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By modification of surfaces of multi-walled carbon nanotubes with ultra-thin monolayer-type films of phosphododecamolybdic acid, H 3 PMo 12 O 40 , an electrode material with improved capacitance properties is produced. It is apparent from three distinct test experiments (based on cyclic voltammetry, galavanostatic charging–discharging and AC impedance) that capacitors utilizing H 3 PMo 12 O 40 -modified carbon nanotubes are characterized by specific capacitances and energy densities on the levels of 40 F g −1 and 1.3 Wh kg −1 , whereas the respective values for the systems built from bare carbon nanotubes are lower, 22 F g −1 and 0.7 Wh kg −1 . It is reasonable to expect that fast and reversible multi-electron transfers of the Keggin-type H 3 PMo 12 O 40 account for the pseudocapacitance effect and significantly contribute to the observed overall capacitance.

Combination of multi-walled carbon nanotubes, cobalt porphyrin, and peroxidase (horseradish, cabbage) enzyme in the film (deposited onto glassy carbon electrode substrate) produces a bio-electrocatalytic system capable of effective reduction of oxygen in such neutral media as 0.1 mol dm −3 KCl and 0.1 mol dm −3 KCl + 0.01 citrate buffer (pH 6). Carbon nanotubes have been modified with ultra-thin layers of 4-(pyrrole-1-yl) benzoic acid, or phosphododecamolybdate, to form stable colloidal suspensions of carbon nanostructures. The resulting inks have been utilized during sequential deposition of components. Co-existence of cobalt porphyrin, peroxidase enzyme together with dispersed carbon nanotubes leads to synergistic effect that is evident from some positive shift of the oxygen reduction voltammetric potentials (more than 50 mV in citrate buffer) and significant (ca. twice) increase of voltammetric currents (relative to those of the enzyme-free system). The multi-component bio-electrocatalytic film has also exhibited relatively higher activity towards reduction of hydrogen peroxide. It is reasonable to expect that the reduction of oxygen is initiated at cobalt porphyrin redox centers, and the undesirable hydrogen peroxide intermediate is further reduced at the horseradish or cabbage peroxidase enzymatic sites. An important function of carbon nanotubes is to improve transport of electrons within the bio-electrocatalytic multi-component film.

Surfaces of bulk platinum and unsupported (Vulcan-free) Pt nanoparticles, that are modified and stabilized with such Keggin type heteropolyacids of molybdenum and tungsten as H3PMo12O40 and H3PW12O40, have been characterized using cyclic voltammetry, FTIR (by reflectance), as well as transmission and scanning electron microscopies. The presence of the polyoxometallate monolayer on platinum results in the partial suppression of the interfacial formation of PtOH/PtO oxides. Both molybdates and tungstates seem to interact with Pt surface via their corner oxygen atoms. The existence of spacious, largely hydrated, polyoxometallate monolayers on platinum does not block access of reactant (oxygen) to the catalytic Pt sites. The electrocatalytic properties of H3PMo12O40 and H3PW12O40 modified Pt nanoparticles towards reduction of oxygen in acid medium have been examined and compared using rotating ring–disk voltammetry. Reactivity of Pt-free H3PMo12O40 and H3PW12O40 with respect to reduction of hydrogen peroxide has also been considered. Our results clearly show that modification of Pt nanoparticles with PW12 (but not with PMo12) results in the enhancement of the electrocatalytic reduction of oxygen. © 2007 Elsevier Ltd. All rights reserved.

Nafion stabilized inks of Vulcan XC-72 supported platinum (20 wt.%) nanoparticles (Pt/XC-72) were utilized to produce electrocatalytic films on glassy carbon. The catalysts were modified (activated) with phosphododecatungstic acid H 3 PW 12 O 40 (PW 12 ). Comparison was made to bare (PW 12 -free) electrocatalytic films. Electroreduction of dioxygen was studied at 25 °C in 0.5 mol dm −3 H 2 SO 4 electrolyte using rotating disk voltammetry. For the same loading of platinum (≈95 μg cm −2 ) and for the approximately identical distribution of the catalyst, the reduction of oxygen at a glassy carbon electrode modified with the ink containing PW 12 proceeded at ca. 30–60 mV more positive potential (depending on the PW 12 content), and the system was characterized by a higher kinetic parameter (rate of heterogeneous electron transfer), when compared to the PW 12 -free electrocatalyst. Gas diffusion electrodes with Pt/XC-72 supported on carbon paper (Pt loading 1 mg cm −2 ) were also tested. Under the same experimental conditions, while the exchange current density and the total resistance contribution to polarization components, computed from the galvanostatic polarization curves were found to be clearly higher and lower, respectively, for the ink modified with PW 12 relative to the unmodified system. The results demonstrate that addition of heteropolytungstatic acid (together with Nafion) enhances the electrocatalytic activity of platinum towards reduction of oxygen.

Platinum nanoparticles of ca. 8 nm diameter (that had been first deposited on glassy carbon) were subsequently modified with ultra-thin films of polyoxometallates through their spontaneous adsorption on solid (platinum and carbon) surfaces. The following polyoxometallates (Keggin type heteropolyacids), H3SiW12O40, H3SiMo12O40, H3PW12O40 and H3PMo12O40, were considered as potential activating agents. Rotating disk voltammetry was used to probe the electroreduction of dioxygen in 0.5 mol dm−3 H2SO4 at 25 °C. For the same loading and the approximately identical distribution of platinum nanoparticles on glassy carbon, the statistically higher (in comparison to bare Pt) electrocatalytic currents for the oxygen reduction were observed upon introduction of monolayers of heteropolyanions. Out of polyoxometallates considered, the system modified with heteropolytungstate (H3PW12O40) seemed to be the most effective in electrocatalysis. Although the possibility of structural changes cannot be excluded, the synergistic effect originated presumably from the bifunctional activity of the electrocatalyst. While Pt retained its usual reactivity towards the oxygen reduction, H3PW12O40 could act as both effective mediator (e.g. for the reduction of the hydrogen peroxide intermediate) and the source of mobile protons at the electrocatalytic interface.

We explore oxidative electrocatalytic properties of a system consisting of bimetallic Pt/Ru nanoparticles dispersed over a nanotubular self-organized TiO2 matrix. The nanotubular TiO2 layers consist of individual tubes of 100 nm diameter, 500 nm length and 15 nm wall thickness. This nanotubular TiO2 support provides a high surface area and it significantly enhances the electrocatalytic activity of Pt/Ru for methanol oxidation (relative to the performance of Pt/Ru at the same loading but immobilized on a conventional compact TiO2 support). Annealed to anatase, the TiO2 nanotubular support exhibits even higher enhancement effect during electrooxidation of methanol than when used in the “as-formed” amorphous structure. The overall electrocatalytic activity of the system can be further increased by illumination with UV-light (wavelength 325 nm). Keywords: Titanium dioxide, Nanotubes, Bimetallic Pt/Ru nanoparticles, Electrocatalysis, Methanol oxidation

To fabricate electrocatalytic network films containing Pt nanoparticles, the ability of a Keggin-type polyoxometallate, phosphododecatungstate (PW 12 O 40 3- ), to form stable anionic monolayers on solid surfaces is explored. Three-dimensional assemblies on electrode (glassy carbon or platinum) surfaces are grown using the layer-by-layer method involving repeated alternate treatments in the solution of PW 12 O 40 3- (or in the colloidal suspension of polyoxometallate-protected Pt-nanoparticles) and in the solution of monomer (e.g., anilinium) cations. In the resulting structured (organic-inorganic) films, the layers of negatively charged polyoxometallate, or polyoxometallate-protected (stabilized) Pt-nanoparticles, interact electrostatically with the ultra-thin layers of such a positively charged conducting polymer as polyaniline. Consequently, physicochemical properties of organic conducting polymers, and reactivities of inorganic polyoxometallate and/or noble metal particles can be combined. The modification of Pt nanoparticles by adsorbing monolayers of phosphododecatungstate tends to activate them towards efficient electrocatalytic reduction of oxygen in acid medium.

A novel polymetacrylate-based redox-conducting polymeric gel, into which Keggin-type polyoxometallate, phosphododecatungstic acid (H 3 PW 12 O 40 ), had been incorporated, was electrochemically characterized in the absence of external liquid supporting electrolyte using an ultramicrodisk-working electrode. The phosphotungstate component (15 wt.% of the gel block) was entrapped as the polar organic solvent solution within pores of the polymer matrix. H 3 PW 12 O 40 plays bifunctional role: it provides well-behaved redox centers and serves as strong acid (source of mobile protons). The solid-state voltammetric properties of the system are defined by the reversible one-electron transfers between phosphotungstate redox centers. The following parameters have been determined from the combination of potential step experiments performed in two limiting (radial and linear) diffusional regimes: the concentration of heteropolytungstate redox centers, 6 × 10 −2 mol dm −3 , and the apparent diffusion coefficient, 5 × 10 −7 cm 2 s −1 . The room temperature ionic (protonic) conductivity of the bulk gel was equal to 1.6 × 10 −3 S cm −1 . The charge propagation mechanism was found to be primarily controlled by physical diffusion of heteropolytungstate units within the gel pores rather than by electron hopping (self-exchange) between mixed-valence sites.

The ability of a polyoxometallate (dodecamolybdophosphate) to form negatively charged monolayers on solid electrode surfaces is explored here to perform immobilization of monomeric (anilinium) units followed by electropolymerization within the monolayer. Consequently, hybrid films containing ultrathin conducting polymer (polyaniline) layers can be formed. By repeated and alternate treatments in solutions of dodecamolybdophosphate anions and anilinium cations, the amount of the material can be increased systematically in a controlled fashion leading to stable three-dimensional multilayer hybrid assemblies. The fact, that formal potentials of the dodecamolybdophosphate redox processes appear in the potential range where polyaniline is conductive, allows the system to operate reversibly and reproducibly in acid electrolyte. Keywords: Phosphomolybdate, Self-assembled monolayer, Nanostructured polyaniline, Ultrathin composite films, Organized multilayers

We propose a composite bifunctional electrocatalytic film that contains platinum microparticles and oxocyanoruthenium microstructures dispersed in the reactive matrix of non-stochiometric tungsten(VI,V) oxides. Polynuclear oxocyanoruthenium can be viewed as a ruthenium analogue of Prussian blue or, more precisely, ruthenium(II,III,IV) hexacyanoruthenate(II,III), that features ruthenium oxo species within the cyanide bridged network. The composite film was fabricated by sequential electrodeposition starting from the preparation of tungsten oxide film into which oxocyanoruthenium and metallic platinum (from chloroplatinate) centers were introduced. To evaluate the reactivity of such a hybrid system toward the oxidation of methanol at fairly low potentials (0.1–0.3 V vs. SCE), cyclic voltammetric and chronoamperometric measurements have been performed at 22 and 60 °C. With approximately the same platinum loading and under analogous conditions, the composite film shows higher catalytic activity than a simple platinized tungsten oxide. It is possible that ruthenium hydroxo species generated within polynuclear oxocyanoruthenium microstructures exhibit an activating effect on neighboring dispersed platinum microparticles. An alternative explanation may involve the possibility of different morphologies of the catalytic films in the presence and absence of oxocyanoruthenium species. At higher potentials, ruthenium(IV)-oxo centers participate directly in the oxidation of methanol. The protonically/electronically conducting and physicochemically stable tungsten oxide/hydrogen bronze matrix is expected to support the activity of platinum and ruthenium centers.

We propose novel composite (hybrid) organic/inorganic systems that can be fabricated as thin and moderately thick (μm level) films on electrode surfaces. During electrodeposition by potential cycling alternate layers of polyanilne and metal hexacyanoferrate are produced. Polyaniline can serve as a robust, conductive, matrix for such polynuclear mixed-valence inorganic redox centers as nickel(II)hexacyanoferrate(III,II). Due to the existence of electrostatic attraction between the negatively charged metal hexacyanoferrate and the positively charge polyaniline (partially oxidized), the composite material cannot be considered as a simple mixture of nickel hexacyanoferrate and the conducting polymer. It comes from atomic force microscopic studies that the morphology of the composite film is granular, but its structure is fairly dense. The fact, that the formal potential of nickel hexacyanoferrate redox process lies in the potential range where polyaniline is conductive, allows the system to operate reversibly and reproducibly in acid electrolytes containing potassium cations. The whole concept may lead to the fabrication of composite (hybrid) films that are capable of effective accumulation of charge and show high current densities at electrochemical interfaces.