Alkaline oxygen reduction/evolution reaction electrocatalysis: A critical review focus on orbital structure, non-noble metal catalysts, and descriptors

The oxygen reduction reaction (ORR) and oxygen revolution reaction (OER) show slow reaction kinetics because of the linear relationship that needs to be solved urgently. Transition metal (TM) based ORR/OER catalysts have satisfactory performance and stability, and their orbital structure is of great importance in adjusting the adsorption energy of intermediates. The 3d orbital of TM-based ORR/OER catalysts and the 2p orbital structure of oxygen-containing reactants and intermediates were analyzed in depth, and the results indicated that there is an optimal value or perfect range in orbital structures for satisfactory ORR/OER electrocatalytic activity because the adsorption energy of the reactant needs stronger and the intermediates are weak. According to the above contents, it was explored that the metal-N4 4 site (M-N4) 4 ) with Fe-N-C catalyst as an example was more likely to react with O2 2 and its intermediates. Besides, the latest developments in bimetallic catalysts, high-entropy alloys, and single-atom catalysts were introduced. Moreover, various descriptors are also summarized since the descriptors can link the ORR performance with the catalytic behavior. The related covalent interactions controlling measures including heteroatom doping, defect engineering, stress-strain, potential regulation, and other component regulation methods to break the linear relationship are also written. Considering the larger research space of hydrogen bond (HB) in the breaking linear relationship, this paper described the regulation means of HB interaction, which provides guidance and reference for the construction of more active and tolerant TM-based ORR/OER catalysts in the future. This work is not only applicable to the ORR/OER non-noble metal catalysts, some theories are also applicable to metal-catalyzed other O-containing electrochemical reactions.