Impedance characteristics of three-phase electrodes on solid electrolytes

Previously obtained theoretical expressions for the impedance associated with the diffusion of neutral species through a permeable electrode and along the electrode/electrolyte interface are discussed as they apply to gas electrodes on solid electrolytes. A new result, describing diffusion under circular electrode contacts, is obtained. Representative impedance-plane, admittance-plane and Bode-type plots are given for each theoretical model. The effects of rate-controlling adsorption-desorption exchange with the gas phase are considered, as is the applicability of the idealized theoretical models to irregular electrodes.     

Effect of surface acoustic waves on activity in heterogeneous catalysis

A synopsis of the recent developments in acoustically influencing and controlling gas-surface interactions is presented. The cleaning effect of ultrasound and its surface activation play an important role for the sonochemical enhancement of reactivity in chemical processes involving solid and liquid phases. So far, there have only been a few studies on the effects of surface acoustic waves on surface chemical reactions under high-vacuum conditions by the application of piezoelectric surface acoustic wave transducers. Very recently, metal films deposited between InterDigital Transducer (IDT) electrodes on a LiNbO₃ substrate have shown a significant inerease in catalytic activity during surface acoustic excitation and Edge-Bonded Transducers (EBT) with a metal single crystal as a substrate have been used to acoustically influence the rate in the oscillatory reaction for CO oxidation. Tunable narrowband surface acoustic excitation is anticipated to be an efficient route to control catalytic processes, and in our work this approach is being used to investigate the physical basis of this process.     

Non-faradaic electrochemical modification of the catalytic activity for propane combustion of Pt/YSZ and Rh/YSZ catalyst-electrodes

The effect of non-faradaic electrochemical modification of catalytic activity (NEMCA effect) or electrochemical promotion (EP) was investigated in the total oxidation of propane on porous Pt and Rh catalyst-electrode films interfaced to 8 mol% Y₂O₃-stabilized-ZrO₂ (or YSZ), in the temperature range 425–520 °C and for sub-stoichiometric O₂ to propane ratios. Application of either positive or negative overpotentials resulted in non-faradaic increase of the catalytic rate, by up to a factor of 4 in the case of Rh and by up to a factor of 1350 in the case of Pt. The rate increase observed in the case of Pt is among the highest ones reported so far in NEMCA studies with oxygen ion conductors as active supports.     

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.     

Investigation of solid sodium reference electrodes for solid-state electrochemical gas sensors

Several types of solid reference electrodes for potentiometric gas sensors based on solid sodium ion conductors (e.g., Na⁺-/-Alumina or NASICON) are described and the observed experimental results are discussed in view of theoretical predictions. Defined sodium activities are established by appropriate equilibria of binary and ternary phases. The binary systems Na-Sb and Na-Bi, and the ternary systems Na-M-O (M is a transition-metal, Co or Ni) were found to show excellent kinetic performance and are capable to fix the chemical potential of sodium well defined over extended periods of time. Some of the sodium reference electrodes are even stable in air. The results are compared with the application of elemental sodium as reference electrode.     

Isotope effects in the interaction of hydrogen and deuterium with a rhodium (110) surface

The adsorption of H₂ and D₂ on a Rh (110) surface at 100 K leads to a sequence of ordered phases, among others 1×2 phases at _H =0.5 and at _H =1.5 which likely involve a partial surface reconstruction consisting of a small perpendicular displacement of Rh surface atoms. The structure of the adsorbate phases is strongly correlated with the binding energy of the adsorbed phases. Three H (D) binding states (₁,₂ and) are populated at saturation as determined by thermal desorption spectroscopy (TDS). Whereas the peak temperature of the state is invariant with the hydrogen isotope, the D ₁ state appears at a 8 Klower and theD ₂ state at a 5 Khigher temperature than the respective H states. Generally the D phases exhibit a better long-range order than the H phases. The rate of adsorption is identical for the first three adsorbed phases but D₂ adsorbs appreciably faster in the 1×2–3H and the final l×1–2H phases.Zero point energy effects as well as a H coverage dependent local interaction model could account for the observed effects.     

Dynamic order in a surface process

Under certain well-defined conditions (p _co, ,T) the rate of catalytic oxidation of CO on a Pt(110) surface may exhibit sustained temporal oscillations with an autonomous frequencyv ₀. Small amplitude modulation of with frequencyv _p causes a variety of phenomena characteristic for systems of nonlinear dynamics which may be identified with temporal order and show formal similarities to spatial order of surface phases: Periodic behavior for certain rational numbers ofv _p/v₀ — corresponding to commensurate surface structures; quasiperiodic behavior characterized by an irrational ratio of the periods of perturbation and response — corresponding to incommensurate structures; and critical slowing down near the boundary of a transition to quasiperiodicity which has its counterpart in the critical fluctuations near a (spatial) phase transition.     

The influence of surface reconstruction on the kinetics of adsorption-desorption processes: A Monte Carlo study of a simple model

The influence of surface reconstruction on the kinetics of adsorption-desorption processes is studied through a simple two-position model by means of Monte Carlo simulation. Effects due to constraints on the translational motion of activated complexes and to heterogeneity are particularly investigated. Heterogeneity emerges as the most important factor to explain the huge variation of the preexponential Arrhenius parameter with coverage observed in the H/W(001) system. In the present model it is conjectured that heterogeneity originates from additional interactions of H with surface or sub-surface W atoms when hydrogen is adsorbed on sites where surface W atoms are farther apart due to reconstruction.     

Synthesis and catalytic activity of hybrid metal/silicon nanocomposites

In this Letter we demonstrate that hydrogen‐terminated porous silicon (PSi) layers and powders can serve as highly efficient reductive templates for noble metal salts. The reduction results in metal nanoparticle (NP) formation in the pores of PSi. Gold NP formation has been monitored in‐situ by measuring the plasmon resonance response. Pt NPs, formed in the PSi matrix, were investigated by transmission electron microscopy and energy‐dispersive X‐ray analysis. Furthermore, hybrid Pt/PSi nanocomposites exhibit a high catalytic activity for CO oxidation.

     

Design parameters for measurements of local catalytic activity on surfaces

Computational fluid dynamics in combination with experiments is used to characterize a gas sampling device for measurements of the local catalytic activity on surfaces. The device basically consists of a quartz capillary mounted concentrically inside an aluminum tube. Reactant gas is blown toward the catalytic surface through the annulus between the tubes, and the gas is sampled close to the surface by the capillary. The influence of various design parameters on the lateral resolution and sensitivity of the measurements is investigated. It is found that the outer diameter of the annulus sets the upper limit for the lateral resolution of the measurement, and that a flow rate of the order of 240 (ml/min)_n is sufficient to achieve this resolution. The sensitivity is reasonable also with high flow rates, due to the presence of a pocket of stagnant gas under the tip of the capillary. Furthermore, the limits of the range in reaction rate, which can be studied are estimated.     

Evaluation of solid electrolyte cells with a versatile electrochemical technique

Voltage losses in fuel cells and other solid electrolyte systems are due to several mass transport and kinetics processes at the electrode/electrolyte interface as well as to ohmic contributions from the electrolyte, electrodes, current collectors and contact resistances. Electrochemical impedance spectroscopy (EIS) has been in use for several decades in fuel cell research and is quite effective in determining the contribution of individual electrode and electrolyte processes. However, data acquisition and analysis can be time-consuming and the technique has many limitations whilst cell performance and operating conditions are varying rapidly with time especially when the cells are under current load. The galvanostatic current interruption (GCI) technique is fast and can be used under a wide range of operating as well as for rapidly varying loads and cell performance conditions. In this paper a totally new and very simple way of adapting commercially available equipment has been described to perform high quality, reliable and fast GCI measurements over a range of different currents in one sequence without having to use an electronic switch or a solid state relay or a separate fast data logging system. Its versatility has been demonstrated with a number of standard RC circuits simulating slow electrode and fast electrolyte processes and by evaluating a number of solid oxide fuel cell materials. The GCI technique has been shown to be able to determine the composition of all standard test circuits within ±1 % of those determined from the EIS technique and actual values of circuit components. The technique has been applied to investigating solid electrolyte cells and produced excellent results.     

Laser-stimulated desorption of potassium atoms

Desorption of K atoms by laser-excitation of surface plasmons in small K particles is reported. The desorption rate has been measured for different laser wavelengths and particle sizes. Time-of-flight measurements reveal a kinetic energy of the desorbed atoms of E_kin=0.13(3) eV. From the experimental data it is concluded that the desorption mechanism is non-thermal in nature. Comparison of the results reported here with our earlier work on Na desorption is made.     

Investigation of native-oxide growth on HF-treated Si(111) surfaces by measuring the surface-state distribution

The evaluation of the surface state distribution of differently HF-treated Si(111) surfaces during the native-oxide growth in air is investigated by the large-signal field-modulated photovoltage technique. The surface state distribution consisting of intrinsic and extrinsic Si dangling bond defects is directly related to the state of oxidation of the Si surface. It is shown that the kind of HF treatment strongly influences the concentration of extrinsic defects with a lower state of oxidation. Special HF preparations for H termination of the Si(111) surface result in a nearly intrinsic surface state distribution. During the oxidation process three typical phases can be distinguished each characterized by specific defect structures. It was found that native-oxide growth is highly sensitive to the concentration of extrinsic defects directly after HF treatment.     

CO-H2-O2 reaction on a catalytic surface: A computer simulation study

The oxidation of carbon monoxide to form carbon dioxide and the oxidation of hydrogen to form water are the reactions of environmental and industrial importance. These two reactions have been studied independently by Monte Carlo computer simulation using Langmuir-Hinshelwood mechanism but no effort has been made to study the combined CO-H₂-O₂ reaction on these lines. Keeping in view the importance of this 3-component system, the surface coverages and production rates are studied as a function of CO partial pressure for different ratios of H₂ and O₂. The diffusion of reacting species on the surface as well as their desorption from the surface is also introduced to include temperature effects. The phase diagrams of the system are drawn to observe the behavior of these atoms/molecules on the surface and the production of CO₂ and H₂O are determined at different concentrations of H₂. The results are compared with 2-component systems.     

On the adsorption and desorption of H2 at metal surfaces

Recent technological developments have made possible measurements of the distribution of internal levels of molecules desorbing from a hot surface. Such measurements provide new information concerning the desorption process and the potential energy surface (PES) that governs it. Associative, or re-combinative desorption is of particular interest because the distributions of internal levels reflect the manner in which the molecular bond is formed as the desorbing species leaves the surface. As the simplest associative desorption systems, H₂ and D₂ adsorbing on and desorbing from metal surfaces deserve special attention and serve as prototypes for systems with a more complex chemistry. In this note I review briefly from the theoretical point of view some features of the interaction of H₂ with metals and their relevance to associative adsorption and dissociative sticking.     

Photocatalytic activity enhancement of TiO2 films by micro and nano-structured surface modification

Titanium oxide thin films were deposited by spin coating using a precursor solution of titanium oxide (IV) acetylacetonate. To increase the contact surface area of the films, TiO₂ microspheres were added to the surface of the films. These spheres were 2 μm in diameter and formed agglomerates on the surface. They did not spread uniformly across the substrate, creating different roughnesses and morphologies along the surface of films. Photocatalytic properties of the samples were tested by the degradation of a methyl orange solution. The degradation performance was compared between plain films, films with microspheres and films covered with commercial TiO₂ P25 powder. The results indicate that the samples that were surface modified with TiO₂ microspheres present a photodegradation reaction rate 62 times higher than that obtained for plain TiO₂ films. The rate of reaction of the samples covered with P25 was 2 times greater than that obtained for the samples with microspheres, but the adhesion to the film was better in the case of microspheres. Moreover, samples with microspheres could be reused several times maintaining the same structural and photocatalytic properties.     

Reactive sputtering deposition and electrochemical characterisation of thin solid electrolyte membranes for solid oxide fuel cells

Solid oxide fuel cells directly convert the chemical energy of a fuel into electricity. To enhance the efficiency of the fuel cells, the thickness of the gastight solid electrolyte membranes should be as thin as possible. Y₂O₃-stabilised ZrO₂ (YSZ) electrolyte films were prepared by reactive sputtering deposition using Zr/Y targets in Ar/O₂ atmospheres. The films were 5 – 8 μm thin and were deposited onto anode substrates made of a NiO/YSZ composite. After deposition of a cathode with the composition La_0.65Sr_0.35MnO₃ the electrochemical properties of such a fuel cell were tested under operating conditions at temperatures between 600 °C and 850 °C. Current-voltage curves were recorded and impedance measurements were performed to calculate apparent activation energies from the fitted resistance data. The conductivity of the YSZ films varied between 4.6·10⁻⁶ S/cm and 2.2·10⁻⁵ S/cm at 400 °C and the fuel cell gave a reasonable power density of 0.4 W/cm² at 0.7 V and 790 °C using H₂ with 3 % H₂O as fuel gas. The gas compositions were varied to distinguish the electrochemical processes of the anode and cathode in the impedance spectra. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Plasma-electrolytic formation, composition and catalytic activity of manganese oxide containing structures on titanium

In the present work, we report the data about formation of TiO₂-rutile or TiO₂ and Mn₂O₃, Mn₃O₄ containing oxide structures on titanium in aqueous electrolytes by means of plasma-electrolytic deposition. The layers formed are characterized by X-ray diffraction, electron probe microanalysis and scanning electron microscopy methods. The PEO coatings on titanium formed in sodium tetraborate solution contain the TiO₂ stabile rutile modification that is important when utilizing such a structure as a catalyst carrier. Manganese acetate adding into the electrolyte leads to formation of layers that contain Mn₂O₃, Mn₃O₄ and TiO₂-rutile in outer region. The manganese content in the surface layer depends on the formation conditions as well as on manganese acetate concentration in the electrolyte. Catalytic activity of the layers in CO → CO₂ reaction is studied in the static and flow conditions. The manganese-containing layers obtained possess the catalytic activity in CO → CO₂ oxidation reaction at the temperature range of 250-350 °C. The catalytic activity depends on the concentration and surface distribution of manganese as well as on the layers morphology.     

Photoemission of molecular adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N₂, and CO₂ adsorbates to discuss the various aspects of photoemission of molecular adsorbates.