Improved Charge Storage Capacity of Supercapacitor Electrodes by Engineering Surfaces: The Case of Janus MXenes

Surface engineering in two-dimensional (2D) materials has turned out to be a useful technique to improve their functional properties. By designing Janus compounds MM'C in the MXene family of compounds M2C, where the two surfaces are constituted by two different transition metals M and M', we have explored their potentials as electrodes in a supercapacitor with an acidic electrolyte. Using density functional theory (DFT) in conjunction with the classical solvation model, we have made an in-depth analysis of the electrochemical parameters of three Janus MXenes, passivated by oxygen: NbVC, MnVC, and CrMnC. Comparisons with the corresponding end-point MXenes Nb2C, V2C, Mn2C, and Cr2C are also made. We find that the surface redox activity enhances due to the formation of Janus, improving the charge storage capacities of MXene electrodes significantly. Our analysis reveals that the improved functionality has its root in the variations in the charge state of one of the constituents in the Janus compound, which, in turn, has its origin in the electronic structure changes due to surface manipulation. Our work, which is the first on the electrochemical properties of Janus MXenes for supercapacitor applications, suggests surface engineering by forming appropriate Janus compounds as a possible route to extract high energy density in MXene electrode-acidic electrolyte-based energy storage devices.