Introduction of surface defects and phase control engineering in the electrocatalytic system of overall water splitting has played a crucial role in significantly enhancing its electrocatalytic activity toward the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in water splitting, but the relationship between structure and electrocatalysis is still elusive. Herein, we report a solid-liquid method to induce surface reorganization (formation of a chalcogenide layer with rich chalcogenide vacancies) and phase transformation (Co₉S₈-to-Co₃S₄) simultaneously on cobalt chalcogenide. Featuring a uniform 2D morphology and the in situ formation of sulfur (S) vacancies, in a 0.1 M KOH solution, it exhibits a low overpotential of 288 mV vs. RHE at 10 mA cm^-2, a low Tafel slope of 43.4 mV dec^-1, and strong cycling stability (35 h), outperforming commercial RuO₂ and most reported OER electrocatalysts. In addition, we also investigate the OER activity of the Co-S-P electrode in 1.0 M KOH solutions. Co_0.37S_0.38P_0.02 NSs only need 257 mV to reach a current density of 10 mA cm^-2. Meanwhile, the Tafel slope of Co_0.37S_0.38P_0.02 NSs (44.0 mV dec^-1) is lower than those of other recently reported electrocatalysts. Also, it shows high HER electrocatalytic activity in alkaline and acidic solutions. Finally, the Co_0.37S_0.38P_0.02 electrode is used as a cathode and anode simultaneously for overall water splitting, which merely requires a cell voltage of 1.59 V at 10 mA cm^-2 with excellent stability (40 h).

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