Surface Co3+-rich engineering of Co(SxSe1–x)2 nanocrystals coated with ultrathin carbon layer for efficient OER/HER

•ZIF-derived Co(SxSe1−x)2@C core-shell nanoparticles with tunable composition were synthesized via a facile two-step approach.•The co-introduced sulfur and selenium induces a Co3+-rich surface and structural defect sites.•The resulting Co(SxSe1−x)2@C displays an advanced bifunctional activity of OER and HER.