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

Finding a viable electrolyte for next-generation super capacitors is of primary importance for developing supercapacitors with high power and energy densities. Herein, we synthesized a conducting salt, tetramethylphosphonium tetrafluoroborate (TMPBF4), and compared the electrochemical characteristics of supercapacitor devices containing TMPBF4 or 1,1-dimethylpyrrolidinium tetrafluoroborate (DMPBF4). The TMPBF4 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 TMPBF4 in acetonitrile (AN) exhibited a slightly lower capacitance (24.6 +/- 0.1 F g-1) than those with DMPBF4 (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 TMPBF4 showed an 18% increase in capacitance relative to those with DMPBF4. Supercapacitors with TMPBF4 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 TMPBF4/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.