Increasing the stability of very high potential electrical double layer capacitors by operando passivation

Abstract

Electrochemical behaviour of titanium carbide-derived carbon (TiC-CDC) electrodes in 1 M (C2H5)3CH3NBF4 + acetonitrile electrolyte has been studied using cyclic voltammetry, constant current charge-discharge, impedance and constant power methods. After step by step cell potential widening and repetitive cell potential cyclization up to 3.4 V, surface passivation, i.e. decrease of faradic reaction currents at potentials higher than 3.4 V, has been detected. Based on Raman spectroscopy data, the passivation effect has been explained by repetitive oxidation-reduction of more active carbon sites from the surface of TiC-CDC and exposition of the more stable graphitic regions. After very careful electrode surface passivation/modification, the region of ideal polarizability can be widened up to 3.7 V and nearly ideal capacitive behaviour with phase angle more negative than −88° and stable series capacitance (∼120 F g−1) have been obtained. Very high experimental energy densities (30 Wh kg−1) at power densities 40 kW kg−1 (ΔE ≥ 3.4 V) have been measured, comparable even with the sol-gel TiC-CDC/(acetonitrile + (C2H5)3CH3NBF4) two-electrode electrical double layer capacitor system. Much higher energy and power values have been achieved at ΔE = 3.7 V, being the maximal cell potential limit, where nearly ideal polarizability for few hundred cycles was achieved.