Sustainable Synthesis of Biomass-Derived Carbon Electrodes with Hybrid Energy-Storage Behaviors for Use in High-Performance Na-Ion Capacitors

Although there have been many efforts to improve the performance of electrical energy storage devices by preparing electrode materials with nanostructures and specific chemical compositions, most of the synthetic pathways developed have not addressed issues of safety, cost, and sustainability. Herein, we have simultaneously realized the sustainable synthesis, nanostructure engineering, and heteroatom doping of two carbon materials by separate tailored strategies using gelatin and phytic acid as biomass precursors. These—together with all the other reagents employed—have high terrestrial abundance with low cost and low toxicity and can be easily mixed at a molecular level in deionized water without using organic solvents. Additionally, all the non-carbonaceous products can be easily removed by water washing and further recycled by heat drying. The tailored syntheses result in porous nanosheet structures and uniform heteroatom doping of the final carbons. Based on their typical porosities and chemical compositions, these two carbons have been specifically used as cathode and anode materials in Na-ion capacitors. Electrochemical characterization and first-principles calculations show that the porous nanosheet structures and heteroatom doping endow the carbon electrodes with battery-capacitive storage features, thus leading to their excellent electrochemical performance in half cells. Beneficial from the compatible kinetics of cathode and anode, the assembled Na-ion capacitor exhibits high energy density (135.3 Wh kg–1) and power density (16.1 kW kg–1) as well as ultralong lifetime (88.6% of the initial capacity after 8000 cycles).