A double-redox aqueous capacitor with high energy output

Abstract

The paper puts forward the concept of a double-redox electrochemical capacitor operating in an aqueous electrolyte. The redox activity of sulphur from insoluble Bi₂S₃ nanocrystals embedded in the negative electrode material (up to 10 wt%) operating in 1 mol L⁻¹ Li₂SO₄ electrolyte is demonstrated. It is also shown that the performance is significantly boosted using MPA (3-mercaptopropionic acid) as a ligand attached to the surface of the nanocrystals, which allows for more efficient use of Bi₂S₃ redox active species. This redox activity is combined with the reactions of iodides, which occur at the opposite electrode with 1 mol L⁻¹ NaI. This enables the formation of a discharge voltage plateau that effectively boosts the capacitance (275 F g⁻¹), and thus specific energy of the device owing to the relatively high cell voltage of 1.5 V. This performance is possible due to the advantageous electrode mass ratio (m₋ : m₊ = 2 : 1), which helps to balance the charge. The rate capability test of the device demonstrates its capacitance retention of 73% at 10 A g⁻¹ of the discharge current. The different states of the redox species ensure their operation at separate electrodes in an immiscible manner without a shuttling effect. The specific interactions of the redox active species with carbon electrodes are supported by operando Raman spectroscopy.