Enhancing Energy Storage via TEA-Dependent Controlled Syntheses: Two Series of Polyoxometalate-Based Inorganic-Organic Hybrids and their Supercapacitor Properties

Six polyoxotungstate-based transition metal compounds for electrochemical capacitor application and a comparative analysis of factors affecting capacitances

Six different polyoxotungstate-based transition metal complexes were synthesized, namely [Cu5(2,2'-bpy)5(μ2-Cl)2(PO4)2(H2O)2][HPW12O40]·2H2O (1), [Cu1.5(2,2'-bpy)1.5(inic)2(H2O)1.5]3[H1.5PW12O40]2·16.25H2O (2), [Cu(2,2'-bpy)2]2[SiW12O40]·10H2O (3), [Zn(phen)3]2[PWVWVI11O40]·5H2O (4), [Zn(phen)2(H2O)]2[SiW12O40]·2H2O (5), and [Zn(2,2'-bpy)2]2[SiW12O40] (6) (2,2'-bpy = 2,2'-bipyridine, inic = isonicotinic acid, phen = 1,10-phenanthroline). Compound 1 is based on [HPW12O40]2- anions, which are accommodated within the open channels of a supramolecular network formed by novel Cu-P-Cl coordination clusters. Compound 2 is constructed from [H1.5PW12O40]1.5- and novel [Cu1.5(2,2'-bpy)1.5(inic)2(H2O)1.5]+ coordination fragments, and polyoxoanions are encapsulated within the pores created by the copper coordination fragments, resulting in a unique three-dimensional supramolecular architecture. Compound 3 is a two-dimensional structure formed through the covalent linkage between [SiW12O40]4- and [Cu(2,2'-bpy)2]2+. Compound 4 is a supramolecular architecture formed by [PWVWVI11O40]4- and [Zn(phen)3]2+ coordination fragments, while compound 5 is a supramolecular structure based on POM bi-supported Zn coordination complexes. Compound 6 is a two-dimensional framework structure constituted by [SiW12O40]4- and [Zn(2,2'-bpy)2]2+via covalent interactions. In addition, electrochemical measurement results show that the copper-based tungstate compounds 1-3 and zinc-based tungstate compounds 4-6 exhibit different performances and durabilities as electrochemical capacitors (compound 1 shows the highest specific capacitance of 94.0 F g-1 at 1.5 A g-1, whereas compound 6 maintains the best cycling stability with the capacity retention of 80.7% after 1000 cycles at 4 A g-1.). This study contributes to the development of POM-based transition metal complexes with high capacitance by providing insights into the design and synthesis process.

Nanomaterial with Core-Shell Structure Composed of {P2W18O62} and Cobalt Homobenzotrizoate for Supercapacitors and H2O2-Sensing Applications

Designing and preparing dual-functional Dawson-type polyoxometalate-based metal-organic framework (POMOF) energy storage materials is challenging. Here, the Dawson-type POMOF nanomaterial with the molecular formula CoK₄[P₂W₁₈O₆₂]@Co₃(btc)₂ (abbreviated as {P₂W₁₈}@Co-BTC, H₃btc = 1,3,5-benzylcarboxylic acid) was prepared using a solid-phase grinding method. XRD, SEM, TEM et al. analyses prove that this nanomaterial has a core-shell structure of Co-BTC wrapping around the {P₂W₁₈}. In the three-electrode system, it was found that {P₂W₁₈}@Co-BTC has the best supercapacitance performance, with a specific capacitance of 490.7 F g^-1 (1 A g^-1) and good stability, compared to nanomaterials synthesized with different feedstock ratios and two precursors. In the symmetrical double-electrode system, both the power density (800.00 W kg^-1) and the energy density (11.36 Wh kg^-1) are greater. In addition, as the electrode material for the H₂O₂ sensor, {P₂W₁₈}@Co-BTC also exhibits a better H₂O₂-sensing performance, such as a wide linear range (1.9 μM-1.67 mM), low detection limit (0.633 μM), high selectivity, stability (92.4%) and high recovery for the detection of H₂O₂ in human serum samples. This study provides a new strategy for the development of Dawson-type POMOF nanomaterial compounds.

Polyoxometalate intercalated MXene with enhanced electrochemical stability

MXene/polyoxometalate (POM) hybrids are useful target materials for a variety of applications. Yet, the goal of preparing simple binary hybrids by intercalation of POMs into MXene has not been achieved. We propose and demonstrate here a method to intercalate POMs (phosphotungstate, PW12) into Ti₃C₂T_x MXene through the interaction between POM anions and pre-intercalated surfactant cations. A variety of quaternary ammonium cations have been used to expand Ti₃C₂T_x interlayer spacing. Cetyltrimethylammonium cations (CTA⁺) lead to an expansion of 2 nm while allowing intercalation of a considerable load (10 wt%) thanks to their tadpole-like shape and size. CTAPW12 has a layered structure compatible with Ti₃C₂T_x. The CTA⁺-delaminated Ti₃C₂T_x keeps the large interlayer spacing after being coupled with PW12. The PW12 clusters are dispersed and kept isolated thanks to CTA surfactant and the confinement into Ti₃C₂T_x layers. The redox reactions in CTA⁺-delaminated Ti₃C₂T_x/PW12 are diffusion-controlled, which proves the well-dispersed PW12 clusters are not adsorbed on the surface of Ti₃C₂T_x particles but within Ti₃C₂T_x layers. The CTA⁺- delaminated Ti₃C₂T_x/PW12 shows superior electrochemical stability (remaining redox active after 5000 cycles) over the other MXene/POM hybrids prepared in this work (inactive after 500 cycles). We associate this improved stability to the effective intercalation of PW12 within Ti₃C₂T_x layers helped by the CTA cations, as opposed to the external aggregation of PW12 clusters into micro or nanocrystals taking place for the other cations. The results provide a solid guide to help develop high-performance MXene/POM hybrid materials for a variety of applications.

"Tree"-like Multidentate Ligand-Assisted Synthesis of Polymolybdate-Based Architectures with Multinuclear Metal Clusters: Supercapacitor and Electrochemical Sensing Performances

In this work, aiming for constructing multinuclear metal cluster-modified polymolybdate-based architectures with novel conformation, the "tree"-like multidentate ligand 5-(3-pyridyl)-1H-tetrazole) (3-ptzH) is introduced into the polymolybdate reaction system. Three new polymolybdate-based architectures with various multinuclear metal clusters, H₄[Cu₆(μ₃-OH)₂(3-ptz)₆(γ-Mo₈O₂₈) (H₂O)₂]·2H₂O (BOHU-1), H₂[Ag₄(3-ptz)₂(Mo₈O₂₆)] (BOHU-2), and H₄[Cu₅(3-ptzH)₂(3-ptz)₂(MnMo₉O₃₂)₂(H₂O)₄] (BOHU-3) (BOHU = Bohai University), have been prepared via the hydrothermal method and structurally characterized. In BOHU-1, a kind of pentanuclear copper cluster unit: [Cu₅(μ₃-OH)₂(3-ptz)₆]²⁺ is formed, which connects to construct a one-dimensional (1D) cluster-based chain. The 1D chains are extended to a two-dimensional (2D) layer via the Cu ions, which are further linked by the 4-connected [γ-Mo₈O₂₈]^8- anions to build a three-dimensional (3D) framework. In BOHU-2, when a Ag^I ion was used as the central metal, the 3-ptz adopts different coordination modes to link the Ag ions, forming hexanuclear [Ag₆(3-ptz)₄]²⁺ cluster and finally 1D chains. These 1D cluster-based chains are connected by the 6-connected [γ-Mo₈O₂₆]^4- anions to establish a 2D layer, which is further extended by [Mo₈O₂₆]_n^4n- 1D chains to a 3D framework. For BOHU-3, the chiral [MnMo₉O₃₂]^6- anions are introduced and coordinated with the Cu ions to build left- and right-handed 1D chains, which are connected via the [Cu₃(3-ptz)₄]²⁺ cluster to form a 1D ladder-like chain. The effects of 3-ptz on the formation of multinuclear clusters, as well as the metals and polymolybdates on the multinuclear clusters and final structures of BOHU-1∼3, are discussed. The electrochemical performances of BOHU-1∼3 as electrode materials for supercapacitors and electrochemical sensors are investigated.

Porous {P6Mo18O73}-type Poly(oxometalate) Metal-Organic Frameworks for Improved Pseudocapacitance and Electrochemical Sensing Performance

{P₆Mo₁₈} poly(oxometalate) (POM) clusters have huge steric hindrance and limited active oxygen atoms, which make them difficult to combine with metal-organic units to form three-dimensional (3D) porous structures. Therefore, functionalization of such POMs has always been a bottleneck that is difficult to break through. In this study, {P₆Mo₁₈} POM was successfully grafted on a lock-like metal-organic chain to generate a multiporous coordination polymer, [{Na(H₂O)(H₂btb)}{Cu₄^I(H₂O)(pz)₅Cl}{H₂Sr⊂P₆Mo₂^VMo₁₆^VIO₇₃}]·3H₂O (1) (pz = pyrazine; btb = 1,4-bis(1,2,4-triazole) butane). Meanwhile, a zero-dimensional (0-D) control compound with only btb ligands as counterions, (H₄btb)[H₄Sr⊂P₆Mo₂^VMo₁₆^VIO₇₃]·3H₂O (2), was also obtained via a hydrothermal reaction. Compound 1 represents the first basket-type 3D poly(oxometalate) metal-organic framework (POMOF) assembly, which possesses interpenetrating pores and complex topology. 1-GO-CPE displays improved supercapacitor (SC) performance (the specific capacitance of 929.4 F g^-1 at a current density of 3 A g^-1 with 94.1% of cycle efficiency after 5000 cycles) compared with 2-GO-CPE and most reported POMOF electrode materials, which may be due to the outstanding redox capability of basket-POM, introduction of metal-organic chains, intersecting pores, and excellent conductivity of graphene. An asymmetric SC device with 1-GO-CPE as the negative electrode exhibits an energy density of 29.7 Wh kg^-1 with a power density of 3148.2 W kg^-1 and long-lasting cycling life. In addition, 1-GO-GCE as an electrochemical sensor responds to dopamine (DA) at a voltage of 0.40 V and shows lower detection limits (0.19 μM (signal-to-noise ratio (SNR) = 3)), higher selectivity, and good reproducibility in the linear range of 0.56 μM to 0.24 mM. The ability to accurately detect the content of DA in biological samples further proves the feasibility of the sensor in practical applications.

Coherent Integration of Organic Gel Polymer Electrolyte and Ambipolar Polyoxometalate Hybrid Nanocomposite Electrode in a Compact High-Performance Supercapacitor

We report a gel polymer electrolyte (GPE) supercapacitor concept with improved pathways for ion transport, thanks to a facile creation of a coherent continuous distribution of the electrolyte throughout the electrode. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was chosen as the polymer framework for organic electrolytes. A permeating distribution of the GPE into the electrodes, acting both as integrated electrolyte and binder, as well as thin separator, promotes ion diffusion and increases the active electrode–electrolyte interface, which leads to improvements both in capacitance and rate capability. An activation process induced during the first charge–discharge cycles was detected, after which, the charge transfer resistance and Warburg impedance decrease. We found that a GPE thickness of 12 μm led to optimal capacitance and rate capability. A novel hybrid nanocomposite material, formed by the tetraethylammonium salt of the 1 nm-sized phosphomolybdate cluster and activated carbon (AC/TEAPMo12), was shown to improve its capacitive performance with this gel electrolyte arrangement. Due to the homogeneous dispersion of PMo12 clusters, its energy storage process is non-diffusion-controlled. In the symmetric capacitors, the hybrid nanocomposite material can perform redox reactions in both the positive and the negative electrodes in an ambipolar mode. The volumetric capacitance of a symmetric supercapacitor made with the hybrid electrodes increased by 40% compared to a cell with parent AC electrodes. Due to the synergy between permeating GPE and the hybrid electrodes, the GPE hybrid symmetric capacitor delivers three times more energy density at higher power densities and equivalent cycle stability compared with conventional AC symmetric capacitors.

Simple preparation of Ag-BTC-modified Co3Mo7O24 mesoporous material for capacitance and H2O2-sensing performances

{Co3Mo7O24}@Ag-BTC-2 was synthesized by a grinding method, and it showed excellent performance in a supercapacitor and H2O2 sensing.

The Intrinsic Charge Carrier Behaviors and Applications of Polyoxometalate Clusters Based Materials

Polyoxometalates (POMs) are a series of molecular metal oxide clusters, which span the two domains of solutes and solid metal oxides. The unique characters of POMs in structure, geometry, and adjustable redox properties have attracted widespread attention in functional material synthesis, catalysis, electronic devices, and electrochemical energy storage and conversion. This review is focused on the links between the intrinsic charge carrier behaviors of POMs from a chemistry-oriented view and their recent ground-breaking developments in related areas. First, the advantageous charge transfer behaviors of POMs in molecular-level electronic devices are summarized. Solar-driven, thermal-driven, and electrochemical-driven charge carrier behaviors of POMs in energy generation, conversion and storage systems are also discussed. Finally, present challenges and fundamental insights are discussed as to the advanced design of functional systems based upon POM building blocks for their possible emerging application areas.

{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.

Wells-Dawson Arsenotungstate Porous Derivatives for Electrochemical Supercapacitor Electrodes and Electrocatalytically Active Materials

Two Wells-Dawson arsenotungstate coordination polymers, [{Cu^II(bim)₂}₃(As₂W₁₈O₆₂)] (1) and [(Cu^I₁₀pz₁₀Cl₄)(As₂W₁₈O₆₂)] (bim = 2,2'-biimidazole; pz = pyrazine), have been assembled via a hydrothermal method and fully characterized. Compound 1 exhibits a 2,6-connected two-dimensional hybrid layer based on asymmetrically modified {As₂W₁₈} anions and {Cu(bim)₂} linkers, which is extended to a three-dimensional network with a special interlayer structure and a one-dimensional tunnel. Compound 2 is a host-guest framework that consists of a Cu-pz-Cl network with 20-member square rings, 16-member irregular rings, and embedded eight-node {As₂W₁₈} guest molecules. Compounds 1 and 2 show uncommon specific capacitance (834.8 and 960.1 F g^-1, respectively, at a current density of 2.4 A g^-1), enduring cycling stability (capacitance retention rates of 89.3% and 91.9%, respectively, after 5000 cycles), and good electrical conductivity, which are superior to those of the unmodified zero-dimensional Dawson arsenotungstate compound and most reported electrode materials in terms of their stable structure, special layer spacing, and orderly channels. Moreover, the title compounds exhibit excellent electrocatalytic activity for oxidizing ascorbic acid and reducing nitrite.

Metal–organic frameworks with different spatial dimensions for supercapacitors

Recent progress in MOF materials for SCs with different spatial dimensions, such as 2D MOFs, including conductive MOFs and nanosheets, and 3D MOFs, categorized as single metallic and multiple metallic MOFs, are reviewed.

Polyoxometalate Metal-Organic Frameworks: Keggin Clusters Encapsulated into Silver-Triazole Nanocages and Open Frameworks with Supercapacitor Performance

To investigate the relationship between the structures of polyoxometalate host-guest materials and their energy-storage performance, three novel polyoxometalate-based metal-organic compounds, [Ag₁₀(C₂H₂N₃)₈][HVW₁₂O₄₀], [Ag₁₀(C₂H₂N₃)₆][SiW₁₂O₄₀], and [Ag(C₂H₂N₃)][Ag₁₂(C₂H₂N₃)₉][H₂BW₁₂O₄₀] are synthesized by a one-step hydrothermal method and further confirmed by single-crystal X-ray diffraction analyses and other numerous characterization techniques. In compound [Ag₁₀(C₂H₂N₃)₈][HVW₁₂O₄₀], the Keggin clusters are intersected into channels formed by a 3D open metal-organic framework. In contrast, in compounds [Ag₁₀(C₂H₂N₃)₆][SiW₁₂O₄₀] and [Ag(C₂H₂N₃)][Ag₁₂(C₂H₂N₃)₉][H₂BW₁₂O₄₀], the Keggin clusters are encapsulated into silver-triazole metal-organic nanocages to construct core-shell structures, which are further fused together by covalent bonds to form 3D polyoxometalate-based metal-organic frameworks. The electrochemical properties of three compound-based electrodes are estimated by cyclic voltammetry, galvanostatic charge-discharge, electrochemically active surface area, and electrochemical impedance spectroscopy. The results of the electrochemical performance tests indicate that these compounds possess high specific capacitance and cycling stability, especially [Ag₁₀(C₂H₂N₃)₈][HVW₁₂O₄₀], showing a specific capacitance of 93.5 F g^-1, which is higher than that of many other polyoxometalate-based electrode materials. A possible mechanism of the electrochemical performance is explored, which is mainly related to the redox capacity of polyoxometalate, the electrochemically active surface area, the electrochemical impedance spectroscopy, and the microstructures of polyoxometalate-based metal-organic frameworks.

Polyoxometalate-Incorporated Metallacalixarene@Graphene Composite Electrodes for High-Performance Supercapacitors

Composites of polyoxometalate (POM)/metallacalixarene/graphene-based electrode materials not only integrate the superiority of the individual components perfectly but also ameliorate the demerits to some extent, providing a promising route to approach high-performance supercapacitors. Herein, first, we report the preparations, structures, and electrochemical performance of two fascinating POM-incorporated metallacalixarene compounds [Ag₅(C₂H₂N₃)₆][H₅ ⊂ SiMo₁₂O₄₀] (1) and [Ag₅(C₂H₂N₃)₆][H₅ ⊂ SiW₁₂O₄₀] (2); (C₂H₂N₃ = 1 H-1,2,4-triazole). Single-crystal X-ray diffraction analyses illustrated that both 1 and 2 possess intriguing POM-sandwiched metallacalix[6]arene frameworks. Nevertheless, our investigations, including the electrochemical cyclic voltammetry, galvanostatic charge-discharge tests, and electrochemical impedance spectroscopy, reveal that the oxidation ability of the Keggin ions is a primary effect in electrochemical performance of these POM-incorporated metallacalixarene compounds. Namely, the electrodes containing Mo as metal atoms in the Keggin POM shows much higher capacitance than the corresponding W-containing ones. Moreover, compound 1@graphene oxide (GO) composite electrodes are fabricated and systematically explored for their supercapacitor performance. Thanks to the synergetic effects of GO and POM-incorporated metallacalixarenes, the compound 1@15%GO-based electrode exhibits the highest specific capacitance of up to 230.2 F g^-1 (current density equal to 0.5 A g^-1), which is superior to majority of the reported POM-based electrode materials.

Mo-Based crystal POMOFs with a high electrochemical capacitor performance

The capacitor performance of newly synthesized crystalline POMOFs was higher than those of the majority of reported POMOF-, state-of-the-art MOF- and POM-based materials.