Layer-by-layer self-assembly and electrocatalytic properties of poly(ethylenimine)-silicotungstate multilayer composite films

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.

In situ anodic dissolution–cathodic deposition route for preparation of the Pt–SiW11Co/SiW11Co–CNP/GC electrode: application as an efficient electrode for the hydrogen evolution reaction

A novel nanohybrid based on carbon nanoparticles, platinum nanoparticles, and SiW₁₁Co polyoxometalate is introduced as an efficient electrocatalyst for the hydrogen evolution reaction (HER).

Dual modification of TiO2 nanorod arrays with SiW11Co and Ag nanoparticles for enhanced photocatalytic activity under simulated sunlight

Well-organized TiO₂ nanorod arrays (TNRs) have increasingly attracted our attention in recent years due to their excellent photocatalytic properties. However, it is of great importance to prepare more efficient photocatalysts using a facile method towards their more widespread use. In this work, K₆SiW₁₁O₃₉Co(ii)(H₂O) (SiW₁₁Co) and Ag nanoparticles were introduced into TNRs using spin-coating and chemical bath deposition methods. It was found that TNRs/SiW₁₁Co/Ag composite films with an active area of only 1 cm² exhibit highly efficient and sustainable properties for the photodegradation of NO₂ and display a significant enhancement compared with P₂₅ and pure TNRs. Photocatalytic measurements demonstrated that both SiW₁₁Co and Ag synergistically improve the light absorption and charge separation efficiency, thus obtaining the most efficient photocatalytic performance. In addition, the probable photocatalytic mechanism and the dominating active species for NO₂ photodegradation were also proposed, in order to testify the effectively enhanced photocatalytic ability of the TNRs/SiW₁₁Co/Ag composite. Hence, the design of these polyoxometalate and metal particle co-modified TNRs may provide a new tactic for developing promising materials for photocatalytic degradation.

How do transition-metal-substituted POMs modify the photoanode of dye-sensitized solar cells? A DFT study

Herein, the electronic structures and absorption spectra of a series of transition-metal-substituted polyoxometalates (TMSPs) were systematically investigated to screen promising candidates for the POM/TiO₂ nanocomposite film used in DSSCs using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods.

Polyoxotungstate@Carbon Nanocomposites As Oxygen Reduction Reaction (ORR) Electrocatalysts

The oxygen reduction reaction (ORR) has a crucial function as the cathode reaction in energy-converting systems, such as fuel cells (FCs), which contributes to a sustainable energy supply. However, the current use of precious Pt-based electrocatalysts (ECs) is a major drawback for the economic viability of fuel cells. Hence, it is urgent to develop cost-effective and efficient electrocatalysts (ECs) without noble metals to substitute the Pt-based ECs. Herein, we report the preparation and application as ORR electrocatalysts of four new nanocomposites based on sandwich-type phosphotungstate (TBA)₇H₃[Co₄(H₂O)₂(PW₉O₃₄)₂] (TBA-Co₄(PW₉)₂) immobilized onto different carbon nanomaterials [single-walled carbon nanotubes (SWCNT), graphene flakes (GF), carbon nanotubes doped with nitrogen (N-CNT), and nitrogen-doped few layer graphene (N-FLG)]. In alkaline medium, the four nanocomposites studied presented comparable onset potentials (0.77-0.90 V vs RHE), which are similar to that observed for Pt/C (0.91 V vs RHE). Higher diffusion-limiting current densities ( j_L,_{0.26V,1600 rpm} = -168.3 mA cm^-2 mg^-1) were obtained for Co₄(PW₉)₂@N-CNT, as compared to Pt/C electrode -130.0 mA cm^-2 mg^-1) and the other ECs (-45.0, -50.7, and -87.5 mA cm^-2 mg^-1 for Co₄(PW₉)₂@SWCNT, Co₄(PW₉)₂@GF, and Co₄(PW₉)₂@N-FLG, respectively). All the Co₄(PW₉)₂@CM ECs showed selectivity toward direct O₂ reduction to water with the exception of Co₄(PW₉)₂@GF where a mixture of the 2- and 4-electron mechanisms is observed. Furthermore, low Tafel slopes were obtained for all the nanocomposites (68-96 mV dec^-1). Co₄(PW₉)₂@CM ECs also showed excellent tolerance to methanol with no significant changes in current density, in contrast to Pt/C (decrease of ≈59% after methanol addition) and good long-term electrochemical stability with current retentions between 75 and 84%.

Single-Molecule Magnet Behavior of Individual Polyoxometalate Molecules Incorporated within Biopolymer or Metal-Organic Framework Matrices

The chemically and structurally highly stable polyoxometalate (POM) single-molecule magnet (SMM) [(FeW9 O34 )2 Fe4 (H2 O)2 ](10-) (Fe6 W18 ) has been incorporated by direct or post-synthetic approaches into a biopolymer gelatin (Gel) matrix and two crystalline metal-organic frameworks (MOFs), including one diamagnetic (UiO-67) and one magnetic (MIL-101(Cr)). Integrity of the POM in the Fe6 W18 @Gel, Fe6 W18 @UiO-67 and Fe6 W18 @MIL-101(Cr) composites was confirmed by a set of complementary techniques. Magnetic studies indicate that the POMs are magnetically well isolated. Remarkably, in Fe6 W18 @Gel, the SMM properties of the embedded molecules are close to those of the crystals, with clear quantum tunneling steps in the hysteresis loops. For the Fe6 W18 @UiO-67 composite, the molecules retain their SMM properties, the energy barrier being slightly reduced in comparison to the crystalline material and the molecules exhibiting a tunneling rate of magnetization significantly faster than for Fe6 W18 @Gel. When Fe6 W18 is introduced into MIL-101(Cr), the width of the hysteresis loops is drastically reduced and the quantum tunneling steps are smeared out because of the magnetic interactions between the antiferromagnetic matrix and the SMM guest molecules.

A chitosan–Pt nanoparticles/carbon nanotubes-doped phosphomolybdate nanocomposite as a platform for the sensitive detection of nitrite in tap water

A polyoxometalate-based composite film decorated with CNTs and Pt–CHIT NPs was constructed on an electrode using the LBL self-assembly method. It acted as an electrochemical nitrite sensor with greatly enhanced electron transfer ability and sensing performance.

Multielectrocatalysis by layer-by-layer films based on pararosaniline and vanadium-substituted phosphomolybdate

Hybrid multilayer films based on the two molecular species pararosaniline (PR) and Keggin-type polyoxometalate K5[PMo11VO40)] (PMo11V) were prepared on different substrates using the electrostatic layer-by-layer (LbL) self-assembly method. The film buildup, monitored by electronic spectroscopy, showed a regular stepwise growth, and X-ray photoelectron spectroscopy data confirmed the presence of both molecular components within the LbL films. Scanning electron microscopy images revealed a completely covered surface with a nonuniform distribution of film components, and atomic force microscopy images confirmed a rough surface. The film electrochemical responses and permeability were studied by cyclic voltammetry. Films revealed three Mo-based redox processes (Mo(VI) → Mo(V)) and one V-based redox process (V(V) → V(IV)) in the potential range between 0.8 and -0.4 V vs Ag/AgCl. Studies with the redox probes [Fe(CN)6](3-/4-) and [Ru(NH3)6](3+/2+) showed that the films maintain the permeability even after six bilayers. Furthermore, the {PR/PMo11V}n multilayer films exhibit excellent Mo-based electrocatalytic activity toward reduction of iodate and V-based electrocatalytic activity toward ascorbic acid oxidation, thus acting as a versatile multielectrocatalyst.

Novel composite material polyoxovanadate@MIL-101(Cr): a highly efficient electrocatalyst for ascorbic acid oxidation

A novel hybrid composite material, PMo10V2@MIL-101 was prepared by the encapsulation of the tetra-butylammonium (TBA) salt of the vanadium-substituted phosphomolybdate [PMo10V2O40](5-) (PMo10V2) into the porous metal-organic framework (MOF) MIL-101(Cr). The materials characterization by powder X-ray diffraction, Fourier transform infrared spectra and scanning electron microscopy confirmed the preparation of the composite material without disruption of the MOF porous structure. Pyrolytic graphite electrodes modified with the original components (MIL-101(Cr), PMo10V2), and the composite material PMo10V2@MIL-101 were prepared and their electrochemical responses were studied by cyclic voltammetry. Surface confined redox processes were observed for all the immobilized materials. MIL-101(Cr) showed one-electron reduction process due to chromium centers (Cr(III) → Cr(II)), while PMo10V2 presented five reduction processes: the peak at more positive potentials is attributed to two superimposed 1-electron vanadium reduction processes (V(V) → V(IV)) and the other four peaks to Mo-centred two-electron reduction processes (Mo(VI) → Mo(V)). The electrochemical behavior of the composite material PMo10V2@MIL-101 showed both MIL-101(Cr) and PMo10V2 redox features, although with the splitting of the two vanadium processes and the shift of the Mo- and Cr- centered processes to more negative potentials. Finally, PMo10V2@MIL-101 modified electrode showed outstanding enhanced vanadium-based electrocatalytic properties towards ascorbic acid oxidation, in comparison with the free PMo10V2, as a result of its immobilization into the porous structure of the MOF. Furthermore, PMo10V2@MIL-101 modified electrode showed successful simultaneous detection of ascorbic acid and dopamine.