Variation of the MnO₂ Birnessite Structure upon Charge/Discharge in an Electrochemical Supercapacitor Electrode in Aqueous Na₂SO₄ Electrolyte

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This paper provides new insight on the charge storage mechanism in crystallized Mg-doped sodium birnessite-type manganese dioxide used as active supercapacitor electrode material that involves the intercalation/deintercalation of cations between the sheets during electrochemical reduction/oxidation, respectively. An increase of the interlayer spacing from 0.710 to 0.720 nm upon electrochemical oxidation in the presence of Na+ cations in the electrolyte is associated with the deintercalation of Na+ and the intercalation of H2O between the layers. A progressive crystallinity loss of the material is also observed upon potential cycling between the oxidized and reduced states. Despite these structural changes, the birnessite electrode exhibits a stable capacitance of 145 F/g over 1100 cycles.