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 metal-supported 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 GO-modified ZIF transformed from typical leaf-like (at GO contents of less than 40 wt%) to elongated hexagonal-shape (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 leaf-like ZIF-Co-L@10%GO and hexagonal-shaped ZIF-Co-L@40%GO precursors were carbonized to Co-embedded N-doped-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-to-rGO 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.