In-Situ Observation of Adsorption Species on Platinum Catalyst in Oxygen Reduction Reaction Using High Energy Resolution XAFS

The oxygen reduction reaction (ORR) on Pt surfaces, for examples, Pt single crystals, polycrystalline Pt electrodes, Pt nanoparticles (NPs), Pt-based alloys, and core–shell Pt nanostructures, has been discussed in the last few decades. In polymer electrolyte fuel cells (PEFCs), hydrogen peroxide and its radicals are known to be fatal attacking species not only for electrode catalysts but also for carbon materials and polymer membranes. A thorough understanding of ORR schemes has led to the development of seamless four-electron reduction cathodes, contributing to improved endurance reliability of both anion-exchange membrane (AEM) and proton-exchange membrane (PEM) fuel cells. In this study, the reaction pathway, and intermediate species such as molecular oxygen, in the ORR were investigated in alkaline and acidic environments, respectively. In this work, High Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy (HERFD-XAS) at the Pt L III-edge, which enhances resolution by partially collecting fluorescent X-ray was employed. X-ray absorption fine structure (XAFS) can detect the electronic structure of Pt, and its high energy resolution makes it possible to distinguish between different adsorption species on the surface of Pt nanoparticles. In addition, photon-in photon-out experiments using hard X-ray with high penetrating ability make it relatively easy to perform in-situ/operando measurements in potential controlled solutions. Carbon-supported Pt nanoparticles as powder catalysts were used for measurement as more realistic conditions, and the species adsorbed on the Pt nanoparticles were analyzed while controlling under the potentials both in acidic and basic electrolyte solutions with N2 or O2 gas bubbling. In N2-saturated solutions, the XANES spectra of the Pt LIII-edge reflected the presence of the adsorption species such as Pt-O (*O), Pt-OH (*OH), Pt-H (*H), as well as PtO2 oxide and Pt metallic states without any adsorption, during the ORR with the potential change. In O2-saturated solutions, one additional adsorption species of molecular oxygen was observed in both acidic and basic electrolyte solutions. It was analyzed that the intermediate species adsorbed to platinum in a side-on configuration as a superoxide anion in an alkaline environment. This intermediate species is identified as hydrogen superoxide, also known as hydroperoxyl radical, with the chemical formula HO2 by being protonated in an acidic environment. The results of research on adsorption species using HERFD-XAS have provided important knowledge for designing catalysts that enable seamless four-electron reduction. The preferred cathode catalyst design will be discussed in detail in an oral presentation at the 244th ECS Meeting. Acknowledgments: The synchrotron radiation experiments were performed at the BL11XU in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Numbers of 2012B3506, 2013A3506, 2014A3506, 2014B3506, 2015B3506, 2016A3556 2019A3508, 2019B3508, 2020A3508, 2021A3508, 2021B3508, 2022A3508 and 2022B3597 at BL11XU). This work was supported by JSPS KAKENHI Grant Number JP22H02188). Figure 1