Effect of a buffer/iodide electrolyte on the performance of electrochemical capacitors†

Abstract

One of the major problems affecting the energetic characteristics and cycle life of electrochemical capacitors (ECs) utilizing aqueous electrolytes is the narrow operating voltage range, which is limited by the thermodynamic stability of water (1.23 V). An improvement in the EC energy can be realized by a capacitance and/or voltage increase. For that purpose, cost-effective and environmentally friendly iodides have been added to aqueous electrolytes to improve the redox activity. Moreover, buffer agents (acetate, citrate, and phosphate) that are well known for adjusting the pH of an electrolyte have been applied, which enabled ECs to reach a stable operating voltage of 1.5 V. After adding iodide to the buffer system, the conductivity increased notably, whereas the pH values remained nearly the same. Galvanostatic charge/discharge test (0.5 A g⁻¹) for ECs operating in buffer electrolytes containing 0.2 mol L⁻¹ NaI showed that this additive noticeably increased the specific capacitance values from 91 to 149 F g⁻¹ for acetate buffer, from 7 to 101 F g⁻¹ for citrate buffer, and from 22 to 132 F g⁻¹ for phosphate buffer. The long-time performance of the ECs was investigated through accelerated potentiostatic floating. The electrochemical performance was studied using various activated carbons. During the floating aging test, the YP50F-based EC working in acetate buffer with 0.2 mol L⁻¹ NaI displayed the best long-term performance (310 h) compared to YP80F and BP2000 carbons, which exhibited 212 h and 126 h, respectively. The highly microporous YP50F carbon in the acetate buffer/iodide electrolyte revealed the best wettability. Interestingly, the citrate buffer/iodide EC system with YP50F demonstrated an extremely long floating performance (1006 h). Thus, this study presents a new strategy for improving the energetic metrics and cycling performance of carbon-based ECs operating in buffer electrolytes with an iodide redox pair.