Interfacial ions in the electrical double layer affect various electrochemical reactions. Recently, non-specific adsorbed ions located at the outer Helmholtz planes (OHPs) have attracted attention, and detailed structures have been determined by surface X-ray diffraction. The activities of oxygen reduction reaction (ORR) and hydrogen oxidation/evolution reactions (HOR/HER) strongly depend on the interfacial hydrophilicity/hydrophobicity of cations at the OHP.
We investigated the effect of hydrophilic/hydrophobic ions on the fuel cell and electrolysis reactions on single crystal Pt electrodes. The high hydrophobicity of tetraalkylammonium cation with a longer alkyl chain enhances the oxygen reduction reaction activity. The activity on Pt(111) in the presence of tetra-n-hexylammonium cation is eight times as high as that without this cation, which is comparable to the activities on Pt₃Co(111) and Pt₃Ni(111) electrodes. Hydrophobic cations and their hydration shells destabilize the OH_ad and adsorbed water. The hydrophobic characteristics of non-specifically adsorbed cations can prevent the adsorption of poisoning species on the Pt electrode and form a highly efficient interface for the oxygen reduction reaction.
We also investigated the HOR and HER activities on single-crystal Pt electrodes modified with hydrophobic/hydrophilic cations and anion-exchange copolymers in alkaline solutions. In alkali metal hydroxide solutions, Pt(110) exhibits the highest HER/HOR activity in the low-index planes of Pt. Detailed interfacial structure during HER was determined using X-ray diffraction. Hydrophilic cations at the interface facilitate hydrogen extraction from the hydrated water. Furthermore, forming a hydrogen-bonding network at the interface activates HER by promoting OH⁻ diffusion.

Masashi Nakamura

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