One-pot synthesis of 2D Ti3C2/Ni2CO3(OH)2 composite as electrode material with superior capacity and high stability for hybrid supercapacitor

Ti3C2Tx MXene reinforcement: a nickel-vanadium selenide/MXene based multi-component composite as a battery-type electrode for supercapacitor applications

Designing innovative microstructures and implementing efficient multicomponent strategies are still challenging to achieve high-performance and chemo-mechanically stable electrode materials. Herein, a hierarchical three-dimensional (3D) graphene oxide (GO) assisted Ti3C2Tx MXene aerogel foam (MXene-GAF) impregnated with battery-type bimetallic nickel vanadium selenide (NiVSe) has been prepared through a hydrothermal method followed by freeze-drying (denoted as NiVSe-MXene-GAF). 3D-oriented cellular pore networks benefit the energy storage process through the effective lodging of NiVSe particles, improving the access of the electrolyte to the active sites, and alleviating volume changes during redox reactions. The 3D MXene-GAF conductive matrix and heterostructured interface of MXene-rGO and NiVSe facilitated the rapid transport of electrical charges and ions during the charge-discharge process. As a result of the synergism of these effects, NiVSe-MXene-GAF exhibited remarkable electrochemical performance with a specific capacity of 305.8 mA h g-1 at 1 A g-1 and 99.2% initial coulombic efficiency. The NiVSe-MXene-GAF electrode delivered a specific capacity of 235.1 mA h g-1 even at a high current density of 12 A g-1 with a 76.8% rate performance. The impedance measurements indicated a low bulk solution resistance (Rs = 0.71 Ω) for NiVSe-MXene-GAF. Furthermore, the structural robustness of NiVSe-MXene-GAF guaranteed long-term stability with a 91.7% capacity retention for successive 7000 cycles. Thus, developing NiVSe-MXene-GAF provides a progressive strategy for fabricating high-performance 3D heterostructured electrode materials for energy storage applications.

Construction of rod-like cobalt-pyridinedicarboxylic acid/MXene nanosheets composites for hydrogen evolution reaction and supercapacitor

Metal-organic frameworks (MOFs) have attracted considerable attention in the field of energy storage and conversion due to their large specific surface area, regulatable pore structure and composition. However, the poor electrical conductivity and few active sites of MOFs impede their application. Herein, highly conductive MXene nanosheets are introduced to modulate the electronic conductivity and structure of rod-like Co-pyridinedicarboxylic acid (Co-PDC), and thus enhancing the electrochemical performance of MOFs. The heterostructural Co-PDC/MXene (CPM) was facily synthesized at room temperature. The as-prepared CPM-30 with 30 % MXene only requires the overpotential of 75.1 mV to achieve a current density of 10 mA cm-2 for hydrogen evolution reaction (HER), and the assembled electrolytic cell with CPM-30 and RuO2 as cathode and anode electrodes can achieve a current density of 10 mA cm-2 at a voltage of 1.65 V. In addition, CPM-10 exhibits a high specific capacitance of 583.1 F g-1 at 0.5 A g-1 and an excellent rate performance of 41.6 % at 50 A g-1. Furthermore, the assembled asymmetric supercapacitor CPM-10//AC exhibited an energy density of 15.55 Wh kg-1 at a power density of 750 W kg-1 and excellent stability with a capacitance retention rate of 95 % after 10,000 cycles. The excellent electrochemical properties of Co-PDC/MXene are attributed to the unique structure and synergistic effect of Co-PDC and MXene.

Cobalt-Nickel Layered Double Hydroxides on Electrospun MXene for Superior Asymmetric Supercapacitor Electrodes

Flexible electrodes for energy storage and conversion require a micro-nanomorphology and stable structure. Herein, MXene fibers (MX-CNF) are fabricated by electrospinning, and Co-MOF nanoarrays are prepared on the fibers to form Co-MOF@MX-CNF. Hydrolysis and etching of Co-MOF@MX-CNF in the Ni2+ solution produce cobalt-nickel layered double hydroxide (CoNi-LDH). The CoNi-LDH nanoarrays on the MX-CNF substrate have a large specific surface area and abundant electrochemical active sites, thus ensuring effective exposure of the CoNi-LDH active materials to the electrolyte and efficient pseudocapacitive energy storage and fast reversible redox kinetics for enhanced charging-discharging characteristics. The CoNi-LDH@MX-CNF electrode exhibits a discharge capacity of 996 F g-1 at a current density of 1 A g-1 as well as 78.62% capacitance retention after 3,000 cycles at 10 A g-1. The asymmetric supercapacitor (ASC) comprising the CoNi-LDH@MX-CNF positive electrode and negative activated carbon electrode shows an energy density of 48.4 Wh kg-1 at a power density of 499 W kg-1 and a capacity retention of 78.9% after 3,000 cycles at a current density of 10 A g-1. Density-functional theory calculations reveal the charge density difference and partial density of states of CoNi-LDH@MX-CNF confirming the large potential of the CoNi-LDH@MX-CNF electrode in energy storage applications.

Nickel hydroxide-decorating potassium-doped graphitic carbon nitride for boosting photocatalytic carbon dioxide reduction

In this study, nickel hydroxide (Ni(OH)2) was employed to modify potassium (K)-doped graphitic carbon nitride (g-C3N4, CN) for enhancing photocatalytic CO2 reduction. The light absorption and charge separation performances of CN were enhanced after modification. Experiments and theoretical calculations indicated that the loaded Ni(OH)2 could gather electrons, facilitate adsorption and activation of CO2. The optimized photocatalyst exhibited high CO2 reductive rate with CO and CH4 yields of 42.6 and 3.5 μmol g-1, respectively after 3 h irradiation in the presence of 0.5 mL water, which was 1.4 and 4.6 times higher than the yields on K-doped CN and Ni(OH)2-decorated CN, respectively. This work provides new thought for enhancing CO2 reductive performance of CN.

In situ synthesis of label-free electrochemical aptasensor-based sandwich-like AuNPs/PPy/Ti3C2Tx for ultrasensitive detection of lead ions as hazardous pollutants in environmental fluids

The monitoring of hazardous pollutants in environmental fluids is one of main stretaegy in investigation of water and soil quality. Metal ions are one of main and dangerius materials in water sampels and one of the main causes of environmental problems. Therefore, many of environmental researchers focused on fabrication of highly sensitive sensor to ion hazardous pollutants environmental fluids. The encapsulation of 2D MXenes with other stable materials has proven to be an effective method for enhancing their stability and electrochemical properties. In this work, a sandwich-like nanocomposite structure, AuNPs/PPy/Ti₃C₂T_x, was designed and synthesized via a facile method of one-step layer-by-layer self-assembly. The morphology and structure of the prepared nanocomposites are characterized with various methods such as scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Ti₃C₂T_x as a substrate played a significant role in the synthesis and alignment of PPy and AuNPs growth. The nanocomposites have maximized the benefits of the inorganic AuNPs and organic PPy materials, enhancing their stability and electrochemical performance. Meanwhile, AuNPs have given the nanocomposite the ability to form covalent bonds with biomaterials via the Au-S bond. Thus, a novel electrochemical aptasensor was developed based on AuNPs/PPy/Ti₃C₂T_x for the sensitive and selective detection of Pb²⁺. It demonstrated a wide linear range from 5 × 10^-14 to 1 × 10^-8 M with a low LOD of 1 × 10^-14 M (S/N = 3). Additionally, the developed aptasensor exhibited excellent selectivity and stability and successfully used to sensing of Pb²⁺ in environmental fluids such as NongFu Spring and tap water.

Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

With a decade of effort, significant progress has been achieved in the synthesis, processing, and applications of MXenes. Metal ions play many crucial roles, such as in MXene delamination, structure regulation, surface modification, MXene composite construction, and even some unique applications. The different roles of metal ions are attributed to their many interactions with MXenes and the unique nature of MXenes, including their layered structure, surface chemistry, and the existence of multi-valent transition metals. Interactions with metal ions are crucial for the energy storage of MXene electrodes, especially in metal ion batteries and supercapacitors with neutral electrolytes. This review aims to provide a good understanding of the interactions between metal ions and MXenes, including the classification and fundamental chemistry of their interactions, in order to achieve their more effective utilization and rational design. It also provides new perspectives on MXene evolution and exfoliation, which may suggest optimized synthesis strategies. In this respect, the different effects of metal ions on MXene synthesis and processing are clarified, and the corresponding mechanisms are elaborated. Research progress on the roles metal ions have in MXene applications is also introduced.

Composite Fe3O4-MXene-Carbon Nanotube Electrodes for Supercapacitors Prepared Using the New Colloidal Method

MXenes, such as Ti3C2Tx, are promising materials for electrodes of supercapacitors (SCs). Colloidal techniques have potential for the fabrication of advanced Ti3C2Tx composites with high areal capacitance (CS). This paper reports the fabrication of Ti3C2TX-Fe3O4-multiwalled carbon nanotube (CNT) electrodes, which show CS of 5.52 F cm-2 in the negative potential range in 0.5 M Na2SO4 electrolyte. Good capacitive performance is achieved at a mass loading of 35 mg cm-2 due to the use of Celestine blue (CB) as a co-dispersant for individual materials. The mechanisms of CB adsorption on Ti3C2TX, Fe3O4, and CNTs and their electrostatic co-dispersion are discussed. The comparison of the capacitive behavior of Ti3C2TX-Fe3O4-CNT electrodes with Ti3C2TX-CNT and Fe3O4-CNT electrodes for the same active mass, electrode thickness and CNT content reveals a synergistic effect of the individual capacitive materials, which is observed due to the use of CB. The high CS of Ti3C2TX-Fe3O4-CNT composites makes them promising materials for application in negative electrodes of asymmetric SC devices.

High areal capacitance of vanadium oxides intercalated Ti3C2 MXene for flexible supercapacitors with high mass loading

Flexible all-solid-state supercapacitors (ASSSs) have caught the scientific attention to meet the explosive demand for portable and wearable electronic devices. However, it is difficult for flexible electrode materials to obtain a high areal capacitance at a high mass loading, which limits their commercial applications. In this study, vanadium oxide (V₂O₅) nanoparticles are introduced into Ti₃C₂ flakes with the aid of cetyltrimethylammonium bromide (CTAB). The intercalation of V₂O₅ particles in the interlayer of Ti₃C₂ establishes a hierarchical structure and facilitates the electrolyte penetration. As a result, the prepared CT-Ti₃C₂@V₂O₅ composite electrode achieves a high areal capacitance of 2065 mF cm^-2 at 3 mA cm^-2 and superior active mass loading (15 mg cm^-2). Meanwhile, over 93% capacitance is maintained after 6000 cycles at 18 mA cm^-2. The ASSS based on CT-Ti₃C₂@V₂O₅ delivers a high areal capacitance of 477 mF cm^-2 at 1 mV s^-1 and exhibits stable performance at different bending states, which reaches to the advanced level for the ASSSs based on MXenes.

MnO1.88/R-MnO2/Ti3C2(OH/F)x composite electrodes for high-performance pseudo-supercapacitors prepared from reduced MXenes

Mixed-valency MnO_x (MnO_1.88 and MnO₂)/Ti₃C₂(OH/F)_x composites electrode materials were in situ growth of MnO_x on the surface of Ti₃C₂(OH/F)_x, where the Ti₃C₂(OH/F)_x acts as both template and reducing agent.

Scalable nanohybrids of graphitic carbon nitride and layered NiCo hydroxide for high supercapacitive performance

The limited number of edge nitrogen atoms and low intrinsic electrical conductivity hinder the supercapacitive energy storage applications of the nitrogen-rich graphitic carbon nitride (g-C3N4). In this study, a novel graphitic carbon nitride/NiCo-layered double hydroxide (CNLDH), a two-dimensional nanohybrid, is prepared by a simple hydrothermal synthesis. The homogeneous interpolation of g-C3N4 nanosheets into NiCo LDH stacked nanosheets effectively increases the overall performances of the g-C3N4/NiCo LDH nanohybrid. The improved morphology of the nanohybrid electrode upon the addition of g-C3N4 to the NiCo LDH yields a specific capacity of 183.43 mA h g-1 in 6 M KOH at 1 A g-1, higher than those of bare g-C3N4 (20.89 mA h g-1) and NiCo LDH (95.92 mA h g-1) electrodes. The excellent supercapacitive performance of the CNLDH nanohybrid is complemented by its low internal resistance, excellent rate capability, and large cycling lifetime. Furthermore, the hybrid supercapacitor is assembled using CNLDH 0.1 as a positive electrode and activated carbon (AC) as a negative electrode. The hybrid supercapacitor device of CNLDH 0.1//AC shows the maximum specific capacity of 37.44 mA h g-1 at 1 A g-1 with remarkable energy density, power density and good cycling performance. This confirms that the CNLDH 0.1 nanohybrid is an excellent electrode material for supercapacitor applications.

Alternative electrodes for HTMs and noble-metal-free perovskite solar cells: 2D MXenes electrodes

The high cost of hole transporting materials (HTMs) and noble metal electrodes limits the application of perovskite solar cells (PSCs). Carbon materials have been commonly utilized for HTMs and noble-metal-free PSCs. In this paper, a more conductive 2D MXene material (Ti₃C₂), showing a similar energy level to carbon materials, has been used as a back electrode in HTMs and noble-metal-free PSCs for the first time. Seamless interfacial contact between the perovskite layer and Ti₃C₂ material was obtained using a simple hot-pressing method. After the adjustment of key parameters, the PSCs based on the Ti₃C₂ electrode show more stability and higher power conversion efficiencies (PCE) (13.83%, 27% higher than that (10.87%) of the PSCs based on carbon electrodes) due to the higher conductivity and seamless interfacial contact of the MXene electrode. Our work proposes a promising future application for MXene and also a good electrode candidate for HTM and the noble-metal-free PSCs.

The 2D Mxene material was successfully used as the counter electrode of the perovskite solar cell and achieved power conversion efficiencies of 13.84%.