Electrocatalytic Activity Caused by the Electric Double Layer Consisting of an Electrolyte Solution and a Single-Atom Catalyst Revealed by 3D-RISM-SCF Study

Abstract

Single-atom catalysts (SACs) have attracted strong attention for reducing the amount of platinum used as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. The ORR activity reportedly depends on the coordination environment, including the interface to the electrolyte solution. Thus, to elucidate the mechanism of electrochemical reactions, it is crucial to investigate the electric double layer (EDL) that consists of a charged electrode and an electrolyte solution. This study investigated ORR using two types of SACs in an aqueous CF3SO3H solution using our original method based on 3D-RISM-SCF (three-dimensional reference interaction-site model self-consistent-field). For the electrocatalyst models of SACs, Pt coordinated to four N atoms (PtN4) and to one N atom (PtN) were used. Computational results demonstrate that H2O2 and H2O are selectively generated using PtN4 and PtN, respectively, which is consistent with experimental observations. The EDL destabilizes all reaction intermediates and thereby governs the H2O generation with PtN. The degree of destabilization varies with the type of SAC and adsorbate; thus, we examined the underlying mechanism. Charge analysis revealed that the EDL suppresses Pt-to-adsorbate electron transfer, thereby contributing to the destabilization. This suppression effect depends on the catalyst–adsorbate combination, which accounts for the variations in destabilization. Furthermore, changes in solvation structure and electrostatic potential distribution indicated that the negatively charged adsorbates undergo repulsive interactions within the spatially nonuniform electrostatic environment generated by the EDL. Consequently, the O* and OH* intermediates using both PtN and PtN4 are destabilized as the electrode potential increases. The EDL effects on electrocatalytic activity are governed by the atomic-scale structural details of the reaction center.