Yttria-stabilized Zirconia Interface 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.

Cracking Behavior of Gd2Zr2O7/YSZ Multi-Layered Thermal Barrier Coatings Deposited by Suspension Plasma Spray

A new multi-layered thermal barrier coating system (TBCs) containing gadolinium zirconate (GZ, Gd2Zr2O7) and yttria-stabilized zirconia (YSZ) was developed using suspension plasma spray (SPS) to improve the overall thermal cycling performance. This study focuses on the cracking behavior of the GZ/YSZ TBC after thermal exposure to find out the key factors that limit its lifetime. Different cracking behaviors were detected depending on the thermal treatment condition (i.e., horizontal cracks within the ceramic layer and at the thermally grown oxide (TGO)/YSZ interface) which can be related to stresses developed through thermal expansion mismatch and increased TGO thickness beyond a critical value, respectively. A reduction in hardness of bond coat (BC) was measured by nanoindentation and linked with the thermally activated grain growth mechanism. The hardness and elastic modulus of ceramic layers (GZ and YSZ) showed an increased trend after treatment that contributed to the interfacial cracks.

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.

Investigation of surficial and interfacial properties of BaCo0.4Fe0.4Zr0.1Y0.1O3-δ cathode directly on yttria-stabilized zirconia electrolyte for solid oxide fuel cell

The cobalt/iron-containing perovskites are the most promising cathodes for the intermediate-temperature solid oxide fuel cell (IT-SOFC). However, these cathodes are generally not directly applied on the currently available commercial yttria-stabilized zirconia (YSZ) electrolyte due to the issues of over-firing, harmful reaction and thermal incompatibility. BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) which contains both zirconium and yttrium ions can theoretically be more compatible with the YSZ electrolyte. This paper focuses on the surficial and interfacial properties of BCFZY directly prepared with YSZ under different conditions. The surface of BCFZY by the glycine-nitrate process (GNP) can form the nanoparticles in contrast to that by solid-state reaction (SSR). The peak power density of the cell with BCFZY-GNP cathode directly on YSZ electrolyte can reach 1250 mW cm−2 at 750 °C, which is hardly achieved on the reported traditional cobalt/iron-containing cathodes. It is proved that BCFZY-GNP cathode fired at 900 °C can show more distinct nanoparticles with extensive specific surface area and better BCFZY|YSZ interface, which can promote both oxygen exchange and ion incorporation processes.

Three dimensional YSZ interface engineering layer for enhancement of oxygen reduction reactions of low temperature solid oxide fuel cells

In this study, we fabricated three dimensional yttria-stabilized zirconia (YSZ) interface engineering layer (IEL) by using a DC sputtering method to enhance oxygen reduction reactions (ORRs) of low temperature solid oxide fuel cells (LT-SOFCs). In order to deposit three dimensional YSZ IEL, a commercial Y–Zr metal alloy target was sputtered under various deposition pressure conditions (4, 8, 12 Pa) and then annealed at 800 °C for 6 h. Phase transformation of metal alloy Y–Zr to YSZ was identified by X-ray diffraction, X-ray photoelectron spectroscopy and high resolution-transmission electron microscope. The roughness of the three dimensional YSZ IELs were strongly related to the deposition pressure of the sputtering technique. After physical and chemical characterization, electrochemical examinations proved that the three dimensional YSZ IELs efficiently improved the ORR activity by geometrical effects. The fuel cell with the IEL fabricated at 8 Pa condition achieved the best power density (37.9 mW/cm2), which was 30.2% improved performance compared to the fuel cell without IEL (29.1 mW/cm2). Electrochemical impedance spectroscopy investigations also proved that rough three dimensional YSZ IELs efficiently enhance the performance of LT-SOFCs.

Accelerated degradation of yttria stabilized zirconia electrolyte during high-temperature water electrolysis

The accelerated degradation of a commercial LSCF/YDC/YSZ/Ni-YSZ solid oxide electrolyzer cell (La0.6Sr0.4Co0.2Fe0.8O3-δ/Y0.1CeO1.95/Y0.08Zr0.92O1.96/Ni-YSZ) contaminated by Si-containing impurities is studied with time under up to − 1.7 A cm−2 applied. Above ~ − 0.6 A cm−2, a new region appears in the polarization curve. This region corresponds to electronic conduction in the yttria-stabilized zirconia (YSZ) electrolyte, induced by the reduction under high current conditions. A shift in the typical frequencies (relaxation times) toward lower frequencies is then observed for the entire impedance spectra. This shift results finally in the disappearance of the positive loop related to the polarization resistance and the appearance of a negative (inductance type) loop which crosses the real axis (Z’) at the lowest frequencies to become positive again. This is characteristic for an electrode process mode in which the electrochemical redox reactions vanish while the cell current becomes mainly electronic due to the reduction of the YSZ electrolyte. This trend increases with time. Such a characterization of the electronic conduction of the YSZ electrolyte by electrochemical impedance spectroscopy has not been reported to date under electrolysis mode, to the best of our knowledge. Post-mortem analysis by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDX) shows detrimental degradation of the electrolyte after only 360 h of overall testing duration with numerous micropores in the YSZ volume, and cracks and delamination at the yttria-doped ceria (YDC)/YSZ interface. EDX analysis reveals (i) a migration of La, Sr, Co, and Fe elements from lanthanum strontium cobalt ferrite (LSCF) anode to YDC layer and YSZ electrolyte and (ii) a very important shift of Ni from Ni-YSZ cathode to YSZ and YDC, and also to LSCF anode in a lower proportion. This study highlights the critical issue that impurities represent for solid oxide electrolysis cell operation.

Computational design of metal oxides to enhance the wetting and adhesion of silver-based brazes on yttria-stabilized-zirconia

Ag–CuO is a broadly used reactive air brazing (RAB) system for effectively bonding ceramics and metal interfaces, especially for sealing yttria-stabilized zirconia (YSZ) to metals in solid-oxide fuel cells (SOFCs). To understand the superior performance of this braze, density functional theory (DFT) calculations were employed to investigate two mechanisms that can potentially increase the work of adhesion (Wadh) and hence reduce the wetting angle of Ag on YSZ. It was found while the formation of Ag–dissolved O clusters at the Ag–YSZ interface can promote wetting, a much greater wetting angle reduction comes from the formation of CuO interlayers between Ag and YSZ. Further, the Wadh of an Ag/CuO and CuO/YSZ interface was found to be significantly higher than that of an Ag/YSZ interface. Based on simulation-obtained insights into metal to oxide bond formation, a simple descriptor was developed to predict the Ag/oxide interface energies, the Ag/oxide Wadh, and to search for potential oxide interlayers capable of promoting the wetting and adhesion of Ag on YSZ. Many simple metal oxides (single cation) were examined, however their Wadh with Ag were less than that of an Ag/CuO interface. Expanding the search to multi-cation oxides led to several promising candidates, such as CuAlO2, CuGaO2, and Cu3TiO4; all of which are also stable in the reducing SOFC conditions. Depending upon their solubility in molten Ag, these newly-identified oxides could either be pre-applied as wetting promoting interlayers or directly incorporated into Ag to form new reactive air brazes.

Activation and failure mechanism of La0.6Sr0.4Co0.2Fe0.8O3−δ air electrode in solid oxide electrolyzer cells under high-current electrolysis

This work investigates the activation and delamination of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) air electrode of solid oxide electrolyzer cells sintered on yttria-stabilized zirconia (YSZ) electrolyte. After polarization with an electrolysis current of 1 A cm−2 for 24 h at 800 °C, the LSCF electrode delaminates accompanied by an increase of ohmic and polarization resistance. Notably, polarization resistance decreases at the beginning. By scanning electron microscopy (SEM), a thin but dense layer is observed at the LSCF–YSZ interface of an as-prepared sample, which is identified as SrZrO3 phase by X-ray diffractometry. This layer causes the initial high polarization resistance due to retarded ionic and electronic conductivity. After the test, SEM reveals that the SrZrO3 layer delaminates from YSZ electrolyte. Moreover, energy dispersive X-ray tests confirm that Co diffuses to the SrZrO3 layer and SrZrO3–YSZ interface. Later, the LSCF electrode with Co-containing SrZrO3 layer is shown to perform better than that with pure SrZrO3 layer. Thus, Co diffusion can be the reason for the initial decrease of polarization resistance and renders the generation of oxygen at SrZrO3–YSZ interface during the electrolysis. Owing to its limited porosity, the SrZrO3 layer traps the generated oxygen. High pressure eventually builds up at the SrZrO3–YSZ interface driving the delamination of SrZrO3 layer, and hence the entire LSCF electrode.

Ion transfer in Ni-containing composite anodes of solid oxide fuel cells: A microstructural study

High-resolution transmission and scanning electron microscopy of the solid oxide fuel cell (SOFC) anodes made of Ni and yttria-stabilized zirconia (YSZ), revealed a substantial microstructural reconstruction induced by the anodic current. These microstructural alterations, primarily the growth of NiO inclusions at the Ni| YSZ interface, the resultant orientation relationship between the NiO and YSZ grains, and Ni grain fragmentation on subsequent reduction, provide a straightforward evidence for the so-called oxygen spillover mechanism of the anodic reaction.

Enhanced performance and endurance of nano-porous platinum solid oxide fuel cell electrodes by oxygen partial pressure cycling

Abstract

On the variability of oxygen exchange kinetics of platinum model electrodes on yttria stabilized zirconia

The electrochemical polarization resistance of the oxygen exchange reaction on yttria stabilized zirconia (YSZ) was investigated by means of geometrically well-defined dense platinum microelectrodes. The polarization resistance was found to be strongly affected by two different phenomena changing the electrode kinetics. One process is only observable in oxidizing atmosphere and was almost reversible (i.e. causes degradation as well as regeneration of the electrode performance). It is attributed to the formation/decomposition of a PtOx “phase”. The second process leads to continuous degradation and is discussed to be related to impurities accumulating at the three phase boundary (TPB). The effects of both time dependent phenomena on the electrochemical oxygen exchange at the TPB and at the Pt|YSZ interface are analyzed by means of equivalent circuits. It can be shown that the PtOx related degradation/regeneration affects both the TPB process as well as the electrochemical process at the Pt|YSZ interface. The impurity related degradation, however, is demonstrated to only influence the TPB kinetics.

The relevance of interfaces for oxide ion transport in yttria stabilized zirconia (YSZ) thin films

Thin YSZ films were prepared on magnesia, sapphire and strontium titanate (STO) single crystals using pulsed laser deposition and, for comparison, by a sol-gel method on STO. The bulk and interfacial mass and charge transport properties of these films were investigated by complementary impedance spectroscopy and tracer diffusion measurements. In this context, a novel two-step tracer diffusion experiment is introduced. For YSZ films on sapphire and magnesia, grain bulk conductivities similar to those of polycrystalline samples were measured in most cases. Strongly blocking grain boundaries could be identified by impedance measurements. The films on sapphire and magnesia also exhibited good agreement between effective transport properties of impedance and tracer measurements. YSZ layers on strontium titanate single crystals, on the other hand, showed a strongly increased effective conductivity in impedance studies. However, in tracer diffusion experiments this could be unambiguously attributed to conduction in the substrate while the diffusion coefficient of YSZ on STO was comparable to that of YSZ films on other substrates. Moreover, the tracer diffusion experiments did not indicate any significant increase of oxide ion mobility on a free YSZ surface compared to a Pt|YSZ interface.

Characteristics of Molten Metals as Anodes for Direct Carbon Solid Oxide Fuel Cells

Sn, Bi, In, Pb and Sb were examined as molten anodes for solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolytes. Cells were operated to characterize the electrochemical oxidation of these metals at the YSZ interface. For Sn and In, which form high-melting oxides, the performance of the cell drops due to formation of a thin solid oxide film on the YSZ interface, which prevents further oxidation. For Bi, such a drop is not observed because its oxide is O2- conductive. For Pb, above the melting point of PbO, the oxide no longer hampers cell performance. For Sb, which forms low melting Sb2O3 (m.p. 929 K), the anode impedance remains low, making efficient fuel cells possible as Sb2O3 is also readily reduced by most types of solid fuels at 973 K. Finally, a high power direct carbon fuel cell (DCFC) is demonstrated with the Sb anode using sugar char fuel.

Interfacial characterization of YSZ-to-steel joints with Ag–Cu–Pd interlayers for solid oxide fuel cell applications

Yttria-stabilized zirconia (YSZ)/stainless steel joints made using two commercial silver-based interlayers containing palladium (58Ag–32Cu–10Pd and 65Ag–20Cu–15Pd), were systematically analyzed for the microstructures of the interlayer matrices, interlayer–steel interface, and interlayer–YSZ interface using scanning electron microscopy (SEM) and transmission electron microscopy coupled with energy dispersive spectroscopy (TEM/EDS). In the interlayer matrix, a face-centered cubic (FCC) Cu-rich phase formed in the vicinity of the YSZ and dissolved a significant amount of Zr from the ZrO2 and minor amounts of Fe and Cr from the steel. The Cu-rich phase in the interlayer matrix in the vicinity of the steel substrate was the ordered phase Cu3Pd with antiphase boundaries (APBs) and the L12 crystal structure. Silver particles precipitated within the Cu3Pd phase in 58Ag–32Cu–10Pd; a Fe(Cr) needle-like phase, instead of Ag particles, precipitated within the Cu3Pd phase in 65Ag–20Cu–15Pd. Although no reaction products at the interlayer–steel interface were found, dislocations appeared within the Ag- and Cu-rich phases. At the interlayer–YSZ interface, two reaction products, SiO2 (impurity in YSZ) and Ti3O5 (from the reaction of YSZ with Ti impurities in the steel) were observed. Diffusion and chemical reactions led to the compositional changes and interface reconstruction, thereby yielding metallurgically sound YSZ/steel joints.

Investigation of the oxygen exchange mechanism on Pt|yttria stabilized zirconia at intermediate temperatures: Surface path versus bulk path

The oxygen exchange kinetics of platinum on yttria-stabilized zirconia (YSZ) was investigated by means of geometrically well-defined Pt microelectrodes. By variation of electrode size and temperature it was possible to separate two temperature regimes with different geometry dependencies of the polarization resistance. At higher temperatures (550–700°C) an elementary step located close to the three phase boundary (TPB) with an activation energy of ∼1.6eV was identified as rate limiting. At lower temperatures (300–400°C) the rate limiting elementary step is related to the electrode area and exhibited a very low activation energy in the order of 0.2eV. From these observations two parallel pathways for electrochemical oxygen exchange are concluded.The nature of these two elementary steps is discussed in terms of equivalent circuits. Two combinations of parallel rate limiting reaction steps are found to explain the observed geometry dependencies: (i) Diffusion through an impurity phase at the TPB in parallel to diffusion of oxygen through platinum – most likely along Pt grain boundaries – as area-related process. (ii) Co-limitation of oxygen diffusion along the Pt|YSZ interface and charge transfer at the interface with a short decay length of the corresponding transmission line (as TPB-related process) in parallel to oxygen diffusion through platinum.

Stability and performance of infiltrated La 0.8Sr 0.2Co xFe 1− xO 3 electrodes with and without Sm 0.2Ce 0.8O 1.9 interlayers

The chemical stability of composite electrodes produced by the infiltration of La 0.8Sr 0.2Co x Fe 1− x O 3 (LSCF) into a porous yttria-stabilized zirconia (YSZ) scaffold were investigated as a function of the Co:Fe ratio in the LSCF and the LSCF calcination temperature. XRD and impedance spectroscopy results indicate that for an LSCF calcination temperature of 1123 K, reactions between the LSCF and YSZ do not occur to a significant extent. Reactions producing La 2Zr 2O 7 and SrZrO 3 at the interface were observed, however, for a calcination temperature of 1373 K and x values greater than 0.2. In addition to determining the conditions for which reactions between LSCF and YSZ occur, the effectiveness of infiltrated SDC interlayers in preventing reactions at the LSCF–YSZ interface and their influence on the overall performance of LSCF/YSZ composite electrodes was studied.

Effect of La2Zr2O7 on Interfacial Resistance in Solid Oxide Fuel Cells

The impact of (LZO) on interfacial resistance at the /yttria-stabilized zirconia interface was studied upon isothermal sintering at for 2–25 h. Quantification of triple phase boundary length was performed by applying focused ion beam/scanning electron microscopy (FIB/SEM) serial-sectioning techniques and classical stereology. Electrochemical impedance spectroscopy was used to characterize the . The effect of LZO formation on microstructural models for was analyzed with respect to previous works that did not include this effect. LZO formation modifies the TPB length, rapidly increases , and needs to be controlled in analytical microstructural models.

A Comparison of Molten Sn and Bi for Solid Oxide Fuel Cell Anodes

Molten Sn and Bi were examined at 973 and for use as anodes in solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolytes. Cells were operated under “battery” conditions, with dry He flow in the anode compartment, to characterize the electrochemical oxidation of the metals at the YSZ interface. For both metals, the open-circuit voltages (OCVs) were close to that expected based on their oxidation thermodynamics, for Sn and for Bi. With Sn, the cell performance degraded rapidly after the transfer of approximately of charge due to the formation of a layer at the YSZ interface. At , the anode impedance at OCV for freshly reduced Sn was approximately but this increased to well over after the transfer of of charge. Following the transfer of at , the formation of a thick layer was confirmed by scanning electron microscopy. With Bi, the OCV anode impedance at was less than and remained constant until essentially all of the Bi had been oxidized to . Some implications of these results for direct carbon fuel cells are discussed.

Improvement in Propene Sensing Characteristics by the Use of Additives to In2O3 Sensing Electrode of Mixed-Potential-Type Zirconia Sensor

In an effort to improve propene selectivity and long-term stability of the mixed-potential-type yttria-stabilized zirconia (YSZ)-based tubular sensor attached with sensing electrode (SE), each of the nanosized noble metals (Pt, Ru, Pd, Rh, or Ir) and YSZ powder was added to SE. It was found that the addition of nanosized Pt gave considerable improvement in selectivity, and the addition of YSZ powder exhibited great improvement in long-term stability of the sensor. Thus, the sensor attached with composite SE consisting of , Pt, and YSZ was fabricated and its sensing characteristics to were examined. As a result, the present sensor exhibited high selectivity and excellent long-term stability to at under the wet condition for the examined period of about . Based on the results obtained from the measurement of catalytic activity for CO oxidation to , the observation of morphology, and the analysis of composition of SEs, it was speculated that the added Pt nanoparticles could promote the oxidation of CO and and then improve the selectivity, while the added YSZ powder could stabilize the morphology of the SE∕YSZ interface and give better long-term stability.