Ethanol and methane fueled solid oxide fuel cells: A comparative study

Ethanol and methane are compared as candidate fuels for generation of electrical power in Solid Oxide Fuel Cells (SOFCs). The thermodynamic analysis of both alternatives was undertaken considering that a SOFC operates with the equilibrium products of the steam reforming of each raw fuel. The comparison was made assuming SOFC operation under atmospheric total pressure in the temperature range of 800–1200K, and results were obtained in terms of the maximun theoretical electromotive force (emf) and the thermodynamic efficiency of total energy conversion. It was found that although methane fueled SOFCs are able to provide slightly higher efficiencies, ethanol is a competitive alternative fuel with suitable characteristics. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Technical and economic evaluation of a methane solid oxide fuel cell

Solid oxide fuel cells offer the possibility of high temperature synthesis of chemical products with cogeneration of electricity, a process known as chemical cogeneration. This research primarily addresses the likelihood of upscaling present bench-scale experimental results of methane fuelled SOFCs. Methane coupling, i.e. production of C₂ hydrocarbons, is one of the possibilities of chemical cogeneration. In evaluating the co-generating SOFC, the methane-coupling design was compared to two other possible competing designs, namely, a regular SOFC plant (complete oxidation of methane to CO₂ and H₂O) and an SOFC plant coproducing synthesis gas. It was found that the rate of return for the regular fuel cell exceeded 25% whereas that for the ethylene plant was about 21%. The synthesis gas plant was well behind at about 17%. The reasons that have so far prohibited large-scale application of such systems are discussed. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995.     

Energy integration strategies for solid oxide fuel cell systems

Solid oxide fuel cells (SOFCs) have operating temperatures ranging from as low as 600 °C for intermediate temperature operation to above 900 °C for higher temperature operation. These high temperatures are often viewed as a considerable disadvantage from a materials point of view because of the occurrence of unwanted interfacial reactions, stresses as a result of thermal expansivity mismatches, etc. However, higher temperatures are also an advantage of SOFC systems. Fuel pretreatment that may involve such processes as reforming is very often highly endothermic in nature. The high operating temperature of an SOFC allows for efficient system energy integration with the waste heat from the fuel cell being used to drive fuel pretreatment processes. Here, we demonstrate this propensity for energy integration by looking at the use of a novel hydrogen-carrier system working with an SOFC.     

Reaction between solid oxide fuel cell materials

In fabrication processes of solid oxide fuel cells, high-temperature heat treatments cannot be avoided. It will give rise to mutual reaction and interdiffusion of the cell component materials: yttria-stabilized zirconia (YSZ, electrolyte), (La, Sr)MnO₃ (cathode), Ni-YSZ cermet (anode) and (La, Ca)CrO₃ (separator). Reactivity of LaMnO₃ and YSZ was estimated by thermodynamic calculations, and it was found that the nonstoichiometry at La site in LaMnO₃ plays an important role on the reaction. Diffusion of Mn into YSZ leads to increase of La activity at the interface and promotes the reaction. Electrical conductivity of YSZ decreases when Mn dissolves in the cubic phase of YSZ. Oxidation state of the dissolving Mn varies with partial pressure of oxygen and affects the electrical properties of YSZ. Migration of Ca from (La, Ca)CrO₃ separator to other cell components is one of the largest problems in the co-firing cell fabrication process because it prohibits the sintering of the separator.     

Fabrication and characterization of Ni-supported solid oxide fuel cell

Metal-supported solid oxide fuel cells (SOFCs) are promising due to their good mechanical and thermal properties. Stainless steels are often used as a supporting metal. In this study, the possible use of Ni as a supporting metal was tested. Ni is also a good model of supporting metal due to its lack of Cr which poisons Ni anode.YSZ electrolyte and Ni-YSZ anode were coated on a porous Ni support to fabricate Ni-supported SOFC. A porous Ni support in thick-film form (t ~ 150 µm) was prepared by the mold casting of Ni powders. An anode and an electrolyte layer were sequentially coated on the Ni support using screen printing and tape casting methods, respectively. The Ni-supported cell (t ~ 200 µm) was sintered at 1400 °C in a reducing atmosphere and the performance was evaluated at 800 °C with a La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cathode. The unit cell showed an open circuit voltage (OCV) of 0.96 V and a peak power density of 470 mW/cm² at 800 °C.     

中温固体氧化物燃料电池的基本性质测量

介绍了固体氧化物燃料电池的工作原理,测量了中温薄膜固体氧化物燃料电池的开路电压、放电曲线及功率曲线,并分析了电池内阻随电流密度的变化.     

Aspects of the internal reforming of methane in solid oxide fuel cells

The tubular concept of the Siemens Westinghouse SOFC is reviewed. The efforts to reduce costs by on-cell reformation of methane on high power density cells are outlined. The problems concerning the internal reformation of methane are discussed from a thermodynamic point of view. Investigations of the steam reformation of methane on Ni-YSZ anodes are reported to clarify the kinetics. The suitability of this material as a catalyst for this reaction is confirmed. On stripes of anodes a steady state utilisation of methane was reached only after many hours, significantly complicating the experiments. This delay was not seen with anodes in cell configurations, suggesting a different process. In neither arrangement the shift reaction was in thermodynamic equilibrium. This reaction seems to be correlated with the reformation. Experiments on stripes with pure mixtures of methane and steam revealed the dissolving of Ni into the gas phase at the leading edge of the sample. This effect can be suppressed by adding a small amount of hydrogen. With gas compositions typical for SOFC operation no degradation of the electrodes was detected during 1000 hours. Assuming an isothermal sample and equilibrium of the shift reaction, the reaction rate was best described being first order in the partial pressures of methane and water for varying lengths of the stripes. But this function failed to describe the variation of the methane utilisation with composition of the input gas, showing that the complexity of the reformation is not fully understood. The reaction rate was found to be too high for on-cell reformation in SOFC stacks. Simulations of a planar design yielded such large temperature gradients that they are likely to result in a disintegration of the cell structure. The same is expected for the tubular design. Tailored catalysts and new cell designs are required to overcome the heat management problem. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

高温固体氧化物燃料电池实验演示

介绍一种高效低污染的新型能源－高温固体氧化物燃料电池的工作原理及其演示实验装置。     

A practice of single layer solid oxide fuel cell

It has been found that the electrical conduction behavior of La_0.9Sr_0.1InO_3−δ varies with oxygen partial pressure. P-type and n-type conduction at high and low oxygen partial pressure have been observed respectively. While at intermediate oxygen partial pressures, the electrical conductivity changes slightly with the oxygen partial pressure. Thus, La_0.9Sr_0.1InO_3−δ may be a possible material for making single layer solid oxide fuel cell (SLFC). The concept of SLFC has been tested using a piece of thick ceramic pellet of La_0.9Sr_0.1InO_3−δ. The maximum current density and power density is 12 mA/cm² and 3 mW/cm² at 800 °C when dilute H₂ and air were used as fuel and oxidizing agent, respectively. The phase stability of La_0.9Sr_0.1InO_3−δ has been studied by Raman spectra and XRD. It is confirmed that secondary phase may appear in La_0.9Sr_0.1InO_3−δ after long term testing in low oxygen partial pressure, and finally it may be decomposed into La₂O₃ and metal Indium. Much attention should be paid to stabilize La_0.9Sr_0.1InO_3−δ and to improve the performance of SLFC.     

Thin porous Ni-YSZ films as anodes for a solid oxide fuel cell

Porous Ni-YSZ (YSZ—yttria-stabilized zirconia) films were fabricated by reactive co-sputtering of a Ni and a Zr-Y target, followed by sequentially annealing in air at 900 °C and in vacuum at 800 °C. The Ni-YSZ films comprised small grains and pores that were tens of nanometers in size. The porous Ni-YSZ films were used as an anode on one side of a YSZ electrolyte disc and a La_0.7Sr_0.3MnO₃ thick film was used as a cathode on the other side of the disc to form solid oxide fuel cells (SOFCs). The voltage-current curves of the SOFCs with single- and a triple-layered porous anodes were measured in a single-chamber configuration, in a mixture of CH₄ and air (CH₄:O₂ volume ratio=2:1). The maximum power density of the SOFC using the single-layered porous Ni-YSZ thin films as the anode was 0.38 mW cm⁻², which was lower than that of 0.76 mW cm⁻², obtained using a screen-printed Ni-YSZ thick anode. The maximum power density of the SOFC with a thin anode was increased, but varied between 0.6 and 1.14 mW cm⁻² when a triple-layered porous Ni-YSZ anode was used.     

Development of anodes for direct electrocatalytic oxidation of methane in solid oxide fuel cells

Gold-doped nickel/zirconia and nickel/ceria cermet anodes incorporating different levels of gold have been prepared and studied as potential anodes for the direct electrocatalytic oxidation of methane in solid oxide fuel cells. The methane conversion activity and selectivity towards synthesis gas products of these anodes have been determined over a range of SOFC operating temperatures and methane/oxidant ratios, and compared with the corresponding undoped nickel cermet. The amount of carbon deposition has been determined using post-reaction temperature programmed oxidation. The influence of gold loading on the methane conversion activity, product selectivity and amount of carbon deposition has been determined. The addition of small amounts of gold to nickel cermets results in a dramatically increased tolerance to carbon deposition and a reduction in the activity of the anode and the selectivity towards partial oxidation compared to the corresponding undoped nickel cermet. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Oxy-combustion of coal in liquid-antimony-anode solid oxide fuel cell system

A novel system based on the indirect oxy-combustion of coal in a liquid Sb anode solid oxide fuel cell (SOFC) has been used to produce electricity for over 48?h. Pulverized anthracite was fed to the liquid-antimony-anode of the fuel cell, and a peak power density of 47?mW cm^?2 was reached at 1023?K and 35?mW cm^?2 at 973?K. The fuel cell was prepared using a porous stainless-steel tube as a support for an LSM cathode, antimony oxide (Sb₂O₃)/yittria stabilized zirconia (YSZ, Y_0.08Z_0.92O_1.96) composite electrolyte (membrane), while liquid antimony acted as the anode. Liquid antimony/antimony oxide served as the intermediate medium for coal oxidation producing mainly carbon dioxide, which evolved as a separate gas stream. The fuel cell will facilitate carbon capture process, and simultaneously convert the chemical energy of coal directly to electricity. The experiment showed that while the fabricated electrolyte was porous, it became dense during the actual operation, preventing nitrogen leakage into the Sb/C side and producing reasonable open circuit voltage. Analysis of the experimental EIS data illustrates that the Ohmic resistance was the primary loss mechanism in the system. It further suggests approaches to improve the design. Continuous operation of this coal fueled oxy-combustion/fuel cell system achieved an overall efficiency of 28.2% despite of its tiny scale. Simple technologies can be employed to scale up this system at relatively low cost of fabrication and materials.     

Performance of a tubular solid oxide fuel cell with model kerosene reformate gas

A performance of an anode-supported tubular Ni–8YSZ/Ni–ScSZ/ScSZ/GDC/LSC cell was investigated at 650–750 °C by feeding model kerosene reformate gas (H₂, H₂O, CO, CO₂, and CH₄) to a Ni–8YSZ/Ni–ScSZ anode. Variations of gas composition were observed not only between inlet and outlet of anode to estimate the degree of internal reforming, but also during current input by online quadrupole mass spectrometry and Fourier-transform infrared spectrometry.The electrochemical performance of the cell was independent of reforming temperature of kerosene, i.e. gas composition (in particular CH₄ concentration) at moderate anode gas flow rates. At open-circuit states, 10% or less methane in the kerosene-reformed gas was readily converted by steam or CO₂ over the Ni–8YSZ/Ni–ScSZ electrode so that gas compositions could almost follow the thermodynamic equilibrium at 650–750 °C. This suggests that the internal reforming should proceed almost completely over the Ni anode. Consumption of H₂ and CO and production of CO₂ were observed during current input. I–V characteristics remained constant at 650 °C as long as anodic W/F was more than 0.2 kg mmol^− 1 s. It was demonstrated that a catalytic activity of an anode electrode for hydrocarbons will be important for SOFCs with liquid fuels such as kerosene in order not to deteriorate cell performance.     

Reaction mechanisms of the copper-manganese spinel cathode in a solid oxide fuel cell

Cu_1.25Mn_1.75O₄ spinel (CMO) was studied as a potential solid oxide fuel cell (SOFC) cathode material at intermediate temperatures. The reaction mechanism of a composite cathode consisting of Cu_1.25Mn_1.75O₄ and yttria-stabilized zirconia (YSZ) was investigated by impedance spectroscopy. The influence of the CMO/YSZ ratio, time exposed to current passage and temperature on the impedance spectra was examined. Activation energy of the corresponding processes was calculated to be near 1 eV and between 1.32 and 1.96 eV for the high and low frequency arcs in the impedance spectra. Comparison between CMO-YSZ and Sr-doped LaMnO₃ (LSM)-YSZ composite cathodes showed they had similar reaction mechanisms. The transport or transfer of oxygen intermediates or oxide ions between the catalyst and electrolyte was suggested to be the rate determining steps between 700 and 800 °C, whereas dissociative adsorption, mass transfer and surface diffusion were rate controlling between 600 and 700 °C.     

Overview on grain-boundary and transport problems in solid oxide fuel cell

An overview is given on the present state of development of fuel cells based on solid oxygen ion conductors. The performance is compared to other types of fuel cells. The employed electrolyte and electrode materials and current problems are described. The possibilities of reduced temperature and the reduction of internal resistances is discussed. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

Spectroelectrochemical cell for in situ studies of solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are able to produce electricity and heat from hydrogen‐ or carbon‐containing fuels with high efficiencies and are considered important cornerstones for future sustainable energy systems. Performance, activation and degradation processes are crucial parameters to control before the technology can achieve breakthrough. They have been widely studied, predominately by electrochemical testing with subsequent micro‐structural analysis. In order to be able to develop better SOFCs, it is important to understand how the measured electrochemical performance depends on materials and structural properties, preferably at the atomic level. A characterization of these properties under operation is desired. As SOFCs operate at temperatures around 1073 K, this is a challenge. A spectroelectrochemical cell was designed that is able to study SOFCs at operating temperatures and in the presence of relevant gases. Simultaneous spectroscopic and electrochemical evaluation by using X‐ray absorption spectroscopy and electrochemical impedance spectroscopy is possible.     

Direct ammonia proton-conducting solid oxide fuel cells prepared by a modified suspension spray

A dense (BCSO) membrane was fabricated by a modified suspension spraying on porous NiO–BCSO anode support. In the process, the suspension was directly prepared by ball-milling the BaCO₃, CeO₂, and Sm₂O₃ powders in ethanol. A dense and uniform electrolyte layer in the thickness of 10 μm was successfully prepared on porous anode support by suspension spray process after co-sintering at 1,400 °C for 5 h. With (NSMO) cathode, a single cell was assembled and tested with hydrogen and ammonia as fuels, respectively. The hydrogen-fueled cell exhibits 1.01 V for open circuit voltage (OCV) and 560 mW/cm² for peak power density at 700 °C. In comparison, the cell in ammonia displays a similar performance (1.02 V for OCV and 530 mW/cm² for output), which indicates the liquid ammonia is a promising substitute for hydrogen. Moreover, the fuel cell displays good interface contacts. To sum up, ammonia-fueled solid oxide fuel cells prepared by this simple suspension spray is an alternative way to promote the commercialization.     

Sites for catalysis and electrochemistry in solid oxide fuel cell (SOFC) anode

Fuel cells represent a challenging overlap of catalysis and electrochemistry. This is illustrated by anode reactions in a solid oxide fuel cell. The sites for catalytic conversion of methane and electrochemical conversion of hydrogen on an SOFC anode appear not to be the same. The fuel (methane, hydrogen, etc.) is activated by chemisorption on the nickel surface of the anode. This is linked to the electrochemical reaction at the interface of the electrolyte and the nickel crystals converting oxygen ions into electrons and water by reactions with adsorbed hydrogen atoms resulting from the activation of the fuel. The sites for these reactions appear not to be the same. This is reflected by different sensitivities of the two steps to sulphur poisoning. The role of different sites on the nickel surface for the steam reforming reaction is well understood in terms of impact on activity for methane activation, carbon formation and sintering. The study is supplemented by an analysis of anodes having been exposed to 13000 of operation using a number of characterisation methods. PACS 82.47.Ed; 82.45.-h; 82.65.-s