Zinc citrate-assisted defect engineering in coal tar pitch-derived porous carbon for high-performance zinc-ion hybrid capacitors.

Aqueous zinc-ion hybrid capacitors (ZIHCs) hold great promise for safe and cost-effective energy storage, yet their practical viability is largely constrained by the insufficient ion-storage capacity of carbonaceous cathodes. Herein, we report a synergistic template-activation strategy to engineer hierarchically porous carbon from coal tar pitch, using zinc citrate as a dual-function self-sacrificial template in conjunction with KOH activation. During pyrolysis, zinc citrate undergoes a distinctive decomposition process in which the in situ generated ZnO nanoparticles serve as a removable hard template, while the concomitantly released gaseous species (CO2, H2O, and hydrocarbons) physically blow and chemically etch the carbon matrix. This cooperative mechanism simultaneously creates an interconnected micro-/meso-/macroporous architecture and implants abundant oxygen-containing functionalities, circumventing the need for exogenous templates or post-synthetic surface modification. The optimized ZCPC-1 cathode features a high specific surface area of 1922.1 m2 g-1 and an optimal balance of pore hierarchy and surface polarity. In aqueous ZIHCs, it delivers a reversible capacity of 171.2 mAh g-1 at 0.2 A g-1, an energy density of 167.85 Wh kg-1, and outstanding long-term durability with 82.9% capacity retention over 25 000 cycles. Beyond aqueous systems, the quasi-solid-state flexible device assembled with PASHE gel electrolyte demonstrates a high capacity of 124.54 mAh g-1 at 0.2 A g-1, retains 80.5% of its initial capacity after 50 000 cycles at 10 A g-1, and exhibits an ultralow self-discharge rate of 4.75 mV h-1, underscoring its practical potential for flexible electronics. This work establishes a conceptually novel and industrially viable approach to upgrading low-cost coal tar pitch into high-performance carbon cathodes through in situ defect engineering and surface chemistry modulation, offering new perspectives for advanced aqueous energy storage systems.