High-Temperature Oxidized Mo2CTx MXene for a High-Performance Supercapacitor.

Molybdenum carbide (Mo2CTx MXene) did not possess suitable properties for supercapacitors. Herein, a short oxidation method of Mo2CTx in air at moderately high temperatures is proposed for fabricating a Mo2C/MoO3 heterostructure. The stability of Mo2CTx in air up to 700 °C and the phase transition at higher temperatures are confirmed. Such a heterostructure is beneficial in reducing the diffusion energy barrier of H+. In the aqueous system, the Mo2C/MoO3 electrode delivers a capacitance of up to 811 F g-1. A fully assembled symmetric solid-state supercapacitor delivers 224 F g-1 with an excellent retention rate of 91.05% after 7500 cycles. Besides, the supercapacitor can work at the low temperature of -60°, showing good low-temperature properties. The approach presented in this work opens a promising way to turn a neglected MXene, assumed to be unsuitable for supercapacitors, into one of the top-performing supercapacitor electrodes.

Metal Ion-Induced Porous MXene for All-Solid-State Flexible Supercapacitors.

MXenes are normally used for energy storage applications. However, large nanosheets and restacking are detrimental to the ion diffusion and thus limit its rate capability. Here, a strategy to prepare flexible and porous MXene-M supercapacitor electrodes can simultaneously enlarge the interlayer spacing between layers and create holes in the layers. As a result, Ti3C2Tx-Mn presents an excellent lifespan, with still 248 F g-1 after 100 000 cycles at a current density of 100 A g-1. Moreover, Ti3C2Tx-Mn-based symmetric all-solid-state supercapacitor exhibits outstanding volumetric energy up to 52.4 mWh cm-3 and retains 38.4 mWh cm-3 at an ultrahigh volumetric power density of 55.3 W cm-3. We believe this work provides an idea for the later regulation of MXene layer spacing and the design of porous structures, and can be widely used in the next-generation high-energy density and power density practical applications.

Solution combustion synthesis of a nanometer-scale Co3O4 anode material for Li-ion batteries.

A novel solution combustion synthesis of nanoscale spinel-structured Co3O4 powder was proposed in this work. The obtained material was composed of loosely arranged nanoparticles whose average diameter was about 36 nm. The as-prepared cobalt oxide powder was also tested as the anode material for Li-ion batteries and revealed specific capacities of 1060 and 533 mAh·g-1 after 100 cycles at charge-discharge current densities of 100 and 500 mA·g-1, respectively. Moreover, electrochemical measurements indicate that even though the synthesized nanomaterial possesses a low active surface area, it exhibits a relatively high specific capacity measured at 100 mA·g-1 after 100 cycles and a quite good rate capability at current densities between 50 and 5000 mA·g-1.

Chlorophyll-Based Organic Heterojunction on Ti3 C2 Tx MXene Nanosheets for Efficient Hydrogen Production.

The Z-scheme process is a photoinduced electron-transfer pathway in natural oxygenic photosynthesis involving electron transport from photosystem II (PSII) to photosystem I (PSI). Inspired by the interesting Z-scheme process, herein a photocatalytic hydrogen evolution reaction (HER) employing chlorophyll (Chl) derivatives, Chl-1 and Chl-2, on the surface of Ti3 C2 Tx MXene with two-dimensional accordion-like morphology, forming Chl-1@Chl-2@Ti3 C2 Tx composite, is demonstrated. Due to the frontier molecular orbital energy alignments of Chl-1 and Chl-2, sublayer Chl-1 is a simulation of PSI, whereas upper layer Chl-2 is equivalent to PSII, and the resultant electron transport can take place from Chl-2 to Chl-1. Under the illumination of visible light (>420 nm), the HER performance of Chl-1@Chl-2@Ti3 C2 Tx photocatalyst was found to be as high as 143 µmol h-1 gcat -1 , which was substantially higher than that of photocatalysts of either Chl-1@Ti3 C2 Tx (20 µmol h-1 g-1 ) or Chl-2@Ti3 C2 Tx (15 µmol h-1 g-1 ).

Electrochemical Actuators Based on Two-Dimensional Ti3C2Tx (MXene).

Electrochemical actuators are devices that convert electrical energy into mechanical energy via electrochemical processes. They are used in soft robotics, artificial muscles, micropumps, sensors, and other fields. The design of flexible and stable electrode materials remains a major challenge. MXenes, an emerging family of 2D materials, have found applications in energy storage. Here, we report an actuator device using MXene (Ti3C2Tx) as a flexible electrode material. The electrode in 1 M H2SO4 electrolyte exhibits a curvature change up to 0.083 mm-1 and strain of 0.29%. Meanwhile, the MXene-based actuator with a symmetric configuration separated by gel electrolyte (PVA-H2SO4) has curvature and strain changes up to 0.038 mm-1 and 0.26% with excellent retention after 10,000 cycles. In situ X-ray diffraction analysis demonstrates that the actuation mechanism is due to the expansion and shrinkage of the interlayer spacing of MXenes. This research shows promise of this new family of materials for electrochemical actuators.

Electrochemical Behavior of Ti3 C2 Tx MXene in Environmentally Friendly Methanesulfonic Acid Electrolyte.

Two-dimensional transition metal carbides, nitrides, and carbonitrides, called MXenes, have gained much attention as electrode materials in electrochemical energy storage devices. In particular, Ti3 C2 Tx has shown outstanding performance in common sulfuric acid (H2 SO4 ) electrolyte. In this work, a more environmentally friendly alternative acidic electrolyte, methanesulfonic acid (MSA), is proposed. The energy storage performance of Ti3 C2 Tx in aqueous and neat MSA ionic liquid electrolytes is investigated. The specific capacitance of 298 F g-1 was obtained at a scan rate of 5 mV s-1 in 4 m MSA and it exhibits excellent cycle stability with retention of nearly 100 % over 10 000 cycles. This electrochemical performance is similar to that of Ti3 C2 Tx in H2 SO4 , but using a greener electrolyte. In situ X-ray diffraction analysis reveals the intercalation charge storage mechanism. Specifically, the interlayer spacing changes by up to 2.58 Šduring cycling, which is the largest reversible volume change observed in MXenes in aqueous electrolytes.

2D MXenes as Co-catalysts in Photocatalysis: Synthetic Methods.

Since their seminal discovery in 2011, two-dimensional (2D) transition metal carbides/nitrides known as MXenes, that constitute a large family of 2D materials, have been targeted toward various applications due to their outstanding electronic properties. MXenes functioning as co-catalyst in combination with certain photocatalysts have been applied in photocatalytic systems to enhance photogenerated charge separation, suppress rapid charge recombination, and convert solar energy into chemical energy or use it in the degradation of organic compounds. The photocatalytic performance greatly depends on the composition and morphology of the photocatalyst, which, in turn, are determined by the method of preparation used. Here, we review the four different synthesis methods (mechanical mixing, self-assembly, in situ decoration, and oxidation) reported for MXenes in view of their application as co-catalyst in photocatalysis. In addition, the working mechanism for MXenes application in photocatalysis is discussed and an outlook for future research is also provided.

Thermally Reduced Graphene/MXene Film for Enhanced Li-ion Storage.

Two-dimensional transition-metal carbides called MXenes are emerging electrode materials for energy storage due to their metallic electrical conductivity and low ion diffusion barrier. In this work, we combined Ti2 CTx MXene with graphene oxide (GO) followed by a thermal treatment to fabricate flexible rGO/Ti2 CTr film, in which electrochemically active rGO and Ti2 CTr nanosheets impede the stacking of layers and synergistically interact producing ionically and electronically conducting electrodes. The effect of the thermal treatment on the electrochemical performance of Ti2 CTx is evaluated. As anode for Li-ion storage, the thermally treated Ti2 CTr possesses a higher capacity in comparison to as-prepared Ti2 CTx . The freestanding hybrid rGO/Ti2 CTr films exhibit excellent reversible capacity (700 mAh g-1 at 0.1 Ag-1 ), cycling stability and rate performance. Additionally, flexible rGO/Ti3 C2 Tr films are made using the same method and also present improved capacity. Therefore, this study provides a simple, yet effective, approach to combine rGO with different MXenes, which can enhance their electrochemical properties for Li-ion batteries.

Flexible MnS-Carbon Fiber Hybrids for Lithium-Ion and Sodium-Ion Energy Storage.

Nanostructures can improve battery capacity and cycle life, especially with sulfide electrodes. In this work, a freestanding flexible electrode, consisting of MnS nanoparticles embedded onto carbon nanofibers, was prepared by electrospinning. The produced hybrid was used as an electrode for lithium-ion and sodium-ion batteries. MnS nanoparticles have a size of about 5 nm and the particles are evenly distributed in the carbon nanofibers. Carbon nanofibers act as electronic conductors and buffer the volume change, while MnS nanoparticles react through rapid electrochemical reaction. As a Li-ion battery anode, this hybrid electrode exhibits specific capacities from 240 mAh g-1 at a high current density of 5 A g-1 , up to 600 mAh g-1 at 200 mA g-1 .

Two-Dimensional Vanadium Carbide (MXene) as Positive Electrode for Sodium-Ion Capacitors.

Ion capacitors store energy through intercalation of cations into an electrode at a faster rate than in batteries and within a larger potential window. These devices reach a higher energy density compared to electrochemical double layer capacitor. Li-ion capacitors are already produced commercially, but the development of Na-ion capacitors is hindered by lack of materials that would allow fast intercalation of Na-ions. Here we investigated the electrochemical behavior of 2D vanadium carbide, V2C, from the MXene family. We investigated the mechanism of Na intercalation by XRD and achieved capacitance of ∼100 F/g at 0.2 mV/s. We assembled a full cell with hard carbon as negative electrode, a known anode material for Na ion batteries, and achieved capacity of 50 mAh/g with a maximum cell voltage of 3.5 V.

Prediction and characterization of MXene nanosheet anodes for non-lithium-ion batteries.

Rechargeable non-lithium-ion (Na(+), K(+), Mg(2+), Ca(2+), and Al(3+)) batteries have attracted great attention as emerging low-cost and high energy-density technologies for large-scale renewable energy storage applications. However, the development of these batteries is hindered by the limited choice of high-performance electrode materials. In this work, MXene nanosheets, a class of two-dimensional transition-metal carbides, are predicted to serve as high-performing anodes for non-lithium-ion batteries by combined first-principles simulations and experimental measurements. Both O-terminated and bare MXenes are shown to be promising anode materials with high capacities and good rate capabilities, while bare MXenes show better performance. Our experiments clearly demonstrate the feasibility of Na- and K-ion intercalation into terminated MXenes. Moreover, stable multilayer adsorption is predicted for Mg and Al, which significantly increases their theoretical capacities. We also show that O-terminated MXenes can decompose into bare MXenes and metal oxides when in contact with Mg, Ca, or Al. Our results provide insight into metal ion storage mechanisms on two-dimensional materials and suggest a route to preparing bare MXene nanosheets.

Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide.

The intercalation of ions into layered compounds has long been exploited in energy storage devices such as batteries and electrochemical capacitors. However, few host materials are known for ions much larger than lithium. We demonstrate the spontaneous intercalation of cations from aqueous salt solutions between two-dimensional (2D) Ti3C2 MXene layers. MXenes combine 2D conductive carbide layers with a hydrophilic, primarily hydroxyl-terminated surface. A variety of cations, including Na(+), K(+), NH4(+), Mg(2+), and Al(3+), can also be intercalated electrochemically, offering capacitance in excess of 300 farads per cubic centimeter (much higher than that of porous carbons). This study provides a basis for exploring a large family of 2D carbides and carbonitrides in electrochemical energy storage applications using single- and multivalent ions.

Intercalation and delamination of layered carbides and carbonitrides.

Intercalation and delamination of two-dimensional solids in many cases is a requisite step for exploiting their unique properties. Herein we report on the intercalation of two-dimensional Ti3C2, Ti3CN and TiNbC-so called MXenes. Intercalation of hydrazine, and its co-intercalation with N,N-dimethylformamide, resulted in increases of the c-lattice parameters of surface functionalized f-Ti3C2, from 19.5 to 25.48 and 26.8 Å, respectively. Urea is also intercalated into f-Ti3C2. Molecular dynamics simulations suggest that a hydrazine monolayer intercalates between f-Ti3C2 layers. Hydrazine is also intercalated into f-Ti3CN and f-TiNbC. When dimethyl sulphoxide is intercalated into f-Ti3C2, followed by sonication in water, the f-Ti3C2 is delaminated forming a stable colloidal solution that is in turn filtered to produce MXene 'paper'. The latter shows excellent Li-ion capacity at extremely high charging rates.