Charge Storage Mechanism of MnO₂ Electrode Used in Aqueous Electrochemical Capacitor

Abstract

The charge storage mechanism in MnO2 electrode, used in aqueous electrolyte, was investigated by cyclic voltammetry and X-ray photoelectron spectroscopy. Thin MnO2 films deposited on a platinum substrate and thick MnO2 composite electrodes were used. First, the cyclic voltammetry data established that only a thin layer of MnO2 is involved in the redox process and electrochemically active. Second, the X-ray photoelectron spectroscopy data revealed that the manganese oxidation state was varying from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. The X-ray photoelectron spectroscopy data also show that Na+ cations from the electrolyte were involved in the charge storage process of MnO2 thin film electrodes. However, the Na/Mn ratio for the reduced electrode was much lower than what was anticipated for charge compensation dominated by Na+, thus suggesting the involvement of protons in the pseudofaradaic mechanism. An important finding of this work is that, unlike thin film electrodes, no change of the manganese oxidation state was detected for a thicker composite electrode because only a very thin layer is involved in the charge storage process.

To Be or Not To Be Pseudocapacitive?

“…), from the electrolyte. 31,32,33,34 The same argument applies to the RuO 2 electrode where proton insertion is the main cause of pseudocapacitance, concomitant with a change in Ru oxidation state over the potential window. 35,36 There are now more than 300 papers that describe Ni-based electrodes as pseudocapacitive, which, as presented above, does not follow the intended definition of the pseudocapacitance term.…”

Development of novel MnO2/nanoporous carbon composite electrodes in capacitive deionization technology

“…Through the high fraction of meso-surface area, the pores in the electrodes are better connected that more pores can be accessed by the ions. Also MnO 2 can provide high pseudo-capacitance [25,27,28]. Therefore, composites can be expected to have a better performance than AC in the capacitive desalination process as higher capacitance suggests better electrosorption of salt ions.…”

Effect of Percolation on the Capacitance of Supercapacitor Electrodes Prepared from Composites of Manganese Dioxide Nanoplatelets and Carbon Nanotubes

“…[22][23][24] Indeed, the theoretical pseudocapacitance of MnO2 is 1370 F/g, which is extremely high. 25 Moreover, a growing selection of layered oxides and hydroxides are showing great promise in this area. 3,[26][27][28][29][30][31][32][33][34][35] To best employ the high intrinsic pseudocapacitance of any material requires the maximum number of redox active sites be in contact with the electrolyte and therefore available for charge storage.…”

“…25,37,38 It has not proven straightforward to transform such promise into practical supercapacitors with high absolute capacitance. One of the reasons for this is that the specific capacitance (i.e.…”