The electrosorption characteristics of simple aqueous ions on loofah-derived activated carbon decorated with manganese dioxide polymorphs: The effect of pseudocapacitance and beyond

Abstract The capability of manganese dioxide (MnO2) supported on loofah-derived activated carbon (AC) in the electrosorptive removal of common inert ions was studied in constant potential mode. Four MnO2 polymorphs were prepared by redox chemical precipitation and characterized by XRD, SEM, BET and XPS. The pseudocapacitance property of the MnO2/AC electrode, significantly affected by the surface- and diffusion-controlled charge storage mechanism, was assessed via voltammetry and modified Langmuir adsorption isotherm. Batch electrosorption experiments were then performed at constant potential in the range of −1.5 V to + 1.5 V (vs. Ag/AgCl) using AC and α-MnO2/AC electrodes in the presence of common simple electrolytes, including NaNO3, Li2SO4, and Ca(ClO4)2. Consequently, the applied working potential (Eapp) positively affected the ion electrosorption rate and capacity. Faradaic processes occurred on MnO2, i.e., Mn(III)/Mn(IV) transition, increased the diffusion capacitance of AC, thus enhancing the first-order rate and monolayer capacity, mainly for the electrosorption of cations. Results showed that ion solvation, controlled by the ionic radius and valence of an ion, which impacted ion intercalation in MnO2 and affected ion adsorption characteristics. The cation sorption capacity of α-MnO2/AC followed the order of Na+ (2.8 × 10-4 mol g−1) > Ca2+ (2.1 × 10-4 mol g−1) > Li+ (0.76 × 10-4 mol g−1) at Eapp = -1.5 V. The differential capacitance as affected by polarized potential and the shift of zero net charge or IEP toward more positively charging potential further highlighted the contribution of pseudocapacitance to ion adsorption on α-MnO2/AC electrode.