From Low- to High-Crystallinity Bimetal–Organic Framework Nanosheet with Highly Exposed Boundaries: An Efficient and Stable Electrocatalyst for Oxygen Evolution Reaction

One critical problem of metal–organic frameworks (MOFs) limiting their applications lies in the inherent instability of the frameworks constructed by the metals ions and organic ligands, which can be destroyed in water or many other solvent environments. It is one of the great disadvantages for the existing MOFs; while here, we first put forward to utilize this property to prepare hierarchically structured two-dimensional (2D) MOFs based on as-developed two-step synthesis including ultrasound-assisted synthesis of 2D MOFs followed with a solvothermal treatment, in which the second-step treatment can remove unstable MOF domains and in situ create continuous mesopores on the 2D MOFs, which generates high crystalline and stable MOFs with abundant boundaries. As exampled with CoFe-MOFs, the as-obtained MOF from the two-step method exhibits hierarchical porosity interpenetrating on the ultrathin crystalline nanosheets (1.3 nm), and more importantly, more active sites were generated, as much of the metal ions transform from M–OOC into M–O/M–OH after the second solvothermal step. By comparison, the MOF made by only ultrasound has no such pores, while the MOF made from direct solvothermal method displays a much larger thickness. The resulting hierarchical 2D CoFe-MOF yields a low overpotential of 277 mV at 10 mA·cm–2, and Tafel slope of 31 mV·dec–1 with respect to the MOFs made from only ultrasonic (300 mV, 40 mV dec–1) and solvothermal method (310 mV, 57 mV dec–1). Apart from the more exposed active sites, the arising of the hierarchical pores can significantly accelerate the charge transport in oxygen evolution reaction based on the mass transport simulation. Additionally, the synthesis method is facile and general, which helps to synthesize a large variety of 2D MOFs desired for hierarchical 2D morphologies and stable structural properties.