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 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.

A parametric analysis is presented using a previously validated 1D model for a cathode catalyst layer (CL). The results show that maximum power density at low Pt loading can be maximized with relatively thin CLs (${\rm thickness}\sim 2\;\mu \rm m$) featuring a high carbon volume fraction (low ionomer-to-carbon weight ratio, I/C) compared to high Pt loading CLs. The shift of the optimal carbon volume fraction (I/C ratio) is caused by the dominant role of the local oxygen transport resistance at low Pt loading, which is lowered by a reduction of the average ionomer film thickness (better ionomer distribution among carbon particles). In contrast, at high Pt loading, higher porosity and pore radius (lower carbon volume fraction) is beneficial due to an increase of bulk effective diffusivity despite thickening of ionomer films. Moreover, the results show that performance at low Pt loading is significantly improved with increasing mass-specific activity. The effect of average saturation and ionomer permeability on performance at low Pt loading is lower compared to dry CL composition and mass-specific activity.

Free volume plays a key role on transport in proton exchange membranes (PEMs), including ionic conduction, species permeation, and diffusion. Positron annihilation lifetime spectroscopy and electrochemical impedance spectroscopy are used to characterize the pore size distribution and ionic conductivity of synthesized PEMs from polysulfone/polyphenylsulfone multiblock copolymers with different degrees of sulfonation (SPES). The experimental data are combined with a bundle-of-tubes model at the cluster-network scale to examine water uptake and proton conduction. The results show that the free pore size changes little with temperature in agreement with the good thermo-mechanical properties of SPES. However, the free volume is significantly lower than that of Nafion®, leading to lower ionic conductivity. This is explained by the reduction of the bulk space available for proton transfer where the activation free energy is lower, as well as an increase in the tortuosity of the ionic network.

A high advancement has been achieved in the design of proton exchange membrane fuel cells (PEMFCs) since the development of thin-film catalyst layers (CLs). However, the progress has slowed down in the last decade due to the difficulty in reducing Pt loading, especially at the cathode side, while preserving high stack performance. This situation poses a barrier to the widespread commercialization of fuel cell vehicles, where high performance and durability are needed at a reduced cost. Exploring the technology limits is necessary to adopt successful strategies that can allow the development of improved PEMFCs for the automotive industry. In this work, a numerical model of an optimized cathode CL is presented, which combines a multiscale formulation of mass and charge transport at the nanoscale (∼10nm) and at the layer scale (∼1μm). The effect of exterior oxygen and ohmic transport resistances are incorporated through mixed boundary conditions. The optimized CL features a vertically aligned geometry of equally spaced ionomer pillars, which are covered by a thin nanoporous electron-conductive shell. The interior surface of cylindrical nanopores is catalyzed with a Pt skin (atomic thickness), so that triple phase points are provided by liquid water. The results show the need to develop thin CLs with bimodal pore size distributions and functionalized microstructures to maximize the utilization of water-filled nanopores in which oxygen transport is facilitated compared with ionomer thin films. Proton transport across the CL must be assisted by low-tortuosity ionomer regions, which provide highways for proton transport. Large secondary pores are beneficial to facilitate oxygen distribution and water removal. Ultimate targets set by the U.S. Department of Energy and other governments can be achieved by an optimization of the CL microstructure with a high electrochemical surface area, a reduction of the oxygen transport resistance from the channel to the CL, and an increase of the catalyst activity (or maintaining a similar activity with Pt alloys). Carbon-free supports (e.g., polymer or metal) are preferred to avoid corrosion and enlarge durability.

Improved modeling of the membrane electrode assembly (MEA) and operation is essential to optimize proton exchange fuel cells (PEFCs). In this work, a hybrid model, which includes a pore network formulation to describe water capillary transport and a continuum formulation to describe gas diffusion, is presented. The model is validated with previous data of carbon-paper gas diffusion layers (GDL), including capillary pressure curve, relative effective diffusivity, g(s), and saturation profile. The model adequately captures the increase of capillary pressure with compression, the nearly cubic dependency of g(s) on average saturation, s^{\rm avg}, and the shape of the saturation profile in conditions dominated by capillary fingering (e.g., running PEFC at low temperature). Subsequently, an analysis is presented in terms of the area fraction of water at the inlet and the outlet of the GDL, A_w^{in} and A_w^{out}, respectively. The results show that gas diffusion is severely hindered when A_w^{in} is exceedingly high (>80%), a situation that can arise due to the bottleneck created by flooded interfacial gaps. Furthermore, it is found that s^{avg} increases with A_w^{out}, reducing the GDL effective diffusivity. Overall, the work shows the importance of an appropriate design of MEA porous media and interfaces in PEFCs.

Polymer electrolyte fuel cells (PEFCs) are promising power sources for stationary, portable and vehicular applications due to its key advantages, including quiet operation, quick start-up and load response, and high efficiency. In the last decades, several fuel cell vehicles have been developed by automakers, such as General Motors, Hyundai and Toyota. However, the widespread commercialization of PEFCs for vehicular applications is still limited by their cost, performance and durability, among which durability is the most challenging aspect. The target lifetimes for PEFCs set by the U.S. Department of Energy (DOE) are 5,000 h for passenger cars, 25,000 h for transit buses and 40,000 h for stationary applications. Therefore, currently there is a need to demonstrate the above lifetimes, while decreasing capital and operating costs. In this context, mathematical modeling is an indispensable tool to examine the effect of material microstructure on PEFC performance and degradation. This task requires the development of multiscale models that incorporate key information from the microscale into the macroscale, while keeping computational cost moderate for engineering applications. In this work, multiblock copolymer membranes composed of sulfonated Polysulfone and Polyphenylsulfone poly(ether sulfone) (SPSU/SPPSU) were synthetized by polycondensation using a “one-pot two-step” route [1]. The performance of membranes with different thicknesses at different temperatures (T) and relative humidities (RH) was examined experimentally (see Figure 1). The data were compared with the predictions of a multiscale, non-isothermal, two-phase model of a PEFC accounting for non-equilibrium water sorption/desorption in the catalyst layers, as well as non-equilibrium water evaporation/condensation in the porous media of the MEA (GDLs, MPLs and CLs) and in the channels. The dominant transport mechanisms of dissolved water in the electrolyte were assumed to be diffusion and electro-osmotic drag. Double-trap kinetics was considered to model the oxygen reduction reaction, while Butler-Volmer kinetics was used to model the hydrogen oxidation reaction [2-5]. The combined experimental and numerical work has revealed key information on the interplay between membrane thickness, performance and water management for the design of high-performance, durable PEFCs. Currently, work is underway to develop thin multiblock copolymer membranes with power densities above 1 W cm-2 and extended durability within DOE targets. [1] N. Ureña, M.T. Pérez-Prior, C. del Río, A. Várez, J.Y. Sánchez, C. Iojoiu, B. Levenfeld, Multiblock copolymers of sulfonated PSU/PPSU Poly(ether sulfone)s as solid electrolytes for proton exchange membrane fuel cells, Electrochim. Acta 302 (2019) 428–440. [2] P.A. García-Salaberri, D.G. Sánchez, P. Boillat, M. Vera, K.A. Friedrich, Hydration and dehydration cycles in polymer electrolyte fuel cells operated with wet anode and dry cathode feed: A neutron imaging and modeling study, J. Power Sources 359 (2017) 634–655. [3] A. Goshtasbi, P.A. García-Salaberri, J. Chen, K. Talukdar, D. García-Sánchez, T. Ersal, Through-the-Membrane Transient Phenomena in PEM Fuel Cells: A Modeling Study, J. Electrochem. Soc. 166 (2019) F3154–F3179. [4] P.A. García-Salaberri, I.V. Zenyuk, G. Hwang, M. Vera, A.Z. Weber, J.T. Gostick, Implications of inherent inhomogeneities in thin carbon fiber-based gas diffusion layers: A comparative modeling study, Electrochim. Acta 295 (2019) 861–874. [5] J. Liu, P.A. García-Salaberri, I.V. Zenyuk, Bridging Scales to Model Reactive Diffusive Transport in Porous Media, J. Electrochem. Soc. 167 (2020) 013524. . Figure 1. (left) Polarization curves and (right) power density curves corresponding to multiblock copolymer membranes with different dry thicknesses and RHs. The operating temperature of the pure oxygen feed PEFC is T=60 oC. . Figure 1

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