A double-redox aqueous capacitor with high energy output†
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.
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