Effects of film structure on electrochromic properties of the multilayer films containing polyoxometalates

Co-assembly of graphene/polyoxometalate films for highly electrocatalytic and sensing hydroperoxide

Graphene oxide (GO) films mixed with polyethylenimine (PEI) were prepared by a layer-by-layer assembly (LBL) method, in which the GO component is then converted to reduced GO (rGO) in situ through an electron transfer interaction with a polyoxometalate (POM) that is assembled on the outer surface. With this, devices were manufactured by spreading composite films of (PEI/rGO)_n-POM with different numbers of PEI/rGO layers on ITO substrates. Cyclic voltammetry (CV) reveals that the catalytic activity for H₂O₂ of (PEI/rGO)_n-POM films was significantly higher than that of similar films of (PEI/GO)_n/PEI/POM manufactured LBL with the same number of layers, although the catalyst POM content of (PEI/rGO)_n-POM was only half that of (PEI/GO)_n/PEI/POM. The catalytic activity of (PEI/rGO)_n-POM films first increases and then decreases as the number of PEI/rGO layers increases. The result shows that (PEI/rGO)₃-POM films with three PEI/rGO layers exhibit the highest efficiency. Amperometric measurements of the (PEI/rGO)₃-POM films showed improved current response, high sensitivity, wide linear range, low detection limit, and fast response for H₂O₂ detection. The enhanced catalytic property of (PEI/rGO)_n-POM films is attributed to the electron transfer interaction and electrostatic interaction between POM and rGO.

Porous polyoxotungstate/MXene hybrid films allowing for visualization of the energy storage status in high-performance electrochromic supercapacitors

Electrochromic supercapacitors (ECSCs) have recently received growing attention for potential smart energy storage components in intelligent electronics. However, in the development of ECSCs, the design and assembly of high-performance electrode materials remain ongoing challenges. In this study, Ti₃C₂T_x MXene and polyoxotungstate (P₂W₁₈) were deposited on TiO₂ nanowires to construct a unique three-dimensional (3D) porous hybrid film, NW@MXene/P₂W₁₈, via a convenient layer-by-layer self-assembly approach. The 3D porous structure of the nanocomposite reduced the aggregation and stacking of Ti₃C₂T_x MXene nanosheets during self-assembly, leading to the formation of unobstructed ion diffusion channels and interfacial charge transfer between adjacent layers, resulting in a good electrochemical performance. Compared to the tightly packed structure, the porous hybrid film demonstrated an enhanced electrochromic energy storage performance with a higher areal capacitance (i.e., 19.0 mF cm^-2 at a current density of 0.6 mA cm^-2), in addition to a high cycling stability (i.e., 90.7% retention rate after 2000 cycles), and an excellent color rendering efficiency. Subsequently, an asymmetric ECSC was fabricated using an NW@MXene/P₂W₁₈ film as the cathode and a TiO₂ nanowire film as the anode. This ECSC exhibited a high areal capacitance of 4.0 mF cm^-2 at a current density of 0.1 mA cm^-2 with a wide operating window of 4.5 V, whilst also achieving high-speed color switching between olive green and dark blue during the charge/discharge processes, ultimately offering new avenues for the development of electrochromic energy storage electrode materials and the design of novel devices.

Polyoxotungstate-based nanocomposite films with multi-color change and high volumetric capacitance toward electrochromic energy-storage applications

A facile fabrication strategy is proposed to construct a POMs-based nanocomposite film. It realizes multi-color transition during charging and discharging process, thereby links electrochromic behavior with energy storage performance.

Electrochemical Reduction-Assisted In Situ Fabrication of a Graphene/Au Nanoparticles@polyoxometalate Nanohybrid Film: High-Performance Electrochemical Detection for Uric Acid

Nanohybrid films had attracted much attention owning to the enhancement of catalytic activity. However, the fabrication time took hours to days, no matter if it was the preparation of nanohybrids or the assembly process. Furthermore, the catalytic efficiency of the nanohybrid film still remained to improve. In this paper, a reduced graphene oxide (rGO)/gold nanoparticles (Au NPs)@polyoxometalate (POM) nanohybrid film was successfully fabricated by combining electrodeposition and electrochemical reduction in situ processes. The assembly process involving no organic or polymer linker molecules [except for a precursor poly(ethylenimine) (PEI) coating for indium tin oxide (ITO)] can be completed within 1 h. The reduced POM K₆[P₂W₁₈O₆₂]·19H₂O (P₂W₁₈) was employed as reducing agents and bridging molecules for rGO and Au nanoparticles and the encapsulating molecules for the Au nanoparticles. The most interesting one is the {rGO/Au@P₂W₁₈} modified electrode loading only the monolayer catalyst of Au@P₂W₁₈ and exhibiting comparable, even better electrochemical detection performance toward uric acid than other sensors with three to eight layers of the catalyst. The amperometric detection displayed a great sensitivity, lower detection limit, wide linear range, good long-time stability, superior selectivity, and reproducibility. The enhanced catalytic property may attribute to the improved conductivity of the film without organic or polymer linker molecules (except for a precursor PEI coating) and the electron transfer in the process of film fabrication.

Ionic Liquid-Derived Imidazolium Cation Linkers for the Layer-by-Layer Assembly of Polyoxometalate-MWCNT Composite Electrodes with High Power Capability

Imidazolium cations derived from ionic liquids were demonstrated as effective linker molecules for the layer-by-layer (LbL) deposition of polyoxometalates (POMs) to increase the charge storage of multi-walled carbon nanotube (MWCNT) electrodes. MWCNTs modified with GeMo12O40(4-) (GeMo12) via an imidazolium cation linker demonstrated highly reversible redox reactions and a capacitance of 84 F cm(-3), close to 4 times larger than bare CNT. Compared to CNT-GeMo12 composites fabricated with a conventional polyelectrolyte linker poly(diallyldimethylammonium chloride), (PDDA), the imidazolium cations resulted in lower POM loading, but higher conductivity and in turn superior performance at fast charge-discharge conditions. A polymerized imidazolium linker (PIL) was also synthesized based on the ethyl-vinyl-imidazolium monomer. CNT-GeMo12 composites fabricated with this PIL achieved high POM loading comparable to PDDA, while still maintaining the good conductivity and high rate capabilities shown by the monomer imidazolium units. The high conductivity imparted by the PIL is especially valuable for the fabrication of multilayer POM composites. Dual-layer GeMo12 O40(4-)-SiMo12O40(4-) (GeMo12-SiMo12) electrodes built with this PIL demonstrated a combined contribution of the individual POMs resulting in a capacitance of 191 F cm(-3), over nine times larger than bare MWCNT. The PIL dual layer composites also maintained 72% of this capacitance at a fast rate of 2 V s(-1), compared to just over 50% retention for similar electrodes fabricated with PDDA.

Hybrid Organic-Inorganic Materials Based on Polyoxometalates and Ionic Liquids and Their Application in Catalysis

An overview of the recent advances in the field of polyoxometalate, ionic liquid hybrids, is proposed with a special attention paid to their application in catalysis, more precisely biphasic and heterogeneous catalysis. Both components of the hybrids are separately outlined pointing to their useful properties and potential for catalysis, followed by the description of the hybrids preparation and synergy between components in a large range of organic transformations. And finally a vision on the future developments is proposed.

Polyoxometalate Surfactants as Unique Molecules for Interfacial Self-Assembly

Whereas, commonly, Langmuir monolayers are structurally dominated by the aliphatic chains, we present here the first case of monolayers where the chains merely serve anchoring at the air/water interface and the organization is dictated by the hydrophilic head group self-assembling in a hexagonal lattice. These head groups are polyoxometalates known for their multifunctional potential. The chain length has been systematically varied, allowing for a general study of the impact of the chain length on the supramolecular structure. These model structures are studied here by a combination of modern techniques, the leading ones being X-ray reflectivity and grazing incidence X-ray diffraction. The quantitative structural insights offered in this Letter might represent a starting point for the rational design and study of a new class of emulsions, including an organic tail and a multifunctional inorganic polar head.

Novel layer-by-layer interfacial [Ni(salen)]-polyelectrolyte hybrid films

A novel multilayer film containing a cationic phosphonium-derivatized Ni(salen)-type complex and poly(sodium-4-styrenesulfonate (NaPSS) was assembled onto quartz, mica, and metal surfaces using the layer-by-layer (LbL) technique. Spectroscopic (UV-vis) and gravimetric (QCM) responses for the multilayer films show regular stepwise growth and the signature of strong electrostatic interactions between the component layers. The gravimetric responses indicate the presence of substantial additional (net neutral) material in the PSS layers, which XPS shows is not polyelectrolyte or salt, so charge compensation is intrinsic; we deduce the presence of space-filling solvent. Direct electrostatic interaction of the two-component layers is enhanced by a secondary noncovalent interaction between the delocalized pi-systems of the two components. Permeability of the film to the redox probe [Fe(CN)(6)](3-/4-) was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Qualitatively similar results were obtained in the absence and presence of a precursor PSS/PAH multilayer, but with a general shift in kinetic and diffusional processes to longer time scales (lower frequencies) in the presence of the precursor layer and with increasing numbers of PSS/[Ni(salen)] bilayers. Quantitatively, the EIS data were interpreted using a capillary membrane model (CMM) to yield values of coverage, apparent charge transfer resistance, double-layer capacitance, pore size, and diffusion coefficient. The coverage values were consistent with a model in which there are no preferential growth sites and the surface charge density is independent of the number of bilayers.