Anion-mediated transition metal electrocatalysts for efficient water electrolysis: Recent advances and future perspectives

The storage of intermittent energies, such as wind and solar energies, in the form of hydrogen gas through electrochemical water splitting, is a fascinating strategy. Transition metal composites have emerged as exceptional electrocatalysts for water splitting; however, their practical implementation is hindered by their low conversion efficiency and poor long-term stability. Tuning the electronic structure of transition metal-based electrocatalysts by introducing additional anions, which possess different electronegativities and sizes as compared to the parent anion, is a rational strategy for enhancing the electrochemical performance. In this review, we attempt to review the recent progress on anion-mediated multi-anion transition metal electrocatalysts for the hydrogen evolution reaction, oxygen evolution reaction, and overall water-splitting process. A brief overview of anion-containing transition metal-based electrocatalysts is presented, followed by recent advance surveys in the design of multi-anion-doped transition metal electrocatalysts for high electrochemical performances. The rationale behind the utilization of anion regulation to tune the electrocatalyst properties is described by combined theoretical and experimental approaches. Finally, we discuss the challenges to be addressed and the steps to be taken toward further advancing this research area to achieve affordable carbon-free hydrogen generation in the future.