Modification of Carbon/Electrolyte Interface for Electrochemical Capacitor Improvement

Electrochemical capacitor (EC) parameters are affected by the amount of charge reversibly accumulated at electrode/electrolyte interface, electrode material, type of electrolyte and its electrochemical stability as well as cell construction. It has been proved that asymmetric configuration, i.e., various electrode materials for positive and negative electrodes or hybrid configuration where different charge storage mechanism is utilised on both electrodes can bring a significant capacitor enhancement. Both approaches, it means typical electrostatic attraction of ions on various electrode materials and pseudocapacitive phenomena originating from electrode materials and/or electrolyte have been considered. Carbon electrode surface has been modified by the ammonia adsorption. Dry and wet preliminary oxidation as well as thermal treatment of carbon materials have been realized for more efficient ammonia trapping into pores. A detailed analysis of nitrogen functionalities and carbon texture was performed. Donor-acceptor nature of modified carbon allows a capacitor voltage to be extended. The equilibria state and protective layers formed at the carbon/electrolyte interface give the possibility to reach operating capacitor voltage of 1.8 V. This unique but simple strategy enables a self-controlled pH gradient within the capacitor system. It is assumed that bulk of electrolyte has a neutral character (lithium sulfate) whereas both electrode interfaces (+) and (-) indicate acidic and basic character, respectively. Expected gradient of pH between positive and negative electrode is at least 6. Excellent reversibility during long-term galvanostatic cycling (10 000 cycles) and floating has been confirmed. Furthermore, this configuration eliminates the need for physical separation of ions (such as an ion-exchange membrane) to decelerate their mixing and improves the power density of the device. Another approach for capacitor operation improvement is selection of suitable ions at electrode/electrolyte interface considering their ‘hard’ and ‘soft’ character.