Hexis Ltd.

Winterthur, Switzerland

Hexis Ltd.

Winterthur, Switzerland
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Mai A.,HEXIS Ltd. | Fleischhauer F.,HEXIS Ltd. | Denzler R.,HEXIS Ltd. | Schuler A.,HEXIS Ltd.
ECS Transactions | Year: 2017

HEXIS is developing and manufacturing SOFC-based micro-CHP systems for single-family or small multi-family houses. Galileo 1000 N with an output of 1 kW electrical power was brought into the market 2013. Since then, HEXIS together with other members of the Viessmann Group have been developing the next generation of the Viessmann/HEXIS SOFC system. This paper reports the newest results of the short-stack tests, full-scale-stack tests as well as system tests based on the established Galileo systems as well as on our next generation systems. These include results of Galileo tests running close to 25'000 h, electrical efficiencies of the new generation of 40 % AC, net and total efficiencies of 95 % (LHV). © The Electrochemical Society.

Holzer L.,ZHAW Zurich University of Applied Sciences | Iwanschitz B.,Hexis Ltd | Hocker T.,ZHAW Zurich University of Applied Sciences | Keller L.,ZHAW Zurich University of Applied Sciences | And 4 more authors.
Journal of Power Sources | Year: 2013

A methodology based on FIB-tomography and image analysis (IA) is proposed which allows quantification of all relevant morphological features that are necessary to predict effective transport properties in porous SOFC electrodes. These morphological features are constrictivity, tortuosity, percolation factor and phase volume fraction. An M-factor can then be calculated which represents the ratio of effective over intrinsic conductivities. The methodology is used to describe effects of microstructure degradation in Ni-YSZ anodes which are caused by redox cycling at 950 C. The so calculated M-factors predict that because of redox cycling the effective electronic conductivity of nickel decreases from 3 to 1.2% which is mainly due to changes of percolation and constriction factors. Based on these results the effective electrical conductivity of nickel is predicted to be 685 S/cm before redox and 243 S/cm after 8 redox cycles. The predictions fit well with the experimental measurements that reveal 600 S/cm before and 200 S/cm after redox cycling at 950 C. For YSZ the M-factors obtained with 3D-analysis predict that the degradation causes a drop of the effective ionic conductivity from 7 to 0.6%, which is due to a change of the bottleneck dimensions. This finding contradicts the frequent interpretation of YSZ as a 'rigid backbone' that is not affected by microstructure degradation. Finally, the effective bulk gas diffusivity increases from 2 to 11% due to an increase of porosity associated with swelling of the anode. © 2013 Elsevier B.V. All rights reserved.

Holzer L.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Munch B.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Iwanschitz B.,Hexis Ltd. | Cantoni M.,Ecole Polytechnique Federale de Lausanne | And 2 more authors.
Journal of Power Sources | Year: 2011

The present study is focusing on the elaboration of the quantitative relationships between the primary microstructural parameters (i.e. the phase volume fractions Φ, the particle and pore sizes and the corresponding size distributions PSD) with the more complex topological features (triple phase boundary length TPBL, and specific surface area SSA) in porous Ni-cermet anodes. These relationships are crucial for the microstructure optimization and for the improvement of the corresponding electrode performance. In the first part of the study a pristine anode material with a graded microstructure is analyzed. The results from the different zonations indicate that the TPBL correlates well with the volume fraction of the coarsest phase which is nickel. However the volume fractions also correlate with the particle size after sintering due to stronger particle coalescence at higher nickel volume fractions. Since the larger particle size leads to a decrease of the TPBL this coarsening effect partly neutralizes the advantage of a higher nickel volume fraction. Furthermore, the TPBL is also linked with the nickel surface area and with the corresponding pore-nickel interface area. The pore-nickel interface is the least abundant internal surface and therefore it represents the microstructural feature which predominantly limits the TPB formation in our nickel-cermet anode. In the second part the effects of microstructural degradation during exposure in a humid gas environment at 950 °C are investigated. During the early period (<200 h) the rapid decrease of the TPBL is directly linked with the kinetics of nickel grain growth. At longer periods (>1000 h) the nickel coarsening ceases whereas the decrease of the TPBL continues at a lower rate. During longer exposure time the reduction of TPBL is most probably caused by a slight volatilization of the nickel phase and by the formation of continuous CGO layers which are shielding the surface of the nickel grains. © 2010 Elsevier B.V. All rights reserved.

Holzer L.,Performance Technology | Iwanschitz B.,Hexis Ltd. | Hocker T.,ZHAW Zurich University of Applied Sciences | Munch B.,Performance Technology | And 7 more authors.
Journal of Power Sources | Year: 2011

The effects of compositional and environmental parameters on the kinetics of microstructural degradation are investigated for porous Ni/CGO anodes in solid oxide fuel cells (SOFC). Improved methodologies of SEM-imaging, segmentation and object recognition are described which enable a precise quantification of nickel grain growth over time. Due to these methodological improvements the grain growth can be described precisely with a standard deviation of only 5-15 nm for each time step. In humid atmosphere (60 vol.% H2O, 40% N2/H2) the growth rates of nickel are very high (up to 140%/100 h) during the initial period (<200 h). At longer exposure time (>1000 h) the growth rates decrease significantly to nearly 0%/100 h. In contrast, under dry conditions (97 vol.% N2, 3 vol.% H2) the growth rates during the initial period are much lower (ca. 1%/100 h) but they do not decrease over a period of 2000 h. In addition to the humidity factor there are other environmental and compositional parameters which have a strong influence on the kinetics of the microstructural degradation. The nickel coarsening is strongly depending on the gas flow rate. Also the initial microstructures and the anode compositions have a big effect on the degradation kinetics. Thereby small average grain sizes, wide distribution of particle size and high contents of nickel lead to higher coarsening and degradation rates. Whereas the nickel coarsening appears to be the dominant degradation mechanism during the initial period (<200 h) other degradation phenomena become more important during long exposure time (>1000 h) in humidified gas. Thereby the evaporation of volatile nickel species may lead to a local increase of the Ni/CGO ratio. Due to the surface wetting of CGO a continuous layer tends to form on the surface of the nickel grains which prevents further grain growth and evaporation of nickel. These phenomena lead to a microstructural reorganization between 1000 and 2300 h of exposure. This complex pattern of degradation phenomena also leads to a change of the amount of active microstructural sites that are important for catalytic reactions at the pore-nickel interfaces and for electrochemical reactions at the triple phase boundaries (TPB). © 2010 Elsevier B.V. All rights reserved.

Fleischhauer F.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Fleischhauer F.,University of Leoben | Tiefenauer A.,ZHAW Zurich University of Applied Sciences | Graule T.,Empa - Swiss Federal Laboratories for Materials Science and Technology | And 3 more authors.
Journal of Power Sources | Year: 2014

For solid oxide fuel cells (SOFCs) one key aspect is the structural integrity of the cell and hence its thermo mechanical long term behaviour. The present study investigates the failure mechanisms and the actual causes for fracture of electrolyte supported SOFCs which were run using the current μ-CHP system of Hexis AG, Winterthur - Switzerland under lab conditions or at customer sites for up to 40,000 h. In a first step several operated stacks were demounted for post-mortem inspection, followed by a fractographic evaluation of the failed cells. The respective findings are then set into a larger picture including an analysis of the present stresses acting on the cell like thermal and residual stresses and the measurements regarding the temperature dependent electrolyte strength. For all investigated stacks, the mechanical failure of individual cells can be attributed to locally acting bending loads, which rise due to an inhomogeneous and uneven contact between the metallic interconnect and the cell.© 2014 Elsevier B.V. All rights reserved.

Fleischhauer F.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Fleischhauer F.,University of Leoben | Terner M.,ZHAW Zurich University of Applied Sciences | Bermejo R.,University of Leoben | And 4 more authors.
Journal of Power Sources | Year: 2015

Zirconia tapes are established for the application as electrolytes in solid oxide fuel cells. One of their functional tasks is to provide the structural integrity of the cell. The mechanical failure of the electrolyte would cause leakage between the electrodes, leading to a reduction of the performance of a whole fuel cell stack. In order to assess the reliability of a certain zirconia tape with respect to a specific stress distribution and environment, the strength has to be properly characterised. In the present study a selection of several commercial highly ionic conductive scandia doped zirconia tapes was investigated regarding their strength under inert and humid conditions at room temperature, which required the determination of the elastic constants and the fracture toughness as well. The strength of the different zirconia tapes ranks according to their fracture toughness, while the strength distributions typically exhibit a Weibull modulus of m = 10. It was found that all tapes suffer from sub critical crack growth, which leads to a limited lifetime at a given static load and a drastic reduction of the room temperature strength at time scales relevant for actual operation. © 2014 Elsevier B.V. All rights reserved.

Iwanschitz B.,Hexis Ltd. | Holzer L.,Performance Technology | Mai A.,Hexis Ltd. | Schutze M.,Dechema Institute
Solid State Ionics | Year: 2012

Ni/CG40-cermet anodes with two different ratios of nickel and ceria-gadolinia-oxide were exposed to seven different reduction temperatures between 555 °C and 1140 °C. For each reduction temperature one sample was analysed by scanning electron microscopy. The continuous particle size distributions were determined from the images. Significant nickel particle growth occurred above a reduction temperature of 850 °C for both anodes. The T-dependent nickel particle growth in the range of several initial particle radii could be fitted well with an Arrhenius type equation. However, no clear trend was obtained for the activation energy of the nickel agglomeration as a function of the initial particle sizes and the Ni/CG40 ratio. This lack of systematic trend may indicate competing material transport mechanisms such as diffusion and/or evaporation/condensation.For Ni/CG40 anodes with higher metallic content, nickel agglomeration was more pronounced for reduction temperatures above 950 °C. Besides the nickel, the ceria-gadolinia-oxide showed an increase in particle size at reduction temperatures above 950 °C as well. © 2012 Elsevier B.V.

Iwanschitz B.,Hexis Ltd | Sfeir J.,Hexis Ltd | Mai A.,Hexis Ltd | Schutze M.,Dechema Institute
Journal of the Electrochemical Society | Year: 2010

In this experimental study, electrolyte-supported solid oxide fuel cells (SOFCs) with two different anodes were investigated. Specifically, the stability of cells with a nickel/8 mol % yttria-stabilized zirconia (Ni/8YSZ) cermet anodes was compared to those based on nickel/40 mol % gadolinia-doped ceria (Ni/CG40). For this, the cells were characterized by impedance spectroscopy as well as by four-point electrical conductivity measurements. A high frequency process was observed in the Ni/8YSZ anode, which was not detected in the Ni/CG40 anode. After eight redox cycles at 950° C, the cell with the Ni/8YSZ anode showed an increase in the polarization resistance mainly in the high frequency domain. However, the cell with the Ni/CG40 anode showed an increase in both ohmic and polarization resistances, the latter mainly in the low frequency domain. Compared with Ni/CG40, the degradation in Ni/8YSZ upon redox cycling was higher at 850°C but lower at 950°C. For the Ni/8YSZ anode, a significant degradation was seen in the first 3 h after a redox cycle. The increase in the ohmic resistance of the Ni/CG40-based cell is believed to correlate with a decrease in the electrical conductivity of the anode. The latter showed a strong decrease upon a subsequent redox cycling at 950°C. For the Ni/CG40 anode, the degradation in both the conductivity and electrochemical performance significantly improved by decreasing the operation temperature from 950 to 850°C. © 2009 The Electrochemical Society.

Mai A.,Hexis Ltd. | Iwanschitz B.,Hexis Ltd. | Weissen U.,Hexis Ltd. | Denzler R.,Hexis Ltd. | And 3 more authors.
ECS Transactions | Year: 2011

Hexis is the developer and manufacturer of the SOFC-based Micro-CHP system Galileo 1000 N. More than 60 Galileo 1000 N systems have been installed up to now and are in operation at customer's sites and in the lab. In the lab, long-term system operation of more than 28,000 hours has been achieved. With beginning of 2010, a cell with increased performance has been established as a new standard. An AC net electrical efficiency of the system (including all peripheral consumptions) of 30 to 35% has been achieved and the overall (electrical + thermal) efficiency of the system is in the range of 90 to 105% (LHV). On stack level, cells with even higher performance were tested, resulting in electrical efficiencies of up to 44% (DC) with CPOx reforming and 57% (DC) with steam reforming. ©The Electrochemical Society.

Fleischhauer F.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Fleischhauer F.,University of Leoben | Bermejo R.,University of Leoben | Danzer R.,University of Leoben | And 3 more authors.
Journal of Power Sources | Year: 2015

Solid-Oxide-Fuel-Cell systems are efficient devices to convert the chemical energy stored in fuels into electricity. The functionality of the cell is related to the structural integrity of the ceramic electrolyte, since its failure can lead to drastic performance losses. The mechanical property which is of most interest is the strength distribution at all relevant temperatures and how it is affected with time due to the environment. This study investigates the impact of the temperature on the strength and the fracture toughness of different zirconia electrolytes as well as the change of the elastic constants. 3YSZ and 6ScSZ materials are characterised regarding the influence of sub critical crack growth (SCCG) as one of the main lifetime limiting effects for ceramics at elevated temperatures. In addition, the reliability of different zirconia tapes is assessed with respect to temperature and SCCG. It was found that the strength is only influenced by temperature through the change in fracture toughness. SCCG has a large influence on the strength and the lifetime for intermediate temperature, while its impact becomes limited at temperatures higher than 650 °C. In this context the tetragonal 3YSZ and 6ScSZ behave quite different than the cubic 10Sc1CeSZ, so that at 850 °C it can be regarded as competitive compared to the tetragonal compounds. © 2014 Elsevier B.V. All rights reserved.

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