Modeling and Characterization of the Ionic Conductivity of Proton-Exchange Membranes Based on Multiblock Copolymers of Sulfonated PSU/Ppsu Poly(Ether Sulfone)s

Abstract

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