Karmen Lust

Electric double layer structure and adsorption of cyclohexanol on single crystal cadmium, antimony and bismuth electrodes

The electric double layer structure in aqueous and non-aqueous surface inactive electrolyte solutions and the adsorption of cyclohexanol on the electrochemically polished Sb, Bi and Cd electrodes were studied by using cyclic voltammetry, impedance and chronocoulometry. The limits of ideal polarizability, the potentials of zero charge and the values of capacity of inner layer were established. As found, the differences between the zero charge potential for various planes depend on the chemical nature and crystallographic structure of the electrode surface, as well as on the chemical nature of the solvent studied. The inner layer capacity at negative surface charges depends on the crystallographic orientation of the single crystal planes and on the electronic properties of the electrode metal. The inner layer capacity increases in the sequence of metals Sb < Bi < Hg < Cd < Zn, as the hydrophility of electrodes increases and as the contribution of metal phase decreases. The values of capacity of metal phase and the effective thickness of the thin metal surface layer were calculated by using various theoretical approximations. The effective thickness of the thin layer of metal surface increases in the sequence of metals Ga < Zn < Cd < Hg < Bi < Sb as the metallic properties of electrodes decrease. The adsorption parameters of cyclohexanol were established by using Frumkin-Damaskin adsorption theory. It was found that the adsorption characteristics depend on the chemical nature and crystallographic structure of electrode surface. The adsorption activity of cyclohexanol increases in sequence of metals Zn < Cd < Bi < Hg < Sb as the adsorption energy of solvent decreases.

Adsorption of halide anions on bismuth single crystal plane electrodes

Adsorption behaviour of Cl - , Br - and I anions on Bi single crystal plane electrodes from aqueous, methanol and 2-propanol solutions has been studied by impedance spectroscopy and chronocoulometry methods. The values of surface excess and the Gibbs energy of adsorption of halide ions on the Bi(111), Bi(001) and Bi(011) single crystal planes have been calculated using the electrode potential E and the surface charge density a as the independent electrical variables. It was found that under comparable conditions the result obtained at constant E and at constant a are coincident. The formal charge transfer coefficient (so called electrosorption valency) and the dipole moment of the dipole formed by an adsorbed halide ion and its image charge in Bi plane have been calculated. It was found that on the Bi(111) plane, where the chemical bonds are all saturated, the formal charge transfer coefficient and the dipole moment of adsorbed anion are independent of electrode potential. For more active Bi planes there is noticeable dependence of both characteristics on electrode charge and at higher positive charges the covalent bonding between Bi(011) surface atoms and I - is probable. It was concluded that the dipole formed is significantly screened by the solvent molecules and the metal electron gas, but the influence of the solvent dielectrical and other characteristics on the effective dipole moment values is comparatively small.

Adsorption of pyridine on the (111), (001) and (001) faces of bismuth

The adsorption of pyridine (PY) on Bi(111), Bi(001) and Bi(011) single crystal electrodes has been studied in 0.1 M aqueous NaF solution for concentrations of PY ranging from 1 to 200 mM. In the range of potentials explored, adsorption maximum coverage of the surface and the desorption of PY at E = −1.8 V (SCE) have been observed. At the negative potentials and polarization close to the potential of zero charge, PY molecules at bismuth single crystal planes assume a tilted orientation, with the hydrocarbon ring facing the metal. The saturation coverage Λmax and the limiting capacity C1 decrease, and the shift of zero charge potential EN, due to the displacement of surface water by a monolayer of PY, rises in the sequence of planes Bi(011) < Bi(001) < Bi(111) as the vertical component of the orientation of the PY molecules increases. Adsorption isotherms, values of the attraction constant a and standard Gibbs energy of adsorption ΔGads0 have been determined. As in the case of other organic compounds studied, the activity of Bi planes increases in the sequence of planes Bi(111) < Bi(001) < Bi(011). The partial change transfer from PY to Bi electrodes increases in the same direction of planes. The attractive interaction between the adsorbed molecules in the sequence of planes Bi(011) < Bi(111) < Bi(001) as the superficial density of atoms decreases. The absolute value of the Gibbs energy of adsorption of Py increases in the sequence of electrodes Ag Hg>Bi(001)>Bi(011).

Influence of surface pretreatment of bismuth and cadmium electrodes to the electric double layer and adsorption characteristics of organic compounds

Abstract Influence of the surface structure of electrodes on the electric double layer properties, as well as on the adsorption characteristics of various organic compounds has been investigated by cyclic voltammetry, impedance and electron microscopic methods at variously prepared Bi, Sb and Cd electrodes. The systematic trends of the influence of surface roughness and energetic inhomogeneity on the electric double layer characteristics (value of differential capacity, potential of diffuse minimum, Parsons-Zobel factor, inner layer capacity, roughness function) have been established. The dependencies of the shape of adsorption-desorption maxima of organic compounds and other adsorption parameters (values of attraction interaction, adsorption equilibrium constant, shape of adsorption isotherm) on the electrode surface structure have been investigated. The Debye-length dependent roughness function has been calculated. The approximate values of the linear parameter of homogeneous regions, which prevail at the surface of polycrystalline electrodes have been calculated using various theoretical models. The established values of linear parameters of homogeneous regions have been compared with the characteristics obtained from the electron microscopic studies.

Adsorption of organic compounds and hydrophilicity of bismuth, cadmium and antimony electrodes

The adsorption behavior of various organic compounds at bismuth, antimony, cadmium, mercury and other “mercury-like” metals has been discussed. The systematic trends of the influence of the chemical nature of electrode metal and adsorbate to the molecular interaction parameter, a; limiting Gibbs adsorption, ΓA, max; and Gibbs energy of adsorption, ΔGA0, have been analyzed. The ingredients of the total Gibbs energy of organic compounds adsorption have been found. A new more general method for obtaining the metal-water; interaction Gibbs energy has been worked out and used. It was found that the dependence of Gibbs energy of metal-water interaction on the chemical nature of metal and aliphatic organic compound studied is weak. Only in the case of chemically very different metals, for example for Sb and Zn, the difference of Gibbs energy of metal-water interaction values is somewhat higher than the exactness of the determination of the experimental values of Gibbs energy of organic compound adsorption.

Electrical Double Layer Capacitance at Bi ( 111 ) ∣ 1 -Ethyl-3-methylimidazolium Tetrafluoroborate Interface as a Function of the Electrode Potential

The influence of the electrode potential on the Bi(111) | 1-ethyl-3-methylimidazolium tetrafluoroborate interface has been studied by cyclic voltammetry and electrochemical impedance. The weak deviation of the system from capacitive behavior for the ideally flat electrode has been established within the moderate electrode potential region from -0.9 to -0.1 V vs Ag | AgCl in 1-ethyl-3-methylimidazolium tetrafluoroborate. The parallel faradaic processes are possible outside the potential region from -0.9 to -0.1 V. The results obtained for the Bi(111) | 1-ethyl-3-methylimidazolium tetrafluoroborate interface have been compared with those for gold, mercury, and glassy carbon electrodes.

Influence of the surface structure of cadmium electrodes on the electric double layer parameters in aqueous surface-inactive electrolyte solutions

The electric double layer structure at the electrochemically polished Cd(0001), Cd(1010), Cd(1120), Cd(1011) and Cd(1121), and chemically treated Cd(0001), Cd(1010) and Cd(1120) electrodes in aqueous surface-inactive electrolyte solutions was studied using cyclic voltammetry, impedance and chronocoulometry. The limits of ideal polarizability, potentials of zero charge and capacity of the inner layer were established. Like those for the respective single crystal planes of Zn, the value of the zero charge potential decreases and the inner layer capacity increases if the superficial density of atoms decreases. The capacity of the metal phase was established using the various theoretical approximations. The effective thickness of the thin metal layer increases in the sequence of planes Cd(1120) < Cd(1010) < Cd(0001) as the surface density of atoms increases.

Adsorption of isomers of butanol on bismuth single crystal plane electrodes

Cyclic voltammetry, impedance and chronocoulometry have been employed for quantitative study of normal butanol (n-BA), iso-butanol (iso-BA), sec-butanol (sec-BA) and tert-butanol (tert-BA) adsorption at the bismuth single crystal plane¦aqueous Na2SO4 solution interface. The adsorption isotherms, Gibbs energies of adsorption, the surface excess and other adsorption parameters have been determined. As found, the adsorption characteristics of the isomers of butanol depend on the crystallographic structure of the electrodes, as well as on the geometrical structure of hydrocarbon chains of adsorbate. The adsorption characteristics obtained from the impedance and chronocoulometric measurements are in good agreement within the limits of surface charge densities — 24 < σ < 3 μC cm−2. The adsorption of n-BA, sec-BA, iso-BA and tert-BA on Bi single crystal planes is physical and is limited by the rate of diffusion of organic molecules to the electrode surface. The adsorption activity of adsorbates at the bismuth¦solution interface increases in the sequence tert-BA < sec-BA < iso-BA < n-BA as the adsorption at the air¦solution interface increases. In the case of all compounds studied, the adsorption activity of planes increases in the sequence (101) ≤ (001) < (211) < (011) as the superficial density of atoms increases. According to the values of the standard Gibbs energy of adsorption, it was established that the hydrophilicity of electrodes increases in the sequence Sb(111) < Bi(011) < Bi(211¯) < Bi(101) ≤ Hg < Bi(001) < Bi(111) < Cd(0001) < Zn(0001) < Ag(111) < Ag(100) < Ga.

Optimization of the Cathode Composition for the Intermediate-Temperature SOFC

Electrochemical impedance, cyclic voltammetry, and chronoamperometry have been used for characterization of the medium-temperature half-cells Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖La 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 1), Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖Pr 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 2), and Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖Gd 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 3) at various electrode potentials and temperatures. Analysis of the impedance data shows that the kinetically mixed process is probable, characterized by the slow electron transfer to an adsorbed and thereafter dissociated oxygen atom O a d s , as well as by slow mass transfer of electroactive species inside the cathode or O a d s at the internal surface of the porous cathode. The total polarization resistance increases with rising the atom mass of the A-site cation in the porous perovskite structure. The values of activation energy decrease slightly with increasing negative polarization and in the order of half-cells Sys 3 > Sys 2 > Sys 1. The transfer coefficient α c over 0.5 indicates the deviation of the mainly charge-transfer-limited process toward the mass-transfer-limited process in the porous cathode with decreasing temperature. The electrochemical characteristics of half-cells Sys 2 and Sys 1 are stable under repetitive potential and thermocycling during long operation times (t > 1200 h for Sys 2 and t > 4800 h for Sys 1).

Adsorption kinetics of uracil on bismuth single crystal planes

The electrochemical impedance method has been used for the quantitative study of uracil adsorption kinetics at the bismuth single crystal plane j aqueous Na2SO4 solution interface. Analysis of impedance data demonstrates that the adsorption process of uracil is complicated and limited by the mixed kinetics, i.e. by the slow adsorption and diffusion steps. Analysis of the Cole � /Cole and other dependences indicates that, at small frequencies and higher uracil concentrations, two-dimensional association of the uracil molecules is possible. Nonlinear regression analysis has been used for fitting the experimental complex plane (Z ƒ, Z ?) plots. The classical Frumkin � /Melik-Gaikazyan circuit in the region of higher ac frequencies and lower uracil concentrations (curacil 5/2 � /10 � 3 M), and the modified Frumkin � /Melik-Gaikazyan equivalent circuit (taking into account the inhomogeneous semi-infinite diffusion) at curacil ]/2 � /10 � 3 M fit with a reasonable accuracy the experimental Z ƒ, Z ?-plots. It was found that the values of the double layer capacitance, partial charge transfer resistance, adsorption capacitance and Warburg-like diffusion impedance depend noticeably on the electrode potential and concentration of uracil. Other more complicated equivalent circuits have been used for fitting the experimental impedance data. # 2003 Elsevier B.V. All rights reserved.