Biomass-derived carbon prepared through a quadruple-functional-salt approach for application in K-ion capacitors

Heteroatom-doped porous carbons are regarded as promising electrodes for K-ion capacitors. However, current synthetic methods suffer from problems of low efficiency and use unsustainable precursors, and this has restricted wide application of these materials. Moreover, the mechanisms of energy-storage by heteroatom-doped porous carbon electrodes in K-ion capacitors are not well understood. In this work, we prepare porous carbons having honeycomb-like hierarchical structures and doped with P/N heteroatoms by using a low-cost fish scale and K3PO4 as precursor and auxiliary, respectively. K3PO4 and its derivatives not only serve as collagen-hydrolysis reagents during the pre-treatment process, but also act as dopants, templates, and activators during the subsequent carbonization process. Deionized water is the only solvent required in the whole synthesis process. The as-prepared carbons were used as electrodes in K-ion capacitors and their energy-storage mechanisms studied by detailed in-situ characterization methods and DFT calculations. The optimized carbon anode and cathode were used to assemble a pouch K-ion capacitor, which showed high energy density (184.6 Wh kg(-1)), high power density (4.8 kW kg(-1)), and long lifetime (retaining 90% of the initial capacity after 10,000 cycles).