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The characterization of solid oxide cells (SOCs) relies mainly on direct-current (DC) and alternating-current (AC) methods that provide only average surface electro-chemical responses. As such, the spatial distribution is not well captured and the complexity of the electrochemical system is not fully represented. Infrared imaging could be used to locally characterize SOCs. This method has only been performed at steady-state, but, analogous to DC and AC characterization methods, applying a thermal perturbation could improve its sensitivity. This work proposes the use of active infrared imaging to characterize an operating SOC. To demonstrate the feasibility of the proposed method, an SOC was exposed to a current pulse while monitoring the infrared response using a CMOS camera. The phase and amplitude representation of the infrared response was then computed by Fourier transform and used for rapid identification of regions of interests. Results showed that less than 10% of the SOC was useful in the electrochemical process.

While high performance of SOFC systems has already been achieved, there is still a need to increase the stack lifetime, while decreasing the system's total cost of ownership, both being linked. The increase in lifetime includes the minimization of performance degradation, but also avoidance of potentially detrimental events during operation, originating from the stack, outside the stack, or a combination of both. The present paper highlights how the detection of faulty operation as early as possible is important to implement adequate mitigation strategies, wherever applicable, according to the fault and its grade of severity. Three types of faults have been considered, fuel starvation, leakage and carbon deposition. Both hardware components and algorithms to monitor, detect, isolate, and finally correct faulty system operation have been developed and will be embedded in a real SOFC system.

Nowadays, the researchers that are working on the SOFC technology, are agree that it is necessary to decrease the operating temperature at 700°C(650-750 °C) of IT-SOFC technology, which currently is around 850-900°C for electrolyte supported cell used by the three main operators –Bloom, Hexis and Sun Fire. At this temperature the reactivity and the component cost of the cell decrease. Nevertheless, some interactions between the conventional electrolyte material (YSZ) and the cathode material (lanthanum based) are still present and it is necessary to include a ceria buffer layer to avoid the possible formation of an insulating pyrochlore structure La2Zr2O7 layer at the electrolyte-cathode interface. In ceria family, the best material regarding the ionic conductivity is the samaria doped ceria. However, the gadolinia doped ceria is the most employed from the cost point of view and the availability of gadolinium in comparison with samarium. This buffer layer avoids the reactivity between electrolyte and cathode, and presents a good anionic conductivity. It must be dense, thin and adherent on electrolyte, in order to be suitable for the surface treatment process. In this paper, we present some recent results obtained on the gadolinia doped ceria coatings deposited by magnetron sputtering from metallic targets in argon-oxygen reactive gas mixtures with different substrate bias voltage. After a description of the experimental device, a first part will be dedicated to their chemical, microstructural and structural characterization (SEM, XRD,…) in relation with the magnetron sputtering deposition parameters. In a second part, the ionic transport characteristics of the half cell (Ni-YSZ/YSZ/GDC) will be investigated via impedance spectroscopy measurements performed at various temperatures up to 800°C under Ar-H2 gas mixtures. Finally, a complete cell test ((Ni-YSZ/YSZ/GDC/LSC) was performed with the Fiaxell Open Flanges test bench. This study is carried out within the framework of the European territorial cooperation program INTERREG V A France-Switzerland. It has benefited from financial support from the EU through the European Regional Development Fund (ERDF- € 139,176) and the Swiss Confederation's contribution of CHF 134,173

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

A Triode Solid Oxide Fuel Cell is a novel cell architecture that aims to control the electrocatalytic activity under low steam reforming conditions, or poisoning conditions. This fuel cell design introduces a third electrode (auxiliary electrode) of the same nature of the cathode while the anode is common between the two circuits (Figure 1). The auxiliary circuit, which electrically connects the anode with the auxiliary electrode, is operated in electrolysis mode while the fuel cell circuit runs in its conventional way. During the triode operation, performance enhancement is recorded especially when a significant electrode overpotential is presented; this can be the case for SOFC anode feeds with natural gas and gasoline fuels, or when it is being poisoned from impurities. A two-dimensional stationary isothermal model is developed and implemented in commercial software, COMSOL Multiphysics (version 4.3b). The model is based on composite electrodes, where the equations for mass, momentum and charge transport, along with electrochemical global kinetics (Butler-Volmer equation) and charge conservation are solved simultaneously. Exchange current densities are fitted from experimental data to reproduce the observed behavior of the cell. The model, able to predict conventional and triode operation, gives guidelines for the electrodes geometry design that needs to be adopted in order to favor the expected improvements. In particular, it is noted that the aspect ratio between the electrodes distance cathode-auxiliary and the electrolyte thickness is a key parameter. The interaction between the main and auxiliary circuits is then discussed in terms of equipotential and current lines in a cross section of the cell for two different geometries: anode and electrolyte supported cells. The improvements achievable in triode operation are also calculated in terms of electric current surplus that is possible to obtain in the fuel cell circuit while imposing an electrolysis voltage of 1.7 V in the auxiliary circuit. 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

The prevalent anode supported thin electrolyte solid oxide fuel cells (ASC) used today are highly susceptible to failure under redox conditions. The expansion caused during the oxidation of Ni to NiO can destroy the support, leading to irreversible cell damage. To counter this problem, Fiaxell SOFC Technologies (Switzerland) has engineered the microstructure of their cells, to confer added redox stability to their anode supported cell (2R-cell™). This commercial cell was investigated within the Eurostars-Roxsolidcell project against thermal and redox cycling. A 2R-cell with a 10cm2 active area LSCF cathode on a CGO buffer layer, was placed between alumina felts, mounted inside a seal-less open flange setup kept in an oven, and operated at a temperature of 780°C. A nickel foam and a gold grid were used for the current collection at the anode and cathode respectively. Dry hydrogen (180 Nml/min) was fed to the anode and air (500 Nml/min) to the cathode. 20 thermal cycles were performed between 780 and 100°C, using 200°C/h heating ramps and natural cooling, followed by 20 redox cycles, during which air was fed to the anode for 1 hour at 780°C, causing the cell potential to drop to zero. V-i polarisation curves and electrochemical impedance spectroscopy (EIS) measurements were performed before and after each consecutive cycle in order to monitor the performance losses. The OCV remained stable during the whole test whereas the area specific resistance (ASR) measured at 0.5 A/cm2 increased by +1.3 mΩcm2 per thermal cycle and by +1.8 mΩcm2per redox cycle. The EIS measurements showed that the degradation during the thermal cycling was mainly due to an increase of the ohmic losses. In the case of the redox-cycling, the cell degradation after stabilisation principally came from high frequency polarisation losses (cf. Figure), which were attributed to changes in the anode functional layer microstructure. The cell was examined after the test by scanning electron microscopy (SEM). No delamination of the cathode was observed, however, fragmentation of the anode active layer was apparent. Figure 1

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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.

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.

SOFCs have the advantage of being able to operate with fuels other than hydrogen due to their high operation temperatures. Among these, biogas from anaerobic digestion is considered to be carbon neutral and highly underused for power generation. Methane-free biogas, which can be produced by inhibition of the methanogenic process, is an alternative high quality fuel, consisting mainly of CO2 (70 to 50%), H2 (30 to 50%) and traces of CO and H2S. The impact of H2S in the ppm range on the performance of anode supported Ni-YSZ SOFC at 750°C is evaluated. An initial voltage drop is observed when the cell is exposed to H2S but the voltage recovers after a few hours of exposure to the impurity. Impedance spectroscopy revealed an increase in the polarization resistance even though the cell voltage was increasing. Furthermore, it was found that this behavior is due to deactivation of the reverse water gas shift reaction (RWGSR); a mathematical model confirmed this conclusion. The poisoning effect is partly reversible under the applied operating conditions. Figure 1

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The degradation of the electrochemical performance of solid oxide fuel cell (SOFC) devices is a major hurdle to overcome before commercialisation. The interplay between the phenomena and the long testing times complicate the research, which highlights the relevance of modelling to propose mitigation approaches. This study comprises two parts. This Part II proposes approaches for the simulation of the degradation induced by: (i) interconnect corrosion, (ii) loss of ionic conductivity of the ion-conducting materials, (iii) nickel particle growth in the anode, (iv) chromium contamination and (v) formation of insulating phases in the cathode. The literature survey highlights the lack of data for a completely consistent calibration of the models, despite the simplifications. The support for the implementation is the electrochemical model validated in Part I and a two-dimensional model of the cell and interconnection system. The cathode largely contributes to the degradation. The local overpotential predominantly governs chromium contamination, which can promote the formation of insulating phases, as operation proceeds. The local electronic current density has comparatively a weak direct influence on the degradation. Qualitative agreement with experimental data from the literature could be achieved, without dedicated adjustments of the parameters. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596435] All rights reserved.

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A rigorous mathematical model is developed for the complex impedance of a solid-state electrochemical cell, which is commonly used for the measurement of oxygen transport, oxygen exchange kinetics and thermodynamic properties of nonstoichiometric mixed conducting oxides. The model leads to a simple equivalent circuit for the cell with unambiguous definition of the physical significance of its components. A method is proposed for the analysis of experimental data. The methodology thus developed is validated by comparing the experimental data measured for a well-studied perovskite (SrCo0.5Fe0.503-delta) with the results obtained from the completely equivalent potential-step technique. In addition, various electrochemical properties of the other cell components, such as Pt electrodes and YSZ electrolyte, also obtainable from measurements, show good agreement with the available literature data. The cell design, which significantly minimizes the gas space in contact with the sample, has a clear advantage over similar relaxation cells in terms of reducing the dominating effect of the gas phase capacitance in numerical data analysis. A possible disadvantage, however, is the large impedance of the oxygen pump at low oxygen partial pressures, which may in a similar manner obstruct deconvolution of the sample properties from the measured data.

Our SOFC stack development technology is based on the unique SOFConnexTM concept, using flexible gas distribution layers between metal sheet interconnect and thin ASE cells. Flexibility is given both in material and design. This ensures proper electrical contact over the whole cell (50 cm2 active), without necessitating restrictive cell fabrication tolerances, and allows easy adaptation and evolution of the flowfields. With the presently used configuration, several multiple cell stacks were assembled and tested. Reproducible stack power density (H2 fuel, λ = 1.5-2, 800°C maximum local temperature) is 0.5 W/cm2 at 0.7 V average cell voltage (1.5 kWe/L), for 67% fuel utilisation (35% LHV electrical efficiency). Performance with simulated POX-syngas (H2/CO/N2) was close to that with H2. Degradation is the focus of attention now that adequate power density and efficiency using the SOFConnex™ approach have been established and reproduced.

Our planar SOFC stacking technology uses unprofiled metallic interconnects (MIC) and thin cells of tape cast anode supported YSZ. The key element is the gas diffusion layer (GDL) between cell and MIC, which consists of so-called SOFConnex™. Using square cells with internal manifolds, 0.5 W/cm2 stack power density (800°C) can be obtained on short stacks. However, this open design configuration limits the assembly of large stacks and the durability of operation, owing to postcombustion and redox cycling occurring at unprotected cell edges. A new design, inspired from modeling work and the adaptability of the SOFConnex™ GDL, led to oblong-shaped cells, assembled in a closed stack casing with external air manifolding and fuel recovery manifolding, avoiding postcombustion. While stack power density in both designs remains similar, the operation at increased fuel utilization has become more stable in the 2nd design. Furthermore, a correlation of performance homogeneity during stack testing was drawn to assembly quality control. A 36-cell stack in dilute H2 at 800°C achieved 625 Wel (28% LHV efficiency, 0.35 W/cm2) under continuous polarisation, with all 6 clusters of 6 cells showing coincident i-V-output.