Marcin A. Malik

Tungsten Oxides as Active Supports for Highly Dispersed Platinum Microcenters: Electrocatalytic Reactivity Toward Reduction of Hydrogen Peroxide and Oxygen

The electrochemical deposition of spatially dispersed platinum centers within a tungsten oxide film onto the surface of a carbon substrate is described. The approach explores cathodic fabrication of metallic Pt microparticles via anodic dissolution of a Pt counterelectrode. Nonstoichiometric tungsten(VI,V) oxides are simultaneously electrodeposited with Pt particles during reductive potential cycles. We describe electrocatalytic properties of the resulting composite films toward reduction of dioxygen. We also demonstrate that a Pt-free tungsten oxide film shows strong electrocatalytic activity for reduction of hydrogen peroxide. The method permits preparation of films containing platinum microcenters at low loadings (ca. 5 μg cm 2 ) within the reactive oxide matrix. We postulate bifunctional activity of the system in terms of promoting catalytic reduction of oxygen (by the traces of platinum) and reduction of any hydrogen peroxide intermediate [by the W(VI,V) oxide matrix].

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.

Microgravimetric monitoring of transport of cations during redox reactions of indium(III) hexacyanoferrate(III,II) Radiotracer evidence for the flux of anions in the film

Abstract We demonstrate here that, primarily, electrolyte cations but also, to some extent, anions are capable of penetrating indium hexacyanoferrate films during redox reactions. We find from electrochemical quartz crystal microbalance measurements that the electrolyte cation (K+) undergoes sorption and desorption during the systems reduction and oxidation, respectively. The formal potential, which has been determined from the systems well-defined voltammetric peaks recorded with the use of an ultramicroelectrode, decreases ∼40 mV per decade of decreasing K+ concentration. The latter value is lower than the 60 mV change expected for the involvement of a cation in the reaction mechanism according to the ideal Nernstian dependence. We also demonstrate, using 35-S labelled sulfate, that anion penetrates the reduced film and its concentration markedly increases during oxidation. Careful examination of cyclic voltammetric responses of the system shows that, in addition to the well-defined peaks, capacitance-like currents appear during oxidation. During reduction anion is largely expelled from the film. This complex ionic penetration and transport in indium hexacynoferrate may be explained in terms of formation of two forms: ‘soluble’, KInIII[FeII(CN)6], and ‘insoluble’ (‘normal’), InIII4[FeII(CN)6]3, during the electrochemical growth or potential cycling of the films. These forms would require cations and anions, respectively, to provide charge balance during reactions. Regardless of the actual mechanism, penetration of anions cannot be neglected completely in the discussion of charging of indium hexacyanoferrate.

Spectroelectrochemical characterization of cobalt hexacyanoferrate films in potassium salt electrolyte

Cobalt(II) hexacyanoferrate(III, II) films show reversible electrochromic behavior in potassium salt electrolyte. Gold-covered foil was used as a conductive, optically transparent substrate onto which cobalt hexacyanoferrate films were deposited by a slow coagulation method. Both voltammetric and spectroelectrochemical results are consistent with the existence of two cobalt hexacyanoferrate forms, presumably KCoII1.5[FeII(CN)6] and K2CoII[FeII(CN)6]. The separate nature of the redox reactions of these forms is clearly evident from spectroelectrochemical measurements, particularly from voltabsorptometry, which involves monitoring of the time-derivative signal of absorbance as a function of the linearly scanned potential. Combination of voltabsorptometry with voltammetry allows determinations of molar absorptivity and film loading, and it permits changes of concentration of colored redox centers vs the applied potential to be monitored.

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.

Interfacial electron transfer and bulk charge propagation in solid-state voltammetry of mixed-valence inorganic material☆

Abstract We demonstrate the usefulness of cyclic voltammetric measurements for characterization of a model, mixed-valence protonically conducting solid, tetragonal phospho-12-tungstic acid single crystal, H3PW12O40·29H2O, in the absence of liquid electrolyte phase. The measurements were performed in an all-solid cell which utilizes a carbon fiber ultramicrodisk (diameter 12 μm) working electrode, a silver disk semi-reference electrode, and a glassy carbon ring counter electrode. Diagnostic experiments at various scan rates aimed at probing the model of mass transport and potential kinetic limitations. Such bulk parameters as the effective diffusion coefficient for charge propagation (kinetic) and concentration of mixed-valence redox sites (analytical) were determined from the combination of voltammetric experiments. They were carried out in two time regimes related to the application of slow (0.5 mV s−1) and fast (34 V s−1) scan rates. A modified Nicholson method was used to estimate the interfacial parameter, standard heterogeneous rate constant for electron transfer between the ultramicroelectrode and tungsten redox sites in the crystal. The results are consistent with high dynamics of bulk charge propagation (4 × 10−7cm2s−1) and fast interfacial electron transfer (10−1cm s−1). Simulation of cyclic voltammograms was performed using the calculated kinetic and analytical parameters. The reliability of estimations is discussed. Comparative determinations were done on the standard, highly concentrated (0.86 mol dm−3) phospho-12-tungstic acid solution.