Pseudocapacitive mechanism of manganese oxide in 1-ethyl-3- methylimidazolium thiocyanate ionic liquid electrolyte studied using X-ray photoelectron spectroscopy

The electrochemical behavior of anodically deposited manganese oxide was studied in pyrrolidinium formate P-HCOO), 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF₆), and 1-ethyl-3-methylimidazolium thiocyanate (EMI-SCN) ionic liquids (ILs). The experimental data indicate that the Mn oxide electrode showed ideal pseudocapacitive performance in aprotic EMI-SCN IL. In a potential window of ~1.5 V, the oxide specific capacitance, evaluated using cyclic voltammetry and chronopotentiometry, was about 55 F/g. The electrochemical energy storage reaction was examined using X-ray photoelectron spectroscopy (XPS). It was confirmed that the SCN^- anions, instead the EMI⁺ cations, were the primary working species that can become incorporated into the oxide and thus compensate the Mn ³⁺/Mn⁴⁺ valent state variation upon the charge-discharge process. According to the analytical results, a pseudocapacitive mechanism of Mn oxide in the SCN^- based aprotic IL was proposed.