Confinement of iodides in carbon porosity to prevent from positive electrode oxidation in high voltage aqueous hybrid electrochemical capacitors

Hybridization of battery-type positive and capacitor-type negative activated carbon (AC) electrodes has been realized in a carbon/carbon electrochemical capacitor by adding potassium iodide (0.5 mol L−1 KI) to aqueous manganese sulfate (2 mol L−1 MnSO4) electrolyte. Raman spectroscopy proved that polyiodides are formed inside the porosity of the positive carbon electrode under polarization in MnSO4+KI electrolyte, and give rise to an interface being at the origin of the battery-like behavior of this electrode. By contrast, the capacitive negative carbon electrode displays a wider potential range, which allows the cell to operate up to 1.5 V. When the hybrid cell is charged at 0.1 A g−1 up to 1.5 V, it exhibits a higher capacitance of 81 F g−1 (expressed per total mass of active carbon material) compared to 41 F g−1 for the symmetric cell in MnSO4. During potentiostatic floating at 1.5 V, the hybrid cell in MnSO4+KI demonstrates remarkably stable capacitance and resistance. Temperature-programmed desorption (TPD) analyses and nitrogen adsorption on the positive electrode of this cell after floating reveal noticeably reduced oxidation as compared to the positive electrode of the cell floated in MnSO4. Such reduced oxidation is owing to the redox couples associated with polyiodides confined in the porosity of the electrode which contribute to stabilize its potential well below the water oxidation limit.

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