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This work presents the poisoning effect of organic oxo-silicon compounds (siloxanes) which are generally found in sewage biogas. Lifetime and durability associated with solid oxide fuel cell (SOFC) technology is strongly related to the amount of contaminants that reach the stack. Several experiments on Ni anode-supported (AS) single cells are performed in order to clarify the mechanism of degradation and also the possibility of cell performance recovery. Three experiments focus on the degradation and recovery of AS Ni-YSZ fed with H-2, co-feeding 5 ppm D4-siloxane (octamethylcyclotetrasiloxane, C8H24O4Si4) as representative compound for the organic silicon species, at 800 degrees C. A fourth experiment focuses on the durability of the AS Ni-YSZ cell with variable concentrations of the impurity (0-5ppm), during steady state polarization (0.25Acm(-2)) for 250 h, using simulated biogas-reformate fuel H-2/CO/CO2/H2O: irreversible degradation was observed with the D4-impurity feed in the anode gas. Post-test scanning electron microscopy (SEM) results indicate formation of SiO2(s) deposits, which block pores and reduce the TPB length.

The present paper investigates the variation in performances in co-electrolysis of steam/CO2 on SOEC single cells caused by the progressive increase of CO2 inlet concentration and current density. Steam electrolysis and co-electrolysis were investigated on circular Ni-YSZ supported cells produced by SOLIDpower (“ASC-700” type cells). The thin film electrolyte is constituted by YSZ and the oxygen electrode by LSCF-GDC. The active area corresponds to 12.56 cm2 (2 cm of radius). The cells were fed with different H2O/CO2 mixtures, while keeping 10% H2 of the inlet molar flow in order to avoid re-oxidation of the Ni-YSZ support. A sensitivity analysis was performed looking in particular at the effects of inlet gas composition, gas flows and temperature on the performances. I-V characterization and EIS measurements at different current densities were carried out for each sequence. In steam electrolysis mode, the reference inlet molar flow was equal to 150 Nml/min (90% H20, 10% H2), at 750°C. The I-V curve showed a voltage of 0.861V at OCV, 1.049V at 0.5 A/cm2 and 1.253V at 1 A/cm2. A sensitivity analysis was performed varying the temperature (700°C, 750°C and 800°C), the inlet air flow (100 Nml/min, 300 Nml/min and 500 Nml/min) and the total inlet fuel flow (from 8 to 16 Nml/min/cm2). In this case, the steam conversion factor was kept constant equal to 50% (adjusting the current imposed). EIS analysis exhibits that ohmic and polarization resistances decrease with current densities lower than 0.4 A/cm2 and then rise with higher values. As expected, the increase of temperature has a positive impact on the performances, while the change of air and fuel flow slightly affects them. In co-electrolysis mode the CO2 and H2O content was varied, keeping constant the total molar flow and the hydrogen fraction as explained before. As shown in the enclosed image, the area specific resistance (ASR) increases with the CO2 concentration in the mixture and gas transport limitation occurs at lower current densities. Also in this case a sensitivity analysis was conducted changing the temperature (from 700°C to 800°C) and the resulting EIS plots demonstrate different ongoing processes favored at high current densities. In particular, the reverse water-gas-shift reaction plays an important role on the overall balance of the gas species, and it directly depends on temperature and inlet fuel composition. Figure 1

This paper presents an experimental study of the impact of toluene on the performance of anode-supported (AS) Ni-YSZ SOFC operating at ~800 °C, 0.25 A/cm2 fed with H2 and reformed biosyngas. Toluene was added to the fuel stream and its concentration increased stepwise from tens to thousands of ppm. Each poisoning test was followed by a recovery step (no toluene in the fuel stream) for the duration of 25 hours. The main goal of this work is to define a concentration threshold of toluene as a model tar compound in the fuel stream and later to identify the degradation mechanism caused by this tar compound. Polarization behavior, current-voltage and electrochemical impedance spectroscopy (EIS) were analyzed to evaluate the cell performance. A linear degradation is observed at concentrations above 1000 ppmv for the cell which was fed with pure H2 (cf Figure) The degradation was due to local C(s) deposition. In contrast, toluene even up to 3500 ppm did not cause a severe degradation when the cell was fed with reformed biosyngas. Figure 1

Lifetime and durability issues connected with Solid Oxide Fuel Cell (SOFC) technology are strongly related to the amount of contaminants that reach the stack. In this study the focus is on organic silicon compounds (siloxanes) and their highly detrimental effects on the performance of SOFC Ni-YSZ anodes. The involved mechanism of degradation is clarified and quantified through several test runs and subsequent post-mortem analysis on tested samples. In particular, experiments on both Ni anode-supported single cells and 11-cell-stacks are performed, co-feeding D4-siloxane (octamethylcyclotetrasiloxane, C8H24O4Si4) as model compound for the organic silicon species which are generally found in sewage biogas. High degradation rates are observed already at ppb(v) level of contaminant in the fuel stream. Post-test analysis revealed that Si (as silica) is mostly deposited at the inlet of the fuel channel on both the interconnect and the anode side of the cell suggesting a relatively fast condensation-type process. Deposition of the Si was found on the interconnect and on the anode contact layer, throughout the anode support and the three phase boundary in the anode, correlating with the observed increase of polarization losses from the EIS analysis of tested cells. (C) 2015 Elsevier B.V. All rights reserved.

The vanadium-dependent chloroperoxidase Mcl24 was discovered to mediate a complex series of unprecedented transformations in the biosynthesis of the merochlorin meroterpenoid antibiotics. In particular, a site-selective naphthol chlorination is followed by a sequence of oxidative dearomatization/terpene cyclization reactions to build up the stereochemically complex carbon framework of the merochlorins in one step. Inspired by the enzyme reactivity, we developed a chemical chlorination protocol paralleling the biocatalytic process. These chemical studies led to the identification of previously overlooked merochlorin natural products.

In this study, two short SOE stacks (6-cells) were characterized in steam-electrolysis mode. The first was based on Ni-YSZ supported cells with LSCF-based air electrodes. The second included LSC-based air electrodes specifically developed within the SOFC600 project. The stacks were fed with a 90% steam, 10% hydrogen mixture, and characterized between 600 and 700 degrees C. For the LSCF-based stack, an average cell voltage of 1.6V was reached at -1Acm(-2) and 700 degrees C, corresponding to 60% steam conversion. The LSC-based stack showed better performances, with 1.25V at -0.8Acm(-2) and 700 degrees C. Durability tests were performed around the thermoneutral voltage (1.35V) for resp. 2,400 (LSCF) and 1,500h (LSC). The average stack degradation rates were resp. +2.8% (kh)(-1) (LSCF) and +1.9% (kh)(-1) (LSC). Twelve load-cycles were also performed on the LSC-stack with no apparent degradation. In addition, co-electrolysis was performed between 750 and 850 degrees C by feeding the stack with a 60% H2O, 30% CO2, and 10% H-2 mixture. Ninety-five percent conversion was reached and the measured outlet syngas composition was close to that predicted by thermodynamics. Steam electrolysis tests were carried out in the same conditions for comparison. The stack performance in the co-electrolysis mode was slightly lower than in the electrolysis mode.

The locally-resolved degradation behavior was studied during 1900 hours in a SOFC repeat-element. In-situ measurements of local electrochemical performance were made on 18 locations over a segmented anode-supported cell. The evolution of local current densities, overpotentials and area-specific resistances was studied, showing a reorganization of the electrochemical reaction with time. The extent and the spatial distribution of degradation were established for different electrochemical reactions steps using impedance spectroscopy. The low-frequency cathode contribution was the mostly altered process, followed by the charge transfer reaction on anode side. Post-experiment analyses allowed to identify three major pollutants on the cathode side (chromium, silicon and sulfur), whose spatial distributions corresponded to the observed local degradation. Chromium was present only near the air inlet, while silica was found to affect a larger area, with decreasing amounts along the flow. Sources of pollutants were identified in system components as well as within the stack repeat-element.

Mixed ionic–electronic conducting $$La_{0.5} Sr_{0.5} Fe_{1 - x} B_x O_{3 - \delta } $$ (B: Al, Cr, Zr, Ga, Ti, Sn, Ta, V, Mg, and In with x = 0, 0.1, 0.2) perovskite materials were produced via solid-state synthesis. In order to study the effect of B-site substitution on the expansion behavior of these materials, their thermal expansion in air up to 900°C and isothermal expansion at the same temperature from air to Ar were measured by dilatometry. Ti and Ta were found to be the most effective substitutions in suppressing the isothermal expansion. The isothermal expansion at 900°C from air to Ar was reduced by 50% by substitution of 20% Ti or 10% Ta. Therefore, these compositions were further characterized by 4-probe total DC conductivity and permeation measurements under air/Ar gradient. The total conductivity of $$La_{0.5} Sr_{0.5} FeO_{3 - \delta } $$ was decreased by more than one order of magnitude at low temperatures and from 430 S/cm, which is the maximum, to around 100 S/cm at 500°C with the addition of Ti and Ta. The normalized oxygen permeation of LSF at 900°C decreased from 0.18 to 0.05 μmol/cm2s and 0.07 μmol/cm2s with the substitution of 20% Ti and 10% Ta, respectively.

Tubular membranes of La0.6Ca0.4Fe0.75Co0.25O3−δ and La0.5Sr0.5Fe1−yTiyO3−δ (y = 0, 0.2) for the application of partial oxidation of methane to syngas were produced by thermoplastic extrusion and investigated by oxygen permeation measurements. The optimum ceramic content in the feedstock for extrusion was found to be 51 vol% as a result of rheology measurements. Tubes with an outer diameter of 4.8–5.5mmand thickness of 0.25–0.47mm were produced with densities higher than 95% of the theoretical density. The oxygen permeation flux of the tubular membranes wasmeasured with air on one side and Ar or Ar +CH4 mixture on the other side. The oxygen permeation rate decreased with Ti-substitution while it was considerably increased by introduction of 5% methane into the system. The normalized oxygen fluxes in air/Ar gradient at 900 ◦C were measured to be 0.06, 0.051, and 0.012 mol cm−2 s−1 for LCFC, LSF, and LSFT2, respectively, and 0.18 mol cm−2 s−1 for LSFT2 with 5% methane.

The influence of partial substitution of Fe with Ti on the oxygen transport properties of La1−x Sr x FeO3 membranes was investigated in view of their application for oxygen separation. Samples of composition % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeGaca % ackaa0lGqaaiaa-XeacaWFHbWaaSbaaSqaaiaaicdacaGGUaGaaGyn % aaqabaGccaWFtbGaa8NCamaaBaaaleaacaaIWaGaaiOlaiaaiwdaae % qaaOGaaeOraiaabwgadaWgaaWcbaGaaGymaiabgkHiTiaadMhaaeqa % aOGaaeivaiaabMgadaWgaaWcbaGaamyEaaqabaGccaqGpbWaaSbaaS % qaaiaaiodacqGHsislcqaH0oazaeqaaaaa!4C93! $$ La_{{0.5}} Sr_{{0.5}} {\text{Fe}}_{{1 - y}} {\text{Ti}}_{y} {\text{O}}_{{3 - \delta }} $$ (y=0, 0.2) were prepared and their oxygen transport properties characterised by potential step relaxation and by oxygen permeation measurement in an air/argon gradient. With the first technique, chemical diffusion % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX!% MathType!MTEF!2!1!+-% feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX% garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz% aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq% Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq% Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaada% qadaqaamaaGaaabaGaamiraaGaay5adaaacaGLOaGaayzkaaaaaa!3AA4! $${( {\widetilde{D}} )}$$ and surface exchange (k S) coefficients were obtained by fitting of the current relaxation data to a single expression valid over the complete time range. The Ti-substituted composition gave slightly larger values of % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX!% MathType!MTEF!2!1!+-% feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX% garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz% aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq% Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq% Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaada% aiaaqaaiaadseaaiaawoWaaaaa!391B! $${\widetilde{D}}$$ and k S. The trend was opposite for the measured oxygen permeation flux. In the latter experience, ordering of oxygen vacancies was observed at lower temperature, reducing significantly the performance of the material.

Electrochemical techniques were applied to a coulometric titration cell to study oxygen nonstoichiometry and transport in the perovskite-type oxide La0.4Ba0.6Fe0.8Co0.2O3-delta. Slow scan voltammetry (3mV/s) was used to obtain oxygen nonstoichiometry vs. the oxygen partial pressure (p(O2)) data. The voltammograms were further analysed using a simple defect model to yield the absolute value of the oxygen nonstoichiometry. Relaxation measurements were performed to obtain chemical diffusion and surface exchange coefficients. In particular, the suitability of converting the relaxation data to the frequency domain for analysis purpose was examined. Impedance spectroscopy measurements were also performed on the same cell to allow direct comparison. A satisfactory agreement was obtained for the chemical diffusion coefficients but the surface exchange coefficient values were systematically different by a factor 2 to 3. This discrepancy was attributed to the short-time extrapolation used in the numerical conversion procedure. Finally, other transport coefficients (ionic conductivity, DV and DO) were calculated from the chemical diffusion and nonstoichiometry data.

Three perovskite-type compounds of composition La0.4Ca0.6Fe1-xCoxO3-d (x=o, o.25 and 0.5) were investigated for use as oxygen separation membranes for the partial oxidation (POX) of methane to syngas. Special attention was given to the question of their stability in real operating conditions. A permeation set-up was specially designed to measure oxygen fluxes through these materials when placed in a strong pO2 gradient. It also facilitated testing the long-term stability of the specimen. Permeation measurements performed in an air/argon gradient between 800 and 1000 °C showed that the highest fluxes were obtained with the highest content of cobalt (La0.4Ca0.6Fe0.5Co0.5 O3-d =La0.4Ca0.6Fe0.75Co0.25 O3-d >La0.4Ca0.6FeO3-d ). In addition, comparison between the fluxes of samples of different thickness gave clear evidence of surface limitations in the oxygen transport. The long-term stability test showed opposite trends: only the two lowest Co containing compounds (x = 0 and 0.25) sustained and air/(Ar + H2) gradient over more than 600 h. The other (x = 0.5) broke shortly after the introduction of H2. In the presence of H2, the oxygen flux was increased by a factor 10 compared to Ar and reached 0.83 mmol/cm2s for La0.4Ca0.6Fe0.75Co0.25O3-d at 900°C. Post-operation SEM examination of the cross-section and both surfaces revealed that the surface exposed to H2 had started to decompose resulting in the formation of a thin porous layer but the bulk of the material remained unchanged.

Chemical diffusion (D) and surface exchange (k) coefficients for SrFeO3-delta were measured using an electrochemical cell combined with electrochemical impedance spectroscopy (EIS) and potential step technique (PS) in the temperature range of 850-915°C. A value of 4x10-5 cm2/s and a k value of 8x10-5 cm/s were obtained at 900°C. Slow scan (0.5-3 microV/s) cyclic voltametry (CV) was performed in the same temperature range, using the same electrochemical cell to obtain oxygen nonstoichiometry data. The oxygen nonstoichiometry (delta) at 900°C in air was determined as 0.4. A plateau corresponding to delta = 0.5 was observed below an oxygen partial pressure (pO2) of 10-6 atm. These results were shown to be consistent with the literature data. Nonstoichiometry data were further analysed using the existing defect models, and the limits of the independent point defect approximation and the necessity of considering interactions between point defects and clusters were established. Keywords: Strontium Ferrate/Ferrite; Oxygen transport; Oxygen nonstoichiometry.

Oxygen diffusion is treated in a dense electronically conducting cobaltate pellet blocked ionically on one surface, electronically on the other, and sealed on its cylindrical periphery. A procedure is developed for extracting the chemical diffusion and surface exchange coefficients for oxygen by use of the asymptotic equations derived for the current response to a potential step at short and long times. It is shown that, while the formation of interfacial phases by reaction between the sample and the electrolyte may affect the surface exchange coefficient, the chemical diffusion coefficient data determined by the present approach are independent of such interfacial phenomena. The consistency of data obtained from several specimens with varying thickness and manner of interfacing with the electrolyte validates the diffusion model and the method used for data analysis. An oxygen permeation cell is also developed in this work as a modification of the diffusion cell. The new cell allows monitoring of the permeation rate by electrochemical means. The steady-state permeation data obtained by the permeation cell are consistent with the chemical-diffusion and surface- exchange coefficients measured by the blocked diffusion cell as long as the assumptions of the related theoretical models are satisfied. This is a further validation of the diffusion model and the related methodology developed here for obtaining the necessary data for characterizing oxygen exchange and transport in such materials.

International audience; The neodymium-deficient nickelate Nd1.95NiO4+δ, mixed conducting K2NiF4-type oxide, was evaluated as cathode for solid oxide fuel cells. The electrochemical properties were investigated on planar Ni–YSZ anode-supported SOFC based on co-tape casted and co-fired HTceramix® cells. Using a layer of strontium doped lanthanum cobaltite as current collector, a current density of 1.31 A cm−2 (at 0.70 V) was obtained at 800 °C using hydrogen fuel with small single cells, after optimizing the cathode sintering temperature. Impedance spectroscopy measurements were performed; the different resistive contributions and the values of the corresponding equivalent capacities are discussed.

Dense planar and tubular oxygen separation membranes of La 0.6Ca0.4Fe0.75Co0.25O3-d were investigated as reactors for the partial oxidation (POX) of methane to syngas. Disk-shaped membranes were prepared by compaction and tubes by extrusion. Their permeation properties were measured in an air/argon pO2 gradient as a function of temperature. At 900°C, the oxygen flux through a 1.26 mm thick membrane was 0.075 micromol/cm2s and through a 0.25 mm thick tube, 0.24 micromol/cm2s. For the POX measurements, a catalyst was added to the membrane and methane was introduced on the argon side. This resulted in a gradual increase of the oxygen flux with increasing concentration of methane, reaching 2micromol/ cm2s at 900°C with pure methane. The catalytic properties of the POX reactors were characterised as a function of the methane content. For the planar reactor, the CO selectivity reached 99% and the CH4 conversion 75% at 918°C with pure methane. For the tubular reactor, the CO selectivity and CH4 conversion were respectively 83 and 99% for the same conditions. In all cases the H2/CO ratio was about 2. After 1’400 hours of operation in a tubular POX-reactor, the membrane was examined revealing phase demixing and local decomposition.

This study experimentally analyzes the performance and degradation issues of an anode supported (AS) Ni-YSZ solid oxide fuel cell exposed to thiophene (C4H4S). The impact of this organic sulfur compound on the performance of SOFC Ni-YSZ anodes is reported as a function of temperature and the impurity concentration. There are extensive investigations on the effect of hydrogen sulfide as inorganic sulfur compound on the performance of SOFC but much less of complex sulfur-containing molecules like thiophene. The main objective of this study is to evaluate the impact of thiophene at constant (5 ppmv) and variable (1-15 ppmv) concentrations in the fuel stream on the SOFC performance and regeneration. With the help of the observed results on polarization with time and impedance spectroscopy, including the use of a new equivalent gas composition of reformed bio-syngas, the degradation mechanism is explained. Post-test characterization (SEM-EDX) helps to clarify carbon formation and degradation due to the sulfur impurity. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

(La,Sr)FeO3 mixed conducting perovskites are considered as interesting candidates for oxygen separation membranes but they suffer from limited structural stability in a large oxygen partial pressure (pO2) gradient, because of their propensity for chemical expansion. Partial substitution of Fe with more stable elements tends to improve the stability while penalizing the electronic and ionic conductivities. In this study, we investigate the effect of 10% Ta substitution on the oxygen transport properties and stability of La0.5Sr0.5FeO3. For this purpose, the material was evaluated as a membrane in a CPOX reactor. The oxygen permeation through a ~ 3 cm2 pellet sample was first measured under air/Ar gradient in the temperature range of 800 to 1000 °C. The measured flux was 0.1 µmol cm− 2 s− 1 at 900 °C, which was a factor of 2 lower than for the Ta-free material. Methane was then introduced into the system and reacted in a catalytic bed with oxygen that has permeated through the membrane to form syngas (H2, CO). As a result, the oxygen flux increased by a factor of 9, reaching 0.9 µmol cm− 2 s− 1 at 900 °C. The reactor was operated at 1000 °C for another 1000 h. During this time, the oxygen permeation flux decayed by ca. 4%/1000 h. The test was stopped after more than 2000 h of operation and the membrane analyzed by electron microscopy.

This paper reports on medium term tests of anode-supported five-cell short stacks, as well as on some separate anode development. Two stacks were operated under steady-state conditions: one with unprotected metal interconnects, H-2 fuel and 0.35 A cm(-2) (40% fuel utilisation) polarisation current showed an average cell voltage degradation of 56 mV per 1000 h for 2750 h; one with coated metal interconnects, synthetic reformate fuel and 0.5 A cm(-2) (60% fuel utilisation) polarisation current showed an averaged cell voltage degradation slope of 6.6 mV per 1000 h for 800 h before a power cut prematurely interrupted the test. A third stack was subjected to 13 complete thermal cycles over 1000 h, average cell voltage degradation was evaluated to -2 mV per cycle for operation at 0.3 A cm(-2), open circuit voltage (OCV) remained stable, whereas area specific resistance (ASR) increase amounted on average to 0.008 Omega cm(2) per cycle. To improve anode reoxidation tolerance, MgO addition was considered. (Mg, Ni)O solid solutions were synthesized from MgO and NiO powders at 1500 degrees C for 2 h in air. Samples were reduced at 1000 degrees C in 4 vol% H-2 in N-2, and their expansion behaviour was observed. XRD and SEM/EDX analysis suggested that the expansion was caused by the segregation of Ni particles between (Mg, Ni)O grains. (c) 2008 Elsevier B.V. All rights reserved.

In this study, we examine the parameters that govern the overall oxygen flux through a permeation membrane. The chemical diffusion (D) and the surface exchange (k) coefficients for oxygen in La0.6Sr0.4Fe0.8Co0.2O3-d were determined as a function of temperature using a specially designed electrochemical cell combined with impedance spectroscopy. Typically, D=1.7 10-5 cm2/s and k=3.6 10-4 cm/s in air at 900°C. These values were compared with literature 18O/16O isotope exchange data. Oxygen permeation measurements were also performed on the same material in an air/Ar gradient, in the temperature range of 800 to 1000°C. At 900°C, the oxygen flux across a 1.53 mm thick membrane was 8.0 10-8 mol/(cm2s). The measured fluxes were compared with fluxes calculated on the basis of the D and k values using expressions derived from Fick’s law. Comparison showed that the flux is controlled by both bulk diffusion and surface exchange, even for such thick membranes, and that the apparent k? varies significantly from one experiment to another.

Oxygen diffusion is treated in a dense electronically conducting perovskite pellet blocked ionically on one surface, electronically on the other, and sealed on the periphery. Oxygen exchange at the electronically blocked surface is assigned first order reaction kinetics. An equivalent circuit model is suggested for the cell impedance by the Laplace transform of Fick’s second law. Asymptotic expressions are employed to extract the slope of the coulometric titration curve and the chemical diffusion coefficient from electrochemical-impedance-spectroscopy (EIS) data. NLLS fit with the theoretical model is performed to evaluate the surface oxygen exchange coefficient at the interface of the electrochemical cell. The methodology is applied to determine the chemical diffusion and surface exchange coefficients of oxygen in SrCo 0·5 Fe 0·5 O 3−δ , interfaced with a YSZ electrolyte. The experimental results are used to link the processes corresponding to the diffusion of oxygen vacancies to the ionic conductivity of the material. The data is applicable to solid oxide fuel cell cathodes, oxygen permeation membranes and related systems.

Oxygen diffusion is treated in a dense electronically conducting perovskite pellet blocked ionically on one surface, electronically on the other, and sealed on the cylindrical surface. Oxygen exchange at the electronically blocked surface is assigned first order reaction kinetics. An equivalent circuit model is suggested for the cell impedance by the Laplace transform of Fick’s second law. The methodology thus developed for this technique is applied to determine the chemical diffusion and surface exchange coefficients of SrCo 0.5 Fe 0.5 O 3− δ interfaced with air in a solid state electrochemical cell employing a YSZ electrolyte as an oxygen pump. These parameters measured by EIS are compared with the values obtained by the potential step (PS) method on the same electrochemical system. The two approaches are equivalent for characterizing diffusion of oxygen vacancies, but EIS is more appropriate for direct measurement of the exchange coefficient. Calculation of this parameter from the PS data may not be straightforward. The two techniques are complementary for complete characterization of oxygen exchange and transport in mixed conducting oxides.

Internal steam reforming (IR) of methane was investigated on Ni-YSZ anode supported cells, looking in particular at the effect of the steam to carbon (S/C) ratio on the degradation rate. The cells were fed with different H2O/CH4 mixtures during 100 hours sequences, alternating with sequences of dry H2 feeding. V-I characterization was performed before and after each sequence, and EIS measurements were performed regularly. A marked degradation was observed during the IR sequences while it was negligible under dry H2 feed. The observed degradation, attributed to carbon deposition on the anode active sites, was partially reversible for S/C >1.5, whereas it became irreversible at lower S/C.