Investigation of dry ionomer volume fraction in cathode catalyst layer under different relative humilities and nonuniform ionomer-gradient distributions for PEM fuel cells

Abstract

The dry ionomer volume fraction in cathode catalyst layer (CCL) of polymer electrolyte membrane (PEM) fuel cells is of great important for determining cell performance. The interaction between the dry ionomer volume fraction in CCL and reactant relative humidity (RH), as well as a CCL with through-plane (TP) and in-plane (IP) nonuniform ionomer-gradient distributions, is numerically investigated by using a three-dimensional multiphase fuel cell model. Results show that cell performance is highly sensitive to the variations in dry ionomer volume fraction in CCL under high current densities with lower RHs, and is less sensitive under low current densities with higher RHs. A higher dry ionomer volume fraction in CCL benefits proton conductivity, but causes a lower porosity and higher mass diffusion resistance under the same content of Pt/C for CCL. Overall, the dry ionomer volume fraction of 0.25 with a moderate RH of 60% for a conventional CCL shows the best cell performance. Therefore, the results of our model support the finds of previous studies showing that an optimal ionomer volume fraction is 0.25 by volume and 0.33 by mass. This study further ascertains that a TP gradient preferable with dry ionomer volume fraction concentrated near the polymer membrane (PM)-CCL interface contributes to an improvement in cell performance. Finally, the underlying mechanism is revealed through the observation of the influence of TP and IP ionomer-gradient distributions under an RH of 60% on the mass diffusion, the proton conductivity and the oxygen reduction reaction (ORR).