Progress of Two-Dimensional Ti3 C2 Tx in Supercapacitors

MXene: A Roadmap to Sustainable Energy Management, Synthesis Routes, Stabilization, and Economic Assessment

MXenes with their wide range of tunability and good surface chemistry provide unique and distinctive characteristics offering potential employment in various aspects of energy management applications. These high-performance materials have attracted considerable attention in recent decades due to their outstanding characteristics. In the literature, most of the work is related to specific methods for the preparation of MXenes. In this Review, we present a detailed discussion on the synthesis of MXenes through different etching routes involving acids, such as hydrochloric acid, hydrofluoric acid, and lithium fluoride, and non-acidic alkaline solution, electrochemical, and molten salt methods. Furthermore, a concise overview of the different structural, optical, electronic, and magnetic properties of MXenes is provided corresponding to their role in supporting high thermal, chemical, mechanical, environmental, and electrochemical stability. Additionally, the role of MXenes in maintaining the thermal management performance of photovoltaic thermal systems (PV/T), wearable light heaters, solar water desalination, batteries, and supercapacitors is also briefly discussed. A techno-economic and life cycle analysis of MXenes is provided to analyze their sustainability, scalability, and commercialization to facilitate a comprehensive array of energy management systems. Lastly, the technology readiness level of MXenes is defined, and future recommendations for MXenes are provided for their further utilization in niche applications. The present work strives to link the chemistry of MXenes to process economics for energy management applications.

In Situ Growth of MnO2 Nanosheets on a Graphite Flake as an Effective Binder-Free Electrode for High-Performance Supercapacitors

In this work, manganese dioxide (MnO2) nanosheets in situ loaded on a high-purity graphite flake (GF) were prepared by one-step hydrothermal deposition. It was found that the specific capacitance value of a single MnO2/GF electrode was 882 F/g at a current density of 1.0 A/g in a KOH electrolyte, and the specific capacitance retention of the MnO2/GF electrode can reach about 90.1% after 5000 charge-discharge cycles at a current density of 10 A/g. Furthermore, a MnO2/GF∥MnO2/GF symmetric supercapacitor device was fabricated with two pieces of MnO2/GF electrodes and ordinary filter paper with a 1 M KOH/PVA gel electrolyte as a separator. The single symmetric device displayed a high energy density of 64.2 Wh/kg at a power density of 400 W/kg within an applied voltage of 1.6 V, and this value was superior to those of previously reported MnO2-based systems. A tandem device consisting of a five-series tandem device (the applied voltage of a single device was 0.7 V) and a three-series tandem device (the applied voltage of a single device was 1.6 V) was prepared to drive a red light-emitting diode (LED). These findings open up application prospects for MnO2-based composite electrode materials for high-performance supercapacitors.

A Rising 2D Star: Novel MBenes with Excellent Performance in Energy Conversion and Storage

As a flourishing member of the two-dimensional (2D) nanomaterial family, MXenes have shown great potential in various research areas. In recent years, the continued growth of interest in MXene derivatives, 2D transition metal borides (MBenes), has contributed to the emergence of this 2D material as a latecomer. Due to the excellent electrical conductivity, mechanical properties and electrical properties, thus MBenes attract more researchers' interest. Extensive experimental and theoretical studies have shown that they have exciting energy conversion and electrochemical storage potential. However, a comprehensive and systematic review of MBenes applications has not been available so far. For this reason, we present a comprehensive summary of recent advances in MBenes research. We started by summarizing the latest fabrication routes and excellent properties of MBenes. The focus will then turn to their exciting potential for energy storage and conversion. Finally, a brief summary of the challenges and opportunities for MBenes in future practical applications is presented.

Simultaneously tuning interlayer spacing and termination of MXenes by Lewis-basic halides

The surface and interface chemistry are of significance on controlling the properties of two-dimensional transition metal carbides and nitrides (MXenes). Numerous efforts have been devoted to the regulation of Ti3C2Tx MXene, however, tuning interlayer spacing and surface halogen termination of other MXenes (besides Ti3C2Tx) is rarely reported while demanded. Here we propose a Lewis-basic halides treatment, which is capable of simultaneously engineering the interlayer spacing and surface termination of various MXenes. Benefited from the abundant desolvated halogen anions and cations in molten state Lewis-basic halides, the -F termination was substituted by nucleophilic reaction and the interlayer spacing was enlarged. Ti3C2Tx MXene treated by this method showed a high specific capacity of 229 mAh g−1 for Li+ storage, which is almost 2 times higher than pristine one. Considering the universality, our method provides an approach to regulating the properties of MXenes, which may expand their potential applications in energy storage, optoelectronics and beyond.

Assessment of Salt Stress to Arabidopsis Based on the Detection of Hydrogen Peroxide Released by Leaves Using an Electrochemical Sensor

Salt stress will have a serious inhibitory effect on various metabolic processes of plant cells, this will lead to the excessive accumulation of reactive oxygen species (ROS). Hydrogen peroxide (H2O2) is a type of ROS that can severely damage plant cells in large amounts. Existing methods for assessing the content of H2O2 released from leaves under salt stress will cause irreversible damage to plant leaves and are unable to detect H2O2 production in real time. In this study, on the strength of a series of physiological indicators to verify the occurrence of salt stress, an electrochemical sensor for the detection of H2O2 released from leaves under salt stress was constructed. The sensor was prepared by using multi-walled carbon nanotube-titanium carbide-palladium (MWCNT-Ti3C2Tx-Pd) nanocomposite as substrate material and showed a linear response to H2O2 detection in the range 0.05-18 mM with a detection limit of 3.83 μM. Moreover, we measured the determination of H2O2 released from Arabidopsis leaves at different times of salt stress by the sensor, which was consistent with conventional method. This study demonstrates that electrochemical sensing is a desirable technology for the dynamic determination of H2O2 released by leaves and the assessment of salt stress to plants.

Design and finite element modeling of two-dimensional nanomechanical biosensors for SARS-CoV-2 detection

SARS-CoV-2 is the causative agent of COVID-19 disease. The development of different variants has increased the prevalence, pathogenicity, and mortality of the SARS-CoV-2. Prompt diagnosis and timely initiation of therapy can undoubtedly minimize the damage caused by this virus. In this study, a wide range of emerging single layer two-dimensional materials (SL2DMs), including graphene, grapheme oxide (GO), reduced graphene oxide (rGO), hexagonal boron nitride (h-BN), Ti3C2Tx MXene, and MoS2that can be used to fabricate highly sensitive biosensors, are analyzed using the finite element method based on antigen-antibody interaction. Important design parameters including sensor size, sensor aspect ratio, number of viruses, and applying in-plane strain on sensor performance are analyzed using frequency shift technique. In the following, an analytical relationship that can predict the limit of detection (LOD) according to the above parameters is proposed. The results show that all the above materials have a good performance in detecting viruses in the sample range of 10-100 viruses. This range can be reduced significantly by applying strains of less than 0.1. Also, applying strain increases shift frequency index by 2 to 3 times, which is a significant result. The maximum and minimum sensor performance are obtained for GO and Ti3C2Tx, respectively. The results of this paper can be used to build a new generation of two-dimensional biosensors for rapid detection of COVID-19 and other viruses.

Compacted N-Doped 3D Bicontinuous Nanoporous Graphene/Carbon Nanotubes@Ni-Doped MnO2 Electrode for Ultrahigh Volumetric Performance All-Solid-State Supercapacitors at Wide Temperature Range

Developing wide temperature range flexible solid-state supercapacitors with high volumetric energy density is highly desirable to meet the demands of the rapidly developing field of miniature consumer electronic devices and promote their widespread adoption. Herein, high-quality dense N-doped 3D porous graphene/carbon nanotube (N-3DG/CNTs) hybrid films are prepared and used as the substrate for the growth of Ni-doped MnO₂ (Ni-MnO₂ ). The integrated and interconnected architecture endows N-3DG/CNTs@Ni-MnO₂ composite electrodes' high conductivity and fast ion/electron transport pathway. Subsequently, 2.4 V solid-state supercapacitors are fabricated based on compacted N-3DG/CNTs@Ni-MnO₂ positive electrodes, which exhibit an ultrahigh volumetric energy density of 78.88 mWh cm^-3 based on the entire device including electrodes, solid-state electrolyte, and packing films, excellent cycling stability up to 10 000 cycles, and a wide operating temperature range from -20 to 70 °C. This work demonstrates a design of flexible solid-state supercapacitors with exceptional volumetric performance capable of operation under extreme conditions.

MXene titanium carbide synthesized by hexagonal titanium aluminum carbide with high specific capacitance and low impedance

The electrochemical properties of the MXene titanium carbide, Ti₃C₂, which has received much attention in the application of electrode materials for supercapacitors, are affected by the different morphologies of its precursor. In particular, the increase of layer spacing and specific capacitance, as well as the decrease of impedance and the dynamics analysis of Ti₃C₂ etched from hexagonal Ti₃AlC₂ precursors, are still not clear, and need to be further studied and explored. In this work, MXene Ti₃C₂ was synthesized efficiently in 2 hours by microwave assisted selective etching with hexagonal Ti₃AlC₂ as the precursor material. The specific capacitance of the Ti₃C₂ electrode is up to 357.85 F g^-1, while the ohmic resistance R_Ω of the whole electrochemical energy storage system is 0.234 ohm and the charge transfer resistance R_ct is 0.875 ohm. By analyzing the structural evolution and electrochemical properties from hexagonal Ti₃AlC₂ to Ti₃C₂, it is revealed that Ti₃C₂ prepared with hexagonal Ti₃AlC₂ as the precursor material has larger atomic layer spacing, more active sites, smaller diffusion impedance and higher energy storage efficiency than that prepared with ordinary Ti₃AlC₂. These lay a structural foundation for improving the energy storage performance of Ti₃C₂ supercapacitor electrodes.

CoS nanowires grown on Ti3C2Tx are promising electrodes for supercapacitors: High capacitance and remarkable cycle capability

Benefitting from the large interlayer spacing, ultrahigh conductivity and abundant surface chemistry, Ti₃C₂T_x has been a promising electrode material for supercapacitors (SCs). CoS has attracted much attention due to its low cost, weak Co-S bond and relatively high theoretical capacity. Herein, CoS nanowires were grown on few-layered Ti₃C₂T_x by one-step solvothermal method as a SC electrode. Within the composite, Ti₃C₂T_x could function as conductive network and buffer matrix to provide ultra-fast electronic transport and relieve volume expansion of CoS nanowires. Simultaneously, the active CoS nanowires with high capacitance act as interlayer spacer to restrain the restacking of Ti₃C₂T_x nanosheets. As a result, CoS/Ti₃C₂T_x-5 electrode exhibits a remarkable improvement specific capacitance of 528 F g^-1 at a current density of 1 A g^-1 and ultrahigh capacitance retention of 99.3% after 20 000 cycles at a current density of 10 A g^-1. The attempts and efforts made in this work provide a prototype for achieving excellent electrochemical properties.

A {BW12O40} hybrid decorated by Ag+ for use as a supercapacitor electrode material and photocatalyst

Through a hydrothermal method, we successfully synthesized a supramolecular compound [{Ag(phen)₂}₄{Ag(phen)}₂(H₂BW₁₂O₄₀)₂]. The as-synthesized material exhibited excellent supercapacitive and photocatalytic performances.