The role of non-covalent interactions in electrocatalytic fuel-cell reactions on platinum

Abstract

The classic models of metal electrode–electrolyte interfaces generally focus on either covalent interactions between adsorbates and solid surfaces or on long-range electrolyte–metal electrostatic interactions. Here we demonstrate that these traditional models are insufficient. To understand electrocatalytic trends in the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) and the oxidation of methanol on platinum surfaces in alkaline electrolytes, non-covalent interactions must be considered. We find that non-covalent interactions between hydrated alkali metal cations M⁺(H₂O)_x and adsorbed OH (OH_ad) species increase in the same order as the hydration energies of the corresponding cations (Li⁺ >> Na⁺ > K⁺ > Cs⁺) and also correspond to an increase in the concentration of OH_ad–M⁺(H₂O)_x clusters at the interface. These trends are inversely proportional to the activities of the ORR, the HOR and the oxidation of methanol on platinum (Cs⁺ > K⁺ > Na⁺ >> Li⁺), which suggests that the clusters block the platinum active sites for electrocatalytic reactions.