Enhancing Specific Energy and Power in Asymmetric Supercapacitors - A Synergetic Strategy based on the Use of Redox Additive Electrolytes

Abstract

The strategy of using redox additive electrolyte in combination with multiwall carbon nanotubes/metal oxide composites leads to a substantial improvements in the specific energy and power of asymmetric supercapacitors (ASCs). When the pure electrolyte is optimally modified with a redox additive viz., KI, ~105% increase in the specific energy is obtained with good cyclic stability over 3,000 charge-discharge cycles and ~14.7% capacitance fade. This increase is a direct consequence of the iodine/iodide redox pairs that strongly modifies the faradaic and non-faradaic type reactions occurring on the surface of the electrodes. Contrary to what is shown in few earlier reports, it is established that indiscriminate increase in the concentration of redox additives will leads to performance loss. Suitable explanations are given based on theoretical laws. The specific energy or power values being reported in the fabricated ASCs are comparable or higher than those reported in ASCs based on toxic acetonitrile or expensive ionic liquids. The paper shows that the use of redox additive is economically favorable strategy for obtaining cost effective and environmentally friendly ASCs.

Figure 1

FESEM and TEM micrographs observed for (a–c) MWZ. (d–f ) MWW composite materials. Inset to (a,d) represent FESEM micrographs for ZrO₂ and WO₃.

Figure 2

N₂ absorption-desorption isotherms and pore size distribution observed for: (a) MWZ, (b) MWW composite materials and (c) MWCNTs (MW).

Figure 3

(a) Two electrode CV curves observed in different voltage ranges at 50 mV s⁻¹ for ASCs assembled in 1 M Li₂SO₄ electrolyte and (b) Energy band diagram to explain maximum achieved operating voltage window.

Figure 4

Two electrode CV curves observed at different scan rates for ASCs assembled after addition of (a) 7.5 mmol (b) 15 mmol (c) 30 mmol (d) 45 mmol and (e) 75 mmol KI in aq. 1 M Li₂SO₄ electrolyte, resp-ectively (f ) Variation of anodic and cathodic peak currents with v^1/2 (v is scan rate) for ASCs assembled in 15 mmol KI added electrolyte system.

Figure 5. Schematic showing the various charge-storage processes occurring at the electrode/electrolyte interface in an ASC.

Figure 6. Galvanostatic charge-discharge curves at 1 A g⁻¹ for ASCs fabricated in only 1 M Li₂SO₄ and after addition of different KI concentrations.

Figure 7

(a) Rate capability and (b) Cycling stability of ASCs in only 1 M Li₂SO₄ and with addition of KI (7.5 and 15 mmol).

Figure 8. A schematic diagram to explain the reduction of cyclic stability at higher KI concentrations.

Figure 9

(a) Nyquist plot, (b) Randles plot and (c) Complex power analysis for ASCs fabricated in 1 M Li₂SO₄ aqueous electrolyte and after the addition of KI (7.5 and 15 mmol).

Figure 10

(a) Ragone plot and (b) Performance comparison of proposed ASCs with previously reported laboratory scale ASCs in aqueous electrolyte.