1.9 V aqueous carbon–carbon supercapacitors with unequal electrode capacitances

The symmetrical aqueous carbon–carbon supercapacitor is commonly perceived to display a narrower working cell voltage than an asymmetrical one, but this study suggests an exception. By controlling the capacitance ratio of the positive and negative electrodes, both of which were made from the same Cabot Monarch 1300 pigment black (CMPB) powder, it was found that a wider negative electrode potential window than that of the positive electrode could be achieved in the neutral aqueous electrolyte of 0.3 mol L−1 K2SO4. This modification is beneficial to extend the overall cell voltage, taking advantage of the high overpotential of hydrogen evolution on the nanostructured carbon electrode. The equi-potentials (for cells discharged to 0 V) and end-potentials (for cells charged to the voltage limit) of the positive and/or negative electrodes with equal or unequal capacitances are reported as a function of the cell voltage limit. The results have confirmed that the carbon–carbon supercapacitor with a positive to negative electrode capacitance ratio of 4:3 can reach beyond 1.90 V in cell voltage, and retain over 85% of the initial capacitance through 10,000 cycles with the cell becoming stable after the 1000th cycle. The specific energy of this cell with unequal electrode capacitances at 1.90 V was increased by over 38% in comparison with a similarly constructed cell with equal electrode capacitances at 1.60 V.