Electrochemical insights into the energy storage mechanism of birnessite in aqueous solutions

The birnessite structure of manganese dioxide makes this compound a promising electrode materials for energy storage devices including, in particular for supercapacitors, thanks to the high theoretical capacitance and pseudocapacitive response of MnO₂. The effect of ionic species, in particular alkali cations, on the structure of birnessite has been intensively discussed, however their role on the charge storage mechanism is still controversial. Taking into consideration the solvation/desolvation process of different alkali cations (Li⁺, Na⁺, K⁺ and Cs⁺), this work unveils relevant new aspects of the pseudocapacitive charge storage mechanism of birnessite. The work also discusses the simultaneous contribution of non-faradaic (double-layer processes) and faradaic redox reactions to the charge storage mechanism based on electrochemical impedance spectroscopy (EIS) analysis. It was found that the relative extension of these processes was dependent on diffusional features related to the cation degree of solvation. Results revealed that the potassium birnessite exhibited an interesting specific capacitance value that reached 175.6 F g⁻¹ at 0.5 A g⁻¹ in a potential window of 0.8 V in 0.5 M Li₂SO₄. This capacitance value includes a contribution resulting from intercalation/deintercalation of cations in the interlayer region and a larger contribution from the double-layer processes (above 76%). This investigation provides new electrochemical insights on the charge storage of birnessite, highlighting the role of alkali ions intercalation in the interlayer structure of birnessite.