Carbon Capacitors Operating at 1.8 V Based on Concentrated Aqueous Electrolytes: Impacts of the Electrolyte Concentration and Carbon Properties

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.