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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. Co0.37S0.38P0.02 NSs only need 257 mV to reach a current density of 10 mA cm−2. Meanwhile, the Tafel slope of Co0.37S0.38P0.02 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 Co0.37S0.38P0.02 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).

A simple one-pot approach to the in situ construction of ultrathin shells is developed to create a new highly efficient and durable Pt-based electrocatalyst for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Herein, the synthesized ultrathin PdPtAg-shells are located on twinned PdAg nanocrystals and deposited onto graphene (G) (denoted by PdAg@PdPtAg/rGO). The PdAg@PdPtAg/rGO nanocrystal shows a low overpotential of 22 mV at 10 mA cm−2 and excellent electrochemical stability with 99.5% retention over either 12 h or 30 000 cycles of HER. Meanwhile, its ORR activities in acidic and alkaline media are very high and better than that of commercially available Pt/C. Both the experimental and calculated results suggest that the presence of an ultrathin PdPtAg alloy shell synthesized on a PdAg twinned support is the main reason for the greatly enhanced activities. Moreover, the twinned PdAg nanocrystal provides a number of twinned grain boundaries and defect sites (such as step, edge and kink sites). This typical structure represents a new strategy for synthesizing high-performance Pt-based catalysts, promoting their practical applications in energy conversion and storage devices.