We demonstrate that surface modified nanocellulose fibers (NCFs) can be used as substrates to synthesize supercapacitor electrodes with the highest full electrode-normalized gravimetric (127 F g<sup>–1</sup>) and volumetric (122 F cm<sup>–3</sup>) capacitances at high current densities (300 mA cm<sup>–2</sup> ≈ 33 A g<sup>–1</sup>) until date reported for conducting polymer-based electrodes with active mass loadings as high as 9 mg cm<sup>–2</sup>. By introducing quaternary amine groups on the surface of NCFs prior to polypyrrole (PPy) polymerization, the macropore volume of the formed PPy-NCF composites can be minimized while maintaining the volume of the micro- and mesopores at the same level as when unmodified or carboxylate groups functionalized NCFs are employed as polymerization substrates. Symmetric, aqueous electrolyte-based, devices comprising these porosity-optimized electrodes exhibit device-specific volumetric energy and power densities of 3.1 mWh cm<sup>–3</sup> and 3 W cm<sup>–3</sup> respectively; which are among the highest values reported for conducting polymer electrodes in aqueous electrolytes. The functionality of the devices is verified by powering a red light-emitting diode with the device in different mechanically challenging states
