Correlation Between Changes in Population of d-orbitals and Electrocatalytic Activity of Complex Transition Metal Oxides

Abstract

The development of advanced catalysts for oxygen electrocatalysis using complex metal oxides presents new opportunities to enhance the efficiency of energy conversion systems, including fuel cells, electrolysers, and metal-air batteries. Among these materials, perovskite and manganese-based oxides, have emerged as promising alternatives to noble metal catalysts due to their earth abundance, cost-effectiveness, and tunable electronic structure. This thesis explores the relationship between the electronic structure associated with the density of d-states in the active sites of complex metal oxides and their electrochemical responses under operational conditions. Hence, we gain insights into the fundamental factors influencing the catalytic performance in oxygen electrocatalysis. The first study investigates the activation of manganese sites in α-MnO₂ nanostructures through nickel substitution to enhance catalytic activity. The hydrothermal synthesis method was used to incorporate Ni into the α-MnO₂ lattice, leading to modifications in the local electronic structure and an increased density of active Mn sites. Electrochemical studies revealed that Ni doping up to 5% significantly impact in the pseudocapacitive response and improved electron transfer kinetics, thereby improve the catalytic performance for the oxygen reduction reaction (ORR). However, higher Ni doping lead to the formation of segregated Ni phases, which decreased ORR performance, while increasing oxygen evolution reaction (OER) activity.

Inspired by the role of the density of Mn 3d state in α-MnO₂ for in enhancing the ORR activity, further effort was made towards perovskite materials based on Mn as the B-site for ORR. The second study examines the AMnO₃ perovskites (A = La, Pr, Nd, Lu, and Y) as catalysts for the ORR, focusing on how A-site cation coordination, ionic radii, and the active 3d electronic structure influence catalytic performance. The results revealed a direct correlation between ORR activity and increasing the density of the active Mn 3d states under operational condition within the energy range relevant to oxygen reduction. Among the investigated materials, NdMnO3 exhibited the highest ORR activity due to its optimised density of Mn 3d states, while LuMnO3 and YMnO showed poor performance. These results provide valuable insights into the relationship between the density of d-state and catalytic activity, offering valuable guidance for the rational design of ORR electrocatalysts. Building on this valuable understanding, perovskite oxides have taken significant interest as active electrocatalysts for OER.

The third study focuses on the La1−xNdxCoO3 perovskite series as electrocatalysts for OER. Structural characterisation revealed a transition from a rhombohedral to an orthorhombic structure at approximately 40% Nd substitution. The catalytic performance was observed to increase with Nd content, peaking at approximately 50% Nd substitution. This enhancement is attributed to the evolution of redox active cobalt sites, driven by changes in the density of Co 3d-states just prior to the OER region. These findings highlight the critical role of A-site composition in tuning the electronic structure of perovskites materials to optimise their OER performance.