Colossal Pseudocapacitance in a High Functionality - High Surface Area Carbon Anode Doubles the Energy of an Asymmetric Supercapacitor

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, 88vol% 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 > 550 F g^-1 at 0.25A g^-1 and > 350 F g^-1 at 10 A g^-1 in their optimized state. These are among the highest values reported in 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 to the much lower specific capacitance of the opposing AC. An asymmetric cell employing HFACs demonstrates a 2X higher specific energy as compared to the one employing a state-of-the-art 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 50 Wh kg^-1 at 230 W kg^-1 and 28 Wh kg^-1 at 1900 W kg^-1 are achieved. In terms of volumetric energy density, the improvement is 4X, with the energy density of an asymmetric cell being 76.3 Wh L^-1 versus 18.3 Wh L^-1 of the symmetric cell. The asymmetric cell performance is among the best in literature for hybrid aqueous systems, and actually rivals cells operating with a much wider voltage window in organic electrolytes.