There is increasing interest in researching materials with nanoconfined fluids for electrochemical energy storage applications due to the interesting modifications confined fluids can have on ion transport, ion desolvation and insertion, or solvent co-intercalation into host materials. Furthermore, layered materials with these confined fluids can demonstrate higher rates of charge storage and transfer due to decreases in the activation energy of charge-transfer at their solvated surfaces. Since their discovery in 2011, the family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides known as MXenes have received significant attention for energy storage and electronics applications due to their tunable properties. The reactivity of their transition metal surfaces enables MXenes to store higher amounts of energy than all reported traditional double layer capacitor materials, and due to their unique chemical composition and layered structures MXenes can intercalate a wide variety of cations such as K+, Na+, Mg2+, Al3+, and even bulky imidazolium and alkylammonium cations. Since the surface of the MXenes has abundant surface functional groups (e.g. O, F, and OH), surface redox reactions with intercalated protons gives Ti3C2 very high volumetric capacitance in acidic aqueous electrolyte. The water confined within the MXene nanosheets in this system also enables ion transport rates that are far superior to any other known pseudocapacitive material. Furthermore, MXenes are not limited to operating in aqueous environments, and reversible intercalation and deintercalation of lithium and sodium ions from organic electrolytes has been demonstrated with competitive energy and power densities and high charge-discharge rates. Interestingly, the confinement of electrolyte in between MXene interlayer space changes with the intercalation of different ions and polarization. For example, when compared to an uncharged state, the interlayer spacing of Ti3C2 becomes larger when negatively charged and then smaller when positively charged in an ionic liquid electrolyte. Different MXenes have been synthesized with a large variety of surface chemistries, all of which have demonstrated promising properties for energy storage applications. By further selecting the proper combination between ions, polarization conditions, and the type of MXene, the changeable interlayer space may allow for tunabl ion intercalation and improved supercapacitor performance.

High-rate energy storage in MXenes with nanoconfined fluids

256th National Meeting (2018)

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