Noble-Metal-Free Bifunctional Electrocatalysts for Overall Water Splitting in Alkaline Medium

The ever-growing energy crisis and environmental pollution motivated scientific researchers to develop a green energy economy. Hydrogen generation through electrochemical water splitting has shown immense potentiality to be an important energy conversion technology towards the said goal. The electrochemical water splitting consists of two half-cell reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The overall water splitting is sluggish as both of these half-cell reactions involve multiple proton and electron transfer steps. Theoretical as well as experimental results have revealed that the noble-metal-based electrocatalysts (such as Pt and RuO₂) possess superior electrocatalytic activity toward water splitting. However, these materials lack commercial-scale applicability due to their high price and lower abundance. Recently, numerous investigations have been conducted to develop cheap and efficient noble-metal-free electrocatalysts. Pure water is a poor conductor of electricity and thus an electrolyte is added to facilitate the overall water-splitting process. The efficiency and cost-effectiveness of water electrolysis are higher in an alkaline medium if all the technical aspects related to it are considered together. An electrocatalyst with sufficient bifunctional activity towards both HER and OER in the same operational condition simplifies the electrolyzer optimization process and consequently, higher efficiency in full water splitting is achieved. So, a cheap and efficient bifunctional noble-metal-free electrocatalyst for water splitting in an alkaline medium is highly desirable. Borides, carbides, pnictides, and chalcogenides of transition metals like Fe, Co, Ni, etc. show significant promise in that context. Few transition metal-based heterostructures and engineered electrocatalysts also exhibit desirable bifunctional activity towards overall water splitting. This chapter provides a brief idea about electrocatalytic water splitting and also summarizes some recent advancements in noble-metal-free bifunctional electrocatalyst development. Finally, the future prospects in the development of efficient and cheap bifunctional electrocatalysts for water splitting have been discussed.