Malgorzata Chojak

Hybrid organic-inorganic nanocomposite materials for application in solid state electrochemical supercapacitors

Integration into a conducting polymer matrix to form a hybrid material is an effective way to harness the electrochemical activity of nanosized oxide clusters. By anchoring them into polyaniline, the reversible redox chemistry of the otherwise soluble polyoxometalate clusters can be combined with that of the conducting polymer and be put to work in energy storage applications. We present here preliminary results that show how the resulting hybrid polymer displays the combined activity of its organic and inorganic components to store and release charge in a solid state electrochemical capacitor device.

Polyoxometallates as inorganic templates for monolayers and multilayers of ultrathin polyaniline

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.

Electrochemical preparation and characterization of hybrid films composed of Prussian blue type metal hexacyanoferrate and conducting polymer

Abstract 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.

Electrochromic features of hybrid films composed of polyaniline and metal hexacyanoferrate

Abstract Hybrid organic/inorganic films, composed of polyaniline (PANI) matrix and Prussian blue-like nickel hexacyanoferrate redox centers, showed reversible electrochromic behavior in acidic potassium salt electrolytes. The systems coloration properties were assessed from various spectroelectrochemical measurements including voltabsorptometry that involved monitoring of the time-derivative signal of absorbance at 700 and 410 nm as a function of linearly scanned potential. Gold-covered foil was used as a conductive, optically transparent, substrate onto which the composite film was electrodeposited by potential cycling in the mixture for modification consisting of aniline monomer, Ni2+, Fe(CN)63− and electrolyte containing K+ and H+ ions. An important feature of hybrid (composite) material was that its electrochromic properties were dominated by color changes occurring in the PANI component. Coloration originating from nickel hexacyanoferrate barely affected the systems electrochromic characteristics. But the cyanometallate redox centers distributed in the PANI matrix behaved reversibly as expected for a system capable of fast charge transport.

Effective Charge Transport in Poly(3,4-ethylenedioxythiophene) Based Hybrid Films Containing Polyoxometallate Redox Centers

Electrodeposition and electrochemical charging of hybrid organic/inorganic films composed of the poly(3,4-ethylenedioxythiophene), PEDOT, conducting polymer matrix, and Keggin type polyoxometallate, phosphododecamolybdate(PMo 1 2 O 3 - 4 0 ) or phosphododecatungstate (PW 1 2 O 3 - 4 0 ), redox centers, are described under conditions of aqueous solutions. The systems are electropolymerized through potential cycling as thin and moderately thick (μm level) films on electrode surfaces. They are capable of fast charge propagation during redox reactions in strong acid medium (0.5 mol dm - 3 H 2 SO 4 ). The high overall physicochemical stability of PEDOT is explored to produce a robust, conductive, matrix for such polynuclear mixed-valence inorganic nanostructures as PMo 1 2 O 3 - 4 0 and PW 1 2 O 3 - 4 0 . The composite (hybrid) materials are stabilized due to the existence of electrostatic attraction between anionic phosphomolybdate or phosphotungstate units and positively charged conducting polymer (oxidized). Charge transport is facilitated by the fact that the reversible and fast redox reactions of polyoxometallate appear in the potential range where PEDOT is conductive. The effective diffusion coefficients are on the level 4 X 10 - 8 cm 2 s - 1 . The whole concept may lead to the fabrication of composite (hybrid) films that are capable of effective accumulation and propagation of charge in redox capacitors.

Protection of Steel Against Corrosion in Aggressive Medium by Surface Modification with Multilayer Polyaniline-Based Composite Film

Robust, polyaniline-based, multilayer composite films containing Prussian blue and hexacyanoferrate anions are considered here for the protection of stainless steel (SS) against pitting corrosion in strongly acid media. Polyaniline serves as a stable host matrix for large inorganic anions. At the interface formed by a composite film with SS, the interaction of hexacyanoferrate(II,III) - with iron(II,III), and some chromium(III), results in the formation of a thin insoluble metal hexacyanoferrate (mostly Prussian blue) layer. This phenomenon leads to improved passivation of the steel, and it strongly inhibits metal dissolution even in the chloride acid media. Because the metal hexacyanoferrate deposits are electronically conducting, further electropolymerization of dense and adherent composite polyaniline layers with hexacyanoferrate has been readily achieved. Due to the existence of electrostatic repulsion between negatively charged hexacyanoferrate sites and pitting-causing chloride anions, their transport through the composite film and access to the surface of SS are largely blocked.

Platinization assisted by Keggin-type heteropolytungstates

Abstract Chronoamperometry and cyclic voltammetry (CV) were used to address the platinum deposition kinetics on glassy carbon and polycrystalline Pt supports from H 2 PtCl 6 -containing solutions with and without additives of different Keggin-type heteropoly acids (HPAs) of tungsten. The effect of addition of these polyoxometallates manifested itself in the pronounced disturbance of instantaneous nucleation and it can be interpreted in terms of strong HPA adsorption on certain nucleation centers followed by the modification of their activity. HPA additives decreased the current efficiency for Pt deposition despite the fact that the reduction of HPA additives was thermodynamically forbidden at the potentials chosen for electrodeposition. The latter fact can be explained as selective inhibition of the intermediate Pt II reduction by tungstates. CV of Pt deposits in sulfuric acid solutions indicated a noticeable increase in the deposit roughness if P- and Si-containing HPA were added in the course of platinization. More complex behavior was found for B -dodecatungstate that most likely originated from its tendency to undergo codeposition with Pt. Ex situ scanning tunneling microscopy (STM) images demonstrate more pronounced globular structure and smaller crystal sizes for HPA-affected deposits as compared with usual platinized platinum. These features can be explained by HPA effects on deposition kinetics assumed on the basis of electrochemical data and demonstrate that HPA can act in template-like manner.

Spatial Control of Polyaniline Electrodeposition by Patterned Polyoxometalate Monolayers

Electro-oxidative polymerization of aniline is facilitated by modification of electrode surfaces with an inorganic polyoxometalate, namely with self-assembled monolayers of dodecamolybdophosphate, PMo 12 O 3- 40 and dodecatungstophosphate PW 12 O 3- 40 . Introduction of a protonated and therefore positively charged 1,12 diaminododecane [NH 2 -(CH 2 ) 12 -NH 2 ] adlayer can invert the interfacial charge and significantly inhibits the access of anilinium monomers. Consequently, the deposition of polyaniline with spatial selectivity is feasible, and preferential growth of the polymer on electrode surfaces modified with polyoxometalate has been observed. Microcontact printed patterns of the polyoxometalate or of 1,12 diaminododecane on top of a full polyoxometalate monolayer allows the electropolymerization of aniline to be triggered or inhibited locally.

New Strategies to Activation and Assembling of Catalytic Metal Nanoparticles through Modification with Polyoxometallate Monolayers

The modification of Pt nanoparticles of ca. 8 nm diameter by adsorbing monolayers of phosphododecatungstic acid (H3PW12O40) on their surfaces tends to activate them towards efficient electrocatalytic reduction of oxygen in acid medium. The modification steps were be performed either before or after introduction of catalytic nanoparticles onto the glassy carbon substrates. Formation of a stable colloidal suspension of phosphotungstate protected Pt-nanoparticles as a source of activated catalytic centers was required in the first case; whereas, in the second case, bare Pt nanoparticles had been deposited on glassy carbon before they were subsequently modified with ultrathin films of H3PW12O40. Rotating disk voltammetry was used to probe the electroreduction of dioxygen in 0.5 mol dm H2SO4 at 25C. In addition to the 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.