Dense Yttria-stabilized Zirconia Electrolyte Research Articles

Solid Oxide Fuel Cells with 3D Inkjet Printing Modified LSM-YSZ Interface

In this study, pillar shaped yttria-stabilized zirconia (YSZ) 3D microstructures with ∼60 to 90 μm diameter and 12 to 20 μm height are fabricated by 3D inkjet printing to improve the topology of the electrolyte/cathode interface. The microstructures increase the surface area of the cell by ∼2.4% to 4.0% and enhance the connection between the dense YSZ electrolyte and mixed YSZ-lanthanum strontium manganite (LSM) cathode. The morphology and microstructure of the YSZ interface are characterized with scanning electron microscopy. Polarization curves confirm that the power density improves by 47% to 107% at 0.55 V, depending on the dimensions of the microstructures, in comparison to a flat interface. The non-linear improvement in power density with the size of microstructures is confirmed by calculating the uncertainty with repeated tests. Based on electrochemical impedance spectroscopy and distribution of relaxation times analysis, the performance improvement is attributed to changes in the oxygen surface exchange kinetics and O2− diffusivity in the cathode.

Preparation of yttria-stabilized zirconia electrolyte via atmospheric plasma spraying for metal-supported solid oxide fuel cells

Plasma spraying under low chamber pressure has been proven to be capable of preparing dense MS-SOFC electrolytes, but it is costly and inefficient. In this study, atmospheric plasma spraying (APS) as a low-cost and high efficiency alternative was attempted to prepare yttria-stabilized zirconia (YSZ) electrolyte. Different spraying distances were adopted to investigate the effects of in-flight particle states on the coating microstructure and properties. The results show that due to the YSZ particle temperature increasing with spraying distance, the YSZ splats have more internal microcracks and more regular morphologies. At the spraying distance of 80 mm, the high particle velocity and plasma heating effect combine to create a dense YSZ coating with a porosity of 3.56 %. Additionally, the nanohardness and elastic modulus of this coating reaches 15.6 ± 0.8 GPa and 209.6 ± 4.5 GPa, respectively. The MS-SOFC with the dense YSZ electrolyte, achieve a maximum power density of 902 mW⋅cm-2 and ohmic resistance of 0.10 Ω⋅cm2 at 900 °C. This work demonstrates the feasibility of APS in the efficient preparation of MS-SOFCs electrolytes through optimized particle deposition.

Solid Oxide Fuel Cells with 3D Inkjet Printing Modified LSM-YSZ Interface

Solid oxide fuel cells (SOFCs) have drawn attention as an alternative power generation technology due to demonstrated high efficiency and low emissions. The lanthanum strontium manganite (LSM)-yttria-stabilized zirconia (YSZ) cathode is utilized widely for SOFCs due to its high performance and stability. The electrochemical oxygen reduction reaction that occurs in the cathode can be enhanced by the porous LSM-YSZ cathode with good electronic and ionic conductivity. With sufficient electrochemically active reaction sites, the oxygen ion transfer processes in the mixed ionic conductor (YSZ) and at the interface of cathode and electrolyte (LSM-YSZ) become critical for the activity of the cathode. Thus, the pillar shape YSZ microstructure with ~ 150 μm×150 μm×10 μm (L×W×H) size can be fabricated by 3D inkjet printing on the interface of electrolyte and cathode to increase the TPB area and enhance the connection between the dense YSZ electrolyte and mixed YSZ ionic conductor. Scanning Electron Microscopy (SEM) is utilized to assess the morphology and microstructure of the YSZ interface. To investigate the influence of the modified interface on the performance of the SOFC, electrochemical performance testing and characterization are conducted for the SOFCs with/without the modified YSZ layer. The microstructures with different pillar central spacing are also fabricated by 3D inkjet printer to optimize the geometry of the LSM-YSZ interface.

Solid Oxide Fuel Cells with 3D Inkjet Printing Modified LSM-YSZ Interface

The geometry of the electrode/electrolyte interface is critical to solid oxide fuel cell (SOFC) performance. Adding 3D microstructures on electrode/electrolyte interface may enhance the performance of SOFCs potentially. In this study, the pillar shape yttria-stabilized zirconia (YSZ) 3D microstructures with ~ 60 m to 90 m diameter were fabricated by 3D inkjet printing on the interface of electrolyte and cathode to increase the TPB area and enhance the connection between the dense YSZ electrolyte and mixed YSZ ionic conductor. Scanning Electron Microscopy (SEM) is utilized to assess the morphology and microstructure of the YSZ interface. To investigate the influence of the modified interface on the performance of the SOFC, electrochemical performance testing and characterization are conducted for the SOFCs with/without the modified YSZ layer. SOFCs performance is investigated with a different number of printed interfacial layers fabricated with the 3D inkjet printer.

A 2D and 3D X-ray μ-diffraction and μ-fluorescence study of a mixed ionic electronic conductor

Due to the mixed ionic electronic conductive properties of the Lanthanum Strontium Cobalt Ferrite (LSCF) La0.6Sr0.4Co0.2Fe0.8O3−δ compound, it is of great scientific and technological interest. Especially in the Solid Oxide Fuel Cell (SOFC) technology, this compound has receiving great attention as a cathode material. However, its chemical reactivity with the Yttria-stabilized Zirconia (YSZ) electrolyte still remains one of the main challenges, which demands a comprehension in the μm and sub-μm range. In order to address the reactivity issues locally in the micrometre scale range, 2D and 3D X-ray μ-diffraction and μ-fluorescence analysis have been performed on a pristine LSCF cathode layer. The cathode was deposited on a dense YSZ electrolyte substrate spaced by a thin Gadolinium doped Ceria Oxide (CGO) barrier layer in between LSCF and YSZ to limit the reactivity. The present approach offers a larger field of view in comparison to electron microscopy techniques. The method can provide a more representative information and may offer some insights on the reactivity distribution along the interfaces. The formation of micro SrZrO3 inclusions in LSCF layer is then indubitably identified, as well as in the CGO/YSZ interface.

Dense thin YSZ electrolyte films prepared by a vacuum slurry deposition technique for SOFCs

A novel vacuum deposition technique based on a slurry coating is proposed for the preparation of thin yttria-stabilized zirconia (YSZ) electrolyte films on a porous NiO-YSZ substrate for solid oxide fuel cells (SOFCs). The vacuum pressure not only improved the packing density of the electrolyte particles but also enhanced the bonding strength between the anode and electrolyte layers. The polarization resistance of the cell prepared via vacuum deposition decreased from 1.27 to 0.89Ωcm2, which resulted in a remarkable increase in the peak power density from 249 to 292mWcm−2 at 800°C. An anode functional layer (AFL) was also fabricated by the vacuum slurry deposition method in an attempt to increase the three-phase boundaries (TPBs) at the anode/electrolyte interface. The scanning electron microscopy (SEM) images revealed that the sintered YSZ electrolyte film has a crack-free, dense and homogeneous microstructure. These experimental results indicate that slurry vacuum deposition is an effective technique for producing dense thin films for SOFCs and other applications.

Self-supported ceramic substrates with directional porosity by mold freeze casting

Manufacture of thin-film ceramic substrates with high permeability and robustness is of high technological interest. In this work thin (green state thickness ∼500μm) porous yttria-stabilized zirconia self-supported substrates were fabricated by pouring stable colloidal aqueous suspensions in a mold and applying directional freeze casting. Use of optimized suspension, cryoprotector additive and mold proved to deliver defect free ceramic films with high dimensional control. Microstructure analysis demonstrated the formation of desirable aligned porosity at macro-structural scale and resulted to be highly dependent on colloidal behaviour and freeze casting conditions. Manufactured green films were joined by lamination at room temperature and sintered to obtain symmetrical cells consisting of two porous self-supported substrate electrodes (∼420μm) and dense yttria stabilized zirconia electrolyte (∼10μm).

The Impact of Strong Cathodic Polarization on SOC Electrolyte Materials

One of the most promising reversible energy conversion/storage technologies is that of Solid Oxide Fuel/Electrolysis Cells (SOFC/SOEC, collectively termed SOC). Long term durability is typically required for such devises to become economically feasible, hence considerable amount of work has and is being done on the degradation and long term durability of these systems. When using a SOC in electrolysis mode, it is economically beneficial to operate the cell at high current density, since it increases the fuel production rate. However, several degradation phenomena, such as segregation of impurities at the grain boundaries, electrode poisoning, delamination or cracks of the electrolyte etc., have been observed in cells operated at such conditions, lowering the lifetime of the cell1,2. High polarizations are observed at the electrolyte/cathode interface of an electrolysis cell operated at high current density. In case of a cell voltage above 1.6 V, p-type and n-type electronic conductivity are often observed at the anode and cathode respectively3. Hence, a considerable part of the current is lost as leakage through the electrolyte, thus lowering the efficiency of the cell considerably. The presented work aims to study the impact of short term strong cathodic polarization in SOC model systems PtIr/YSZ and Ni/YSZ. The study is mainly focused on electrolyte materials such as yttria stabilized zirconia (YSZ). Since impurity segregation is one of the suggested degradation mechanisms, electrolyte materials with silica additions are investigated likewise. The electrochemistry of the electrolytes is studied in a Controlled Atmosphere High Temperature Scanning Probe Microscope (CAHT-SPM)4,5, using PtIr (8:2) or Ni probes as working electrodes. Different cathodic polarizations ranging from -0.1 V to -2 V vs. a macroscopic platinum reference electrode in the same atmosphere are applied to the YSZ electrolyte at 650 °C through the well-defined tip of the probe. The atmosphere used for these measurements is a mixture of 9% H2 in N2 saturated with water vapor at room temperature. The electrochemical impedance spectroscopy measurements, in addition to the expected change in the DC resistance, reveal a decrease in the series resistance (Rs) with increasing cathodic polarization (Figure 1a, 1b). Such decrease in the ohmic resistance of the electrolyte indicates the introduction of electronic conductivity in the electrolyte material. The measurements presented in Figure 1 are obtained from stepwise cathodic polarization ranging from -0.2 V to -2 V with a conditioning period of 120 s before each impedance scan on a dense YSZ electrolyte using a nickel probe as working electrode. Similar behavior of the Rshas been obtained using PtIr probes. In addition to the electrochemical study, the chemistry, microstructure and surface conductance of the region impacted from the polarization are investigated using Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Scanning Electron Microscopy (SEM) and CAHT-SPM. The surface conductance measurements for the PtIr-YSZ model system show that the impact of the polarization is larger than the tip-electrolyte contact area. The presence of the high and low conductance regions is attributed to the introduction of electronic conductivity, the segregation of impurities/low conducting phases and the microstructural changes in the material. The affected area is strongly dependent on the time and strength of the polarization. Such trends are also observed in the chemistry and microstructure of the surface. The nickel ion image obtained with ToF-SIMS (Figure 1c) gives an indication of the tip-sample contact area, since the formation of Ni-Zr compounds has been previously observed. The siliconion image on the other hand, demonstrates that silicon species segregate in a much larger region surrounding the contact area. The silicon ion signal is attributed to silica, since the polarization applied is lower than the silica reduction potential. Thus, by combining all of the above techniques, valuable information is obtained about multiple degradation mechanisms caused from strong cathodic polarization.

Enhancing the Stability of Infiltrated Ni/YSZ Anodes

One of the challenges related to Ni-based anodes in solid oxide fuel cells (SOFCs) is their dimensional instability, especially when they are exposed to air and then back to fuel, which is called redox cycling. One possible solution to this problem is the fabrication of a pre-sintered porous electrolyte matrix, followed by infiltration of 10-20 vol% of the active materials into this scaffold. However, this can lead to instability of the infiltrated particles as a result of sintering of the infiltrated phase at SOFC working temperatures. This problem is very severe when depositing nano-sized infiltrated metal particles on the oxide ceramic backbone. In this study, Ni-containing solutions were infiltrated into a symmetrical tubular half-cell, fabricated by slip-casting, composed of two porous yttria-stabilized zirconia (YSZ) layers, separated by a 50 μm dense YSZ electrolyte. A combination of electrochemical impedance spectroscopy (EIS) and electron microscopy imaging has been used to determine long term stability of the cells. It was found that the amount of infiltrated Ni has a significant impact on the long term stability of the Ni/YSZ anodes. This can be explained by the better connectivity between Ni particles even after long term testing when there is more Ni in the structure. It was also demonstrated that high temperature treatment of the infiltrated Ni/YSZ anodes just after the first few infiltrations, followed by several further Ni infiltration steps, has a significant effect not only on the stability of the anode at 800 C, but also on the anode performance. As the YSZ backbone has the ability to dissolve NiO at higher temperatures, the dissolved NiO can be ex-soluted in the form of nano-sized Ni particles at cell working temperatures and under reducing atmospheres. This effect was also investigated using electron microscopy, revealing the size and shape of the ex-soluted Ni particles, especially after long term testing. Finally, the effect of infiltration of cerium oxide prior to Ni infiltration on the stability of the anodes was also investigated. As the wettability of Ni particles on a reduced ceria surface is different from non-reduced ceria, the sintering behaviour of Ni on these two surfaces was found to be different as well.

Electrophoretic Deposition of Zirconia Thin Film on Nonconducting Substrate for Solid Oxide Fuel Cell Application

Electrophoretic deposition (EPD) of 8 mol% yttria‐stabilized zirconia (YSZ) electrolyte thin film has been carried out onto nonconducting porous NiO‐YSZ cermet anode substrate using a fugitive and electrically conducting polymer interlayer for solid oxide fuel cell (SOFC) application. Such polymer interlayer burnt out during the high‐temperature sintering process (1400°C for 6 h) leaving behind a well adhered, dense, and uniform ceramic YSZ electrolyte film on the top of the porous anode substrate. The EPD kinetics have been studied in depth. It is found that homogeneous and uniform film could be obtained onto the polymer‐coated substrate at an applied voltage of 15 V for 1 min. After the half‐cell (anode + electrolyte) is co‐fired at 1400°C, a suitable cathode composition (La0.65Sr0.3MnO3) thick film paste is screen printed on the top of the sintered YSZ electrolyte. A second stage of sintering of such cathode thick film at 1100°C for 2 h finally yield a single cell SOFC. Such single cell produced a power output of 0.91 W/cm2 at 0.7 V when measured at 800°C using hydrogen and oxygen as fuel and oxidant, respectively.

Decomposition of carbon monoxide and carbon dioxide using cathode-supported dense yttria-stabilized zirconia electrolyte cell

CO, CO2 and CO–CO2 mixed gas were electrochemically decomposed at 800°C using a cell of Ni–YSZ (8mol% yttria-stabilized zirconia) cathode (5mm thick), dense YSZ electrolyte (40μm thick) and Ru–YSZ anode (30μm thick). The decomposition rate of CO gas (2CO→2C+O2) was in the range of 9–15% at an applied voltage of 1.0V and increased to 67–100% at 2.0–8.0V. Carbon was deposited significantly near the electrolyte interface in the cathode. In the case of CO2 gas decomposition (2CO2→2CO+O2, CO2→C+O2), CO2 gas of 6–8vol% was converted to CO (6–8vol%), O2 (2–9vol%) and C. Deposition of a small amount of C was observed in the cathode. The decomposition of CO–CO2 mixed gas produced O2 gas of 2–10vol% in the anode but carbon deposition hardly occurred. Carbon deposited in the cathode from CO decomposition was easily oxidized to CO gas again by CO2 mixed (C+CO2→2CO).

Slip-cast and hot-solution infiltrated porous yttria stabilized zirconia (YSZ) supported tubular fuel cells

Hot solution infiltration was investigated as a flexible and rapid method to incorporate anode and cathode components into fully sintered, porous ceramic tubular templates for use as solid oxide fuel cells (SOFC). Composed of either a porous 8 mol% yttria-stabilized zirconia (YSZ) or 5 wt% NiO–YSZ support structure, a thin Ni–YSZ anode functional layer and an outer ca. 10 μm dense YSZ electrolyte, closed end tubes were first hot solution (ca. 100 °C) infiltrated on the inside with NiO–SDC (Sm0.2Ce0.8O1.9) to serve as the anode. Cathodes were either LSM (nominally La0.8Sr0.2MnO3+δ) infiltrated into a thin porous YSZ layer on the outer electrolyte surface, or an LSCF–GDC composite (Gd0.1Ce0.9O1.95–La0.6Sr0.4Co0.2Fe0.8O3−δ) on a thin GDC buffer layer. Although hot solution infiltration of the Ni, Ce and Sm salts into the anode support structure did not result in complete penetration (with the Ni contents in the tube wall ranging between 4 and 10 vol.%), well-sealed full cells produced power densities as high as 275, 196 and 153 mW cm−2 at 800, 750 and 700 °C, respectively. Hot solution infiltration of active SOFC electrode materials is thus shown to be a very flexible approach for the evaluation of their performance.

Modeling Power Production in a Tubular Carbon Fuel Cell

Introduction As developing countries consume increasing quantities of fossil fuels to elevate their standard of living, there is growing concern about greenhouse gas emissions, and an increased need for mitigating emissions during power generation. It is clear that in the foreseeable future renewable energy technologies will be unable to eliminate fossil fuel dependence, and hence efforts to reduce the environmental impact of carbon-based fuels are required.Carbon Fuel Cells (CFCs) offer the double benefit of efficient utilization of carbonaceous fuels such as coal or biomass, and the production of a concentrated stream of CO2 that can be easily stored or sold as a marketable product.Previous work in our laboratory has demonstrated solid-oxide based CFC for efficient electricity production from various types of carbons [1-3]. Our CFC utilizes a bed of solid carbon fuel at the anode compartment and air at the cathode compartment, which are separated by a dense yttria-stabilized zirconia electrolyte (YSZ) for selective transport of oxide ions. Carbon dioxide in the anode compartment reacts with the carbon to produce CO via the Boudouard reaction.C + CO2 → 2CO (1) As shown in Figure 1, the cell oxidizes CO to generate electricity, producing an outlet stream of CO2, part of which can be recycled to the anode and the reminder sent for storage. Modeling the Tubular Carbon Fuel Cell Geometry In this presentation we provide a comprehensive operational model that couples heat transfer with chemical and electrochemical processes as well as mass transport in a tubular carbon fuel cell. In the carbon bed, the Boudouard gasification reaction of the solid carbon fuel is endothermic, while CO oxidation at the anode surface is exothermic. As the kinetics of both reactions is temperature dependent, it is important to understand the coupled relationship between reaction rates, heat release and local temperature. Thus, consideration of heat transfer effects is necessary to develop a realistic understanding of the overall cell operation.The operation of the cell results in an anode exhaust containing largely CO2 with the remaining balance of unreacted CO. Any CO in the exhaust is carbon fuel still capable of undergoing oxidation to produce electricity at the cell. Thus, fuel utilization improves with increasing CO2/CO ratio at the exhaust, and this influences the overall conversion efficiency of the cell.In this work, an operational model for a tubular CFC was developed that takes into account heat transfer and temperature distributions within the cell. The parameters in the model were determined experimentally. The model was then used to map out the operational space for power density and cell efficiency. Furthermore, geometrical parameters such as fuel bed height and tubular placement can be tuned to minimize the mole fraction of CO in the exhaust. The model was implemented for multiple tubular geometries and spacing between tubes in order to determine how these parameters affect overall fuel cell performance.This presentation will address the dependence of cell efficiency and power density on the cell and carbon bed geometries, which will then aid in the optimization of tubular cell design for the air-carbon fuel cell. Optimal operation conditions are identified for maintaining high efficiencies while also achieving realistic power densities.

Suspension chemistry and electrophoretic deposition of zirconia electrolyte on conducting and non-conducting substrates

Suspensions of 8mol% yttria stabilized zirconia (YSZ) particulates in isopropanol medium are prepared using acetylacetone, iodine and water as dispersants. The effect of dispersants concentration on suspension stability, particle size distribution, electrical conductivity and pH of the suspensions are studied in detail to optimize the suspension chemistry. Electrophoretic deposition (EPD) has been conducted to produce thin and dense YSZ electrolyte films. Deposition kinetics have been studied in depth and good quality films on conducting substrate are obtained at an applied voltage of 15V for 3min. YSZ films are also fabricated on non-conducting NiO-YSZ anode substrate using a steel plate on the reverse side of the substrate. Upon co-firing at 1400°C for 6h a dense YSZ film of thickness ∼5μm is obtained. Such a half cell (anode+electrolyte) can be used to fabricate a solid oxide fuel cell on applying a suitable cathode layer.

Combinatorial deposition of a dense nano-thin film YSZ electrolyte for low temperature solid oxide fuel cells

This paper presents a combinatorial deposition of a nano-thin film yttria-stabilized zirconia (YSZ) electrolyte deposited on a nanoporous anodic aluminum oxide (AAO) membrane, which serves as a supporting and gas-permeable substrate. The dense YSZ electrolyte was deposited by combining atomic layer deposition (ALD) and sputtering in an attempt to realize a pinhole-free electrolyte on a porous structure with a minimum electrolyte thickness. The YSZ electrolyte with an overall thickness of about 390 nm was successfully fabricated on porous AAO with this combined deposition method, and the open circuit voltage (OCV) of the cell with this electrolyte was verified to reach a high value of 1.14 V at 350 °C under an atmosphere of pure hydrogen and ambient air. The fuel cell performance reached power densities of 88, 129, and 180 mW cm−2 at 350, 400, and 450 °C, respectively, and has been so far the highest attainable for a nanoscale thin film electrolyte on a porous substrate at such low temperatures. The addition of the pre-ALD YSZ layer also serves to confine the anode morphology and thus enhance the thermal stability of the metal electrodes.

Impact of Cofiring Ceria in Ni/YSZ SOFC Anodes for Operation With Syngas and n-Butane

This study explores cofiring ceria (CeO2) with NiO and 8 mol% yttria-stabilized zirconia (YSZ) to form Ni-based cermet anodes for high-temperature solid oxide fuel cells (SOFCs) operating on syngas and n-butane/steam fuel feeds. Particular attention is paid to the suppression of carbon deposit growth in Ni-based anodes with carbonaceous fuel feeds. CeO2 was cofired with NiO and YSZ to form a porous Ni cermet anode support layer after reduction in H2 at 800°C. The porous anode support layer (1 mm thick) was combined with a Ni/YSZ functional layer (∼25 μm thick), a dense YSZ electrolyte (10–20 μm thick), and porous La0.8Sr0.2MnO3−x (LSM)/YSZ cathodes (∼50 μm thick) to form anode-supported button cells for electrochemical characterization. The button cells were tested from 700 °C to 800 °C on various fuels including syngas and n-butane/H2O mixtures at steam-to-carbon (S/C) ratios of 1.0 and 1.5. Electrochemical testing revealed that CeO2 addition provided stable performance at 800 °C without compromising power densities—up to 0.6 W/cm2 on syngas and 0.35 W/cm2 on direct butane feeds. Furthermore, the addition of CeO2 suppressed significant carbon deposition as observed for Ni/YSZ anode support layers without CeO2. Testing with syngas at different H2 and CO partial pressures indicated that high power densities can be maintained along an anode channel for up to 50% fuel conversion. The results indicate that cofiring CeO2 in Ni/YSZ anode support layers presents a viable option for stable SOFC operation on either prereformed or internally reformed light-hydrocarbon fuel feeds.

A newly-developed effective direct current assisted sintering technique for electrolyte film densification of anode-supported solid oxide fuel cells

In order to lower the sintering temperature and shorten firing time, a novel, effective and facile technique has been developed for sintering Yttria-stabilized zirconia (YSZ) electrolyte thin film. Herein this technique, which employs a weak direct current (DC), is used for the first time in the manufacture of the anode-supported solid oxide fuel cell (SOFC). A single cell is directly assembled using a pre-sintered anode/electrolyte and screen printed cathode and subsequently sintered under electric assistance from room temperature to 800 °C at a heating rate of 5 °C min−1. A fully dense YSZ electrolyte film can be observed by scanning electron microscopy (SEM) and the open circuit voltages (OCVs) are in accordance with theoretical values, confirming that the cell possesses a dense YSZ film. Using hydrogen fuel, the maximum power density of this cell was 0.8, 1.1 and 1.4 W cm−2 at 650, 700 and 750 °C, respectively. We believe this DC assisted sintering technique (DC-AST) may not only reduce the cost, but also maintain the anode nanostructure, thus offering a potentially useful manufacturing technique for SOFCs.

Single solution spray pyrolysis of La0.6Sr0.4Co0.2Fe0.8O3 − δ–Ce0.8Gd0.2O1.9 (LSCF–CGO) thin film cathodes

Thin film (200–300nm) (X)La0.6Sr0.4Co0.2Fe0.8O3−δ–(1−X)Ce0.8Gd0.2O1.9 (LSCF–CGO, X=0 to 100 vol.%) composite cathodes were produced by spraying a single precursor solution of nitrate salts onto dense yttria stabilized zirconia electrolyte substrates at temperatures of 235 and 275°C. Segregation of the films into distinct LSCF and CGO phases was observed by X-ray diffraction after annealing above 650°C for 4h. Grain growth was suppressed in the LSCF–CGO composites relative to pure LSCF, the grain size in 59LSCF–41CGO ranging from 15 to 50nm for annealing temperatures between 700 and 900°C. The polarization resistance of films fired at 700°C were modeled using a Gerischer impedance with a characteristic polarization resistance Rc and time constant tc. Relative to pure LSCF films, the LSCF–CGO composites exhibited lower activation energies resulting in a smaller polarization resistance at operating temperatures below 600°C.

Fabrication and characterization of Sm0.2Ce0.8O2−δ–Sm0.5Sr0.5CoO3−δ composite cathode for anode supported solid oxide fuel cell

The Sm0.5Sr0.5CoO3−δ (SSC) with perovskite structure is synthesized by the glycine nitrate process (GNP). The phase evolution of SSC powder with different calcination temperatures is investigated by X-ray diffraction and thermogravimetric analyses. The XRD results show that the single perovskite phase of the SSC is completely formed above 1100°C. The anode-supported single cell is constructed with a porous Ni–yttria-stabilized zirconia (YSZ) anode substrate, an airtight YSZ electrolyte, a Sm0.2Ce0.8O2−δ (SDC) barrier layer, and a screen-printed SSC–SDC composite cathode. The SEM results show that the dense YSZ electrolyte layer exhibits the good interfacial contact with both the Ni–YSZ and the SDC barrier layer. The porous SSC–SDC cathode shows an excellent adhesion with the SDC barrier layer. For the performance test, the maximum power densities are 464, 351 and 243mWcm−2 at 800, 750 and 700°C, respectively. According to the results of the electrochemical impedance spectroscopy (EIS), the charge-transfer resistances of the electrodes are 0.49 and 1.24Ωcm2, and the non charge-transfer resistances are 0.48 and 0.51Ωcm2 at 800 and 700°C, respectively. The cathode material of SSC is compatible with the YSZ electrolyte via a delicate scheme employed in the fabrication process of unit cell.

Polarization Characteristics and Chemistry in Reversible Tubular Solid-Oxide Cells Operating on Mixtures of H2, CO, H2O , and CO2

This paper reports the results of combined experimental and modeling studies of reversible solid-oxide cells. The tubular cells are fabricated using a Ni–YSZ (yttria-stabilized zirconia) fuel-electrode support, a dense YSZ electrolyte membrane, and a strontium-doped lanthanum manganate–YSZ composite air electrode. Experiments are designed to systematically vary gas-phase species partial pressures and operating temperatures. The fuels are mixtures of , CO, , , and Ar. Performance is measured under anodic (fuel cell) and cathodic (electrolysis) polarization. The models consider reactive porous-media transport within the composite electrodes, thermal chemistry on Ni and YSZ surfaces, and charge-transfer chemistry. All chemistry is modeled with elementary reversible reactions. Close coupling between experimental measurements and model-based interpretation provides a basis for establishing reaction pathways and rates. In addition to advancing fundamental understanding, the resulting detailed reaction mechanisms are valuable for incorporation into predictive models that can be used for design and optimization of fuel-cell and electrolysis systems.