Nickel Yttria-stabilized Zirconia Research Articles

Degradation Issues in Solid Oxide Electrolysers

One of the crucial aspects to achieving long lifetime and high-efficient SOCs (Solid oxide cells) is to enhance durability of the current devices.Degradation issues are typically associated with problems in both electrodes. For example, nickel-yttria stabilized zirconia (Ni-YSZ) hydrogen electrodes present under extreme conditions of high steam partial pressures, a low durability due to Ni oxidation resulting in lowered electronic conductivity and catalytic activity of the electrode. Ni agglomeration, depleting and micro crack in Ni-containing hydrogen electrode can also occur leading to a lower activity of the Ni-based electrodes as an SOEC fuel electrode. Furthermore, carbon deposition and sulfur poisoning on the Ni surface are also lead to cell performance degradation and poor durability.In addition, the performance of the oxygen electrode is also particularly more important in electrolysis mode than in fuel cell mode, as it is well established that electrochemically induced oxygen pressure increase at the electrolyte-oxygen electrode interface and subsequent membrane failure have been theoretically predicted and experimentally observed in SOEC mode. For example, several examples in the literature reported the presence on voids at the YSZ electrolyte, leading to delamination of the oxygen electrode. In this sense, lanthanide nickelates (Ln = La, Nd, Pr) have received considerable interest as materials for IT-SOFC electrodes and oxygen separation membranes, and they seem to be very attractive for electrolysis applications. The hyperstoichiometry of some oxygen electrode materials such as these Ruddlesden-Popper phases is believed to be favourable for effective oxygen evolution, as performance of these electrodes is enhanced in SOEC mode.Different strategies to develop optimized electrode structures as well as controlled operating conditions will be discussed in order to improve the durability of SOCs.

Carbon Deposition Mechanisms on Ni-Based Anode for SOFC: A Comparison Between Non-Discharge and Discharge Modes

The nickel/yttria-stabilized zirconia (Ni/YSZ) and nickel/scandia-stabilized zirconia (Ni/ScSZ) have widely used as an anode for solid fuel cells (SOFCs). When hydrocarbon gas is supplied to the Ni-based fuel electrode of SOFCs, carbon is deposited on the anode, and it causes anode destruction. Therefore, carbon deposition control is required.The purpose of this study is to clarify carbon deposition mechanisms to understand how the carbon resistance appears on the Ni-based electrode in non-discharge and discharge modes. NiO/8XSZ powder with a binder was uniaxially pressed to form 0.7mm thick pellet (Ni : XSZ=50:50 vol%, X(dopant)=Y, Sc) which were subsequently sintered. After a reduction in H2 atmosphere, Ni/8XSZ sintered cells were heated in CH4/Ar (S/C ratio (Steam to Carbon) =0, CH4/Ar=0.25) and CH4/H2O/Ar atmosphere (S/C ratio=0.15, CH4/Ar=0.25) using a thermobalance, and the carbon deposition rates were measured from the weight change of the cell at 1173 K and 973 K. In this experiment, the S/C ratio was adjusted by bubbling CH4/Ar gas to set the saturated water vapor pressure at a given temperature. In addition, a power generation experiment of SOFC was conducted at 1073 K to investigate the effect of electrochemical reactions on the carbon deposition. The SOFC cells were prepared as follows. YSZ suspension was spin-coated on the NiO/8YSZ substrate. The electrolyte/electrode bilayer was co-sintered at 1673K. Then, a paste of LSM/YSZ was coated on electrolyte and sintered at 1473K. The cell was sealed to the alumina tube in the electric furnace. The anode side was fed with CH4/Ar/H2O (S/C ratio = 0.15) and the cathode side was exposed to O2. Current density was set to 126mA/cm2 for 30 min. After discharge, the anode surface was observed using SEM/EDS.As a result, the carbon deposition rate of Ni/ScSZ was lower than that of Ni/YSZ by 34 % at 1173 K and S/C=0 under non-discharge mode. This result indicated that the Ni/ScSZ exhibited superior carbon deposition tolerance than the Ni/YSZ at 1173 K. Steam reforming reaction was also different between the Ni/YSZ and Ni/ScSZ. Previous studies showed that carbon deposition tended to progress easily at the metal/solid electrolyte interface. The difference in carbon deposition behavior between Ni/YSZ and Ni/ScSZ was caused by the difference in the interface structure. On the other hand, the carbon deposition rate of Ni/ScSZ was higher than that of Ni/YSZ at 973 K which was opposite to that at 1173 K. Moreover, the rate of carbon reforming reaction (C+H2O→CO+H2) of Ni/YSZ was higher than that of Ni/ScSZ. In fact, the Ni/YSZ anode showed superior carbon tolerance than the Ni/ScSZ at lower temperature. XRD (X-ray Diffraction) analysis showed that the ScSZ with cubic structure was partially changed to the rhombohedral structure after the carbon deposition at high temperature while YSZ cubic structure was unchanged. This implied that the rhombohedral structure at the interface inhibited the carbon deposition reaction, suggesting that the electrolyte structure influenced the carbon deposition and steam reforming reactions. Therefore, it was shown that the electrolyte structure can control the carbon deposition while the same Ni was used. These findings give an useful hint to design the carbon tolerance electrode.Meanwhile, carbon deposition was not found on the Ni/YSZ surface after discharge at 1073 K. This indicated O2- ion improved the carbon tolerance during discharge. Comparison of carbon deposition between Ni/YSZ and Ni/ScSZ will be discussed using DFT (Density Functional Calculation).

Oxidation Process of Ni Cathode in CO2 Electrolysis in SOEC: X-Ray and DFT Study

Solid oxide electrolysis cells (SOECs) can convert H2O and CO2 to syngas(H2/CO) at high temperature, promising a way in the power-to-gas process. A Ni electrode such as nickel/yttria-stabilized zirconia (Ni/YSZ) and nickel/scandia-stabilized zirconia (Ni/ScSZ) have widely used as a cathode for SOECs. Meanwhile, the cathode faced severe oxidizing condition due to a high steam and CO2 content. H2, a reduction agent, can inhibit the cathode oxidation; however, an additional H2 decreases a total energy efficiency. Oxidation tolerance is an important parameter for the material candidate as the cathode. A combination study of X-ray and DFT study is a promising way to understand oxidation mechanisms of Ni cathode, which provides an useful hint in the cathode design with oxidation tolerance.In this study, oxidation process of Ni cathode in CO2 electrolysis was studied using X-ray and density functional theory (DFT). Ni cathodes (Ni/YSZ, Ni/ScSZ) were used in H2/CO2 environment under the electrolysis mode. After CO2 electrolysis, Ni cathode surface was analyzed using SEM/EDS and X-ray diffraction (XRD). Influence of H2/CO2 ratio and operating condition on cathode oxidization was studied in the electrolysis mode. All calculations here are performed employing the plane wave density functional theory (DFT) method implemented in the Vienna Ab-Initio Simulation Package (VASP).XRD results showed that the cathode oxidation was progressed at H2/CO2 ratio of 0, whereas the cathode oxidation was inhibited when the H2/CO2 ratio was higher than 0.75 at 800 oC. In addition, it was suggested that the electrolysis mode, in which oxygen was moved from the cathode to the anode, had a role in inhibiting the cathode oxidation. In the DFT calculation, an activation energy of CO2 decomposition at the triple phase boundary (CO2→CO+O,vacancy) was much lower than that at the Ni surface (CO2→CO+O,Ni), indicating that CO2 electrolysis was more preferable than Ni oxidation. It was shown that the electrolysis mode can enhance the oxidation tolerance of Ni cathode. Moreover, Ni/YSZ and Ni/ScSZ cathode were also studied in terms of CO2 electrolysis and oxidation. The activation energy of CO2 decomposition of Ni/ScSZ was lower than that of Ni/YSZ mainly because O can easily enter the vacancy in the ScSZ. It was shown that the oxidation can be controlled from the electrolyte structure in the electrolysis mode.

Characterization of a Nickel / Gadolinia Doped Ceria Fuel Electrode for Co-Electrolysis

Modelling of the high temperature co-electrolysis process requires understanding of the underlying reaction pathways. These include the electrochemical steam reduction, the electrochemical CO2 reduction and their coupling via the catalytic (reverse-)water-gas-shift reaction (RWGS). The assumption of a very fast water-gas-shift reaction and therefore neglectable CO electro-oxidation is widely used. The validity of this assumption has been previously investigated and proven to be sufficient by Kromp et al. [1] for an anode supported cell (ASC) with a nickel / yttria-stabilized zirconia (Ni/YSZ) fuel electrode.Within this contribution we aim to extend the approach by Kromp et al. [1] towards an electrolyte-supported cell (ESC) with a nickel / gadolinium-doped ceria (Ni/CGO) fuel electrode. Previously, it has been proposed, that the electro-oxidation /-reduction of CO / CO2 can be present under reformate gas mixtures [2,3] on Ni/CGO fuel electrodes. However, a detailed comparison of pure activation resistance contributions during different operating modes has not been performed yet.We present results from a complex variation of operating parameters (such as various gas compositions, exemplarily shown in fig. 1) for process identification by the use of electrochemical impedance spectroscopy and the subsequent impedance analysis by the distribution of relaxation times (DRT). Visualization and deconvolution of gas diffusion impedance contributions is done via the evaluation of the polarization resistance differences with different inert gases. This has been successfully applied for binary gas mixtures of hydrogen and steam [4] and also CO2 and CO. [5] The applicability of this method towards more complex multicomponent gas mixtures incorporating hydrogen, steam, CO2 and CO will be shown. By subtracting the gas diffusion impedance contributions, it is possible to evaluate reaction kinetics of the underlying activation polarization and a meaningful comparison of operation modes is facilitated. The ratio of direct CO2electro-reduction and CO2-conversion by RWGS will be discussed for different compositions and temperatures to show a non-neglectable influence of the CO2 electroreduction at even lower pCO2 / pH2 ratios than previously reported. Furthermore, we compare our results with ones obtained from Ni/YSZ fuel electrodes, to investigate the influence of fuel electrode structure and material.

Solid oxide electrolysis cells – Interplay between operating conditions, fuel electrode overpotential and degradation

With the integration of increased share of energy from renewable, but fluctuating, energy sources there will be an increased demand for large-scale flexible conversion of electrical energy into easily storable chemical fuels. Electrolysis will be a key component in the future of energy conversion and storage solutions. Solid oxide cells (SOC) can provide the most efficient electrolysis technology. Furthermore, solid oxide electrolysis cell (SOEC) technology has improved tremendously in terms of initial performance, long-term durability, and up-scaling of cell and stack sizes; and with respect to commissioning and operation of SOEC plants. Nevertheless, it is evident that further improvement of electrochemical performance and especially long-term durability is crucial for the coming up-scaling and integrated SOEC systems as part of our energy grid. This calls for research within 1) thorough mapping of degradation mechanisms, 2) determination of thresholds for the onset of irreversible degradation processes, 3) safe operating conditions and means to mitigate known degradation processes, etc. However, degradation is not a simple singular property and there is often a puzzling interplay between parameters such as: the chosen electro-catalysts, the manufacturing of cell components, obtained microstructures, the resulting initial electrochemical performance, operating conditions, and the nature and extent of different degradation processes. This work will present11This work has been presented as invited keynote speak at symposium E01.07 as part of the 23rd International Conference on Solid State Ionics, MRS Meeting, Boston (US), July 2022. an overview of the puzzling interplay between initial electrochemical performance and long-term durability. It will then provide examples of multiple complementary characterization techniques applied for the analyses of these features and processes (e.g., SEM, TEM, Raman spectroscopy and electrochemical impedance spectroscopy). Furthermore, strategies to mitigate and/or limit specific degradation processes and insight into data required to establish degradation wise safe operating conditions are discussed. The present work will touch upon multiple performance characteristics and degradation processes: 1) triple-phase-boundaries (TPB) and critical micro/nano-structural parameters of the porous nickel/yttria-stabilized-zirconia (Ni/YSZ) electrode; 2) the perception of electrode overpotential and why this parameter is more important than the set current density or operating cell/stack voltage, 3) the effect of impurities, 4) the interplay between impurities and carbon deposition and 5) the degradation caused by Ni migration and actions to mitigate this degradation process.

The properties of the fuel electrode of solid oxide cells under simulated seawater electrolysis

In this study, electrolysis of seawater in flat-tube nickel-yttria-stabilized zirconia (Ni-YSZ) electrode-supported solid oxide electrolysis cells (SOECs) were modeled and the effects of variations in electrical conductivity and microstructure of Ni-YSZ electrode support were investigated. When the current density was greater than 700 mA·cm−2, the conductivity of the electrode support decreased slightly with an increase in current density at 800 °C in hydrogen reduction environment; the conductivity of the electrode support decreased with an increase in the current density when the current density was greater than 400 mA·cm−2 at 800 °C in the seawater electrolysis environment. During long-term durability experiment of seawater electrolysis, the degradation rates in area specific resistance (ASR) were 0.096 mΩ·cm2/100 h and 0.207 mΩ·cm2/100 h with a current density of 300 mA·cm−2 (i.e., ≤400 mA·cm−2) and 1000 mA·cm−2 (i.e., ≥400 mA·cm−2), respectively. Besides, the various ions commonly present in seawater did not contaminate the Ni-YSZ support during the long-term durability test. The degradation mechanism of seawater electrolysis in flat-tube SOECs is discussed and clarified.

Ni-doped CeO2 nanoparticles to promote and restore the performance of Ni/YSZ cathodes for CO2 electroreduction

Despite the considerable efforts to develop innovative electrode materials, nickel/yttria-stabilized zirconia (Ni/YSZ) electrodes are still employed in nearly all commercial and industrial solid oxide cell units. This is because Ni/YSZ cermets have good electrocatalytic activity in both electrolysis and fuel cell modes, while they are cost-effective as compared to alternative electrodes under development at the lab scale. Infiltration of nanoparticles has immersed as a promising concept to enhance the performance and robustness of conventional Ni/YSZ. Herein, we apply a relatively simple procedure to infiltrate Ni/YSZ electrodes with hexane solution containing prefabricated Ni-doped CeO2 nanoparticles. Cells with modified Ni/YSZ electrodes show a great improvement in the electrocatalytic activity and stability towards direct CO2 reduction as compared to unmodified cathodes. Besides, a single infiltration step is sufficient to achieve the optimum cell performance, streamlining the preparation process. More importantly, we show that this strategy can be equally efficient to fully reactivate previously degraded Ni/YSZ electrodes and restore their electrochemical performance at levels even higher than the initial ones. In addition, due to the high viscosity of the hexane solution, Ni-doped CeO2 nanoparticles access the entire electrode volume all the way close to the YSZ interface, while it penetrates the micro cracks between Ni and YSZ particles. In this way the electrocatalytic reaction zone is recovered, and the degraded Ni/YSZ electrode is reactivated. Physicochemical, microstructural and electrochemical characterization of the cells evinces several effects that contribute to the performance improvement of Ni/YSZ after infiltration, such as the enhanced surface reducibility of Ni and the amelioration of charge transfer processes within the electrode. The simplicity of the proposed infiltration method and the significant promotion in CO2 electroreduction activity, advocate it as a cost-efficient strategy to improve or regenerate conventional Ni/YSZ electrodes without significantly altering the already well-established fabrication process.

(Invited) Gradient Ni-SDC Anode By Reactive Co-Sputtering for Low Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are attracting attention as next-generation energy conversion devices due to their high efficiency, eco-friendliness, and scalability. However, the high operation temperature of conventional SOFCs (> 800°C) has posed several practical issues such as thermal degradation and limited material choice. To overcome this limitation, low-temperature SOFC (LT-SOFC, operation temperature <600°C) is actively being researched these days. Ni-YSZ (nickel-yttria stabilized zirconia) is commonly used as anode material in SOFCs. Ni-YSZ anodes are reasonably conductive at high operating temperatures and stable under harsh reducing conditions. However, at lower operating temperatures, the low conductivity of Ni-YSZ becomes an issue for use as an anode. Using doped ceria instead of YSZ is considered an alternative because the doped ceria has higher conductivity compared to YSZ due to the co-existence of multivalent states, i.e., Ce3+ and Ce4+, which can be further improved by doping. Moreover, if one adopts a carefully designed anode structure, e.g., gradient structure, the electrochemical performance of Ni-doped ceria anode can be further improved. Employing nanoscale components, e.g., thin films, are also one of the strategies to improve the electrode kinetics at the low operating temperature of LT-SOFCs. Thin film electrodes with short electronic and ionic paths as well as high surface area contribute to low ohmic and activation resistances. While high uniformity, reproducibility and throughout are practical concerns in thin film fabrication, reactive sputtering, which can produce uniform ceramic thin films from metal target in reactive gas environment in high reproducibility ad deposition rate, is a suitable candidate for thin film components for SOFCs. In this study, we fabricate a Ni-Samaria doped ceria(SDC) gradient anode by reactive co-sputtering. We prepare and characterize the cells with homogeneous and gradient-structured Ni-SDC thin film anodes. The gradient Ni-SDC thin film anode is composed of three layers, whose compositions are graded in a way that the Ni content increases from bottom (near electrolyte-anode interface) to top (anode top-surface). Possibly due to the combination of facilitated ionic conduction near electrolyte-anode interface and higher catalytic activity at anode surface, we observe the improved anode kinetics with lower activation resistance in the cell with gradient-structured Ni-SDC anode compared to the one with homogeneous anode.

Interphase mechanical behaviors of nickel/yttria-stabilized zirconia cermets: A study by nanoindentation and molecular dynamics simulations

The metal/ceramic interface is of great importance in a variety of basic science research and technological applications. In this paper, the interphase mechanical behaviors of the nickel/yttria-stabilized zirconia (Ni/YSZ) system serving as an anode material in solid oxide fuel cells are investigated by nanoindentation experiments and molecular dynamics (MD) simulations. The experimental results indicate that the interface effect can greatly enhance the Young's modulus and hardness of Ni near Ni/YSZ interface, while a reduction is observed in the aforementioned mechanical properties of YSZ near the interface. In addition, the change in the mechanical properties and the affected region of the YSZ component due to the interface effect are both much larger than those of its Ni counterpart. Our MD simulation results reveal that the different effects of Ni/YSZ interface on the mechanical properties of Ni and YSZ near the interface can be ascribed to different transport capabilities of deformations generated in these two components across the interface. Based on MD simulation results, an interface barrier model and a model considering the structural compliance are proposed here for theoretically predicting the interphase hardness and Young's modulus of the Ni/YSZ system.

Three-dimensional topology optimization of Ni-YSZ anode for solid oxide fuel cells via multiphase level-set method

In the present study, numerical model is developed to optimize the 3D multiphase electrode microstructure for SOFC, e.g. nickel-yttria-stabilized zirconia (Ni-YSZ) anode. During the optimization, Ni-YSZ microstructure is deformed in order to maximize the total reaction current. Multiphase level-set (MLS) model is applied to represent the Ni-YSZ anode microstructure, and adjoint method is applied for the sensitivity analysis. In addition, in order to improve the computational accuracy and to make the optimized microstructure manufacturable, a radius constraint is imposed to ensure that the local particle radii are kept larger than a target value (Rtarget), where the interface is updated only when the local radius value is larger than Rtarget. It is also confirmed that the optimized microstructure and performance of the Ni-YSZ anode are independent of the initial structures and the phase division schemes. According to the optimization results, the optimized electrochemical performances are improved, and the particle and pore radii of the microstructure successfully remained larger than the target values. The optimized Ni-YSZ microstructures are composed of Ni particles embedded into a YSZ scaffold which has a pillar-like shape along the anode thickness direction.

Effect of particle size on surface roughness and morphology of heat-treated electroless Ni-YSZ coating

The paper discusses the surface characterisation of electroless nickel-yttria-stabilised zirconia (Ni-YSZ) coating with varying YSZ particle sizes and undergoes heat treatment at a temperature between 300-400°C for 1-2 hours for wear resistance purposes. This finding will be helpful to the application of Ni-YSZ as an alternative coating for cutting tools. The surface characterisation was analysed using JOEL Scanning Electron Microscope (SEM) coupled with Energy Dispersive X-ray (EDX) JSM 7800F. The crystallographic structure of materials was analysed by X-ray diffraction (XRD) Bruker D8 Advance instrument. The Ni-YSZ coating was deposited using electroless nickel co-deposition of 8YSZ ceramic particles with a nano, mixed and microparticle sizes onto a high-speed steel (HSS) substrate. The coatings were heat treated at temperature 300-400°C and time 1-2 hours. The surface roughness was measured using Mitutoyo surface roughness tester SJ-301. The electroless Ni-YSZ coating deposited has an average thickness of 30 µm. It is found that the coating morphology electroless coating without YSZ particle incorporation (EN) and Ni-YSZ nano (N) is smoother compared to the Ni-YSZ mixed (NM) and Ni-YSZ micro (M). The EDS composition analysis shows the YSZ content in the electroless Ni-YSZ coating for N samples is the lowest, whereas NM samples are the highest. This resulted in the surface roughness behaviour where the mixed-size YSZ particle gives the highest roughness at all temperatures. The XRD analysis shows that heating temperatures above 300°C caused the precipitation of Ni3P crystalline. Previous studies in the surface characterisation of electroless nickel composite are scarce; thus, the study has limitations in finding supporting data. The surface characterisation especially related to the surface roughness of the electroless nickel, either the Ni-P or composites or alloys are rarely reported. Thus, this study enlightened the effect of particle size on surface roughness and morphology of heat-treated coatings.

An adaptive thermo-mechanical peridynamic model for crack analysis in anode-supported solid oxide fuel cell

Nickel–yttria-stabilized zirconia (Ni–YSZ) composite cermet is widely used as the conventional anode material in solid oxide fuel cells (SOFCs) for both functional and supporting layers under high-temperature operating conditions. Although with high energy conversion efficiency and multi-fuel compatibility, the cell always suffers irreversible gradual degradation and sudden deterioration, which can be attributed to mechanical damages formed during thermal cycling and long-time operation. In this study, the three-dimensional (3D) real microstructures of various porous Ni–YSZ anodes are reconstructed using the focused ion beam-scanning electron microscopy (FIB-SEM) dual-beam technique. Based on the reconstructions, a weakly coupled thermo-mechanical peridynamic (PD) model is proposed to predict the possible mechanical damages under severe conditions for different anodes associated with the single-cell design under thermal shocks. The relevant equivalent mechanical and thermal properties of the composite anodes are analyzed and correlated to the real microstructures. The work provides a trans-scale model to simulate the generation and propagation of cracks based on the microstructural characteristics, and the simulation results show great potential in providing supporting information for trans-scale optimization of SOFCs.

Ceria nanoparticles as promoters of CO2 electroreduction on Ni/YSZ: An efficient preparation strategy and insights into the catalytic promotion mechanism

Since many decades nickel yttria-stabilized zirconia cermet (Ni/YSZ) has been the most frequently used fuel electrode material for high temperature solid oxide cells (SOCs). However, in recent years there has been considerable effort to improve the Ni/YSZ performance through surface engineering. In this work, we report a simple strategy to apply nanosized un-doped (CeOx) and Ni-doped (NiCeOy) ceria particles into porous Ni/YSZ cermet electrodes via infiltration from hexane solution. Detailed characterization of the particles in their solution revealed differences in the ease of agglomeration, with NiCeOy nanoparticles being better dispersed and thus forming smaller aggregates. This property is critical for the effectiveness of the solution in filling the pores of Ni/YSZ cermet and the consequent ceria deposition. In particular, morphological and microstructural characterization reveals that NiCeOy nanoparticles decorate uniformly the pores of Ni/YSZ backbone, deep up to the interface with the electrolyte. More importantly, this can be done with relatively high ceria loading per infiltration/co-firing step. Electrochemical tests demonstrate that infiltrated Ni/YSZ fuel electrodes have improved I-V performance in CO2 electrolysis as compared to pristine Ni/YSZ. Synchrotron-based operando NAP-XPS experiments using both soft and tender X-rays revealed the formation of an ultrathin Ni-Ce3+ layer on the electrode surface, which can rationalize the ameliorated CO2 electrolysis performance.

Effect of heat treatment on microhardness of electroless Ni-YSZ cermet coating

The paper discussed the effect of heat treatment on electroless nickel-yttria-stabilised zirconia (Ni-YSZ) cermet coating. Ni-YSZ cermet coating has potential applications such as cutting tools, thermal barriers, solid oxide fuel anode, and various others. The compatibility of ceramic YSZ and metallic nickel in terms of the mechanical properties such as hardness by varying the heating temperature, time and ceramic particle size is highlighted. Ni-YSZ cermet coating was deposited onto a high-speed steel substrate using the electroless nickel co-deposition method. The temperature and time were varied in a range of 300-400°C and 1-2 hours, respectively. The microhardness measurements were carried out using a Vickers microhardness tester (Shimadzu) according to ISO 6507-4. The surface characterisation of the cermet coating was carried out using JOEL Scanning Electron Microscope (SEM) coupled with Energy Dispersive X-ray (EDX) JSM 7800F. The crystallographic structure of materials was analysed by X-ray diffraction (XRD) Bruker D8 Advance instrument. It was found that the microhardness of Ni-YSZ cermet coating with the ratio of 70:30, respectively, is directly proportional to the heating temperature and time. Heating the Ni-YSZ cermet coating at 300°C from room temperature (rtp) to 1 hour shows a 12% microhardness increment, while from 1 to 2 hours gives a 19% increment. Compared to heating at 350°C and 400°C, the increment is more significant at 33% and 49% for rtp to 1 hour and 8% and 16% for 1 to 2 hours, respectively. In addition, the effect of varying YSZ particle size in the Ni-YSZ cermet gave response differently for heating temperature and heating time. The paper is only limited to the discussion of the heat treatment effect on Ni-YSZ cermet coating hardness property. The tribological effect will be in future work. The microhardness data may vary due to the Vickers microhardness force applied and the amount of ceramic particle incorporation and phosphorus content in the nickel matrix. The value of this work is the compatibility of the ceramic YSZ and metallic nickel matrix in terms of mechanical properties, such as hardness, upon heat treatment.

Effects of doping by copper on electrical properties of LaCrO3 based perovskite

The use of nickel-yttria-stabilized zirconia (Ni/YSZ) as anode material for solid oxide fuel cell (SOFC) applications, presents some limitations such as poor stability in reduction/oxidation cycling, and poor sulphur tolerance which results in carbon deposition when utilizing hydrocarbon fuels. Even though lanthanum chromite (LaCrO3 noted LCO) is an excellent alternative to Ni/YSZ, it is rarely used in SOFC applications due to its poor conductivity. In order to further progress in developing LCO materials, this work was conducted to improve the conductivity of LCO by copper doping. In particular, LaCr1-xCuxO3 compounds with x = 0, 0.1, and 0.3 were synthesized via a solid-solid method. The followed preparation process allows obtaining a cubic structure with Pm-3m space group. The substitution of Cr with Cu resulted in a slight increase in lattice parameter a, but no change in structure or space group. Whereas, the band gap energy decreased from 3.68 eV for x = 0–3.06 eV for x = 0.3. The crystallite size increases with doping level. With regard to the obtained data from electrical measurements, a thermally activated and frequency depended conductivity was obtained for all samples. Furthermore, the correlated barrier hopping mechanism was proposed as a conduction mechanism, and an electrical transition from semiconductor to metallic behavior was observed. The conductivity increased also monotonously with Cu content and shows a maximum value of 4.5 S/m for x = 0.3. Besides, the tangent loss shows a maximum value of around 104 for x = 0.1. The LaCr0.7Cu0.3O3 compound could be a good candidate for SOFC applications, while LaCr0.9 Cu0.1O3 can be used as a heating element.

Numerical Investigations on the Damage Behaviour of a Reconstructed Anode for Solid Oxide Fuel Cell Application

This paper addresses the damage behaviour of a nickel/yttria-stabilised zirconia (Ni-YSZ) anode, in order to understand microstructural degradation processes of Solid Oxide Fuel Cells (SOFCs) during long-term operation. Numerical investigations are carried out to analyse the failure mechanisms in detail. For this purpose, finite element (FE) models are generated from focused ion beam-scanning electron microscopy 3D image data, representing the anode microstructure with varying phase compositions. A brittle model and a ductile material model were assigned to the YSZ phase and the nickel phase, respectively. The porosity is found to affect the strength of the microstructure significantly, leading to low compressive strength results. A high Ni content generally increases the toughness of the overall structure. However, the orientation and the geometry of the nickel phase is essential. When the Ni phase is aligned parallel to the loading direction, a supporting effect on the microstructure is observed, resulting in a significant high toughness. On the contrary, a rapid failure of the sample occurs when the Ni phase is oriented perpendicular to the loading direction. Two main failure mechanisms are identified: (i) cracking at the Ni/YSZ interface and (ii) cracking of struts at the location of the smallest diameter.

Degradation Comparison of Cyclic and Linear Siloxane Contamination on Solid Oxide Fuel Cells Ni-YSZ Anode

The solid oxide fuel cell (SOFC) nickel-yttria stabilized zirconia (Ni-YSZ) anode degradation due to different types of siloxane contamination is investigated. A cyclic structure siloxane, octamethylcyclotetrasiloxane (D4), and a linear structure siloxane, decamethyltetrasiloxane (L4), are mixed with H2+N2 as the fuel for SOFCs at 750°C. The electrochemical characterization results after stability experiments suggest that the SOFC contaminated with cyclic siloxane, D4, had higher degradation. Pure YSZ pellets with different surface hydroxylation extents were also tested to investigate the D4/L4 adsorption and deposition process. Postmortem SEM/WDS, XRD and Raman analysis all indicate that cyclic siloxane has more deposition than linear siloxane on the anode. Further analysis demonstrates that high adsorption and low desorption rates of cyclic siloxane on YSZ are linked to the degradation. Besides the silicon deposition, SiC and amorphous carbon deposition were also observed from the XRD and Raman analysis.

Effect of Oxygen Partial Pressure on Reactive-Sputtered Ni-YSZ Anode Properties for Low-Temperature Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is a promising next-generation energy conversion device because of its high efficiency, cleanliness, and fuel flexibility. Recently, SOFC operating at low- to intermediate-temperature (LT to IT) range (< 600 ℃) has been studied to overcome the limitations of conventional SOFC, e.g., thermal stability, cost, and limited application. In lowered temperature range, however, charge transfer and transport rates slow down; therefore, SOFCs design with thin film stacks, namely thin film SOFC (TF-SOFC) fabricated on porous anode support have been usually adopted for sufficient power density even at this temperature regime.TF-SOFC usually uses Ni-YSZ (nickel-yttria-stabilized zirconia) cermet for anode functional layer (AFL) to accommodate the deposition of dense electrolyte layers on anode support with relatively large pores because of its reasonable cost, catalytic activity, and electrical conductivity. While various thin film techniques have been reported for AFL fabrication, e.g., pulsed layer deposition (PLD), RF sputtering, etc. Among such techniques, reactive sputtering method, which uses metal target in controlled gas environment (e.g., oxygen environment for oxide deposition), is a versatile method to deposit various types of thin films with fast speed (up to > 1um/hr), which is beneficial in manufacturabilily regarding the usual thickness range of AFL (a few microns). Furthermore, the reactive co-sputtering method can control anode properties such as composition ratio, deposition speed, and morphology in wide ranges.In this study, we fabricate reactive sputtered Ni-YSZ cermet anodes, as prototypes for AFLs of TF-SOFCs, by using a single metal target with various oxygen partial pressure conditions, and characterize their physical and chemical properties. Oxygen partial pressure significantly affects deposition speed, surface morphology, composition, electrochemical performance and its thermal durability at 450 ℃. The Ni-YSZ anode sputtered at the environment of O2/Ar partial pressure ratio of 0.1 sample shows the lowest activation resistance when applied to LT-SOFC’s anode. It is also shown that excessive O2/Ar partial pressure ratio (> 0.2) significantly reduces the deposition speed by approximately one order of magnitude compare to the lower ratio case.

Investigating the Solid Oxide Fuel Cell Anode Degradation Under Siloxane Contamination

Siloxane, a typical contaminant component in biogas from wastewater treatment plants and landfills, can negatively affect prime movers such as solid oxide fuel cells (SOFCs) and engines due to its deposition. In order to investigate the solid oxide fuel cell nickel-yttria stabilized zirconia (Ni-YSZ) anode degradation mechanism because of siloxane contamination, three different experimental methods were conducted. First, fuels with different components were utilized for SOFCs with ppmv level siloxane. The degradation rates among different experiments were analyzed and compared to assess the influence of reactants on the siloxane degradation process. Then, Pure YSZ and Ni pellets were utilized to investigate their roles in siloxane deposition process individually. The pellet experiments can also reduce the complexity of deposition composition analysis by various techniques used in this study such as scanning electron microscopy, wavelength-dispersive X-Ray spectroscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy and Raman spectroscopy. Acceleration degradation experiments for SOFCs were accomplished with percentage level siloxane contamination. In this case, the exhaust gas for the siloxane deposition process was analyzed by gas chromatography (GC) for gas phase product identification. After considering the experimental results from the three methods mentioned before, a new anode degradation mechanism is proposed. The chemical adsorption process of siloxanes on the anode is hypothesized as an essential process in deposition reactions. Beside silicon deposition, carbon deposition is also assessed in this study, and it is also considered to play a critical role in SOFCs’ degradation process.

Correlation between microstructures and macroscopic properties of nickel/yttria-stabilized zirconia (Ni-YSZ) anodes: Meso-scale modeling and deep learning with convolutional neural networks

A deep learning based homogenization framework is proposed to link the microstructures of porous nickel/yttria-stabilized zirconia anodes in solid oxide fuel cells (SOFCs) to their effective macroscopic properties. A variety of microstructures are generated by the discrete element method and the meso‑scale kinetic Monte Carlo method. Then, the finite element method and the homogenization theory are used to calculate the effective elastic modulus (E), Poisson's ratio (υ), shear modulus (G) and coefficient of thermal expansion (CTE) of representative volume elements. In addition, the triple-phase boundary length density (LTPB) is also calculated. The convolutional neural network (CNN) based deep learning model is trained to find the potential relationship between the microstructures and the five effective macroscopic properties. The comparison between the ground truth and the predicted values of the new samples proves that the CNN model has an excellent predictive performance. This indicates that the CNN model could be used as an effective alternative to numerical simulations and homogenization because of its accurate and rapid prediction performance. Hence the deep learning-based homogenization framework could potentially accelerate the continuum modeling of SOFCs for microstructure optimization.