2D layered double hydroxides and transition metal dichalcogenides for applications in the electrochemical production of renewable hydrogen

The effects of Climate Change are now clearly evident across the world, with increasing storms, precipitation, rising sea levels and draughts, with an ever increasing need to develop energy storage and conversion approaches and devices to alleviate these issues. Hydrogen has emerged as a clean fuel with no environmental issues and it can be coupled with renewable energy sources such as wind, solar and wave energy. The excess energy from renewables can be employed to power electrolysis cells giving rise to green or renewable hydrogen. This approach is attracting considerable attention as it has the potential not only to provide a very clean energy source, but it also enables more efficient use of renewable energy resources. One of the limiting factors in achieving high volumes of pure hydrogen is the electrocatalytic materials used for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In this review, we cover the progress made largely within the last six years in the development of two materials that satisfy the conditions of being cost-effective, earth-abundant, simple to form with little or no environmental concerns. The two materials are transition metal dichalcogenides (TMDs) and layered double hydroxides (LDHs), with the latter often described as clays. Following a short overview of the main synthetic methods used to form the TMDs and LDHs, and a short introduction to the HER and OER, the performance of different LDHs, TMDs and their composites in the electrochemical splitting of water, and as bifunctional electrodes is discussed. Finally, the ability of these materials to act as electrocatalysts in seawater electrolysis and to serve as electrocatalytic materials for the oxidation of alternative half-reactions to the sluggish OER is reviewed. Although more studies are required, it is clear that these earth-abundant materials are promising electrocatalysts for the generation of renewable hydrogen.