Green and Solvent-Free Mechanochemical Route to N,P Co-Doped Tannin-Derived Nanoporous Carbons for Electrochemical Capacitors

Heteroatom doping of nanoporous carbons is a promising strategy to enhance electrochemical energy storage by simultaneously improving electric double-layer capacitance and introducing pseudocapacitive contributions. However, conventional doping strategies typically rely on corrosive activating agents or procedures that are difficult to scale. This study reports a sustainable route to N,P co-doped nanoporous carbon from tannin via mechanochemical blending with non-corrosive ammonium phosphate salts, carbonization under N2, and CO2 physical activation. X-ray photoelectron spectroscopy (XPS) reveals that this green route establishes stable surface speciation dominated by pyridinic and graphitic nitrogen (N ∼ 2 at %) and phosphate-type P groups (P ∼ 4 at %). The best-performing material achieves a specific surface area and pore volume of up to 1615 m2 g–1 and 1.4 cm3 g–1, respectively, with a micro-mesoporous texture confirmed by N2/H2 adsorption isotherms. In symmetric two-electrode cells using 1 M H2SO4, the best-performing material delivers an electrode capacitance of ca. 254 F g–1 at 0.5 A g–1, retains 95.5% of its initial capacitance after 30,000 charge–discharge cycles, and provides 8.8 W h kg–1 at 164 W kg–1. These results demonstrate that renewable polyphenolic precursors processed by solvent-free mechanochemistry yield competitive carbon electrodes, offering a scalable and environmentally responsible pathway for electrochemical capacitors.