Measurement of Proton Conductivity and Water Diffusivity of Nafion Membrane at Subzero Temperatures

Abstract

Water management is critical for the high performance of proton exchange membrane fuel cells (PEMFCs). The knowledge of PEM properties such as water content, proton conductivity, and water diffusivity is necessary for water management. In literature, PEM properties are very well documented at temperatures above room temperatures but at subzero temperatures PEM properties are scarcely reported. The acquisition of PEM properties at sub-zero temperatures is very important to the quick start-up of fuel cell electric vehicles in below freezing conditions.The objective of this study is to obtain the water transport characteristics of a membrane at subzero temperatures for automotive applications. In this study, three different properties of a Nafion membrane are quantified: water content, proton conductivity, and water diffusivity.In sub-zero condition at different RHs, the membrane water content and proton conductivity were quantified from gravimetric and electrochemical measurements. A novel measurement technique using segmented cell and methodology were established to estimate water diffusion coefficient as a function of water content and temperature (see figure below). Further, particular to the Nafion membrane, an empirical relationship between water diffusivity and water content in temperature range of 243K to 303K was developed. Figure 1