Oxygen Reduction Reaction Mechanisms on Al-Doped X-Graphene (X = N, P, and S) Catalysts in Acidic Medium: A Comparative DFT Study

Density functional theory (DFT) is applied to study the oxygen reduction reaction (ORR) mechanisms on Al-doped X-graphene (X = N, P, and S) electrocatalyst in acidic medium in a fuel cell cathode comparatively. In order to study the catalytic properties of Al-doped X-graphene (X = N, P, and S), we calculate the adsorption properties of the ORR intermediates O-2, O, OOH, OH, H2O, and H2O2. We also examine 2e and 4e pathways during the ORR process in terms of adsorption energy of each ORR step. Our calculated results reveal that each Al-doped X-graphene (X = N, P, and 5) catalyst follows a 4e transfer pathway with favorable (exothermic) reaction energies. We observe that both Al-doped N-graphene and Al-doped P-graphene are energetically more favorable than Al-doped S-graphene catalysts for enhanced and stable ORR via 4e pathways in an acidic environment. Such analysis is quite useful in choosing the appropriate catalyst in applications of a polymer electrolyte fuel cell cathode.