Hydrogen crossover in high-temperature PEM fuel cells

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 H2 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 H2 permeability coefficients as a function of measured H2 crossover rates and controlled H2 partial pressures. The model analysis suggests that the dominant factor in the overall H2 crossover is the step of H2 diffusing through the PEM. The H2 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 H2 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 H2 permeability coefficient, based on which other parameters such as maximum permeability coefficients and activation energies for H2 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 H2 crossover mechanism.