NSRRC Activity Report 2023

Chemical Science 025 E lectrolysis of water is a technique with the potential to convert electricity into clean hydrogen for energy storage and conversion applications. To increase the production efficiency of hydrogen for the growing demands of energy and environmental issues, the development of efficient and reliable catalysts is crucial. The anode materials responsible for the oxygen evolution reaction (OER) during water electrolysis have been identified as a bottleneck due to their intrinsically sluggish kinetics. Many groups are devoted to developing and optimizing these catalysts through morphology engineering and modification of the electronic structure to improve the OER efficiency. To understand the origin of the enhancing catalytic reactivity, different techniques, such as X-ray absorption spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS), are applied to probe these novel materials. Among these techniques, XPS is a powerful technique that can provide the elemental composition, chemical state, and electronic state of materials within nanometers of the sample surface. Nowadays, ambient pressure X-ray photoelectron spectroscopy (APXPS) integrated with ingenious sample cells provides scientific opportunities to probe the change of chemical composition between phases during the reaction process. Fig. 1 : PMEC cells at TLS 24A1 . (a) Schematic representation of the PEMC cell with the static electrolyte. (b) Photographic image of the PMEC cell with a continuous electrolyte flow system connected to the APXPS endstation. (c) Photographic image of the PMEC cell head with a continuous electrolyte flow system within the analysis chamber of the APXPS endstation. [Reproduced from Ref. 1] Fig. 2 : Schematic of surface oxide species evolution of the Pt electrocatalyst during redox polarization. [Reproduced from Ref. 1] Probing the Liquid–Solid Interface by Ambient Pressure X-ray Photoelectron Spectroscopy Novel polymer membrane-based electrochemical liquid cells allowed the investigation of surface species evolution under electrochemical conditions through APXPS measurements at TLS 24A1 . Recently, Chueh-Cheng Yang and Chia-Hsin Wang (NSRRC) have developed a polymer membrane-based electrochemical liquid cells (PMEC cells) system combined with the APXPS chamber at the beamline TLS 24A1 ( Fig. 1 ) , which can obtain in operando XPS spectra under electrochemical conditions. Using the ion exchange membranes as the separators between the electrolyte and the vacuum, ions and water can diffuse through the membrane onto the electrocatalysts. The thin layers of the ion and water on the electrocatalysts’ surface serve as the reactant for the electrochemical reactions and allow the electron energy analyzer to detect the escaping photoelectrons from the surface of the catalysts. Using these novel liquid cells with the APXPS, changes in the surface species on the catalyst’s surface during the reaction can be probed to reveal the active sites and reaction mechanism. The group studied the surface species evolution of a low- loading Pt catalyst deposited on a Nafion membrane in an acidic environment. 1 Employing operando APXPS measurements, the dynamic transformation of oxide species (Pt δ+ , Pt 2+ , and Pt 4+ ) at different potentials within the water layer formed by a 0.05 M H 2 SO 4 solution from the back of the membrane. Their findings revealed that the