Viscosity mechanism of perfluorosulfonic acid-based materials and their application in proton exchange membrane fuel cells

C Feng and J Zheng and Y Wang and CM Zhang and PW Ming, APPLIED MATERIALS TODAY, 34, 101896 (2023).

DOI: 10.1016/j.apmt.2023.101896

Perfluorosulfonic acid (PFSA) ionomers and their composites are popular electrolyte materials for proton exchange membrane fuel cells (PEMFCs), which subject to the combined actions associated with humidity change, temperature cycling, and clamp force during long-time operation. Thus, their time-related mechanical behaviour can greatly affect their proton conductivity and material degradation. Because viscosity is a vital factor controlling the mechanical deformation in the entire life cycle of a material, it is important to accurately predict for guiding the structural designation and operation condition. Currently, most of the literature discusses the PFSA viscosity based on large-strain-rate experiments. Microscopic driving mechanisms and macroscopic responses to small strain rates that occur during PEMFC operation remain largely unclear. In this study, the characterises of PFSA viscosity were investigated in terms of molecular interactions and macroscopic rheological behaviour. A cross-scale model was developed to describe the shear thinning phenomenon to obtain a zero-shear viscosity, which captures the linear viscous behaviour for small strain rates. Subsequently, a modified generalised Maxwell model consisting of elastic-plastic elements and zero-shear viscosity-dependent viscous elements was proposed and calibrated by the tensile and stress relaxation experiments for Nafion 117 membranes. In addition, the proposed visco-elastoplastic model, after validated by small-strain rate experiments, was used to investigate the response to cyclic loading during start-up and shutdown, which provides insights on the mechanical damage due to the hydrothermal cycles.

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