MXenes for Energy Harvesting

Two-dimensional nanomaterial MXenes for efficient gas separation: a review

Transition metal carbides/nitrides (MXenes) are emerging two-dimensional (2D) materials that have been widely investigated in recent years. In general, these materials can be obtained from MAX phase ceramics after intercalation, etching, and exfoliation to obtain multilayer MXene nanosheet structures; moreover, they have abundant end-group functional groups on their surface. In recent years, the excellent high permeability, fine sieving ability and diverse processability of MXene series materials make the membranes prepared using them particularly suitable for membrane-based separation processes in the field of gas separation. 2D membranes enhance the diversity of the pristine membrane transport channels by regulating the gas transport channels through in-plane pores (intrinsic defects), in-plane slit-like pores, and planar to planar interlayer channels, endowing the membrane with the ability to effectively sieve gas energy efficiently. Herein, we review MXenes, a class of 2D nanomaterials, in terms of their unique structure, synthesis method, functionalization method, and the structure-property relationship of MXene-based gas separation membranes and list examples of MXene-based membranes used in the field of gas separation. By summarizing and analyzing the basic properties of MXenes and demonstrating their unique advantages compared to other 2D nanomaterials, we lay a foundation for the discussion of MXene-based membranes with outstanding carbon dioxide (CO₂) capture performance and outline and exemplify the excellent separation performances of MXene-based gas separation membranes. Finally, the challenges associated with MXenes are briefly discussed and an outlook on the promising future of MXene-based membranes is presented. It is expected that this review will provide new insights and important guidance for future research on MXene materials in the field of gas separation.

Constructing oxygen vacancy-rich MXene @Ce-MOF composites for enhanced energy storage and conversion

Oxygen vacancies can regulate the coordination structure and electronic states of atoms, thus promoting the formation of surface-active sites and increasing the conductivity of the electrode material. This work presents a design for MXene@Ce-MOF composites with abundant oxygen vacancies. The hydroxyl groups on the surface of monolayer MXene attract cerium ions, which create surface defects in Ce-MOF and further promote the formation of oxygen vacancies. This results in a significant improvement in energy storage capacity, as well as performance in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The MXene@Ce-MOF composite exhibits a specific capacity of 496 F g^-1, which is 1.8 times higher than that of pure Ce-MOF and 3.5 times higher than MXene alone. At a current density of 10 mA cm^-2, the overpotential for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is as low as 270 and 220 mV, respectively, and the composite exhibits excellent cycling stability. Oxygen vacancy-based MOF composites play a crucial role in electrocatalysis and energy conversion.

Chalcogen (S, Se, and Te) decorated few-layered Ti3C2Tx MXene hybrids: modulation of properties through covalent bonding

2D carbides and nitrides of transition metals (MXenes) have shown great promise in a variety of energy storage and energy conversion applications. The extraordinary properties of MXenes are because of their excellent conductivity, large carrier concentration, vast specific surface area, superior hydrophilicity, high volumetric capacitance, and rich surface chemistry. However, it is still desired to synthesize MXenes with specific functional groups that deliver the required characteristics. This is due to the fact that a considerable amount of metal atoms is exposed on the surface of MXenes during their synthesis through an etching procedure; hence, other anions and cations are uncontrollably implanted on their surfaces. Because of this situation, the first invented Ti₃C₂T_x MXene suffers from low photoresponsivity and detectivity, large overpotential, and small sensitivity in photoelectrochemical (PEC) photodetectors, hydrogen evolution reaction (HER), and sensing applications. Therefore, surface modification of the MXene structure is required to develop the device's performance. On the other hand, there is still a lack of understanding of the MXene mechanism in such cutting-edge applications. Thus, the manipulations of MXenes are highly dependent on understanding the device mechanism, suitable modification elements, and modification methods. This study for the first time reveals the conjugation effect of pre-selected S, Se, and Te chalcogen elements on a few-layered Ti₃C₂T_x MXene to synthesize new composites for PEC photodetector, HER, and vapor sensor applications. Also, the mechanism of the chalcogen decorated few-layered Ti₃C₂T_x MXene composites for each application is discussed. The selection of a few-layered Ti₃C₂T_x MXene is due to its fascinating characteristics which make it capable to be considered as an appropriate substrate and incorporating chalcogen atoms. The Te-decorated few-layered Ti₃C₂T_x MXene composite provides better performances in PEC photodetector and vapor sensing applications. Although the potential value of the Se-decorated few-layered Ti₃C₂T_x composite is slightly lower than that of the Te-decorated sample in HER application, its overpotential is still greater than that of the Te-decorated sample. The acquired results show that the S-decorated few-layered Ti₃C₂T_x composite demonstrates the lowest performance in all three examined applications in comparison with the other two samples.

A Dual-Functional Triboelectric Nanogenerator Based on the Comprehensive Integration and Synergetic Utilization of Triboelectrification, Electrostatic Induction, and Electrostatic Discharge to Achieve Alternating Current/Direct Current Convertible Outputs

Traditional alternating current (AC) and direct current (DC) triboelectric nanogenerators (TENGs), which are implemented via the pairwise coupling of triboelectrification, electrostatic induction, and electrostatic discharge, have been widely explored in various fields. In this work, the comprehensive integration and synergetic utilization of triboelectrification, electrostatic induction, and electrostatic discharge in a single device for the first time is realized, achieving a dual-functional TENG (DF-TENG) to produce an AC/DC convertible output. Distinguishing from the conventional TENGs, the coupling of triboelectrification and electrostatic discharge enables charge circulation between the dielectric tribo-layers, while electrostatic induction realizes charge transfer in the external circuit. This novel energy conversion mechanism has been proven to be applicable to a variety of materials, including polymers, fabrics, and semiconductors. The output mode of the DF-TENG can be tuned by adjusting the slider motion state, and its constant output current and power density can reach 1.51 mA m^-2 Hz^-1 and 398 mW m^-2 Hz^-1 , respectively, which are the highest records reported for constant DC-TENGs to date. This work not only provides a paradigm shift to achieve AC/DC convertible output, but it also exhibits high potential for extending the TENG design philosophy.

Selective Ion Transport in Two-Dimensional Lamellar Nanochannel Membranes

Precise and ultrafast ion sieving is highly desirable for many applications in environment-, energy-, and resource-related fields. The development of a permselective lamellar membrane constructed from parallel stacked two-dimensional (2D) nanosheets opened a new avenue for the development of next-generation separation technology because of the unprecedented diversity of the designable interior nanochannels. In this Review, we first discuss the construction of homo- and heterolaminar nanoarchitectures from the starting materials to the emerging preparation strategies. We then explore the property-performance relationships, with a particular emphasis on the effects of physical structural features, chemical properties, and external environment stimuli on ion transport behavior under nanoconfinement. We also present existing and potential applications of 2D membranes in desalination, ion recovery, and energy conversion. Finally, we discuss the challenges and outline research directions in this promising field.

Ultrasound-mediated piezoelectric nanoparticle modulation of intrinsic cardiac autonomic nervous system for rate control in atrial fibrillation

Rate control is a cornerstone of atrial fibrillation treatment. Barium titanate nanoparticles (BTNPs) are piezoelectric nanomaterials that can generate local electromagnetic fields under ultrasound activation, stimulating nearby neuronal tissue. This study aimed to modulate the inferior right ganglionated plexus (IRGP) of the heart and reduce the ventricular rate during rapid atrial pacing (RAP)-induced atrial fibrillation using ultrasound-mediated BTNPs. Adult male beagles were randomly divided into a phosphate-buffered saline (PBS) group (n = 6) and a BTNP group (n = 6). PBS or nanoparticles were injected into the IRGP of both groups before RAP. The biological safety of the material was evaluated according to electrophysiology recordings, thermal effects and level of inflammation. Compared to the PBS group, the BaTiO₃ piezoelectric nanoparticle group had reduced ventricular rates in the sinus rhythm and atrial fibrillation models after stimulating the IRGP by applying ultrasound. In addition, transient stimulation by BTNPs did not lead to sustained neuronal excitation in the IRGP. The activation of the BTNPs did not induce inflammation or thermal damage effects in the IRGP. Ultrasound-mediated BTNP neuromodulation can significantly reduce the ventricular rate by stimulating the IRGP. Thus, ultrasound-mediated BTNP neuromodulation is a potential therapy for atrial fibrillation rate control.

Magnetic moment impact on spin-dependent Seebeck coefficient of ferromagnetic thin films

Magnetic materials may be engineered to produce thermoelectric materials using spin-related effects. However, clear understanding of localized magnetic moments (µ_I), free carriers, and Seebeck coefficient (S) interrelations is mandatory for efficient material design. In this work, we investigate µ_I influence on the spin-dependent S of model ferromagnetic thin films, allowing µ_I thermal fluctuations, ordering, and density variation influence to be independently investigated. µ_I influence on free carrier polarization is found to be of highest importance on S: efficient coupling of free carrier spin and localized magnetic moment promotes the increase of S, while spin-dependent relaxation time difference between the two spin-dependent conduction channels leads to S decrease. Our observations support new routes for thermoelectric material design based on spin-related effects in ferromagnetic materials.

MXene-Based Porous Monoliths

In the past decade, a thriving family of 2D nanomaterials, transition-metal carbides/nitrides (MXenes), have garnered tremendous interest due to its intriguing physical/chemical properties, structural features, and versatile functionality. Integrating these 2D nanosheets into 3D monoliths offers an exciting and powerful platform for translating their fundamental advantages into practical applications. Introducing internal pores, such as isotropic pores and aligned channels, within the monoliths can not only address the restacking of MXenes, but also afford a series of novel and, in some cases, unique structural merits to advance the utility of the MXene-based materials. Here, a brief overview of the development of MXene-based porous monoliths, in terms of the types of microstructures, is provided, focusing on the pore design and how the porous microstructure affects the application performance.

Harvesting Hydropower via a Magnetoelastic Generator for Sustainable Water Splitting

Energy consumption and the resulting climate change are the two major challenges to human sustainability. Hydrogen (H₂) is a form of environmentally friendly renewable energy with an extremely high energy density of 143 MJ kg^-1. Water splitting is a practical and cost-effective approach to generate H₂ through the decomposition of H₂O by electrolysis with an external power supply. Herein, we introduce a compelling platform technology for self-powered water splitting by using a soft magnetoelastic generator to convert hydropower into electricity as a sustainable power supply for electrolysis. At a rotating speed of 469 rpm, the hydropower harvester is able to convert flowing kinetic energy into electricity and produce a high current density of 2.99 mA cm^-2 at a low resistance of 60 Ω. The magnetoelastic generator is intrinsically waterproof since the magnetic field can penetrate the water molecules. As a demonstration, the device maintained a stable electrical output even in underwater situations after over 7,000 cyclic operations. The generated electricity from hydropower could produce H₂ at a rate of 1.93 × 10^-3 mL min^-1. In conclusion, this work provides a compelling method for self-powered water splitting by using flowing kinetic energy.

Nb2CTx MXene Cathode for High-Capacity Rechargeable Aluminum Batteries with Prolonged Cycle Lifetime

Aluminum-ion batteries have garnered significant interest as a potentially safer and cheaper replacement for conventional lithium-ion batteries, offering a shorter charging time and denser storage capacity. Nonetheless, the progress in this field is considerably hampered by the limited availability of suitable cathode materials that can sustain the reversible intercalation of Al³⁺/[AlCl₄]^- ions, particularly after long cycles. Herein, we demonstrate that rechargeable Al batteries embedded with two-dimensional (2D) Nb₂CT_x MXene as a cathode material exhibit excellent capacity and exceptional long cyclic performance. We have successfully improved the initial electrochemical performance of Nb₂CT_x MXene after being properly delaminated to a single-layered microstructure and subjected to a post-synthesis calcining treatment. Compared to pristine Nb₂CT_x MXene, the Al battery embedded with the calcined Nb₂CT_x MXene cathode has, respectively, retained high capacities of 108 and 80 mAh g^-1 after 500 cycles at current densities of 0.2 and 0.5 A g^-1 in a wide voltage window (0.1-2.4 V). Noteworthily, the cyclic lifetime of Nb₂CT_x MXene was extended from ∼300 to >500 times after calcination. We reveal that attaining Nb₂CT_x nanosheets with a controllable d-spacing has promoted the migration of the [AlCl₄]^- and Al³⁺ ions in the MXene interlayers, leading to enhanced charge storage. Furthermore, we found out that the formation of niobium oxides and amorphous carbon after calcination probably benefits the electrochemical performance of Nb₂CT_x MXene electrode in Al batteries.

Two-dimensional ultrathin Ti3C2 MXene nanosheets coated intraocular lens for synergistic photothermal and NIR-controllable rapamycin releasing therapy against posterior capsule opacification

Posterior capsule opacification (PCO) is one of the most frequent late-onset complications after cataract surgery. Several kinds of drug-eluting intraocular lenses (IOL) were designed for sustainable drug release to suppress ocular inflammation, the proliferation of lens epithelial cells (LECs) and the development of PCO after cataract surgery. Despite previous advances in this field, the drug-loaded IOLs were limited in ocular toxicity, insufficient drug-loading capacity, and short release time. To prevent PCO and to address these drawbacks, a novel drug-loaded IOL (Rapa@Ti3C2-IOL), prepared from two-dimensional ultrathin Ti3C2 MXene nanosheets and rapamycin (Rapa), was fabricated with a two-step spin coating method in this study. Rapa@Ti3C2 was prepared via electrostatic self-assembly of Ti3C2 and Rapa, with a loading capacity of Rapa at 92%. Ti3C2 was used as a drug delivery reservoir of Rapa. Rapa@Ti3C2-IOL was designed to have the synergistic photothermal and near infrared (NIR)-controllable drug release property. As a result, Rapa@Ti3C2-IOL exhibited the advantages of simple preparation, high light transmittance, excellent photothermal conversion capacity, and NIR-controllable drug release behavior. The Rapa@Ti3C2 coating effectively eliminated the LECs around Rapa@Ti3C2-IOL under a mild 808-nm NIR laser irradiation (1.0 W/cm−2). Moreover, NIR-controllable Rapa release inhibited the migration of LECs and suppressed the inflammatory response after photothermal therapy in vitro. Then, Rapa@Ti3C2-IOL was implanted into chinchilla rabbit eyes, and the effectiveness and biocompatibility to prevent PCO were evaluated for 4 weeks. The Rapa@Ti3C2-IOL implant exhibited excellent PCO prevention ability with the assistance of NIR irradiation and no obvious pathological damage was observed in surrounding healthy tissues. In summary, the present study offers a promising strategy for preventing PCO via ultrathin Ti3C2 MXene nanosheet-based IOLs with synergistic photothermal and NIR-controllable Rapa release properties.

Manufacturing Technics for Fabric/Fiber-Based Triboelectric Nanogenerators: From Yarns to Micro-Nanofibers

Triboelectric nanogenerator (TENG), as a green energy harvesting technology, has aroused tremendous interest across many fields, such as wearable electronics, implanted electronic devices, and human-machine interfaces. Fabric and fiber-structured materials are excellent candidates for TENG materials due to their inherent flexibility, low cost, and high wearing comfort. Consequently, it is crucial to combine TENG with fabric/fiber materials to simultaneously leverage their mechanical energy harvesting and wearability advantages. In this review, the structure and fundamentals of TENG are briefly explained, followed by the introduction of three distinct methods for preparing fabric/fiber structures: spinning and weaving, wet spinning, and electrospinning. In the meantime, their applications have been discussed, focusing primarily on energy harvesting and wearable self-powered sensors. Finally, we discussed the future and challenges of fabric and fiber-based TENGs.

Energy Harvesting Using Cotton Fabric Embedded with 2D Hexagonal Boron Nitride

Continuous health monitoring through sensitive physiological signals (using a wearable device) is crucial for the early detection of heart diseases and breathing problems. Here, we have developed a flexible hBN/cotton hybrid device that can detect minor signals such as heartbeat and breathed-out air pressure. Systematic observation of the real-time motion sensing showed a peak-to-peak voltage output of ∼1.5 V for each heart rate pulse. The as-fabricated device showed a high voltage output of up to ∼10 V upon applying a pressure of ∼3 MPa. The FTIR results and DFT calculation suggested a chemical interaction between hBN and cellulose, giving rise to flat band characteristics and partially filled σ-bonding (sp²) hybridization. The atomic-scale chemical interface between atomically thin hBN and surface functional groups present on cotton resulted in charge localization and enhanced output voltage. An hBN/cotton hybrid device can bring new insights and opportunities to develop a self-charging and health-monitoring energy-harvesting cloth.

Advances of MXenes; Perspectives on Biomedical Research

The last decade witnessed the emergence of a new family of 2D transition metal carbides and nitrides named MXenes, which quickly gained momentum due to their exceptional electrical, mechanical, optical, and tunable functionalities. These outstanding properties also rendered them attractive materials for biomedical and biosensing applications, including drug delivery systems, antimicrobial applications, tissue engineering, sensor probes, auxiliary agents for photothermal therapy and hyperthermia applications, etc. The hydrophilic nature of MXenes with rich surface functional groups is advantageous for biomedical applications over hydrophobic nanoparticles that may require complicated surface modifications. As an emerging 2D material with numerous phases and endless possible combinations with other 2D materials, 1D materials, nanoparticles, macromolecules, polymers, etc., MXenes opened a vast terra incognita for diverse biomedical applications. Recently, MXene research picked up the pace and resulted in a flood of literature reports with significant advancements in the biomedical field. In this context, this review will discuss the recent advancements, design principles, and working mechanisms of some interesting MXene-based biomedical applications. It also includes major progress, as well as key challenges of various types of MXenes and functional MXenes in conjugation with drug molecules, metallic nanoparticles, polymeric substrates, and other macromolecules. Finally, the future possibilities and challenges of this magnificent material are discussed in detail.

Recent Research Progress in the Structure, Fabrication, and Application of MXene-Based Heterostructures

Two-dimensional (2D) materials have received increasing attention in the scientific research community owing to their unique structure, which has endowed them with unparalleled properties and significant application potential. However, the expansion of the applications of an individual 2D material is often limited by some inherent drawbacks. Therefore, many researchers are now turning their attention to combine different 2D materials, making the so-called 2D heterostructures. Heterostructures can integrate the merits of each component and achieve a complementary performance far beyond a single part. MXene, as an emerging family of 2D nanomaterials, exhibits excellent electrochemical, electronic, optical, and mechanical properties. MXene-based heterostructures have already been demonstrated in applications such as supercapacitors, sensors, batteries, and photocatalysts. Nowadays, increasing research attention is attracted onto MXene-based heterostructures, while there is less effort spent to summarize the current research status. In this paper, the recent research progress of MXene-based heterostructures is reviewed, focusing on the structure, common preparation methods, and applications in supercapacitors, sensors, batteries, and photocatalysts. The main challenges and future prospects of MXene-based heterostructures are also discussed to provide valuable information for the researchers involved in the field.

Scaled Deposition of Ti3C2Tx MXene on Complex Surfaces: Application Assessment as Rear Electrodes for Silicon Heterojunction Solar Cells

Two-dimensional transition metal carbides (MXenes) are of great interest as electrode materials for a variety of applications, including solar cells, due to their tunable optoelectronic properties, high metallic conductivity, and attractive solution processability. However, thus far, MXene electrodes have only been exploited for lab-scale device applications. Here, to demonstrate the potential of MXene electrodes at an industry-relevant level, we implemented a scalable spray coating technique to deposit highly conductive (ca. 8000 S/cm, at a ca. 55 nm thickness) Ti₃C₂T_x films (T_x: surface functional groups, i.e., -OH, -O, -F) via an automated spray system. We employed these Ti₃C₂T_x films as rear electrodes for silicon heterojunction solar cells as a proof of concept. The spray-deposited MXene flakes have formed a conformal coating on top of the indium tin oxide (ITO)-coated random pyramidal textured silicon wafers, leading to >20% power conversion efficiency (PCE) over both medium-sized (4.2 cm²) and large (243 cm², i.e., industry-sized 6 in. pseudosquare wafers) cell areas. Notably, the Ti₃C₂T_x-rear-contacted devices have retained around 99% of their initial PCE for more than 600 days of ambient air storage. Their performance is comparable with state-of-the-art solar cells contacted with sputtered silver electrodes. Our findings demonstrate the high-throughput potential of spray-coated MXene-based electrodes for solar cells in addition to a wider variety of electronic device applications.