Rapid progress on developing smart materials and design of hybrids is motivated by pressing challenges associated with energy crisis and environmental remediation. While emergence of versatile classes of nanomaterials has been fascinating, the real excitement lies in the design of hybrid materials with tunable properties. Metal-organic frameworks (MOFs) are the key materials for gas sorption and electrochemical applications, but their sustainability is challenged by limited chemical stability, poor electrical conductivity, and intricate, inaccessible pores. Despite tremendous efforts towards improving the stability of MOF materials, little progress has made researchers inclined toward developing hybrid materials. MXenes, a family of two-dimensional transition-metal carbides, nitrides and carbonitrides, are known for their compositional versatility and formation of a range of structures with rich surface chemistry. Hybridization of MOFs with functional layered MXene materials may be beneficial if the host structure provides appropriate interactions for stabilizing and improving the desired properties. Recent efforts have focused on integrating Ti3C2Tx and V2CTx MXenes with MOFs to result in hybrid materials with augmented electrochemical and physicochemical properties, widening the scope for emerging applications. This review discusses the potential design strategies of MXene@MOF hybrids, attributes of tunable properties in the resulting hybrids, and their applications in water treatment, sensing, electrochemical energy storage, smart textiles, and electrocatalysis. Comprehensive discussions on the recent efforts on rapidly evolving MXene@MOF materials for various applications and potential future directions are highlighted.

Emerging MXene@Metal-Organic Framework Hybrids: Design Strategies toward Versatile Applications.

Owing to the favorable energy efficiency and environmental compatibility, capacitive deionization (CDI) has been greatly developed as a potential technology to overcome the ever-growing global water shortage. Herein, an effective and simple strategy is reported to boost the desalination performance by 30 percent through an expansion of interlayer spacing. The final redox active hybrid CDI material consists of WS2 nanoflowers embedded in a highly conductive free-standing rGO-CNT (WS2/rGO-CNT) aerogel as an anode and rGO-CNT aerogel as a cathode. WS2 is applied as a redox-active material for sodium ion intercalation, and three-dimensional (3D) rGO-CNT facilitates the diffusion of the ions to boost the reaction kinetics. Upon charging/discharging, sodium ions intercalate/deintercalate into/from the WS2 lattice structure which is confirmed via in situ XRD measurements, and chloride ions are physically adsorbed/desorbed by the rGO-CNT aerogel. Benefiting from the synergistic effect between the highly conductive and porous rGO-CNT framework and WS2 nanoflowers with a tuned structure, the as-assembled HCDI device enables stable desalination performance with a superior removal capacity of 80 mg g−1, and an excellent removal rate of 3.9 mg g−1 min−1.

Tungsten disulfide-reduced GO/CNT aerogel: a tuned interlayer spacing anode for efficient water desalination

Capacitive deionization is an emerging and rapidly developing electrochemical technique for water desalination across the globe with exponential growth in publications. There are various architectures and materials being explored to obtain utmost electrosorption performance. The symmetric architectures consist of the same material on both electrodes, while asymmetric architectures have electrodes loaded with different materials. Asymmetric architectures possess higher electrosorption performance as compared with that of symmetric architectures owing to the inclusion of either faradaic materials, redox‐active electrolytes, or ion specific pre‐intercalation material. With the materials perspective, faradaic materials have higher electrosorption performance than carbon‐based materials owing to the occurrence of faradaic reactions for electrosorption. Moreover, the architecture and material may be tailored in order to obtain desired selectivity of the target component and heavy metal present in feed water. In this review, we describe recent developments in architectures and materials for capacitive deionization and summarize the characteristics and salt removal performances. Further, we discuss recently reported architectures and materials for the removal of heavy metals and radioactive materials. The factors that affect the electrosorption performance including the synthesis procedure for electrode materials, incorporation of additives, operational modes, and organic foulants are further illustrated. This review concludes with several perspectives to provide directions for further development in the subject of capacitive deionization.

Recent progress in materials and architectures for capacitive deionization: A comprehensive review

Recently, the rational design and development of efficient faradaic deionization electrodes with high theoretical capacitance, natural abundance, and attractive conductivity have shown great promise for outstanding capacitive deionization (CDI)‐based desalination applications. Herein, the construction of novel FeOOH hybrid heterostructures with Na and Cl dopants (e.g., Na‐FeOOH and Cl‐FeOOH) via a robust hydrothermal strategy is reported, and an asymmetric CDI cell (Na‐FeOOH//Cl‐FeOOH) comprising Na‐FeOOH and Cl‐FeOOH working as the cathode and anode, respectively, is assembled. The multiple coupling effects of the specific structural features (e.g., enriched porosity, hierarchical pore alignment, and highly open crystalline framework), enhanced electrochemical conductivity, and optimized ion‐transfer property endow the FeOOH hybrid electrode with improved electrochemical performance. Impressively, the Na‐FeOOH//Cl‐FeOOH cell demonstrates a superior salt adsorption capacity (SACNaCl) of 35.12 mg g−1 in a 500 mg L−1 NaCl solution, a faster removal rate, and remarkable cycling stability. Moreover, the pseudocapacitive removal mechanism from the synergetic contribution of the Na‐FeOOH cathode and Cl‐FeOOH anode account for the significant desalination promotion of the Na‐FeOOH//Cl‐FeOOH cell.

Efficient and Durable Sodium, Chloride‐doped Iron Oxide‐Hydroxide Nanohybrid‐Promoted Capacitive Deionization of Saline Water via Synergetic Pseudocapacitive Process

Constructing titanium carbide MXene/reduced graphene oxide superlattice heterostructure via electrostatic self-assembly for high-performance capacitive deionization.

Simultaneously regulating the physical properties and chemical environment of interlayer is a critical need for enhancing the capacity and stability of layered electrode materials in energy and env...

Enhanced Desalination Capacity and Stability of Alkylamine-Modified Na0.71CoO2 for Capacitive Deionization

Hydrated ammonium metal phosphates (AMPs) with numerous electractive sites and intercalated water are the current research focus in the field of supercapacitors. Herein, we have developed a novel approach where Co-MOF(ZIF-67) as a precursor was introduced into the confined space of an MXene layer, and the as-prepared MOF/Ti3C2Tx was subsequently etched in situ in a solution of (NH4)2HPO4 to transform into NH4CoPO4·H2O nanoparticles with a uniform distribution on the surface of Ti3C2Tx. NH4CoPO4 nanoparticles were embedded into MXene nanosheets, while the Ti3C2Tx MXene endowed AMPs with high electronic conductivity. Benefiting from the synergistic effect of NH4CoPO4·H2O and MXene, this material showed good rate performance (601 F g−1 at 1.0 A g−1 and 535 F g−1 at 20.0 A g−1) and long cycle life (87% capacitance retention up to 5000 cycles at 5 A g−1) in a three-electrode system, which can be ascribed to excellent electronic conductivity and short ion transport path. When NH4CoPO4·H2O/Ti3C2Tx and activated carbon are used as the positive and negative electrodes of asymmetric supercapacitors (ASCs), the ASC simultaneously shows high energy density (47 W h kg−1 at 1350 W kg−1) and power density (18 W h kg−1 at 5000 W kg−1), which can be considered as a potential candidate for next-generation energy storage devices.

A solution-assisted etching preparation of an MOF-derived NH4CoPO4·H2O/Ti3C2Tx MXene nanocomposite for high-performance hybrid supercapacitors

Atomic substituents effect on boosting desalination performances of Zn-doped NaxCoO2

Defect engineering is one of the effective strategies to regulate and control catalyst properties. Constructing appropriate catalytically active centers effectively tunes the electronic and surface properties of the catalyst to achieve further enrichment of photogenerated electrons, enhances the electronic feedback of the catalytically active center to the anti-bonding orbitals of the nitrogen molecule, and enhances N2 adsorption while weakening the NN bond. In this study, titanium vacancy (VTi)-rich undoped anatase p-TiO2 was successfully synthesized to investigate the effect of its metal vacancies on photocatalytic nitrogen reduction reaction (NRR) performance. The cation vacancies of VTi-rich p-TiO2 lead to local charge defects that enhance carrier separation and transport while trapping electrons to activate N2, allowing effective reduction of the excited electrons to NH3. This work provides a viable strategy for driving the efficiency of photocatalytic nitrogen fixation processes by altering the structural properties of semiconductors through cationic vacancies, offering new opportunities and challenges for the design and preparation of titanium dioxide-based materials.

Cationic vacancy engineering of p-TiO2 for enhanced photocatalytic nitrogen fixation.

A rapid, productive, and efficient process was invented to produce hybrid catalysts for transition metal oxide water electrolysis. The microwave-assisted hydrothermal method was applied to synthesize transition metal oxide catalysts by controlling the amount of cobalt and iron. This work solves the cracking problem for the catalytic layer during the water electrolysis. It uses Fe2O3 as the support and covers a catalytic layer outside it and a nanoscale gap between each catalyst, which can help to remove the gas and fill up the water. The unique structure of the catalysts can prevent them from accumulating gas and increasing their efficiency for long-term water electrolysis. By using unique catalysts in the water electrolyzer, the current density reaches higher than 200 mA cm−2 at 2.0 V and does not show a significant decay even after 200 h.

/pdf/island-type-hybrid-catalysts-applied-for-anion-exchange-utxsvbs8.pdf

Wenhui Wei

Papers

Enhanced Desalination Capacity and Stability of Alkylamine-Modified Na0.71CoO2 for Capacitive Deionization

A solution-assisted etching preparation of an MOF-derived NH4CoPO4·H2O/Ti3C2Tx MXene nanocomposite for high-performance hybrid supercapacitors

Atomic substituents effect on boosting desalination performances of Zn-doped NaxCoO2

Cationic vacancy engineering of p-TiO2 for enhanced photocatalytic nitrogen fixation.

Island-Type Hybrid Catalysts Applied for Anion Exchange Membrane Water Electrolysis