Noble Metal Catalysts for Electrocatalytic Water Splitting in Acidic Environments

Abstract

This thesis aims to design electrocatalysts with high activity, long-term stability and cost-efficiency for proton exchange membrane (PEM) water electrolyzers. This technique is of vital importance for fast hydrogen fuel generation, but the catalysts generally suffer from rapid degradation due to the harsh acidic and oxidative conditions. Firstly, a rational design strategy is reported for the fabrication of heterostructured oxygen evolution reaction (OER) electrocatalyst (Ru@IrOx), in which a strong charge redistribution exists between highly strained ruthenium core and partially oxidized iridium shell across the metal-metal oxide heterojunction. The increased valence of iridium shell and decreased valence of ruthenium core activate a synergistic electronic and structural interaction, resulting in the simultaneously enhanced activity and stability of the catalyst as compared to most of the state-of-the-art Ru- and Ir-based materials. The electrocatalysts with bifunctionality for efficient OER and hydrogen evolution reaction (HER) in acidic environments towards PEM water electrolyzers have been further investigated. A bifunctional electrocatalyst (M-RuIr, M = Co, Ni, Fe) is designed by doping RuIr alloy nanocrystals with transition metals that modify electronic structure and binding strength of reaction intermediates. Applying the Co-RuIr as the catalyst for overall water splitting, a very small cell voltage of 1.52 V is required to achieve current density of 10 mA cm-2. More importantly, the catalytic activity is correlated with the chemical/valence states of catalysts and a novel composition-activity relationship is established. Although noble metals exhibit potential towards energy conversion reactions in acidic environments, the wide application of noble metals is unfortunately restricted by high cost and limited choice of geometric structures spanning single atoms, clusters, nanoparticles, and bulk crystals. Therefore, it is proposed to overcome this limitation by integrating noble metal atoms with the lattice of transition metal oxides to create a new type of hybrid structure. The third study shows that various kinds of noble metal atoms can be accommodated into the cationic sites of cobalt spinel oxide with short-range order and an identical spatial correlation with the host lattice. Among them, the iridium-incorporated hybrid exhibits higher electrocatalytic activity than the parent oxide by two orders of magnitude and significantly improved corrosion resistance towards the challenging OER under acidic conditions. This strategy can be extended to other oxide systems and would greatly diversify the topologies of noble metal structures for a variety of applications. To further explore the effect of spatial structure of noble metal substitutions on their catalytic performance, a series of Pt-substituted Co spinel oxide catalysts with different amounts of Pt substitutions are synthesized. It is shown that the catalyst with optimized spatial correlation of Pt substitutions outperforms the commercial Pt/C catalyst towards HER and hydrogen oxidation reaction (HOR) in acidic environments by over one order of magnitude. These systematic works open a new horizon for developing efficient electrocatalysts towards a wide range of applications in acidic environments by rational promotion of the chemical composition, interfacial structure and atomic spatial structure of noble metals to achieve a balance among activity, stability and cost of catalysts. 2