Size Effect of Electrolyte Ions on the Electric Double-Layer Structure and Supercapacitive Behavior

Finding a viable electrolyte for next-generation supercapacitors is of primary importance for developing supercapacitors with high power and energy densities. Herein, we synthesized a conducting salt, tetramethylphosphonium tetrafluoroborate (TMPBF 4 ), and compared the electrochemical characteristics of supercapacitor devices containing TMPBF 4 or 1,1-dimethylpyrrolidinium tetrafluoroborate (DMPBF 4 ). The TMPBF 4 salt was selected based on the generally accepted design rule that smaller ions and solvation shells contribute to higher energy and power densities. However, at a low current density (0.1 A g –1 ), supercapacitors with TMPBF 4 in acetonitrile (AN) exhibited a slightly lower capacitance (24.6 ± 0.1 F g –1 ) than those with DMPBF 4 (25.0 ± 0.2 F g –1 ). Clarification of this behavior using electrochemical and theoretical methods revealed that the total capacitance is influenced by not only the size of the ions but also the degree of cation–anion interactions near the electrodes. By contrast, at a high current density of 10 A g –1, supercapacitors with TMPBF 4 showed an 18% increase in capacitance relative to those with DMPBF 4 . Supercapacitors with TMPBF 4 exhibited both high capacitance and a remarkably long cycle life (10,000 cycles) at a high working voltage (3.2 V) and high current density (10 A g –1 ). Furthermore, supercapacitors utilizing TMPBF 4 /AN provided a sufficiently fast frequency response for 60 Hz alternating current (ac) line filtering. These insights into the influence of the investigated electrolyte salts on the formation of electric double-layer structures offer design rules for developing electrolytes for supercapacitors.