Yttria Stabilized Zirconia Backbone Research Articles

Optimizing infiltration parameters of nanostructured anode electrode in solid oxide fuel cells

This study focuses on the development and optimization of nanostructured anode electrodes for solid oxide fuel cells (SOFCs) by infiltration method. Ni (nickel) catalyst in the form of a nickel nitrate solution is infiltrated into porous YSZ (yttria stabilized zirconia) backbone fabricated by tape casting. Numerous electrolyte-supported cells are fabricated to investigate the effects of several significant infiltration fabrication parameters such as catalyst loading and infiltration sintering temperature. Conventional cell with screen printed Ni-YSZ anode is also fabricated for comparison. Electrochemical performances and microstructural properties of the cells are examined and evaluated. The best peak performance of 0.398 W/cm2 at 800 °C is obtained from the cell, which is infiltrated 9 times with 2 M solution followed by firing at 800 °C. The conventional cell, on the hand, exhibits only 0.174 W/cm2 under the same testing conditions in spite of the relatively higher Ni catalyst content in the anode. Furthermore, the optimized cell produces 0.169 W/cm2 maximum power density at 700 °C. The overall results reveal that nickel catalyst infiltration is a very effective method to improve the cell performance by providing increased number of electrochemical reaction zones within the anode electrode due to nanostructured nickel catalyst formed around the main YSZ phase.

Amperometric type NO2 sensor based on La0.75Sr0.25Cr0.5Fe0.5O3-δ-Bi2O3 sensing electrode prepared by self-demixing

Bi2O3 modified La0.75Sr0.25Cr0.5Fe0.5O3-δ (LSCF) composites have been successfully prepared by self-demixing and used as the sensing electrode (SE) of an amperometric NO2 sensor with yttria stabilized-zirconia (YSZ) electrolyte. The Bi2O3 modified LSCF composites were formed from precursor La0.75Sr0.25BixCr0.5Fe0.5O3+δ (x = 0, 5%, 10%, 20%, 30% and 40%) after calcining at 1200 °C. The particle size of LSCF was increased with the Bi2O3 content increasing. The YSZ porous framework structures were fabricated on YSZ to prolong TPB and the LSCF-xBi2O3-SE with porous structure was prepared on the porous YSZ backbone after calcining at 1000 °C. It is found that the sensors attached with LSCF-xBi2O3-SE are more sensitive to different concentrations of NO2 (25–300 ppm) than the LSCF-based sensor. Among them, LSCF-20% Bi2O3 based sensor exhibits the highest sensitivity (0.301 μA /ppm at 550 °C). The relationship between ΔIs and NO2 concentrations of the sensor varies linearly at a given temperature. Meanwhile, the sensor shows well anti-interference capability for co-existent gases.

Fracture of Porous Ceramics: Application to the Mechanical Degradation of Solid Oxide Cell During Redox Cycling

Solid Oxide Cell (SOC) is a high-temperature electrochemical device that can be operated in both fuel and electrolysis modes. It is constituted of a dense electrolyte in Yttria Stabilized Zirconia (YSZ) sandwiched between two porous electrodes. The hydrogen electrode is classically made of a cermet composed of Nickel and YSZ (Ni-YSZ). Because of system failures in operation (e.g. a fuel shortage or air re-introduction during the system shutdown), the Nickel is liable to be re-oxidized inducing its volume expansion. The Ni swelling within the cermet leads to a mechanical damage in the YSZ backbone [1] resulting in a degradation of the overall cell performances [2]. Therefore, it is still required to enhance the hydrogen electrode robustness for a higher redox tolerance. For this purpose, it is essential to predict accurately the formation and distribution of micro-cracks in the YSZ network during the re-oxidation. Nevertheless, the modeling of the initiation and propagation of cracks in complex microstructures of porous ceramics is still a subject of investigation. Moreover, there is also a lack of data on fracture properties for the porous YSZ material. In this frame, the present study was focused to propose a relevant model for the initiation and the growth of cracks in porous ceramics in order to simulate the fracture induced by Ni re-oxidation. For this purpose, a coupled experimental and modeling approach has been adopted (Fig.1). For the experimental part, the mechanical characterizations have been performed on thin YSZ membranes of different porosities representative of SOC microstructures. The compression test has been chosen as it generates a quasi-uniform stress field in the tested samples. To this end, micro-pillars of few tens of micro-meters in section have been milled in the YSZ membrane using a xenon plasma focused ion beam. For each investigated porosity, the compressive fracture strength has been measured on several pillars. The tested samples have been carefully characterized to assess the damage in the microstructure. As expected, the compressive strength was found to decrease with increasing the porosity. Moreover, a transition has been observed from a brittle behaviour at low porosity towards a local damage at high porosity [3]. All these experimental data have been used to validate a numerical tool to simulate the fracture in porous microstructure. In this study, a model based on the Phase Field Method (PFM) has been developed. In this approach, the crack is modeled implicitly through a smooth scalar damage variable and its width is controlled by a length scale parameter. Therefore, this method is well adapted to solve the fracture problem in complex and heterogeneous materials since it is mesh independent. The implemented model corresponds to the one proposed by C. Miehe et al. [4] who proposed a staggered scheme for the numerical resolution. A special attention has been paid in this work to evaluate the model capability to accurately predict the crack initiation for porous electrode material. For this purpose, the results of the PFM simulations have been compared to the mixed criterion proposed by D. Leguillon [5]. The study has been conducted on notched samples with different opening angles and for macro-cracks blunted by different cavity sizes. It has been highlighted that the two approaches provide consistent predictions for the crack initiation. Moreover, it has been checked that the PFM model is able to reproduce accurately the expected decrease of the fracture toughness when increasing the material porosity. Once these studies completed, the PFM model has been used to simulate the micro-compression test using a multiscale approach. It has been found that the model is able to reproduce correctly the decrease of the measured compressive fracture strength with the sample porosity. Moreover, the distribution of cracks in the simulations has enabled analysing the transition from a brittle to a local damage detected during the experiments. The validated model has been applied to simulate the fracture induced by the Ni re-oxidation. The repartition and density of micro-cracks in the YSZ backbone has been computed during the Ni reoxidation and the results have been confronted with an available tomographic data set. Finally, the impact on the cermet redox tolerance of the Yttrium content in YSZ has been discussed thanks to the modelling. [1] Faes, A. et al. J. Power Sources 193, 55–64. [2] Laurencin, J. et al. J. Power Sources 192, 344–352. [3] Meille, S. Et al . J. Eur. Ceram. Soc. 32, 3959–3967. [4] Miehe, C. et al. Comput. Methods Appl. Mech. Eng. 199, 2765–2778. [5] Leguillon, D. J. Mech. – A/Solids 21, 61–72. Figure 1

Impact of Nickel agglomeration on Solid Oxide Cell operated in fuel cell and electrolysis modes

Long-term experiments have been carried out to investigate the impact of Nickel (Ni) coarsening on the performance of Solid Oxide Cell. Durability tests have been performed with H2 electrode supported cells at 850 °C and 750 °C in fuel cell and electrolysis modes. Microstructural changes in the composite electrode of Nickel and Yttria Stabilized Zirconia (YSZ) have been characterized by synchrotron X-ray nanotomography. Analysis of the reconstructions have revealed that Ni coarsening induces a significant decrease of both the density of Triple Phase Boundary lengths (TPBls) and the Ni/gas specific surface area. However, the contact surface between Ni and YSZ is not changed upon operation, meaning the Ni sintering is inhibited by the YSZ backbone. Moreover, the Ni coarsening rate is independent of the electrode polarization. The evolution of TPBls in operation has been fitted by a phenomenological law implemented in an electrochemical model. Simulations have shown that microstructural changes in the H2 electrode explain ∼30% of the total degradation in fuel cell mode and ∼25% in electrolysis mode at 850 °C after 1000–2000 h. Moreover, it has been highlighted that the temperature at which the degradation is estimated after the durability experiment plays a major role on the result.

Continuous conversion of biomass wastes in a La0.75Sr0.25Cr0.5Mn0.5O3–δ based carbon–air battery

In this study, a carbon–air battery based on a solid oxide fuel cell (SOFC) is demonstrated to continuously generate electric power through a direct oxidation of the biomass wastes. Due to inevitable poisoning gases such as H2S, SO2, CO2 in wastes, more stable La0.75Sr0.25Cr0.5Mn0.5O3–δ (LSCM) instead of Ni is used as the anode material for SOFC. Nano– and/or micro–particles of LSCM are prepared onto a 3D porous yttria–stabilized zirconia (YSZ) scaffold via an infiltration process, in which YSZ backbone provides rich pathways for O2− which can remedy the major defect of the LSCM, negligible O2− conductivity, and thus expanding the triple–phase boundary (TPB) to the whole anode. When falling leaves are tucked into the anode chamber, an attractive performance of 330 mW cm−2 can be reached for the battery at 800 °C, which is among the best in the published open literature. There is no visible degradation in cell performance during stability-test for 1000 min. This work provides a novel waste fed SOFC system that has a wide range of application potentials due to its unique advantages of low cost, low emission and high efficiency.

Electrical Performance of Nano-Structured La0.6Sr0.4Co0.2Fe0.8O3-δ Impregnated onto Yttria-Stabilized Zirconia Backbone

Perovskite structure La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) nano-particles are deposited onto porous yttria-stabilized zirconia frameworks via an ion impregnation/infiltration process. The apparent conductivity of the impregnated LSCF nano-structure is investigated regarding the fabricating parameters including LSCF loading, heat-treatment temperature, heating rate, and annealing at 750°C for 400 hr. The conductivity and the intrinsic conductivity of the impregnated LSCF is estimated from the apparent conductivity using the conductive model for the impregnated network. The conductivity increases with LSCF loading while the interfacial polarization resistance exhibits the lowest value at an optimal loading of about 5 vol.%, which corresponds to the largest three-phase boundary as predicted using the numerical infiltration methodology. At the optimal loading, the area specific ohmic resistance of the impregnated LSCF is about 0.027 Ω cm2 at 700°C for a typical impregnated cathode of 30 μm thick. It is about 20% of the cathode interfacial polarization resistance and 3.5% of the total resistance for a single cell consisting of a Ni-YSZ support, a 10 μm thick electrolyte and a 30 μm thick cathode.

Electrical performance of nanostructured strontium-doped lanthanum manganite impregnated onto yttria-stabilized zirconia backbone

Strontium-doped lanthanum manganite (LSM) nanoparticles are deposited onto porous yttria-stabilized zirconia frameworks via an ion impregnation/infiltration process. The apparent conductivity of the impregnated LSM nanostructure is investigated regarding the fabricating parameters including LSM loading, heat treatment temperature, heating rate, and annealing at 750 °C for 400 h. Besides, the conductivity, the intrinsic conductivity as well as Bruggeman factor of the impregnated LSM is estimated from the apparent conductivity using the analytical model for the three-dimensional impregnate network. The conductivity increases with LSM loading while the interfacial polarization resistance exhibits the lowest value at an optimal loading of about 5 vol.%, which corresponds to the largest three-phase boundary as predicted using the numerical infiltration methodology. At the optimal loading, the area specific ohmic resistance of the impregnated LSM is about 0.032 Ω cm2 at 700 °C for a typical impregnated cathode of 30 μm thick. It is only 5.5% of the cathode interfacial polarization resistance and 3.3% of the total resistance for a single cell consisting of a Ni-YSZ support, a 10 μm thick electrolyte and a 30 μm thick cathode, demonstrating that the ohmic resistance is negligible in the LSM impregnated cathode for SOFCs.

Enhancing the Stability of Infiltrated Ni/YSZ Anodes

One of the challenges related to Ni-based anodes in solid oxide fuel cells (SOFCs) is their dimensional instability, especially when they are exposed to air and then back to fuel, which is called redox cycling. One possible solution to this problem is the fabrication of a pre-sintered porous electrolyte matrix, followed by infiltration of 10-20 vol% of the active materials into this scaffold. However, this can lead to instability of the infiltrated particles as a result of sintering of the infiltrated phase at SOFC working temperatures. This problem is very severe when depositing nano-sized infiltrated metal particles on the oxide ceramic backbone. In this study, Ni-containing solutions were infiltrated into a symmetrical tubular half-cell, fabricated by slip-casting, composed of two porous yttria-stabilized zirconia (YSZ) layers, separated by a 50 μm dense YSZ electrolyte. A combination of electrochemical impedance spectroscopy (EIS) and electron microscopy imaging has been used to determine long term stability of the cells. It was found that the amount of infiltrated Ni has a significant impact on the long term stability of the Ni/YSZ anodes. This can be explained by the better connectivity between Ni particles even after long term testing when there is more Ni in the structure. It was also demonstrated that high temperature treatment of the infiltrated Ni/YSZ anodes just after the first few infiltrations, followed by several further Ni infiltration steps, has a significant effect not only on the stability of the anode at 800 C, but also on the anode performance. As the YSZ backbone has the ability to dissolve NiO at higher temperatures, the dissolved NiO can be ex-soluted in the form of nano-sized Ni particles at cell working temperatures and under reducing atmospheres. This effect was also investigated using electron microscopy, revealing the size and shape of the ex-soluted Ni particles, especially after long term testing. Finally, the effect of infiltration of cerium oxide prior to Ni infiltration on the stability of the anodes was also investigated. As the wettability of Ni particles on a reduced ceria surface is different from non-reduced ceria, the sintering behaviour of Ni on these two surfaces was found to be different as well.

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).

Electrochemical stability of Sm(0.5)Sr(0.5)CoO(3-δ)-infiltrated YSZ for solid oxide fuel cells/electrolysis cells.

Composite SSC (Sm(0.5)Sr(0.5)CoO(3-δ))-YSZ (yttria stabilized zirconia) oxygen electrodes were prepared by an infiltration process. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) of the composite electrodes showed the formation of SSC perovskite and a well-connected network of SSC particles in the porous YSZ backbone, respectively. The electrochemical performance of the cell was investigated under both fuel cell and steam electrolysis modes using polarization curves and electrochemical impedance spectroscopy (EIS). The cell experienced a large degradation rate at 700 °C with a constant voltage of 0.7 V for over 100 h under power generation operation. The subsequent post-cell SEM micrograph revealed that agglomeration of the infiltrated SSC particles was possibly the cause for the performance deterioration. Furthermore, the long-term stability of the cell was examined at 700 °C with a constant voltage of 1.3 V under steam electrolysis mode. SEM associated with energy dispersive X-ray spectroscopy (EDS) was employed to characterize the post-test cell after the long-term electrolysis operation and it indicated that besides the agglomeration of SSC particles, the delamination of the SSC-YSZ oxygen electrode from the YSZ electrolyte, as well as segregation of cobalt-enriched particles (particularly cobalt oxides) at the interface, was probably responsible for the cell degradation under the steam electrolysis mode.

High Performance Infiltrated Backbones for Cathode-Supported SOFC's

The concept of using highly ionic conducting backbones with subsequent infiltration of electronically conducting particles has widely been used to develop alternative anode-supported SOFC´s. In this work, the idea was to develop infiltrated backbones as an alternative design based on cathode-supported SOFC. The cathodes are obtained by infiltrating LSM into a sintered either thick (300 μm) yttria stabilized zirconia (YSZ) backbone or a thin YSZ backbone (10-15 μm) integrated onto a thick (300 μm) porous strontium substituted lanthanum manganite (LSM) and YSZ composite.Fabrication challenges, microstructural characterization and electrochemical testing are discussed. Data on polarization resistance, Rp, are obtained from impedance spectra recorded on quasi-symmetrical cells (YSZ backbones/YSZ/LSM-YSZ (screen printed)). The backbones are infiltrated with LSM and compared to a standard LSM-YSZ screen printed symmetrical cells. Samples with LSM/YSZ composite and YSZ backbones made with graphite+PMMA as pore formers exhibited comparable Rp values to the screen printed LSM/YSZ cathode. This route was chosen as the best to fabricate the cathode supported cells.SEM micrograph of a cathode supported cell with infiltrated LSM nanoparticles is shown in Fig. 1. Figure 1. Cross section of LSM infiltrated cathode supported cell.

Theoretical model for surface diffusion driven Ni-particle agglomeration in anode of solid oxide fuel cell

The agglomeration of Ni particles in nickel–yttria stabilized zirconia (YSZ) anode is an important degradation mechanism for the solid oxide fuel cell and is widely believed to be driven by surface diffusion. This work aims to develop a quantitative model to describe the agglomeration kinetics. The model treats the anode as a system of random packing Ni and YSZ particles. Surface diffusion occurs between the connected Ni particles of different sizes characterized by two representative radii, but is influenced by the YSZ network. The Fick's law for diffusion, the Gibbs–Thomson relation for vacancy concentration and the coordination number theory for percolating Ni network are employed in the mathematical derivation. The growth kinetics is expressed as an analytical function consisting of two model parameters, one for the Ni-particle size distribution and the other for the influence of the YSZ backbone. The model is in excellent agreement with the available experiments. The influence of the YSZ backbone is further considered to obtain a model with just one fitting parameter. The one-parameter model is also in good agreement with the experiments and the fundamental physics for the Ni-particle growth is therefore believed to be well characterized.