Controlled Hydrolysis of Metal–Organic Frameworks: Hierarchical Ni/Co-Layered Double Hydroxide Microspheres for High-Performance Supercapacitors

Abstract

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