Selective Formation of Polyaniline Confined in the Nanopores of a Metal-Organic Framework for Supercapacitors

Effect of a Zr-Based Metal-Organic Framework Structure on the Properties of Its Composite with Polyaniline

Composites of polyaniline (PANI) and Zr-based metal-organic frameworks (MOFs), UiO-66 and UiO-66-NH₂, were synthesized by the oxidative polymerization of aniline in the presence of MOF templates with the MOF content in the resulting materials (78.2 and 86.7 wt %, respectively) close to the theoretical value (91.5 wt %). Scanning electron microscopy and transmission electron microscopy showed that the morphology of the composites was set by the morphology of the MOFs, whose structure was mostly preserved after the synthesis, based on the X-ray diffraction data. Vibrational and NMR spectroscopies pointed out that MOFs participate in the protonation of PANI and conducting polymer chains were grafted to amino groups of UiO-66-NH₂. Unlike PANI-UiO-66, cyclic voltammograms of PANI-UiO-66-NH₂ showed a well-resolved redox peak at around ≈0 V, pointing at the pseudocapacitive behavior. The gravimetric capacitance of PANI-UiO-66-NH₂, normalized per mass of the active material, was also found to be higher compared to that of pristine PANI (79.8 and 50.5 F g^-1, respectively, at 5 mV s^-1). The introduction of MOFs into the composites with PANI significantly improved the cycling stability of the materials over 1000 cycles compared to the pristine conducting polymer, with the residual gravimetric capacitance being ≥100 and 77%, respectively. Thus, the electrochemical performance of the prepared PANI-MOF composites makes them attractive materials for application in energy storage.

Advances of Electroactive Metal-Organic Frameworks

The preparation of electroactive metal-organic frameworks (MOFs) for applications of supercapacitors and batteries has received much attention and remarkable progress during the past few years. MOF-based materials including pristine MOFs, hybrid MOFs or MOF composites, and MOF derivatives are well designed by a combination of organic linkers (e.g., carboxylic acids, conjugated aromatic phenols/thiols, conjugated aromatic amines, and N-heterocyclic donors) and metal salts to construct predictable structures with appropriate properties. This review will focus on construction strategies of pristine MOFs and hybrid MOFs as anodes, cathodes, separators, and electrolytes in supercapacitors and batteries. Descriptions and discussions follow categories of electrochemical double-layer capacitors (EDLCs), pseudocapacitors (PSCs), and hybrid supercapacitors (HSCs) for supercapacitors. In contrast, Li-ion batteries (LIBs), Lithium-sulfur batteries (LSBs), Lithium-oxygen batteries (LOBs), Sodium-ion batteries (SIBs), Sodium-sulfur batteries (SSBs), Zinc-ion batteries (ZIBs), Zinc-air batteries (ZABs), Aluminum-sulfur batteries (ASBs), and others (e.g., LiSe, NiZn, H⁺ , alkaline, organic, and redox flow batteries) are categorized for batteries.

In Situ Growth of Ni-MOF Nanorods Array on Ti3C2Tx Nanosheets for Supercapacitive Electrodes

For the energy supply of smart and portable equipment, high performance supercapacitor electrode materials are drawing more and more concerns. Conductive Ni-MOF is a class of materials with higher conductivity compared with traditional MOFs, but it continues to lack stability. Specifically, MXene (Ti₃C₂T_x) has been employed as an electrochemical substrate for its high mechanical stability and abundant active sites, which can be combined with MOFs to improve its electrochemical performance. In this paper, a novel Ni-MOF nanorods array/Ti₃C₂T_x nanocomposite was prepared via a facile hydrothermal reaction, which makes good use of the advantages of conductive Ni-MOF and high strength Ti₃C₂T_x. The high density forest-like Ni-MOF array in situ grown on the surface of Ti₃C₂T_x can provide abundant active electrochemical sites and construct a pathway for effective ion transport. The formation of a "Ti-O···Ni" bond accomplished during an in situ growth reaction endows the strong interfacial interaction between Ni-MOF and Ti₃C₂T_x. As a result, the Ni-MOF/Ti₃C₂T_x nanocomposite can achieve a high specific capacitance of 497.6 F·g^-1 at 0.5 A·g^-1 and remain over 66% of the initial capacitance when the current density increases five times. In addition, the influence of the Ti₃C₂T_x concentration and reaction time on the morphology and performance of the resultant products were also investigated, leading to a good understanding of the formation process of the nanocomposite and the electrochemical mechanism for a supercapacitive reaction.

Metal-organic framework functionalized poly-cyclodextrin membranes confining polyaniline for charge storage

Crystals of a metal-organic framework, UiO-66, are grown on electrospun crosslinked poly-cyclodextrin (poly-CD) fibrous membranes with an ultrahigh coverage, and polyaniline (PANI) is further confined within the MOF pores. The obtained PANI@UiO-66/poly-CD membranes are used as free-standing electrodes towards use in wearable energy-storage devices.

Proton-Conductive Cerium-Based Metal-Organic Frameworks

In this study, proton-conducting behaviors of a cerium-based metal-organic framework (MOF), Ce-MOF-808, its zirconium-based isostructural MOF, and bimetallic MOFs with various Zr-to-Ce ratios are investigated. The significantly increased proton conductivity (σ) and decreased activation energy (E_a) are obtained by substituting Zr with Ce in the nodes of MOF-808. Ce-MOF-808 achieves a σ of 4.4 × 10^-3 S/cm at 25 °C under 99% relative humidity and an E_a of 0.14 eV; this value is among the lowest-reported E_a of proton-conductive MOFs. Density functional theory calculations are utilized to probe the proton affinities of these MOFs. As the first study reporting the proton conduction in cerium-based MOFs, the finding here suggests that cerium-based MOFs should be a better platform for the design of proton conductors compared to the commonly reported zirconium-based MOFs in future studies on energy-related applications.

Cerium-Based Metal-Organic Framework Nanocrystals Interconnected by Carbon Nanotubes for Boosting Electrochemical Capacitor Performance

In this study, nanocrystals of a cerium-based metal-organic framework (Ce-MOF), Ce-MOF-808, are directly grown on the surface of carboxylic acid-functionalized carbon nanotubes (CNTs) by a facile one-step solvothermal synthesis method. Ce-MOF-CNT nanocomposites with various Ce-MOF-to-CNT ratios are synthesized, and their crystallinity, morphology, porosity, and electrical conductivity are examined. The redox-hopping and electrochemical behaviors of the pristine Ce-MOF in aqueous electrolytes are investigated, suggesting that the pristine Ce-MOF is electrochemically active but possesses a limited charge-transport behavior. As a demonstration, all the Ce-MOF, CNT, and nanocomposites are used as active materials for application in aqueous-based supercapacitors. The capacitive performance of the CNT can be significantly boosted with the help of redox-active Ce-MOF-808 nanocrystals.