A hexagonal 2D ZIF-Co-L variant: Unusual role of graphene oxide on the water-regulated morphology of ZIF hybrid and their derived Co@N-doped carbon electrocatalyst for hydrogen evolution reaction

Designing unique metal-organic framework (MOF) precursors is crucial for the development of efficient metalsupported carbon-based electrocatalysts for the hydrogen evolution reaction (HER). Herein, the atypical role of graphene oxide (GO) in the morphology of 2D Co-based leaf-like zeolitic imidazolate framework (ZIF-Co-L) is reported. We demonstrated that GO regulated water accessibility in the ZIF reaction medium. The shape of GOmodified ZIF transformed from typical leaf-like (at GO contents of less than 40 wt%) to elongated hexagonalshape (at GO contents of >= 40 wt%). This was attributed to the trapping of water molecules into the interlayers of GO. The water-deficient medium restricted crystal growth along the a direction of ZIF-Co-L and caused the formation of hexagonal shapes by distorting the water-driven hydrogen-bonding interactions between monodentate methylimidazole (mIm) in the ab direction and free mIm in the c direction. Furthermore, the leaflike ZIF-Co-L@10%GO and hexagonal-shaped ZIF-Co-L@40%GO precursors were carbonized to Co-embedded Ndoped-carbon/reduced GO (rGO) electrocatalysts, denoted as Co-NC@10rGO-leaf and Co-NC@40rGO-Hex, respectively. The Co-NC@10rGO-leaf catalyst (-3% rGO) presented fast kinetics and high durability (-10 h) in KOH electrolyte. Moreover, the overpotential of Co-NC@10rGO-leaf at 10 mA cm-2 in the KOH electrolyte (220 mV) was lower than that of Co-NC@40rGO-Hex. (230 mV). This was ascribed to the lower ratio of Co-torGO contents in the Co-NC@40rGO-Hex. (-13% rGO) inhibiting its HER activity despite their excellent shape benefits of the ZIF precursor. The superior performance of Co-NC@rGO electrocatalysts was attributed to the unique shape features of ZIF precursor; abundant active sites; and synergistic effect of Co nanoparticles, N doping, and conducting carbon. The proposed synthesis approach offers promising prospects for the development of transition-metal-supported carbon-based catalysts from shape-variant MOF precursors for efficient electrochemical water splitting.

Automated summary

Unique metal-organic framework (MOF) precursors are crucial for efficient metal-supported carbon-based electrocatalysts for the hydrogen evolution reaction (HER). In this study, graphene oxide (GO) was found to regulate water accessibility in the reaction medium, leading to changes in the morphology of ZIF-Co-L and ultimately affecting the electrocatalytic performance.