To generate “green hydrogen,” electrochemical water splitting is regarded as the most promising method, which requires highly efficient bifunctional electrocatalysts to accelerate the hydrogen and oxygen evolution reactions (HER and OER). Herein, a “d‐electron complementation” principle to develop such candidates is proposed. By taking advantage of the filling characteristics of 3d orbitals, vanadium‐cobalt phosphide (V‐CoP) is synthesized. According to X‐ray absorption fine structure and X‐ray photoelectron spectroscopy investigation, V can disorder the coordination environment of Co, leading to increased/decreased charge density in Co(P)/Co(O). As a result, V‐CoP shows impressive bifunctional activity, which only needs overpotentials of 46 and 267 mV to deliver 10 mA cm−2 for the HER and OER, respectively. Density functional theory calculations reveal that the accelerated H2O dissociation process on the VOx/CoOx/CoP surface and H conversion kinetics on the V‐neighboring CoCo bridge site result in promoted HER activity. Furthermore, the up‐shifted d‐band center of V‐CoOOH endows the V‐neighboring Co site with a lowered energy barrier for the OER. This work serves as a proof‐of‐concept for the application of the “d‐electron complementation” principle, which can be a generalized guideline to design and develop novel highly efficient catalysts.

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A new pH-activatable ratiometric fluorescent probe (RhMP) has been developed based on FRET. This probe displayed good selectivity, and excellent reversibility. In addition, RhMP has low cytotoxicity and has been successfully applied in HeLa cells.