Selection and optimization of carbon precursor to prepare porous graphitic carbon for zinc-ion hybrid capacitor

Constructing porous graphitic carbon (PGC) materials with sufficient active sites and good electrical conductivity is crucial for Zn2+ storage in zinc-ion hybrid capacitors (ZIHCs). However, precise control over the pore structure and graphitization degree remains challenging due to the trade-off between these two characteristics. Here, a simple yet effective strategy is proposed by fabricating a honeycomb N-doped PGC, utilizing tannin acid (TA) and glucose as carbon precursors, K2FeO4 as the activating-graphitizing agent, and (NH4)2C2O4 as the nitrogen source and foaming agent. The aromatics in TA and its coordination with Fe6+ are favorable for the formation of graphitic structure, while the small molecules of glucose and (NH4)2C2O4 facilitate the development of pores within the graphitic skeleton. As a results, the obtained N-doped PGC (NPGC-TA/G) exhibits a large specific surface area of 2171 m2 g-1, well-developed pore structures with abundant micro- and mesopores, a satisfactory graphitization degree (IG/ID = 1.06), and a moderate amount of N/O-doping. These features enable NPGC-TA/G with sufficient active sites, fast ion/electron transport kinetics, and good wettability. In practical applications, NPGC-TA/G based aqueous ZIHCs demonstrate impressive performances, with an excellent specific capacity of 203.5 mA h g-1 at 0.5 A g-1, a high energy density of 164.6 W h kg-1 at 400 W kg-1, and good cycling stability (94 % of the specific capacity retained after 10,000 charge-discharge cycles). Moreover, the assembled flexible ZHICs exhibit significant flexibility and stability when bent at angles from 0 degrees to 90 degrees and excellent cycling stability with 82.6 % capacity retention after 1000 cycles. An assembled pouch cell successfully powers an electric fan for 2.5 h. This work provides valuable insights into the precise control of porous and graphitic structures for PGC materials at the molecular level, which is beneficial for ZHIC applications.