Electrochemical Anion-Exchanged synthesis of porous Ni/Co hydroxide nanosheets for Ultrahigh-Capacitance supercapacitors.

The commonly reported calcination strategy usually requires high temperature to crack the metal-organic frameworks (MOFs) particles, which often lead to uncontrollable growth of nanomaterials. Here, for the first time, we utilize an electrochemical anion-exchanged method to control the hydrolysis of MOFs and synthesize porous Ni/Co hydroxide nanosheets. After the electrochemical anion-exchange, the organic ligands of MOFs nanosheets can be recycled and reused. Applying an electric field to the MOFs bulk in alkaline solution can accelerate the nucleation rate of hydroxide and change the migration behavior of charged ions/molecules, which can tailor the microstructure of derivatives and improve deep charge and discharge capability of the electrodes. As a result, the hydroxide with the optimized Ni:Co molar ratio of 7:3 and electric-field application time of 1000 cycles [NiCo(OH)-1000c] provides much better electrochemical properties than the materials synthesized without electric-field assistance: a high specific capacitance of 2115C g (4230F g). A hybrid supercapacitor with the NiCo(OH)-1000c electrode shows a high energy density of 74.7 Wh kg, an improved power density (5,990.6 W kg), and an excellent cyclic stability (8,000 cycles). This study not only provides a novel strategy for the preparation of low-cost, deep-discharge electrodes for supercapacitors, but also proposes an unconventional method for mild synthesizing MOFs materials into porous nanoscale derivatives with tailored micromorphology.