Toward Understanding the Enhanced Pseudocapacitive Storage in 3D SnS/MXene Architectures Enabled by Engineered Surface Reactions

The optimization of three-dimensional (3D) MXene-based electrodes with desired electrochemical performances is highly demanded. Here, a precursor-guided strategy is reported for fabricating the 3D SnS/MXene architecture with tiny SnS nanocrystals (≈5 nm in size) covalently decorated on the wrinkled Ti₃C₂T_x nanosheets through Ti−S bonds (denoted as SnS/Ti₃C₂T_x-O). The formation of Ti−S bonds between SnS and Ti₃C₂T_x was confirmed by extended X-ray absorption fine structure (EXAFS). Rather than bulky SnS plates decorated on Ti₃C₂T_x (SnS/Ti₃C₂T_x-H) by one-step hydrothermal sulfidation followed by post annealing, this SnS/Ti₃C₂T_x-O presents size-dependent structural and dynamic properties. The as-formed 3D hierarchical structure can provide short ion-diffusion pathways and electron transport distances because of the more accessible surface sites. In addition, benefiting from the tiny SnS nanocrystals that can effectively improve Na⁺ diffusion and suppress structural variation upon charge/discharge processes, the as-obtained SnS/Ti₃C₂T_x-O can generate pseudocapacitance-dominated storage behavior enabled by engineered surface reactions. As predicted, this electrode exhibits an enhanced Na storage capacity of 565 mAh g⁻¹ at 0.1 A g⁻¹ after 75 cycles, outperforming SnS/Ti₃C₂T_x-H (336 mAh g⁻¹), SnS (212 mAh g⁻¹), and Ti₃C₂T_x (104 mAh g⁻¹) electrodes.     

Electrochemical and in-situ X-ray diffraction studies of Ti3C2Tx MXene in ionic liquid electrolyte

2D titanium carbide (Ti₃C₂T_x MXene) showed good capacitance in both organic and neat ionic liquid electrolytes, but its charge storage mechanism is still not fully understood. Here, electrochemical characteristics of Ti₃C₂T_x electrode were studied in neat EMI-TFSI electrolyte. A capacitive behavior was observed within a large electrochemical potential range (from − 1.5 to 1.5 V vs. Ag). Intercalation and de-intercalation of EMI⁺ cations and/or TFSI⁻ anions were investigated by in-situ X-ray diffraction. Interlayer spacing of Ti₃C₂T_x flakes decreases during positive polarization, which can be ascribed to either electrostatic attraction effect between intercalated TFSI⁻ anions and positively charged Ti₃C₂T_x nanosheets or steric effect caused by de-intercalation of EMI⁺ cations. The expansion of interlayer spacing when polarized to negative potentials is explained by steric effect of cation intercalation.     

Aptamer-functionalized Ti3C2-MXene Nanosheets with One-step Potentiometric Detection of Programmed Death-ligand 1

This communication describes a simple sensitive one-step potentiometric aptasensing method for quantitative detection of a referenced therapeutic biomarker (programmed death-ligand 1, PD−L1). The aptasensor is constructed by modifying PD−L1-specific aptamer on Ti₃C₂-MXene nanosheets-functionalized electrode. Introduction of PD−L1 induces the specific reaction between PD−L1 and aptamer, thereby resulting in the change of spatial structures. The surface electric potential of modified electrode is shifted upon addition of PD−L1 proteins before and after the reaction of aptamer with the analyte. Interestingly, potentiometric aptamer with Ti₃C₂-MXene nanosheets can achieve a higher sensitivity and a lower detection limit toward target PD−L1 relative to aptamer-modified electrode. Experimental results indicated that the linear range and detection limit of using Ti₃C₂-MXene nanosheets were 0.01–100 ng mL⁻¹ and 7.8 pg mL⁻¹ PD−L1, respectively. Meanwhile, the specificity, reproducibility, storing stability and accuracy of potentiometric aptasensor are acceptable for the screening of PD−L1 in human serum samples.     

Fluorine‐Free Synthesis of High‐Purity Ti3C2Tx (T=OH,O) via Alkali Treatment

MXenes, 2D compounds generated from layered bulk materials, have attracted significant attention in energy‐related fields. However, most syntheses involve HF, which is highly corrosive and harmful to lithium‐ion battery and supercapacitor performance. Here an alkali‐assisted hydrothermal method is used to prepare a MXene Ti₃C₂T_x (T=OH, O). This route is inspired from a Bayer process used in bauxite refining. The process is free of fluorine and yields multilayer Ti₃C₂T_x with ca. 92 wt % in purity (using 27.5 m NaOH, 270 °C). Without the F terminations, the resulting Ti₃C₂T_x film electrode (ca. 52 μm in thickness, ca. 1.63 g cm⁻³ in density) is 314 F g⁻¹ via gravimetric capacitance at 2 mV s⁻¹ in 1 m H₂SO₄. This surpasses (by ca. 214 %) that of the multilayer Ti₃C₂T_x prepared via HF treatments. This fluorine‐free method also provides an alkali‐etching strategy for exploring new MXenes for which the interlayer amphoteric/acidic atoms from the pristine MAX phase must be removed.     

A Hybrid Assembly of MXene with NH2−Si Nanoparticles Boosting Lithium Storage Performance

A facile hybrid assembly between Ti₃C₂T_x MXene nanosheets and (3‐aminopropyl) triethoxylsilane‐modified Si nanoparticles (NH₂?Si NPs) was developed to construct multilayer stacking of Ti₃C₂T_x nanosheets with NH₂?Si NPs assembling together (NH₂?Si/Ti₃C₂T_x). NH₂?Si/Ti₃C₂T_x exhibits a significantly enhanced lithium storage performance compared to pristine Si, which is attributed to the robust crosslinking architecture and considerably improved electrical conductivity as well as shorter Li⁺ diffusion pathways. The optimized NH₂?Si/Ti₃C₂T_x anode with Ti₃C₂T_x: NH₂?Si mass ratio of 4 : 1 displays an enhanced capacity (864 mAh g^?1 at 0.1 C) with robust capacity retention, which is significantly higher than those of NH₂?Si NPs and Ti₃C₂T_x anodes. Furthermore, this work demonstrates the important effect of the MXene‐based electrode architecture on the electrochemical performance and can guide future work on designing high‐performance Si/MXene hybrids for energy storage applications.     

Functional MXene Materials: Progress of Their Applications

Nowadays, two‐dimensional materials have many applications in materials science. As a novel two‐dimensional layered material, MXene possesses distinct structural, electronic, and chemical properties; thus, it has potential applications in many fields, including battery electrodes, energy storage materials, sensors, and catalysts. Up to now, more than 70 MAX phases have been reported. However, in contrast to the variety of MAX phases, the existing MXene family merely includes Ti₂C, Ti₃C₂, (Ti_1/2, Nb_1/2)₂C, (V_1/2, Cr_1/2)₃C₂, Nb₂C, Ti₃CN, Ta₄C₃, V₂C, and Nb₄C₃. Among these materials, the Ti₃C₂T_x MXene exhibits prominently high volumetric capacitance, and the rate at which it transports electron is suitable for electrode materials in batteries and supercapacitors. Hence, Ti₃C₂T_x is commonly utilized as an electrode material in ion batteries such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Al³⁺ batteries. What is more, Ti₂C has the biggest specific surface area among all of these potential MXene phases, and therefore, Ti₂C has remarkably high gravimetric hydrogen storage capacities. In addition, Ti₂CO₂ materials display extremely high activity for CO oxidation, which makes it possible to design catalysts for CO oxidation at low temperatures. Furthermore, Ti₃C₂T_x with O, OH, and/or F terminations can be used for water purification owing to excellent water permeance, favorable filtration ability, and long‐time operation ability. This review supplies a relatively comprehensive summary of various applications of MXenes over the past few years.     

零维/二维MXene复合膜的制备及其超级电容器性能

采用水热法制备了0D/2D复合Ti₃C₂T_x MXene,利用X射线衍射、动态光散射和荧光光谱表征了其结构与形貌,结果表明形成了量子点吸附于纳米片的Ti₃C₂T_x复合结构（QDT）。相比未引入量子点的Ti₃C₂T_x,由QDT组装得到的自支撑膜电极的电化学性能有了显著提高：在三电极体系中,扫速为5 mV·s^-1时,比电容为338 F·g^-1,当扫速达到2 000 mV·s^-1,电容保持率达到46%;在两电极体系中,0.5 A·g^-1时的比电容达到216 F·g^-1,10 000次循环后电容保持率为87%。以上性能可归结于：量子点提供了更多的离子吸附位点,且纳米片尺寸减小,缩短了离子传输路径。     

Chlorophyll-Based Organic Heterojunction on Ti3C2Tx MXene Nanosheets for Efficient Hydrogen Production

The Z-scheme process is a photoinduced electron-transfer pathway in natural oxygenic photosynthesis involving electron transport from photosystem II (PSII) to photosystem I (PSI). Inspired by the interesting Z-scheme process, herein a photocatalytic hydrogen evolution reaction (HER) employing chlorophyll (Chl) derivatives, Chl-1 and Chl-2, on the surface of Ti₃C₂T_x MXene with two-dimensional accordion-like morphology, forming Chl-1@Chl-2@Ti₃C₂T_x composite, is demonstrated. Due to the frontier molecular orbital energy alignments of Chl-1 and Chl-2, sublayer Chl-1 is a simulation of PSI, whereas upper layer Chl-2 is equivalent to PSII, and the resultant electron transport can take place from Chl-2 to Chl-1. Under the illumination of visible light (>420 nm), the HER performance of Chl-1@Chl-2@Ti₃C₂T_x photocatalyst was found to be as high as 143 μmol h⁻¹ g_cat⁻¹, which was substantially higher than that of photocatalysts of either Chl-1@Ti₃C₂T_x (20 μmol h⁻¹ g⁻¹) or Chl-2@Ti₃C₂T_x (15 μmol h⁻¹ g⁻¹).     

Molecular Ligand-Mediated Assembly of Multicomponent Nanosheet Superlattices for Compact Capacitive Energy Storage

Inspired by the self-assembly of nanoparticle superlattices, we report a general method that exploits long-chain molecular ligands to induce ordered assembly of colloidal nanosheets (NSs), resulting in 2D laminate superlattices with high packing density. Co-assembly of two types of NSs further enables 2D/2D heterostructured superlattices. As a proof of concept, co-assembly of Ti₃C₂T_x and graphene oxide (GO) NSs followed by thermal annealing leads to MXene-rGO superlattices with tunable microstructures, which exhibit significantly higher capacitance than their filtrated counterparts, delivering an ultrahigh volumetric capacitance of 1443 F cm⁻³ at 2 mV s⁻¹. Moreover, the as-fabricated binder-free symmetric supercapacitors show a high volumetric energy density of 42.1 Wh L⁻¹, which is among the best reported for MXene-based materials in aqueous electrolytes. This work paves the way toward rational design of 2D material-based superstructures for energy applications.     

Regulating Fast Anionic Redox for High‐Voltage Aqueous Hydrogen‐Ion‐based Energy Storage

The ionic conductivity and small size of the hydrogen ion make it an ideal charge carrier for hydrogen‐ion energy storage (HES); however, high‐voltage two‐electrode configurations are difficult to construct as the result of the lack of efficient cathodic energy storage. Herein, the high potential fast anionic redox at the cathode of reduced graphene oxide (rGO) was applied by introducing redox additive electrolytes. By coupling the storing hydrogen ion in the Ti₃C₂T_x at the anode, a HES with a voltage of 1.8 V and a plateau voltage at 1.2 V was constructed. Compared with 2.2 Wh kg^?1 for the low‐voltage Ti₃C₂T_x//Ti₃C₂T_x, the specific energy of asymmetric rGO//Ti₃C₂T_x reaches 34.4 Wh kg^?1. Furthermore, it possesses an energy density of 23.7 Wh kg^?1 at high power density of 22.5 kW kg^?1. Thus, this study provides a novel guideline for constructing high‐voltage fast HES full cells.     

Titanium Carbide (Ti3C2Tx) MXene Ornamented with Palladium Nanoparticles for Electrochemical CO Oxidation

Titanium carbide (Ti₃C₂T_x) MXene possesses various unique physicochemical and catalytic properties. However, the electrochemical CO oxidation performance is not yet addressed experimentally. Herein, Ti₃C₂T_x (T_X=OH, O, and F) ordered and exfoliated two-dimensional nanosheets ornamented with semi-spherical palladium nanoparticles (2.5 Wt. %) with an average diameter of (10±1 nm) (denoted as Pd/Ti₃C₂T_x) is rationally designed for the electrochemical CO oxidation. The fabrication process is based on the selective chemical etching of Ti₃AlC₂ and delamination under sonication to form Ti₃C₂Tx nanosheets that are used as a substrate and reducing agent for supporting in situ growth of Pd nanoparticles via impregnation with Pd salt. Interestingly, Pd-free Ti₃C₂T_x displayed inferior CO oxidation activity, while Pd/Ti₃C₂T_x enhanced the CO oxidation activity substantially. This is attributed to the combination of outstanding physicochemical properties of Ti₃C₂T_x and the catalytic merits of Pd nanoparticles.     

Ti3C2Tx/MnO2正极在水系锌电池中的电化学性能

二氧化锰(MnO₂)材料具有比容量大、电极电位高、储量丰富以及价格低廉等优势,成为水系锌电池正极最受关注的一类材料,然而其仍然存在着结构稳定性差和电化学储存机理复杂的问题。因此,我们通过两步合成法制备了一种花苞状结构的MnO₂负载在Ti₃C₂T_x表面形成Ti₃C₂T_x/MnO₂复合材料,通过X射线粉末衍射(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)和高分辨透射电子显微镜(HRTEM)对复合样品的结构、成分和形貌进行表征。通过将Ti₃C₂T_x/MnO₂复合材料作为正极,与锌负极匹配组装成水系锌电池,研究了其分别在2 mol·L^-1 ZnSO₄、2 mol·L^-1 ZnSO₄+0.1 mol·L^-1 MnSO₄、30 mol·L^-1三氟甲基磺酸四乙基铵(TEAOTf)+1 mol·L^-1三氟甲烷磺酸锌(ZnOTf)和3 mol·L^-1 ZnOTf四种电解液中的电化学性能。结果表明,Ti₃C₂T_x/MnO₂在2 mol·L^-1 ZnSO₄中的比容量较高,但循环稳定性很差。将TEAOTf盐和ZnOTf盐共溶于水中,设计了一种新型的含惰性阳离子的超高浓度盐包水电解液(30 mol·L^-1 TEAOTf+1 mol·L^-1 ZnOTf),不仅提高了Ti₃C₂T_x/MnO₂材料的可逆性,而且有效抑制了电极材料在循环过程中的溶解。     

Simultaneous Delamination and Rutile Formation on the Surface of Ti3C2Tx MXene for Copper Adsorption

In this work, we studied the formation of the rutile phase of titanium dioxide (TiO₂) on delaminated MXene (d‐Ti₃C₂T_x) flakes by the reaction of Ti₃C₂T_x with amino acids in water. Three types of amino acids with varied side‐chain polarity were used to delaminate Ti₃C₂T_x. d‐Ti₃C₂T_x flakes formed stable colloidal solutions due to the negative surface charges of chemisorbed amino acids on the d‐Ti₃C₂T_x. Rutile formed on d‐Ti₃C₂T_x at room temperature upon the intercalation of aromatic amino acids and subsequent sonication of the solution, while flakes intercalated with aliphatic amino acids did not oxidize. X‐Ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy revealed the nanosize rutile formation on the surface of Ti₃C₂T_x flakes. The XPS results indicated the surface functionalization of histidine on d‐Ti₃C₂T_x flakes. As‐synthesized histidine functionalized rutile TiO₂@d‐Ti₃C₂T_x hybrid was used for adsorption of Cu²⁺ ions from aqueous solution with a maximum uptake of 95 mg g^?1.     

Achieving high-rate capacitance of multi-layer titanium carbide (MXene) by liquid-phase exfoliation through Li-intercalation

The stacks of multi-layer Ti₃C₂T_x and other types of MXene materials limit their electrochemical performance. Herein, we report a facile exfoliation technique to improve the exfoliation efficiency through Li-intercalation into Ti₃C₂T_x interlayers in isopropyl alcohol (IPA) with LiOH as intercalant. This de-intercalation method presented here not only effectively delaminates the stacked Ti₃C₂T_x multi-layers into separate few-layer MXene sheets, but also achieves high-rate supercapacitive performance of Ti₃C₂T_x electrode. The as-produced delaminated Ti₃C₂T_x shows highly improved electrochemical capacitive properties from 47 to 115 F g^− 1 at 200 mV s^− 1. Even at extremely high scan rate of 1000 mV s^− 1, a specific capacitance of 82 F g^− 1 is still obtained. The high-rate capability can be attributed to improved ions accessibility into the few-layer structures. This study offers a new and simple exfoliation pathway for MXenes materials to exploit their full potential in energy storage applications.     

Fluorine‐Free Synthesis of High‐Purity Ti3C2Tx (T=OH,O) via Alkali Treatment

MXenes, 2D compounds generated from layered bulk materials, have attracted significant attention in energy‐related fields. However, most syntheses involve HF, which is highly corrosive and harmful to lithium‐ion battery and supercapacitor performance. Here an alkali‐assisted hydrothermal method is used to prepare a MXene Ti₃C₂T_x (T=OH, O). This route is inspired from a Bayer process used in bauxite refining. The process is free of fluorine and yields multilayer Ti₃C₂T_x with ca. 92 wt % in purity (using 27.5 m NaOH, 270 °C). Without the F terminations, the resulting Ti₃C₂T_x film electrode (ca. 52 μm in thickness, ca. 1.63 g cm^?3 in density) is 314 F g^?1 via gravimetric capacitance at 2 mV s^?1 in 1 m H₂SO₄. This surpasses (by ca. 214 %) that of the multilayer Ti₃C₂T_x prepared via HF treatments. This fluorine‐free method also provides an alkali‐etching strategy for exploring new MXenes for which the interlayer amphoteric/acidic atoms from the pristine MAX phase must be removed.     

Fast self-assembled microfibrillated cellulose@MXene film with high-performance energy storage and superior mechanical strength

The trade-off between the electrochemical performance and mechanical strength is still a challenge for Ti₃C₂T_x free-standing electrode. Herein, a facile approach was proposed to fabricate a Microfibrillated cellulose@Ti₃C₂T_x (MFC@Ti₃C₂T_x) self-assembled microgel film by means of hydrogen bonding linkage. Benefiting from the rich hydroxyl groups on the MFC, the Ti₃C₂T_x nanosheets coated on the MFC in a time scale of minutes (within 1 min) instead of hours. The ultralong 1D frame of MFC effectively mitigated the re-aggregation of Ti₃C₂T_x nanosheet. The fluffy MFC@Ti₃C₂T_x film structure and the constructed 1D/2D conducting Ti₃C₂T_x pathways in horizontal and vertical directions endowed the fast ion transport of the electrolytes and the improved accessibility to the Ti₃C₂T_x surface. As a result, the freestanding MFC@Ti₃C₂T_x microgel film delivered a high specific capacitance of 451F/g. And the rate performance was increased to 71% from the 64% of that of pristine Ti₃C₂T_x film. Furthermore, the tensile strength of MFC@Ti₃C₂T_x film was also promoted to 46.3 MPa, 3 folds of that of the pristine Ti₃C₂T_x film, due to the high strength of MFC and the hydrogen bonding effect.     

A Two‐Dimensional Lamellar Membrane: MXene Nanosheet Stacks

Two‐dimensional (2D) materials are promising candidates for advanced water purification membranes. A new kind of lamellar membrane is based on a stack of 2D MXene nanosheets. Starting from compact Ti₃AlC₂, delaminated nanosheets of the composition Ti₃C₂T_x with the functional groups T (O, OH, and/or F) can be produced by etching and ultrasonication and stapled on a porous support by vacuum filtration. The MXene membrane supported on anodic aluminum oxide (AAO) substrate shows excellent water permeance (more than 1000 L m⁻² h⁻¹ bar⁻¹) and favorable rejection rate (over 90 %) for molecules with sizes larger than 2.5 nm. The water permeance through the MXene membrane is much higher than that of the most membranes with similar rejections. Long‐time operation also reveals the outstanding stability of the MXene membrane for water purification.     

Preparation of Ultrathin Two-Dimensional TixTa1−xSyOz Nanosheets as Highly Efficient Photothermal Agents

Although two-dimensional (2D) metal oxide/sulfide hybrid nanostructures have been synthesized, the facile preparation of ultrathin 2D nanosheets in high yield still remains a challenge. Herein, we report the first high-yield preparation of solution-processed ultrathin 2D metal oxide/sulfide hybrid nanosheets, that is, Ti_xTa_1−xS_yO_z (x=0.71, 0.49, and 0.30), from Ti_xTa_1−xS₂ precursors. The nanosheet exhibits strong absorbance in the near-infrared region, giving a large extinction coefficient of 54.1 L g⁻¹ cm⁻¹ at 808 nm, and a high photothermal conversion efficiency of 39.2 %. After modification with lipoic acid-conjugated polyethylene glycol, the nanosheet is a suitable photothermal agent for treatment of cancer cells under 808 nm laser irradiation. This work provides a facile and general method for the preparation of 2D metal oxide/sulfide hybrid nanosheets.     

MXene/枝状Co/聚偏氟乙烯复合光热膜制备及其界面蒸发性能

使用湿法刻蚀方式将Ti₃AlC₂刻蚀剥离成单/少层Ti₃C₂T_x MXene纳米片,采用电化学还原法制备枝状Co,然后以亲水的聚偏氟乙烯(PVDF)膜为基底通过真空抽滤制备Ti₃C₂T_x/枝状Co/PVDF复合光热膜。对复合材料的结构和形貌进行表征,研究了复合光热膜的光吸收性能和界面蒸发性能。结果表明,在模拟1个太阳光照下(光照强度为1 kW·m^-2),Ti₃C₂T_x/枝状Co/PVDF复合光热膜的光吸收率达到95.3%,纯水蒸发速率达到1.78 kg·m^-2·h^-1,界面蒸发效率高达97.5%。此外,还测试了在模拟海水中的界面蒸发性能,蒸发冷凝得到的水达到世界卫生组织(WHO)和美国环境保护署(EPA)饮用水标准,蒸发速率达到1.61 kg·m^-2·h^-1,循环5次后稳定在1.59 kg·m^-2·h^-1。     

二维层状Ti3C2Tx-MXene@VS2用于高性能锂离子电池负极

为探索一种高性能的锂离子电池负极材料,采用酸刻蚀法制备了高导电性、高稳定性的二维层状Ti₃C₂T_x,通过溶剂热法制备了具有高理论比容量的花瓣状VS₂纳米片,再经过简单的液相混合得到了二维层状Ti₃C₂T_x-MXene@VS₂复合物。通过扫描电子显微镜、透射电子显微镜、X射线光电子能谱、X射线衍射和能谱分析对复合材料的形貌和结构进行了表征,采用循环伏安、恒流充放电、长循环和交流阻抗谱对复合材料的电化学性能进行了研究。结果表明：VS₂纳米片均匀地分布在Ti₃C₂T_x的层间及表面,该复合物具有高的可逆容量(电流密度为0.1A·g^-1时,比容量为610.5mAh·g^-1)、良好的倍率性能(电流密度为2A·g^-1时,比容量为197.1mAh·g^-1)和良好的循环稳定性(电流密度为0.2 A·g^-1时,循环600圈后比容量为874.9 mAh·g^-1;电流密度为2 A·g^-1时,循环1 500圈后比容量为115.9mAh·g^-1)。