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Infiltration of nanoscale electrocatalysts into Ni/yttria-stabilized zirconia (Ni-YSZ) cermets has been shown to improve the electrochemical performance of solid oxide fuel cell (SOFC) anodes. While infiltrated electrodes in SOFCs result in improved cell performance, long-term operation leads to coarsening of the infiltrated nanoparticles and negates the short-term performance improvements. This study explores the roles of humidity, temperature, and number of cycles of nanocatalyst infiltration in improving mixed conduction within the Ni-YSZ electrode. Two mixed conduction phases were studied: Gd0.1Ce0.9O2-δ (GDC) as an infiltrant into Ni-YSZ electrodes, and Ni/transition metal doped-YSZ electrodes infiltrated with Ni. Analysis of impedance data from these cells shows improved electrochemical performance in infiltrated cells with mixed conduction compared with infiltrated cells containing purely ionic and electronic conducting phases. Improved anode performance is attributed to the availability of electronic pathways through predominantly ionic-conducting phases to connect distant Ni nanoparticles and/or Ni grains.

The performance of solid oxide fuel cell (SOFC) anodes infiltrated with different nanocatalysts are compared. In traditional Ni/YSZ cermet anodes, reactions occur at triple phase boundaries (TPBs). Infiltration of Ni nanoparticles increases the TPB density. However, unless the added TPBs created by infiltration have electronically conducting pathways to the percolated Ni in the Ni/YSZ cermet, the added TPBs are not electrochemically active. Different strategies to activate these added TPBs by providing such a conducting pathway, have been explored. These include, spreading the Ni nanoparticles to form a percolating network at temperature, co-infiltration of Ni with GDC, a mixed ionic and electronic conducting (MIEC) phase, and doping the YSZ in the Ni/YSZ cermet to promote electronic conductivity between the Ni nanoparticles. Electrochemical I-V and EIS studies of full and symmetric cells, SEM analysis of fracture cross-sections of infiltrated cells, and FIB-SEM based 3-D reconstruction of the infiltrated microstructures will be discussed.

The performance of solid oxide fuel cells (SOFCs) can be degraded by “chromium poisoning” where thermally grown Cr2O3 on metallic surfaces forms volatile Cr-containing species that are redeposited on active regions of the cathode. This phenomenon is further exacerbated for porous metallic interconnects and metal-supported electrodes due to their large surface-to-volume ratios. In this study, electrophoretic deposition (EPD) of CuNi0.2Mn1.8O4 spinel powders on porous SUS430 metallic substrates using alternating current (AC) was explored. Two-step densification heat treatment was used to form a thin, uniform, protective spinel coating. The area-specific resistance (ASR) and weight gain were tracked during 100-h oxidation tests at 700°C in air. The results showed that, despite the considerable complexity of the sample shape, AC EPD was able to form a protective coating layer that significantly limited the growth rate of the thermally grown oxide (TGO) by reducing the kg value by a factor of 25.

We report a study investigating the effects of thermal annealing on the optical properties of Si-Ge alloy-core silica-cladded fibers. Low temperature fiber draw was performed with a laboratory-made draw tower at 1760 °C that minimizes impurity diffusion from cladding to the core. As a post-drawing process, Si–Ge core fibers were annealed in a box furnace to alter the core structure. Microstructural and optical properties of fibers were investigated, and transmission losses were measured as 28 dB/cm at 6.1 µm. Numerical studies were performed to analyze the experimental results and to find the optimum structure for low loss semiconductor-core glass-cladded fibers.

Protonic ceramic fuel or electrolysis cells (PCFC/PCEC) have shown promising performance at intermediate temperatures. However, these technologies have not yet been demonstrated in a stack, hence the oxidation behavior of the metallic interconnect under relevant operating environments is unknown. In this work, ferritic stainless steels 430 SS, 441 SS, and Crofer 22 APU were investigated for their use as interconnect materials in the PCFC/PCEC stack. The bare metal sheets were exposed to a humidified air environment in the temperature range from 450 °C to 650 °C, to simulate their application in a PCFC cathode or PCEC anode. Breakaway oxidation with rapid weight gain and Fe outward diffusion/oxidation was observed on all the selected stainless steel materials. A protective coating is deemed necessary to prevent the metallic interconnect from oxidizing. To mitigate the observed breakaway oxidation, state-of-the-art protective coatings, Y2O3, Ce0.02Mn1.49Co1.49O4, CuMn1.8O4 and Ce/Co, were applied to the stainless steel sheets and their oxidation resistance was investigated. Dual atmosphere testing further validated the effectiveness of the protective coatings in realistic PCFC/PCEC environments, with a hydrogen gradient across the interconnect. Several combinations of metal and coating material were found to be viable for use as the interconnect for PCFC/PCEC stacks.

Lanthanum nickelate, La2NiO4+δ (LNO), is studied as a cathode material for use in solid oxide fuel cells with the objective of mitigating chromium poisoning. Under current load, both electrochemical and chemical reactions cause chromium poisoning, and high current density and humidity accelerate the poisoning. However, compared with a standard strontium-doped lanthanum manganite cathode, the LNO cathode has a much higher tolerance for chromium poisoning. This can be ascribed to a greatly reduced chromium deposition in LNO.

Liquid-phase infiltration of nickel (Ni) nanoparticles in Ni/yttria-stabilized-zirconia (YSZ) cermet anodes for solid oxide fuel cells can improve anode performance provided that the infiltrated nanoparticles on YSZ connect to form conducting pathways and the Ni nanoparticles do not coarsen significantly. This study explores liquid phase co-infiltration of Ni with mixed conducting oxides, the latter providing microstructural stability and conductive pathways between Ni nanoparticles. Two mixed conducting oxides have been studied: Gd0.1Ce0.9O2−δ (GDC), a predominantly ionic conductor, and La0.6Sr0.3Ni0.15Cr0.85O3−δ (LSNC), a predominantly electronic conductor. Experimental results show that both oxides improve the nickel nanoparticle stability and charge transfer kinetics. However, the electrochemical performance of the Ni-GDC-infiltrated electrode is much better than that of the Ni-LSNC-infiltrated electrode. This is attributed to the citrate–nitrate combustion reaction required to form LSNC, which fills the pores of the anode and inhibits gas diffusion, reducing the performance of the Ni-LSNC-infiltrated electrode.

Date of Conference: 10-12 August 2020 Conference name: 3rd IEEE Research and Applications of Photonics in Defense Conference, RAPID 2020 Si-Ge alloy-core silica-cladded fibers were drawn at a low temperature to minimize impurity diffusion. The elemental segregation in the as-drawn fibers was overcome by a thermal treatment. The transmission losses of the fibers were calculated as 28 dB/cm at 6.1 μm wavelength.

Nickel – yttria stabilized zirconia (Ni-YSZ) anodes for solid oxide fuel cells were infiltrated with nickel nanoparticles with the objective of improving its performance by increasing the density of triple phase boundaries (TPBs). However, the nanoparticles deposited on the YSZ grains are not connected to one another, so the additional TPBs are not electrochemically active. It is postulated that at highly humid conditions (>95% H2O) created by high current density, nickel nanoparticles near the anode-electrolyte interface will spread and percolate, becoming electrochemically active. To study this behavior, infiltrated nickel nanoparticles were exposed to low, high and extreme humidity conditions during testing. When exposed to low humidity, no performance improvement or changes to the nanoparticle morphology was observed. At high humidity, the cell showed performance improvement and when cooled they showed nanoparticle coarsening. Exposure to extreme humidity (≈100%) caused performance to degrade back towards that of an un-infiltrated cell and severe nickel nanoparticles coarsening was observed in the cooled cell. This study validates the hypothesis that exposure to controlled high local humidity conditions can lead to the right amount of spreading of the deposited nickel nanoparticles to achieve percolation, thereby activating their TPBs and improving cell performance.

Strontium-doped lanthanum manganite is widely used as a cathode material for solid oxide fuel cells (SOFCs). In this study, the oxygen diffusivity in A-site-deficient (La0.8Sr0.2)0.95MnO3±δ (LSM-20) was measured as a function of temperature (600°C, 700°C, and 800°C) and oxygen partial pressure (0.03 atm, 0.16 atm, and 0.30 atm). Tracer (O-18) concentration profiles were measured in single-crystal epitaxial thin films of LSM-20 by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS). The profile was fit to a diffusion equation to obtain the diffusivity of oxygen in bulk LSM-20 as a function of temperature and oxygen partial pressure. A defect model was used in conjunction with the experimental data to calculate the activation energies of oxygen migration, Schottky reaction, redox reaction, and charge disproportionation reaction in LSM-20. These calculated data were then used to calculate the Brouwer diagram of (La0.8Sr0.2)0.95MnO3±δ at 800°C, a typical operating temperature of SOFCs.

Liquid infiltration of NiO followed by reduction to form Ni nanoparticle catalysts in solid oxide fuel cell (SOFC) can produce a high density of electrochemical reaction sites. In recent years, electrode architectures utilizing porous oxide substrates with ionic conductivity or mixed ionic-electronic conductivity and connected networks of nickel produced by liquid infiltration have become a popular approach to improve SOFC anode catalytic performance, especially for operating temperatures less than 800 °C. However, infiltrated nickel structures suffer from poor durability, demonstrating significant loss in performance during the first 100 h of use. In contrast, traditional Ni-yttria stabilized zirconia (Ni-YSZ) cermet SOFC anodes exhibit long-term performance stability. However, Ni-YSZ cermet anodes have micron sized structures, and consequently have a significantly lower density of electrochemical reaction site density than infiltrated nickel structures, which have dimensions of around 100 nm. In this study, the performance impact of liquid phase infiltration of nickel nanoparticles into Ni-YSZ cermet anode supported SOFCs is studied by measuring the electrochemical behavior of infiltrated cells at 800 °C, 700 °C, and 600 °C, and comparing them to the performance of an uninfiltrated cell. Durability of the nanoparticles after electrochemical testing is also assessed using a method for quantifying particle statistics from fracture cross sections.

The emergence of nanostructured and composite materials has resulted in significant advancements in energy conversion and storage. The design and development of low-dimensional nanomaterials and composites include photocatalysts for photoelectrochemical devices for solar fuel production; semiconductor nanomaterials for new-generation solar cells, high specific surface area electrodes for efficient energy storage systems including batteries and supercapacitors, and nanocatalysts for increased triple phase boundaries in fuel cells, among others.

Chromium poisoning is one of the major reasons for cathode performance degradation in solid oxide fuel cells (SOFCs). To mitigate the effect of Cr-poisoning, a protective coating on the surface of interconnect for suppressing Cr vaporization is necessary. Among the various coating materials, Cu-Mn spinel coating is considered to be a potential candidate due to their good thermal compatibility, high stability and good electronic conductivity at high temperature. In this study, Crofer 22 H meshes with no protective coating, those with commercial CuMn2O4 spinel coating and the ones with lab-developed CuMn1.8O4 spinel coating were investigated. The lab-developed CuMn1.8O4 spinel coating were deposited on Crofer 22 H mesh by electrophoretic deposition and densified by a reduction and re-oxidation process. With these different Crofer 22 H meshes (bare, CuMn2O4-coated, and CuMn1.8O4-coated), anode-supported SOFCs with Sr-doped LaMnO3-based cathode were electrochemically tested at 800 °C for total durations of up to 288 h. Comparing the mitigating effects of the two types of Cu-Mn spinel coatings on Cr-poisoning, it was found that the performance of the denser lab-developed CuMn1.8O4 spinel coating was distinctly better, showing no degradation in the cell electrochemical performance and significantly less Cr deposition near the cathode/electrolyte interface after the test.

Cr-poisoning of the cathodes due to the presence of metallic interconnects is detrimental to the performance of intermediate temperature solid oxide fuel cell stacks. Applying a protective coating on the interconnect is an effective solution to preventing Cr-poisoning. In this study, the application of a protective CuMn1.8O4 spinel coating is explored. Dense coatings are deposited on both metallic flat plates and meshes by electrophoretic deposition followed by thermal densification steps. The coating is found to be a mixture of Mn3O4 and cubic spinel phases at room temperature but is a pure cubic spinel phase between 750 °C and 850 °C. A reaction layer between the Cr2O3 scale at the coating/interconnect interface and CuMn1.8O4 coating is found to be a mixture of (Cu,Mn,Cr)3-xO4 cubic spinel phases with Cr-rich precipitates believed to be Cr2O3, indicating that the coating layer acts as a Cr getter. Solubility experiments show that 1 mol of the CuMn1.8O4 phase can getter at least 1.83 mol of Cr2O3 at 800 °C. Electrochemical testing of cells in the presence of coated interconnects show that the CuMn1.8O4 coating getters Cr effectively for 12 days at 800 °C, leading to no performance loss of the cell due to Cr-poisoning.

Increasing the density of percolated triple phase boundaries (TPBs) by infiltrating nanoscale electrocatalysts can improve the performance of solid oxide fuel cell (SOFC) anodes. However, the complex microstructure of these infiltrated nanocatalysts creates challenges in quantifying their role in anode performance improvements. In this research, scanning electron microscopy of fractured cross-sections of a Ni-nanocatalyst infiltrated anodic symmetric cell along with three-dimensional (3-D) reconstruction of the same anode have been used to quantify the changes in percolated TPB densities due to infiltration. This change in percolated TPB density has been compared to the improvement in anode activation polarization resistance measured by electrochemical impedance spectroscopy (EIS). It was found that increased TPB densities only partially accounted for the measured performance improvement. Distribution of relaxation times (DRT) analyses showed that a reduction in the time constants of the catalytic processes in the anode also play a role, suggesting that the added nanoscale percolated TPB boundaries are more electrochemically active as compared to the cermet TPB boundaries.

To prevent Cr-poisoning in solid oxide fuel cells (SOFCs) and to decrease area specific resistance by limiting the rate of chromia growth on Crofer 22 APU interconnects, a protective coating layer on the interconnects is necessary. In this study, the glycine nitrate process (GNP) was used to synthesize Cu1.3Mn1.7O4 submicron powders, which then were deposited on Crofer 22 APU by electrophoretic deposition (EPD), followed by post-deposition thermo-mechanical treatments. The phase purity, uniformity, thickness and porosity of the coating layers were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effectiveness of the coating layer to prevent oxygen and chromium diffusion was evaluated by thermogravimetric analysis and energy dispersive X-ray spectroscopy (EDX), respectively. The area specific resistance (ASR) of the coated samples was measured as a function of temperature.

Performance degradation of cathodes caused by chromium deposition and poisoning is one of the major challenges to overcome for long-term operation of solid oxide fuel cells (SOFCs). To fundamentally understand the mechanisms of the degradation phenomenon, it is necessary to investigate the roles of humidity and cathodic current in chromium poisoning. In this study, anode-supported SOFCs, with Sr-doped LaMnO3 (LSM) based cathode are employed. These cells are electrochemically tested at 800 °C with and without chromia-forming interconnect. On identical cells, different cathode atmospheres (dry air or 10% humidified air) and current conditions (no current or 0.75 A/cm2 cathodic current) are imposed. It is found that both humidity and cathodic current promote chromium poisoning. Microstructural characterizations also confirmed that larger amounts of chromium-containing deposits are present at the cathode/electrolyte interfaces of the cell tested with cathodic current and/or humidity. Free energy minimization calculations and thermogravimetric experiments are performed to determine the chromium vapor species that form over chromia-forming alloy interconnect and result in chromium deposition. Based on the experimental and computational results, the roles of humidity and cathodic current in chromium poisoning are evaluated, and a mechanism associated to chromium vapor species deposition at the cathode/electrolyte interface is proposed.

The performance of SOFCs degrades with increasing fuel utilization, since the fuel oxidation reaction faces increased competition for the three-phase-boundary (TPB) sites in the anode from adsorbed water vapor species. Introduction of addition TPB sites by infiltration of Ni nanoparticles into the anode is an effective way to alleviate this problem. Both liquid and vapor phase infiltration of Ni nanoparticles into commercially available Ni/YSZ cermet anodes have been explored. The microstructure of the anode has been examined by µ-CT and FIB/SEM 3-D reconstructions. The cells have been electrochemically characterized by I-V and EIS techniques in a temperature range of 600°C-800°C. The infiltrated Ni nanoparticles have been characterized by image analysis of SEM micrographs from fracture cross-sections, before and after electrochemical testing. The effect of the nanoparticles on cell electrochemical performance as a function of temperature and anode input water vapor content, the stability of the nanoparticles, as well as the thermodynamics of vapor phase infiltration of Ni will be presented.

Infiltrating nanoparticle electrocatalysts into solid oxide fuel cells (SOFCs) has been shown to improve their intermediate-temperature performance by increasing the density of reaction sites, known as triple phase boundaries (TPBs), in the electrode. By increasing TPB density with infiltration, anode activation polarization is mitigated and cell performance improves relative to an uninfiltrated cell, especially at low temperatures. In this study, electrochemical impedance spectroscopy and distribution of relaxation times analyses were performed on baseline and Ni-infiltrated NiYSZ anode symmetric cells. A transmission line model (TLM) was used to deconvolute the electrode processes. The effects of temperature and humidity on TLM parameters, and the relative polarization contributions of various charge transfer processes will be discussed.

Chromium poisoning of the cathode remains a significant obstacle to the long-term performance of solid oxide fuel cells. Previously, a quick, easy method was introduced that requires no material modifications, major thermal cycling, nor exposes the cell to any species it does not already encounter during operation. The method, electrochemical cleaning, reverses the electrochemical deposition reaction of chromium-containing species by applying a mild anodic bias. Chromium vapor species are formed, freeing active sites in the cathode. The method has been shown to completely recover cell performance via this current-voltage technique. The present work investigates the mechanism of electrochemical cleaning by assessing the effect of various operating parameters on the rate of chromium removal. Using a design of experiments method, eight cells were poisoned and then cleaned using differing combinations of the following cleaning parameters: cell temperature, air humidity, anodic current density, and fuel humidity. Regression analysis of two comparative metrics, cell performance via current voltage curves and post-test chromium content determined via Energy Dispersive Spectroscopy, demonstrated that cell temperature has the highest effect on the rate of chromium removal, followed by air humidity. A refined parameter space for future study is proposed for further optimization of the parameters. Previously, the electrochemical cleaning method was shown to be effective in the removal of Cr2O3 deposited species, but not Cr-Mn spinel deposits. The effects of operating parameters on the removal of the two different types of Cr deposits is discussed. An additional process for the removal of Cr-Mn spinel is demonstrated, which again makes use of the cell operating parameters. Together, the two methods present a facile, periodic process to greatly extend solid oxide fuel cell lifetime.

To improve the intermediate-temperature performance of solid oxide fuel cells (SOFCs) and expand their applications requires mitigating anode activation polarization while maintaining long-term stability during use. The infiltration of nanoscale electrocatalysts into SOFC anodes has been shown to increase the number of reaction sites, thereby decreasing activation polarization. This study explores the impacts of liquid-phase infiltration of Ni and gadolinium-doped ceria (GDC), a Mixed Ionic and Electronic Conductor (MIEC), into both Ni-YSZ and Ni-MIEC anodes, where the YSZ has been doped with titanium dioxide to facilitate electronic conduction. Distribution of relaxation times (DRT) analysis is performed on Electrochemical Impedance Spectroscopy (EIS) spectra to deconvolute the kinetics of various charge transfer processes. Figure 1

In this study, a strontium doped lanthanum cobalt ferrite thin film with 30% Sr on A-site, denoted as La 0.7 Sr 0.3 Co 0.2 Fe 0.8 O 3-δ or LSCF-7328, is investigated before and after annealing at 800 °C under CO 2 containing atmosphere for 9 h. The formation of secondary phases on surface of post-annealed LSCF-7328 is observed using atomic force microscope (AFM) and scanning electron microscope (SEM). The extent of Sr segregation at the film surface is monitored using the synchrotron-based total reflection X-ray fluorescence (TXRF) technique. The bonding environment of the secondary phases formed on the surface is investigated by synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Scanning transmission electron microscopy (STEM) and related spectroscopy techniques are used for microstructural and quantitative elemental analyses of the secondary phases on surface. These studies reveal that the secondary phases on surface consist of SrO covered with a capping layer of SrCO 3 . The formation of Co-rich phases is observed on the surface of post-annealed LSCF-7328.

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1–x SrxCo0.2Fe0.8O3−δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO3, SrTiO3, and GdScO3 substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on t...

The search for low-loss and robust optical fibers in the infrared spectrum has always been an important research topic for many investigators. Over the years, fibers of various materials groups have been proposed to obtain ‘the ultimate infrared fiber’. Recently, a new competitive alternative has emerged: the semiconductor-core glass-cladded optical fibers. The excellent bulk materials properties and integrated circuit applications reveals the potential of semiconductors as fiber materials. One of the important photonic materials that has been proposed as a fiber-core material is germanium. In this paper, the development of Ge-based fibers and their optical properties in the mid-infrared spectrum are discussed. The performance of Ge-based fibers has been compared with other semiconductor-core fibers. Recent developments in the area of semiconductor fibers and the future prospects of semiconductors as infrared fiber materials are also discussed.

Electrochemical cleaning, a recently proposed mitigation strategy for chromium poisoning in solid oxide fuel cell (SOFC) cathodes, involves rapid in-situ removal of Cr2O3 deposits from LSM-YSZ cathodes accompanied by a recovery of a large fraction of the cell performance originally lost due to Cr poisoning. By operating the cell briefly as a solid oxide electrolyzer cell (SOEC), the cleaning method effectively reverses the Cr deposition reactions, reforming Cr-containing vapor species, thereby freeing up electrochemically active sites and restoring cell performance. In practice, this method can be periodically applied to the system after a specified amount of degradation due to chromium poisoning has occurred. The current study investigates the efficacy of this method by cycling a single cell through a stage of accelerated poisoning followed by electrochemical cleaning for a total of three times. Current-voltage measurements demonstrate repeated loss in performance due to Cr poisoning and recovery in performance due to electrochemical cleaning, reinforcing the utility of this cleaning method over the lifetime of the cell operation.

Polarization modeling based on the measured I-V curves is a useful tool for analyzing SOFC performance and diagnosing performance limitations. Using the polarization model, the overpotential of a cell is usually separated into different polarization contributions, including area specific ohmic polarization, activation polarization, and anodic and cathodic concentration polarizations. In this study, we aim to experimentally validate the accuracy of the polarization model. As a case study, anode-supported cells with LSM-YSZ/YSZ/Ni-YSZ configuration are used. By curve-fitting the I-V curves of a cell tested under different cathode oxygen partial pressures into the polarization model, the contributions of different polarization losses are quantified. To validate the polarization model, independent experiments/studies including electrochemical impedance spectroscopy, cathode kinetics study, ex-situ diffusivity measurement, and tortuosity measurement using 3D reconstructed anode are conducted. The results from polarization modeling and from validation studies are in good agreement, thereby validating the polarization model for SOFC performance analysis.

Degradation mechanisms of chromium poisoning have been extensively investigated for last two decades on various cathode materials for solid oxide fuel cells. While most researchers used half-cell experiments to investigate the effects of chromium impurities, this paper compares the degradation phenomena of LSM-YSZ and LSF-GDC cathodes in full cells under different operating conditions, namely polarized current density and humidity over the cathode. Under cathodic load, performance degradation of LSF-GDC cathode is significantly less than that of LSM-YSZ cathode. Most interestingly, under humidified air condition, LSF-based cell hardly experiences performance degradation while LSM-based cell shows a catastrophic degradation. Cr deposition in LSM-based cells occurs mainly near the electrolyte under both dry and humidified air conditions. In contrast, Cr deposition in LSF-based cell concentrates at the cathode surface under dry air condition, but concentrates both at cathode surface and near electrolyte under humidified air condition. Microstructure analysis reveals Mn-associated Cr deposits in LSM-based cathode, and Sr- and Fe-associated Cr deposits in LSF-based cathode. With the help of distribution of relaxation time analysis, the different electrochemical behaviors of LSM- and LSF-based cells are carefully interpreted. The effects of electrochemical deposition, Sr surface segregation, and humidity on LSM- and LSF-based cathodes are discussed.

Chromium poisoning is one of the primary factors that affect solid oxide fuel cell (SOFC) durability. In this study, an electrochemical cleaning method has been employed to reverse the effects of chromium poisoning. Two cells were fabricated and poisoned under identical conditions by operating them at 800 °C in the presence of a Cr source. Both poisoned cells exhibited a significant reduction in maximum power density and an increase in polarization resistance. One of the poisoned cells, designated as the baseline cell, was cooled to room temperature after poisoning. Microstructural observation of the baseline cell showed significant contamination of the cathode active layer by chromium oxide and (Cr,Mn) spinel deposits. The other poisoned cell was electrochemically cleaned for 2 h under a mild electrolytic condition. As a result of the electrochemical cleaning process, the chromium oxide deposits were substantially removed leading to a significant reduction of Cr concentration in the cathode, especially at the cathode/electrolyte interface. A substantial fraction of the cell performance loss was also reversed. To the best of our knowledge, this is a first demonstration of reversing the effects of chromium poisoning using a rapid in-situ process.

Cr2O3 scales formed on ferritic steel interconnects in intermediate temperature solid oxide fuel cell (SOFC) stacks have the problem of Cr-poisoning that can deteriorate cell performance. Applying a protective coating on the surface of the interconnects is an effective strategy to solve this problem. (CuMn)3O4 spinel is a potential candidate-coating material. In this study, the performance of the CuMn1.8O4 at 750 °C, the conductivities of the reaction layer formed between the coating and Cr2O3 scale, and the reduction of Cr ion diffusivity in Ni-doped (CuMn)3O4 were explored. CuMn1.8O4 coatings were found to be good Cr diffusion barriers at 750 °C, with the coatings also acting as an excellent Cr getter. The conductivities of the reaction layer were found to be at least two orders of magnitude higher than that of Cr2O3, indicating the formation of the reaction layer is favorable compared to the thickening of the Cr2O3 scale at the coating/alloy interface. Ni-doped of (CuMn)3O4 decreased the diffusivity of Cr, making (CuMnNi)3O4 a promising candidate-coating material.

We present a systematic synchrotron radiation photoemission spectroscopy study of the interaction of O2 with defective La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surfaces at low temperatures. First, the surface chemical evolution during low energy Ar+ ion sputtering is investigated and is found to create oxygen vacancies. Subsequently, the interaction of the O2 molecules with La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surface is observed to create an insulating phase. We performed a detailed core-level peak-fitting analysis to identify the chemical nature of this phase and to probe the role of vacancies in the formation of this phase.

The ultra-high vacuum surface preparation of heteroepitaxial lanthanum strontium cobalt ferrite thin films has been studied using soft X-ray photoelectron spectroscopy. Specifically, the effect of annealing the films at low temperatures in low partial pressures of oxygen and argon has been investigated. We find that atmospheric surface carbon contamination of the films can be removed in select anneal temperature regimes in argon, but remains bound to the surface with oxygen annealing at any temperature. Irrespective of the gas used, an insulating phase transition occurs near 300 °C due to strontium segregation at the surface. The surface develops more insulating character if annealed with oxygen. Different species are proposed to be responsible for the discrepancy in insulating character.

A basic zirconia oxygen sensor was utilized to monitor the gas–solid oxidation reaction characteristics of metal/metal-oxide powder beds. The method described allowed for simple determination of chemical reaction rate constants for the oxidation reactions. The signal output of the sensor was analyzed for oxidation of the powder bed and diffusion of oxygen into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the powder bed, and a discussion of relevant thermodynamic and kinetic theory was provided. Two metal/metal-oxide systems were examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility, and the complex W/WO3 system was chosen to demonstrate versatility of the sensor. Combining the experimental data with relevant kinetic theory, chemical reaction rate constants were calculated from plots of sensor ocv versus time for each reaction step during the metal oxidation sequences.

SiC/SiC ceramic matrix composites (CMCs) are being used increasingly in the hot-sections of gas turbines, especially for aerospace applications. These CMCs are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to molten alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al 2 O 3 •2SiO 2 ) deposited by chemical vapor deposition (CVD), and the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The stability of this system is investigated, which includes the adhesion between the two coating layers and the substrate, the physical and chemical stability of each layer at high temperature, and the performance under severe thermal shock and during exposure to hot corrosion. The effect of vertical cracks in the TBC on the EBC layer below it is also examined.

The reduction kinetics of tungsten trioxide (WO 3 ) powder using hydrogen gas was studied with titanium dioxide (TiO 2 ) or 8 mol% yttria-stabilized zirconia (8YSZ) mixed into the powder bed. TiO 2 was selected as an inert volume filler, and 8YSZ was selected to increase the ionic conductivity of the powder bed. The reduction sequence of tungsten trioxide in hydrogen is known to take place in the following steps: WO 3 → WO 2.9 → WO 2.72 → WO 2 → W. When the electronic and ionic conductivities of the mixed powder bed were both sufficiently high, reduction rates were found to increase. Specifically, reaction kinetics improved with the addition of 8YSZ to the WO 3 when compared to experiments in which TiO 2 was added to the WO 3 powder bed. Chemical reaction rates were determined by first recording changes in mass as a function of time during reduction using thermogravimetry, and then reaction rates were calculated using relevant kinetic theory.

Mid-infrared (IR) fibers have been extensively investigated due to their applicability in chemical sensing and remote laser delivery, among others. Materials such as chalcogenides and fluoride glasses transmit mid-IR wavelengths with low practical losses. However, their low glass transition temperatures make them chemically unstable, even at room temperatures, resulting in performance degradation over time. Semiconductors, such as germanium, have a wide transmission window in the mid-IR region, and offer significantly improved chemical stability. In this research, germanium-core, borosilicate-cladded fibers were drawn by a ‘rod in tube’ method using a mini draw tower assembled in-house at 1000°C, which is significantly lower than the drawing temperatures of 2000-2200°C for conventional silica fibers. Typical drawn fibers had a 40 μm core diameter and 177 μm cladding diameter. Transmission electron microscopy (TEM) studies showed that diffusion of oxygen and silicon from the cladding to the core during the drawing process was minimal, with diffusion distances of the order of 10s of nm. This is encouraging for mid-IR transmission, since the presence of oxygen in the fiber core is known to increase transmission losses in the mid-IR spectrum. This low diffusivity is presumably due to the relatively low drawing temperature. Transmission losses through the fibers were measured with a quantum cascade laser (QCL) and the losses were found to be in the 3-9 dB/cm range in the spectral range of 5.75-6.3 μm.

Mitigating activation polarization in the anode is one of the major challenges in intermediate-temperature operation of solid oxide fuel cells (SOFCs). Liquid phase infiltration of nanoscale electrocatalysts has been shown to result in significant reductions in activation polarization in SOFC anodes. In this study, we explore liquid-phase infiltration of nickel, gadolinium doped ceria (GDC), and Ni/GDC electrocatalysts into two different types of cermet anodes: one with a conventional Ni-YSZ composition, and the other with a Ni-MIEC cermet anode where the YSZ has been doped with 3 mol% TiO2 to impart electronic conductivity. The principal goal of this study is to explore the role of electronic transport in the MIEC phase in effective utilization of the infiltrated nanoscale electrocatalysts. The role of temperature, infiltration cycles and the type of electrocatalysts have been experimentally studied in symmetric cells using electrochemical impedance spectroscopy (EIS). Distribution of relaxation times (DRT) modeling has been used to elucidate the contributions of various charge transfer processes.

The infiltration of nanoparticle electrocatalysts into solid oxide fuel cell (SOFC) electrodes has been proven to produce a high density of electrochemically active sites, and reduce charge transfer polarization losses for SOFC electrodes. This is crucial for intermediate temperature operation, as these losses increase greatly at lower temperatures. Nickel-yttria stabilized zirconia (Ni-YSZ) cermets are low-cost, and exhibit excellent stability, but their main disadvantage stems from nickel coarsening and performance loss over their operational lifetimes. Infiltration of electrocatalyst nanoparticles has been shown to mitigate nickel coarsening and the consequent anode degradation. In this study, the effects of these infiltrants are observed in a standard Ni-YSZ electrode. In addition to nickel, mixed ionic and electronic conducting (MIEC) phases were infiltrated into Ni-YSZ scaffolds and their performances were characterized using electrochemical impedance spectroscopy (EIS). Cross-sectional microscopy of fractured cells was used to compare electrode microstructure and particle statistics. A model is proposed for how the nanoparticle electrocatalysts improve the anode performance.

Infiltration of nickel nanoparticles into nickel – yttria-stabilized-zirconia (Ni-YSZ) cermet anodes improves anode performance by reducing the average nickel feature size of the electrode, thus increasing the density of triple phase boundaries (TPBs). However, the TPBs of nickel nanoparticles are not fully utilized because there is no conductive pathway between nanoparticles, and nickel nanoparticles suffer from rapid coarsening during operation. This work explores the infiltration of Gd0.1Ce0.9O2-δ (GDC) as a material for connecting and stabilizing infiltrated nickel nanoparticles. Anodes were infiltrated with Ni, GDC, and Ni-GDC to study their stability and electrochemical performance. Measurements show that the simultaneous infiltration of nickel and GDC results in a greater performance improvement than either nickel or GDC alone. From this result, it is speculated that GDC provides a conducting pathway between nickel nanoparticles, better utilizing their TPBs. Fitting of electrochemical impedance spectra with an equivalent circuit model is used to measure individual cell resistances, revealing that infiltration of GDC reduces the activation energy of the anodic charge transfer reaction from 1.29 eV to 0.74 eV. The Ni-GDC nanoparticles are also more durable than nickel nanoparticles, demonstrating microstructural stability after 120 h of constant current at 800 °C, while nickel alone shows extensive coarsening.

Solar grade silicon can be formed using a YSZ solid oxide membrane (SOM). The SOM membrane is exposed to a complex fluoride flux with dissolved silica at high temperature and electrochemically separated into silicon and oxygen. A failure mode of the SOM membrane by the formation of ‘inner cracks’ was studied, and attributed to yttria depletion in the YSZ, leading to phase transformation from cubic to tetragonal phase. The roles of silica and YF 3 in the flux were studied, and it was shown that silica attacks the SOM membrane, while YF 3 retards the attack. A detailed mechanism of the yttrium depleted layer (YDL) formation, and its role in the formation of inner cracks is proposed. Based on this study, a new flux composition was designed and tested. The flux composition did not attack the SOM membrane, and Si crystals were produced, demonstrated long-term viability of the Si–SOM process.

a b s t r a c t Formation of oxygen vacancies by introducing various mixed-valent cation dopants is a common practice to improve the cathode performance in solid oxide fuel cells. A computational procedure is developed in this work to predict the equilibrium oxygen vacancy non-stoichiometric concentrations at experimen- tally relevant temperatures and oxygen partial pressures for both bulk and surface oxide phases. The calculations are based on the first-principles density functional theory and a constrained free-energy functional. Quantitative agreements are found by direct comparisons to the thermogravimetry and solid electrolyte coulometry measurements for the strontium-doped lanthanum cobalt iron oxides at different compositions. Our results indicate that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and all oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures.