Controlled\nHydrolysis of Metal–Organic Frameworks: Hierarchical Ni/Co-Layered\nDouble Hydroxide Microspheres for High-Performance Supercapacitors

Abstract

Pseudomorphic\nconversion of metal–organic frameworks (MOFs) enables the fabrication\nof nanomaterials with well-defined porosities and morphologies for\nenhanced performances. However, the commonly reported calcination\nstrategy usually requires high temperature to pyrolyze MOF particles\nand often results in uncontrolled growth of nanomaterials. Herein,\nwe report the controlled alkaline hydrolysis of MOFs to produce layered\ndouble hydroxide (LDH) while maintaining the porosity and morphology\nof MOF particles. The preformed trinuclear M₃(μ₃-OH) (M = Ni²⁺ and Co²⁺) clusters in\nMOFs were demonstrated to be critical for the pseudomorphic transformation\nprocess. An isotopic tracing experiment revealed that the ¹⁸O-labeled M₃(μ₃-¹⁸OH) participated\nin the structural assembly of LDH, which avoided the leaching of metal\ncations and the subsequent uncontrolled growth of hydroxides. The\nresulting LDHs maintain the spherical morphology of MOF templates\nand possess a hierarchical porous structure with high surface area\n(BET surface area up to 201 m²·g^–1), which is suitable for supercapacitor applications. As supercapacitor\nelectrodes, the optimized LDH with the Ni:Co molar ratio of 7:3 shows\na high specific capacitance (1652 F·g^–1 at\n1 A·g^–1) and decent cycling performance, retaining\nalmost 100% after 2000 cycles. Furthermore, the hydrolysis method\nallows the recycling of organic ligands and large-scale synthesis\nof LDH materials.