Additive Manufacturing of Two-Dimensional Conductive Metal-Organic Framework with Multidimensional Hybrid Architectures for High-Performance Energy Storage

Zeolitic imidazolate framework-67-derived chalcogenides as electrode materials for supercapacitors

With the rapid development of new energy technologies, hybrid supercapacitors have received widespread attention owing to their advantages of high power density, fast charging/discharging rate and long cycle life. In this case, the selection and design of electrode materials are the key to improving the energy storage performance of supercapacitors. Herein, zeolitic imidazolate framework-67 (ZIF-67) is presented as a good candidate material for the fabrication of supercapacitor electrodes because of its controllable pore size, constant cavity size and large specific area. Moreover, pristine ZIF-67 and ZIF-67-derived porous carbon have shown exemplary performances in supercapacitors. However, they belong to the class of electric double layer capacitor materials and have a lower magnitude of energy storage compared with pseudocapacitor materials. Therefore, to improve the energy density of hybrid supercapacitors, other ZIF-67 derivatives need to be explored, especially chalcogenides. This review mainly reports the application of ZIF-67-derived transition metal chalcogenides (TMCs, C including Oxide, Sulfide, Selenide, Telluride) in supercapacitors. Moreover, the strategies for the preparation of ZIF-67-derived TMCs and their electrochemical performance in supercapacitors are further discussed. Finally, the remaining challenges and future perspectives are highlighted.

Interfacial modulation strategy using poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT:PSS) and ultrathin two-dimensional metal-organic framework nanosheets for wearable supercapacitors: Solution engineering

Two-dimensional metal-organic frameworks (2D MOFs) hold great promise as electrochemically active materials. However, their application in MOF nanocomposite electrodes in solution engineering is limited by structural self-stacking and imperfect conductive pathways. In this study, we used meso-tetra(4-carboxyphenyl) porphine (TCPP) with off-domain π-bonds to reconstitute Zn-TCPP (ZMOF) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) through an interfacial modulation strategy involving electrostatic coupling and hydrogen bonding, creating a conductive composite with a nanosheet structure. The negatively charged PSS and ZMOF formed a three-dimensional interconnected conductive network with excellent interfaces. The positively charged PEDOT, fine tuned with the lamellar structure, established strong π-π stacking interactions between the porphyrin and thiophene rings. ZMOF also induced changes in the PEDOT chain structure, weakening PSS entanglement and enhancing charge-transport properties. The specific capacitance of the prepared supercapacitor was as high as 967.8 F g-1. Flexible supercapacitors produced on a large scale using dispensing printing technology exhibited an energy density of 1.85 μWh cm-2 and a power density of 7.08 μW cm-2. This interfacial modulation strategy also exhibited excellent wearable properties, with 96 % capacitance retention at a 180° bending angle and stable cycling performance. This study presented a significant advancement in the functionalization of 2D materials, highlighting their potential for device-grade capacitive architectures.

Metal-Organic Framework and Covalent-Organic Framework-Based Aerogels: Synthesis, Functionality, and Applications

Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs)-based aerogels are garnering significant attention owing to their unique chemical and structural properties. These materials harmoniously combine the advantages of MOFs and COFs-such as high surface area, customizable porosity, and varied chemical functionality-with the lightweight and structured porosity characteristic of aerogels. This combination opens up new avenues for advanced applications in fields where material efficiency and enhanced functionality are critical. This review provides a comparative overview of the synthetic strategies utilized to produce pristine MOF/COF aerogels as well as MOF/COF-based hybrid aerogels, which are functionalized with molecular precursors and nanoscale materials. The versatility of these aerogels positions them as promising candidates for addressing complex challenges in environmental remediation, energy storage and conversion, sustainable water-energy technologies, and chemical separations. Furthermore, this study discusses the current challenges and future prospects related to the synthesis techniques and applications of MOF/COF aerogels.

Significantly Enhanced Oxidation Resistance and Electrochemical Performance of Hydrothermal Ti3C2Tx MXene and Tannic Acid Composite for High-Performance Flexible Supercapacitors

The electrochemical performances of Ti3C2Tx MXene are severely restricted by the easy oxidation and restacking. Herein, tannic acid (TA) is introduced into Ti3C2Tx dispersion, and the mixed dispersion is further subjected to a simple hydrothermal treatment to prepare the hydrothermal Ti3C2Tx and TA composite (h-Ti3C2Tx@h-TA). Due to the decomposition of TA into gallic acid (GA), hydrothermal TA (h-TA) is a mixture of TA and GA. The strong interaction between h-TA and MXene mainly involves chemical interaction between the hydroxyl groups in h-TA and the surface/edge Ti atoms, along with numerous hydrogen bonds. The h-TA intercalation weakens MXene restacking and increases interlayer spacing, thereby improving ion transport pathways and accessibility. The chemical interaction between the hydroxyl groups of GA and the Ti atoms significantly enhances oxidation resistance and pseudocapacitive active sites. Therefore, the h-Ti3C2Tx@h-TA film electrode shows significantly enhanced capacitance (848 F·g-1 at 1 A g-1) and cycling stability (100% retention after 20 000 cycles). Moreover, flexible sandwiched supercapacitors with symmetrical h-Ti3C2Tx@h-TA electrodes exhibit a high energy density of 30.1 Wh kg-1 at a high power density of 300 W kg-1, outperforming those of Ti3C2Tx-based film electrodes and sandwiched supercapacitors reported so far. The extrusion-printed microsupercapacitors with h-Ti3C2Tx@h-TA electrodes demonstrate high areal capacitance (135 mF cm-2 at 5 mV s-1) along with energy storage performance (6.74 μWh cm-2 at 506 μW cm-2) and cycling stability (98.8% retention after 41 460 cycles), all while maintaining excellent flexibility. These impressive results indicate the great application potential of the hydrothermal Ti3C2Tx MXene and tannic acid composite in flexible energy storage devices.

Conjugated Coordination Nanosheets with Molecular Rotors for Pseudocapacitors: Nanoarchitectonics and Enhanced Performance

High-level pseudocapacitive materials require incorporations of significant redox regions into conductive and penetrable skeletons to enable the creation of devices capable of delivering high power for extended periods. Coordination nanosheets (CNs) are appealing materials for their high natural electrical conductivities, huge explicit surface regions, and semi-one-layered adjusted pore clusters. Thus, rational design of ligands and topological networks with desired electronic structure is required for the advancement in this field. Herein, we report three novel conjugated CNs (RV-10-M, M=Zn, Ni, and Co), by utilizing the full conjugation of the terpyridine-attached flexible tetraphenylethylene (TPE) units as the molecular rotors at the center. We prepare binder-free transparent nanosheets supported on Ni-foam with outstanding pseudocapacitive properties via a hydrothermal route followed by facile exfoliation. Among three CNs, the high surface area of RV-10-Co facilitates fast transport of ions and electrons and could achieve a high specific capacity of 670.8 C/g (1677 F/g) at 1 A/g current density. Besides, the corresponding flexible RV-10-Co possesses a maximum energy density of 37.26 Wh kg-1 at a power density of 171 W kg-1 and 70 % capacitance retention even after 1000 cycles.

Challenges and Opportunities in Preserving Key Structural Features of 3D-Printed Metal/Covalent Organic Framework

Metal-organic framework (MOF) and covalent organic framework (COF) are a huge group of advanced porous materials exhibiting attractive and tunable microstructural features, such as large surface area, tunable pore size, and functional surfaces, which have significant values in various application areas. The emerging 3D printing technology further provides MOF and COFs (M/COFs) with higher designability of their macrostructure and demonstrates large achievements in their performance by shaping them into advanced 3D monoliths. However, the currently available 3D printing M/COFs strategy faces a major challenge of severe destruction of M/COFs' microstructural features, both during and after 3D printing. It is envisioned that preserving the microstructure of M/COFs in the 3D-printed monolith will bring a great improvement to the related applications. In this overview, the 3D-printed M/COFs are categorized into M/COF-mixed monoliths and M/COF-covered monoliths. Their differences in the properties, applications, and current research states are discussed. The up-to-date advancements in paste/scaffold composition and printing/covering methods to preserve the superior M/COF microstructure during 3D printing are further discussed for the two types of 3D-printed M/COF. Throughout the analysis of the current states of 3D-printed M/COFs, the expected future research direction to achieve a highly preserved microstructure in the 3D monolith is proposed.

Microfluidic-Assisted 3D Printing Zinc Powder Anode with 2D Conductive MOF/MXene Heterostructures for High-Stable Zinc-Organic Battery

Zinc powder (Zn-P) anodes have significant advantages in terms of universality and machinability compared with Zn foil anodes. However, their rough surface, which has a high surface area, intensifies the uncontrollable growth of Zn dendrites and parasitic side reactions. In this study, an anti-corrosive Zn-P-based anode with a functional layer formed from a MXene and Cu-THBQ (MXene/Cu-THBQ) heterostructure is successfully fabricated via microfluidic-assisted 3D printing. The unusual anti-corrosive and strong adsorption of Zn ions using the MXene/Cu-THBQ functional layer can effectively homogenize the Zn ion flux and inhibit the hydrogen evolution reaction (HER) during the repeated process of Zn plating/stripping, thus achieving stable Zn cycling. Consequently, a symmetric cell based on Zn-P with the MXene/Cu-THBQ anode exhibits a highly reversible cycling of 1800 h at 2 mA cm-2 /1 mAh cm-2 . Furthermore, a Zn-organic full battery matched with a 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl organic cathode riveted on graphene delivers a high reversible capacity and maintains a long cycle life.

3D printing of hierarchically micro/nanostructured electrodes for high-performance rechargeable batteries

3D printing, also known as additive manufacturing, is capable of fabricating 3D hierarchical micro/nanostructures by depositing a layer-upon-layer of precursor materials and solvent-based inks under the assistance of computer-aided design (CAD) files. 3D printing has been employed to construct 3D hierarchically micro/nanostructured electrodes for rechargeable batteries, endowing them with high specific surface areas, short ion transport lengths, and high mass loading. This review summarizes the advantages and limitations of various 3D printing methods and presents the recent developments of 3D-printed electrodes in rechargeable batteries, such as lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries. Furthermore, the challenges and perspectives of the 3D printing technique for electrodes and rechargeable batteries are put forward. This review will provide new insight into the 3D printing of hierarchically micro/nanostructured electrodes in rechargeable batteries and promote the development of 3D printed electrodes and batteries in the future.

Negative electrodes for supercapacitors with good performance using conductive bismuth-catecholate metal-organic frameworks

Metal-organic frameworks (MOFs) have attracted increasing research interest in various fields. Unfortunately, the poor conductivity of most traditional MOFs considerably hinders their application in energy storage. Benefiting from the full charge delocalization in the atomic plane, two-dimensional conductive coordination frameworks achieve good electrochemical performance. In this work, π-π coupling conductive bismuth-catecholate nanobelts with tunable lengths, Bi(HHTP) (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), are synthesized by a simple hydrothermal reaction and their length-dependent electrochemical properties are also investigated. The Bi(HHTP) nanobelts (about 10 μm in length) possess appropriate porosity, numerous redox active sites and good electrical conductivity. Being a negative electrode for supercapacitors, Bi(HHTP) nanobelts display a high specific capacitance of 234.0 F g^-1 and good cycling stability of 72% after 1000 cycles. Furthermore, the mechanism of charge storage is interpreted for both battery-type and surface-capacitive behavior. It is believed that the results of this work will help to develop battery-type negative electrode materials with promising electrochemical performance using some newly designed π-π coupling conductive coordination frameworks.

Flexible high-performance microcapacitors enabled by all-printed two-dimensional nanosheets

Chemically exfoliated nanosheets have exhibited great potential for applications in various electronic devices. Solution-based processing strategies such as inkjet printing provide a low-cost, environmentally friendly, and scalable route for the fabrication of flexible devices based on functional inks of two-dimensional nanosheets. In this study, chemically exfoliated high-k perovskite nanosheets (i.e., Ca₂Nb₃O₁₀ and Ca₂NaNb₄O₁₃) are well dispersed in appropriate solvents to prepare printable inks, and then, a series of microcapacitors with Ag and graphene electrodes are printed. The resulting microcapacitors, Ag/Ca₂Nb₃O₁₀/Ag, graphene/Ca₂Nb₃O₁₀/graphene, and graphene/Ca₂NaNb₄O₁₃/graphene, demonstrate high capacitance densities of 20, 80, and 150 nF/cm² and high dielectric constants of 26, 110, and 200, respectively. Such dielectric enhancement in the microcapacitors with graphene electrodes is possibly attributed to the dielectric/graphene interface. In addition, these microcapacitors also exhibit good insulating performance with a moderate electrical breakdown strength of approximately 1 MV/cm, excellent flexibility, and thermal stability up to 200 ℃. This work demonstrates the potential of high-k perovskite nanosheets for additive manufacturing of flexible high-performance dielectric capacitors.

Nickel(II) Cluster-Based Pillar-Layered Metal-Organic Frameworks for High-Performance Supercapacitors

Most metal-organic frameworks (MOFs) cannot be used as electrode materials for supercapacitors because of their high costs, poor stabilities in aqueous solutions, inferior intrinsic electrocatalytic activities, and poor conductivities. Herein, the application of two nickel(II) cluster-based pillar-layered MOFs, Ni-mba-Na ([Ni₈(mba)₆(Cl)₂Na(OH^-)₃]_n, H₂mba is 2-mercaptobenzoic acid) and Ni-mba-K ([Ni₈(mba)₆(Cl)₂K(OH^-)₃]_n), as electrode materials are reported. They differ from conductive MOFs because they are insulators with small specific surface areas (<10 m² g^-1), and H²mba is an inexpensive raw material. The conductivities of Ni-mba-Na and Ni-mba-K at 30 °C were 4.002 × 10^-10 and >10^-11 S cm^-1, respectively. They showed excellent supercapacitor performance and stabilities and high inherent densities and specific capacitances. The specific powers of their asymmetric supercapacitors could reach up to 16,000 W kg^-1; the specific energies of Ni-mba-Na and Ni-mba-K were 16.9 and 21.8 Wh kg^-1, respectively. Design recommendations for these MOFs are provided based on their structure and performance differences. This paper shows a novel application of nonconductive MOFs in the energy storage field and design of high-performance electrode materials for supercapacitors.