Supercapacitors can provide high rate capability and long-term cycleability, profited by their simple mechanism based on the adsorption and desorption of ions on the surface of electrodes. It is possible to charge or discharge supercapacitor cells in a minute for 10,000 cycles without notable degradation. Based on the high power characteristics, supercapacitors can augment the batteries or fuel cells for electric vehicles or heavy equipment. So far, most of supercapacitors have symmetric configuration with activated carbon electrodes as the positive and negative electrodes (Fig. 1a). And they follow Daniell-type mechanism, where the electrolyte is depleted of and replenished with ions upon charging and discharging, respectively (Fig. 1b). In this mechanism, electrolyte serves as an ionic reservoir, from which cations and anions are separated to negative and positive electrodes, respectively. As a result, sufficient volume of electrolyte is indispensable for the operation of supercapacitors. This leads to a limitation in energy density and an increase in the internal resistance as the starvation of the concentration of salts in the electrolyte decreases during operation.

Herein, we introduce a hybrid supercapacitor that follows ‘rocking-chair’-type mechanism for the first time. This hybrid supercapacitor is composed of magnesium metal and activated carbon as the negative and positive electrodes, respectively (Fig. 1c). Mg²⁺ ions are replenished at the negative electrode while they are adsorbed into the pores of the positive electrode upon the discharging of the cell (Fig. 1d). On the other hand, Mg²⁺ ions are released from the positive electrode while Mg metal is deposited on the negative electrode upon the charging of the cell. As a result, only a minimum amount of electrolyte is needed to operate the cell since the electrolyte serves as a medium of Mg²⁺ ions. The use of Mg metal electrode can increase the energy density of the cell significantly by virtue of the large volumetric capacity of 3832 mAh/cc and the low redox potential of -2.2 V vs activated carbon’s open circuit potential. Therefore, the ‘rocking-chair’-type magnesium hybrid supercapacitor can double the energy density of a supercapacitor, and the internal resistance can be kept constant during the operation of the cell. The electrochemical results show highly reversible reactions at the positive and negative electrodes thanks to newly-developed Mg²⁺ electrolytes. Charge-discharge cycles at rates up to 5 mA/cm² were possible, whereas cycling tests showed 64% retention of the capacity after 400 cycles. Detailed studies of the mechanisms of degradation and evidence of the ‘rocking-chair’-type mechanism will be presented.