(Invited) Theoretical Study on the Energy Storage Mechanism of Transition-Metal-Carbide MXene By Quantum-Classical Hybrid Simulation

Abstract

Two-dimensional transition-metal carbide MXenes has been known as a promising material of high-performance electrode for energy storage applications with both non-aqueous and aqueous electrolytes because it has electronic conductivity, ion capability in interlayer nanospace, and the redox activity of Ti. Especially, as an electrode of electrochemical capacitors, MXenes have a unique dependence of capacitance on intercalated cation species and a distinctive behavior that is both capacitive and pseudocapacitive depending on the electrolyte. To better understand electrochemical mechanism of these behaviors, we performed a microscopic theoretical analysis of the electric-double layer (EDL) in the interlayer nanospace of the MXene electrode by combining first-principles calculations based on density functional theory (DFT) and classical implicit solvation theory named three dimensional reference-site interaction model (3D-RISM) method [1]. By using the quantum-classical hybrid simulation, solvation effect at the interlayer nanospace of MXene electrode was taken into consideration appropriately with low computational cost comparable with conventional DFT simulation.For the unique dependence of capacitance on intercalated cation species, our simulation results showed that the capacitance of EDL capacitors can be enhanced by a factor of 1.8, when the water molecules are strongly confined into the two-dimensional nanoslits of titanium carbide MXene electrode. This is because that dipolar polarization of strongly confined water overscreens an external electric field applied on the EDL and enhances capacitance with a characteristically negative dielectric constant of a water molecule [2, 3]. Another investigated feature is the distinctive behavior that is both capacitive and pseudocapacitive depending on the electrolyte. As a result of their electronic states simulated via the quantum-classical hybrid simulation, it was found that the hydration shell of intercalated ions prevents orbital coupling between MXene and the intercalated ions, which leads to the formation of an EDL and capacitive behavior. However, once the cations are partially dehydrated and adsorbed onto the MXene surface directly, because of orbital coupling of the cation states with the MXene states, particularly for surface-termination groups, charge transfer occurs and results in a pseudocapacitive behavior [4].