Theoretical investigation of quantum capacitance of Co-doped α-MnO2 for supercapacitor applications using density functional theory

The rapid depletion of fossil fuels and ever-growing energy demand have led to a search for renewable clean energy sources. The storage of renewable energy calls for immediate attention to the fabrication of efficient energy storage devices like supercapacitors (SCs). As an electrode material for SCs, MnO2 has gained wide research interest because of its high theoretical capacitance, variable oxidation state, vast abundance, and low cost. However, the low electric conductivity of MnO2 limits its practical application. The conductivity of MnO2 can be enhanced by tuning the electronic states through substitution doping with cobalt. In the present work, first principles analysis based on density functional theory (DFT) has been used to examine the quantum capacitance (C-QC) and surface charge (Q) of Co-doped MnO2. Doping enhanced the structural stability, electrical conductivity, potential window, and quantum capacitance of alpha-MnO2. The shortened band gap and localized states near the Fermi level improve the CQC of alpha-MnO2. For the narrow potential range (-0.4 to 0.4 V), the C-QC is observed to increase with doping concentration. The highest C-QC value at +0.4 V is observed to be 2412.59 mu F cm(-2) for Mn6Co2O16 (25% doping), five times higher than that of pristine MnO2 (471.18 mu F cm(-2)). Mn6Co2O16 also exhibits better C-QC and 'Q' at higher positive bias. Hence, it can be used as an anode material for asymmetric supercapacitors. All these results suggest better capacitive performance of Co-doped alpha-MnO2 for aqueous SCs and as an anode material for asymmetric supercapacitors.