A Dual-Carbon Potassium-Ion Capacitor Enabled by Hollow Carbon Fibrous Electrodes with Reduced Graphitization

The large size of K⁺ ions (1.38 Å) sets a challenge in achieving high kinetics and long lifespan of potassium storage devices. Here, a fibrous ZrO₂ membrane is utilized as a reactive template to construct a dual-carbon K-ion capacitor. Unlike graphite, ZrO₂-catalyzed graphitic carbon presents a relatively disordered layer arrangement with an expanded interlayer spacing of 0.378 nm to accommodate K⁺ insertion/extraction. Pyridine-derived nitrogen sites can locally store K-ions without disrupting the formation of stage-1 graphite intercalation compounds (GICs). Consequently, N-doped hollow graphitic carbon fiber achieves a K⁺-storage capacity (primarily below 1 V), which is 1.5 time that of commercial graphite. Potassium-ion hybrid capacitors are assembled using the hollow carbon fiber electrodes and the ZrO₂ nanofiber membrane as the separator. The capacitor exhibits a high power of 40 000 W kg^-1, full charge in 8.5 s, 93% capacity retention after 5000 cycles at 2 A g^-1, and a low self-discharge rate of 8.6 mV h^-1. The scalability and high performance of the lattice-expanded tubular carbon electrodes underscores may advance the practical potassium-ion capacitors.