Micro‐modeling of porous composite anodes for solid oxide fuel cells

A new anode micromodel for solid oxide fuel cells to predict the electrochemical performance of hydrocarbon‐fuelled porous composite anodes with various microstructures is developed. In this model, the random packing sphere method is used to estimate the anode microstructural properties, and the complex interdependency among the multicomponent mass transport, electron and ion transports, and electrochemical and chemical reactions is taken into account. As a case study, a porous Ni–YSZ composite anode operated with biogas fuel is simulated numerically and distributions of the current density, polarization, and mole fraction and rate of flux of the fuel components along the thickness of the anode are determined. The effect of the anode microstructural variables including the porosity, thickness, particle‐size ratio, and particle size and volume fraction of Ni particles on the anode electrochemical performance is also studied. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1893–1906, 2012     

Basic characteristics of spinel type manganese mixed oxide/titanium composite anodes for electroorganic redox catalysis

Manganese mixed oxide composite layers of about 1 μm thickness on titanium sheet as substrate were fabricated by firing of the corresponding nitrates at a typical temperature of 400°C in air. The activity of these anodes was evaluated by cyclic voltammetry (10 mV/s) and the stability was determined by chronopotentiometry (2.5 mA/cm²) in 1M H₂SO₄. The oxidation of 2-propanol was examined as a simple electroorganic model reaction. The quality of a first category of mixed oxides with general composition MnMe₂O₄ decresed in the order Me = Co, Ni, Fe. In a second group with the general formula MeMn₂O₄ a decrease in the order Me = Co, Cu, Fe, Ni, Ti, Zn, Cd, Ca, Mg, (Zn, Ge), Li was observed. The corresponding candidates of the second group were superior to those of the first. The anode service life τ of the optimum spinel anode CoMn₂O₄/Ti is dependent on the current density, according to j^λ τ = const. (λ = 1.7). Thus high current densities are precluded. The mechanism has been discussed in terms of a heterogeneous redox catalysis: surface Mn(VII) states are formed in a slow electrochemical step. In a subsequent fast chemical oxidation of the organic molecule the original reduced state is regenerated. This also explains the comparatively good service life of these anodes.     

Oxide anodes in electro-organic oxidation. Oxidation of maleic addon tungsten oxide anodes

The electrochemical oxidation of maleic acid on tungsten anodes has been investigated. Glyoxal and carbon dioxide were the main products together with tartaric acid and acetaldehyde. Glyoxal is also obtained as the main product from the oxidation ofd-tartaric acid. Under the same conditions succinic acid is completely oxidized to carbon dioxide and water. The anodic dissolution of tungsten and the oxidation of water to oxygen become predominant in the final stages of the electrolyses.     

Electrochemical properties of composite anodes for cathodic protection

Comparative results of electrochemical investigations of polymeric composites with different contents of carbon black and graphite activated with ruthenium oxides are presented. Electrochemical impedance measurements (EIS), enabling determination of electrical and electrochemical properties of polymeric composites, were accompanied by potential measurements performed during a 21-day galvanostatic polarization at 5 A m^–2 current density. Gravimetric measurements were also made to determine the extent of anode consumption. It was demonstrated that the presence of ruthenium oxides characterised by high electrocatalytic activity in the polymeric composites causes an improvement in the polarisation characteristics, a decrease in the value and improvement in stability of the potential during long-term operation, and also a decrease in the anode consumption, as compared to composites exclusively containing activated carbon black.     

Preparation and evaluation of electrocatalytic oxide coatings on conductive carbon-polymer composite substrates for use as dimensionally stable anodes

This work investigates the feasibility of developing a lower cost dimensionally stable anode based on a polymer substrate by examining possible methods of applying coatings, such as MnO₂, to catalyse the oxygen evolution reaction. The conductive polymer is based on a blend of polypropylene and a rubber, to provide the mechanical strength and acid resistance of the electrode, with graphite fibre and carbon black dispersed through the composite to ensure a reasonable conductivity. Of the different compositions analysed, the best incorporated Degussa XE-2 carbon black. This material not only displays an overpotential at its bare surface which was comparable to the lead electrode, but also it provides excellent contact with the catalyst coatings. Two methods of applying a catalytic coating, thermal decomposition and pressed oxide coating, are evaluated. Coatings prepared by thermal decomposition displays an overpotential greater than the standard lead electrode, although the poor performance of the catalysts is found to be due to poor contact with the conductive sites of the substrate and to incomplete thermal decomposition of the metal chlorides to the oxide. The pressed oxide coating method is the technique developed during this work to take advantage of the properties of the polymer composites. Polymer substrates are heated until soft and the MnO₂ catalyst (in its active form) is pushed into the surface. Electrodes coated in this manner with MnO₂ all display excellent overpotentials which, on average, are 0.2 V less than the lead electrodes. Extension of this technique to the coating of polymeric substrates with other catalytic oxides shows great potential.     

贱金属氧化物不溶性阳极的研究进展

介绍了 PbOZ.SnO2.MnO2.CO3O4等贱金属氧化物不溶性阳极的研究和应用状况以及存在的问题,认为有效改善贱金属氧化物阳极的使用性能特别是表面催化性能以及如何搭配各种贱金属氧化物将是其替代贵重金属氧化物阳极的关键.     

Development of novel anodes for solid oxide fuel cells

Ni cermet anodes pose considerable problems for SOFC operation in natural gas fuels, particularly with regard to carbon deposition due to hydrocarbon cracking. Oxide anodes offer a good alternative, particularly if a material combining good electronic and ionic transport properties can be utilised. In our search for alternative anode materials, we have investigated fluorite-based systems containing reducible early transition metal dopants. The extent of phase stability has been investigated by solid-state chemical techniques and electrical properties have been investigated by ac impedance techniques as a function of both temperature and oxygen partial pressure. The Nb---Zr---Y---O system has been found to provide a good model system exhibiting reasonable electrical properties. Niobium pentoxide exhibits a wide range of solid solubility in the yttria-zirconia cubic fluorite system and the fluorite structure is retained under reducing conditions. Electronic conductivity increases as niobium concentration increases; however oxide-ionic conductivity decreases with extent of niobium substitution. The defect chemistry of this system, which determines the electrical properties, is dominated by the high concentration of oxide vacancies necessary to stabilise the cubic structure, hence electronic conductivity exhibits a P(O₂)^-1/4 dependence on oxygen partial pressure.     

Advanced Sn/C composite anodes for lithium ion batteries

Metallic tin was deposited in fine particulate form on the surface of carbonaceous mesophase spherules (CMS) and in the pores of porous carbon by the decomposition and reduction of tin(II) 2-ethylhexanoate at 450 °C. The Sn/C composite powders obtained were used as anode materials for lithium ion cells. Electrochemical cycling tests of coin cells show that the dispersion of tin into the carbonaceous materials enhances the reversible capacity of the electrodes. The capacity retention at the 50th cycle is 91 % for Sn/CMS composite containing 22% tin, against 428 mAh g^–1 at the first cycle. With further increase in tin content, the capacity fade upon cycling is more rapid.     

Electrochemical performance of gel-cast NiO-SDC composite anodes in low-temperature SOFCs

NiO-Ce_0.8Sm_0.2O_1.9 (SDC) composites were synthesized using gel-casting technique. The electrochemical performance of the gel-cast (GC) Ni-SDC cermet as anode was investigated contrast with that fabricated from traditional mechanical mixing (MM) technique using fuel cells with about 35 μm-thick SDC electrolyte and Sm_0.5Sr_0.5CoO₃-SDC cathode. Maximum power density of the cell with GC anode achieved 491 mW cm⁻² at 600 °C, over 100 mW cm⁻² larger than that with MM anode, inferring high catalytic activity of the GC anode. Impedance measurements on the fuel cell at open circuit showed that the anodic interfacial polarization resistance of the GC anode was 0.1 Ω cm² lower than that of the MM anode. Long-term stability of the cell with GC anode in hydrogen was also performed, which showed that it can stabilize at least 7 days.     

Characterization of silicon- and carbon-based composite anodes for lithium-ion batteries

In recent years development of active materials for negative electrodes has been of great interest. Special attention has been focused on the active materials possessing higher reversible capacity than that of conventional graphite. In the present work the electrochemical performance of some carbon/silicon-based materials has been analyzed. For this purpose various silicon-based composites were prepared using such carbon materials as graphite, hard carbon and graphitized carbon black. An analysis of charging-discharging processes at electrodes based on different carbon materials has shown that graphite modified with silicon is the most promising anode material. It has also been revealed that the irreversible capacity mainly depends on the content of Si. An optimum content of Si has been determined with taking into account that high irreversible capacity is not suitable for practical application in lithium-ion batteries. This content falls within the range of 8-10 wt%.The reversible capacity of graphite modified with 8 wt% carbon-coated Si was as high as 604 mAh g⁻¹. The irreversible capacity loss with this material was as low as 8.1%. The small irreversible capacity of the material allowed developing full lithium-ion rechargeable cells in the 2016 coin cell configuration. Lithium-ion batteries based on graphite modified with silicon show gravimetric and volumetric specific energy densities which are higher by approximately 20% than those for a lithium-ion battery based on natural graphite.     

Electro-organic synthesis without supporting electrolyte: Possibilities of solid polymer electrolyte technology

The application of ion exchange membranes as solid polymer electrolytes (SPE) in fuel cells is state-of-the-art. This technology needs no supporting electrolyte; consequently it can be applied for electro-organic syntheses in order to save process steps. In this case the process is not predetermined to a maximized energy efficiency so that the selection of the cell design, of the electrode materials and of the operating conditions can be focused on a high selectivity of the electrode reactions. The electro-osmotic stream, which is caused by the solvation shells of the ions during their migration through the membrane, and hence is a typical property of SPE technology, has a significant effect on the electrode reactions. It generates enhanced mass transfer at the electrodes, which is beneficial for reaction selectivity. It can be influenced by the choice of, and possibly by the preparation of, the membrane. An additional remarkable advantage of SPE technology is the exceptional long durability of oxide coated electrodes. By combination of several process engineering methods stable operation of SPE cells has been realized, even for examples of non-aqueous reaction systems. Experiments up to 6000 h duration and in cells of up to 250 cm² membrane area show the potential for industrial application.     

Nanostructured Sn–Ti–C composite anodes for lithium ion batteries

Nanostructured Sn–Ti–C composites have been synthesized by a facile, inexpensive high energy mechanical milling process and investigated as an anode material for lithium-ion cells. Characterization data collected with X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) reveal an uniform dispersion of Sn nanoparticles within the conductive, amorphous (or poorly crystalline) TiC + C matrix. Among the three Sn–Ti–C compositions investigated, the Sn₁₁Ti₃₁C₅₈ composite exhibits the best electrochemical performance, with a capacity of ∼370 mAh/g and excellent capacity retention over 300 cycles studied. It also exhibits excellent cycle life with LiMn₂O₄ spinel cathode, suggesting a tolerance of the Sn–Ti–C anodes toward poisoning by the manganese leached out from the spinel cathode. The superior electrochemical performance of Sn₁₁Ti₃₁C₅₈ composite is attributed to a homogeneous distribution of the electrochemically active amorphous Sn, suppression of Sn grain growth, and the mechanical buffering effect provided by the conductive TiC + C matrix toward the volume expansion-contraction occurring during cycling.     

Long‐term microstructural changes in solid oxide fuel cell anodes: 3D reconstruction

Microstructural changes in solid oxide fuel cell anodes after long‐term operation have been characterized by sequential sectioning with a focused ion beam, followed by scanning electron microscopy imaging and three‐dimensional reconstruction. The anodes were porous composites of Ni and Y₂O₃‐stabilized ZrO₂ (YSZ). The cells were operated at 800°C for 2, 4, and 8 kh, and at 925°C for 2 and 4 kh. For each specimen, the volume fraction, surface area, particle diameter, and tortuosity have been calculated for each phase (Ni, YSZ, and pores). The dependence of these microstructural parameters on the volume of sample analyzed was monitored; sufficiently large volumes were analyzed so as to eliminate any effect of sample volume. Gradients in volume fraction of Ni and porosity developed during fuel cell operation, with Ni fraction increasing, and pore fraction decreasing, at the electrolyte/anode interface. The magnitudes of these gradients increased with time.     

Processing and properties of oxide matrix/oxide fibre composite

Abstract

An all-oxide composite was fabricated. Single crystal alumina fibres were coated with a carbon/zirconia slurry, dried, and uniaxially aligned by winding. Matrix material, alumina with 5 vol.-% unstabilised zirconia added, was tape cast on top of the fibres. Pre-pregs were cut, stacked, and laminated to cross-ply material. Final sintering was done by hot isostatic pressing. A heat treatment was added to remove the carbon and create a porous zirconia interphase. Flexure strengths around 200 MPa were obtained for composites at room temperature while a strength of 124 MPa was recorded at 1200°C. The mechanical properties and non-brittle behaviour was sustained after aging at 1400°C for 1000 h in air.     

Oxidation studies of anodes layer microstructures in metal supported solid oxide fuel cells

The effect of oxidation on the microstructural and mechanical stability of ceramic layers in metal supported solid oxide fuel cells is reported. Half-cells that are produced with a reduced nickel based anode are oxidized for different times and temperatures in order to assess stability limits. Samples are analyzed in terms of the effective cell curvature and microstructure, where further insight is obtained via the observation of microstructures before and after oxidization. The interpretation is aided by a comparison to the behavior of structures without electrolyte layer. Electrolyte cracking and anode delamination are observed after oxidation, where the latter is absent in case of oxidation experiments without electrolyte layer, highlighting the failure relevance of strain induced by electrolyte deposition.     

Activity of anodic oxide films on metal and cermet anodes in cryolite-alumina melts

The activity of anodic oxide films on nickel, iron and copper metal anodes in cryolite-alumina melts was calculated using values of the reversible potential obtained from a polarization scan of the corresponding metal electrode. The best results were obtained with prepolarized electrodes. The anodic oxide layer formed on nickel becomes thick and dense and remains adherent during the period of prepolarization. Similar activity calculations were made for selected nickel-, iron- and copper-cermet compositions containing either a MnZn ferrite or a nickel ferrite ceramic phase. Large activities were observed for a NiO type corrosion product on both the nickel and nickelcermet electrodes. The results suggest that a dense surface layer containing a NiO phase is formed on nickel-cermet electrodes. This layer may help lower corrosion by minimizing electrolyte penetration of the anode surface.