A study on Ca-substituted LaCoO3 perovskite oxides for alkaline water and ethanol oxidation reactions

Abstract

The unusual natural disasters have prompted the need to develop ‘green hydrogen’ technology, which produces hydrogen by splitting water using electricity generated from renewable energy sources. In water electrolysis, the oxygen evolution reaction (OER) has a sluggish reaction rate and requires a large overpotential. Therefore, it is required to develop a high-performance catalyst that expedites OER. Replacing OER with ethanol oxidation reaction (EOR) that operates at a lower potential and can generate high-value acetate has received great attention. However, unlike OER, there is little research on translating the catalytic performance in terms of the electronic structure. Perovskite oxide is a promising electrocatalyst because it is a non-platinum-group metal catalyst and possesses structural stability, excellent catalytic activity, and compositional flexibility. Specifically, substituting an aliovalent element in the A-site can indirectly affect the B-site transition metals and oxygen ions, which are highly correlated with electrochemical behaviors. Here, the effect of Ca substitution in the A-site of LaCoO3 is highlighted. The series of La1-xCaxCoO3-δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 1) possessing both highly crystallinity and similar morphologies is successfully prepared. With an increase in Ca concentration, crystal structure symmetry enhances and holes are created on the oxygen ligand and Co 3d t2g orbitals. These modifications result in an evolution of electronic structure by increasing the overlap of Co 3d and O 2p orbitals and creating hole states in the band structure. This ultimately (1) rises density of states below the Fermi level, (2) activates lattice oxygen, (3) decreases charge-transfer energy, and (4) induces a semiconductor-to-metal transition. Consequently, both OER and EOR activity increases with Ca substitution with a facilitated charge transfer. This study aims to provide a guide for translating electrochemical catalysts based on their electronic structure, and it would bring new insights into the development of EOR catalysts.