Nickel-yttria Stabilized Zirconia Anode Research Articles

Modeling and Analysis of Polarization Losses in Solid Oxide Fuel Cells with Linear Siloxane Contamination

In this study, the degradation of solid-oxide fuel cells (SOFC) nickel-yttria stabilized zirconia anode under siloxane contamination is examined. The experiments were conducted inside a furnace heated to 800°C with an Ni-YSZ-supported (Nickel-yttria-stabilized zirconia) fuel cell. A fuel source with a flow rate of 20 mL/min of hydrogen gas (66.3%), 10 mL/min of nitrogen gas (33.2%) and 0.15 mL/min (0.5%) of L4 siloxane was fed to the anode. Air was supplied to the cathode. Polarization losses are recorded at intervals of 30 minutes over a period of 3 hours. The polarization losses (activation, ohmic, and mass transport) are modeled with the parameters α and δ. The model is fitted to the experimental polarization curve, to understand how the siloxane degrades the SOFC performance with time. Ohmic losses are dominant in the anode due to much higher anode thickness (380 μm) compared to electrolyte thickness (13 μm). Activation losses vary with α, which decreases from 0.31 to 0.25, significantly lower than the accepted value of 0.5 found in existing literature. The current density is limited by mass transport losses at higher current densities (limiting current density j). Mass transport losses vary with both δ and α. The value of δ, which represents the diffusion layer thickness, varies from 45 μm to 66 μm. The results of the model indicate that the total polarization losses increase for each subsequent time interval, with a total increase of approximately 45% from 0 to 180 min. Activation loss dominates the polarization losses initially, but decrease over time while mass transport losses increase. With current density being constant at 500 mA/cm2, the activation losses decrease from 58% to 55%, ohmic losses decrease from 28% to 20% and mass transport losses increase from 14% to 21% of the total polarization loss over a period of 180 mins.

Modeling and Analysis of Polarization Losses in Solid Oxide Fuel Cells with Linear Siloxane Contamination

In this study, the degradation of the solid-oxide fuel cell (SOFC) nickel-yttria stabilized zirconia anode under decamethyltetra-siloxane (L4) contamination is examined. A model is developed for the polarization losses based on the charge transfer coefficient, α, and diffusion layer thickness, į, and fitted to the experimental data to understand how the siloxane degrades the SOFC performance with time. Ohmic losses are dominant in the anode due to much higher anode thickness (380 µm) compared to electrolyte thickness (13 µm). Activation losses vary with α, which decreases from 0.31 to 0.25. Mass transport losses vary with both į and α, with įincreasing from 28 µm to 45 µm. The results of the model indicate that the total polarization losses increase for each subsequent time interval, with a total increase of approximately 45% from 0 to 180 min. Activation losses dominate the polarization losses initially but decrease in their total contribution while mass transport losses increase.

(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.

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.

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.

Methane steam reforming reaction in solid oxide fuel cells: Influence of electrochemical reaction and anode thickness

The influence of operation temperature, inlet gas composition, current density and the anode thickness on the methane steam reforming reaction over nickel yttria-stabilized zirconia anodes was experimentally studied in solid oxide fuel cells. The experimental results were analyzed using data fitting in Power-Law and Langmuir–Hinshelwood kinetic models. Similar trends of dependence of methane and steam partial pressures were observed in both models. The methane reaction order is positive. Negative influence of steam partial pressure on the methane steam reforming reaction rate are found. The electrochemical reaction and anode thickness affect the reforming kinetics parameters. The anodes thickness shows particular influences on the steam reaction order, and the activation energy when a current is produced. The model evaluation suggests that the two models are comparable and the extra parameters within the Langmuir–Hinshelwood kinetic model are contributing to the lower mean absolute percentage error and higher coefficient of determination R 2 . • Current promotes the methane steam reforming reaction rate. • Anode thickness influences the fitted activation energy. • More experimental data is helpful in getting a reliable rate expression. • Importance of studying effect of mass-transfer on the global MSR kinetics.

Siloxane Deposition on the Ni-YSZ Solid Oxide Fuel Cell Anode Exposed to Bio-Syngas

Siloxane, a common contaminant present in biogas, is known for adverse effects on cogeneration prime movers. In this study, the siloxane deposition products and mechanism in the solid oxide fuel cell nickel-yttria stabilized zirconia (Ni-YSZ) anode are investigated analytically and experimentally. An SOFC with Ni-YSZ anode and pure Ni/YSZ pellets were exposed to a simulated biogas-reformate fuel with octamethylcyclotetrasiloxane (D4) contamination at 750 °C. The electrochemical characterization results show that the SOFCs performance degradation caused by D4 contamination is irreversible. Morphology and XRD results illustrate that silicon and carbon deposition can both be detected in the anode and pellets. Graphite, SiC and SiO2 are all possible products based on the results of XRD test. According to the formation of graphite and SiC, the new mechanism suggests that carbon is also an essential factor in siloxane contamination of Ni-YSZ anodes besides silicon, which can be explained by the catalytic and electrochemical analysis.

Investigating the degradation mechanism of the solid oxide fuel cell nickel-yttria stabilized zirconia anode under siloxane contamination

In order to investigate the solid oxide fuel cell nickel-yttria stabilized zirconia (Ni-YSZ) anode degradation mechanism due to poisoning by Octamethylcyclotetrasiloxane (D4), four different experiments utilizing H2+H2O, H2+H2O+D4, H2+D4 and H2+CO+D4 as fuels at 750 °C are conducted in this study. The electrochemical characterization and morphology results are analyzed and compared for anode degradation phenomenon. The results contradict the previously proposed degradation mechanism as the experimental results show that water can inhibit the silicon deposition and anode degradation. Based on the experimental results in this study and previous studies about siloxane deposition on metal oxides, a new anode degradation mechanism is proposed. The proposed degradation mechanism for siloxane is due to siloxane adsorption resulting in silicon deposition and carbon deposition after siloxane decomposition.

(Invited) Building Resilience into Ni-YSZ Anodes with Secondary Phases

Solid oxide fuel cells anodes are typically composed of a porous nickel-yttria stabilized zirconia (Ni-YSZ) ceramic metallic (cermet) composite. The advantages of a Ni-YSZ anode are many: the materials are inexpensive, Ni is an exceptionally effective CH bond activation catalyst and these electrodes can operate with a wide variety of fuels. However, Ni-YSZ anodes are also susceptible to material degradation and deteriorating device performance, especially when subjected to repeated reduction/oxidation cycling and/or carbon accumulation and metal dusting. Recent work in our laboratory has sought to improve the durability and resilience of Ni-YSZ anodes by adding small amounts of secondary materials to the anode stock prior to fabrication and processing. One secondary material that has proven particularly effective is aluminum titanate (Al2TiO5or ALT) During anode processing, ALT (4% by mass) reacts with Ni-YSZ to form nickel aluminate (NiAl2O4) and zirconium titanate (Zr5Ti7O24). Upon anode reduction, NiAl2O4 forms Ni particles decorated with Al2O3 nanoparticles. These secondary phases improve device performance and appear to preserve anode microstructure even after 100 hours of continuous operation. Furthermore, these secondary phases enhance anode resilience to redox cycling. We have used a variety of electrochemical and operandooptical methods to test the resilience of ALT-enhanced anodes subject to both electrochemically-induced oxidative stress and environmentally driven anode oxidation. In both cases, ALT-enhanced anodes can withstand more than twice as many redox cycles as anodes that have not had ALT added. In addition, adding ALT to the anode reduces its susceptibility to carbon accumulation when operating with hydrocarbon fuels.

Improving sulfur tolerance of Ni-YSZ anodes of solid oxide fuel cells by optimization of microstructure and operating conditions

In this study, we propose an improvement in the sulfur tolerance of nickel-yttria stabilized zirconia (Ni-YSZ) anodes for solid oxide fuel cells (SOFCs) by simultaneously employing optimized operating conditions and microstructural modifications. An electrolyte supported SOFC is operated at 20 ppm H2S impurity at 750 °C for 20 h degradation and 10 h recovery test. The current cycles with a higher amplitude and small pulse time during the constant current operation are beneficial for the mitigation of sulfur poisoning. The effect of humidity on the sulfur degradation of Ni-YSZ anode is also studied. The synergetic effect of microstructure modification and current cycling conditions improves the sulfur tolerance of Ni-YSZ anode. It has been found that, when an anode with a modified microstructure by infiltrated CeO2 and Yb2O3 nanoparticles is operated on 20 ppm H2S poisoned gas at 10% relative humidity and the optimum pulsed current cycling conditions, about 7 times less degradation of the SOFC performance is observed. This study shows that at lower H2S concentration, a stable operation of a SOFC with minimum degradation can be achieved with the combination of optimization of operating conditions and modification of the anode microstructure.

Effect of nano-Al2O3 addition on mechanical durability of nickel-yttria stabilized zirconia anode support of solid oxide fuel cells

For anode supported solid oxide fuel cells (SOFCs), the strength and durability of the porous cermet support during long-term operation is critical for the reliability of the cells and stacks. The effect of adding nano-Al2O3 on the long-term durability of the mechanical strength of nickel-yttria stabilized zirconia (Ni-YSZ) based SOFC anode supports was investigated in this study. The SOFC anode support materials, Ni-3YSZ (3 mol% YSZ), Ni-8YSZ (8 mol% YSZ), and nano-Al2O3 added Ni-YSZ composites, were subjected to a long-term degradation test at 900 °C and 30% humidity in an H2 environment and after 1000 h exposure, the flexural strength, phase, and microstructural changes were analyzed. The results suggest that the addition of nano-Al2O3 to Ni-3YSZ improves the long-term strength and durability of the cermet while its addition to Ni-8YSZ cermet did not show any significant changes in the strength degradation behavior of the Ni-8YSZ based anode support. Further analysis by SEM and XRD shows that the tetragonal to monoclinic phase transformation in partially stabilized zirconia was suppressed due to the addition of nano-Al2O3 in Ni-3YSZ resulting in improved long-term stability and higher mechanical strength. Replacing the Ni-8YSZ anode support with nano-Al2O3 added Ni-3YSZ might significantly reduce the material cost of SOFCs without compromising long-term strength durability, thus bringing SOFCs a step closer to commercialization.

Three-dimensional thermal stress analysis of the re-oxidized Ni-YSZ anode functional layer in solid oxide fuel cells

Nickel-yttria-stabilized zirconia (Ni-YSZ) cermet is widely used as an anode material in solid oxide fuel cells (SOFCs); however, Ni re-oxidation causes critical problems due to volume expansion, which causes high thermal stress. We fabricated a Ni-YSZ anode functional layer (AFL), which is an essential component in high-performance SOFCs, and re-oxidized it to investigate the related three-dimensional (3D) microstructural and thermo-mechanical effects. A 3D model of the re-oxidized AFL was generated using focused ion beam-scanning electron microscope (FIB-SEM) tomography. Re-oxidation of the Ni phase caused significant volumetric expansion, which was confirmed via image analysis and calculation of the volume fraction, connectivity, and two-phase boundary density. Finite element analysis (FEA) with simulated heating to 500–900 °C confirmed that the thermal stress in re-oxidized Ni-YSZ is concentrated at the boundaries between YSZ and re-oxidized NiO (nickel oxide). NiO is subjected to more stress than YSZ. Stress exceeding the fracture stress of 8 mol% YSZ appears primarily at 800 °C or higher. The stress is also more severe near the electrolyte-anode boundary than in the Ni-YSZ cermet and the YSZ regions. This may be responsible for the electrolyte membrane delamination and fracture that are observed during high-temperature operation.

Investigation of Microstructural Change of Nickel-Yttria Stabilized Zirconia Anode during Oxidation and Reduction Based on Three-Dimensional Reconstruction

Mechanisms of oxidation and reduction processes were investigated based on two dimensional observation by scanning electron microscopy and three-dimensional reconstruction using focused ion beam-scanning electron microscopy (FIB-SEM). Oxidation process was governed by outward cation diffusion, accompanying isolated pore formation. Reduction started from NiO/pore and NiO/YSZ interfaces with formation of pores inside the reduced Ni phase. The volume expansion during oxidation, pore formations during oxidation and reduction, and sintering during reduction resulted in thin and elongated Ni particle shape. Enhancement of Ni connectivity and densification of anode were promoted during the oxidation process.

Razvoj kristalnih faz in mikrostrukture LSCM perovskitnih materialov za SOFC anode, pripravljenih s karbonatno metodo koprecipitacije

Most SOFC development has been based on nickel yttria-stabilized zirconia anodes. Such materials have excellent catalytic properties for fuel oxidation, high electrical conductivity, good mechanical strength and an appropriate thermal expansion coefficient compatible with other cell components. Unfortunately, cermet anodes based on doped zirconia exhibit some disadvantages, e.g., the catalysing side reaction of carbon deposition during hydro-carbon fuel oxidation and a susceptibility to sulphur poisoning. Perovskite-type compounds based on lanthanum-strontium-manganese-chromium oxide (LSCM) can serve as an alternative material. Since the optimal perovskite composition is still not known, La1–xSrxMnyCr1.yO3± (x from 0 to 0.3 and y from 0.4 to 0.6) ceramics were prepared with the co-precipitation method. Crystalline phase formation was followed by X-ray powder diffraction and Rietveld refinement. Quantitative microstructure analysis of the samples sintered at various temperatures was performed on SEM micrographs using Axiovision 4.8 software.

Evaluation of nickel-yttria stabilized zirconia anode degradation during discharge operation and redox cycles operation by electrochemical calculation

Abstract Degradation of Solid Oxide Fuel Cell (SOFC) anode during discharge operation and redox cycles operation were evaluated by three-dimensional electrochemical calculations using a Lattice Boltzmann method (LBM). Three dimensional microstructures were obtained by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) reconstruction. In the electrochemical calculations, changes in exchange current density and ionic conductivity of Yttria stabilized Zirconia (YSZ) during the operations were assumed and their values were calculated by fitting the calculated overpotential values to the experimental ones. Changes in triple phase boundary density calculated from the reconstructed microstructures were inconsistent with the gradual degradation observed during repeated redox-discharge cycles. Changes of the fitted exchange current density and YSZ ionic conductivity values in both discharge operation and redox cycle operation showed same tendency as the experimental results. Change in exchange current density or YSZ ionic conductivity should be considered as an essential factor which governs the cell performance change regardless of the redox treatment.

A Study on Degradation of Ni-YSZ Anode Materials in Terms of Redox Stability for Solid Oxide Fuel Cells

▪ Introduction Solid oxide fuel cells (SOFCs) have lately attracted considerable attention as the source of new energy considering environmental pollution and exhaustion of underground resources. Nickel-yttria stabilized zirconia (Ni-YSZ) cermet has been used as an anode material for SOFCs for more than 20 years. Although Ni-YSZ has various advantages such as high catalytic activity for H2 oxidation and methane reforming, stability in reducing atmosphere, high electrical conductivity, it exhibits various types of degradations such as poor redox stability, sulfur poisoning, carbon deposition. Ni-YSZ anode still suffers from exposure to oxidizing condition caused by sudden interruption of fuel or sealing problem like leakage. When exposed in oxidizing atmosphere, Ni-YSZ experiences significant volume change due to the oxidation of Ni to NiO, which causes internal cracks and consequent destruction of the cell. In this study, the redox stability of Ni-YSZ cermet according to various conditions such as porosity, pore size distribution, and composition has been studied systematically. ▪ Experimental NiO-YSZ composite powders have been prepared by mixing of commercial NiO (High purity chemicals, 99.97%), 8YSZ (TZ-8YS: 8 mol% Y2O3-stabilized ZrO2, 99.9% Tosoh, Japan) and PMMA as a pore former. The NiO-YSZ bulk samples were fabricated by the typical die compaction method and sintering at 1450 °C for 4 h in air. The sintered NiO-YSZ samples were reduced at 800 °C for 3 h in H2 in order to make Ni-YSZ. The phase change was confirmed by the X-ray diffraction (XRD). The morphology of Ni-YSZ was investigated by scanning electron microscopy (SEM). Degree of oxidation (D.o.O) of Ni was calculated from thermogravimetric analysis (TGA) data. The amount of volume change of Ni powders during oxidation was measured using a helium pycnometer. The both porosity and pore size distribution of Ni-YSZ bulk samples were analyzed with mercury porosimeter. The linear length change of Ni-YZ during oxidation was measured using dilatometer. ▪ Results and Discussion If we know both temperature and time of exposure in oxidizing atmosphere, we can predict the D.o.O of Ni at a certain condition using the D.o.O equation obtained from TGA data. The percentage weight gain per specific surface area can explain the oxidation kinetics of the Ni-YSZ anode supports, which obeys a multi-stage parabolic law expressed by Kp. Kp can be expressed by Kp1 and Kp2. Kp1 and Kp2 correspond to fast oxidation reaction at early oxidation stage and slow oxidation reaction at the next oxidation stage, respectively. The volume change of Ni-YSZ caused by oxidation can be the result of two competing factors: expansion caused by NiO formation and shrinkage caused by continuous sintering of both Ni and NiO. Ni-YSZ cermet with high porosity showed higher tolerance to the cumulative length change due to providing the more relaxation space.

Modeling of gas transport with electrochemical reaction in nickel-yttria-stabilized zirconia anode during thermal cycling by Lattice Boltzmann method

Abstract This work reports an investigation of the impact of microstructure on the performance of solid oxide fuel cells (SOFC) composed of nickel yttria-stabilized zirconia (Ni YSZ). X-ray nano computed tomography (nano-CT) was used to obtain three-dimensional (3D) models of Ni-YSZ composite anode samples subjected to different thermal cycles. Key parameters, such as triple phase boundary (TPB) density, were calculated using 3D reconstructions. The electrochemical reaction occurring at active-TPB was modeled by the Lattice Boltzmann Method for simulation of multi-component mass transfer in porous anodes. The effect of different electrode geometries on the mass transfer and the electrochemical reaction in anodes was studied by TPB distributions measured by nano CT for samples subjected to different thermal cycles. The concentration polarization and the activation polarization were estimated respectively. The results demonstrate that a combined approach involving nano-CT experiments in conjunction with simulations of gas transport and electrochemical reactions using the Lattice Boltzmann method can be used to better understand the relationship between electrode microstructure and performance of nickel yttria-stabilized zirconia anodes.

Nanostructured Ni-YSZ Anode By Atomic Layer Deposition for Solid Oxide Fuel Cells

Nickel yttria-stabilized zirconia (Ni-YSZ) is the most commonly used anode for solid oxide fuel cells (SOFCs). The electrochemical performance of Ni-YSZ anodes can be improved when the size of constituent particles is reduced. However, under SOFC operating environment, nano-sized Ni particles suffer from sintering; nanoscale Ni-YSZ anodes can undergo structure changes, which constrain the development of nanoscale Ni-YSZ anodes. In this work, nanoscale nanostructured Ni-YSZ anode was successfully fabricated by combining atomic layer deposition (ALD) and glancing angle deposition (GLAD). Prior to atomic layer deposition of YSZ, the porous Ni nanorod electrode was firstly prepared by GLAD. ALD was applied to deposit a uniform nanoscale YSZ coating layer over the nickel nanorods. The morphology of the nanostructured Ni-YSZ anode was examined with scanning electron microscopy (SEM). The surface area was increased to more than 3 times as large as plane area and the triple phase boundary (TPB) extended to the entire nickel surface. The surface composition was examined with X-ray photoelectron spectroscopy (XPS). The atomic composition of YSZ film was controlled by the ALD process and not affected by the post-deposition annealing process. ALD coating approach combined ionic conductive YSZ electrolyte phase with electrical conductive Ni phase flexibly and controlled the porosity of the nano-composite electrode just through deposition and post-deposition annealing process. Electrical conductivity measurements were performed using a 4-point probe method. The Ni-YSZ anodes maintained resistivity in the 10-4 cm·S-1 magnitude, which is sufficiently conductive as an SOFC anode. The ALD YSZ coating layer showed sintering-resistance on the nanostructured electrode with an as thin as 1.9 nm YSZ coating layer and excellent anode performance under SOFC operating conditions.

Nanostructured Ni-YSZ by Atomic Layer Deposition

Nickel yttria-stabilized zirconia (Ni-YSZ) is used as anode material for solid oxide fuel cells (SOFCs). The electrochemical performance of Ni-YSZ anodes can be improved when the size of constituent particles of nickel is reduced. However, at SOFC high temperature operating environments, nano-sized Ni particles suffer from sintering. Nanoscale Ni-YSZ anodes can undergo severe structure changes. In this work, a unique approach of fabricating a nanostructured Ni-YSZ anode is demonstrated by combining atomic layer deposition (ALD) and glancing angle deposition (GLAD) techniques whereby nickel nanoparticle sintering is prevented at high temperatures with as thin as 1.9 nm YSZ ALD coating on the nickel; the surface area of the resulting Ni-YSZ anode was found to increase more than 3 times that of a planar electrode. In addition, the novel ALD/GLAD coating approach used here provides an ionic conductive YSZ electrolyte phase combined with an electrical conductive Ni phase whereby porosity can be controlled through deposition and post-deposition annealing. In addition, the surface roughness of Ni-YSZ anode decreases with increasing YSZ coating layer thickness and it is unaffected by the post-deposition annealing process. Conductivity measurements of the Ni-YSZ anodes at room temperature show a resistivity in the order of 10−4 cm·S−1.