The evolution of transition metal hydroxide electrocatalysts in alkaline media: A three-generation roadmap from 2D nanosheets to metal hydroxide organic frameworks (MHOFs)

The transition from fossil-fuel-based energy systems to sustainable hydrogen production via alkaline water electrolysis remains constrained by the sluggish kinetics of the anodic oxygen evolution reaction (OER). To mitigate the high overpotentials inherent to this four-electron transfer process, the development of high-performance electrocatalysts remains essential. Transition metal hydroxides (TMHs) have recently emerged as a promising class of catalysts due to their electronic tunability and greater synthetic flexibility compared to traditional oxides. However, bulk TMHs have low conductivity and limited stability, hindering their practical application in alkaline water electrolysis. To overcome these problems, 2D LDHs, high-entropy materials and heterostructure are utilised to enhance the structural stability and maximise active sites. Furthermore, the development of metal hydroxide organic frameworks (MHOFs), a subset of metal organic frameworks (MOFs) that integrates high-porosity organic architectures with the intrinsic catalytic activity of metal hydroxide layers, demonstrates higher degree of flexibility in electrocatalyst design to further improve their anodic OER performance. This review categorises TMH research into three generations: the first generation (simple transition metal hydroxides), the second generation (LDHs, high entropy LDHs, heterostructures), and the third generation (MHOFs). This classification illustrates the clear structural progression of TMH-based materials and serves as a roadmap for the ongoing development of advanced OER electrocatalyst for TMHs. Finally, this review outlines recent advances within each generation of TMHs, providing a comprehensive discussion of the latest MHOF developments and their future outlooks.