Synthesis and Application of Advanced Electrocatalysts toward Oxygen Evolution and Oxygen Reduction Reactions

Abstract

Electrochemical processes provide attractive pathways in energy conversion and storage technologies. Among them, the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are two key half-reactions in several typical energy-related devices, including water electrolyzers, fuel cells, and metal-air batteries. However, as both OER and ORR involve four-electron transfer, sluggish kinetics needs to be overcome by large overpotential, leading to low energy efficiency and further affecting the performance of the overall device. Thus, over the past decades, numerous works have focused on constructing high-performance and cost-effective electrocatalysts for OER/ORR.

Transition metals, including Fe, Co, Ni, etc., have been extensively involved in OER/ORR catalysts and exhibited promising activity. Their high performance, abundant reserves, and low price make them suitable for large-scale applications in the future. However, there is still a long way to go in promoting the activity and stability of transition metal-involved catalysts to meet the requirements of the industry. Various strategies have been developed for the rational design of catalysts, and it is noted that the effect of doping is profound in many studies.

Single-atom catalysts (SACs), possessing the merits of high metal utilization efficiency and adjustable local coordination of metal centers, have been reported to have remarkable performance in many electrochemical processes. Especially for ORR, they have displayed comparable and even higher activity than Pt-based catalysts. Nevertheless, the adsorption configuration of reactant molecules (O₂ for ORR) on active sites in SACs is limited, thus the adsorption-energy scaling relationships are unavoidable for the reactions involving multiple intermediates. In this respect, dual-metal catalysts (DACs) allow tunable adsorption configurations on catalytic centers, providing the possibility of alternative reaction pathways and achieving higher intrinsic reaction activity.

In this thesis, advanced electrocatalysts for oxygen evolution reaction and oxygen reduction reaction are proposed and studied, and the origin of intrinsic activity is investigated and disclosed. In chapter 2, the effect of Fe doping into CeO₂ on OER activity has been studied, and the structure-activity relationship has been investigated. It was shown that the incorporation of Fe element and favorable long-range disorder structure could generate increased oxygen vacancies, not only ensuring fast electron transport, but also favoring direct coupling of lattice oxygen. In chapter 3, three Co-based SACs have been prepared as model electrocatalysts for ORR under alkaline conditions. The number of electrochemically accessible Co sites was determined by an electrochemical method, and the intrinsic activity of metal centers with various coordination structures was compared. Furthermore, to achieve higher ORR performance, a Co dual-atom catalyst (Co DAs/CB) was designed in Chapter 4. The excellent ORR activity of Co DAs/CB was ascribed to the synergistic effect of the two adjacent Co atoms.

Date of Award	28 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yung-kang PENG (Supervisor), Jianlin CHEN (Co-supervisor) & Zonglong ZHU (Co-supervisor)