The Nickel Age in Electrocatalytic Anodic Reactions

In the last three decades, the superior performance of Ni-based electrocatalysts over other transition-metal (TM)-derived materials has been established for different anodic reactions, such as the oxygen evolution reaction (OER), organic oxidation, biomass valorization, plastic reforming, small molecule activation, etc. In the alkaline electrolyte, Ni-based catalysts are transformed into different active crystal phases of Ni-(O)-OH (β, γ, or their mixture) with a wide variation in the local geometry of Ni (octahedral, square pyramidal, square planar, etc.), coordination environment, oxidation state (+2, + 3, and +4), spin state, and Ni-O bond length. The dynamic redox behavior of Ni ions in different reactions has been found to be crucial for OER and other anodic oxidation processes. Therefore, designing efficient Ni-based electrocatalysts for anodic oxidation reactions requires a comprehensive understanding of their structure-activity relationships and underlying reaction mechanisms. In this perspective, the mechanistic aspects of different anodic reactions with Ni-based catalysts have been described with the help of in situ and ex situ spectroscopic, microscopic, and analytical techniques. Further, theoretical calculations have been introduced for a clearer understanding of the structure-activity relationship.