Disappearance of electric double layer effects on electrochemical reactions: the case of a chemisorbed small species on a metal surface at the electrode/electrolyte–solution interface†

The electric double layer (EDL) consisting of a charged electrode and an electrolyte solution hosts numerous electrochemical reactions. In this study, the oxygen reduction reaction (ORR) at the interface between a Pt electrode and a HClO₄ aqueous solution was investigated using a hybrid method combining density functional theory and a statistical mechanical theory for molecular liquids, the three-dimensional reference interaction site model (3D-RISM) theory. This method can reveal the EDL structure at the atomic scale by explicitly considering the EDL charging owing to the electrode potential variation. Our calculation clarified that the solvation effect changes the ORR energy profile because of the local interaction between the adsorbate and the electrolyte solution. The charge distribution and solvation structure remarkably change depending on the electrode potential owing to the EDL formation; nevertheless, the energy profile of the dissociative mechanism is unaffected by the EDL formation. To elucidate the disappearance of the EDL effect on the energy profile, we analyzed the electrostatic potential change by the EDL formation and found that the O and OH adsorbates merge with the charged electrode surface. Therefore, the EDL does not affect the stability of the adsorbates. Detailed analysis further indicates that the EDL effect on electrochemical reactions will depend on the adsorption structure and size of adsorbates on the electrode surface. This study affords atomic-scale insights into the relationship between the EDL structure and electrochemical reactions.