Modulating effect of urea/melamine on Co2+/Co3+ ratio of Co3O4 microplates for rapid hydrogen generation via NaBH4 hydrolysis

The oxidation state of an active metal is vital in the hydrolysis of NaBH4 for rapid hydrogen generation since the electron-rich and electron-deficient sites facilitate BH4− and H2O adsorption. Herein, we systematically design a tunable oxidation state of Co in Co3O4 by in situ N-doing using the combination of melamine (M) and urea (U) as nitrogen precursors during thermal treatment. The oxidation state of Co was tuned via electronic interaction between N and Co. This interaction was optimized by varying the amount of N doping and calcination temperature. The types of heterocyclic nitrogen, mainly pyridinic- and graphitic-N, and their carbon matrix content are critical in controlling the Co oxidation state. Various Co2+/Co3+ ratios showed a different catalytic performance, and the catalytic activity reached its peak at the highest ratio of 2.74 due to the synergetic effect of the maximum adsorption of BH4− and H2O by Co2+ and Co3+, respectively. Consequently, an excellent hydrogen generation rate (2042 mL g−1 min−1) was achieved with a low activation energy of 46.9 kJ mol−1 of M2.5U10Co3O4-400. Hence, N doping from a dual N source significantly enhanced the hydrogen performance compared to the reported cobalt-based catalyst. Furthermore, M2.5U10Co3O4-400 is highly recyclable, implying its stability and practical application.