Phosphorus Filling Sulfur Vacancies in Co₉S₈ Optimizes the Electronic Structure and Boosts the Overall Water Splitting in Alkaline Media

The defective cobalt sulfide (Co9S8-x) core-shell hollow spheres are modified by homogeneously distributed phosphorus (P) atoms. P atoms can not only effectively stabilize sulfur vacancies but also form the Co-P coordination with Co active site in the sulfur vacancies, which can modulate the electronic structure of the active site, enabling P-Co9S8 to exhibit much superior oxygen evolution reaction (OER) activity. Furthermore, Co9S8 has been developed as a highly efficient OER electrocatalyst but has poor activity for hydrogen evolution reaction (HER), thus limiting the overall performance of water splitting. Excitedly, the filling of P in the vacancies can efficiently improve the HER activity for Co9S8-x core-shell hollow spheres. Thus, we successfully transformed Co9S8-x into a HER-OER electrocatalyst by filling the sulfur vacancies in Co9S8-x with P. P-Co9S8 affords a current density of 10 mA cm-2 at an overpotential of 271 and 157 mV for OER and HER, respectively, and thus exhibits a high water-splitting activity with a cell voltage of 1.71 V at 10 mA cm-2 and excellent stability with maintaining 93.6% of the original current output after 10 h. Theoretical calculations indicate that P filling optimizes the electronic structure and intermediate binding of Co9S8, significantly enhancing its OER and HER activities.