Colossal pseudocapacitance in a high functionality–high surface area carbon anode doubles the energy of an asymmetric supercapacitor

Abstract

Here we demonstrate a facile template-free synthesis route to create macroscopically monolithic carbons that are both highly nitrogen rich (4.1–7.6 wt%) and highly microporous (SA up to 1405 m² g⁻¹, 88 vol% micropores). While such materials, which are derived from common chicken egg whites, are expected to be useful in a variety of applications, they are extremely promising for electrochemical capacitors based on aqueous electrolytes. The Highly Functionalized Activated Carbons (HFACs) demonstrate a specific capacitance of >550 F g⁻¹ at 0.25 A g⁻¹ and >350 F g⁻¹ at 10 A g⁻¹ in their optimized state. These are among the highest values reported in the literature for carbon-based electrodes, including for systems such as templated carbons and doped graphene. We show that HFACs serve as ideal negative electrodes in asymmetric supercapacitors, where historically the specific capacitance of the oxide-based positive electrode was mismatched with the much lower specific capacitance of the opposing AC. An asymmetric cell employing HFACs demonstrates a 2× higher specific energy and a 4× higher volumetric energy density as compared to the one employing a high surface area commercial AC. With 3.5 mg cm⁻² of HFAC opposing 5.0 mg cm⁻² of NiCo₂O₄/graphene, specific energies (active mass normalized) of 48 W h kg⁻¹ at 230 W kg⁻¹ and 28 W h kg⁻¹ at 1900 W kg⁻¹ are achieved. The asymmetric cell performance is among the best in the literature for hybrid aqueous systems, and actually rivals cells operating with a much wider voltage window in organic electrolytes.