Mixed-Fuels Fuel Cell Running on Methane-Air Mixture

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.

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

To improve the performance of the anode-supported Solid Oxide Fuel Cells (SOFCs) which can be operated at an intermediate temperature, the functional layer (FL) is introduced on a Ni- YSZ anode substrate. And the scandia-stabilized zirconia (ScSZ) and the samaria-doped ceria (SDC) which have the high ionic conductivity and the chemical stability as well as the yttria- stabilized zirconia (YSZ) are used as material for the anode FL with the Ni. The fabrication process of anode-supported single cell with the anode FL was established and the power density of those was appreciated. As a result, the sample with anode FL (Ni-YSZ) has higher power density than normal cell. The single cell which was consist of the FL(Ni-YSZ) and a electrolyte(YSZ) showed about 550mW/cm2 as the maximum power density and 0.41Ω cm2 as a lower internal resistance at 650{degree sign}C. In case of the single cell using the ScSZ and the SDC as the anode FL, the performance of samples decrease rapidly and those showed unstable voltages during long-term test.

Long term durability was demonstrated on high-performance direct ethanol solid oxide fuel cell operating in gradual internal reforming. Anode supported single cells, fabricated at Forschungszentrum Jülich, composed of (La,Sr)MnO3 cathode, 8 mol% yttria-stabilized zirconia (YSZ) electrolyte, and YSZ-Ni anode were used for long term testing on anhydrous (direct) ethanol. Gadolinia-doped ceria (CGO), with 0.1 wt.% of Ir, catalyst was produced by the impregnation of Ir salt in commercial CGO (Praxair) and mixed with organic additives. The CGO-Ir suspension was deposited onto the surface of the YSZ-Ni anode support with an air brush. Such a catalytic layer acts both as a physical barrier to avoid direct contact of the fuel with Ni-based anode and to promote the gradual internal reforming of ethanol. The CGO-Ir catalyst was designed for the reforming of the main compounds resulting from the thermal decomposition of ethanol at high temperature and to inhibit the formation of ethylene, which is a known source of carbon deposits. After deposition the sample was heated at 850ºC in inert atmosphere to ensure good adhesion between the catalytic layer and the anode. Gold grid and paint were used as current collectors in both cathode and anode sides. The single fuel cells were mounted in the test apparatus, sealed with gold rings and alumina cement and heated to the measuring temperature (850°C) under inert atmosphere. After gas tightness was checked by flowing air on the cathode side and argon on the anode side, the anode was reduced under hydrogen. The fuel cell was kept at 0.6 V while monitoring the current drained from the cell at 850 °C. Samples were initially operated in hydrogen for a few hours and fuel was switched to dry ethanol, carried by nitrogen. It is important to mention that no oxidizing agent was added to the fuel and that water produced by the electrochemical reaction of hydrogen at the anode promoted the reforming of the ethanol in the CGO-based layer, in the so-called gradual internal reforming. In such conditions the current output was recorded and observed to not be significantly different form that under hydrogen. Moreover, the direct ethanol fuel cell was continuously operated for 650 hours without major losses due to carbon formation. The experimental results demonstrate that the water released by the electrochemical oxidation of reformate hydrogen at the electrolyte/anode active interfaces is sufficient for the reforming of the primary fuel in the catalytic layer, provided that a minimum fuel utilization is respected. After the durability tests, energy dispersive X-ray spectroscopy and scanning electron microscopy analyses of the samples revealed no carbon deposit formation in the anode. The experimental results provided compelling evidence for the viability of gradual internal reforming of ethanol in high performance solid oxide fuel cells.

In this chapter, the construction, principle of operation, types and production technologies of SOFCs are presented and characterized. Three main types of SOFC are described according to the type of support layer responsible for the mechanical strength of the cell: electrolyte-supported solid oxide fuel cells (ES-SOFC), anode-supported solid oxide fuel cells (AS-SOFC), and metal-supported solid oxide fuel cells (MS-SOFC). Base and raw materials for particular functional layers of solid oxide fuel cells are characterized, i.e., anode, electrolyte, and cathode layers. Thick- and thin-film technologies for the production of particular elements of SOFC fuel cells are presented. Using the example of technology developed in the Institute of Power Engineering Ceramic Department CEREL, the production technology of anode-supported solid oxide fuel cells and the method of characterizing the microstructure and electrochemical properties of the produced cells are presented.

The objective of this work was to investigate how fuel utilization and method of fuel delivery for a large planar fuel cell with co-flow configuration will affect the degradation rate and mechanism of phosphine poisoning of a solid-oxide fuel cell (SOFC). Coal syngas, a potential fuel source for SOFCs, contains gas phase impurities such as PH3, which rapidly degrade Ni-based SOFC anodes. Researchers have shown significant reconstruction of Ni-anodes in button cell configurations with ~0.5 mV hr-1 degradation rates, but it is not evident that these rates will occur in actual stack applications. A singular planar stack repeat unit was constructed using a Haynes 242 interconnect manifold with a cobalt-oxide coating. The cell was operated at 800°C with 10 ppm PH3 in dry H2. Cell performance was evaluated over 500 hours by means of voltage-current measurements and impedance spectroscopy. PH3 was measured entering the fuel cell and then exiting the fuel cell in the exhaust. The post-run material analysis of the contaminated cell was conducted via x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron micrographs (SEM). In this work, there was no appreciable degradation attributable to PH3 poisoning of the anode. No reconstruction of the Ni-anode was observed. This result is in contrast to the many papers published on the subject, where anode-supported SOFC in a button cell configuration degraded rapidly. It is believed that the increased fuel utilization of the large planar cell compared to the published literature using button cells contributed to increased H2O generation which led to side reactions that prevented the phosphorus from interacting with the Ni-anode; platinum components may have catalyzed these reactions.

It is always an important research direction to improve the performance of solid oxide fuel cells (SOFCs) through structural optimization. For anode-supported SOFC, the thin cathode thickness results in an uneven distribution of oxygen concentration in the porous electrode, which limits the output performance. In this paper, a three-dimensional model of the anode-supported planar SOFC was established, and the thickness of the cathode diffusion layer in the area covered by the connector was increased to improve the uniformity of oxygen distribution in the cathode. The results show that increasing the thickness of the cathode diffusion layer under the connector can significantly improve the overall output performance of SOFC. The uniformity of oxygen concentration distribution in the cathode is improved, the local current density under the connector is increased, and the maximum output power density of the cell is increased by 29.14%. The results show that the output performance of the SOFC and the uniformity of gas distribution in the porous electrode can be improved by increasing the thickness of the cathode diffusion layer under the connector in the structure design and practical application of the SOFC. The research of this paper provides a reference for the optimization design of SOFC.

Microtubular anode-supported solid oxide fuel cells (μt-SOFC) were created with a metallic coil embedded in the anode to act as current collector. The electrochemical performance was experimentally examined by comparing the power density (PD) of μt-SOFC with embedded coils of different turns per unit length and composition (nickel and palladium). It is shown that an increase in the turns per unit length results in a proportional current density increase and in a quadratic increment of PD. Additional performance improvement is found for the cell with palladium current collector due to the higher catalytic activity for hydrogen oxidation.

Solid oxide fuel cells (SOFCs) are attractive devices for power generation because they can operate under a variety of fuels, including H2, hydrocarbon fuels, biogas, and syngas (CO + H2). Syngas is an important fuel since it is a major product of reforming hydrocarbon logistics fuels (i.e. JP-8, JP10, S8, etc.). However, the mechanism of electrochemical oxidation of CO in SOFCs is still not well understood. This is due, in part, to the difficulty in probing CO consumption and product formation under operating conditions in real time. Herein, the operando methods Fourier transform infrared emission spectra (FTIRES) of the SOFC headspace together with thermal imaging data of the Ni-YSZ anode surface to provide data relevant to understanding these reaction mechanisms. The studies are carried out on anode-supported SOFCs (Ni-YSZ/GDC-LSC button cells) operating on simulated syngas mixtures. The contamination and subsequent degradation of SOFC operation at operational temperatures of 600° C to 800° C, in the presence of sulfur concentrations below 30 ppm, is followed using potentiostatic spectroelectrochemistry, chronocoulometry, electrochemical impedance spectroscopy, and operando optical methodologies to provide a comprehensive picture of the processes occurring in the fuel oxidation. Together, the data obtained indicate that sulfur contaminants 1) competitively inhibit formation of carbon over CO catalytic and electrocatalytic sites, and 2) induce changes of the Ni-YSZ anode permanently perturbing their catalytic and electrocatalytic function.

Stress in an anode-supported type solid oxide fuel cell (SOFC) was measured by in-situ measurement using micro Raman scattering spectroscopy. The stress conditions evaluation method focuses on Raman spectra of samarium-doped ceria (SDC) which was used as an interlayer in SOFC. The shift of ceria Raman peak and peak width in SDC interlayer were measured to evaluate stress, temperature, and Sm concentration. We performed this method in the purchased cells of conventional anode-supported type SOFC under thermal cycles, which simulate start-up and shout down. Observed results suggested that not only thermal stress relaxation but also diffusion of samarium from an interlayer to an electrolyte, and stress relaxation by plastically deformation were occurred.

Development and testing of anode-supported solid oxide fuel cells with slurry-coated electrolyte and cathode

A systematic study was initiated of anode-supported solid oxide fuel cells (SOFCs) with Nd2NiO4 cathodes. This type of cathode, a mixed conducting K2NiF4-type material, can be considered as a candidate for SOFC applications. In this study, the influence of (1) the presence of a Ce0.8Gd0.2O1.9 (CGO) interlayer between the electrolyte and the cathode, (2) sintering conditions, and (3) the grain size of the powder on the performance of SOFCs with Nd2NiO4 cathodes was investigated in more detail. Results from current density-voltage characteristics and permeation and gas diffusion measurements showed that the electrochemical performance was promising for SOFCs including a CGO interlayer and a Nd2NiO4 cathode (prepared with a powder with a d50 of 0.5A∕cm2 or 0.8μm) sintered at 1100°C or higher; current density at 800°C and 700mV between 1.2 and 1.3A∕cm2. The microstructure of the cathode was such that no gas diffusion problems occurred.

The effects of both temperature and applied bias power during the sputtering of gadolinia-doped ceria (GDC) interlayers used as diffusion barriers in anode-supported solid oxide fuel cells (SOFCs) were studied. Scanning electron microscopy analysis revealed that increasing the applied bias power, in the 0–300 W range, increasingly promotes the deposition of continuous and dense interlayers. Such feature was mirrored in the electrochemical performance of single cells that exhibited enhancement of the power density of an SOFC with bias-assisted sputtered interlayers. In addition, fuel cells having interlayers deposited in the temperature range exhibited similar microstructure and electrochemical performances, indicating that the applied bias allows for the sputtering of GDC protective interlayers at relatively lower temperatures than unbiased depositions. The presented results evidenced that bias-assisted sputtering is an effective technique for the fabrication of high performance anode-supported SOFCs.

Ni-Gd doped ceria (Ni-GDC) anodes exhibit high durability during direct butane utilization in solid oxide fuel cells (SOFCs). However, the cells containing Ni-GDC anodes showed less performance compared to those containing conventional Ni-Y stabilized zirconia (Ni-YSZ) anodes, when zirconia-ceria solid solution formed at the interface between the YSZ electrolyte and Ni-GDC anode during co-sintering at high temperature in anode-supported SOFCs. The formation of the solid solution increases electrochemical impedance at low frequencies due to oxygen nonstoichiometric variation process. To prevent the formation of this solution, La doped ceria (LDC) was infiltrated into the Ni-YSZ anode after sintering. The maximum power density of the cell containing Ni-YSZ anode with LDC infiltration (Ni-YSZ+LDC) was higher than those containing Ni-YSZ and Ni-GDC anodes. Distribution of relaxation times (DRT) analysis revealed that the polarization resistance of fuel oxidation process decreased, and the oxygen nonstoichiometric variation process was not observed after LDC infiltration. Power was continuously generated over 100 h using butane fuel for the Ni-YSZ+LDC. No carbon deposition was observed after the durability test, because the LDC nanoparticles enhanced the catalytic activity toward carbon oxidation. The LDC infiltration is an method to obtain the comparability of performance and durability for direct butane utilization.

The most commonly used solid oxide fuel cell (SOFC) anode material is a two phase nickel and yttria stabilized zirconia (Ni/YSZ) cermet. During typical fuel cell operation, this material remains a cermet; however, in a commercial SOFC system seal leakage, fuel supply interruption, or other situations may cause the anode to reoxidize. The cyclic reduction and oxidation (redox) of nickel will result in large bulk volume changes, which may have a significant effect on the integrity of interfaces within the fuel cell and thus may cause significant performance degradation. Following initial study of the degradation mechanisms and redox kinetics, a number of solutions have been devised. These can be broadly categorised into system solutions and unit cell or materials solutions. The former includes purge gas, other means to produce a reducing gas, cell reversal and passive solutions such as hydrides, oxygen getters and steam purging. The latter includes alternative anode materials, microstructural modification of anode functional layers (AFLs), graded porosity and composition of AFL and redox barrier layers. This paper discusses the pros and cons of each approach and some initial feasibility testing in short stack repeat units.