A nonlinear viscoelastic–viscoplastic constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells

This paper describes a phenomenological constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells (PEMFCs). Unlike the existing approaches of elasto-plastic, viscoelastic, and viscoplastic model, the proposed model was inspired by micromechanisms of polymer deformation. The constitutive model is a combination of the nonlinear visco-elastic Bergström–Boyce model and hydration–temperature-dependent empirical equations for elastic modulus of ionomer membranes. Experiment results obtained from an uniaxial tension test for Nafion NR-111 membrane under well controlled environments were compared with simulated results by the finite element method (FEM) and the proposed model showed fairly good predictive capabilities for the large deformation behavior of the Nafion membrane subjected to the uniaxial loading condition in a wide range of relative humidity and temperature levels including liquid water.

Research highlights▶ We have developed a nonlinear viscoelastic-viscoplasticconstitutive model for describing the finite deformation of the ionomer membrane for PEMFCs. ▶ The stress in the constitutive relationship is obtained from summation of two molecular networks acting in parallel; the model isstrain-rate, temperature, and hydration dependent. ▶ The model can accurately describe the stress-strain behavior of both vapor and liquid water equilibratedionomer membranes after choosing appropriate material parameters.