**4.2 Chemistry and electrochemistry of Co3O4**

The next superior electrocatalyst with competitive activity for bifunctional reaction is cobalt oxide (Co3O4) and is an extensive studied material especially for OER due to its excellent redox behavior, inexpensiveness, high abundance and great stability in alkaline medium. It is a spinel oxide, where oxygen atoms are arranged in a cubic close packed (CCP) matrix composed of two Co3+ and one Co2+ cations located in the octahedral (Oh) and tetrahedral (Td) lattice sites, respectively [34, 35]. The catalytic performance of spinel Co3O4 is deeply rooted in two factors such as exposed crystal facets and population of Td Co2+ and Oh Co3+ on it. The (110) facet of Co3O4 exhibits enhanced activity compared to other facets due to the existence of substantially higher density of the dangling bond and catalytically active Co3+ sites [36]. But the higher surface energy and existence of higher density of dangling bonds of this facet results in less possibility of the formation of this facet. The bifunctional activity of Co3O4 is oxidation state dependent; the higher occurrence of Co3+ in the octahedral environment is responsible for the superior performance of OER, whereas Co2+ in the tetrahedral environment is capable of releasing electrons under applied potential, which results in the formation of CoOOH, μ-OO peroxide intermediates over the surface of Co3O4, which is capable of acting as an active site in ORR. Although there are many efforts on Co3O4, the intrinsically poor electrical conductivity and relatively smaller surface area of the material still remain inadequate, which significantly drag the overall activity.
