Urea-Assisted Dual-Shell Hollow Zn-MOF Microsphere-Derived Carbon for a Zinc Ion Capacitor

In the zinc ion capacitors (ZICs), Zn2+ storage is limited by the common carbon-based cathodes due to the sluggish kinetics of zinc ion diffusion. Porous carbon derived from metal–organic frameworks (MOFs) is a good candidate with the abundant micropores and mesopores. By using a one-step template-free solvothermal method, double-shell hollow Zn-MOF microspheres were directly synthesized, eliminating the traditional multistep activation process using KOH/HCl. During the carbonization procedure, urea has been used to provide an additional carbon source and activate the derived carbon, which is due to urea pyrolysis products, such as NH3, cyanuric acid, and ammelide. The derived porous carbon (U-MPC) exhibits a high specific surface area of 1695.66 m2 g–1. The ZIC device assembled with U-MPC as the cathode has a high specific capacitance of 199.0 F g–1 and a high energy density of 70.8 Wh kg–1 at 0.1 A g–1. The Zn//U-MPC ZICs demonstrate a good antiself-discharge performance, with a low self-discharge rate of only 3.1 mV h–1 over 300 h, and can maintain 59.7% of the initial capacitance. Moreover, the ZICs exhibit good cyclic stability, showing a slight decrease after 20,000 cycles at 5 A g–1, and the Coulombic efficiency is 90%.