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Hydrothermal degradation deteriorates the fracture toughness and strength of tetragonal stabilised zirconia. The phase transformation to the monoclinic phase is particularly critical for materials with lower stabiliser content such as 2 mol% and 3 mol% yttria stabilised zirconia (2YSZ and 3YSZ). In this work, two routes in two-step sintering and co-doping with calcium oxide are analysed to mitigate the degradation. Both strategies show a reduction in average grain size of the 2YSZ while maintaining a comparable densification. The achieved smaller grains suppress the hydrothermal degradation rates. In addition, a mitigating effect beyond the reduction in grain size of YSZ is found for CaO doping. 1.6 mol% CaO co-doped 2YSZ shows less than 4% of monoclinic phase after 50 h in an autoclave with H 2 O at 134 °C. Pure 2YSZ contrarily reaches 100% monoclinic phase after 20 h at the same conditions. A suppression of degradation by CaO doping was also observed for the composite of nickel oxide and 3 mol% yttria stabilised zirconia. Hence, CaO co-doping can be an interesting strategy to increase resistivity against hydrothermal degradation for both biomedical and renewable energy applications. The findings further outline a route to achieve tetragonal YSZ with lower yttria contents than 2 mol%. • Two approaches were analysed to mitigate hydrothermal degradation in 2YSZ and 3YSZ. • Co-doping with CaO and two-step sintering reduced the grain growth and degradation. • CaO co-doping mitigated the degradation beyond the effect of the grain size. • A complete suppression of degradation was achieved by the addition of 1.6 mol% CaO. • The results show a route to ageing-resistant tetragonal YSZ with lower yttria contents. [ABSTRACT FROM AUTHOR]

This study concerns numerical investigation of a solid oxide electrolysis stack, in which the cells are pressurized on only the fuel side (single-sided mode) or both fuel and air sides (double-sided mode). A 90-cell stack model is used to study the effects of the operating pressure on the polarization curves, area-specific-resistance (ASR), temperature, and pressure. The model predicts higher open-circuit voltage (OCV), lower ASR, and lower pressure drops over the flow fields at higher operating pressures. The latter leads to flexibility in the flow field design. The results show that the single-sided mode hinders the OCV increase and reduces the ASR. Nonetheless, the ASR reduction under the double-sided mode is more significant than the single-sided mode. Therefore, the double-sided mode performs better at high current densities since its higher ASR reduction provides a lower increase in the stack voltage (/input power). Moreover, the results indicate a nonlinear relationship between the temperature distribution and the operating pressure under the double-sided mode due to its counteracting effects on the ohmic heat sources and reaction heat sink. It is illustrated that the temperature difference over the stack decreases under the double-sided mode at higher operating pressures and lower current densities. • Investigation of single- and double-sided higher-pressure operations of SOCE stacks. • Single-sided mode inhibits OCV increase but has lower ASR reduction. • At high current densities, double-sided mode performs better than single-sided mode. • High-pressure operation lowers pressure drop, leading to flexible flow field design. • Double-sided mode decreases temperature difference in the stack. [ABSTRACT FROM AUTHOR]

Long-term, reliable operation of Solid Oxide fuel and electrolysis Cell (SOCs) necessitates cells with high strength and resistance to mechanical degradation. In this work, strength, Young's modulus and Low-Temperature Degradation (LTD) of several zirconia-based SOCs supports are studied. The study shows that replacing 3YSZ with a tetragonal zirconia compound having lower stabilizer contents can improve the strength of porous supports. An enhancement up to 30 % over the-state-of-the-art support (NiO‒3YSZ) can be achieved for example with using NiO‒2.5YSZ. It is further evidenced that tetragonal zirconia-based SOC components (both Y-doped and Ce-Y co-doped) are susceptible to LTD in the studied range of grain sizes (i.e. ≈ 200−300 nm). Addition of small amount of alumina (0.5 wt%) is observed to increase the LTD resistance of porous supports. The susceptibility to LTD must be considered in designing materials for SOCs, also during materials processing, storage, handling and operation where a dry atmosphere is recommended. [ABSTRACT FROM AUTHOR]

To assess the risk of failure of various components in solid oxide cell (SOC) stacks, the temporal evolution of stresses from sintering and thermal gradients in the operating stacks must be known. In this work it is shown experimentally that the residual stresses in a solid oxide cell are relaxed and, in most cases, go to zero at the point of chemical reduction of the structurally dominant fuel electrode from NiO-YSZ to Ni-YSZ. This is essential for understanding and modeling the stresses during the SOC stack assembly and after. In part I, the in-plane macro-strain and stresses in each layer is determined by in-situ X-ray diffraction at different temperatures and during the chemical reduction. The stresses are also analyzed by a multilayer model of the cell. The relaxation of stresses is explained and attributed to so-called accelerated creep occurring in the nickel phase of fuel electrode. • Stress relaxation in solid oxide cells measured in-situ during reduction of Ni(O). • Relaxation results in practically zero stress in cells after stack assembly. • Explained by chemo-thermo-mechanical accelerated creep. • Provides the initial condition for thermo-mechanical models. [ABSTRACT FROM AUTHOR]

Solid oxide fuel and electrolysis cells (SOCs) are promising devices for efficiently generating electricity and hydrogen/syngas, respectively. The preferred cell configuration by many industrial developers consists of a Ni(O)-3YSZ fuel electrode support or a 3YSZ electrolyte support, which allows the manufacturing of large-area cells. 3YSZ, like many other transformable tetragonal zirconia ceramics, is well-known to be susceptible to hydrothermal degradation at temperatures below 400 °C (LTD) but is widely believed to withstand this degradation at higher temperatures. In this work, we present our recent finding that 3YSZ is, in fact, susceptible to high-temperature degradation (HTD) after operation over 8500–10,000 h. The degradation leads to substantial embrittlement of the cells. In addition, we report on the excellent resistance to LTD and HTD of a newly developed fine-grained Ni-3YSZ support. This study highlights the importance of using HTD-resistant tetragonal zirconia ceramics in developing the next generation of strong and durable ceramic-supported SOCs. [ABSTRACT FROM AUTHOR]

Solid oxide electrochemical cells (SOCs) can store electrical energy in the form of chemical fuels with high efficiency by electrolysis of CO 2 and H 2 O . However, achieving commercially relevant lifetime is hindered by degradation mechanisms such as carbon deposition, which can even destroy the cell especially during electrolysis where carbon formation is electrochemically driven at the electrode-electrolyte interface. Here we used a three-dimensional multiphysics model to simulate a SOC performing CO 2 electrolysis and determine the operating conditions and locations in the porous nickel-based electrodes where carbon deposition is expected based on local conditions (gas composition, temperature and overpotential) crossing local thermodynamic thresholds. It is found that CO/ CO 2 gas diffusion gradients and cooling from the endothermic electrolysis reaction are important driving forces for carbon deposition to occur locally when it is not expected based on the outlet CO concentration. Furthermore, correlation with a set of experimentally determined threshold operating points indicates that carbon deposition occurs primarily by the Boudouard reaction rather than by direct electrochemical reduction of CO or CO 2 to carbon for the studied cell type. Variation of fuel electrode porosity and thickness shows that these methods of reducing gas diffusion limitations widen the operating window that avoids carbon deposition. [ABSTRACT FROM AUTHOR]

3YSZ specimens with variable open porosity (1–57%) were fabricated, and the stiffness, strength and fracture properties (fracture toughness and R-curve) were measured to investigate their potential use as support structures for solid oxide fuel or electrolysis cells. The ball-on-ring test was used to characterize Young's modulus and Weibull strength. The variation of fracture toughness with porosity was investigated and modelled using the results from fracture mechanical testing. A distinct R-curve behaviour was observed in dense 3YSZ specimens, in samples with a porosity around 15% and in some of the highly porous samples (porosities ∼45%) reflecting a transformation toughening in the material. For the most porous samples, the “R-curve behaviour” disappeared and subcritical crack growth was observed. The studies indicate that even highly porous 3YSZ structures (porosities exceeding 40%) are feasible supports for SOFC/SOECs from a mechanical point of view. [ABSTRACT FROM AUTHOR]

Fuel recirculation is a system configuration designed to improve the fuel utilization of fuel cells by recirculating the unreacted fuels at the outlet to the fuel inflow stream. In this study, fuel recirculation of a solid oxide fuel cell (SOFC) operated with ammonia is investigated. Recirculation operation results in a notable elevation of nitrogen content within the cell, originating from the decomposition of ammonia within the cell and subsequently reintroduced into the inflow stream. Therefore, the fuel flow rate needs to be increased to maintain the fuel utilization level the same as the base operation, which is an ammonia-fueled operation without recirculation. A multiscale model of an SOFC stack is used to compare the base operation and saturated level of the recirculation operation cases based on their polarization curves and distributions of ammonia, hydrogen, current density, temperature, first principal stress, and nitriding in the fuel electrode. The results show that overall stack performance, as reflected in the polarization curves, remains comparable between the base and recirculation operation cases. The power densities exhibit a difference of less than 2% between the base and recirculation operation cases. Moreover, it is indicated that the recirculation operation cases lead to more uniform hydrogen and current density distributions, lower temperature gradients, and lower thermal stresses. The results demonstrate that the recirculation operation cases lead to a reduction of up to 38% in the maximum tensile stresses within the fuel support layer. Nevertheless, these recirculation operation cases also induce nitriding across a larger portion of the active area. But as shown here this is a challenge that can effectively be addressed by applying nickel coatings to the inlet headers. • Overall stack performance remains comparable for the base and AOR operation cases. • AOR operation results in more uniform hydrogen and current density distributions. • AOR operation decreases temperature gradients, reducing thermal stresses. • AOR operation increases nitriding across a broader portion of the active area. • Mitigation of nitriding is achievable through nickel coatings over inlet headers. [ABSTRACT FROM AUTHOR]

Metal-supported Solid Oxide Fuel Cells (SOFCs) are developed as a durable and cost-effective alternative to the state-of-the-art cermet SOFCs. This novel technology offers new opportunities but also new challenges. One of them is corrosion of the metallic support, which will decrease the long-term performance of the SOFCs. In order to understand the implications of the corrosion on the mass-transport through the metallic support, a corrosion model is developed that is capable of determining the change of the porous microstructure due to oxide scale growth. The model is based on high-temperature corrosion theory, and the required model parameters can be retrieved by standard corrosion weight gain measurements. The microstructure is reconstructed from X-ray computed tomography, and converted into a computational grid. The influence of the changing microstructure on the fuel cell performance is evaluated by determining an effective diffusion coefficient and the equivalent electrical area specific resistance (ASR) due to diffusion over time. It is thus possible to assess the applicability (in terms of corrosion behaviour) of potential metallic supports without costly long-term experiments. In addition to that an analytical frame-work is proposed, which is capable of estimating the porosity, tortuosity and the corresponding ASR based on weight gain measurements. [ABSTRACT FROM AUTHOR]

The cost-effectiveness of Solid Oxide Cells production can be improved by introducing “multilayer-tape-casting” (MTC: sequential casting of the layers) and co-sintering of the half-cells. MTC additionally results in more homogeneous layers with strong interfaces. However, the thermal expansion coefficient (TEC) mismatch between the layers, cumulated from high temperature, induces significant residual stresses in the half-cells. Furthermore, it has been observed that MTC half-cells with 4 layers (MTC4: support, fuel electrode, electrolyte and barrier layer) are sometimes more fragile to handle than those with 3 layers (MTC3: without barrier layer). The bending strength of MTC3 and MTC4 under various loading orientations (electrolyte on the tensile or compressive side of the loading) is compared. The analysis, by taking residual stresses into account, shows that the strength of the half-cells with the electrolyte on the compressive side corresponds to the strength of the support. With the loading in the other direction (electrolyte on the tensile side), the origin of the failure is in a different layer for MTC3 (fuel electrode) and for MTC4 (barrier layer). In order to decrease the tensile residual stresses, especially in the outer barrier-layer, possible changes to the layer properties are discussed and some optimization guidelines proposed. [ABSTRACT FROM AUTHOR]

Abstract: In solid oxide fuel cell (SOFC) technology a major challenge lies in balancing thermal stresses from an inevitable thermal field. The cells are known to creep, changing over time the stress field. The main objective of this study was to assess the influence of creep on the failure probability of an SOFC stack. A finite element analysis on a single repeating unit of the stack was performed, in which the influence of the mechanical interactions, the temperature-dependent mechanical properties and creep of the SOFC materials are considered. Moreover, stresses from the thermo-mechanical simulation of sintering of the cells have been obtained and were implemented into the model of the single repeating unit. The significance of the relaxation of the stresses by creep in the cell components and its influence on the probability of cell survival was investigated. Finally, the influence of cell size on the failure probability was investigated. [Copyright &y& Elsevier]

Abstract: In the development of structural and functional ceramics for high temperature electrochemical conversion devices such as solid oxide fuel cells, their mechanical properties must be tested at operational conditions, i.e. at high temperature and controlled atmospheres. Furthermore, characterization of the strength of ceramic components requires testing of a high number of samples due to stochastic failures. Successive heating and testing a single sample is very time consuming and consequently this hinders thorough studies with sufficient samples at operational conditions. This work presents a methodology for testing multiple samples at operational conditions providing a high throughput and thus the possibility achieve high reliability. Optical methods are used to measure deformations contactless, frictionless load measuring is achieved, and multiple samples are handled in one heat up. The methodology is validated at room temperature, and exemplified by measurement of the strength of solid oxide fuel cell anode supports at 800 °C. [Copyright &y& Elsevier]

Highlights: [•] First treatment of the effective volume of the ball-on-ring specimen. [•] First analytical derivation of effective volume for a multiaxial stressed specimen. [•] Handling of multiple terms in effective volume integral with multinomial theorem. [•] Analytical solution found to match FEM for a range of geometric parameters. [•] Analytical solution compares well with other multiaxial failure theorems. [ABSTRACT FROM AUTHOR]

Abstract: Shape distortions during constrained sintering experiment of bi-layer porous and dense cerium gadolinium oxide (CGO) structures have been modeled. Technologies like solid oxide fuel cells require co-firing thin layers with different green densities, which often exhibit differential shrinkage because of different sintering rates of the materials resulting in undesired distortions of the component. An analytical model based on the continuum theory of sintering has been developed to describe the kinetics of densification and distortion in the sintering processes. A new approach is used to extract the material parameters controlling shape distortion through optimizing the model to experimental data of free shrinkage strains. The significant influence of weight of the sample (gravity) on the kinetics of distortion is taken in to consideration. The modeling predictions indicate good agreement with the results of sintering of a bi-layered CGO system in terms of evolutions of bow, porosities and also layer thickness. [Copyright &y& Elsevier]

Improving the power density of solid oxide fuel cell stacks would significantly enhance this technology for transportation. Using a monolithic structure to downsize the stack dimension offers a key to elevate the power density of solid oxide fuel cell stacks. This innovative design is promising but manufacturing is a challenge. The monolith is co-sintered in one firing step, and the gas channels are formed by burning off sacrificial organic materials. Structure distortion or fracture was observed in post-mortem investigations. In this work a multiscale, multiphysics modelling approach is proposed to describe and resolve this challenge in the debinding process occurring in a monolithic stack, i.e. the burning of organics and transportation of gases through the gradually opening microstructure, as well as the pressure build-up in the microstructure due to gas development. Simulation results show that a prominent pressure peak is experienced in the stack when a plasticiser (polyethylene glycol) and a pore-former (polymethyl methacrylate) are decomposed simultaneously. To reduce the high pressures, we investigate two possible strategies: (i) changing the mixture of organic additives; (ii) modifying the debinding temperature profile. Three tapes with different pore-formers are prepared, and the generated pressures during debinding of the three stacks are compared. The corresponding stack shapes after debinding are recorded. Numerical investigations show a good agreement with the post-mortem observations. By changing the composition of organics the distortion or fracturing of the stack can be avoided. Furthermore, to facilitate stack manufacturing, the high pressures can also be reduced by adjusting the heating rates and dwell temperatures of debinding. By using the new temperature profile suggested by the simulation study, the duration of debinding can also be reduced. [ABSTRACT FROM AUTHOR]

Abstract: The ball-on-ring experiment is used for testing of the biaxial strength of ceramics. In this work the solution for the stress distribution and displacements of the disc specimen in the ball-on-ring experiment are determined on closed form. The solution comprises the displacement field and its derivatives, the shear force distribution, the bending moments distribution for the entire specimen. From these the stress distributions are obtained. Solutions for this problem already exist in the literature, but these are incorrect and they have been shown to deviate from both experiments and finite element solutions. The correctness of the solution in this work is validated with more than 3000 finite element simulations for a large range of geometric parameters of the experimental setup. It is shown that the solution is accurate and only deviates in limiting cases. [Copyright &y& Elsevier]

The fracture toughness and slow crack growth behaviour of two load-bearing metal ceramic components adopted in negatrode-supported electrochemical devices, namely Ni-BCZY27 and Ni-BCZY442, are investigated via double torsion technique. The investigation showed that the two metal-ceramic composites are considerably less sensitive to slow crack growth when compared to oxide ceramics such as stabilized zirconia and alumina, with K I 0 / K IC equal to approximately 0.8. On the other hand, they also showed lower fracture toughness than Ni-3YSZ, with Ni-BCZY442 and Ni-BCZY27 having a fracture toughness of respectively 1.74 and 3.04 MPa*m1/2 for porosities equal to 25.8% and 19.1% respectively, compared to Ni-3YSZ reported in previous work with similar Ni content and porosity of approximately 30% having a fracture toughness equal to 3.4 MPa*m1/2. Furthermore, the analysis of samples presenting extensive barium depletion indicated fast fracture propagation upon application of loads significantly lower than the critical fracture load. [ABSTRACT FROM AUTHOR]

Limiting the degradation of solid oxide fuel cells is an important challenge for their widespread use and commercialization. The computational expense of long-term simulation of a full stack with conventional models is immense. In this study, we present a multiscale three-dimensional model of a degrading full stack of solid oxide cells, where we integrate degradation phenomena of nickel particle coarsening in the anode electrode, chromium poisoning of the cathode electrode, and oxidation of the interconnect into a multiscale model of the stack. This approach makes this type of simulation computationally feasible, and 38 thousand hours of the stack operation can be simulated in 1 h and 15 min on a high-end workstation. Hereby one can start to explore the optimum operating conditions for a range of parameters. The model is validated with experimental data from an 18-cell Jülich Mark-F stack experiment and predicts common trends reported in the literature for evolutions of the stack performance, degradation phenomena, and the related model variables. Moreover, it captures how different regimes in the full stack degrades at different rates and how the various degradation phenomena interact over time. The model is used to investigate the effects of galvanostatic and potentiostatic operation modes, operating conditions, and flow configurations on the long-term performance of the stack. Results demonstrate, as expected, that potentiostatic operation mode, moderate temperature, lower load current, and counter-flow configuration improve the long-term performance of the stack. • A multiscale degradation model for a full solid oxide fuel cell stack is developed. • The model simulates 38 thousand hours of the stack lifetime in 1 h and 15 min. • The model is validated with experimental data and predicts common long-term trends. • Nickel coarsening, chromium poisoning, and interconnect oxidation are considered. • Degradations under potentiostatic and galvanostatic operation modes are compared. [ABSTRACT FROM AUTHOR]

This study investigates utilizing hydrogen produced via water electrolysis to produce green ammonia. Routes are benchmarked based on employing either alkaline electrolysis (AEC) or solid oxide electrolysis (SOEC). Both existing and possible improvements are modeled for the AEC and SOEC technologies coupled with the Haber-Bosch process to synthesize ammonia. The cost of green ammonia is estimated considering the cost of electrolyzers for both today and future projections and are compared with that of "fossil" ammonia synthesized from natural gas. Threshold CO 2 taxes required to achieve cost parity between green and "fossil" ammonia are determined based on the price of natural gas and the levelized cost of electricity. It is found that whereas the green ammonia produced from a system based on AEC is cheaper today, SOEC shows to be more cost-effective, when basing the comparison on the projected future cost of the electrolyzers. Green ammonia from an SOEC-based plant is estimated to have accost of 495 €/t by 2050 with an assumed electricity price of 30 €/MWh. In the SOEC-based ammonia plants, approximately 57 % and 31 % of the steam needed for the electrolyzers can be generated through heat integration between the electrolyzer and Haber-Bosch process, for low-pressure and high-pressure electrolyzers, respectively. A good fraction of the heat can also be covered by the intercoolers of the compressors. With the projected cost for SOEC, reducing the levelized cost of electricity from 60 to 10 €/MWh would decrease the cost of green ammonia from 690 to 340 €/t by 2050. [Display omitted] • Green ammonia production based on AEC and SOEC are thermodynamically assessed. • Cost of green ammonia is benchmarked against fossil and blue ammonia. • The effect of anticipated development of AEC and SOEC on ammonia price is examined. • The threshold CO 2 taxes to bring grey and green ammonia on par is estimated. • Using high-pressure SOEC, the cheapest projected ammonia cost is 495 €/ton in 2050. [ABSTRACT FROM AUTHOR]

Galvanostatic operation of solid oxide electrolysis cells (SOECs) at high current densities and low temperatures would enable faster implementation and enhance the competitiveness of SOEC technology. However, the fuel electrode of Ni-yttria stabilized zirconia-based cells (Ni-YSZ) experiences considerable degradation at high current densities. This study investigates the long-term durability of Ni-YSZ fuel-electrode supported SOECs operated galvanostatically for steam electrolysis at different current densities and temperatures. Detailed electrochemical evaluation of the cells reveals that: The short-term degradation of the fuel electrode is reflected on the rapid increase of the fuel electrode polarization resistance (R Fuel) while in the long-term, its degradation primarily stems from a continuous increase of ohmic resistance (R Ohmic). Microstructure analysis suggests that physical detachment of Ni from YSZ is not a prerequisite for Ni migration but constitutes an additional degradation phenomenon. More specifically, the Ni-YSZ electrode exhibits accelerated degradation, when the fuel electrode overpotential (η Fuel) exceeds −205 mV (at 800 °C) and −285 mV (at 750 °C). Upon surpassing these overpotential thresholds, a low-frequency inductive contribution emerges in the Nyquist plots of EIS. Consequently, this operando emergence should be related to the accelerated degradation of the Ni-YSZ electrode, initiated by the Ni/YSZ detachment and impurities inclusion phenomena. [Display omitted] • SOECs long-term tested at different temperatures and current densities. • Overpotential degradation thresholds for safe operation of SOECs are determined. • Detachment of Ni from YSZ does not appear to be a prerequisite for Ni migration. • Ni migration increases ohmic resistance at a defined overpotential rate. • Low-frequency inductive contribution in EIS indicates accelerated degradation. [ABSTRACT FROM AUTHOR]

Hydrogen produced from H 2 O electrolysis works as an energy carrier and helps to overcome the challenges of intermittent renewable energy sources. At present, no comprehensive environmental impact assessment is available for three commercially H 2 O electrolysis technologies, namely solid oxide electrolysis cell (SOEC), polymer electrolyte membrane electrolysis cell (PEMEC), and alkaline electrolysis cell (AEC). The study aimed to provide potential environmental impacts of the electrolysis technologies based on life cycle assessment. Among the investigated 16 impact categories, the stage of critical material use of three H 2 O electrolysis stacks was identified as the hotspot of environmental impacts. The critical materials were stainless steel and nickel from SOEC, platinum and iridium from PEMEC, and nickel from AEC. Life cycle impact results from PEMEC stack were much higher than these from SOEC and AEC stacks, while electricity played a more important role in the life cycle impact of hydrogen production. The sensitivity analysis indicated that the most effective approach to reducing potential impacts would be to reduce critical materials use on the current status of electrolysis technologies. • Performances an LCA of H 2 O electrolysis technologies of SOEC, PEMEC, and AEC. • Provides an environmental burden pertaining to H 2 O electrolysis stacks. • Provides an inventory based mainly on primary data. • Compares LCIA of hydrogen production from three electrolysis technologies. [ABSTRACT FROM AUTHOR]

Ceria-Yttria co-doped tetragonal zirconia is an attractive class of materials having high strength, toughness and thermal stability. In this study, several co-doped zirconia powders with different stabilizers contents were synthesized via continuous hydrothermal flow synthesis (CHFS). The CHFS was concluded as a suitable method in synthesizing ultrafine tetragonal zirconia particles with controlled morphology. The synthesized powders as well as some commercial powders were heat-treated both in the form of powders and pellets between 1150 and 1500 °C and their crystalline structure after cooling to room temperature was studied. The results were used to map out the stability range of the tetragonal phase. The developed diagrams are useful tools to select the appropriate amounts of stabilizers applicable for different sintering temperatures and for samples with different target densities. [ABSTRACT FROM AUTHOR]

In electrochemical devices such as solid oxide cell stacks, many physical phenomena are interacting on many different length scales in an intricate geometry. Modeling is a strong tool to understand the interior of such devices during operation, enhance their design and investigate long-term response (degradation). Computations can however be challenging as the many geometric details and coupled physical phenomena require a significant computational power, and in some cases, even state-of-the-art clusters will not be sufficient. This hinders the use of the models for the further development of the technology. In this work, we present an original type of solid oxide cell stack model, which is highly computationally efficient, resulting in computations which are two orders of magnitude faster than the conventional type of stack models with all geometric details explicitly represented. In the model presented here, the geometric details are implicitly represented by using the so-called homogenization. The resulting homogeneous anisotropic media provides the correct overall response (temperature, species, molar fractions, etc.). Local details as the mechanical stress in the electrolyte are not represented explicitly. These can be retrieved by localization through sub-models (multiscale model), in some cases without loss of computational efficiency, as demonstrated. • Homogenization decreases the computational times by two orders of magnitude. • All relevant physics and couplings for solid oxide cells stack are homogenized. • Local conditions can be retrieved by localization, i.e. using sub-models. • Sub- and homogenized models constitute a multi-scale model. • The multi-scale model provides full detail with fast computations. [ABSTRACT FROM AUTHOR]

Operating solid oxide fuel cell (SOFC) stacks with ammonia induces the risk of nitriding in the fuel electrode and other metallic surfaces in the stack. This study aims to investigate the nitriding in the fuel electrode of an ammonia-fueled SOFC stack using a 3D Multiphysics model of a full stack. Based on the resulting species concentrations, a so-called nitriding potential is determined and compared to its critical level for various operating conditions and design modifications. The effects of the gas inflow temperatures, counter-flow configuration, and nickel coating over the inlet header of the stack are investigated. The results show that nitriding occurs in the first few centimeters of the fuel electrode for all investigated operating conditions considered in this study. Moreover, it is indicated that higher gas inflow temperatures and counter-flow configuration reduce the nitriding in the fuel electrode. Furthermore, the model illustrates the nitriding in the fuel active electrode for the gas inflow temperatures up to 700 °C. Finally, a significant reduction in nitriding in the fuel electrode is shown for a proposed nickel coating over the metallic inlet header due to a spreading of the ammonia decomposition. • Numerical investigation of Ni nitriding in an ammonia-fueled SOCF stack. • Nitriding occurs in the first few cm of the fuel electrode for base case operation. • Higher temperatures and counter-flow configuration reduce the nitriding. • Nitriding happens in the fuel active electrode for inflow temperatures up to 700 °C. • Nickel coating over the fuel inlet header reduces nitriding significantly. [ABSTRACT FROM AUTHOR]

Ammonia-fueled operation of solid oxide fuel cells is a promising alternative to their hydrogen-fueled operation. However, high ammonia decomposition rates at elevated operating temperatures of the solid oxide cells lead to a significant temperature drop at the stack inlet, causing increased thermal stresses. A multi-scale model is used in this study to investigate stack performance under direct feed and external pre-cracking of ammonia. Additionally, the effects of co- and counter-flow configurations, gas inflow temperatures, current density, and air flow rate on the stack performance under direct ammonia feed are examined. The simulation results show that for gas inlet temperatures above 750 °C, the power densities with direct feed and external cracking of ammonia differ by less than 5%. Moreover, it is indicated that the thermal stresses are lowest for the co-flow case, which decrease with decreasing gas inlet temperature and current density and with increasing air flow. Finally, this study shows that under practically applicable operating conditions, the risk of mechanical failure of the cells under direct ammonia feed operation is small. • Ammonia cools down the stack but leads to high tensile stresses. • Stack performs closely under direct and pre-reformed ammonia at higher temperatures. • Counter-flow configuration and higher inflow temperatures increase tensile stresses. • Lower load current density and higher air mass flow rate decrease tensile stresses. • Mechanical failure is not expected for operating condition considered in this study. [ABSTRACT FROM AUTHOR]

Stresses and probability of failure during operation of solid oxide fuel cells (SOFCs) is affected by the deformational properties of the different components of the SOFC stack. Though the overall stress relaxes with time during steady state operation, large stresses would normally appear through transients in operation including temporary shut downs. These stresses are highly affected by the transient creep behavior of metallic components in the SOFC stack. This study investigates whether a variation of the so-called Chaboche's unified power law together with isotropic hardening can represent the transient behavior of Crofer 22 APU, a typical iron-chromium alloy used in SOFC stacks. The material parameters for the model are determined by measurements involving relaxation and constant strain rate experiments. The constitutive law is implemented into commercial finite element software using a user-defined material model. This is used to validate the developed constitutive law to experiments with constant strain rate, cyclic and creep experiments. The predictions from the developed model are found to agree well with experimental data. It is therefore concluded that Chaboche's unified power law can be applied to describe the high temperature inelastic deformational behaviors of Crofer 22 APU used for metallic interconnects in SOFC stacks. [ABSTRACT FROM AUTHOR]

Ni-YSZ (nickel - yttria stabilized zirconia) is a material widely used for electrodes and supports in solid oxide electrochemical cells. The mechanical and electrochemical performance of these layers, and thus the whole cell, depends on their microstructure. During the initial operation of a cell, NiO is reduced to Ni. When this process is conducted under external load, like also present in a stack assembly, significant deformations of NiO/Ni-YSZ composite samples are observed. The observed creep is orders of magnitude larger than the one observed after reduction during operation. This phenomenon is referred to as accelerated creep and is expected to have a significant influence on the microstructure development and stress field present in the Ni-YSZ in solid oxide electrochemical cells (SOCs), which is highly important for the durability of the SOC. In this work we present energy selective neutron imaging studies of the accelerated creep phenomenon in Ni/NiO-YSZ composite during reduction and also during oxidation. This approach allowed us to observe the phase transition and the creep behavior simultaneously in-situ under SOC operation-like conditions. [ABSTRACT FROM AUTHOR]

Currently thermo-mechanical analysis of the entire solid oxide fuel cell (SOFC) stack at operational conditions is computationally challenging if the geometry of metallic interconnects is considered explicitly. This is particularly the case when creep deformations in the interconnect are considered in addition to elasticity. In this work, this problem is addressed using homogenization, whereby the effect of the geometry is build into an effective anisotropic material law for a continuum block of material, which then represents the interconnect in the stack model. The study presents a finite element model to calculate the homogenized mechanical response of corrugated metallic interconnects at high temperatures. Thereafter, a constitutive law for the homogenized structure (effective material law) is developed. In order to properly describe the mechanical behavior of the interconnect at high temperature, deformations involving the elastic, creep as well as effect of changes in the geometry due to contact should be accounted for. The constitutive law can be applied using 3D modeling, but for simple presentation of the theory, 2D plane strain formulation is used to model the corrugated metallic interconnect. Finally, the developed constitutive law is verified by comparing its predictions for creep strain with results from the original 2D finite element model for different loading conditions. The constitutive law is found to satisfactorily describe the mechanical behavior of corrugated metallic interconnect with computational feasibility and significant speed gain. [ABSTRACT FROM AUTHOR]

Ammonia is a promising carbon-free fuel for solid oxide fuel cells (SOFCs). However, direct feeding of ammonia into the SOFC may lead to serious degradation due to the nitriding. Conversely, the pre-cracking of ammonia introduces complexity to the system design and causes increased power losses in the system. Therefore, it is crucial to explore all these aspects simultaneously. In this context, this study introduces a novel multiscale modeling approach by integrating a 3D multiphysics simulation of the ammonia-fueled SOFC stack with system-level modeling to investigate the reliability and performance of the stack and system. Two different cell technologies developed for low temperature (LT) and high temperature (HT) operation are investigated at LT (600 – 700 ° C) and HT (700 – 800 ° C) ranges. The results indicate that fuel inlet temperature should be 55 ° C and 18 ° C higher than the minimum temperature in 0% pre-cracking case for HT and LT cases, respectively. The increase in the required air flow rate for cooling in the 100% pre-cracking case compared to the 0% pre-cracking case is around 100% and 216% for the HT and LT cases, respectively. However, stack power production and power losses in the system components are comparable for LT and HT cases which leads to similar system performance. A larger share of the active area is affected by nitriding in LT cases than HT ones. However, a smaller cracking ratio at LT (∼ 82%) compared to HT conditions (∼ 92%) is needed for elimination of nitriding. While the LT and HT cases are comparable in terms of system power production, the lower stack outlet temperatures in LT cases require novel and more expensive catalysts for ammonia pre-cracking and HT cases need more expensive steels. • A multiscale multiphysics modeling approach of ammonia-fueled SOFCs is developed. • Two cell technologies developed for low and high temperature operation are compared. • Inlet temperature and air flow rate should be adjusted to control stack temperature. • Both low and high temperature SOFC stacks need pre-cracking to avoid nitriding. • Systems working with low and high temperature cells have comparable performances. [ABSTRACT FROM AUTHOR]

Abstract: Mechanical strength at elevated temperatures and operating atmospheres needs to be characterized during development of tubular ceramic components for advanced energy technologies. Typical procedures are time-consuming because a large number of tests are required for a reliable statistical strength characterization and every specimen has to be subjected to the process conditions individually. This paper presents an efficient strength characterization methodology for tubular ceramics. The methodology employs flexure of semi-cylindrical specimens as the strength test and implements the tests within a facility capable of loading multiple specimens in controlled environments. The stress field is analyzed in detail with finite element analyses. The validity of the test is found to depend on specimen dimensions and the valid range is established. The methodology is demonstrated by experimental measurements conducted on oxygen transport membrane materials at room temperature and 850°C. [Copyright &y& Elsevier]

Creating a tough bond for the electrical contact between metallic interconnects and ceramic solid oxide cells (SOC) in a stack is challenging due to restrictions on the assembly temperature. The reactive oxidation bonding in the formation of Co 2 MnO 4 (CoMn) and Cu 1.3 Mn 1.7 O 4 (CuMn) spinel oxides from metallic precursors could provide a potential solution for achieving tough and well-conducting contact layers. These contact layers are deposited from metallic precursors onto CoCe-coated AISI441 substrates to achieve high toughness even after aging for 3000 h at typical operating temperatures for SOCs. The interface fracture energy of CoMn and CuMn contact layers was measured for as-sintered and aged samples by using a modified four-point bending test. After the fracture test, X-ray diffraction, electron microscopy, and energy-dispersive X-ray spectroscopy were used to determine phase evolution and possible reactions at the contact layer/interconnect interface. The results show that the interface fracture energy of sintered CoMn contact layer (6.1 J/m2) decreased to 2.9 J/m2 after aging at 850 ○C for 3000 h while the fracture energy for CuMn increased from 6.4 J/m2 to 19.7 J/m2. [ABSTRACT FROM AUTHOR]

Chromium poisoning of the air electrode is a primary degradation mechanism for solid oxide cells (SOCs) operating under fuel cell mode. Recent experimental findings show that reversed pulse operation for SOCs operated as electrolyser cells can reverse this degradation and extend the lifetime. Here, we use a multiphysics model of an SOC to investigate the effects of reversed pulse operation for alleviating chromium poisoning of the air electrode. We study the effects of time fraction of the operation under fuel cell and electrolysis modes, cyclic operation starting after a certain duration, and fuel cell and electrolysis current densities on the cell lifetime, total power, and hydrogen production. Our modeling shows that reversed pulse operation enhances cell lifetime and total power for all different cases considered in this study. Moreover, results suggest that the cell lifetime, total power, and hydrogen production can be increased by reversed pulse operation at longer operation times under electrolysis mode, cyclic operation starting from the beginning, and lower electrolysis current densities. All in all, this paper documents and establishes a computational framework that can serve as a platform to assess and quantify the increased profitability of SOCs operating under a co-production operation through reversed pulse operation. • Numerical investigation of benefits of co-production operation of SOCs. • SOC lifetime and performance improvements by reversed pulse operation. • Alleviation of chromium poisoning. • Cyclic operation with longer electrolysis mode improve SOC lifetime and performance. • Mild electrolysis starting from the beginning enhance SOC lifetime and performance. [ABSTRACT FROM AUTHOR]

A deep understanding of underlying processes and possible challenges is required for the development of ammonia-fueled solid oxide fuel cell (SOFC) systems. In this context, a detailed 3D multiphysics model of an ammonia-fueled SOFC is developed in the current study. Anode off-gas recirculation and nickel nitriding as two of the main aspects of ammonia-fueled SOFCs are investigated in the current study. The developed model is validated with experimental data under various conditions including direct and pre-cracked ammonia cases for various flow rates at different temperatures (700–850 ° C) as well as different recirculation rates. A good agreement is achieved between the model results and the measurement indicating that the physics are well captured. Furthermore, nickel nitriding as one of the main challenges in the direct ammonia-fueled SOFCs is addressed and qualitatively validated with experimental data. The model can explain the reduction of performance difference between direct ammonia-fueled SOFC compared to its pre-cracked counterpart by reducing the fuel flow rate (increasing fuel utilization) and increasing the operating temperature. The highest performance difference (5.87%) is observed at 700 ° C and fuel utilization of 14%, and the lowest difference (0.05%) belongs to 850 ° C and fuel utilization of 70% at 0.5 A/cm 2. Anode off-gas recirculation is found to yield significant savings in ammonia fuel around 21–27% while a slight power reduction (0.34–0.88%) is observed by increasing the recirculation rate from 70 to 90% compared to the case without recirculation. Excessive amount of nitrogen in the high recirculation rates does not show a significant effect on the performance. Nickel nitriding is reduced by increasing the temperature and fuel utilization. • A detailed 3D model of an ammonia-fueled solid oxide fuel cell (SOFC) is developed. • Developed multiphysics model is validated under direct and pre-cracked ammonia. • Excessive amount of N2 in the recirculation has a negligible effect on performance. • Using anode off-gas recirculation saves a considerable amount (21%–27%) of ammonia. • Nickel nitriding in the model is qualitatively validated with the experimental data. [ABSTRACT FROM AUTHOR]

Further development and upscaling of the Solid Oxide fuel and electrolysis Cell (SOCs) technologies would significantly benefit from improvement of their mechanical robustness. In this work, microstructure, crystalline phase composition, fracture toughness and susceptibility to low- and high-temperature degradation of six different Ni(O)‒Zirconia fuel electrode supports, manufactured from six different stabilized zirconia compounds, are investigated. In the oxidized state, tetragonal zirconia-based supports have higher fracture toughness than cubic zirconia-based substrate, due to the transformation toughening effect and a finer grained microstructure. The NiO‒1.5CeO 2 4.5YO 1.5 -SZ support exhibits the highest fracture toughness, showing a 30 and 10 % improvement compared to the state-of-the-art NiO‒5.8YO 1.5 -SZ support at room temperature and 800 °C, respectively. In the reduced state on the other hand, the tetragonal and cubic zirconia-based substrates have comparable fracture toughness. The Ceria-Yttria co-doped materials possess superior resistance to hydrothermal degradation due to the stabilizing effect of Ce3+ formed during reduction. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) multi-physics models of solid oxide cells (SOC) reproduce distributions inside the cell during operation, and allow to implement interdependencies of heat transfer, mass transport, charge transfer, and electrochemistry. So far, this approach has mostly been utilised for transient and steady-state problems, preventing widespread application to electrochemical impedance spectroscopy, one of the main methods in experimentally analysing SOCs. In the present work, a computationally efficient alternative is outlined, surpassing these shortcomings by transforming the set of equations from the transient into the frequency form. The coupled multi-physics implemented by partial differential equations are solved numerically over a 2D domain by the finite element method, reproducing the cross section of the SOC. The model is validated with experimentally obtained polarisation curves and impedance spectra. The 2D model reproduces the experimental results and further visualises frequency-dependent oscillations of gas phase and potentials. These insights separate between impedance features from electrochemistry, gas diffusion, and gas conversion. In addition, overlapping gas conversion and diffusion impedance features at low frequencies are deconvoluted and transition frequency regions are discussed. • A 2D multi-physics model was developed to analysis impedance spectroscopy of SOCs. • The approach was validated with 31 impedance spectra and 16 iV-curves. • Experimentally unattainable spatially varying oscillations are analysed in the SOC. • The cause of gas conversion and diffusion impedance is analysed. • A separation between gas conversion and diffusion impedance is achieved. [ABSTRACT FROM AUTHOR]

The continuous introduction of renewable electricity and increased consumption through electrification of the transport and heating sector challenges grid stability. This study investigates load shifting through demand side management as a solution. We present a four-month experimental study of a low-complexity, hierarchical Model Predictive Control approach for demand side management in a near-zero emission occupied single-family house in Denmark. The control algorithm uses a price signal, weather forecast, a single-zone building model, and a non-linear heat pump efficiency model to generate a space-heating schedule. The weather-compensated, commercial heat pump is made to act smart grid-ready through outdoor temperature input override to enable heat boosting and forced stops to accommodate the heating schedule. The cost reduction from the controller ranged from 2-17% depending on the chosen comfort level. The experiment demonstrates that load shifting is feasible and cost-effective, even without energy storage, and that the current price scheme provides an incentive for Danish end-consumers to shift heating loads. However, issues related to controlling the heat pump through input-manipulation were identified, and the authors propose a more promising path forward involving coordination with manufacturers and regulators to make commercial heat pumps truly smart grid-ready. • We conduct a long-term control demonstration in a real occupied single-family house with heat pump and floor heating. • The demonstration employs a price responsive, mixed-Integer, hierarchical model predictive control algorithm. • The domestic air-to-water heat pump is retrofitted to be smart-grid-ready. • We demonstrate the actual load shifting potential of the heat pump in a nearly zero emission building. • Results show that a single-zone model based on an average indoor temperature can maintain a comfortable indoor environment. [ABSTRACT FROM AUTHOR]

Solid Oxide Fuel Cells are subjected to significant stresses during production and operation. The various stress-generating conditions impose strength requirements on the cell components, and thus the mechanical properties of the critical load bearing materials at relevant operational conditions need to be characterized to ensure reliable operation. In this study, the effect of reduction temperature on microstructural stability, high temperature strength and elastic modulus of Ni−YSZ anode supports were investigated. The statistical distribution of strength was determined from a large number of samples (∼30) at each condition to ensure high statistical validity. It is revealed that the microstructure and mechanical properties of the Ni−YSZ strongly depend on the reduction temperature. Further studies were conducted to investigate the temperature dependence of the strength and elastic modulus for both the unreduced and reduced Ni(O)−YSZ anode supports. With increasing temperature, the strength and elastic modulus of the reduced Ni−YSZ specimens drop almost linearly. In contrast, the strength and elastic modulus of the unreduced NiO−YSZ remain almost constant over the investigated temperature range. Compared to the NiO-YSZ, a significantly lower strength and elastic modulus of the reduced Ni-YSZ is observed; while reduction leads to a remarkable increase in failure strain of the anode support. [ABSTRACT FROM AUTHOR]

The effective steady-state creep response of porous metals is studied by numerical homogenization and analytical modeling in this paper. The numerical homogenization is based on finite element models of three-dimensional microstructures directly reconstructed from tomographic images. The effects of model size, representativeness, and boundary conditions on the numerical results are investigated. Two analytical models for creep rate of porous bodies are derived by extending the Hashin–Shtrikman bound and the Ramakrishnan–Arunchalam model in linear elasticity to steady-state creep based on nonlinear homogenization. The numerical homogenization prediction and analytical models obtained in this work are compared against reported measurements and models. The relationship between creep rate and porosity computed by homogenization is found to be bounded by the Hodge–Dunand model and the Hashin–Shtrikman creep model, and closely matched by the Gibson–Ashby compression and the Ramakrishnan–Arunchalam creep models. [ABSTRACT FROM AUTHOR]

Constrained sintering of tubular bi-layered structures is being used in the development of various technologies. Densification mismatch between the layers making the tubular bi-layer can generate stresses, which may create processing defects. An analytical model is presented to describe the densification and stress developments during sintering of tubular bi-layered samples. The correspondence between linear elastic and linear viscous theories is used as a basis for derivation of the model. The developed model is first verified by finite element simulation for sintering of tubular bi-layer system. Furthermore, the model is validated using densification results from sintering of bi-layered tubular ceramic oxygen membrane based on porous MgO and Ce 0.9 Gd 0.1 O 1.95− d layers. Model input parameters, such as the shrinkage kinetics and viscous parameters are obtained experimentally using optical dilatometry and thermo-mechanical analysis. Results from the analytical model are found to agree well with finite element simulations as well as measurements from sintering experiment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cost of green hydrogen from water electrolysis is estimated for 2020, 2030 and 2050. • Low- and high-temperature pressurized alkaline electrolyzer plants are modeled. • Low-and high-pressure solid oxide electrolyzer plants are modeled. • Carbon dioxide tax to bring grey and green hydrogen on par is estimated. • Dynamic operation is considered for both alkaline and solid oxide electrolyzers. Power-to-X technologies will play a key role in the future energy market, converting renewable electricity to chemicals. The first step in most Power-to-X schemes is the production of hydrogen. Several electrolysis technologies capable of producing hydrogen by water/steam splitting are currently available, each characterized by specific advantages and drawbacks and more are under development. This work presents a techno-economic analysis of green hydrogen production via alkaline electrolysis and solid oxide electrolysis technologies. Their current state of development and anticipated improvements are also considered; an alkaline electrolyzer operating at high pressure and temperature and a solid oxide electrolyzer operating at high pressure. The projected costs of electrolytic hydrogen via the different routes are compared to the cost of hydrogen derived from natural gas with and without CO 2 capture. Threshold CO 2 taxes needed for different electrolysis technologies to be cost-competitive are derived as a function of natural gas price and levelized cost of electricity. With the projected capital expenditure for solid oxide electrolyzers, reducing the levelized cost of electricity from 60 to 30 €/MWh would decrease the cost of hydrogen from 3.2 to 1.9 €/kg by 2050. With today's capital expenditure, natural gas price of 30 €/MWh and electricity price of 30 €/MWh, a CO 2 tax of 90 €/tCO 2 would make electrolytic hydrogen from alkaline electrolyzers cheaper than hydrogen derived from natural gas. It was found that feeding free steam increases the efficiency of the low-pressure solid oxide electrolyzer from 79 to 94%. [ABSTRACT FROM AUTHOR]

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. • A novel operation method for SOECs is developed for dynamic power-to-X systems. • The method works by rapidly (ms) cycling between fuel cell and electrolysis mode. • Stack temperature variation and associated thermomechanical stress is minimized. • Lower degradation is observed, possibly due to higher impurities tolerance. • Less nickel migration and agglomeration is observed relative to reference cells. [ABSTRACT FROM AUTHOR]

Solid oxide cells can deliver highly efficient energy conversions between electricity and fuels/chemicals. A central challenge of upscaling solid oxide cells is the probability of failure of the brittle ceramic components. The failures of the ceramic components may lead to significant degradation or eventual failure of a stack. To predict mechanical failures in a stack, a full stack model is needed, together with a local assessment of stresses at the vicinity of failing regions, e.g. the contact points between the cells and interconnects. A conventional three-dimensional model requires a very fine discretization of the mesh to capture stress intensities. Computational resources needed for such a model are therefore immense, and it is highly unlikely to compute at stack scale, as well describe the evolution over time. In this work, the homogenization modelling framework for solid oxide cell stacks is extended to identify local mechanical failures. Thus, the fracturing within a local failing point is examined by using a localization approach, where stresses in the stack model are linked to the local stresses and the energy release rate at the crack tip of the relevant interface. This is done in a general manner, such that the local stresses and the energy release rate can be evaluated at every point in the stack at every instant of time without loss of computational efficiency. A 100-cell stack can be modelled in three dimensions with all coupled multiphysics in steady state within 3 min on a current workstation computer. • Local mechanical failures in a fuel cell stack are modelled for the first time. • This is made possible by using a novel multiscale multiphysics modelling approach. • The impact of stack design and flows on local mechanical failures are modelled. • Exceptional computing speed achieved: 100-cell stack in 3 minutes for steady-state. [ABSTRACT FROM AUTHOR]