**Abstract**

Transition metal nanoparticles and metal oxide have been used extensively for a wide range of applications in electrochemical reactions (HER, ORR, OER) and energy storage (supercapacitors batteries). To make less expensive, the use of transition metal at minimum metal contents without compromising the catalytic activity could be one way. Most of the catalytic process takes place on the surface and reaction dynamic can be manipulated by changing the particle size and shape. For a long time, single metal atom organometallic compounds have been used as a catalyst at the industrial level. However, problems with the homogeneous catalyst to recover back at the end of the process lead to development of heterogeneous single-atom catalysts with equal activity like a homogeneous catalyst. Cobalt single atom has received a tremendous interest of the scientific community due to its excellent catalytic activity and recyclability. Cobalt single-atom catalyst has shown better performance compared with sub-nanometer nanoparticles catalyst for ORR, OER, and HER. This chapter is conferring method of preparation of carbon-based single Co atoms heterogeneous catalyst, their application for ORR, OER, HER reactions, and mechanistic investigations through DFT calculations. The role of single Co metal atoms and anchoring using *N* or heteroatoms is discussed and compared.

**Keywords:** single metal atom heterogeneous catalyst, transition metal catalyst, homogeneous, heterogeneous, oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)
