Electrical degradation of porous and dense LSM/YSZ interface

Electrochemical cells formed by the interface between dense and porous lanthanum strontium manganate (LSM) and yttria stabilized zirconia (YSZ) were submitted to annealing temperatures varying from 1373 K to 1673 K for 200 h and studied by Impedance Spectroscopy (IS) in order to investigate how the high annealing temperature can modify the contact between LSM/YSZ and to which extension these changes influence the electrical behavior of dense and porous LSM electrodes before and after the formation of insulating phases. Up to 1473 K the annealing process did not lead to substantial electrical behavior modifications at the LSM/YSZ interfaces for both porous and dense electrodes. IS measurements show two capacitive semicircles, the best fitting of impedance data brings to an equivalent circuit constituted by a serial combination of the electrolyte resistance and two parallel combinations of a resistance and a constant phase element, CPE. The higher frequency semicircles, HF, were attributed to the diffusion of oxide ions from the interface LSM/YSZ to the oxide ion vacancies located at the electrolyte surface. The semicircle at lower frequency, LF, will be ascribed to the oxygen species adsorption and diffusion in the LSM. At 1473 K the only changes recorded are related with the sinterization process of the porous electrodes. Over of 1473 K, the resistance contributions increased largely, especially for porous electrodes, and one additional semicircle was observed. This semicircle was associated to the oxygen diffusion process at the new insulating phases formed from YSZ and LSM solid state reactions. Porous and dense electrodes exhibited different rates for the degradation process. The porous electrode degraded faster than the dense one, probably because of the morphological effects as grain growth and their coalescence during annealing at higher temperatures.     

Photoemission study of oxygen adsorbed on coldly deposited silver films

UPS spectra of coldly deposited silver films differ from those of films deposited at room temperature by electronic states localized at surface defects with an energy about 4.2 eV below E_F. Changes after exposure at 140 K to oxygen only occur in the presence of these defects, demonstrating that oxygen is only adsorbed at defects. Raman vibrational spectroscopy shows that oxygen is adsorbed nominally as O₂^? and O₂^2?. Possible assignments of the oxygen related UPS structures are discussed.     

Effect of starting powder on screen-printed YSZ films used as electrolyte in SOFCs

Screen-printing technology was developed to fabricate dense YSZ electrolyte films onto NiO–YSZ porous anode substrates. A single fuel cell of Ni-YSZ/YSZ (31 μm)/LSM-YSZ was successfully prepared by screen-printing technology. Using humidified hydrogen as fuel and ambient air as oxidant, the fuel cell provided the maximum power densities of 0.18, 0.33, 0.58, 0.97 and 1.3 W/cm² at 650, 700, 750, 800 and 850 °C, respectively. The properties of the starting YSZ powder exerted a significant effect on the characteristics of the screen-printed YSZ electrolyte films. The aggregates of the powder could be partially broken by ball milling. The YSZ powder with a small particle size and a narrow particle size distribution helped to obtain dense YSZ films. The films prepared from the YSZ powder with high aggregates were very porous, which resulted in a low open circuit voltage, a high ohmic resistance, a high polarization resistance and thus a poor cell performance.     

Sensors measuring oxygen activity in melts: Development of impedance method for“in-situ” diagnostics and control electrolyte/liquid-metal electrode interface

An impedance method for periodic“in-situ” diagnostics of the solid electrolyte/liquid-metal electrode interface during the lifespan of yttria-stabilized zirconia (YSZ)-based sensors measuring oxygen partial pressure in melts was developed. It was found that the impact of polarization effects on YSZ, stipulated by corrosive measuring environments (molten alkaline metals), is increasing with increase of the working temperature, which may lead to the appearance blocking reaction layers at the electrolyte/liquid-metal electrode interface. The proposed impedance method allows obtaining necessary information about the electrolyte/liquid-metal electrode interface and about the character and the level of polarization of the liquid-metal electrode.     

Preparation of thin film SOFCs working at reduced temperature

SOFCs are expected to become competitive devices for electrical power generation, but successful application is dependent on decreasing working temperature from 1000 to 800 °C, without detrimental effects on resistance and on electrode processes. This requires a reduction of the stabilised zirconia electrolyte thickness and an optimisation of the electrodes and interfaces. We have studied the preparation of a thin film SOFC device working at intermediate temperatures (less than 850 °C). The electrolyte must be very dense and as thin as possible to avoid ohmic losses. Electrodes, anode and cathode, must be porous enough to enable the gas fluxes to go through. Ni/YSZ cermet anodes have been prepared by a conventional ceramic method and support the cells. Thin Yttria Stabilised Zirconia electrolytes (YSZ) have been deposited by RF sputtering, DC sputtering and spray pyrolysis onto a Ni/YSZ cermet. Thin film La_0.7Sr_0.3MnO₃ (LSM) cathodes have been deposited onto the electrolytes by a spray-pyrolysis method. We present here the preparation and the characterisation of each component and the electrochemical performance of such cells at 850 °C. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Porous electrolyte-supported tubular micro-SOFC design

Due to the poor redox cycling resistance of the second generation of μ-SOFCs, a new generation of SOFC has been recently developed using a porous electrolyte-supported structure to overcome this problem. In this research, the porous structure was successfully fabricated with slip casting using calcined YSZ (ZrO₂ + 8 mol% Y₂O₃) with or without graphite as a pore former. Calcination of YSZ powder at 1300-1500 °C prior to making the slip leads to growth of YSZ crystals and particle size which results in a decrease in surface area and powder sinterability. This was found to be an important criterion in developing the porous structure as, due to the high sinterability of non-calcined YSZ, even the addition of graphite is inadequate to generate sufficient open porosity. A dense YSZ electrolyte layer was immediately coated on the porous structure using YSZ calcined at 1300 °C with a sequential slip casting method. Sample thickness was found to be a function of both graphite content as well as YSZ calcination temperature. Physical properties of the porous YSZ supports and SEM analysis of the support and coated electrolyte are presented.     

Photoemission studies of H2S,H2 and S adsorbed on Ru(110): Evidence for an adsorbed SH species

H₂S, H₂ and S adsorbed on Ru(110) have been studied by angle-integrated ultraviolet photoemission (UPS) as part of a study of the effect of adsorbed sulfur, a common catalytic poison, on this Ru surface. For low exposures of H₂S at 80 K, the work function rises to a value 0.16 eV above that of clean Ru(110) while the associated UPS spectra (hν = 21.2 eV) exhibit features similar to those of H(ads) and S(ads) and different from those of molecular H₂S. We conclude that H₂S dissociates completely at low coverages on Ru(110) at 80 K. At intermediate exposures the work function drops and the UPS spectra show new features which are attributed to the presence of an adsorbed SH species. This appears to be the first direct observation of this surface complex. At higher exposures the work function saturates at a value 0.36 eV below the clean value; the UPS spectra change markedly and indicate the adsorption of molecular H₂S. Heating adsorbed H₂S leaves a stable layer of S(ads) on Ru(110). The surface with adsorbed sulfur strongly modifies the adsorption at 80 K of a number of molecules relative to the clean Ru(110) surface.     

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     

Photoelectrochemical behaviour of ruthenium disulphide electrodes in contact with aqueous electrolytes

RuS₂, which is a semiconductor, has been prepared as a sintered material. As an electrode, it has a high catalytic activity for oxygen evolution from aqueous solution in the dark. On illumination, an oxidation reaction occurs at potentials more negatively by 1 V. This photoreaction with water was studied as a function of the photon energy, the pH of the electrolyte, and the electrochemical history of the electrode. The interpretation of the behaviour is complicated by the apparent dependence of the double layer potential on the total electrode potential and the pH which leads to a considerable shift of the bands of the semiconductor with respect to the redox levels of the electrolyte. Kinetic evidence also indicates the involvement of surface states in a reduction of the oxidation products of water. The remarkable photoelectrochemical stability of RuS₂ is discussed in terms of a high concentration of Ru d-states in the valence band and the presence of S₂^2? ions in the crystal lattice. The prospects for RuS₂ as a photocatalytic material for oxygen evolution seem promising, but a full evaluation is at present hampered by the occurrence of unfavourable side reactions, probably caused by the polycrystalline and porous nature of the available material. In this first investigation we study the microcrystalline sintered product which would be more likely to be a practical electrode. Later we will present a more fundamental study using single crystals of RuS₂.     

Measurement of electrode overpotentials for direct hydrocarbon conversion fuel cells

Cathodic and anodic overpotentials were measured using current interruption and AC impedance spectroscopy for two separate solid oxide fuel cells (SOFCs). The fuel cells used yttria-stabilized zirconia (YSZ) as the electrolyte, strontium-doped lanthanum manganite (LSM) as the cathode, and a porous YSZ layer impregnated with copper and ceria as the anode. The Cu/CeO₂/YSZ anode is active for the direct conversion of hydrocarbon fuels. Overpotentials measured using both current interruption and impedance spectroscopy for the fuel cell operating at 700 °C on both hydrogen and n-butane fuels are reported. In addition to providing the first electrode overpotential measurements for direct conversion fuel cells with Cu-based anodes, the results demonstrate that there may be significant uncertainties in measurements of electrode overpotentials for systems where there is a large difference between the characteristic frequencies of the anode and cathode processes and/or complex electrode kinetics.     

Behaviour of a dense In2O3/ YSZ electrode in oxygen containing atmospheres

A study of electrical and electrochemical properties of a dense In₂O₃ electrode in contact with a single crystal YSZ electrolyte was carried out by d.c. and a.c. methods. As a result, it was found that dense In₂O₃ electrodes have high electrical conductivity but very low electrochemical activity. In a vicinity of the equilibrium potential and under the anodic polarisation, the rate of Faraday reaction at the In₂O₃ electrodes was as low as to consider the electrode a blocking one. The blocking properties of the In₂O₃ electrodes were used to measure the hole conductivity of the YSZ electrolyte in the temperature range between 795 to 1163 K and oxygen partial pressure from 1 to 10⁵ Pa. A comparison with the literature data confirmed that the dense In₂O₃ electrode blockes the ionic transfer through the YSZ. A set of experiments indicated that the oxygen exchange between the indium oxide surface presented to the oxygen containing gaseous phase and this phase is very poor. A route of the electrode process at O₂, In₂O₃ / YSZ electrode was proposed a limiting stage of which is the discharge of the oxygen ions to the atomic oxygen adsorbed on the electrode surface: $$O_0 ^x \left( {In_2 O_3 } \right)_s = V_0 ^{ \bullet \bullet } \left( {In_2 O_3 } \right)_s + O_{ad} \left( {In_2 O_3 } \right)_s + 2e'\left( {In_2 O_3 } \right).$$ The polarisation resistance decreases when platinum or the praseodymium oxide is deposited on the surface of the In₂O₃ electrodes. The cathodic polarisation also increases the electrochemical activity of the electrodes. Both the establishment of the steady state of the electrode under polarisation and the recovery of the equilibrium state by the electrode are very long processes, which are probably related to the diffusion mechanism by which the stoichiometry of the indium oxide is changed.     

Evidence from photoelectron spectroscopy for dissociative adsorption of oxygen on nickel oxide

XPS and UPS spectra from oxygen adsorption at high temperature on polycrystalline nickel oxide surfaces pre-heated at 700°C and 1450°C are presented. Adsorption results in complete loss of surface charge on both surfaces. There is an increase in intensity of the 0(1s) 529.7 eV peak, attributed to O^2? ions, and the $\text{Ni}\text{(2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ spectra show a shift of intensity to the 854.6 eV peak due to Ni²⁺ ions. The results are compared with previous data from kinetic, conductivity and electrochemical measurements. Agreement with a model of O^2? and nickel vacancy production is demonstrated. UPS spectra indicate considerable reorganisation of electronic charge in the surface of the 1450°C pre-heated oxide, after oxygen adsorption, giving an almost stoichiometric surface similar to that of a cleaved single crystal.     

砷化镓上银和金膜的价带光电子谱

木文测量了光子能量为21.2eV,40.8eV和1486.6eV的光电子谱,得到了关于GaAs(110)解理面上银膜的价带能谱新数据,并得到金膜价带能谱的补充数据,蒸发的银膜和金膜的厚度范围为0.l?到1000?,实验上发现并讨论了下面的现象:GaAs(110)面上金膜和银膜的紫外价带光电子谱的强度与膜厚的关系曲线中出现极大峰。 关键词：     

Work function measurements on indium tin oxide films

We determined the work function of indium tin oxide (ITO) films on glass substrates using photoemission spectroscopy (PES). The ITO coated glass substrates were chemically cleaned ex-situ, oxygen plasma treated ex-situ, or sputtered in-situ. Our results suggest that the performance of ultraviolet photoemission spectroscopy (UPS) measurements can induce a significant work function reduction on the order of 0.4–0.5 eV, on ex-situ chemically and oxygen-plasma treated ITO samples. This was demonstrated by the use of low intensity X-ray photoemission spectroscopy (XPS) work function measurements before and after the UPS measurements were carried out.     

Absorption,fluorescence, and SERS spectra of sanguinarine at different pH values

We have studied the absorption, fluorescence, and surface-enhanced Raman scattering (SERS) spectra of sanguinarine using a silver hydrosol and an electrochemical cell with a silver working electrode for different pH values in the medium. We carried out quantum chemical calculations in order to interpret the electronic and vibrational spectra and to establish their correlations with the structure of the molecules. We optimized the structure and determined the spectral characteristics of the cationic and neutral forms of the sanguinarine molecules in solution and adsorbed on the surface of an anodized Ag electrode for different potentials. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 604–609, September–October, 2007.     

Oxygen reduction on lanthanum strontium manganites investigated by the use of cone shaped electrodes

The oxygen reduction kinetics has been investigated by impedance spectroscopy on two different cone shaped electrodes of La_0.85Sr_0.15MnO₃ in contact with either YSZ or CGO electrolyte pellets. The contact area has been varied by varying the mechanical load on the electrode on both polarized and unpolarized electrodes. The experiments showed the activation of the electrode observed after a cathodic polarization to be caused by the creation of an active zone on the electrolyte close to the triple phase boundary. As a result of this, the oxygen reduction on polarized electrodes probably takes place by several parallel routes making a kinetic investigation complicated. Impedance analysis of unpolarized electrodes at different oxygen partial pressures showed a charge transfer process at high frequencies involving oxygen defects and a mass transport limited reaction at lower frequencies. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Reaction mechanisms of the copper-manganese spinel cathode in a solid oxide fuel cell

Cu_1.25Mn_1.75O₄ spinel (CMO) was studied as a potential solid oxide fuel cell (SOFC) cathode material at intermediate temperatures. The reaction mechanism of a composite cathode consisting of Cu_1.25Mn_1.75O₄ and yttria-stabilized zirconia (YSZ) was investigated by impedance spectroscopy. The influence of the CMO/YSZ ratio, time exposed to current passage and temperature on the impedance spectra was examined. Activation energy of the corresponding processes was calculated to be near 1 eV and between 1.32 and 1.96 eV for the high and low frequency arcs in the impedance spectra. Comparison between CMO-YSZ and Sr-doped LaMnO₃ (LSM)-YSZ composite cathodes showed they had similar reaction mechanisms. The transport or transfer of oxygen intermediates or oxide ions between the catalyst and electrolyte was suggested to be the rate determining steps between 700 and 800 °C, whereas dissociative adsorption, mass transfer and surface diffusion were rate controlling between 600 and 700 °C.     

Adsorption of thiophene on Al(1 1 1) and deposition of aluminum on condensed thiophene

The adsorption of thiophene on clean Al(1 1 1) at 90 and 130 K has been studied with X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and work function measurements. Relatively weak chemisorption compared to adsorption on transition metals is indicated by minor changes in the valence spectrum in progressing from monolayer to multilayer thiophene, a modest work function change of −0.50 eV due to saturation dosing, and return of the work function and valence spectrum to that of clean Al(1 1 1) upon annealing at 210 K. The complementary experiment in which aluminum is thermally deposited on multilayer thiophene condensed on gold at 130 K has also been performed. XPS peak area analysis shows that metal doses less than 14×10¹⁵ atoms/cm² result in penetration through the physisorbed thiophene, but higher doses lead to the gradual build up of metal throughout the organic layer. Persistence of the thiophene UPS valence features for metal doses of 50×10¹⁵ atoms/cm² is consistent with penetration and aluminum island formation. For aluminum deposition on thiophene, charge transfer from aluminum is evidenced by metal-induced low binding energy components in the C 1s and S 2p spectra at 282.6 and 162.5 eV, respectively, and a shift in the Al 2p spectrum of 0.5 eV to higher binding energy compared to metallic aluminum. UPS also indicates progression of the frontier orbital toward the Fermi level as aluminum is deposited.     

An investigation of the adsorption of oxygen and oxygen containing species on platinum by photoelectron spectroscopy

It is shown that XPS can detect 0.01 monolayers of adsorbed carbon or oxygen and can identify the chemical state of the adsorbed atom(s). Two states of adsorbed oxygen were resolved by thermal desorption spectroscopy and by XPS. The O 1s binding energies (^FE_B) were 530.2 and 533 eV below the platinum Fermi level for the strongly and weakly adsorbed states respectively. (^FE_B) did not vary with coverage. The resulting apparent variation of (^VE_B), the vacuum level referenced value, is discussed in terms of a simple model for the work function Φ which was measured in situ. UPS indicated that the weakly adsorbed state is probably molecular, with levels at 6.1, 9.3, 10.4 and l2.4 eV below the Fermi level. The main change in the UPS spectra produced by the strongly adsorbed state was a reduction of a peak close to the Fermi level.     

Potential-dependent structure of the interfacial water on the gold electrode

Doubly tunable sum frequency generation (SFG) spectra demonstrate that the water molecules at gold/electrolyte interface change their orientation with applied potential. At negative potentials, water molecules in the double layer align with their oxygen atom pointing to the solution. As potential became positive to be close to the potential of zero charge (PZC), the SFG signal decreased, suggesting the OH groups of the water molecule are either in random orientation or parallel to the electrode. As potential became more positive than the PZC, the SFG signal increased again with the oxygen-up orientation as same as in the negative potential region, indicating that water molecules interact with the adsorbed sulfate anions. The peak position of the SFG spectra indicates a relatively disordered state of water molecules at the gold electrode surface, in contrast to the previously observed ice-like structure of water at electrolyte/oxide interfaces.