Bias-induced surface reconstruction of a MOF-derived bimetallic (Co & V) oxide as an electrocatalyst for water oxidation

Abstract

The four-electron oxygen evolution reaction (OER) is the bottleneck in the overall water splitting process, which needs to be addressed for the better hydrogen economy of an electrolyzer. In this work, a bimetallic metal–organic framework with the formula [Co(C₄H₄N₂)(VO₃)₂] was annealed at 500, 600, and 700 °C in an argon atmosphere to yield CoV@500, CoV@600, and CoV@700, respectively. The high-temperature annealing leads to the generation of heterostructured nanoparticles of crystalline spinel oxide Co₂VO₄ and amorphous VO_x wrapped with a carbon and nitrogen matrix. CoV@600 exhibits superior OER performance to afford the state-of-the-art current density of 10 mA cm⁻² at a lower overpotential of 273 mV and a small Tafel slope of 54.32 mV dec⁻¹ that outperformed the benchmark electrocatalyst RuO₂ and other catalysts used here. The superior OER performance could be ascribed to the lower charge transfer resistance, presence of oxygen vacancy, and lattice defects. Interestingly, the long-term chronopotentiometric stability exhibits a lowering in the overpotential with time, and a detailed investigation of the catalyst after OER manifests the leaching of the amorphous vanadium oxide and oxidation of Co(II) to Co(III) induced by the potential bias. Leaching triggers the surface decoration of cobalt oxyhydroxide (CoOOH) with core spinel oxide (Co₂VO₄) as confirmed from XPS, TEM, FESEM, and PXRD techniques. Further, accelerated degradation studies and the LSV after the 1000^th cycle present a lowered overpotential of 241 mV at 10 mA cm⁻² current density. The synergistic effect between the active CoOOH surface and defect-rich core spinel oxide could be the origin of the enhanced OER performance. This work not only presents superior OER performance of the annealed product of the framework but also demonstrates a strategy to obtain in situ bias-induced surface modification and emphasizes the importance of the postmortem analysis of a catalyst after catalysis.