Three-phase Boundary Density Research Articles

In-Operando Observations of Ni-YSZ Patterned Fuel Electrodes Under SOFC and SOEC Operations

Solid oxide cells (SOCs), both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC), have being attracting more and more attention due to its high energy conversion efficiency and fuel flexibility. However, degradation of fuel electrodes after long-term operation remains as one of the main challenges for their commercial application. In particular, migration and coarsening of nickel (Ni) in Ni - yttria-stabilized zirconia (Ni-YSZ) fuel electrodes have strong impacts on the decreases in both active three-phase boundary (TPB) and Ni network connectivity, which result in performance deterioration. One of the mechanisms proposed as an explanation of Ni migration is Ni transport via volatile Ni(OH)x species, so-called evaporation-condensation mechanism [1]. On the other hand, Jiao and Shikazono [2] proposed a model to explain Ni spreading on YSZ surface based on the variation of Ni-YSZ wettability which is modified by the oxygen adsorption. Nevertheless, further investigations are required to understand the mechanisms of Ni migration. In the present study, Ni-film patterned electrodes which can provide well defined TPB configurations are used to investigate the Ni-YSZ degradation under SOFC, SOEC and sequential SOFC and SOEC operations, i.e. reversible operation (RSOC). The edge of sputtered Ni-film contacting with open YSZ surface can be treated as active TPB during operation. Confocal laser scanning microscopy (CLSM, Keyence, VK-X 1000) is used, which enabled in-operando observation of the real-time local morphological change at active TPB [3]. The in-operando potentiostatic experiment was carried out inside a high temperature chamber which was kept at 800oC with 4% H2O-4% H2-92% N2 as fuel gas and 30% O2-70% N2 as oxidant as shown in Fig. 1(a). The correlations between the Ni morphological changes and cells’ electrochemical performances were investigated. The Ni phase dynamically spread and de-percolated only under the SOFC operation, while such a phenomenon was not observed during the SOEC operation, as shown in Figs. 1(c), (d) and (e). Ni coarsening occurred regardless of the operation modes. The competition between Ni-film spreading and breaking-up finally determined the complicated local morphological change and corresponding cell performance variation. The time variations of active TPB and current density showed good correlation as shown in Fig. 1(b). Real-time in-operando observations revealed different Ni migration under SOFC, SOEC and RSOC operations, which may provide new insights for the understanding of Ni-YSZ fuel electrode degradation.Fig. 1. (a) Schematic illustration of in-operando setup. (b) Correlation between current density and Ni morphological evolution. (c)-(e) CLSM images and 3D surface height contour maps of the yellow-frame-enclosed areas after SOFC, SOEC and RSOC modes, respectively.

Potentiometric Hydrogen Sensing of Ordered SnO2 Porous Thin Films

Introduction The sensing performance of mixed-potential gas sensors is highly dependent on the electrode microstructure. Recently, much effort has been made to improve the sensor performance by engineering of the electrode microstructure [1, 2]. Nevertheless, the different microstructure parameters, such as thickness, porosity, and three phase boundary (TPB), contribute jointly to the sensing response, and their respective contributions are usually difficult to identify. Thus, deviation of the apparent sensor behavior from the reality is usually unable to judge due to the uncertain effect of electrode catalytic reaction. In order to unravel the true response behavior, it would be necessary to reduce or minimize impact of the electrode diffusion-reaction process and control the microstructure, which could be achieved by using a very thin electrode with well-defined and controllable morphology. Herein, we fabricated thin ordered SnO2 sensing electrodes on YSZ substrate by employing a Polystyrene (PS) sphere-template method [3]. By varying the PS sphere diameter, three sensors of different electrode thickness and electrode/electrolyte interface were obtained. The gas sensing characteristics of the sensors were systematically studied. The sensing behavior was discussed in relation to variation of the electrode process and TPB density. Sensor fabrication and characterization As depicted in Figure 1, highly-ordered porous thin film sensors were prepared by using PS sphere templates in 200 nm, 500 nm and 1000 nm diameters. SEM, TEM and AFM images of SnO2 porous film electrodes were presented in Figure 2. Periodically ordered films of inverse opal structure were observed, with uniform circular pore openings and interconnected walls as the skeleton. Each film was porous and a homogenous assembly of 10-20 nm sized particles. The wall height was roughly half of the PS sphere size, which increased from 121 nm to 248 nm and 448 nm for the three PF films. Sensing performance Figure 3a and b show the response values and response time of the three PF sensors as a function of hydrogen concentration at 500 °C. Obviously, the response significantly increased, i.e., became more negative, with decreasing template diameters, while the response time of the three sensors was short and generally rather close to each other, with a value of 4.5 s-6 s for 500 ppm H2 at 500 °C. Figure 3c displays the cross-sensitivities of the PF sensors to 100 ppm various gases at 450 °C. The PF sensors exhibited much poorer H2 selectivity than regular thick film sensors [4].The small film thickness and porous nature of the PF SE are highly favorable for the gas transport from the gas phase to the TPB, which are beneficial to the response kinetics. That the three PF sensors exhibited very close response time despite their minor difference in the film thickness, suggesting absence of significant electrode process and large H2 concentration reduction. This may allow us to study the effect of the TPB density on the sensing response. The TPB length was thus estimated for the PF sensors (Figure 4a). Clearly, the TPB density increased from 56.8 μm-1 to 89.7 μm-1 with the decreasing template diameters. It can also be seen that for each H2 concentration, the response increased (became more negative) monotonically with increasing TPB density (Figure 4b-c). It should be noted that thin film favors fast response kinetics and large response but may be disadvantageous to the selectivity. To achieve good overall sensing performance, e.g., fast response kinetics, large response, and high selectivity, careful tuning of the film thickness and TPB density would be demanded.

Topology optimization of microstructure of solid‐oxide fuel cell anode to minimize thermal mismatch

Thermomechanical reliability and lifetime of solid-oxide fuel cells are significantly influenced by thermal mismatch between anode and electrolyte layers. This study presents a numerical analysis of topology optimization of the microstructure of Ni–8YSZ anode to minimize the thermal mismatch of the components. We obtain two 2D microstructures by taking minimum thermal mismatch as object function. The 3D microstructures become fibrous and orthogonal by stretching the 2D microstructures. Results show that the coefficients of thermal expansion of microstructures in the plane parallel to the electrolyte layer are almost equal to those of electrolytes from room temperature to 800°C, which almost completely removes the thermal mismatch. Both microstructures have high three-phase boundary density, which is almost twice or five times that of a typical anode. Compared with the typical anodes, the microstructures have higher Ni–pore interfacial areas and ion conductivities. Optimization results are helpful in the design of future electrodes.

Optimization of Cathode Functional Layers of Solid Oxide Electrolysis Cells.

Sluggish CO2 reduction on the cathodes of solid oxide electrolysis cells greatly affects electrolysis performance. However, there is no study systematically investigating the cathode functional layer (CFL), where the reduction occurs. Cathode supports equipped with fast gas diffusion channels were employed as a platform to investigate the CFL, including porosity, NiO/(Y2O3)0.08Zr0.92O2 (YSZ) ratio, and thickness. The porosity was adjusted by pore former content, and a higher porosity generated a higher electrolysis current density, while the porosity improvement is limited by the fabrication process. The three-dimensional microstructure of the CFL with different NiO/YSZ ratios was reconstructed by distance correlation functions to estimate three-phase boundary density, which can explain the optimal NiO/YSZ weight ratio of 60:40 for CO2 electrolysis. Increasing CFL thickness can provide more active sites until the optimal thickness of 35 μm. Further increasing the thickness results in gas diffusion limitation. Based on the channeled cathode supports, the CFL was optimized according to CO2 electrolysis performance.

Potentiometric Hydrogen Sensing of Ultrathin Highly-Ordered SnO2 Films

Introduction The sensing performance of mixed-potential gas sensors is highly dependent on the electrode microstructure. Recently, much effort has been made to improve the sensor performance by engineering of the electrode microstructure [1, 2]. Nevertheless, the different microstructure parameters, such as thickness, porosity, and three phase boundary (TPB), contribute jointly to the sensing response, and their respective contributions are usually difficult to identify. In order to shed light on the role of TPB, it is crucial to minimize the influence of the diffusion-reaction process occurring inside the electrode, which could be achieved by using a very thin electrode. Moreover, a highly-ordered electrode morphology may help obtain well-defined TPB. Herein, we fabricated sub-micron thick ordered SnO2 sensing electrodes on YSZ substrate by employing a Polystyrene (PS) sphere-template method [3]. Three sensors were obtained by using PS spheres of different diameters in order to vary the TPB density. The gas sensing characteristics of the sensors were systematically studied. The sensing behavior was discussed in relation to the electrode microstructure. Sensor fabrication and characterization As depicted in Figure 1, ultrathin highly-ordered porous film sensors were prepared by using PS sphere templates in 200 nm, 500 nm and 1000 nm diameters. SEM, TEM and AFM images of SnO2 porous film electrodes were presented in Figure 2. Periodically ordered films of inverse opal structure were observed, with uniform circular pore openings and interconnected walls as the skeleton. Each film was porous and a homogenous assembly of 10-20 nm sized particles. The wall height was roughly half of the PS sphere size, which increased from 121 nm to 248 nm and 448 nm for the three PF film electrodes. Sensing performance Figure 3a and b show the response values and response time of the three PF sensors as a function of hydrogen concentration at 500°C. Obviously, the response significantly increased, i.e., became more negative, with decreasing template diameters, while the response time of the three sensors was generally rather close to each other, with a value of 4.5 s-6 s for 500 ppm H2 at 500°C. Figure 3c displays the cross-sensitivities of the PF sensors to 100 ppm various gases at 450°C. The PF sensors exhibited much poorer H2 selectivity than regular thick film sensors [4].The ultra-small film thickness and porous nature of the PF SE are highly favorable for the gas transport from the gas phase to the TPB. The three PF sensors exhibited very close response time despite their minor difference in the film thickness (in the order of hundreds of nanometer), suggesting absence of significant electrode process and of large H2 concentration reduction. This may allow us to study the effect of the TPB density on the sensing response. The TPB length was thus estimated for the PF sensors. Clearly, the TPB density values increased with the decrease of the PS template diameter. It can also be seen that for each H2 concentration, the response indeed increased (became more negative) monotonically with increasing TPB density (Figure 4). It should be noted, however, that the ultra-thin film electrode was disadvantageous to the selectivity. To achieve good overall sensing performance, e.g., fast response kinetics, large response, and high selectivity, careful tuning of the film thickness and TPB density would be demanded.

Effect of Infiltration on Performance of Ni-YSZ Fuel Electrodes

Ni-YSZ electrodes for solid oxide cells (SOCs) can have a diverse range of compositions, porosities, and particle sizes – factors that impact electrochemical performance. A typical Ni-YSZ structure in an anode-supported cell fired at 1400oC has feature sizes of ~ 0.5 μm that yield desirably low polarization resistance values < 0.1 Ω cm2 at 800oC in H2-H2O fuel, along with good stability. Decreasing feature size increases three-phase boundary density, thereby reducing polarization resistance and improving low-temperature cell performance. However, the feature size that can be reached in anode-supported cells is limited by the relatively high co-firing temperature. Furthermore, decreased feature sizes can exacerbate coarsening effects that degrade performance. This paper discusses an alternative method for enhancing the low-temperature performance of Ni-YSZ anodes – infiltration of Gd-doped Ceria (GDC). Since GDC is introduced after the high-temperature firing, nano-scale particles can be achieved. A single-step GDC infiltration into Ni-YSZ is studied with different solution concentrations. The optimal infiltration is found to reduce polarization resistance by a factor of 3 times at 600oC.

Asymmetric anode substrate fabricated by phase inversion process and its interface modification for solid oxide fuel cells

NiO/8YSZ anode supports for planar solid oxide fuel cells are prepared via phase inversion method. With the asymmetric anode structures, two approaches are applied to modify the interface active zones: One is eliminating the dense layer of the anode by mesh-assisted phase inversion process and laser ablation. The other is the introduction of an anode functional layer to further improve the electrolyte/anode interface. Furthermore, different anode structures fabricated by mesh mould and laser ablation are compared. The results show dense layers are effectively removed by mesh mould and laser ablation, and the peak power densities of the single cells all increase. Compared to the two methods on eliminating the dense layer, laser ablation is more effective way to modify the cell performance. For further improving interface active areas of anode support which is modified by laser ablation, an anode functional layer (AFL) is introduced to enlarge the three-phase boundary density. And the peak power density increases from 169 to 370 mW cm−2 at 800 °C. These results imply that the effective structural modification and the introduction of the AFL may be promising approaches for enhancing the electrochemical performance for SOFCs.

Investigation of La0.6Sr0.4CoO3 − δ-Gd0.1Ce0.9O2 − δ composite cathodes with different volume ratios by three dimensional reconstruction

In the present study, the polarization characteristics of La0.6Sr0.4CoO3−δ (LSC)-Gd0.1Ce0.9O2−δ (GDC) composite cathodes for solid oxide fuel cell (SOFC) with different volume ratios were experimentally evaluated. The target volume ratios of LSC:GDC=20:80, 30:70, 50:50, 70:30, 100:0% cathodes were fabricated by screen printing method. Current-voltage curve and electrochemical impedance spectroscopy (EIS) measurements were conducted at 700°C under 100% oxygen. The best performance was achieved at a volume ratio of LSC:GDC=30:70%. Three dimensional microstructures of composite cathodes were reconstructed by focused ion beam scanning electron microscope (FIB-SEM). Microstructural parameters were quantified and correlated with the polarization characteristics. Neither LSC surface nor three phase boundary (TPB) reactions can individually explain the total performance of the composite cathode. Effective ionic conductivity is monotonously enhanced as the volume ratio of GDC is increased due to its high ionic conductivity. Compared to La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)-GDC composite cathodes, overpotentials of LSC-GDC composite cathodes are lower regardless of the volume ratio. Pure LSC cathode shows small surface area due to high sinterability of LSC. Compared to LSCF-GDC composite, LSC:GDC=50:50 and 70:30vol% samples showed higher active TPB density due to the larger LSC-GDC interface area.

Guidelines for the Rational Design and Engineering of 3D Manufactured Solid Oxide Fuel Cell Composite Electrodes

The growth of 3D printing has opened the scope for designing microstructures for solid oxide fuel cells (SOFCs) with improved power density and lifetime. This technique can introduce structural modifications at a scale larger than particle size but smaller than cell size, such as by inserting electrolyte pillars of ∼5–100 μm. This study sets the minimum requirements for the rational design of 3D printed electrodes based on an electrochemical model and analytical solutions for functional layers with negligible electronic resistance and no mixed conduction. Results show that this structural modification enhances the power density when the ratio keff between effective conductivity and bulk conductivity of the ionic phase is smaller than 0.5. The maximum performance improvement is predicted as a function of keff. A design study on a wide range of pillar shapes indicates that improvements are achieved by any structural modification which provides ionic conduction up to a characteristic thickness ∼10–40 μm without removing active volume at the electrolyte interface. The best performance is reached for thin (<∼2 μm) and long (>∼80 μm) pillars when the composite electrode is optimised for maximum three-phase boundary density, pointing toward the design of scaffolds with well-defined geometry and fractal structures.

(Invited) Three Dimensional Microstructure Measurements of SOFCs: Electrode Microstructural Evolution and Long-Term Durability

Quantitative three-dimensional (3D) analysis of solid oxide fuel cell (SOFC) electrode microstructure has been shown to be a key means for connecting materials processing and electrochemical performance. In addition, 3D imaging is being used to help observe and understand degradation mechanisms, important for developing degradation models that can predict long-term cell stability. This paper will describe 3D electrode tomography measurements, of various electrodes including Ni-Y-stabilized Zirconia (Ni-YSZ), (La,Sr)MnO3-YSZ, and (La,Sr)(Fe,Co)O3 (LSCF), made using focused ion beam scanning electron microscopy and transmission x-ray microscopy. Results on electrode structural evolution, and how the microstructure impacts electrochemical performance, will be described. For the Ni-YSZ case, measured particle coarsening during high-temperature ageing is correlated with decreases in three-phase boundary density and increases in polarization resistance and a model developed to predict long-term degradation. For LSCF, depending on ageing time and temperature, either particle coarsening or SrO surface segregation can impact electrode performance degradation.

Anode-Supported Solid Oxide Fuel Cells Fabricated by Single Step Reduced-Temperature Co-Firing

Anode-supported solid oxide fuel cells (SOFCs) consisting of NiO-Y0.16Zr0.92O2-δ (YSZ) anode support layer, NiO-YSZ anode functional layer, YSZ electrolyte and (La0.8Sr0.2)0.98MnO3-δ (LSM)-YSZ cathode were successfully fabricated by single-step co-firing at 1250°C. Cells were prepared by tape casting, with Fe2O3 sintering aid used to obtain a nearly dense YSZ electrolyte. Scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDS) showed no evidence of reactions or interdiffusion between layers during co-firing. The cells yielded area specific resistance of 0.44 Ωcm2 and a maximum power density of 0.91 W/cm2 at 800°C. Impedance spectroscopy measurements showed that the LSM-YSZ cathode polarization resistance was higher for the co-fired cathodes than for a cathode that was fired separately at an optimized temperature of 1175°C. However, reducing the cell co-firing time decreased cathode polarization resistance and increased cell power output. Analysis of SEM images showed that co-firing caused more sintering and coarsening than in the optimally-fired LSM-YSZ, reducing three-phase boundary density and explaining the increased cathode resistance.

Evaluation of Exchange Current Density for LSM Porous Cathode Based on Measurement of Three-Phase Boundary Length

The microstructure of porous electrodes has a strong impact on a power generation performance of solid oxide fuel cells. Sufficient gas diffusion, electron transport and oxide-ion transport to the reaction sites is necessary for active electrochemical reactions. Recent advancement of 3-D microstructure observation by a focused ion beam and scanning electron microscope (FIB-SEM) or micro X-ray computed tomography enabled quantification of electrode microstructure. The three dimensional datasets of porous electrodes are also applied to numerical simulations of the electrode's polarization characteristics. Understanding the correlation between the electrode performance and microstructure is important to obtain guidelines for optimal microstructure. The numerical simulation requires a rate equation for the electrochemical reaction occurring at the reaction sites. It is often expressed by Butler-Volmer like form that includes the density of three-phase or two-phase boundary and the exchange current density per unit reaction site, i.e. unit triple phase boundary length for three phase boundary (TPB) reactions and unit area for two phase boundary reactions. Generally, the exchange current density per unit TPB length was derived from experiments using flat and dense patterned electrodes since there is an advantage that TPB length can be determined from its geometric shapes. However, we often observe a discrepancy between the polarization characteristics obtained from the simulation and the experimental measurements. One of the possible reasons for the discrepancy is the rate equation evaluated from the patterned electrodes which structure is very different from the real porous electrodes. This study focuses on LSM (Lanthanum Strontium Manganite) porous cathode. Although the number is still limited, we can find reports on the exchange current density per unit TPB length obtained from patterned LSM electrodes typically with a thickness of several hundred nanometers. It is also reported in literature that the thickness of patterned LSM electrodes affects activation overpotential. Since LSM has small oxide ion conductivity, oxide ion can be transferred through LSM phase, and the charge transfer at interface of LSM phase and pore phase (two phase boundary) can occur. This phenomenon may bring overestimation of the exchange current density per unit TPB length in the experiment with patterned LSM. In this study, we derive a model of exchange current density per unit TPB length considering its temperature and oxygen partial pressure dependency from experiments using LSM porous cathode with a particle diameter of approximately 3 micrometers. We assume that the charge transfer occurs only at TPB in this large particle. We first conducted power generation experiments using LSM/YSZ/Ni-YSZ button cells. For the reference electrode, a platinum wire was attached so as to surround the side edge of the electrolyte disk. Therefore the potential difference between the cathode and the reference electrode can be obtained. Activation overpotentials were obtained by impedance spectroscopy varying temperature (1073 - 1223 K) and oxygen partial pressure (5 - 21 %). After the power generation experiment, the cathode microstructure was observed by FIB-SEM and three-dimensional microstructure dataset with dimensions of 18 μm / 25 μm / 12 μm was obtained. From the geometry of its microstructure, TPB length was extracted and quantified. The exchange current densities per unit TPB length were calculated by substituting current density, activation overpotential and TPB density into Butler-Volmer like form. Eventually, obtained exchange current densities per unit TPB length are fitted into a power-law equation that has temperature and oxygen partial pressure term. As a result, obtained exchange current densities per unit TPB length in this study is found to be smaller than the ones estimated from patterned electrode at any condition. The discrepancy between them increased with decreasing temperature and/or with increasing oxygen partial pressure. For instance, the exchange current density derived in this study was less than half of that derived by Radhakrishnan et al. from patterned electrode at 1023 K for 21 % of oxygen. For actual LSM or LSM/YSZ composite cathodes using relatively large LSM particles, the equation of exchange current density per unit TPB length derived in this study is appropriate. Figure 1

Numerical Assessment of SOFC Anode Polarization with Microstructure Evolution

A phase field method was introduced to simulate the microstructural evolution of nickel-yttria stabilized zirconia composite anode of solid oxide fuel cell in high temperature sintering based on three-dimensional microstructures reconstructed by focused ion beam-scanning-electron-microscopy technique. Nickel phase morphology and crystal structure evolutions and the change of nickel network percolation in operation were simulated simultaneously. In order to quantitatively study the correlation between anode microstructure change and the anode electrochemical performance degradation, the evolution of three-phase boundary density and nickel specific surface area were calculated. Corresponding degradation experiments have been conducted to compare with the simulation results. The experimental results showed good agreement to the simulation results. Figure 1

Estimation of 3D Effective Properties from 2D Cross Sections in Porous Electrodes

The study summarizes the correlations between 2D and 3D microstructural properties of SOFC electrodes within the framework of stereology. 3D geometrical properties, such as the mean particle diameter, the pore size, the specific surface area and the three-phase boundary density are correlated by simple relations to 2D properties evaluated in a cross section, such as the mean diameter of the sectioned particles, the mean chord length, the particle perimeter exposed to the porous phase and the number of three-phase boundary points per unit area, respectively. The strong correlation between 2D and 3D properties predicted by stereological relations is verified by using packing algorithms in a wide range of porosity, particle size and electrode composition. The results provide practical indications that can be easily applied to SEM images of real samples for a short-cut estimation of the effective properties and of the range of confidence of the prediction.

Three-dimensional microstructure of high-performance pulsed-laser deposited Ni-YSZ SOFC anodes.

The Ni-yttria-stabilized zirconia (YSZ) anode functional layer in solid oxide fuel cells produced by pulsed laser-deposition was studied using three-dimensional tomography. Anode feature sizes of ~130 nm were quite small relative to typical anodes, but errors arising in imaging and segmentation were shown using a sensitivity analysis to be acceptable. Electrochemical characterization showed that these cells achieved a relatively high maximum power density of 1.4 W cm(-2) with low cell resistance at an operating temperature of 600 °C. The tomographic data showed anode three-phase boundary density of ~56 μm(-2), more than 10 times the value observed in conventional Ni-YSZ anodes. Anode polarization resistance values, predicted by combining the structural data and literature values of three-phase boundary resistance in an electrochemical model, were consistent with measured electrochemical impedance spectra, explaining the excellent intermediate-temperature performance of these cells.

Simulation of Nickel Morphological and Crystal Structures Evolution in Solid Oxide Fuel Cell Anode Using Phase Field Method

In the present study, a phase field method is used to simulate the morphological and crystal structural evolution of nickel-yttria stabilized zirconia composite anode of solid oxide fuel cell based on three-dimensional microstructures reconstructed by focused ion beam-scanning-electron-microscopy technique. A three-dimensional reconstruction of the anode sample after initial reduction is used as the initial microstructure. In order to simulate the nickel crystal grain growth and the corresponding morphological change at the same time, both material composition and crystallographic orientation order parameters are introduced in the model. In order to quantitatively study the nickel coarsening effect on anode performance, the evolution of three-phase boundary density and nickel specific surface area are quantitatively calculated in the simulations. Simulation results reproduced well the initial microstructural evolution in the experiments.

Investigation of the active thickness of solid oxide fuel cell electrodes using a 3D microstructure model

A 3D microstructure model is used to investigate the effect of the thickness of the solid oxide fuel cell (SOFC) electrode on its performance. The 3D microstructure model, which is based on 3D Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three phase boundary (TPB) length. The electrode model can also be used to perform transport and electrochemical modelling such that the performance of the synthetic electrode can be predicted. The dependence of the active electrode thickness, i.e. the region of the anode, which is electrochemically active, on operating over-potential, electrode composition and particle size is observed. Operating the electrode at an over-potential of above 200 mV results in a decrease in the active thickness with increasing over-potential. Reducing the particle size dramatically enhances the percolating TPB density and thus the performance of the electrode at smaller thicknesses; a smaller active thickness is found with electrodes made of smaller particles. Distributions of local current generation throughout the electrode reveal the heterogeneity of the 3D microstructure at the electrode/electrolyte interface and the dominant current generation in the vicinity of this interface. The active electrode thickness predicted using the model ranges from 5 μm to 15 μm, which corresponds well to many experimental observations, supporting the use of our 3D microstructure model for the investigation of SOFC electrode related phenomena.

Firing Temperature Effect on 3D Microstructure and Performance of LSM-YSZ Composite SOFC Cathodes

(La0.8Sr0.2)MnO3-Y-stabilized Zirconia (LSM-YSZ) cathodes fired at various temperatures were studied using 3D Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) tomography and Impedance Spectroscopy (IS). The lowest resistance cathode, fired at 1175°C, had nearly the highest active three-phase boundary density. This optimum was where the firing temperature was sufficient to yield good LSM-particle percolation but not so high as to cause substantial particle coarsening and densification. Increasing the firing temperature above 1175°C caused a shift in the main impedance response from ~100-1000 Hz to 1-10 Hz; this low-frequency response was primarily an electrochemical process, with gas diffusion becoming significant only for low oxygen pressure. Focusing on the low frequency peak for cathodes fired at 1325°C, A-site-deficient LSM yielded lower resistance than stoichiometric LSM. Finally, cathodes were annealed at 1000-1100°C in order to accelerate structural evolution; changes in the impedance response were similar to those observed with increasing firing temperature.

Quantitative evaluation of solid oxide fuel cell porous anode microstructure based on focused ion beam and scanning electron microscope technique and prediction of anode overpotentials

A three-dimensional microstructure of a solid oxide fuel cell (SOFC) anode is directly observed by a focused ion beam and scanning electron microscope (FIB–SEM) technique. Microstructural parameters, which are closely related to transport phenomena and electrochemical reaction in a porous anode, are quantitatively evaluated, such as volume fraction, percolation probability, tortuosity factor, surface-to-volume ratio, and three-phase boundary density. A random-walk-based diffusion simulation is effectively used for quantification. As an application of the quantified parameters, 1D numerical simulation of a SOFC anode is conducted. The predicted anode overpotential agrees well with the experimental counterparts in the condition of 3.0% H2O–97% H2, 1273K, while it is overestimated at high humidified and low temperature conditions.

J0802-2-1 SOFCのアノード微構造定量化と電極性能([J0802-2]SOFC(1))

Solid Oxide Fuel Cells (SOFCs) have great advantage of high efficiency and fuel flexibility. For the SOFC electrodes, porous structure is commonly used and its microstructure is considered to have a significant impact on the performance and durability. In this study, we focused on the SOFC anode composed of Ni and YSZ. Two anodes with different performance were prepared and were supplied for microstructure observation by a focused-ion beam and scanning electron microscope (FIB-SEM). 3D microstructure is reconstructed from the obtained images to quantify the microstructural parameters, such as volume fraction, surface-to-volume ratio, contact area density, and three-phase boundary density. The relationship between the anode performance and the microstructure are discussed. Obtained parameters are applied for 1D numerical simulation. The obtained results showed that the performance difference can be explained by the difference of TPB density and can qualitatively be reproduced by 1D simulation.