Around the thermodynamic limitations of supercapacitors operating in aqueous electrolytes

This paper reports on a primary study of the capacitor system that can successfully overcome the main drawback of aqueous electrolytes, i.e. low max. voltage and then low energy density of the device. Our idea employs hybridization of electrolytes, i.e. use of two separated electrolytes with different pH values, each for one electrode. In such a way, the capacitor voltage is extended to 2.1 V. 6 mol L−1 KOH plays the role of electrolyte for the negative electrode whereas 1 mol L−1 H2SO4 or 5 mol L−1 LiNO3 are electrolytic solutions for a positive one. This idea allows us to take advantage of very negative hydrogen evolution potential for KOH and high overpotential of oxygen evolution, especially for LiNO3 solutions. Accordingly, it was possible to maintain almost 100 F g−1 after 25 000 cycles at 1 A g−1, gaining the energy more than 10 Wh kg−1 along the average power of 1 kW kg−1.