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Solid-state sodium batteries are currently gaining enormous interest as a lower-cost and more environmentally friendly alternative to lithium batteries. They contain significantly less critical rare elements in both the electrolyte and the active material. However, to date, there is no efficient material combination of metallic anode, cathode, and solid electrolyte for room temperature applications that does not require an additional liquid electrolyte while maintaining high energy density. NaPSiO-based glass-ceramics show high ionic conductivity at room temperature, good corrosion resistance against ambient humidity and CO2, and stability against metallic sodium. However, the conductivity mechanisms of this promising class of materials are currently poorly understood. Herein, high frequency impedance measurements up to 108 Hz shed light on the contributions of grains and grain boundaries to the total impedance, including the distribution of relaxation time constants of the fully crystallized material. In addition, analysis of the temperature dependence allows separation of electrode contributions and determination of activation energies for grain and grain boundary conductivities. Our study provides the basis for fine-tuning the stoichiometry of NaPSiO-based glass-ceramics in terms of maximizing the conductive phase fraction to optimize the performance of future solid-state sodium batteries.

All solid-state batteries offer the possibility of increased safety at potentially higher energy densities compared to conventional lithium-ion batteries. In an all-ceramic oxide battery, the composite cathode consists of at least one ion-conducting solid electrolyte and an active material, which are typically densified by sintering. In this study, the reaction of the solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) and the active material LiNi0.6Co0.2Mn0.2O2 (NCM622) is investigated by cosintering at temperatures between 550 and 650 °C. The characterization of the composites and the reaction layer is performed by optical dilatometry, X-ray diffractometry, field emission scanning electron microscopy with energy dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, as well as scanning transmission electron microscopy (STEM). Even at low sintering temperatures, elemental diffusion occurs between the two phases, which leads to the formation of secondary phases and decomposition reactions of the active material and the solid electrolyte. As a result, the densification of the composite is prevented and ion-conducting paths between individual particles cannot be formed. Based on the experimental results, a mechanism of the reactions in cosintered LATP and NCM622 oxide composite cathodes is suggested.

A solid electrolyte is a core component for the development of low temperature sodium batteries with metallic Na-anode. The Na5YSi4O12 (N5) composition in the Na2O-Y2O3-SiO2 system shows a high ionic conductivity comparable to NASICON or β-Al2O3. Up to date glass ceramic solid electrolytes of this type have been mainly prepared by crystallization of monolithic, molten glass components. We show that this material can be processed via the glass-ceramic powder route starting with a glass powder. A glass with a composition according to the stoichiometry of the Na5YSi4O12-phase and tailored by addition of P2O5 allows the separation of sintering and crystallization of the glass powder resulting in dense microstructure. Thermophysical properties and phase content have been correlated with ionic conductivity. The densification and crystallization are completed at temperatures below 1100 °C. Grain conductivities up to 0,18 mS cm−1 at room temperature in sintered glass ceramic microstructures are demonstrated.

New sodium-based battery concepts require solid electrolytes as ion conducting separators. Besides NaSICON and β-Al2O3 in the Na2O-R2O3-SiO2 system (R = rare earth), a rarely noticed glass-ceramic solid electrolyte with the composition Na5RSi4O12 (N5-type) exists. The present study addresses the investigation of the ionic conductivity of Na5RSi4O12 solid electrolytes sintered from pre-crystallized glass-ceramic powders. The sintering behavior (optical dilatometry), the microstructure (SEM/EDX), and phase composition (XRD), as well as electrochemical properties (impedance spectroscopy), were investigated. To evaluate the effect of the ionic radii, Y, Sm and Gd rare elements were chosen. All compositions were successfully synthesized to fully densified compacts having the corresponding conducting N5-type phase as the main component. The densification behavior was in agreement with the melting point, which decreased with increasing ionic radii and specific cell volume. Alternatively, the ionic conductivities of N5-phases decreased from Y to Gd and Sm containing samples. The highest ionic conductivity of 1.82 × 10−3 S cm−1 at 20 °C was obtained for Na5YSi4O12 composition. The impact of grain boundaries and bulk conductivity on measured values is discussed. A powder-based synthesis method of this glass-ceramic solid electrolyte using different rare earth elements opens possibilities for optimizing ionic conductivity and scalable technological processing by tape casting.

The principle of the electrochemical decomposition of water in an electrolysis cell has already been known for more than 230 years. The first generation of hydrogen by electricity was done as early as 1789 by van Troostwijk and Deiman using an electrostatic generator as the direct current source. Shortly after Volta invented the voltaic pile in 1800, Carlisle and Nicholson used such a device to decompose water into hydrogen and oxygen. In the same year, Ritter performed comparable experiments in Jena, Germany. Moreover, Cruickshank used a voltaic pile for the electrochemical decomposition of NaCl to hydrogen and chlorine at the beginning of the 19th century. Nevertheless, it took decades before the processes were used in the first technical applications. Around 1890 Charles Renard constructed a water electrolysis unit to generate hydrogen for French military airships. It is estimated that around 1900 more than 400 industrial alkaline water electrolyzers were in operation worldwide and large-scale deployment of the chlor-alkali process started. Later, different types of commercial alkaline water electrolyzers were developed in the 20th century to generate hydrogen needed to produce ammonia fertilizers based on low-cost hydroelectricity. As the 20th century progressed, the more cost-effective production of hydrogen by steam reforming of methane increasingly replaced water electrolysis, and by the end of the 20th century, the process was only used in niche applications. In the late 1960s the development of proton exchange membrane electrolysis began at General Electric using an acidic fluorinated ionomer as a solid electrolyte. But the technology became established in the following decades only in laboratory, military, and space applications due to the high material costs. At about the same time, General Electric, and the Brookhaven National Laboratory also started to develop high-temperature electrolysis with solid oxide cells. In Germany, Dornier followed in the HOT ELLY project this technology until the mid-1980s. But despite of all technical progress made commercialization of the membrane and solid oxide electrolysis could not be launched in that time. But new interest in water electrolysis was stimulated already in the 1990s as hydrogen was regarded as a green energy carrier for renewable energy sources like wind and solar power and to power fuel cells, e.g., in automotive applications. However, it is only in the last 10 years that we have seen a significant increase in global interest for water electrolysis, with the adoption of ambitious national climate protection programs. Water electrolysis is regarded as the central element for sector coupling and is expected to make an important contribution to reducing greenhouse gas (GHG) emissions close to net-zero by 2050.

Crystallization of highly ionic conductive N5 (Na5YSi4O12) phase from melted Na3+3x-1Y1-xPySi3-yO9 parent glass provides an attractive pathway for cost-effective manufacturing of Na-ion conducting thin electrolyte substrates. The temperature-dependent crystallization of parent glass results in several crystalline phases in the microstructure (N3 (Na3YSi2O7), N5 and N8 (Na8.1Y Si6O18) phases) as well as in rest glass phase with temperature dependent viscosity. The electrical properties of dense parent glass and of compositions densified and crystallized at 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C are investigated by impedance spectroscopy and linked to their microstructure and crystalline phase content determined by Rietveld refinement. The parent glass has high isolation resistance and predominantly electrons as charge carriers. For sintering at ≥ 900 °C, sufficient N5 phase content is formed to exceed the percolation limit and form ion-conducting pathways. At the same time, the highest content of crystalline phase and the lowest grain boundary resistance are observed. Further increase of the sintering temperature leads to a decrease of the grain resistance and an increase of grain boundary resistance. The grain boundary resistance increases remarkably for samples sintered at 1100 °C due to softening of the residual glass phase and wetting of the grain boundaries. The conductivity of fully crystallized N5 phase (grain conductivity) is calculated from thorough impedance spectra analysis using its volume content estimated from Rietveld analysis, density measurements and assuming reasonable tortuosity to 2.8 10−3 S cm−1 at room temperature. The excellent conductivity and easy processing demonstrate the great potential for the use of this phase in the preparation of solid-state sodium electrolytes.

Li-ion all-solid-state batteries (ASSBs) employing solid electrolytes (SEs) can address the energy density and safety issues that plague the current state-of-the-art Li-ion battery (LIB) architecture. To that end, intimate physical and chemical bonding has to be established between high-performance cathodes and high-voltage stable SEs to facilitate high Li+ transfer. The production of intimate interfaces in oxide cathode–solid electrolyte composites requires high-temperature (>1000 °C) processing, which results in a range of degradation products. Here, we report the morphological, structural, and chemical changes that occur in commercial Ni-rich layered LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode in contact with oxide SE Li1.3Al0.3Ti1.7(PO4)3 (LATP) when cosintered between 550 °C and 650 °C. The structural evolution of pristine NCM622 heat-treated at a temperature of 650 °C is contrasted with the NCM622 from the composites using aberration-corrected scanning transmission electron microscopy (AC-STEM). At high spatial resolutions, the degradation of NCM particles in the composites proceeds via phase transitions from R3̅m (layered) to Fd3̅m (spinel) to Fm3̅m (rocksalt) to amorphous at the grain boundaries and via pit formations and intragranular crack nucleation and propagation in the bulk. Automated crystal orientation mapping (ACOM) in combination with low-dose TEM was used to investigate the beam-sensitive cathode–solid electrolyte interfaces. To provide statistical relevance to the investigations undertaken, ACOM-TEM was used in combination with time-of-flight secondary ion mass spectroscopy (ToF-SIMS). By combining these techniques, we show that the phase transitions of the NCM particles are correlated with simultaneous lithium transfer from NCM regions to LATP regions with evolving temperature.

New sodium-based battery concepts require solid electrolytes as ion conducting separators. Besides NaSICON and β-Al

All solid-state batteries offer the possibility of increased safety at potentially higher energy densities compared to conventional lithium-ion batteries. In an all-ceramic oxide battery, the composite cathode consists of at least one ion-conducting solid electrolyte and an active material, which are typically densified by sintering. In this study, the reaction of the solid electrolyte Li

A zirconia-based potentiometric solid electrolyte gas sensor with internal solid state reference was used to study the response behavior of platinum cermet and indium tin oxide sensing electrodes. Target gases included both oxygen and carbon monoxide in nitrogen-based sample gas mixtures. It was found that with the indium tin oxide sensing electrode, the low-temperature behavior is mainly a result of incomplete equilibration due to contaminations of the electrode surface. On the other hand, some of these contaminant species have been identified as being pivotal for the higher carbon monoxide sensitivity of the indium tin oxide sensing electrode as compared to platinum cermet electrodes.

In order to be a real competitor to other industrial-scale hydrogen production technologies, solid oxide electrolysis cells (SOEC) must present essential features such as high hydrogen production rates, load flexibility and high system efficiency. However, operating conditions necessary to achieve these objectives can induce undesired performance deterioration and microstructural degradation negatively influencing the long-term stability of SOECs. This work provides the results of experimental investigations on the performance of SOECs operated under different operating conditions. In order to induce and accelerate specific degradation phenomena high steam partial pressures, fast load changes, high voltages, high steam conversion rates and alternating mode were applied. Different kinds and degrees of impact on performance and microstructure were observed and analyzed in detail. Correlations between performance changes and their underlying processes were investigated and useful connections between them were discovered. The major findings are summarized in the form of a suggestion for accelerated stress test (AST) protocols for SOECs. The developed ASTs enable deeper understanding of several degradation phenomena induced by different operating conditions and present methods for identification thereof. Further, the ASTs were estimated to have potential to predict performance changes over long-term operating periods.

The combination of aerosol-jet printing and laser micromilling technology makes it possible to fabricate the ceramics MEMS platforms and SMD packages by using their digital models. It is shown that combination of platinum metallization for the heater and ceramic materials is the best choice for the microelectronic MEMS actuator and sensor package which should be installed in devices aimed for work under harsh environmental conditions. The developed metal-oxide semiconductor gas sensors in miniaturized package with low power consumption can be extremely useful in modern agriculture facilities. The idea of their integration into livestock facilities, food-storage, forestry, fish farming and horticulture seems quite promising for developing smart agriculture, where devices must work under harsh environmental conditions such as high humidity level and temperature. To fulfill the demand of these types of sensors their mass production should be cheap and not require a clean room. In this work we present a way of fabrication of all parts of gas sensor including a package using only one setup, which allows us to reduce the cost of the final product and simplify the whole production process in order to make the technology available for many different agricultural applications.

Introduction The progress of the Internet of Things stimulates the development of sensors of small size and low power consumption. Miniaturized metal-oxide semiconductor (MOX) gas sensors (e.g. methane, hydrogen or carbon monoxide detection) can be integrated into agro-industrial facilities such as livestock facilities, fish farming, forestry, food-storage and horticulture, where they support future-oriented plant production (smart agriculture). The central part of a MOX gas sensor is a micro-hotplate, which is mainly responsible for the sensor power consumption at operating temperatures of 450 to 600°C. Under harsh environmental conductions, ceramic materials are the best choice for the micro-hotplate substrate and sensor housing (ceramic MEMS) in combination with platinum metallization for the heater. To realize such gas sensors with low power consumption (< 200 mW@4500C) the development of miniaturized printable heaters on ultra-thin ceramic membranes is needed. Methods Yttria-stabilized zirconia (3YSZ) powders were evaluated for the preparation of casting slurries and tape casting of thin tapes < 40 µm (present on Fig.1). After thermal treatment the substrates were characterized by density, thickness, flatness, roughness and mechanical bending stability. To reduce the thermal mass of the hotplate, the ceramic substrate was cut by a developed micro-milling process to realize free standing membranes of 280 µm size in diameter (present on Fig.2). Platinum particle (20 wt.-% and 30 wt.-%) as well as Pt-SiO2-composite inks were synthesized and ink properties like viscosity, surface tension and sedimentation stability characterized. Printing tests of miniaturized heater layouts (2.0x0.5 mm2 and 40 µm line width in hot spot) were performed by aerosol-jet (Optomec M3D 150 µm nozzle) and piezoDoD inkjet (Dimatix DMP 10 pL). The printed microheater were characterized (photo of experiment present on Fig.5) after sintering by film thickness, resistivity and microstructure (SEM). The applicability is demonstrated by heating tests, where temperature and power consumption is monitored in dependence of the heater driving voltage. The all parts of gas sensor was fabricated by totally digital technological flow with using pre-developed in STL format 3D model like as is customary in rapid prototyping way. For material of sensor package was used inexpensive Al2O3 monolithic ceramics. As a form factor for sensor was using SOT-23 package (3.0x1.4x1.0 mm), which give possibility to dissipate 350 mW heating power at room temperature. Parts of sensor package was fabricated by 20W fiber laser with a wavelength of 1.064 μm and tunable pulse duration from 50 to 200 ns with using especially developed software (present on Fig.7) combined process of micronilling and on-line comparisons fabricating geometrical parameters with 3D model. Results and Conclusions Mechanical flexible 3YSZ substrates of 5x5 cm2 size with thickness of 20 to 40 µm and a high density of 6.1 g/cm3 (equals theoretical density) were achieved (Fig.1a). The choice of raw material powders correlates to the substrates surface roughness (250 to 600 nm) and mechanical stability. The developed Pt-particle inks show a good print compatibility and a high sedimentation stability of < 0.2 mm per month. Miniaturized heater layouts with a narrow line width down to 40 µm were only achieved by aerosol-jet printing (Fig.4). Too large ink droplets and a pronounced ink wetting on the substrates leads to line width > 100 µm by using inkjet. The sintered Pt-heater show a suitable resistance of 10 to 30 Ohm and were successfully evaluated by heating tests up to 450-600°C by micro melting technique [1] with using micro powder of polyamide with dependence of power consumption vs. working temperature on surface of microhotplate present on Fig.6. Acknowledgement This research was sponsored by the Federal Ministry of Education and Research (BMBF) in Germany founding No. 02P15B520, the Israel Innovation Authority and the Ministry of Science and Higher Education of the Russian Federation founding with unique identifier RFMEFI58718X0054 in frame of MANUNET project MNET17/ADMA-1147. References [1] Biró, F., Dücso, C., Hajnal, Z., Riesz, F., Pap, A.E., Bársony, I. Thermo-mechanical design and characterization of low dissipation micro-hotplates operated above 500 °c (2015) Microelectronics Journal, 45 (12), pp. 1822-1828. Figure 1

The progress of the Internet of Things stimulates the development of sensors of small size and low power consumption. Miniaturized metal-oxide semiconductor (MOX) gas sensors (e.g. methane, hydrogen or carbon monoxide detection) can be integrated into agro-industrial facilities such as livestock facilities, fish farming, forestry, food-storage and horticulture, where they support future-oriented plant production (smart agriculture). The central part of a MOX gas sensor is a micro-hotplate, which is mainly responsible for the sensor power consumption at operating temperatures from 450 °C to 600 °C. Under harsh environmental conductions, ceramic materials are the best choice for the micro-hotplate substrate and sensor housing (ceramic MEMS) in combination with platinum metallization for the heater. To realize such gas sensors with low power consumption (

All-solid-state batteries (ASSB) are considered promising candidates for future energy storage and advanced electric mobility. When compared to conventional Li-ion batteries, the substitution of Li-ion conductive, flammable liquids by a solid electrolyte and the application of Li-metal anodes substantially increase safety and energy density. The solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) provides high Li-ion conductivity of about 10&minus, 3 S/cm and is considered a highly promising candidate for both the solid electrolyte-separator and the ionically conductive part of the all-solid state composite cathode, consisting of the cathode material, the solid electrolyte, and an electron conductor. Co-sintering of the composite cathode is a sophisticated challenge, because temperatures above 1000 &deg, C are typically required to achieve the maximum ionic conductivity of LATP but provoke reactions with the cathode material, inhibiting proper electrochemical functioning in the ASSB. In the present study, the application of sintering aids with different melting points and their impact on the sinterability and the conductivity of LATP were investigated by means of optical dilatometry and impedance spectroscopy. The microstructure of the samples was analyzed by SEM. The results indicate that the sintering temperature can be reduced below 800 &deg, C while maintaining high ionic conductivity of up to 3.6 &times, 10&minus, 4 S/cm. These insights can be considered a crucial step forward towards enable LATP-based composite cathodes for future ASSB.

In this study, a ten-layer SOC-stack, consisting of large planar electrolyte supported cells (ESC) and CFY interconnects, was operated in both fuel cell and electrolysis modes considering steam-, co-, and CO2-electrolysis. Different gas mixtures containing H2, H2O, CO, CO2, and N2 were applied for this purpose, and an extensive analyses were performed at two temperatures, 800°C and 850°C. Under these operating conditions, independent Electrochemical Impedance Spectroscopy (EIS) measurements were performed on the stack as a whole, and several cells within the stack concurrently. In order to analyze the losses that occurred during operation, measured impedance values were analyzed, and an electrical equivalent circuit (EEC) for ESCs was applied to the measured impedance spectra. Finally, the electrochemical impedance measurements were performed at both OCV and elevated current densities.

Despite long and successful development history of Solid Oxide Cells (SOC) continuous improvement in performance, longevity, manufacturing and system integration is necessary to bring this highly efficient technology to the market. The activities on material development and optimization at IKTS are focused mainly on enhancement of durability in SOFC, SOEC and rSOC operation and on boosting the power density for electrolysis operation. The recent results show considerable enhancement in cell longevity and power density. The SOC priorities in applied research are moving from materials to industrialization and Fraunhofer as applied R&D service provider is following this trend. During recent years considerable efforts on simplification and automation of cell and stack manufacturing processes has been addressed at IKTS. The manufacturing processes for electrode manufacturing have been adjusted for high yield automated printing on thin electrolytes with integrated quality control measures. The efficient ways for reduction of time and energy consumption for sealing process of SOC stacks have been found and shown in pilot production as well as automated assembling of components to stacks demonstrated. Furthermore the stack integration into modules utilizing stacks of different providers has been addressed during last years and reliable solutions for building of compact stack modules of higher power (2 to 10 kW) has been developed and tested with industrial partners. The increasing interest in “green hydrogen” generated multiple opportunities for SOEC technology to be considered as inherent part of industrial and chemical processes. IKTS pioneering work on coupled operation of SOEC module with Fischer-Tropsch reactor provided first demonstration of feasibility of this approach for waxes production. The process design and analysis of steel, using SOEC technology, has been performed and resulted in multiple demonstration activities in last four years. Recently novel approaches for ammonia production are addressed at IKTS showing opportunity for transportable chemicals/fuel production without need of CO2 source. High power electrolysis applications (>10 MW) will need new approaches for stack design and put higher requirements on durability. High power electrolysis applications will also open a new window of opportunities for industrial SOFC power plants. This vision is considered as outline for our future research.

A stationary SOFC system utilizing gas from a biomass updraft gasifier is under development within the project “HiEffBioPower” by MAWERA, BIOS, Bosal, AVL, Calida Cleantech, Fraunhofer IKTS, Wuppertal Institute and University Utrecht (European Union Horizon2020 No. 727330). The system is based on Mk352-CFY-stack with Chromium based (CFY) interconnects and electrolyte supported cells. These stacks are robust, redox stable and demonstrated low degradation rates. The stacks are arranged to modules of eight 30-cell stacks and are electrically connected in series. The stack module was pre-tested at the test rig at IKTS and showed performance in accordance with system requirements. Stationary power points were measured to compare the behaviour of the stack module with the system tests at MAWERA. AVL designed and installed the overall SOFC system (heat exchangers, stack, catalytic afterburner, process control and visualisation) with a nominal capacity of 6 kWel. After shipping all parts of the SOFC system to MAWERA in Hard (AT), the coupling of the biomass gasifier, the gas cleaning unit and the SOFC system was conducted. The system is operated by an automated process control. The highest efforts needed the novel gas cleaning unit, consisting of a high temperature filter, a desulphurisation unit as well as a novel catalytic gas reforming stage developed by BIOS. The biomass gasifier has been operated with wood chips of different quality. The high tar contents of the product gas make a tar cracking essential to reach a high calorific value of the gas for the SOFC system. The novel gas cleaning system shows good performance and reaches the defined SOFC requirements. The SOFC system developed and operated by AVL reached stable conditions for several days. The operating results of the stack module at lab conditions and at the real system are in a good agreement. The SOFC stack module performance in terms of cell voltages, stack temperature distribution and fuel utilization under biomass reformate fuel will be shown and supported by several simulation to get an overall energy balance. Examples of optimization for several components by CFD simulation and component tests will be given.

The efficient high temperature electrolysis is of high interest for potential application in a renewable energy system. The possibility to use high temperature heat for the electrolysis reaction makes the SOEC technology an attractive option for the integration in process concepts for the production of valuable chemicals from renewable energies. Moreover, the SOEC allows the electrolysis of water and carbon dioxide in a so-called Co-electrolysis. The produced syngas can be used in a subsequent synthesis step, e.g. a Fischer-Tropsch synthesis. The development of these power-to-product processes as well as of necessary components are in focus of current research activities at IKTS. The main component is the high temperature electrolyzer (SOEC). In collaboration with Plansee SE, the CFY stack technology has been developed. This stack technology can be applied for SOFC, SOEC and reversible operation (rSOC) and establishes an universal platform for different applications. Performance maps of a MK352 CFY stack in steam- (H2O) and Co- (H2O/CO2) electrolysis operation show a difference in electric power demand of less than 5 %. This low additional power demand of Co-electrolysis together with the advantages of direct syngas production makes it promising for highly efficient integration within a process concept for the production of valuable hydrocarbons. The output syngas can be directly adjusted by choosing appropriate operating conditions for the SOEC to typical compositions used for reactions like Fischer-Tropsch synthesis. Based on this approach the operating conditions were set and durability tests with stacks (>1500 h) were conducted. The results raised several scientific questions which were answered by splitting the “black box” of the stack in separate parallel investigated experiments in the fields of cell tests, tests of interconnect materials, contact resistances, protective layer and glasses. The surprising behavior of reversible degradation of the stacks was analyzed by single cell experiments and could be reduced by adjusting of operating conditions and resulted in a fine tuning of the electrode design. Nevertheless still an irreversible degradation of about 1%/1000 h was detected. Several material tests consolidated the common understanding of degradation phenomena and will be shown with comparison to the real stack tests. The results show the excellent suitability of the CFY stack for electrolysis applications and give an outlook on further improvements. Figure: 30-cell MK352 stack for SOFC/SOEC and rSOC operation Figure 1

A new alkali-containing diopside based glass-ceramic sealant for solid oxide cells was synthesized, characterized and tested. The composition was designed to match the coefficient of thermal expansion (CTE) of Crofer22APU interconnect. The sealant has a glass transition temperature of 600 °C, a crystallization peak temperature of 850 °C and a maximum shrinkage temperature of 700 °C, thus suggesting effective densification prior to crystallization. The CTE of the glass-ceramic is 11.5 10−6 K−1, a value which is compatible with the CTE for Crofer22APU stainless steel. Crofer22APU/glass-ceramic/Crofer22APU joined samples were tested in simulated real-life operating conditions at 800 °C in dual atmosphere under an applied voltage, monitoring the electrical resistivity. The effect of two different applied voltages (0.7 V and 1.3 V) was evaluated. A voltage of 1.3 V led to a rapid decrease in the electrical resistivity during the test; such a drop was due to the formation of Cr2O3 ""bridges"" that connected the two Crofer22APU plates separated by the sealant. There was no decrease in the resistivity when a voltage of 0.7 V was applied. Instead, resistivity value remained stable at around 105 O cm for the 100 h test duration. The degradation mechanisms, due to both the alkali content and the applied voltage, are investigated and discussed.

The electrode processes governing response behavior of a solid electrolyte CO2 sensor have been studied using electrochemical impedance spectroscopy on symmetric cells exposed to various temperatures and gas compositions. It was shown that the electrochemical processes leading to potential formation at the Au/Na2CO3 electrode can be described by adapting models commonly used in the field of molten carbonate fuel cells.

The operation of solid oxide fuel cells requires gas tight and stable sealing materials. Glass ceramic materials have been established to seal different stack designs and can be optimized to match the requirements of components and joining conditions. It is known that glass based sealants are sensitive to the formation of undesired chromia species in contact with ferritic steels used as interconnects. Their formation depends on the earth alkaline type and content in the glass as well as the chromium content of the interconnect. A robust interconnect material is the chromium based sinter alloy CFY with > 93 wt.% chromium what in turn enforces the formation of chromia phases. SOFC stacks with chromium based interconnects and electrolyte supported cells (ESC) with 10SCSZ electrolyte are a candidate for SOFC systems with high efficiency in a wide power range. This study presents results on the development of sealing glasses adapted for the CFY by optimizing the glass components BaO, CaO, SiO2 and Al2O3. Depending properties like crystallization behavior, thermal expansion, adhesion and reactivity in contact with CFY have been regarded. The glasses have been tested under operating conditions of SOFC (dual atmosphere, electric potentials) in samples as well as in real stacks. It has been shown that optimized glass compositions especially regarding a limited BaO-content combine a good joining behavior and minimized BaCrO4 formation in contact with the CFY alloy.

In this study the gas-tightness of reactive air brazed YSZ-steel joints and their long-term stability at high temperatures in air and dual atmospheres are investigated. Silver-based braze compositions like Ag-4CuO are used for reactive air brazing of gas-tightness samples. During aging in air at 850 °C for 800 h all tested samples remained hermetic. However, microstructural analysis reveals the formation of voids within the brazing zones and severe changes of the interfacial oxide layers. It is shown, that due to the diffusion of chromium and manganese from the steel substrate the thickness of the reaction layers increases and the phase composition changes. Furthermore, in previous work a decrease of the bending strength of reactive air brazed YSZ-steel joints due to the microstructural changes is observed. In contrast, the aging in dual atmospheres at 850 °C causes strong degradation and the total loss of gas-tightness. Reducing atmospheric conditions lead to different reactions profiles of the silver based brazes in contact with the YSZ electrolyte. Complex reaction products are identified and reported.

The 3YSZ substrates with thickness between 90-200 μm are excellent from mechanical and economical points of view, but its main disadvantage is rather low ionic conductivity when compared with scandia-doped zirconia or fully stabilized 8YSZ. With decreasing electrolyte thickness between 40-60 μm, it is possible to significantly improve the electrochemical performance of the 3YSZ based electrolyte-supported cell (ESC) thereby fully utilizing the available mechanical stability. Development and progress in manufacturing of high power density electrolyte supported cell based on a thin (50 µm) 3YSZ substrate is presented. The cells with improved cathode based on LSMM’/ScSZ and the multilayer anode based on Ni/GDC cermet show very good electrochemical performance. The maximum power density increases up to 750 mW/cm2 for developed cell without additional contact layer at 0.7V@860 °C and is greater than one of the commercial cells based on partially scandia stabilized zirconia electrolytes as well as 95 µm 3YSZ. The changes in polarization resistance of tested cells under different operating conditions as well as during redox-cycling and durability tests are discussed on basis of analysis of impedance spectra. The developed cells show a good long-term stability (proved for >1300 h) under high current density (500 mA/cm²@850 °C, N2:H2:H2O=55:40:5, air). The estimated power degradation rate is lower than 0.5%/1300 h. By using Ni/GDC anode the redox cycle ability of cell under real operating conditions is considerably improved. To reduce the contacting losses in stack different contacting layers have been tested and optimized in test bench. The influence of different parameters on electrochemical performance of the cell as well as first results for cells integration in stack are presented and discussed.

Membrane-electrode-assembly (MEA) is a heart component of SOFC and defines the limits for power density, efficiency and durability available for exploitation in stacks and systems. MEA performance is often given as measure for possibilities of fuel cell technology. Most widespread way to communicate the MEA performance is to present the I-U-characteristic at a constant temperature and to estimate the area specific resistance from the slope of this curve. Other methods for characterization of performance like current density, impedance and local resistance measurement under defined operating conditions also have been used for this purpose. The fuel composition worldwide used for MEA tests (from dry hydrogen to H2:H2O=50:50) is connected with open circuit voltage which has great impact on measured performance. Moreover the realized fuel utilization strongly depends on operating conditions and varies in broad range. For these reasons it is difficult to compare results achieved by different researchers. It is beneficial to define unified value which would characterise electrochemical MEA performance. To resolve this challenge the voltage drop during the cell performance have been mathematically separated in two parts: (i) voltage drop due to change of Nernst voltage as a result of fuel utilization (and humidification) by current flow and (ii) voltage drop due to ohmic and polarization losses in the cell. This approach allows to separate the influence of non-linearity of Nernst voltage dependence from current on estimated cell resistance. It was found that at temperatures above 750°C the area specific resistance of the cell corrected to the Nernst voltage drop (fuel utilization) is a constant value, which only weakly depend on fuel composition. Favourable operating conditions as well as influence of operating conditions for estimation of cell resistance will be presented.

The stack concept of electrolyte supported cells (ESC) is applicable for both, solid oxide fuel cells as well as solid oxide electrolysis cells. High power densities and efficiencies comparable to anode supported cells (ASC) can be achieved, however, for the stack design presented in this paper, in unison with high robustness. Incorporating an electrolyte made of fully scandia stabilized zirconia paired with chromium-based CFY (Cr5FeY) interconnects of matched CTE (coefficient of thermal expansion), a feasible stack concept was created with the focus on long-term stability in regard to high temperature corrosion and thermal cycling. Stack data of more than 20.000 hours of a single stack is presented in addition to industrial onward development of the stack design. Using the same material combinations and the same footprint as for the MK351-design, the newly developed MK352-design was simulated, drafted and produced and allows for easier stack setup and system integration. Minor changes in interconnect (IC) design enable lower pressure drops over the stack, which, paired with enhanced tolerance chains lead to enhanced stack performance, especially in regard to thermal cycling. Cross-checked, test results for the MK352-stack are in good agreement with the simulation.