Understanding surface charge effects in electrocatalysis. Part 2: Hydrogen peroxide reactions at platinum

Electrocatalytic activity is influenced by the surface charge on the solid catalyst. Conventionally, our attention has been focused on how the surface charge shapes the electric potential and concentration of ionic reactant(s) in the local reaction zone. Taking H 2 O 2 redox reactions at Pt(111) as a model system, we reveal a peculiar surface charge effect using ab initio molecular dynamics simulations of electrified Pt(111)-water interfaces. In this scenario, the negative surface charge on Pt(111) repels the O–O bond of the reactant (H 2 O 2 ) farther away from the electrode surface. This leads to a higher activation barrier for breaking the O–O bond. Incorporating this microscopic mechanism into a microkinetic-double-layer model, we are able to semi-quantitatively interpret the pH-dependent activity of H 2 O 2 redox reactions at Pt(111), especially the anomalously suppressed activity of H 2 O 2 reduction with decreasing electrode potential. The relevance of the present surface charge effect is also examined in wider scenarios with different electrolyte cations, solution pHs, crystal facets of the catalyst, and model parameters. In contrast with previous mechanisms focusing on how surface charge influences the local reaction condition at a fixed reaction plane, the present work gives an example in which the location of the reaction plane is adjusted by the surface charge. The activation barrier of breaking the oxygen-oxygen bond of the hydrogen peroxide molecule is higher at negatively charged surface, resulting in the abnormal experimental observation that the reduction current is suppressed with decreasing the electrode potential.