Microstructural Characterization of Solid Oxide Fuel Cell Anodes Operating on Hydrocarbon Fuels

Introduction For many potential mobile robot applications, current power supply technology (such as the lithium ion battery technology) is in fact a key limiting factor due to its low energy density. Solid oxide fuel cells (SOFCs) are a promising alternative and can overcome limitations of current battery technology. This is because SOFCs can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density such as gasoline and so on.Ni-YSZ cermet is the most promising anode material for SOFCs because of its low cost, chemical stability, and closely matched thermalexpansion coefficients with the dense YSZ electrolyte. Ni is an excellent catalyst for hydrocarbon fuel reforming and cracking, which leads to rapid carbon deposition inside the SOFC anode and causes coking and deactivation1). In principle, the carbon deposition on Ni can be depressed by adding steam in the hydrocarbon fuel2,3), whereas few detailed investigations of SOFCs that operate on gasoline fuel have been reported.In this work, we studied the effect of steam content and demonstrated the direct operation of SOFCs by octane which is a main ingredient of gasoline. In the case of the Ni-YSZ anode, the operation temperature must be high in order to avoid sulfur poisoning4 -6 ), because most commercially available fuels contain sulfur. Therefore, in this study, the SOFCs operation was conducted at high temperature of 950 oC. Experimental Surface-polished YSZ pellets of 2 mm in thickness were used for the electrolyte. Pt cathode electrode and Pt reference electrode were prepared on the surface and side of the pellets, respectively, by firing Pt paste at 1300 oC. Then, NiO powder and GDC powder (NiO/YSZ=6/4 (wt)) were thoroughly mixed in a vacuum using the planetary ball mill (200 rpm, 12 h), and made into NiO-YSZ pastes using ethyl-cellulose and organic solvent. The NiO-YSZ paste was screen-printed in diameter of 8 mm on the YSZ pellets and sintered at 1200 oC for 1 h.Octane and steam were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Octane bubbler temperature was 30 oC, and Steam/Carbon ratio (S/C) were controlled by changing the temperature of the water bubbler from 40 to 70 oC. Electrochemical properties (open circuit voltage, anode electrode resistance, I - V and I - P characteristics, stable power generation property) were measured over a temperature range of 950-700 oC. During the constant current generation, exhaust gas was analyzed by gas chromatography. Results and Discussion Figure 1 shows I - V and I - P characteristics for the cells at 950 oC in wet octane. Maximum power density of 35 mW/cm2 was obtained by S/C of 8.9. The power density decreased with S/C quickly.Figure 2 shows the time course of cell voltage at a drawn current density of 5.2 mA/cm2 for S/C of 3.5. The cell voltage was stable. That is, carbon deposit does not arise even in the cell of the Ni-YSZ anode in low S/C of 3.5 when the operation temperature is high of 950 oC. References 1) S. Mclntosh and R.J. Gorte, “Direct Hydrocarbon Solid Oxide Fuel Cells,” Chemical Reviews, 104 (2004) 4845- 4865.2) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: I. Equilibrium Compositions and Reforming Conditions,” Journal of Electrochemical Society, 850 (7) (2003) A878-A884.3) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: II. The C-H-O Ternary Diagrams,” Journal of Electrochemical Society, 850 (7) (2003) A885-A888.4) K. Sasaki, K. Susuki, A. Iyoshi, M. Uchimura, N. Imamura, H. Kusaba, Y. Teraoka, H. Fuchino, K. Tsujimoto, Y. Uchida, N. Jingo, “H2S Poisoning of Solid Oxide Fuel Cells,” J. Electrochem. Soc., 153 (2006) A2023–A2029.5) P. Lohsoontorn, D.J.L. Brett, N.P. Brandon, “Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells,” Journal of Power Sources, 175 (2008) 60–67.6) H. Kishimoto, Y-P Xiong, K. Yamaji, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, “Stability of Ni base anode for direct hydrocarbon SOFCs, ” Journal Chemical Engineering of Japan, 40 (13) (2007) 1178-1182.

Experts have long projected that in the future solid oxide fuel cells (SOFCs) could provide a viable path for the replacement of existing power sources and the production of clean, secure, and sustainable energy. Today, not only have SOFCs matured as a technology with a proven ability to provide high density, high efficiency power with long term stability at low cost and environmental impact, but with the theoretical capacity to utilize a variety of naturally sourced fuels. This theoretical capacity makes their potential use attractive in many additional applications and positions SOFCs to be the technology which can bridge the gap between current hydrocarbon fueled technologies and future hydrocarbon fueled electric technologies. That said, there remains many technical hurdles associated with the incorporation of naturally sourced fuels, to include thermal stress cracking and catalyst coarsening, delamination, and poisoning. Catalyst poisoning, specifically arising from sulfur impurities in naturally sourced fuels, remains one of the largest SOFC technical challenges, since their presence, even at low concentrations, have now shown permanent performance degradation. As the U.S. Army begins to investigate SOFCs to enable additional capabilities like silent watch, advanced radios, and exportable power in their fleet, catalyst poisoning from sulfur impurities presents a concerning technology gap which must be addressed, as the entire fleet operates on JP-8 kerosene-based fuel, which by specification can contain up to 3000 ppm of sulfur, via AR-70 single fuel policy.As SOFC technologies migrate to lower temperature operation to resolve physical degradation issues, the development catalysts materials with greater sulfur tolerance are being explored to resolve issues of chemical degradation related to sulfur poisoning. In this study, a previously reported, high temperature, sulfur tolerant SOFC catalyst, La0.7Sr0.3VO3.86-⸹ (LSV), was further experimentally investigated via Optical Microscopy, X-ray Diffraction, Scanning Electron Microscopy and Energy Dispersive Spectroscopy for its sulfur tolerance at currently targeted SOFC intermediate operating temperatures (400-600°C) in wide range of hydrogen sulfide (30ppm, 300ppm and 10vol%) balance hydrogen gas environments for up to 100 hours. When compared against the industry standard Ni-YSZ anode, LSV was observed to have significantly lower (287x) sulfur adsorption rates, with the lowest rates observed between 600-700°C. This behavior is attributed the cubic structure the material presents in this temperature range. Higher adsorption rates were observed in the monoclinic/tetragonal structure the material presents at temperatures between 400-500°C. This study was further supplemented by a theoretical investigation of hydrogen sulfide adsorption on low index LSV surfaces via periodic Density Functional Theory. When compared against Ni-YSZ, which is known to accumulate sulfur from hydrogen sulfide via two-step dissociative adsorption, LSV was observed to accumulate sulfur through weak chemisorption (0.43 eV max) of undissociated molecular hydrogen sulfide only. The weak chemisorption behavior is attributed to structural surface changes due to oxygen vacancies near strontium defects, wherein the strongest adsorption energies were observed, and in direct contrast to similar calculations of hydrogen sulfide adsorption with low index surfaces of LVO3, wherein no adsorption was observed.Since it is likely SOFCs will operate on hydrocarbon fuels, as well as compressed hydrogen gas in the future, this study is also currently experimentally investigating sulfur accumulation under the same operating conditions above with hydrogen sulfide mixtures in methane gas. Methane is a small hydrocarbon chain, which will help minimize the accumulation of carbon deposition. Initial comparisons of sulfur adsorption between hydrogen and methane gas mixtures, using low hydrogen sulfide concentrations, show they are statistically similar, although methane gas exposure was observed to result in the formation of two activation energies, which was not observed with hydrogen mixture previously. The methane gas was also observed to promote LSV to take the monoclinic/tetragonal structure at lower operating temperatures than hydrogen mixtures. This observation may lead to elevated sulfur adsorption rates at higher hydrogen sulfide concentrations. Finally, it was originally observed during experimental exposure experiments with hydrogen sulfide/hydrogen gas balance mixtures that higher concentrations of lattice oxygen, present at lower temperatures, had a positive correlation with sulfur accumulation in the LSV material. This study is therefore currently theoretically investigating the effect of lattice oxygen content on the LSVs structure via Density Functional Theory, which could lead to further derivation from optimal cubic configuration, which may lead to configurations which increase sulfur adsorption. Figure 1

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).

Temperature-dependent chemical incompatibility between NiO-YSZ anode and alkaline earth metal oxides: Implications for surface decoration of SOFC anode

Solid oxide fuel cells (SOFCs) are capable of utilizing energy-dense hydrocarbon fuels such as jet fuels for highly efficient power generation. Currently, a major challenge for using the jet fuel (e.g. JP8) in SOFC power systems is its sulfur content, which poisons the SOFC catalyst and anode. Sulfur contents in jet fuels could be as high as 3000 ppm although typical sulfur contents could be in the range from 550 and 750 ppm. The reduction of sulfur to <1 ppm is generally desired and required to ensure reliable SOFC operation. Various technologies have been developed to remove sulfur from jet fuel. However, most of these technologies are designed for large-scale desulfurization at industrial facilities or for stationary applications. Due to their complexity and substantial weight and size, these systems are not suitable for onboard integration with portable fuel cells. Therefore, our research aimed at developing a lightweight, compact desulfurization reactor for onboard applications. After several versions of improvements, including the use of selective laser sintering (SLS) 3D printing method, we have developed a miniature one-piece stainless-steel reactor which is capable of removing >99.5% sulfur in the jet fuel with the final sulfur concentrations close to a sub-ppm level. The reactor could be adapted to the desulfurization of other liquid hydrocarbon fuels such as gasoline and diesel. The reactor designs, desulfurizing materials, operation conditions and effectiveness of the technology will be discussed in this paper.

Comprehensive investigation of methane conversion over Ni(111) surface under a consistent DFT framework: Implications for anti-coking of SOFC anodes

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.

The presently limited understanding of solid oxide fuel cell (SOFC) anode mechanisms is due to a lack of experimental techniques that can probe species and component materials directly during operation, or in operando. Previous investigations on the efficiency of Ni-YSZ-based electrode materials in different reforming conditions have been performed using post-mortem and in situ electrochemical analyses with little information on specific mechanisms that lead to fuel oxidation and cell degradation. An ensemble of non-invasive optical methods consisting of near infrared thermal imaging (NIRTI), Raman spectroscopy, and Fourier-transform infrared emission spectroscopy (FTIRES) is utilized to correlate the chemistry of fuel oxidation and carbon formation with traditional electrochemical measurements for methane and simulated biogas fuels in operating SOFCs. Different reforming conditions for methane including plain, wet (steam), and dry (carbon dioxide) are investigated on anode- and electrolyte-supported cells. NIRTI results demonstrate that cooling on the anode surface due to endothermic hydrocarbon cracking for dry reforming or simulated biogas is significantly greater than for unreformed methane on both types of cells. Raman and FTIRES both show that more carbon is deposited with methane than with biogas. The small amount of carbon formed is limited to lower temperatures with simulated biogas whereas carbon formation occurs to a great extent at all temperatures with methane. Exhaust gas analysis with ex situ FTIR absorption measurements and mass spectrometry are combined with in operando measurements to further characterize and quantify the species involved. These findings serve as benchmark data for a better understanding of the mechanisms responsible for performance and degradation of SOFCs.

Application of NixCo1-x catalyst on solid oxide fuel cell anode for biogas dry reforming

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.

Identifying the corresponding degradation mechanisms at the early stage of operation is important for the long-term stable operation of solid oxide fuel cells (SOFCs). This paper uses total harmonic distortion analysis (THDA) to identify the anode carbon deposition mechanism in the early stage of SOFC operation with carbon-containing fuels. An accelerated calculation model for the electrochemical impedance spectrum (EIS) and a total harmonic distortion calculation model were constructed to study the carbon deposition mechanism and carbon removal strategy of SOFC anodes based on a one-dimensional transient elementary reaction kinetic model of an SOFC fueled by a CH4/H2O mixture. It is shown that the THDA calculation model can identify the primary stages of carbon deposition on SOFC anodes at specific frequencies. The THDA profiles at frequencies greater than 2 kHz contain essentially the same key information as at lower frequencies, suggesting that THDA can significantly reduce the test time for identifying performance degradation in the early stages of SOFC operation compared to EIS. Using a mixture of hydrogen and water vapor to remove the solid carbon generated by the cell anode and restore it to its initial performance, the THDA computational model can successfully respond to this process.

Solid oxide fuel cells (SOFCs) operate under harsh environments, which cause the deterioration of anode material properties and service life. In addition to electrochemical performance, the structural integrity of the SOFC anode is essential for successful long-term operation. The SOFC anode is subjected to stresses at high temperature, thermal/redox cycles, and fuel gas contaminant effects during long-term operation. These mechanisms can alter the anode microstructure and affect its electrochemical and structural properties. In this research, anode material degradation mechanisms are briefly reviewed and an anode material durability model is developed and implemented in finite-element analysis. The model takes into account thermomechanical and fuel gas contaminant degradation mechanisms for predicting the long-term structural integrity of the SOFC anode. The proposed model is validated experimentally using a NexTech Probostat™ SOFC button cell test apparatus integrated with a Sagnac optical setup for simultaneously measuring electrochemical performance and in situ anode surface deformation.

Carbon tolerance, electrochemical performance and stability of solid oxide fuel cells with Ni/yttria stabilized zirconia anodes impregnated with Sn and operated with methane

The anode-supported solid oxide fuel cell (SOFC) degrades when the anode is subjected to redox cycling. The degradation has qualitatively been related to microstructural changes in the nickel-yttria stabilized zirconia anode of the tested cells. In this work, the microstructural changes were observed in situ using environmental scanning electron microscopy. In the reduced state, a dynamic rounding of the nickel particles occurred. The oxide growth upon re-oxidation depended on the oxidation kinetics. During rapid oxidation, the NiO particles divided into 2-4 particles, which grew into the surrounding voids. For slower oxidation, an external oxide layer was seen to develop around the individual particles.

Solid Oxide Fuel Cells (SOFCs) represent one of the most promising technologies for clean and efficient energy conversion due to their high efficiency and fuel flexibility [1]. However, carbon deposition, which can occur due to the reaction of hydrocarbons in the fuel within the anode, leads to significant reduction of durability. Understanding the mechanisms of carbon-induced degradation is critical for improving the long-term stability and performance of SOFCs. Currently, some studies have conducted experimental tests on the microstructure of anode under carbon deposition [2,3] or simulated the deformation of electrode materials under long-term operation [4-7], but there is still a lack of microstructure simulation considering carbon deposition reaction kinetics.In this study, we employed a phase-field modeling (PFM) approach to investigate the carbon evolution in the nickel-based anode and combined it with a one-dimensional transient elementary reaction kinetic model. The PFM model incorporates a set of coupled partial differential equations, which describe the growth and diffusion of carbon species in the anode material. The PFM model accounts for the interactions between the carbon and Ni-YSZ crystal grains, using a free energy functional that includes the bulk free energy and gradient energy. The time evolution of the system is governed by the time-dependent Ginzburg-Landau equation and Cahn-Hilliard equations. The kinetic model incorporates the coupling effect of heterogeneous elementary chemical and electrochemical reactions, the electrode microstructure and the charge and mass transport processes. The carbon deposition source term is calculated by the heterogeneous elementary chemical reactions using methane and water as fuels. Then to ensure the accuracy and reliability, the model is validated by experimental I-V curves. After validation, the size and distribution of carbon, the porosity of the anode, and the three phase boundary area can be analyzed under different temperatures and fuel compositions. This study provides valuable insights into the mechanisms of carbon-induced degradation in SOFCs and offers a predictive tool for optimizing operating conditions to enhance the long-term stability of SOFCs.