3D imaging and quantification of interfaces in SOFC anodes

High operating temperature affords solid oxide fuel cells (SOFCs) fuel flexibility, favorable kinetics, enhanced transport properties and high quality heat for secondary thermodynamic cycles.1 Elevated temperatures, however, present challenges to ensuring cell durability.2 Coarsening of nickel in the Ni-YSZ anode is one such challenge that can lead to significant performance degradation. The fundamental driving force Δμ for coarsening phenomena is given by Eq. 1.3 The expression, which assumes isotropic interfacial energy, indicates that the chemical potential μ of a curved interface differs from that of a flat interface μ 0 by the factor VmγH, where H is mean curvature, γ interfacial energy and Vm molar volume. Δμ = μ - μ 0 = VmγH (1) Eq. 1 makes evident that variations in interface curvature, i.e., interface shape, directly correspond to variations in chemical potential. Furthermore, it follows from Eq. 1 that microstructural coarsening is fundamentally related to variations in interface shape. The link between interface shape and coarsening propensity is well known and methods have been developed to directly measure interfacial curvature distributions for complex morphologies in 3-D.4,5 In this work, these techniques are applied to 3-D images obtained by x-ray nanotomography to study coarsening in microstructures obtained from Ni-YSZ solid oxide fuel cell anodes aged for varying lengths of time. This work builds on a previous characterization of the same samples.6 The curvature measurements collected suggest that a combination of coarsening and morphological instability, which leads to channel pinch-off, severely diminish the connectivity of the anode’s Ni network and, in addition, lead to a significant loss of both nominal and effective triple-phase boundary (TPB) density. Both the loss of connectivity and TPB density coincide with cell performance degradation that was observed in the aging experiment7 from which the anode samples were taken. References Singhal, S. C.; Kendall, K. High Temperature Solid Oxide Fuel Cells, Elsevier, 2003.Yokokawa, H.; Tu, H.; Iwanschitz, B.; Mai, A. Fundamental mechanisms limiting solid oxide fuel cell durability. J. Power Sources 2008, 400-412.Voorhees, P. W. The Theory of Ostwald Ripening. J. Stat. Phys. 1985, 231-252.Glicksmann, M. E.; Voorhees, P. W. Ostwald Ripening and Relaxation in Dendritic Structures. Metall. Trans. A, 1984, 995-1001.Mendoza, R.; Savin, I.; Thornton, K.; Voorhees, P. W. Topological complexity and the dynamics of coarsening. Nat. Mater., 2004 385-388.Nelson, G. J.; Grew, K. N.; Izzo, J. R. Jr.; Lombardo, J. J.; Harris, W. M.; Faes, A.; Hessler-Wyser, A.; Van herle, J.; Wang, S.; Chu, Y. S.; Virkar, A. V.; Chiu, W. K. S. Three-dimensional microstructural changes in the Ni-YSZ solid oxide fuel cell anode during operation. Acta Mater., 2012 3491-3500. 7. Faes, A.; Hessler-Wyser, A.; Presvytes, D.; Vayenas, C. G.; Van herle, J. Nickel-Zirconia Anode Degradation and Triple Phase Boundary Quantification from Microstructural Analysis. Fuel Cells, 2009 841-851. Acknowledgements Financial support from the National Science Foundation (Award CBET-1134052) and an Energy Frontier Research Center on Science Based Nano-Structure Design and Synthesis of Heterogeneous Functional Materials for Energy Systems (HeteroFoaM Center) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Award DE-SC0001061) is gratefully acknowledged. X-ray nanotomography was performed with Dr. Steve Wang at the Advanced Photon Source supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357. Ni-YSZ samples were provided by MER Dr. J. Van herle (Ecole Polytechnique Federale de Lausanne, Switzerland).

Polarization resistance of solid oxide fuel cell electrodes consists of resistances due to mass transfer and chemical reaction. Therefore, higher ion conductivity, electron conductivity, and gas diffusivity and larger triple-phase boundary (TPB) density lead to lower polarization resistance. To increase effective conductivities and diffusivity, tortuosity factors in each mass-transfer path should be reduced. In this study, magnetic alignment method is developed to align the mass-transfer paths in the electrode microstructure. Since Ni as an electron-conducting material in anode has high magnetic permeability, the particles are polarized and aligned by a magnetic field. Therefore, the tortuosity factors can be reduced. In addition, this Ni alignment is expected to reduce the tortuosity factors of yttria-stabilized zirconia (YSZ) and pore phases as well. However, there is a risk for a decrease in TPB density. The effect of magnetic field on resultant microstructure and polarization resistance of Ni-YSZ anode are investigated by impedance measurement of anode symmetrical cells and by reconstruction of three-dimensional microstructure. The tortuosity factors of Ni and YSZ decreased and TPB density decreased by the magnetic field. As a result, the polarization resistance of anode was kept nearly unchanged. The effect of reducing Ni volume fraction is also investigated.

Microstructural stability of supported metal catalysts: A phase field approach

A modeling study of porous composite microstructures for solid oxide fuel cell anodes

Design of optimal microstructures for infiltrated solid oxide fuel cell (SOFC) electrodes is a complicated process because of the multitude of the electrochemical and physical phenomena taking place in the electrodes in different temperatures, current densities and reactant flow rates. In this study, a stochastic geometric modeling method is used to create a range of digitally realized infiltrated SOFC electrode microstructures to extract their geometry-related electrochemical and physical properties. Triple Phase Boundary (TPB), active surface density of particles along with the gas transport factor is evaluated in those realized models to adapt for various infiltration strategies. Recently, additive manufacturing or freeze type casting methods enable researchers to investigate the performance of directional electrodes to get the maximum electrochemical reaction sites, gas diffusivity and ionic conductivity simultaneously. A series of directional backbones with different amount of virtually deposited electrocatalyst particles are characterized in the first step. The database of microstructural parameters (inputs) and effective geometric properties (outputs) is used to train a range neural network. A microstructure property hull is created using the best neural network model to discover the range of effective properties, their relative behaviour and optimum microstructure. The characteristics of models is shown that there is not any contradiction between the high level of TPB and contact surface density of particles, but the highest amount of gas diffusivity can be found in the microstructures with lower level of reaction sites. Also increasing the contact surface density has a negative effect on gas transport but the high level of TPB density is feasible in the full range of microstructures. In the other hand, TPB density and gas diffusion into the models are inversely related, although there are a limited number of microstructures with high level of reaction sites and acceptable gas diffusivity. Finally, using a simple optimization process, the microstructures with the highest level of reaction sites and gas transport factor are identified which have the backbone porosity of about 50%, and extremely higher gain growth rate normal to the electrolyte. Additive manufacturing and 3D printing methods will enhance researchers in the future to create the real directional electrodes on the base of these proposed models.

Direct comparison between X-ray nanotomography and scanning electron microscopy for the microstructure characterization of a solid oxide fuel cell anode

Biomass gasification has acquired considerable interest as a sustainable and environmentally friendly way to produce heat, hydrogen or electricity from agro-industrial wastes or other kinds of biomass. A very effective solution for the achievement of high electrical efficiency (up to 55%) is the integration of biomass gasification with solid oxide fuel cell (SOFC) technology, including the necessary gas cleanup to avoid degradation of the SOFC. For this reason, this paper first shows in detail the risk and the impact of carbon deposition, exposure to tar, hydrogen sulphide, hydrogen chloride and alkali metals on SOFC anode and then, considering the tolerance limit for inorganic and organic contaminants (<1 ppmv for H2S, HCl and alkali and <10 ppmv for tar), offers an overview of the most relevant and effective technologies to remove these contaminants and to feed safely a SOFC system.

Solid oxide fuel cells are among the most thermodynamically efficient energy conversion devices. They are critical for the future if we are to meet rising energy demands and increasing selectivity over our future energy mix. Traditionally, solid oxide fuel cell performance has been assessed using bulk parameters, and the microstructure-bulk properties relationship is yet to be well understood. Here, we use time-lapse 3D imaging to probe 3D electrode structure and move towards intelligently designing structures with specific performance attributes. Using 3D imaging allows for characterisation and quantification of parameters that are not well described in 2D, such as triple-phase boundary lengths, tortuosity and phase percolation. In this work, the parameters for an optimised porous nickel-scandia stabilized zirconia anode (Ni-ScSZ) were calculated using an electrochemical model to maximise triple-phase boundary length and density, gas permeation and electronic and ionic transport. Ni-ScSZ scaffolds were then produced using screen-printing and impregnation and characterized with FIB-SEM to obtain 3D microstructural parameters such as actual percolation of the phases, tortuosity and connectivity of the phases, and the length and density of triple-phase boundaries (Figure 1). The experimental microstructural parameters were then compared to the modelled optimised microstructure, allowing for an examination of the accuracy of the manufacturing parameters. Symmetrical cells were set up to compare the anode performance with the performance predicted by the electrochemical model. Early iterations of impregnated scaffolds show triple-phase boundary densities increased by approximately an order of magnitude and allowed for a critical assessment of the electrochemical model. The degraded electrode morphology was also characterized in 3D to link its changes during cycling to performance degradation. This approach enables us to move towards designing optimised electrodes with specific performance attributes. Figure 1

In recent years great effort has been taken to understand the effect of gas transport on the performance of electrochemical devices. This study aims to characterize the diffusion regimes and the possible inaccuracies of the mass transport calculation in Solid Oxide Fuel Cell (SOFC) anodes when a volume-averaged pore diameter is used. 3D pore size distribution is measured based on the extracted pore phase from an X-ray CT scan, which is further used for the calculation of a Knudsen number (Kn) map in the porous medium, followed by the voxel-based distribution of the effective diffusion coefficients for different fuel gases. Diffusion fluxes in a binary gas mixture using the lower boundary, upper boundary and average effective coefficients are compared, and the impact on overpotential is analyzed. The results show that pore diameters from tens to hundreds of nanometers result in a broad range of Knudsen number (1.1 ∼ 4.8 and 0.6 ∼ 3 for H2 and CH4 respectively), indicative of the transitional diffusion regime. The results highlight that for a porous material, such as an SOFC anode where Knudsen effects are non-negligible, using a volume-averaged pore size can overestimate the mass flux by ±200% compared to the actual value. The characteristic pore size should be chosen sensibly in order to improve the reliability of the mass transport and electrochemical performance evaluation.

This study presents a physically-based model for the simulation of impedance spectra in solid oxide fuel cell (SOFC) composite anodes. The model takes into account the charge transport and the charge-transfer reaction at the three-phase boundary distributed along the anode thickness, as well as the phenomena at the electrode/electrolyte interface and the multicomponent gas diffusion in the test rig. The model is calibrated with experimental impedance spectra of cermet anodes made of nickel and scandia-stabilized zirconia and satisfactorily validated in electrodes with different microstructural properties, quantified through focused ion beam SEM tomography. Besides providing the material-specific kinetic parameters of the electrochemical hydrogen oxidation, this study shows that the correlation between electrode microstructure and electrochemical performance can be successfully addressed by combining physically-based modelling, impedance spectroscopy and 3D tomography. This approach overcomes the limits of phenomenological equivalent circuits and is suitable for the interpretation of experimental data and for the optimisation of the electrode microstructure.

Quantitative estimation of triple phase boundaries in solid oxide fuel cell electrodes via artificial neural network

Solid oxide fuel cells (SOFCs) are being extensively researched for clean power generation from coal-derived syngas. Some of the contaminants in syngas such as phosphine (PH3) may interact with the SOFC anode material, and degrade its electrochemical performance and material properties. In this paper, a modified Sagnac interferometry method is utilized to monitor the anode surface transient temperature as a function of applied current densities under hydrogen and simulated coal syngas. Moreover, the poisoning effects of PH3 contaminant on the SOFCs performance are investigated in dry and moist conditions. The experimental results indicate that the Ni-cermet-based SOFC anode is more susceptible to degradation due to PH3 in the presence of steam than under dry conditions. These experiments are valuable for the validation and the development of SOFC electrochemical models, and understanding the anode-contaminant interaction.

We report here on the development of a novel type of solid oxide fuel cell anodes that are based on Mn containing perovskites and prepared by infiltration of electrode precursor solutions into Yttria stabilized zirconia scaffolds. The presence of Mn, even in small amount, on the B-site of the ABO3 structure leads to electrode microstructures that offer a large surface area that is available for electrochemical reactions. The oxides form a remarkable thin coating on the electrode scaffold after high temperature sintering. The evolution of those films in reducing environments creates arrays of interconnected ~10nm scale particles covering the electrode scaffold. Such electrodes, based on LSCM with small amounts of added catalyst, provide outstanding performance, both for the direct oxidation of methane in solid oxide fuel cells, and the reduction of pure CO2 in solid oxide electrolyzers.

Manufacturing cost remains one of the major issues facing the solid oxide fuel cell (SOFC) industry. In the anode supported SOFC design, the cermet anode constitutes around 90% of the total material required to build a cell, making the technology very sensitive to anode raw material price. A new patent-pending process called “nickel yttria reaction-sintered zirconia (NiYRSZ)” has been developed for manufacturing SOFC anodes at a fraction of the cost. Typically, the solid component of the anode consists of about 50/50 volume percent nickel and 8 mole percent yttria stabilized zirconia, the latter being a rather costly material. It was discovered that zirconia and yttria powders sintered in the presence of nickel oxide readily form the cubic phase at moderate temperature. Cells manufactured using this process show excellent microstructures for anode supports: a strong bond between the electrolyte and the anode, and a high porosity without addition of pore formers. The strength of the anode was 100 MPa making the material equivalent or slightly superior to an anode fabricated with the traditional NiO/8YSZ material of similar porosity. The resistivity of the material was measured at 850°C and found to be less than 2 mΩ·cm. Cell performance was also compared to cells manufactured with traditional material. Every indication is that SOFC anodes fabricated with this new method perform as well as anodes made with the conventional material set.

The electrochemical reduction of CO2 was studied via solid oxide electrolysis cell (SOEC), a type of device that can work reversely into a solid oxide fuel cell (SOFC) to generate electricity. Application of SOEC for CO2 electrolysis possesses potential rewards both in energy and environmental aspects, as it offers a way to recycle CO2 into chemicals and value-added fuels, which helps to reduce the accumulation of atmospheric CO2 and realize the carbon neutral cycling of fuels. Secondly, SOEC techniques provide a means to utilise the intermittent renewable sources, such as wind, tide, etc., as energy input to store excess electricity in the form of H2, CO and hydrocarbons and use these chemicals when necessary. Yet it is a challenging task to realize an efficient reduction of CO2 by SOEC due to the non-polar nature of CO2 fuels which are hard to be chemically absorbed and activated in high temperature range. The CO product from CO2 reduction is also demanding for the choice of fuel electrode (i.e. cathode in SOEC and anode in SOFC) materials, which has been an issue for the CO/hydrocarbon-fuelled SOFCs. To date, the CO2 electrolysis by SOEC is still at the starting point, and the mechanisms on the electrochemical reduction of CO2through SOECs are not fully understood. Extensive efforts need to be dedicated to the material developments, mechanism study, and system designs etc. Effort was made in our lab to find a highly performed, long-term durable cathode material for electrochemical reduction of CO2 by SOEC and to obtain more understandings of the mechanism of CO2 electrochemical reduction process. Different cathode materials were employed, including Ni-8 mol% yttria stabilised zirconia (YSZ) cermet, (La,Sr)(Cr,Mn)O3 (LSCM)-YSZ composite, and LSCM-(Gd, Ce)O2 (GDC) composite. Focus was casted on LSCM based cathodes which were found to be carbon-resistive, and the microstructure of LSCM-based cathodes was tuned to high performance and efficient CO2 electrolysis by the strategy of applying a gradient composite cathode and of adopting wet impregnation as cell fabrication procedures. The electrochemical performance of CO2 electrolysis was characterized in various CO2-CO mixtures and applied potentials in 900-750oC with the aid of impedance spectroscopy, on YSZ electrolyte supported three-electrode SOECs. In this paper, impedance behaviour of the electrochemical reduction reaction of CO2 from different cathode SOECs was correlated with the variations in gas composition, operating temperature and loading potential. As both charge transfer and surface adsorption/desorption equilibration and surface diffusion of activated species were found to be dominant processes taking place in the scope of cathodes under examination, with the later being significant in CO2-rich fuels, areas of discussion will be the effects of operational conditions on the impedance arcs associated with these steps, and how the cathode microstructure impacts the impedance behaviour and the corresponding elementary steps from CO2electrochemical reduction. By introducing a LSCM-YSZ 30-70/LSCM-YSZ 60-40 graded cathode and by incorporating extra catalyst into LSCM-GDC composite, the cathode performance was greatly improved, and the surface activity was profoundly accelerated. However, the most effective way to promote cathode performance was to introduce the cathode components in separate steps via wet impregnation. A competitive CO2 electrolysis performance to Ni-YSZ cermet was obtained from the GDC impregnated LSCM-YSZ cathode with 0.5% Pd extra catalyst, which also showed a comparable performance between SOEC and SOFC when operating in CO2-CO 50-50 mixture.