High-Performance Yet Sustainable Triboelectric Nanogenerator Based on Sulfur-Rich Polymer Composite with MXene Segregated Structure.

State-of-the-art triboelectric nanogenerators (TENGs) typically employ fluoropolymers, highly negative chargeable materials in triboelectric series. However, many researchers nowadays are concerned about environmental pollution caused by poly-and per-fluoroalkyl substances (PFAS) due to their critical immunotoxicity as fluoropolymers are likely to release PFAS into the ecosystem during their life cycle. Herein, a sulfur-rich polymer (SRP)/MXene composite, offering high-performance yet sustainable TENG is developed. Value-addition of sulfur into SRP-based TENG has huge advantages since sulfur is abundant waste from petroleum refining and possesses the highest electron affinity (-200 kJ mol-1) among polymerizable atoms. MXene segregated structure is introduced into SRP to achieve homogeneous distribution without electrical percolation by utilizing below 0.5 wt% of MXene, resulting in a significantly enhanced dielectric constant without a drastic increase of dielectric loss. Due to homogeneous MXene distribution, SRP/MXene composite-based TENG demonstrates 2.9 times and 19.5 times enhances peak voltage and peak current compared to previous SRP-based TENGs. Additionally, it exhibits reusability without critical reduction of modulus and TENG performance due to dynamically exchangeable disulfide bonds. Finally, after the corona discharging and scaling-up process to a 4-inch wafer size, SRP/MXene composite-based TENG exhibits an 8.4 times improvement in peak power density, reaching 3.80 W m-2 compared to previous SRP-based TENGs.

Surface nitrided MXene sheets with outstanding electroconductivity and oxidation stability.

Two-dimensional Ti3C2Tx MXenes are promising candidates for a wide range of film- or fiber-based devices owing to their solution processability, high electrical conductivity, and versatile surface chemistry. The surface terminal groups (Tx) of MXenes can be removed to increase their inherent electrical performance and ensure chemical stability. Therefore, understanding the chemical evolution during the removal of the terminal groups is crucial for guiding the production, processing, and application of MXenes. Herein, we investigate the effect of chemical modification on the electron-transfer behavior during the removal of the terminal groups by annealing Ti3C2Tx MXene single sheets under argon (Ar-MXene) and ammonia gas (NH3-MXene) conditions. Annealing in ammonia gas results in surface nitridation of MXenes and preserves the electron-abundant Ti3C2 structure, whereas annealing MXene single sheets in Ar gas results in the oxidation of the titanium layers. The surface-nitrided MXene film exhibits an electrical conductivity two times higher than that of the Ar-MXene film. The oxidation stability is quantified by calculating the oxidation rate constants for severe reactions with H2O2. The surface-nitrided MXene is 13 times more stable than Ar-MXene. The investigation of MXene single sheets provides fundamental insights that are valuable for designing electrically conductive and chemically stable MXenes.

Impermeable Graphene Skin Increases the Heating Efficiency and Stability of an MXene Heating Element.

A Joule heater made of emerging 2D nanosheets, i.e., MXene, has the advantage of low-voltage operation with stable heat generation owing to its highly conductive and uniformly layered structure. However, the self-heated MXene sheets easily get oxidized in warm and moist environments, which limits their intrinsic heating efficiencies. Herein, an ultrathin graphene skin is introduced as a surface-regulative coating on MXene to enhance its oxidative stability and Joule heating efficiency. The skin layer is deposited on MXene using a scalable solution-phased layer-by-layer assembly process without deteriorating the excellent electrical conductivity of the MXene. The graphene skin comprises narrow and hydrophobic channels, which results in ≈70 times higher water impermeability of the hybrid film of graphene and MXene (GMX) than that of the pristine MXene. A complementary electrochemical analysis confirms that the graphene skin facilitates longer-lasting protection than conventional polymer coatings owing to its tortuous pathways. In addition, the sp2 planar carbon surface with a low heat loss coefficient improves the heating efficiency of the GMX, indicating that this strategy is promising for developing adaptive heating materials with a tractable voltage range and high Joule heating efficiency.

Grafting Behavior of Amine Ligands for Surface Modification of MXene.

Surface modification to improve the oxidation stability and dispersibility of MXene in diverse organic media is a facile strategy for broadening its application. Among the various ligands that can be grafted on the MXene surface, oleylamine (OAm), with amine functionalities, is an advantageous candidate owing to its strong interactions and commercial viability. OAms are grafted onto MXene through covalent bonds induced by nucleophilic reactions and H bonds in liquid interface reactions at room temperature. In addition, this grafting behavior of the ligand was characterized by a reduction in the slope with an increase in the ligand concentration (Cl), confirming that the OAms were grafted via Langmuir-like behavior, and the monolayer of OAms was developed via two distinct steps (I: lying-down phase; II: ordered monolayer). MXene nanosheets modified by OAm (OAm-MX) are highly dispersible in a wide range of organic solvents owing to the alkyl chain of the OAms, which induces hydrophobic properties on the surface of MXene. The OAm-MX dispersion exhibits outstanding oxidation and dispersion stability and remarkable coating performance on a wide range of substrates owing to their excellent solution processability. Therefore, this study provides fundamental insights into the adsorption behavior and interaction between amine ligands and MXene nanosheets for the surface chemistry of MXene.

Sub-Second Joule-Heated RuO2-Decorated Nitrogen- and Sulfur-Doped Graphene Fibers for Flexible Fiber-type Supercapacitors.

Graphene-based fiber-shaped supercapacitors (FSSCs) have received considerable attention as potential wearable energy storage devices owing to their simple operating mechanism, flexibility, and long-term stability. To date, energy storage capacities of supercapacitors have been significantly improved via strategies such as heteroatom doping and the incorporation of pseudocapacitive metal oxides. Herein, we develop a novel and scalable direct-hybridization method that combines heteroatom doping and metal oxide hybridization for the fabrication of high-performance FSSCs. Using porous and highly conductive nitrogen and sulfur co-doped graphene fibers (NS-GFs) as self-heating units, we successfully convert ruthenium hydroxide anchored to the surface into ruthenium oxide nanoparticles after programmed sub-second electrothermal annealing without structural damage of the fibers. The resulting fibers show an increased gravimetric capacitance of 68.88 F g-1 compared to that of the pristine NS-GF (8.32 F g-1), excellent cyclic stability maintaining 96.67% of the initial capacitance after 20 000 continuous charging/discharging cycles, and good mechanical flexibility. The findings of this work advocate a successful Joule heating strategy for preparing high-performance graphene-based metal oxide hybrid FSSCs for use in energy storage applications.