Effect of Stratification of Cathode Catalyst Layers on Durability of Proton Exchange Membrane Fuel Cells

energies

Article Effect of Stratiﬁcation of Cathode Catalyst Layers on Durability of Proton Exchange Membrane Fuel Cells

Zikhona Nondudule 1, Jessica Chamier 2,* and Mahabubur Chowdhury 1

1 Department of Chemical Engineering, Faculty of Engineering, Cape Peninsula University of Technology, Cape Town 7530, South Africa; [email protected] (Z.N.); [email protected] (M.C.) 2 Department of Chemical Engineering, Centre for Catalysis Research, HySA Catalysis, University of Cape Town, Cape Town 7700, South Africa * Correspondence: [email protected]; Tel.: +27-(0)-21-650-2515

Abstract: To decrease the cost of fuel cell manufacturing, the amount of platinum (Pt) in the catalyst layer needs to be reduced. In this study, ionomer gradient membrane electrode assemblies (MEAs) were designed to reduce Pt loading without sacrificing performance and lifetime. A two-layer stratification of the cathode was achieved with varying ratios of 28 wt. % ionomer in the inner layer, on the membrane, and 24 wt. % on the outer layer, coated onto the inner layer. To study the MEA performance, the electrochemical surface area (ECSA), polarization curves, and electrochemical impedance spectroscopy (EIS) responses were evaluated under 20, 60, and 100% relative humidity (RH). The stratified MEA Pt loading was reduced by 12% while maintaining commercial equivalent performance. The optimal two-layer design was achieved when the Pt loading ratio between the layers was 1:6 (inner:outer layer). This MEA showed the highest ECSA and performance at 0.65

V with reduced mass transport losses. The integrity of stratiﬁed MEAs with lower Pt loading was evaluated with potential cycling and proved more durable than the monolayer MEA equivalent.

Citation: Nondudule, Z.; Chamier, J.; The higher ionomer loading adjacent to the membrane and the bi-layer interface of the stratified Chowdhury, M. Effect of Stratification catalyst layer (CL) increased moisture in the cathode CL, decreasing the degradation rate. Using of Cathode Catalyst Layers on ionomer stratification to decrease the Pt loading in an MEA yielded a better performance compared Durability of Proton Exchange to the monolayer MEA design. This study, therefore, contributes to the development of more durable, Membrane Fuel Cells. Energies 2021, cost-effective MEAs for low-temperature proton exchange membrane fuel cells. 14, 2975. https://doi.org/10.3390/ en14102975 Keywords: proton exchange membrane fuel cell; cathode catalyst layers; ionomer loading; stratified cathode catalyst layers Academic Editor: Jin-Soo Park

Received: 5 February 2021 Accepted: 10 May 2021 1. Introduction Published: 20 May 2021 Proton exchange membrane fuel cells (PEMFCs) have attracted great attention in ◦ Publisher’s Note: MDPI stays neutral research and development due to their simplicity, low-temperature operation (50–100 C), with regard to jurisdictional claims in absence of pollutants, and higher power density (40–60%) compared to conventional inter- published maps and institutional affil- nal combustion gasoline engines [1]. PEMFCs currently use platinum (Pt) (nanoparticles iations. dispersed on carbon support) as a catalyst for the energy driving hydrogen oxidation reactions (HORs) and oxygen reduction reactions (ORRs), which significantly increases their manufacturing cost [2]. The ORR occurring at the cathode is about six orders of magnitude slower than the HOR requiring a higher Pt loading and is, therefore, the focal point for Pt reduction. Research and development activities are aimed at improving the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. catalyst activity and utilization in the cathode catalysts layer (CCL) without compromising This article is an open access article durability and stability [3]. distributed under the terms and A promising MEA design includes the development of a graduated catalyst multilayer conditions of the Creative Commons in the catalyst-coated membrane (CCM). A graduated CCL structure can increase the Pt Attribution (CC BY) license (https:// catalyst use efficiency by increasing the available Pt surface area. Recent studies have creativecommons.org/licenses/by/ shown that the O2 mass transport resistance is inversely proportional to Pt loading and 4.0/). highly dependent on the available Pt surface area [4].

Energies 2021, 14, 2975. https://doi.org/10.3390/en14102975 https://www.mdpi.com/journal/energies Energies 2021, 14, 2975 2 of 17

Many studies have focused on the CCL design to improve the MEA structure and materials’ distribution in the CL [5–8]. They demonstrated that non-uniform CL structures can increase electrode performance compared to monolayer CLs, using the same catalyst and Pt loadings [9–13]. The non-uniform multilayer cathode proposed by Yoon et al. [5] improved the proton and electron conductivity and therefore the oxygen reduction in the CL, which increased the MEA performance. The double-layer design consisted of electrolyte-rich and -poor layers. The best of both was sought, in which the electrolyte- induced electron conduction resistance was minimized where the largest mass transport occurred, enabling increased access to catalyst sites. The ionomer level was increased closer to the membrane, where hydration and proton transfer were key to performance. Using an ionomer gradient in the catalyst layer design increases the pore size distribution and improves water management in an MEA. Roshandel et al. [14] conducted a study on multilayer cathodes showing that the fuel cell performance is affected by the porosity in a gas diffusion electrode (GDE). Xie et al. [6] designed GDEs containing gradient Nafion (Aldrich) layers and found that cathode performance improved when the Nafion content was higher in the GDE towards the CL/membrane interface and lower towards the CL/gas diffusion layer (GDL) interface. The design maximized proton transport in the regions of greatest ion flux and porosity in the regions with increased gas transport. Kim et al. [15] designed anode and cathode CLs with gradient Nafion® content (EW1100, DuPont Inc., Wilmington, Delaware, USA) and varied Pt loading. The dual catalyst layer coated MEA showed better cell performance at the high current density region than the monolayer MEA did, illustrating the need for porosity or mass transport in the outer layer. Jung et al. [16] proposed an electrode composed of a highly phase-separated 1-methyl-2-pyrrolidinone (NMP)-based external layer and a lowly phase-separated glycerol-based inner layer. The proposed electrode resulted in increased cell performance in the high-current region. They later improved on their work [17], showing that controlled porosity and water movement can increase Pt use efficiency. The Pt loading in the inner and outer layers was reduced 2 to 0.16 and 0.04 mgPt/cm , producing a current density at 0.6 V, four times larger than a monolayer equivalent MEA. The ionomer (Nafion, Du Pont Inc., Wilmington, DE, USA)/catalyst (Pt/C) gradient design by Chen et al. [18] reported a 28.40 to 135.7% higher power density compared to the conventional single-layer CL MEA depending on the relative humidity. An increased power density was observed at lower relative humidity, illustrating the increased hydration offered by higher ionomer loadings closer to the membrane. Shahgaldi et al. [8] designed an ionomer gradient CCL that improved the Pt utilization by approximately 15% as well as reducing the mass and proton transport resistances compared to the monolayer CCL design. It is evident that gradual stratification of the CCL increases MEA performance [6, 8,11,13,15–18] by building the CL structure to consider mass movement, hydration, and access to active sites. This study investigates whether the durability of the MEA can be increased for the same considerations. Potential cycling of MEAs between 0 and 1.5 V (N2/H2) decreases the electrochemical surface area significantly due to both Pt dissolution and carbon corrosion [19]. Support corrosion in CCLs is one of the major contributors to the reduced stability and lifetime of PEMFCs [20]. The durability of the MEA decreases with decreasing CCL thickness and Pt loading [20], simply due to the lower availability of the support fuel (carbon) and the accelerated degradation of the catalyst layer’s structural integrity. Stratification of the catalyst layer into electrolyte-rich and -poor cathode sublayers can increase its durability towards carbon corrosion. Superior water management closer to the membrane and reduced hydration in the outer layer closer to the gas diffusion layers will limit the movement of dissolved Pt into the membrane. The electrolyte-poor catalyst layer will likely be less prone to carbon corrosion due to the lower retained water content [19]. Energies 2021, 14, 2975 3 of 17

2. Materials and Methods 2.1. MEA Preparation The catalyst ink was prepared by dispersing HySA-K40 (HyPlat®) Pt on carbon (Pt/C 40 wt. % Pt) in 99.9% isopropyl alcohol, deionized water, and Aquivion® ionomer (25.0 wt. % solids, D79-25BS Solvay, Princeton, NJ, USA). The catalyst slurry was coated onto a 50-cm2 M820.15 gore membrane using the ultrasonic spray method. The ionomer content of the monolayer CCL design was 24 wt. %. For a stratified CCL design, two catalyst inks containing 24 and 28 wt. % ionomer contents were prepared. The first ink slurry, containing 28 wt. % ionomer, was sprayed onto the membrane to form the first layer until a targeted Pt loading was reached, as shown in Table1. The second ink mixture, containing 24 wt. % ionomer, was deposited onto the first layer of catalyst to form the 2 outer layer (Table1 ). The cathode Pt loading was 0.4 mgPt/cm for the monolayer CCL 2 and 0.35 mgPt/cm for the stratified CCL MEA design. The anode Pt loading was fixed at 2 0.1 mgPt/cm as a monolayer for both the monolayer and stratified CCL designs. After coating, the CCM was fitted with a gasket and hot pressed at 90 ◦C and 10,000 kPa for 1 min using a hot press to ensure complete sealing. The CCM fitted with a gasket was then sandwiched between two double-layer GDLs (Avcarb MB30) to form an MEA. The MEA thickness was then measured using a thickness gauge.

Table 1. Speciﬁcations of monolayer and stratiﬁed CCL designs.

Cathode Pt Loading Total Cathode Pt 2 2 Ionomer Loading (wt. %) (mgPt/cm ) Loading (mgPt/cm ) Inner Layer Outer Layer Inner Layer Outer Layer MEA #1M 0.40 - 0.40 24 - MEA #2M 0.35 - 0.35 24 - MEA #3 0.050 0.30 0.35 28 24 MEA #4 0.10 0.25 0.35 28 24 MEA #5 0.15 0.20 0.35 28 24 MEA #6 0.20 0.15 0.35 28 24

2.2. Physical Characterization Techniques The Brunauer–Emmett–Teller (BET) technique and a scanning electron microscopy (SEM) analysis were used to characterize the MEAs. The SEM images were taken with an FEI Nova NanoSEM 230 using a backscatter detector at 20.0 keV at 2000, 5000, and 10,000× magniﬁcation. The BET analysis was performed using a Micrometrics TriStar II 3020 to determine the surface area, pore size, and cumulative pore volume of the CLs.

2.3. Electrochemical Measurement The MEA was placed into a 50-cm2 single cell fuel cell hardware consisting of two graphite bipolar plates with parallel flow field channels, current collectors, and two end plates [21]. The assembly was compressed to a torque of 5 Nm. All single cell tests were conducted with a fully automated FuelCon (C50-LT) fuel cell test station in the humidity range of 20–100% relative humidity (RH). The MEA was conditioned prior to polarization measurements: the cell was heated up to 80 ◦C at 100% RH, with hydrogen and airflow in the potential range between 0.3 and 1 V in potential 3 steps of 0.05 V. Pure H2 gas was supplied at 0.00111 Nm /min to the anode and air at 0.00265 Nm3/min to the cathode compartment. The conditioning cycle was repeated 12 times for a 2-h period. The single cell was activated at 0.3 V using H2 and air at 80% RH, a 74.6 ◦C operating temperature, and 2-bar back pressure for both the anode and cathode. Polarization (I–V) curves were measured at 80 ◦C while the cathode and anode were fed with air and pure H2 with stoichiometries of 2.5 and 1.5, respectively. I–V curves were recorded at 20, 60, and 100% RH. Cyclic voltammograms (CVs) were collected under gas ◦ fluxes of 500 mL/min N2 at the cathode and 300 mL/min H2 at the anode at 80 C from 20 to 100% RH. The potential scan range in the CV was 0.04–0.9 V, scanned at 50 mV/s. The hydrogen adsorption peak was then used to determine the electrochemical surface Energies 2021, 14, 2975 4 of 17

area (ECSA). The EIS was measured in a frequency range from 20 kHz to 100 mHz with an amplitude of 5 mV while the cell was operated at 500 mA/cm2 and 80 ◦C.

2.4. Accelerated Stress Test An accelerated stress test (AST) was used to study the durability of the support during potential cycling. The cell was conditioned at the conditions listed in Table2.

Table 2. Fuel cell operating conditions for the carbon corrosion AST.

Cell Temperature Gas Flow Rates Feed Gas Back Pressures RH (%) (◦C) (Nl/min) Temperature (◦C) (bar) Standard operating 1.11 H (anode) 50 ◦C anode (H ) 100 (anode) 80 2 2 2 conditions 2.65 Air (cathode) 50 ◦C cathode (Air) 80 (cathode) ◦ Carbon corrosion 0.2 H2 (anode) 83 C anode (H2) 100 (anode) 80 ◦ 1 test conditions 30 N2 (cathode) 83 C cathode (N2) 100 (cathode)

During the carbon corrosion test, carbon and water oxidation were incurred by cycling voltage across the MEA. The potential was cycled repeatedly between 0 and 1.2 V at 50 mv/s. The process was repeated for up to 6000 cycles. The experimental cycles were divided into the following segments: beginning of life (BoL), 20, 180, 200, 600, 1000, 2000, and 6000 cycles. Polarization curve, hydrogen crossover, and CV were measured after each segment.

3. Results and Discussion 3.1. Reducing Pt Loadings in Stratified MEAs 2 The loading in the CCL was reduced to 0.35 mgPt/cm in a monolayer as well as stratified configurations. The thicknesses of the MEAs are presented in Table3. The reduced Pt loading MEAs (MEA #2M- #6) were 16–40% thinner than the high Pt loading monolayer MEA #1M, mostly due to the decreased Pt loading. MEAs with a thicker 28 wt. % I loading layer (MEA #5 and #6) were thinner and relatively less porous than those with more 24 wt. % I (MEA #3 and #4). Increasing the ionomer loading in the CL impacted the agglomerate and aggregate size and distribution in the CL. Consequently, the stratified CCLs had dual Pt/C-ionomer layer structures due to their varying Pt/C to ionomer ratios as well as an interface boundary. Increasing the ionomer content increased the ionomer thin film around the agglomerate structures and decreased the pore volume between the agglomerate structures. The 28 wt. % I ionomer layer presented with overall denser catalyst layers.

Table 3. The thickness and pore characteristics of the monolayer and stratiﬁed CCL MEAs.

MEA Thickness Bulk Density Cumulative Pore BET Surface Area % Porosity (mm) (g/cm3) Volume (cm3/g) (m2/g) MEA #1M 0.05180 0.8157 0.1443 70.18 11.77 MEA #2M 0.03067 1.144 0.1040 70.43 11.85 MEA #3 0.04200 1.002 0.09620 67.27 9.640 MEA #4 0.04330 0.9566 0.1867 57.77 17.89 MEA #5 0.03700 1.141 0.04209 54.50 4.800 MEA #6 0.03800 1.111 0.09623 47.22 10.68

The BET surface areas of the monolayer MEAs were significantly higher compared to the stratified layers, which is primarily attributed to the nature of the deposition method. During monolayer coating, the layer is sprayed continuously in a single timeframe to achieve the desired thickness. While manufacturing stratified layers, a single ionomer ink is coated until the desired loading is reached. Thereafter, it rests until all the MEAs are completed. This provides a settling time for the first layer until a second layer, with a decreased ionomer content, can be deposited onto it. The second layers can therefore Energies 2021, 14, x FOR PEER REVIEW 5 of 18

timeframe to achieve the desired thickness. While manufacturing stratified layers, a single Energies 2021, 14, 2975 5 of 17 ionomer ink is coated until the desired loading is reached. Thereafter, it rests until all the MEAs are completed. This provides a settling time for the first layer until a second layer, with a decreased ionomer content, can be deposited onto it. The second layers can there- penetratefore penetrate the first the tofirst a largerto a larger extent, extent, reducing reducing the layerthe layer thickness thickness and and porosity porosity as well as well as surfaceas surface area. area. TheThe BETBET surface surface area area of of the the stratified stratified MEAs MEAs decreased decreased with with the the increasing increasing thickness thickness of theof the first first layer layer containing containing 28 wt. 28 %wt. ionomer. % ionomer. Thus, Thus, an increasing an increasing ionomer ionomer content content increases in- thecreases ionomer the ionomer coverage coverage in the CL in and, the CL therefore, and, therefore, the surface the area. surface area.

3.1.1.3.1.1. ElectrochemicalElectrochemical SurfaceSurface AreaArea FigureFigure1 1presents presents the the ECSAs ECSAs of of the the stratiﬁed stratified and and non-stratiﬁed non-stratified MEAs MEAs at at full full and and 12% 12% reducedreduced PtPt loading.loading.

100 MEA #1M MEA #2M

) 80 MEA #3 gPt / 2 60 MEA #4 MEA #5 40 MEA #6

ECSA (cm 20

0 20 60 100 Relative Humidity (%)

Figure 1. The ECSAs for the stratified and monolayer MEAs determined at 20–100% RH. Figure 1. The ECSAs for the stratiﬁed and monolayer MEAs determined at 20–100% RH.

TheThe monolayer monolayer MEA MEA #2M, #2M, with with reduced reduced Pt loading,Pt loading, demonstrated demonstrated the largest the largest and most and most consistent ECSA under all RH conditions (94.112 m2/gPt, 91.522 m2/gPt, and 89.19 m2 2/gPt), consistent ECSA under all RH conditions (94.11 m /gPt, 91.52 m /gPt, and 89.19 m /gPt), owingowing toto its highly porous porous agglomerate agglomerate struct structure.ure. No No trend trend in inECSA ECSA was was observed observed for forthe theincrease increase or decrease or decrease in ionomer-loaded in ionomer-loaded layer layer thickness. thickness. The The stratified stratified CCL CCL MEA MEA with with the thelargest largest BET BET surface surface area area and and more more 24 24 wt. wt. % % I Iloading loading (MEA (MEA #3) demonstrateddemonstrated aa largerlarger 22 2 2 ECSAECSA fromfrom 2020 toto 60%60% RH conditions (92.92 m /g/gPtPt andand 88.04 88.04 m m/gPt/g) andPt) and low low porosity porosity and 2 2 andsurface surface area. area. MEA MEA #5 showed #5 showed the the highest highest ECSA ECSA (81.81 (81.81 m m/g/gPt) Ptof) ofthe the stratified stratified MEAs MEAs at at100% 100% RH. RH. The The increase increase in ECSA in ECSA with with RH was RH wasalso reported also reported by Fan by et Fan al. [22], et al. who [22 ],mainly who mainlyattributed attributed this trend this to trend the improved to the improved contact area contact between area between Pt particles Pt particles and water and domains water domainsinstead of instead the formation of the formation of new transport of new transport pathways. pathways. The low Theporosity low porosityand denser and agglom- denser agglomerateerate structure structure of MEA of #5 MEA could #5 have could played have a played role in a increasing role in increasing the contact the area contact between area betweenwater domains water domains and ionomer/catalyst and ionomer/catalyst aggregates. aggregates. ItIt isis evidentevident thatthat thethe increasedincreased 28 28 wt. wt. %% II loadingloading asas wellwell asas thethe interfaceinterface boundary boundary of of thethe dualdual catalyst catalyst layer layer structure structure impacted impacted the ECSA.the ECSA. The higherThe higher ionomer ionomer content content increased in- thecreased surface the coverage surface ascoverage well as wateras well adsorption as water at adsorption higher relative at higher humidity, relative which humidity, reduced accesswhich to reduced Pt. The access impact to of Pt. the The interface impact boundary of the interface layer, in boundary which the layer, second in layerwhich partially the sec- intrudesond layer into partially the first, intrudes is also into likely the to first, have is analso impact likely onto have the overall an impact access on tothe catalyst overall activeaccess areas. to catalyst This canactive be observedareas. This through can be theobserved decreases through in surface the decreases area and porosityin surface of area the stratifiedand porosity layers of comparedthe stratified to thelayers monolayer compared structures. to the monolayer structures.

3.1.2.3.1.2. MEAMEA PerformancePerformance ToTo determine determine the the effect effect of ionomerof ionomer stratiﬁed stratifi layersed layers on the on electrochemical the electrochemical performance perfor- ofmance the resulting of the resulting MEA, the MEA, polarization the polarizati curveson arecurves compared are compared in Figure in2 .Figure 2.

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1 MEA #1M 0.9 MEA #2M 0.8 MEA #3 0.7 MEA #4 0.6 MEA #5 0.5 Voltage (V) MEA #6 0.4 0.3 0.2 0 200 400 600 800 1000 1200 Current density (mA/cm2)

(a) 1 MEA #1M MEA #2M 0.9 MEA #3 MEA #4 0.8 MEA #5 MEA #6 0.7 Voltage (V)

0.6

0.5 0 200 400 600 800 1000 1200

Current density (mA/cm2)

(b) 1 MEA #1M MEA #2M 0.9 MEA #3 0.8 MEA #4 MEA #5 0.7 MEA #6

Voltage (V) 0.6

0.5

0.4 0 200 400 600 800 1000 1200 1400 Current density(mA/cm2)

(c)

Figure 2. PolarizationFigure curves 2. Polarization for monolayer curves (MEA for monolayer #1M and #2M) (MEA and #1M stratified and #2M) MEAs and (MEA stratiﬁed #3- MEAs (MEA #3- #6) #6) obtained for (obtaineda) 20% RH, for (b (a)) 60% 20% RH, RH, and (b) 60%(c) 100% RH, RH. and (c) 100% RH.

The polarizationThe curves polarization in Figure curves 2 show in that Figure the2 showoutput that voltage the output performance voltage de- performance decreased as the amountcreased of as current the amount drawn of currentfrom the drawn single from cell was the singleincreased. cell wasThe increased.output The output voltage was loweredvoltage by was various lowered resistances by various experienced resistances through experienced the MEA through with increasing the MEA with increasing current density.current At low density.current densities At low current(<200 mA/cm densities2), a (<200sharp mA/cmdecrease2 ),in aoutput sharp volt- decrease in output age is observed due to activation losses. The intermediate region is the ohmic loss region

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voltage is observed due to activation losses. The intermediate region is the ohmic loss 2 2 region(<600 (<600 mA/cm mA/cm), and2 ),the and high the current high current density density region region (>1000 (>1000 mA/cm mA/cm) is the2 )mass is the transport mass transportloss region. loss region.Ohmic losses Ohmic represent losses represent a combination a combination of ionic ofand ionic electronic, and electronic, membrane, mem- and brane,contact and resistances. contact resistances. Mass transport Mass transport losses in lossesthe high in thecurrent high density current region density are region associated are associatedwith gas withdiffusion gas diffusion limitations limitations from the fromsupply the channels supply channelsto the CL toactive the CL sites active where sites elec- wheretrochemical electrochemical reactions reactions occur [23] occur. The stratified [23]. The CCL stratified MEAs CCL performed MEAs performedbetter than the better load- thaning the equivalent loading equivalentmonolayer monolayerMEA #2M MEAdid in #2M the high did in current the high density current region. density This region. perfor- Thismance performance improvement improvement is attributed is attributed to the higher to the ionomer higher loading ionomer in loading the first in layer, the firstwhich layer,increased which proton increased conductivity, proton conductivity, and a lower and ionomer a lower loading ionomer in loadingthe second in the layer, second which layer,reduced which mass reduced transport mass losses transport [8,11]. losses [8,11]. AtAt low low current current densities densities (200 (200 mA/cm mA/cm2 and2 and lower), lower), the the activation activation losses losses of of the the mono- mono- layerlayer MEA MEA #2M #2M were were higher higher than than those those of theof the stratified stratified MEAs, MEAs, indicating indicating limitations limitations for for chargecharge transfer. transfer. Under Under dry dry conditions, conditions, the the performance performance of theof the stratified stratified MEAs MEAs improved improved withwith the the increasing increasing thickness thickness of of the the 28 28 wt. wt. % % layer layer due due to to the the increased increased ability ability to retainto retain water.water. At At low low RH, RH, MEA MEA #6, #6, which which had had the the thickest thicke 28st 28 wt. wt. % % I layer, I layer, demonstrated demonstrated superior superior performanceperformance owing owing to itsto its higher higher proton proton conductivity. conductivity. From From medium medium to high to high current current densi- den- ties,sities, the voltagethe voltage drop drop was was greater greater for MEAfor MEA #2M #2 underM under all RHall RH conditions conditions due due to increasedto increased ionicionic transport transport resistances, resistances, which which limited limited its performance. its performance. At high At current high densities,current densities, MEAs #4MEAs and #6 #4 experienced and #6 experienced higher voltage higher losses, voltage indicating losses, massindicating transport-related mass transport-related losses. It is evidentlosses. It that is evident the boundary that the interface boundary between interface the between catalyst the layers catalyst did not layers have did a uniformnot have a impactuniform on the impact performance on the performance of the MEAs, of save the forME improvementAs, save for improvement in the lower current in the lower density cur- region.rent density The lower region. losses The indicate lower an losses increase indicate in electronic an increase connectivity, in electronic despite connectivity, the overall de- decreasespite the in ECSAoverall for decrease the stratified in ECSA MEAs. for the Figure stratified3 compares MEAs. the Figure performances 3 compares of the the lower perfor- 2 Ptmances loading of MEAs the lower (MEA Pt #2M-loading #6) MEAs and the (MEA benchmark #2M- #6) MEA and the #1M, benchmark with 0.4 mgMEAPt/cm #1M, Ptwith loading.0.4 mg ThePt/cm performance2 Pt loading. wasThe determinedperformance at was 0.65V—the determined voltage at 0.65V—the at which the voltage benchmark at which performancethe benchmark for automotive performance design for systemsautomotive from de ambientsign systems to saturated from conditionsambient to (0–100% saturated RH)conditions is determined (0–100% [24 ].RH) is determined [24].

1000 MEA #1M ) 2 MEA #2M 800 MEA #3 MEA #4 600 MEA #5 MEA #6 400

200 Current density (mA/cm 0 20 60 100 Relative Humidity (%)

Figure 3. The current density (mA/cm2) for the stratified and non-stratified MEAs determined at Figure 3. The current density (mA/cm2) for the stratiﬁed and non-stratiﬁed MEAs determined at 20–100% RH and 0.65 V. 20–100% RH and 0.65 V.

UnderUnder dry dry conditions, conditions, MEA #2MMEA demonstrated #2M demonstr veryated poor very performance poor performance (460 mA/cm 2(460), 2 achievingmA/cm 52.27%), achieving of MEA 52.27% #1M’s of performance. MEA #1M’s The perf Ptormance. utilization The may Pt have utilization dropped may signifi- have cantlydropped for the significantly low Pt content for 24the wt. low % Pt MEA content due to24 thewt. inefficient% MEA due I/C to ratio. the inefficient As expected, I/C theratio. performanceAs expected, of the stratifiedperformance MEAs of the decreased stratified with MEAs a decrease decreased in the with first a layer’sdecrease thickness, in the first aslayer’s a higher thickness, ionomer loadingas a higher decreases ionomer the ionicloading resistance decreases under thedry ionic conditions. resistance At under higher dry RH,conditions. the ionomer At absorbshigher RH, moisture the ionomer [25] and absorbs elongates moisture ionic pathways. [25] and elongates A higher ionomerionic path- loadingways. meansA higher more ionomer hydrophilic loading zones means and more greater hydrophilic ionomer swelling,zones and which greater increases ionomer chargeswelling, transfer which resistances increases at charge higher transfer RH. As resi a result,stances when at higher the RH RH. increased, As a result, the perfor-when the RH increased, the performance of MEAs with more 24 wt. % I loading increased (relative

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to higher 28 wt. % I). Lower ionomer loading on the secondary layer clearly contributed mance of MEAs with more 24 wt. % I loading increased (relative to higher 28 wt. % I). to improved mass transport, preventing water accumulation in the active area. Generally, Lower ionomer loading on the secondary layer clearly contributed to improved mass the performance of the reduced Pt loading MEAs was comparable to the benchmark mon- transport, preventing water accumulation in the active area. Generally, the performance of olayer MEA the#1M. reduced It is, therefor Pt loadinge, possible MEAs to was reduce comparable the Pt loading to the benchmarkby 12% and monolayerstill achieve MEA #1M. It a similar benchmarkis, therefore, performance possible to under reduce various the Pt loadingRH conditions by 12% in and stratified still achieve catalyst a similar lay- benchmark ers. Sasikumarperformance et al. [26] understudied various the dependence RH conditions of ionomer in stratiﬁed content catalyst on Pt layers. loading Sasikumar and et al. [26] found that thatstudied the optimal the dependence ionomer of content ionomer depends content on on the Pt loadingPt loading. and This found study that thathas the optimal confirmed thationomer the performance content depends in various on the RHs Pt loading. is strongly This dependent study has conﬁrmedon the I/C thatloading the performance ratio. MEA #3in variouswith a Pt RHs ratio is strongly= 1:6 (0.05 dependent 1st layer onand the 0.3 I/C 2nd loading layer) ratio.gave MEAthe best #3 withperfor- a Pt ratio = 1:6 mance at higher(0.05 RH 1st layerand MEA and 0.3#5 2ndwith layer) a 1:0.75 gave (0.15 the 1st best layer performance and 0.2 2nd at higherlayer) ratio RH and per- MEA #5 with formed best aunder 1:0.75 dry (0.15 conditions. 1st layer and 0.2 2nd layer) ratio performed best under dry conditions.

3.1.3. EIS Analysis3.1.3. EIS Analysis To further understandTo further understandthe performance the performance of the monolayer of the and monolayer stratified and MEAs, stratified EIS MEAs, EIS was used towas determine used to the determine key resistances. the key The resistances. resulting TheNyquist resulting plots Nyquistare presented plots in are presented Figure 4, showingin Figure the4 imaginary, showing impedance the imaginary (Z’’) impedanceas a function (Z”) of real as aimpedance function of(Z’). real In impedance the high-frequency(Z’). In range, the high-frequency ohmic resistances range, dominate, ohmic resistances while the intermediate dominate, while frequency the intermediate region is dominatedfrequency by region kinetic is and dominated charge transfer by kinetic resistances and charge (Rct transfer). Charge resistances transfer re- (Rct). Charge sistances dependtransfer on resistancesinterfacial reaction depend kinetics on interfacial resulting reaction from kineticsthe three-phase resulting boundary from the three-phase zone [27]. Theboundary magnitude zone of [27 total]. The ohmic magnitude resistance of total (RΩ ohmic) of the resistance cell is determined (RΩ) of the by cell the is determined intercept of bythe thehigh-frequency intercept of thecurve high-frequency with the Z’ real curve axis. with The theEIS Z’spectrum real axis. generally The EIS spectrum shows two distinctivegenerally showsarcs representing two distinctive the charge arcs representing transfer resistance the charge (Rct transfer) and the resistance mass (Rct) and transport resistancethe mass (R transportmt) [27]. Figure resistance 4 shows (Rmt)[ the27]. Nyquist Figure4 plotsshows obtained the Nyquist for the plots mono- obtained for the layer and stratifiedmonolayer MEAs. and The stratified mass transfer MEAs. Theresistance mass transferarc was resistancereasonably arc small was to reasonably insig- small to nificant in theinsignificant Nyquist plots in the because Nyquist the plotsstudy because was conducted the study at was low conducted current density at low (500 current density 2 mA/cm2), where(500 charge mA/cm transfer), where and charge ohmic transfer resistances and dominate. ohmic resistances The resistance dominate. to charge The resistance to transfer arc variescharge significantly transfer arc at varies higher significantly RH, suggesting at higher that RH,the higher suggesting humidity that theimpacts higher humidity the electron impactstransfer, the most electron likely transfer,by the formation most likely of bywater the barriers. formation At of 100% water RH, barriers. the At 100% smallest arcsRH, were the demonstrated smallest arcs by were the demonstratedmonolayer MEAs, by the indicating monolayer that MEAs, the boundary indicating that the interface in boundarythe stratified interface layers inhas the an stratified impact layerson the has charge an impact transfer on pathways. the charge Mean- transfer pathways. while, ohmicMeanwhile, resistance decreased ohmic resistance with increasing decreased RH with for all increasing MEAs. At RH low for RH, all MEAs.the ohmic At low RH, the resistance variedohmic significantly resistance varied between significantly MEAs, with between the difference MEAs, with decreasing the difference with decreasing in- with creasing humidity.increasing This humidity. is due to Thisa reduction is due to in a reductionionic resistance in ionic through resistance the throughmembrane the membrane and the ionomerand thein the ionomer catalyst in layer. the catalyst At lower layer. humidity, At lower the humidity, contribution the contribution of the ionomer of the ionomer to hydration is more evident. to hydration is more evident.

5 MEA #1M 4.5 MEA #2M

2) 4 3.5 MEA #3 /cm Ω 3 MEA #4 2.5 MEA #5 2 1.5 MEA #6 1 Z’’ imaginary ( 0.5 0 0 5 10 15 20 Z’real (Ω/cm2)

(a)

Figure 4. Cont.

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4 MEA #1M

3.5 MEA #2M 2) 3 MEA #3 /cm

Ω 2.5 MEA #4

2 MEA #5

1.5 MEA #6

1 Z’’ imaginary ( 0.5

0 0 5 10 15 Z’real (Ω/cm2)

(b) 6 MEA #1M

5 MEA #2M 2) MEA #3

/cm 4 MEA #4 Ω MEA #5 3 MEA #6 2 Z’’ imaginary ( 1

0 0 5 10 15 Z’real (Ω/cm2)

(c) Figure 4. Nyquist plots of benchmark monolayer MEA (MEA #1M) and reduced Pt loading MEAs Figure 4. Nyquist plots of benchmark monolayer MEA (MEA #1M) and reduced Pt loading MEAs (MEA #2M- #6), under (a) 20%, (b) 60%, and (c) 100% RH. (MEA #2M- #6), under (a) 20%, (b) 60%, and (c) 100% RH.

The NyquistThe plots Nyquist were fitted plots with were a ﬁtted Randles with equivalent a Randles circuit equivalent model circuit [28] and model the [ 28] and the results are shownresults in are Table shown 4. R1 in Tablerepresents4. R1 representsthe ohmic resistance the ohmic and resistance the charge and thetransfer charge transfer resistance isresistance given by R2. is given by R2.

Table 4. The EIS fitting parameters obtained from the Randles equivalent circuit model. Table 4. The EIS fitting parameters obtained from the Randles equivalent circuit model. MEA #1M MEA #2M MEA #3 MEA #4 MEA #5 MEA #6 MEA #1M MEA #2M MEA #3 MEA #4 MEA #5 MEA #6 20% RH 2.019 1.759 2.748 2.854 2.683 1.328 R1 (Ω cm2) 60% RH 1.325 1.600 20% RH 1.655 2.019 1.759 1.560 2.748 1.253 2.854 1.263 2.683 1.328 R1 (Ω cm2) 60% RH 1.325 1.600 1.655 1.560 1.253 1.263 100% RH 1.115 1.560 100% RH 1.438 1.115 1.560 1.334 1.438 1.320 1.334 1.336 1.320 1.336 20% RH 10.56 8.157 20% RH 9.004 10.56 8.157 8.561 9.004 8.090 8.561 7.146 8.090 7.146 2 R2 (Ω cm ) 60% RH 5.873 R2 (Ω cm2 6.278) 60% RH 6.712 5.873 6.278 6.503 6.712 7.274 6.503 7.339 7.274 7.339 100% RH 6.004 6.228 100% RH 7.565 6.004 6.228 7.788 7.565 7.339 7.788 7.177 7.339 7.177 At 20% RH, the ohmic and charge transfer resistances were the highest for all MEAs due to the lower Atwater 20% content RH, the in ohmic the membra and chargene. At transfer lower humidity, resistances the were stratified the highest MEAs for all MEAs due to the lower water content in the membrane. At lower humidity, the stratified MEAs demonstrated lower charge transfer resistances compared to the 0.4 mgPt/cm2 monolayer 2 MEAs (MEAdemonstrated #1M) due to the lower increased charge transferionomer resistancesloading in comparedthe first layer. to the In 0.4the mg stratifiedPt/cm monolayer MEAs, chargeMEAs transfer (MEA and #1M) ohmic due resistances to the increased both decreased ionomer loadingwith the in increasing the first layer. ionomer In the stratified MEAs, charge transfer and ohmic resistances both decreased with the increasing ionomer thickness of the first layer. Increasing the thickness of the first layer increased proton

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thickness of the first layer. Increasing the thickness of the first layer increased proton conductivity and thus reduced ohmic resistance. The lowest charge transfer and ohmic resistances of 7.146 and 1.328 Ω cm2 were demonstrated by MEA #6, respectively, which confirmed the better performance of this MEA in dry conditions. A sublayer containing 28 wt. % I reduced the resistance experienced in dry conditions. At 60% RH, charge transfer resistances decreased with the increasing 24 wt. % I layer thickness in stratified MEAs. This can be attributed to the reduced effect of ionomer swelling and covering of electrochemical conductive areas [29–31]. Monolayer MEAs had the smallest charge transfer resistances in fully humidified conditions. From 20 to 100% RH, charge transfer and ohmic resistances were significantly lower for the benchmark MEA #1M as it had a higher Pt loading, which increased ORR kinetics. MEA #2M exhibited larger ohmic resistance under all RH conditions, likely contributing to its poor performance. The results suggest that a monolayer with 24 wt. % I loading is not optimal for the 2 0.35 mgPt/cm loaded MEAs; a higher ionomer loading may be required.

3.2. Durability Comparison of Stratified and Monolayer MEAs While the stratified CCL MEAs performed better than the monolayer equivalent MEA #2M in certain conditions, the durability of stratified MEAs is yet to be established. An AST which incurs carbon degradation was performed to compare the durability of the stratified MEAs with monolayer MEA #1M and MEA #2M. MEA #6, which was more consistent in reaching performance targets under the studied RH conditions, was selected for comparison. Cyclic voltammetry, SEM, and polarization analyses were performed to determine the MEA durability during the AST, which caused carbon and water oxidation through potential cycling.

3.2.1. Catalyst Layer Degradation Cross-sectional images of the stratified and monolayer CCL MEAs before and after the 30-h AST are shown in the SEM images of Figure5. The MEAs used for SEM imaging and those used in the AST differ due to the nature of the sample preparation. A 10% size variation is expected between MEAs of the same set due to the nature of the production process. The thicknesses are therefore qualitative values and only serve as an estimation of the catalyst layer integrity after the AST. The CCL of the unused MEA #1M is thicker than those of MEA #2M and #6. The MEA #6 CCL was also 19.29% thicker than that of MEA #2M before the degradation test, owing to the high ratio of 28 wt. % ionomer layer on the inner layer of MEA #6. Figure5b,d,f show the thinning of the CL structure of MEA #1M, #2, and #6 due to severe carbon degradation. The MEA thicknesses are summarized in Table5. After the carbon degradation test, thinning of both the anode and cathode CLs was observed for all MEAs. The anode CL thinned by 83.88% for the stratified MEA and 86.55% for the reduced Pt loading monolayer MEA #2M. There was no significant change in the anode thickness of MEA #1M (1.72%) over the degradation period. The CCL thickness of MEA #1M was reduced by 54.32%, that of MEA #2M was reduced by 79.69%, and the stratified CCL of MEA #6 by 59.16%. The Pt/C, ionomer, and membrane, together, comprise the mechanical strength of a CCM. Decreasing the amount of any of these materials decreases the mechanical strength of an MEA. The degradation of the CLs and membrane was greater for the reduced Pt loading MEAs compared to the higher Pt loading MEAs. MEA #2M and MEA #6 experienced membrane dehydration/thinning, while the higher Pt loading MEA #1M experienced membrane swelling. The PEM of MEA #1M expanded during the carbon corrosion test due to isotropic and anisotropic swelling [32]. The water content in the membrane imposed swelling and resulted in high mechanical stresses, which could have resulted in membrane failure and gas crossover [33–35]. The thinning of the PEM membrane observed for lower Pt loading MEAs resulted from thermal and chemical degradation of the membrane during the carbon degradation process. This degradation could result in both gas crossover and electrical shortening [36–38]. The PEM Energies 2021, 14, 2975 11 of 17

thickness decreased by 49.61% for MEA #6 and 71.93% for MEA #2M. Sethruman et al. [37] showed that inadequate water content and high temperature accelerate membrane thinning. Therefore, the high ionomer loading adjacent to the membrane of the stratiﬁed MEA #6 Energies 2021, 14, x FOR PEER REVIEWplayed a role in reducing PEM degradation by retaining moisture closer to the11 of PEM 18 and improving heat dissipation.

Figure 5. Cross-sectional images of an unused (a) CCL MEA #1M; (c) CCL MEA #2M; (e) CCL stratified MEA #6. After the Figure 5. Cross-sectional images of an unused (a) CCL MEA #1M; (c) CCL MEA #2M; (e) CCL stratiﬁed MEA #6. After the carbon corrosion test, SEM images of the degraded (b) MEA #1M, (d) MEA #2M, and (f) MEA #6 were captured. Magnified carbonimages corrosion of the degraded test, SEM (g images) MEA #1M, of the (h degraded) MEA #2M, (b and) MEA (i) MEA #1M, #6 (d show) MEA that #2M, Pt migrated and (f) MEA from #6the were CL into captured. the PEM. Magniﬁed images of the degraded (g) MEA #1M, (h) MEA #2M, and (i) MEA #6 show that Pt migrated from the CL into the PEM. The CCL of the unused MEA #1M is thicker than those of MEA #2M and #6. The MEA Table#6 CCL 5. Thewas thicknessesalso 19.29% ofthicker MEAs than before that and of afterMEA the #2M carbon before degradation the degradation test. test, owing to the high ratio of 28 wt. % ionomer layer on the inner layer of MEA #6. Figure 5b,d,f Thickness Before Thickness After show the thinning of the CLMEA structure Components of MEA #1M, #2, and #6 due to severe carbon degradation. The MEA thicknesses are summarized in DegradationTable 5. (µm) Degradation (µm) CCL 17.49 7.990 Table 5.MEA The thicknesses #1M of MEAs beforePEM and after the carbon degradation 13.96 test. 16.28 ThicknessACL Before Degradation 5.210 Thickness After Degradation 5.120 MEA Components CCL(µm) 12.48(µm) 5.029 MEA #2M CCL PEM17.49 11.85 7.990 3.324 MEA #1M PEM ACL13.96 3.875 16.28 0.5210 ACL CCL5.210 14.53 5.120 2.951 MEA #6 CCL PEM12.48 13.47 5.029 6.785 MEA #2M PEM ACL11.85 3.972 3.324 0.6400 ACL 3.875 0.5210 CCL 14.53 2.951 MEAPt migration #6 intoPEM the PEM was observed13.47 for all MEAs, but 6.785 less for MEA #2M (Figure5h ) and MEAACL #6 (Figure5i) compared3.972 to MEA #1M (Figure 0.64005g). Pt aggregates After the carbon degradation test, thinning of both the anode and cathode CLs was observed for all MEAs. The anode CL thinned by 83.88% for the stratified MEA and

Energies 2021, 14, x FOR PEER REVIEW 12 of 18

86.55% for the reduced Pt loading monolayer MEA #2M. There was no significant change in the anode thickness of MEA #1M (1.72%) over the degradation period. The CCL thickness of MEA #1M was reduced by 54.32%, that of MEA #2M was reduced by 79.69%, and the stratified CCL of MEA #6 by 59.16%. The Pt/C, ionomer, and membrane, together, Energies 2021, 14, 2975 comprise the mechanical strength of a CCM. Decreasing the amount of any of these12 of ma- 17 terials decreases the mechanical strength of an MEA. The degradation of the CLs and membrane was greater for the reduced Pt loading MEAs compared to the higher Pt load- appearing MEAs. as white MEA particles #2M and onthe MEA PEM #6 shown experience in Figured membrane5g,i, which dehydration/thinning, suggests that Pt migrated while fromthe higher the catalyst Pt loading layer. MEA As the #1M carbon experienced support degradation,membrane swelling. Pt particles The dissolve, PEM of MEA migrate, #1M andexpanded precipitate during into the the carbon PEM, resultingcorrosion intest a lossdue ofto electrochemicalisotropic and anisotropic activity in swelling the CL. [32]. Pt inThe the water membrane content is electronicallyin the membrane and imposed ionically swelling isolated and cannotresulted be in accessed high mechanical by gas reactants,stresses, whichwhich leadscould to have electrical resulted performance in membrane degradation failure and [20 ,gas38]. crossover [33–35]. The thinning of the PEM membrane observed for lower Pt loading MEAs resulted from ther- 3.2.2.mal ECSAand chemical Loss degradation of the membrane during the carbon degradation process. ThisThe degradation loss of Pt could was evaluated result in both periodically gas crossover during and the electrical AST to shortening examine the [36–38]. effect The of electrochemicalPEM thickness carbondecreased corrosion by 49.61% on the for ECSA.MEA #6 The and ECSA 71.93% losses for MEA were normalized#2M. Sethruman to its et beginning-of-lifeal. [37] showed that (BoL) inadequate ECSA and water the following content and equation high temperature was used: accelerate membrane thinning. Therefore, the high ionomer loading adjacent to the membrane of the stratified MEA #6 played a role in reducingECSA PEM BoLdegradation− ECSAn −byth cycleretaining moisture closer to the % ECSA loss = × 100, (1) PEM and improving heat dissipation. ECSABoL Pt migration into the PEM was observed for all MEAs, but less for MEA #2M (Figure where ECSA is the ECSA at the beginning of life, and ECSA is the measured 5h) and MEABoL #6 (Figure 5i) compared to MEA #1M (Figure 5g).n-th Pt cycle aggregates appear as ECSA after the n-th cycle. The comparison of ECSA in Figure6 shows a decrease in ECSA white particles on the PEM shown in Figure 5g,i, which suggests that Pt migrated from for all MEAs with progressive cycling. the catalyst layer. As the carbon support degradation, Pt particles dissolve, migrate, and The ECSA decreased over the degradation period due to carbon support corrosion and precipitate into the PEM, resulting in a loss of electrochemical activity in the CL. Pt in the Pt dissolution [39]. There was an accelerated decay in the ECSA for MEA #1M compared membrane is electronically and ionically isolated and cannot be accessed by gas reactants, to MEAs #2M and #6, congruent with the results reported by Speder et al. [40]. This is which leads to electrical performance degradation [20,38]. likely due to the extensive loss of Pt resulting from extensive Pt dissolution, as observed in Figure5g. MEA #1 reached a 100.0% ECSA loss after only 2000 cycles, while MEA #2M had 3.2.2. ECSA Loss a total loss of 80.10% after 6000 cycles and MEA #6 had only a 59.55% total ECSA loss. The ECSAThe of MEA loss #2Mof Pt decreased was evaluated signiﬁcantly periodically faster thatduring of thanthe AST MEA to #6. examine The extensive the effect loss of ofelectrochemical ECSA in MEA #2Mcarbon resulted corrosion from on a drasticthe ECSA PEM. The degradation, ECSA losses asobserved were normalized in Figure 5tod its, whichbeginning-of-life possibly increased (BoL) ECSA hydrogen and crossoverthe following [41]. equation was used:

% = × 100, (1) 3.2.3. Decrease in Electrochemical Performance Polarization curves were drawn with 59% RH on both the anode and cathode, at where ECSABoL is the ECSA at the beginning of life, and ECSAn-th cycle is the measured ECSA periodicafter the intervals n-th cycle. during The comparison the degradation of ECSA period. in Figure Figure 76 shows thea decrease performance in ECSA curves for all ofMEAs MEA with #6 and progressive the monolayer cycling. MEAs over the degradation period.

100 90 80 70 60 50 MEA #1M 40 MEA #2M

% ECSA loss 30 MEA #6 20 10 0 0 1000 2000 3000 4000 5000 6000 Cycle number

Figure 6. ECSA loss during cycling of the stratiﬁed MEA #6 and monolayer MEA #2M and MEA #1.

Energies 2021, 14, x FOR PEER REVIEW 13 of 18

Figure 6. ECSA loss during cycling of the stratified MEA #6 and monolayer MEA #2M and MEA #1.

The ECSA decreased over the degradation period due to carbon support corrosion and Pt dissolution [39]. There was an accelerated decay in the ECSA for MEA #1M compared to MEAs #2M and #6, congruent with the results reported by Speder et al. [40]. This is likely due to the extensive loss of Pt resulting from extensive Pt dissolution, as observed in Figure 5g. MEA #1 reached a 100.0% ECSA loss after only 2000 cycles, while MEA #2M had a total loss of 80.10% after 6000 cycles and MEA #6 had only a 59.55% total ECSA loss. The ECSA of MEA #2M decreased significantly faster that of than MEA #6. The extensive loss of ECSA in MEA #2M resulted from a drastic PEM degradation, as observed in Figure 5d, which possibly increased hydrogen crossover [41].

3.2.3. Decrease in Electrochemical Performance Energies 2021, 14, 2975 Polarization curves were drawn with 59% RH on both the anode and cathode, at pe- 13 of 17 riodic intervals during the degradation period. Figure 7 shows the performance curves of MEA #6 and the monolayer MEAs over the degradation period.

1 Beginning of life 0.9 20 cycles 0.8 180 cycles 0.7 0.6 200 cycles 0.5 600 cycles 0.4 1000 cycles Voltage (V) Voltage 0.3 2000 cycles 0.2 0.1 6000 cycles 0 0 200 400 600 800 1000 Current density (mA/cm2)

(a)

0.9 Beginning of life 0.8 20 cycles

0.7 180 cycles 0.6 200 cycles 0.5 600 cycles 0.4 1000 cycles

Voltage (V) Voltage 0.3 0.2 2000 cycles 0.1 6000 cycles 0 0 200 400 600 800 1000 Current density (mA/cm2) Energies 2021, 14, x FOR PEER REVIEW 14 of 18

(b)

1 Beginning of life 0.9 20 cycles 0.8 0.7 180 cycles 0.6 200 cycles 0.5 600 cycles 0.4

Voltage (V) 1000 cycles 0.3 0.2 2000 cycles 0.1 6000 cycles 0 0 200 400 600 800 1000

Current density (mA/cm2)

(c)

Figure 7. PerformanceFigure 7.curvesPerformance taken during curves potential taken during cycling potential for (a) cyclingMEA #1M, for( a(b)) MEA MEA #1M, #2M, ( b(c)) MEA #2M, (c) and and the stratifiedthe CCL stratiﬁed MEA CCL#6. MEA #6.

The performance curves show a decrease in cell potential after every following potential cycling period. The initial performance of the MEAs is comparable, but their performance degradation is distinctive. The output voltage decreased from the low to the high current density region with progressive cycling due to CL morphology changes caused by carbon degradation, mainly ECSA loss. Carbon degradation reduced the electron conduction, contributing to ohmic losses and charge transfer losses, while PEM degradation led to proton conductivity losses, all limiting performance. CL surface roughening due to carbon corrosion also contributes to the CL hydrophilicity and adds oxide groups resulting in oxygen diffusion and water management challenges, thus drastically reducing performance [42,43]. The OCV of MEA #2M drastically decreased after 6000 cycles from 0.8300 to 0.101 V, whereas that of MEA #6 OCV decreased from 0.9100 to 0.6800 V and that of MEA #1M from 0.8900 to 0.5850 V. These OCV losses can be correlated to increased hydrogen crossovers associated with membrane swelling/thinning [41,44] and Pt surface area losses during potential cycling. To compare the performance losses obtained in each MEA, the voltage losses were normalized to the BoL cell voltage and the following equation was used [45]:

% = × 100, (2)

where VBoL is the cell voltage at BoL, and Vn-th cycle is the voltage after the n-th cycle. Figure 8 presents the percentage performance loss of each MEA versus the cycling period measured at 200 mA/cm2.

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The performance curves show a decrease in cell potential after every following potential cycling period. The initial performance of the MEAs is comparable, but their performance degradation is distinctive. The output voltage decreased from the low to the high current density region with progressive cycling due to CL morphology changes caused by carbon degradation, mainly ECSA loss. Carbon degradation reduced the electron conduction, contributing to ohmic losses and charge transfer losses, while PEM degradation led to proton conductivity losses, all limiting performance. CL surface roughening due to carbon corrosion also contributes to the CL hydrophilicity and adds oxide groups resulting in oxygen diffusion and water management challenges, thus drastically reducing performance [42,43]. The OCV of MEA #2M drastically decreased after 6000 cycles from 0.8300 to 0.101 V, whereas that of MEA #6 OCV decreased from 0.9100 to 0.6800 V and that of MEA #1M from 0.8900 to 0.5850 V. These OCV losses can be correlated to increased hydrogen crossovers associated with membrane swelling/thinning [41,44] and Pt surface area losses during potential cycling. To compare the performance losses obtained in each MEA, the voltage losses were normalized to the BoL cell voltage and the following equation was used [45]:

VBoL − Vn−th cycle % Performance loss = × 100, (2) VBoL

where VBoL is the cell voltage at BoL, and Vn-th cycle is the voltage after the n-th cycle. Figure8 Energies 2021, 14, x FOR PEER REVIEW 15 of 18 presents the percentage performance loss of each MEA versus the cycling period measured at 200 mA/cm2.

100 MEA #1M 90 80 MEA #2M 70 60 MEA #6 50 40 30 20 % Performance loss % Performance 10 0 0 1000 2000 3000 4000 5000 6000 Number of cycles

Figure 8. Percentage performance losses of the MEAs during potential cycling, measured at 200 Figure 8. Percentage performance losses of the MEAs during potential cycling, measured at mA/cm2. 200 mA/cm2.

PerformancePerformance loss loss began began to to increase increase sharply sharply after after 1000 1000 cycles cycles for for all all MEAs. MEAs. The The higher higher 2 PtPt loading loading MEA MEA #1M #1M experienced experienced lower lower performance performance losses losses at at 200 200 mA/cm mA/cm2 compared compared to to thethe reduced reduced Pt Pt loading loading MEAs. MEAs. At At 2000 2000 cycles, cycles MEA, MEA #2M #2M reached reached 100.0% 100.0% performance performance loss, loss, whilewhile MEA MEA #6 #6 demonstrated demonstrated 48.35% 48.35% loss loss and and MEA MEA #1M, #1M, 35.25% 35.25% loss. loss. PerformancePerformance degradation degradation was was substantial substantial for for both both the the monolayer monolayer and and stratified stratified CCL CCL MEAs,MEAs, but but the the degradation degradation of of the the CL CL after after prolonged prolonged corrosion corrosion was was slower slower in in the the MEAs MEAs withwith a a stratified stratified CCL CCL with with equivalent equivalent loading. loading. This This was was corroborated corroborated by by the the higher higher ECSA, ECSA, lowerlower kinetic, kinetic, and and OCV OCV losses losses after after carbon carbon degradation. degradation. The The high high ionomer ionomer coverage coverage in in the the firstfirst layer layer of of MEA MEA #6 #6 may may have have reduced reduced carbon carbon corrosion corrosion by by retaining retaining moisture moisture closer closer to to thethe membrane, membrane, preventing preventing severe severe membrane membrane dryness, dryness, which which causes causes PEM PEM degradation. degradation. The bilayerThe bilayer interface interface also played also played a role a inrole enhancing in enhancing the durability the durability of the of MEAthe MEA by disrupting by disrupting the rate of mass transport during the carbon degradation test, thus reducing the carbon corrosion rate.

4. Conclusions Using an ionomer gradient approach, stratified CCL MEAs were designed for low- temperature PEMFCs. The goal of the project was to reduce Pt loading in the stratified layers without sacrificing performance and lifetime. This study has shown that using ionomer stratification to decrease the Pt loading in an MEA can yield better performance compared to the monolayer MEA equivalent, depending on the RH. Compared to the benchmark MEA at 0.4 mgPt/cm2, the stratified MEA showed a ±2.00% performance change from 20 to 60% RH and about a 13.00% performance increase in fully humidified conditions. This is in stark contrast to the 13.00 to 47.72% decrease in performance observed in the reduced Pt monolayer MEA compared to the commercial benchmark. Not only did the stratified MEA perform better than the monolayer equivalent MEA, but it also proved to be more durable. The AST resulted in a decrease in the performance for each of the MEAs, which was correlated to a decrease in CCL thickness, membrane degradation, and ECSA loss. The stratified MEAs proved to be more durable than monolayer MEAs at equivalent Pt loadings. The high ionomer loading adjacent to the membrane of the stratified MEAs increases moisture in the CL, hydrating the PEM, and, therefore, slows the degradation process. Compared to the benchmark high Pt loading MEA, the stratified MEA showed increased durability by decreasing the ECSA loss by 41.83% and the OCV losses by 26.25%. How- ever, a slight increase of 13.10% in kinetic losses was observed. The AST results showed that catalyst loading plays a major role in the durability of an MEA. Therefore, it is

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the rate of mass transport during the carbon degradation test, thus reducing the carbon corrosion rate.

4. Conclusions Using an ionomer gradient approach, stratified CCL MEAs were designed for low- temperature PEMFCs. The goal of the project was to reduce Pt loading in the stratified layers without sacrificing performance and lifetime. This study has shown that using ionomer stratification to decrease the Pt loading in an MEA can yield better performance compared to the monolayer MEA equivalent, depending on the RH. Compared to the 2 benchmark MEA at 0.4 mgPt/cm , the stratified MEA showed a ±2.00% performance change from 20 to 60% RH and about a 13.00% performance increase in fully humidified conditions. This is in stark contrast to the 13.00 to 47.72% decrease in performance observed in the reduced Pt monolayer MEA compared to the commercial benchmark. Not only did the stratified MEA perform better than the monolayer equivalent MEA, but it also proved to be more durable. The AST resulted in a decrease in the performance for each of the MEAs, which was correlated to a decrease in CCL thickness, membrane degradation, and ECSA loss. The stratified MEAs proved to be more durable than monolayer MEAs at equivalent Pt loadings. The high ionomer loading adjacent to the membrane of the stratified MEAs increases moisture in the CL, hydrating the PEM, and, therefore, slows the degradation process. Compared to the benchmark high Pt loading MEA, the stratified MEA showed increased durability by decreasing the ECSA loss by 41.83% and the OCV losses by 26.25%. However, a slight increase of 13.10% in kinetic losses was observed. The AST results showed that catalyst loading plays a major role in the durability of an MEA. Therefore, it is imperative to always consider the impact of reducing Pt loading on MEA durability. These findings are anticipated to contribute to the development of more durable MEAs for low-temperature PEMFCs. It is suggested that increasing the ionomer gap in stratified CCL MEAs and optimizing the ionomer content for low PGM MEAs could yield even better MEA performance and further increase MEA durability.

Author Contributions: Conceptualization, J.C. and Z.N.; methodology, Z.N. and J.C.; validation, Z.N. and J.C.; formal analysis, Z.N. and J.C.; investigation, Z.N. and J.C.; resources, J.C. and M.C.; data curation, Z.N. and J.C.; writing—original draft preparation, Z.N. and J.C.; writing—review and editing, Z.N., J.C. and M.C.; visualization, J.C. and Z.N.; supervision, J.C. and M.C.; project administration, J.C., Z.N. and M.C.; funding acquisition, J.C., Z.N. and M.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Research Foundation (NRF) Free Standing Inno- vation grant and HySA Catalysis funded by the South African Department of Science and Innovation. Data Availability Statement: The data presented in this study are available on request from the corresponding author, J.C, upon request. Acknowledgments: Special thanks to HyPlat (Pty) Ltd. for providing all materials used for the experiments in this study. Conﬂicts of Interest: The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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