Low-dimensional carbon and MXene-based electrochemical capacitor electrodes

A Portable Integrated Electrochemical Sensing System for On-Site Nitrite Detection in Food

Ensuring an appropriate nitrite level in food is essential to keep the body healthy. However, it still remains a huge challenge to offer a portable and low-cost on-site food nitrite analysis without any expensive equipment. Herein, a portable integrated electrochemical sensing system (IESS) is developed to achieve rapid on-site nitrite detection in food, which is composed of a low-cost disposable microfluidic electrochemical patch for few-shot nitrite detection, and a reusable smartphone-assisted electronic device based on self-designed circuit board for signal processing and wireless transmission. The electrochemical patch based on MXene-Ti3C2Tx/multiwalled carbon nanotubes-cyanocobalamin (MXene/MWCNTs-VB12)-modified working electrode achieves high sensitivity of 10.533 µA mm-1 and low nitrite detection limit of 4.22 µm owing to strong electron transfer ability of hybrid MXene/MWCNTs conductive matrix and high nitrite selectivity of VB12 bionic enzyme-based ion-selective layer. Moreover, the portable IESS can rapidly collect pending testing samples through a microfluidic electrochemical patch within 1.0 s to conduct immediate nitrite analysis, and then wirelessly transmit data from a signal-processing electronic device to a smartphone via Bluetooth module. Consequently, this proposed portable IESS demonstrates rapid on-site nitrite analysis and wireless data transmission within one palm-sized electronic device, which would pave a new avenue in food safety and personal bespoke therapy.

Progress and Perspective in harnessing MXene-carbon-based composites (0-3D): Synthesis, performance, and applications

MXene is recognized as a promising catalyst for versatile applications due to its abundant metal sites, physicochemical properties, and structural formation. This comprehensive review offers an in-depth analysis of the incorporation of carbon into MXene, resulting in the formation of MXene-carbon-based composites (MCCs). Pristine MXene exhibits numerous outstanding characteristics, such as its atomically thin 2D structure, hydrophilic surface nature, metallic electrical conductivity, and substantial specific surface area. The introduction of carbon guides the assembly of MCCs through electrostatic self-assembly, pairing positively charged carbon with negatively charged MXene. These interactions result in increased interlayer spacing, reduced ion/electron transport distances, and enhanced surface hydrophilicity. Subsequent sections delve into the synthesis methods for MCCs, focusing on MXene integrated with various carbon structures, including 0D, 1D, 2D, and 3D carbon. Comprehensive discussions explore the distinctive properties of MCCs and the unique advantages they offer in each application domain, emphasizing the contributions and advancements they bring to specific fields. Furthermore, this comprehensive review addresses the challenges encountered by MCCs across different applications. Through these analyses, the review promotes a deeper understanding of exceptional characteristics and potential applications of MCCs. Insights derived from this review can serve as guidance for future research and development efforts, promoting the widespread utilization of MCCs across a broad spectrum of disciplines and spurring future innovations.

2D MXene-Based Biosensing: A Review

MXene emerged as decent 2D material and has been exploited for numerous applications in the last decade. The remunerations of the ideal metallic conductivity, optical absorbance, mechanical stability, higher heterogeneous electron transfer rate, and good redox capability have made MXene a potential candidate for biosensing applications. The hydrophilic nature, biocompatibility, antifouling, and anti-toxicity properties have opened avenues for MXene to perform in vitro and in vivo analysis. In this review, the concept, operating principle, detailed mechanism, and characteristic properties are comprehensively assessed and compiled along with breakthroughs in MXene fabrication and conjugation strategies for the development of unique electrochemical and optical biosensors. Further, the current challenges are summarized and suggested future aspects. This review article is believed to shed some light on the development of MXene for biosensing and will open new opportunities for the future advanced translational application of MXene bioassays.

Three-dimensional N-doped mesoporous carbon–MXene hybrid architecture for supercapacitor applications

NMC@MXene exhibits excellent rate capability as electrode material for supercapacitors.

Tunable Structured MXenes With Modulated Atomic Environments: A Powerful New Platform for Electrocatalytic Energy Conversion

Owing to their rich surface chemistry, high conductivity, tunable bandgap, and thermal stability, structured 2D transition-metal carbides, nitrides, and carbonitrides (MXenes) with modulated atomic environments have emerged as efficient electrochemical energy conversion systems in the past decade. Herein, the most recent advances in the engineering of tunable structured MXenes as a powerful new platform for electrocatalytic energy conversion are comprehensively summarized. First, the state-of-the-art synthetic and processing methods, tunable nanostructures, electronic properties, and modulation principles of engineering MXene-derived nanoarchitectures are focused on. The current breakthroughs in the design of catalytic centers, atomic environments, and the corresponding structure-performance correlations, including termination engineering, heteroatom doping, defect engineering, heterojunctions, and alloying, are discussed. Furthermore, representative electrocatalytic applications of structured MXenes in energy conversion systems are also summarized. Finally, the challenges in and prospects for constructing MXene-based electrocatalytic materials are also discussed. This review provides a leading-edge understanding of the engineering of various MXene-based electrocatalysts and offers theoretical and experimental guidance for prospective studies, thereby promoting the practical applications of tunable structured MXenes in electrocatalytic energy conversion systems.

Two Dimensional MOene: From Superconductors to Direct Semiconductors and Weyl Fermions

The number of semiconducting MXenes with direct band gaps is extremely low; thus, it is highly desirable to broaden the MXene family beyond carbides and nitrides to expand the palette of desired chemical and physical properties. Here, we theoretically report the existence of the single-layer (SL) dititanium oxide 2H-Ti₂O MOene (MXene-like 2D transition oxides), showing an Ising superconducting feature. Moreover, SL halogenated 2H- and 1T-Ti₂O monolayers display tunable semiconducting features and strong light-harvesting ability. In addition, the external strains can induce Weyl fermions via quantum phase transition in 2H-Ti₂OF₂ and Ti₂OCl₂ monolayers. Specifically, 2H- and 1T-Ti₂OF₂ are direct semiconductors with band gaps of 0.82 and 1.18 eV, respectively. Furthermore, the carrier lifetimes of SL 2H- and 1T-Ti₂OF₂ are evaluated to be 0.39 and 2.8 ns, respectively. This study extends emerging phenomena in a rich family of 2D MXene-like MOene materials, which provides a novel platform for next-generation optoelectronic and photovoltaic fields.

Strain modulation on the spin transport properties of PTB junctions with MoC2 electrodes

Based on MoC₂ nanoribbons and poly-(terphenylene-butadiynylene) (PTB) molecules, we designed MoC₂-PTB molecular spintronic devices and investigated their spin-dependent electron transport properties by using spin-polarized density functional theory and the non-equilibrium Green's function method. As a typical MXene material, it is found that the magnetic contribution of MoC₂ nanoribbons mainly comes from the delocalized 3d electron of edge Mo atoms. Owing to the obvious spin-splitting near the Fermi level of the MoC₂ nanoribbon electrode, the spin states can be effectively injected into the central scattering region under an external bias voltage. In addition, we also studied the effects of z-axis strain on the spin transport properties of the PTB molecular device, where the strain was controlled within the range of -9% to 9%. Under a compressed strain, spin current increases obviously, and the spin-filtering efficiency (SFE) decreases slightly. Nevertheless, under a tensile strain, we found that the SFE increases but spin current decreases. Moreover, z-axis strain can induce a negative differential resistance (NDR) effect at a high bias point. This work would expand the potential applications of new two-dimensional (2D) materials in the field of molecular spintronic devices.

Computational insights into modulating the performance of MXene based electrode materials for rechargeable batteries

MXene, a still-growing large family of two-dimensional (2D) materials, has aroused enormous attention in the scientific community. Owing to their high specific surface area, good electronic conductivity, stability, and hydrophilicity, MXene has found a wide application involving electromagnetic interference shielding, sensors, catalysis, and energy storage, etc. In the field of energy storage, MXenes are promising electrode materials for various metal-ion batteries and they are also effective anchoring materials for Li-S batteries. One of the most unique features of MXene is its abundant compositions, which renders us large room to modulate its properties. Besides, other effective approaches applicable to traditional 2D materials can also be used to optimize the performance of MXene. Theoretical calculations have played a significant role in predicting and screening high-performance MXene based electrode materials. So far, theoretical researchers have made much progress in optimizing the performance of MXene as electrode materials for various rechargeable batteries. In the present review, started by a brief introduction of the involved mechanism and basic calculation methods, we comprehensively overview the latest theoretical studies of modulating the performance of MXene based electrode materials for rechargeable batteries.

A wearable battery-free wireless and skin-interfaced microfluidics integrated electrochemical sensing patch for on-site biomarkers monitoring in human perspiration

In this study, an ultra-high sensitive, flexible, wireless, battery-free, and fully integrated (no external analysis equipment) electrochemical sensing patch system, including a microfluidic-sweat collecting unit, was newly developed for the on-site monitoring of the [K⁺] concentration in human sweat. Multiwalled carbon nanotube (MWCNT) and MXene-Ti₃C₂T_X based hybrid multi-dimensional networks were applied to obtain a high surface activation area and faster charge transfer rate, strongly adsorbing the valinomycin membrane to protect the ionophore for effective transshipment and immobilization of the [K⁺]. Furthermore, the controllable porosity of carbon-based materials can accelerate the kinetic process of ion diffusion. This hybrid nanonetwork structure effectively enhanced electrochemical stability and sensitivity, addressing the noise and signal drifting problems experienced with low concentration detection. The fabricated sensor exhibited a high ion concentration sensitivity of 63 mV/dec with excellent selectivity, amplified to 173 mV/dec with the integrated amplification system. The Near Field Communication (NFC) is used to transmit measurements to a smartphone wirelessly. A microfluidic channel was integrated with the electrochemical sensor patch to efficiently collect sweat on the human skin surface and mitigate the sensor surface contamination problem. Furthermore, the developed sensing patch can also be applied to other biomarkers on-site detection after modifying the working electrode with the corresponding selective membranes.

Microstructure and Mechanical Properties of Alumina Composites with Addition of Structurally Modified 2D Ti3C2 (MXene) Phase

This study presents new findings related to the incorporation of MXene phases into ceramic. Aluminium oxide and synthesised Ti₃C₂ were utilised as starting materials. Knowing the tendency of MXenes to oxidation and degradation, particularly at higher temperatures, structural modifications were proposed. They consisted of creating the metallic layer on the Ti₃C₂, by sputtering the titanium or molybdenum. To prepare the composites, powder metallurgy and spark plasma sintering (SPS) techniques were adopted. In order to evaluate the effectiveness of the applied modifications, the emphasis of the research was placed on microstructural analysis. In addition, the mechanical properties of the obtained sinters were examined. Observations revealed significant changes in the MXenes degradation process, from porous areas with TiC particles (for unmodified Ti₃C₂), to in situ creation of graphitic carbon (in the case of Ti₃C₂-Ti/Mo). Moreover, the fracture changed from purely intergranular to cracking with high participation of transgranular mode, analogously. In addition, the results obtained showed an improvement in the mechanical properties for composites with Ti/Mo modifications (an increase of 10% and 15% in hardness and fracture toughness respectively, for specimens with 0.5 wt.% Ti₃C₂-Mo). For unmodified Ti₃C₂, enormously cracked areas with spatters emerged during tests, making the measurements impossible to perform.

Ti3C2-Based MXene Oxide Nanosheets for Resistive Memory and Synaptic Learning Applications

MXene, a new state-of-the-art two-dimensional (2D) nanomaterial, has attracted considerable interest from both industry and academia because of its excellent electrical, mechanical, and chemical properties. However, MXene-based device engineering has rarely been reported. In this study, we explored Ti₃C₂ MXene for digital and analog computing applications by engineering the top electrode. For this purpose, Ti₃C₂ MXene was synthesized by a simple chemical process, and its structural, compositional, and morphological properties were studied using various analytical tools. Finally, we explored its potential application in bipolar resistive switching (RS) and synaptic learning devices. In particular, the effect of the top electrode (Ag, Pt, and Al) on the RS properties of the Ti₃C₂ MXene-based memory devices was thoroughly investigated. Compared with the Ag and Pt top electrode-based devices, the Al/Ti₃C₂/Pt device exhibited better RS and operated more reliably, as determined by the evaluation of the charge-magnetic property and memory endurance and retention. Thus, we selected the Al/Ti₃C₂/Pt memristive device to mimic the potentiation and depression synaptic properties and spike-timing-dependent plasticity-based Hebbian learning rules. Furthermore, the electron transport in this device was found to occur by a filamentary RS mechanism (based on oxidized Ti₃C₂ MXene), as determined by analyzing the electrical fitting curves. The results suggest that the 2D Ti₃C₂ MXene is an excellent nanomaterial for non-volatile memory and synaptic learning applications.

First-Principles Study of the Ferromagnetic Properties of Cr2CO2 and Cr2NO2 MXenes

Massive attention has been paid to MXenes due to their intriguing properties and potential diverse applications. Extensive studies using first-principles calculations on the electronic structures of MXenes Cr₂CO₂ and Cr₂NO₂ were performed in this paper. Based on the accurate Heyd-Scuseria-Ernzerhof (HSE) calculations, Cr₂CO₂ is clarified to be a ferromagnetic semiconductor; meanwhile, Cr₂NO₂ is a half-metallic material, which is consistent with previous results. In particular, by analyzing the contribution of the orbitals to the band structures and density of states, the basic mechanism of ferromagnetism was analyzed in detail. Our theoretical work might promote the spintronics study and application of Cr-contained MXenes.

An Ultrafast Conducting Polymer@MXene Positive Electrode with High Volumetric Capacitance for Advanced Asymmetric Supercapacitors

Pseudocapacitors or redox capacitors that synergize the merits of batteries and double-layer capacitors are among the most promising candidates for high-energy and high-power energy storage applications. 2D transition metal carbides (MXenes), an emerging family of pseudocapacitive materials with ultrahigh rate capability and volumetric capacitance, have attracted much interest in recent years. However, MXenes have only been used as negative electrodes as they are easily oxidized at positive (anodic) potential. To construct a high-performance MXene-based asymmetric device, a positive electrode with a compatible performance is highly desired. Herein, an ultrafast polyaniline@MXene cathode prepared by casting a homogenous polyaniline layer onto a 3D porous Ti₃ C₂ T_x MXene is reported, which enables the stable operation of MXene at positive potentials because of the enlarged work function after compositing with polyaniline, according to the first-principle calculations. The resulting flexible polyaniline@MXene positive electrode demonstrates a high volumetric capacitance of 1632 F cm^-3 and an ultrahigh rate capability with 827 F cm^-3 at 5000 mV s^-1 , surpassing all reported positive electrodes. An asymmetric device is further fabricated with MXene as the anode and polyaniline@MXene as the cathode, which delivers a high energy density of 50.6 Wh L^-1 and an ultrahigh power density of 127 kW L^-1 .

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

Functional MXene Materials: Progress of Their Applications

Nowadays, two-dimensional materials have many applications in materials science. As a novel two-dimensional layered material, MXene possesses distinct structural, electronic, and chemical properties; thus, it has potential applications in many fields, including battery electrodes, energy storage materials, sensors, and catalysts. Up to now, more than 70 MAX phases have been reported. However, in contrast to the variety of MAX phases, the existing MXene family merely includes Ti₂ C, Ti₃ C₂ , (Ti_1/2 , Nb_1/2 )₂ C, (V_1/2 , Cr_1/2 )₃ C₂ , Nb₂ C, Ti₃ CN, Ta₄ C₃ , V₂ C, and Nb₄ C₃ . Among these materials, the Ti₃ C₂ T_x MXene exhibits prominently high volumetric capacitance, and the rate at which it transports electron is suitable for electrode materials in batteries and supercapacitors. Hence, Ti₃ C₂ T_x is commonly utilized as an electrode material in ion batteries such as Li⁺ , Na⁺ , K⁺ , Mg²⁺ , Ca²⁺ , and Al³⁺ batteries. What is more, Ti₂ C has the biggest specific surface area among all of these potential MXene phases, and therefore, Ti₂ C has remarkably high gravimetric hydrogen storage capacities. In addition, Ti₂ CO₂ materials display extremely high activity for CO oxidation, which makes it possible to design catalysts for CO oxidation at low temperatures. Furthermore, Ti₃ C₂ T_x with O, OH, and/or F terminations can be used for water purification owing to excellent water permeance, favorable filtration ability, and long-time operation ability. This review supplies a relatively comprehensive summary of various applications of MXenes over the past few years.

Versatile porous graphene flakes derived from alkali metal carbonates using an ultrafast and sulfuric acid-free solid-state oxidation reaction

Herein, we report on an unprecedented synthetic method for single-layered GO that takes just a few tens of minutes. This rationally designed solid-state oxidation based on alkali metal carbonates (Li2CO3, Na2CO3, K2CO3) involves a molten salt reaction, which enables the effective exfoliation and oxidation of graphene layers without using H2SO4 and KMnO4. The advantage of this approach is not only the ability to avoid the introduction of strong acid reactants in the reaction process, but this approach also leads to a 4.2 times larger specific surface area than conventional GO. For these reasons, we anticipate that this green, safe, fast and effective approach enables practical applications in graphene-based energy storage and light-absorbing black materials.

Clay-Inspired MXene-Based Electrochemical Devices and Photo-Electrocatalyst: State-of-the-Art Progresses and Challenges

MXene, an important and increasingly popular category of postgraphene 2D nanomaterials, has been rigorously investigated since early 2011 because of advantages including flexible tunability in element composition, hydrophobicity, metallic nature, unique in-plane anisotropic structure, high charge-carrier mobility, tunable band gap, and favorable optical and mechanical properties. To fully exploit these potentials and further expand beyond the existing boundaries, novel functional nanostructures spanning monolayer, multilayer, nanoparticles, and composites have been developed by means of intercalation, delamination, functionalization, hybridization, among others. Undeniably, the cutting-edge developments and applications of clay-inspired 2D MXene platform as electrochemical electrode or photo-electrocatalyst have conferred superior performance and have made significant impact in the field of energy and advanced catalysis. This review provides an overview of the fundamental properties and synthesis routes of pure MXene, functionalized MXene and their hybrids, highlights the state-of-the-art progresses of MXene-based applications with respect to supercapacitors, batteries, electrocatalysis and photocatalysis, and presents the challenges and prospects in the burgeoning field.