Toward full simulation of the electrochemical oxygen reduction reaction on Pt using first-principles and kinetic calculations

Abstract

Present fuel cells must increase the activity of the oxygen reduction reaction (ORR) on platinum (or Pt alloy) electrodes. Detailed simulation analyses can direct future investigations by providing a better understanding of the ORR. We adopted a density functional theory (DFT)-based, first-principles molecular dynamics simulation for the elementary steps of the electrochemical ORR on Pt(111). The two-step process involves successive protonation of O and OH, which are adsorbed on Pt. The relevant redox potentials were estimated by changing the coverage of OH(ad) and O(ad). The reaction energy profiles were determined along the reaction coordinate using the Blue-Moon ensemble method and a constant-bias scheme in the DFT calculations. These profiles at different biases were then used to generate activation energies and symmetry factors. Cyclic voltammetry (CV) and linear sweep voltammetry profiles were then calculated from the Butler–Volmer rate, Nernst equilibrium, and mass diffusion equations using these obtained parameters, literature values and appropriate prefactors in the rate equations. The experimentally observed reversible and irreversible peaks in CV were obtained. The irreversibility of the protonation of O(ad), (R2), attributed to its higher activation energy, affects the ORR potential and thus fuel cell performance. It is therefore necessary not only to tune the adsorption energy of the O(ad) and OH(ad) intermediates, which are the origin of the “volcano plot”, but also to tune (R2)'s activation energy to elevate the performance above that of the volcano-top.