ICYS Annual Report 20231. Outline of Research2. Research ActivitiesTomoaki KUMEDAIn order to develop highly performance electrocatalysts for electrochemical energy conversion and synthesis, understanding the microscopic reaction kinetics and mechanisms based on the elementary proton and electron transfer processes and the atomic-scale structure of the reaction fields at the electrode−electrolyte interface is essential. This study aims not only to increase electrocatalytic activity or selectivity but also to explore unique electrochemical phenomena caused by specific interfacial structures and extreme temperatures. The research is based on the atomic-scale structure of electrocatalysts and the characteristics of electrolyte materials by highly accurate electrochemical experiments using well-defined single-crystal model electrodes.(1) Cation effects on oxygen reduction reaction mechanism on Pt(111)In this study, the electron/proton transfer mechanism and selectivity of inner-sphere (IS) and outer-sphere (OS) processes for the oxygen reduction reaction (ORR) on Pt(111) depending on electrolyte cations was discovered collaborating with the theoretical research group of the University of Jyväskylä in Finland.1 In the IS process, the reactant O2 is adsorbed on the electrode surface, and electrons and protons are transferred to form adsorbed intermediates (*OOH, *O, and *OH), finally producing OH−. On the other hand, the OS process occurs away from the surface. Solvated O2 receives an electron from the −. The product of the surface via long-range transfer, forming O2−. Both the IS and OS processes can proceed OS process is HO2on Pt electrodes in alkaline electrolytes. However, the factors governing the selectivity between the IS and OS processes are open to debate.We performed ORR linear sweep voltammetry and rotating ring disk electrode (RRDE) experiments in alkaline solutions with different cations (Li+, Na+, K+, and tetramethylammonium (TMA+)) to identify the IS and OS processes. The results clearly showed that the OS process is promoted by the Li-containing electrolyte, while the IS process is dominant in the other three electrolytes. With DFT-MD simulations, we proposed the cation-dependent mechanism of the IS and OS processes for the ORR, as shown in Fig. 1. Li+ cation stabilizes adsorbed OH species on Pt surface and interfacial water molecules by non-covalent interactions, which are critical descriptors for OS electron and proton transfer processes. In addition, Li+ can interact with OS − and HO2, which also promotes the OS intermediates such as O2pathway.fuel Fig. 1. Schematic models of cation-dependent (a) inner- and (b) outer-sphere processes for oxygen reduction reaction.1References 1) T. Kumeda, L. Laverdure, K. Honkala, M. M. Melander, K. Sakaushi, Angew. Chem. Int. Ed. 62, e202312841 (2023). 2) K. Sakaushi, Faraday Discuss. 221, 428 (2020).(2) Cation effects on quantum proton tunneling for hydrogen evolution reaction on Au(111)Quantum proton tunneling (QPT) has been suggested to occur in electrochemical reactions under certain conditions. A recent study has revealed that the transition of classical-to-quantum proton transfer occurs during hydrogen evolution reaction (HER) on a Au electrode, depending on the electrode potential.2 We are studying the effect of electrolyte cations (Li+ vs K+) on QPT in HER on the Au(111) electrode. The classical and quantum proton transfers are identified by the kinetic isotope effect (KIE) analysis, which compares kinetics in H2O and D2O systems. The kinetic isotope effect value (KH/D) in KOH increases with overpotential, indicating the QPT at higher overpotentials. In contrast, QPT was not observed in LiOH. The water structure at the electrode and electrolyte interface will be a key factor for QPT. Li+ forms a stable hydrogen-bonded water layer, while K+ approaches the surface and breaks the water layer. We are investigating the relationship between the interfacial water structure and the proton transfer mechanism using vibrational spectroscopy and theoretical calculations with collaboratorsResearch Digest 15Microscopic Investigation of High-PerformanceElectrocatalytic Fields Using Single-CrystalModel Electrodes
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