Bimetallic Co-Mo sulfide/carbon composites derived from polyoxometalate encapsulated polydopamine-decorated ZIF nanocubes for efficient hydrogen and oxygen evolution

The increased call on carbon neutrality by 2050 makes it compelling to develop emission-free alternative energy sources. Green hydrogen produced from water electrolyzer by renewable electricity is of great importance, and the development of efficient transition-metal-based materials for electrolysis hydrogen production is highly desirable. In this report, a new approach to produce electrocatalysts that are highly active for hydrogen and oxygen evolution reactions (HER and OER), defect-rich ultra-fine bimetallic Co-Mo sulfides on carbon composites from polyoxometalates@ZIF-67@polydopamine nanocubes via carbonization/sulfurization have been successfully developed. The coating of polydopamine (PDA) on the surface of the acid-sensitive ZIF-67 cubes can prevent the over-dissociation of ZIF-67 caused by encapsulated phosphomolybdic acid (PMA) etching through PDA chelating with the PMA molecules. Meanwhile, the partially dissociated Co2+ from the ZIF-67 can be captured by the coated PDA via chelation, resulting in more evenly dispersed active sites throughout the heterogeneous composite after pyrolysis. The optimized bimetallic composite CoMoS-600 exhibits a prominent improvement in HER (with overpotential of -0.235 V vs. RHE at current density of 10 mA cm-2) and OER performance (with overpotential of 0.350 V vs. RHE at current density of 10 mA cm-2), due to the synergistic effect of ultra-fine defect-rich Co-Mo-S nanoparticle active sites and N, S-doped porous carbons in the composites. Moreover, this synthesis approach can be readily expanded to other acidic polyoxometalates to produce HER and OER active bimetallic Co-W sulfide/carbon composites by replacing PMA with phosphotungstic acid. This new synthesis strategy to modify acid-sensitive ZIFs with selected compounds offers an alternative approach to develop novel transition metal sulfide/carbon composites for various applications.