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Onsite production of oxygen and hydrogen is an essential contribution to any manned mission to the Moon or to Mars. In both environments there are elements consisting of bound oxygen, such as water ice-bearing consolidated regolith on the moon and carbon dioxide on Mars. These resources can be used to generate oxygen gas through electrolysis, together with energy carriers, namely hydrogen and CO, as by-products. Solid Oxide Electrolysis (SOE) technology possesses the highest potential for this application, due to the high operation temperature ranging from, 650°C to 900°C. Therefore, it can split each or both H2O and CO2, into H2, CO and O2 without compromising the integrity of the electrodes by poisoning, while the process can be further integrated with other thermal processes, such as the reduction of mineral oxide to provide overall very high system efficiencies. The solid oxide cell (SOC) technology is also fully reversible (i.e., the cell works both as electrolyser and as fuel cell) and can be used for unitised reversible fuel cell systems (URFCS) for energy storage and generation. This paper describes the developments under a European Space Agency (ESA) activity for the development of a high-pressure steam electrolyser system for exploration surface missions. The base case scenario considered is a hydrogen and oxygen production unit for lunar missions, where water for the electrolyser is generated from ice-bearing consolidated regolith. The development of the electrolyte supported cells was based in an all-ceramic approach. Specifically, electrolyte (8YSZ, 3YSZ, 6Sc1CeSZ and 10Sc1CeSZ) supported cells were fabricated comprising substituted lanthanum chromite La0.75Sr0.25Cr0.9Fe0.1O3 (LSCrF), as fuel electrode, and (La0.80Sr0.20)0.95MnO3-x (LSM) or (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x (LSCoF), as oxygen electrodes. The electrochemical characterization and performance assessment in button cells (with an electrode footprint of ~0.8 cm2) was conducted for (i) steam electrolysis and (ii) reversible cell operation (fuel cell and electrolysis modes). All cells exhibited stable performance within the whole range of potentials, thus revealing the fact that Solid Oxide technology is in principle reversible and versatile in functionalities. Regarding steam electrolysis, the effect of hydrogen co-fed was investigated. The lack of reducing agent (H2) in the fuel side results in a non-trivial evolution of the current-potential curve, as the Nernst potential is not fixed and the production of a substantial concentration of hydrogen is required for the IV curve to follow the typical linear relation. However, within the linear IV region the performance of the cells is similar in presence or in absence of hydrogen, exhibiting ~2 A/cm2 at 1.5 V. Following these key developments, a high-pressure short stack is designed and constructed, comprising five (5) cells with an active area of 42 cm2. The stack is enclosed in a hot box consisting of the mechanical and thermal environment for the stack. To operate at pressure up to 10 bars, the hot box is placed inside a stainless-steel chamber made of standard piping elements. The hot box is complemented with all necessary balance of plant (steam generator, water system, gas storage) to realise a complete a steam electrolysis system which will be validated in operation against a load profile representing energy storage in Lunar environment for 3 lunar days (~2,126 h). Figure 1

In energy systems, multi-generation including co-generation and tri-generation has gained tremendous interest in the recent years as an effective way of waste heat recovery. Solid oxide fuel cells are efficient power plants that not only generate electricity with high energy efficiency but also produce high quality waste heat that can be further used for hot and chilled water production. In this work, we present a concept of combined cooling, heating and power (CCHP) energy system which uses solar power as a primary energy source and utilizes a reversible solid oxide fuel cell (R-SOFC) for producing hydrogen and generating electricity in the electrolyser (SOEC) and fuel cell (SOFC) modes, respectively. The system uses “high temperature” metal hydride (MH) for storage of both hydrogen and heat, as well as “low temperature” MH's for the additional heat management, including hot water supply, residential heating during winter time, or cooling/air conditioning during summer time. The work presents evaluation of energy balances of the system components, as well as heat-and-mass transfer modelling of MH beds in metal hydride hydrogen and heat storage system (MHHS; MgH2), MH hydrogen compressor (MHHC; AB5; A = La + Mm, B Ni + Co + Al + Mn) and MH heat pump (MHHP; AB2; A = Ti + Zr, B Mn + Cr + Ni + Fe). A case study of a 3 kWe R-SOFC is analysed and discussed. The results showed that the energy efficiencies are 69.4 and 72.4% in electrolyser and fuel cell modes, respectively. The round-trip COP's of metal hydride heat management system (MHHC + MHHP) are close to 40% for both heating and cooling outputs. Moreover, the tri-generation leads to an improvement of 36% in round-trip energy efficiency as compared to that of a stand-alone R-SOFC.

A method, based on tape-casting, is developed for the manufacture of ceramic interconnects for SOFCs. The development of an optimized process is shown with a discussion of the effects on the precursor-slurry quality and viscosity of, among other things, the concentrations of ammonia, binder and solid particles. Furthermore the effects of some processing modifications, such as the drying rate in the tape casting machine, the speed of the transport band and the doctor blade height on the quality (density and homogeneity) of the green tapes and the finished product are discussed. Improving the degree of dispersion of the primary particles in the slurry was found to make possible improved densification and quality of the thick plates produced as did reducing the drying rate of the wet tapes during the casting process.

Temperature distribution over a Solid oxide fuel cell (SOFC) surface is a crucial parameter for design of a SOFC stack. The selection of both materials and the operating point of a stack is heavily affected by temperature gradient. Temperature distribution can also be used for control and monitoring purposes. An experimental set-up consisting of a cross flow type stack of six cells was built to measure the temperature distribution in different current densities and in two oven temperatures. Five thermocouples were inserted inside the middle cell to measure temperatures in four corners and in the middle of the cell. Voltage was also measured for different cells using platinum wires. Low fuel utilization (meaning low current density) and high excess air caused maximum temperature at the fuel inlet-air outlet corner. Higher oven temperature caused more uniform temperature distribution, while increasing the current density resulted in higher temperature gradient over the cell surface. This paper provides measurement data and analysis of the results from the test runs.

Electrolyte supports for SOFC applications were obtained by water-based tape-casting. Slurries of commercial 3YSZ and 8YSZ and mixtures with a mass ratio of 25/75 and 75/25, respectively, were prepared. A commercial acrylic polymer binder with dispersive properties was used. The drying conditions for obtaining crack-free green tapes were studied. Crack formation during drying was greatly affected by the carrier speed at a fixed temperature. Thermal analysis was performed on the green tapes to study the binder burn out properties. Weight loss of the green tapes occurred until 400 °C and a pre-heating step was therefore included in the sintering program. The microstructure of the green and sintered tapes were investigated by SEM. Fully densified tapes were obtained after sintering at 1450 °C. The ionic conductivity of pressed pellets were characterized by impedance spectroscopy in the temperature range of 200–1000 °C. Phase and crystal structure of pressed pellets were analyzed by XRD. The crystallinity varied from a majority of monoclinic structure (pure 3YSZ) to a majority of tetragonal structure with increasing the 8YSZ content. A cubic single phase structure was obtained by further increasing the 8YSZ content. It was shown that the ionic conductivity of the composite samples were modified with respect to the pure materials. At high temperatures the pure cubic structure gave rise to high conductivity. At low temperatures the conductivity was affected by the grain boundary resistance which was the lowest for the pure 3YSZ, the highest for the pure 8YSZ and decreased as a function of increasing 8YSZ content.

A dip-coating method was used to obtain 8YSZ thin films formed from nanoparticles on anode support for SOFC applications. 8YSZ nanopowder was obtained via a modified sol–gel method using sucrose and pectin as organic precursors. The nanoparticles were suspended in ethanol using phosphate ester (PE) and polyvinyl butyral (PVB) as dispersant and binder, respectively. The microstructure of the sintered films were analyzed with SEM. The suspension preparation conditions were tuned in order to achieve high initial green density of the 8YSZ films subsequently determining the final sintered microstructure. In addition, the effect of the linear shrinkage of the anode substrate on the 8YSZ film microstructure was studied.

The electronic charge carrier concentration in La1−x Sr x FeO3−δ was shown to depend on the partial pressure of O2 (pO 2). Chemical diffusion coefficient and surface exchange coefficient, k chem, were determined by conductivity relaxation in O2/N2 and CO/CO2 mixtures. k chem was proportional to pO 2 1.06 in O2/N2, while in CO/CO2 k chem was controlled by a reaction mechanism involving both CO and CO2.

Preparation conditions for obtaining dense 8YSZ electrolyte films by dip-coating on NiO–YSZ anode electrode substrates for SOFC application were investigated with focus on the co-sintering process. The NiO–YSZ substrates were prepared from nanoparticles which were obtained by a single modified sol–gel process using sucrose and pectin as organic precursors. The microstructure and the morphology of the product powders were analyzed by X-ray diffraction and TEM. A thin film of 8YSZ was obtained by a single-step dip-coating process using phosphate ester (PE) as dispersant and polyvinyl butyral (PVB) as binder. The microstructure of the sintered films was analyzed with SEM. The effect of the sinterability of the anode substrate on the 8YSZ film quality was studied. Fully dense electrolyte films were obtained on anode substrates which held a linear shrinkage of ∼23%. The use of substrates with a linear shrinkage below 23%, resulted in a poorly densified 8YSZ microstructure.

In this work 4YSZ nanoparticles were produced with a variant of the sol–gel method, the particles were sintered using two different heating schemes, and the properties of the resulting pellets investigated. XRD spectra show the crystal structure of the particles to be mainly cubic, and TEM pictures reveal their size to be around 20 nm. The relative densities of the sintered pellets are found with the Archimedes method to lie between 87 and 97% of the theoretical material density, and the grain size, using thermal etching and SEM imaging, found to lie between 0.2 and 5 μ m, depending on the sintering temperature and the presence of impurities. The pellets were subjected to electrical impedance spectroscopy making it possible to distinguish the internal crystal and grain boundary impedances due to their different time-constants. With the “brick layer model” the approximate grain boundary thickness was found to lie between 4 and 8 nm, depending on the sintering conditions and on the presence of impurities. The specific resistances of the grain interiors and the grain boundaries, and the activation energies for ion diffusion in both, are presented. The ionic conductivities of the pellets are compared with literature values, and are found to compare favorably with these, at some temperatures even with data for 8YSZ, which is the standard material used for electrolytes due to its higher ionic conductivity, but which is also mechanically weaker than 4YSZ.

Cation tracer diffusion coefficients in La 1− x Sr x FeO 3− δ , ( x = 0 and 0.1) at temperatures ranging from 1173 to 1373 K have been investigated by secondary ion mass spectrometry (SIMS) for both A-site and B-site diffusion applying Y 3+ and Cr 3+ as tracer ions. B-site impurity (represented by Cr) diffusion was found to be faster than A-site impurity (represented by Y) diffusion for LaFeO 3 in the whole temperature interval investigated and for La 0.9 Sr 0.1 FeO 3 at temperatures less than 1273 K. The obtained activation energies for bulk diffusion in La 0.9 Sr 0.1 FeO 3− δ were 168 ± 24 kJ/mol for the Y tracer and 150 ± 30 kJ/mol for the Cr tracer, which are significantly lower than activation energies reported from diffusion couple experiments. The large variation in reported activation energies can be explained in terms of the difference in cation vacancy populations between the two experimental techniques. The diffusion coefficients were also clearly affected by a first order phase transition at approx. 1263 K in pure LaFeO 3. The obtained grain boundary diffusion coefficients were about five order of magnitude higher than the bulk diffusion coefficients, with an activation energy of 128 ± 28 kJ/mol.

This paper presents a comprehensive model for a unit solid oxide fuel cell (SOFC). It covers all parts of the cell including fluid and porous solid phases. The electrochemical and transport processes involved in an SOFC can thereby be taken into account. The commercial computational fluid dynamic package Star-CD along with built-in user-defined subroutines is adapted for an anode-supported unit cell. Some results such as gas concentration, temperature distribution and current density are presented at the end of the paper.

Short-term tests showing the influence sulfur has on solid oxide fuel cell (SOFC) performance have been performed. The experiments were performed using two single-cell SOFC setups operated at 800 d ...

The electrical conductivity of polycrystalline LaFeO 3 is reported as a function of temperature. The conductivity of LaFeO 3 was shown to be affected by annealing for extended periods (about one week) at temperatures above 1000 °C prior to the conductivity measurements. Higher annealing temperatures resulted in higher conductivity. This effect was explained by the formation of Schottky cation defects, which act as electron acceptors, leading to an increasing conductivity with increasing annealing temperature. Below 1000 °C, the conductivity was unaffected by the annealing temperature, probably due to the low mobility of cation vacancies. A model for the defect chemistry and conductivity of LaFeO 3 , including the formation of Schottky defects is proposed. The model reproduces quantitatively the dependence of the conductivity with respect to the partial pressure of oxygen at 1000 °C. The electronic mobilities of the charge carriers in the material were determined by the conductivity of La 0.99 Sr 0.01 FeO 3 , where the charge carrier concentration is given by the dopant level. The model also predicts that the concentration of p -type charge carriers in air is increasing with temperature due to the formation of Schottky defects, for a limited temperature interval above 1000 °C.

Crystal structure and thermal properties of La 1-x Sr x FeO 3-δ (x = 0, 0.1, 0.3, 0.4, 0.5, and 0.75) have been studied by high-temperature X-ray diffraction and thermal analysis in air and nitrogen (p(O 2 ) 10 -3 atm) atmosphere. The first-order phase transition from orthorhombic-to-rhombohedral La 1-x -Sr x FeO 3-δ (x = 0, 0.1) was strongly shifted to lower temperatures with increasing Sr content. The phase-transition temperature was observed significantly lower in polycrystalline ceramics compared with fine powders. The temperature depression of the phase transition in the ceramics was qualitatively explained by stresses induced both by the anisotropic thermal expansion of LaFeO3 and the observed volume contraction of the phase transition. Rhombohedral La 1-x Sr x -FeO 3-δ (x = 0.3, 0.4, 0.5) were observed to transform to the cubic perovskite structure during heating. The second-order phase-transition temperature decreased with increasing Sr content and decreasing partial pressure of oxygen. On the basis of the present findings, a pseudobinary phase diagram of the LaFeO 3 -SrFeO 3-δ system is presented. Finally, a severely nonlinear thermal expansion was observed for the Sr-rich materials at high temperature. The high thermal expansion in this region is due to a chemical expansion resulting from a reduction of the valence state of Fe.

Heteroepitaxial LaFeO 3 (1 1 0) thin films with a thickness of 150 nm were grown on LaAlO 3 (0 0 1) by reactive sputtering in an inverted cylindrical magnetron geometry. Equilibrium conductivity was measured as a function of partial pressure of oxygen at T =1000 °C, and log σ plotted vs. log P (O 2 ) showed a minimum in conductivity for P (O 2 )=10 −11 atm and a linear response between 10 −10 and 1 atm. This linear response makes thin films of LaFeO 3 a promising material for oxygen sensor applications. We have also measured the time response of the film conductivity upon an abrupt change in the partial pressure of ambient oxygen from 10 −2 to 10 −3 atm, which was determined at 60 s for T =700 °C and T =1000 °C.

During the recent years, SOFC research has mainly been aimed at reducing the capital cost to effectively compete with other power generation technologies. CMR Prototech, Norway is a provider of technical solutions, product designs and tools for various application areas. Currently, CMR Prototech is planning to develop 2 MW modular fuel cell systems for subsea operation. Each module consists of 200 stacks, each stack producing 10kW. In order to address the technical challenges faced during this development and to make SOFC technology feasible for offshore applications more research on fuel cells in marine environment is necessary. This research was aimed at determining the effects of a) reducing partial pressure of oxygen and b) NaCl contamination on the performance of the SOFC cathode of a single SOFC cell. This present paper focuses on the effect of oxygen partial pressure. Single cell tests were performed in a versatile test station, fully automated and controlled using LabVIEW programming. The fluid handling system with mass flow controllers and four-points measurement probes along with field-point connections were installed in the test station. A set of experiments were conducted, reducing the partial pressure of oxygen supplied to the cathode stopwise (at thre relevant temperatures, namely 800, 850 and 900oC). All the cells were characterized and analyzed by I-V curves. The grain structures of the cells were characterized by scanning electron microscopy (SEM) before and after the experiments.

One of the fundamental impediments to the wide-spread commercialization of fuel cell technology is questions about the robustness and durability of this type of installation. In the course of a larger experimental program designed to answer these questions, specifically for operation in a marine environment, a series of tests were carried out to determine the sensitivity of SOFC cathodes to salt contamination in the gaseous feed stream. The salt content in the feed stream was varied by adding crystalline salt in the inlet piping, allowing it to sublimate at temperatures designed to give the desired salt concentration in the feed to the cathode. Both short-term and long-term degradation tests were performed, and in addition to characterizing the performance using the I-V curves, the microstructure of the cathodes were investigated using scanning electron microscopy before and after each run. The tests showed that there is degradation of the cathode due to the salt contamination, both in the short and the long term. While it was difficult to determine the reason for the short-term degradation, the long-term degradation was much more severe, and can be attributed to two effects: the salt appeared as crystals in the cathode microstructure and had a coarsening effect, and the presence of the salt gave rise to delamination of the cell at the cathode-electrolyte interface, something that was not observed under otherwise like operation conditions and -duration without salt contamination.

Tracer diffusion coefficients in La 1− x Sr x FeO 3 , x =0 and 0.1 from 900 to 1100 °C for both A- and B-site diffusions were examined with secondary ion mass spectrometry (SIMS) applying Y 3+ and Cr 3+ as tracers. In both compositions, the B-site diffusion, represented by Cr, was found to be faster than the A-site diffusion, represented by Y. The obtained activation energies for bulk diffusion in La 0.9 Sr 0.1 FeO 3 were 168±24 kJ/mol for the Y tracer and 150±30 kJ/mol for the Cr tracer. The 18 O-tracer diffusion coefficients were found to be significantly higher in La 0.9 Sr 0.1 FeO 3 compared with LaFeO 3 .

LaCoO3 and LaFeO3 based perovskites have been studied as model materials for oxygen permeable membranes. To avoid gas leakages it is of crucial importance to obtain dense membranes, hence the densification behavior in terms of temperature, stoichiometry (AB-ratio), presence of secondary phases and substituents (Ca and Sr) have been investigated. Only a narrow temperature window for the sintering is available and it is important to avoid the formation of secondary phases. The upper temperature to obtain high density materials is limited by a severe swelling. Mechanical properties such as bending strength and fracture toughness have been determined both at ambient- and elevated temperatures. Non elastic behaviour due to ferroelasticity is reported for pure LaFeO3, LaCoO3 and La0.8Ca0.2CoO3. The ferroelasticity is found to influence the behavior of the fracture toughness and fracture strength as a function of temperature. The electrical conductivity of pure LaFeO3 at 1000°C is determined in terms of partial pressure of oxygen. Cation diffusion is found to be rate controlling for the establishment of a time independent conductivity in the p-conductivity regime.

High temperature SOFC with operation temperature from 800 - 1000oC is suitable for high efficiency fuel cell systems based on heat integration with other technologies, such as SOFC-GT, gasification or hydrogen production. This requires alternatives to the commonly used metallic interconnects. Prototech is currently developing an all ceramic SOFC suitable for operation in this temperature region in larger systems with heat integration.