An active contribution: The specific capacitance of carbon‐based supercapacitors can be significantly improved by the addition of an electrochemically active compound (hydroquinone) to the supporting electrolyte. The strong contribution of the redox‐active electrolyte to the pseudocapacitance of the system results in the largest capacitance values obtained for an activated carbon‐based supercapacitor.

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).

Ein aktiver Beitrag: Die spezifische Kapazität kohlenstoffbasierter Superkondensatoren lässt sich durch Zugabe einer elektrochemisch aktiven Verbindung (Hydrochinon) zum Leitsalz deutlich erhöhen (siehe Bild; Molekülstrukturen: grau C, weiß H, rot O). Der starke Beitrag des redoxaktiven Elektrolyten zur Pseudokapazität des Systems führt zu den größten Kapazitätswerten, die bisher für einen Superkondensator auf Basis aktivierten Kohlenstoffs erhalten wurden.

The proliferation of novel types and designs of electrochemical capacitors makes it necessary to obtain a better understanding of the behavior of these systems together with a more systematic classification of them. In this study a rational classification of supercapacitors based on the charge storage mechanism and the active material of each electrode is proposed. The internationally accepted terminology - the terms symmetric, asymmetric and hybrid - is also clarified in an attempt to standardize the current definitions and facilitate the systematic classification of each device. Additionally, the selection of suitable mathematical expressions to calculate the capacitance of each kind of system is rationalized throughout the discussion taking into account the behavioral characteristics of each electrode. An examination of the potential evolution profile of each electrode during the galvanostatic cycling of the supercapacitor is presented as a key tool for understanding the fundamental behavior of these devices.