Dense YSZ Electrolyte Research Articles

Analysis of Structural Degradation Effects in a Solid Oxide Fuel Cell after Long-Term Operation

The sustainable production of hydrogen as well as its re-conversion into electricity will be a key aspect in a future green energy environment that is depending on regenerative resources. In order to ensure the eminent position of solid oxide cells (SOC) in this future hydrogen economy their reliability and long-term performance still needs to be improved. Having this in mind, we performed detailed investigations on samples from cells that were operated for 100,000h to get a profound understanding of structural degradation effects after long-term operation.The investigated samples originate from a two layer stack of fuel electrode supported solid oxide cells that were operated at 700°C and 0.5 A/cm2 in fuel cell mode. The cells consist of a Ni/Y-stabilized ZrO2 (YSZ) fuel electrode, a dense YSZ electrolyte layer, a Gd doped CeO2 (GDC) barrier layer and a La0.58Sr0.4Co0.2Fe0.8O3-d (LSCF) air electrode. Furthermore, an additional contact layer of La0.97Mn0.4Co0.3Cu0.3O3-d was applied on the air electrode surface and, in order to inhibit Cr-poisoning the interconnector material was coated with a manganese oxide layer by wet powder spraying. The stack represents the usual material status from 2009 when the stack operation has been starting.In the post-test analysis the microstructure and chemical composition was investigated by scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDS). The EDS analysis revealed localized, correlated enrichments of Sr and Cr in the bulk of the LSCF air electrode as well as at the LSCF / GDC interface. Moreover, non-correlated, localized Co enrichments could also be detected at the LSCF / GDC interface. In order to get an insight into localized structural changes and to identify impurity phases after long-term operation Raman spectroscopy measurements were performed. Hereby, it was for example possible to visualize, in accordance with the EDS analysis, the formation of a SrCrO4 impurity phase on the LSCF air electrode surface as well as inside the bulk of the LSCF by running Raman measurements on the air electrode surface as well as on embedded and polished cross-section samples. To study the structural degradation in detail with high spatial resolution, additional µ-beam Laue diffraction measurements were performed at the CRG-IF BM32 beamline at European Synchrotron Radiation Facility (ESRF).

Fabrication and optimization of LSM infiltrated cathode electrode for anode supported microtubular solid oxide fuel cells

In this study, anode supported microtubular solid oxide fuel cells (SOFCs) with LSM (lanthanum strontium manganite) catalyst infiltrated LSM-YSZ (yttria stabilized zirconia) cathodes are developed to increase the density of triple/three phase boundaries (TPBs) in the cathode, thereby to improve the cell performance. For this purpose, two different porous YSZ layers are formed on the dense YSZ electrolyte, i.e., one is with co-sintering while the other one is not. Incorporation of LSM into these porous YSZ layers is achieved via dip coating of a sol-gel based infiltration solution. The effects of the fabrication method for porous YSZ, LSM solution dwelling time and the thickness of the porous YSZ layer on the cell performance are experimentally investigated and optimized in the given order. A reference cell having a conventional dip coated cathode prepared by mixing the commercial LSM and YSZ powders is also fabricated for comparison. The results show that among the cases considered, the highest peak power density of 0.828 W/cm2 can be obtained from the cell, whose single dip coated porous electrolyte layer co-sintered with the dense electrolyte is impregnated with LSM for a dwelling time of 45 min. On the other hand, the peak power density of the reference cell is measured as only 0.558 W/cm2. These results reveal that ∼50% increase in the maximum cell performance compared to that of the reference cell can be achieved by LSM infiltration after the optimizations.

Detailed Mechanistic Studies of Electrochemical Reactions on Pt and Au Electrodes in Solid Oxide Cells Via EIS Data Analysis

In order to reduce the environmental impact of fossil fuels, a range of sustainable technologies for power generation and storage are currently being developed. In addition, efficient and low-cost methods for CO2 capture and conversion to form useful products are critically needed. One of the promising technologies towards achieving these goals involves the use of solid oxide cells (SOC), which are unique devices that can be employed in both the fuel cell mode for clean power generation and in the electrolysis mode to achieve CO2 conversion through its electrochemical reduction (‘CO2RR’).One of the burning problems that is impeding SOC implementation is catalyst selection, with many promising materials having been reported over the last 50 years, including metals and mixed ionic and electronic conducting (MIEC) metal oxide materials. However, the reasons for better or worse activity, as well as the mechanism of the CO2RR at these catalysts remains unknown and it is often unclear what factors limit the catalyst activity during cell operation and how this can be improved. This is due, in part, to the limited number of techniques that can be used to evaluate these performance metrics, especially at high operating temperatures. One of the most useful techniques is electrochemical impedance spectroscopy (EIS). However, data interpretation is often quite complex and thus other methods, such as CNLS fitting or the determination of the distribution of relaxation times (DRT), are needed.While CNLS fitting allows the fit parameters to be correlated with specific physical properties, a precise model of the system is required. Since SOC systems are relatively unexplored and good models are not yet available, this could lead to the mis-interpretation of the EIS data. DRT, on the other hand, can be a useful tool since it does not require any pre-existing models of the system. Unlike CNLS fitting, DRT can provide the correct number of time constants present (from the number of peaks obtained), as well as the associated resistance and capacitance values. According to the literature, each peak is typically related to a specific reaction step with a unique capacitance value, identified by changing conditions such as temperature, polarization or gas composition.In the present work, DRT analysis of Pt and Au electrodes on YSZ electrolytes was carried out in order to determine the origin of each resistance and to determine the effect of temperature, electrode polarization and gas composition on both the resistance and capacitance values. Here, both porous and point metal electrodes were investigated as no double phase boundary activity or chemical capacitance effects should be present, making data interpretation less complex. Furthermore, to avoid the challenges of porous electrodes, point electrodes, produced by pressing the metal wire to a flat YSZ surface by applying pressure with a spring, followed by softening at 1100 °C to achieve better contact, were used to achieve a controllable triple phase boundary (TPB) length. This allowed the true activity of the metal/YSZ TPB to be measured as a function of reactive interface length, rather than just per geometric surface area. Porous electrodes were also studied, made by depositing Pt or Au paste onto YSZ, followed by sintering at 600 °C for 20 min. Polymeric YSZ precursors were then infiltrated into the metallic backbone, then sintered at 750 °C for 2 hours. Then, a thin layer of the same metal paste was applied in order to provide better conductivity. The point electrodes were made by attaching 0.3 mm thick Pt and Au wires onto dense and flat YSZ electrolyte by a spring-loaded cell holder, then held at 1050 °C for 1 hour to soften and produce a fixed TPB length. Electrochemical testing of the porous electrodes was conducted at 750 °C in both half- full cell modes in air, CO2 and CO2/CO mixtures, while the wires were tested at 650-750°C in the same gases, but in half-cell mode.8 individual process were seen for both the porous and point electrodes, including both electrodes in each cell. The time constant with the smaller capacitors are likely related to the electrode-electrolyte interface, while the mid-range capacitors are associated with electrode surface processes, such as surface diffusion, molecular species dissociation, and adsorption. The high capacitance steps are likely related to gas-phase processes, such as gas diffusion and the possible presence of a conversion layer. More detailed analysis of polarization and gas atmosphere dependence is ongoing in order to confirm the nature of each process.

Time-Resolved Characterization of Electrochemically-Induced Solid Oxide Cell Microstructure Evolution

Solid oxide cells (SOC) are exciting both for their capability of fuel-flexible electricity generation and their promise in the production of green H2; however, their degradation and long-term performance remain important questions. Operating individual button cells to characterize these changes is prone to cell-to-cell variations, whereas large-area cell/stack life testing requires considerable resources. In both cases, full cell characterization usually must wait until the completion of the life test, which may delay getting results for months or years. While progress has been made in observing microstructural changes over time (termed here time-resolved characterization) using techniques like non-destructive transmission x-ray microscope tomography, such experiments are difficult to implement, especially the effect of current is unexplored due to geometrical and gas environmental limitations of the system.Presented here is a method to rapidly obtain time-resolved microstructural information of the same cell in tandem with electrochemical measurements. While SOC degradation arises due to a range of processes, this work focuses on the microstructural evolution of Ni-YSZ fuel electrodes that occurs during electrolysis operation, which is known to be a key issue. Two key experimental features allowed time-resolved characterization of multiple cells simultaneously: (1) the use of Ni-YSZ symmetric cells allows multiple cells of different size in a single test by avoiding the need for gas seals, and (2) the use of laser milling to allow controlled removal of a portion of each cell during the test.Here we illustrate the method for the case of low steam content and high current density, designed to exacerbate and accelerate electrolysis degradation due to the highly reducing conditions achieved. Tape-cast planar electrode-supported Ni-YSZ symmetric cells were laser cut into precise geometries with well-defined projected areas and easily removable sections. Four cells with projected areas of 1, 1, 0.75, and 0.5 cm2 were connected in series within the same furnace at 800 C and a gas environment of 97% H2 and 3% H2O. A current of 0.75 A was run through three of the cells, resulting in current densities of 0.75, 1.0, and 1.5 A/cm2, while one was maintained with no current. The life test lasted for 500 h, and the microstructure was observed at 0, 200, and 500 hours.Figure 1 shows the microstructures observed at the highest current density by polished cross-sectional SEM and FIB-polished sections under conditions that provided Ni/YSZ contrast. Microstructural degradation had occurred by 200 h and became immense at 500 h, due to high current density in these reducing conditions. By 200 hours, grain boundaries become enriched with Ni, likely due to the formation of Ni-Zr intermetallics in the ultra-low pO2 induced by local overpotential as reported by Chen[1] and Szasz[2]. Additionally, the electrode-electrolyte interface becomes nanoporous and a Ni enriched fracture is present ~2 μm into the electrolyte. By 500 hours, the Ni rich nanoporosity has progressed more than 10 μm into the electrolyte and created islands of large grains surrounded by Ni-rich deposits.Electrochemical impedance spectroscopy measurements show a clear initial decrease in polarization resistance followed by an overall impedance increase by 200 hours. These correspond to the initial electrochemical boost from the production of a nanoporous structure, followed by the deactivation of areas of the cell due to the large fractures through the electrolyte.[1] M. Chen et al., “Microstructural Degradation of Ni/YSZ Electrodes in Solid Oxide Electrolysis Cells under High Current,” J. Electrochem. Soc., vol. 160, no. 8, pp. F883–F891, 2013, doi: 10.1149/2.098308jes.[2] J. Szász et al., “High-Resolution Studies on Nanoscaled Ni/YSZ Anodes,” Chem. Mater., vol. 29, no. 12, pp. 5113–5123, 2017, doi: 10.1021/acs.chemmater.7b00360.Figure 1: Microstructural and electrochemical changes of a symmetric Ni-YSZ SOC undergoing 1.5 A/cm2 in reducing conditions at 0, 200, and 500 hours. Cathode is on the left in all images; anode is on the right. a, b, c) Backscatter electron imaging reveals destruction of the dense YSZ electrolyte over the lifetime of the experiment. Lines of brighter phases on the anode side of fractures are Ni deposits. d, e, f) Focused Ion Beam cross sections reveal nanoporosity forming in the first 10 μm from the cathode-electrolyte interface at 200 and 500 hours. Charging is evident as a result of curtaining from the nanoporous features in the electrolyte. g) Electrochemical impedance spectroscopy shows the initial decrease in resistance from 0 to 48 hours followed by increasing resistance due to microstructure evolution. Figure 1

Ni migration in solid oxide cell electrodes: Review and revised hypothesis

AbstractSevere degradation of Ni‐YSZ (yttria stabilized zirconia) electrodes of solid oxide cells (SOCs) due to Ni migration is well known, but the literature contains apparent contradictions. The mechanisms are still under debate. Fine structured Ni‐YSZ composite electrodes often degrade at operation temperature (700–950°C), because Ni particles lose electrical contact with each other as larger Ni‐particles grow on the expense of smaller ones. Another type of Ni migration, which may be very damaging, is the relocation of Ni in the most active part of the Ni‐YSZ cermet electrode next to the dense YSZ electrolyte. Emphasis is put on the migration of Ni away from the YSZ electrolyte in solid oxide electrolysis cells (SOECs). This is seen as an important obstacle to the commercialization of SOC systems.Apart from temperature, degradation of Ni‐YSZ electrodes in SOCs is related to Ni‐YSZ electrode overpotential and the local redox potential of the gas mixture inside the porous Ni‐YSZ electrode. A unifying Ni migration mechanism is proposed, and methods of alleviating the electrode degradation are discussed. The hypothesis is that Ni migrates via surface diffusion of Ni(OH)x species below ca. 800°C and via Ni(OH)x species in gas phase above ca. 900°C.

Comparative Study of the Performance of Syngas-Fueled Carbon-Air Solid Oxide Fuel Cells Using Anodes Based on M-Doped CeO2 (M: Mo or Mo-Cu)

The use of syngas to feed solid oxide fuel cells (SOFCs) appears as an attractive alternative due to the possibility of being obtained from renewable sources such as biomass or from other solids of carbonaceous composition such as solid wastes through reforming reactions. The use of syngas in SOFCs has been studied in several previous works, directing the efforts to obtain an anode material with the appropriate electrocatalytic activity for the electrooxidation of syngas, able to tolerate carbon deposition and avoid sulfur poisoning [1]. The Ni-YSZ anode has been studied in the electrooxidation of syngas, being one of its advantages the catalysis of the water gas shift (WGS) reaction, generating more H2 with respect to CO and therefore improving the reaction kinetics [2]. One of the disadvantages presented by Ni-YSZ is the decrease of its performance in long term tests, due to the catalytic activity of Ni towards the reverse Boudouard reaction, so that carbon deposits block the triple phase boundaries (TPBs) of the anode [3]. CeO2-based materials are suitable for use as anodes in fuel cells fed with carbonaceous fuels due to their resistance to carbon deposition and sulfur poisoning [4, 5]. Recently it has been reported that the addition of molybdenum to CeO2 (Mo-doped CeO2 or CMO) confers catalytic activity to the carbon gasification reaction and an increase in the electrical conductivity of CeO2 [6]. Furthermore, it has been reported that the addition of copper nanoparticles to Mo-doped CeO2 increases about 10 times its electrical conductivity [7].Two nanomaterials for SOFC anodes were synthesized: Mo-doped CeO2 (CMO) and Cu,Mo-doped CeO2 (CMCuO). Both nanomaterials were synthesized by the combustion method with 10 wt.% total metal loading following the experimental procedures detailed in [6] and [7] correspondingly. The SOFCs were fabricated using a CMO-YSZ or CMCuO-YSZ composite as anode, a dense YSZ electrolyte and a LSM-YSZ cathode. Both SOFCs were fed with air at the cathode and humidified syngas at the anode (syngas + 3 vol.% H2O). The morphology of the SOFCs was studied by SEM, while the crystal structure and chemical stability of the materials were studied by XRD and EDS. The cells were characterized by EIS at OCV condition at different temperatures and syngas concentrations, while the cell performance was studied by linear sweep voltammetry.An ADIS analysis obtained from the EIS data for both fuel cells in Figure 1 clearly shows the presence of two phenomena at high frequencies related to the electrooxidation reactions of H2 and CO, these phenomena are sensitive to temperature changes, the differences in their charge transfer resistance values suggest that both reactions are carried out with different kinetics. The processes shown at low frequencies are related to diffusional limitations of each gas towards triple phase boundaries (TPBs), processes sensitive to changes in syngas concentration. Cell tests, operating at 800°C, showed a maximum electrical power density of 114 mW cm-2 at a current density of 208 mA cm-2 and a cell voltage of 549 mV for SOFC with CMO-YSZ anode and a maximum electrical power density of 120 mW cm-2 at a current density of 200 mA cm-2 and a cell voltage of 600 mV for SOFC with CMCuO-YSZ. The addition of copper to the CMO produces a slight improvement in the electrocatalytic activity of the material and reduced ohmic resistance by almost 50% (see Figure 2), however the ASR of the cell with CMCuO anodes does not show a substantial improvement with respect to the cell with CMO anodes. In addition, the ohmic resistance is the most important component in the ASR of both cells. SEM images of the cells after tests show that the CMCuO anode surface exhibits fractures and delamination of the material, which is not observed on the CMO anode surface. The X-ray diffraction spectra do not show the presence of new phases in any of the two anode materials so the delamination of the CMCuO anode is related to the differences in thermal expansion coefficients (TECs) of the materials whose value must be measured to prove this hypothesis.[1] T.M. Gür (2013) Chem. Rev., 113(8), 6179-6206.[2] O. Costa Nunez et al. (2005) J. Power Sources, 141(2), 241-249.[3] T.M. Gür et al. (2010) J. Power Sources, 195(4), 1085-1090.[4] I. Sreedhar et al. (2019), J. Electroanal. Chem., 848, 113315.[5] N. Mahato et al. (2015) Prog. Mater. Sci., 72, 141-337.[6] I. Díaz‐Aburto et al. (2019) Fuel Cells, 19(2), 147-159.[7] I. Díaz-Aburto et al. (2021), J. Electrochem. Sci. Technol., [epub ahead of print: https://doi.org/10.33961/jecst.2020.01571]. Figure 1

Multiphysical modelling of planar solid oxide fuel cell stack layers

Anode supported (ASC) and electrolyte supported cells (ESC) represent the most common cell concepts in solid oxide fuel cell (SOFC) technology. In ASCs, mechanical manageability is provided by a porous nickel/yttria-stabilized zirconia (Ni/YSZ) substrate, whereas in ESCs a self-supporting dense YSZ electrolyte is applied. Naturally, the electrical loss contributions arising in ASCs and ESCs differ in quantity, leading to different temperature profiles within planar SOFC stacks.A two-dimensional, finite element method model was developed which considers the underlying chemical and physical processes, and calculates both the electrical performance and the thermal distribution of planar SOFC stack layers operated with reformate fuels. It was then validated by comparing simulation results with extensively measured (i) temperature profiles in SOFC stacks, (ii) gas composition changes along the fuel gas channel of planar ASCs, and (iii) current-voltage characteristics in a temperature range from 650 °C to 800 °C.The subsequent numerical study reveals (i) the different performances of ASC and ESC, (ii) the impact of operation conditions on performance and temperature profile and (iii) how the individual loss contributions generate temperature distributions in the stack layer.

Morphological of Yttrium Stabilized Zirconia (YSZ) Thin Film of Electrolyte in Solid Oxide Fuel Cell Application

Recently, the performance of solid oxide fuel cell is focused on lowering operating temperature (400°C-650°C). To achieve the goal, the thickness of commonly used electrolyte, YSZ was reduced. The dip-coating technique was used for preparing dense YSZ electrolyte thin films on glass substrate. The suspensions were prepared by sol-gel method. Polyvinyl alcohol (PVA) and polyethylene glycol (PEG) was used as a ceramic binder and plasticizer, respectively. Crystalline structure and morphology of thin films were analyze by X-ray diffraction (XRD) and Atomic Force Microscope (AFM). The XRD reveals that crystallization of YSZ phase does not occur at any sintering temperature but can only have a small peak at 2θ = 30° as the thickness of layer increase. The roughness and morphology of the film with different thickness were observed. The roughness increased as the thickness increased.

Densification of an YSZ electrolyte layer prepared by chemical/electrochemical vapor deposition for metal-supported solid oxide fuel cells

Abstract A densification process based on chemical/electrochemical vapor deposition (CVD/EVD) was successfully performed to produce a dense and gas-tight YSZ electrolyte on a metal support for solid oxide fuel cell applications. Micro Ni/YSZ (7:3 wt%) was deposited by screen printing and YSZ was deposited by an atmospheric plasma spray (APS) process on a metal support prior to the CVD/EVD refinement process. The initial nitrogen permeation flux through the YSZ layer prepared by the APS process was in the range of 1.8–2.7×10 −7 mol/s cm 2 at 25 °C, which shows that residual pores/pinholes existed in the YSZ layer. After YSZ density refinement by the CVD/EVD process, a dense and gas-tight YSZ layer can be obtained after five hours of deposition. An additional 4–7 µm of YSZ was observed after the refinement process was finished. The average film growth rate during CVD/EVD was approximately 1.14 µm/h. From XRD analysis, the YSZ layer prepared after CVD/EVD showed a dominant cubic structure; nonetheless, a secondary phase was also observed. From the SEM and elemental mapping analyses, the YSZ layers showed a homogeneous distribution on the surface of the metal support. The present results showed that the CVD/EVD process is capable of refining the YSZ electrolyte density/tightness by plugging residual pores/pinholes, along with increasing the YSZ thickness, for application in metal-supported solid oxide fuel cells.

La and Ca-Doped A-Site Deficient Strontium Titanates Anode for Electrolyte Supported Direct Methane Solid Oxide Fuel Cell

Nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes for solid oxide fuel cells (SOFC) possesses excellent catalytic properties and stability for H2 oxidation but not for hydrocarbons as it results in fast carbon deposition in absence of excess steam. In the present work, A-site deficient porous LSCTA- (La0.2Sr0.25Ca0.45TiO3) anode has been fabricated using the environment friendly, aqueous tape casting method followed by the same procedure for the dense YSZ electrolyte and YSZ porous scaffold as cathode matrix. The anode, electrolyte, and porous cathode matrix have been laminated together and sintered up to 1350°C. After sintering, nitrate precursors of La, Sr, Co and Fe are infiltrated inside the porous YSZ cathode matrix to form the perovskite phases of La0.8Sr0.2CoO3 (LSC) and La0.8Sr0.2FeO3 (LSF). The as fabricated electrolyte supported SOFCs have been tested in H2 and CH4 fuel at 800°C. The electrolyte supported cell 15%LSF-5% LSC-YSZ/YSZ/4%Ni-6%CeO2-LSCTA- gives maximum power density of 328 mW cm−2 for 3 h in H2, but in CH4 the performance decreased to 165 mW cm−2 even though a sustained open circuit voltage of ∼1 V obtained during H2 and CH4 operation. The morphology of the anode before and after cell testing has been analyzed using scanning electron microscope followed by X-ray diffraction studies to understand phase changes during fabrication and testing.

Elementary kinetics of the oxygen reduction reaction on LSM-YSZ composite cathodes

A multi-physics based transient, continuum model of a solid oxide half-cell comprising of a porous LSM-YSZ composite cathode sintered to a dense YSZ electrolyte was developed to investigate the oxygen reduction reaction kinetics. The model coupled species, electron and ion transport through the porous cathode to surface and electro-chemistry. The electrochemical reduction of O2 was modeled using three candidate elementary kinetic mechanisms. Each mechanism included parallel surface and bulk pathways for O2 reduction and was driven by three different electric phase potentials. The mechanisms were compared against three sets of electrochemical impedance spectra and polarization curves measured by Barbucci et al. (2009), Cronin et al. (2012) and Nielsen and Hjelm (2014) over a wide range of operating temperatures (873–1173K), inlet O2 concentrations (5–100%) and overpotentials (−1V to +1V). Two of the three mechanisms were able to quantitatively reproduce the three sets of experiments by only tweaking the microstructural parameters for each individual set. Yet on analyzing their kinetic and thermodynamic parameters, the mechanism postulating the chemisorption of gas-phase O2 on LSM to form the superoxo-like adsorbate O2- was determined to be the most realistic. A model-based sensitivity analysis revealed that ionic transport in the YSZ phase, O2 dissociation in conjunction with surface to bulk charge transfer in the LSM phase and charge transfer at the three phase boundary were the rate limiting steps throughout the operating space. Additionally, the bulk pathway was found to be insignificant.

Anode-Supported Solid Oxide Fuel Cells Fabricated by Single Step Reduced-Temperature Co-Firing

Anode-supported solid oxide fuel cells (SOFCs) consisting of NiO-Y0.16Zr0.92O2-δ (YSZ) anode support layer, NiO-YSZ anode functional layer, YSZ electrolyte and (La0.8Sr0.2)0.98MnO3-δ (LSM)-YSZ cathode were successfully fabricated by single-step co-firing at 1250°C. Cells were prepared by tape casting, with Fe2O3 sintering aid used to obtain a nearly dense YSZ electrolyte. Scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDS) showed no evidence of reactions or interdiffusion between layers during co-firing. The cells yielded area specific resistance of 0.44 Ωcm2 and a maximum power density of 0.91 W/cm2 at 800°C. Impedance spectroscopy measurements showed that the LSM-YSZ cathode polarization resistance was higher for the co-fired cathodes than for a cathode that was fired separately at an optimized temperature of 1175°C. However, reducing the cell co-firing time decreased cathode polarization resistance and increased cell power output. Analysis of SEM images showed that co-firing caused more sintering and coarsening than in the optimally-fired LSM-YSZ, reducing three-phase boundary density and explaining the increased cathode resistance.

Cracking Reaction of Octane Fuel and Electrochemical Reaction on Ni-YSZ Cermet Anode of Internal Steam Reforming Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density. Then, SOFC is a promising power source for many potential mobile robot applications. Octane is a good candidate as fuel due to high boiling point enough to be in the liquid phase even at high temperatures in summer. Performance of the internal reforming SOFC is governed by anode electrode material, internal reforming method, and operating conditions. Ni-yttria stabilized zirconia (Ni-YSZ) cermet is the most promising anode material for SOFC because of its low cost, chemical stability, and closely matched thermal expansion coefficients with the dense YSZ electrolyte. Steam reforming is the first choice for the internal reforming method because it has been frequently-studied for the direct use of methane fuel. Then, operation of SOFC by internal steam reforming of octane has been studied. However, cracking reactions on the YSZ anode, which is affected by the operating conditions, have not been elucidated sufficiently. The purpose of this study is to clarify the cracking reaction of octane fuel and the electrochemical reaction of the Ni-YSZ anode electrode under various operating conditions. In this study, the fuel-cracking reaction on the Ni-YSZ anode electrode is estimated by gas chromatography with thermal conductivity detector. Anode electrode performance is studied by electrochemical measurement. Two types of octane (n-octane; linear alkane, iso-octane; branched chain alkane) are used in order to elucidate the effect of difference in carbon-chain structure. Octane and steam of equilibrium vapor pressure were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Temperature of the octane bubbler and the carrier gas flow rate was 30 oC and 10 ml/min, respectively. The carrier gas flow rate of the water bubbler was 83 ml/min. Steam/carbon ratio (S/C) was controlled over an S/C ratio from 0.5 to 5.5 by changing the temperature of the water bubbler from 30 to 80 oC. The cell temperature range was 700-950 oC. The octane internal steam reforming operation of SOFC was demonstrated by I-V and I-P characteristics and stable power generation property by constant current. The effects of operating conditions (S/C ratio and cell temperature) on the open circuit voltage (OCV) and power generation property were investigated. The chemicals generated by the cracking reaction of octane at various operating conditions were analyzed by gas chromatography and compared. The difference in outlet gas compositions from the cell of open circuit condition and power generation condition revealed the main species consumed for the power generation reaction on the anode electrode. The electrochemical reaction resistance of the Ni-YSZ anode was measured by ac impedance method and the rate-determining process was identified. In this study the following was revealed. When the S/C and/or the operating temperature are high, the reforming reaction rate is large. By the reforming reaction of n-octane, H2, CO, CO2, and CH4 is mainly produced. When the iso-octane is used as the fuel, small amount of C2H6 is also generated in addition to them. Only H2 and CO contribute to the power generation directly. Rate-determining process of the electrode reaction is a cracking reaction to H2 and CO.

Evaluation of the Influence of Nickel Addition on LST-CGO Based Solid Oxide Fuel Cell Anodes Performance

In this paper we present the combine modeling and experimental study of electrochemical hydrogen oxidation at an alternative perovskite based mixed-conducting SOFC anode. Two types of button cells without and with addition of nickel (Ni) were fabricated based on La0.1Sr0.9TiO3-α(LST)-CGO composite anodes and dense YSZ electrolytes. Electrochemical experiments were performed using symmetrical cell configuration in H2/H2O fuel mixture systematically varying applied potentials and operating temperatures. The previously developed model, which includes thermal chemistry at each surface, charge-transfer processes and reactive porous media transport, was employed. It was found that in the case of conventional LST based anodes heterogeneous and charge transfer chemistry at LST surface has capacitive behavior and alters the impedance spectra. However, if nickel is added the influence of LST surface chemistry is diminished leading to an improvement of cell performance.

Impedance Spectroscopy Analysis of Ni/YSZ Interfaces Prepared by Liquid Precursor Deposition

The state-of-the-art material for SOFC anodes is a Ni-yttria-stabilized zirconia (Ni-YSZ) based cermet, due primarily to the excellent electrocatalytic activity of Ni towards H2oxidation. To design and construct Ni-YSZ anodes with maximum performance, it is important to understand the reaction mechanism at the Ni/YSZ interface. Therefore, model anode designs, in which dense Ni films were deposited on dense YSZ electrolytes in the form of regularly spaced strips, have been developed by multiple groups [1, 2] to control the Ni/YSZ interfacial length, with impedance spectroscopy (EIS) used to evaluate performance. However, a consensus has not been reached yet on the exact reaction steps and how they change with temperature. Charge transfer at the Ni/YSZ interface, hydrogen diffusion on Ni surface and hydrogen adsorption/desorption reaction, have all been suggested as the rate-limiting steps in the literature [1, 2]. Thus, a clear identification of the rate limiting step at the Ni/YSZ interface and its correlation with the Ni-YSZ composite anodes is still required. In this work, solution precursor deposited Ni thin films were deposited on dense Zr oxide discs for comparison with Ni-YSZ composite anodes that were prepared by the infiltration of the Ni solution precursor into porous YSZ scaffolds. EIS analysis was carried out, and a transmission line model, which allows the separation of the porous YSZ scaffold ionic resistance and the Ni/YSZ interfacial impedance, was used to fit the impedance data. The dominant impedance arcs in the thin film Ni anodes and the Ni/YSZ interfacial impedance, extracted from the transmission line modelling of the Ni-YSZ composites, both exhibited an activation energy of ~1.3 eV. The same activation energy obtained for the Ni/YSZ interfacial impedance obtained from both Ni thin film anodes and infiltrated Ni-YSZ anodes validates the transmission line fitting approach while the obtained activation energy value of ~1.3 eV indicates that the charge transfer process occurring at the triple phase boundary is the rate determining reaction step in Ni-YSZ anodes. Acknowledgements: The authors gratefully acknowledge the Eyes High PDF Program at the University of Calgary and Alberta Innovates – Technology Futures (AITF) for the support of AB, as well as the Natural Sciences and Engineering Research Council of Canada (NSERC) for the overall financial support of this work.

Effect of Operating Conditions on the Electrochemical Performance of Ni-YSZ Cermet Anode for Solid Oxide Fuel Cells Using Octane Fuel

Introduction For many potential mobile robot applications, current power supply technology (such as the lithium ion battery technology) is in fact a key limiting factor due to its low energy density. Solid oxide fuel cells (SOFCs) are a promising alternative and can overcome limitations of current battery technology. This is because SOFCs can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density such as gasoline and so on.Ni-YSZ cermet is the most promising anode material for SOFCs because of its low cost, chemical stability, and closely matched thermalexpansion coefficients with the dense YSZ electrolyte. Ni is an excellent catalyst for hydrocarbon fuel reforming and cracking, which leads to rapid carbon deposition inside the SOFC anode and causes coking and deactivation1). In principle, the carbon deposition on Ni can be depressed by adding steam in the hydrocarbon fuel2,3), whereas few detailed investigations of SOFCs that operate on gasoline fuel have been reported.In this work, we studied the effect of steam content and demonstrated the direct operation of SOFCs by octane which is a main ingredient of gasoline. In the case of the Ni-YSZ anode, the operation temperature must be high in order to avoid sulfur poisoning4 -6 ), because most commercially available fuels contain sulfur. Therefore, in this study, the SOFCs operation was conducted at high temperature of 950 oC. Experimental Surface-polished YSZ pellets of 2 mm in thickness were used for the electrolyte. Pt cathode electrode and Pt reference electrode were prepared on the surface and side of the pellets, respectively, by firing Pt paste at 1300 oC. Then, NiO powder and GDC powder (NiO/YSZ=6/4 (wt)) were thoroughly mixed in a vacuum using the planetary ball mill (200 rpm, 12 h), and made into NiO-YSZ pastes using ethyl-cellulose and organic solvent. The NiO-YSZ paste was screen-printed in diameter of 8 mm on the YSZ pellets and sintered at 1200 oC for 1 h.Octane and steam were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Octane bubbler temperature was 30 oC, and Steam/Carbon ratio (S/C) were controlled by changing the temperature of the water bubbler from 40 to 70 oC. Electrochemical properties (open circuit voltage, anode electrode resistance, I - V and I - P characteristics, stable power generation property) were measured over a temperature range of 950-700 oC. During the constant current generation, exhaust gas was analyzed by gas chromatography. Results and Discussion Figure 1 shows I - V and I - P characteristics for the cells at 950 oC in wet octane. Maximum power density of 35 mW/cm2 was obtained by S/C of 8.9. The power density decreased with S/C quickly.Figure 2 shows the time course of cell voltage at a drawn current density of 5.2 mA/cm2 for S/C of 3.5. The cell voltage was stable. That is, carbon deposit does not arise even in the cell of the Ni-YSZ anode in low S/C of 3.5 when the operation temperature is high of 950 oC. References 1) S. Mclntosh and R.J. Gorte, “Direct Hydrocarbon Solid Oxide Fuel Cells,” Chemical Reviews, 104 (2004) 4845- 4865.2) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: I. Equilibrium Compositions and Reforming Conditions,” Journal of Electrochemical Society, 850 (7) (2003) A878-A884.3) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: II. The C-H-O Ternary Diagrams,” Journal of Electrochemical Society, 850 (7) (2003) A885-A888.4) K. Sasaki, K. Susuki, A. Iyoshi, M. Uchimura, N. Imamura, H. Kusaba, Y. Teraoka, H. Fuchino, K. Tsujimoto, Y. Uchida, N. Jingo, “H2S Poisoning of Solid Oxide Fuel Cells,” J. Electrochem. Soc., 153 (2006) A2023–A2029.5) P. Lohsoontorn, D.J.L. Brett, N.P. Brandon, “Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells,” Journal of Power Sources, 175 (2008) 60–67.6) H. Kishimoto, Y-P Xiong, K. Yamaji, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, “Stability of Ni base anode for direct hydrocarbon SOFCs, ” Journal Chemical Engineering of Japan, 40 (13) (2007) 1178-1182.

In-Situ Raman Spectroscopy Probing of Solid Oxide Electrolysis Cells

Solid oxide electrolysis cell (SOEC) technology is a promising route for sustainable recycling of carbon dioxide and steam into synthetic fuels and commodity chemicals via syngas (H2+ CO) production. This technology is based on the reverse polarity of the prominent solid oxide fuel cell (SOFC) and commonly consists of a porous Ni/YSZ cathode, dense YSZ electrolyte and porous LSM/YSZ anode.High temperature co-electrolysis of CO2 and H2O is a complicated process and has not yet been fully understood. This is due to a number of reactions that take place simultaneously: water electrolysis, carbon dioxide electrolysis and the reverse water-gas shift reaction (rWGS CO2 + H2 ↔ CO + H2O). Other possible reactions driven by the equilibrium may include the Bosch reaction (CO + H2 ↔ C(s) + H2O) and Boudouard reaction (2CO ↔ CO2 + C(s)).Carbon deposition on the nickel-cermet surface of SOECs can severely degrade the performance of the cell by deactivating the reaction sites. Recent work by Tao et al. has shown that, at high current densities and conversion efficiencies of CO2 and H2O to CO and H2, carbon formation on the surface of Ni/YSZ fuel electrode is likely to occur [1].SOECs are typically analyzed in-situ using electrochemical impedance spectroscopy and current-voltage characteristics and ex-situ using scanning electron microscopy to obtain information about cell’s surface topography and composition. However, these techniques are not able to provide conclusive information about the processes occurring during operation of the cell.In this work we aim to use in-situ Raman spectroscopy alongside traditional electrochemical techniques to probe the surface species of the SOEC electrodes for carbon impurities deposition and provide insight into the fundamental processes and reaction pathways that occur during high temperature (700 – 850 oC) co-electrolysis of CO2 and steam. In-situ Raman spectroscopy is also applied to characterize structural changes and material transformations during SOEC operation.The particular area of interest is the triple phase boundary (TPB), where the electrochemical reactions take place. Accessing the TPB is challenging due to high operation temperatures and the depth of laser penetration into the sample.The experimental setup with optical access for the Raman microscope will be discussed in detail as well as the correlation of in-situ spectroscopic data with the electrochemical information obtained from the SOECs.

Metal-ceramic/ceramic nanostructured layered composites for solid oxide fuel cells by spark plasma sintering.

In this work, bi-layered Fe-Ni-Co-YSZ/YSZ nanostructured composites for solid oxide fuel cells were obtained using the spark plasma sintering (SPS) technique. The microstructures of the anode and electrolyte were controlled by optimization of SPS consolidation parameters. The resulting bilayers have a full dense YSZ electrolyte and porous Fe-Ni-Co/YSZ anode as well as crack-free and well-bonded anode/electrolyte interface. On the other hand, SPS under non-optimized processing parameters cannot yield the desired results. The high resistance to thermal stresses of the fabricated half-cells was achieved with Fe-Ni-Co/YSZ anode. The developed anode showed higher thermal compatibility with YSZ electrolyte than usual Ni/YSZ cermet. Thus, with the successful combination of SPS parameters and anode material, we have obtained bi-layers for SOFCs with required microstructure and thermal compatibility.

Carbon-resistant Ni-YSZ/Cu–CeO2-YSZ dual-layer hollow fiber anode for micro tubular solid oxide fuel cell

A NiO-YSZ/porous YSZ dual-layer hollow fiber with an asymmetric structure was fabricated by a co-spinning-sintering method. A dense YSZ electrolyte film was prepared on NiO-YSZ layer by dip-coating method and calcined at 1450 °C; subsequently a porous cathode was dip-coated on the dense YSZ electrolyte film using LSM-YSZ (in the weight ratio 4:1) ink to fabricate a micro tubular solid oxide fuel cell (MT-SOFC). Cu–CeO2 catalyst was impregnated into the porous YSZ layer to form the second anode composition. The power output of the MT-SOFC with Ni-YSZ/Cu–CeO2-YSZ graded anode was up to 242 mW cm−2 operated at 850 °C using CH4 as fuel and air as oxidant. Little carbon deposition was observed on the double anode using methane as the fuel after 60 h' stable operation.

Microstructure and Performance of Anode for Microtubular Solid Oxide Fuel Cells

NiO-YSZ hollow fibers were fabricated via a phase-inversion spinning technique. An YSZ electrolyte film was dip-coated on the outer surface of the fiber and then co-sintered at 1450°C to form electrolyte/anode hollow fiber half cell. The microstructure of the NiO-YSZ anode was modulated by controlling NMP/ethanol ratio of the inner coagulant during spinning process. Experimental results showed that, with the 1-Methy 1-2-Pyrrolidinone (NMP) content increase from 0 to 30wt%, 50wt%, 70wt% and 100wt%, the microscopy of the anode hollow fibers evolved from a sandwiched structure, i.e. finger-like pore/sponge voids/finger-like pore morphology, to a finger-pore-penetrating structure, leading to porosity increase of the anode. Meanwhile, gas tightness of YSZ electrolyte film, mechanical strength of the reduced dual-layer hollow fibers, and conductivity of anode could decrease. Microtubular fuel cells were fabricated by coating a porous Ag cathode onto the dense YSZ electrolyte film. The concentration polarization in the H2/air cell decreased with increasing the length of the finger-like pores. The microtubular SOFC made from the anode hollow fibers with 70wt% of NMP-ethanol as the inner coagulant demonstrated the minimum polarization resistance and the highest output performance with 662 mW/cm power density.