NiO Yttria-stabilized Zirconia Research Articles

Electrophoretic deposition of YSZ layers on pyrolytic graphite and a porous anode substrate based on NiO-YSZ

Solid oxide fuel cells are promising hydrogen energy devices. The goal of this research was to create ceramic layers for SOFCs based on yttria-stabilized zirconia (YSZ) and to investigate their parameters. YSZ ceramic layers with a thickness of 5.14 μm on a porous NiO–YSZ substrate and 7 μm on pyrolytic graphite were obtained by electrophoretic deposition. X-ray diffraction and electron microscopy were used to determine the composition, structure, and morphological features of ceramic layers. The effects of the substrate's nature, the degree of dispersion of the initial YSZ powder, and the heat treatment conditions on the properties of the ceramic layer YSZ were considered.

Low leakage rate of silicate glass modified with Al2O3 for solid oxide fuel cell

The gas tightness of glass sealing materials is a big challenge for the solid oxide fuel cell (SOFC) stacks operating at high temperature. Thermal, sintering, crystallization behavior and gas tightness properties of the glass-based with two different Al2O3 contents sealants are evaluated and discussed. The study showed that the sealants avoid cracks at the interface on NiO-YSZ (NiO-yttria stabilized zirconia) and SUS430 stainless steel interconnect substrates. The Al2O3 embedded in the glass matrix as a second phase, and promoted crystallization of K[AlSi3O8] at the early stage. This may because some ultrafine Al2O3 particles whose structure is destroyed by prolonged high temperature treatment according XRD and TEM analysis. Especially, the sealant containing 5 wt% Al2O3 undergoes a thermal cycle and maintains a stable leakage rate below 10−4 sccm∙cm−1 for about 1000 h at 750 °C. The above results prove the possibility of using the Al2O3-doped sealing glass for SOFC stacks.

Highly conductive hematite electrolyte for low‐temperature solid oxide fuel cell

Hematite (HEM) is a kind of cheap natural mineral with ion transport ability. In this paper, some basic properties of HEM were studied to determine its potential as an electrolyte for low-temperature solid oxide fuel cell (SOFC). A composite (HEM-GDC) formed by Gd0.1Ce0.9O2-δ (GDC) and HEM was developed to improve the performance of the HEM. The result of the X-ray fluorescence spectrometer confirms that the main components of HEM are Fe2O3 and SiO2. The X-ray diffraction (XRD) results show that the chemical compatibilities of HEM and HEM-GDC are good with La2NiO4 + δ (LNO) and NiO-yttria-stabilizedzirconia (NiO-YSZ). Moreover, the thermal expansion coefficients (TECs) of HEM and HEM-GDC are 11.83 × 10−6 K−1 and 12.1 × 10−6 K−1, permitting that they have good physical compatibility with Ni-yttria-stabilized zirconia (Ni-YSZ) and LNO. According to the results of the differential scanning calorimetry with thermal gravimetric analysis (DSC-TGA), HEM and HEM-GDC are stable at high temperatures. The ionic conductivities of HEM and HEM-GDC are 1.07 × 10−2 S·cm−1 and 7.23 × 10−2 S·cm−1 at 450°C, which meet the requirement of the low-temperature SOFC electrolytes. In addition, electrochemical impedance spectra (EIS) tests of GDC, HEM, and HEM-GDC half-cell with LNO as cathode show that HEM-GDC performs best.

Long-term stability of SOFC glass sealant under oxidising and reducing atmospheres

The behaviour of the 59.6SiO2–11.0Al2O3–6.6ZrO2–3.4CaO–15.4Na2O–4Y2O3 (wt. %) non-crystallising glass sealant was studied while in contact with NiO–YSZ (yttria-stabilized zirconia) ceramics and an Fe–Ni alloy after exposure to reducing and oxidising atmospheres at 850 °C and while on a Pt substrate. The sealant was completely amorphous after exposure on a Pt substrate for 1000 h in an air atmosphere. After exposure on a NiO–YSZ substrate in both oxidising and reducing atmospheres, no interaction on the NiO(Ni)–YSZ|sealant interface was observed. For the Fe–Ni alloy substrate, the formation of an Fe2SiO4 layer was observed after exposure in an oxidising atmosphere. In the reducing atmosphere, the Fe–Ni alloy substrate intensified sealant crystallisation, leading to the formation of numerous crystalline phases, but no interaction on the Fe–Ni alloy|sealant interface was observed.

Cellular Nickel-Yttria/Zirconia (Ni-YSZ) Cermet Foams: Manufacturing, Microstructure and Properties.

Open-celled ceramic composite foams were prepared from NiO and yttria-stabilized zirconia (YSZ) powders by the polymer sponge replication (Schwartzwalder) technique using the respective aqueous dispersions. Mechanically stable NiO–YSZ foams with an average porosity of 93 vol.% were obtained. After chemical reduction of the NiO phase with hydrogen, cellular Ni–YSZ cermet structures were obtained. They are characterized by an electric conductivity up to 19∙103 S∙m−1 which can be adjusted by both, the Ni volume fraction, and the sintering/reduction procedure. The NiO–YSZ ceramic foams, as well as the cellular Ni–YSZ cermets prepared therefrom, were characterized with respect to their microstructure by scanning electron microscopy, confocal Raman microscopy and X-ray diffraction with Rietveld analysis. In addition, the compressive strength, the electric conductivity and the thermal conductivity were determined. The collected data were then correlated to the sample microstructure and porosity and were also applied for modelling of the mechanical and electric properties of the bulk Ni–YSZ strut material.

Performance of NiO/YSZ anode-supported solid oxide fuel cell fueled with landfill gas stream

Generating electrical energy from landfill gas (LFG) is a challenge due to its low conversion efficiency. In this study, performance of a NiO/yttria-stabilized zirconia (NiO-YSZ) anode-supported cell operating with LFG feed stream was evaluated. This study investigated the potential of solid oxide fuel cells (SOFC) to produce electricity from LFG generated in pilot scale anaerobic municipal solid waste bioreactors. During the initial experiments, power generation was achieved in the SOFC with direct feeding of the LFG. Different feed flow rates (10 25 mL/min) and varying temperature conditions (700 800 °C) were also investigated to define the optimal conditions. Experiments were carried out at different feed rates and the successful results obtained from 10 mL/min and 20 mL/min feed speeds. It was also observed that the maximal power values were between 0.10 0.11 Watt/cm2 for all experiments. This study showed that SOFCs can provide significantly higher energy efficiencies than steam engines for LFG conversion into electrical energy.

Porous NiO–YSZ and Ni–YSZ composites fabricated using NiO–YSZ composite nanopowders and WO3 additive

NiO–YSZ composite nanopowders were synthesized through a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Porous Ni–yttria stabilized zirconia (YSZ) composites, Ni–YSZ composites, with high porosity (25.0–32.8%) were fabricated by reducing NiO-YSZ composites in (94% Ar + 6% H2) gas. The electrical conductivity (14,221 cm−1 at 30 °C) and bending strength (130 MPa) of Ni–YSZ composites were enhanced by the addition of WO3 (0.1 wt% WO3) to NiO–YSZ composite nanopowders. Furthermore, the Y2(WO4)3 with a negative coefficient of thermal expansion (CTE) reduced the CTE of Ni–YSZ composites. The CTE values of 0 and 0.6 wt% WO3–added Ni–YSZ composites were 13.8 × 10−6 and 12.4 × 10−6 °C−1, respectively. We demonstrate that the fabrication of NiO–YSZ composite nanopowders and the addition of WO3 to the NiO–YSZ composite nanopowders are highly effective methods for the fabrication of high-quality Ni–YSZ anode materials in solid oxide fuel cells.

Multilayer tape casting of large-scale anode-supported thin-film electrolyte solid oxide fuel cells

Tape casting is conventionally used to prepare individual, relatively thick components (i.e., the anode or electrolyte supporting layer) for solid oxide fuel cells (SOFCs). In this research, a multilayer ceramic structure is prepared by sequentially tape casting ceramic slurries of different compositions onto a Mylar carrier followed by co-sintering at 1400 °C. The resulting half-cells contains a 300 μm thick NiO–yttria-stabilized zirconia (YSZ) anode support, a 20 μm NiO–YSZ anode functional layer, and an 8 μm YSZ electrolyte membrane. Complete SOFCs are obtained after applying a Gd0.1Ce0.9O2 (GDC) barrier layer and a Sm0.5Sr0.5CoO3 (SSC) -GDC cathode by using a wet-slurry spray method. The 50 mm × 50 mm SOFCs produce peak power densities of 337, 554, 772, and 923 mW/cm2 at 600, 650, 700, and 750 °C, respectively, on hydrogen fuel. A short stack including four 100 mm × 150 mm cells is assembled and tested. Each stack repeat unit (one cell and one interconnect) generates around 28.5 W of electrical power at a 300 mA/cm2 current density and 700 °C.

Freeze Casting of Tubular Anode Supports for High-Performance Solid Oxide Fuel Cells

Freeze casting method has received significant attention in recent years for the fabrication of porous materials with unique pore geometries. A typical freeze casting process involves freezing of a liquid suspension forming solvent crystals that grow in the direction of solidification. When these solvent crystals are removed by sublimation under a reduced pressure, a porous structure with unidirectional and gradient pores is achieved. Such a porous structure is considered ideal for solid oxide fuel cell (SOFC) electrodes because it allows for rapid gaseous diffusion through the bulk of the electrode and a large number of active sites toward the electrolyte–electrode interface for electrochemical reactions. In the present work, we developed a freeze casting and freeze drying process to fabricate tubular SOFC anode supports using an aqueous NiO–yttria-stabilized zirconia (YSZ) slurry. Freeze casting was performed in an aluminum mold that contained peripheral coils for flowing a coolant. After complete freezing of the slurry, the mold was connected to a vacuum chamber for freeze drying. With a proper control of the experimental setup and adjustments in the freeze drying profile, we were able to reduce the total drying time from several days to a few hours. The fabricated anode supports were coated with the thin layers of YSZ electrolyte and strontium-doped lanthanum manganite (LSM)-YSZ cathode to obtain the single cells. The electrochemical performance of the single cells was evaluated by the I–V polarization curves and electrochemical impedance spectra. Owing to the favorable gas transport properties of the freeze-cast anode, the concentration polarization was found to be greatly reduced and thus a high power density was achieved. Figure 1

Mitigating Carbon Formation with Al2TiO5-Enhanced Solid Oxide Fuel Cell Anodes

Spectroscopic and electrochemical techniques were used to examine the benefits of adding small amounts of aluminum titanate (Al2TiO5 or ALT) to standard NiO–yttria-stabilized zirconia (YSZ)-based solid oxide fuel cell anodes operating with methane at 800 °C. Combining small amounts (4% by mass) of ALT with NiO–YSZ leads to the formation of secondary phases that improve anode carbon tolerance via multiple reaction mechanisms. Raman data show that carbon forms on both ALT-doped and undoped anodes at open-circuit voltage and with applied bias as evidenced by the vibrational band of highly ordered graphite at 1560 cm–1. However, ALT-doped anodes limited the amount of carbon that forms on both the surface and within the bulk, resulting in improved performance for up to 12 h of exposure to CH4. Results show that ALT-enhanced anodes accumulate ∼25% less carbon on the surface under open-circuit conditions and that increasing polarizations to 50% of the maximum current and beyond results in no observable carbon ac...

Densification behavior of yttria-stabilized zirconia powders for solid oxide fuel cell electrolytes

Yttria-stabilized zirconia (YSZ) is the most common electrolyte material for solid oxide fuel cells. Herein, we conducted a comparative study on the densification behavior of three different kinds of commercial 8 mol% YSZ powders: (i) TZ-8Y (Tosoh, Japan), (ii) MELox 8Y (MEL Chemicals, UK), and (iii) YSZ-HT (Huatsing Power, China). The comparison was made on both the self-supporting pellets and thin-film electrolytes coated onto a NiO–YSZ anode support. For the pellets, MELox 8Y showed the highest densification at lower sintering temperatures with 93% and 96% of the theoretical density at 1250 and 1300 °C, respectively. Although YSZ-HT showed a higher sintering rate than TZ-8Y, a sintering temperature of 1350 °C was required for both the powders to reach 95% of the theoretical density. For the thin-film electrolytes, on the other hand, YSZ-HT showed the highest sintering rate with a dense microstructure at a co-sintering temperature of 1250 °C. Our results indicate that besides the average particle size, other factors such as particle size distribution and post-processing play a significant role in determining the sintering rate and densification behavior of the YSZ powders. Additionally, a close match in the sintering shrinkage of the electrolyte and anode support is important for facilitating the densification of the thin-film electrolytes.

Porosity, microstructure and mechanical behavior of NiO–YSZ composite anode for solid oxide fuel cells

Abstract In this study, NiO–yttria stabilized zirconia (YSZ) composite anodes were prepared by using ball-milling followed by a sintering process. For this purpose, two different weight percent ratios of NiO and YSZ powders (50 : 50 and 60 : 40) were mixed by the cost-effective ball-milling method. The morphology and particle size distribution of the mixed powders were investigated using scanning electron microscopy and dynamic light scattering methods, respectively. The powders were then uniaxially pressed to form pellets with a thickness and diameter of around 1 and 22 mm, respectively. Sintering of the pellets was conducted at 1 100 °C and the microstructure and porosity, as well as phase analysis of bulk composites, were then investigated using scanning electron microscopy and X-ray diffraction, respectively. More importantly, the microhardnesses of the prepared pellets were obtained and compared with literature values to evaluate the accuracy of the sintering process. The results showed that the open porosity of the samples varied between 30 to 40 percent, which makes them suitable for anode application in solid oxide fuel cells. The prepared NiO–YSZ composites with a uniform distribution of Ni particles showed a value of microhardness above 0.5 GPa, and these results were discussed in detail.

In situ time-of-flight neutron imaging of NiO–YSZ anode support reduction under influence of stress

This article reports on in situ macroscopic scale imaging of NiO–YSZ (YSZ is yttria-stabilized zirconia) reduction under applied stress – a phase transition taking place in solid oxide electrochemical cells in a reducing atmosphere of a hydrogen/nitrogen mixture and at operation temperatures of up to 1073 K. This process is critical for the performance and lifetime of the cells. Energy-resolved neutron imaging was applied to observe the phase transition directly with time and spatial resolution. Two different approaches are presented for using this imaging technique for the investigation of chemical and physical processes requiring controlled atmosphere and elevated temperature. The first type of measurement is based on alternating stages of short-term partial chemical reaction and longer neutron image acquisition, and the second type is a real in situ neutron imaging experiment. Results of applying energy-resolved neutron imaging with both approaches to the NiO–YSZ reduction investigation indicate enhancement of the reduction rate due to applied stress, which is consistent with the results of the authors' previous research.

Performance Investigation of Dual Layer Yttria-Stabilized Zirconia–Samaria-Doped Ceria Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

The performance of yttria-stabilized zirconia (YSZ)–samaria-doped ceria (SDC) dual layer electrolyte anode-supported solid oxide fuel cell (AS-SOFC) was investigated. Tape-casting, lamination, and co-sintering of the NiO–YSZ anode followed by wet powder spraying of the SDC buffer layer and BSCF cathode was proposed for fabrication of these cells as an effective means of reducing the number of sintering stages required. The AS-SOFC showed a significant fuel cell performance of ∼1.9 W cm−2 at 800 °C. The fuel cell performance varies significantly with the sintering temperature of the SDC buffer layer. An optimal buffer layer sintering temperature of 1350 °C occurs due to a balance between the YSZ–SDC contact and densification at low sintering temperature and reactions between YSZ and SDC at high sintering temperatures. At high sintering temperatures, the reactions between YSZ and SDC have a detrimental effect on the fuel cell performance resulting in no power at a sintering temperature of 1500 °C.

Steam electrolysis in a solid oxide electrolysis cell fabricated by the phase-inversion tape casting method

In this study, an asymmetric NiO–yttria-stabilized zirconia (YSZ) cathode substrate has been successfully fabricated by the phase-inversion tape casting method. 3-dimensional X-ray microscopy and subsequent analysis demonstrate that the NiO–YSZ substrates exhibit dual-layer structure: a thin sponge-like pore layer is supported on a thick finger-like macro-void layer. The electrochemical performance of the Ni–YSZ/YSZ/YSZ–LSM electrolyzer with this novel dual-layer cathode has been significantly enhanced. The electrolyzer exhibits an excellent current density of 2.3Acm−2 and a high H2 production rate of 958mLcm−2h−1 at 800°C and 1.5V, and a polarization resistance of 0.25Ωcm2 at the open circuit voltage when the cathode and anode are exposed to 33vol.% H2O–67vol.% H2 and ambient air, respectively, demonstrating that the thin sponge-like pore layer in the dual-layer cathode can provide sufficient electrochemical reaction sites for H2O reduction, while the thick finger-like macro-void layer can serve as the support for the electrolyzer and facilitate H2/H2O transport in the cathode, reducing the cathode polarization resistance and enhancing the cell 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.

Fabrication and Performance of La0.6Sr0.4Co0.2Fe0.8O3−δ Infiltrated-Yttria-Stabilized Zirconia Cathode on Anode-Supported Solid Oxide Fuel Cell

The three layers with porous yttria-stabilized zirconia (YSZ) backbone/dense YSZ/porous NiO–YSZ were fabricated by tape-casting process, respectively, then laminated together and co-fired at 1300 °C for 5 h. The cathode material La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) was loaded by infiltrating the precursor of metal ions into porous YSZ backbone. As a result, LSCF nanoparticles with the size of 60–100 nm were uniformly distributed on YSZ backbone. The power density was 1.046 W cm−2 and the polarization resistance was 0.17 Ω cm2 at 800 °C in humidified H2 (3 vol.% H2O). But the stability was not good enough, especially in early operating stage, e.g., 20 h. After that, it showed good stability for the following 70 h operating under a constant voltage of 0.7 V at 750 °C. This is due to the growth and agglomeration of LSCF nanoparticles at early steps, which reduced the three phase boundaries (TPBs).

Microstructure tailoring of the nickel oxide-Yttria-stabilized zirconia hollow fibers toward high-performance microtubular solid oxide fuel cells.

NiO-yttria-stabilized zirconia (YSZ) hollow fiber anode support with different microstructures was prepared using a phase-inversion method. The effect of the solid loading of the phase-inversion suspensions on the microstructure development of the NiO-YSZ anode support was investigated. Solid loading in the suspension was found to have an important influence on the microstructure of the NiO-YSZ anode support and viscosity-related viscous fingering mechanism can be adopted to explain the pore formation mechanism of the as-prepared hollow fibers. NiO-YSZ anode-supported microtubular solid oxide fuel cells (SOFCs) with different anode microstructures were fabricated and tested, and the correlation between the anode support microstructures, porosity, gas permeability, electrical conductivity, and the cell electrochemical performance was discussed. Microtubular SOFCs with a cell configuration of Ni-YSZ/YSZ/YSZ-LSM (LSM = (La(0.8)Sr(0.2))(0.95)MnO(3-x)) and optimized anode microstructure show cell output power density of 833.9 mW cm(-2) at 750 °C using humidified H2 as fuel and ambient air as oxidant.

Slip-cast and hot-solution infiltrated porous yttria stabilized zirconia (YSZ) supported tubular fuel cells

Hot solution infiltration was investigated as a flexible and rapid method to incorporate anode and cathode components into fully sintered, porous ceramic tubular templates for use as solid oxide fuel cells (SOFC). Composed of either a porous 8 mol% yttria-stabilized zirconia (YSZ) or 5 wt% NiO–YSZ support structure, a thin Ni–YSZ anode functional layer and an outer ca. 10 μm dense YSZ electrolyte, closed end tubes were first hot solution (ca. 100 °C) infiltrated on the inside with NiO–SDC (Sm0.2Ce0.8O1.9) to serve as the anode. Cathodes were either LSM (nominally La0.8Sr0.2MnO3+δ) infiltrated into a thin porous YSZ layer on the outer electrolyte surface, or an LSCF–GDC composite (Gd0.1Ce0.9O1.95–La0.6Sr0.4Co0.2Fe0.8O3−δ) on a thin GDC buffer layer. Although hot solution infiltration of the Ni, Ce and Sm salts into the anode support structure did not result in complete penetration (with the Ni contents in the tube wall ranging between 4 and 10 vol.%), well-sealed full cells produced power densities as high as 275, 196 and 153 mW cm−2 at 800, 750 and 700 °C, respectively. Hot solution infiltration of active SOFC electrode materials is thus shown to be a very flexible approach for the evaluation of their performance.

Tubular solid oxide fuel cells fabricated by slip-phase inversion combined with a vacuum-assisted coating technique

Anode-supported tubular solid oxide fuel cells with diameters of ~4mm based on yttria-stabilized zirconia (YSZ) electrolytes were fabricated by slip-phase inversion combined with a vacuum-assisted coating technique. NiO–YSZ anode tubes were prepared by slip-phase inversion and the YSZ electrolyte membranes were prepared on the NiO–YSZ anodes by the vacuum-assisted coating method. The effect of pre-sintering treatment of anodes on the densification of the supported electrolytes was studied. Dense electrolyte membranes were obtained based on the anodes which were pre-sintered at 1000°C with the co-sintering temperature of 1400°C. Furthermore, an anode functional layer between anode and electrolyte was also fabricated by the vacuum-assisted coating method to enhance the performance of the cell. With La0.8Sr0.2MnO3−δ–YSZ (LSM–YSZ) as the composite cathode, a maximum power density of 178mWcm−2 was obtained at 700°C for the single cell with an anode functional layer and a 22-µm-thick YSZ electrolyte. Resistances of the tested cells were investigated under open circuit conditions at different operating temperatures by impedance spectroscopy.