LOW-TEMPERATURE, ANODE-SUPPORTED HIGH POWER DENSITY SOLID OXIDE FUEL CELLS WITH NANOSTRUCTURED ELECTRODES

High performance yttria-stabilized zirconia based intermediate temperature solid oxide fuel cells with double nano layer composite cathode

One-step synthesized Co1.5Mn1.5O4-Ce0.8Sm0.2O1.9 composite cathode material for intermediate temperature solid oxide fuel cells

Anode-supported solid oxide fuel cells (SOFCs) with Ni+yttria-stabilized zirconia (YSZ) anode, YSZ-samaria-doped ceria (SDC) bilayer electrolyte, and Sr-doped (LSC)+SDC cathode were fabricated. Fuel used consisted of diluted with He, or mixtures of and CO, and mixtures of CO and Cell performance was measured at 800°C with the above-mentioned fuel gas mixtures and air as oxidant. For a given concentration of the diluent, cell performance was higher with He as the diluent than with as the diluent. Mass transport through porous Ni-YSZ anode for binary systems, and -diluent gas ternary systems was analyzed using multicomponent gas diffusion theory. At high concentrations of diluent, the maximum achievable current density was limited by the anodic concentration polarization. From this measured limiting current density, the corresponding effective gas diffusivity was estimated. Highest effective diffusivity was estimated for fuel gas mixtures containing mixtures (∼0.55 cm2/s), and the lowest for mixtures (∼0.07 cm2/s). The lowest performance was observed with mixture as a fuel, which in part was attributed to the lowest effective diffusivity of the fuels tested and higher activation polarization. © 2003 The Electrochemical Society. All rights reserved.

A comparison of La 0.75Sr 0.25Cr 0.5Mn 0.5O 3− δ and Ni impregnated porous YSZ anodes fabricated in two different ways for SOFCs

La 0.4Ce 0.6O 1.8–La 0.8Sr 0.2MnO 3–8 mol% yttria-stabilized zirconia composite cathode for anode-supported solid oxide fuel cells

Most of the ceramic ionic conductors typically conduct one ion. For example, AgI conducts Ag+ ion, Sodium β”-alumina conducts Na+ ion and Yttria-stabilized zirconia (YSZ) conducts O2- ions. Some selected materials such as alkaline earth cerates or zirconates conduct both protons and O2- ions. In such materials also, conductivity of one species is not independent of the other species. Thus, temperature and atmosphere determine the total conductivity. All of such materials are single phase materials. We have been investigating two phase materials such that one phase conducts one ion (Na+) while the other phase conducts another ion (O2-). The materials in such cases is a composite with the two phases forming an interpenetrating network. The volume ratios of the two phases can be adjusted as necessary. Since the two phases essentially constitute a mixture, conduction of one species occurring through one phase is independent of the conduction of the other species through the second phase. Yet, the condition of electroneutrality is applicable globally across multiple phases. Transport equations can then be written down in the usual manner but accounting for different volume fractions for the two phases. A mixed Na+ ion and O2- ion two phase conductor is fabricated by first making a sintered dense composite containing α-alumina and YSZ such that both phases are contiguous. YSZ and α-alumina are immiscible. This allows the fabrication of fine-grained, dense, strong composites. Subsequently, the sample is packed in Na-β”-alumina + YSZ powder and heat treated in air over a range of temperatures from 1200oC to 1400oC. During this process, α-alumina is converted into Na-β”-alumina by the incorporation of Na2O into α-alumina by coupled transport in which Na+ transports through the formed Na-β”-alumina and O2- ions transport through YSZ. The reaction front over time sweeps through the sample forming a two phase composite of Na-β”-alumina + YSZ. Such a material is now capable of conducting two ions; Na+ and O2-. Materials capable of transporting other alkali ions can be made by an ion exchange process in molten salts. For example, by ion exchanging in lithium salts, it is possible to make mixed Li-ion and O2- ion conducting materials. The oxygen ion conductivity is negligible at low temperatures. Thus, the materials at room temperature are predominantly alkali ion conductors with possible use in all solid state lithium ion and sodium ion batteries. Another mixed ion conductor was made by a similar process using a composite of Ga2O3 and YSZ. It was similarly packed in Na-β”-alumina powder and heat treated. In this case a two phase mixture containing YSZ and sodium zirconium gallate formed, the latter with a chemical formula of Na0.7Ga4.7Zr0.3O8 with a monoclinic structure. This compound is isostructural with Na0.7Ga4.7Ti0.3O8 which is known to be one dimensional sodium ion conductor. Measurement of ionic conductivity on Na0.7Ga4.7Zr0.3O8 + YSZ showed it to be a mixed Na+ ion and O2- ion conductor. Sodium ion conductivity of Na0.7Ga4.7Zr0.3O8 is much lower than that of Na-β”-alumina. The activation energy for Na+ ion transport is much lower than for O2- ion transport in YSZ. As a result, the two phase mixture is predominantly a Na+ ion conductor at low temperatures and an O2- ion conductor at elevated temperatures. Both Na-β”-alumina + YSZ and Na0.7Ga4.7Zr0.3O8 + YSZ are mixed Na+ ion and O2- ion conductors. This allows one two explore electrochemical transport of two species and examine the applicability of Goldman-Hotchkin-Katz trype of an equation, routinely used in aqueous electrochemistry, for solid-state systems. Preliminary results of experiments conducted by maintaining different chemical potentials of Na2O at the two electrodes and different chemical potentials of oxygen (by testing in a fuel cell mode) will be described. Possible degradation of Na-β”-alumina in reducing atmosphere will also be addressed. Acknowledgements: This work was supported by the National Science Foundation under Grant Number DMR-1407048.

Composite cathodes of Cu1.25Mn1.75O4 (CMO) spinel and yttria stabilized zirconia (YSZ) were made by infiltration of a precursor Cu–Mn gel into a porous YSZ matrix. Structural characterization showed no evidence of secondary phases but revealed a continuous network of CMO in the YSZ framework. Impedance spectroscopy and overpotential measurements revealed that the CMO–YSZ composite cathode had better electrochemical performance than the traditional LSM–YSZ cathode between 650 and 800 °C. A 50 wt% CMO impregnated YSZ cathode had a polarization resistance of 0.3 Ω cm2 at 750 °C, compared to 8.5 Ω cm2 for LSM–YSZ. Further investigation showed that charge transfer is the probable rate determining step for oxygen reduction in the CMO–YSZ cathode above 750 °C.

Solid oxide fuel cell (SOFC) is well known as power and heat generation device which converts chemical energy directly from fuel into electricity. SOFC operate at high temperature becomes obstacle for SOFC which reducing ionic conductivity material of current electrolyte, reduce lifetime of cell components, high fabrication cost, limited durability and performance issues. This introduce to environment pollution and decrease the SOFC lifetime. The fabrication of durability and stability composite cathode are comprised from mixing of perovskite La0.6Sr0.4CO0.2Fe0.8 (LSCF) powders with nanoscale ionically conducting ceramic electrolyte materials, SDC-carbonate (SDCc) was overcome this problems. Powder preparation and composite cathode fabrication must consider which as main factors in the development of durability and stability of LSCF-SDCc composite cathode. Powders must in nanoscale to enhance the conductivity and decrease the interfacial polarization resistance and the composite cathode should in nanoporous morphology for achieve high power density over than 500 h and remarkable durability. Calcination also plays in important role and its operations will effects to the SOFC durability and performance. The necessary to prolong the lifetime and increase the SOFC performance has lead to development of durability and stability of SOFC. This paper reviews the durability and stability of the composite cathode and focus on the challenges in material technology.

The oxygen reduction kinetics was investigated at the porous (LSM)–yttria-stabilized zirconia (YSZ) composite electrodes with various YSZ grain sizes by employing ac impedance spectroscopy and the potentiostatic current transient (PCT) technique. For this purpose, LSM-YSZ composite electrodes were coated on a YSZ pellet by sintering at in air. The grain size of YSZ was controlled by varying the initial particle size of YSZ. From analyses of the ac impedance spectra and the cathodic PCTs measured on the electrodes as functions of temperature and ionic conductivity, it was first recognized that the overall cathodic reaction is mainly controlled by the oxygen-exchange reaction coupled with ion migration at low operating temperatures below . Second, from the discrete Fourier transformation analysis of impedance spectra, it was confirmed that the distribution of relaxation times for oxygen-ion migration becomes wider with increasing YSZ grain size in the range from due to the larger effective migration length . Finally, from the theoretical and experimental analyses of the cathodic PCTs based upon the equivalent circuit in consideration of the relaxation time distribution for ion migration, it is suggested that as the YSZ grain size decreases, the higher density of the three-phase-boundary (TPB) length leads to an increase in the overall current density and a reduction in the time to reach the steady-state current density , thus causing high power density.

The performance of solid oxide fuel cells is affected by various polarization losses, usually grouped in ohmic, activation and concentration polarization. Under typical operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometry: as an example, the performance of a tubular cell is strongly limited by the ohmic polarization due to the long current path of electrons, while in a planar cell each of these losses has a comparable effect. It is therefore of interest, in case of planar geometry, to investigate the performance limiting factors. In this paper, a performance evaluation of planar circular-shaped seal-less SOFC cells from InDEC® was performed, with an outline of the limiting factors at reduced temperature. Two different designs of planar cells are considered: both have porous NiO-YSZ anode as mechanical support, NiO-YSZ anode active layer, yttria-stabilized zirconia (YSZ) electrolyte, and only differ for the cathode design: (1) strontium doped lanthanum manganate (LSM)-YSZ cathode functional layer (CFL) and LSM cathode current collector layer (CCCL); (2) yttria doped ceria (YDC) blocking layer and lanthanum strontium cobalt ferrite oxide (LSCF) functional layer. The characterization was performed by taking V-I measurements over a range of temperatures between 650°C and 840°C with hydrogen as fuel, and air as oxidant. The dependence of the cell performance on the various polarization contributions was rationalized on the basis of a analytical model, through a parameter estimation on the experimental data, devoted to the determination of the temperature dependence of the area specific resistance (ASR) and of the cathode exchange current density: in particular, the performance limitation at low temperature is due to activation polarization for ASC1 and ohmic polarization for ASC2. Based on the results of the investigation, it is concluded that LSCF cathodes are really effective for decreasing the cathode activation polarization, allowing the reduction of operating temperature. Finally, a microstructural analysis with SEM and optical microscopy has been performed on the ASC2 cell after the polarization testing. The aim of this investigation was in particular to evaluate the degradation phenomena occurring in the anodic structure and over the interfaces between the various active layers. The ASC2 elastic modulus has also been estimated before and after polarization testing in order to evaluate the decreasing of the mechanical strength of the cell after a complete thermal cycle. The results describe a mechanical degradation of the structure and of the distribution of the phases.

YSZ ( yttria-stabilized zirconia)-modified LSM composite cathodes were fabricated by formation of YSZ film on triple phase boundary (TPB) of LSM/YSZ/gas. The YSZ coating film greatly enlarged electrochemical reaction sites from the increase of additional TPB. The composite cathode was formed on thin YSZ electrolyte (about 30 Um thickness) supported on an anode and then I-V characterization and AC impedance analyses were performed at temperature between . As results of the impedance analysis on the cell at with humidified hydrogen as the fuel and air as the oxidant, R1 around the frequency of 1000 Hz represents the anode Polarization. R2 around the frequency of 100Hz indicates the cathode polarization, and R3 below the frequency of 10 Hz is the resistance of gas phase diffusion through the anode. The cell with the composite cathode produced power density of at air and oxygen atmosphere, respectively. The I-V curve could be divided into two parts showing distinctive behavior. At low current density region (part I) the performance decreased steeply and at high current density region (part II) the performance decreased gradually. At the part I the performance decrease was especially resulted from the large cathode polarization, while at the part H the performance decrease related to the electrolyte polarization.

Solid oxide fuel cells (SOFCs) are expected to be a future power source. Simulation analyses of SOFCs can help to understand well the interactive functions among the multiphysics phenomena in the SOFC system. A three-dimensional multiphysics finite-element model was used to simulate the performance of a half-cell SOFC with (Pr0.7Sr0.3)MnO3±δ∕8mol% yttria-stabilized zirconia (8YSZ) composite cathode on one side of the 8YSZ electrolyte before and after aging. Multiphysics phenomena in the SOFC were considered in the modeling. The current/voltage curves simulated matched the experimental data before and after aging. The average current density was found to have a linear relationship to the logarithm of the effective exchange current density. The effect of the effective ionic conductivity of the composite cathode was more apparent for small total effective ionic conductivity values than for large ones.

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

In situations common to high-temperature SOFC (Solid-Oxide Fuel Cell) operation at steady-state, the dependence of the activity of oxide ions on the oxygen partial pressure (p_O2) becomes relatively unimportant for p_O2>0.15 bar. Nevertheless, one might question what would be the indirect effect (if any) of surface ad-ions on the evaluation of concentration polarization due to changes on the oxygen partial pressure near to the grain boundary between electronic, ionic, and gas phases. From thermostatics considerations and macroscale-continuum phenomenological equations it is shown that the definition of concentration polarization, as commonly found in the literature, may contain a series of limitations and, among these limitations, it is discussed the relative importance of oxygen surface ad-ions on the formation of concentration cells,when it is compared to the concentration gradients of “bulk” gas species in the porous channels of SOFC cathodes. This theoretical discussion about the influence of contact chemistry near the TPB (three-phase boundary) region indicates that concentration polarization could in fact either increase or decrease, depending on whether the chemical activity of oxide ions is affected by the local chemical activity of O2 near the gas/solid interface. In order to verify this, one needs to know how the differences of interfacial potential step and chemical activity relate to changes in the contact chemistry and space charge zone near this interface. Particularly, the evaluation of the potential step between the electronic conductor and the ion adsorbed layer requires the prescription of certain phenomenological parameters. In the present study, one of these parameters is the density of “active” surface sites near the TPB region, which represents just a fraction of the total surface site density available over the entire electrode phase. It is a fraction because only those sites sufficiently close to the TPB “hot spot” are considered “active” for the adsorption of charged species, which are then subsequently able to undergo electrochemical reduction at the TPB. So to calculate this density of “active” surface sites and thus the corresponding electrostatic potential step, a phenomenological governing equation for oxygen ad-ions on the electrode surface is proposed (i.e., in terms of surface coverage of ad-ions) to account for electrocatalytic and interfacial transport processes (i.e., competing surface diffusion and adsorption processes near the TPB “hot spot”) within a composite porous cathode.

The performance characteristics of high-performance Ni-YSZ (yttria-stabilized zirconia) anode-supported solid oxide fuel cells (SOFCs) are examined under typical fuel cell operating conditions over a period of ∼3000 h when hydrogen contaminated with different concentrations of H2S was used as the fuel. Analyses reveal some new insights into the sulfur poisoning behavior of a Ni-YSZ anode. First, an initial drop in power output upon exposure to H2S-contaminated fuels is more dramatic in an anode-supported cell (with small cell resistance) than in electrolyte-supported cell (with large cell resistance), creating an illusion that a high-performance cell appears to be less sulfur tolerant than a low-performance cell. Second, the transition time for this rapid performance drop to cease is much longer for an anode-supported cell (with a thicker anode) than an electrolyte-supported cell (with a thinner anode). Third, the degree of sulfur poisoning (or the cell resistance increase due to sulfur poisoning) diminishes with operating cell current density, suggesting that the water produced at the active sites on anode surface from electro-oxidation of hydrogen (or oxygen ions) may promote the oxidation and removal of adsorbed sulfur. This effect becomes less pronounced at higher concentration of H2S, but still significant at ∼1 ppm of H2S. Fourth, the subsequent slow degradation in performance after the rapid performance drop upon initial exposure to H2S can be avoided, indicating that the previously reported slow degradation is unlikely the inherent behavior of a Ni-YSZ anode, but associated with other complications. This further implies that sulfur poisoning may be reversible and the performance drop could be fully recovered when the fuel is switched back to clean hydrogen.