In this work, the ionic conductivity of proton-exchange membranes (PEMs) based on sulfonated multiblock copolymers composed of Polysulfone (PSU) and Polyphenylsulfone (PPSU) poly(ether sulfone) segments (SPSU/SPPSU) is evaluated. The copolymers were synthesized for the first time by polycondensation in a “one-pot two-step synthesis” of commercial monomers, followed by sulfonation reaction with trimethylsilyl chlorosulfonate (TMSCS) [1]. The morphology of the membranes was examined by Field Emission Scanning Electron Microscopy (FE-SEM). Electrochemical Impedance Spectroscopy (EIS) was employed to measure the ionic conductivity of the membranes. The FE-SEM analysis of the samples revealed a random nanostructure of hydrophilic/hydrophobic domains with high and low content of ionic groups, respectively. Thus, no microphase separation was observed, even though the PSU block showed a greater affinity to be sulfonated. The ionic conductivity and water uptake of membranes with three different degrees of sulfonation, DS=0.45, 0.70 and 0.79, were characterized at 80 oC and relative humidities ranging from RH=10% to RH=100%. For a given RH, the ionic conductivity increased non-linearly with DS, showing a strong rise when DS was varied from 0.45 to 0.70, although the water uptake of the membranes remained nearly the same. In contrast, the increase of the ionic conductivity between DS=0.70 and DS=0.79 was significantly lower, but the water uptake increased sharply. The behavior of the membranes was modeled using percolation theory concepts. The numerical model was implemented in the finite volume-based code ANSYS Fluent [2,3]. The nanostructure of the membranes was divided into three types of sites in a random cubic network: sulfonated and well-hydrated sites, sulfonated and weakly hydrated sites and non-sulfonated sites (see Figure 1). The volume fraction of sulfonated sites was varied according to the DS of the samples, while the relative volume fraction of hydrated sites (i.e., the ratio between the volume fraction of hydrated sites and the volume fraction of sulfonated sites) was varied depending on RH. Isolated, hydrated sites not connected to the edges of the domain were removed from the network, since water was assumed to form a continuous network connected to the edges of the membrane according to the experimental conditions. Good agreement was found between the experimental data and numerical results, thus providing a fundamental explanation of the behavior of the multiblock copolymer membranes. The validated model will be used in future work to assist the design of high-performance and durable multiblock copolymer membranes for proton-exchange fuel cells, and related electrochemical devices. [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, I.V. Zenyuk, J.T. Gostick, A.Z. Weber, Modeling Gas Diffusion Layers in Polymer Electrolyte Fuel Cells Using a Continuum-based Pore-network Formulation, ECS Trans. 97 (7) 615. [3] P.A. García-Salaberri, Modeling diffusion and convection in thin porous transport layers using a composite continuum-network model: Application to gas diffusion layers in polymer electrolyte fuel cells, Int. J. Heat Mass Trans. (2020), accepted. . Figure 1. Random cubic network composed of sulfonated and well-hydrated sites, sulfonated and weakly hydrated sites and non-sulfonated sites used to model the ionic conductivity of the multiblock copolymer membranes. . Figure 1
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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