Capacitive Performance of Two-Dimensional Metal Carbides

Recently a new family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. Unlike graphene, whose chemistry is restricted to carbon, MXenes allow a variety of chemical compositions and are establishing themselves as a large new class of two-dimensional materials. MXenes combine the metallic conductivity of transition metal carbide layers with the hydrophilic nature of their mostly hydroxyl or oxygen terminated surfaces. In essence, they behave as "conductive clays" and have shown much of promise as electrode materials for Li-ion batteries. Prior to the initiation of this study, there have been no reports on the capacitive properties of MXenes. In this work the potential was explored of the new family of the two-dimensional carbides, MXenes, as electrode materials for electrochemical capacitors. This study was focused on Ti3C2Tx. It was established that variety of single- and multiply charged cations (such as Li+, Na+, K+, NH4+, Mg2+) can intercalate MXenes (chemically or electrochemically) and participate in energy storage. Highly reversible electrochemical insertion of the same cations has been demonstrated for Ti3C2Tx in aqueous electrolytes. Perfect capacitive behavior was observed for Ti3C2Tx MXene even at quite high charge and discharge rates, all coupled with excellent cyclability; no drop in capacitance was observed even after 10 000 cycles. Further investigation showed that surface chemistry has significant effect on the resulting capacitance, i.e. by creating predominantly oxygen-containing functionalities the capacitance can be substantially boosted in comparison to the as-received material. It was also demonstrated that Ti3C2Tx clay produced using LiF-HCl mixturewith predominantly oxygen-containing functionalities, showed outstanding capacitance up to 900 F/cm3 and can be manufactured in to electrodes in less than 10 min without need of binder or conductive additive. Electrochemical in-situ XAS measurements detected changes in Ti oxidation state during cycling, which matched closely the observed experimental values of the material's capacitance. Therefore it was concluded that mechanism of electrochemical storage of the Ti3C2Tx MXene clay is predominantly pseudocapacitive. . Also concept of all-solid-state asymmetrical supercapacitor (freestanding and current collector free) based on Ti3C2Tx was developed. Among other applications, using in-situ AFM the potential of the use of MXenes in electrochemical actuators was demonstrated. It was also shown that MXenes other than Ti3C2Tx also demonstrated a lot of promise for electrochemical capacitors: Nb2CTx/CNT paper electrodes showed high volumetric capacitance of 325 F/cm3 when tested in a Li-ion capacitor configuration.Ph.D., Materials Science and Engineering -- Drexel University, 201

Separation and Liquid Chromatography Using a Single Carbon Nanotube

Use of a single template-grown carbon nanotube as a separation column to separate attoliter volumes of binary mixtures of fluorescent dyes has been demonstrated. The cylindrical nanotube walls are used as stationary phase and the surface area is increased by growing smaller multi-walled carbon nanotubes within the larger nanotube column. Liquid-liquid extraction is performed to separate selectively soluble solutes in a solvent, and chromatographic separation is demonstrated using thin, long nanotubes coated inside with iron oxide nanoparticles. The setup is also used to determine the diffusion coefficient of a solute at the sub-micrometer scale. This study opens avenues for analytical chemistry in attoliter volumes of fluids for various applications and cellular analysis at the single cell level

Selective Calixarene Directed Synthesis of MXene Plates, Crumpled Sheets, Spheres and Scrolls

Fully exploiting the electronic and mechanical properties of 2D laminar materials not only requires efficient and effective means of their exfoliation into low dimensional layers, but also necessitates a means of changing their morphology so as to explore any enhancement that this may offer. MXenes are a rapidly emerging new class of such laminar materials with unique properties. However, access to other morphologies of MXenes has not yet been fully realised. To this end we have developed the synthesis of MXenes (Ti2C) as plates, crumpled sheets, spheres and scrolls, which involves selective intercalation of p-phosphonic calix[n]arenes, with control in morphology arising from the choice of the size of the macrocycle, n = 4, 5, 6 or 8. This opens up wider avenues of discovery/design for new morphologies from the wider family of MXenes beyond Ti2C, along with opportunities to exploit any new physico-chemical properties proffered

Extent of Pseudocapacitance in High‐Surface Area Vanadium Nitrides

Early transition‐metal nitrides, especially vanadium nitride (VN), have shown promise for use in high energy density supercapacitors due to their high electronic conductivity, areal specific capacitance, and ability to be synthesized in high surface area form. Their further development would benefit from an understanding of their pseudocapacitive charge storage mechanism. In this paper, the extent of pseudocapacitance exhibited by vanadium nitride in aqueous electrolytes was investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The pseudocapacitance contribution to the total capacitance in the nitride material was much higher than the double‐layer capacitance and ranged from 85 % in basic electrolyte to 87 % in acidic electrolyte. The mole of electrons transferred per VN material during pseudocapacitive charge storage was also evaluated. This pseudocapacitive charge‐storage is the key component in the full utilization of the properties of early‐transition metal nitrides for high‐energy density supercapacitors.Double‐layer capacitance vs. pseudocapacitance: the electrostatic double‐layer and pseudocapacitive charge storage mechanisms in high‐surface‐area vanadium nitride are investigated. The magnitude of the pseudocapacitive charge storage capacity and mole of electrons transferred are reported. The pseudocapacitive charge‐storage mechanism is the key component in maximizing the energy density of supercapacitors based on transition‐metal nitrides.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146597/1/batt201800050.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146597/2/batt201800050_am.pd

2D metal carbides and nitrides (MXenes) for energy storage

The family of 2D transition metal carbides, carbonitrides and nitrides (collectively referred to as MXenes) has expanded rapidly since the discovery of Ti3C2 in 2011. The materials reported so far always have surface terminations, such as hydroxyl, oxygen or fluorine, which impart hydrophilicity to their surfaces. About 20 different MXenes have been synthesized, and the structures and properties of dozens more have been theoretically predicted. The availability of solid solutions, the control of surface terminations and a recent discovery of multi-transition-metal layered MXenes offer the potential for synthesis of many new structures. The versatile chemistry of MXenes allows the tuning of properties for applications including energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, gas- and biosensors, lubrication, and photo-, electro- and chemical catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties have also been shown. In this Review, we present the synthesis, structure and properties of MXenes, as well as their energy storage and related applications, and an outlook for future research

Large Intercalation Pseudocapacitance in 2D VO2(B): Breaking through the Kinetic Barrier

VO2 (B) features two lithiation/delithiation processes, one of which is kinetically facile and has been commonly observed at 2.5 V versus Li/Li+ in various VO2 (B) structures. In contrast, the other process, which occurs at 2.1 V versus Li/Li+, has only been observed at elevated temperatures due to large interaction energy barrier and extremely sluggish kinetics. Here, it is demonstrated that a rational design of atomically thin, 2D nanostructures of VO2 (B) greatly lowers the interaction energy and Li+‐diffusion barrier. Consequently, the kinetically sluggish step is successfully enabled to proceed at room temperature for the first time ever. The atomically thin 2D VO2 (B) exhibits fast charge storage kinetics and enables fully reversible uptake and removal of Li ions from VO2 (B) lattice without a phase change, resulting in exceptionally high performance. This work presents an effective strategy to speed up intrinsically sluggish processes in non‐van der Waals layered materials

Proton Ion Exchange Reaction in Li3 IrO4 : A Way to New H3+ x IrO4 Phases Electrochemically Active in Both Aqueous and Nonaqueous Electrolytes

Progress over the past decade in Li‐insertion compounds has led to a new class of Li‐rich layered oxide electrodes cumulating both cationic and anionic redox processes. Pertaining to this new class of materials are the Li/Na iridate phases, which present a rich crystal chemistry. This work reports on a new protonic iridate phase H3+xIrO4 having a layered structure obtained by room temperature acid‐leaching of Li3IrO4. This new phase shows reversible charge storage properties of 1.5 e− per Ir atom with high rate capabilities in both nonaqueous (vs Li+/Li) and aqueous (vs capacitive carbon) media. It is demonstrated that Li‐insertion in carbonate LiPF6‐based electrolyte occurs through a classical reduction process (Ir5+ ↔ Ir3+), which is accompanied by a well‐defined structural transition. In concentrated H2SO4 electrolyte, this work provides evidence that the overall capacity of 1.7 H+ per Ir results from two additive redox processes with the low potential one showing ohmic limitations. Altogether, the room temperature protonation approach, which can be generalized to various Li‐rich phases containing either 3d, 4d or 5d metals, offers great opportunities for the judicious design of attractive electrode materials