MXenes are a class of two-dimensional (2D) nanomaterials comprised of transition metal carbides, nitrides, and carbonitrides, which exhibit a rare combination of hydrophilicity and metallic conductivity. Their compositional variability, transition metal redox chemistry, tunable interlayer spacing, and controllable surface chemistry make MXenes broadly applicable in energy storage systems. MXenes exhibit intercalation pseudocapacitance, where bulk redox reactions take place with ultrafast ion kinetics, which is distinctly different from both the sluggish ion diffusion kinetics of battery electrodes and the formation of electrical double layers at the surfaces of porous carbon electrodes. The ability of MXenes to rapidly intercalate a variety of cations is key for developing high rate electrodes using these materials for pseudocapacitive energy storage, hybrid metal-ion capacitors, and multi-valent batteries. Titanium carbide (Ti3C2), the first reported and best studied MXene, has high electronic conductivity (~10000 S/cm), good packing density (4 g/cm3), and proton induced pseudocapacitance, which results in high volumetric capacitance, upwards of 1500 F/cm3 at high rates. This high capacitance is mainly due to the active -O surface functional groups. However, the dense stacking of MXene sheets when they are reassembled into free standing films impedes ion transport, limiting the overall rate performance. This restacking issue was circumvented through liquid crystalline alignment of MXene electrodes, in which thickness independent capacitive behavior was demonstrated for industrial scale thicknesses (~250 µm). Furthermore, methods using spacers such as carbon nanotubes, polymers, or the introduction of holes into the MXene sheets by chemical etching were also used to improve rate performance. A new, facile in situ anodic oxidation method has also been developed for improving the rate performance of restacked MXene electrodes. The capacitance retention of Ti3C2 at 2 V/s (relative to that at 5 mV/s) increases from 38% to 66% when the MXene electrode is cycled to an anodic potential of 0.1 V vs. Hg/Hg2SO4. This process introduces holes in the MXene structure, without damaging the electrochemical active sites, which improves the rate performance of Ti3C2. These studies have helped the design of advanced high rate MXene based energy storage devices such as microsupercapacitors, wearable/textile energy storage devices, electrochromic, and transparent devices.

High rate charge storage mechanisms in MXenes

2019 Fall National Meeting

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