Mechanisms of Energy Storage in Carbon-Based Supercapacitors Modified with a Quinoid Redox-Active Electrolyte

The mechanisms involved in the storage of energy in carbon-based supercapacitors modified by the addition of an electrochemically active compound (quinone/hydroquinone, Q/HQ) into the electrolyte (H2SO4) are investigated. Besides the charging of the double-layer characteristic of carbon materials, galvanostatic cycling experiments performed on each electrode revealed a battery-type behavior in the anode and a pseudocapacitive hydrogen electrosorption process in the cathode as a consequence of an asymmetric split of voltage between the electrodes after the incorporation of HQ. Both the hydrogen electrosorption and Q/HQ redox reactions were studied in depth from the cyclic voltammograms obtained for both electrolytes in a three-electrode cell. An outstanding specific capacitance value of 5017 F g–1 was attained by the anode due to the development of the quinoid redox reactions on its surface. Meanwhile, the cathode capacitance also increased significantly with respect to the value obtained by the supercapacitor without HQ (from 290 to 477 F g–1). As a result of the concurrence of all these mechanisms of energy storage, the energy density of the HQ-containing SC is significantly greater than that of the original SC (30.6 W h kg–1 vs 10.1 W h kg–1).