Layer-by-Layer Construction of a Nanoarchitecture by Polyoxometalates and Polymers: Enhanced Electrochemical Hydrogen Evolution Reaction

In this contribution, a nanoarchitectural approach was employed to produce a nanolayer of polyoxometalate (POM) on the surface of a glassy carbon electrode (GCE) to achieve a higher surface area with higher electrocatalytic activity toward the electrochemical hydrogen evolution reaction (HER). To accomplish this, the well-known layer-by-layer (LbL) technique was employed, which involved the alternate adsorption of the POM, Na0.3[N(C4H9)4]7.7 [(Mo3O8)4(O3PC(O)(C3H6NH2CH2C4H3S)PO3)4], abbreviated as [(TBA)Mo12(AleThio)4], and polyethyleneimine (PEI) polymer. This nanolayered electrode exhibited catalytic properties toward the HER in 0.5 M H2SO4 with the resulting polarization curves indicating an increase in the HER activity with the increasing number of POM layers, and the overpotential required for this reaction was lowered by 0.83 V when compared with a bare GCE. The eighth PEI/[(TBA)Mo12(AleThio)4] bilayer exhibited a significantly lower HER overpotential of −0.077 V at a current density of 10 mA cm–2. Surface characterization of the LbL-assembled nanolayers was carried out using X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy. We believe that the synergetic effect of the positively charged PEI polymer and the catalytically active molybdate POM is the cause for the successful response to the electrochemical HER

Investigations into the Electrochemical, Surface, and Electrocatalytic Properties of the Surface-Immobilized Polyoxometalate, TBA₃K[SiW₁₀O₃₆(PhPO)₂]

Surface anchoring of an organic functionalized POM, TBA₃K­[SiW₁₀O₃₆(PhPO)₂] was carried out by two methods, the layer-by-layer (LBL) assembly technique by employing a pentaerythritol-based ruthenium­(II) metallodendrimer as a cationic moiety and also by entrapping the POM in a conducting polypyrrole film. The redox behavior of the constructed films was studied by using cyclic voltammetry and electrochemical impedance spectroscopy. The surface morphologies of the constructed multilayers were examined by scanning electron microscopy and atomic force microscopy. X-ray photoelectron spectroscopy was conducted to confirm the elements present within the fabricated films. The multilayer assembly was also investigated for its catalytic efficiency towards the reduction of nitrite

Redox switching of polyoxometalate-doped polypyrrole films in ionic liquid media

The surface immobilization of the parent Dawson polyoxometalate (POM) as a counter-ion for the electropolymerization of polypyrrole (PPy) or as an electrode-adhered solid was utilized for voltammetric studies of the surface adhered POM in room temperature ionic liquids (RTIL). Illustrating the efficiency of intermediate stabilization, voltammetry at POM-modified electrodes in a PF6-based RTIL revealed richer redox behaviour and higher stabilization in comparison with aqueous electrolytes and with BF4-based RTIL, respectively. High stability of the POM-doped PPy towards continuous charge-discharge voltammetric redox cycles was confirmed by minor changes in film morphology observed after the cycling in RTILs

Electrochemical characterisation of Ni(II)-Crown type Polyoxometalate doped polypyrrole films for the catalytic reduction of bromate in water

The nickel substituted (Ni4[(P8W48O184)(WO2)]28–, crown type polyoxometalate was electrochemically polymerised with conducting polymer pyrrole for the electrocatalytic reduction of bromate in water. The immobilized films of different thickness were characterised by electrochemical and surface based techniques. The resulting films were found to be extremely stable towards redox switching between the various redox states associated with the incorporated POM. This system was found to effectively electrocatalyse the reduction of bromate in water without the interference of other common anions. The detection limit was found to be 0.2 µm, with a linear region from 0.1 mm up to 2 mm bromate. The resulting POM doped polypyrrole films were found to be highly conductive by AC impedance. Surface characterization of the polymer films was carried out by using X‐ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy

Transition metal ion-substituted polyoxometalates entrapped into polypyrrole as electrochemical sensor for hydrogen peroxide

A conducting polymer was used for the immobilization of various transition metal ion-substituted Dawson-type polyoxometalates (POMs) onto glassy carbon electrodes. Voltammetric responses of films of different thicknesses were stable within the pH domain 2-7 and reveal redox processes associated with 10 the conducting polymer, the entrapped POMs and incorporated metal ions. The resulting POM doped polypyrrole films were found to be extremely stable towards redox switching between the various redox states associated with the incorporated POM. An amperometric sensor for hydrogen peroxide detection based upon the POM doped polymer films was investigated. The detection limits were 0.3 and 0.6 uM, for the Cu2+- and Fe3+-substituted POM-doped polypyrrole films respectively, with a linear region from 15 0.1 up to 2mM H2O2. Surface characterization of the polymer films was carried out using atomic force microscopy, x-ray photoelectron spectroscopy and scanning electron microscopy

Organic−Inorganic Hybrid Films of the Sulfate Dawson Polyoxometalate, [S2W18O62] 4−, and Polypyrrole for Iodate Electrocatalysis

The Dawson-type sulfate polyoxometalate (POM) [S2W18O62]4− has successfully been entrapped in polypyrrole (PPy) films on glassy carbon electrode (GCE) surfaces through pyrrole electropolymerization. Films of varying POM loadings (i.e., thickness) were grown by chronocoulometry. Film-coated electrodes were then characterized using voltammetry, revealing POM surface coverages ranging from 1.9 to 11.7 × 10−9 mol·cm−2 , and were stable over 100 redox cycles. Typical film morphology and composition were revealed to be porous using atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, and the effects of this porosity on POM redox activity were probed using AC impedance. The hybrid organic− inorganic films exhibited a good electrocatalytic response toward the reduction of iodate with a sensitivity of 0.769 μA·cm−2 ·μM−1 . </p

Nitrate and nitrite electrocatalytic reduction at layer-by-layer films composed of Dawson-type heteropolyanions mono-substituted with transitional metal Ions and silver nanoparticles

A series of Dawson-type heteropolyanions (HPAs) mono-substituted with transitional metal ions (α2-[P2W17O61FeIII]8−, α2-[P2W17O61CuII]8− and α2-[P2W17O61NiII]8−) have exhibited electrocatalytic properties towards nitrate and nitrite reduction in slightly acidic media (pH 4.5). The immobilization of these HPAs into water-processable films developed via layer-by layer assembly with polymer-stabilized silver nanoparticles led to the fabrication of the electrocatalytic interfaces for both nitrate and nitrite reduction. The LBL assembly as well as the changes in the HPA properties by immobilization has been characterized by electrochemical methods. The effects of the substituent ions, outer layers and the cationic moieties utilized for the films assembly of the developed film on the performances of nitrate electrocatalysis has been elucidate

Electrocatalysis by crown-type polyoxometalates multi-substituted by transition metal ions; comparative study

The difference in electrochemical properties of three crown-type polyoxometalates multi-substituted by Fe3+, Ni2+ or Co2+ ions and their precursor has been rationalized with respect to their electrocatalytic performances studied in solution and in the immobilized state within the layer-by-layer film formed with a positively charged pentaerythritol-based Ru(II)-metallodendrimer. The film assembly was monitored with electrochemical methods and characterized by surface analysis techniques. An influence of the terminal layer on the electrode reaction and on film porosity has been observed. The electrocatalytic performance of the compounds on nitrite reduction was assessed in solution and in the immobilized state. (C) 2015 Elsevier Ltd. All rights reserved

Surface Immobilization of a Tetra-Ruthenium Substituted Polyoxometalate Water Oxidation Catalyst Through the Employment of Conducting Polypyrrole and the Layer-by-Layer (LBL) Technique

A tetra Ru-substituted polyoxometalate Na₁₀[{Ru₄O₄(OH)₂(H₂O)₄}­(γ-SiW₁₀O₃₆)₂] (Ru₄POM) has been successfully immobilised onto glassy carbon electrodes and indium tin oxide (ITO) coated glass slides through the employment of a conducting polypyrrole matrix and the layer-by-layer (LBL) technique. The resulting Ru₄POM doped polypyrrole films showed stable redox behavior associated with the Ru centres within the Ru₄POM, whereas, the POM’s tungsten-oxo redox centres were not accessible. The films showed pH dependent redox behavior within the pH range 2–5 whilst exhibiting excellent stability towards redox cycling. The layer-by-layer assembly was constructed onto poly­(diallyldimethylammonium chloride) (PDDA) modified carbon electrodes by alternate depositions of Ru₄POM and a Ru­(II) metallodendrimer. The resulting Ru₄POM assemblies showed stable redox behavior for the redox processes associated with Ru₄POM in the pH range 2–5. The charge transfer resistance of the LBL films was calculated through AC-Impedance. Surface characterization of both the polymer and LBL Ru₄POM films was carried out using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Initial investigations into the ability of the Ru₄POM LBL films to electrocatalytically oxidise water at pH 7 have also been conducted

Redox, surface and electrocatalytic properties of layer-by-layer films based upon Fe(III)-substituted crown polyoxometalate [P8W48O184Fe16(OH)28(H2O)4]20-

The electrocatalytic ability of the iron-substituted crown-type polyoxometalate (POM)Li4K16[P8W48O184Fe16(OH)28(H2O)4]·66H2O·2KCl (P8W48Fe16) towards the reduction of both nitriteand hydrogen peroxide reduction has been studied in both the solution and immobilized states for thePOM. P8W48Fe16was surface immobilised onto carbon electrode surfaces through employment of thelayer-by-layer technique (LBL) using pentaerythritol-based Ru(II)-metallodendrimer [RuD](PF6)8as thecationic layer within the resulting films. The constructed multilayer films have been extensively studiedby various electrochemical techniques and surface based techniques. Cyclic voltammetry and impedancespectroscopy have been utilized to monitor the construction of the LBL film after the deposition of eachmonolayer. The electrochemical behaviour of both a cationic and anionicredox probes at the LBL films has been undertaken to give indications as to the film’s porosity. The ele-mental composition and the surface morphology of the LBL films was conifmrde through the employmentof AFM, XPS and SEM