Capacitance enhancement of hybrid electrochemical capacitor with asymmetric carbon electrodes configuration in neutral aqueous electrolyte

A hybrid electrochemical capacitor in 1 mol L−1 Li2SO4 + 0.2 mol L−1 KI aqueous solution (pH = 6.5, conductivity = 86 mS cm−1 at 24 °C) has been developed by implementing highly microporous carbon (KAC, average micropore size L0 = 0.88 nm) as negative electrode and a micro-/mesoporous carbon (PAC, average micropore size L0 = 1.01 nm) as positive electrode. Under negative polarization, KAC demonstrates a high electrical double-layer (EDL) capacitance (187 F g−1), owing to strictly microporous texture making it suitable as negative electrode in a hybrid cell. Besides, under positive polarization, a well-defined faradaic redox signature (in narrow potential range) related with the iodine/iodide system is observed with PAC, giving rise to much higher capacity (62 mAh g−1) than with KAC (37 mAh g−1) and making the former a good candidate to be implemented as positive electrode in a hybrid cell. As a result, the asymmetric (−)KAC/PAC(+) hybrid cell in aqueous Li2SO4+KI exhibits a high capacitance of 74 F g−1 (per total active mass of the two electrodes) compared to 66 F g−1 and 50 F g−1 for the (−)KAC/KAC(+) and (−)PAC/PAC(+) hybrid cells, respectively. Moreover, for the asymmetric (−)KAC/PAC(+) capacitor, the potential of the positive electrode is lower than the thermodynamic limit of water oxidation, and the potential of the negative electrode is higher than the kinetic limit of hydrogen evolution. Consequently, the asymmetric (−)KAC/PAC(+) hybrid cell demonstrates the lowest capacitance drop and resistance increase during floating. At 2.5 A g−1, the (−)KAC/PAC(+) capacitor exhibits a high specific energy of 15.4 Wh kg−1 compared to 10.1 Wh kg−1 and 8.1 Wh kg−1 for the (−)KAC/KAC(+) and (−)PAC/PAC(+) capacitors, respectively. Raman spectra on the PAC positive electrode of the asymmetric (−)KAC/PAC(+) cell demonstrate upshifting of D and G bands by 12 cm−1 and 8 cm−1, respectively, suggesting a charge transfer between carbon and polyiodides confined in the porosity.