T

Ta

b

a

ARAA

KPDAAC

C

1

hw

1d

Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

journa l homepage: www.e lsev ier .com/ locate / rser

he potential of PEM fuel cell for a new drinking water source

aeyoung Kima,b, Seungjae Leea, Heekyung Parka,∗

Department of Civil and Environmental Engineering, KAIST, Guseong-dong, Yuseong-gu, Daejeon, Republic of KoreaFuel Cell Research Center, Korea Institute of Energy Research, 102, Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea

r t i c l e i n f o

rticle history:eceived 21 December 2010ccepted 24 June 2011vailable online 6 August 2011

eywords:EM fuel cellrinking water sourceir stoichiometryir pressureell temperature

a b s t r a c t

The worldwide water scarcity, especially in the developing countries and arid regions, forces people torely on unsafe sources of drinking water. There is a pressing need for these regions to develop decen-tralized, small-scale water utilities. However, more than 50% of the total operating costs associated withsuch small-scale, water-utility operations are the cost of providing electricity to run water pumps. Wethink that advances in a variety of renewable and sustainable energy technologies offer considerablepromise for reducing the energy required for the production and distribution of water by small-scalewater utilities. This paper provides a comprehensive review of the potential for using proton exchangemembrane (PEM) fuel cells to provide an alternative supply of drinking water. This system can eliminatethe excessive energy requirements that are currently associated with water production. Such alternativewater production processes are designed to increase the production rate of drinking water by reducingthe amount of water required to humidify the reactant gases during stable cell performance. The principal

operational components of PEM fuel cells are reviewed and evaluated, including air stoichiometry, pres-sure, and cell temperature. Hydrogen-fed fuel cell systems provide sufficient water to meet the potablewater needs of a typical household. Furthermore, it is concluded that PEM fuel cells have great promisefor decentralized, small-scale, water-production applications, because they are capable of generatingsufficient quantities of potable water by operating at maximum power and by increasing the number ofpolymer membranes.

© 2011 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36762. Production of drinking water from fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36773. Scenario for water and energy production from PEM fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36784. Mass balance for reactant and product species in a PEM fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36795. Review on parameters for increasing the amount of water source produced from PEM fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3680

5.1. Humidification of the inlet air and hydrogen feed streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36815.2. Polymer membrane thickness and material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36825.3. Stoichiometry, pressure, and temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3682

6. Drinking water production rate from PEM fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36857. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3688

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3688References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3688

. Introduction worse as the population increases, which results in increases inthe need for water in households and industries. When potable

Water is an essential resource that is required for life and goodealth. Yet, 33% of the people in the world do not have enoughater to meet their daily needs. Globally, the problem is getting

∗ Corresponding author. Tel.: +82 42 350 3620; fax: +82 42 350 3610.E-mail address: hpark57@kaist.edu (H. Park).

364-0321/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.rser.2011.06.006

water is scarce, people are forced to rely on unsafe sources of drink-ing water. Poor water quality can increase the risk of diarrhoealdiseases, such as cholera, typhoid fever, and dysentery, which arethe second leading cause of death in children under five years old.

Impure water can also cause many other water-borne infectionsthat are life-threatening. One quarter of the global population livesin developing countries that face water shortages due to the lack of

T. Kim et al. / Renewable and Sustainable En

Nomenclature

a capture efficiency of effluent water in condenser (%)A total membrane area (cm2)EW equivalent weight of polymer membraneF Faraday constant (96,485 C mol−1)i current density (A cm−2)I current (A)MEA membrane electrode assemblyma,d mass of dry hydrogen at anode inlet (kg s−1)mc,d mass of dry air at cathode inlet (kg s−1)ma,w mass of water present in anode inlet (kg s−1)mc,w mass of water present in cathode inlet (kg s−1)mH2O molecular mass of water (18.02 g mol−1)MC amount of captured water at condenser per day

(kg d−1)MD amount of available drinking water per day (kg d−1)MH water amount supplied for internal humidification

per day (kg d−1)MP amount of water production through electrochem-

ical reaction (kg d−1)pa total pressure at anode inlet (atm)pc total pressure at cathode inlet (atm)pa,w,in water partial pressure at anode inlet (atm)pc,w,in water partial pressure at cathode inlet (atm)pa,w,out water partial pressure at anode outlet (atm)pc,w,out water partial pressure at cathode outlet (atm)psat saturated water vapor pressure of inlet gas (atm)psat

cellsaturated water vapor pressure at given cell tem-perature (atm)

RHa,in hydrogen relative humidity at anode inlet (%)RHc,in air relative humidity at cathode inlet (%)RHa,out hydrogen relative humidity at anode outlet (%)RHc,out air inlet relative humidity at cathode outlet (%)T1 entry temperature of the air at compressor (K)xH2O mole fraction of water vapor in the exhaust air gas

Greek symbols˛ net water drag coefficient (mol s−1 cm−2)�c air stoichiometry number�a hydrogen stoichiometry number�c compressor efficiency

isfmawdaaesabb

eut[c

�m efficiency of the electric drive for the compressor

nfrastructure that acquire water from rivers and aquifers. Watercarcity occurs even in areas where there is plenty of rainfall orreshwater. In arid and semi-arid regions, where water scarcity is

ore serious and endemic by definition, groundwater has playedmajor role in meeting domestic and irrigation demands. Ground-ater mining and the lack of adequate planning have opened a newebate within legal frameworks and governance on the sustain-bility of the intensive use of groundwater. Desalination systemsre also a viable alternative for solving the water shortage. How-ver, conventional desalination systems are usually large-scaleystems, which are more suitable for energy-rich and economicallydvanced regions. Also, they cause environmental degradationecause they are fossil-fuel driven and because of the problem ofrine disposal [1].

Furthermore, we are not fully aware of the linkages betweennergy, water, and sanitation service. In general, the electricity

sage to convey, treat, and distribute water for all uses amountso more than 20% of the total electricity usage in the United States2]. Quite often, more than 50% of the total operating costs asso-iated with small-scale water utility operations are the cost of the

ergy Reviews 15 (2011) 3676–3689 3677

electricity required to run the water pumps. Furthermore, unre-liable power supply results in increased energy costs due to theneed for back-up power and for installation of protection systems toprevent damage to electrical systems. Inefficiency in the operationof water utilities results in high levels of non-revenue-producingwater and unaccounted for water. The use of old equipment thatis past its operational lifespan results in higher energy costs andinefficiency in system operations. The lack of energy-efficient sys-tem designs increases the operational and maintenance costs forthe utilities, and their operational costs are increased further bydamage to the electrical installations in water supply and sanita-tion systems caused by an unreliable power supply. Thus, we thinkthat advances in a variety of renewable and sustainable energytechnologies offer considerable promise for reducing the energyrequired for the production and distribution of water by small-scale water utility companies. In particular, hydrogen-fed fuel cellsgenerate pure water, electricity, and heat energy through electro-chemical reactions. Among the hydrogen-fed fuel cells, the protonexchange membrane (PEM) fuel cells are widely used due to highpower density, quick response to changes in current demand, lowoperating temperature, and relatively low cost. Hristovski et al. [3]indicated that PEM fuel cells, as electrical energy and safe waterproduction systems, could make a significant contribution to thefuture hydrogen economy. If appropriate renewable energy tech-nologies, such as wind and solar, are designed for stable hydrogensupply, the fuel cell systems will eliminate the hundreds of miles ofpipelines required to convey water from sensitive aquatic ecosys-tems. Furthermore, they may be advantageous for inland regionswhere water distribution is uneven. In the following sections, PEMfuel cell systems are introduced as viable water production systems,and useful guidelines for suitable ranges of operational parametersare provided to increase the amount of drinking water generatedby PEM fuel cells.

2. Production of drinking water from fuel cell

The Gemini spacecraft was the first to use an alkaline electrolytefuel cell to provide electricity as well as drinking water for the astro-nauts [4]. In an alkaline electrolyte fuel cell, hydroxyl ions (OH−)are mobile, and, as they approach the anode, they react with hydro-gen, releasing electrons; then, water is produced at the anode asa byproduct through electrochemical reaction. However, problemsare encountered in the use of water generated by fuel cells, becausethe water contains sulfobenzoic acid, p-benzaldehyde sulfonic acid,and formaldehyde due to the degradation of the organic electrode[5]. Even though the water can be treated by means of filtration,carbon sorption, and ion exchange resins, none of these methodshas proven to be sufficiently effective to make safe, potable water.These problems were resolved in the Apollo program by replacingthe organic electrodes with sintered nickel electrodes, which donot degrade. Consequently, the fuel cell systems aboard the Apollospacecraft produced safe water of extremely high quality at a peakrate of 1 kg h−1. In 1998, Orta et al. [6] conducted a study of the qual-ity of water generated by fuel cells aboard the U.S. Space Shuttle andthe Russian Mir Space Station. According to their analytical results,the water generated aboard both spacecraft was high quality, withonly a few anions and cations at �g/L concentrations. In 2009, Hris-tovski et al. [3] examined the quality of the water generated byvarious PEM fuel cells with different external humidification sys-tems. Water samples were collected from six different PEM fuelcell systems, and their constituents were measured. The qualityof the water generated by fuel cells was directly related to the

quality of the input water supplied from different external humid-ification systems. However, the quality parameters of the watergenerated by PEM fuel cells were below the maximum contami-nant levels with the exception of zinc, lead, and antimony, which

3678 T. Kim et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.2

0.4

0.6

0.8

1.0

1.2

Current density / A cm-2

0.0

0.1

0.2

0.3

0.4

0.5(a)

Cel

l vol

tage

/ V

Power density / W

cm-2

0 1000 2000 3000 4000 5000 60000

10

20

30

40

50

Water production Power

Current / A

0.0

0.5

1.0

1.5

2.0

2.5

Scenario I Scenario III

Wat

er p

rodu

ctio

n (k

g d-1

)Pow

er / kW

Scenario II(b)

F f PEMs 0]. It i1

mofdewhiS

3c

t

W

ig. 1. (a) Polarization (filled symbols) and power density curves (open symbols) oymbols). The fuel cell performance is estimated from published values of Ref. [100 cm−2, respectively.

ay be related to leaching of these materials from the plumbingr from fuel cell materials. They concluded that the water collectedrom PEM fuel cell systems may be ‘safe’ as an alternative source ofrinking water. These results showed that the quality of water gen-rated by PEM fuel cells was higher than the quality of typical tapater and that the quality was in compliance with regulations thatave been established by National Aeronautics and Space Admin-

stration (NASA) and the U.S. Environmental Protection Agency (U.. EPA).

. Scenario for water and energy production from PEM fuelell

In a PEM fuel cell, water is produced at the rate of 1 mol for every

wo electrons and can be expressed by Eq. (1):

ater production = iAMw

2F(kg s−1) (1)

fuel cell. (b) Plot of the water production rate (dash line) and power density (opens assumed that the number of polymer membrane and active area are 50 EA and

where i is the current density (A cm−2), A is the total areaof the polymer membrane (cm2), Mw is the molecular massof water (18.02 × 10−3 kg mol−1), and F is the Faraday constant(96,485 C mol−1).

Hristovski et al. [3] showed that a commercial PEM fuel cellstack (i.e., Mark 1020 ACS) manufactured by Ballard Power Sys-tems, Inc., which is compromised of 46 membranes with a totalcurrent of 2392 A and a power of 1.63 kW, can produce approx-imately 19.3 kg d−1 of water, assuming two-phase water captureof 100%. While this water production rate is insufficient to meetthe non-internal, consumptive uses of water in a typical household(e.g., bodily contact, washing, outdoor use, and commercial activi-ties), it is sufficient for meeting the potable water needs of a typicalU.S. household [7]. Additionally, it is possible to generate sufficient

potable water quantity by running the fuel cell at maximum powerand increasing the number of polymer membrane.

Fig. 1(a) shows that the electrical power increases to a maxi-mum and then decreases due to oxygen mass-transport resistance

T. Kim et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689 3679

ell wi

boipscnme

wddfcTwasistp

PaGaorioftnawMmdhsofr

fm

Fig. 2. Schematic of water cycle in a fuel c

y water flooding at high current. In order to increase the amountf product water from PEM fuel cell, it needs to increase the limit-ng current density without water flooding. Additionally, Fig. 1(b)resents that water production increases linearly with current den-ity. These show that water and electrical power productions arelassified according to electrical power produced by stack (i.e., Sce-ario I-stage before maximum electrical power, Scenario II-stage ataximum electrical power, and Scenario III-stage after maximum

lectrical power).Scenario II and III can produce maximum electrical power and

ater, respectively. However, scenario III is in an unfavorable con-ition for production of electrical power due to severe potentialrop by water flooding. A main limitation in scenario III resultsrom the transport of reactants from the gas flow channel to theatalyst layer, referred to as the oxygen mass-transport limitation.his limitation is further amplified by the presence of liquid water,hich blocks some of the open pores in the gas diffusion layer (GDL)

nd thus reduces the available paths for the transport of reactantpecies. This phenomenon, commonly referred to as water flooding,s more severe in the cathode at high current density because thelower oxygen reduction reaction is more susceptible to the nega-ive impact caused by flooding. Thus, in point of water and energyroduction, Scenario II is the better than Scenario III.

In order to increase the production of water and electricity byEM fuel cells, the limiting current density must be increased whilevoiding the flooding induced by accumulated liquid water in theDL and gas flow channel. The main limitation that prevents thechievement of high current density results from the transportf reactant gases from the gas flow channel to the catalyst layer,eferred to as the oxygen mass-transport limitation. This limitations exacerbated by the presence of liquid water, which blocks somef the open pores in the GDL and thus reduces the available pathsor the transport of reactant species. Consequently, it is importanto remove excessive liquid water in the GDL and gas flow chan-el in order to increase the limiting current density. The materialnd operational parameters of PEM fuel cells can effectively reduceater flooding and increase cell potential at high current density.aterial parameters are generally divided into polymer electrolyteembrane, GDL, micro-porous layer (MPL), and gas flow channel

esigns. Operational parameters are associated with the relativeumidity, reactant flow rate (i.e., gas stoichiometry), and pres-ure of the inlet reactant gases and the operational temperaturef the cell. All of these parameters have strong impacts on the per-ormance of the fuel cell, and they are interrelated by non-linear

elationships.

In addition to increasing the limiting current density in PEMuel cells, the provision of input water for hydration of the poly-

er membrane influences the amount of drinking water produced.

th internal humidification at steady state.

Usually, PEM fuel cells should be fully hydrated by an externalhumidification system to ensure high proton conductivity, whichincreases the performance of the cell and its water productionat the cathode. However, a PEM fuel cell that does not requireexternal humidification of the reactant gases is highly desirablefrom the engineering perspective of power generation, becausethe extra cost of humidification equipment can exceed the sav-ings produced by a smaller and lighter fuel cell [8]. Additionally,it is possible to eliminate the parasitic losses attributed to heat-ing the gas humidifiers. Also, non-humidified gases are richer infuel or oxidant per unit volume compared to the equivalent wetgas streams, which favors higher cell efficiency [9]. In attemptsto overcome these problems, internal humidification by recycledexhaust gas has been used for polymer membrane hydration inPEM fuel cells, as shown in Fig. 2. However, if the input watermass (i.e., MH) for humidification of the reactant gases increasesdramatically during the cell operation, there is a sharp decreasein the amount of drinking water (i.e., MD) generated by the PEMfuel cell because the efficiency of the condenser in capturing efflu-ent water is not perfect. Thus, minimal use of humidification (i.e.,dry or slightly humidified gas streams) for reactant gases is desir-able in order to produce more drinking water without affectingelectrical power productivity. However, the performance of PEMfuel cells that are operated with dry or slightly humidified gasstreams is significantly decreased due to the increase of ohmicresistance compared to cells with analogous, well-humidified gasstreams. Technically, the performance of a PEM fuel cell in termsof voltage and power density can be greatly influenced by var-ious operating parameters, such as reactant gas stoichiometry,operating pressure, and cell temperature. In addition, these opera-tional parameters can affect the relative humidity of the exit gases,which must be within a proper range for stable cell performance.Therefore, it is essential to quantify the effects of operationalparameters on the performance and water production rate of PEMfuel cells.

4. Mass balance for reactant and product species in a PEMfuel cell

At steady-state conditions in a PEM fuel cell, mass balances forreactant and product species can be described by the law of conser-vation of mass, as depicted in Fig. 3. The mole numbers associatedwith each reactant and product are related to the operational con-ditions and are provided in Table 1. To derive the formula of mass

balance, pressure drop on the cathode side and the anode sidewere neglected and that the water generated by the electrochem-ical reaction exists in the vapor phase. The relationship betweenthe molar flux of reactant and product species and the partial pres-

3680 T. Kim et al. / Renewable and Sustainable En

Fig. 3. Schematic diagram of the mass balance for reactant and product species ina PEM fuel cell.

Table 1Mole numbers associated with reactant and product during PEM fuel cell operation.

mol s−1

Oxygen flow rate at cathodeinlet

a = 0.21�ciA4F

Hydrogen flow rate at anodeinlet

d = �aiA2F

Nitrogen flow rate at cathodeinlet and outlet

b = w = 0.79�ciA4F

Input water supplied forcathode humidification

c = �cRHc,inpsat

c(pc−RHc,inpsat

c )iA4F

Input water supplied for anodehumidification

e = �aRHa,inpsat

a(pa−RHa,inpsat

a )iA2F

Unused oxygen flow rate v = (0.21�c − 1) iA4F

Unused hydrogen flow rate y = (�a − 1) iA2F

Water content at cathode x = �cRHc,inpsat

c(pc−RHc,inpsat )

iA4F + iA

2F + ˛ iAF

se

wcppactarFnpla

o

R

R

outlet c

Water content at anode outlet z = �aRHa,inpsat

a(pa−RHa,inpsat

a )iA2F − ˛ iA

F

ure at the exits of the cathode and the anode, respectively, can bexpressed as:

x

v + w= pc,w,out

pc − pc,w,out

= �c(RHc,inpsatc /(pc − RHc,inpsat

c ))(iA/4F) + (iA/2F) + ˛(iA/F)(�c − 1)(iA/4F)

(2)

z

y= pa,w,out

pa − pa,w,out

= �a(RHa,inpsata /(pa − RHa,inpsat

a ))(iA/2F) − ˛(iA/F)(�a − 1)(iA/2F)

(3)

here pc,w,out and pa,w,out are partial pressures of the water at theathode and anode outlets, respectively; pc and pa are the totalressures at the cathode and anode inlets, respectively; psat

c andsata are the saturated water vapor pressures at the cathode andnode inlets, respectively; �c and �a are the air and hydrogen stoi-hiometry numbers (i.e., ratio of the actual flow rate of reactant athe fuel cell inlet to the consumption rate of that reactant); RHc,innd RHa,in are the air and hydrogen relative humidities at the inlets,espectively; i is the current density; A is the total membrane area;is the Faraday constant; ˛ is the net water drag coefficient. (Theet water drag coefficient, ˛, represents the net water molecule perroton flux ratio through the polymer membrane. When ˛ value is

ess than zero, there is a net water transport towards the anode. Fordetailed explanation of ˛, please see Ref. [10])

Thus, the relative humidity of the gases at the cathode and anodeutlets are, respectively, defined as:

Hc,out = pc,w,out

psatcell

= pc[�c(RHc,inpsatc /(pc − RHc,inpsat

c )) + 2 + 4˛]

psatcell

[�c(pc/(pc − RHc,inpsatc )) + 1 + 4˛]

(4)

Ha,out = pa,w,out

psatcell

=pa[�H2 (RHin,apsat

in,a/(pa − RHin,apsat

in,a)) − 2˛]

psatcell

(�H2 [pa/(pa − RHin,apsatin,a

)] − 2˛ − 1)(5)

ergy Reviews 15 (2011) 3676–3689

where RHc,out and RHa,out are the relative humidities of air andhydrogen, respectively, at the outlet, and psat

cellis the saturated water

vapor pressure at the given cell temperature.To calculate the mass of water to be added to the air and hydro-

gen inlet gases to achieve a specific humidity at a given pressureand cell temperature, it must be noted that the mass of any speciesin a mixture is proportional to the product of the molecular massand the partial pressure [8].

mc,w

mc,d= 18 × pc,w,in

28.97 × (pc − pc,w,in)= 0.622

pc,w,in

(pc − pc,w,in)

= 0.622RHc,inpsat

c

(pc − RHc,inpsatc )

(6)

ma,w

ma= 18 × pa,w,in

2.016 × (pa − pa,w,in)= 8.929

pa,w,in

(pa − pa,w,in)

= 8.929RHa,inpsat

a

(pa − RHa,inpsata )

(7)

where mc,w and ma,w are the masses of water present in air andhydrogen gases at inlets, respectively (i.e., required input watermass for humidification); mc,d and ma,d are the masses of dry airand dry hydrogen gases, respectively; pc,w,in and pa,w,in are the par-tial pressures of vapor-phase water at cathode and anode inlets,respectively.

The required masses of water for inlet air humidity and inlethydrogen humidity can be calculated from Eqs. (6) and (7), respec-tively, when the masses of dry air and hydrogen gases per secondare expressed as Eqs. (8) and (9) at any current I of a PEM fuel cell:

mc,d = 3.57 × 10−7 × �c × I (8)

ma,d = 1.05 × 10−8 × �a × I (9)

As shown in Fig. 2, if a % of the effluent water from the condensercan be captured, the amount of available drinking water per day(MD, kg d−1) from a PEM fuel cell with an internal humidificationsystem can be calculated as follows:

MD = MC − MH =(

MH + mH2OI

2F

)a

100− MH

= mH2OI

200Fa − MH

(100 − a

100

)(10)

where Mc is the amount of captured vapor-phase water at the con-denser per day; MH is the amount of water supplied for internalhumidification per day; mH2O is the molecular mass of water; anda is the capture efficiency of the effluent water from the condenser.

5. Review on parameters for increasing the amount ofwater source produced from PEM fuel cell

In order to get more drinking water from PEM fuel cells withinternal humidification system, it is desirable to increase the lim-iting current density for high water production or/and reducethe amount of water feeding for humidification of reactant gases.However, PEM fuel cells operated with dry or slightly humidifiedgas streams for the reduction of input water show significantlydecreased cell performance due to increase of ohmic resistancecompared to cells with analogous well-humidified gas streams.

In this work, first, the effects of humidification of the cathodeand anode on current characteristics are described for a particu-

lar configuration of a PEM fuel cell that uses an MPL only on thecathode side. This initial work is being conducted in an attempt tounderstand the effects the humidification of each feed stream oncell performance and to effectively reduce the amount that enters

able En

aP(fpdriqcEpb

5

pmpbtttcSaiaif

fbcfitiwfimtTo[gtwo7(botlfiaico1cact

T. Kim et al. / Renewable and Sustain

PEM fuel cell. Additionally, based on the overall reactions in aEM fuel cell at steady state under the single-phase flow conditioni.e., the relative humidity of the exiting gas is less than 100%), use-ul guidelines for suitable ranges of operational parameters wererovided. For the single-phase flow condition, the limiting currentensity can be increased due to the reduction of oxygen-transportesistance induced by excessive liquid-phase water accumulatedn the GDL and the gas flow channel. Second, assuming that theuality water produced by the PEM fuel cell is higher than typi-al tap water and complies with regulations established by the U.S.PA, the models are designed to identify the optimum operationalarameters for increasing the amount of drinking water producedy PEM fuel cells.

.1. Humidification of the inlet air and hydrogen feed streams

In general, the water inside the polymer membrane is trans-orted mainly by electro-osmotic drag (i.e., the dragging of waterolecules from the anode to the cathode by the current-carrier

rotons), back-diffusion (i.e., the transfer of water into the mem-rane due to the water concentration gradient from the cathode tohe anode), and convection (i.e., water movement that occurs dueo the pressure gradients between the cathode and the anode inhe fuel cell). However, the convection effect is generally negligibleompared to the effects of electro-osmotic drag and back diffusion.ince a PEM fuel cell is normally supplied with pure hydrogen gasnd air at same pressure, the water flow due to convection can begnored because there is no pressure difference between the anodend cathode. Thus, electro-osmotic drag and back diffusion dom-nate the water transport inside the polymer membrane of a PEMuel cell.

To increase the amount of available drinking water from a PEMuel cell, the limiting current density in a given catalyst area shoulde increased since the water production rate is proportional to theurrent. A higher current density results in greater water transportrom the anode to the cathode due to electro-osmotic drag. Thus,f back diffusion of water from the cathode to the anode throughhe polymer membrane is insufficient to maintain sufficient watern the polymer membrane at the dry hydrogen condition (i.e.,

ater concentration at the cathode side is very low due to insuf-cient humidification of the cathode), the performance of the cellay be significantly decreased due to the increase of ohmic resis-

ance due to the low water content in the polymer membrane.hese phenomena are consistent with other studies of the effectsf cathode humidification on cell performance [11,12]. Yan et al.11] tested the effect of various humidity conditions of reactantases on cell performance with Nafion 117 (e.g., this membranehickness is 178 �m) polymer membrane. Cell performance testsith I–V curves were conducted for RHc,in levels at the cathode

f 10, 50, 70, and 100% and for RHa,in levels at the anode of 60,0, 80, 90, and 100%. Their results showed that low RHc,in valuese.g., RHc,in = 30 and 50%) can exacerbate membrane dehydrationy reducing back diffusion from the cathode at cell temperaturef 80 ◦C and �c/�a = 2/2 conditions. Humidification of the gas athe anode helped to counteract membrane dehydration at highevels of anode humidification. They concluded that there was suf-cient back diffusion with medium and high RHc,in values (e.g., 70nd 100%) to keep the membrane hydrated, and further humid-fication of the anode did not improve the performance of theell significantly. Cai et al. [12] also compared performance andhmic resistance at different humidity conditions using a Nafion12 (e.g., this membrane thickness is 51 �m) membrane and a

onstant current density of 0.5 A cm−2 at cell temperature of 60 ◦Cnd �c/�a = 2.5/1.1 conditions. When the RHa,in value at the anodehanged from 0 to 100% with an RHc,in value of 56% at the cathode,he both average outputs of the cell voltage was 0.660 V under dry

ergy Reviews 15 (2011) 3676–3689 3681

and saturated hydrogen conditions and ohmic resistance slightlydecreased from 0.221 � cm−2 to 0.216 � cm−2. It was shown that,when RHa,in increased from 0 to 100% at RHc,in value of 56%, therewere insignificant changes in membrane resistance and cell per-formance. However, when the RHc,in value at the cathode waschanged from 56% to 36% with an RHc,in value of 0% at the anode,the average output of the cell voltage decreased from 0.660 V to0.619 V and ohmic resistance slightly increased from 0.221 � cm−2

to 0.267 � cm−2. They insisted that, when the value of RHc,in at thecathode was changed from 56% to 35% at dry RHa,in condition, backdiffusion was weaker and ohmic resistance was increased signif-icantly due to low water content in the polymer membrane. As aresult, Yan et al. [11] and Cai et al. [12] concluded that the effect ofcathode humidification on cell performance was more importantthan the effect of anode humidification.

In this work, we also evaluated the effects of the systematicdehydration of the feed streams on the performance of a PEM fuelcell using Gore-PRIMEA-18 membrane electrode assembly (MEA).The MEA consists of a membrane with a thickness of 18 �m anda catalyst layer with a thickness of 12 �m with a Pt loading of0.4 mg cm−2. The surface area of the MEA was 25 cm2. The SGL 10BC(i.e., GDL with MPL, SGL Carbon Group, USA) and 10BA (i.e., GDLwithout MPL, SGL Carbon Group, USA) were used for cathode andanode GDLs, respectively. The gas flow channel used for the anodeand the cathode sides were identical single-serpentine, parallelchannels. The width, depth, and land width in the gas flow chan-nel are 1 mm. The experiments described below follow a generalprotocol of monitoring cell current at a fixed cell potential of 0.6 V.In order to verify the effects of humidification levels of hydrogenand air feed gases on cell performance, the relative humidity of thehydrogen feed-gas stream was set to 10, 30, 50, 80, and 100% with afully saturated air feed-gas stream at a cell temperature of 80 ◦C andtotal air pressure of 1 atm. The relative humidity of the air feed-gasstream was set to be 10, 30, 50, 80, and 100% with a fully saturatedhydrogen feed-gas stream. The H2/air stoichiometries were 1.46and 2.5, respectively. The counter-flow mode was used. The pipesbetween the humidifier and the fuel cell were heated to 90 ◦C toavoid the condensation of water vapor.

Fig. 4(a) presents the variations of the current in the cell for a25 cm2 active area that is subject to various levels of RHa,in whenthere is 100% relative humidity at RHc,in. Fig. 4(a) shows that thecurrent levels were stable for the three segments of the cell whenit was operating with RHa,in values of 50, 80, and 100%. However, cellperformance diminished slightly as the humidification level of thehydrogen gas stream decreased (i.e., low RHa,in values of 10% and30%). For instance, when slightly humidified hydrogen gas with anRHa,in value of 10% was fed, the average output current of the cellwas approximately 21.5 A at a constant voltage of 0.6 V, for whichcurrent fluctuations were small.

However, as is shown in Fig. 4(b), the average current decreasedmarkedly from approximately 26.3 A to 9.7 A for a cell with satu-rated hydrogen gas in which the RHc,in conditions decreased from100% to 10% when RHa,in was fully humidified. As a consequence,these results also indicate that the influence of the humidificationcondition at the cathode on variations in output current is more sig-nificant than the influence of humidification condition at the anode.This means that the performance of the cell is affected significantlyby back diffusion, which is controlled by the humidification condi-tions of the feed gases. Thus, if the amount of water at the cathode isincreased by well-humidified air gas, the water transport by backdiffusion will be enough to effectively retain the proton conduc-tivity when the humidification at the anode is low at high current

density. However, for the dry air condition, insufficient back dif-fusion was responsible for the dehydration of the membrane athigh current density. When the air is dry, there will be a signifi-cant change in the current due to membrane dehydration. Thus,

3682 T. Kim et al. / Renewable and Sustainable En

Fig. 4. Current variation as functions of time and the RHin of inlet reactant gases atcnv

wsp

5

ohpiiwtmw[Niubttmp

onstant 0.6 V cell potential for a 25 cm2 Gore-PRIMEA-18 MEA (membrane thick-ess of 18 �m): (a) variation of anode RHa,in at constant cathode RHc,in = 100%; (b)ariation of cathode inlet RHc,in at constant anode inlet RHa,in = 100%.

hen dry hydrogen gas is supplied to the fuel cell, it is important toupply well-humidified air for effective water and electrical powerroduction at high current density.

.2. Polymer membrane thickness and material

The thickness of the polymer membrane has a major impactn the performance of the cell and on water production at lowumidification. If the value of RHc,in is reduced to increase theroduction of drinking water, the membrane thickness is a very

mportant consideration for stable cell operation at low humid-fication conditions. For a thin membrane, the back diffusion of

ater may be sufficient to counteract the anode-drying effect dueo electro-osmotic drag. However, for a thicker membrane, drying

ay occur on the anode side due to insufficient back diffusion ofater. This was demonstrated very vividly by Büchi and Scherer

13], who created thick membranes by combining several layers ofafion membranes. They showed that the membrane resistance is

ndependent of current density variation for polymer membranesp to a thickness of 120 �m, but it increases for thicker mem-ranes. Janssen and Overvelde [14] also studied the net water

ransport in an operating fuel cell with Nafion 105 (e.g., membranehickness = 127 �m) and 112 (e.g., membrane thickness = 51 �m)

embranes and determined the net water transport across theolymer membrane. As expected, the Nafion 112 membrane exhib-

ergy Reviews 15 (2011) 3676–3689

ited somewhat lower net water transport than the thicker Nafion105 membrane. Values of −0.3 to +0.1 were reported when theNafion membrane was changed from Nafion 112 to Nafion 105.(The negative values refer to the case in which back diffusion isgreater than electro-osmotic drag.) Additionally, they showed thatno significant dependence was found on the stoichiometry of thereactants or pressure differential, indicating that hydraulic perme-ation may be negligible for these membranes. Also, Cai et al. [12]indicated that, for the dry hydrogen condition, the cell with theNafion 115 membrane (e.g., membrane thickness = 127 �m) hadlower performance than the cell with the Nafion 112 membrane(e.g., membrane thickness = 51 �m) due to the increased resistanceof the membrane. They suggested that a thin membrane was moreproper for the operation of a fuel cell with dry hydrogen. Con-sequently, when a PEM fuel cell is operated to produce drinkingwater at a dry hydrogen condition, a thin polymer membrane andhigh RHc,in (e.g., RHc,in = 50–100%) can be expected to alleviate thedehydration of the membrane due to sufficient back diffusion.

The decrease of membrane thickness reduces water shortageproblems in PEM due to increase of the water back-diffused fromthe cathode to the anode. However, this usually accelerates thecrossover of H2 and O2 through the thin polymer membrane, whichdecreases the cell performance and the fuel utilization by the chem-ical short-circuit reaction [15]. Hence, the management of watercontent and reactant crossover is recognized as a key technologyto enhance the cell performance and to suppress the degradationof PEM. Attempting to overcome these problems, Watanabe et al.[16–18] developed the self-humidifying PEM with highly dispersednanometer sized Pt and/or metal oxides (i.e., TiO2, SiO2). The metaloxide particles that have hygroscopic property were expected toadsorb the water produced at Pt particles together with that pro-duced at the cathode. The cells with these PEMs exhibited superiorcell voltage at high current density and decreased crossover evenunder a dry or low-humidified condition. Gnana Kumar et al. [19]fabricated a modified Nafion membrane with silica and silica sul-furic acid for the lower humidity. Their results show that with thehydroscopic effort, high water molecules retention provokes theself humidification of modified Nafion membrane.

5.3. Stoichiometry, pressure, and temperature

The water mass balance in the polymer membrane can bedescribed as follows. At the anode, water gain is caused by thehumidified hydrogen gas and back diffusion, while water loss iscaused by electro-osmosis and the evaporated water that is in theeffluent gases. At the cathode, water gain is from the humidified airgas, electro-osmosis, and the water that is produced by the oxygenreduction reaction. Water is removed by back diffusion and evap-oration into the unsaturated air. At both sides, these water gainsand losses dominate the water distribution and water content inthe polymer membrane, and they are of considerable importancein understanding the performance of PEM fuel cells.

Weber and Newman [20] established a water transport model inMEA and investigated the water distribution in MEA. They indicatedthat, due to electro-osmosis and water production at the cathodesite, moles of water per mole of sulfonic acid at the interface of themembrane/cathode catalyst layer is higher than that at the anodesite, even though the relative humidities for both the anode andthe cathode are the same. Thus, if the number of moles of waterper mole of sulfonic acid at the cathode site decreases due to lowwater gain at the cathode site, the polymer membrane at the anodesite will be dehydrated dramatically due to the decreased back dif-

fusion. As a result, the water content of the polymer membrane isprimarily dominated by the water activity at the cathode site or bythe relative humidity (RHc,out) of the exit air, which is determinedby total water gain and loss at the cathode. Larminie and Dicks [8]

T. Kim et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689 3683

Fig. 5. (a) Variation of proton conductivity and water content equilibrated withv(a

mctG

impiwtt8csadtwbtigabwet

2 4 6 8 10

40

60

80

100

120

140

160

180

200

220

240

3.59

3.94

2.84 7.56

5.95

RHc,in=80% RHc,in=50% RHc,in=30%

Rel

ativ

e hu

mid

ity o

f the

eix

t air,

RH c,

out (%

)

Air stoichiometry number

apor-phase water according to changes in the relative humidity of the exit airRHc,out), (b) relationship between the proton conductivity of a polymer membranend its water content (Equivalent weight (EW) of polymer membrane = 950).

ore clearly suggested that the suitable range of RHc,out for stableell operation should be above 80% to prevent membrane dehydra-ion, but it must be less than 100% to avoid water flooding in theDL, electrode, and gas flow channel.

Thampan et al. [21] proposed a proton conductivity (�) modeln polymer membranes as a function of the water content (�). (For

ore detailed information about the proton conductivity model,lease see Ref. [21]) Using the aforementioned proton conductiv-

ty model, we represent the variation of proton conductivity andater content equilibrated with vapor-phase water according to

he change of RHc,out, as shown in Fig. 5(a). It can be seen that pro-on conductivity decreases rapidly as the RHc,out decreases below0%. Using the suitable range of RHc,out from 80 to 100% for stableell operation, a proton conductivity is presented in Fig. 5(a) underuitable RHc,out range (i.e., � > 10 and � = 0.025–0.333 S cm−1). Inddition, we represent the relationship between the proton con-uctivity of the polymer membrane and its water content at a cellemperature of 80 ◦C as shown in Fig. 5(b). It can be seen that,hen the water content is less than 2, the polymer membrane

ehaves like an insulator. Above this threshold, the proton conduc-ivity of the membrane increases significantly as the water contentn the polymer membrane increases. When the water content isreater than 10, the proton conductivity of the membrane reachesplateau. As shown in Fig. 4(a), this means that the PEM fuel cell can

e stably operated in the range of proper water content (i.e., � > 10hen RHc,out is in the suitable range of 80–100%). As mentioned

arlier, the operational parameters (i.e., RHc,in, air stoichiometry,otal air pressure, and cell temperature) have strong impacts on the

Fig. 6. Relative humidity of the exit air stream vs. air stoichiometry number forvarious relative humidities at the cathode inlet with a total air pressure of 1.1 atm.

suitable range of RHc,out and are related among themselves by non-linear relationships. Eq. (4) can be used to analyze the influences ofthe various operational parameters on RHc,out.

Fig. 6 shows the relationship between air stoichiometry andRHc,out under various RHc,in conditions at a given cell temperature of80 ◦C. Based on previous experimental results from a study by Kimet al. [10], in which the MPL was only on the cathode, the net waterdrag coefficient was assumed to be ˛ = −0.27. It can be seen in Fig. 6that RHc,out decreases significantly as the air flow rate increases at agiven RHc,in. Additionally, the curves of proper range of RHc,out vs. airstoichiometry number are presented in the shaded zone of Fig. 5,according to the suggestion of Larminie and Dicks [8]. This indi-cates that, when the curves of proper RHc,out vs. air stoichiometrynumber fall in the shaded zone, the PEM fuel cell can be operatedstably. Above the shaded zone, vapor-phase water would condensein the cell components, causing water flooding at the cathode side.Below the shaded zone, it would be difficult to keep the membranehydrated.

In general, the air stoichiometry (�c) is normally determined inthe range of 2–4, which is dependent on the operational conditionof the fuel cell and material design. A lower air stoichiometry (e.g.,�c < 2) will cause poor cell operation due to the reduction of theconcentration of available oxygen, which results in increased con-centration over-potential, whereas a higher air stoichiometry (e.g.,�c > 4) will result in the reduction of oxygen utilization as well asthe rapid evaporation of water due to rapid flow of oxygen gas.Therefore, under various RHc,in conditions, the appropriate air stoi-chiometry range should be determined for better cell performance,as shown in Fig. 6. For the case in which RHc,in = 80%, the appropriateair stoichiometry range is too high to operate under these operationconditions. If the air stoichiometry range for RHc,in = 80% is from 2to 4, water condensation in the fuel cell may impair the operationof the cell due to the resistance to oxygen transport caused by thehindrance of liquid water at a lower stoichiometry number. In addi-tion, although air stoichiometry at RHc,in = 30% is within the optimalrange, low RHc,in values of 10% and 30% are expected to affect theperformance of the cell adversely, as shown in Fig. 4(b). This maybe limited primarily due to low proton conductivity. Furthermore,a dry air environment can also slow down the charge transfer pro-cess. Zhang et al. [22] showed that, with dry reactant gases, the

resistance to charge transfer is 1.5–2.0 times higher than the resis-tance when the reactant gases are fully saturated. This could bedue to the reduction of the Pt surface area in the dry catalyst layers.Even though the oxygen and hydrogen concentrations were high

3684 T. Kim et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

60

80

100

120

140

160

180

200

220(a)

RHc,in=80% RHc,in=50% RHc,in=30%

Rel

ativ

e hu

mid

ity o

f the

eix

t air,

RH c,

out(%

)

Pressure of air gas (atm)

Too wet

Too dry

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

60

80

100

120

140

160

180

200

220(b)

RHc,in=80% RHc,in=50% RHc,in=30%

Rel

ativ

e hu

mid

ity o

f the

eix

t air,

RH c,

out(%

)

Pressure of air gas (atm)

Too dry

Too wet

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

60

80

100

120

140

160

180

200

220(c)

RHc,in=80% RHc,in=50% RHc,in=30%

Rel

ativ

e hu

mid

ity o

f the

eix

t air,

RH c,

out(%

)

Pressure of air gas (atm)

Too dry

Too wet

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

60

80

100

120

140

160

180

200

220(d)

RHc,in=80% RHc,in=50% RHc,in=30%

Rel

ativ

e hu

mid

ity o

f the

eix

t air,

RH c,

out(%

)

Pressure of air gas (atm)

Too dry

Too wet

F e at dp ◦C.

dfdoaticraaa

RIWtcgavTlTfh

ig. 7. Calculated relative humidity of the exit air stream as a function of air pressurhysical parameters: net water drag coefficient of −0.27 and cell temperature of 80

ue to high gas stoichiometry, a large resistance to mass trans-er occurred at dry or slightly humidified reactant gas conditionsue to the limited proton transfer process to Pt catalysts. This wasbserved mainly in dry ionomer medium within a dry electrode,nd it occurred in addition to the increase in charge transfer resis-ance at dry or slightly humidified air conditions. Consequently, its important to determine the proper air stoichiometry range of 2–4onsidering the aspect of RHc,in = 50% for the reductions of variousesistances as well as the amount of inlet water amount supplied forir humidification. If the RHc,in condition is less than 50%, the totalir pressure should be increased to keep the membrane hydratedt low humidity conditions at the cathode.

Fig. 7 presents a further evaluation of the relationship betweenHc,out and total air pressure at various air stoichiometry numbers.

t can be seen that RHc,out is proportional to the total air pressure.hen the saturation partial pressure of water vapor is only a func-

ion of cell temperature and the cell temperature is constant duringell operation, the mole fraction of water vapor in the exhaust airas is determined primarily by total air pressure. This is because,s the total air pressure becomes higher, the mole fraction of waterapor in the exhaust air gas would decrease (i.e., xH2O = Psat/Pc).hus, the amount of water leaving with the exhaust air gas becomes

ower, and the number of moles of water in the fuel cell increases.his results in the increase of the vapor pressure of the water in theuel cell. This leads to the important conclusion that a system withigher air pressure requires less added water to achieve the same

ifferent air stoichiometry numbers: �c = (a) 2.5, (b) 3.5, (c) 4.5, and (d) 5.5. Common

humidity of exit air and increases the amount of drinking waterproduced by the PEM fuel cell due to the reduction of amount ofwater used for the humidification of reactant gases.

Furthermore, cell operation at higher air pressure increases cellperformance due to various causes. Zhang et al. [22] showed thatcell performance increases significantly with increasing gas pres-sure at dry reactant gas conditions due to the increase of the partialpressure of the reactant gases and the reduction of the volumetricflow rate of the reactant gases. These factors lead to increases inthe amounts of water retained in both the cathode and the anode.Additionally, the increased pressure raises the exchange currentdensity, which has the apparent effect of increasing the open circuitvoltage (OCV). In addition to these benefits, there is also a reduc-tion in the mass transport losses due to the increase in the partialpressure of oxygen in the catalyst layer [11]. However, additionalpower is consumed to increase the air pressure when a fuel cellmust be operated at higher air pressure. Thus, voltage gains andlosses associated with increasing the air pressure should be con-sidered. Consequently, the pros and cons of adding the extra aircompressor apparatus must be considered. These problems will bediscussed later.

Additionally, there is usually a strong relationship between the

cell temperature and water production rate in a fuel cell, since celltemperature has a major impact on relative humidity (RH). The RH isdefined as the ratio of the partial pressure of water to the saturatedvapor pressure of water at a given temperature. The saturated vapor

T. Kim et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689 3685

Table 2Input water mass rate in appropriate range of air stoichiometry at different RHc,in

conditions.

RHc,in (%) �c MH (kg d−1) Inputwater/productwater

ptdfwtia6w

6

aiatoaPMcam1tibaf

stcFtTatsRcctibiaiifii

wa

Fig. 8. Calculated input water mass rate (MH) for cathode humidification and input

30 2.84–3.94 18.8–26.0 0.97–1.3550 3.59–5.95 43.7–72.4 2.26–3.7580 7.56– 175< 9.07<

ressure increases more and more rapidly above 60 ◦C. This meanshat, when the partial pressure of vapor water is constant, RHc,inecreases to a significant extent as the internal temperature of theuel cell exceeds 60 ◦C. This is because the pressure of the saturatedater vapor must increase significantly to achieve the same rela-

ive humidity in the inlet stream to the cathode if the temperatures above 60 ◦C. Thus, the input water mass rate for the appropri-te cathode humidification increases more and more rapidly above0 ◦C, which results in decrease of the amount of available drinkingater from PEM fuel cell with internal humidification.

. Drinking water production rate from PEM fuel cell

The mass of water to be added to the air stream depends togreat extent on operational parameters such as RHc,in, air sto-

chiometry, total air pressure, and cell temperature. Therefore,nother objective of the present work is to conduct technical quan-ification and assessment of the effect of operational parametersn input water mass rate (i.e., MH, kg d−1) for air humidificationnd on the drinking water production rate (i.e., MD, kg d−1) of theEM fuel cell. To calculate the amount of water production (i.e.,P, kg d−1) generated by the oxidation-reduction reaction at the

athode, it is assumed that a PEM fuel cell stack is operated withtotal current of 2392 A (i.e., 52 A per polymer membrane and 46embranes in a fuel cell stack [23] and can produce approximately

9 kg of water per day. As mentioned earlier, this water produc-ion rate from commercial fuel cell stack can be increased by themprovement of the current density as well as increase of the num-er of polymer membrane. However, we only focus on increase ofvailable drinking water from PEM fuel cell by reduction of watereeding for humidification of reactant gases.

Assuming that no humidification at the anode (dry H2), Fig. 8hows the MH for cathode humidification and ratio of input watero water produced as function of air stoichiometry at different RHc,inonditions at a total air pressure of 1.1 atm. As one would expect,ig. 8 indicates that the MH is proportional to the air stoichiome-ry. Furthermore, the MH increases significantly as RHc,in increases.able 2 shows the ratio of input water to product water (i.e., MH/MP)nd the MH values within the appropriate range of air stoichiome-ry values in Fig. 6 at different RHc,in conditions. Except for low airtoichiometry at RHc,in = 30%, the MH/MP is always greater than 1 atHc,in = 50% and 80%. This means that, if the capture efficiency at theondenser is not 100%, the product water generated by the electro-hemical reaction at the cathode will be consumed for humidifyinghe cathode. This leads to a sharp reduction of MD whereas MH

ncreases significantly. Thus, other operational parameters shoulde considered to reduce MH for humidification of the cathode. Fig. 9

ndicates the reduction of MH for humidification of the cathodes total air pressure increases at different RHc,in conditions. Fornstance, it can be seen that, at an air pressure of 1.5 atm, the MH/MP

s less than 1, except for RHc,in = 80%. However, the MH for humidi-cation of the cathode is not greatly reduced until the air pressure

s 3 atm.As mentioned earlier, running a fuel cell at higher pressure

ill increase the electrical power due to reductions in the cathodectivation overvoltage as well as oxygen-transport loss. However,

water/product water ratio (MH/MP) as a function of air stoichiometry number atdifferent RHc,in conditions: RHc,in = (a) 30%, (b) 50% and (c) 80%. Common operationalparameters: cell temperature of 80 ◦C and total air pressure of 1.1 atm.

operating the fuel cell at higher pressure involves the expenditureof parasitic power taken up by the compression equipment. To con-

sider the pros and cons of increasing the air pressure, the voltageloss and gain, as well as the increase of MD at higher total air pres-sure, must be considered more quantitatively. The voltage gain andloss are changed according to the increase of the operational pres-

3686 T. Kim et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689

Fig. 9. Calculated input water mass rate (MH) for cathode humidification and inputwhp

svc

N

weI

Fig. 10. Net voltage change and amount of available drinking water (MD) resultingfrom initial ambient air pressure (1 atm) and higher pressures: RHc,in = (a) 30%, (b)50% and (c) 80%. Common operational parameters: cell temperature of 80 ◦C, airstoichiometry of 2, total current of 2392 A. The capture efficiency, a, is assumedto be approximately 70%. Actually, Chu and Jiang [26] were able to capture only

ater/product water ratio (MH/MP) as a function of air pressure at different relativeumidity of cathode inlet: RHc,in = (a) 30%, (b) 50% and (c) 80%. Common operationalarameters: cell temperature of 80 ◦C, air stoichiometry of 2, total current of 2392 A.

ure from an original value of P1 to a higher pressure P2. The netoltage change resulting from operating at higher pressure can bealculated as follows [8]:

et�V = �Vgain − �Vloss = C ln(

P2

P1

)− 3.58 × 10−4

× T1

�c�m

((P2

P1

)0.286− 1

)�c (11)

here C is a constant the value of which depends on how thexchange current density is affected by pressure and temperature.t is not clear what value should be used for C. Parsons, Inc. has

approximately 66% of the theoretical amount of effluent water under STP conditionsdue to evaporation and other losses.

reported figures ranging from 0.03 to 0.06 V [24]. Hirschenhoferet al. [25] used a constant value of 0.06 V. In this work, we used0.06 V as a constant value. The last term on the right side of Eq. (11)can be written for the voltage loss by the compressor in terms ofcompressor efficiency �c (�c = 0.7) [8], the efficiency of the electricdrive for the compressor �m (�m = 0.9) [8], and the entry tempera-ture of the air stream, T .

1

Eqs. (10) and (11) were used to plot the graph of net voltagechange and MD at different RHc,in conditions, as shown in Fig. 10.The data presented in Fig. 10 show that there is always a net voltage

T. Kim et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3676–3689 3687

F king wc Comms

lebnwa(pWcH

ig. 11. Input water mass rate for humidification (MH) and amount of available drinonditions: (a) and (b) RHc,in = 30%, (c) and (d) RHc,in = 50%, (e) and (f) RHc,in = 80%.toichiometry of 2.

oss as a result of the higher pressure. This means that the additionallectrical power needed to drive the compressor is always exceededy the power gained. Furthermore, it can be expected that a higheret voltage loss at a higher cell temperature and air stoichiometryill be observed due to the greater voltage losses associated with

ir compression, which result in a larger �Vloss, as given by Eq.11). For this reason, the practice of operating above atmosphere

ressure is by no means universal, even with larger PEM fuel cells.ith smaller fuel cells, i.e., less than about 5 kW, are rare, since the

ompressors are likely to be less efficient and more expensive [8].owever, this pressurization process is needed to provide sufficient

ater (MD) as a function of cell temperature at different total air pressure and RHc,in

on physical properties: capture efficiency of 70%, total current of 2392 A, and air

drinking water for a household, as shown in Fig. 10. With higherRHc,in, the MD will increase as the total air pressure increases sincethe amount of water required at the cathode inlet to achieve thesame humidity will be less at higher pressure. Therefore, at highRHc,in, it is recommended that the fuel cell be operated at the higherpressure of 3 atm to achieve a greater water production rate. For anRHc,in value of 30%, however, the pressurization process does not

materially affect the production of drinking water from the PEMfuel cell.

As mention earlier, the cell temperature above 60 ◦C signifi-cantly affects the amount of feeding water for the humidification

3 able En

oofg1ahttcrtcdeittiiftadwsagtsabsamctnp

itotiaai(ti

7

rmpoihaiwr

688 T. Kim et al. / Renewable and Sustain

f reactant gases. This results in a sharp increase of MH for cath-de humidification at cell temperatures above 60 ◦C. It can be seenrom Fig. 11(a) that, at a total air pressure of 1 atm, MD is alwaysreater than zero over the full range of cell temperatures from 10 to00 ◦C. In Fig. 11(c) and (e), however, it can be seen that, at temper-tures above 60 ◦C, MD decreases very markedly during the internalumidification process. Thus, in order to produce MD values greaterhan 10 kg d−1 at different RHc,in conditions, it is desirable to main-ain the temperature of the fuel cell at less than 60 ◦C. At lowell temperatures, small amounts of water are needed to satu-ate the air, and MD is more than sufficient. However, a low cellemperature results in lower cell potential. This is because lowerell temperature results in exponentially lower exchange currentensity and significantly hinders the oxygen-mass transport prop-rties at the cathode side, whereas increased cell temperatures canncrease the kinetics of both oxygen reduction and hydrogen oxida-ion reactions. However, increased cell temperatures can acceleratehe evaporation of water evaporation at low RHc,in values, resultingn a reduction of water retention in the polymer membrane and anncrease in ohmic resistance. This has a negative effect on the per-ormance of the cell. Furthermore, if more MH should be suppliedo a PEM fuel cell in order to maintain proper proton conductivityt high cell temperature, the high cell temperature significantlyecreases the MD. Thus, the MD, as well as a trade-off betweenater retention in the polymer membrane and reaction kinetics,

hould be considered. Zhang et al. [22] studied the effect of 23 ◦Cnd 60 ◦C cell temperatures on cell performance with dry reactantases. They showed that, at low current density (i.e., <0.3 A cm−2),he cell performances at both cell temperatures were almost theame, with a higher performance relative to high current densityt 60 ◦C. This indicates that water retention in the polymer mem-rane was higher at 23 ◦C than at 60 ◦C, but the reaction rates werelower at 23 ◦C than at 60 ◦C. The trade-off between water retentionnd reaction kinetics may be responsible for the similar cell perfor-ances for low current densities less than 0.3 A cm−2. At higher

urrent densities, however, the positive impact of increased reac-ion kinetics at 60 ◦C on the cell performance may overwhelm theegative impact of less water retention and result in higher cellerformance.

As a result, at low current density with low cell temperature,t is possible to maintain the cell’s electrical and water produc-ion performances. However, at high current density, the amountf drinking water available will be significantly decreased to main-ain the same RHc,in, even though the cell’s electrical performances higher than it is at low cell temperature. In order to reduce themount of water that must be added to humidify the reactant gasest high cell temperature as well as to maintain high current density,ncreasing the air pressure is more effective, as shown in Fig. 11(b),d), and (f). It can be seen that, even though the RHc,in condition andhe cell temperature increase, the MD at a total air pressure of 2 atms greater than that at ambient pressure.

. Conclusions

In this work, useful guidelines were developed along with theirespective operational parameters for maintaining stable perfor-ance of the cell and for increasing the amount of drinking water

roduced by the PEM fuel cell. These guidelines were based on theverall reaction in a PEM fuel cell at steady state. As the resultsndicate, the minimal use of humidification (i.e., dry or slightlyumidified gas streams) is desirable for significantly increasing the

mount of drinking water from the PEM fuel cell without affect-ng electrical power productivity by low humidification. However,

hen a PEM fuel cell operates with dry or low-humidity gases, theesult is significantly decreased cell performance due to increased

ergy Reviews 15 (2011) 3676–3689

ohmic resistance. Specifically, the influence of the humidificationcondition at the cathode on current variation was more importantthan the influence of the humidification condition at the anode.Thus, for stable cell performance, the humidity of the air at theoutlet (RHc,out) should be above 80% to prevent membrane dehy-dration, but it must be kept less than 100% to avoid flooding the PEMfuel cell with water. The proper RHc,out is dependent to a significantextent on operational parameters, such as air inlet humidification(RHc,in), air stoichiometry, air pressure, and operating cell tempera-ture. When the inlet gas humidity at the anode is very low (dry gas)and air stoichiometry is in the normal range of 2–4, the appropri-ate RHc,in is 50% to ensure sufficient water content in the polymermembrane and stable cell performance. If RHc,in is less than 50%,air pressure should be increased to maintain sufficient water con-tent in the polymer membrane. This is important because the molefraction of water vapor in the exhaust air decreases as total air pres-sure increases. In addition, cell operation at high air pressure willincrease the performance of the cell due to the reduction of cathodeactivation overvoltage. However, when a fuel cell must be operatedat higher air pressure, additional power is consumed to increasethe air pressure. Thus, voltage gains and losses associated withincreasing the air pressure should be considered. There is alwaysa net voltage loss when air pressure increases, but the pressuriza-tion process is required to provide sufficient drinking water (i.e.,>10 kg d−1) for a household. For the low RHc,in condition, the pres-surization process does not materially affect the amount of drinkingwater produced by a PEM fuel cell. However, for a high RHc,in con-dition, the mass of the input water is decreased significantly as airpressure increases. However, the water required for humidificationat a high RHc,in condition is not greatly reduced for an air pressuregreater than 3 atm. Furthermore, it is clear that, above a cell tem-perature of 60 ◦C, the amount of available drinking water decreasesmarkedly, since saturated water vapor pressure increases more andmore rapidly above this temperature, which means that the par-tial pressure of dry air and the RHc,out both decrease significantly.Consequently, in order to produce more than 10 kg d−1 of drinkingwater at different RHc,in conditions, it is desirable to decrease theoperating cell temperature to less than 60 ◦C. The water productionrate from a PEM fuel cell is dependent on limiting current den-sity and the number of polymer membrane. In general, at limitingcurrent density, the water yield from PEM fuel cells would not besufficient to meet the internal consumptive uses of water in typicalhouseholds. Thus, an increase in the number of polymer membraneat low current density is an alternative solution for increasing theamount of drinking water from a PEM fuel cell.

Acknowledgement

This research has been performed as Project No. WI08STU06and was supported by the Korea Water Resources Corporation (K-Water).

References

[1] Narayan GP, Sharqawy MH, Summers EK, Lienhard JH, Zubair SM, Antar MA.The potential of solar-driven humidification–dehumidification desalinationfor small-scale decentralized water production. Renewable and SustainableEnergy Reviews 2010;14:1187–201.

[2] U.S. Geological Survey 2008. Residential energy consumption survey. Availableat: http://ga.water.usgs.gov/edu/wupt.html/.

[3] Hristovski KD, Dhanasekaran B, Tibaquirá JE, Posner JD, Westerhoff PK. Produc-ing drinking water from hydrogen fuel cells. Journal of Water Supply: Researchand Technology – AQUA 2009;58(5):327–35.

[4] Sauer RL, Calley DJ. Biomedical results of Apollo-Potable water supply. Rep.

No. LC-75-600030; NASA-SP-368. Houston, TX: Johnson Space Center; 1975.Available at: http://lsda.jsc.nasa.gov/books/apollo/S6CH4.htm.

[5] Collier A, Wang H, Yuan XZ, Zhang J, Wilkinson D. Degradation of poly-mer electrolyte membranes. International Journal of Hydrogen Energy2006;31(13):1838–54.

able En

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

T. Kim et al. / Renewable and Sustain

[6] Orta D, Mudgett PD, Ding L, Drybread M, Schultz JR, Sauer RL. Analysis ofwater from the space shuttle and Mir space station by ion chromatogra-phy and capillary electrophoresis. Journal of Chromatography A 1998;804:295–304.

[7] Aquacraft, Inc. Residential end uses of water. American Water WorksAssociation Research Foundation; 1999. Available at: http://www.aquacraft.com/Publications/resident.htm (retrieved February 18, 2010).

[8] Larminie J, Dicks A. Fuel cell systems explained. 2nd ed. John Wiley & Sons, Ltd.;2003.

[9] Atkins JR, Savett SC, Creager SE. Large-scale current fluctuations in PEM fuelcells operating with reduced feed stream humidification. Journal of PowerSources 2004;128:201–7.

10] Kim T, Lee S, Park H. A study of water transport as a function of the micro-porous layer arrangement in PEMFCs. International Journal of Hydrogen Energy2010;35:8631–43.

11] Yan Q, Toghiani H, Wu J. Investigation of water transport through membranein a PEM fuel cell by water balance experiments. Journal of Power Sources2006;158:316–25.

12] Cai Y, Hu J, Ma H, Yi B, Zhang H. Effect of water transport properties ona PEM fuel cell operation with dry hydrogen. Electrochimica Acta 2006;51:6361–6.

13] Büchi FN, Scherer GG. Investigation of the transversal water profile in Nafionmembranes in polymer electrolyte fuel cells. Journal of the ElectrochemicalSociety 2001;148(3):A181–8.

14] Janssen GJM, Overvelde MLJ. Water transport in the proton-exchange-

membrane fuel cell: measurements of the effective drag coefficient. Journalof Power Sources 2001;101:117–25.

15] Hagihara H, Uchida H, Watanabe M. Preparation of highly dispersed SiO2 andPt particles in Nafion® 112 for self-humidifying electrolyte membranes in fuelcells. Electrochimica Acta 2006;51:3979–85.

[

[

ergy Reviews 15 (2011) 3676–3689 3689

16] Watanabe M, Uchida H, Seki Y, Emori M, Stonehart P. Self-humidifying poly-mer electrolyte membrane for fuel cells. Journal of the Electrochemical Society1996;143:3847–52.

17] Watanabe M, Uchida H, Emori M. Polymer electrolyte membranes Incorporatedwith nanometer-size particles of Pt and/or metal-oxides: experimental analysisof the self-humidification and suppression of gas-crossover in fuel cells. TheJournal of Physical Chemistry 1998;102(17):3129–37.

18] Watanabe M, Uchida H, Emori HM. Analyses of self-humidification and sup-pression of gas crossover in Pt-dispersed polymer electrolyte membranes forfuel cells. Journal of the Electrochemical Society 1998;145(4):1137–41.

19] Gnana Kumar G, Kim AR, Nahm KS, Elizabeth R. Nafion membranes modifiedwith silica sulfuric acid for the elevated temperature and lower humidity opera-tion of PEMFC. International Journal of Hydrogen Energy 2009;34(24):9788–94.

20] Weber AZ, Newman J. Transport in polymer-electrolyte membrane III Modelvalidation in a simple fuel-cell model. Journal of the Electrochemical Society2004;151:A326–39.

21] Thampan T, Malhotra S, Tang H, Datta R. Modeling of conductive transportin proton-exchange membranes for fuel cells. Journal of the ElectrochemicalSociety 2000;147(9):3242–50.

22] Zhang J, Tang Y, Song C, Cheng X, Zhang J, Wang H. PEM fuel cells operated at 0%relative humidity in the temperature range of 23–120 ◦C. Electrochimica Acta2007;52:5095–101 (2007).

23] Ballard Power Systems, Inc. Available at: http://www.ballard.com/Stationary Power/Backup Power Fuel Cells/Product Specifications.htm.

24] Parsons Inc, EG&G Services. Fuel cells: a handbook. 5th ed. U.S. Department of

Energy; 2000. p. 3–10.

25] Hirschenhofer JH, Stauffer DB, Engelman RR. Fuel Cells: A Handbook, Revision3, Business and Technology Books; 1995.

26] Chu D, Jiang R. Comparative studies of polymer electrolyte membrane fuel cellstack and single cell. Journal of Power Sources 1999;80:226–34.