Modeling the Effect of Low Pt Loading Cathode Catalyst Layer in Polymer Electrolyte Fuel Cells. Part II: Parametric Analysis

A model for the cathode catalyst layer (CL) is presented, which is validated with previous experimental data in terms of both performance and oxygen transport resistance. The model includes a 1D macroscopic description of proton, electron and oxygen transport across the CL thickness, which is locally coupled to a 1D microscopic model that describes oxygen transport toward Pt sites. Oxygen transport from the channel to the CL and ionic transport across the membrane are incorporated through integral boundary conditions. The model is complemented with data of effective transport and electrochemical properties extracted from multiple experimental works. The results show that the contribution of the thin ionomer film and Pt/ionomer interface increases with the inverse of the roughness factor. Whereas the contribution of the water film and the water/ionomer interface increases with the ratio between the geometric area and the surface area of active ionomer. Moreover, it is found that CLs diluted with bare carbon provide lower performance than non-diluted samples due to their lower electrochemical surface area and larger local oxygen transport resistance. Optimized design of non-diluted samples with a good distribution of the overall oxygen flux among Pt sites is critical to reduce mass transport losses at low Pt loading.

The pressing need for highly-performing and environmentally benign energy technologies continues to spur on research on polymer electrolyte fuel cells (PEFC). Among the components that are needed for a well-functioning, durable, and affordable PEFC, the cathode catalyst layer (CCL) continues to stand out. The main function to be provided by the CCL is to facilitate the oxygen reduction reaction (ORR). However, this function entails an intricate interplay of microscopic kinetics with the transport of electrons, protons, oxygen molecules, and water. A hierarchy of structural effects must be considered, as illustrated in Figure 1. Due to the complex composition and multiscale nature of the CCL, physical modeling has gained high importance in efforts to rationalize the dynamic interplay of structure, properties, and performance. The first part of the presentation will briefly review the capabilities of model-based analyses of experimental data to deconvolute and quantify voltage loss contributions,1 extract kinetic and transport parameters from fitting and discover systematic trends and correlations among these parameters,2 provide an activity map of the layer, and evaluate the overall effectiveness factor of Pt utilization.3 Macrohomogeneous modelling can propose or predict the optimal CCL thickness or macroscopic effective composition (for the target range of operating conditions).Recognizing the importance of an optimal water-distribution for a well-functioning CCL, recent efforts in CCL modeling have been focusing on the consistent treatment of aspects like pore size distributions and pore network morphologies as well as wettability properties. The ionomer inclusions in the CCL play a crucial role in this context. At the microscopic scale, the ionomer film that forms an interface with the water-covered catalyst-support surface strongly impacts the local reaction environment that determines the rate of the ORR as well as that of platinum dissolution. Moreover, the structure and distribution of ionomer inclusions determine the wetting behaviour of pores and thus the water sorption properties of the porous composite CCL, which in turn affect the transport properties for oxygen and water. Recent forays in modelling that strive to unravel the intertwined impacts of ionomer and water will be presented. As a final consideration, a CCL cannot be understood and optimized as a stand-alone component. Overarching balances at the PEFC level in terms of reactant, charge, water, and heat fluxes must be considered. Comprehensive modeling approaches must account for the coupling of the corresponding local equilibria and transport phenomena across the whole cell, including polymer electrolyte membrane, diffusion media and flow fields. Efforts focusing on the coupled water fluxes across the cell are underway with promising results to look out for. References. 1 M. Baghalha, J. Stumper and M. Eikerling, ECS Transactions 28, 159–167 (2010). 2 T. Muzaffar, T. Kadyk and M. Eikerling, Sustainable Energy & Fuels 2, 1189–1196 (2018). 3 M. Eikerling and A.A. Kulikovsky, Polymer Electrolyte Fuel Cells – Physical Principles of Materials and Operation, CRC Press Taylor & Francis Group, 2014. Figure 1

Modelling the reactive transport processes in different reconstructed agglomerates of a PEFC catalyst layer

In polymer electrolyte fuel cells (PEFCs), the cathode catalyst layer (CL) needs a large amount of Pt because of the slow oxygen reduction reaction. Since electron, proton, and oxygen are necessary for the cathode reaction in PEFCs, achieving the optimum structure of cathode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. Some researches reported that when Aquivion is used as the ionomer of the CL, the output voltage increases as compared with the CL using Nafion in the intermediate current density region (1). However, dominant factors improving the cell performance have not been elucidated quantitatively. In this study, the oxygen transport resistance at the Pt surface, which is thought to have a large contribution to the oxygen transport resistance inside the CL (2), was investigated using CLs fabricated with different ionomers. In this study, catalyst coated membranes was fabricated with various cathode structure: 6 types of cathode side CLs using three kinds of Pt-supported carbons (TEC10E30E, TEC10E50E, TEC10E70TPM) having different Pt carrying densities and two types of ionomer, Nafion and Aquivion. All of these CLs were fabricated with I/C = 0.8. First, to distinguish the oxygen transport resistance in the CL from the other resistance such channels, I-V measurement experiments were conducted with different total pressures (0.10MPa, 0.15MPa, 0.20MPa), and the limiting current density method (2) was applied. In the I-V measurement, the flow rate of cathode gas (oxygen and nitrogen mixture) was 2000 sccm with different oxygen concentration (1%, 2%, 3%) and the flow rate of anode hydrogen was 100 sccm with RH 80% (80°C). Next, the oxygen transport resistance at the Pt surface was evaluated. (The details will be described later). In this evaluation, the electrochemical surface area (ECSA [m2/gPt]) in the CL is needed. The ECSA was estimated by CV measurement. In the CV measurement, cathode was filled with N2 gas with RH 100% (80°C). The flow rate of anode hydrogen was 100 sccm with RH100% (80°C). Figure 1 shows I-V characteristics of the CL with Pt loadings of approximately 0.20 mg/cm2. The output current density for the Aquivion based CL is higher than that of the Nafion based CL at a voltages around 0.5 V, whereas the Nafion based CL has superior performance in the operation at 0.1 V where oxygen supply is limited by various transport resistances. In this paper, we focus on the oxygen transport resistance at low voltage, in particular the resistance at the Pt surface, because the Pt surface resistance is considered to be dominant part of oxygen transport resistance under the low Pt loading CL. Figure 2 shows the ECSA estimated from the CV measurement. The values of ECSA are similar with Nafion and Aquivion based CLs. As the Pt carrying density increases, the ECSA decreases. This is because the Pt particles aggregate on the carbon and the surface area becomes smaller. Using the simplified evaluation formula of oxygen transport resistance (2), the total oxygen transport resistance in the CL can be expressed as RCL = RPt + Rdiss + Rpore . The RPt means the resistance at the Pt surface, Rdiss means the resistance of oxygen dissolves into ionomer, and Rpore means the oxygen diffusion resistance in the CL pores. The RPt is formulated as RPt = 1 / (APt × γ × kPt ) (2). Here, APt is the reactive surface area of catalyst Pt per unit area of the CL, γ is ratio of oxygen concentration in the ionomer and CL pore, and kPt [m/s] is the oxygen transfer rate constant at the Pt surface. Figure 3 shows the result evaluating RPt with both ionomer based CLs. The ordinate represents RCL and the abscissa represents 1/APt . Using this result, kPt can be calculated as kPt = 1.5×10 -2 s/m with the Nafion based CL and kPt = 9.1×10 -3 s/m with the Aquivion based CL. As the oxygen transport resistance at the Pt surface is inversely proportional to the kPt , the resistance of Aquivion based CL is estimated to be larger. This indicates a possibility that Aquivion based CL is inferior due to its larger Pt surface resistance. The larger Pt surface resistance may be due to the characteristics of Aquivion: the shorter side chain and the hydrophilic property induce strongly condensed structure of Aquivion at the surface of Pt, and that results in inhibiting the transport of oxygen. Reference (1)C. Park, et al., J. Power Sources, 315,179 (2016). (2)Tabe, et al., ECS Trans., 80(8), 205 (2017). Figure 1

Self-startup of polymer electrolyte fuel cell (PEFC) without external heating is advantageous for the system. It is generally recognized that the produced ice accumulating in the cathode catalyst layer (CL) reduce the electrochemically active area (ECA) for oxygen reduction reaction (ORR) and the open pore volume for oxygen diffusion. Visualization results obtained from a transparent PEFC shows the importance of the catalyst layer design and gas purge to freeze-starts.1,2 A cyclic voltammetry (CV) technique was developed to investigate the effect of ice formation in the cathode CL on electrochemically active Pt area during and post subzero startup of a PEFC.3Mechanical stress built in the cell during cold start may limit its lifetime. To our knowledge, no work has been done about mechanical stress during cold start in PEFC.A cyclic voltammetry (CV) technique is developed to further investigate the effect of membrane thickness and startup current density on cumulative product water, rate of ice formation in electrode and rate of water absorption by ionomer and membrane in PEFC during cold start. Figure 1a shows typical CVs of the cathode prior to and post cold start. Obviously, the electrochemically active Pt surface area of the cathode CL post cold start is lower than that prior to. It is found that thick membrane offers much more capacity for water to be absorbed by the membrane at −10oC and −20oC. The product water almost does not increase with membrane thickness for starting from −30oC because water absorption by the membrane is negligible. The rate of ice formation in cathode catalyst layer increases with current density but decreases with the startup temperature. The rate of water absorption by the membrane increases with the startup temperature, current density and membrane thickness. A mathematical model is developed to investigate mechanical stress during cold start in PEFC. Ice produced in the catalyst layer forces the catalyst layer to expand. Figure 1b shows the effect of cold-start temperature on stress during cold start. Cold starting from low temperature brings high compress mechanical stress. Cold starting from low temperature also bring high strain in CL, but low strain in the membrane. The membrane is compressed during cold start from -30oC. That is because the amount of water absorbed by membrane is negligible but CL expands a lot. This work demonstrates the effect of thickness of gas diffusion layer (GDL), micro-porous layer (MPL) and CL on the mechanical stress built in the membrane. Membrane constraint is considered in the model.4The simulation result shows that mechanical stressed during purge, cool down and cost start operation do not vary serious due to elastic and porous MPL and GDL. The model is to help elucidate the fundamental physics of ice formation in the PEFC during cold start.Figure 1. (a) Cyclic voltammograms of the cathode prior to and post cold start at startup temperature of −10oC and −30oC. (CCM-25 mm. m Pt=0.4 mg/cm2. Startup conditions: i start=0.05 A/cm2, x H2=14.35, x air=12.06, RHH2=RHair=0%, p H2=p air=1 atm. CV conditions: T CV=T start, n=50 mV/s, F N2=F H2=40 mL/min, RHN2=RHH2=0%.) and (b) Effect of cold-start temperature on stress during cold start (lInitial= 6.0).

Polymer electrolyte fuel cells (PEFCs) are expected as a next-generation power sources due to their low emissions and high efficiencies. However, it is required to improve the power density at lower Pt loading. It is known that the losses of the power density are caused by activation losses, ohmic losses and mass transport losses, and that the mass transport losses are dominant at higher current density. Moreover, it is suggested that one of the causes of the mass transport losses is the shortage of oxygen in the cathode side of PEFC. In the cathodic catalyst layer, there are Pt catalysts on supported carbon microparticles, and those particles are covered with ionomer films which are composed of polymer electrolytes and water molecules. The ionomer has two properties for the water-generating reactions in the cathodic catalyst layer: the proton conductivity and the oxygen permeability. In particular, the dependence of the oxygen permeability on water content has not been clear. In this study, we analyzed water content dependence of oxygen permeation properties in ionomer on Pt surface using molecular dynamics simulations. Regarding the calculation system, Nafion was adopted to the polymer electrolyte in the ionomer, and water content, λ, was defined as the ratio of the number of sulfo groups in Nafion to the number of water molecules. We set λ = 3, 7, 11 and constructed a system of an equilibrium state of ionomer on Pt surface at each water content. Then oxygen molecules were inserted above the ionomer and permeate the ionomer along the thickness direction at steady state. Firstly, the density distributions were obtained to evaluate the structural properties of the ionomer. As a result, the region of the ionomer is divided into three region: ionomer/gas interface, bulk region, ionomer/Pt interface. Next, the number of permeated oxygen molecules through the ionomer was counted, and the oxygen permeability of the ionomer was estimated at different water content. As a result, the oxygen permeability decreases with increasing water content. Gas permeability is estimated using the permeation coefficient which is obtained by the product of the diffusion coefficient and solubility coefficient of gas molecules, indicating that the diffusivity and solubility are governing factors of the permeability. Therefore, the oxygen diffusivity and solubility was evaluated to discuss the oxygen permeation mechanism in the ionomer. The oxygen diffusion coefficient was obtained by the density distribution of the oxygen molecules along the thickness direction using the Fick’s law. As a result, the oxygen diffusivity in each region of the ionomer decreases at lower water content while slightly increases at higher water content, with the increase in water content. On the other hand, the oxygen solubility coefficient was calculated using the test-particle-insertion method, and the solubility in each region of the ionomer decreases with increasing water content. Finally, the oxygen permeability was evaluated using the diffusivity and solubility in each region. Consequently, the oxygen permeability decreases with increasing water content in each region, and the permeability in the ionomer/Pt interface is the smallest, which indicates that the oxygen dissolution in the ionomer/Pt interface is dominant in the oxygen permeation through the ionomer.

A parametric study of cathode catalyst layer structural parameters on the performance of a PEM fuel cell

Polymer electrolyte fuel cells (PEFCs) are suitable for fuel cell vehicles (FCVs) because the PEFCs have high efficiency, high responsibility and high performance, but the critical issues of PEFCs are the cost and the durability of the fuel cell stack. The development of PEFCs requires increased operating temperature, over 100 °C, for high efficiency FCV systems. In addition, the PEFCs of fuel cell buses and trucks need to have higher durability of the fuel cell stack than that of passenger cars. With increasing operating temperature under start-up/shut-down condition, carbon black supports for platinum undergo accelerated corrosion, while niobium-doped tin oxide supports undergo negligible corrosion. We invented a highly durable SnO2 nanoparticle support with a “fused aggregate network structure” as a new candidate to replace carbon [1-4] and found a novel platinum anti-dissolution mechanism of Pt/Nb-SnO2 cathode catalyst layers (CLs) during load cycling under an oxygen atmosphere, which was not found in the case of Pt/GCB (graphitized carbon black) CLs. The load cycle durability of Pt/Nb-SnO2 CLs increased with increasing cathode oxygen concentration, while that of Pt/GCB CLs decreased. When the Pt nanoparticles (NPs) on the Nb-SnO2 support are oxidized by oxygen in the cathode at the open circuit voltage (OCV), the availability of free electrons in the Pt NPs decrease, and thus the supply of free electrons to the depletion layer on the surface of Nb-SnO2 becomes difficult. This effect would be particularly evident for the high cathode oxygen concentration [5]. The Pt loading amount for both the Pt/Nb-SnO2 cathode catalyst and the Pt/CB anode catalyst was 0.1 mg cm-2. The cell performances were evaluated under H2/O2 at 80 °C and 100% RH before and after the durability evaluations. Fig. 1 shows the protocols for the durability evaluation as a function of OCV holding time (2 s, 60 s) using galvanostatic operation under H2/air conditions, and upper cell potential holding time (2 s, 60 s) using potentiostatic operation under H2/N2 conditions. The protocols of Figs. 1(c) and (d) simulate the durability evaluations of Figs. 1(a) and (b). Fig. 2 shows the normalized cell performances at 1.0 A cm-2 after the durability evaluations under H2/O2 conditions at 80 °C and 100% RH. The cell performances were nearly the same before and after the durability evaluations with OCV (upper cell potential) holding time of 2 s during H2/N2 and H2/air operation. In the case of the durability evaluations with OCV (upper cell potential) holding time of 60 s, the cell performance deterioration after the durability evaluations during H2/air operation was suppressed as compared with that under H2/N2 operation. Fig. 3 shows the decrease rate of ohmic resistance during the durability evaluations at lower cell voltage as a function of OCV (upper cell potential) holding time. The decrease rate was the highest of all during the durability evaluations with OCV holding time of 60 s during H2/air operation. We reported that the degradation of Pt NPs on the Nb-SnO2 support is suppressed by means of the formation of a depletion layer existing on the surface of the Nb-SnO2 support with increasing cathode oxygen concentration at the OCV [5]. The OCV holding time in the presence of the oxygen would influence the growth of the depletion layer on the surface of the Nb-SnO2 support during load cycling. We will discuss in detail the effects of OCV/load holding time, relative humidity, and operating temperature on the load cycle durability of Pt/Nb-SnO2 cathode catalyst layers. Acknowledgement This work was partially supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)” Project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).K. Kakinuma, Y. Chino, Y. Senoo, M. Uchida, T. Kamino, H. Uchida, S. Deki, and M. Watanabe, Electrochim. Acta, 110, 316 (2013).Y. Chino, K. Kakinuma, D. A. Tryk, M. Watanabe, and M. Uchida, J. Electrochem. Soc., 163(2), F97 (2016).K. Kakinuma, R. Kobayashi, A. Iiyama, and M. Uchida, J. Electrochem. Soc., 165(15) J3083 (2018).C. Takei, R. Kobayashi, Y. Mizushita, Y. Hiramitsu, K. Kakinuma, and M. Uchida, J. Electrochem. Soc., 165(16), F1300 (2018). Figure 1

We have been researching manufacturing methods of catalyst layers in order to improve the effective utilization rate of Pt in polymer electrolyte fuel cells. In our previous research1-3, we demonstrated experimentally that an electrospray (ES) method was able to make great progress in the interface formation between the electrolyte polymer ionomer and the Pt supported on carbon1,3 or conductive ceramics2, and that the immediate drying of the catalyst layer ink also greatly contributes to the improvement of Pt effectiveness. Since the ink droplets had already dried when these were deposited on the membrane, we proposed that the ink drying process can be reduced and that the cost of producing the catalyst layer is also greatly reduced. However, since the amount of catalyst applied is very small with a single nozzle, we also proposed that a multi-nozzle system is indispensable in order to adapt to the actual catalyst layer formation process.Therefore, we are jointly developing the ES manufacturing system with a multi-nozzle device (Fig. 1a) in a collaboration between the University of Yamanashi and Meiko Co., Ltd. We developed the first prototype machine, MES-Lab.α, for R & D, which features options such as nozzle material, multi-nozzle structure, coating treatment of nozzle tip, nozzle fixing adhesive, suck-back method, and ink energization method, among others. We have completed a new design for a basic 72-nozzle block (Fig. 1b), which incorporates all of the above new materials and new structures and is compatible with the production of industry-standard electrodes (5 cm x 5 cm).The performance of the PEFC cell using the cathode catalyst layer produced by this ES device was superior to that of a cell using the conventional pulse-swirl-spray device (PSS). We prepared the catalyst layer according to two different ionization methods (positive (ESP), and negative (ESN) ionization), and compared these with that prepared by the PSS method.3 The catalyst layers prepared by ESN successfully mitigated oxygen transport resistance. The highest porosity, which induced better water management, revealed superior performance at high current. Moreover, we confirmed the importance of the proton conduction in the catalyst layer through the ionomer contents. Thinly coated ionomer film on the catalyst surface revealed that lower resistance originates from the ionomer film on the Pt surface, RCL, film , but poor σ H+cath. , as evaluated for low contents of ionomer in the catalyst layer. These results also suggest that we will be able to achieve superior actual cell performance for catalyst layers prepared via ES using modified ionomer chemical structures, for which there is enhanced H+ conductivity, σ H+cath. , via, e.g., increased ion exchange capacity, and oxygen permeability3 (Fig. 2). Acknowledgement This work was partially supported by funds for the “Superlative, stable, and scalable performance fuel cell” (SPer-FC) and the “Electrolytes, catalysts and catalyst layers with extraordinary efficiency, power and durability for PEFCs to 2030” (ECCEED’30-FC) projects from the New Energy and Industrial Technology Development Organization (NEDO) and the “Yamanashi Frontier for Innovation and Ecosystem” (FCyFINE) project from the Regional Innovation Ecosystems Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). References Takahashi, K. Kakinuma, and M. Uchida, J. Electrochem. Soc., 163, F1182-F1188 (2016).Takahashi, R. Koda, K. Kakinuma, and M. Uchida, J. Electrochem. Soc., 164, F235-F242 (2017).Cho, K. Tamoto, and M. Uchida, Energy Fuels, 34, 14853−14863 (2020). Figure 1

Several emerging and growing applications like biosensors, gas separation membranes, protective coatings, photonics, nanocomposites and microelectronics depend on polymers of thicknesses under 100nm[1]. Ion conducting polymers (ionomers) of thickness <100nm have induced significant advances in the performance of polymer-electrolyte fuel cells (PEFCs) by serving as binder in the catalyst layers (CLs), aiding proton conduction and extending the reaction zone of the CL porous electrode.[2] Thin and ultra-thin polymer films provide ease of processing, application specific tunable properties and reduction in material cost. However, due to deviation of nano-confined polymers from bulk behavior, they pose a challenge of predictability. This is evident in PEFCs where thin-film ionomers are large contributors to mass-transport losses at low catalyst loadings required for commercialization. In-situ investigations over the past 10 years have provided a significant amount of insight into these losses in CLs.[3] As a result, it would be misleading to employ the same principles of understanding used for bulk polymers for nano-confined polymers without modification. In our study, we aim to understand nano-confined ionomers in two ways. First, looking at impact of interfaces by using ionomers supported on different substrates and tuning substrate-ionomer interaction via reducing and oxidizing environment. Second, by examining the impact of cooperative motion and chain dynamics using thermal expansion study of nano-confined ionomers. Combining these two studies we can gain new understanding of effect of confinement on performance. Understanding ionomer-catalyst surface interface and interaction can elucidate not only how ionomers impact catalyst performance but also how catalyst surface impacts ionomer performance. In our study we use water uptake and modulus to probe the effect of platinum surface on thin-film ionomers under oxidizing(Air) and reducing(H2) conditions. Several in-situ and ex-situ methods have demonstrated that oxide formation on the active platinum catalyst surface can reduce the catalytic activity, further hampering activity in the cathode catalyst layer[4]. Despite this, our studies show continued growth of oxides on platinum surface results in higher density of water maintained in the ionomer. This indicates that nano-confined ionomers have greater water stability in the cathode than on the anode catalyst layer, enhancing proton transport in the cathode catalyst layer. Using transition temperature as a proxy for material behavior, we also looked at relaxation behavior of nano-confined ionomers under slow heating conditions. For glass-forming polymers like perfluorinated sulfonic-acid (PFSA) ionomers (such as Nafion and 3M-ionomer) cooperative motion and dynamics of chains have been used to explain changes in modulus, thermal expansion, relaxation behavior, and gas transport. Thermal transition temperatures are often used as a broad physical description for these kinetically and thermodynamically-driven changes in polymer behavior[1]. In this study, we used heated cell ellipsometry to explore the presence of transition temperatures and their dependence on thickness and substrate in thin-film ionomers. We are able to see both α and β transition temperatures using ion exchanged nano-confined ionomers supported on silicon substrates. Our results demonstrate increased rate of expansion for thin films as compared to bulk and dramatic change of transition temperatures around 50-70nm indicating impact of ionomer-surface interaction and influence of electrostatic interaction on chain relaxation and gas transport. Acknowledgement We would like to thank Douglas Kushner for providing assistance and guidance in this study. This work made use of facilities at the Joint Center for Artificial Photosynthesis at Lawrence Berkeley National Laboratory. This work was funded in part by the University of California Chancellor's Graduate Fellowship, National Science Foundation Graduate Fellowship and the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U. S. Department of Energy under contract number DE-AC02-05CH11231.

The effect of water management on oxygen transport plays a critical role in the design of high-performance polymer electrolyte fuel cells (PEFCs) for the automotive industry, especially at low cathode platinum loading [1,2]. In this regard, optimizing the hierarchical pore structure of the membrane electrode assembly (MEA) is crucial to alleviate cathode flooding while ensuring good membrane hydration [3]. However, this task is complicated due to the small dimensions involved in the problem. Mathematical modeling has turned out to be a key tool to understand transport in the thin layered assembly used in PEFCs, including the catalyst layer (CL), microporous layer (MPL) and gas diffusion layer (GDL).In this work, oxygen, heat and two-phase water transport in the MEA are examined by means of a 1-dimension continuum bundle of capillary tubes model. Experimental pore size distributions (PSDs) are considered for the GDL/MPL/CL assembly. The model takes especial care in analyzing thermal and water behavior in catalyst layer by considering micro (< 20 nm of pore radius), meso (between 20 and 300 nm) and macroscale (> 300 nm) structures as different interconnected domains, each with their own effective thermal conductivity and water saturation. In this aspect, taking into account pore hydrophilicity or hydrophobicity is determinant as water behavior is extremely different.The model predictions are validated against previous experimental data for various operating conditions and pore structures (Figure 1a). The results offer precise information on the coupling between heat and water transport in both liquid and gas phases, and the impact of operating conditions and transport properties on the saturation distribution through the MEA layers (Figure 1b). Then the continuum bundle of capillary tubes model is also used to run a parametric analysis centered on the obtention of optimal PSD-dependent effective transport properties to produce improved MEA microstructures.[1] A. Sánchez-Ramos, J.T. Gostick, P.A. García-Salaberri, Modeling the Effect of Low Pt loading Cathode Catalyst Layer in Polymer Electrolyte Fuel Cells: Part I. Model Formulation and Validation, Journal of the Electrochemical Society 168 (2021) 124514.[2] A. Sánchez-Ramos, J.T. Gostick, P.A. García-Salaberri, Modeling the Effect of Low Pt loading Cathode Catalyst Layer in Polymer Electrolyte Fuel Cells: Part II. Parametric Analysis, Journal of the Electrochemical Society 169 (2022) 074503.[3] M. Gomaa, A. Sánchez-Ramos, N. Ureña, M.T. Pérez-Prior, B. Levenfeld, P.A. García-Salaberri, M. Rabeh, Characterization and modelling of free volume and ionic conduction in copolymer proton exchange membranes, Polymers 14(9) (2022) 1688. Figure 1

This paper presents a comprehensive evaluation of metal foam flow field application within polymer electrolyte fuel cells (PEFCs) and compares it with conventional serpentine channels from both computational fluid dynamics simulation and experimental viewpoints. The experiments are designed to investigate the effects of material, area density, compression ratio, and final thickness of metal foam. Additionally, the influence of housing plate material and relative humidity (RH) is tested here. The results reveal that at RH = 75%–100%, the best flow field design is nickel foam with a compression ratio of 70%, a final thickness of 0.5 mm, and an SS-304 housing plate, which delivers a great limiting current density. In comparison with the serpentine channel case, the PEFC with this foam flow field shows a 10% improvement in maximum power density (901 vs 989 mW cm−2) and a 45% improvement in limiting current density (2140 vs 3110 mA cm−2). While at RH = 30%, the same foam flow field with a final thickness of 1 mm is a superior option. The experiments also indicate that maximum power density increases by 23% (from 684 to 841 mW cm−2) as the compression ratio rises from 0% to 70% while reducing final thickness from 1 to 0.5 mm causes a 5.8% enhancement in (from 935 to 989 mW cm−2) cell performance. Simulation results reveal that metal foam is more effective in evenly distributing reactants, resulting in an average oxygen mass fraction at the cathode catalyst layer that is 38% higher than the serpentine channel case.

Proper water management in cathode porous electrode of polymer electrolyte fuel cells (PEFCs) is essential for achieving better performance. Especially at high current densities, excessive water produced by oxygen reduction reaction (ORR) on cathode side is rapidly condensed in catalyst layer (CL) and gas diffusion layer (GDL). When the open pores in cathode CL and the interface between CL and GDL are filled with liquid water, oxygen cannot be supplied to the reaction sites. This phenomenon, called “water flooding”, is a critical barrier for high efficiency and high power density operation. To alleviate this issue, it is necessary to understand and control liquid water transport inside cathode porous electrode. It is well known that microporous layer (MPL) plays a critical role in suppressing liquid water flooding in PEFCs [1,2]. MPL is a much thinner and finer porous layer, which is typically composed of carbon black (CB) particles and PTFE, and inserted between cathode CL and GDL. Nam et al. [2] experimentally and numerically investigated the vapor condensation and liquid water breakthrough in porous layers, and showed that inserting MPL between CL and GDL reduces the size and saturation level of interfacial water droplets on CL surface. However, there is a risk that MPL addition on cathode side increases oxygen transport resistance through porous layers. In general, MPL is formed by coating the slurry of CB powder and PTFE onto GDL substrate and then heat-treated. Its pore structure is complicatedly affected by various fabrication parameters such as CB loading, PTFE/CB ratio, coating and dry processes. To control interfacial water transport between cathode CL and GDL and minimize water flooding, it is essential to optimize the fabrication parameters of MPL determining its microstructure. In this study, we focused on CB loading as one of the major fabrication parameters of MPL, and analyzed the pore structure of MPLs with different CB loadings using SEM-EDX and mercury porosimetry. Furthermore, the operation test and the optical visualization of liquid droplet behavior in the cathode electrode were conducted using a transparent cell, and the influence of CB loading of MPL on its microstructure, the interfacial water transport and the performance characteristics of PEFC was discussed. In the experiment, the MPLs were coated on the hydrophobized carbon paper (Toray, TGP-H-060) by using the doctor-blade technique. The SEM-EDX observation revealed that the average thickness of two MPLs with 1.0 and 4.0 mg/cm2 CB are 33.7 and 123.6 um, respectively. When the CB loading is increased from 1.0 to 4.0 mg/cm2, the thickness of MPL is enlarged approximately 3.7 times. Subsequently, the operation test of a PEFC was carried out at 70 oC under high-humidity conditions, and the effect of CB loading of MPL on the I-V characteristics was investigated. The MPL addition on the cathode side drastically improves the limiting current density, because it suppresses the formation of liquid water film at the CL/GDL interface and reduces the concentration overpotential. In addition, the cell voltage in the high current density range increases with an increase in CB loading of MPL. Since the thick MPL raises the temperature near the cathode reaction sites, the rate of condensation of product water at the CL decreases in the case of 4.0 mg/cm2CB. Therefore, the thick MPL with high CB loading beneficially reduces water flooding at the cathode CL/MPL interface and improves the cell performance.

Polymer electrolyte fuel cell (PEFCs) is a potential power source for new-generation automobiles like fuel cell vehicles that run on hydrogen energy, it has been studied a lot for last decades in order to use as the power sources of automotive. The most important characteristics of PEFCs are low operation temperature and quick start and shutdown. However, the properties of PEFCs are still limited due to its high cost and insufficient durability and performance.Since oxygen reduction reaction (ORR) at the cathode is the dominant reaction in PEFCs, oxygen proton and electron have to be transferred smoothly in porous catalyst layer. Under normal operating conditions, oxygen transfer is the dominant factor of limiting current density, therefore, oxygen transfer must be increased from the gas channel to the Pt surface through various porous media and various components.On the other hand, inkjet printing has been considered to be an economical and easy scale-up technology for the micro-scale patterning and fabrication as one of the on-demand methods in a variety of field because it allows for the patterning of various ink. In addition, it can reduce the process time, cost and the toxic waste created during the manufacturing process. Also, this method can be easily applied to perform ink composition studies. Especially for fuel cell applications, inkjet printing resolves many of the problems associated with previous methods of catalyst deposition by allowing a uniform distribution of catalyst ink onto the surface of GDL or membrane and by loading a very low amount of catalyst.So in this research, in order to increase the performance of polymer electrolyte cells, the oxygen diffusion resistance was tested based on the difference of surface of catalyst layer in cathode by using inkjet method. Firstly, we mixed carbon supported Pt catalyst, distilled water and 20wt% Nafion solution and NPA together and then use homogenizer to disperse the liquid, then we got the catalyst ink. The Second step was fabrication of cathode catalyst layer (CCL) by using inkjet method. Third step was to observe the morphology for CCL with catalyst ink, in order to know the length and the height of a drop of ink, we made a sample, and then we took this sample to observe through laser microscope. When we got the basic information about morphology of CCL, we fabricated catalyst layer by inkjet, and took it to do the MEA evaluation and characterization to investigate their electrochemical properties, such as I-V curves, limiting current densities and impedance. And in order to compare with the experimental results, a theoretical model of the 3D structure was also circulated and discussed. From this model and measured data, it was confirmed that the better electrochemical properties results from 3D structure of catalyst layer in cathode.

Oxygen transport resistance, one of the causes of large polarization in the cathode catalyst layer (CL), is intensified in low‐Pt‐loaded polymer electrolyte membrane fuel cells (PEMFCs). In order to explore operation strategies and cathode design to mitigate the large oxygen transport resistance of low‐Pt‐loaded fuel cells, the influence of operating conditions and ionomer structure on oxygen transport in the CL is investigated. Remarkably, the oxygen transport resistance data for different operation conditions and ionomer structures lie on a single curve when they are plotted as a function of the water partial pressure of the feed. At a high water partial pressure of 80 kPa, the oxygen transport resistance of the low‐Pt‐loaded CL (0.14±0.03 mg−Pt cm−2) becomes comparable to that of the high‐Pt‐loaded CL (0.40±0.04 mg−Pt cm−2) as a result of the opposing influences of Pt loading on Knudsen and ionomer film diffusion. This emphasizes the importance of the water uptake in the ionomer film for reducing oxygen transport in the CL. From a fuel cell design perspective, the operation strategy and CL design to maintain high water partial pressure in the cathode CL are extremely important for realizing low‐Pt‐loaded fuel cells.