The Mechanisms of the Fuel Cell Oxygen Reduction Reaction on Pt and Other 8-11 Column Metal Surfaces

Abstract

To better understand and improve the cathode process for Proton exchange membrane fuel cell, we studied systematically the mechanism of oxygen reduction reaction (ORR) on group 8–11 metals and their alloys using density functional theory calculations. To address the contribution of solvent effect, we developed a practical implicit solvation model based on Poisson-Boltzmann equation. We discovered that solvation changed greatly the reaction barriers and hence the pathways preferences. The two well known mechanisms O2-diss and OOH-form mechanisms become impossible with water solvation. Instead, we found three new alternative mechanisms, namely, O2-diss-hydr, OOH-form-hydr, and high-H mechanisms. We showed that the oxygen hydrolysis Oad + H2Oad -> 2OHad plays an important role in the ORR which leads to the preferred O2-diss-hydr mechanism. We also developed a method to study the processes involving electron transfer between the solvent and the electrode. We found that direct OH formation from Oad and H3O+ has a high barrier of 0.70eV and is hence unlikely to be the dominant way of forming OHad at the operating potential of 0.8V. The potential dependent barrier leads to an overall optimal operating potential of 0.68V. We also studied the ORR on Pt3Ni alloys and found that the sublayer Ni atoms imposes an inhomogeneity in surface binding sites. The different binding energies make the barriers coverage dependent. Pt3Ni can only outperform Pt at higher coverage. We also showed the general approach of studying an unknown alloying system using Pd-Cu system as an example. We studied the structural, surface cleavage, and binding site preferences for various types of PdCu alloys. We predicted that 1:1 PdCu alloy with L11 structure and layered surface is a better catalyst than pure Pd and Cu, which agrees with later experiments.