Karl Doblhofer

Polypyrrole-based electrode coatings switchable electrochemically between the anion- and cation-exchanger states

Dodecylsulphate was incorporated into a polypyrrole matrix during anodic polymerization of pyrrole. The concentration was adjusted such that in the oxidized film about one half of the polypyrrole cationic groups was charge compensated by the sulphate groups from the incorporated dodecylsulphate, the second half by anions from the electrolyte. Upon electrochemical oxidation/reduction, the film is reversibly “switched” between a cation-exchanger (at negative electrode potentials), and an anion exchanger (at more positive potentials) state. The different states of the film are characterized by electrochemical, EDAX, and in particular by Volta-potential measurements. The Donnan potentials determined with the latter techniques as a function of electrode potential and electrolyte concentration were in good agreement with calculated values.

Autocatalytic mechanism of H2O2 reduction on Ag electrodes in acidic electrolyte: experiments and simulations

It is shown that the cathodic reduction of hydrogen peroxide (H2O2) on silver electrodes in acidic electrolyte can proceed by two parallel mechanisms: first, by the ‘normal’ mechanism that has been discussed in the literature, second, by a novel mechanism proceeding at a significantly more positive potential. It is proposed that the second mechanism involves the activating adsorbate (OH)ad, that forms in the course of the H2O2 reduction reaction as an unstable intermediate. The coverage of the electrode with (OH)ad increases with the rate of H2O2 reduction, i.e., the process is autocatalytic. At more negative potentials the coverage decreases as the rate of adsorbate reduction/desorption rises. This leads to a potential region of negative differential charge-transfer resistance and thus to complex dynamic phenomena, in particular to electrochemical oscillations. Model calculations based on these considerations yield the potential dependent OH-adsorption, the N-shaped current/voltage curves and current oscillations that agree well with the experimental findings.

Copper ion reduction catalyzed by chloride ions

Abstract The cathodic reduction of copper(II) is studied in the absence and in the presence of chloride ions (1×10−3≤cCl≤8×10−3 M) in an electrolyte that is comparable to galvanic copper baths (2.2 M H2SO4+0.3 M CuSO4). Experiments conducted with the electrochemical quartz-microbalance, largely under conditions of cyclic voltammetry, demonstrate the formation and reduction of CuCl on the copper surface. A mechanism of Cu(II) reduction with intermediate formation of CuCl is postulated. Its rate is comparable to that of chloride-free Cu(II) reduction. The enhancement of the reduction current observed in the presence of chloride is proposed to result from the fact that the mechanism involving CuCl mediation proceeds in parallel with the chloride-free path.

In situ Raman spectroscopy studies of the interface between silver(111) electrodes and alkaline NaF electrolytes

Abstract The electrochemical interface between Ag(111) single crystal and NaF + NaOH electrolytes at various pH values has been studied using cyclic voltammetry and in situ Raman spectroscopy. Submonolayer oxidation starts well below bulk silver oxide formation at potential about −0.6 V vs. Hg|HgO electrode at pH 11. Two potential-dependent Raman bands ν 1 at 540 to 560 cm −1 and ν 2 at 803 to 819 cm −1 in basic NaF electrolytes are attributed to AgOH stretching and AgOH bending vibrations of electrochemisorbed hydroxide species. Strong isotope effect is observed in D 2 O solutions, ν 2 being shifted to values of 550 to 570 cm −1 . Fluoride stabilizes the adsorption of hydroxide species. A multistep scheme is proposed that describes the mechanism of formation of hydroxide/oxide species at the considered silver-electrode surface.

The membrane properties of Prussian Blue films on electrodes

Abstract The membrane properties of the “Prussian Blue” [iron(III, II)hexacyanoferrate(II, III)] system, deposited as thin films onto electrodes, were studied with conventional electrochemical techniques and with Volta-potential measurements conducted on the emersed coated electrodes. In particular, the Volta-potential (work function) measurements demonstrate that the coating has cation-exchange properties, and the potential drop across the film/electrolyte interface can be considered to be a Donnan potential. Up to the highly oxidized mixed-valence form “Berlin Green” (at 0.9 V/SCE) the polynuclear microstructure carries negative fixed charges and thus contains mobile cations as counter ions. The results support the formulas K in1 3 {Fe III [Fe III (CN) 6 ]} in2 3 {Fe III [Fe II (CN 6 ]} in1 3 for Berlin Green at 0.9 V/SCE, and K{Fe III [Fe II (CN) 6 ]} for Prussian Blue, with K + as the exchangeable counter ion.

The effect of incorporated negative fixed charges on the membrane properties of polypyrrole films

Polypyrrole (PPy) and poly-N-methylpyrrole (PMPy) films have been prepared on electrodes in NaCl, polystyrenesulphonate (PSS), and dodecylsulphate (ROSO3–) electrolytes. The redox-switching of these coatings has been studied, mainly in NaCl electrolytes. At various electrode potentials the coated electrodes were withdrawn from the solutions to determine the composition of the films. Volta potential measurements, were conducted to obtain information on the Donnan potential. Based on these measurements, the effect of the negative fixed charge groups incorporated into the PPy or PMPy matrix on the membrane properties of the films was analysed. The anion exchange, cation exchange, and neutral states of the various coatings are defined over a range of electrode potentials. In one situation (PPy/PMPy/ROSO3–), the reversible and complete electrochemical redox transition between anion- and cation-exchange character is demonstrated.

An EQCM Study of the Electrochemical Copper(II)/Copper(I)/Copper System in the Presence of PEG and Chloride Ions

The charge-transfer reaction between copper(II) and copper electrodes is studied in electrolytes that are similar to galvanic copper baths, 2.2 M H 2 SO 4 + 0.3 M CuSO 4 + chloride ions (c C1 ≤ 1 × 10 -2 M), and polyethyleneglycol 1500 (PEG, c PEG ≤ 4 X 10 -3 M). Electrochemical quartz crystal microbalance (EQCM) measurements are conducted, mainly under conditions of cyclic voltammetry. The formation and dissolution of CuCI on the electrode surface at c C1 ≥ 2 mM is demonstrated, a notable shift of the pseudo-equilibrium potential associated with CuCI deposition is analyzed, and the inhibition of the charge-transfer reaction by the PEG/Cl - surface layer is characterized. It is shown that the inhibiting layer forms by reaction between the adsorbate-covered copper electrode and PEG, i.e., neither Cu + nor Cu ++ from the electrolyte are required. Numerical simulations of the processes as well as parallel experiments conducted with electrolytes not containing Cu(II) support the proposed mechanisms, in particular the role of the intermediate Cu + .

On the mechanism of Ag(111) sub-monolayer oxidation: a combined electrochemical, in situ SERS and ex situ XPS study

Abstract In the present work in situ surface enhanced Raman spectroscopy (SERS), ex situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) are used to study the interface between a Ag(111) electrode and an alkaline electrolyte. Formation of a number of potential-dependent adsorbates is observed above the point of zero charge ( E pzc ) of the Ag electrode. These are: OH groups (OH ads γ − ) and oxide-like species (O ads δ − ). Electrochemisorbed hydroxide species show by the appearance of Raman bands at 540–560 cm −1 and at 803–819 cm −1 , attributed to Ag–OH stretching and AgO–H bending vibrations respectively. Strong isotope shift of the Raman bands towards lower frequencies is observed in D 2 O solutions, proving their assignment. The O ads δ − and OH ads γ − species are characterised by the O 1s peaks at ca. 529.5 and 531.6, respectively. Formation of the above-mentioned species is verified also by the UP spectra of the emersed electrodes, showing the bands at 3.0 eV typical for the oxide-like adsorbates and 9.0 and 11.1 eV for hydroxo-groups. The OH ads γ − and O ads δ − species are negatively charged, as evidenced by the adsorption of Na + on the Ag electrode positive to the E pzc . A mechanism of the Ag(111) sub-monolayer oxidation is suggested on the basis of combined evidence from cyclic voltammetry, in situ SERS, ex situ XPS and UPS.

Two-Dimensional Imaging of Potential Waves in Electrochemical Systems by Surface Plasmon Microscopy

The potential dependence of resonance conditions for the excitation of surface plasmons was exploited to obtain two-dimensional images of the potential distribution of an electrode with high temporal resolution. This method allows the study of spatiotemporal patterns in electrochemical systems. Potential waves traveling across the electrode with a speed on the order of meters per second were observed in the bistable regime of an oscillatory electrochemical reaction. This velocity is close to that of excitation waves in nerve fibers and is far greater than the velocity of reaction-diffusion waves observed in other chemical systems.

Kinetics and mechanism of charge-transfer reactions between conducting polymers and redox ions in electrolytes

Charge-transfer reactions of selected ionic redox couples were studied on rotating glassy carbon electrodes covered with conducting polymers of different membrane properties: poly-N-methylpyrrole as the anion exchanger or poly-N-methylpyrrole with immobilized poly(4-styrenesulfonate) ions as the cation exchanging matrix. The electrochemical and spectroscopic (EDAX) results obtained with the redox systems: Fe(CN)3−4−6, Ru(NH3)3+2+, Eu3+/Eu2+, Co(en)3+2+3 and Fe(C2O4)3−4−3 pointed to reactions proceeding at the polymer-solution interface. The data were analysed on the basis of a model in which the charge transfer was regarded as a redox reaction between polymeric sites in the film and the redox species in the solution. The rate of electron transfer was found to be: (i) proportional to the concentration of the oxidized or reduced polymeric sites; (ii) dependent to some extent on the Donnan potential prevailing at the interface; and (iii) correlated with the thermodynamic driving force of the reaction between the polymer and redox species.