Design of metal oxide/hydroxide electrocatalysts towards electrochemical energy conversion

Abstract

Sustainable electrochemical synthesis of “green” energy molecules has received considerable interest, due to their potential to replace traditional fossil-fuel-based energy utilization cycles. This includes hydrogen generation via. (sea) water splitting and ammonia production from nitrate. Here, electrocatalysts play a pivotal role in overall efficiency, durability, and techno-economic feasibility. However, electrocatalysts reported to date still fail to meet the requirements for industrial applications. The fundamental challenge is the binding energy scaling of close-coupled reaction intermediates that often gives rise to large overpotential and poor efficiency. Thus, rationally designing cost-effective and active electrocatalysts is both economically desirable and technically important. Hence, the project focuses on the integrated theoretical and experimental design of metal oxide/hydroxide electrocatalysts for energy conversion of renewable electricityinto valuable chemicals. The efforts aim to construct dual-active site metal oxide/hydroxide electrocatalysts to bypass the theoretical activity ceiling under the intermediates scaling limitation and rationalize the structure engineering of dual-active site electrocatalysts by combining in situ characterizations with theoretical calculations. Specific achievements are outlined below. Chapter 2 aims to de-couple the rate-limiting multi-electron transfer process of Oxygen Evolution/Reduction reactions (OER and ORR) in the generation/utilization of water. By the introduction of Mn into RuO2, the oxygen-bridged asymmetrical “Ru– O–Mn” metal pairs are achieved, which promotes the relatively efficient Lattice Oxygen Mechanism (LOM) and weakens the *OOH adsorption on the Mn site for enhanced ORR, to break the key intermediate, *OOH, involved in the linear-scaling relationship. These findings rationalize the atomic engineering of Ru–Mn dual active sites for bifunctional oxygen electrocatalysis and provide a new strategy to avoid the *OH−*OOH scaling relations. Chapter 3 shows the potential of a hetero-structural interface electrocatalyst, Pt/NiO, to enact dual-active sites thus minimizing the energy barrier for water dissociation and H–H coupling in hydrogen evolution reaction (HER). Impressively, the Pt/NiO electrocatalyst yields only 122 mV overpotential at 100 mA cm-2. The results point out the importance of co-modulation of adsorption of *H and *OH towards high-performance HER. Chapter 4 shows dual-species (Coδ+/Co2+) catalyst, Co/β-Co(OH)2 which is developed to promote nitrate reduction (NO3−RR) into ammonia. The disorderedCo/β−Co(OH)2 interface exhibits optimal NO3−RR performance with ammonia yield of 20 mg h-1 cm-2 and FE of 96.0% at −0.4 V. Our work highlights the key role of water dissociation for NO3−RR and provides insight into the synergistic effect of deoxygenation and hydrogenation on the interface. Chapter 5 shows the development of lithium rich nickel iron (LiNiFe) hydroxide for promoting OER in seawater splitting. The LiNiFe electrocatalyst exhibits high activity in simulated seawater water, which only requires 281, 335 and 376 mV overpotential to deliver current density of 100, 500 ,1000 mA cm-2, respectively. General discussions and conclusions are provided in Chapter 6, along with future perspectives, to guide continued research in the field