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Research was performed on the development of metal-supported solid oxide fuel cell (SOFC) electrolytes by suspension and solution precursor plasma spraying (SPS and SPPS). Experiments were conducted to understand the effects of many plasma-, feedstock-, and substrate-related process parameters on the microstructure, permeability, and conductivity of the resulting coatings. Most work was performed with yttria-stabilized zirconia (YSZ), but samaria-doped ceria (SDC) was also considered. The plasma-to-substrate heat flux behaviour of the process is particularly relevant for producing dense electrolytes with low segmentation cracking. Heat flux profiles for various processing conditions were experimentally determined and then used to model temperature distributions in the electrolyte and substrate during deposition. The results showed a strong correlation between segmentation crack severity and the peak temperature difference between the electrolyte surface and the metal support during deposition. Building on these findings, two strategies were developed for improving electrolyte performance. The first strategy is to use a bi-layer electrolyte structure, in which one layer is dense but has segmentation cracks and the other layer is more porous but contains relatively few segmentation cracks. A cell with a bi-layer electrolyte achieved a peak power density of 0.718 W cm-2 at 750 °C using hydrogen as fuel. The second strategy involves reducing the thickness and roughness of the electrode on which the electrolyte is deposited, which first required the development of improved metal supports. A thinner electrode reduces the thermal stresses that drive segmentation cracking and a smoother surface minimizes the formation of concentrated porosity. A cell with a 16 μm thick anode and a 21 μm thick electrolyte achieved an open circuit voltage (OCV) of 1.053 V, a series resistance of 0.284 Ω cm2, and a peak power density of 0.548 W cm-2 at 750 °C using hydrogen as fuel. A separate cell with a 28 μm thick electrolyte achieved an OCV of 1.068 V. At the end of the thesis, cell performance is compared to that of state-of-the-art cells produced in other facilities and using other production methods.

Research was performed on the development of metal-supported solid oxide fuel cell (SOFC) electrolytes by suspension and solution precursor plasma spraying (SPS and SPPS). Experiments were conducted to understand the effects of many plasma-, feedstock-, and substrate-related process parameters on the microstructure, permeability, and conductivity of the resulting coatings. Most work was performed with yttria-stabilized zirconia (YSZ), but samaria-doped ceria (SDC) was also considered. The plasma-to-substrate heat flux behaviour of the process is particularly relevant for producing dense electrolytes with low segmentation cracking. Heat flux profiles for various processing conditions were experimentally determined and then used to model temperature distributions in the electrolyte and substrate during deposition. The results showed a strong correlation between segmentation crack severity and the peak temperature difference between the electrolyte surface and the metal support during deposition. Building on these findings, two strategies were developed for improving electrolyte performance. The first strategy is to use a bi-layer electrolyte structure, in which one layer is dense but has segmentation cracks and the other layer is more porous but contains relatively few segmentation cracks. A cell with a bi-layer electrolyte achieved a peak power density of 0.718 W cm-2 at 750 °C using hydrogen as fuel. The second strategy involves reducing the thickness and roughness of the electrode on which the electrolyte is deposited, which first required the development of improved metal supports. A thinner electrode reduces the thermal stresses that drive segmentation cracking and a smoother surface minimizes the formation of concentrated porosity. A cell with a 16 μm thick anode and a 21 μm thick electrolyte achieved an open circuit voltage (OCV) of 1.053 V, a series resistance of 0.284 Ω cm2, and a peak power density of 0.548 W cm-2 at 750 °C using hydrogen as fuel. A separate cell with a 28 μm thick electrolyte achieved an OCV of 1.068 V. At the end of the thesis, cell performance is compared to that of state-of-the-art cells produced in other facilities and using other production methods.

A numerical model of a solid oxide fuel cell electrode is presented. Using an already established algorithm for dropping spheres as a base, alterations are made to the algorithm to increase the realism of the model. Two changes are analyzed in detail: the ability to drop pore former particles, and the use of pre-agglomerated solid particles. These changes are characterized by their impact on mean pore size, tortuosity, and effective diffusivity. As pore former volume fraction is increased, so too are mean pore size and tortuosity. A higher mean pore size has a beneficial effect on effective diffusivity due to Knudsen effects, while a higher tortuosity has a detrimental effect on effective diffusivity. The impact of mean pore size and tortuosity on diffusivity generally balances and if the impact of porosity is ignored, pore former volume fraction does not greatly affect effective diffusivity. As pore former particle size is increased, mean pore size and tortuosity also increase. Similarly to before, the effects of mean pore size and tortuosity balance. However, effective diffusivity is shown to decrease slightly with an increasing pore former particle size, suggesting a change in tortuosity has greater impact on diffusivity than a change in mean pore size. For a domain constructed with pre-agglomerated particles, the tortuosity and mean pore size were both noticeably larger than when no pre-agglomerated particles are used. Effective diffusivity was only slightly higher for a domain constructed with pre-agglomerated particles than with no pre-agglomerated particles. It is also shown that the relationship of effective diffusivity with porosity for a domain constructed with pre-agglomerated particles does not fit the correlation proposed by Berson et al. [1] for low porosity structures. A secondary goal of this work is to examine pore size measurement techniques, and present a novel technique that allows the determination of a local pore size, and therefore, a local Knudsen number. Results from the local pore size technique do not match those of the random walk method and so although the novel technique may prove to be a good starting point, it is deemed not yet suitable for use. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2011-06-13 15:30:00.25

Solid oxide fuel cells (SOFCs) are the most efficient electrochemical devices to directly convert stored chemical energy to usable electrical energy. The infiltration of ceramic conductors and catalytic metals (e.g. Ni, Pt and Pd) into porous scaffolds that had been pre-sintered onto the electrolyte is regarded as an effective way of promoting the electrode performance via producing nano-scale particles by in-situ sintering at relatively low temperatures. Large-scale fuel cells (5 cm x 5 cm) are prepared with this method and tested to demonstrate its scalability so as to achieve industrial applications. Four configurations are examined in respect of variation in the thickness of cathode, anode and electrolyte to investigate their effect on the infiltration process and electrochemical losses. To further improve infiltration as a method of fabricating high-performance electrodes, much effort is also devoted to optimising and understanding the microstructure of pre-sintered scaffold and its effect on infiltration using image analysis and electrochemical impedance. First, we have prepared the nano-structured electrodes on the 200-μm thick electrolyte-supported planar fuel cell with a 5 x 5 cm dimension. The 8YSZ scaffold is impregnated with La₀.₈Sr₀.₂Cr₀.₅Mn₀.₅O₃ (LSCM) for the anode and La₀.₈Sr₀.₂FeO₃ (LSF) for the cathode. The large planar cell achieved a maximum power density of 116 mWcm⁻² at 700°C and 223 mWcm⁻² at 800°C in humidified hydrogen. Moreover, with the addition of catalyst of 10 wt.% CeO₂ and 1 wt.% Pd, the cell performance reached 209 mWcm⁻² at 700°C and 406 mWcm⁻² at 800°C. Compared to the cell without catalysts, ceria and Pd are efficient in decreasing the electrochemical reaction resistance but making the diffusion resistance more obvious. Second, supported thin electrolytes are prepared by scalable tape casting to reduce the ohmic losses as that in electrolyte-supported cell. The cell with thick LSF-infiltrated support is very efficient in decreasing the ohmic loss thanks to the high solubility of its nitrate precursors in water and fairly high electric conductivity, but the thick cathode causes higher diffusional losses, especially at 800°C. Even though with thinner electrolyte, the ohmic loss from the cell with thick infiltrated anode is comparable to that of 200-μm electrolyte supported cell. The extra ohmic loss can be attributed to the compositional segregation of La₀.₇Sr₀.₃VO₃ (LSV) in the infiltration process in the anode, and lower loading, ca. 25 wt %. A trade-off between the diffusional loss from the cathode and the extra ohmic loss from the thick anode can be achieved by sandwiching the electrolyte between electrodes with identical thickness. A flat large area cell prepared with this method can achieve a high performance of 300 mW cm⁻² and 489 mW cm⁻² at 700°C and 800°C, respectively, if Pd-ceria is added to the anode LSV as catalyst. Third, image analyses and modelling are performed on the constrained sintering of porous thin film on a rigid substrate to study the evolution of pores at different stages. Result shows that both the anisotropy of the pore former/pores in the green body and transport of materials during the sintering process have effect on the orientation of the final microstructure. Specifically, the in-plane orientation of large-scale pores will be intensified during the constrained sintering process, while those small pores whose shape are subjected to materials transport during sintering tend to erect during the constrained sintering process at 1300°C. Fourth, image analyses and semi-quantification are used to predict the correlation between the microstructure and performance of the LSF-infiltrated electrode. Two types of YSZ powders, Unitec 1-μm powder with a broad particle-size distribution having two maxima at ~ 0.1 μm and 0.8 μm, and Unitec 2-μm powder with only one at ~1 μm are selected to fabricate the porous scaffold for infiltration. The porous structure using Unitec 2-μm powder shows finer YSZ grains and a higher boundary length than the 2-μm powder. Ac impedance on symmetrical cells was used to evaluate the performance of the electrode impregnated with 35-wt.% La₀.₈Sr₀.₂FeO₃. At 700°C, the electrode from Unitec 2-μm powder shows a polarization resistance (Rp) of 0.21 Ω cm², and series resistance (Rs) of 8.5 Ω cm², lower than the electrode from Unitec 1-μm powder does. The quantitative study on image indicates that Unitec 2-μm powder is better in producing architecture of high porosity or long triple phase boundary (TPB), which is attributed as the reason for the higher performance of the LSF-impregnated electrode. Finally, oxides of transition metals are doped into the YSZ-infiltrated LSF electrode and the impedances of symmetrical cells are tested to evaluate their effect on the ohmic and polarization resistance. Cobalt oxides are able to reduce the ohmic resistance and polarization resistance only when it is calcined at 700°C, but nickel oxide can reduce both the ohmic and polarization resistance if it is well-mixed and fully reacted with the previously infiltrated LSF. Doping of manganese oxide into LSF-YSZ electrode slightly changes the ohmic resistance but significantly increases the polarization resistance. Detailed analyses of the impact of infiltration process on the impedance data and oxygen reduction process are also presented.

A numerical model of the steady state and alternating current behaviour of a solid-oxide fuel cell is presented to explore the possibilities to diagnose and identify degradation mechanisms in a minimally invasive way using impedance spectroscopy. This is the first report of an SOFC impedance model to incorporate degradation, as well as the first one to include the ribbed interconnect geometry, using a 2-D approximation. Simulated degradation modes include: electrode/electrolyte delamination, interconnect oxidation, interconnect/electrode interface detachment, and anode sulfur poisoning. Detailed electrode-level simulation replaces the traditional equivalent circuit approach, allowing the simulation of degradation mechanisms that alter the shape of the current path. The SOFC impedance results from calculating the cell response to a small oscillatory perturbation in potential. Starting from the general equations for mass and charge transport, and assuming isothermal and isobaric conditions, the system variables are decomposed into a steady-state component and a small perturbation around the operating point. On account of the small size of the imposed perturbation, the time dependence is eliminated, and the original equations are converted to a new linear, time independent, complex-valued system, which is very convenient from a numerical viewpoint. Geometrical and physical modifications of the model simulate the aforementioned degradation modes, causing variations in the impedance. The possibility to detect unique impedance signatures is discussed, along with a study of the impact of input parameter inaccuracies and parameter interaction on the presented results. Finally, a study of pairs of concurrent degradation modes reveals the method’s strengths and limitations in terms of its diagnosis capabilities.

Solid oxide fuel cells with graded electrodes consisting of multiple composite layers yield generally lower polarization resistances than single layer composite electrodes. Optimization of the performance of solid oxide fuel cells with graded electrode composition and/or microstructure requires an evaluation of both the three phase boundary length per unit volume and the effective diffusion coefficient in order to provide insight into how these properties vary over the design space. A numerical methodology for studying the three phase boundary length and effective diffusivity in composite electrode layers with controlled properties is developed. A three dimensional solid model of a sintered composite electrode is generated for which the mean particle diameter, composition, and total porosity may be specified as independent variables. The total three phase boundary length for the modeled electrode is calculated and tomographic methods are used to estimate the fraction of this length over which the electrochemical reactions can theoretically occur. Furthermore, the open porosity of the modeled electrode is identified and the effective diffusion coefficient is extracted from the solution of the concentration of the diffusing species within the open porosity. Selected example electrode models are used to illustrate the application of the methods developed, and the resulting connected three phase boundary length and diffusion coefficients are compared. A significant result is the need for thickness-specific effective diffusivity to be determined, rather than the general volume averaged property, for electrodes with porosity between the upper and lower percolation thresholds. As the demand for current increases, more of the connected three phase boundaries become active, and therefore a greater fraction of the electrode layer is utilized for a given geometry, resulting in a higher apparent effective diffusivity compared to the same electrode geometry operating at a lower current. The methods developed in this work may be used within a macroscopic electrode performance model to investigate optimal designs for solid oxide fuel cell electrodes with stepwise graded composition and/or microstructure.

In this study, deposition of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) oxide films on Gd-doped ceria (CGO) substrates by an electrostatic assisted ultrasonic spray pyrolysis (EAUSP) method was demonstrated for the first time. The electrostatic field employed for directing the aerosol stream towards the substrate was shown to be indispensable for film deposition. The XRD result indicates that a single phase of cubic perovskite was obtained in the calcined films. SEM examination reveals that the electric field strength had a profound effect on film porosity with weaker field resulting in higher porosity. The results of impedance measurement on LSCF//CGO//LSCF cells indicate that the area specific resistance (ASR) values of current LSCF films and their activation energies are comparable to that obtained by conventional sample preparation routes. In view of the simplicity, efficiency and economy of film deposition and the sound electrochemical characteristics of the obtained films manifested in current work, it is concluded that EAUSP method is a promising method for preparation of SOFC electrode films. Besides the EAUSP method, electrostatic spray deposition (ESD) method was also employed to deposit LSCF films. The growth mechanism of LSCF films deposited on silicon wafer was studied by examining a series of films obtained with increasing deposition durations. The results show that the film formation mechanism in the initial stage depends on the deposition temperature, and films with a unique porous structure were obtained when a deposition temperature lower than the boiling point of the precursor solution was used. Deposition parameters were also varied systematically to deposit LSCF cathode films on CGO substrates to obtain symmetrical cells. The microstructure and morphology of obtained films were investigated by X-ray diffraction and SEM, and the area specific resistances of the symmetrical cells measured by electrochemical impedance spectroscopy (EIS). The minimum interfacial ASR value associated with the LSCF cathodes was 0.25 ohm¡Ecm2 at700 ¢XC. NiO-SDC (Sm0.2Ce0.8O1.9)/SDC/LSCF (La0.6Sr0.4Co0.2Fe0.8O3-£_) cells with either single layer or double layer cathode were also fabricated and tested. The single layer LSCF cathode was made by stencil printing while the double layer one was prepared by depositing a thin porous layer on the SDC electrolyte by ESD before stencil printing LSCF. The maximum power density increased from 1.04 to 1.18 Wcm-2 at 700¢XC when the LSCF inter-layer was introduced. The results showed that the ASRs of the cells reduced to half with the addition of the LSCF inter-layer.

Research was performed on the development of relationships between the microstructure of nickel and yttria-stabilized zirconia (YSZ) coatings and the processing parameters used for their deposition by atmospheric plasma spraying (APS). Research was also performed on the development of relationships between the microstructure of plasma sprayed Ni-YSZ coatings and the electrochemical performance of metal-supported solid oxide fuel cells (SOFCs) incorporating these coatings as anodes. Three APS processes were used to deposit Ni-YSZ coatings: dry-powder plasma spraying (DPPS), suspension plasma spraying (SPS), and solution precursor plasma spraying (SPPS). These processes differ in the form of the feedstock injected into the plasma. The composition of the Ni-YSZ coatings deposited with each spray process could be controlled through adjustment of the plasma gas composition and stand-off distance, as well as adjustment of feedstock properties including agglomerate size fraction for DPPS, NiO particle size and suspension feed rate in SPS, and the enthalpy of decomposition of the precursors used in SPPS. The porosity of the Ni-YSZ coatings could be controlled through the addition of a sacrificial pore forming material to each feedstock, with coating porosities up to approximately 35% being achieved for each coating type. Metal-supported SOFCs were fabricated to each have anodes deposited with a different plasma spray process, where all anodes had nominally identical composition. The microstructures obtained for each anode type were distinctly different. SPPS led to the most uniform mixing of the smallest Ni and YSZ particles. These anodes most resembled typical structures from anodes fabricated using conventional methods. It was found that the polarization resistance, Rp, associated with the high frequency (> 1 kHz) range of the impedance spectrum correlated to the three phase boundary length (TPBL) density of each anode, with lower Rp values corresponding to higher TPBL densities. It was also found that the Knudsen diffusion coefficient and effective ordinary diffusion coefficient of the porous anodes correlated with the Rp associated with the low frequency (< 1 kHz) range of the impedance spectrum. Therefore, the impedance spectrum can be used to compare microstructural differences among plasma sprayed Ni-YSZ anodes.

Solid oxide fuel cells with graded electrodes consisting of multiple composite layers yield generally lower polarization resistances than single layer composite electrodes. Optimization of the performance of solid oxide fuel cells with graded electrode composition and/or microstructure requires an evaluation of both the three phase boundary length per unit volume and the effective diffusion coefficient in order to provide insight into how these properties vary over the design space. A numerical methodology for studying the three phase boundary length and effective diffusivity in composite electrode layers with controlled properties is developed. A three dimensional solid model of a sintered composite electrode is generated for which the mean particle diameter, composition, and total porosity may be specified as independent variables. The total three phase boundary length for the modeled electrode is calculated and tomographic methods are used to estimate the fraction of this length over which the electrochemical reactions can theoretically occur. Furthermore, the open porosity of the modeled electrode is identified and the effective diffusion coefficient is extracted from the solution of the concentration of the diffusing species within the open porosity. Selected example electrode models are used to illustrate the application of the methods developed, and the resulting connected three phase boundary length and diffusion coefficients are compared. A significant result is the need for thickness-specific effective diffusivity to be determined, rather than the general volume averaged property, for electrodes with porosity between the upper and lower percolation thresholds. As the demand for current increases, more of the connected three phase boundaries become active, and therefore a greater fraction of the electrode layer is utilized for a given geometry, resulting in a higher apparent effective diffusivity compared to the same electrode geometry operating at a lower current. The methods developed in this work may be used within a macroscopic electrode performance model to investigate optimal designs for solid oxide fuel cell electrodes with stepwise graded composition and/or microstructure.