Nickel Oxide-yttria Stabilized Zirconia Research Articles

(Digital Presentation) Phase-Field Modeling of the Reduction Process in Solid Oxide Fuel Cell Anode

It is well-known that the reduction process of nickel oxide-yttria-stabilized zirconia (NiO-YSZ) composite anode influences the initial performances and durability of solid oxide fuel cell. In this work, phase field method is used to simulate the reduction process of nickel oxide (NiO) in anode-supporting solid oxide fuel cell (SOFC), which captures the competition effects among diffusion, crystal phase transition of NiO to Ni, and sintering. A real three-dimensional microstructure of NiO-YSZ composite anode reconstructed using focused ion beam-scanning electron microscopy (FIB-SEM) technique is used to initialize the simulation. The simulation results are qualitatively compared to experimental results under different reduction conditions. The results reveal that the stress distribution in SOFC anode can be largely influenced by the interactions between the residual stress in yttria-stabilized zirconia (YSZ) and the reduction process. The results explain the phenomena of mechanical damage formed in SOFC single cell during reduction. Figure 1

Kinetic Monte Carlo (KMC) simulation of sintering of nickel oxide-yttria stabilized zirconia composites: Model, parameter calibration and validation

This study proposes a kinetic Monte Carlo (KMC) model for the sintering of two-phase composites using nickel oxide-yttria stabilized zirconia (NiO-YSZ) as a case study. An artificial neural network (ANN) assisted method is used to calibrate the KMC model parameters by comparing the simulated microstructures and the real microstructure reconstructed by focused ion beam scanning electron microscopy (FIB-SEM). It is demonstrated that the ANN calibrated KMC model can predict quantitatively the microstructure evolution of NiO-YSZ composite. The microstructural parameters such as volume fraction, specific surface area, and triple-phase boundary length density of sintered NiO-YSZ are well predicted within acceptable errors. The proposed framework can be adapted for the simulation of the microstructure evolution of any other composite electrodes for solid oxide fuel cells and other composites fabricated by sintering.

Tailoring the solid oxide fuel cell anode support composition and microstructure for low‐temperature applications

AbstractIn this research, the performance of a tubular fuel cell based on a nickel oxide–yttria‐stabilized zirconia (Ni‐YSZ) anode support containing 90 wt% NiO ≈ 82 vol.% of Ni (Ni82) is compared with a cell containing the conventional Ni‐YSZ support with 50 vol.% Ni. A Ni‐YSZ buffer layer with a tailored microstructure was added to the Ni82 support layer to provide intermediate porosity and to reduce the thermal expansion mismatch with the anode functional layer. Both cells were tested using infiltrated Nd2NiO4+δ cathodes. High peak power densities of 790 and 478 mW/cm2 were achieved at 600 and 550°C, respectively, for the Ni82 cell which was 25% and 87% higher than the performances for the conventional cell at respective temperatures. In addition, no degradation was found during four redox cycles at 550°C, making this support an attractive candidate for low‐temperature solid oxide fuel cell applications.

Fabrication of Solid Oxide Electrolysis Single Cell Using NiO-YSZ/YSZ/LSM-YSZ via Drop-Coating Method

Solid oxide electrolysis cell (SOEC) is a highly efficient and environmentally friendly technology for future hydrogen generation. In this study, electrolyte-supported SOEC single cell was fabricated via a simple and facile drop-coating technique. Thin film electrodes of nickel oxide/yttria stabilized zirconia (NiO-YSZ) cathode and strontium-doped lanthanum manganite/ytrria-stabilized zirconia (LSM-YSZ) anode were deposited onto yttria-stabilized zirconia (YSZ) solid electrolyte substrate. Scanning electron microscopy (SEM) with energy dispersive analysis (EDS) was used to study the microstructural properties of the heat-treated samples and revealed a successful thin film deposition of porous electrodes onto the dense YSZ substrate. XRD patterns showed the desired crystal structure of the deposited electrode thin films. Distinct phases of cubic YSZ and monoclinic LSM were observed for the LSM-YSZ anode while cubic NiO and YSZ phases were observed for the deposited cathode. Electrochemical conductivity of the cell was investigated using electrochemical impedance spectroscopy analysis (EIS) which revealed a total conductivity of about 2.0 mS/cm at 700 °C.

Pore Former Addition in the Preparation of Highly Porous Anode Using Phase-inversion Technique for Solid Oxide Fuel Cell

A detailed study on the anode structure of solid oxide fuel cell (SOFC) is very crucial in developing high performance power generating devices, given how porous electrodes are known to provide a number of active sites for reaction, consequently accelerate fuel conversion. This present study investigates the feasibility of pore former addition to nickel oxideyttria stabilized zirconia (NiO-YSZ) anode, fabricated via phase inversion-based casting and sintering technique. The loading of two types of pore former (i.e. polyetheretherketone [PEEK] and corn starch) was varied from 0 to 10 wt% of total suspension. The effects of pore former loading and its types were examined based on anode morphology using scanning electron microscopy (SEM), crystal phase by X-ray diffraction (XRD), apparent porosity based on standard ASTM C373-88, and bending strength using three-point bending test. Results had shown that higher loading of pore former increased the porosity, yet significantly reduced the bending strength. SEM images of anode displayed that by generating more connected open pores through the addition of 2 wt.% PEEK, it produced an effective porous structure. In fact, the sample recorded to be 67 % mechanically stronger compared to the best loading which is corn starch (4 wt.%). Based on these findings, PEEK can be used as pore former in anode fabrication that creates appropriate porosity, which benefits gas diffusion and also provides good mechanical strength as an anode support.

Effect of magnetron sputtered anode functional layer on the anode-supported solid oxide fuel cell performance

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) thin films were reactively sputter-deposited by pulsed direct current magnetron sputtering from the Ni and ZrY targets onto heated commercial NiO-YSZ substrates. The microstructure and composition of the deposited films were investigated with regard to application as thin anode functional layers (AFLs) for solid oxide fuel cells (SOFCs). The pore size, microstructure and phase composition of both as-deposited and annealed at 1200 °C for 2 h AFLs were studied by scanning electron microscopy and X-ray diffractometry and controlled by changing the deposition process parameters. The results show that annealing in air at 1200 °C is required to improve structural homogeneity of the films. NiO-YSZ films have pores and grains of several hundred nanometers in size after reduction in hydrogen. Adhesion of deposited films was evaluated by scratch test. Anode-supported solid oxide fuel cells with the magnetron sputtered anode functional layer, YSZ electrolyte and La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O2 (LSCF/CGO) cathode were fabricated and tested. Influence of thin anode functional layer on performance of anode-supported SOFCs was studied. It was shown that electrochemical properties of the single fuel cells depend on the NiO volume content in the NiO-YSZ anode functional layer. Microstructural changes of NiO-YSZ layers after nickel reduction-oxidation (redox) cycling were studied. After nine redox cycles at 750 °C in partial oxidation conditions, the cell with the anode NiO-YSZ layer showed stable open circuit voltage values with the power density decrease by 11% only.

Joining of solid oxide fuel/electrolysis cells at low temperature: A novel method to obtain high strength seals already at 300 °C

A novel method using a combination of surface three-dimensional structuring, silver nanoparticles and a silver-foam interlayer is developed for joining solid oxide fuel/electrolysis cell components at low temperature. Joints with a high mechanical strength of 24 MPa are obtained with this method already at 300 °C. The interfaces of a sealed assembly comprising ferritic stainless steel interconnect and nickel oxide-yttria stabilized zirconia (NiO-YSZ) support are analyzed by scanning electron microscopy and transmission electron microscopy to study the interfacial sintering mechanism. It is shown that the high strength of the joint is due to a combination of an optimized three-dimensional nickel/gold nanosheet array deposited on the substrates which facilitates mechanical interlocking, and a silver-foam interlayer which enhances the resistance to crack propagation. The long-term stability of the joint is evaluated by aging in a reducing atmosphere at 800 °C for 250 h. No defects indicating a possible failure are observed in the joint after this aging. An oxide layer forms along the silver/steel interface and parts of the steel are transformed to austenite due to nickel diffusion from the nanosheet array, but this does not deteriorate the joint stability.

Reduction kinetics of undoped and aluminum titanate (Al2TiO5) doped NiO-YSZ solid oxide fuel cell anodes

Undoped and 1–10 wt% aluminum titanate, Al2TiO5, (ALT) doped nickel oxide/yttria-stabilized zirconia (NiO/YSZ) anode materials were kinetically evaluated during isothermal (800 °C) H2 reduction (5% H2-95% N2 and pure H2 gas). It was found that the two parameter Avrami-Erofe’ev model (AEn) was the most successful in representing reduction kinetics. The reduction mechanisms have been identified as the formation and growth of nuclei and is maintained after aluminum titanate, Al2TiO5, (ALT) doping and when the H2 concentration in the gas flow is increased. ALT doped samples result in the formation of NiAl2O4; which, when present in sufficient amounts, slowly reduces causing a shift of the model from the experimental data. This phenomenon has been correlated, through microstructural evaluation, to the formation of Ni nanoparticles. In all investigated cases, the reduction kinetics of the system was accelerated when the H2 concentration was increased from 5% to 100%.

Characterisation of NiO-YSZ Porous Anode-Support for Solid Oxide Fuel Cells Fabricated by Ceramic Injection Moulding

Ceramic injection moulding (CIM) has advantages for a cost effective fabrication of large-scale, near-net-shape products. In this work, CIM is carried out to prepare porous anode-support for solid oxide fuel cells (SOFC) applications. The CIM process started with a preparation of feedstocks by mixing powder with binder. The feedstock is then injected into the mould of desired shapes. The mouldings were subsequently undergo the removal of the binder (debinding) and, finally, sintering. It is shown that porous nickel oxide-yttria stabilized zirconia (NiO-YSZ) anode-support for SOFC were successfully prepared by CIM technique. In addition, a water-soluble based binder system, consisted mainly of polyethylene glycol (PEG), has been used in this work. This is to avoid the use of organic solvents when wax-based binder was used. Therefore, it can promote more environmentally friendly process. The removal of binder was carried out using water debinding technique. The porous anode for SOFC was subjected to systematic characterisation. The effect of processing parameters, such as powder characteristics and powder/binder ratio has been investigated. Rate of binder removal was also studied. The porous anode specimens were characterised for their properties and microstructure. It was also found that the porosity of the specimens can be controlled by adjusting the sintering temperatures and holding times.

Fabrication and Electrochemical Characterization of Freeze-Cast Tubular Solid Oxide Fuel Cells

NASA and others have shown improved power density of planar solid oxide fuel cells (P-SOFCs) fabricated via the freeze casting technique. As a manufacturing process, freeze casting is capable of producing ceramic, metal, and cermet components with unique and tailorable microstructural features that enhance SOFC performance. Low thermo-cyclic durability and slow startup times are common problems for P-SOFCs and have similarly limited the development of freeze cast P-SOFCs. Tubular SOFCs (T-SOFCs) have demonstrated superior thermal cycling capability and relatively fast startup times but have been reported to have lower volumetric power density compared to P-SOFCs. The utilization of freeze casting for fabrication of tubular geometries is suggested as a solution to significantly increase the volumetric power density of T-SOFCs by enhancing gas diffusivity and triple phase boundary reactions and to be at par or better than the power densities of state-of-art P-SOFCs. This paper reports the fabrication and characterization of freeze cast tubular anode supports for SOFCs. NiO-8YSZ (nickel oxide yttria-stabilized zirconia) anode supports were fabricated using three methods: gelation casting, center pin method, and a freeze and drain method. Slurry rheology was modified for vertical dip coating, and a dual purpose freezing and drying chamber was designed and manufactured. The effects of slurry properties and process parameters were studied with respect to the resulting microstructures. The freeze and drain method was determined to be optimum for producing highly porous tubular anode supports with hierarchical, acicular or dendritic micro-pore channels. With an optimized freeze-drying cycle, drying time was reduced significantly with minimal residual moisture in the green bodies. Additionally, the electrochemical performance of the T-SOFCs with freeze cast anode was evaluated. Figure 1

(Invited) Numerical Simulation of Sintering Process in Solid Oxide Fuel Cell Anode

Solid oxide fuel cell (SOFC) is expected as a promising energy conversion device because of its high efficiency and fuel flexibility. It is widely known that electrode microstructures are of great importance for attaining high performance as well as high reliability of SOFCs. In the SOFC fabrication processes, sintering of sub-micron scale ceramic and metallic powders is one of the most important step to achieve efficient and reliable electrodes. In addition, microstructural changes in SOFC anodes associated with particle sintering during high temperature operation may lead to severe performance degradation. Thus, in order to develop more reliable electrodes of SOFC, a method that can offer quantitative information of sintering behavior at the sub-micron scale is highly demanded.Accurate prediction of microstructure evolution during sintering, even in single-phase systems, is still very challenging. This is because sintering behavior is deeply affected by the three-dimensional (3D) complex geometries of the powder compacts, such as grain and pore size distributions, their shapes, relative density and coordination number, etc. Among many types of numerical approaches, a mesoscale kinetic Potts Monte Carlo (KMC) method has the potential to overcome this difficulty [1]. This method is capable of capturing three dimensional microstructural evolution from the initial to the final stages of solid-state sintering. However, the underlying kinetics at the sub-micron scale are not yet completely understood. Validation of the results using quantitative 3D microstructure data is indispensable to develop a numerical simulation tool which can reproduce proper sintering trajectory of real powder sintering. In the present study, KMC simulations of nickel (Ni) and nickel oxide - yttria stabilized zirconia (NiO-YSZ) composite are carried out starting from the green powder microstructure obtained using focused ion beam - scanning electron microscopy (FIB–SEM). Quantitative topological changes during sintering of real powder compacts are obtained by FIB–SEM. This technique offers a great advantage because of its high resolution, which is sufficient to capture the 3D geometric nature of sub-micron-sized powder compacts (typically < 1/10 of the powder size). The simulated temporal trajectories of microstructural parameters are compared with the experimental data from FIB-SEM to verify the numerical modeling results. [1] Hara, S., Ohi, A. and Shikazono, N., Sintering Analysis of Sub-Micron-Sized Nickel Powders: Kinetic Monte Carlo Simulation Verified by FIB-SEM Reconstruction, J. Power Sources, 276, pp. 105-112 (2015).

Effects of reduction process on the electrochemical and microstructural properties for electrolyte-supported SOFC

Reduction process plays an important role before solid oxide fuel cells (SOFCs) start to work. The reduced anode must have enough porosity for transferring the fuel gases and good conductivity for decreasing the area specific resistance. To investigate how the reduction process affects the electrochemical and microstructural properties for electrolyte-supported SOFC with a nickel oxide/yttria-stabilized zirconia (NiO/YSZ) anode, three different reduction processes are applied to the same batch of cells in this paper. Process I for one of tested cells is to introduce a dilute gas of 20% hydrogen and 80% nitrogen to the anode side at room temperature. Process II and III for the others are to introduce the same dilute gas while the cells are heated up to 800 and 700 °C respectively. Although the Process I provides much time to reduce the anode of the cell, the resulting performance is lower than the others. AC impedance analyses show that the cell reduced at higher temperature has lower ohmic and polarization resistance. Moreover, the differences of nickel connection and particle size on the anode are observed in microstructure of the cells, which proves that reduction process has a vital effect for solid oxide fuel cells.

Nanocrystalline La0.84Sr0.16MnO3 and NiO-YSZ by Combustion of Metal Nitrate-Citric Acid/Glycine Gel – Phase Evolution and Powder Characteristics

Phase evolution behaviour of lanthanum strontium manganite (LSM) and nickel oxide-yttria stabilized zirconia (NiO-YSZ) composite from the respective oven dried fuel – metal nitrate gels (citrate-metal nitrate for LSM and glycine-metal nitrate for NiO-YSZ) was studied using TG-DTA, XRD and FTIR. The TG-DTA revealed sharp exotherms around 200° and 221°C attributed to the setting in of combustion of gels of LSM and NiO-YSZ respectively. The XRD results indicated formation of nanocrystalline LSM and NiO-YSZ phases above 200° and 221°C respectively. The FTIR spectra of LSM and NiO-YSZ gels heated above 200° and 221°C exhibited characteristic absorption bands corresponding to metal-oxygen vibrations in LSM and NiO-YSZ indicating formation of the respective compounds. However, chemical and phase pure LSM and NiO formed above 700°C indicate the minimum calcination temperature to be used in the processing. The role of calcination treatment on powder characteristics in the temperature range of 700° to 1350°C was studied. With increasing calcination temperature of the as-formed powders, the characteristics such as crystallite size, median particle size (D50), green density of the compacts increased while specific surface area decreased. Typical sintering studies at 1400° and 1450°C revealed that sinterability and per cent linear shrinkage decreased with increasing calcination temperature. However, the required amount of porosity for SOFC application could be obtained with incorporation of a pore former in case of LSM and varying the sintering time in case of NiO-YSZ composite.

Electrochemical characteristics and performance of anode-supported SOFCs fabricated using carbon microspheres as a pore-former

This paper evaluates the influence of carbon microspheres (CMSs) as an electrode pore-former on the fabrication and electrochemical properties of the anode-supported solid oxide fuel cells (SOFCs). The anode supports are fabricated by dry-pressing of CMS and NiO/YSZ (nickel-oxide/yttria-stabilized zirconia) composite powder, and the YSZ electrolyte layer is prepared by the electrophoretic deposition technique. The ohmic and polarization resistances for NiO/YSZ–YSZ half cells at different testing temperatures (650–850 °C) are analyzed by electrochemical impedance spectroscopy (EIS). The polarization ASR (area specific resistance) for the fabricated half cells increases from 0.583 Ω cm2 to 3.047 Ω cm2 when the temperature decreases from 850 °C to 650 °C. The electrochemical performance of single cells is measured at different temperatures (700–850 °C) and the results indicate that the cells fabricated using CMS as the pore-former exhibit much higher electrochemical performance than those without using CMS. A maximum power density of 207.7 mW cm−2, 431.2 mW cm−2, and 571.6 mW cm−2 is recorded at 850 °C for the cells fabricated by adding 0 wt. %, 2.5 wt. % and 5 wt. % of CMS, respectively. The maximum fuel utilization efficiency is also found to increase from 26.5% for the cell prepared without CMS to 47.0% and 59.6% for the cells prepared with 2.5 wt. % and 5 wt. % of CMS, respectively. The increase in the electrochemical performance by adding CMS as pore-former to anode-supports is attributed to higher porosity and pore size of the electrode.

Modeling the Influence of Carbon Spheres on the Porosity of SOFC Anode Materials

The influence of fabrication pressure and other ceramic processing variables, including volumetric loading fraction and the particle size ratio of pore-forming agents, on the porosity of fabricated ceramic anodes, was investigated using an integrated experimental approach with mathematical modeling to differentiate the impacts of each parameter. Despite historic observation of the properties of ceramic bodies, to date, there is a lack of available models to accurately interpret the ceramic properties as a function of the processing variables. Herein, we focus on the open porosity of the solid oxide fuel cell anode prepared from NiO/YSZ (nickel oxide/yttria stabilized zirconia) as the ceramic powder and using CMS (carbon microspheres) as a pore-forming agent. A range of pore-former volumetric blend ratios (4.4%–44.6%) and different particle size ratios between NiO/YSZ and CMS (11.27, 4.29, and 0.26) were used, whereas the influence of the applied uni-axial fabrication pressure on the open porosity of sintered anode was studied in a range of atmospheric pressure up to 40 MPa. A good agreement was observed between the proposed model and the experimental data, implying that the approach could be used to determine the significant processing parameters to fabricate ceramics with desired porosity. The model could also be used to interpret the physical basis of pore formation when using a pore-forming agent in a fabricated porous ceramic body.

A new empirical viscosity model for ceramic suspensions

This paper presents a new predictive viscosity model for ceramic suspensions. Starting from Einstein's model (1906), various theoretical, empirical, and phenomenological models have been proposed for different suspension systems. However, there is still a lack of reliable model for ceramic suspensions used in colloidal ceramic shape-forming methods. Here, the rheological properties of ceramic suspensions comprising NiO/YSZ (nickel oxide/yttria stabilized zirconia) as the ceramic powder, and furfuryl alcohol as the suspending media were measured over a range of shear-rates (between 1 and 1000 s −1) and different solid volume fractions from 0 to 0.4010. An empirical equation was then developed for the ceramic suspensions using the mobility parameter ( ϕ/( ϕ m − ϕ)), which links Einstein's model with the more recent relative viscosity models. The proposed model was used to predict the relative viscosity data, showing excellent agreement to the experimental data from this study and with reported data in literature for other ceramic systems. The model was also used to estimate the maximum solid volume fraction for the ceramic suspensions ( ϕ m =0.571), with better accuracy than those estimated by existing models.

Imaging Chemical Aggregation of Ni/NiO Particles from Reduced NiO-YSZ

Energy dispersive X-ray spectroscopy (EDS) mapping of nickel oxide yttria-stabilized zirconia (NiO-YSZ) was carried out after various hydrogen reducing and methane steam reforming conditions. Nickel aggregation was visualized after methane steam reforming by correlating Ni K{sub {alpha}} map with scanning transmission electron microscopy (STEM) images. From the reduced O K{sub {alpha}} intensities in the Ni K{sub {alpha}} dominated regions after methane steam reforming, NiO reduction in to Ni can be interpreted. From correlation between Zr K{sub {alpha}} and O K{sub {alpha}} maps, high stability of YSZ was also realized. Examples of NiO-YSZ overlapped particles are considered to discuss chemical imaging of a single particle.

Synthesis of Nickel Oxide - Zirconia Composites by Coprecipitation Route Followed by Hydrothermal Treatment

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) for use as solid oxide fuel cell anode were synthesized by coprecipitation to obtain amorphous zirconia and crystallized β-nickel gels of the corresponding metal hydroxides. Hydrothermal treatment at 200°C and 220 psi from 2 up to 16 hours, under stirring, was performed to produce nanocrystalline powder. The as-synthesized powders were uniaxially pressed and sintered in air. Powders were characterized by X-ray diffraction, laser scattering, scanning and transmission electron microscopy (SEM/TEM), gas adsorption technique (BET) and TG-DTA thermal analysis. Ceramic samples were characterized by dilatometric analysis and density measurements by Archimedes method. The characteristics of hydrothermally synthesized powders and compacts were compared to those produced without temperature and pressure application. Crystalline powders were obtained after hydrothermal process, excluding the calcination step from this route. The specific surface area of powders decreases with increasing time of hydrothermal treatment while the agglomerate mean size is not affected by this parameter.

Microwave processing: A potential technique for preparing NiO-YSZ composite and Ni-YSZ cermet

In the present study, microwave energy (2·45 GHz) has been used to prepare nickel oxide-yttria stabilized zirconia (NiO-YSZ) composites of composition, mNiO-(1 − m) Zr0·9Y0·1O1·95 (m = 0·2, 0·3, 0·4, 0·5 and 0·6), from a precursor obtained by mixing NiO, Y2O3 and monoclinic ZrO2 in their stoichiometric ratio. The composites have been prepared by conventional processing also to compare the products with those of microwave processed products. During comparison, it was observed that NiO-YSZ composites of each composition obtained by microwave processing had cubic phase of YSZ while in the conventionally prepared composites of compositions, m = 0·2 and 0·3, monoclinic, tetragonal and cubic phases of zirconia existed instead of its pure cubic phase. The composites were reduced to yield Ni-YSZ.

Combustion Synthesis of NiO/YSZ Composite

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) anode materials were synthesized by combustion process involving metal nitrate-urea decomposition. The precursor was heated to evaporate water undergoing dehydration, and then the mixtures froth and swell. Further heating produces large amounts of gases that ignite at ambient atmosphere. Different nitrate/urea initial molar ratios were employed in order to investigate the influence of the reaction temperature in the physical characteristics of the powder composite. The flame temperature was measured by infrared optical pyrometer. The combustion was rapid and self-sustaining, with flame temperatures ranging from 870 to 1330°C. The as-synthesized powders were uniaxially pressed and sintered in air. Powders were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), helium picnometry for density measurements and gas adsorption technique (BET). The ceramic samples were evaluated by SEM and Archimedes density measurements.