Strain-tunable electronic properties and quantum capacitance of ScHfCO$lt;sub$gt;2$lt;/sub$gt; MXene as supercapacitor electrodes

MXenes have wide applications in energy storage devices because of their compositional diversity. Electronic and optical properties, Bader charge and quantum capacitance of Janus ScHfCO2 MXene under biaxial strain are studied by density functional theory (DFT). The substitution of Hf atoms induces the decrease of the band gap of ScHfCO2, which changes from direct semiconductor into indirect semiconductor. Band gap generally increases with the increase of the tensile strain because of the blueshift of Sc-d and Hf-d orbits, and ScHfCO2 changes to M→K indirect semiconductor at +5% strain. ScHfCO2 under strains from −5% to +4% maintains the indirect bandgap characteristics. The appearance of built-in electric field in ScHfCO2 under strain improves the charge redistribution across Janus layer. ScHfCO2 under compressive strain has better conductivity than ScHfCO2 under tensile strain. ScHfCO2 under strains are all promising cathode materials. Larger voltage improves the character of cathode materials because of their much larger |Qc| when compared with those at aqueous system.