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Abstract Nickel – yttria stabilized zirconia (Ni-YSZ) anodes for solid oxide fuel cells were infiltrated with nickel nanoparticles with the objective of improving its performance by increasing the density of triple phase boundaries (TPBs). However, the nanoparticles deposited on the YSZ grains are not connected to one another, so the additional TPBs are not electrochemically active. It is postulated that at highly humid conditions (>95% H 2 O) created by high current density, nickel nanoparticles near the anode-electrolyte interface will spread and percolate, becoming electrochemically active. To study this behavior, infiltrated nickel nanoparticles were exposed to low, high and extreme humidity conditions during testing. When exposed to low humidity, no performance improvement or changes to the nanoparticle morphology was observed. At high humidity, the cell showed performance improvement and when cooled they showed nanoparticle coarsening. Exposure to extreme humidity (≈100%) caused performance to degrade back towards that of an un-infiltrated cell and severe nickel nanoparticles coarsening was observed in the cooled cell. This study validates the hypothesis that exposure to controlled high local humidity conditions can lead to the right amount of spreading of the deposited nickel nanoparticles to achieve percolation, thereby activating their TPBs and improving cell performance. Highlights • Nickel nanoparticles improve Ni-YSZ cermet performance after spreading. • High humidity in the electrode causes infiltrated nanoparticles to spread. • Nickel spreading percolates infiltrated nanoparticles, improving performance. • Cells without nanoparticle spreading do not show improved performance. [ABSTRACT FROM AUTHOR]

Liquid infiltration of NiO followed by reduction to form Ni nanoparticle catalysts in solid oxide fuel cell (SOFC) can produce a high density of electrochemical reaction sites. In recent years, electrode architectures utilizing porous oxide substrates with ionic conductivity or mixed ionic-electronic conductivity and connected networks of nickel produced by liquid infiltration have become a popular approach to improve SOFC anode catalytic performance, especially for operating temperatures less than 800 °C. However, infiltrated nickel structures suffer from poor durability, demonstrating significant loss in performance during the first 100 h of use. In contrast, traditional Ni-yttria stabilized zirconia (Ni-YSZ) cermet SOFC anodes exhibit long-term performance stability. However, Ni-YSZ cermet anodes have micron sized structures, and consequently have a significantly lower density of electrochemical reaction site density than infiltrated nickel structures, which have dimensions of around 100 nm. In this study, the performance impact of liquid phase infiltration of nickel nanoparticles into Ni-YSZ cermet anode supported SOFCs is studied by measuring the electrochemical behavior of infiltrated cells at 800 °C, 700 °C, and 600 °C, and comparing them to the performance of an uninfiltrated cell. Durability of the nanoparticles after electrochemical testing is also assessed using a method for quantifying particle statistics from fracture cross sections. [ABSTRACT FROM AUTHOR]

For several decades an open question in many X-ray photoelectron spectroscopy (XPS) studies was whether or not the results obtained in ultra-high vacuum conditions (UHV) were representative of the sample state in gas atmospheres. As a consequence, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was received by surface scientists as an important tool for in situ characterization of the gas-solid interactions. However, it is not yet clear how, if at all, the surface state formed in contact with the gas is modified when this gas is evacuated. In this work we compare synchrotron-based XPS results recorded at 300 °C on Ni/yttria- stabilized zirconia cermet and La 0.75 Sr 0.25 Cr 0.9 Fe 0.1 O 3 perovskite, under 3.5 mbar O 2 and UHV environments. We found that the surface state formed in O 2 is maintained to a large extent under vacuum. In addition, we demonstrate that the correlation of XPS spectra recorded in the two conditions can provide information regarding the electrical conductivity of the specific surface sites of these complex oxides. Our findings suggest that comparison of XPS measurements in gas and in vacuum environments might be particularly useful in applications where the electronic conductivity at the surface plays a crucial role, as for example in solid oxide electrochemical devices. [ABSTRACT FROM AUTHOR]

The change in porosity of the Ni-3.5YSZ (ZrO stabilized with 3.5 mol.% YO) composite, when produced, and the effect of porosity on the strength and electrical conductivity is studied. The porosity was provided by adding granular starch to the mixture of NiO and 3.5YSZ powders. The content of 18 vol.% pore-forming agent in the Ni-3.5YSZ cermet provides open porosity 47.1%, while its strength and electrical conductivity are 74.3 MPa and 0.93 · 10 S/m, respectively. [ABSTRACT FROM AUTHOR]

Ni-YSZ (nickel - yttria stabilized zirconia) is a material widely used for electrodes and supports in solid oxide electrochemical cells. The mechanical and electrochemical performance of these layers, and thus the whole cell, depends on their microstructure. During the initial operation of a cell, NiO is reduced to Ni. When this process is conducted under external load, like also present in a stack assembly, significant deformations of NiO/Ni-YSZ composite samples are observed. The observed creep is orders of magnitude larger than the one observed after reduction during operation. This phenomenon is referred to as accelerated creep and is expected to have a significant influence on the microstructure development and stress field present in the Ni-YSZ in solid oxide electrochemical cells (SOCs), which is highly important for the durability of the SOC. In this work we present energy selective neutron imaging studies of the accelerated creep phenomenon in Ni/NiO-YSZ composite during reduction and also during oxidation. This approach allowed us to observe the phase transition and the creep behavior simultaneously in-situ under SOC operation-like conditions. [ABSTRACT FROM AUTHOR]

Tubular SOFC have the advantage over planar SOFC of the low temperature sealing and more resistance to thermal shock. On the other hand the volumetric power density of tubular Fuel Cells goes with the inverse of the tube diameter which added to the faster warm-up kinetics makes low diameter tubular SOFC favorable for low power applications. Anode supported tubular SOFC of 3mm diameter and 150 mm length with YSZ electrolyte were fabricated and tested by V-I measurements using H2-Ar (5, 10, 100 vol%) as fuel and air for the cathode. The NiO-YSZ tubes of about 400 μm thickness were produced by hydrostatic pressure and then coated with an YSZ film of 15-20 μm. The electrolyte was deposited using a manual aerograph. After sintering either Pt paste or LSF (with YSZ or SDC) coatings of about 20-50 μm thickness were deposited for the cathode. The OCV of the cells were excellent, very close to the expected Nernst law prediction indicating that there were not gas leaks. The maximun electrical power of the cell was near to 500mW/cm2 at 850ºC operation temperature. Complex impedance measurements of the cells were performed in order to determine the resistance of the different cell components.La principal ventaja de las SOFC tubulares frente a las planares es el sellado de la cámara anódica y catódica a bajas temperaturas. Además la densidad de energía volumétrica de las pilas tubulares es inversamente proporcional al diámetro del tubo, que añadido a los tiempos cortos de encendido y apagado hacen que las mini-tubulares sean interesantes para usos de baja potencia. Se han fabricado y caracterizado SOFC tubulares soportadas en ánodo de 3mm de diámetro y de 150 mm de longitud, 400μm de espesor, con electrolito de YSZ depositado por spray de 15-20 μm. Los tubos de NiO-YSZ son producidos por prensado isostático. La caracterización eléctrica se ha realizado empleando H2-Ar como combustible anódico (5, 10, 100%vol) y aire para el cátodo. Los cátodos empleados son pintura de Pt o capas de LSF (con YSZ o SDC) de entre 20-50μm. Los OCV medidos ajustan con la predicción prevista de la ley de Nernst lo que indica que las capas de YSZ son estancas al paso de gas. Las curvas I-V muestran una corriente eléctrica máxima próxima a 500mW/cm2 operando a 850ºC. También se han realizado medidas de impedancia compleja para determinar la resistencia de los diversos componentes de la pila.

Abstract: Microstructure of a cermet Ni–YSZ bi-layer is analysed on the basis of three dimensional reconstructions obtained on both functional layer and cell support. Microstructural parameters of gas, ionic and electronic phases are determined in terms of phase connectivity, mean particles diameter, particles size distribution, specific surface area, tortuosity factor and density of TPBls. Microstructural properties are introduced in an SOEC cathode micro model that takes into account the specific configuration of the Ni–YSZ composite bi-layer. Simulations show that the extent of the electrochemical reaction in the support is very limited. Moreover, it is found that electrode apparent activation energy is a combination of effective ionic conduction and charge transfer in the active functional layer. [Copyright &y& Elsevier]

Holographic and near-field ptychographic X-ray computed tomography techniques have been compared by characterizing a typical solid oxide cell hydrogen electrode using a high-energy X-ray beam. The main advantages and drawbacks of both methods are discussed regarding the possibility to image the Ni-YSZ cermet, a complex porous electrode microstructure composed of X-ray absorbent materials. The same innovative sub-pixel alignment algorithm, based on tomographic consistency, was applied to align the different tomographic projections for each technique. It has been shown that a better signal-to-noise ratio (SNR) is obtained using near-field ptychographic tomography, whereas holographic tomography can be faster with similar spatial resolution. Moreover, quantitative electron density maps have been obtained with the two techniques. The quality of the phase identification has also been assessed and compared in both cases using a classical grey-level class separability criterion. After the segmentation, a set of typical microstructural properties describing the electrode morphology was computed. The comparison of the results allowed validating the complementarity of the two X-ray imaging techniques. Despite the more time-consuming data acquisition and processing than holographic tomography, near-field ptychographic tomography is especially well adapted to image samples without any insight on their composition or when the sample is highly absorbent. Yet, holographic X-ray tomography, using high-energy X-rays to reduce the sample absorption, remains a faster 3D imaging technique with spatial resolution and contrast sensitivity sufficient for the characterization of solid oxide cell materials. • 3D reconstruction of a Solid Oxide Cell electrode synchrotron X-ray holography and near-field ptychography using hard X-rays. • Comparison of the two reconstruction techniques in terms of acquisition time, computational intensity, quality of the 3D images. • Quantitative electron density 3D maps obtained for the sample with the two methods • Validation of the the microstructural parameters extracted from the raw 3D reconstructions. [ABSTRACT FROM AUTHOR]

This paper describes briefly the use of Ni-YSZ (yttrium oxide doped with 8 % in mol of zirconium oxide) for methanol oxidation in 1.0 mol L KOH solution. The Ni-YSZ has a great performance for methanol oxidation reaching a current density around 160 mA cm. This value is at least four times greater than in platinum electrode. The high performance of Ni-YSZ compared with Pt electrode can also be viewed in measures electrochemical impedance spectroscopy. The Ni-YSZ shows the charge transfer resistance around 1.80 Ohm cm. Thus, in our material, the effect of NiO(OH) barrier is drastically reduced. The chronoamperograms for methanol oxidation at potential equal to 1.0 V shows that the charge density for Ni-YSZ electrode is about four times larger than in Pt electrode. [ABSTRACT FROM AUTHOR]

Abstract: Due to their high operating temperatures, SOFCs can be directly fed with biogas, mainly composed of CH4 and CO2. In this work, experiments was performed with a classical Ni-YSZ cermet//YSZ//LSM cell fed either with a synthetic simulated biogas (CH4/CO2 ratio equal to 1 with 6% humidity), or with humidified H2. In both cases, the performances are found to be very similar, which confirms the ability of SOFCs to operate with internal reforming of biogas. Nevertheless, carbon formation in these operating conditions needs to be considered because of durability concerns. Thermodynamic calculations and modelling are carried out to evaluate the risk of carbon deposition depending on operating parameters. In the ternary diagram Ch name="sbnd" />O, the limits for carbon deposition are plotted, allowing the determination of “safe” operating conditions in terms of CH4 inlet flow rate and cell voltage. First experiments confirm these modelling results. [Copyright &y& Elsevier]

Abstract: The NiO–YSZ powder was prepared by the combustion of nitrate–glycine gel, a novel and versatile technique to form the nano-composite. The phase purity of the as-formed powder was confirmed by XRD. Onset sintering temperature was measured and sintering of NiO–YSZ specimens was carried out at 1450°C for different soaking times. Porosity of the specimens decreases with increasing soaking time. Ni–YSZ cermet with varying open porosities between 23 and 41% was obtained by the in situ reduction of NiO–YSZ. Electrical conductivity, thermal expansion coefficient, and SEM microstructure of the Ni–YSZ specimens possessing varying porosities were evaluated. The electrical conductivity decreased while thermal expansion coefficient remained essentially the same with increasing porosity. SEM results confirmed the presence of interconnected open pores. [ABSTRACT FROM AUTHOR]

Abstract: NiO–YSZ composites and Ni–YSZ cermets were successfully performed for solid oxide fuel cell applications. These composites must have enough porosity and appropriate microstructure for transferring the fuel gases. In this study, ball-milling was used as a simple, cost-effective method for the purpose of mixing the raw materials. The homogeneity of NiO–YSZ composites was examined by Map mode of SEM. NiO–YSZ composites were reduced at the high temperature under the controlled atmosphere to fabricate Ni–YSZ cermet. Variations in the anode phases were investigated by XRD and microstructure and porosity of composites were observed by SEM. Effective parameters like temperatures and the amount of pore former were investigated on open porosity, bulk density, electrical conductivity as well as electrochemical impedance of NiO–YSZ composites and Ni–YSZ cermet. A thin layer of YSZ was deposited by EPD as an electrolyte on NiO–YSZ composites which had various amount of open porosity, to study its effect on the performance of semi-cells by electrochemical impedance. [ABSTRACT FROM AUTHOR]

Abstract: The oxidation of Ni–YSZ cermet as well the reduction of re-oxidized Ni–YSZ cermet was investigated by using temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR) and scanning electron microscope (SEM). The scanning electron microscope (SEM) photographs and temperature-programmed reduction (TPR) profiles indicated that the sintering of smaller nickel oxide crystallites to larger aggregates occurred concurrently with the formation of smaller nickel oxide crystallites from the oxidation of nickel at 800°C, and the sintering of smaller nickel oxide crystallites at 600°C was slower than that at 800°C. The SEM results showed that each Ni particle was separated into a lot of smaller NiO particles during oxidation. The TPO profiles showed that two kinds of nickel particles exist in the anode reduced at 800 and 600°C, one with high activity towards oxidation for the nickel crystallites directly from reduction, and another one with low activity towards oxidation for the sintered nickel particles. The Ni–YSZ anodes reduced at higher temperature showed higher re-oxidation temperature than the one reduced at lower temperature because of the accelerated passivating and sintering of the smaller nickel particles at higher temperature. The re-oxidation profiles were almost unchanged during redox cycling at 600°C, whereas the re-oxidation peak temperature decreased during redox cycling at 800°C, indicating that the primary nickel grains split to smaller ones upon cyclic reduction at higher temperature. [Copyright &y& Elsevier]

Abstract: Ammonia is a possible candidate as the fuel for SOFCs. In this work, the influence on the performance of a tubular SOFC running on ammonia is studied. Analysis of open circuit voltages (OCVs) on the cell indicated the oxidation of ammonia within a SOFC is a two-stage process: decomposition of the inlet ammonia into nitrogen and hydrogen, followed by oxidation of hydrogen to water. For comparison, cell was also tested with hydrogen as the fuel and air as oxidant at different temperatures showing a similar behaviour. The performance of the cell tested under various conditions shows the high potential of ammonia as fuel for SOFCs. [Copyright &y& Elsevier]

Abstract: Atmospheric plasma spray (APS) deposition of a high-performance anode coating, which is essential for obtaining high power density from a solid oxide fuel cell (SOFC), is developed. A conventional, micron-sized, nickel-coated graphite – yttria stabilized zirconia (YSZ) – graphite blend feedstock leads to a non-uniform layered coating microstructure due to the difference in the physical and thermo-physical properties of the components. In this research, new types of feedstock material received from a spray-drying method, which includes nano-components of NiO and YSZ (300nm), are used. The microstructure and mechanical properties of a coating containing a nano composite that is prepared from spray-dried powders are evaluated and compared with those of a coating prepared from blended powder feedstock. The coating microstructures are characterized for uniformity, mechanical properties and electrical conductivity. The coatings prepared from spray-dried powders are better as they provide larger three-phase boundaries for hydrogen oxidation and are expected to have lower polarization losses in SOFC anode applications than those of coatings prepared from blended feedstock. [Copyright &y& Elsevier]

Abstract: The thermoplastic coextrusion process was used to fabricate YSZ/Ni-YSZ bi-layered composites. In this method, an initial feedrod was prepared by assembling YSZ and NiO-YSZ thermoplastic compounds to produce a monolithic body with a cross-section of 24mm×24mm and then coextruded through various orifices having dimensions of 24×2, 1 and 0.6mm at 120°C. This simple process produced thin plates with a remarkable reduction in thickness, while preserving the YSZ:NiO-YSZ thickness ratio of 1:39. After binder removal, the samples were co-fired at 1350°C for 3h in air, followed by reduction at temperatures ranging from 700 to 900°C for 5h in an Ar-5% H2 atmosphere. Using this method, it was possible to fabricate a YSZ/Ni-YSZ bi-layered composite without any noticeable defects, in which a 11-μm thick dense YSZ layer was coated onto a 400-μm thick Ni-YSZ cermet. Furthermore, the fabricated samples showed reasonable electrical conductivities, thus rendering them suitable for use as the basic components of planar solid oxide fuel cells (SOFCs). [Copyright &y& Elsevier]

Abstract: A novel multilayer Ni–YSZ anode structure, which had a gradient in particle size and Ni content, was introduced in this paper. This gradient anode was co-fabricated with electrolyte by a symmetrical uniaxial die-pressing method. The anode functional layer (AFL) was developed to several sub-layers. The influences of different composition and various thickness of the AFL were investigated. The thickness of anode effective electrochemical reaction zone was ascertained to about 40–50μm. A unit cell with gradient in both particle size and Ni content demonstrated a maximum power density of 467.14mWcm−2. EIS result demonstrated that the surface dissociation/diffusion step is easier than charge-transfer reaction, which indicated that the microstructure of the anode, especially anode functional layer, was well fabricated and the distribution of the two phases was homogeneous thus the interfacial resistance was dampened. [Copyright &y& Elsevier]

Abstract: Ni–YSZ cermet was exposed to hydrogen containing different concentrations of H2S to identify the phases formed under various conditions using Raman spectroscopy and X-ray diffraction (XRD). For Ni–YSZ samples exposed to hydrogen containing 100ppm H2S at 727°C for 5 days, thermodynamic calculations indicate that Ni–YSZ would be stable and XRD analysis was unable to detect any changes. However, the vibration modes of Ni3S2 were detected using Raman spectroscopy, suggesting that Raman spectroscopy could be a powerful tool for in situ study of sulfur poisoning of SOFC anodes. For Ni–YSZ cermet exposed to hydrogen containing 10% H2S at 950°C for 5 days, Ni was converted to nickel sulfide, and vibration modes of NiS were detected using Raman spectroscopy. [Copyright &y& Elsevier]

Abstract: The particle size distribution and the specific surface area of yttria stabilized zirconia (YSZ) powders could be changed by pre-calcining. Accordingly, the performances of Ni–YSZ cermet might be changed too. In this paper, the effects of pre-calcined YSZ powders on Ni–YSZ cermet were investigated. The performances of Ni–YSZ cermet, including the basic physical characterization, sintering shrinkage and electrochemical properties, were studied in connection with the quality of YSZ powders pre-calcined at 900°C, 1100°C and 1200°C, respectively. The results indicated that the crystal structure was not changed, but the porosity of open holes in NiO–YSZ pellets was increased with the pre-calcining temperature increasing. In addition, the total shrinkage of different anode materials was changed by using different YSZ powders, the value of Ni–YSZ cermet doping with YSZ pre-calcined at 900°C and 1100°C was closer to that of YSZ electrolyte. The cermet with YSZ powders pre-calcined at 900°C showed the highest electric conductivity, and its polarization performance was closed to that of cermet with raw YSZ powders. [Copyright &y& Elsevier]

Abstract: Anode-supported cells made of conventional materials were tested in single-chamber conditions under various CH4/air gas mixtures. Methane-to-oxygen ratio (R MIX) and nominal temperature between 600 and 800°C both affect the performance of the cell. At a flow rate of 350sccm, maximum values of power density (260mWcm−2) and cell voltage (1.05V) were obtained for R MIX =2 at 800°C. However, short term ageing experiments show that the stability of the cells depends on R MIX as well as the flow of current. Scanning electron micrographs (SEM) reveal some important changes in anode microstructure close to the fuel inlet that may be, assign to the volatilization of the nickel contained in the Ni–YSZ cermet. [Copyright &y& Elsevier]

Abstract: Effect of redox cycling on a Ni–YSZ anode prepared from 50 wt.% NiO and 50 wt.% YSZ was investigated by using temperature-programmed reduction (TPR), XRD and SEM techniques. XRD results showed that NiO was formed during re-oxidation. Both the XRD and TPR results depicted that the conversion of nickel to NiO depended on the re-oxidation temperature. The oxidation of Ni to NiO occurred quickly in the initial several minutes and then reached a quasi equilibrium. The TPR profiles tracing the redox cycling showed that it brought continuous changes in the NiO micro-structure at 800 °C, whereas at 600 °C it had only little effects on the reduction of NiO. Re-oxidation resulted in the formation of spongy aggregates of NiO crystallites. Redox cycling at 800 °C led to a continuous decrease in the primary crystallite size of NiO and a high dispersion of the Ni particles. A continuous expansion of the slice sample was observed in both of the oxidized and reduced states during the redox cycling at 800 °C, whereas this process did not occur during the redox cycling at 600 °C. [Copyright &y& Elsevier]

The most important requirements to electrodes for solid oxide fuel cells and similar devices are listed, and it is explained how the requirements usually lead to a composite electrode. Three examples of composite electrodes are described in some details, namely the ceria based hydrogen electrode, the Ni-YSZ cermet anode and the LSM-YSZ. Here YSZ means yttria stabilized zirconia, and LSM is lanthanum strontium manganate. [ABSTRACT FROM AUTHOR]