YSZ Surface Research Articles

(Invited) On Degradation Mechanisms of Ni-YSZ Fuel Electrodes in Solid Oxide Cells

The solid oxide cell (SOC) is reversible. It has about equally good performance both in solid oxide fuel cell (SOFC) and in solid oxide electrolyzer cell (SOEC) mode. The classical Ni-YSZ cermet SOC fuel electrode has an excellent initial performance provided that it has a good structure in terms of particle size of both Ni and YSZ, and a suitable porosity with sufficient contact between Ni-Ni, YSZ-Ni, YSZ-YSZ particles, and in absence of certain impurities such as silica and sulfur. The essential entity of the Ni-YSZ electrode is the length of the three phase boundary (3pb) between the three phases of Ni electron conductor, YSZ oxide ion conductor, and H2-H2O gas, which have electronic contact to the main Ni electrode, ionic contact to the bulk YSZ electrolyte, and access to the the main gas atmosphere, respectively.Nano-particular Ni is an excellent electrocatalyst for the reduction of H2O to H2 + O2- and for the oxidation. It seems that it is generally accepted that at operation temperatures (above 650 °C and above) the initial nano-sized Ni particles, which are in electric contact with each other, will over time continue to sinter into larger and larger Ni particles until all the Ni has become one dense body or the growth of the Ni particles has been blocked by particles of another phase like YSZ. Thus, the relative high mobility of Ni continues to pose a problem towards the lifetime of a highly functional Ni-YSZ electrode.A most serious type of degradation of Ni-YSZ electrodes in solid oxide electrolysis cells (SOEC) seems to be the one coupled to Ni-migration, which is regarded as an important obstacle for the commercialization of SOEC. Post mortem scanning electron microscopy investigations of the degraded Ni-YSZ electrode reveal that a thin (up to 3 – 5 µm) zone of the original nano-structured active Ni-YSZ cathode has been significantly been depleted in Ni. This degradation is clearly driven by the electrochemical polarization of the Ni-YSZ electrode [1], but the mechanism is highly discussed. Several researchers have the hypothesis that the Ni depletion is a result of Ni migration in a gradient in Ni/YSZ interface energy, see e.g. [2,3]. However, a review of relevant literature points out that this hypothesis cannot explain several reported clear experimental results, whereas our hypothesis based on Ni+ ions from Ni particles, which have lost electrochemical contact, migrate as NiOH across the YSZ particles to new active 3pbs formed further away from the bulk electrolyte. This hypothesis may qualitatively explain all reported results from steam electrolysis cells and H2 fuel cells so far [1]. However, our hypothesis does not directly explain the similar migration of Ni away from the YSZ bulk electrolyte in case of CO2 electrolysis.Therefore, a revision of our hypothesis will be presented. The essential change is simply to propose that it is the Ni+-ion that migrates in the YSZ-surface layer as either NiOH in case of steam electrolysis, or as “Ni2CO3” in case of CO2 electrolysis. A monolayer of “Ni2CO3” on a YSZ surface is not assumed to be any kind of crystalline Ni2CO3, but rather a layer of adsorbed Ni+ and “CO3 1-“ in which CO3 2- has one of its oxygen atoms incorporated into the YSZ surface crystalline structure which is formed by reaction between CO2 from gas and an unsaturated surface oxygen with a “dangling” electron with similarity to CO2 reduction on ceria [4]. Furthermore, the possible role of SiO2 impurities in the loss of contact between Ni and YSZ at negatively polarized Ni will be presented as another revision of the hypothesis together with further new details.References M.B. Mogensen et al., Fuel Cells, (2021), 1–15; DOI: 10.1002/fuce.202100072, and references therein.M. Trini et al., Acta Materialia, 212 (2021) 116887; DOI: 10.1016/j.actamat.2021.116887.L. Rorato et al., J. Electrochem. Soc., 170 (2023) 034504, DOI: 10.1149/1945-7111/acc1a3, and references therein.E.M. Sala et al., Phys. Chem. Chem. Phys. (2023), DOI: 10.1039/d2cp05157e.

Influence of Transition Metal Elements on Ni Migration in Solid Oxide Cell Fuel Electrodes

In the present study, the electrochemical performance and microstructure evolutions of Ni-M (M = Fe, Cu, Mn) bimetallic fuel electrodes were investigated under SOFC and SOEC operations. Ni-Fe, Ni-Mn, Ni-Cu and pure Ni patterned fuel electrodes were sputtered on YSZ pellets. During the SOFC tests, the electrochemical performance of all Ni-M bimetallic fuel electrodes were lower than pure Ni electrode, while the degradation rates of Ni-Fe and Ni-Mn electrodes were smaller than the others. The spreading of Ni film on YSZ surface was observed for all samples, and such Ni migration was suppressed by Fe and Mn addition, whereas it was enhanced by Cu addition. During the SOEC tests, the cell performance degraded with Cu and Fe addition, but improved with Mn addition. The adhesion between Ni film and YSZ substrate was enhanced by doping Fe and Mn, which correlated well with the inhibited degradation in both fuel cell and electrolysis operations. The Ni migration phenomenon is discussed by the strength of Ni-O bond, surface tension and melting point, which are influenced by the addition of transition metal elements.

Effects of Transition Metal Elements on Ni Migration in Solid Oxide Cell Fuel Electrodes

Solid oxide cells (SOCs), both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC), have emerged and attracted more and more attentions due to its high energy conversion efficiency and fuel flexibility. However, degradation of fuel electrodes after long-term operation remains as one of the main challenges for their commercial application. Two major types of microstructure evolution, nickel (Ni) migration and Ni coarsening in Ni - yttria-stabilized zirconia (Ni-YSZ) fuel electrodes have been widely reported, which have strong impacts on both cell performance and durability. Therefore, materials-driven research has focused on developing more robust fuel electrodes with good electrocatalytic ability. Utilization of Ni alloys has immersed as a promising concept to enhance the performance and robustness of conventional Ni/YSZ. In the present study, Ni-M (M = Fe, Cu) bimetallic fuel electrodes are applied to investigate the effects of transition metal elements on the morphological evolutions under SOFC and SOEC operations. Ni-Fe, Ni-Cu and pure Ni patterned fuel electrodes are sputtered on YSZ pellets. The electrochemical performance of these Ni-M bimetallic fuel electrodes decreases compared with pure Ni fuel electrode, while the performance degradation rate of Ni-Fe fuel electrodes is smaller than others. Besides, the spreading of Ni film on YSZ surface is observed for all samples under anodic polarization and such Ni migration is suppressed by Fe doping, whereas enhanced by Cu doping. On the other hand, the adhesion is weakened at the electrode / electrolyte interface for Ni-Cu and pure Ni fuel electrodes under cathodic polarization, while good adhesion at the interface is maintained for Ni-Fe, which correlates with the smaller performance degradation rate.

First-Principles Studies for Optimal Model of the Ni/YSZ Triple Phase Boundary in Solid Oxide Cells

Non-Faradaic electrochemical modification of catalytic activity (NEMCA) with electric field applications in solid oxide cells (SOCs) is thought to be induced by spillover effects of lattice oxygen from the bulk, although the detailed mechanism has not still been clear. In SOCs, important phenomena such as fuel decomposition, charge transfer, etc. occur at the triple phase boundary (TPB) as a highly active site that consists of catalyst, electrolyte, and gas phases. NEMCA is expected to be also induced strongly by the surface mechanism on TPB, and understanding surface reactions on TPB is essential for improving catalyst and cell performances. However, the reliable TPB model has not been still uniquely defined to discuss the property of TPB although various studies have been reported. Therefore, in this study, we have focused on the TPB model comprising Ni catalyst cluster; YSZ electrolyte; and gas phase, and aimed to identify a reliable TPB model for theoretical studies by using first-principles calculations as an initial step. In concrete, we identified firstly the stable structures of YSZ surface models by using DFT calculations taking into account oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and other factors. Thereafter, we discussed a reliable Ni/YSZ interface model based on the most stable YSZ model proposed above results by evaluating the Ni structure, interface stability, and so on.In this study, DFT calculations with a plane-wave basis set were implemented using CASTEP, and GGA-PBE exchange-correlation functional was used. The plane-wave cutoff energy was set as 489.8 eV, the OTFG-ultrasoft was used as the pseudopotentials, and the spin-polarization is considered because YSZ is a ferromagnetic substance. In YSZ surface models, yttria concentrations are set to 4.35 mol% and 9.1 mol% which shows the maximum ion conductivity of ZrO2. The three-layer YSZ (111) slabs with 15 Å vacuum layer with 2×2 and 2×4 unit cells were used for repeated slab models. The DFT+U method is used to obtain the correct electric structure of metal oxides with partially filled d or f-orbital shells, and k-points were set to 4×2×1. In Ni/YSZ interface models, we considered Ni cluster and Ni belt type models based on the most stable 2×4 YSZ surface model (9.1 mol%) proposed in this study. We also considered both cases of (111) and (100) facets for the contact interface. In the case of Ni/YSZ interface models, the DFT+U method was not considered to improve the calculation convergence and k-points were set to 4×2×1. A schematic diagram of the Ni/YSZ interface model is shown in the attached Figure.In the case of the 2×2 YSZ model with the yttria concentration of 9.1 mol%, the YSZ model is stabilized when oxygen vacancy is on the second O atom layer and the second neighbor to Y atoms, indicating that improvement in the geometry instability of ZrO2 for 8-coordination is more important than keeping local electron neutrality. We have also found that oxygen vacancy positions are more sensitive to the YSZ surface stability than Y atom arrangements. In the case of the 2×4 YSZ model with the yttria concentration of 4.35 mol%, the YSZ model where there are Y atoms on the first layer is stabilized. The crystal structure achieves a more stable structure by varying bond lengths when Y3+ with the larger ionic radius is replaced with Zr4+. Therefore, the structure is easier to stabilize when the Y atom exists on the surface than in the bulk due to the higher degree of freedom of the Y atom. The most stable structure of the 2×4 YSZ model with the yttria concentration of 9.1 mol% given based on the above results is 0.18 eV more stable than the previously reported structure [1]. This is because the number of Y atoms on the first layer with the second neighbor from oxygen vacancy is larger than the previously reported structure. We evaluated then the structural stability of the Ni/YSZ interface model based on the above YSZ model. As a result, we have found that adhesion energy between Ni and YSZ is independent of the relative position of Ni atoms. In addition, the larger the number of Ni atoms is, the more stable the structure is. This is because the electronic property of Ni atoms approaches the metal as increasing the number of Ni atoms. Other results and a detailed discussion will be reported in our meeting publications (ECS Transactions) and the presentation.[1] M. Shishkin and T. Ziegler, Phys. Chem. Chem. Phys., 16, 1798-1808 (2014). Figure 1

Effects of Transition Metal Elements on Ni Migration in Solid Oxide Cell Fuel Electrodes

In the present study, Ni-M (M = Fe, Cu) bimetallic fuel electrodes are applied to investigate the effects of transition metal elements on nickel (Ni) migration and Ni coarsening under SOFC and SOEC operations. Ni-Fe, Ni-Cu and pure Ni patterned fuel electrodes are sputtered on YSZ pellets. The electrochemical performance of these Ni-M bimetallic fuel electrodes are lower than pure Ni fuel electrode, while the degradation rate of Ni-Fe fuel electrodes is smaller than the others. The spreading of Ni film on YSZ surface is observed for all samples under anodic polarization, and such Ni migration is suppressed by Fe doping, whereas it was enhanced by Cu doping. The adhesion between the electrode/electrolyte interface is weakened for Ni-Cu and pure Ni fuel electrodes under cathodic polarization, while good adhesion at the interface is maintained for Ni-Fe, which correlates with the smaller performance degradation rate.

Modeling Ni redistribution in the hydrogen electrode of solid oxide cells through Ni(OH)2 diffusion and Ni-YSZ wettability change

Ni redistribution in the hydrogen electrodes of solid oxide cells is an important degradation mechanism. Its driving force is still under debate. This work focuses on the interplay between Ni(OH)2 diffusion and Ni-YSZ wettability change. With a Ni(OH)2 diffusion model developed in our previous work, we further employ three models for the Ni-YSZ contact angle to quantify the Ni-YSZ wettability change. The microstructure evolutions in a reconstructed Ni-YSZ electrode are simulated under selected experimental operating conditions. It is shown that the phenomenological model can capture the driving force of Ni spreading/detachment on YSZ surface and the Ni migration reported in experiments can be qualitatively reproduced through the competition between Ni(OH)2 diffusion and Ni-YSZ wettability change. It is also found that the initial microstructure of the active layer and the microstructure characteristics in the support layer can strongly affect the distribution of steam partial pressure and overpotential, hence the Ni redistribution, in the active layer, which may explain the inconsistencies in experiments. Our results show that there are two missing pieces in current theory of Ni redistribution: a mechanism of fast Ni diffusion under humid atmosphere and the physical reason behind the Ni spreading/detachment.

Detailed Mechanistic Studies of Electrochemical Reactions on Pt and Au Electrodes in Solid Oxide Cells Via EIS Data Analysis

In order to reduce the environmental impact of fossil fuels, a range of sustainable technologies for power generation and storage are currently being developed. In addition, efficient and low-cost methods for CO2 capture and conversion to form useful products are critically needed. One of the promising technologies towards achieving these goals involves the use of solid oxide cells (SOC), which are unique devices that can be employed in both the fuel cell mode for clean power generation and in the electrolysis mode to achieve CO2 conversion through its electrochemical reduction (‘CO2RR’).One of the burning problems that is impeding SOC implementation is catalyst selection, with many promising materials having been reported over the last 50 years, including metals and mixed ionic and electronic conducting (MIEC) metal oxide materials. However, the reasons for better or worse activity, as well as the mechanism of the CO2RR at these catalysts remains unknown and it is often unclear what factors limit the catalyst activity during cell operation and how this can be improved. This is due, in part, to the limited number of techniques that can be used to evaluate these performance metrics, especially at high operating temperatures. One of the most useful techniques is electrochemical impedance spectroscopy (EIS). However, data interpretation is often quite complex and thus other methods, such as CNLS fitting or the determination of the distribution of relaxation times (DRT), are needed.While CNLS fitting allows the fit parameters to be correlated with specific physical properties, a precise model of the system is required. Since SOC systems are relatively unexplored and good models are not yet available, this could lead to the mis-interpretation of the EIS data. DRT, on the other hand, can be a useful tool since it does not require any pre-existing models of the system. Unlike CNLS fitting, DRT can provide the correct number of time constants present (from the number of peaks obtained), as well as the associated resistance and capacitance values. According to the literature, each peak is typically related to a specific reaction step with a unique capacitance value, identified by changing conditions such as temperature, polarization or gas composition.In the present work, DRT analysis of Pt and Au electrodes on YSZ electrolytes was carried out in order to determine the origin of each resistance and to determine the effect of temperature, electrode polarization and gas composition on both the resistance and capacitance values. Here, both porous and point metal electrodes were investigated as no double phase boundary activity or chemical capacitance effects should be present, making data interpretation less complex. Furthermore, to avoid the challenges of porous electrodes, point electrodes, produced by pressing the metal wire to a flat YSZ surface by applying pressure with a spring, followed by softening at 1100 °C to achieve better contact, were used to achieve a controllable triple phase boundary (TPB) length. This allowed the true activity of the metal/YSZ TPB to be measured as a function of reactive interface length, rather than just per geometric surface area. Porous electrodes were also studied, made by depositing Pt or Au paste onto YSZ, followed by sintering at 600 °C for 20 min. Polymeric YSZ precursors were then infiltrated into the metallic backbone, then sintered at 750 °C for 2 hours. Then, a thin layer of the same metal paste was applied in order to provide better conductivity. The point electrodes were made by attaching 0.3 mm thick Pt and Au wires onto dense and flat YSZ electrolyte by a spring-loaded cell holder, then held at 1050 °C for 1 hour to soften and produce a fixed TPB length. Electrochemical testing of the porous electrodes was conducted at 750 °C in both half- full cell modes in air, CO2 and CO2/CO mixtures, while the wires were tested at 650-750°C in the same gases, but in half-cell mode.8 individual process were seen for both the porous and point electrodes, including both electrodes in each cell. The time constant with the smaller capacitors are likely related to the electrode-electrolyte interface, while the mid-range capacitors are associated with electrode surface processes, such as surface diffusion, molecular species dissociation, and adsorption. The high capacitance steps are likely related to gas-phase processes, such as gas diffusion and the possible presence of a conversion layer. More detailed analysis of polarization and gas atmosphere dependence is ongoing in order to confirm the nature of each process.

Bonding of Resin Cements to Ultra-Translucent Zirconia After Aging for 24 Hours and 1 Year.

To evaluate the effect of different luting techniques on the shear bond strength (SBS) of ultra-translucent multi-layered zirconia (UTML) after 24 hours and 1 year of water storage and to analyze the influence of primers on the wettability of the UTML MTU-6 surface. Four commercial resin cements were evaluated: Variolink Esthetic LC (Ivoclar Vivadent); RelyX Ultimate (3M); Estecem II (Tokuyama); and Panavia V5 (Kuraray Noritake). Panavia V5 was tested with and without Panavia V5 Tooth Primer (Kuraray Noritake). The SBS immediately after cement/primer application and after aging, scanning electron microscopy of the surfaces, and the contact angle of the primers on the UTML surfaces were analyzed. At 24 hours, Panavia V5 with Tooth Primer exhibited the highest SBS. After 1 year, the SBS of Variolink Esthetic LC and Panavia V5 with Tooth Primer decreased, and RelyX Ultimate increased. There was no statistical difference between RelyX Ultimate and Panavia V5 with Tooth Primer at 1 year. For all groups, the rate of adhesive failures increased after 1 year. The highest contact angle was observed on the control (no primer) group, and the lowest contact angles were obtained when the YSZ surface was treated with Scotchbond Universal and the combination of Clearfil Ceramic Primer Plus and Tooth Primer. Clinicians should prefer 10-MDP-based cementation systems that can be properly polymerized to achieve a stable long-term bond strength to YSZ restorations. Also, the use of ceramic primers improves the capability of the cement to establish an intimate contact with the intaglio of the restoration.

Doped effect of Gd and Y elements on corrosion resistance of ZrO2 in CMAS melt: First-principles and experimental study

The effects of Gd and Y doping on the corrosion resistance of ZrO2 in CMAS (CaO, MgO, Al2O3, SiO2) melts were investigated via first-principle calculations and experimental investigations. It is found that, although Gd2Zr2O7 with single Gd-doped ZrO2 has the lowest cohesive energy Ecohesive and formation enthalpy Hformation, YSZ(Gd) exhibits the lowest chemical activity with Y and Gd composite doping. The small Griffith’s rupture work (W = −3.498 J m−2) is beneficial to the flow of the CMAS melt on the YSZ(Gd) surface. Furthermore, the low Fermi energy and low O and Y electronegativities cause the weakest chemical activity. Additionally, the diffusion coefficients of Y, Zr, and O elements are also decreased through Gd-doping in YSZ. EDS investigations also demonstrate that the CMAS/YSZ(Gd) has the smallest reaction depth (6.65 μm) and the lowest elements penetration ability. Thus, the best corrosion resistance is originated from Y and Gd composite doping in YSZ(Gd) TBCs.

First-Principles Studies of the Surface and Interface between/Among Metal Oxides Used in SOFCs

On the surface and interface, all materials change the property such as stability and electronic states from them on bulk. Human being continue to apply the phenomena to the key technologies from early times. For example, in solid oxide fuel cells (SOFCs), important phenomena such as fuel decomposition, charge transfer etc., occur at surface or triple phase boundary (TPB) that consists of catalyst, electrolyte, and gas phase. To solve remaining issues of SOFCs such as degradation or efficiency improvement, important is to know the detail of property changes of materials used in SOFCs on the surface and interface. Theoretical study is efficient approach to investigate material property, especially electronic states. Therefore, in this study, we have focused on surface and interface of metal oxides used as electrolytes in SOFCs and investigated theoretically their property using two calculation models with first-principles calculation. In concrete, one is density functional theory (DFT) calculations with the TPB model that consists of Ni catalyst cluster, YSZ electrolyte, and gas phase. In this case, we have tried to decide the reliable structure to discuss properties on TPB and evaluated adsorption reactivity etc. The other is DFT calculations with the hetero-structure interface model that consists of zirconia (ZrO2) and ceria (CeO2). In this case, we have tried to evaluated electronic states on that interface and discussed in terms of conductivity control. Here, only the results of optimized YSZ (111) surface model and density of state (DOS) change in ZrO2/CeO2 hetero-structure are shown as examples for word limitation. The CASTEP software was used for all calculations.Geometry optimization of YSZ (111) surface model have been performed with some arrangements of yttrium (Y) atoms and oxygen vacancy in order to identify the most stable structure. Our model consists of (111) three layers includes 2×2 or 2×4 ZrO2 unit cells, 15 Å vacuum layer, and 9.1 % substituted Y atoms. In the most stable YSZ 2×2 model, one Y atom is on the top Zr atoms layer and another Y atom is on the middle Zr atoms layer nearest to the top Y atom, and oxygen vacancy is on the 2nd O atoms layer nearest to the top Y atom. We have found the position of oxygen vacancy against Y atoms and surface is sensitive to the YSZ surface stability. Oxygen vacancy does not prefer to stick around two Y atoms. It indicates that improvement for the geometry unstability of cubic ZrO2 is more important than keeping local electroneutrality even near on the surface. By applying this finding, we have performed geometry optimization of YSZ 2×4 unit cell model with changing Y atom arrangements in the same layer and keeping the stable structure in 2×2 unit cell model. The most stable YSZ 2×4 surface model finally-obtained is shown in attached figure (A).Our 2nd topic is for the electronic states change on the interface of metal oxides. We have calculated DOS with pure ZrO2, pure CeO2, and several ZrO2/CeO2 models hetero-connected with (111) surfaces. DOS changes by hetero-connection are shown in attached figure (B). A new small peak not appeared in the cases of pure ZrO2 and pure CeO2 is observed in 01 eV near Fermi level. It is considered that the peak is caused by the interaction between 4f orbital shown in 13 eV of Ce atoms and 2p orbital shown in -50 eV of O atoms in ZrO2 on the interface. In addition, the peak height is influenced only by the thickness of CeO2 layers, not that of ZrO2 layers. Our results suggest that we can control electronic conductivity of these metal oxides with hetero-structuration in atomic level especially at low temperature. Other results and detailed discussion will be reported at our meeting publications (ECS Transactions) and presentation in this conference. Figure 1

Structural Stability of Nickel Electrodes in Solid Oxide Cells

1. IntroductionLong-term durability is an issue of solid oxide cells (SOCs). Degradation of SOCs is most pronounced in a fuel electrode, where length of three-phase boundary, that is active reaction site, decreases. The major cause of the decrease in the three-phase boundary is nickel aggregation. The mechanism of Ni aggregation has been proposed to be Ni transport through the gas phase by nickel hydroxide. However, experimental facts did not support the hypothesis, suggesting that Ni transport through the gas phase should not be major factor [2].Because Ni-YSZ fuel electrode has a complex three-dimensional structure, it has been difficult to elucidate the mechanism of Ni transport. Therefore, it is natural to use nickel model electrode to simplify the evaluation of microstructure change of nickel, which will lead to the elucidation of the degradation behavior. Jiao et al. observed the microstructure change of Ni using Ni grid electrodes [3]. The structural change of Ni was attributed to change in wettability of Ni by applied voltage, where the wettability increased under anodic polarization and decreased under cathodic polarization.In this study, we aim to establish a method to observe the structure change of Ni inside the electrodes by modeling the complex structure of Ni-YSZ porous electrodes using patterned electrodes.2. ExperimentalFig. 1 (a) shows the simulated cross section of a symmetrical Ni-YSZ porous electrode cell. To model the cross section of porous Ni-YSZ fuel electrode, dense nickel-film patterned electrodes were fabricated on the YSZ substrate as shown in Fig. 1 (b) and (c). In this model cell, YSZ locating between two opposed nickel electrodes simulates the electrolyte, while the comb-shaped Ni pattern electrode simulates the porous Ni; YSZ surface between Ni fingers represents porous YSZ. Using this cell, transport of Ni in the direction of the electrode cross-section can be observed from top. After certain period of polarization to the cell, microstructure change of nickel was observed by microscope.3. Results and discussionAfter SOFC operation, the cross-sectional area of nickel electrode was the largest at the tip of the model electrode, suggesting that Ni migrated toward the tip. On the other hand, after SOEC operation, the cross-sectional area was larger toward the root, suggesting that Ni migrated toward the root. These migration directions were consistent with those assumed by wettability. The results confirmed that wettability contributed to Ni migration in the model electrode and that wettability-induced Ni migration also occurred in the porous electrode.

In-Operando Observations of Ni-YSZ Patterned Fuel Electrodes Under SOFC and SOEC Operations

Solid oxide cells (SOCs), both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC), have being attracting more and more attention due to its high energy conversion efficiency and fuel flexibility. However, degradation of fuel electrodes after long-term operation remains as one of the main challenges for their commercial application. In particular, migration and coarsening of nickel (Ni) in Ni - yttria-stabilized zirconia (Ni-YSZ) fuel electrodes have strong impacts on the decreases in both active three-phase boundary (TPB) and Ni network connectivity, which result in performance deterioration. One of the mechanisms proposed as an explanation of Ni migration is Ni transport via volatile Ni(OH)x species, so-called evaporation-condensation mechanism [1]. On the other hand, Jiao and Shikazono [2] proposed a model to explain Ni spreading on YSZ surface based on the variation of Ni-YSZ wettability which is modified by the oxygen adsorption. Nevertheless, further investigations are required to understand the mechanisms of Ni migration. In the present study, Ni-film patterned electrodes which can provide well defined TPB configurations are used to investigate the Ni-YSZ degradation under SOFC, SOEC and sequential SOFC and SOEC operations, i.e. reversible operation (RSOC). The edge of sputtered Ni-film contacting with open YSZ surface can be treated as active TPB during operation. Confocal laser scanning microscopy (CLSM, Keyence, VK-X 1000) is used, which enabled in-operando observation of the real-time local morphological change at active TPB [3]. The in-operando potentiostatic experiment was carried out inside a high temperature chamber which was kept at 800oC with 4% H2O-4% H2-92% N2 as fuel gas and 30% O2-70% N2 as oxidant as shown in Fig. 1(a). The correlations between the Ni morphological changes and cells’ electrochemical performances were investigated. The Ni phase dynamically spread and de-percolated only under the SOFC operation, while such a phenomenon was not observed during the SOEC operation, as shown in Figs. 1(c), (d) and (e). Ni coarsening occurred regardless of the operation modes. The competition between Ni-film spreading and breaking-up finally determined the complicated local morphological change and corresponding cell performance variation. The time variations of active TPB and current density showed good correlation as shown in Fig. 1(b). Real-time in-operando observations revealed different Ni migration under SOFC, SOEC and RSOC operations, which may provide new insights for the understanding of Ni-YSZ fuel electrode degradation.Fig. 1. (a) Schematic illustration of in-operando setup. (b) Correlation between current density and Ni morphological evolution. (c)-(e) CLSM images and 3D surface height contour maps of the yellow-frame-enclosed areas after SOFC, SOEC and RSOC modes, respectively.

Multiferroic h-LuFeO3 Thin Films on (111) and (100) Surfaces of YSZ Substrates: An Experimental and Theoretical Study

Thin films of in-bulk unstable multiferroic hexagonal LuFeO3 were synthesized on coherent (111) and for the first time on incoherent (100) YSZ and Pt/YSZ surfaces by the metal–organic chemical vapor deposition (MOCVD) technique. The obtained films were thoroughly studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), atomic force microscopy (AFM), piezoresponse force microscopy (PFM), and theoretical simulations. The substrate surface symmetry has a crucial role in the formation of the epitaxial film’s structure. Also, the molecular mechanics calculations were adapted for film/substrate interface simulation and, for the first time, the number of variants was predicted by the number of minima on the energy profile as well as it was proved that that the formation of h-LuFeO3 is more energetically preferable than o-LuFeO3, even on the incoherent surface. It was shown that h-LuFeO3 films deposited on the YSZ(111) surface have formed a single in-plane rotational variant structure, while those deposited on the YSZ(100) surface have formed a bivariant structure. PFM results of bivariant h-LuFeO3(001)//Pt(111)//YSZ(100) show half the size of ferroelectric domains (∼100 nm) and twice as large the values of piezoelectric response compared to h-LuFeO3(001)//Pt(111)//YSZ(111).

Low-resistance electrical contact between epitaxially grown thermoelectric oxide material (Ca2CoO3)0.62CoO2 and iridium

For the integration of the thermoelectric and magneto resistive active material (Ca2CoO3)0.62CoO2 (Ca3Co4O9) into silicon technology it is mandatory to provide, apart from diffusion barriers and epitaxial growth, electrical contacts with minimal electrical resistance between this oxide and the metal contact. Epitaxial Ca3Co4O9 thin films were grown by pulsed laser deposition on yttria stabilized zirconia (Y2O3)0.09(ZrO2)0.91 (YSZ) and Ir metallized YSZ buffered single crystalline (001)-Si substrates. X-ray diffraction pole figures of these multilayer systems reveal that the Ca3Co4O9 thin film exhibit a 12-fold in-plane rotational symmetry grown on a (001)-Ir and (001)-YSZ surface, but mutually rotated by 15∘. Ir-Ca3Co4O9 thin film electrical contacts were fabricated by lateral structuring and overgrowth to allow 4-wire measurements for determining the pure Ir-Ca3Co4O9 contact current-voltage characteristic and the area specific contact resistance (ASR). We demonstrate, that the Ca3Co4O9/Ir/YSZ and Ca3Co4O9/YSZ layer system allows to realize low ASR, low in-plane electrical resistivity of the Ca3Co4O9 thin film while maintaining a high Seebeck coefficient.

The spreading behavior of CMAS melt on YSZ single crystal with low index orientation

The spreading behavior of CMAS melt on cubic crystalline YSZ with low index orientation is studied by observing the variation of contact angles of CMAS melt on YSZ surface and the microstructure change caused by thermal chemical corrosion. In this study, the empirical formula of CMAS spreading on YSZ is established based on the experimental results. Whether in the vacuum or in the air, the tendency of the equilibrium contact angle of molten CMAS on the surface of single crystalline cubic YSZ is 〈1 0 0〉 > 〈1 1 1〉 > 〈1 1 0〉, and the spreading speed increases as 〈1 0 0〉 < 〈1 1 0〉 < 〈1 1 1〉. However, the specific spreading behavior of sectional samples varies with the atmosphere. Compared to the air environment, YSZ 〈1 0 0〉 obviously shows faster spreading speed and smaller equilibrium contact angle than in the vacuum. In this spreading process, the microstructures of invert trapezoid formed at the interface between CMAS and YSZ 〈1 0 0〉 and the semicircle (or the rounded triangle) formed on YSZ 〈1 1 0〉 and 〈1 1 1〉 play more important role than the surface energy which is calculated by first-principles.

Hot corrosion of yttria-stabilized zirconia by liquid antimony and antimony oxide

Direct carbon solid oxide fuel cells (DC-SOFCs) with a liquid Sb anode have many advantages over other types of SOFCs; however, their electrolyte faces corrosion by liquid Sb and Sb2O3. To evaluate the tolerance of the electrolyte to the liquid Sb and Sb2O3, 8 mol.% Y2O3–ZrO2 electrolyte (YSZ) discs are prepared and evaluated at 750 °C for up to 100 h in both chemical and electrochemical modes, simulating DC-SOFC operating conditions. In the chemical mode, the corrosion by liquid Sb causes Y dissolution from YSZ into the liquid Sb, while the YSZ experiences a phase transformation from cubic to tetragonal structure and grain boundary crack on the surface. Liquid Sb2O3 is more corrosive than liquid Sb, penetrating additionally the grain boundaries into a depth around 15 μm. In the electrochemical mode, liquid Sb is separated from YSZ by a layer (~150 μm) of liquid Sb2O3 formed during cell operation. The liquid Sb2O3 layer so formed corrodes YSZ in the same way as in the chemical mode, but the corrosion is intensified with the presence of a current, resulting in a top porous layer (~15 μm) on YSZ surface with a grain boundary penetration layer (~100 μm) underneath.

Effect of Laser-derived Surface Re-melting of YSZ Electrolyte on Performance of Solid Oxide Fuel Cells

A simple and effective technique to reduce the overall fuel cell polarization resistance was introduced on a conventionally sintered YSZ electrolyte by creating a hybrid microstructure using low-intensity CO2 laser scanning methods. After laser modification of the YSZ electrolyte surface on the cathode side, the overall polarization resistance was reduced from 513 to 124 Ω cm2, and the maximum power density was increased by 28% from 1.86 to 2.38 mW/cm2 at 450 °C. This improvement was due to the enhanced surface kinetics by higher grain boundary density on the YSZ surface from recrystallization after laser scanning.

CO2 and H2 Adsorption and Reaction at Nin/YSZ(111) Interfaces: A Density Functional Theory Study

To recycle CO2 into sustainable fuel and chemicals, co-electrolysis of CO2 and H2O can be achieved in solid oxide electrolysis cells, where the molecules are supplied to the Ni/YSZ electrode (YSZ = yttria-stabilized zirconia). Oxygen diffusion along the electrode has been identified as the critical step in the process, where YSZ is the common catalyst support. We have investigated the interaction of a CO2 molecule with the clean YSZ(111) surface and with Nin/YSZ(111) (n =1, 4-7, 10, 20) interfaces, using spin polarized density functional theory (DFT) and long-range dispersion correction. Here, we have considered up to six initial adsorption sites and two orientations for the CO2 molecule, which showed that the adsorption is stronger at the Nin/YSZ(111) (n =4-7, 10, 20) interface than on the clean YSZ(111) and Ni1/YSZ(111) systems. Additionally, we have determined that the preferential adsorption site of CO2 is at the interface between the Ni clusters and the YSZ(111) surface. We have observed a bending and stretching of the molecule, demonstrating its activation upon adsorption, due to charge transfer between the metal cluster and the molecule and a mixing between Ni orbitals and CO2 orbitals. In this work, we show that, although the electronic structure of the clusters depends on the cluster size, the interaction strength of CO2 with the interface is independent of the size of the supported nickel particle. Finally, we have considered the reverse water gas shift reaction and determined the hydrocarboxylic intermediate in the reaction mechanism over Ni5/YSZ(111).

Evolution of porous YSZ surface morphology in YSZ‐MnOx system

AbstractA unique porous YSZ surface morphology is observed and reported, for the first time, during the sintering of YSZ‐MnO2 composite. The porous YSZ morphology largely depends on YSZ/MnO2 ratio and sintering temperature, time, and atmosphere. The structural evolution is hypothesized to be due to the presence of Mn especially related to Mn diffusion, dissolution, precipitation, and evaporation. A series of experiments were designed and conducted to investigate the porous morphology formation with emphasis on performing quench experiments that showed the initiation of porous surface evolution during cooling. Computational thermodynamics has been applied to investigate the role of Mn on the surface porosity formation.

CHARGE TRANSPORT FEATURES IN THE COMPOSITE ANODES OF SOLID OXIDE FUEL CELLS: MICROSTRUCTURAL AND IN-SITU RAMAN SPECTROSCOPY ANALYSES

In this work, we carry out the high-resolution electron microscopy of microstructure of grains boundaries of anion and electronic conductors in composite Ni/YSZ anodes before and after study of the current–voltage characteristics of model SOFCs. We propose a mechanism of 2-stage reaction of hydrogen oxidation occurring in the vicinity of triplephase boundary of Ni/YSZ SOFC anodes. On the first stage, metallic nickel is oxidized to nickel oxide by oxygen anion coming from the solid electrolyte membrane. On the second one, hydrogen reduces nickel oxide to metallic nickel, and water is formed. Decrease of the Ni grains size in the vicinity of contact with anion conductor grains is shown to be the result of NiO nano-grains appearance and their consequent reduction to metallic Ni during SOFC operation. High-resolution electron microscopy analysis demonstrates the significant changes in microstructure of grains boundaries of anion and electronic conductors in composite Ni/YSZ anodes after application of load current to SOFC. Nano-sized NiO grains appear in near-boundary regions of Ni grains after current tests. Orientation alignment between YSZ and nano-sized NiO lattices is unambiguous evidence of epitaxial growth of nickel oxide at YSZ surface as on a substrate that is possible only as a result of oxygen anion transport from anion conductor YSZ to the metal surface during current passage through the solid oxide fuel cell. We study the chemical transformations in the electrochemical reaction zone in SOFC composite electrodes depending on the current density passing the SOFC by new “in-situ” Raman spectroscopy technique. Increase of the current passing is shown to lead to growth in the intensity of Raman peak connected with symmetric oscillations of CeO 2 group. We connect this result with the change of the cerium cations charge state from Ce 3+ to Ce 4+ and consider this to be direct proof of the charge transfer in composite anode via oxygen anion transfer.