Considering the depletion of hydrocarbon reservoirs and adverse effects of their combustion, providing a renewable and environmentally friendly resource of energy is necessary for human societies. Photoelectrochemical (PEC) and electrochemical (EC) water splitting into O₂ and H₂ at the surface of a proper material (photoactive semiconductor or electrocatalysts) is a promising solution to produce solar fuel as a green and sustainable source of energy. In both these approaches, each half-reaction, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), must be optimized to achieve the highest efficiency.
The challenging step is the OER since its Gibbs free energy is 1.23 eV per electron while kinetic barriers (known as overpotential) may increase this amount up to ~2 eV. In order to reduce the overpotential, a proper electrocatalyst should be applied to facilitate surface hole transfer and raise reaction kinetics. In this context, materials employed as cocatalysts in PEC systems must be chosen among earth abundant and chemically stable compounds. According to extensive researches in this area, iridium and ruthenium-based co-catalysts are found to exhibit the lowest overpotential but not applicable due to their high cost and rarity. As proposed by well-known volcano plot, the most attracting candidates for oxygen evolution reaction are first row transition metals such as Co and Ni in the form of oxide/hydroxide nanostructures which are abundant with variety of low cost preparation methods. Recently, efficient electrocatalysts for HER are also proposed based on 2D MoS₂ nanosheets. In the following review papers, basic information about parameters as well as recent trends can be found.

1- Thorarinsdottir et l. Self-healing oxygen evolution catalysts, Nature Communications, 2022.
2- Roger et al. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting, 2017.
3- Kanan et al. Cobalt–phosphate oxygen-evolving compound, 2009.
4- Chen et al. Recent Progress on Nickel‐Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction, 2018.
5- Lyu et al. Noble‐Metal‐Free Electrocatalysts for Oxygen Evolution, 2018.
6- Guo et al. Phosphate‐Based Electrocatalysts for Water Splitting: Recent Progress, 2018.
7- Zhao et al. Heterostructures for Electrochemical Hydrogen Evolution Reaction: A Review, 2018.
8- Wang et al. Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting, 2018.