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

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 ZnI 2 into aqueous ZnSO 4 electrolyte helps to significantly increase the capacity of ZHC due to the redox reactions. It has been demonstrated that the Zn//ZnSO 4 /ZnI 2 //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//ZnSO 4 //NPMC ZHC. The mechanisms for this superb performance have been explored in terms of the Zn 2+ cations deposition/stripping, SO 4 2− /I − anions adsorption/desorption, Zn 4 SO 4 (OH) 6 ·0.5H 2 O precipitation/dissolution and the redox reactions (3I − /I 3 − , 2I − /I 2 ). This novel work may pave a way to the exploration of high-energy ZHCs. • NPMCs were prepared by template-guided growth coupled with activation strategy. • NPMCs feature 3D framework, hierarchical pores and tunable N/P species. • The energy density of ZHC increased from 90.7 to 324.8 Wh kg −1 by ZnI 2 additive. • The working mechanisms of ZHC based on ZnSO 4 /ZnI 2 electrolyte were explored.