Oxo-Replaced Polyoxometalates: There Is More than Oxygen

The presence of oxo-ligands is one of the main required characteristics for polyoxometalates (POMs), although some oxygen ions in a metallic environment can be replaced by other nonmetals, while maintaining the POM structure. The replacement of oxo-ligands offers a valuable approach to tune the charge distribution and connected properties like reducibility and hydrolytic stability of POMs for the development of tailored compounds. By assessing the reported catalytic and biological applications and connecting them to POM structures, the present review provides a guideline for synthetic approaches and aims to stimulate further applications where the oxo-replaced compounds are superior to their oxo-analogues. Oxo-replacement in POMs deserves more attention as a valuable tool to form chemically activated precursors for the synthesis of novel structures or to upgrade established structures with extraordinary properties for challenging applications.

Surficial grafting of organoimido moieties enhances the capacity performance of oxometallic clusters

Molybdenum trioxide (MoO₃) with a theoretical specific capacity of 1117 mA h g^-1 is widely considered a promising anode material for lithium-ion batteries. However, the irreversible conversion reactions, low electrical conductivity, and detrimental volume expansion upon Li intercalation between the one-dimensional layered structures of MoO₃ hinder its practical implementation. Herein, we report a facile synthetic protocol that allows surficial modification by replacing the terminal and bridging oxo groups of molybdenum oxide clusters. Successful organoimido functionalization resulted in a large cathodic shift in Mo(VI/V) reduction by 0.6 V, pronounced electronic communication between the organic moiety and the metal-oxide unit, and significant increase in electrical conductivity (80-100 Ω interfacial charge-transfer resistance). Combined with the enlarged active surface area due to the structural hindrance induced by the organic functionality, the steady specific capacity of the organoimido-modified molybdenum oxide clusters was greater than 1200 mA h g^-1 at 900 mA g^-1 at the end of 360 cycles, where the best value of 1653 mA h g^-1 was achieved for the nitroaniline-substituted species. The steady capacity of 480 mA h g^-1 was achieved in the fast charge-discharge process (3000 mA g^-1) over 1400 cycles. The results indicate that the surficial modification of metal oxides with organo moieties using our facile synthetic method has broad application potential for metal oxides to be used as high-capacity electrode materials in the future.

Insights into organic-inorganic hybrid molecular materials: organoimido functionalized polyoxomolybdates

Polyoxometalates (POMs) are polyatomic anions that comprise transition metal group 5 (V, Nb, Ta) or group 6 (Mo, W) oxyanions connected together by shared oxygen atoms. POMs are fascinating because of their exclusive and remarkable characteristics. One of the most interesting features of POMs is their capability to function as an electron relay by performing stepwise multi-electron redox reactions while maintaining their structural integrity. Functionalization of POMs with amino organic compounds results in organoimido derivatives of polyoxometalates, which have aroused interest due to augmentation of their properties. Comprehensive study has shown that the synthesis methodologies to obtain desired organoimido derivatives of POMs by employing various imido-releasing reagents have progressed drastically in recent decades, particularly the innovative DCC-dehydrating technique. These organoimido functionalized POMs have been used as major building blocks to develop unique nanostructured organic-inorganic hybrid molecular materials. Many conventional organic synthesis processes such as Pd-catalyzed carbon-carbon coupling and esterification reactions have been performed with organoimido functionalized POMs where the presence of POM triggered the reaction process. Thus, investigation of the reactivity of organoimido derivatives of POMs foreshadows the intriguing future of POMs chemistry.

Electrocatalytic performance of tyrosinase detection in Penaeus vannamei based on a [(PSS/PPy)(P2Mo18/PPy)5] multilayer composite film modified electrode

Polyoxometalates (POMs) are widely used in the preparation of sensors that detect the content of substances because of their excellent electron transfer capabilities. In this paper, a [(PSS/PPy)(P₂Mo₁₈/PPy)₅] multilayer composite film modified electrode was prepared by the potentiostatic deposition method. The electrochemical performance of the modified electrode was studied by cyclic voltammetry under the conditions of different modified layers, different supporting electrolytes and different sweep rates. Different concentrations of tyrosinase were catalyzed by the modified electrode under a suitable supporting electrolyte, and the electrochemical sensing of tyrosinase by the modified electrode was studied. The research results show that the modified electrode has good stability and reproducibility for electrochemical sensing of tyrosinase, and the response current has a good linear relationship with the amount of tyrosinase added. Taking peak III as an example, the detection limit (S/N = 3) was 2.7649 U mL^-1. It can be known from the timing ampere curve that as the concentration of tyrosinase in the reaction system continues to increase, its response current increases stepwise, providing a linear curve in the range of 3.66 U mL^-1 to 26.87 U mL^-1, and the minimum detection limit (S/N = 3) reaches 0.0021 U mL^-1. The [(PSS/PPy)(P₂Mo₁₈/PPy)₅] multilayer composite membrane modified electrode was used to detect tyrosinase in Penaeus vannamei. The spiked recovery of the sample was 96.3-100.8%, indicating that the modified electrode has high accuracy and can be used for the detection of tyrosinase in actual samples.

In Operando X-ray Studies of High-Performance Lithium-Ion Storage in Keplerate-Type Polyoxometalate Anodes

Polyoxometalates (POMs) have emerged as potential anode materials for lithium-ion batteries (LIBs) owing to their ability to transfer multiple electrons. Although POM anode materials exhibit notable results in LIBs, their energy-storage mechanisms have not been well-investigated. Here, we utilize various in operando and ex situ techniques to verify the charge-storage mechanisms of a Keplerate-type POM Na₂K₂₃{[(Mo^VI)Mo^VI₅O₂₁(H₂O)₃(KSO₄)]₁₂ [(V^IVO)₃₀(H₂O)₂₀(SO₄)_0.5]}·ca200H₂O ({Mo₇₂V₃₀}) anode in LIBs. The {Mo₇₂V₃₀} anode provides a high reversible capacity of up to ∼1300 mA h g^-1 without capacity fading for up to 100 cycles. The lithium-ion storage mechanism was studied systematically through in operando synchrotron X-ray absorption near-edge structure, ex situ X-ray diffraction, ex situ extended X-ray absorption fine structure, ex situ transmission electron microscopy, in operando synchrotron transmission X-ray microscopy, and in operando Raman spectroscopy. Based on the abovementioned results, we propose that the open hollow-ball structure of the {Mo₇₂V₃₀} molecular cluster serves as an electron/ion sponge that can store a large number of lithium ions and electrons reversibly via multiple and reversible redox reactions (Mo⁶⁺ ↔ Mo¹⁺ and V⁵⁺/V⁴⁺↔ V¹⁺) with fast lithium diffusion kinetics (D_Li⁺: 10^-9-10^-10 cm² s^-1). No obvious volumetric expansion of the microsized {Mo₇₂V₃₀} particle is observed during the lithiation/delithiation process, which leads to high cycling stability. This study provides comprehensive analytical methods for understanding the lithium-ion storage mechanism of such complicated POMs, which is important for further studies of POM electrodes in energy-storage applications.

Organic phosphomolybdate: a high capacity cathode for potassium ion batteries

This work firstly introduces tetra-n-butylammonium phosphomolybdate into potassium ion batteries as a high capacity cathode.

A Comprehensive Study on the Dye Adsorption Behavior of Polyoxometalate-Complex Nano-Hybrids Containing Classic β-Octamolybdate and Biimidazole Units

Six new hybrids based on β-[Mo₈O26]4- polyoxometalates, [Ni(H₂biim)₃]₂[β-Mo₈O26]•8DMF(1); (DMA)₂[M(H₂biim)₂(H₂O)₂][β-Mo₈O26]•4DMF (M = Ni (2), Co (3)), DMA = dimethyl-ammonium, H₂biim=2,2'-biimidazole); [M(H₂biim)(DMF)₃]₂[β-Mo₈O26]•2DMF (M = Zn (4), Cu (5)); [(DMA)₂[Cu(DMF)₄][β-Mo₈O26]•2DMF]n (6), have been successfully synthesized and characterized. Compounds 2⁻5 show favorable capacity to adsorb methylene blue (MB) at room temperature, and they can selectively capture MB molecules from binary-mixture solutions of MB/MO (MO = Methyl Orange), or MB/RhB (RhB = Rhodamine B). Compound 3 can uptake up to 521.7 mg g-1 MB cationic dyes rapidly, which perform the maximum adsorption in an hour among the reported materials as far as we know. The compounds are stable and still work very efficiently after three cycles. For compound 3, the preliminary adsorption mechanism studies indicated that the adsorption is an ion exchange process and the adsorption behavior of polyoxometalate-complex can be benefited from additional exchangeable protons in the complex cations.

Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions

Hydrogen evolution reaction (HER) in alkaline medium is currently a point of focus for sustainable development of hydrogen as an alternative clean fuel for various energy systems, but suffers from sluggish reaction kinetics due to additional water dissociation step. So, the state-of-the-art catalysts performing well in acidic media lose considerable catalytic performance in alkaline media. This review summarizes the recent developments to overcome the kinetics issues of alkaline HER, synthesis of materials with modified morphologies, and electronic structures to tune the active sites and their applications as efficient catalysts for HER. It first explains the fundamentals and electrochemistry of HER and then outlines the requirements for an efficient and stable catalyst in alkaline medium. The challenges with alkaline HER and limitation with the electrocatalysts along with prospective solutions are then highlighted. It further describes the synthesis methods of advanced nanostructures based on carbon, noble, and inexpensive metals and their heterogeneous structures. These heterogeneous structures provide some ideal systems for analyzing the role of structure and synergy on alkaline HER catalysis. At the end, it provides the concluding remarks and future perspectives that can be helpful for tuning the catalysts active-sites with improved electrochemical efficiencies in future.

[V4Mo3O14(NAr)3(μ2-NAr)3]2−: the first polyarylimido-stabilized molybdovanadate cluster

This work presents the first polyarylimido-stabilized molybdovanadate cluster, which will enrich polyoxometalate chemistry and lead to more potential POM-based materials.

Sandwich-type silicotungstate modified TiO2 microspheres for enhancing light harvesting and reducing electron recombination in dye-sensitized solar cells

Sandwich-type silicotungstates have been applied in DSSCs where an 0.8% Ni₄ modified photoanode exhibited the best performance in enhancing light harvesting and reducing electron recombination.

Development of ionic liquid mediated novel polymer electrolyte membranes for application in Na-ion batteries

Polymer electrolyte membranes based on polymer PEO, ionic liquid, 1-butyl-3-methylimidazolium methylsulfate (BMIM-MS), and salt, sodium methylsulfate (NaMS), were prepared and characterized by SEM, XRD, TGA/DTGA, DSC, ac impedance spectroscopy and cyclic voltammetry.

Ultrasound promoted facile one pot synthesis of highly substituted pyran derivatives catalyzed by silica-coated magnetic NiFe2O4nanoparticle-supported H14[NaP5W30O110] under mild conditions

Application of ultrasonic method as a green source of energy to reduce the reaction times compared to conventional processes. NiFe₂O₄@SiO₂–H₁₄[NaP₅W₃₀O₁₁₀] was used as magnetically separable catalyst for synthesis of pyrans with minimum work up.

Polyoxometalate-conductive polymer composites for energy conversion, energy storage and nanostructured sensors

The exchange of electric charges between a chemical reaction centre and an external electrical circuit is critical for many real-life technologies. This perspective explores the "wiring" of highly redox-active molecular metal oxide anions, so-called polyoxometalates (POMs) to conductive organic polymers (CPs). The major synthetic approaches to these organic-inorganic hybrid materials are reviewed. Typical applications are highlighted, emphasizing the current bottlenecks in materials development. Utilization of the composites in the fields of energy conversion, electrochemical energy storage, sensors and nanoparticle "wiring" into conductive materials are discussed. The outlook section presents the authors' views on emerging fields of research where the combination of POMs and CPs can be expected to provide novel materials for groundbreaking new technologies. These include light-weight energy storage, high-sensitivity toxin sensors, artificial muscles, photoelectrochemical devices and components for fuel cells.

N-alkylation of organo-imido substituted polyoxometalates: an efficient and stoichiometric approach for the easy post-modification of polyoxometalates

A high-yield post-functionalization protocol developed here provides an easy inclusion of the desired organic groups on a POM's highly negative surface.

Microwave assisted synthesis of a mono organoimido functionalized Anderson polyoxometalate

The synthesis of an aliphatic organoimido functionalized polyoxometalate has been achieved through a microwave assisted reaction protocol in the absence of any activating reagents.

Electrode Nanostructures in Lithium-Based Batteries

Lithium-based batteries possessing energy densities much higher than those of the conventional batteries belong to the most promising class of future energy devices. However, there are some fundamental issues related to their electrodes which are big roadblocks in their applications to electric vehicles (EVs). Nanochemistry has advantageous roles to overcome these problems by defining new nanostructures of electrode materials. This review article will highlight the challenges associated with these chemistries both to bring high performance and longevity upon considering the working principles of the various types of lithium-based (Li-ion, Li-air and Li-S) batteries. Further, the review discusses the advantages and challenges of nanomaterials in nanostructured electrodes of lithium-based batteries, concerns with lithium metal anode and the recent advancement in electrode nanostructures.