An account of the strategies to enhance the water splitting efficiency of noble-metal-free electrocatalysts

The electrolysis of water for hydrogen generation has shown immense promise as an energy conversion technology for the green energy economy. Two concurrently occurring electrochemical reactions in water electrolysis (hydrogen and oxygen evolution reactions) are sluggish in nature and therefore the employment of electrocatalysts is highly essential. Noble-metal-based electrocatalysts (Pt, RuO2, IrO2, etc.) have shown superior activity towards these reactions. However, their lower natural abundance and inferior stability make the cost to performance ratio of water electrolysis too high. Thus, huge amount of research efforts are being carried out to develop electrocatalysts consisting of earth abundant elements (transition metals, carbon etc.) as the replacement of these noble-metal-based materials. Transition metal compounds, carbonaceous and hybrid materials have shown promise as efficient electrocatalysts but there is still huge gap between the activities of these materials and the noble-metal-based electrocatalysts. Several strategies like morphology modulation, elemental doping, defect engineering etc. are being deployed to enhance the activity of these noble-metal-free electrocatalysts. This review summarizes these strategies and thoroughly discusses the reason behind the changes in activity of the electrocatalysts owing to these modifications. Finally, the remaining research gaps and future prospects in this field are also discussed in detail.