Search

SOEC operation using ammonia as a fuel was investigated. At high temperatures (T>800oC) cell performance closely resembles performance in equivalent gas mixtures of N2 and H2. Between 600 oC < T < 750 oC, however, a mild impeding effect of presence of ammonia on the hydrogen oxidation rate is observed. Further, the fuel cell durability when operating on ammonia was tracked over test periods between 2000 and 6000 hours at different temperatures. In general, degradation rates observed in ammonia were similar to degradation rates observed when operating in a 1:3 gas mixture of N2/H2. Some morphological changes in the fuel electrode support and auxiliary Ni-meshes were observed after test. These indicate Ni-nitride formation and whereas this is detrimental to the Ni-mesh, it did not affect overall cell performance. Experiments with ammonia operation at stack level were also conducted illustrating the potential to realize a stack cooling by internal ammonia cracking.

In the last decades, Solid Oxide Fuel Cells (SOFCs) have received a lot of attention due to their ability to efficiently convert hydrogen and other fuels to electricity and heat. Cells with different designs (planar, tubular, anode-, electrolyte-, metal-supported) have been intensively studied in terms of performance, costs and lifetime. Still, technical challenges such as limited thermal cycling stability and cost-efficient paths to up-scaling need to be solved to make the SOFC technology more commercially attractive. This study presents the design, fabrication and testing of a novel monolithic metal-supported SOFC with the aim to achieve thermal cycling robustness and a high volumetric power density using cost-competitive and scalable manufacturing methods. The study presents preliminary electrochemical performances of the cells and key parameters of the manufacturing process that were optimized to increase the stability/durability of the monolith by a factor of 100.

The ongoing green energy transition is increasing the need for dynamic and efficient Power-to-X (PtX) systems to convert surplus wind and solar power to high-value products. The solid oxide electrolysis cell (SOEC) technology offers the highest energy conversion efficiency. However, high degradation and thermal variations that cause thermomechanical stress hinders up-scaling of the SOEC technology. Here we present a novel operation method that alleviates temperature variations and minimize degradation caused by impurities and nickel migration. By rapidly switching between electrolysis mode and brief periods in fuel cell mode, a flat thermal profile is obtained. Our results thus establish a new, simple way to achieve increased SOEC stack and module size and extended lifetime. The new operation method enables dynamic operation of large SOEC modules for renewable energy powered PtX systems which could drastically decrease costs associated with production of high-value green fuels and chemicals from wind and solar power.

The transportation sector is currently undergoing a technology shift from internal combustion engines to electric motors powered by batteries. However, their limited range and long charging times limit wide-spread adoption. Electrified transportation powered by solid oxide fuel cells (SOFCs) offer an attractive alternative especially for heavy freight and long-range transportation, as this technology can provide high-efficiency and flexible fuel choices. Thus far, the technology is mostly used for stationary applications owing to the high operating temperature, low volumetric and gravimetric power density, and poor robustness towards thermal cycling and mechanical vibrations of conventional ceramic-based cells. Here, we present a novel metal-based monolithic fuel cell design to overcome these issues. Highly cost-competitive and scalable manufacturing methods are employed for fabrication, and only a single heat treatment is required, as opposed to two or three for conventional SOFCs. The design is further optimised through three-dimensional multiphysics modelling, nanoparticle infiltration, and corrosion-mitigating treatments. The monolithic fuel cell shows exceptionally high power density (5.6 kW/L) and excellent thermal cycling robustness, revealing the vast potential of SOFC technology for transport applications.

Electrochemical impedance spectroscopy (EIS) is a powerful diagnostic tool used in a wide range of applications, including solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). While EIS measurements on single cell level are commonplace, implementation of EIS on stack level remains limited, primarily due to the extremely low impedance of large-footprint cells, which imposes stringent demands on the impedance spectrometer and experimental setup (1). Furthermore, compared to single-cell EIS data, stack-level data reported in the literature are rarely subjected to the same degree of equivalent circuit modeling, making a detailed analysis of underlying electrochemical processes difficult. Here, we report a framework for analyzing stack-level impedance spectroscopy data. The quality of the acquired data is first verified using Kramers-Kronig transformations (2), followed by the application of distribution of relaxation times (DRT) and analysis of the difference of impedance spectra (ADIS) methods to experimental data collected under a set of pre-determined operating conditions (electrochemical test procedure) (3-4). DRT and ADIS results were incorporated into the complex non-linear least squares (CNLS) fitting procedure and the validity of the chosen equivalent circuit model was confirmed by reconstruction of Nyquist, Bode, the individual electrochemical contributions, and DRT plots of the fitted data. The developed framework was successfully implemented on a commercial 75-cell SOEC stack (TSP-1) developed by Haldor Topsoe (5). An electrochemical impedance spectrum is recorded simultaneously on all 13 cell groups in the stack using Zahner instrumentation. In this work, the electrochemical impedance data obtained from a state-of-the-art TSP-1 stack, containing three different types of Ni/YSZ based cells, is reported. The stack was initially investigated by EIS, using an electrochemical test procedure including CO2 electrolysis under constant conversion, H2O electrolysis under constant conversion, variations in the pO2 on the oxygen electrode, and finally temperature variations in CO2 electrolysis operation. This electrochemical test procedure was performed to develop an equivalent circuit model for the stack. Afterward, the stack was subjected to a degradation in steam electrolysis under 580 mA/cm2 (70% conversion), with an impedance spectrum being recorded every 24 hours. The operational point for this degradation test was chosen to match the real-life operation point of these stacks and hence to give a realistic image of any potential degradation. The developed framework and the equivalent circuit model developed for the stack was finally used to model the EIS data obtained during the degradation test. The modeling of these EIS data utilizes the full potential of electrochemical impedance spectroscopy and help identify any degradation mechanisms taking place in the stack, an analysis not previously reported on stack level. (1) R. R. Mosbæk, J. Hjelm, R. Barfod, and P. V. Hendriksen. ECS transactions, 35, 1 (2011). (2) B. Boukamp, J. Electrochem. Soc., 142, 1885 – 1894 (1995). (3) S. H. Jensen, A. Hauch, P.V. Hendriksen, M. Mogensen, N. Bonanos, and T. Jacobsen. Journal of the Electrochemical Society, 154(12), B1325-B1330, (2007) (4) H. Schichlein, and E. Ivers-Tiffee. Journal of Applied Electrochemistry, 32, 875-882, (2002). (5) J. Rass-Hansen, P. Blennow, T. Heiredal-Clausen, R. Küngas, T. Holt Nørby, and S. Primdahl, Proceedings of the 12th European SOFC & SOE Forum, 318-325 (2016). Figure 1

Experimental SOFC stacks with 10 SOFCs (LSM-YSZ/YSZ/Ni-YSZ) were infiltrated with CGO and Ni-CGO on the air and fuel side, respectively in an attempt to counter degradation and improve the output. The electrochemical performance of each cell was characterized (i) before infiltration, (ii) after infiltration on the cathode side, and (iii) after the infiltration of the anode side. A significant performance enhancement was observed after the infiltration with CGO on the cathode, while the infiltration of the anode side with Ni-CGO had no significant effect on the electrochemical performance. After testing the cells were characterized by SEM and TEM/EELS. A thin layer of CGO nanoparticles around the LSM-YSZ back bone structure was found after infiltration. On the anode side nano sized Ni particles were found embedded in a CGO layer formed around the Ni-YSZ structure. EELS analysis showed that the oxidation state of the Ce ions is identical on the air and the fuel side.

State-of-the-art (SoA) solid oxide fuel cell (SOFC) stacks are tested using profiles relevant for use in micro combined heat and power (CHP) units. Such applications are characterised by dynamic load profiles. In order to shorten the needed testing time and to investigate potential acceleration of degradation, the profiles are executed faster than required for real applications. Operation with fast load cycling, both using hydrogen and methane/steam as fuels, does not accelerate degradation compared to constant operation, which demonstrates the maturity of SoA stacks and enables transferring knowledge from testing at constant conditions to dynamic operation. 7.5 times more cycles than required for 80,000 h lifetime as micro CHP are achieved on one-cell-stack level. The results also suggest that degradation mechanisms that proceed on a longer time-scale, such as creep, might have a more dominating effect for long life-times than regular short time changes of operation. In order to address lifetime testing it is suggested to build a testing program consisting of defined modules that represent different application profiles, such as one module at constant conditions, followed by modules at one set of dynamic conditions etc.

In this work, co-electrolysis of steam and carbon dioxide was studied in a Topsoe Fuel Cell (TOFC®) 10-cell stack, containing three different types of Ni/yttria stabilized zirconia (YSZ) electrode supported solid oxide electrolysis cells with a footprint of 12 × 12 cm. The stack was operated at 800 °C and –0.75 A cm–2 with 60% conversion for a period of 1,000 h. One type of the cells showed no long term degradation but actually activation during the entire electrolysis period, while the other two types degraded. The performance and durability of the different cell types is discussed with respect to cell material composition and microstructure. The results of this study show that long term electrolysis is feasible without notable degradation at 800 °C and a current density of –0.75 A cm–2.

As solid oxide fuel cell (SOFC) technology is moving closer to a commercial break through, lifetime limiting factors, and methods to measure the “state-of-health” of operating cells and stacks are becoming of increasing interest. This requires application of advanced methods for detailed electrochemical characterization during operation. An experimental stack with low ohmic resistance from Topsoe Fuel Cell A/S was characterized in detail using electrochemical impedance spectroscopy (EIS). An investigation of the optimal geometrical placement of the current feeds and voltage probes was carried out in order to minimize measurement errors caused by stray impedances. Three different stack geometries were investigated by impedance spectroscopy and the stack geometry with the minimum effect of stray impedances was selected. A 13-cell experimental SOFC stack was tested during 2,500 h of operation with hydrogen as fuel with 52% fuel utilization and constant current load (0.2 A cm–2) at 750 °C. Stack interconnects were coated with six different coatings to prevent chromium poisoning on the cathode side. Four repeating units (RUs) with different coatings were selected for detailed impedance analysis. EIS allowed a distinction to be made in terms of the degradation between the four RU types that is not possible from IV-data only.

The performance of SOFC (solid oxide fuel cell) anodes is influenced negatively by impurities. In the present study segregation of impurities is minimized by using high purity materials at relatively low temperatures to prevent fast segregation. Ni point electrodes on polished single crystals of stabilized zirconia (SZ) with 10, 13 and 18 mol% yttria and one with 6 mol% scandia plus 4 mol% yttria were studied at open circuit voltage at 400–500 °C in mixtures of H 2 /H 2 O over 46 days. The polarization resistances (R p ) for all samples increased significantly during the first 10–20 days at 500 °C. No effect of the electrolyte composition on R p was found. Surface sensitive techniques were used to analyze the composition of the nickel and the electrolytes before and after the electrochemical experiment. Impurities were found to segregate to the surfaces/interfaces, and they are believed to impede the electrode processes and hence to cause the increase in polarization resistance.

The development of low temperature solid oxide fuel cell (SOFC) anodes by infiltration of Pd/Gd-doped cerium oxide (CGO) electrocatalysts in Nb-doped SrTiO3 (STN) backbones has been investigated. Modification of the electrode/electrolyte interface by thin layer of spin-coated CGO (400–500 nm) contributed to a significant improvement in performance of the STN backbones and infiltrated electrodes. The improvement is due to the result of CGO enrichment at the interface. The impedance analysis showed that addition of Pd further increased the electrode reaction rate with a factor 10 with respect to the CGO electrocatalysts. Very low electrode polarization resistances of 0.055 Ω cm2 (after excluding the gas diffusion limitation contribution) and 1.2 Ω cm2 at 600 °C and 400 °C, respectively, have been obtained in 3% H2/H2O. A gradual decrease in polarization resistance was achieved with increasing loading of Pd-CGO electrocatalyst. The microstructural analysis of the infiltrated Pd-CGO electrocatalyst on STN revealed a homogenous coating of Pd and CGO nanoparticles

Electrocatalyst precursor of various combinations: Pt, Ru, Pd, Ni and Gd-doped CeO 2 (CGO) were infiltrated into a porous Sr 0.94 Ti 0.9 Nb 0.1 O 3 (STN) backbone, to study the electrode performance of infiltrated ceramic anodes at low temperature ranges of 400–600 °C. The performance of the binary electrocatalyst infiltrated ceramic backbones are Pt–CGO>Ru–CGO>Pd–CGO>Ni–CGO. Ternary electrocatalyst of Ni–Pd–CGO and Ni–Pt–CGO showed the lowest polarization resistance of 0.31 and 0.11 Ωcm 2 , respectively at 600 °C in H 2 /3% H 2 O. The average particle size of the ternary electrocatalyst was larger than the binary Pd–CGO and Pt–CGO due to the particle coarsening of Ni nanoparticles. High resolution transmission electron microscopic analysis on the best performing Ni–Pt–CGO electrocatalyst infiltrated anode reveals the formation of Ni–Pt nanocrystalline alloy and a homogenous distribution of nanoparticles on STN backbone.

Preparation and electrochemical characterization of WO3-infiltrated 0.48W–0.52Cu–ScYSZ (WCS) anode for solid oxide fuel cell are reported. The DC conductivity of a WO3 ceramic was 1,200 and 24 S cm–1 in reducing and oxidizing atmospheres, respectively, at 650 °C. WCS porous backbones in the form of symmetric cells were prepared by screen printing of WO3–CuO–ScYSZ ink and subsequent sintering at 1,300 °C for 1 h in 9% H2/N2. Analysis of the sintered backbone by X-ray diffraction showed the metallic W and Cu phases. Precursor solutions of WO3 or CuO were infiltrated into porous WCS backbones to form the anode. The electrochemical performance of these anodes measured by impedance spectroscopy showed polarization resistances of 11 and 6.5 Ω cm2 for WO3 and CuO infiltrated anodes, respectively, at 600 °C in humidified hydrogen. Activation energy values of 86.8 and 96.5 kJ mol–1 were obtained for WO3 and CuO infiltrated WCS anodes, respectively. The microstructure of the tested anodes showed well-dispersed sub-micron particles of WO3 in the WCS backbone whereas CuO infiltration resulted in a dense microstructure.

This report concentrates on high performance anodes appropriate for SOFCs operating at low temperatures (400–600 °C). Symmetrical cells were made by screen printing of Nb-doped SrTiO3 (STN) on both sides of a dense ScYSZ electrolyte. Backbones I (36% porosity) and II (47% porosity) were obtained by sintering STN in air and reducing atmosphere, respectively. The porous microstructures were then infiltrated with Ni and Gd-doped CeO2 (CGO) precursors to incorporate the electrocatalytically active sites. The electrochemical performance of the anode was improved with the increment of Ni-CGO loadings. The gas diffusion impedance was investigated with He and N2 in H2/H2O gas mixtures and by varying the H2O content at 655 °C. This study indicated that the gas diffusion in these electrodes are influenced by Knudsen and bulk diffusion.

Electrolysis of steam and co-electrolysis of steam and carbon dioxide was studied in Solid Oxide Electrolysis Cell (SOEC) stacks composed of Ni/YSZ electrode supported SOECs. The results of this study show that long-term electrolysis is feasible without notable degradation in these SOEC stacks. The degradation of the electrolysis cells was found to be influenced by the adsorption of impurities from the applied inlet gases, whereas the application of chromium containing interconnect plates and glass sealings do not seem to influence the durability when operated at 850 °C. Cleaning the inlet gases to the Ni/YSZ electrode resulted in operation without long-term degradation, and may therefore be a solution for operating these Ni/YSZ based SOEC stacks without degradation.

An electrochemical study of SOFC cathode degradation, due to poisoning by chromium oxide vapours, was performed applying 3-electrode set-ups. The cathode materials comprised LSM/YSZ and LSCF/CGO composites, whereas the electrolyte material was 8YSZ. The degradation of the cathode performance was investigated as a function of time under a current load of 0.2 or 0.4 A cm–2 and in the presence of Cr2O3 at 850 and 750 °C in air, dry or water saturated at room temperature, and compared to that of non-Cr exposed reference specimens tested under, otherwise, the same conditions. This involved continuous logging of the DC current, and the voltage between the reference electrode and the working electrode in the 3-electrode set-up combined with frequent AC impedance measurements under current load which would allow the deduction of the cathode polarisation resistance (Rp). The duration of the tests ranged from 300 to 2,970 h. Both LSM/YSZ and LSCF/CGO cathodes were sensitive to chromium poisoning; LSCF/CGO cathodes to a lesser extent than LSM/YSZ. Humid air aggravated the degradation of the cathode performance. Post-mortem electron microscopic investigations revealed several Cr-containing compounds filling up the microstructure of the cathodes.

Minimising transport of chromium from the metallic interconnect (e.g. of Crofer 22APU) in a planar solid oxide fuel cell to the fuel cell cathode is done by application of a coating between the two parts. The coating is applied by slurry coating, and taken through stack initialisation it transforms into a stable and densely grown barrier layer, which minimises both the evaporation of chromium from the interconnect surface and the electrical contact resistance between the interconnect and the cathode. Between comparable stack element tests with and without coatings at 750ºC, the degra-dation rate in terms of power density decreased from 6.5% per 1000h to very low levels (

Various models of the H2/H2O/Ni/SZ (SZ = stabilized zirconia) electrode kinetics have been presented in the literature in order to explain the reported experimental data. However, there has been a strong tendency of using a limited set of data to verify a given model, disregarding other data sets, which do not fit the model. We have inspected some models in the literature, and problems (e.g. no quantitative model has explained the large variation in reported values of apparent activation energy of the electrode kinetics) as well as strengths of the models are discussed. We point out experimental findings that a useful model must be able to explain such as difference in sensitivity to poisoning by H2S due to differences in the detailed composition of the SZ and large change in apparent activation energy by change in cermet preparation. Finally, we will point out some elements, which seem important for any realistic and useful mathematical model of the H2/H2O/Ni/SZ electrode.

In this study, nickel pattern electrodes were electrochemically investigated in a three-electrode setup, operating both with H-2/H2O and CO/CO2 atmospheres. Heating introduced structural differences in the nickel layer among the pattern electrodes, which appear to affect the electrode performance. Both dense and porous nickel pattern electrodes were formed by heating. Holes appeared in the nickel layer of the porous pattern electrodes, where the open cavity triple phase boundaries exhibited different limiting processes than open triple phase boundary electrodes of the dense electrode. As the temperature was raised in the experiment, the electrodes stabilized, with a degraded behavior that seemed to be strongly coupled to the structural changes in the electrode. It was possible to compare literature results with high temperature impedance measurements in H-2/H2O presented here, while new results at lower temperatures in H-2/H2O are also presented. Impedance spectroscopy measurements were performed, and the gas dependence of the polarization resistance was observed as the mixture ratios and temperatures were varied in both atmospheres. A positive relation between the polarization resistance and the partial pressure of CO was determined for the dense nickel pattern electrode, which agrees with previous results using nickel point electrodes. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3484091] All rights reserved. (Less)

In the current efforts of moving energy production to renewable sources, wind and solar energy are widely considered as the key technologies to cover our growing demands. However, the fluctuating nature of these sources requires a flexible energy system and storage technologies to ensure that energy services can be covered in a stable and affordable manner. One promising solution is the synthetic fuel production by solid oxide electrolyzers. Electricity can be stored in a power-to-gas process during times of excess electricity production and then further converted to liquid fuels for e.g. transportation, or at high demands converted back to electricity by either conventional power plants or fuel cells. One of today’s biggest hurdles for a successful commercialization of solid oxide electrolyzers is the stack’s lifetime with current industry targets in the order of five to ten years. To identify and quantify the different degradation mechanisms sensitive in-situ analysis tools are needed. On single cell level, electrochemical impedance spectroscopy (EIS) is a well-established diagnostics tool. On stack level EIS has been shown to be more difficult because of geometrical restrictions of the stack design and significantly lower resistances due to the larger active cell area. Nevertheless, it is becoming a more and more important technique for stack diagnostics. Here we present impedance spectroscopy results of two solid oxide stacks provided by Haldor Topsoe A/S. The first stack was a 14-cell stack (Delta design) specifically optimized for EIS measurements, while the other stack was an 8-cell stack (TSP-1 design), where impedance measurements were carried out without major modifications to the stack. The individual cell voltages were monitored simultaneously by EIS during up to 2000 hours in electrolysis within the ForskEL project 2015-1-12276 “Towards Solid Oxide Electrolysis Plants in 2020”, funded by Danish Energet.dk. The analysis provides valuable insight into the degradation processes which could not have been obtained by current-voltage-data alone.

The electrode performance of solid oxide fuel cell anode with Pd nanoparticles at the interface of ScYSZ electrolyte and Sr 0.94 Ti 0.9 Nb 0.1 O 3 (STN) electrode introduced in the form of metal functional layer have been investigated at temperatures below 600 °C. A metal functional layer consisting of Pd was deposited by magnetron sputtering. Effecting from heat treatments, Pd nanoparticles with particle sizes in the range of 5–20 nm were distributed at the interface, and throughout the backbone. The polarization resistance of the modified STN reduced to 30 Ωcm 2 at 600 °C, which is three times less than an unmodified STN backbone. In order to improve the anode performance further, Pd and Gd-doped CeO 2 electrocatalysts were infiltrated into the STN backbone. The modified interface with Pd nanoparticles in combination with nanostructured electrocatalyst by infiltration resulted in polarisation resistances of 0.35 Ωcm 2 at 600 °C in H 2 /3% H 2 O fuel.

Segregated impurities seem to be detrimental for the performance of a SOFC (solid oxide fuel cell) anode. In this study the performance of a model system of the SOCF anode was measured. It is sought to minimize the segregation of impurities by using high purity materials at relatively low temperature to prevent fast segregation. Bent Ni wires (99.999 %) were pressed against polished single crystals of stabilized zirconia (SZ), thereby forming point electrodes. Four single crystals stabilized with: 10, 13 and 18 mol% yttria and one stabilized with 6mol% scandia and 4 mol% yttria were used as electrolytes. The polarization resistances (Rp) at OCV (open circuit voltage) of the electrodes were measured from 400-500°C in mixtures of H 2 /H 2 O over 46 days. The Rp for all electrodes increased significantly during the first 10-20 days at 500°C after which they became relatively constant. An effect on the Rp of the different electrolytes was not evident. The Ni wires and single crystals were analyzed before and after the electrochemical experiment using SEM (scanning electron microscopy) to study morphology, and surface sensitive techniques for determination of composition and distribution of elements. After the electrochemical experiment the analysis showed segregation of impurities to the surfaces/interfaces, which means that a pure model system was not achieved even though high purity materials were used at a relatively low temperature. These impurities are believed to impede the electrode processes and hence to cause the increase in Rp.

A novel modified anode/electrolyte structure for a solid oxide electrochemical cell is an assembly comprising (a) an anode consisting of a backbone of electronically conductive perovskite oxides selected from the group of doped strontium titanates and mixtures thereof, (b) a scandia and yttria-stabilised zirconium oxide electrolyte and (c) a metallic and/or a ceramic electrocatalyst in the shape of interlayers incorporated in the interface between the anode and the electrolyte. This assembly is first sintered at a given temperature and then at a lower temperature in reducing gas mixtures. These heat treatments resulted in a distribution of the metallic and/or ceramic interlayers in the electrolyte/anode backbone junction taking place. The structure is prepared by (a) depositing a ceramic interlayer onto one side of the electrolyte, (b) optionally applying a metallic interlayer thereon, (c) repeating steps (a) and (b), (d) applying a layer of the selected anode backbone onto the electrolyte with applied interlayers, (e) sintering the raw assembly and (f) infiltrating the electrocatalyst precursor into the sintered assembly and heat treating the assembly to incorporate additional electrocatalyst into the anode backbone.

As solid oxide fuel cell (SOFC) technology is moving closer to a commercial break through, lifetime limiting factors, determination of the limits of safe operation and methods to measure the “state-of-health” of operating cells and stacks are becoming of increasing interest. This requires application of advanced methods for detailed electrochemical characterisation during operation. An operating stack is subject to steep compositional gradients in the gaseous reactant streams, and significant temperature gradients across each cell and across the stack, which makes it a complex system to analyse in detail. Today one is forced to use mathematical modelling to extract information about existing gradients and cell resistances in operating stacks, as mature techniques for local probing are not available. This type of spatially resolved information is essential for model refinement and validation, and helps to further the technological stack development.Further, more detailed information obtained from operating stacks is essential for developing appropriate process monitoring and control protocols for stack and system developers. An experimental stack with low ohmic resistance from Topsoe Fuel Cell A/S was characterised in detail using electrochemical impedance spectroscopy. An investigation of the optimal geometrical placement of the current probes and voltage probes was carried out in order to minimise measurement errors caused by stray impedances. Unwanted stray impedances are particularly problematic at high frequencies. Stray impedances may be caused by mutual inductance and stray capacitance in the geometrical set-up and do not describe the fuel cell. Three different stack geometries were investigated by electrochemical impedance spectroscopy. Impedance measurements were carried out at a range of ac perturbation amplitudes in order to investigate linearity of the response and the signal-to-noise ratio. Separation of the measured impedance into series and polarisation resistances was possible.

Electrolysis of steam and co-electrolysis of steam and carbon dioxide was studied in stacks composed of Ni/YSZ electrode supported Solid Oxide Electrolysis Cells. The results of this study show that long term electrolysis is feasible in these solid oxide electrolysis stacks. The degradation of the electrolysis cells was found to be influenced by the adsorption of impurities from the gasses, whereas the application of chromium containing interconnect plates and glass sealings do not seem to influence the durability. Cleaning the inlet gasses to the Ni/YSZ electrode resulted in operation without any long term degradation, and may therefore be a solution for operating these Ni/YSZ based solid oxide electrolysis stacks without any long term cell stack degradation.