3D microstructure effects in Ni-YSZ anodes : prediction of effective transport properties and optimization of redox stability

This study investigates the influence of microstructure on the effective ionic and electrical conductivities of Ni-YSZ (yttria-stabilized zirconia) anodes. Fine, medium, and coarse microstructures are exposed to redox cycling at 950 ºC. FIB (focused ion beam)-tomography and image analysis are used to quantify the effective (connected) volume fraction (Φeff), constriction factor (β), and tortuosity (τ). The effective conductivity (σeff) is described as the product of intrinsic conductivity (σ0) and the so-called microstructure-factor (M): σeff = σ0 x M. Two different methods are used to evaluate the M-factor: (1) by prediction using a recently established relationship, Mpred = ε β^0.36/τ^5.17, and (2) by numerical simulation that provides conductivity, from which the simulated M-factor can be deduced (Msim). Both methods give complementary and consistent information about the effective transport properties and the redox degradation mechanism. The initial microstructure has a strong influence on effective conductivities and their degradation. Finer anodes have higher initial conductivities but undergo more intensive Ni coarsening. Coarser anodes have a more stable Ni phase but exhibit lower YSZ stability due to lower sintering activity. Consequently, in order to improve redox stability, it is proposed to use mixtures of fine and coarse powders in different proportions for functional anode and current collector layers

3D microstructure effects in Ni-YSZ anodes : influence of TPB lengths on the electrochemical performance

3D microstructure-performance relationships in Ni-YSZ anodes for electrolyte-supported cells are investigated in terms of the correlation between the triple phase boundary (TPB) length and polarization resistance (Rpol). Three different Ni-YSZ anodes of varying microstructure are subjected to eight reduction-oxidation (redox) cycles at 950 °C. In general the TPB lengths correlate with anode performance. However, the quantitative results also show that there is no simplistic relationship between TPB and Rpol. The degradation mechanism strongly depends on the initial microstructure. Finer microstructures exhibit lower degradation rates of TPB and Rpol. In fine microstructures, TPB loss is found to be due to Ni coarsening, while in coarse microstructures reduction of active TPB results mainly from loss of YSZ percolation. The latter is attributed to weak bottlenecks associated with lower sintering activity of the coarse YSZ. The coarse anode suffers from complete loss of YSZ connectivity and associated drop of TPBactive by 93%. Surprisingly, this severe microstructure degradation did not lead to electrochemical failure. Mechanistic scenarios are discussed for different anode microstructures. These scenarios are based on a model for coupled charge transfer and transport, which allows using TPB and effective properties as input. The mechanistic scenarios describe the microstructure influence on current distributions, which explains the observed complex relationship between TPB lengths and anode performances. The observed loss of YSZ percolation in the coarse anode is not detrimental because the electrochemical activity is concentrated in a narrow active layer. The anode performance can be predicted reliably if the volume-averaged properties (TPBactive, effective ionic conductivity) are corrected for the so-called short-range effect, which is particularly important in cases with a narrow active layer

Reporte de caso: Paciente Peruano de 52 años con Fibrosis Quística

The Cystic fibrosis (CF) is an autosomal and recessive genetic disease. It affects much more frequently the population of Caucasian origin, where their incidence ranges from 1 in 3,000 in Caucasians to 1 in 8,000 in newly Hispanic live births. By 2018, 30,775 patients with cystic fibrosis, of them less than 10% of patients worldwide had more 40 years and none had been diagnosed after 40 years of age. We present below the case of a 52-year-old Peruvian adult with CF. This would be one of the patients with the latest diagnosis worldwide and the longest patient in Peru and one of the longest in Latin America.La fibrosis quística (FQ) es una enfermedad genética autosómico y recesiva. Afecta con mucha más frecuencia a la población de origen caucásico, donde su incidencia varía de 1 entre 3 000 en caucásicos a 1 entre 8 000 en hispanos recién nacidos vivos. Para el 2018 se encontraban registrados 30 775 pacientes con fibrosis quística, de ellos menos del 10% de pacientes a nivel mundial tenían más de 40 años y ninguno había sido diagnosticado luego de los 40 años de edad. Presentamos a continuación el caso de un adulto peruano de 52 años con FQ. Este sería uno de los pacientes con el diagnóstico más tardío a nivel mundial y el paciente más longevo del Perú y uno de los más longevos de América Latina

3D Microstructure Effects in Ni-YSZ Anodes: Influence of TPB Lengths on the Electrochemical Performance

3D microstructure-performance relationships in Ni-YSZ anodes for electrolyte-supported cells are investigated in terms of the correlation between the triple phase boundary (TPB) length and polarization resistance (Rpol). Three different Ni-YSZ anodes of varying microstructure are subjected to eight reduction-oxidation (redox) cycles at 950 °C. In general the TPB lengths correlate with anode performance. However, the quantitative results also show that there is no simplistic relationship between TPB and Rpol. The degradation mechanism strongly depends on the initial microstructure. Finer microstructures exhibit lower degradation rates of TPB and Rpol. In fine microstructures, TPB loss is found to be due to Ni coarsening, while in coarse microstructures reduction of active TPB results mainly from loss of YSZ percolation. The latter is attributed to weak bottlenecks associated with lower sintering activity of the coarse YSZ. The coarse anode suffers from complete loss of YSZ connectivity and associated drop of TPBactive by 93%. Surprisingly, this severe microstructure degradation did not lead to electrochemical failure. Mechanistic scenarios are discussed for different anode microstructures. These scenarios are based on a model for coupled charge transfer and transport, which allows using TPB and effective properties as input. The mechanistic scenarios describe the microstructure influence on current distributions, which explains the observed complex relationship between TPB lengths and anode performances. The observed loss of YSZ percolation in the coarse anode is not detrimental because the electrochemical activity is concentrated in a narrow active layer. The anode performance can be predicted reliably if the volume-averaged properties (TPBactive, effective ionic conductivity) are corrected for the so-called short-range effect, which is particularly important in cases with a narrow active layer

Quantitative relationships between microstructure and effective transport properties based on virtual materials testing

The microstructure influence on conductive transport processes is described in terms of volume fraction ε, tortuosity τ, and constrictivity β. Virtual microstructures with different parameter constellations are produced using methods from stochastic geometry. Effective conductivities σeff are obtained from solving the diffusion equation in a finite element model. In this way, a large database is generated which is used to test expressions describing different micro-macro relationships such as Archie's law, tortuosity, and constrictivity equations. It turns out that the constrictivity equation has the highest accuracy indicating that all three parameters (ε, τ, β) are necessary to capture the microstructure influence correctly. The predictive capability of the constrictivity equation is improved by introducing modifications of it and using error-minimization, which leads to the following expression: σeff = σ0^2.03ε^1.57β^0.72/τ^2 with intrinsic conductivity σ0. The equation is important for future studies in, for example, batteries, fuel cells, and for transport processes in porous materials

FIB-tomography data of porous Zr-oxide fabricated with varying sintering conditions

This dataset contains 3D data acquired with FIB-tomography from porous Zr-oxide. The pore structures of the ceramic materials were investigated in context with optimization of transport properties (aimed for low viscous flow/permeability and high electric conduction via liquid electrolyte in the pores), so that the Zr-oxide is suitable for application as liquid junction (diaphragm) in pH-sensors. Further details on the scientific background are given in Holzer et al., 2016, Materials & Design, 99, 314–327. (http://doi.org/10.1016/j.matdes.2016.03.034). The tomographs represent 9 ceramic materials that were produced with different sintering temperatures (1250, 1300, 1325 and 1350°C) and different sintering times (1, 2 and 3 hours). The study includes quantitative descriptions of relevant microstructure characteristics (porosity, tortuosity, constrictivity, hydraulic radius) and corresponding effective transport prorperties (permeability, conductivity). Each tomograph is presented as stack of 2D-tiff-images in two different versions: as gray-scale images (raw data) and as binarized images (segmented, with solid=black and pores=white). The voxel resolution is 10x10x10 nm for all 18 image stacks

Microstructure-property relationships in a gas diffusion layer (GDL) for polymer electrolyte fuel cells, Part II : pressure-induced water injection and liquid permeability

The performance of polymer electrolyte fuel cells (PEFC) strongly depends on a controlled water management within the porous layers. For this purpose we investigate liquid water transport in a commercial gas diffusion layer (SGL 25BA) on the pore scale. X-ray tomography experiments combined with pressure-induced water injection provide 3D images of the liquid water distribution inside the GDL at incremental pressure steps between 0 and 100 mbar. The breakthrough behavior of the liquid phase is highly anisotropic. In through-plane (tp) direction first bubble points appear at the outlet plane already at 5 mbar and the ‘breakthrough' then evolves continuously over an extended pressure range up to > 30 mbar. For in-plane (ip) direction the breakthrough is discontinuous and takes place at 27 mbar. Simulations of the intrusion process reveal that the different breakthrough behaviors are mainly triggered by different ip- and tp-transport distances. Short tp-transport distances through the thin gas diffusion layer (ca. 100 μm) are responsible for the characteristic continuous tp-breakthrough behavior, which is thus attributed to a so-called short-range effect. Dedicated methods for 3D-image analysis adapted to fibrous GDL microstructures were presented in part I. With these methods we quantify all microstructure characteristics that are relevant for liquid permeability. These characteristics of pore and liquid phases include size distributions of bulges and bottlenecks, connectivity, effective volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. Quantitative relationships are established between these microstructure characteristics and the liquid permeability, which provide a better understanding of the underlying microstructure limitations for injection and liquid transport. For the in-plane direction the liquid permeability is limited to roughly a similar extent by tortuosity, constrictivity and effective volume fraction. In contrast, for through-plane direction relatively low volume fractions of the liquid phase put stronger limitations to the liquid permeability than tortuosity, constrictivity and hydraulic radius. The curves for relative permeability vs. saturation (and vs. capillary pressure, respectively) achieved from 3D-analysis reveal complex but characteristic (reproducible) shapes with concave, linear and convex segments. The shape of these segments can be attributed to distinct microstructure effects. In contrast, the conventional macroscopic descriptions from literature cannot capture these complex shapes and the underlying microstructure effects. Future investigations with different GDL materials are necessary in order to understand whether these complex shapes for the relative permeability represent a general feature of gas diffusion layers or if they are specific to the investigated SGL material

Ohmic resistance of nickel infiltrated chromium oxide scales in solid oxide fuel cell metallic interconnects

Oxide scale formation on metallic interconnects contributes to the overall degradation of solid oxide fuel cell (SOFC) stacks. On the anode side, thermally grown oxide scale might contain additional nickel, nickel oxide, or nickel chromium spinel phases – depending on the applied operation conditions. Ni originates from Ni-meshes, often applied as current collector, from Ni-containing anodes or from Ni-containing coatings. Ni particles released during thermo redox cycles from adjacent Ni-containing components might be interspersed into the oxide scale. This study aims at investigating the influence of Ni on the electrical conductivity of oxide scales. For this purpose pellets of Cr2O3 were mixed with different amounts of Ni and then investigated in-situ under both reducing and oxidizing gas atmospheres at 850 °C. The formed crystals were analyzed using X-ray diffraction, whereas the resulting microstructures were quantified using scanning electron microscopy. During oxidation Ni is converted into NiO, and the latter interacts with Cr2O3 to form a NiCr2O4 spinel phase. Subsequent exposure to reducing conditions leads to an almost instantaneous decomposition of NiCr2O4 spinel, resulting in finely dispersed elementary Ni. This rearrangement of Ni by spinel decomposition leads to a significant improvement of the electrical conductivity of the Cr2O3 pellets compared to their initial state

Influence of strontium-rich pore-filling phase on the performance of La0.6Sr0.4CoO3−δ thin-film cathodes

Nanoporous 1-μm thin La0.6Sr0.4CoO3−δ (LSC) layers are deposited by spray pyrolysis and subsequently sintered at 600°C, 800°C, and 1000°C. A strontium- and oxygen-rich phase can be found within the pore network, which appears at low sintering temperatures. This so-called “secondary phase” occupies up to 20.7 vol.% of the LSC films for the 600°C annealing process. It does not hinder the electrochemical activity towards oxygen reduction of such layers that exhibit an area-specific resistance (ASR) as low as 0.13 Ω cm2 at 600°C in air. This result makes the spray pyrolysed LSC thin films promising candidates as intermediate-temperature solid oxide fuel cell cathodes. For higher sintering temperatures the secondary phase progressively disappears. A correlation between the inverse of the ASR and the whole LSC surface area (regardless of the presence of the secondary phase or not) is also evidenced. The increase of ASR with increasing sintering temperature is found to be primarily related to the exchange neutral flux density of the Sr-deficient LSC