Wearable electronics are some of the most promising technologies with the potential to transform many aspects of human life such as smart healthcare and intelligent communication. The design of self-powered fabrics with the ability to efficiently harvest energy from the ambient environment would not only be beneficial for their integration with textiles, but would also reduce the environmental impact of wearable technologies by eliminating their need for disposable batteries. Herein, inspired by classical Archimedean spirals, we report a metastructured fiber fabricated by scrolling followed by cold drawing of a bilayer thin film of an MXene and a solid polymer electrolyte. The obtained composite fibers with a typical spiral metastructure (SMFs) exhibit high efficiency for dispersing external stress, resulting in simultaneously high specific mechanical strength and toughness. Furthermore, the alternating layers of the MXene and polymer electrolyte form a unique, tandem ionic-electronic coupling device, enabling SMFs to generate electricity from diverse environmental parameters, such as mechanical vibrations, moisture gradients, and temperature differences. This work presents a design rule for assembling planar architectures into robust fibrous metastructures, and introduces the concept of ionic-electronic coupling fibers for efficient multimodal energy harvesting, which have great potential in the field of self-powered wearable electronics. In this work, a concept of ionic-electronic coupling fibers by integrating a 2D MXene and a polymer electrolyte to fabricate spiral metastructures is proposed to realize multimodal power generation from various sources simultaneously.

The creation of vacancies and/or pores into two-dimensional materials, like graphene and MXenes, has shown to increase their performance for sustainable applications. However, a simple and affordable method with controlled and tailorable vacancy concentration and/or pores size remains challenging. Herein, a simple and reproducible method is presented for controlled synthesis of Mo1.74CTz MXene with randomly distributed vacancies and pores, obtained from selective etching of both Ga and Cr in the Cr-alloyed MAX-phase like precursor Mo1.74Cr0.26Ga2C. Structural and compositional analysis of the 3D alloy show approximate to 13% Cr on the metal site, homogeneously distributed between different particles and within the atomic structure. After etching, it translates to Mo1.74CTz MXene, exhibiting defect-rich sheets. Notably, the incorporation of Cr facilitates a shorter etching time with an improved yield compared to Mo2CTz. The Mo1.74CTz MXene displays excellent electrochemical properties, almost doubling the capacitance values (1152 F cm(-3) and 297 F g(-1) at 2 mV s(-1) scan rate), compared to its pristine counterpart Mo2CTz. The presented method and obtained results suggest defect engineering of MXenes through precursor alloying as a pathway that can be generalized to other phases, to further improve their properties for various applications.

Assembly of two-dimensional (2D) nanomaterials into well-organized architectures is pivotal for controlling their function and enhancing performance. As a promising class of 2D nanomaterials, MXenes have attracted significant interest for use in wearable electronics due to their unique electrical and mechanical properties. However, facile approaches for fabricating MXenes into macroscopic fibers with controllable structures are limited. In this study, we present a strategy for easily spinning MXene fibers by incorporating polyanions. The introduction of poly(acrylic acid) (PAA) into MXene colloids has been found to alter MXene aggregation behavior, resulting in a reduced concentration threshold for lyotropic liquid crystal phase. This modification also enhances the viscosity and shear sensitivity of MXene colloids. Consequently, we were able to draw continuous fibers directly from the gel of MXene aggregated with PAA. These fibers exhibit homogeneous diameter and high alignment of MXene nanosheets, attributed to the shear-induced long-range order of the liquid crystal phase. Furthermore, we demonstrate proof-of-concept applications of the ordered MXene fibers, including textile-based supercapacitor, sensor and electrical thermal management, highlighting their great potential applied in wearable electronics. This work provides a guideline for processing 2D materials into controllable hierarchical structures by regulating aggregation behavior through the addition of ionic polymers.

i-MXenes, a new family of 2D transition metal carbides with in-plane ordered vacancies, have shown great potential in aqueous supercapacitor (SC) applications due to their high volumetric capacitances and energy densities. However, how vacancies affect their electrochemical performance, in general, and their self-discharge (SD) behavior in particular, remains unexplored. Herein, we compare the electrochemical performance of vacancy rich, ordered Mo_1.33CT_z i-MXene to that of Mo₂CT_z (with much less vacancies) in a 1 M sulfuric acid (H₂SO₄) or 15 M of lithium bromide (LiBr) electrolyte. The Mo_1.33CT_z exhibits higher volumetric capacitances and energy densities, but at the cost of a higher SD rate. Specifically, the Mo_1.33CT_z symmetric SCs deliver an energy density as high as 25.4 mWh cm(-3) at 152.4 mW cm^-3, with 65.4% voltage retention after 10 h in 15 M LiBr. In comparison, the Mo₂CT_z symmetric SCs have a maximum energy density of 20.8 mWh cm^-3 at 124.9 mW cm^-3, with 73.1% voltage retention after 10 h in the same electrolyte. The SD rates in the H₂SO₄ electrolyte are quite rapid.

MXenes have been introduced as a high energy and power density electrochemical supercapacitor material owing to their high specific capacitance and electrochemical stability. The operating potential window and, in turn, energy density of MXene based symmetric and asymmetric supercapacitors can be effectively enhanced by the proper choice of aqueous electrolyte. Herein, we investigate the electrochemical behavior of vacancy-containing 𝑖-MXene (Mo_1.33CT_z) in sulfate based aqueous electrolytes with univalent (Li⁺, Na⁺, or K⁺) or divalent (Mg²⁺, Mn²⁺, or Zn²⁺) cations. The results show that the Mo_1.33CT_z MXene electrodes can be operated in a potential window higher than 1 V without degradation in these sulfate electrolytes. The Mo_1.33CT_z MXene electrodes deliver a high volumetric capacitance up to ~677 F cm^-3 as measured in 1.0 M MnSO₄ solution. Furthermore, symmetric (Mo_1.33CT_z//Mo_1.33CT_z) and asymmetric (Mo_1.33CT_z//nitrogen-doped activated carbon (NAC)) devices in 0.5 M K₂SO₄ solution can be operated with a cell voltage of about 1.1 V and 1.8 V, respectively. The asymmetric devices retain about 97% of their initial capacitance after 5000 charge/discharge cycles. Overall, the results reveal that the choice of the intercalating cations is a viable route to boost the performance of Mo_1.33CT_z MXene and to construct energy storage devices.

Recently, a 2D metal boride - boridene - has been experimentally realized in the form of single-layer molybdenum boride sheets with ordered metal vacancies, through selective etching of the nanolaminated 3D parent borides (Mo_2/3Y_1/3)₂AlB₂ and (Mo_2/3Sc_1/3)₂AlB₂. The chemical formula of the boridene was suggested to be Mo_4/3B_2-xT_z, where T_z denotes surface terminations. Here, the termination composition and material properties of Mo_4/3B_2-xT_z from both theoretical and experimental perspectives are investigated. Termination sites are considered theoretically for termination species T = O, OH, and F, and the energetically favored termination configuration is identified at z = 2 for both single species terminations and binary termination mixes of different stoichiometries in ordered and disordered configurations. Mo_4/3B_2-xT_z is shown to be dynamically stable for multiple termination stoichiometries, displaying semiconducting, semimetallic, or metallic behavior depending on how different terminations are combined. The approximate chemical formula of a freestanding film of boridene is attained as Mo_1.33B_1.9O_0.3(OH)_1.5F_0.7 from X-ray photoelectron spectroscopy (XPS) analysis which, within error margins, is consistent with the theoretical results. Finally, metallic and additive-free Mo_4/3B_2-xT_z shows high catalytic performance for the hydrogen evolution reaction, with an onset potential of 0.15 V versus the reversible hydrogen electrode.

Aqueous asymmetric supercapacitors (AASCs) can have high voltages and high energy densities. However, the rational design of AASCs with proper negative and positive electrodes remains a challenge. Herein, we report on an AASC using Mo1.33CTz MXene films as the negative electrode, and tetramethylammonium cation intercalated birnessite (TMA(+)-MnO2) films as the positive electrode in a 21 mol kg(-1) lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) electrolyte. Benefiting from a high, stable voltage of 2.5 V, an energy density of 86.5 Wh L-1 at 2 mV s(-1) and a power density of 10.3 kW L-1 at 1 Vs(-1) were achieved. The cells also exhibit excellent cycling stability (>98% after 1,0000 cycles at 100 mV s(-1)) and a 51.1 % voltage retention after 10 h. This good performance is attributed to the high stable potential window and high volumetric capacitances of both Mo1.33CTz and TMA(+)-MnO2 electrodes in highly concentrated electrolytes. This work provides a roadmap for developing high performance AASCs with high voltages and high energy/power densities, with relatively slow self-discharge rates.

The pursuit of new and better battery materials has given rise to numerous studies of the possibilities to use two-dimensional negative electrode materials, such as MXenes, in lithium-ion batteries. Nevertheless, both the origin of the capacity and the reasons for significant variations in the capacity seen for different MXene electrodes still remain unclear, even for the most studied MXene: Ti3C2Tx. Herein, freestanding Ti3C2Tx MXene films, composed only of Ti3C2Tx MXene flakes, are studied as additive-free negative lithium-ion battery electrodes, employing lithium metal half-cells and a combination of chronopotentiometry, cyclic voltammetry, X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy experiments. The aim of this study is to identify the redox reactions responsible for the observed reversible and irreversible capacities of Ti3C2Tx- based lithium-ion batteries as well as the reasons for the significant capacity variation seen in the literature. The results demonstrate that the reversible capacity mainly stems from redox reactions involving the Tx-Ti-C titanium species situated on the surfaces of the MXene flakes, whereas the Ti-C titanium present in the core of the flakes remains electro-inactive. While a relatively low reversible capacity is obtained for electrodes composed of pristine Ti3C2Tx MXene flakes, significantly higher capacities are seen after having exposed the flakes to water and air prior to the manufacturing of the electrodes. This is ascribed to a change in the titanium oxidation state at the surfaces of the MXene flakes, resulting in increased concentrations of Ti(II), Ti(III), and Ti(IV) in the Tx-Ti-C surface species. The significant irreversible capacity seen in the first cycles is mainly attributed to the presence of residual water in the Ti3C2Tx electrodes. As the capacities of Ti3C2Tx MXene negative electrodes depend on the concentration of Ti(II), Ti(III), and Ti(IV) in the Tx-Ti-C surface species and the water content, different capacities can be expected when using different manufacturing, pretreatment, and drying procedures.

Materials with tailored properties are crucial for high performance electronics applications. Hybrid materials composed of inorganic and organic components can possess unique merits for broad application by synergy between the advantages the respective material type offers. Here we demonstrate a novel electrochemical polymerization (EP) enabled by a 2D transition metal carbide MXene for obtaining conjugated polymer-MXene composite films deposited on conducting substrates without using traditional electrolytes, indispensable compounds for commonly electrochemical polymerization. The universality of the process provides a novel approach for EP allowing fast facile process for obtaining different new polymer/MXene composites with controlled thickness and micro-pattern. Furthermore, high performance microsupercapacitors and asymmetric microsupercapacitors are realized based on the excellent composites benefiting from higher areal capacitance, better rate capabilities and lower contact resistance than conventional electropolymerized polymers. The AMSCs exhibit a maximum areal capacitance of 69.5 mF cm(-2), an ultrahigh volumetric energy density (250.1 mWh cm(-3)) at 1.6 V, and excellent cycling stability up to 10000 cycles. The excellent electrochemical properties of the composite polymerized with MXene suggest a great potential of the method for various energy storage applications.

tThe chemical bonding in the carbide core and the surface chemistry in a new group of transition-metalcarbides Tin+1Cn-Tx(n = 1,2) called MXenes have been investigated by surface-sensitive valence bandX-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thick-nesses are analyzed in connection to known hybridization regions of TiC and TiO2that affect elastic andtransport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s– Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, theO 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorptionsites. The effect of surface oxidation and Ar+sputtering on the electronic structure is also discussed.