MXene Electrodes Research Articles

Boosting the capacity of MXene electrodes in neutral aqueous electrolytes

Boosting the capacity of MXene electrodes in neutral aqueous electrolytes

Fluidic MXene Electrode Functionalized with Iron Single Atoms for Selective Electrocatalytic Nitrate Transformation to Ammonia.

The growth of renewable energy industries and the ongoing need for fertilizer in agriculture have created a need for sustainable production of ammonia (NH3) using low-cost, environment-friendly techniques. The electrocatalytic nitrate (NO3-) reduction reaction (NO3RR) has the potential to improve both the management of environmental nitrogen and the recycling of synthetic nutrients. However, NO3RR is frequently hindered by the incomplete NO3- conversion, sluggish reaction kinetics, and suppression of the hydrogen evolution reaction (HER). Inspired by specific local electronic structures that are adjustable for single-atom catalysts, this work presents a nanohybrid electrocatalytic filter with iron single atoms (FeSA) immobilized on MXene. The fabricated FeSA/MXene filter exhibited maximum NH3 Faradaic efficiency and selectivity (82.9 and 99.2%, respectively) that were higher than those for filters made of Fe nanoparticles anchored on MXene (FeNP/MXene) (69.2 and 81.3%, respectively) and MXene alone (32.8 and 52.4%, respectively), measured at an initial pH of 7 and an applied potential of -1.4 V vs Ag/AgCl. Density functional theory calculations revealed that, compared to the FeNP/MXene filter, the FeSA/MXene filter prevented the competition from the HER and reduced the activation energy of the potential-limiting step (*NO to *NHO) that made the NH3 synthesis thermodynamically favorable . This work highlights an alternative strategy for achieving a synergistic NO3- removal and nutrient recovery with durable catalytic activity and stability.

Influence of binder and solvents on the electrochemical performance of screen-printed MXene electrodes

The present study is concerned with the use of binders and solvents in fabricating MXene electrodes, which play a vital role in influencing supercapacitive performance. The electrodes were prepared by screen printing MXene on flexible stainless steel mesh (FSSM) substrate, which is a straightforward, efficient, and cost-effective approach. The influence of binder and solvent on the electrochemical performance was investigated by fabricating them with and without using a binder i.e. only organic solvents ethanol and n-methyl-2-pyrrolidone (NMP). The electrode with the binder is abbreviated as MX-B@FSSM and was prepared with the composition of acetylene black conducting material, polyvinylidene fluoride (PVDF) polymer binder, and MXene (MX) as active material. While electrodes without binder were prepared by a slurry of MXene using organic solvent ethanol and NMP and are abbreviated as MX-E@FSSM and MX-N@FSSM, respectively. The electrochemical performance of these MX-B@FSSM, MX-E@FSSM and MX-N@FSSM electrodes was examined by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy. The influence of the binder altered the electrochemical performance. The samples MX-B@FSSM, MX-E@FSSM, and MX-N@FSSM show the specific capacitance of 35.60, 490.80, and 339.6 F g−1, respectively at 2 mA cm−2 current density. The MX-E@FSSM electrode exhibited marginally the best electrochemical performance. Furthermore, MnO2/MXene//MX-E asymmetric supercapacitor device exhibits 252 F g−1 specific capacitance at 35.2 Wh kg−1 energy density demonstrating a promising electrode for the supercapacitor.

Structural, spectral, and dynamical behaviors of biodegradable choline-based ionic liquids (CBILs) confined inside neutral and charged mxene nanopores for supercapacitor applications

This study explores the potential of using biodegradable choline-based ionic liquids (CBILs) as a more sustainable and eco-friendly alternative to classical ILs in the field of electrolyte media for energy storage devices. While previous studies have focused on electrostatic and capacitance properties, it is important to also consider dynamics and structural behavior when choosing adjustable electrolytes and electrodes. In this regard, molecular dynamics simulations were used to study the structural, spectral, and dynamical behaviors of two CBILs ([Ch][Sa] and [Ch][β-Ala]) inside MXene nanopores with varying pore size and surface charge density. As the pore size decreases, the number of hydrogen bonds in the first layer decreases from 0.447 to 0.122 and 0.154–0.063 in [Ch][Sa] and [Ch][β-Ala], respectively. Also, the presence of an aromatic ring benefits packing efficiency, causing [Sa]- to have a parallel orientation relative to MXene surface. Applying a fixed charge (1.8–7.2 μC/cm2) on MXene electrodes results in the formation of an electric double layer (EDL). At higher surface charge densities, there is slower dynamics and lower structural correlations between anions and cations. Moreover, Infrared (IR) spectra show blue and red shifts for the O-H bond in choline and carbonyl C-O stretching vibrational modes in anions, respectively, due to the interaction of anions with the MXene surface in the first layer reducing their interaction with cations. The results of our research may potentially lead to the creation of novel energy storage devices that are eco-friendly, utilizing MXene electrodes and CBILs as electrolytes.

Phosphorus Doping Strategy-Induced Synergistic Modification of Interlayer Structure and Chemical State in Ti3C2Tx toward Enhancing Capacitance

Heteroatom doping is considered an effective method to substantially improve the electrochemical performance of Ti3C2Tx MXene for supercapacitors. Herein, a facile and controllable strategy, which combines heat treatment with phosphorous (P) doping by using sodium phosphinate (NaH2PO2) as a phosphorus source, is used to modify Ti3C2Tx. The intercalated ions from NaH2PO2 act as “pillars” to expand the interlayer space of MXene, which is conducive to electrolyte ion diffusion. On the other hand, P doping tailors the surface electronic state of MXene, optimizing electronic conductivity and reducing the free energy of H+ diffusion on the MXene surface. Meanwhile, P sites with lower electronegativity owning good electron donor characteristics are easy to share electrons with H+, which is beneficial to charge storage. Moreover, the adopted heat treatment replaces –F terminations with O-containing groups, which enhances the hydrophilicity and provides sufficient active sites. The change in surface functional groups increases the content of high valence-stated Ti with a high electrochemical activity that can accommodate more electrons during discharge. Synergistic modification of interlayer structure and chemical state improves the possibility of Ti3C2Tx for accommodating more H+ ions. Consequently, the modified electrode delivers a specific capacitance of 510 F g−1 at 2 mV s−1, and a capacitance retention of 90.2% at 20 A g−1 after 10,000 cycles. The work provides a coordinated strategy for the rational design of high-capacitance Ti3C2Tx MXene electrodes.

Hierarchical BiFeO3/Cr2CTx MXene composite as a multifunctional catalyst for hydrogen evolution reaction and as an electrode material for energy storage devices

A multifunctional hierarchical Bismuth ferrite/chromium carbide (BiFeO3/Cr2CTx) MXene has been employed as both electrode material for supercapacitors as well as an electrocatalyst for electrocatalytic water splitting. Here, a facile method is suggested for synthesizing Cr2CTx MXene from the chromium aluminum carbide (Cr2AlC) MAX Phase. X-ray diffraction studies, Scanning electron microscopy, and high-resolution transmission electron microscopy indicate that the aluminum atomic layers are removed from Cr2AlC MAX structure. The electrochemical test reveals that the BiFeO3/Cr2CTx MXene composite, which is produced with less Al2O3, performs well in the hydrogen evolution reaction (HER) with a low overpotential of 128 mV in 1 M potassium hydroxide. 53.3 mV dec−1, and 0.16 Ω cm−2, respectively, are the values of the calculated Tafel slope and charge transfer resistance. In a dielectrode electrolysis system, BiFeO3/Cr2CTx MXene electrode needs only 1.81 V of cell potential to provide 10 mA cm−2 with long-term stability. The specific capacity of 671.2 C g−1 at a current density of 1 A g−1 is obtained for BiFeO3/Cr2CTx MXene electrode with 90% capacitance retention after 3000 cycles. The potential use of BiFeO3/Cr2CTx MXene towards HER and supercapacitor application is demonstrated by this study, which offers a gentle path for the fabrication of Cr2CTx MXene composites for energy storage and HER applications.

(Invited) Air-Permeable MXene Electrode with Miura-ori Structure for Biosensing

In this work, we present an air-permeable MXene electrode capable of recording surface electromyographic (sEMG) signals on the skin surface based on the Miura-ori structure. MXene is a two-dimensional (2D) nanomaterial composed of transition metal carbides, nitrides, and carbonitrides developed in recent years. Taking advantage of the Miura-ori structure, MXene-based electrodes have high breathability as well as low and stable electrode-skin interfacial impedances, facilitating long-term reliable electrophysiological monitoring. The electrolytic gel on conventional wet Ag/AgCl gel electrodes increases the impedance between the electrode and the skin as it gradually dries, inhibits sweat evaporation and affects signal acquisition. Therefore, we provide a fascinating electrode design that can be used to obtain high-quality sEMG signals for assessing body movements such as swallowing.

MXene and Carbon-Based Electrodes of Thermocells for Continuous Thermal Energy Harvest.

Low-grade heat represents a significant form of energy loss; thermocells (TECs) utilizing the thermogalvanic effect can convert thermal energy into electricity without generating vibrations, noise, or waste emissions, making them a promising energy conversion technology for efficiently harvesting low-grade heat. Despite recent advancements, the reliance on high-cost platinum electrodes in TECs has considerably hindered their widespread adoption. Developing cost-effective electrodes that maintain the same thermoelectrochemical performance is crucial for the successful application of TECs. In this review article, the exploration of MXene materials as TEC electrodes is discussed first, emphasizing the immense potential of the MXene family for low-grade heat harvesting applications. Next, recent research on carbon-based electrodes is summarized, and morphological and structural optimizations are comprehensively discussed aiming at enhancing the thermoelectrochemical performance of TECs. In the concluding section, the challenges are outlined and future perspectives are offered, which provide valuable insights into the ongoing development of high-performance TEC electrodes using MXene and carbon-based materials.

Regulating d-Band Center of Ti2 C MXene Via Nb Alloying for Stable and High-Efficient Supercapacitive Performances.

Ti2 C MXene with the lowest formula weight is expected to gain superior advantages in gravimetric capacitances over other heavier MXenes. Nevertheless, its poor chemical and electrochemical stability is the most fatal drawback and seriously hinders its practical applications. Herein, an alloy engineering strategy at the transition metal-sites of Ti2 C MXene is proposed. Theoretical calculations reveal that the electronic redistribution of the solid-solution TiNbC MXene improves the electronic conductivity, induces the upward d-band center, tailors the surface functional groups, and increases the electron loss impedance, resulting in its excellent capacitive performance and high chemical stability. The as-prepared flexible TiNbC film delivers specific capacitance up to 381Fg-1 at a scan rate of 2mVs-1 and excellent electrochemical stability without capacitance loss after 10000 charge/discharging cycles. This work provides a universal approach to develop high-performance and chemically stable MXene electrodes.

A late-model electrode structure for intrinsically flexible stretchable supercapacitor

In this work, a late-model intrinsically flexible stretchable supercapacitor (IFSS) was proposed, using liquid metal with excellent electron transfer capability as flexible collector, combined with stretchable silicone and well-designed kirigami MXene electrode based on laser cutting process. Systematic mechanical and electrochemical characterization suggested that the steady electrochemical output can still be maintained when the assembled device undergoes different degrees of stretching range from 0 to 100% even under bending or twisting. These results demonstrate that an effective integration of printed liquid metal and silicone with excellent flexibility and stretchability provides a potential avenue for the development of intrinsically stretchable devices.

Highly Air-Stable, Flexible, and Water-Resistive 2D Titanium Carbide MXene-Based RGB Organic Light-Emitting Diode Displays for Transparent Free-Form Electronics.

Flexible see-through displays are considered to be the next generation smart display, providing improved information flow, safety, situational awareness, and overall user experience in smart windows, automotive displays, glass-form biomedical displays, and augmented reality systems. 2D titanium carbides (MXenes) are promising material as electrodes of the transparent and flexible displays due to their high transparency, metallic conductivity, and flexibility. However, current MXene-based devices have insufficient air stability and lack engineering schemes to develop matrix-addressable display forms with sufficient pixels to display information. Here, we develop an ultraflexible and environmentally stable MXene-based organic light-emitting diode (OLED) display by combining high performance MXene electrodes, flexible OLEDs, and ultrathin and functional encapsulation systems. The MXene material was synthesized and used to fabricate a highly reliable MXene-based OLED that can stably operate in air condition for over 2000 h, endure repetitive bending deformation of 1.5 mm radius, and maintain environmental stability for 6 h when exposed to wet surroundings. The RGB MXene-based OLEDs were fabricated, (1691 cd m-2 at 40.4 mA cm-2 for red, 1377 cd m-2 at 4.26 mA cm-2 for green, and 1475 cd m-2 at 18.6 mA cm-2 for blue) and a matrix-addressable transparent OLED display was demonstrated that could display letters and shapes.

A comprehensive overview of MXene‐based anode materials for univalent metal ions (Li+, Na+, and K+) and bivalent zinc ion capacitor application

AbstractThe excellence of MXene electrode material for high performance battery and supercapacitor devices represents immense possibility to construct MXene‐based metal ion capacitors (MICs) with a high energy/power density and long cyclic life. Various reports indicate that the MXene and its composites/heterostructures can be a suitable anode material to assemble MICs in non‐aqueous/aqueous electrolytes. The pillaring of MXenes with Sn4+, Fe2O3, TiO2, Fe3O4/C, SnO2, NaNbO3, MoS2, CNTs, graphene, etc. effectively alleviates restacking of nanosheets and provides large interlayer spacing as well as additional active sites. Formation of three‐dimensional network with porous structures and heteroatom doping are also advantageous for high performance electrode materials. Results indicate that the MICs can achieve a high energy density (up to 241 Wh kg−1) like rechargeable battery, a high power density (up to 25,000 W kg−1) like supercapacitor, and a satisfactory cyclic stability. This article reviews state‐of‐the art MXene‐based anode materials for various MICs. After a brief introduction about the MICs, its storage principle, historical background, and insights of the MXene synthesis, property, and storage mechanism, the article summarizes/evaluates the electrochemical performance of various MICs assembled by the MXene‐based anode materials. Finally, we outline challenges/future prospects of the MICs using MXene‐based anode materials.

Multi-ion competition mechanism within MXene interlayer and Li ion intercalated electrode function

2D layered MXene material offers atomic level intervals that can expand the function of ion intercalated electrode. Challenges remain in modulating the interlayer environment and overcome the problem of lamellar stacking. Herein, to accurate control the interlayer structure of MXene and understand these ions confined by MXene, Li+, Na+ and K+ are pre-inserted into the inner space, which is the spaced voids of two adjacent Ti3C2Tx MXene layers with Al3+ ions, and the relationship between the interlayer spacing and the multi-ion interaction mechanism is explored. Furthermore, the electrochemical behavior and performance induced by multi-ion intercalated MXene electrodes are evaluated. The results show that the capacity of MXene electrodes with Na+ pre-intercalation obtained a capacity of 250.80 mAh g−1, and the capacity is maintained after 200 cycles is 179.67 mAh g−1, which attenuation is 28.36%. In addition, the pre-intercalated Na+ ions can maintain a more stable structure and benefit to electrochemical cycling performance than these pre-intercalated Li+ ions occupying the transport channels of the electrolyte ion. It provides data support for in-depth understanding of the mechanism and derivation of ion pre-intercalated MXene.

Tuning the pore size distribution of Ti3C2Tx porous film for high capacity supercapacitor electrode

The closely restacked MXene nanosheets obstruct electrolyte ion transfer in electrodes, limmiting the electrochemical performance at a high charging-discharging rate. Although various 3D porous structures have been constructed, size effect of porous MXenes electrodes has been rarely studied. Herein, the classical metal ion-assisted method was used to fabricate 3D porous MXene films. The pore size distribution was changed by adjusting the cation valency. Noteworthy, when the mesoporous pore size distribution is in the range of 5 ∼ 10 nm, the porous structure can significantly improve the electrochemical performance and the presence of micropores can increase the proportion of surface capacitance. As a result, the porous Ti3C2Tx film treated by Mg2+ showed outstanding electrochemical performance, with both high capacity (420F/g at 2 mV/s) and good cyclic stability (94.4% retention after 5000 cycles). This work presents a study on how pore size distribution of porous MXene film effects the electrochemical performance and provides a reference for the design of 3D porous MXene electrodes.

The effect of electrode thickness and electrode/electrolyte interface on the capacitive deionization behavior of the Ti3C2Tx MXene electrodes

2D MXene sheets have been regarded as promising capacitive deionization (CDI) electrodes owing to their high conductivity, hydrophilic surface, excellent adsorption capacity, and reversible intercalation of metal cations. The 2D structure characteristic enables MXene sheets to provide massive spaces to catch Na+ and Cl- ions during the CDI process. In this work, the CDI behavior of Ti3C2Tx MXene has been comprehensively investigated by constructing a symmetrical device. The effect of electrode thickness and electrode/electrolyte on the CDI behavior of the Ti3C2Tx MXene electrodes is deeply investigated. The thin electrode could facilitate the electrolyte (NaCl solution) penetration in the electrode, provide more electrode/electrolyte interface contact, and shorten the ion diffusion pathway from the electrolyte to the internal electrode, leading to faster ion diffusion and better desalination capacity. The optimized MXene‖MXene CDI device with an electrode thickness of 18.41 µm exhibits a high salt removal capacity of 100.9 mg g−1 in NaCl solution at 1.2 V.

Conformal Passivation of Self-Cleanable, Flexible, and Transparent Polytetrafluoroethylene Thin Films on Two-Dimensional MXene and Three-Dimensional Ag Nanowire Composite Electrodes

Despite the excellent performance of MXene–Ag nanowire (MA) composite transparent conductive electrodes (TCEs), they rapidly and severely degrade in ambient and humid environments. To address this critical issue, we developed conformal polytetrafluoroethylene (PTFE) passivation thin films prepared using magnetron sputtering at room temperature. Compared to the PTFE passivated electrode, the nonpassivated bare MA-composite electrode exhibited a considerably increased sheet resistance, and the electrode flexibility and electromagnetic interference (EMI) shielding efficiency (SE) degraded rapidly over time because the moisture and impurities severely oxidized the MXene layer in the harsh testing environment. However, the MA-composite electrode with PTFE passivation shows constant sheet resistance, transmittance, flexibility, and EMI SE, even after the 85 °C–85% relative humidity (RH) test. Compared to the nonpassivated MA-electrode-based thin film heaters (TFHs), the PTFE-passivated MA-composite electrode-based TFHs exhibited improved stability and reliability, indicating that the sputtered PTFE film effectively prevented the moisture and impurity penetration of the MA-composite electrodes.

Boosting the electrochemical activities of MnO2 for next-generation supercapacitor application: Adaptation of multiple approaches

A novel Ag-MnO2/MXene on nickel foam (Ag-MnO2/MXene@NF) electrode has been developed by incorporating hydrothermal and post-sonication processes. Ag-doping, MXene reinforcement, nanotechnology approaches, and highly porous current collector (Nickel foam) play a decisive role in boosting the overall activity of the Ag-MnO2/MXene@NF. The Ag-doping tunes the band structure of MnO2 and intrinsically improves its specific conductivity. At the same time, the sandwiching of Ag-MnO2 NWs between the MXene sheet's voids and their dispersion over the MXene sheet’s surface led to the formation of a hetero-structured composite with superb conductivity, a high surface area, lower crystallinity, and structural openings. The nanostructured nature of MnO2 (NWs) and their addition to MXene, a conductive and porous matrix, resulted in better capacitance retention and faster ion diffusion. The nano-sized and spongy structure of the Ag-MnO2/MXene@NF not only exposes the bulk of the electrode for charge storage but also buffers the electrode from pulverization as a result of tedious cyclic tests and facilitates the electrolyte ions' mobility. These induced features enabled the Ag-MnO2/MXene@NF to show a higher capacitance of 1188 F g−1 @ 1Ag−1, impressive rate capability (85.8 % @9 A g−1), and superb cyclic activity of 96.4 % after 6000 tests. The combination of various techniques boosts the overall electrochemical performance of our developed Ag-MnO2/MXene electrode, making it an acceptable option for use in advanced energy storage devices.

Synthesis of hierarchical structure MXene/PANI composite hybrid electrodes for supercapacitors

An MXene (Ti3C2Tx)/polyaniline (PANI) composite was successfully synthesized by the etching of Ti3AlC2 MAX and a simple polymerization process. In the obtained MXene/PANI composite, a dense covering of PANI appears on the surface and interlayers of two-dimensional MXene owing to electrostatic interaction. The electrochemical properties of Ti3C2Tx MXene, PANI, and MXene/PANI composite electrodes were investigated. The crystal phase data and morphology of the samples were observed by X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy. The electrochemical properties of the electrode materials were measured in a three-electrode system using cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The results demonstrate that the MXene/PANI electrode has a higher specific capacitance of 458.3 Fg−1 at a scan rate of 5 mVs−1 compared to the pure Ti3C2Tx MXene and PANI electrodes and a capacitance retention of 95.5 % after 1000 cycles at a current density of 5 Ag−1, indicating that MXene/PANI can be regarded as a promising electrode material for supercapacitors.

Rational design of new, efficient, and suitable nickel phthalocyanine reinforced MXene electrodes for supercapacitors

2D transition metal carbides and nitrides (MXenes) are emerging as a novel class of pseudocapacitive materials. However, 2D MXene sheets usually easily restack and aggregate, which greatly hinder their practical application in high performance energy storage devices. Herein, the reinforced MXene electrode is rationally designed and constructed by inserting 18-π aromatic electron system of Phaseolus vulgaris-like MPc among MXene sheets, this system constitutes a new functionalization strategy through π-π interactions for stabilizing MXene. Further, in this study, MXenes/NiPc achieves a superior specific capacitance of 792 F g−1 at 1 A g−1, and the assembled ASC provides a high energy density of 84.58 Wh kg−1 at a power density of 749.97 W kg−1, while also produces the excellent cycling stability of 95.1 % retention over 5000 cycles. Notably, the assembled ASC can easily power the green LED light strip. Our research findings provide a novel strategy and paradigm to design high-performance MXene-based energy storage materials.

Hydrogen-Bond Restructuring of Water-in-Salt Electrolyte Confined in Ti3C2Tx MXene Monitored by Operando Infrared Spectroscopy.

Highly concentrated water-in-salt aqueous electrolytes exhibit a wider potential window compared to conventional, dilute aqueous electrolytes. Coupled with MXenes, a family of two-dimensional transition metal carbides and nitrides with impressive charge storage capabilities, water-in-salt electrolytes present a potential candidate to replace flammable and toxic organic solvents in electrochemical energy storage devices. A new charge storage mechanism was recently discovered during electrochemical cycling of Ti3C2Tx MXene electrodes in lithium-based water-in-salt electrolytes, attributed to intercalation and deintercalation of solvated Li+ ions at anodic potentials. Nevertheless, direct evidence of the state of Li+ solvation during cycling is still missing. Here, we investigate the hydrogen bonding of water intercalated between MXene layers during electrochemical cycling in a water-in-salt electrolyte with operando infrared spectroscopy. The hydrogen-bonding state of the confined water was found to change significantly as a function of potential and the concentration of Li+ ions in the interlayer space. This study provides fundamentally new insights into the electrolyte structural changes while intercalating Li+ in the MXene interlayer space.