Hydrogen crossover in high-temperature PEM fuel cells

Abstract

In this paper, hydrogen crossover was measured in an environment of high-temperature proton exchange membrane (PEM) fuel cells using a steady-state electrochemical method at various temperatures ( T) (80–120 °C), backpressures ( P) (1.0–3.0 atm), and relative humidities (RH) (25–100%). An H 2 crossover model based on an MEA consisting of five layers – anode gas diffusion layer, anode catalyst layer, proton exchange membrane (Nafion 112 or Nafion 117), cathode catalyst layer, and cathode gas diffusion layer – was constructed to obtain an expression for H 2 permeability coefficients as a function of measured H 2 crossover rates and controlled H 2 partial pressures. The model analysis suggests that the dominant factor in the overall H 2 crossover is the step of H 2 diffusing through the PEM. The H 2 permeability coefficients as a function of T, P, and RH obtained in this study show that the increases in both T and P could increase the H 2 permeability coefficient at any given RH. However, the effect of RH on the permeability coefficient seems to be more complicated. The T effect is much larger than that of P and RH. Through experimental data simulation an equation was obtained to describe the T dependencies of the H 2 permeability coefficient, based on which other parameters such as maximum permeability coefficients and activation energies for H 2 crossover through both Nafion 112 and 117 membranes were also evaluated. Both Nafion 112 and Nafion 117 showed similar values of such parameters, suggesting that membrane thickness does not play a significant role in the H 2 crossover mechanism.