Filling the oxygen vacancies in Co3O4 with phosphorus: an ultra-efficient electrocatalyst for overall water splitting

Abstract

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. Co₃O₄ 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 Co₃O₄ 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 Co₃O₄ while reserving the excellent activity. Filling the oxygen vacancies with heteroatoms in the V_O-rich Co₃O₄ 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 Co₃O₄ into an HER-OER electrocatalyst by filling the in situ formed V_O in Co₃O₄ with phosphorus (P-Co₃O₄) by treating Co₃O₄ with Ar plasma in the presence of a P precursor. The relatively lower coordination numbers in V_O-Co₃O₄ than those in pristine Co₃O₄ 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-Co₃O₄ than those in V_O-Co₃O₄ and nearly same coordination number as that in pristine Co₃O₄ 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 Co₃O₄ consist of octahedral Co³⁺(O_h)–O and tetrahedral Co²⁺(T_d)–O (Oh, octahedral coordination by six oxygen atoms and T_d, tetrahedral coordination by four oxygen atoms). More Co³⁺(O_h)–O are broken when oxygen vacancies are formed in V_O-Co₃O₄, 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 Co²⁺(T_d) than Co³⁺(O_h) are left in P-Co₃O₄. These results suggest that the favored catalytic ability of P-Co₃O₄ is dominated by the Co²⁺(T_d) site. P-Co₃O₄ shows superior electrocatalytic activities for HER and OER (among the best non-precious metal catalysts). Owing to its superior efficiency, P-Co₃O₄ 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 Co₃O₄.