Ultrahigh N-doped carbon with hierarchical porous structure derived from metal-organic framework for high-performance zinc ion hybrid capacitors

As an emerging electrochemical energy storage device, zinc ion hybrid capacitors (ZHCs) achieve high energy density and power density simultaneously by combining energy storage mechanism of supercapacitors and rechargeable batteries. However, the storage capability of the common carbon-based cathodes is inferior owing to the poor zinc ion diffusion kinetics. Herein, we choose a nitrogen (N)-rich metal–organic frameworks (MOFs), i.e., Zn-based metal-triazolate (Zn-MET), as both carbon precursor and N source to prepare N-doped porous carbon through a facile direct pyrolysis strategy. The obtained Zn-MET-x (x = pyrolysis temperature) is featured with high N-doping level up to 16.2 at% originating from the N-rich ligands, and hierarchical porous structure with the coexistence of micro-/meso-/macro-pores, both of which effectively accelerate the zinc ion diffusion kinetics. Consequently, the optimal Zn-MET-800 electrode not only affords impressive EDLC performance, but also exhibits impressive performance as cathode for ZHC. The assembled ZHC achieves a high capacity of 164.2 mAh/g at 0.1 A/g, outstanding rate capability (64.4 mAh/g at 10 A/g), and admirable cycle stability with 90.3 % capacity retention after 30 000 cycles at 10 A/g. Theoretical calculations prove that the Zn ion adsorption can be enhanced by the presence of N sites in graphene framework. The Zn ion storage mechanism related to the formation/dissolution of Zn4SO4(OH)6·5H2O in the cathode surface is also investigated. This work presents an attractive route to develop advanced porous carbon materials with ultrahigh N-doping level from N-rich MOFs for ZHCs.