It is of essential importance to design an electrocatalyst with excellent performance for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting. It is of essential importance to design an electrocatalyst with excellent performance for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting. Co 3 O 4 has been developed as a highly efficient OER electrocatalyst, but it has almost no activity for HER. In a previous study, it has been demonstrated that the formation of oxygen vacancies (V O ) in Co 3 O 4 can significantly enhance the OER activity. However, the stability of V O needs to be considered, especially under the highly oxidizing conditions of the OER process. It is a big challenge to stabilize the V O in Co 3 O 4 while reserving the excellent activity. Filling the oxygen vacancies with heteroatoms in the V O -rich Co 3 O 4 may be a smart strategy to stabilize the V O by compensating the coordination numbers and obtain an even surprising activity due to the modification of electronic properties by heteroatoms. Herein, we successfully transformed V O -rich Co 3 O 4 into an HER-OER electrocatalyst by filling the in situ formed V O in Co 3 O 4 with phosphorus (P-Co 3 O 4 ) by treating Co 3 O 4 with Ar plasma in the presence of a P precursor. The relatively lower coordination numbers in V O -Co 3 O 4 than those in pristine Co 3 O 4 were evidenced by X-ray adsorption spectroscopy, indicating that the oxygen vacancies were created after Ar plasma etching. On the other hand, the relatively higher coordination numbers in P-Co 3 O 4 than those in V O -Co 3 O 4 and nearly same coordination number as that in pristine Co 3 O 4 strongly suggest the efficient filling of P in the vacancies by treatment with Ar plasma in the presence of a P precursor. The Co–O bonds in Co 3 O 4 consist of octahedral Co 3+ (O h )–O and tetrahedral Co 2+ (T d )–O (Oh, octahedral coordination by six oxygen atoms and T d , tetrahedral coordination by four oxygen atoms). More Co 3+ (O h )–O are broken when oxygen vacancies are formed in V O -Co 3 O 4 , and more electrons enter the octahedral Co 3d orbital than those entering the tetrahedral Co 3d orbital. Then, with the filling of P in the vacancy site, electrons are transferred out of the Co 3d states, and more Co 2+ (T d ) than Co 3+ (O h ) are left in P-Co 3 O 4 . These results suggest that the favored catalytic ability of P-Co 3 O 4 is dominated by the Co 2+ (T d ) site. P-Co 3 O 4 shows superior electrocatalytic activities for HER and OER (among the best non-precious metal catalysts). Owing to its superior efficiency, P-Co 3 O 4 can directly catalyze overall water splitting with excellent performance. The theoretical calculations illustrated that the improved electrical conductivity and intermediate binding by P-filling contributed significantly to the enhanced HER and OER activity of Co 3 O 4 .
