Ni Yttria-stabilized Zirconia Research Articles

Placement of Reference Electrode, Electrolyte Thickness and Three-Electrode Cell Configuration in Solid Oxide Fuel Cells: A Brief Review and Update on Experimental Approach

Cell configuration and placement of reference electrode (RE) have been a subject of intensive interests to the solid oxide fuel cells (SOFCs) communities. Here, the relationship between the position of RE, electrolyte thickness, electrode/electrolyte contact and reliability of 3-electrode cell configuration in electrode polarization and resistance separation is briefly reviewed with particular emphasis on the experimental approach. Numerous modelling studies show that distortion to the electrochemical impedance measurement of electrode reactions on thin electrolyte cells is not avoidable due to the misalignment of working and counter electrodes (WE and CE) and the differences in their electrochemical characteristics. This has been verified experimentally that the asymmetric contact at the electrode/electrolyte interface can cause the shift of equipotential line/surface away from the electrolyte region, leading to the significant distortion in the polarization measurements. However, the distortion effect of misalignment on the equipotential line/surface depends strongly on the electrolyte thickness. In the case of SOFCs with model La0.8Sr0.2MnO3 cathode and Ni-yttria-stabilized zirconia (Ni-YSZ) cermet anode, the threshold for YSZ electrolyte thickness with equal partitioning of the cell resistance is ∼250 μm. In conclusion, for reliable and accurate polarization and impedance measurements in electrolyte-supported cells, it is suggested that minimum YSZ electrolyte thickness should be 250 μm for plate cells with electrode areas of ∼10 cm2 and 500 μm for button cells with electrode areas less than 1 cm2 for symmetric 3-electrode cell configurations with RE positioned on the inactive surface of electrolyte.

One-step synthesized Co1.5Mn1.5O4-Ce0.8Sm0.2O1.9 composite cathode material for intermediate temperature solid oxide fuel cells

A Co1.5Mn1.5O4-Ce0.8Sm0.2O1.9 (CMO-SDC) composite material has been prepared by the one-step hydrothermal synthesis method. The phase structure and morphology of the polycrystalline powders have been characterized by X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM). The results show the coexistence of the spinel and the fluorite-type structure which is uniformly distributed throughout the prepared samples. The fluorite phase SDC is well compatible with the spinel phase CMO. This proposed CMO-SDC composite cathode is successfully assembled with a traditional yttria stabilized zirconia (YSZ) electrolyte to fabricate intermediate temperature solid oxide fuel cells (IT-SOFCs) with the structure Ni–YSZ|YSZ|CMO-SDC. The CMO-SDC (50:50wt%) cell with the YSZ film of ~60µm in thickness achieve power densities of 932 and 646mWcm-2 at 800 and 750℃, respectively, in humidified H2 (3vol%H2O), demonstrating its promising application in IT-SOFCs. The high performance is attributed to the intimate contacts between CMO and SDC nanoparticles in the three-phase boundaries in the cathode.

SrZrO3 Formation in SOFCs : Microstructural Characterization and Cell Performance

One of the major phenomena to shorten SOFC durability is the formation of insulated phases, such as SrZrO3, between the cathode and the electrolyte. It is known that SrZrO3 is formed and grown during sintering processes as well as during long-term operation. A systematical study is therefore desired to clarify the SrZrO3 formation mechanisms and their influence on electrochemical properties. In this study, four parameters are considered and systematically varied: sintering temperatures of cathode, electrolyte, and buffer layer, and buffer layer thickness. The cells tested consist of yttria stabilized zirconia (YSZ) as the electrolyte, Ni-yttria stabilized zirconia (Ni-YSZ) as the anode, La(Sr)Co(Fe)O3（LSCF）as the cathode, and Ce0.9Gd0.1O2 (GDC) as the buffer layer. The GDC layer was sintered at 1300oC with 5μm in thickness, while the YSZ electrolyte was sintered at 1400oC. Cathode sintering temperature and time were varied. The electrochemical performance of the cells was tested under various conditions of testing temperature, current density, and testing time by feeding humidified fuel: H2 (97 cc/min) and H2O (3 cc/min). Microstructure of model cells after sintering and testing was observed in details by FE-SEM (Field Emission Scanning Electron Microscope) and STEM (Scanning Transmission Electron Microscope). Microstructural observation has been revealed (1) no SrZrO3 formation during sintering LSCF at 1000oC, and (2) a partial formation of SrZrO3 between the cathode and the electrolyte after sintering LSCF at 1100oC. (3) After the sintering of the LSCF cathode at 1200oC, the interface was fully covered by the SrZrO3 layer, clarified by the microstructural observation with EDX elementary mapping. This shows that cathode sintering temperature depends on SrZrO3 formation. Microstructure of model cells after sintering for (a) 0.5h, (b) 1h, (c) 2h, (d) 16h was observed in details by STEM. Figure 1 shows that sintering time of cathode depends on SrZrO3 formation. Figure 1 indicates SrZrO3 formation mechanisms that SrZrO3 was partially formed between the YSZ and the GDC by grain boundary diffusion or surface diffusion along pore walls and covered fully the interface. Such SrZrO3 formation led to a decrease in electrochemical performance (Figure 2). These results indicate the importance of the SrZrO3 formation controlling the SOFC performance. Figure 1

Effects of nickel oxide impurities on the microstructure and electrical properties of a nickel–yttria-stabilized zirconia anode

Understanding the impact of raw materials on electrode microstructures is important for improving the electrical performance of fuel cells. In this study, two similar NiO powders with different amounts of impurities were used to prepare an anode support for Ni–yttria-stabilized zirconia (Ni-YSZ) anodes. The anodes fabricated with high-purity NiO and with commercial grade NiO with a high impurity concentration are denoted anode-1 and anode-2, respectively. The nano-CT technique was used to reconstruct the anode support and AFL microstructures and to calculate several structural parameters, such as the triple phase boundary, Ni and YSZ particle sizes and pore sizes. In addition, the electrical properties of the cells fabricated with anode-1 and anode-2 were measured and compared. The electrical performance of the cell fabricated with anode-1 is higher than that of the cell fabricated with anode-2 because the anode-1 microstructure is more suitable for this application. Further investigations indicate that the differences in the microstructures and electrical properties of anode-1 and anode-2 are primarily due to anode-2 over-sintering. The anode-2 sintering temperature is lower than the anode-1 sintering temperature due to MgO impurities introduced by the anode-2 raw material, which results in the observed over-sintering phenomenon. When anode-2 is sintered at a lower temperature, its microstructure and electrical performance is similar to that of anode-1.

Feasibility of tubular solid oxide fuel cells directly running on liquid biofuels

Biomass derived liquid fuels, bio-oil and biofuels, are attractive as a mix of future energy supply, and its combination with solid oxide fuel cells (SOFCs) would substantially enhance the energy conversion efficiency as compared to the conventional direct combustion. Here we present the preliminary results on the performance and operation stability of tubular SOFCs with conventional Ni-yttria-stabilized zirconia (Ni-YSZ) cermet anode and (La,Sr)MnO3-YSZ (LSM-YSZ) composite cathode running on mallee wood derived bio-oil and biofuels for the electricity generation. The tubular SOFCs show high performance for liquid bio-oil and biofuels, achieving a peak power density ~600mWcm-2 at 800°C, close to that produced in pure hydrogen. Significant carbon deposition due to the coke formation of bio-oil and biofuels was observed, leading to the blocking of the reaction sites and structural damage of the cell. However, the addition of CO2 dramatically suppresses the coke formation and significantly enhances the cell operating stability. The results show that it is feasible to directly use biomass derived liquid bio-oil and biofuels in tubular SOFCs for the onsite electricity generation. The cell performance and stability can be enhanced by further optimisation and modification of the composition and microstructure of the anodes of the tubular cells.

Stochastic 3D modeling of complex three-phase microstructures in SOFC-electrodes with completely connected phases

A parametric stochastic 3D model for the description of complex three-phase microstructures is developed. Such materials occur for example in anodes of solid oxide fuel cells (SOFC) which consist of pores, nickel (Ni) and yttria-stabilized zirconia (YSZ). The model is constructed using tools from stochastic geometry. More precisely, we model the backbones of the three phases by a certain class of random geometric graphs called beta-skeletons. This allows us to reproduce complete connectivity of all three phases as observed in experimental image data of a pristine Ni–YSZ anode as well as the prediction of volume fractions by model parameters. Finally a slightly generalized version of this model enables a good fit to experimental image data with respect to transport relevant microstructure characteristics and the length of triple phase boundary. Model validation is performed by comparing effective transport properties from finite element (FE) simulations based on 3D-data from the stochastic model and from tomography of real Ni–YSZ anodes. Moreover, the virtual, but realistic Ni–YSZ microstructures can be used for investigating the quantitative influence of microstructure characteristics on various physical properties and consequently on the performance of the anode material.

(Invited) The Stability of Solid Oxide Fuel Cells: Insights through in Situ X-Ray Tomography

With the advent of high energy x-ray sources it is now possible to follow microstructural evolution in three dimensions and as a function of time. The ability to observe and quantify the the evolution processes provides fundamentally new insights into factors controlling the microstructure of materials. We illustrate this through observation of the high-temperature microstructural evolution of a solid oxide fuel cell Ni-Yttria-stabilized zirconia (Ni-YSZ) anode. Unlike prior studies that compared microstructural differences between different anodes, this three-dimensional measurement directly shows the changes occurring in the same region of an anode, enabling a new understanding of evolutionary processes. High-temperature ageing for 48 h at 1050 °C yielded substantial structural changes in the Ni, YSZ and pore networks, including coalescence of Ni particles, leading to a three-fold decrease in three-phase boundary length. The implications for long-term degradation of fuel cells will be discussed, as well as time resolved x-ray tomography.

Impact of nanostructured anode on low-temperature performance of thin-film-based anode-supported solid oxide fuel cells

The impact of a nanostructured Ni–yttria-stabilized zirconia (Ni–YSZ) anode on low-temperature solid oxide fuel cell (LT-SOFC) performance is investigated. By modifying processing techniques for the anode support, anode-supported SOFCs based on thin-film (∼1 μm) electrolytes (TF-SOFCs) with and without the nanostructured Ni–YSZ (grain size ∼100 nm) anode are fabricated and a direct comparison of the TF-SOFCs to reveal the role of the nanostructured anode at low temperature is made. The cell performance of the nanostructured Ni–YSZ anode significantly increases as compared to that of the cell without it, especially at low temperatures (500 °C). The electrochemical analyses confirm that increasing the triple-phase boundary (TPB) density near the electrolyte and anode interface by the particle-size reduction of the anode increases the number of sites available for charge transfer. Thus, the nanostructured anode not only secures the structural integrity of the thin-film components over it, it is also essential for lowering the operating temperature of the TF-SOFC. Although it is widely considered that the cathode is the main factor that determines the performance of LT-SOFCs, this study directly proves that anode performance also significantly affects the low-temperature performance.

Effect of Ni content on the morphological evolution of Ni-YSZ solid oxide fuel cell electrodes

The coarsening of Ni in Ni–yttria-stabilized zirconia (YSZ) anodes is a potential cause of long term solid oxide fuel cells (SOFC) performance degradation. The specifics of the Ni-YSZ structure—including Ni/YSZ ratio, porosity, and particle size distributions—are normally selected to minimize anode polarization resistance, but they also impact long-term stability. A better understanding of how these factors influence long-term stability is important for designing more durable anodes. The effect of structural details, e.g., Ni-YSZ ratio, on Ni coarsening has not been quantified. Furthermore, prior measurements have been done by comparing evolved structures with control samples, such that sample-to-sample variations introduce errors. Here, we report a four dimensional (three spatial dimensions and time) study of Ni coarsening in Ni-YSZ anode functional layers with different Ni/YSZ ratios, using synchrotron x-ray nano-tomography. The continuous structural evolution was observed and analyzed at sub-100 nm resolution. It is shown quantitatively that increasing the Ni/YSZ ratio increases the Ni coarsening rate. This is due to both increased pore volume and a decrease in the YSZ volume fraction, such that there is more free volume and a less obtrusive YSZ network, both of which allow greater Ni coarsening. The results are shown to be in good agreement with a power-law coarsening model. The finding is critical for informing the design of SOFC electrode microstructures that limit coarsening and performance degradation.

P25] Assessment of temperature and pressure dependence of molar volume and phase diagrams of Al, Cu, Si, and Zn

The performance of a novel micro-tubular solid oxide fuel cell (SOFC) with an inert support and an integrated current collector for the inner electrode was improved by controlling its microstructural features. Multi-step dip coating and co-sintering methods were used to fabricate the cell containing porous yttria-stabilized zirconia (YSZ), Ni, Ni–YSZ, YSZ, strontium-doped lanthanum manganite (LSM)–YSZ, and LSM as the inert support, anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. To enhance gas diffusion through the YSZ support by properly tailoring its porosity, a combination of micro-crystalline cellulose and polymethyl methacrylate pore formers was used. Additionally, the porosity of the Ni current collector was improved and the LSM–YSZ cathode was sufficiently thick for high oxygen reduction activity. Owing to its optimized microstructure, the micro-tubular SOFC delivered excellent power output with maximum power densities of 710, 591, 445, and 316 mW cm−2 at 850, 800, 750, and 700 °C, respectively. The effect of redox cycling on cell performance was investigated by alternately exposing the anode to fuel and air atmospheres. The cell had good tolerance toward the redox phenomenon with no apparent degradation in its performance up to 10 redox cycles.

Comparative study on ammonia oxidation over Ni-based cermet anodes for solid oxide fuel cells

In the current work, we investigate the performance of solid oxide fuel cells (SOFCs) with Ni‒yttria-stabilized zirconia (Ni–YSZ) and Ni‒gadolinia-dope ceria (Ni–GDC) cermet anodes fueled with H2 or NH3 in terms of the catalytic activity of ammonia decomposition. The cermet of Ni–GDC shows higher catalytic activity for ammonia decomposition than Ni–YSZ. In response to this, the performance of direct NH3–fueled SOFC improved by using Ni–GDC anode. Moreover, we observe further enhancement in the cell performance and the catalytic activity for ammonia decomposition with applying Ni−GDC anode synthesised by the glycine–nitrate combustion process. These results reveal that the high performance of Ni–GDC anode for the direct NH3–fueled SOFC results from its mixed ionic–electronic conductivity as well as high catalytic activity for ammonia decomposition.

Observing the microstructural evolution of Ni-Yttria-stabilized zirconia solid oxide fuel cell anodes

This report describes a transmission X-ray microscopy observation of the high-temperature microstructural evolution of a solid oxide fuel cell Ni-Yttria-stabilized zirconia (Ni-YSZ) anode. Unlike prior studies that compared microstructural differences between different anodes, this three-dimensional measurement directly shows the changes occurring in the same region of an anode, enabling a new understanding of evolutionary processes. High-temperature ageing for 48 h at 1050 °C yielded substantial structural changes in the Ni, YSZ and pore networks, including coalescence of Ni particles, leading to a three-fold decrease in three phase boundary length. Implications for fuel cell long-term degradation are discussed.

Solid oxide fuel cell with Ni–Al support

Metal-supported solid oxide fuel cells are considered as next generation of fuel cells. In this configuration, the use of metal support allows lowering the cost of fuel cell fabrication and provides enhanced durability. The paper presents the results concerning the manufacture and research of metal-supported solid oxide fuel cells with YSZ (yttria-stabilized zirconia) electrolyte. As a metal support the porous Ni–Al plates manufactured by combustion synthesis were used. The effects of synthesis conditions and composition of the Ni–Al samples on their properties were investigated. Solid oxide fuel cells with the structure of Ni–YSZ anode/YSZ electrolyte/LSCF cathode were fabricated by the combination of screen printing, magnetron sputtering, and electron beam treatment. The cell performance was analyzed by current–voltage measurements during operation at 700, 750, 800 and 850 °C. The obtained results are very promising at such an early stage of research for the future development of this technology.

Electrochemical and microstructural properties of Ni–(Y2O3)0.08(ZrO2)0.92–(Ce0.9Gd0.1)O1.95 anode-supported microtubular solid oxide fuel cells

The nickel–zirconia cermet is widely used as an anode of solid oxide fuel cells (SOFCs). On the other hand, the nickel–ceria based anode indicates high electrochemical activity for hydrogen oxidation and hydrocarbon reforming. In this study, electrochemical and microstructural properties of microtubular SOFCs with Ni-based composite anodes containing yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) are investigated electrochemically using impedance spectroscopy (EIS) and microstructurally using focused ion beam-scanning electron microscopy (FIB-SEM). The solid solution of YSZ and GDC was easily formed after mechanical mixing and sintering at 1400°C. The electrical conductivity and mechanical strength for the Ni–YSZGDC composite anodes are low relative to Ni–YSZ due to poor sinterability. The GDC-containing anodes show improved electrochemical activity for hydrogen oxidation, despite having lower three-phase boundary densities. Distribution of relaxation times (DRT) analysis of the EIS data shows that the concentration polarization is lower for the Ni–GDC anode, due to a higher measured pore volume. The maximum power density for the cell with the Ni–YSZGDC composite anode was higher than those with the Ni–YSZ and Ni–GDC anodes.

Performance improvement and redox cycling of a micro-tubular solid oxide fuel cell with a porous zirconia support

The performance of a novel micro-tubular solid oxide fuel cell (SOFC) with an inert support and an integrated current collector for the inner electrode was improved by controlling its microstructural features. Multi-step dip coating and co-sintering methods were used to fabricate the cell containing porous yttria-stabilized zirconia (YSZ), Ni, Ni–YSZ, YSZ, strontium-doped lanthanum manganite (LSM)–YSZ, and LSM as the inert support, anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. To enhance gas diffusion through the YSZ support by properly tailoring its porosity, a combination of micro-crystalline cellulose and polymethyl methacrylate pore formers was used. Additionally, the porosity of the Ni current collector was improved and the LSM–YSZ cathode was sufficiently thick for high oxygen reduction activity. Owing to its optimized microstructure, the micro-tubular SOFC delivered excellent power output with maximum power densities of 710, 591, 445, and 316 mW cm−2 at 850, 800, 750, and 700 °C, respectively. The effect of redox cycling on cell performance was investigated by alternately exposing the anode to fuel and air atmospheres. The cell had good tolerance toward the redox phenomenon with no apparent degradation in its performance up to 10 redox cycles.

Boundary Observation and Contrast Tuning of Ni/YSZ Anode by TEM and FIB-SEM

The most widely used anode material in SOFC is Ni/yttria stabilized zirconia (Ni/YSZ). Its microstructure depends on the fabrication techniques, and determines its performance. Electron microscopy is often adopted to characterize the microstructure in wide scales. In this report, we focus on the microstructure observation of conventional Ni/YSZ anodes in two open issues: atomic structure of boundary by transmission electron microscopy (TEM) and phase contrast tuning by focused ion beam-scanning electron microscopy (FIB-SEM). It is well-known that Ni/YSZ interface and Ni/YSZ/pore triple-phase boundary (TPB) play important roles in the stability and reaction activity of Ni/YSZ anodes. However, most of the microstructure researches related to interface and TPB lie in micro- or nano-scales. In atomic scale, few reports on the interfaces of conventional Ni/YSZ anode can be found. Moreover, there is no experimental study on the real atomic structures of TPB. So, high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) were adopted to observe the atomic structures at the interfaces and TPBs. Before observation, the pores of the porous electrode were infiltrated by low viscosity epoxy resin under vacuum, so that they could be easily distinguished in a microscopy and the bulk cell could be readily handled. The specimen for TEM was lifted out in a focused ion beam-scanning electron microscopy (FIB-SEM). After observation, high-index contact surfaces (e.g. Ni(15 13 1)/YSZ(4 4 10)) were found between Ni and YSZ. At TPB, YSZ(200) was found to be the contact surface while Ni(111) shows an included angle of ~13° with YSZ(200). 3D reconstruction is becoming popular with the help of several techniques, such as FIB-SEM and TEM. In case of FIB-SEM, many of previous works showed indistinct contrast among different phases, which will make the analysis difficult and the results distorted in some extent. Herein, we used a high-resolution machine (HITACHI MI4000L) and tuned many conditions to improve the contrast. The related mechanism will be discussed. For Ni/YSZ anode, distinct contrast was obtained among Ni, YSZ and pore (actually resin) by using InLens detector. The three phases could be automatically segmented by Avizo software. Then the 3D microstructure is reconstructed.

Cracking Reaction of Octane Fuel and Electrochemical Reaction on Ni-YSZ Cermet Anode of Internal Steam Reforming Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density. Then, SOFC is a promising power source for many potential mobile robot applications. Octane is a good candidate as fuel due to high boiling point enough to be in the liquid phase even at high temperatures in summer. Performance of the internal reforming SOFC is governed by anode electrode material, internal reforming method, and operating conditions. Ni-yttria stabilized zirconia (Ni-YSZ) cermet is the most promising anode material for SOFC because of its low cost, chemical stability, and closely matched thermal expansion coefficients with the dense YSZ electrolyte. Steam reforming is the first choice for the internal reforming method because it has been frequently-studied for the direct use of methane fuel. Then, operation of SOFC by internal steam reforming of octane has been studied. However, cracking reactions on the YSZ anode, which is affected by the operating conditions, have not been elucidated sufficiently. The purpose of this study is to clarify the cracking reaction of octane fuel and the electrochemical reaction of the Ni-YSZ anode electrode under various operating conditions. In this study, the fuel-cracking reaction on the Ni-YSZ anode electrode is estimated by gas chromatography with thermal conductivity detector. Anode electrode performance is studied by electrochemical measurement. Two types of octane (n-octane; linear alkane, iso-octane; branched chain alkane) are used in order to elucidate the effect of difference in carbon-chain structure. Octane and steam of equilibrium vapor pressure were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Temperature of the octane bubbler and the carrier gas flow rate was 30 oC and 10 ml/min, respectively. The carrier gas flow rate of the water bubbler was 83 ml/min. Steam/carbon ratio (S/C) was controlled over an S/C ratio from 0.5 to 5.5 by changing the temperature of the water bubbler from 30 to 80 oC. The cell temperature range was 700-950 oC. The octane internal steam reforming operation of SOFC was demonstrated by I-V and I-P characteristics and stable power generation property by constant current. The effects of operating conditions (S/C ratio and cell temperature) on the open circuit voltage (OCV) and power generation property were investigated. The chemicals generated by the cracking reaction of octane at various operating conditions were analyzed by gas chromatography and compared. The difference in outlet gas compositions from the cell of open circuit condition and power generation condition revealed the main species consumed for the power generation reaction on the anode electrode. The electrochemical reaction resistance of the Ni-YSZ anode was measured by ac impedance method and the rate-determining process was identified. In this study the following was revealed. When the S/C and/or the operating temperature are high, the reforming reaction rate is large. By the reforming reaction of n-octane, H2, CO, CO2, and CH4 is mainly produced. When the iso-octane is used as the fuel, small amount of C2H6 is also generated in addition to them. Only H2 and CO contribute to the power generation directly. Rate-determining process of the electrode reaction is a cracking reaction to H2 and CO.

Microstructural Characterization of SrZrO3 Formation and the Influence to SOFC Performance

One of the major phenomena to shorten SOFC durability is the formation of insulated phases, such as SrZrO3, between the cathode and the electrolyte. It is known that SrZrO3 is formed and grown during sintering processes as well as during long-term operation. A systematical study is therefore desired to clarify the SrZrO3formation mechanisms and their influence on electrochemical properties. In this study, four parameters are considered and systematically varied: sintering temperatures of cathode, electrolyte, and buffer layer, and buffer layer thickness. The cells tested consist of yttria stabilized zirconia (YSZ) as the electrolyte, Ni- yttria stabilized zirconia (Ni-YSZ) as the anode, La(Sr)Co(Fe)O3（LSCF）as the cathode, and Ce0.9Gd0.1O2 (GDC) as the buffer layer. The GDC layer was sintered at 1300oC with 5μm in thickness, while the YSZ electrolyte was sintered at 1400oC. Cathode sintering temperature was varied from (1) 1000oC to (2) 1100oC and (3) 1200oC. The electrochemical performance of the cells was tested at 800oC, 0.2A/cm2 for 100h by feeding humidified fuel: H2 (97 cc/min) and H2O (3 cc/min). Microstructure of the three types of model cells after sintering and testing for 100h was observed in details by FE-SEM (Field Emission Scanning Electron Microscope) and STEM (Scanning Transmission Electron Microscope). Microstructural observation has been revealed (1) no SrZrO3 formation during sintering LSCF at 1000oC, and (2) a partial formation of SrZrO3 between the cathode and the electrolyte after sintering LSCF at 1100oC. (3) After the sintering of the LSCF cathode at 1200oC, the interface was fully covered by the SrZrO3 layer, clarified by the microstructural observation with EDX elementary mapping. Such SrZrO3 formation led to a decrease in electrochemical performance. These results indicate the importance of the SrZrO3 formation controlling the SOFC performance.

In-situ electron microscopy investigation of reduction-induced microstructural changes in NiO

The structural characteristics of NiO–YSZ (YSZ: Yttria-stabilized zirconia) composites and the redox stability of NiO have been explored by in-situ and high-resolution transmission electron microscopy (TEM). NiO has been reduced on alumina support while doing in-situ microscopy, and the observations on redox instability and microstructural changes have been extrapolated to understand the behavior of NiO/Ni–YSZ system. In the composite, the NiO and YSZ grains are strain free and no reaction product is observed at the grain boundaries after long term annealing. Under a low partial pressure of oxygen, NiO may begin to reduce to metallic Ni at temperatures as low as ~200°C. The volume change associated with such a reduction process is ~40.3%. Although metallic Ni nucleates in NiO coherently, such a huge volume contraction may lead to porosity generation and mechanical instability. Long term exposure of metallic Ni to high temperatures leads to coarsening, which reduces the available surface area. This in turn has a direct, negative consequence on the catalytic properties of the metallic Ni particles. The coarsened particles may even undergo faceting, and the facets may not be the most active catalytically. Synthesis of finely dispersed and percolatively connected Ni–YSZ anodes for SOFCs and core–shell or compositionally graded electrodes for batteries may be viable solutions to enhance the lifetime and structural integrity of such electrodes.

Boundary Observation and Contrast Tuning of Ni/YSZ Anode by TEM and FIB-SEM

Ni/yttria stabilized zirconia (Ni/YSZ) anode was investigated in the aspect of microstructure. Atomic resolution transmission electron microscopy and high resolution focused ion beam-scanning electron microscopy (FIB-SEM) were used to study the atomic structure of boundaries and phase contrast, respectively. Images of Ni/YSZ interface and Ni/YSZ/pore triple-phase boundary were obtained. High-Index contact surfaces were found. Distinct phase contrast among Ni, YSZ and resin was observed during the slice and view process in FIB-SEM.