Ruth Knibbe

Solid Oxide Electrolysis Cells: Degradation at High Current Densities

The degradation of Ni/yttria-stabilized zirconia (YSZ)-based solid oxide electrolysis cells operated at high current densities was studied. The degradation was examined at 850 degrees C, at current densities of -1.0, -1.5, and -2.0 A/cm(2), with a 50:50 (H(2)O:H(2)) gas supplied to the Ni/YSZ hydrogen electrode and oxygen supplied to the lanthanum, strontium manganite (LSM)/YSZ oxygen electrode. Electrode polarization resistance degradation is not directly related to the applied current density but rather a consequence of adsorbed impurities in the Ni/YSZ hydrogen electrode. However, the ohmic resistance degradation increases with applied current density. The ohmic resistance degradation is attributed to oxygen formation in the YSZ electrolyte grain boundaries near the oxygen electrode/electrolyte interface

High Performance Cathodes for Solid Oxide Fuel Cells Prepared by Infiltration of La0.6Sr0.4CoO3−δ into Gd-Doped Ceria

Cathodes prepared by infiltration of La0.6Sr0.4CoO3-delta (LSC40) into a porous Ce0.9Gd0.1O1.95 (CGO10) backbone have been developed for low temperature solid oxide fuel cells. The CGO10 backbone has been prepared by screen printing a CGO10 ink on both sides of a 180 mu m dense CGO10 electrolyte-tape followed by firing. LSC40 was introduced into the CGO10 porous backbone by multiple infiltrations of aqueous nitrate solutions followed by firing at 350 degrees C. A systematic study of the performance of the cathodes was performed by varying the CGO10 backbone firing temperature, the LSC40 firing temperature and the number of infiltrations. The cathode polarization resistance was measured using electrochemical impedance spectroscopy on symmetrical cells in ambient air, while the resulting structures were characterized by scanning electron microscopy (SEM) and high temperature X-ray diffraction (HT-XRD). The firing temperature of 600 degrees C for the LSC40 infiltrate was found to provide a balance between LSC40 material formation and high surface area micro/nanostructure. The lowest polarization resistances measured at 600 and 400 degrees C were 0.044 and 2.3 Omega cm(2) in air, respectively. During degradation tests at 600 degrees C, the cathode polarization resistance levels out after about 450 h of testing, giving a final polarization resistance of 0.07 Omega cm(2)

Recent progress on advanced materials for solid-oxide fuel cells operating below 500 °C

Solid-oxide fuel cells (SOFCs) are electricity generators that can convert the chemical energy in various fuels directly to the electric power with high efficiency. Recent advances in materials and related key components for SOFCs operating at ≈500 °C are summarized here, with a focus on the materials, structures, and techniques development for low-temperature SOFCs, including the analysis of most of the critical parameters affecting the electrochemical performance of the electrolyte, anode, and cathode. New strategies, such as thin-film deposition, exsolution of nanoparticles from perovskites, microwave plasma heating, and finger-like channeled electrodes, are discussed. These recent developments highlight the need for electrodes with higher activity and electrolytes with greater conductivity to generate a high electrochemical performance at lower temperatures.

New composites of nanoparticle Cu (I) oxide and titania in a novel inorganic polymer (geopolymer) matrix for destruction of dyes and hazardous organic pollutants.

New photoactive composites to efficiently remove organic dyes from water are reported. These consist of Cu2O/TiO2 nanoparticles in a novel inorganic geopolymer matrix modified by a large tertiary ammonium species (cetyltrimethylammonium bromide, CTAB) whose presence in the matrix is demonstrated by FTIR spectroscopy. The CTAB does not disrupt the tetrahedral geopolymer structural silica and alumina units as demonstrated by (29)Si and (27)Al MAS NMR spectroscopy. SEM/EDS, TEM and BET measurements suggest that the Cu2O/TiO2 nanoparticles are homogenously distributed on the surface and within the geopolymer pores. The mechanism of removal of methylene blue (MB) dye from solution consists of a combination of adsorption (under dark conditions) and photodegradation (under UV radiation). MB adsorption in the dark follows pseudo second-order kinetics and is described by Freundlich-Langmuir type isotherms. The performance of the CTAB-modified geopolymer based composites is superior to composites based on unmodified geopolymer hosts, the most effective composite containing 5wt% Cu2O/TiO2 in a CTAB-modified geopolymer host. These composites constitute a new class of materials with excellent potential in environmental protection applications.

Electrochemical Characterization of Planar Anode Supported SOFC with Strontium-Doped Lanthanum Cobalt Oxide Cathodes

The electrochemical performance of anode supported cells with screen-printed strontium-doped lanthanum cobalt oxide cathodes was studied using electrochemical impedance spectroscopy and polarization measurements. In order to obtain information on the resistance contribution originating from the cathode alone, symmetrical cell measurements were carried out. Analysis of the symmetrical cell data by the distribution of relaxation times method, aided in the selection of an appropriate equivalent circuit for the cathode. Cell performance depends on the interdiffusion barrier layer and it is shown that the concentration polarization strongly overlap with the cathode losses, thus complicating the deconvolution of the impedance spectra into individual process and / or electrode specific contributions.

Relating Critical Currents to Defect Populations in Superconductors

Analyzing critical currents from an information theoretic or statistical point of view allows one to identify distinct populations of microstates contributing to the critical current under particular conditions of temperature and applied field. We show how this knowledge can be correlated with the known microstructure of a sample to identify how different physical populations of pinning centers are contributing to these statistical populations of microstates. We will then show that by tracking the variation of critical current with temperature, field, and field angle we can construct a picture of the relative contributions of different defect populations under different conditions. We particularly focus our analysis on YBCO thin film coated conductors with potential commercial application.

Structure property relationships in a nanoparticle-free SmBCO coated conductor

We examine the temperature, field and field angle dependence of the critical current of a SmBa2Cu3Oy coated conductor produced by reactive co-evaporation. A transmission electron microscopy based microstructural analysis shows the film contains extended c-axis defects, stacking faults, and two different species of inclined defects. By applying a maximum entropy decomposition of the field angle dependent critical current I-c(theta) datasets we are able to identify the individual contributions of these defects to the critical current even though they do not produce distinct peaks but rather an anisotropy in I-c(theta). We are able to confirm the structure property relationships by determining the matching fields where each of the individual defect contributions are a maximum and showing that these are consistent with the observed microstructure. For a critical current component having a maximum magnitude at an intermediate temperature we propose a model of thermally activated depinning to explain the behaviour.

Oxygen Deficiency, Stacking Faults and Calcium Substitution in MOD YBCO Coated Conductors

The variation in transition temperature <i>T</i>_c with oxygen deficiency or calcium substitution in Y_1-xCa_xBa₂Cu₃O_7-δ (YBCO) is a well-known manifestation of the generic hole-doping phase diagram governing the superconducting cuprates. Less well understood is the role that microstructural defects can play in determining hole doping. We have investigated the formation of Y124-type stacking faults in metal-organic deposited (MOD) YBCO coated conductors and shown that these defects also act to reduce the hole concentration in YBCO. With low stacking-fault density, <i>T</i>_c can be as low as 89 K when fully oxygen loaded and can increase to a maximum of 94 K when partially unloaded. The presence of stacking faults limits the degree to which fully oxygen loaded YBCO can be overdoped. The critical current density <i>J</i>_c is optimized by full oxygen loading but with a moderate density of stacking faults. We have demonstrated substitution of calcium for yttrium in YBCO by modification of the MOD precursor resulting in an increase in hole concentration and consequent decrease in <i>T</i>_c. <i>J</i>_c is initially depressed by calcium substitution but partially recovers upon incorporation of a moderate density of stacking faults.

Origin of Polarization Losses in Solid Oxide Electrolysis Cells under High Current Density

Under high current density, polarization losses in a solid oxide cell are higher in electrolysis mode than in fuel cell mode. Although part of these differences under high current density can be attributed to gas diffusion differences between steam and hydrogen. Diffusion coefficient differences can only partially explain the higher polarization losses during electrolysis operation.

An integrated approach towards enhanced performance of Lithium Sulfur Battery and its fading mechanism

To fulfil the potential of Li-S batteries (LSBs) with high energy density and low cost, multiple challenges need to be addressed simultaneously. Most research in LSBs has been focused on the sulfur cathode design, although the performance is also known to be sensitive to other parameters such as binder, current collector, separator, lithium anode, and electrolyte. Here, an integrated LSB system based on the understanding of the different roles of binder, current collector, and separator is developed. By using the cross-linked carboxymethyl cellulose-citric acid (CMC-CA) binder, Toray carbon paper current collector, and reduced graphene oxide (rGO) coated separator, LSBs achieve a high capacity of 960 mAh g-1 after 200 cycles (2.5 mg cm-2 ) and 930 mAh g-1 after 50 cycles (5 mg cm-2 ) at 0.1 C. Moreover, the failure mechanism at a high sulfur loading with characteristics of fast capacity decay and infinite charging is discussed. This work highlights the synergistic effect of different components and the challenges towards more reliable LSBs with high sulfur loading.