Krzysztof Miecznikowski

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.

Quartz crystal microbalance monitoring of mass transport during redox processes of cyanometallate modified electrodes: complex charge transport in nickel hexacyanoferrate films

The electrochemical quartz crystal microbalance has been employed to monitor the growth of nickel hexacyanoferrate films and to study the mechanism of mass transfer during the systems redox reactions in supporting electrolytes containing the same concentrations of potassium ions and various anions, sulfates, chlorides or nitrates. Although the results are consistent with the general view that charge compensation during electron transfer is primarily achieved through the flux of potassium cations, the actual mechanism of charge propagation in potassium salt electrolytes seems to be more complex at higher potentials where a second redox reaction of nickel hexacyanoferrate occurs. Following careful analysis of microgravimetric data, which includes determination of apparent molar masses and estimation of transport numbers, we postulate the existence of the counterflux of either solvent molecules or anions. The involvement of anions seems to be the most pronounced in the presence of sulfates. The microgravimetric results have also been supported with kinetic data such as effective diffusion coefficients (obtained using chronocoulometry) and with the Nernst plot analysis of voltammetric mid-peak potentials (obtained using modified ultramicroelectrodes) giving insight into the ion-exchange properties of nickel hexacyanoferrate. In all cases, the systems characteristics, which also include the dynamics of the film growth, are different in the presence of sulfates when compared to chlorides or nitrates.

Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium.

Mixed nickel/palladium hexacyanoferrates have been prepared both as thin films and bulk precipitates (powders) attached to electrode surfaces. The mixed material does not seem to be a simple mixture of hexacyanoferrates of nickel and palladium, and it shows unique voltammetric and electrochromic characteristics when compared with the respective single-metal hexacyanoferrates. Electrodeposition of a mixed film is achieved by potential cycling in the solution for modification containing nickel(II), palladium(II) and hexacyanoferrate(III). It comes from elemental analysis that, in general, the stoichiometric ratios of nickel to palladium in mixed metal hexacyanoferrate films reflect relative concentrations of Pd(II) and Ni(II) in the solutions for modification. In the case of the films that have been electrodeposited from the solutions containing palladium ions in amounts lower or comparable with those of nickel ions, the mechanism of film growth seems to involve formation of nickel hexacyanoferrate during negative potential scans followed by simultaneous insertion of palladium ions as countercations into the system. In such cases, palladium ions tend to substitute potassium countercations at interstitial positions in the electrodeposited nickel hexacyanoferrate microstructures. We have determined the following stoichiometric formula, K1.74−2yPdIIyNiII1.13[FeII(CN)6] (where y<0.72) for such films. At higher molar fractions of palladium in solutions for modification, the formation of a mixed phase of nickel/palladium hexacyanoferrate (in which both nickel(II) and palladium(II) are nitrogen-coordinated within the cyanometallate lattice) is expected. This seems to be more probable than simple codeposition of separate palladium hexacyanoferrate and nickel hexacyanoferrate microstructures during the film growth. Mixed (composite) nickel/palladium hexacyanoferrate films show long-term stability as well as promising charge storage and transport capabilities during voltammetric potential cycling. Well-defined and reversible cyclic voltammetric responses have been obtained in lithium, sodium and potassium electrolytes.

Countercation‐Sensitive Electrochromism of Cobalt Hexacyanoferrate Films

Cobalt(II) hexacyanoferrate(III,II) a system analogous to prussian blue, is a unique electrochromic material: its color is not only dependent on the oxidation potential, but also on the nature of the countercations sorbed from electrolyte during reduction. The electrodeposition of cobalt hexacyanoferrate thin films, their voltammetric behavior and spectroelectrochemical identity are reported here in potassium and sodium electrolytes. The oxidized film is purple brown in both electrolytes, but following reduction, the system turns olive-brown in 1 M KCl and becomes green in 1 M NaCl.

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.

Enhancement of the Electrocatalytic Oxidation of Methanol at Pt ∕ Ru Nanoparticles Immobilized in Different WO3 Matrices

The importance of morphology and electrochemical characteristics of tungsten oxide as a support for the catalytic platinum/ ruthenium nanoparticles during electro-oxidation of methanol in acid medium is addressed here. Bimetallic Pt/Ru nanoparticles were immobilized at the same loadings in three different WO 3 matrices. Application of high voltages (up to 40 V at 1 V s -1 ) to tungsten foil in H 2 SO 4 solution containing a small amount of NaF (0.15 wt %) resulted in the formation of highly ordered nanoporous WO 3 . Compact tungsten oxide was produced by application of high voltages to tungsten foil in the absence of NaF. Parallel measurements were done with conventional thin films of microporous tungsten oxide electrodeposited on a glassy carbon substrate from the colloidal Na 2 WO 4 solution in H 2 SO 4 . While redox reactions of the structures generated by high voltage anodization were characteristic of poorly hydrated WO 3 (that could be reduced partially to substoichiometric oxides, WO 3-v ), the conventionally electrodeposited microporous structures behaved more like hydrated WO 3 (that could be reduced to nonstoichiometric hydrogen bronzes, H x WO 3 ). Contrary to compact structures (that were blocking Pt/Ru reactivity), both ordered nanoporous and electrodeposited microporous WO 3 matrices, that were more open and characterized by high surface areas, significantly enhanced the electrocatalytic (chronoamperometric, voltammetric) currents for methanol oxidation. Among important advantages of the system produced by assembling Pt/Ru in nanoporous WO 3 are its rigidity, long-term stability, and the ability to oxidize methanol at less positive potentials.

Hybrid Metal Cyanometallates Electrochemical Charging and Spectrochemical Identity of Heteronuclear Nickel/Cobalt Hexacyanoferrate

Hybrid metal nickel/cobalt hexacyanoferrate (Ni/CoHCNFe) has been prepared in the form of thin films on electrode surfaces and as bulk precipitates (powders). This heteronuclear metal hexacyanoferrate cannot be considered a simple mixture of hexacyanoferrates of nickel and cobalt, and it shows different voltammetric and electrochromic characteristics in comparison to the respective single-metal hexacyanoferrates. On the basis of X-ray absorption, elemental analysis, and comparative voltammetric measurements, the following approximate formulas, K 2 Ni 0.5 II Co 0.5 II [Fe II (CN)] 6 and KNi 0.5 II Co 0.5 II [Fe III (CN) 6 ], have been proposed for the predominant reduced and oxidized forms of hybrid Ni/CoHCNFe. The results are consistent with the existence of linear units -Fe-CN-Co-NC-Fe-CN-Ni- in the structure. Hybrid nickel/cobalt hexacyanoferrate film has unique electrochromic properties, and it shows long-term stability and promising charge storage/transport capabilities during voltammetric potential cycling.

Quartz crystal microbalance study of the growth of indium hexacyanoferrate films during electrodeposition and coagulation

Abstract An electrochemical quartz crystal microbalance and cyclic voltammetry have been used to monitor fabrication of indium(III) hexacyanoferrate(III,II) films by electrodeposition through potential cycling and by interfacial coagulation during ageing in the modification solution containing potassium electrolyte, indium(III) and hexacyanoferrate(III). We have chosen indium hexacyanoferrate as a model system since its cyclic voltammetry shows a single set of reversible, well-defined peaks significantly separated from the solution responses. Simultaneous estimations of charge and mass have been used to characterize the efficiency of the deposition processes. Microgravimetry is particularly attractive for monitoring the film growth due to interfacial coagulation (at open circuit). Electrodeposition by potential cycling permits fast preparation of thin films. The coagulation method is useful for fabrication of thick films. We also discuss the response of the resulting indium hexacyanoferrate film in the supporting electrolyte. The data are consistent with the insertion and exclusion of K+ countercations during the systems reduction and oxidation respectively.