with cells having a mem-brane electrolyte with the gas permeation characteristics of Nafion117 energetically outperform todayrsquos best mechanical compressorsup to tens of MPa with similar compression losses at 100 MPa

With respect to industrially relevant application the intermedi-ate differential pressure strategy respects the challenges and costsof compressed oxygen which will be limited to close to those forconventional differential pressure PEWE with oxygen production at

F1194 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 8 (A) intermediate differential pressure PEWE iR-free iE-curves at 50C at indicated pressure levels (B) ra-tio of the measured iR-free cell voltage difference betweenpressurized and ambient pressure operation (E(iR-free))and the theoretical thermodynamic cell voltage increase(E0

cell(pH2T)) considering only hydrogen compressionie 100 represents an isothermal compression behavioraccording to Equation 1

Figure 9 Total compression efficiencies for differential balanced and intermediate differential pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50Cfor 1 and 3 A middot cmminus2 as a function of the operating pressure For comparison the isothermal compression behavior at 50C and best mechanical compressiontechnology11 are added The PEWE data is based on Figures 3 5B and 8

atmospheric pressure If the technical risk to have highest pressurehydrogen in the electrolyzer in close vicinity to pure oxygen is consid-ered too high then a strategy with a combination of intermediate dif-ferential PEWE up to intermediate pressures of about 10 MPa and finalcompression to highest pressures (100 MPa) by EHCs seems a promis-ing concept with respect to both energetic and safety properties

Acknowledgments

Funding by the Swiss Federal Office of Energy (SFOE under grantno SI500904-01) Belenos Clean Power Holding Ltd and the En-ergy System Integration (ESI) platform at PSI as well as technicalsupport by Martin Ammann and Thomas Gloor (both PSI) are grate-fully acknowledged TJS thanks the Commission for Technology andInnovation Switzerland and the Swiss Competence Center for EnergyResearch Heat amp Electricity Storage

References

1 httpeceuropaeueurostatwebenergydatadatabase (accessed on 31012017)2 Erneuerbare Energien in Zahlen - Nationale und internationale Entwicklung im Jahr

2015 in Bundesministerium fur Wirtschaft und Energie (BMWi) (2016)3 C J Barnhart M Dale A R Brandt and S M Benson Energy amp Environmental

Science 6 2804 (2013)

4 Monitoringbericht 2016 in G Bundesnetzagentur fur Elektrizitat Telekommu-nikation Post und Eisenbahnen Bundeskartellamt Editor Bundesnetzagentur furElektrizitat GasTelekommunikation Post und Eisenbahnen Bundeskartellamt(2016)

5 T Smolinka E T Ojong and J Garche in Electrochemical Energy Storage forRenewable Sources and Grid Balancing P T Moseley and J Garche Editors p 103(2015)

6 httpsenergygoveerefuelcellsdoe-technical-targets-hydrogen-production-electro-lysis (accessed on 10032017)

7 J Alkire Technology Validation Program Area - Plenary Presentation U S Depart-ment of Energy 2017 Annual Merit Review and Peer Evaluation Meeting (2017)

8 httpwwwenergiepark-mainzde (accessed on 10032017)9 M Kopp D Coleman C Stiller K Scheffer J Aichinger and B Scheppat Inter-

national Journal of Hydrogen Energy 42(19) (2017)10 S Metz Fuel Cells Bulletin 2014 12 (2014)11 M Stefan in 20th World Hydrogen Energy Conference (WHEC) KDJ Convention

Center Gwangju South Korea (2014)12 H Ishikawa E Haryu N Kawasaki and H Daimon Honda RampD Technical Review

28 (2016)13 P J Bouwman J Konink D Semerel L Raymakers M Koeman W Dalhuijsen

E Milacic and M Mulder ECS Transactions 64 1009 (2014)14 D G Bessarabov H Wang H Li and N Zhao PEM Electrolysis for Hydrogen

Production Principles and Applications CRC Press Boca Raton FL (USA) (2015)15 L Bertuccioli A Chan D Hart F Lehner B Madden and E Standen Study on

Development of Water Electrolysis in the EU by E4tech Slsquoarl with Element EnergyLtd for the Fuel Cells and Hydrogen Joint Undertaking in (2014)

16 M Carmo D L Fritz J Mergel and D Stolten International Journal of HydrogenEnergy 38 4901 (2013)

F1194 Journal of The Electrochemical Society 164 (12) F1187-F1195 (2017)

Figure 8 (A) intermediate differential pressure PEWE iR-free iE-curves at 50C at indicated pressure levels (B) ra-tio of the measured iR-free cell voltage difference betweenpressurized and ambient pressure operation (E(iR-free))and the theoretical thermodynamic cell voltage increase(E0

cell(pH2T)) considering only hydrogen compressionie 100 represents an isothermal compression behavioraccording to Equation 1

Figure 9 Total compression efficiencies for differential balanced and intermediate differential pressure PEWE using (A) Nafion 112 and (B) Nafion 117 at 50Cfor 1 and 3 A middot cmminus2 as a function of the operating pressure For comparison the isothermal compression behavior at 50C and best mechanical compressiontechnology11 are added The PEWE data is based on Figures 3 5B and 8

atmospheric pressure If the technical risk to have highest pressurehydrogen in the electrolyzer in close vicinity to pure oxygen is consid-ered too high then a strategy with a combination of intermediate dif-ferential PEWE up to intermediate pressures of about 10 MPa and finalcompression to highest pressures (100 MPa) by EHCs seems a promis-ing concept with respect to both energetic and safety properties

Acknowledgments

Funding by the Swiss Federal Office of Energy (SFOE under grantno SI500904-01) Belenos Clean Power Holding Ltd and the En-ergy System Integration (ESI) platform at PSI as well as technicalsupport by Martin Ammann and Thomas Gloor (both PSI) are grate-fully acknowledged TJS thanks the Commission for Technology andInnovation Switzerland and the Swiss Competence Center for EnergyResearch Heat amp Electricity Storage

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