Ru-catalyzed anode materials for direct hydrocarbon SOFCs

A solid oxide fuel cell using a thin ceria-based electrolyte film with a Ru-catalyzed anode was directly operated on hydrocarbons, including methane, ethane, and propane, at 600 °C. The role of the Ru catalyst in the anode reaction was to promote the reforming reaction of the unreacted hydrocarbons by the produced steam and CO₂, which avoided interference from steam and CO₂ in the gas-phase diffusion of the fuels. The resulting peak power density reached 750 mW cm⁻² with dry methane, which was comparable to the peak power density of 769 mW cm⁻² with wet (2.9 vol.% H₂O) hydrogen. More important was the fact that the cell performance was maintained at a high level regardless of the change in the methane utilization from 12 to 46% but was significantly reduced by increasing the hydrogen utilization from 13 to 42%. While the anodic reaction of hydrogen was controlled by the slow gas diffusion, the anodic reaction of methane was not subject to the onset of such a gas-diffusion process.     

Ni-Fe bimetallic anode as an active anode for intermediate temperature SOFC using LaGaO3 based electrolyte film

Effects of additives to Ni anode were studied and it was found that the anodic overpotential can be suppressed by addition of Fe. La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) film was deposited on the dense anode substrate consisting of NiO(Fe₃O₄)-Sm doped CeO₂. After in-situ reduction of NiO and Fe₃O₄ in the dense substrate, the substrate turned to be porous, however, change in size was not large by mixing with SDC. As a result, LSGM dense film with few micrometer thickness was successfully obtained on the porous Ni based anode substrate. By optimizing the thickness of the LSGM film and application of SDC interlayer, the high power density of SOFC single cell using LSGM/SDC bi-layer film as electrolyte at decreased temperature was fabricated and the electrical power generating property was measured as a function of temperature. The high maximum power density could be achieved to a value of 2 W/cm² at 873 K. Even at 673 K, the maximum power density of ca. 80 mW/cm² is exhibited and this high power density was a result of the low electrolyte resistance and the small anodic overpotential of Ni-Fe bimetallic anode.     

Optimization of anode structure for intermediate temperature solid oxide fuel cell via phase‐inversion cotape casting

A functional layer and a porous support that together constitute an anode for a solid oxide fuel cell were simultaneously formed by the phase‐inversion tape casting method. Two slurries, one composed of NiO and yttria‐stabilized zirconia (YSZ) powders and the other of NiO, YSZ, and graphite were cocasted and solidified by immersion in a water bath via the phase‐inversion mechanism. The as‐formed green tape consisted of a sponge‐like thin layer and a fingerlike thick porous layer, derived from the first slurry and the second slurry, respectively. The former acted as the anode functional layer (AFL), while the latter was used as the anode substrate. The AFL thickness was varied between 20 and 60 μm by adjusting the blade gap for the tape casting. Single cells based on such NiO‐YSZ anodes were prepared with thin YSZ electrolytes and YSZ‐(La_0.8Sr_0.2)_0.95MnO_3?δ (LSM) cathodes, and their electrochemical performance was measured using air as oxidant and hydrogen as fuel. The maximum power densities obtained at 750°C were 720, 821, and 988 mW cm^?2 with the AFL thickness at 60, 40, and 20 μm, respectively. The satisfactory electrochemical performance was attributed to the dual‐layer structure of the anode, where the sponge‐like AFL layer provided plenty of triple‐phase boundaries for hydrogen oxidation, and the fingerlike thick porous substrate allowed for facile fuel transport. The phase‐inversion tape casting developed in this study is applicable to the preparation of other planar ceramic electrodes with dual‐layer asymmetric structure.     

A new family of Cu-doped lanthanum silicate apatites as electrolyte materials for SOFCs: Synthesis,structural and electrical properties

La_9.67Si_6-xCu_xO_26.5-x (LSC, x = 0, 0.1, 0.3 and 0.5) are synthesized by a citric-nitrate method. Substitution Si with Cu promotes the densification process of silicate apatite. Unit cell parameters and volume increase linearly with Cu content. The Rietveld refinement reveals a much more distorted (Si,Cu)O₄ tetrahedra in the oxygen stoichiometric La_9.67Si_5.5Cu_0.5O₂₆ sample. The structural observation from high temperature XRD implies a second-order phase transition in La_9.67Si_5.5Cu_0.5O₂₆. Cu-doping decreases the activation energy of oxygen ion conduction and increases the conductivity of LSC materials in the temperature range of 550–800 °C. La_9.67Si_5.5Cu_0.5O₂₆ shows the conductivity values of 29.3 and 12.3 mS cm⁻¹ at 800 °C and 650 °C, respectively. The oxygen ion transference number of La_9.67Si_5.5Cu_0.5O₂₆ is higher than 0.99. These attractive properties make the La_9.67Si_5.5Cu_0.5O₂₆ a promising oxygen ion conducting electrolyte for applications of solid oxide fuel cells, oxygen sensors, oxygen separation membranes, etc.     

Tape casting of ferroelectric,dielectric, piezoelectric and ferromagnetic materials

Tape casting is a feasible method for preparing ceramic tapes with different electrical and magnetic properties for multilayer ceramic devices. This paper describes the tape casting process for several different electroceramic materials (BST, PZT, NZF and ZSB) utilising similar organic additive and solvent systems. The properties of tapes with different ceramic compositions before and after sintering are investigated, including surface roughness, shrinkage and microstructures. The parameters affecting the casting, shrinkage, lamination, thickness and tensile strength of green tape are also presented. This enables process design for tape which can be used in devices with true integration of dielectric and piezoelectric, ferroelectric and ferromagnetic layers in 3-dimensional multilayer structures.     

Use of refractory materials in casting and intermediate ladles

The experience in using refractories in steel-casting and intermediate ladles at the open-hearth plant of the Magnitogorsk Iron and Steel Works is described. __________ Translated from Novye Ogneupory, No. 9, pp. 3–5, September, 2006.     

Electrode materials for intermediate temperature proton-conducting fuel cells

Some electrode materials for intermediate temperature proton-conducting fuel cells are analysed from the perspective of surface reaction and ionic conductivity type. The performance of H₂/O₂ fuel cells using these materials as electrodes with LiNaSO₄–Al₂O₃ as the electrolyte indicates that Ni–Al alloy, Ni–Al₂O₃ catalyst and Ni–YSZ cermet are potential candidates for anode materials and that LiNiO₂, LiCoO₂, Ag–SnO₂ and La_0.8Sr_0.2MnO₃ are good candidates for cathode materials. Among the tested electrode materials, for the same electrolyte, the LiNiO₂/Ni–Al₂O₃ electrode pair gives the best cell performance.     

Tape casting and characterizations of MgO ceramics

We report a high density MgO ceramic substrate produced by the tape casting technology. The tape casting formulation and process produced a uniform tape free of cracking. Y₂O₃ and SiO₂ were used as the sintering aid for the pressureless sintering of the green tape. X-ray diffraction phase identification indicates that MgO is the main phase, while both Y₂O₃ and SiO₂ sintering aids react with MgO to form MgY₄Si₃O₁₃ as the second phase. Liquid phase sintering occurs in the temperature range from 1030°C to ~1500°C, which is confirmed by the simultaneous Thermal Gravitation Analysis/Differential Scanning Calorimeter (TGA/DSC) and the percent linear shrinkage and densification. A 96.5% theoretical density was achieved by presureless sintering at 1650°C for 2 hours, which was further increased to a fully dense structure using hot-isostatic-pressing(HIP) at 1650°C and 207 MPa in argon. Scanning electron microscopy (SEM) and energy dispersive(EDS) spectroscopic analysis on the HIP’ed sample show that MgY₄Si₃O₁₃is located at the MgO grain boundary and the sample has a fully dense structure. The refractive indices and extinction coefficient were measured on the HIP’ed sample along with thermal properties and dielectric properties. Thermal diffusivity and heat capacity were measured to calculate the thermal conductivity.     

Fabrication and performance of PEN SOFCs with proton-conducting electrolyte

A positive-electrolyte-negative (PEN) assembly solid oxide fuel cell (SOFC) with a thin electrolyte film for intermediate temperature operation was fabricated. Instead of the traditional screen-printing method, both anode and cathode catalysts were pressed simultaneously and formed with the fabrication of nano-composite electrolyte by press method. This design offered some advantageous configurations that diminished ohmic resistance between electrolyte and electrodes. It also increased the proton-conducting rate and improved the performance of SOFCs due to the reduction of membrane thickness and good contact between electrolyte and electrodes. The fabricated PEN cell generated electricity between 600°C and 680°C using H₂S as fuel feed and air as oxidant. Maximum power densities 40 mW·cm⁻² and 130 mW·cm⁻² for the PEN configuration with a Mo-Ni-S-based composite anode, nano-composite electrolyte (Li₂SO₄+Al₂O₃) film and a NiO-based composite cathode were achieved at 600°C and 680°C, respectively.     

A high performance solid oxide fuel cells operating at intermediate temperature with a modified interface between cathode and electrolyte

By adding 1% Bi₂O₃ (mol%) into LSCF (La_0.54Sr_0.44Co_0.2Fe_0.8O_3?δ), a layer of dense LSCF film is introduced to the upside of yttria stabilized zirconia (YSZ) electrolyte. The dense film increases the interface contact area and reduces the interface ion transfer resistance between cathode and electrolyte remarkably. As a result, the cell performance is greatly elevated from 492 to 901 mW cm^?2 at 650 °C. Besides, on the basis of careful observation of the cathode surface by FE-SEM, the function of Bi₂O₃ to promote the cathode sintering is speculated. The Bi₂O₃ and the LSCF come into being a kind of eutectic liquid. The eutectic liquid flows down from the cathode bulk to the interface between the cathode and the electrolyte where it accumulates to form a dense layer. This dense layer illuminates the function of adding Bi₂O₃ into LSCF cathode.     

Evaluation of solution combustion synthesized NiO/GDC ceramic powders for anode substrate and anode functional layers of intermediate temperature solid oxide fuel cell

Solution combustion (SC) is a versatile technique for the preparation of composite anode powders of solid oxide fuel cell (SOFC). In the present investigation, NiO/GDC ceramic powders, an anode material for intermediate temperature solid oxide fuel cell (IT-SOFC), has been prepared by SC method by using oxalyl di hydrazide (ODH) and hexamethylnetetramine (HMT) as fuels. Reaction with ODH as fuel resulted in the powders consisting of small particle and crystallite size, whereas, powders prepared using HMT as fuel consisted of agglomerated particles which led to the porous structures in the fabricated anode layers. Through combined microstructural analysis, porosity measurements and conductivity measurements, it was inferred that powders prepared using ODH as fuel are ideal for anode functional layers. On the other hand, powders prepared using HMT as fuels are best suited for anode substrate fabrication.     

Tape casting of strontium and cobalt doped lanthanum chromite suspensions

Lanthanum chromite (LaCrO₃) is currently the most widely studied material as interconnector layers for solid oxide fuel cells (SOFC). The complexity of microstructures and geometries of SOFC devices, which are usually built-up by lamination of the different constitutive layers, make it necessary a precise control of processing parameters to achieve the desired combination of properties. Much effort has been devoted to the processing of electrodes and electrolytes but the other layers, such as that of interconnecting material, have received scarce attention. This work deals with the preparation and optimisation of the rheological behaviour of concentrated suspensions of Sr- and Co-doped LaCrO₃ and the subsequent tape casting to produce homogeneous thin sheets to be used in the SOFC stack.The starting powder was produced by combustion synthesis from the corresponding nitrates and urea as a fuel, and had a final composition of La_0.80Sr_0.20Cr_0.92Co_0.08O₃. These powders were dispersed in ethanol with commercial copolymers (Hypermer, KD6) to solids loading of up to 58 wt%. The binding system (BS) consisted of a mixture of a binder, polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate (PVA-PVAc), and two plasticizers, polyethyleneglycol (PEG400) and benzylbutylphthalate (BBP). The effect of the binding system content and the binder-to-plasticizer ratio on the tape casting performance and the characteristics of the green and the sintered tapes, were studied, as well as the influence of the casting parameters (casting speed and blades height).     

Application of a thin intermediate cathode layer prepared by inkjet printing for SOFCs

The application of an inkjet printing process for fabricating solid oxide fuel cell (SOFC) cathodes was investigated. Stably-dispersed LSCF–GDC inks were prepared by ball milling, and the composition was easily controlled by the preparation process. Fabrication of an LSCF–GDC layer was successfully carried out by depositing dots and the thickness was easily controlled by repeating printing process. A planar SOFC single cell with a double-layered cathode (comprised of a paste painted cathode layer and an inkjet printed interlayer) achieved a maximum power density of 0.71 W/cm² at 600 °C. This is the preliminary work for fabricating the cathode layer of a SOFC single cell via inkjet printing.     

Creep properties of high dense La9.33Si6O26 electrolyte for SOFCs

High density La_9.33Si₆O₂₆ polycrystals were fabricated by conventional and spark plasma sintering starting from nanopowders synthesized by freeze-drying. The materials exhibit a homogeneous microstructure formed by equiaxed grains with average sizes of 1.1 μm and 0.2 μm-diameter depending on the sintering route. Compressive mechanical tests were performed in air at constant strain rate between 900 and 1300 °C. A gradual brittle-to-ductile transition was found with increasing temperature and/or decreasing strain rate. Grain boundary sliding is the main deformation mechanism in the ductile region, characterized by a stress exponent n = 1 for the conventional sintered (large-grained) material and n = 2 for the spark plasma sintered (fine-grained) material; in both cases, the activation energy for creep was 360 kJ/mol. Effective cation diffusivities have been derived from mechanical data by comparison with appropriate models. The creep properties of lanthanum silicates are reported here for the first time.     

One‐pot synthesis of silver‐modified sulfur‐tolerant anode for SOFCs with an expanded operation temperature window

To develop solid oxide fuel cells (SOFCs) capable of operating on sulfur‐containing practical fuels at intermediate temperatures, further improvement of the sulfur tolerance of a Ni + BaZr_0.4Ce_0.4Y_0.2O_3‐δ (BZCY) anode is attempted through the addition of some metal modifiers (Fe, Co, and Ag) by a one‐pot synthesis approach. The effects of these modifiers on the electrical conductivity, morphology, sulfur tolerance, and electrochemical activity of the anode are systematically studied. As a result, the cell with Ag‐modified Ni + BZCY anode demonstrates highest power output when operated on 1000 ppm H₂S‐H₂ fuel. Furthermore, the Ag‐modified anode displays much better stability than Ni + BZCY with 1000 ppm H₂S‐H₂ fuel at 600°C. These results suggest that the addition of Ag modifier into Ni + BZCY is a promising and efficient method for improving the sulfur tolerance of SOFCs. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4287–4295, 2017     

Synthesis and densification of lanthanum silicate apatite electrolyte for intermediate temperature solid oxide fuel cell via co-precipitation method

Lanthanum silicate apatite (LSA, La_9.33+xSi₆O_26+1.5x, x = 0–0.67) has been widely investigated as a promising electrolyte material for intermediate temperature solid oxide fuel cell (SOFC). In this work, a facile and low-cost co-precipitation method is used to synthesize LSA precursor powders. The well dispersed nanopowders (ca. 70 nm) with pure hexagonal LSA phase are obtained by calcining the precursor at 900 °C. Impurity of La₂SiO₅, caused by the different precipitation productivities of La(NO₃)₃ and TEOS, can be eliminated through lowering the La/Si ratio in the starting mixtures. The dispersant (PEG200) plays a crucial role in co-precipitation processes, which can effectively mitigate the agglomeration and therefore significantly improve the sinterability of the nanoparticles. Dense LSA ceramic with relative density of 98% is obtained after sintering at 1550 °C, which exhibits a conductivity of 0.13 mS cm⁻¹ at 500 °C.     

Optimization of a ScCeSZ/GDC bi-layer electrolyte fabrication process for intermediate temperature solid oxide fuel cells

Cost-effective wet ceramic coating techniques for fabricating ScCeSZ/GDC bi-layer electrolyte anode-supported button cells were investigated in this study. Aqueous ceramic slurries were prepared by ball milling and then used for Ni/ScCeSZ half cell fabrication by tape casting and spin coating. Prepared cells were tested at operating temperature between 700 and 800°C with a fuel composition of hydrogen:nitrogen 3:1 and air at the cathode. The cell with a spin coated GDC film showed the maximum power density of 1.142, 1.012, 0.813 W?cm^?2 at 800, 750, and 700°C, respectively. It was also able to produce power output around 0.7 W?cm^?2 for 500 h at 750°C, which confirms the cell operational stability. More importantly, the GDC film prepared by spin coating effectively avoided the formation of the (Zr,Ce)O₂^?based solid solution at the ceria/zirconia interface compared with the other cells with the co-casted and sintered GDC film.     

Tape casting of polysiloxane-derived ceramic with controlled porosity and surface properties

Tape casting has been applied to produce porous hybrid and SiOC ceramic tapes using ceramic precursors and commercially available polysiloxanes as polymeric binders. SiC particles of two different mean sizes (4.5 or 6.5?μm) were used as inert fillers to prevent shrinkage and increase mechanical stability. Macroporosity was adjusted by varying the azodicarbonamide (ADA) content from 0 to 30?wt.%. Decomposition of the polysiloxanes at 600?°C resulted in the generation of micropores with high specific surface area (187–267 m²?g^?1) and a predominant hydrophobic behavior. At 1000?°C mainly meso/macroporosity were observed (SSA: 32–162 m²?g^?1) accompanied by increased hydrophilicity. The influence of ADA content, SiC size, and pyrolysis temperature on open porosity (2.5–37%), average pore size (<0.01–1.76?μm), surface characteristics, and flexural strength (10.5–121?MPa) were investigated. The porous tapes with different surface characteristics and controlled structure are highly promising for applications involving membrane processes, particularly microfiltration systems (0.1–10?μm).