Unravelling the oxygen reduction reaction selectivity of single-atom catalysts on N-doped graphene

Abstract

A systematic investigation of the oxygen reduction reaction (ORR) under acidic conditions on a series of transition metal-based single-atom catalysts (SACs) on N-doped graphene is presented, focusing on the competing four-electron (4e⁻) and two-electron (2e⁻) pathways. The Gibbs adsorption free energy of key ORR intermediates is evaluated from density functional theory (DFT), and the potential-limiting steps and the corresponding limiting potentials are determined. For the 4e⁻ pathway, Co-based SACs exhibit the most favourable thermodynamic profile, with a thermodynamic overpotential of only 0.38 V, making it highly promising for complete oxygen reduction to water. Conversely, the 2e⁻ pathway leading to hydrogen peroxide (H2O2) production, is energetically favoured on Pt and Pd SACs. Notably, Ni and Cu SACs also demonstrate high selectivity toward the 2e⁻ mechanism but suffer from substantial overpotentials due to the sluggish *OOH reduction step towards H2O2. Activity trends were further assessed using the Gmax(U) descriptor, which quantifies the largest free energy span for each reaction pathway. The results reveal that while some SACs, such as Cu, exhibit potential for both ORR pathways, whereas Pt and Pd clearly favour the 2e⁻ pathway under acidic conditions. These findings offer valuable insights into the design of metal-specific SACs optimized for either complete ORR or selective H2O2 generation.