Polyoxometalates-Based Metal-Organic Frameworks Made by Electrodeposition and Carbonization Methods as Cathodes and Anodes for Asymmetric Supercapacitors

Approaches to Enhancing Electrical Conductivity of Pristine Metal-Organic Frameworks for Supercapacitor Applications

Metal-organic frameworks (MOFs), known as porous coordination polymers, have attracted intense interest as electrode materials for supercapacitors (SCs) owing to their advantageous features including high surface area, tunable porous structure, structural diversity, etc. However, the insulating nature of most MOFs has impeded their further electrochemical applications. A common solution for this issue is to transform pristine MOFs into more stable and conductive metal compounds/porous carbon materials through pyrolysis, which however losses the inherent merits of MOFs. To find a consummate solution, recently a surge of research devoted to improving the electrical conductivity of pristine MOFs for SCs has been carried out. In this review, the most related research work on pristine MOF-based materials is reviewed and three effective strategies (chemical structure design of conductive MOFs (c-MOFs), composite design, and binder-free structure design) which can significantly increase their conductivity and consequently the electrochemical performance in SCs are proposed. The conductivity enhancement mechanism in each approach is well analyzed. The representative research works on using pristine MOFs for SCs are also critically discussed. It is hoped that the new insights can provide guidance for developing high-performance electrode materials based on pristine MOFs with high conductivity for SCs in the future.

POMCPs with Novel Two Water-Assisted Proton Channels Accommodated by MXenes for Asymmetric Supercapacitors

To develop high-performance supercapacitors, the negative electrode is at present viewed as one of the most challenging tasks for obtaining the next-generation of energy storage devices. Therefore, in this study, a polyoxometalate-based coordination polymer [Zn(itmb)₃ H₂ O][H₂ SiW₁₂ O₄₀ ]·5H₂ O (1) is designed and prepared by a simple hydrothermal method for constructing a high-capacity negative electrode. Polymer 1 has two water-assisted proton channels, which are conducive to enhancing the electrical conductivity and storage capacity. Then, MXene Ti₃ C₂ T_x is chosen to accommodate coordination polymer 1 as the interlayer spacers to improve the conductivity and cycling stability of 1, while preventing the restacking of MXene. Expectedly, the produced composite electrode 1@Ti₃ C₂ T_x shows an excellent specific capacitance (1480.1 F g^-1 at 5 A g^-1 ) and high rate performance (a capacity retention of 71.5% from 5 to 20 A g^-1 ). Consequently, an asymmetric supercapacitor device is fabricated using 1@Ti₃ C₂ T_x as the negative electrode and celtuce leaves-derived carbon paper as the positive electrode, which demonstrates ultrahigh energy density of 32.2 Wh kg^-1 , and power density 2397.5 W kg^-1 , respectively. In addition, the ability to illuminate a red light-emitting diode for several minutes validates its feasibility for practical application.

A hybrid borotungstate-coated metal-organic framework with supercapacitance, photocatalytic dye degradation and H2O2 sensing properties

The compounding of polyoxometalates (POMs) with structurally well-defined and porous metal-organic frameworks (MOFs) has become a hot research topic. Here, a core-shell type hybrid, {Ag₅BW₁₂O₄₀}@[Ag₃(μ-Hbtc)(μ-H₂btc)]_n (called {Ag₅BW₁₂O₄₀}@Ag-BTC-2, where BTC = 1,3,5-benzyl carboxylic acid), was successfully prepared via a simple grinding method. IR, XRD, SEM, TEM, and XPS analysis was used to confirm the structure. The specific capacitance is 179.1 F g^-1 when the current density is 1 A g^-1, using nickel foam as the collector, and the capacitance retention is 97.4% after 5000 cycles. The resulting aqueous-based symmetric supercapacitor has a power density of 496 W kg^-1 and an energy density of 12.4 W h kg^-1. In addition, the degradation rates using {Ag₅BW₁₂O₄₀}@Ag-BTC-2 toward methylene blue (MB), rhodamine B (RhB), and methyl orange (MO) exceeded 90% in 140 min and remained essentially unchanged over five replicate experiments, showing high photocatalytic activity. Meanwhile, when {Ag₅BW₁₂O₄₀}@Ag-BTC-2 acts as a H₂O₂ biosensor, it has a low detection limit (0.19 μM), a wide linear range (0.4 μM-0.27 mM) and high anti-interference properties. This shows that the synthesis of POMOFs via a grinding method is an effective strategy to improve the performance.

POM-based metal organic frameworks with a woven fabric structure for lithium storage

A novel POMOFs with fabric structure possesses high lithium storage capacity, and its long life cycle performance is further enhanced by CNTs.

{BW12O40} Hybrids Modified by in Situ Synthesized Rigid Ligand with Supercapacitance and Photocatalytic Properties

Organic rigid ligand-modified polyoxometalate-based materials possess complex and diverse structures, promising electrochemical energy storage properties and outstanding photocatalytic capabilities. Hence, two new [BW₁₂O₄₀]^5-(abbreviated as {BW₁₂O₄₀})-based inorganic-organic hybrids [{Cu(en)₂(H₂O)}][{Cu(pdc)(en)}{Cu(en)₂}(BW₁₂O₄₀)]·2H₂O (1) and [{Cu^I₅(pz)₆(H₂O)₄}(BW₁₂O₄₀)] (2) (pdc = 2-picolinate, en = ethylenediamine, pz = pyrazine) were successfully synthesized through a hydrothermal method. Among them, pdc and pz were obtained by in situ transformation from 2,6-pyridinedicarboxylic acid (H₂ pydc) and 2,3-pyrazinedicarboxylic acid (H₂pzdc), respectively. In compound 1, the {BW₁₂O₄₀} clusters as an intermediate junction connect with {Cu(pdc)(en)}{Cu(en)₂} and {Cu(en)₂(H₂O)} to form monomers, which in turn form supramolecular chains, sheets, and space network via hydrogen bonding. The {BW₁₂O₄₀} clusters are packed into copper-pyrazine frameworks in compound 2, and a unique polyoxometalate-based metal organic frameworks (POMOFs) structure with a new topology of {12}₂{6.12³.14²}₂{6².12.14².18}{6².12³.16}{6}₆ is formed via covalent bonds. When used as electrode materials for supercapacitors, the values of specific capacitance are 651.56 F g^-1 for 1-GCE and 584.43 F g^-1 for 2-GCE at a current density of 2.16 A g^-1 and good cycling stability (90.94%, 94.81% of the initial capacity after 5000 cycles at 15.12 A g^-1, respectively). The kinetic analysis reveals that surface capacitance plays a major role. Furthermore, both compounds can effectively degrade Rhodamine B (RhB) and Methylene blue (MB), showing the outstanding photocatalytic performance.

Unlocking the potential of metal organic frameworks for synergized specific and areal capacitances via orientation regulation

Metal organic frameworks (MOFs) with numerous potential pseudocapacitive sites are quite appealing to supercapacitors with high specific and areal capacitances. However, MOFs suffer from a low conductivity nature, resulting in a mediocre electrochemical performance. Herein, we propose a template-growth strategy of MOF to enhance both specific and areal capacitances of MOF as the electrode material for supercapacitor. As the loading mass of MOF increases from 2 to 5 mg, the specific capacitance also increases from 937 to 2387 Fg^-1 (431 to 1098 Cg^-1) and areal capacitance 1.87 to 11.94 Fcm^-2 (0.86 to 5.49 Ccm^-1). Accordingly, a hybrid MOF//AC supercapacitor can deliver a maximum energy density of 67 Wh kg^-1 and a maximum power density of 6000 W kg^-1. These results point to a way to fabricate MOF electrode of supercapacitor with high specific and areal capacitance, which leads them to a more practical future.

Electrochemical investigation and amperometry determination iodate based on ionic liquid/polyoxotungstate/P-doped electrochemically reduced graphene oxide multi-component nanocomposite modified glassy carbon electrode

A novel modified glassy carbon electrode (GCE) was successfully fabricated with a tetra-component nanocomposite consisting of (1,1′-(1,4-butanediyl)dipyridinium) ionic liquid (bdpy), SiW₁₁O₃₉Ni(H₂O) (SiW₁₁Ni) Keggin-type polyoxometalate (POM), and phosphorus-doped electrochemically reduced graphene oxide (P-ERGO) by electrodeposition technique. The (bdpy)SiW₁₁Ni/GO hybrid nanocomposite was synthesized by a one-pot hydrothermal method and characterized by UV-vis absorption, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) analysis, thermogravimetric-differential thermal analysis (TGA/DTA), and transmission electron microscopy (TEM). The morphology, electrochemical performance, and electrocatalysis activity of the nanocomposite modified glassy carbon electrode ((bdpy)SiW₁₁Ni/P-ERGO/GCE) were analyzed by field emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), cyclic voltammetry (CV), square wave voltammetry (SWV), and amperometry, respectively. Under the optimum experimental conditions, the as-prepared sensor showed high sensitivity of 28.1 μA mM⁻¹ and good selectivity for iodate (IO₃⁻) reduction, enabling the detection of IO₃⁻ within a linear range of 10–1600 μmol L⁻¹ (R² = 0.9999) with a limit of detection (LOD) of 0.47 nmol L⁻¹ (S/N = 3). The proposed electrochemical sensor exhibited good reproducibility, and repeatability, high stability, and excellent anti-interference ability, as well as analytical performance in mineral water, tap water, and commercial edible iodized salt which might provide a capable platform for the determination of IO₃⁻.

Constructing a sensitive electrochemical sensor based on (bdpy)SiW₁₁Ni/P-ERGO/GCE for IO₃⁻ detection at the nanomolar level with noticeable selectivity.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.