Advances in ORR Catalysis Promoted by Graphene-Supported Low-Cost Metal Clusters: A DFT Study.

The oxygen reduction reaction (ORR), which converts molecular oxygen (O) into water (HO), is critical for renewable energy transformation processes. However, its industrial application is hindered by long conversion times. Recent studies suggest that transition metal clusters deposited on graphene are promising candidates for ORR catalysis. In this work, we employed density functional theory (DFT) calculations to explore the thermodynamically most stable energy profile of the ORR on pentamer metal clusters (Fe, Co, and Pt) supported on undoped graphene and nitrogen-doped graphene (for Fe), under standard electrochemical conditions (pH = 0 and = 0). Both the "standard" intermediates (*OOH, *O, *OH) and the "unconventional" intermediates (OOH, OHOH) were studied, analyzing thermodynamic stability, adsorption energies, and the influence of the implicit water solvent. Our results reveal that the inclusion of "unconventional" intermediates significantly alters the reaction thermodynamics, presenting a new pathway that is energetically more favorable than the classical one. Catalytic performance predictions, based on the theoretical overpotential (η), indicate that the four catalysts exhibit good stability and high activity in both reduction mechanisms. In particular, Fe@NGr shows the best catalytic performance in the "unconventional" mechanism, with an η close to zero. This study, for the first time, demonstrates how the metal cluster and the support's electronic and structural properties influence the stability of ORR intermediates and catalytic performance. The improved performance of Fe@NGr in the "unconventional" mechanism highlights the importance of selecting the right metal and engineering the graphene support, particularly through N-doping, for the rational design of low-cost, high-performance catalysts.