Galvanostatic investigation of electrochemical processes at the LaF<Subscript>3</Subscript>/M (M: Ag,Bi) interface

The electrochemical processes at the interface between solid fluorine-conducting electrolyte LaF₃(Eu²⁺ 0.8 mol %) and silver or bismuth electrodes in the two-electrode cell with nonpolarizable reference electrode are studied using the galvanostatic method. The anodic galvanostatic transients of LaF₃: Eu²⁺/Ag and LaF₃: Eu²⁺/Bi interfaces are linearized on the log(η ? η_max), vs. t coordinates, i.e. the rate of LaF₃|MF _n |M electrode formation is limited by slow surface diffusion of metal adions. The initial portions of cathodic galvanostatic transients in the range of solid-electrolyte lanthanum reduction are approximated by the linear dependence of η on log(1 ? √t/τ). The plots of logI vs. 1/η are linear both for the lanthanum reduction and for silver and bismuth oxidation involving mobile fluoride ion of solid electrolyte, which is typical for two-dimensional growth of new phase.     

Kinetics of intermediate phase growth at the interface of fluoride-conducting solid electrolytes with metal electrodes

The effect of the reference electrode (Ag, AgCl∣KF + KCl or Sn, SnF₂) on the kinetics of new phase growth at interfaces formed by LaF₃:Eu²⁺ or CeF₃:Sr²⁺ single crystal solid electrolytes with metal electrodes of Ag and Bi is studied by chronoamperometry and linear scan voltammetry. It is shown that when a solid-phase reference electrode is used, the current transients reflect hindrances associated with instantaneous two- or three-dimensional nucleation as well as limitations for fluoride ion diffusion in the solid electrolyte.     

Oxygen reduction at the interface of nonstoichiometric fluoride single crystals with metal electrodes

Electrochemical measurements are carried out on three-phase boundaries LaF₃:Eu²⁺/Me, oxygen gas, where Me = Ti, Be, Ta, Au and also on the boundaries of CeF₃:Sr²⁺ with Ag-supported Ag and Bi microelectrodes by the methods of linear scan voltammetry, chronoamperometry, and cyclic voltammetry. The conditions beneficial for reduction of oxygen gas, transformations of chemisorbed oxygen, and the appearance of its mobile forms are considered. Results on the interaction of gaseous oxygen with metals in contact with LaF₃:Eu²⁺ single crystals are generalized. The correlation is found between the oxygen reduction potential and the difference of electronegativities Δχ Me-O. The relationship is revealed between the oxygen reduction potential and the Me-Me bond energy by taking into account the structural factor f/k _n, where f is the number of binding electrons and k _n is the coordination number.     

Electrochemical Behavior of the LaF3:Eu2+/Me Interface (Me = Sm,Ce, Gd,V, Ag)

The electrochemical behavior of the LaF₃ : Eu²⁺| interface (Me = Sm, Ce, Gd, V, Ag) is studied using stripping voltammetry. Current–voltage curves for cells Ag, AgCl|KCl and KF|LaF₃ : Eu²⁺|Sm (or Ce) may be viewed as background curves for analyzing solid-phase reactions with other metallic electrodes. The substrate material is found to affect properties of interfaces LaF₃ : Eu²⁺/Gd (or V), which are characterized either by solid-phase reactions involving the fluoride ion of a solid electrolyte or by reactions involving oxygen. Regularities, obtained for silver in contact with LaF₃ : Eu²⁺, link electrochemical signals of the reaction of the silver fluoride formation with the contact area.     

The dependence of the kinetics of some simple outer-sphere electrode reactions on the nature of the electrode material

The kinetics of a number of simple inorganic electrode reactions that are known or expected to follow outer-sphere pathways have been examined at mercury, silver, platinum, and gold-aqueous interfaces in order to explore the effects of varying the electrode material on outer-sphere reactivity. The electroreduction kinetics of Co(III) ammine complexes exhibited only mild dependences on the nature of the electrode material which were compatible with the expected variations in double-layer effects. However, the electrooxidation of Cr²⁺ proceeded at strikingly higher overpotentials on the solid surfaces compared with mercury electrodes. Similar effects were also seen for the electrooxidation of V²⁺, Eu²⁺ and Ru²⁺ in the presence of Cr²⁺. Much larger rate constants were observed for these aquo reactions at solid surfaces in the absence of Cr²⁺, although Cr²⁺ had no influence on Co(NH₃)₆³⁺ electroreduction, or any reaction at mercury electrodes. It is speculated that the very large substrate effects upon the electrode kinetics of aquo couples arise from the influence of the inner-layer water structure on the reactant-solvent interactions experienced by these “structure-making” reactants at their plane of closest approach. The inhibiting influence of Cr²⁺ may be due to its ability to efficiently remove adsorbed catalytic contaminants by incorporation into a substitutionally inert Cr(III) electrooxidation product by means of a ligand-bridge mechanism.     

Interphase-exchange processes at the interface of fluorine-conducting solid electrolyte with alkaline solutions

Kinetics of fluorideion exchange at the interface between a LaF₃:Eu²⁺ single crystal and solution with different pF and pH is studied by the galvanostatic and potentiometric methods and also by impedance spectroscopy. Anodic galvanostatic transients are linearized in the coordinates ln (η–η_max) vs. t, which suggests that the exchange rate in alkaline solutions is limited by surface diffusion of fluorine adions. The surface concentration of fluorine adions с ₀ and the surface diffusion flow ν ₀ are assessed. Impedance spectroscopy studies of the exchange processes indicate that the charge-transfer resistance R _F and the heterogeneous reaction resistance R _P increase with the increase in the рН of the fluoride-containing solution and also with the increase in the time of exposure of the fluoride-selective electrode membrane in a neutral solution with pH 6.4 and the high (pF 2) content of fluorides.     

Hybrid system of nickel<Emphasis Type="Bold">–</Emphasis>cobalt hydroxide on carbonised natural cellulose materials for supercapacitors

Advanced carbon materials formed from abundant biomass are an exciting and promising class for energy devices due to the clear advantages of low cost, sustainability and good physical and electrochemical properties. However, these materials typically do not compete well with their metal functionalised counterparts. In this work, we demonstrate that xCo(OH)₂–(1?x)Ni(OH)₂ with various Ni:Co ratios can be deposited onto biomass-derived carbon to make a hybrid inorganic-carbon electrode with tuneable physical features and electrochemical performance. These features were tuned by adjusting the Ni:Co ratio within precursor solutions. The electrodes had shown a capacitance ranging from 780.7 to 2041 F g^?1, which is very close to the theoretical value for Ni(OH)₂ (2365 F g^?1). A hypothesis is presented to help explain this performance for a modified, biomass-derived carbon electrode.     

Cyclic voltammetry and impedance spectroscopy analysis for graphene-modified solid-state electrode transducers

The transducer of solid-state electrodes based on an epoxy-graphite composite was modified by two different methods, such as direct mixed and layer deposition of graphene (commercial and synthesized by electrochemical exfoliation of graphite). The modified electrodes were characterized by cyclic voltammetry and impedance spectroscopy. Also, scanning electron microscopy (SEM) was carried out to acquire information concerning the morphology of the composite electrode. Voltammetric measurements, in presence of [Fe(CN)₆]^3? as electroactive standard, determined a quasi-reversible electrochemical behavior under linear diffusion control. Electronic transference for modified and unmodified electrodes was compared. Solid-state electrode modified by inclusion of synthesized graphene showed a better electronic transference at electrode surface, due to the lower potential difference between anodic and cathodic peaks (ΔE = 125 mV) with respect to unmodified electrode (ΔE = 160 mV). Impedance spectroscopy characterization of electrode bodies in solid-state it was revealed a higher electronic conductivity and a supercapacitive behavior for the modified composites (values of intrinsic capacitances in the order of nanofarads) due to inclusions of graphite and graphene in the epoxy matrix. These inclusions were verified through SEM microscopy. The electronic conductivity and the supercapacitive character contributed both to the enhancement of electronic transference at electrode surface.     

Varieties in structures of Co(II), Ni(II) and Cu(II) coordination compounds based on dimethyl pyridin-2-ylcarbamoylphosphoramidate

A series of new 3d metal complexes based on dimethyl pyridin-2-ylcarbamoylphosphoramidate (HL) was synthesized. The compounds with general formula M(HL)₂Cl₂·nH₂O and M(L)₂·nH₂O (M=Co²⁺, Cu²⁺, Ni²⁺) were characterized by means of single-crystal X-ray analysis and IR spectroscopy. The organic ligands in all complexes are coordinated via oxygen atom of the carbonyl group and nitrogen atom of the heterocycle. The coordination environment of the central atoms is a distorted octahedron. The axial positions in the Co(II) and Ni(II) complexes with deprotonated ligands are occupied by water molecules. The Co(II) and Cu(II) complexes with phosphoryl ligands in a neutral form have different ligands in the axial positions: in the Co(II) complex, the positions are occupied by two water molecules, whereas in the Cu(II) complex, the positions are occupied by two chlorine anions. The structure of HL was experimentally and theoretically obtained by utilizing single-crystal X-ray analysis and DFT calculations. The computationally optimized geometric parameters for HL show a good agreement with the experimental results.     

Synthesis and Luminescence of Europium-Containing Compositions Based on Yttrium Oxide and Oxyfluorides

By heating the products isolated from an ethyl acetate medium, luminescent europium-containing compositions based on yttrium oxyfluorides and oxide have been synthesized. The concentration of Eu³⁺ ions in the compositions was from 0.10 to 10 at % of the yttrium content. It has been shown that the (Eu_хY_1–х)_nO_n–1F_{n + 2}, (Eu_хY_1–х)OF, (Eu_хY_1–х)₂O₃, and (Eu_хY_1–х)5O₄F₇ phases are formed during the synthesis. The luminescence of the compositions is related to the ⁵D₀ → ⁷F_j electron transitions of Eu³⁺ ions. It depends on the synthesis conditions, the type of matrix, the position of the Eu³⁺ ions in the crystal lattice, the excitation wavelength, and other factors.     

Effect of the activation of a graphite-epoxy composite electrode and the buffer capacity of a solution on the reversibility of the hydroquinone electrode reaction

Peak potentials and the kinetics of hydroquinone oxidation at an electrode from a graphite-epoxy composite in activated and passivated states were studied in supporting electrolytes of different buffer capacities and pH varying from 0.1 to 8.8, using methods of direct-current and cyclic voltammetry. The electrode was activated before its polarization by mechanically cutting a 0.2–4-μm surface layer directly in a test solution. The electrode was passivated by storing in air for two or more days. The behavior of hydroquinone in its oxidation at the passivated and activated electrodes was compared using diagnostic criteria for the following functions: I _a?v ^1/2, logI _a?logv, I _a/v ^1/2?v ^1/2, and I _a/c, where v is the rate of the potential sweep and c is the volumetric concentration of hydroquinone. The potential difference of anodic and cathodic peaks in cyclic voltammograms indicated the reversibility of the electrode reaction in all supporting electrolytes.     

Electrochemical formation of a low-dimensional phase on the boundary between the LaF<Subscript>3</Subscript>:Eu<Superscript>2+</Superscript> single crystal and the metal

The kinetics of the formation of a new phase at the interface between the LaF₃:Eu²⁺ single crystal and the (Sn, Bi, or Sb) metallic electrodes was studied using potentiostatic transient measurements and voltammetry with a linear variation of voltage. A comparison of the theoretical and experimental reduced transients that define two-dimensional instantaneous nucleation on a plot of I/I _m vs. t/t _m and three-dimensional growth of the instantaneous and progressive types of nucleation on a plot of I ²/I _m² vs. t/t _m showed that the model was not fully consistent with the experiment. The dependence of the stationary current logI _A(max) of the potentiostatic transients on 1/η during the formation of the intermediate phase on the boundaries of LaF₃:Eu²⁺|Sn and LaF₃:Eu²⁺|Bi was found to be linear, which corresponds to two-dimensional nucleation and growth of the new phase.     

Voltammetry with a Fluoride-Selective Electrode with Solid-Phase Reference System

Experimental results obtained in a study of the voltammetric response of an all-solid fluoride-selective electrode based on LaF₃ (Eu²⁺ 0.8 mol %), LaF₃ (Sr²⁺ 5 mol %) and CeF₃ (Sr²⁺ 5 mol %) single crystals brought in contact with Ag, Bi, and Sn metal samples are presented. The method of cathodic inversion voltammetry was applied to study the reduction of La³⁺ and Ce³⁺-cations from the rigid sublattice of solid electrolytes, which determines the threshold of the electrochemical stability of a membrane. Anodic inversion voltammetry was used to investigate the characteristics of solid-phase generation of metal fluorides at the interface between the fluoride-selective electrode and metals.     

Activation of oxygen reaction by praseodymium oxide film on platinum electrode in contact with YSZ electrolyte

The method of impedancemetry is used to study electrochemical activity of platinum electrodes in contact with the solid electrolyte of YSZ (ZrO2+10 mol % Y₂O₃). Electrodes are activated by small additives of pradeodymium oxide PrO _x (0.1–0.3 mg/cm²) by their impregnation by praseodymium nitrate solution with the further thermal treatment at 850°C and are studied in the cases when PrO _x forms a film on the electrolyte and when no activator film is formed. The effect of the number of electrode activations on the electrochemical activity of platinum electrodes is studied.     

Polarization of the contact between fluorine-conducting solid electrolyte and metal alloys

The kinetics of low-sized phase formation on polarization of the interface between LaF₃:Eu²⁺ and Pb–Sn, Sn–Bi, and Sb–Bi alloys is studied by the potentiostatic and linear voltammetry methods. The analysis of anodic transients shows that the new phase formation involving fluoride ions from the solid electrolyte proceeds by the mechanism of instantaneous two-dimensional or three-dimensional nucleation. Comparison of calculated and experimental transients describing the instantaneous nucleation with either two-dimensional growth on the I/I _m vs. t/t _m coordinates or three-dimensional growth on the I ²/I _m ² vs. t/t _m coordinates shows adequate agreement between the model and the initial regions of experimental curves. The properties of phases formed depend on the alloy composition and the polarization conditions as well as on the energy of interaction between components in the alloy and in the new phase.     

Thermal annealing of LaF3:Eu3+ nanocrystals synthesized by a solvothermal method and their luminescence properties

In this work, we used a low temperature solvothermal method to synthesize Eu³⁺-doped LaF₃ (LaF₃:Eu³⁺) nanocrystals. The effect of thermal annealing on their phase structures and luminescence properties was studied. Transformation from LaF₃ to LaOF was observed after the annealing, and the initial transformation process was studied using a rapid thermal annealing technique. It was found that a sufficiently high annealing temperature is required for the transformation of LaF₃ to LaOF. LaOF phase started to be formed after annealing at 500 °C for as short as 5 min, and higher annealing temperatures and longer annealing time led to a larger amount of LaOF formed. With the increase of the formation of LaOF, the luminescence was greatly enhanced. Strong O^2? → Eu³⁺ charge transfer band was present in these samples annealed at 500 °C and higher temperatures, and greatly enhanced ⁷F₀ → ⁵D₂ transition of Eu³⁺ was also observed.     

Effect of the Electrode Material on the Reduction of La<Superscript>3+</Superscript> and Ce<Superscript>3+</Superscript> in Fluoride-Conducting Solid Electrolytes

The reduction of immobile cations La³⁺ and Ce³⁺ in fluoride-conducting solid electrolytes (FSE) LaF₃ (Eu²⁺ 0.8 mol %), LaF₃ (Sr²⁺ 5 mol %), and CeF₃ (Sr²⁺ 5 mol %) in contact with Ag, Bi, Si, La, Ce, and Sm working electrodes is studied by chronoamperometry and voltammetry with linear potential scan. Discovered is linear dependence of initial segments of potentiostatic transients of cathodic current on t ^1/2 at FSE interfaces with Ag, Bi, La, Ce, and Sm. The dependence is due to diffusion-controlled instantaneous nucleation of Ln and Ce. The La³⁺ and Ce³⁺ reduction at the FSE/Ag interface is reversible in a narrow region. The reduction and oxidation of La³⁺ and Ce³⁺ (cations of the FSE rigid lattice) at the FSE/Me (Me = La, Ce and Sm, Bi, Si) interface is irreversible and involves a chemical reaction.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 662–672.Original Russian Text Copyright © 2005 by Turaeva, Kot, Urchukova, Murin.     

Cyclic Voltammetry of a Fluoride-Selective Electrode in Contact with Metals

Electrochemical behavior of the interface between a LaF₃ : Eu²⁺ single-crystal fluoride-selective membrane and Bi, Sn, Pb, Sb, and Cd metallic microelectrodes is considered with the aim to apply a fluoride-selective electrode in solid-state voltammetry. The ionization-discharge processes on the metals in air, involving mobile fluoride ions from the LaF₃ membrane, were studied by cyclic voltammetry.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 2, 2005, pp. 265–272.Original Russian Text Copyright © 2005 by Turaeva, Pegova, Vasilevskii.     

Oxygen Reactions at the Interface of Nonstoichiometric Fluorides LaF3 and CeF3 with Chemically Modified Electrodes

The oxygen reaction at the interface LaF₃(CeF₃)|electrode,O₂(g) is studied in air on Pt and C electrodes and electrodes modified by supported oxides of Co, Sr, and/or La on C, Si, or Pt substrates. The experimental methods used are voltammetry with a linear potential change and stripping voltammetry and cyclic voltammetry. The voltammetric response is dependent on the nature of solid electrolyte and electrode, the balance between the substrate and active agent, and on the conditions of thermal treatment of electrodes. Mobile oxygen forms are discovered at interfaces CeF₃|C (Co _x O _y ), ₂ ([C] : [Co] = 1 : 10, 1 : 20); CeF₃|C (Co _x O _y ), ₂([C] : [Co] = 1 : 10); and CeF₃|Pt (Co _x O _y ), O₂([Pt] : [Co] = 1 : 40). The peak current I _pa in the anodic curves increases with extending the limit of cathodic potential scan E _rc. The peak current I _pc in the cathodic curves increases with extending the limit of anodic potential scan E _ra.     

Determination of tyrosine using <Emphasis Type="Italic">p</Emphasis>-sulfonated calix[<Emphasis Type="Italic">n</Emphasis>]arene modified electrode

Cyclic voltammetry was used to detect p-sulfonated calix[n]arenas (SCnA) how to immobilize on gold surface. P-sulfonated calix[n]arenes not only increased the specific surface area of the modified electrode, but also improved the enrichment ability of tyrosine. This led to a significant increase in peak current, and improved the sensitivity of tyrosine determination on the p-sulfonated calix[n]arenes-modified electrode. The modified electrode showed good catalytic ability of l-tyrosine oxidation reaction. The peak current of l-tyrosine increased and the oxidation peak potential shifted negatively with cavity size of the SCnA-modified electrode, which indicated that the catalytic ability of the modified electrode to l-tyrosine oxidation reaction was also enhanced. For the tyrosine guest molecule, the order of electrochemical activity and magnitude of catalytic ability of the oxidation reaction of the three modified gold electrodes was as follows: SC4A?>?SC6A?>?SC8A. The electrode had high selectivity and stability for the determination of tyrosine, with a wide linear range, low detection limit and high sensitivity at different concentrations under different pH values. This resulted in an accurate, fast, sensitive electrochemical method for the determination of tyrosine. The sensor was used for the determination of tyrosine in human urine with satisfactory results.