Si-Based Water Oxidation Photoanodes Conjugated with Earth-Abundant Transition Metal-Based Catalysts

The development of a carbon-free hydrogen production method by photoelectrochemical water splitting is a promising pathway to deal with the increased energy demands and deleterious environmental issues derived from the usage of fossil fuels. Silicon, which is the second most earth-abundant element and a small band-gap material, is an applicable candidate for an efficient solar water splitting photoelectrode. However, the stability of Si-based photoelectrode hampers efficient water splitting, especially for the Si-based photoanodes because of its thermodynamic instability and etching of silicon surface. Until now, much research has been conducted to deal with the challenges of using Si to fabricate efficient and stable photoanodes. Over the decades, to expedite the oxygen evolution reaction, a complicated 4-electron transfer process and requires large overpotential, cheap and earth-abundant transition metal-based electrocatalysts have been investigated. In this Review, we briefly introduce the photoelectrochemistry and important parameters for evaluating the performance of the Si photoanodes. We summarize transition metal-based catalysts, focusing on Ni-, Co-, and Fe-based oxygen evolving catalysts. Then, we present various strategies to overcome the challenges of silicon by combining the advantages of transition metal-based oxygen evolving catalysts, which are cost-effective, stable, and highly active for oxygen evolution reaction. Finally, to realize spontaneous water splitting, we introduce Si-based tandem cells combined with transition metal-based materials.