Polarization phenomena at the metal/proton conducting electrolyte interphase

The electrokinetics at the H₂ - Pd/SrCe_0.95Yb_0.05O_3−α interphase are presented. Current-overpotential (I-η) curves were obtained at 400–700 °C and at hydrogen partial pressures from 1 kPa to 100 kPa, using a three electrode single chamber cell. The present data were assumed to be free of mass transfer effects. The anodic and cathodic charge transfer coefficients as well as the exchange current density were calculated from the experimental results by using the Butler-Volmer equation. The high and low field approximations were used to simplify the analysis. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Determination of the exchange current in the SOFC composite cathode

To evaluate the exchange current I _o of the oxygen electrode reaction at the O₂ strontium-doped lanthanum manganite (LSM)/yttria stabilized zirconia (YSZ) composite/YSZ interface, the variation of the current I vs overvoltage η is measured at low cathodic and anodic polarizations (−50 to 30 mV). A linear behavior is observed within this potential domain with a unique slope. Taking into account of the dissociative adsorption of the oxygen molecule on the LSM electrocatalyst surface and the charge transferred at the triple-phase boundary, the exchange current is evaluated to 2.93 mA in air at 747 °C.     

Sensitive voltammetric determination of diclofenac using room-temperature ionic liquid-modified carbon nanotubes paste electrode

An ionic liquid-modified carbon nanotubes paste electrode (IL/CNTPE) has been fabricated using hydrophilic ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) as a binder. This electrode showed enhanced electrochemical response and strong analytical activity towards the direct electrochemical oxidation of diclofenac (DCF). The electron transfer coefficient, α, and charge transfer resistance (R _ct) of DCF at the modified electrode were calculated. Under optimal conditions at pH 7.0, the anodic peak currents increased linearly with the concentration of DCF in the range of 0.5–300 μmol L^?1 with a detection limit of 0.2 μmol L^?1 (3σ). The interferences of foreign substances were investigated. Differential pulse voltammetry was used to check the applicability of the proposed sensor to the determination of DCF in real samples with satisfactory results.     

The effects of anodic and cathodic processes on the characteristics of ceramic coatings formed on titanium alloy through the MAO coating technology

This paper reports on the investigation into the effects of the different anodic j_a and cathodic j_c current densities on the variations of the anodic and cathodic processes and the associated changes in the characteristics of the coatings synthesized on Ti-6Al-4V alloy substrate by microarc oxidation technique. The coated samples were subjected to coating thickness and cross-section fracture observation. Phase and elemental composition at different depth of the coatings were evaluated through X-ray diffraction and energy dispersive spectrum analyses. The experimental results indicate that the increase of j_a leads to the increasing slope of anodic voltage U₊ versus oxidation time plots, the larger coating thickness and the more coarse surface morphology of MAO coatings; while the aggrandizement of the cathodic process results in the lower growth rate and more uniform structure of coatings. The changes of the elements distribution from the interface towards the coating surface with variation of j_a and j_c are affected by the Ti anodic dissolution and the electromigration of electriferous particles, such as Al(OH)₄⁻, in electrolytes.     

Kinetic studies of the electrochemical nitrogen reduction and incorporation into yttria stabilized zirconia

The kinetics of the electrochemical reduction of molecular nitrogen at gold micro electrodes on yttria stabilized zirconia (YSZ) solid oxide electrolyte is studied by steady state polarization measurements. From the η / lg i plot for both cathodic and anodic polarization the apparent transfer coefficients α_a and α_c are evaluated. The sum of α_a + α_c exceeds unity and thus a multistep electron transfer process is suggested. The concept of the stoichiometric number is applied to the electrode reaction N₂ + 6e⁻ = 2N³⁻ supposing that the overall process involves at least two intermediate species. On the basis of the evaluation of the experimental results the reaction N₂⁻ + e⁻ ⇌ N₂²⁻ is suggested as the rate determining reaction step for the cathodic nitrogen reduction and nitride formation.     

Switching effect at a platinum electrode on stabilized zirconia during oxygen reduction in the presence of NO

A galvanic cell based upon the use of stabilized zirconia as solid oxygen ion conductor has been used to measure cathodic reduction currents at a porous platinum electrode in both nitrogen-oxygen gas mixtures with and without small amounts of NO (up to 5450 ppm). Adding small amounts of NO to the N₂/O₂ mixture induced a considerable cathodic current peak at the working electrode in the first moment after addition. After interruption of the NO exposure, the opposite effect, a high current pulse in anodic direction, was observed. The switching effect is reproducible and its magnitude depends on the concentrations of oxygen and nitrogen monoxide in the gas. As the main contribution to the current results from the reduction of the excess oxygen in the gas, it must be concluded that the presence of NO strongly affects the steady state, in particular the adsorbed oxygen at the electrode/electrolyte interface. These experimental results are interpreted in terms of a reversible change of the interface which may be due to a reconstruction of the platinum surface in the presence of NO and corresponding drastic change in the amount of adsorbed oxygen at the platinum surface. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

In situ XPS study of Pd(1 1 1) oxidation at elevated pressure, Part 2: Palladium oxidation in the 10 mbar range

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 0.4 mbar O₂. The in situ XPS data were complemented by ex situ TPD results. A number of oxygen species and oxidation states of palladium were observed in situ and ex situ. At 430 K, the Pd(1 1 1) surface was covered by a 2D oxide and by a supersaturated O_ads layer. The supersaturated O_ads layer transforms into the Pd₅O₄ phase upon heating and disappears completely at approximately 470 K. Simultaneously, small clusters of PdO, PdO seeds, are formed. Above 655 K, the bulk PdO phase appears and this phase decomposes completely at 815 K. Decomposition of the bulk oxide is followed by oxygen dissolution in the near-surface region and in the bulk. The oxygen species dissolved in the bulk is more favoured at high temperatures because oxygen cannot accumulate in the near-surface region and diffusion shifts the equilibrium towards the bulk species. The saturation of the bulk “reservoir” with oxygen leads to increasing the uptake of the near-surface region species. Surprisingly, the bulk PdO phase does not form during cooling in 0.4 mbar O₂, but the Pd₅O₄ phase appears below 745 K. This is proposed to be due to a kinetic limitation of PdO formation because at high temperature the rate of PdO seed formation is compatible with the rate of decomposition.     

Influence of TiO2 nanotube morphology and TiCl4 treatment on the charge transfer in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ nanoparticles (NPs), TiO₂ nanotube arrays (NTAs), and surface-modified NTAs with a TiCl₄ treatment. The photovoltaic efficiencies of the DSSCs using TiO₂ NP, NTA, and TiCl₄-treated NTA electrodes are 4.25, 4.74, and 7.47 %, respectively. The highest performance was observed with a TiCl₄-treated TiO₂ NTA photoanode, although in the case of the latter two electrodes, the amounts of N719 dye adsorbed were similar and 68 % of that of the NP electrode. Electrochemical impedance measurements show that the overall resistance, including the charge–transfer resistance, was smaller with NTA morphologies than with NP morphologies. We suggest that a different electron transfer mechanism along the one-dimensional nanostructure of the TiO₂ NTAs contributes to the smaller charge–transfer resistance, resulting in a higher short circuit current (J _sc), even at lower dye adsorption. Furthermore, the TiCl₄-treated NTAs showed even smaller charge–transfer resistance, resulting in the highest J _sc value, because the downward shift in the conduction band edge improves the electron injection efficiency from the excited dye into the TiCl₄-treated TiO₂ electrodes.     

The influence of α-ZrP coat on proton transfer resistance of cation exchange membrane-solution interface

In this work, the effect of surface modification on proton transfer resistance of the membrane and/or membrane surface interface is investigated. Cation exchange membranes, PE01 and Nafion1135, were modified by zirconium phosphate (ZrP) and inorganic-organic composite membranes were prepared. α-ZrP (α-Zr(HPO₄)₂ · H₂O) was deposited on the composite membrane surface as verified by XRD and SEM. Zeta potential of the composite membrane decreases with the increase of immersion time, indicating that the deposition α-ZrP results in a decrease of membrane surface charge density. The proton transfer resistance measured by AC impedance technique in 0.05 mol/L H₂SO₄ solution shows an interesting result: the proton transfer resistance of membrane-solution (Z_e) sharply reduces while proton transfer resistance of membrane (Z_m) increases slightly with the immersion time for both of the membranes. The slight increase of Z_m is due to the deposit of ZrP on membrane surface, and the sharp decrease of Z_e is attributed to the decrease in the static electrical interaction strength between counterions and the charged groups of the membrane, which is caused by interact of α-ZrP with the ion exchange groups resulting in a reduction of membrane surface charge density. The equivalent circuit models for electrode/solution/membrane/solution/electrode system and electrode/solution system were examined. The results observed in this work seem very interesting in the ion transfer study between phase interfaces.     

Effects of CeO2-ZrO2 present in Pd/Al2O3 catalysts on the redox behavior of PdOx and their combustion activity

The ceria-zirconium-modified alumina-supported palladium catalysts are prepared using impregnation method with H₂PdCl₄ as Pd source, hydrazine hydrate as reducing agent. The physicochemical properties of these catalysts are characterized by BET surface area (BET), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (H₂-TPR) and temperature programmed oxidation (O₂-TPO) techniques, and their catalytic activities for the combustion of methane are examined. The results show that the palladium mainly exist in a highly dispersed PdO species on Ce-Zr-rich grains as well as Al₂O₃-rich grains surfaces, and a stable PdO species due to the strong interaction between PdO and CeO₂-ZrO₂ on the Ce-Zr/Al₂O₃ surfaces. The catalytic activity is strongly related to the redox behavior of PdO species highly dispersed on Ce-Zr-rich grains and Al₂O₃-rich grains surfaces, and the higher the reducibility of the PdO species, the higher the catalytic activity. The presence of Ce-Zr in Pd/Al₂O₃ catalyst would inhibit the site growth of PdO_x particles and decomposition of PdO to Pd⁰, and the reoxidation property of Pd⁰ to PdO_x is significantly improved, which obviously increases thermal stability and catalytic activity of Pd/Ce-Zr/Al₂O₃ catalyst for the methane combustion.     

Enhancement of DD-Reaction yields from the Pd/PdO:Dx heterostructure by N+ and Ne+ ion beams using the GELIS setup

The results on DD-reaction yield enhancement from the Pd/PdO:D_x heterostructure by N⁺ and Ne⁺ ion beams in the energy range of 10–25 keV are presented. Neutron and proton fluxes were measured using a neutron detector based on He-3 counters and a CR-39 plastic track detector. Measurements showed significant DD-reaction yield enhancement effects. The screening potential for this heterostructure under these experimental conditions was determined as U _e = 897 eV.     

Evaluation and use of the Pd | SrCe0.95Yb0.05O3| Pd electrochemical reactor for equilibrium-limited hydrogenation reactions

The current — overpotential characteristics of the H₂ - Pd - SCY interphase have been studied at atmospheric total pressure and temparatures between 400 – 550 °C in the single chamber reactor — cell: Pd | SCY | Pd. The results of I−η measurements indicate that the apparent anodic and cathodic charge tranfer coefficients are equal to: α_a=α_c=0.5. The present results are compared to those obtained with the single — chamber reactor cell: Ag | SCY | Ag. A solid state proton (H⁺) conducting reactor — cell with Pd electrodes was tested for the ammonia synthesis from its elements at atmospheric pressure. At 570 °C, over 75% of the ectrochemically supplied hydrogen was converted into NH₃. The thermodynamic requirement for a high pressure process was eliminated. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Formation of monoclinic zirconia at the anodic face of tetragonal zirconia polycrystalline solid electrolytes

The stability of yttria tetragonal zirconia polycrystalline (Y-TZP) materials with and without the addition of alumina has been investigated during charge flow in solid electrolyte cells. A considerable amount of monoclinic zirconia is formed (up to 50–60%) on the anodic side of the solid electrolyte discs during current flow. The thickness of the surface layer in which maximum transformation takes place was determined to be 3–4.5 m. On the cathodic side, the amount of monoclinic zirconia detected was relatively small (< 5%). The amount of monoclinic formed on the anodic side varied with the microstructure of the ceramic and was considerably less in materials free of pores and with uniform grain size distribution. Relaxation experiments indicate that the tetragonal to monoclinic zirconia phase transformation is related to the oxygen evolution reaction and is not due to oxygenion transport within the solid electrolyte. The observed behaviour has been explained in terms of the creation of space charge layers at the electrode/electrolyte interface leading to the saturation of vacancies by oxygen ions and instability of the tetragonal phase in the surface region on the anodic side of the solid electrolyte.     

L α emission from Ar++H2 collisions in the threshold energy region

In the energy range 2.4 to 33.3 eV_CM, relative cross sections have been measured forL _α emission from impact of Ar⁺ ions in a beam on a H₂ gas target. Absolute cross sections, obtained by normalization to literature data, are 1–10×10^?16cm² for metastable Ar⁺ and 1–20×10^?18cm² for ground state Ar⁺. In the former case, the dominant mechanism is probably dissociative electronic energy transfer, while in the latter case dissociative charge transfer is the most likely process. In addition, at the lowest energiesL _α resulting from a chemiluminescent rearrangement Ar⁺+H₂→ArH⁺+H(2p) has been observed.     

Electronic structure of PdO studied by photoemission (XPS,UPS)

In this paper we present the results of photoemission studies (XPS and UPS) performed on a polycrystalline surface of PdO. The electron density of states (EDOS) deduced both from XPS and UPS (He_I and He_II) are very similar. The valence band of PdO, which differs significantly from the Pd one, can be built up by four structures located at 0.5 eV, 2.2eV, 4.5 eV and 6.5 eV below E_F. The various electronic contributions (p or d) in the band are considered and, in order to explain our spectra, we discuss several hypothesis taking into account the possible existence of satellite lines or crystal field effects. Our XPS and UPS spectra show that the energy bands of PdO are narrow (～ 2–3 eV), moreover the energy shift of the core levels (|ΔE^F_B| = 2 eV) is important : these results suggest that the correlations between the d electrons may be important in PdO.     

The influence of nonpolar organics adsorption on the electrochemical behaviour of powdered activated carbon electrodes in aqueous electrolytes

Adsorption and electrochemical studies were carried out on three activated carbon samples first oxidized, then heat-treated under vacuum (at 180, 500 and 900 °C). The investigations were performed with aqueous electrolyte (Na₂HPO₄ and H₃PO₄) solutions containing selected nonpolar organics (benzene and n-hexane). Adsorption measurements were carried out on solution with a wide range of organics concentration (up to saturation point). Cyclovoltammetric curves of powdered electrodes prepared from the activated carbon samples were recorded for the organics in saturated solutions. The electric double layer capacities of the anodic and cathodic parts were estimated, and the surface anodic and cathodic charge was calculated both in absence and presence of organics in the electrochemical systems. The relative surface charge (in relation to systems without organics) was found to decrease with a reduction in the concentration of surface oxygen-containing groups. Other physicochemical parameters characterizing the degree of surface oxidation (total oxygen concentration, primary water adsorption centres) were also taken into consideration. The correlation between adsorption capacity towards the nonpolar organic compounds (obtained from adsorption isotherms) and change of surface charge was analyzed.     

Hydrogenation of carbon dioxide in a proton conducting cell reactor

The CO₂-H₂ reaction was studied at 723–973 K and atmospheric total pressure using a proton conducting double-chamber cell-reactor. The proton conductor was a strontia-ceria-ytterbia (SCY) perovskite of the form: SrCe_0.95Yb_0.05O_3−α. Palladium films served as cathodic and anodic electrodes. CO₂/He mixtures were introduced over the cathodic electrode while high-purity hydrogen was passing over the anode. The effects of both the imposed current and the inlet gas composition on the reaction rates and product distribution were examined. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Electro-optical response due to mixed conduction electrodes, compared to ferroelectric ones, in asymmetric nematic liquid crystal cells

The usual liquid crystal cells show an electro-optical response symmetric to the applied voltage. On the contrary, when mixed conduction films such as tungsten trioxide or vanadium pentoxide are inserted on one side of the cell, the electro-optical response is asymmetric with respect to the applied voltage. Depending on the structural phase of the inserted film the electro-optical response is in phase (transmission ON during the anodic polarization on metal oxide electrode, transmission OFF during the cathodic one) or in opposition of phase (reverse combination). The first case has been explained by the formation of a double charge layer at the interface metal oxide–liquid crystal, because of migration of protons contained in the oxide films. The other case has been found after high-temperature treatments of metal oxides deposited on glass–ITO substrates, and it appears quite similar to the response collected by using films of ferroelectric materials such as lead zirconium titanate (PZT). This fact suggests a possible interpretation in terms of dielectric response, when the mixed conduction films undergo high-temperature treatments. A comparative study of films of WO₃, V₂O₅, and PZT is carried out in this work as a function of the annealing temperature. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Initial growth at the F16CoPc/Ag(111) interface

An extended, combined (STM/STS)–(UPS/XPS) study was carried out towards a comprehensive understanding of the mechanism responsible for the F₁₆CoPc/Ag(111) interface formation. The evolution of the morphology and the electronic properties at the organic/metal interface is investigated for the early-stage growth of the ultrathin molecular film. Template-guided molecular structures are formed via a strong molecule–substrate interaction which leads to the formation of a new adsorption-induced interface state close to the Fermi energy (E_F). With increasing the thickness the molecular coupling to the metal surface states becomes less important while the more dominant molecule–molecule interaction governs the second layer formation. The quenching of the interface state upon increasing the molecular thickness, together with the changes observed in the Co 2p and F 1s core levels, is explained based on a charge transfer at the interface and a corresponding charge redistribution within the molecular ligand. A detailed “picture” of the energy level alignment close to E_F is achieved by correlating the high resolution UPS and highly localized STS data.     

Probing electric field distribution in organic double-layer diode by electric field induced optical second harmonic generation

Analyzing spectroscopic optical properties of an organic double-layer diode comprised of α-NPD and Alq₃ layers, we studied the selectively probing of electric field distribution in one of the two layers by using the microscopic electric field induced optical second harmonic generation (EFISHG) measurement. Spectroscopic SHGs from Indium–Zinc-Oxide/N,N-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine/tris(8-quinolinolato) aluminium/Al (IZO/α-NPD/Alq₃/Al) diodes were measured. Results showed that the SHG peaks were generated at 940 and 1050 nm from the α-NPD and Alq₃ layers, respectively, due to the EFISHG process, and the electric field in each layer can be selectively probed. The contribution of the accumulated charge at the double-layer α-NPD and Alq₃ interface was also identified by the d.c. voltage dependence on the EFISHG intensity.