Energy Conversion and Management 226 (2020) 113545

Available online 22 October 20200196-8904/© 2020 Elsevier Ltd. All rights reserved.

Spatially graded porous transport layers for gas evolving electrochemical energy conversion: High performance polymer electrolyte membrane electrolyzers

Jason K. Lee a, ChungHyuk Lee a, Kieran F. Fahy a, Pascal J. Kim a, Jacob M. LaManna b, Elias Baltic b, David L. Jacobson b, Daniel S. Hussey b, Svenja Stiber c, Aldo S. Gago c, Kaspar A. Friedrich c,d, Aimy Bazylak a,*

a Thermofluids for Energy and Advanced Material Laboratory, Department of Mechanical and Industrial Engineering, Institute for Sustainable Energy, Faculty of Applied Science and Engineering, University of Toronto, Toronto, ON M5S 3G8, Canada b Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA c Institute of Engineering Thermodynamics, German Aerospace Center, Pfaffenwaldring 38-40, Stuttgart 70569, Germany d Institute of Building Energetics, Thermal Engineering and Energy Storage (IGTE), University of Stuttgart, Keplerstraße 7, 70550 Stuttgart, Germany

A R T I C L E I N F O

Keywords: Polymer electrolyte membrane electrolyzer Porous transport layer Porosity gradient Neutron radiography Vacuum plasma spray Pore network modeling

A B S T R A C T

Decarbonizing society’s energy infrastructure is foundational for a sustainable future and can be realized by harnessing renewable energy for clean hydrogen and on-demand power with fuel cells. Here, we elucidate how graded porous transport layers (PTLs) are instrumental for high performance gas evolving electrochemical en-ergy conversion devices, with an emphasis on polymer electrolyte membrane (PEM) electrolyzers. Spatially graded PTLs fabricated by vacuum plasma spraying are examined via in operando neutron imaging, electro-chemical characterization, and pore network modelling. The results reveal the staggering benefits of positioning the lower porosity region adjacent to the catalyst layer and the higher porosity region adjacent to the flow field, which lead to current densities up to 4.5 A/cm2 with a 29 % reduction in cell potential, 38 % reduction in mass transport overpotential, and 50 % reduction in PTL gas saturation. The liquid water permeability of the PTL also enhances by an order of magnitude, with a drastic reduction in gas saturation adjacent to the catalyst layer. Custom graded PTLs have the potential to transform performance levels for a broad array of gas evolving electrochemical energy conversion devices.

1. Introduction

Global warming is an international crisis that is disproportionately affecting jurisdictions, with dramatic discrimination for socio- economically depressed populations [1]. Decarbonizing energy infra-structure globally by harnessing renewable energy sources is vital for controlling anthropogenic global warming, but the realization of a renewable energy infrastructure is facing a massive barrier primarily due to prohibitive costs [2–4]. Clean electrochemical energy conversion technology is a promising means for transforming renewable energy into a reliable source for on-demand power. Specifically, energy storage is needed to bridge intermittent renewable energy sources to electricity grids that service customers who require stable, on-demand electricity. Polymer electrolyte membrane (PEM) electrolyzers powered by

renewable energy will enable energy storage and the subsequent dispatch of renewable energy by producing high purity hydrogen from water in the absence of carbon emissions [5–11]. Hydrogen fuel can then be used in fuel cells to produce quickly responding, dispatchable elec-tricity to the grid or power for transportation applications. However, state-of-the-art PEM electrolyzer efficiencies have plateaued at ~ 75 % in recent years [12], and the cost of electrolyzed hydrogen is < $6.5/ kg∙H2, which is 3 times the cost of hydrogen produced through steam methane reforming (< $2/kg∙H2) [13]. Furthermore, in the rise of our global water crisis [14], water cannot be assumed to be always plentiful and cheap; therefore, the efficient delivery and utilization of liquid water must also be held paramount.

New materials for electrochemical energy conversion devices pro-vide a game changing opportunity to drive up performance efficiencies.

* Corresponding author. E-mail address: abazylak@mie.utoronto.ca (A. Bazylak).

Contents lists available at ScienceDirect

Energy Conversion and Management

journal homepage: www.elsevier.com/locate/enconman

https://doi.org/10.1016/j.enconman.2020.113545 Received 14 July 2020; Received in revised form 16 September 2020; Accepted 11 October 2020

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For PEM electrolyzers, producing competitively priced hydrogen hinges on controlling efficiency loss mechanisms, which include ohmic, kinetic, and mass transport losses [15]. The ohmic overpotentials are mainly due to the ionic conductivity of the membrane and scale linearly with the current density. The kinetic overpotentials are related to the charge transfer in the oxygen evolution reaction, and these overpotentials are significant at low current densities, and are relatively constant when current density increases. Mass transport overpotentials are arguably the most important loss mechanism to overcome since they inhibit the high current densities that are vital for producing cost effective hydrogen; reducing mass transport losses directly translates into higher production volumes of hydrogen with reduced power demands.

New materials are key for addressing mass transport losses, since these losses stem from a variety of coupled phenomena, such as reaction site coverage [16], interfacial contact between the catalyst layer (CL) and porous transport layer [17], and catalyst utilization [18,19]. Although there are a number of recent studies that focus on enhancing CL and membrane-electrode assembly designs [20–23] and bipolar plate designs [24], the impact of graded pore structure on mass transport mechanisms in novel porous transport layers requires further under-standing to design optimal PTLs exhibiting enhanced ohmic and mass transport performances and to strategize the production of cost effective electrolyzed hydrogen.

The porous transport layer (PTL) is a key component in variety of gas evolving electrochemical conversion technologies from fuel cells to carbon dioxide reduction [25–27], and the attention they deserve is commensurate with the capital cost they impose on these technologies (up to 18% for PEM water electrolyzers [28]). The PTL serves several purposes in a PEM electrolyzer, including thermal and electrical con-duction, mechanical support, pathways for reactant liquid water de-livery to the catalyst layer, and pathways for effective gas removal from the reaction sites to the outlet. The importance of the PTL is demon-strated in a number of previous studies [29–32]. For example, Letten-meier et al. [33] observe drastic changes in high frequency resistance (HFR) in relation to the PTL surface roughness. Majasan et al. [34] report the reduction in HFR for PTLs exhibiting smaller titanium pow-ders, thereby indicating the importance of the CL-PTL interfacial contact with respect to ohmic losses. The interfacial contact of the CL-PTL interface also has an impact on mass transport behavior [17–19,35–37]. Lee et al. [35,36] use a pore network modelling approach to determine the impact of poor contact at the CL-PTL inter-face. A film of oxygen accumulates in the larger interfacial pores, which leads to reduced liquid water permeability in the PTL. Moreover, Schuler et al. [18,19] demonstrate that due to membrane deformation at the CL- PTL interface and limited electron transport in the catalyst coated membrane, only a partial fraction of the catalyst layer is utilized during electrolyzer operation. A new PTL is crucially needed whereby the CL- PTL interfacial contact is controlled to minimize undesired ohmic and mass transport losses.

Novel PTL designs with advanced features such as multi-layer porosity are recently emerging for PEM electrolyzers [33,37–41]. Let-tenmeier et al. [40] apply a porous plasma sprayed coating of Ti on sintered Ti-PTL to produce the first functional backing layer at the interface between the CL and PTL. Their coating enhances PEM elec-trolyzer performance significantly, particularly at high current densities. Subsequently, by using the same coating technique, Lettenmeier et al. [33] produce PTLs with porosity gradients, which exhibit reduced ohmic and mass transport losses compared to low-cost PTLs such as Ti meshes. Schuler et al.[37] also fabricate PTLs that exhibit a bilayer structure, which further improves catalyst utilization. However, the underlying mechanisms concerning how these graded pore structures affect mass transport losses remain unclear. The understanding of two-phase transport phenomena in these graded pore structures is essential for maximizing the effectiveness of these novel PTLs.

PTLs must be prescribed with an optimized range of porosities; pore- network modeling predicts that extremely low porosities (<25%) at the

CL-PTL interface can lead to reduced two-phase permeabilities, while the opposite can lead to high gas saturations [36]. Additionally, Lee et al. [42] demonstrate via ex situ microfluidic experiments that pore size gradients facilitate the unidirectional growth and removal of product gas. Although advantages of having PTLs with graded porosities in PEM electrolyzers are seen in the literature [33,36,42], the underlying transport mechanisms responsible for their performance have not been explained. The discovery of how graded porosities impact PEM elec-trolyzer is vitally needed in order to strategically design new PTL ma-terials that fully harness the opportunities for reaching the never before seen current densities compulsory for cost-effective hydrogen production.

In this work, the underlying transport mechanisms in the graded pore structured PTLs are revealed, thereby elucidating the properties that can be used to tailor and customize PEM electrolyzers for desired high current density operation. Specifically, the impact of a porosity gradient in the PTL on mass transport mechanisms is examined by employing in operando neutron imaging. The microscale distributions of product gas saturation in the custom designed PTL via vacuum plasma spraying is presented at high current densities up to 4.5 A/cm2. Furthermore, the directionality of this porosity gradient is examined, and pore network modelling is used to explain the microscale mass transport mechanisms in these PTLs.

2. Experimental section

2.1. PEM Electrolyzer: Hardware and PTL

Our single cell PEM electrolyzer is custom designed for in operando neutron visualizations. The flow field plates are composed of Grade-II Titanium and electroplated with Au and Pt (0.5 µm and 2.5 µm, respectively) to provide stability against corrosion. The flow field plates feature 55 parallel channels with dimensions measuring 12.50 mm ×500 µm × 500 µm. We use a commercially available catalyst coated membrane1 (HYDRion N115, Ion Power) with an iridium areal density of 1.0 mg/cm2 at the anode and a platinum areal density of 0.3 mg/cm2

at the cathode. An active area of 0.3 cm2 (0.5 cm × 0.6 cm) is selected to ensure sufficient neutron penetration through the PTL to accurately detect anode oxygen gas saturation.

A custom PTL with graded porosity is used at the anode, and a commercially available sintered titanium powder PTL (SIKA-T 10, GKN Sinter Metals) with a porosity of 35% is used at the cathode. The PTLs are compressed to 90% of their original thickness using rigid poly-tetrafluoroethylene gaskets. The PTL with a graded porosity is fabricated via vacuum plasma spraying, following a procedure reported elsewhere [33,40]. In summary, titanium particles with grain diameters of 45 μm and 125 μm are used as the feedstock materials (Grade-1 Titanium, TLS Technik Spezialpulver). The high porosity (HP) region of the PTL is fabricated via 32 iterative plasma torch sweeps on top of a mild steel substrate using a 125 μm feedstock, and the low porosity region of the PTL is fabricated by applying eight iterative plasma torch sweeps on top of the 32 layers with a 45 μm feedstock. The mild steel substrate is later removed from the sprayed titanium PTLs by immersing the samples in 0.5 M H2SO4 for 1 h. The thickness of the resultant PTL with graded porosity is approximately 455 µm. The detailed characterizations of the PTL with graded porosity, including pore size distribution, cross sectional SEM images, and performance comparison to commercial materials, are available in detail elsewhere [33,40].

1 Certain trade names and company products are mentioned in the text or identified in an illustration in order to adequately specify the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily the best avail-able for the purpose.

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We examine both configurations of the PTL porosity gradient to investigate the impact of the directionality of the porosity gradient, as shown in Fig. 1. Herein, the high to low porosity gradient from the catalyst layer (CL) interface to the flow field is noted as (HP → LP), and low to high porosity gradient is noted as (LP → HP). The high porosity region is fabricated by removing a mild steel substrate from the VPS coated titanium particles. During the process of removing the mild steel substrate, some titanium particles near the substrate are also removed, resulting in a higher porosity in this region (see Fig. 1). We expect this higher porosity to facilitate a greater improvement in mass transport through a larger gradient in porosity.

The CL-PTL interfacial contact is characterized with the root mean square roughness (Sq) for both the HP → LP and LP → HP configurations. The Sq is calculated as follows:

Sq =

1

M × N

∑M

i=1

∑N

j=1t2ij

√

(1)

where M and N are the number of pixels equaling the length and width of the PTL [− ], respectively, where i and j are the corresponding pixels in the y- and z-directions [− ], respectively. The surface height at i and j is denoted by tij [m].

2.2. Neutron radiography for quantifying gas saturation

We investigate the relationship between the porosity gradient and the gas accumulation within the PTL via in operando neutron imaging experiments. Neutron imaging is strategically employed due to its high sensitivity to water and relatively low sensitivity to metals [43–45]. Visualizations are performed at the Neutron Imaging Facility (BT-2) of National Institute of Standards and Technology (NIST) located in Gai-thersburg, Maryland [46]. The neutron beam from the reactor core penetrates the electrolyzer cell along the axis parallel to the membrane, as illustrated in Fig. 2. The attenuated neutron beam is converted into visible light by the Gd2O2S:Tb scintillator and captured using a high

speed camera (Andor Neo sCMOS, Oxford Instruments). The experiment setup is arranged to achieve a pixel size of 6.5 μm and a temporal res-olution of 120 s, and we achieve a spatial resolution of approximately 20 µm. To quantify the gas saturation in the anode PTL during constant current operation, the reference image at a fully water-saturated refer-ence state (i = 0 A/cm2) is obtained. Operational images (i > 0 A/cm2) are obtained during constant current operation after steady state is established (last 6 min of the 16 min constant current hold). The modified Beer-Lambert law developed by Hussey et al. [46] is used to quantify gas thickness in the PTL:

tgas =

−ln(I/I0)

βw+

Σw2

4βw2

√

−Σw

2βw(2)

where tgas reflects the gas thickness of an individual pixel [mm], I is the intensity of the images obtained during operation [count⋅s− 1], and I0 is the intensity of the reference image [count⋅s− 1].

∑w and βw are fitting

parameters [0.38483 mm− 1] and [− 0.00947 mm− 2], respectively. We further calculate the gas saturation using the following relation:

sgas =tgas

lPTL × ε (3)

where lPTL is the length of the anode PTL in the beam path [5 mm], and ε is the anode PTL porosity of an individual pixel [− ]. The porosity of the anode PTL (Fig. 1) is obtained using ex-situ microcomputed to-mography (SkyScan 1172, Bruker-micro CT, Belgium) [47]. A sample VPS PTL with domain size of 2.9 mm × 2.9 mm × 455 µm is scanned at an energy level of 100 kV at 100 mA, and a resolution of 2.96 µm per pixel is achieved. The NRECON software is used for reconstruction, and the PTL is segmented using a segmentation method developed in-house [47]. The reconstructed PTL is cropped to 1.45 mm × 1.45 mm × 455 µm to avoid edge effects.

Several image processing procedures are applied to enhance our image quality. We correct for gamma spots (image artefact due to the high energy photons produced from neutrons), hot spots (image artefact

Fig. 1. (a) The schematic and through-plane porosity profile of the PTL used for electrolyzer experiments. The high porosity region is shaded red, and the low porosity region is shaded blue. Schematics are shown for the (b) LP → HP and (c) HP → LP PTL porosity configurations. The white region in the PTL indicates the titanium particles and the black region indicates the pore space. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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from device noise), and beam intensity fluctuations (image artefact arising from the nature of the BT-2 beamline [46]. Gamma spots are removed with a median-combination of three sequential images, and hot spots are removed with a 2-dimensional 3-pixel by 3-pixel area median filter. Lastly, the image registration process developed by Hussey et al. [46] is used to resolve the beam fluctuation artefact.

2.3. Pore network modelling

An open source pore network modelling package, OpenPNM, [48] is used in this work to calculate the permeability of liquid water in the presence of accumulated gas in the PTL. Pore network modelling is strategically applied due to its proven capabilities for accurately pre-dicting gaseous invasion into the PTL and estimating PTL gas saturation [26,36,49,50]. The displacement of the wetting phase (liquid water) by the non-wetting phase (oxygen gas) in the anode PTL is simulated using an invasion percolation algorithm. Invasion percolation is governed by the capillary pressures of the local throats (i.e. local constrictions be-tween pores) [51]. Specifically, during this quasi-static process, the invading fluid is advanced from one pore to the neighbouring pore that exerts the lowest entry throat capillary pressure. For a more detailed overview of the pore network modelling simulation, the readers are referred to our previous pore network modelling works [26,36,50,52].

In this work, both HP → LP and LP → HP PTL configurations are simulated. The PTL structure is characterized via micro-computed to-mography (with domain of the PTL: 1.45 mm × 1.45 mm × 455 μm), and we obtain the porous structure information and produce a network of pores connected by throats by applying a pore network extraction al-gorithm [53]. The inlet pores for the invasion percolation simulation are defined as the surface pores located at the CL-PTL interface. The inva-sion of the non-wetting phase is halted once the invading front displaces the pores at the PTL-flow field interface (i.e. breakthrough event). The gas saturation profile is obtained at the moment of breakthrough, and the two-phase permeability of liquid water is calculated. The perme-ability of liquid water is a crucial parameter for PTLs because liquid water permeates from the channel interface through the PTL to the catalyst layer interface [36]. We direct our attention to the two-phase permeability, which accounts for the blockage of reactant pathways in the PTL due to the presence of by-product gas. The invasion percolation simulated in this work assumes single-phase pore filling. Therefore, the pores are either gas-filled or liquid water filled, and partial filling of

pores are not considered. The gas-filled pores are considered as closed pores, through which liquid water does not transport. This assumption is based on the observations of microfluidic drainage experiments con-ducted on representative 2D PTLs, where the gas fully occupied a pore before invading the next pore [42,54].

2.4. In operando PEM electrolyzer cell testing

The effect of the porosity gradient on the electrochemical perfor-mance of the PEM electrolyzer is investigated via galvanostatic polari-zations and Tafel approximations. The electrochemical performance is measured concurrently with neutron imaging to relate electrochemical performance to gas saturation in the PTL. Deionized liquid water is supplied at a flow rate of 5 mL/min to both the anode and cathode using a commercial peristaltic pump (Masterflex L/S precision variable-speed console drive, Cole-Parmer), and a pulse dampener is used to stabilize the reactant flow. The heated water is circulated through both anode and cathode end plates to maintain the temperature of the electrolyzer cell at 59 ◦C via a heated water bath (Isotemp™ 4100R20, Fisher Sci-entific Co.). A potentiostat with a booster (Gamry Instruments) is used to supply current for the galvanostatic polarizations. Constant current densities ranging from open circuit to 4.5 A/cm2 are applied for both the HP → LP and LP → HP porosity gradient configurations. Each step is held for 16 min, and the average value of the last 6 min is used to characterize steady-state conditions.

A Tafel approximation is conducted after each polarization curve to estimate the mass transport overpotential, and this is conducted for PTLs in both configurations: HP → LP and LP → HP. The high frequency resistance (HFR) and the Tafel slope from 20 mA/cm2 to 80 mA/cm2 are measured while holding each current step for 20 s using the potentiostat. The mass transport overpotentials are estimated as follows:

ηmasstransport = Ecell − Erev − ηohmic − ηkinetic (4)

where ηmass transport is the mass transport overpotential [V], Ecell is the measured cell potential [V], Erev is the reversible potential [V], ηohmic is the ohmic overpotential [V], and ηkinetic is the kinetic overpotential [V]. Ecell is acquired from the polarization curve data. Erev is calculated using an empirical correlation demonstrated by LeRoy et al. [55]:

Fig. 2. A schematic of the in operando neutron imaging experiment setup performed in this work.

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Erev = 1.5184 − 1.5421 × 10− 3 × T + 9.523 × 10− 5 × T × lnT + 9.84 × 10− 8

× T2

(5)

where T is the operating temperature of the electrolyzer [K]. This is an empirical equation developed by LeRoy et al. [55], which accurately describes the Erev based on the operating temperature of the electrolyzer. The ohmic overpotential, ηohmic, is calculated with HFR measurements obtained at applied current densities:

ηohmic = i × Rohmic (6)

where i is the current density [A/cm2], and Rohmic is the measured HFR [Ω⋅cm2]. ηkinetic is calculated using the following equation after fitting to the HFR-corrected polarization curve:

ηkinetic = b × log10

(ii0

)

(7)

where b is the Tafel slope [V/dec], and i0 is the apparent exchange current density [A/cm2]. The apparent exchange current density, i0, is calculated for each PTL configuration using the Tafel relation. With all these variables defined, we estimate the mass transport overpotential for each porosity gradient configuration.

3. Results and discussion

3.1. Impact of porosity gradients on electrochemical performance of a PEM electrolyzer

Positioning the lower porosity region of the PTL adjacent to the catalyst layer leads to superior electrolyzer performance, as demon-strated in the polarization curve (Fig. 3). The electrolyzer cell reaches a constant current of 4.5 A/cm2 with the LP → HP PTL configuration, while the electrolyzer with the HP → LP PTL configuration fails before reaching this current density. The cell potential with the HP → LP PTL configuration is 3.63 V at 3.5 A/cm2, which is notably higher than the cell potential of the LP → HP PTL configuration at 4.5 A/cm2 (3.19 V). The difference between the ohmic overpotential exhibited by the two electrolyzers is also significant. The HFR increases concomitantly with the current density for the HP → LP PTL configuration (Fig. 3b). The HFR measured via the current interrupt method is 233 mΩ⋅cm2 at 0.5 A/cm2

and increases up to 405 mΩ⋅cm2 at 3.5 A/cm2 (maximum current den-sity prior to cell failure). The increasing HFR with current density observed for the HP → LP PTL is attributed to membrane dehydration. There are larger pore diameters associated with the high porosity region at the CL-PTL interface in the HP → LP PTL configuration compared to the LP → HP PTL configuration. During gas invasion into the PTL, gas preferentially percolates from larger pores to smaller pores, thereby flooding the CL-PTL interface in the HP → LP PTL configuration. At higher current densities, this interfacial gas accumulation is exacerbated and results in both mass transport limitations and membrane dehydra-tion (manifesting as an increase in HFR) [37,56,57]. Previous studies report increases in HFR with current density when high porosity PTLs are used (such as mesh-PTLs [33]) or near starvation conditions [56]. Lettenmeier et al. [33] observe a gradual increase in ohmic resistance with increasing current density for an electrolyzer operating with a Ti- mesh PTL. On the other hand, the HFR for the LP → HP PTL configu-ration is constant up to 4.5 A/cm2. The HFR only increases from 217 mΩ⋅cm2 to 250 mΩ⋅cm2 when increasing the current density from 0.5 A/cm2 to 4.5 A/cm2. It is important to note that in the near future the nominal operating target for PEM electrolyzers will be > 4 A/cm2

[58], and new CL-PTL interfaces exhibiting low contact resistances are prerequisite for achieving these commercially relevant high current densities.

We further investigate the effect of the porosity gradient configura-tion on mass transport behaviour via a Tafel approximation (Fig. 3c, d).

The two PTLs configurations exhibit similar electrolyzer performance at the Tafel region (20 mA/cm2 to 80 mA/cm2), indicating comparable kinetics. The exchange current densities of the two configurations also show comparable values; the HP → LP PTL exhibits i0 = 1.68 × 10− 5 A/ cm2 and the LP → HP PTL exhibits i0 = 1.05 × 10− 5 A/cm2. However, the LP → HP PTL configuration results in a remarkably lower mass transport overpotential compared to the HP → LP PTL configuration especially towards higher current densities (Fig. 3c). The mass transport

Fig. 3. (a) Polarization curve comparing the performance of the LP → HP and HP → LP PTL configurations. (b) High frequency resistance (HFR) as a function of current density for the LP → HP and HP → LP PTL configurations. (c) Mass transport overpotential estimated from a Tafel approximation. (d) Tafel mea-surements and iR-corrected electrolyzer potential.

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overpotentials with both PTL configurations increase with increasing current density, as previously reported in the literature with the com-mercial PTLs [15,59]. However, the LP → HP PTL configuration exhibits a considerably lower mass transport overpotential compared to the HP → LP PTL configuration. Specifically, the mass transport overpotential at 4.5 A/cm2 with the LP → HP PTL configuration is 0.41 V, while the mass transport overpotential at 2.5 A/cm2 with the HP → LP PTL configura-tion is 0.49 V. At a current density of 3.5 A/cm2, the mass transport overpotential is 38 % lower for the LP → HP PTL configuration, compared to the HP → LP PTL. However, the dependence of mass transport overpotential with current density is not linear (in contrast to its relationship with the HFR), and this suggests that the pore distribu-tion in the PTL can be further optimized to improve the efficiency of PEM electrolyzers.

3.2. Gas transport through the PTL with porosity gradients

The LP → HP PTL exhibits significantly lower maximum gas satu-ration during operation, as seen in Fig. 4. A maximum gas saturation of 16 % is measured near the CL-PTL interface for the LP → HP configu-ration, which was one half of the HP → LP configuration (Fig. 4b). The maximum gas saturation is located relatively closer to the CL for LP → HP configuration compared to the HP → LP configuration. We also observe that the through-plane position of the maximum gas saturation corresponds to the transition from the high-porosity region to the low- porosity region (x/tPTL = 0.1 for (Fig. 4a) and 0.25 for (Fig. 4b)). The generated gas preferentially accumulates at this transition region for PTLs with graded structures. This accumulation is also apparent in the radiographic image shown in Fig. 5, where higher gas content is distributed near the transition region in the HP → LP PTL.

Similar gas saturation behavior is observed through pore network

modelling by Lee et al. [36], where they observe higher saturation for a HP → LP PTL configuration. During the invasion percolation, the gas invades an adjacent pore with the lowest capillary pressure, which is determined by the diameter of a throat. Since the HP regions have larger pores and throats compared to LP regions, and gases preferentially invade pores in the higher porosity region prior to invading smaller pores. Therefore, higher gas saturation is observed for the HP → LP PTL configuration.

The change in gas saturation is less sensitive to increasing current density compared to the study of Sewyrn et al. [43]. Although the gas saturation slightly increases with increasing current density for the HP → LP configuration, the same trend is not significant for the LP → HP configuration. While gas saturation stays fairly constant with increasing current density, the mass transport overpotential continues to increase since the rate of reactants consumed increases with increasing current density.

3.3. Two-phase transport simulations on PTLs with porosity gradient

Pore network modelling is used to determine the two-phase perme-ability of liquid water for the LP → HP and HP → LP PTL configurations. Fully oxygen flooded inlet conditions are assumed for representing high current density operation, and we correspondingly compare our simu-lation results to the experimentally determined gas saturation profiles obtained at the highest current density tested for both PTLs (i.e., 3.5 A/ cm2). We observe that gas saturation profiles obtained from the PNM simulation closely agree with the in operando neutron imaging results for the LP → HP configuration, as illustrated in Fig. 6. The gas saturation profile from the PNM simulation exhibits similar profiles as the gas saturation profile for the LP → HP configuration. However, the PNM simulation predicts a higher gas saturation compared to the experiments for the HP → LP configuration, as shown in Fig. 6b. The PNM simulation exhibits a gas saturation of 98 % near the CL-PTL interface in the HP → LP PTL configuration, while the maximum gas saturation measured via neutron imaging is approximately 30 %. The difference between the gas saturation observed with our PNM simulation and neutron imaging is attributed to the membrane intrusion into the pores at the CL-PTL interface, which we expect to occur during experiments but have not accounted for in the PNM simulation. The membrane swelling and mechanical compression from the cell assembly leads to membrane intrusion into the pores at the CL-PTL interface. For example, Schuler et al. [19] observe a maximum membrane deformation of 57 µm with their PTL. The membrane intrusion become more severe for PTLs exhibiting higher porosity [19]. The membrane intrusion has likely occurred during in operando neutron imaging when the reference im-ages are being acquired. Since we cannot quantify the amount of membrane intrusion that occurred, the gas saturation measurement is underestimated during our simulations since our porosity values (used for calculating the gas saturation (Eq. (3))) do not account for pore volumes lost from membrane intrusion. Whether it is the membrane or product gas occupying the pore space at the CL-PTL interface, these interfacial pore become inaccessible by reactant liquid water. Therefore, the change in two-phase permeability of liquid water is a highly useful parameter for characterizing mass transport in the PTL.

The pore network modelling results demonstrate that the LP → HP PTL configuration exhibits a higher two-phase permeability of liquid water compared to the HP → LP PTL configuration (Table 1). The single- phase permeability is a structural property of a PTL that does not change based on the PTL configuration. However, the two-phase permeability changes based on the PTL configuration because two-phase permeability is a transport property impacted by the transport of gas in the PTL. The PNM simulation demonstrates that the two-phase permeability of liquid water is an order of magnitude lower for the HP → LP PTL configuration. We observe that reversing the direction of the porosity gradient (from HP → LP to LP → HP) significantly reduces the gas saturation, which leads to a substantial increase in the two-phase permeability. Therefore,

Fig. 4. The gas saturation profiles from in operando neutron imaging the (a) LP → HP PTL configuration and (b) HP → LP PTL configuration. The normalized distance depicts the position across thickness of the PTL (x/tPTL). The error bars in the saturation profile indicate the standard error at 95% confidence interval.

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tailoring the porosity gradient of the PTL is essential in controlling the mass transport behaviour in the PTL.

Garcia-Navarro et al. [60] discusses the permeability of gas as a crucial property for effective gas removal in an electrolyzer PTL, as lower shear stresses are experienced with PTLs exhibiting higher gas

permeabilities. In the simulation work presented here, even with a significantly lower gas saturation in the PTL, we find that the LP → HP PTL configuration exhibits a higher two-phase permeability of gas compared to the HP → LP PTL configuration in the through-plane di-rection (i.e, from reaction site to the channel interface). Therefore, the LP → HP PTL configuration provides both a higher two-phase perme-ability of liquid water for effective reactant transport and a higher two- phase permeability of gas, which is critical for facilitating gas removal.

4. Conclusion

For the first time, we explain the underlying microscale transport behaviour of custom porosity graded PTL materials for high perfor-mance PEM electrolysis. In operando neutron imaging is used to demonstrate that the gas saturation near the CL-PTL interface is reduced by a factor of two when the lower porosity region is placed adjacent to the catalyst layer (LP → HP PTL configuration). Moreover, concurrent electrochemical testing reveals that the cell potential is 29 % lower with the LP → HP PTL configuration due to lower HFR and mass transport overpotentials. The HP → LP PTL configuration leads to significant in-creases in HFR and mass transport overpotentials with increasing cur-rent density, which significantly reduces the performance of the PEM electrolyzer.

Pore network modelling (PNM) is used to further validate our mass transport behaviour observations. The predicted gas saturation profile exhibits strong agreement with the gas saturation profile obtained from the in operando neutron imaging for the LP → HP PTL configuration. By comparing the numerical modelling results and neutron imaging for the HP → LP PTL configuration, we determine that reactant transport is unfavorable due to undesired membrane deformation and gas saturation at the CL-PTL interface. Although both configurations exhibit nearly identical single-phase permeabilities, the two-phase permeability of liquid water is an order of magnitude lower with the HP → LP PTL configuration, further reinforcing that the HP → LP PTL configuration is less desirable for promoting mass transport behaviour. Our work dem-onstrates the importance of using functional spatially graded PTLs in PEM electrolyzers with a low porosity region adjacent to the catalyst layer for reducing both ohmic and mass transport losses. These PTLs will enable the development of high performance PEM electrolyzers that can

Fig. 5. The radiographic image of the (a) LP → HP and (b) HP → LP PTL configuration acquired in operando.

Fig. 6. Gas saturation profiles measured via in operando neutron radiography (hollow circles) and pore network modelling simulations (solid line) for PTL configurations: (a) LP → HP (b) HP → LP.

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operate at high current densities, thus reducing the cost of hydrogen production with renewable electricity. New designs informed by our new insights will have broad impact on the next generation design of gas evolving electrochemical energy conversion technologies for high cur-rent density operation.

CRediT authorship contribution statement

Jason K. Lee: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization, Project administration. ChungHyuk Lee: Meth-odology, Investigation, Writing - review & editing. Kieran F. Fahy: Investigation, Writing - review & editing. Pascal J. Kim: Investigation, Writing - review & editing. Jacob M. LaManna: Resources, Writing - review & editing. Elias Baltic: Resources, Writing - review & editing. David L. Jacobson: Resources, Writing - review & editing. Daniel S. Hussey: Resources, Writing - review & editing. Svenja Stiber: Re-sources, Writing - review & editing. Aldo S. Gago: Resources, Writing - review & editing. Kaspar A. Friedrich: Resources, Writing - review & editing. Aimy Bazylak: Supervision, Methodology, Formal analysis, Funding acquisition, Resources, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants Program and the Canada Research Chairs Program. The Canada Foundation for Innova-tion is also gratefully acknowledged. Graduate scholarships awarded to Jason Keonhag Lee from NSERC Alexander Graham Bell Canada Grad-uate Scholarships – Doctoral Program, Queen Elizabeth II/Edward Rygiel Graduate Scholarship in Science and Technology, Ontario Grad-uate Scholarship and Glynn Williams Fellowships are also gratefully acknowledged. S. Stiber and A.S. Gago thank the European Union’s Horizon 2020 research and innovation programme for funding the project PRETZEL under grant agreement No 779478 and it is supported by FCH JU. A portion of this work was supported by the National Institute of Standards and Technology and the NIST Physical Measure-ment Laboratory.

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