Hydrogen is an environmentally friendly energy carrier. The production of hydrogen, as well as hydrogen usage. should be accomplished using environmentally sustainable methods to achieve its ultimate objective. Water electrolysis, which produces hydrogen by decomposing water with the use of power generated from renewable energy sources, has gained recognition for its efficacy and has been continuously developed. It exhibits an inherent issue that the anode reaction, oxygen evolution reaction (OER), has sluggish reaction kinetics and necessitates a large voltage. Consequently, there is a requirement for efficient catalysts that can facilitate OER. Meanwhile, replacing OER with a biomass upgrading reaction that requires less operating voltage and generates high-value products is receiving great attention. An example is ethanol oxidation reaction (EOR) which produces acetate by oxidizing ethanol. However, unlike OER, there is little research translating the catalytic performance to the electronic structure.

  Perovskite oxide is a promising, non-platinum-group metal catalyst due to its structural stability, excellent catalytic activity, and compositional and structural flexibility. Specifically, substituting an aliovalent element in the A-site can improve its performance by indirectly changing the states of B-site transition metals and oxygen ions, which actually affect the electrochemical activities. In OER field, several studies have been reported on substituting Sr²⁺ for La³⁺ in LaCoO₃, but there is a lack of research on substituting Ca²⁺. Specifically, there is no research probing fundamental material properties to unravel the exact effect of Ca substitution to catalytic performance. Furthermore, it has solely been applied to conventional water electrolysis reactions.

  Here, we highlight the effect of Ca substitution in the A-site of LaCoO₃ perovskite for OER and EOR in alkaline electrolytes. The perovskite series of La_1-xCa_xCoO_3-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, which enables an accurate study comparing the intrinsic substitution effect. Both OER and EOR activity gradually increases with Ca substitution due to a facilitated charge transfer. With an increase in Ca concentration, the crystal structure symmetry is enhanced, and holes are created on the oxygen ligand and Co 3d t_2g orbitals. These modifications increase the overlap of Co 3d and O 2p orbitals and create hole states, respectively. The DFT calculation result verifies that the changes ultimately lead to an electronic structure that is active in electrocatalysis. Beyond phenomena previously reported, further DFT calculation unravels that partial Co atoms in the perovskite oxide undergo a transition in their spin state; from an intermediate spin state to a high spin state with a down-spin orientation. We suggest, for the first time, that Ca substitution in LaCoO₃ triggers the activation of the spin state of local Co atoms, thereby enhancing the electrocatalytic activities. This study is believed to offer a framework for translating electrochemical catalysts based on their electronic structure and spin state. Furthermore, it has the potential to provide new insights into the design of EOR catalysts.