Showing papers by "Yury Gogotsi published in 2019"
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1,114 citations
TL;DR: Examples indicate that nanostructured materials and nanoarchitectured electrodes can provide solutions for designing and realizing high-energy, high-power, and long-lasting energy storage devices.
Abstract: Lithium-ion batteries, which power portable electronics, electric vehicles, and stationary storage, have been recognized with the 2019 Nobel Prize in chemistry. The development of nanomaterials and their related processing into electrodes and devices can improve the performance and/or development of the existing energy storage systems. We provide a perspective on recent progress in the application of nanomaterials in energy storage devices, such as supercapacitors and batteries. The versatility of nanomaterials can lead to power sources for portable, flexible, foldable, and distributable electronics; electric transportation; and grid-scale storage, as well as integration in living environments and biomedical systems. To overcome limitations of nanomaterials related to high reactivity and chemical instability caused by their high surface area, nanoparticles with different functionalities should be combined in smart architectures on nano- and microscales. The integration of nanomaterials into functional architectures and devices requires the development of advanced manufacturing approaches. We discuss successful strategies and outline a roadmap for the exploitation of nanomaterials for enabling future energy storage applications, such as powering distributed sensor networks and flexible and wearable electronics.
941 citations
TL;DR: In this article, the best practices for measuring and reporting metrics such as capacitance, capacity, coulombic and energy efficiencies, electrochemical impedance, and the energy and power densities of capacitive and pseudocapacitive materials are discussed.
Abstract: Due to the tremendous importance of electrochemical energy storage, numerous new materials and electrode architectures for batteries and supercapacitors have emerged in recent years. Correctly characterizing these systems requires considerable time, effort, and experience to ensure proper metrics are reported. Many new nanomaterials show electrochemical behavior somewhere in between conventional double‐layer capacitor and battery electrode materials, making their characterization a non‐straightforward task. It is understandable that some researchers may be misinformed about how to rigorously characterize their materials and devices, which can result in inflation of their reported data. This is not uncommon considering the current state of the field nearly requires record breaking performance for publication in high‐impact journals. Incorrect characterization and data reporting misleads both the materials and device development communities, and it is the shared responsibility of the community to follow rigorous reporting methodologies to ensure published results are reliable to ensure constructive progress. This tutorial aims to clarify the main causes of inaccurate data reporting and to give examples of how researchers should proceed. The best practices for measuring and reporting metrics such as capacitance, capacity, coulombic and energy efficiencies, electrochemical impedance, and the energy and power densities of capacitive and pseudocapacitive materials are discussed.
642 citations
TL;DR: Highly concentrated, additive-free, aqueous and organic MXene-based inks that can be used for high-resolution extrusion and inkjet printing are reported.
Abstract: Direct printing of functional inks is critical for applications in diverse areas including electrochemical energy storage, smart electronics and healthcare. However, the available printable ink formulations are far from ideal. Either surfactants/additives are typically involved or the ink concentration is low, which add complexity to the manufacturing and compromises the printing resolution. Here, we demonstrate two types of two-dimensional titanium carbide (Ti3C2Tx) MXene inks, aqueous and organic in the absence of any additive or binary-solvent systems, for extrusion printing and inkjet printing, respectively. We show examples of all-MXene-printed structures, such as micro-supercapacitors, conductive tracks and ohmic resistors on untreated plastic and paper substrates, with high printing resolution and spatial uniformity. The volumetric capacitance and energy density of the all-MXene-printed micro-supercapacitors are orders of magnitude greater than existing inkjet/extrusion-printed active materials. The versatile direct-ink-printing technique highlights the promise of additive-free MXene inks for scalable fabrication of easy-to-integrate components of printable electronics.
611 citations
TL;DR: Two-dimensional transition metal carbides and nitrides (MXenes) have emerged as highly conductive and stable materials, of promise for electronic applications, and in situ electric biasing and transmission electron microscopy are used to investigate the effect of surface termination and intercalation on electronic properties.
Abstract: MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes' electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes' Fermi level density of states and thereby MXenes' electronic conductivity. Here, we directly correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes.
605 citations
TL;DR: In this article, MXene flakes are added into PAN solutions at a weight ratio of 2':'1 (MXene' :'PAN) in the spinning dope, producing fiber mats with up to 35 wt% MXene.
Abstract: Free-standing Ti3C2Tx MXene/carbon nanofiber electrodes are prepared via electrospinning Ti3C2Tx MXene flakes with polyacrylonitrile (PAN) and carbonizing the fiber networks. Using this simple fabrication method, delaminated MXene flakes are embedded within carbon nanofibers and these fiber mats are used as electrodes without binders or additives. Unlike coated electrodes, which may suffer from the active material delaminating from the substrate during folding or bending, composite electrodes are stable and durable. Previous attempts to incorporate Ti3C2Tx MXene into electrospun fibers resulted in low mass loadings, ∼1 wt% Ti3C2Tx MXene. In this work, MXene flakes are added into PAN solutions at a weight ratio of 2 : 1 (MXene : PAN) in the spinning dope, producing fiber mats with up to 35 wt% MXene. Composite electrodes have high areal capacitance, up to 205 mF cm−2 at 50 mV s−1, almost three times that of pure carbonized PAN nanofibers (70 mF cm−2 at 50 mV s−1). Compared with electrospun nanofibers spray-coated with Ti3C2Tx, these composite fibers exhibit double the areal capacitance at 10 mV s−1. This method can be used to produce MXene composite fibers using a variety of polymers, which have potential applications beyond energy storage, including filtration, adsorption, and electrocatalysis, where fibers with high aspect ratio, accessible surface, and porosity are desirable.
383 citations
TL;DR: The discovery of two-dimensional transition metal carbides and nitrides (MXenes) in 2011 has expanded the pool of nanomaterials available for a variety of applications.
340 citations
TL;DR: In this paper, the authors show how simply changing the solvent of an electrolyte system can drastically influence the pseudocapacitive charge storage of the two-dimensional titanium carbide, Ti3C2 (a representative member of the MXene family).
Abstract: Pseudocapacitive energy storage in supercapacitor electrodes differs significantly from the electrical double-layer mechanism of porous carbon materials, which requires a change from conventional thinking when choosing appropriate electrolytes. Here we show how simply changing the solvent of an electrolyte system can drastically influence the pseudocapacitive charge storage of the two-dimensional titanium carbide, Ti3C2 (a representative member of the MXene family). Measurements of the charge stored by Ti3C2 in lithium-containing electrolytes with nitrile-, carbonate- and sulfoxide-based solvents show that the use of a carbonate solvent doubles the charge stored by Ti3C2 when compared with the other solvent systems. We find that the chemical nature of the electrolyte solvent has a profound effect on the arrangement of molecules/ions in Ti3C2, which correlates directly to the total charge being stored. Having nearly completely desolvated lithium ions in Ti3C2 for the carbonate-based electrolyte leads to high volumetric capacitance at high charge–discharge rates, demonstrating the importance of considering all aspects of an electrochemical system during development.
313 citations
TL;DR: In this article, the electronic properties of Ti3C2Tx for different surface terminations, as achieved by different annealing temperatures, with the help of photoelectron spectroscopy, inverse photo-electron, and density functional theory calculations, were investigated.
Abstract: MXenes, an emerging family of 2D transition metal carbides and nitrides, have shown promise in various applications, such as energy storage, electromagnetic interference shielding, conductive thin films, photonics, and photothermal therapy. Their metallic nature, wide range of optical absorption, and tunable surface chemistry are the key to their success in those applications. The physical properties of MXenes are known to be strongly dependent on their surface terminations. In this study, we investigated the electronic properties of Ti3C2Tx for different surface terminations, as achieved by different annealing temperatures, with the help of photoelectron spectroscopy, inverse photoelectron spectroscopy, and density functional theory calculations. We find that fluorine occupies solely the face-centered cubic adsorption site, whereas oxygen initially occupies at least two different adsorption sites, followed by a rearrangement after fluorine desorption at high annealing temperatures. The measured electroni...
290 citations
TL;DR: In this paper, the surface terminations of three transition metal carbide MXenes (Ti3C2Tx, Mo2CTx, and Nb2CTX) were investigated up to 1500 °C under a He atmosphere.
Abstract: Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) have attracted significant attention due to their electronic, electrochemical, chemical, and optical properties. However, understanding of their thermal stability is still lacking. To date, MXenes are synthesized via top-down wet chemical etching, which intrinsically results in surface terminations. Here, we provide detailed insight into the surface terminations of three carbide MXenes (Ti3C2Tx, Mo2CTx, and Nb2CTx) by performing thermal gravimetric analysis with mass spectrometry analysis (TA–MS) up to 1500 °C under a He atmosphere. This specific technique enables probing surface terminations including hydroxyl (−OH), oxy (═O), and fluoride (−F) and intercalated species, such as salts and structural water. The MXene hydrophilicity depends on the type of etching (hydrofluoric acid concentration and/or mixed acid composition) and subsequent delamination conditions. We show that the amount of structural water in Ti3C2Tx increases with decre...
250 citations
TL;DR: It is shown that two-dimensional titanium carbide or carbonitride nanosheets, known as MXenes, can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives.
Abstract: The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated the quest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a critical thickness, solution-processed films typically encounter electrical/mechanical problems, limiting the achievable areal capacity and rate performance as a result. Herein, we show that two-dimensional titanium carbide or carbonitride nanosheets, known as MXenes, can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives. The nanosheets form a continuous metallic network, enable fast charge transport and provide good mechanical reinforcement for the thick electrode (up to 450 µm). Consequently, very high areal capacity anodes (up to 23.3 mAh cm−2) have been demonstrated.
TL;DR: The hybrid aerogel with high electrical conductivity, good mechanical strength, and superior EMI shielding performance is a promising material for inhibiting EMI pollution.
Abstract: Lightweight materials with high electrical conductivity and robust mechanical properties are highly desirable for electromagnetic interference (EMI) shielding in modern portable and highly integrated electronics. Herein, a three-dimensional (3D) porous Ti3C2Tx/carbon nanotube (CNT) hybrid aerogel was fabricated via a bidirectional freezing method for lightweight EMI shielding application. The synergism of the lamellar and porous structure of the MXene/CNT hybrid aerogels contributed extensively to their excellent electrical conductivity (9.43 S cm-1) and superior electromagnetic shielding effectiveness (EMI SE) value of 103.9 dB at 3 mm thickness at the X-band frequency, the latter of which is the best value reported for synthetic porous nanomaterials. The CNT reinforcement in the MXene/CNT hybrid aerogels enhanced the mechanical robustness and increased the compressional modulus by 9661% relative to that of the pristine MXene aerogel. The hybrid aerogel with high electrical conductivity, good mechanical strength, and superior EMI shielding performance is a promising material for inhibiting EMI pollution.
TL;DR: In this article, the authors proposed a pseudocapacitive anode material for asymmetric supercapacitors using MXenes, a rapidly growing large group of 2D transition metal carbides, nitrides and carbonitrides.
Abstract: DOI: 10.1002/aenm.201802917 Compared to other pseudocapacitive materials, CPs are attractive due to their facile synthesis, intrinsic conductivity, good flexibility, and high redox contributions.[7] Polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT) are among the commonly studied polymers for supercapacitors.[11] However, in spite of focus on improving the performance of CPs as anode materials,[7,11,12] efforts to match them with pseudocapacitive cathode materials are still underway. It is important to note that the target material needs to be stable under negative potentials in acidic electrolytes, because CPs show the best electrochemical performance in acidic electrolytes under positive potentials.[6] Currently available choices for negative electrodes are mostly carbons and metal oxides.[13] However, majority of carbon-based and metal oxide electrodes show hydrogen evolution under modest negative potentials in low-pH media.[14] Although carbons with a low content of surface functionalities may have an extended voltage window, they cannot match CP in capacitance due to lack of redox contribution to energy storage. Finding cathode materials with high pseudocapacitance in acidic electrolytes is still a challenge, which hinders practical use of CP in electrochemical capacitors. In search of negative electrodes for CP-utilizing asymmetric devices, MXenes may offer a viable option due to their metallic conductivity and high redox capacitance under negative potential in acidic electrolytes.[15] MXenes, a rapidly growing large group of 2D transition metal carbides, nitrides, and carbonitrides, have shown a great potential for a range of applications, particularly, for supercapacitors.[16,17] Specifically, delaminated Ti3C2Tx has shown outstanding capacitance values up to ≈1500 F cm−3 (380 F g−1) in aqueous electrolytes in a three-electrode configuration.[15] By using a set of electrochemical experiments coupled with in situ X-ray absorption spectroscopy (XAS), it has been shown that the changes in electrode potential correlate almost linearly to variations in the titanium oxidation state, confirming the predominantly pseudocapacitive charge storage mechanism in acidic electrolyte (1 m H2SO4). In another effort,[19] when charge storage mechanism was investigated using in situ Raman spectroscopy in three different sulfatecontaining aqueous electrolytes, it became clear that bonding between oxygen functional groups and hydronium ion from H2SO4 electrolyte occurs upon charging. During discharge, the reversible bonding/debonding alter the oxidation state of Ti, resulting in high redox capacitance in acidic electrolyte. In Conducting polymers (CPs) are attractive pseudocapacitive materials which show the highest capacitance under positive potentials in aqueous protic electrolytes. One way to expand their voltage window (thus energy density) in aqueous electrolytes is to manufacture asymmetric supercapacitors using distinctly different anodes. However, CPs lack matching pseudocapacitive anode materials that can perform well in protic electrolytes (e.g., sulfuric acid). 2D titanium carbide (Ti3C2Tx), MXene, as a universal pseudocapacitive anode material for a range of CPs, such as polyaniline, polypyrrole, and poly(3,4-ethylenedioxythiophene) deposited on reduced graphene oxide (rGO) sheets, is reported here. All-pseudocapacitive organic–inorganic asymmetric devices with MXene cathodes and rGO–polymer anodes can operate in voltage windows up to 1.45 V in 3 m H2SO4. Most importantly, these devices show outstanding cycling performance, outperforming many reported asymmetric pseudocapacitors.
TL;DR: It was found that aqueous solutions of Ti3C2Tx MXene can be chemically stable for more than 39 weeks when the storage temperature (-80 °C) is sufficiently low to cease the oxidation processes, and if the Ti3 C2Tx flakes are dispersed in ethanol, the degradation process can be significantly delayed even at 5 °C.
Abstract: Two-dimensional (2D) transition metal carbides (MXenes) exhibit outstanding performances in many applications, such as energy storage, optoelectronics, and electrocatalysts. However, colloidal solutions of Ti3C2Tx MXene flakes deteriorate rapidly under ambient conditions due to the conversion of the titanium carbide to titanium dioxide. Here, we discuss the dominant factors influencing the rate of oxidation of Ti3C2Tx MXene flakes, and present guidelines for their storage with the aim of maintaining the intrinsic properties of the as-prepared material. The oxidation stability of the Ti3C2Tx flakes is dramatically improved in a system where water molecules and temperature were well-controlled. It was found that aqueous solutions of Ti3C2Tx MXene can be chemically stable for more than 39 weeks when the storage temperature (−80 °C) is sufficiently low to cease the oxidation processes. It was also found that if the Ti3C2Tx flakes are dispersed in ethanol, the degradation process can be significantly delayed even at 5 °C. Moreover, the oxidation stability of the Ti3C2Tx flakes is dramatically improved in both cases, even in the presence of oxygen-containing atmosphere. We demonstrate practical applications of our approach by employing Ti3C2Tx in a gas sensor showing that when oxidation is inhibited, the device can retain the original electrical properties after 5 weeks of storage.
TL;DR: The advantages of Ti3C2T2 host material could be well retained even at high S loading, suggesting the potential of surface-modified MXene for confining sulfur in Li-S battery cathodes.
Abstract: Multiple negative factors, including the poor electronic conductivity of sulfur, dissolution and shuttling of lithium polysulfides (Li2Sn), and sluggish decomposition of solid Li2S, seriously hinder practical applications of lithium-sulfur (Li-S) batteries. To solve these problems, a general strategy was proposed for enhancing the electrochemical performance of Li-S batteries using surface-functionalized Ti3C2 MXenes. Functionalized Ti3C2T2 (T = N, O, F, S, and Cl) showed metallic conductivity, as bare Ti3C2. Among all Ti3C2T2 investigated, Ti3C2S2, Ti3C2O2, and Ti3C2N2 offered moderate adsorption strength, which effectively suppressed Li2Sn dissolution and shuttling. This Ti3C2T2 exhibited effective electrocatalytic ability for Li2S decomposition. The Li2S decomposition barrier was significantly decreased from 3.390 eV to ∼0.4 eV using Ti3C2S2 and Ti3C2O2, with fast Li+ diffusivity. Based on these results, O- and S-terminated Ti3C2 were suggested as promising host materials for S cathodes. In addition, appropriate functional group vacancies could further promote anchoring and catalytic abilities of Ti3C2T2 to boost the electrochemical performance of Li-S batteries. Moreover, the advantages of a Ti3C2T2 host material could be well retained even at high S loading, suggesting the potential of surface-modified MXene for confining sulfur in Li-S battery cathodes.
TL;DR: In this paper, the authors developed strongly interacting, porous MXene/conducting polymer hybrids for large-scale flexible alternating current filtering symmetric ECs with high areal and volumetric capacitances of 0.56 mF cm−2 and 24.2 F cm−3 at 120 Hz, respectively.
TL;DR: An in situ sulfur-doping strategy to functionalize MXene nanosheets by introducing heteroatomic sulfur into the MXene structure form the MAX phase precursor is reported.
Abstract: Sodium-sulfur batteries using abundant elements offer an attractive alternative to currently used batteries, but they need better sulfur host materials to compete with lithium-ion batteries in capacity and cyclability. We report an in situ sulfur-doping strategy to functionalize MXene nanosheets by introducing heteroatomic sulfur into the MXene structure form the MAX phase precursor. By employing the vacuum freeze-drying method, a three-dimensional (3D) wrinkled MXene nanoarchitecture with the high specific surface area was prepared. The tailor-made wrinkled sulfur-doped MXene (S-Ti3C2Tx) nanosheets were applied as an electrode host material in room temperature sodium-sulfur batteries. The S-Ti3C2Tx matrix shows high polarity with sodium polysulfides, restricting the diffusion of sodium polysulfides. The MXene/sulfur electrode can achieve high areal sulfur loading up to 4.5 mg cm-2 as well as good electrochemical performance (reversible capacity of 577 mAh g-1 at 2 C after 500 cycles).
TL;DR: In this article, a fully knitted textile-based capacitive pressure sensor is also prepared, which offers high sensitivity (gauge factor of ≈6.02), wide sensing range of up to ≈20% compression, and excellent cycling stability (2000 cycles at ≈14% compression strain).
Abstract: Textile-based electronics enable the next generation of wearable devices, which have the potential to transform the architecture of consumer electronics. Highly conductive yarns that can be manufactured using industrial-scale processing and be washed like everyday yarns are needed to fulfill the promise and rapid growth of the smart textile industry. By coating cellulose yarns with Ti3C2Tx MXene, highly conductive and electroactive yarns are produced, which can be knitted into textiles using an industrial knitting machine. It is shown that yarns with MXene loading of ≈77 wt% (≈2.2 mg cm−1) have conductivity of up to 440 S cm−1. After washing for 45 cycles at temperatures ranging from 30 to 80 °C, MXene-coated cotton yarns exhibit a minimal increase in resistance while maintaining constant MXene loading. The MXene-coated cotton yarn electrode offers a specific capacitance of 759.5 mF cm−1 at 2 mV s−1. A fully knitted textile-based capacitive pressure sensor is also prepared, which offers high sensitivity (gauge factor of ≈6.02), wide sensing range of up to ≈20% compression, and excellent cycling stability (2000 cycles at ≈14% compression strain). This work provides new and practical insights toward the development of platform technology that can integrate MXene in cellulose-based yarns for textile-based devices.
TL;DR: In this paper, 2D carbide flakes of Ti3C2Tx MXene can be used as multifunctional conductive binders for flexible hard carbon (HC) electrodes.
Abstract: Hard carbon (HC) is a promising anode material for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs), but the volume change during the insertion/extraction of Na+ or K+ limits the cycle life, especially for PIBs due to the large ion size of K+. Moreover, the conventional anodes fabricated through the coating method cannot satisfy the requirement of flexible devices. Here, it is shown that 2D carbide flakes of Ti3C2Tx MXene can be used as multifunctional conductive binders for flexible HC electrodes. The use of MXene nanosheets eliminates the need for all the electrochemically inactive components in the conventional polyvinylidene fluoride–bonded HC electrode, including polymer binders, conductive additives, and current collectors. In MXene-bonded HC electrodes, conductive and hydrophilic MXene 2D nanosheets construct a 3D network, which can effectively stabilize the electrode structure and accommodate the volume expansion of HC during the charge/discharge process, leading to an enhanced electrode capacity and excellent cycle performance as anodes for both SIBs and PIBs. Benefiting from the 3D conductive network, the MXene-bonded HC film electrodes also present improved rate capability, indicating MXene is a very promising multifunctional binder for next-generation flexible secondary rechargeable batteries.
University of Hamburg1, University of Melbourne2, University of Strasbourg3, National Institute for Materials Science4, Ghent University5, University of Michigan6, Drexel University7, Massachusetts Institute of Technology8, Northwestern University9, University of California, Los Angeles10, National Research Council11, Peking University12, Ikerbasque13, City University of Hong Kong14, University of Liverpool15, Imperial College London16, Queen Mary University of London17, Nankai University18
TL;DR: The developments and applications that are to come in LbL assembly, focusing on coatings, bulk materials, membranes, nanocomposites, and delivery vehicles, are discussed.
Abstract: Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings. In this Perspective, dedicated to the memory of ACS Nano associate editor Prof. Dr. Helmuth Mohwald, we discuss the developments and applications that are to come in LbL assembly, focusing on coatings, bulk materials, membranes, nanocomposites, and delivery vehicles.
TL;DR: In this article, the research and development of carbon-based catalysts in supercapacitors and batteries for clean energy storage as well as in air/water treatments for environmental remediation are reviewed.
Abstract: Owing to their high earth-abundance, eco-friendliness, high electrical conductivity, large surface area, structure tunability at the atomic/morphological levels, and excellent stability in harsh conditions, carbon-based metal-free materials have become promising advanced electrode materials for high-performance pseudocapacitors and metal-air batteries. Furthermore, carbon-based nanomaterials with well-defined structures can function as green catalysts because of their efficiency in advanced oxidation processes to remove organics in air or from water, which reduces the cost for air/water purification and avoids cross-contamination by eliminating the release of heavy metals/metal ions. Here, the research and development of carbon-based catalysts in supercapacitors and batteries for clean energy storage as well as in air/water treatments for environmental remediation are reviewed. The related mechanistic understanding and design principles of carbon-based metal-free catalysts are illustrated, along with the challenges and perspectives in this emerging field.
13 May 2019
TL;DR: In this article, a potential factor to the property difference between different studies is found: measured properties vary widely from study to study; however, measured properties varied widely from one study to another.
Abstract: Ti3C2Tx MXene is an attractive two-dimensional (2D) material for a wide variety of applications; however, measured properties vary widely from study to study. A potential factor to the property dif...
TL;DR: This work adapts the state-of-the-art positive and unlabeled machine learning framework to predict which theoretically proposed 2D materials have the highest likelihood of being successfully synthesized, and identifies 18 MXene compounds that are highly promising candidates for synthesis.
Abstract: Growing interest in the potential applications of two-dimensional (2D) materials has fueled advancement in the identification of 2D systems with exotic properties. Increasingly, the bottleneck in this field is the synthesis of these materials. Although theoretical calculations have predicted a myriad of promising 2D materials, only a few dozen have been experimentally realized since the initial discovery of graphene. Here, we adapt the state-of-the-art positive and unlabeled (PU) machine learning framework to predict which theoretically proposed 2D materials have the highest likelihood of being successfully synthesized. Using elemental information and data from high-throughput density functional theory calculations, we apply the PU learning method to the MXene family of 2D transition metal carbides, carbonitrides, and nitrides, and their layered precursor MAX phases, and identify 18 MXene compounds that are highly promising candidates for synthesis. By considering both the MXenes and their precursors, we ...
TL;DR: In this article, the use of the Ti3C2 MXene in organic-inorganic lead halide perovskite solar cells (PSCs) was explored, and the electron transport layers were used as ETLs in low-temperature processed planar-structured PSCs.
Abstract: MXenes, a class of two-dimensional (2D) transition metal carbides and nitrides, have a wide range of potential applications due to their unique electronic, optical, plasmonic, and other properties. Herein, we explore the use of the Ti3C2 MXene in organic–inorganic lead halide perovskite solar cells (PSCs). SnO2–Ti3C2 MXene nanocomposites with different contents of Ti3C2 (0, 0.5, 1.0, 2.0, and 2.5 wt‰) were used as electron transport layers (ETLs) in low-temperature processed planar-structured PSCs. Mixing SnO2 with 1.0 wt‰ Ti3C2 effectively increases the power conversion efficiency (PCE) from 17.23% to 18.34%, whereas the device prepared with pristine Ti3C2 as the ETL achieves a PCE of 5.28%. Photoluminescence and electrochemical impedance spectroscopy results reveal that metallic Ti3C2 MXene nanosheets provide superior charge transfer paths, enhancing electron extraction, electron mobility, and decreasing the electron transfer resistance at the ETL/perovskite interface, and thus leading to higher photocurrents. This work proposes a new field of application for MXenes and a promising method to increase the efficiency of solar cells.
TL;DR: In this article, in situ X-ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti3C2Tx in 1 m H2SO4.
Abstract: Since the discovery of Ti3C2Tx in 2011, the family of two‐dimensional transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has quickly attracted the attention of those developing energy storage applications such as electrodes for supercapacitors with acidic aqueous electrolytes. The excellent performance of these MXenes is attributed to a pseudocapacitive energy storage mechanism, based on the non‐rectangular shape of cyclic voltammetry curves and changes in the titanium oxidation state detected by in situ X‐ray absorption spectroscopy. However, the pseudocapacitive mechanism is not well understood and no dimensional changes due to proton insertion have been reported. In this work, in situ X‐ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti3C2Tx in 1 m H2SO4. Results reveal that a 0.5 A expansion and shrinkage of the c‐lattice parameter of Ti3C2Tx occur during cycling in a 0.9 V voltage window, showing that the charge storage mechanism is intercalation pseudocapacitance with implication for MXene use in energy storage and electrochemical actuators.
TL;DR: In this article, the effects of synthesis methods on optoelectronic properties of 2D titanium carbonitride, Ti3CNTx, were systematic studied and the impact of the C and N site composition on electronic transport has not been explored.
Abstract: MXenes, a relatively new class of two-dimensional (2D) transition-metal carbides, carbonitrides, and nitrides, exhibit unique properties such as high electronic conductivity, a wide range of optical characteristics, hydrophilicity, and mechanical stability. Because of the high electronic conductivity, MXenes have shown promise in many applications, such as energy storage, electromagnetic interference shielding, antennas, and transparent coatings. 2D titanium carbide (Ti3C2Tx, where Tx represents surface terminations), the first discovered and most studied MXene, has the highest electronic conductivity exceeding 10 000 S cm–1. There have been several efforts to alter the conductivity of MXenes, such as manipulation of the transition-metal layer and control of surface terminations. However, the impact of the C and N site composition on electronic transport has not been explored. In this study, the effects of synthesis methods on optoelectronic properties of 2D titanium carbonitride, Ti3CNTx, were systematic...
TL;DR: The results demonstrate that controlling the interlayer transport of Ti3C2T x MXene is essential for enhancing the selective sensing of gas molecules.
Abstract: Gas molecules are known to interact with two-dimensional (2D) materials through surface adsorption where the adsorption-induced charge transfer governs the chemiresistive sensing of various gases. ...
TL;DR: In this paper, the authors acknowledge the core research facilities at Drexel University for providing access to characterization tools, and thanks Prof. Leslie Lamberson and Steve Pagano for organizing the IExE REU.
TL;DR: In this article, the optoelectronic and pseudocapacitive properties of titanium carbide (Ti3C2Tx) are combined to create a MXene electrochromic device, with a visible absorption peak shift from 770 to 670 nm and a 12% reversible change in transmittance with a switching rate of <1 s when cycled in an acidic electrolyte under applied potentials of less than 1 V.
Abstract: MXenes, a large family of 2D transition metal carbides and nitrides, have shown potential in energy storage and optoelectronic applications. Here, the optoelectronic and pseudocapacitive properties of titanium carbide (Ti3C2Tx) are combined to create a MXene electrochromic device, with a visible absorption peak shift from 770 to 670 nm and a 12% reversible change in transmittance with a switching rate of <1 s when cycled in an acidic electrolyte under applied potentials of less than 1 V. By probing the electrochromic effect in different electrolytes, it is shown that acidic electrolytes (H3PO4 and H2SO4) lead to larger absorption peak shifts and a higher change of transmittance than the neutral electrolyte (MgSO4) (Δλ is 100 nm vs 35 nm and ΔT770 nm is ≈12% vs ≈3%, respectively), hinting at the surface redox mechanism involved. Further investigation of the mechanism by in situ X-ray diffraction and Raman spectroscopy reveals that the reversible shift of the absorption peak is attributed to protonation/deprotonation of oxide-like surface functionalities. As a proof of concept, it is shown that Ti3C2Tx MXene, dip-coated on a glass substrate, functions as both transparent conductive coating and active material in an electrochromic device, opening avenues for further research into optoelectronic and photonic applications of MXenes.