Materials and reaction mechanisms at anode/electrolyte interfaces for SOFCs

The present paper reviews anodic reaction mechanisms of porous cermet and model anodes at metal/oxide interfaces in solid oxide fuel cells (SOFCs). Some analytical results, electrochemical methods, and reaction models were presented at Ni–YSZ cermets and well defined model anodes. Isotope labeling/secondary ion mass spectrometry (SIMS) analysis techniques were applied to determine the oxygen surface reactivity of oxide electrolytes in reducing atmospheres. The technique was also applied to determine the catalytic activity of metal/oxide interfaces for CH₄ decomposition and reactivity with the reformed gases at the mesh or stripe shaped anodes on different oxides. Observed SIMS images and the electrochemical analyses were compared at the model anode/electrolyte interfaces.     

Non-conventional solid state fuel cells: Materials & technology

Investigation of “Non-conventional material” for fuel cells, such as oxide-salt-ceramic composites and ceria based or perovskite oxides with different dopants, leads to a much lower fuel cell operating temperature compared to conventional high temperature, ∼1000 °C, solid oxide fuel cells (SOFCs), which provides the new possibilities for facilitating SOFC commercialisation. This work is essentially an effort to develop new types of solid oxide ion and proton fuel cells (SOFC and SPFC) at fairly low temperatures, <800 °C, or intermediate temperature, 400 to 800 °C. The conventional high temperature SOFCs using yttria-stabilised zirconia (YSZ) materials, and low temperature SPFCs (<200 °C) using polymer membrane electrolytes have complex material and system problems from either special high temperature requests or expensive technology and reforming systems. This research is intended to provide materials and technology along new routes for so-called non-conventional fuel cell systems, to facilitate solid state fuel cell cmmercialisation. The fuel cell research on these non-conventional systems is promising. This paper, based on recent achievements in research on materials and technology, summaries the developnt of material systems and new fuel cell devices regarding their potential marketability in the near future. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

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.     

Electrical and electrochemical properties of SrBiMTiO<Subscript>6</Subscript> (M = Fe,Mn, Cr) as potential cathodes for solid oxide fuel cells

A series of perovskite oxides SrBiMTiO₆ (M = Fe, Mn, Cr) have been synthesized and characterized towards application as cathode materials for solid oxide fuel cells (SOFCs). X-ray diffraction (XRD) patterns reveal that all samples are stabilized in \( \mathrm{Pm}\ \overline{3}\mathrm{m} \) space group. Electrical conductivity, AC impedance characteristics, and thermal and chemical stability have been studied in order to assess their possible use as SOFC cathode materials. In comparison with other low electrical conductivity cathodes of SOFC, our results suggest that SrBiMnTiO₆, which has the highest electrical conductivity (4.02 S cm^?1) and moderate polarization resistance (0.104 Ω cm²) at 850 °C, is the most promising candidate among the three perovskite oxides for further study and optimization as a SOFC cathode material.     

Improvement of Cu–CeO2 anodes for SOFCs running on ethanol fuels

Ethanol is considered to be an attractive green fuel for solid oxide fuel cells (SOFCs) due to several advantages. In this paper, we presented recent progress of our group in Cu–CeO₂ anodes for SOFCs with ethanol steam as a fuel. Cu–CeO₂–ScSZ (scandia stabilized zirconia)anodes with different ratios of copper versus ceria were fabricated and the impedance spectra of symmetric cells were measured to optimize the anode composition. Area specific resistance (ASR) of these anodes was examined to prove the thermal stability of them, and possible reasons for degradation were analyzed. Furthermore, a Ni–ScSZ interlayer was added between Cu–CeO₂–YSZ (yttria stabilized zirconia) anode and ScSZ electrolyte to improve the anode performance, and the three-layer structure was fabricated by acid leaching of nickel and wet impregnation method. The maximum power density of the single cell reached 604 mW cm^? 2 and 408 mW cm^? 2 at 800 °C in hydrogen and ethanol steam respectively, and the cell obtained stable output in ethanol steam over an operation period of 50 h.     

Development and investigation of perovskite (ABO3)-type oxides for power generation

Critical problems of the present approach of electrolytes for Solid Oxide Fuel Cells (SOFCs) for commercialization are discussed. Major progress is expected from the development of materials based on the “SEA (Single Element Arrangement)” concept. The galvanic cell consists in this case basically of a single chemically homogeneous compound, which functions as electrodes at high and low activity and as electrolyte at intermediate activities of the electroactive component. In view of the large structural flexibility of the chemical nature of the constitutents, we explored perovskite (ABO₃)-type compounds to be used as SEAs for SOFCs. Results of studies on Pr-substituted LSGM and Fe-substituted SrSnO₃ perovskite-type oxides are presented. For instance, SrSn_1-xFe_xO_3-δ with x=0.1 exhibits p and n-type electronic conduction at the cathode and anode sides of the SOFC, respectively, while oxide ion conduction prevails at intermediate oxygen partial pressures. The SEA concept is also applicable for other devices in the field of Ionics.     

固体氧化物燃料电池模式阳极内传输与电化学反应耦合机理

模式电极因其结构可控、电化学/化学反应活性位和物质传输路径明确等优势,被广泛应用于固体氧化物燃料电池新型电极研究.现有研究多采用模式电极研究新材料电化学特性、表界面催化反应机理等,尚未涉及几何结构对其内部传输与电化学反应耦合机理的影响,限制了模式电极的应用.本文建立了固体氧化物燃料电池阳极内电荷传输与电化学反应过程的格子玻尔兹曼模拟方法,明确了控制电极过程的关键无量纲参数及其对电极性能的影响规律,研究了模式阳极几何结构的影响机理.根据电极性能对无量纲参数的敏感程度,绘制了指导模式阳极设计与运行的相图,指出相图过渡区(电极性能随操作参数显著变化区域)为进行反应机理研究的最佳操作参数取值范围.同时,研究发现模式阳极电子导体内电子的快速迁移虽不限制阳极性能,其几何结构显著影响过渡区范围;离子导体内离子迁移为影响阳极性能的限速步骤,但其几何结构几乎不影响过渡区范围.本文的数值方法与机理研究结果可为固体氧化物燃料电池模式电极的设计提供重要理论依据.     

Apatite type lanthanum silicate and composite anode half cells

Apatite type rare earth silicates are being extensively studied as electrolyte material for intermediate temperature solid oxide fuel cells (SOFC). In this paper we presents results on synthesis of Al and/or Fe-doped ATLS, the design of compatible anode materials, thermal expansion properties and co-sintering of half-cells from expansion matched materials using the advanced pulsed electric current sintering (PECS) technique. The issues related to the co-sintering of half cells have been addressed successfully by the combined use of nano powders and PECS.     

Measurement of electrode overpotentials for direct hydrocarbon conversion fuel cells

Cathodic and anodic overpotentials were measured using current interruption and AC impedance spectroscopy for two separate solid oxide fuel cells (SOFCs). The fuel cells used yttria-stabilized zirconia (YSZ) as the electrolyte, strontium-doped lanthanum manganite (LSM) as the cathode, and a porous YSZ layer impregnated with copper and ceria as the anode. The Cu/CeO₂/YSZ anode is active for the direct conversion of hydrocarbon fuels. Overpotentials measured using both current interruption and impedance spectroscopy for the fuel cell operating at 700 °C on both hydrogen and n-butane fuels are reported. In addition to providing the first electrode overpotential measurements for direct conversion fuel cells with Cu-based anodes, the results demonstrate that there may be significant uncertainties in measurements of electrode overpotentials for systems where there is a large difference between the characteristic frequencies of the anode and cathode processes and/or complex electrode kinetics.     

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.     

A Ni/YSZ composite containing Ce0.9Ca0.1O2−δ particles as an anode for SOFCs

A composite material (hereafter referred to as NYC) containing Ni, Y₂O₃-stabilized ZrO₂ (YSZ) and Ce_0.9Ca_0.1O_2−δ (CC10) particles was prepared and used as the anode of solid oxide fuel cells (SOFCs). The performance of NYC was better than that of conventional Ni/YSZ anodes in terms of anodic overpotential and interface impedance. The additional CC10 particles improved the anode properties. XRD results suggest that a solid solution of YSZ and CC10 was produced. From impedance measurements, it is concluded that the solid solution exhibits substantial electronic conduction. Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ anodes exhibited the best properties over the experimental temperature range. A SOFC with an anode of Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ provided the maximum power density and current density. Addition of CC10 with an average particle size of 0.3 μm was more advantageous than that with an average size of 3 μm.     

固体氧化物燃料电池

高效、洁净、全固态结构、高温运行的固体氧化物燃料电池(SOFC)是把反应物的化学能直接转化为电能的电化学装置，这种新型发电技术是目前发展最快的能源技术之一，有望在近年内走向商业化应用。SOFC单体电池由致密的电解质和多孔的阳极、阴极组成，现在主要发展了管状结构和平板式结构两种形式，单体电池通过致密的连接体材料以各种方式组装成电池组，广泛应用于大型发电厂、热电耦合设备、小型供能系统和交通工具等，市场前景广阔。     

Performance and sulfur resistance of doped yttrium chromite-ceria composite as anode material for the SOFCs operating on H<Subscript>2</Subscript>S-containing fuel

The sulfur resistance and performance of Sr- and Mn-doped yttrium chromite (YSCM)-samaria-doped ceria (SDC) composite material were investigated for potential use as anodes in solid oxide fuel cells (SOFCs). The anode was well adhered to the electrolyte, which was ascribed to their similar coefficient of thermal expansion (TEC). The electro-catalytic activity of YSCM-SDC anodes in yttria-stabilized zirconia (YSZ) electrolyte-supported cells toward hydrogen oxidation was superior to that of the La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ (LSCM) anode. Sr depletion in the YSCM structure and the formation of SrSO₄ in the presence of sulfur led to performance degradation of the anode. Irreversible and reversible performance degradation suggested that YSCM and SDC played a respective role during the anode deactivation process. The voltage decreased at a rate of 31 mV/h and stabilized at 0.49 V under a 3000 ppm H₂S atmosphere. In addition, the sulfur tolerance of YSCM-SDC anode was better than SDC under strictly identical conditions.     

Internal reforming in intermediate temperature solid oxide fuel cells running on natural gas

Methane steam reforming has been studied over a range of nickel/ceria-gadolinia cermet anodes, over the temperature range 500–700 °C appropriate for intermediate temperature ceriagadolinia based SOFCs. The influence of operating temperature and methane/steam ratio on the reforming characteristics, methane conversion and product selectivity, and the carbon deposition on the anode during reforming, has been determined for each anode. Nickel/ceria powders made from gas atomised intermetallic precursors have been studied as potential anode materials for intermediate temperature ceria-gadolinia based SOFCs running on natural gas. The powders have been characterised structurally and evaluated for their methane reforming characteristics and their resistance to carbon deposition during internal reforming, especially at low steam/methane ratios, over the relevant operating temperature ranges of ceriagadolinia based SOFCs. Their performance has been compared to conventionally prepared anodes and anodes generated from cast alloys, with very favourable results. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Material transport and degradation behavior of SOFC interconnects

The SOFC interconnect materials, both lanthanum chromite based oxides and Fe–Cr ferritic alloys, are discussed from the viewpoint of material transport which causes the degradation in conductivity or chemical stability. The controlling factors, such as effect of oxygen chemical potential gradient, interaction with other cell components, and surrounding gaseous atmospheres are evaluated. The role grain boundary is important in the transport of metal components in oxide materials such as lanthanum chromites, or oxide scales on alloy. The diffusivity of metal components in alloy is much faster, which causes the interdiffusion on nickel and chromium between alloy and anode current collector. The reaction of alloy and sealing materials would be more significant, since the chromium component in alloy easily reacts with alkaline earth components in sealing materials. The slight amount of water vapor in air may greatly enhance the chromium vaporization rate from chromium oxide (Cr₂O₃).     

R & D for intermediate temperature solid state fuel cells

Although some solid oxide fuel cells (SOFCs) are in the process of ommercialization, this technology still faces serious technical challenge due to the special high temperature (1000 °C) requirement resulting in high costs as well. It has thus been a strong tendency to develop intermediate temperature (400 to 800 °C) solid state fuel cells (ITSSFCs). ITSSFCs use either oxide ionic conductors, e.g. fluorite or perovskite oxides or proton conducting oxyacid salts and salt-oxide composites as electrolytes that have a high protonic or oxide ionic conductivity of 10⁻² to 10⁻¹ S/cm in the IT region. The ITSSFCs may provide new opportunities for SSFC (including SOFC) commercialization. This paper gives a brief overview of the current ITSSFC status and activities of research and development mainly based on our work. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

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.     

Electrochemical performance of mixed ionic–electronic conducting oxides as anodes for solid oxide fuel cell

Mixed ionic–electronic conducting (MIEC) oxides, SrFeCo_0.5O_x, SrCo_0.8Fe_0.2O_3−δ and La_0.6Sr_0.4Fe_0.8Co_0.2O_3−δ have been synthesized and prepared on yttria-stabilized zirconia as anodes for solid oxide fuel cells. Power output measurements show that the anodes composed of such kinds of oxides exhibit modest electrochemical activities to both H₂ and CH₄ fuels, giving maximum power densities of around 0.1 W/cm² at 950°C. Polarization and AC impedance measurements found that large activation overpotentials and ohmic resistance drops were the main causes for the relative inferior performance to the Ni-YSZ anode. While interlayered with an Ni-YSZ anode, a significant improvement in the electrochemical performance was observed. In particular, for the SrFeCo_0.5O_x oxide interlayered Ni-YSZ anode, the maximum power output reaches 0.25 W/cm² on CH₄, exceeding those of both SrFeCo_0.5O_x and the Ni-YSZ, as anodes alone. A synergetic effect of SrFeCo_0.5O_x and the Ni-YSZ has been observed. Future work is needed to examine the long-term stability of MIEC oxide electrodes under a very reducing environment.     

Influence of lattice oxygen stoichiometry on the mechanism of methane oxidation in SOFC anodes

There is growing interest in developing oxide materials for direct hydrocarbon solid oxide fuel cell anodes. In addition to electronic and ionic conductivities, the electrocatalytic activity of these materials is a critical requirement for a high performance anode. In this paper, we present evidence for the important role of variable lattice oxygen stoichiometry and anode geometry in dictating the activity and reaction mechanism of La_0.75Sr_0.25Cr_1 − xMn_xO_3 − δ-based anodes for CH₄ oxidation. Total oxidation of CH₄ is favored by low oxygen vacancy concentration and availability of reducible B-site cations. The non-linear dependence of electrode polarization resistance with current density is attributed to dynamic changes in lattice oxygen vacancy concentration. The relatively high open circuit potential of porous anodes compared with thin films is attributed to an increase in secondary reactions of the fuel within the porous anode.     

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