Single/double atom catalysts for electrochemical energy conversion and storage applications
Increasing energy demands along with environmental crises have motivated the extensive investigation of alternative high efficiency energy conversion and storage devices such as metal-air batteries, fuel cells and water electrolysers. Oxygen reduction reaction (ORR) is a key reaction in fuel cells and metal-air batteries, etc., whereas oxygen evolution reaction (OER) is a key reaction in metal-air batteries and water electrolysers. For the easy facilitation of the aforementioned chemical reactions, the efficient electrocatalysts are required. Therefore, the rational design of the highly efficient, cost effective and environmentally benign catalysts is of utmost importance. To date, noble metal Pt based catalysts have been identified as benchmark catalyst for ORR, whereas Ir and Ru based oxides are the benchmark electrocatalysts for OER. However, due to the scarcity and high price of Pt, Ir and Ru, considerable efforts have been made for non-precious stable and efficient electrocatalysts. Recently, transition metal based single atom catalysts (M-N-C) have evolved as efficient candidates for ORR because of its well-defined active sites with very high atom utilization efficiency. We have designed Co-N-C single atom catalysts (SACs) for ORR as the unique electronic and geometric structures of Co-N4 moieties are responsible for high intrinsic activity. We have extended our study further to non-3D transition metal i.e. W based SACs and elucidated its high intrinsic activity and stability due to the WN2C2 active moieties. Atempt has also been made to synthesize N coordinated Mg atoms which have optimal bonding strength with intermediate oxygen species for efficient ORR. Generally, SACs exhibit well defined active sites with very high atom utilization efficiency but selective mainly towards ORR. To improve the functionality of a catalyst, we have designed the carbon-nitrogen-coordinated Fe-Mo double atom catalyst with high atom utilization efficiency which shows remarkable bifunctional ORR and OER. The importance of electronic modulation around Fe-Mo bimetallic moieties has been elucidated which is responsible for improved activity and stability for ORR and OER. We believe that the present work can provide basic understanding on the designing of efficient single and double atom catalysts for various electrochemical energy storage and conversion devices.