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

Abstract 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.