Sol–gel-derived nanostructured electrocatalysts for oxygen evolution reaction: a review

Abstract

In the search for alternative energy sources due to the depletion of fossil fuels, green hydrogen energy produced from the water splitting reaction is considered to be a promising candidate owing to its high energy density and zero carbon emission. This reaction involves two electrons for the production of hydrogen at the cathode and four electrons for the production of oxygen at the anode. Therefore, the oxygen evolution reaction (OER) is as crucial as the hydrogen evolution reaction (HER). However, due to the sluggish kinetics of OER and the low abundance and high cost of the state-of-the-art RuO₂ and IrO₂ catalysts (noble metals), it is necessary to develop alternative low-cost, highly abundant, reasonably active, and stable catalysts to realize their industrial application in the water splitting reaction. In this case, the sol–gel process involves wet chemical synthesis using low-cost transition metal precursors for the production of various nanostructures, specifically oxide materials. In recent decades, a plethora of sol–gel-synthesized materials including metal oxides, hydroxides, phosphides, sulfides, and their derivatives have demonstrated remarkable efficiency as heterostructure catalysts in promoting water splitting activities including oxygen evolution reactions. In this method, the precursor molecules are mainly dissolved in alcohol or water, forming a sol, and then converted to gel via various approaches, and eventually dried to obtain the resultant catalyst. Accordingly, anticipating continuous growth in the research on these materials due to their cost-effective, easy fabrication, and ability to realize industrial applications, a comprehensive review addressing the enhancement of sol–gel-synthesized composites becomes imperative. This review aims to provide a thorough analysis of the recent research advancements in the sol–gel approach and resulting materials for OER. Particularly, we focus on the recent developments, showcasing their potential applications and reasons why the prepared materials display enhanced catalytic properties. Furthermore, a detailed discussion of prospective avenues for future research is presented.