A comprehensive review on advances in polyoxometalate based materials for electrochemical water splitting

Recent years have witnessed a growing global demand for green, sustainable, and reliable energy sources in response to the effects of climate change, pollution, and the energy crisis. Consequently, there has been a significant focus on utilizing water, the most abundant and accessible chemical compound, for renewable energy generation. Electrocatalysis has a pivotal role in the area of green energy production, serving as a fundamental component for sustainable and exceptionally efficient techniques. To achieve this, the design and preparation of practical, functional groups and host/guest compatible materials that are stable, reusable, noble-metal-free, and possess structural and functional diversities have been of paramount importance in electrochemical water splitting (EWS) systems. Polyoxometalates (POMs), have emerged as promising electrocatalysts in the past two decades, offering abundant opportunities for achieving the aforementioned goals. POMs exhibit exceptional qualities such as redox stability, multiredox capability, tunable intrinsic and extrinsic properties (e.g., structure design, surface engineering, and interface engineering), as well as desirable ionic conductivity. However, the practical applications of POMs face substantial challenges, including issues like leaching, agglomeration, and low stability. The confinement of POMs and controlling their composition has proven to be an effective strategy for tackling the aforementioned issues, garnering significant attention in recent years. This review focuses on recent progress in the advancement and application of POM-based materials. It provides a thorough exploration of diverse host/substrate materials, comprising carbon nanotubes, graphdiyne, graphene, carbon nitrides, conductive polymers, MOFs, COFs, CTFs, LDHs, polymers, metal carbide and MXenes, which categorize into tubular, layered, and porous structures. The synthesis strategies of these materials are examined, elucidating their intrinsic and extrinsic catalytic activities. Notably, the review highlights recent research advancements in POM-based compounds and their mechanisms, particularly in EWS applications such as overall water electrolysis, overall tandem water electrolysis, anodic hybrid systems, cathodic hybrid systems, and decoupled hybrid