Aqueous electrolytes with high concentrations of salt
are of great
interest in increasing the energy density and safety of carbon supercapacitors.
Herein, electrolytes based on potassium acetate salt in different
concentrations (20–60 wt %) are studied, and a concentration
of 40 wt % results in the highest capacitance and rate capability.
This behavior is a compromise between the electrolyte ion adsorption,
which is small for low concentrations, and the resistance, which is
large for high-concentration electrolytes. An increase in the voltage
window of 1.8 V is achieved, which significantly improves the capacitance
to 162 F g–1 at 0.1 A g–1 and
the energy density to 22.9 W h kg–1 at a power density
of 45 W kg–1. In addition, the long cycling retention
shows the best retention for 1.5 V (104%), which decreases for 1.8
V (72%), however, without significantly increasing the electrode resistance.
At 2.0 V, the capacitance drops, and the resistance increases considerably.
Subsequently, a series of microporous and mesoporous carbons with
different characteristics are investigated with 40 wt % potassium
acetate in water at 1.8 V. The development of the specific surface
area to 2300 m2 g–1 enables the capacitance
to increase (to 182 F g–1 at 0.1 A g–1). At high current density, mesoporosity proved to play a crucial
role in enhancing the capacitance retention. Along with the presence
of a high oxygen content, the electrolyte diffusion and interactions
are favored, which is beneficial for enhanced rate capability. Other
properties, such as pore connectivity, material structure, and morphology,
are determined to achieve the optimal performance.