Synthesis of N/P co-doped monolithic hierarchical porous carbon for zinc-ion hybrid capacitors with boosted energy density in ZnSO4/ZnI2 redox electrolyte

Abstract

Zinc-ion hybrid capacitors (ZHCs) are of great potential as a new type of energy storage device, yet how to increase the energy density remains a challenge, which is greatly restricted by cathode materials and electrolytes. Herein, we report on the synthesis of N/P co-doped monolithic hierarchical porous carbon (NPMC) materials from fluorene molecules via template-guided growth coupled with in situ activation strategy. The as-prepared NPMCs feature 3D carbonaceous framework with good electron conduction, well-developed macro/meso/micropores for ion transfer and tunable N/P species with additional pseudocapacity. Meanwhile, the introduction of ZnI2 into aqueous ZnSO4 electrolyte helps to significantly increase the capacity of ZHC due to the redox reactions. It has been demonstrated that the Zn//ZnSO4/ZnI2//NPMC ZHC with NPMC as cathode material delivers an ultrahigh energy density of 324.8 Wh kg−1, which is 3.5 times higher than Zn//ZnSO4//NPMC ZHC. The mechanisms for this superb performance have been explored in terms of the Zn2+ cations deposition/stripping, SO42−/I− anions adsorption/desorption, Zn4SO4(OH)6·0.5H2O precipitation/dissolution and the redox reactions (3I−/I3−, 2I−/I2). This novel work may pave a way to the exploration of high-energy ZHCs.