Multiscale Modeling of the Effect of Membrane Thickness on the Performance of Proton-Exchange Membranes Based on Multiblock Copolymers of Sulfonated PSU/Ppsu Poly(Ether Sulfone)s: Towards High-Performance Fuel Cells

Abstract

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