A “MOFs plus MOFs” strategy toward Co–Mo2N tubes for efficient electrocatalytic overall water splitting

Abstract

The rational combination of early-transition metal nitrides (ETMNs) with late-transition metals (LTMs) is a promising method for the design of highly efficient catalysts. Herein, we report the design of Co–Mo₂N tubes based on a “MOFs plus MOFs” strategy for efficient electrocatalytic water splitting in 1 M KOH. The 1-D Mo-MOFs (Mo source) were prepared in large quantities by the reaction of imidazole with MoO₃. ZIF-67 (Zeolitic Imidazolate Framework-67) (Co source) was then grown on the 1-D Mo-MOFs (Mo-based metal–organic frameworks) to obtain ZIF-67/Mo-MOFs by a solution-phase assembly method. The effective combination is due to the similarity of the ligands in the two MOFs. After nitriding, the ZIF-67 was converted into Co metal, and Mo-MOFs were converted into hollow Mo₂N, thus giving a hollow Co–Mo₂N hybrid with a Co outer layer on the Mo₂N inner core. The ratio of Co and Mo in Co–Mo₂N can be easily tuned to give optimized catalysts. The tests indicated that the combination of Co and Mo₂N can significantly boost the HER performance of materials. At the current density of 10 mA cm⁻², the overpotential (η₁₀) on Co–Mo₂N was only 76 mV, which is much lower than 296 mV for Mo₂N derived from Mo-MOFs and 180 mV for Co–carbon from ZIF-67. The hybrid has also shown improved OER activity in comparison with Mo₂N and Co; therefore, the Co–Mo₂N can be used as both anode and cathode for an overall water-splitting cell with η₁₀ of 1.576 V. The improved HER activity can be ascribed to the intimate contact between Co and Mo₂N for optimized adsorption on reagents, the tube-like structure for easy mass transfer and the good conductivity of Mo₂N, which is favourable for transmitting charge.