Experimental and model refinement of water content and membrane conductivity in reinforced composite proton exchange membranes

Abstract

The ionic conductivity of proton exchange membranes (PEMs) directly determines the ohmic resistance of proton exchange membrane fuel cells (PEMFCs), which is largely dependent on the membrane's hydration level. The structural differences between reinforced composite membranes (RCMs) and homogeneous membranes lead to distinct relationships between their hydration levels and ionic conductivity. In this study, Gore788.12 and Nafion®117 were selected as representatives of RCMs and homogeneous membranes, respectively. Experiments were designed to achieve operational conditions using a temperature and humidity chamber, wherein the PEM was weighed at equilibrium to calculate water uptake and water content, while EIS was employed to measure the membrane's ionic resistance and subsequently determine its conductivity, and HFR was utilized to compare the ohmic impedance of MEAs composed of two kinds of membrane under varying temperature and humidity conditions. In this study, the impacts of water activity on water uptake, water content, and ionic conductivity of the reinforced membrane are quantitatively analyzed. The results show that RCMs exhibit lower water content and ionic conductivity compared to homogeneous membranes, particularly at high water activity. The RCMs demonstrate lower ionic resistance and reduce dependence on water activity, resulting in lower ohmic resistance in MEAs using reinforced membranes. An empirical equation for ionic conductivity as a function of water activity is derived from experimental data. The semi-empirical equation between water content and water activity is modified based on the theoretical study of membrane water absorption mode under different water activity. This study can provide valuable insights for optimizing steady-state PEMFC simulation models.