Synchrotron-based investigation of nanoscaled water-splitting electrocatalysts for clean energy storage

Abstract

The global need for sustainable and carbon-neutral energy storage methods has driven the exploration of various avenues; water splitting for hydrogen gas production is particularly promising. This electrochemical reaction is split into two half reactions: the hydrogen evolution reaction and the oxygen evolution reaction (OER). The OER is a major bottleneck for water splitting and requires potent catalysts to be driven at an appreciable rate. This thesis is aimed towards exploring and characterizing metal oxide-based materials as OER electrocatalysts. The benchmark RuOx is presented first, examining through electrochemical and spectroscopic analyses the importance of the synthetic route on structure and OER performance. It was found that while precursor choice has minimal impact on OER activity, the deposition method employed did. This is followed by an examination of first-row transition metal OER catalysts, 12 compositions of amorphous (Fe,Co,Ni)Ox thin films. Literature acknowledges the superior performance of binary materials to single-element oxides, and this synergistic effect is evaluated in this thesis through a combination of X-ray techniques as well as electrochemical methods. The amorphous nature of the catalyst materials were confirmed, and it was found that they are chemically homogeneous without phase separation. The electronic structure of the surface and bulk of the catalyst materials were found in Chapter 4 to be identical, and findings of pre- and post-operation analyses indicate surface and bulk mutual activity during OER operation. Soft X-ray analysis also allowed for insight into the enhancement of hybridization effects between (Fe,Co,Ni) metal 3d and oxygen 2p, evidenced by an O K-edge feature which could be used as an indicator and descriptor of OER activity. In-operando analyses in Chapter 5 investigated changes in oxidation state through XANES analysis and bond distances through EXAFS throughout operation, and results support that a dual-site mechanism for binary (Fe,Co,Ni)Ox OER catalysts is at play. The insights obtained from this research will provide valuable guidance in establishing design principles for future optimized OER catalyst materials, contributing to the advancement of sustainable energy solutions and leading towards a greener future.