Porous YSZ Research Articles

Mechanical and thermal properties of porous yttria-stabilized zirconia

ABSTRACTThis paper reports on analyses of the compressive Young’s modulus (Ep), compressive strength (σp) and thermal conductivity (κ) of sintered porous yttria-stabilized zirconia compacts (YSZ). The Ep and σp values are affected by the number of grains and the grain boundary area in sintered YSZ ceramics. The measured porosity dependence of the Ep and σp values was compared with that of the theoretical Ep and σp values derived for the open pore structure. The tendency of Ep and σp values to increase at low porosity was clarified by the proposed theoretical porosity dependence of Ep and σp. The strain at fractures also increased with increases in the grain boundary area and depended on the crack propagation mode affected by the degree of grain growth. The κ values for porous YSZ, which decreased at high porosity, were analyzed theoretically with two model structures: a pore-dispersed YSZ continuous phase system (F model) and a YSZ-dispersed pore continuous phase system (G model). The measured κ values at 0–30% porosity were in good agreement with the κ values calculated for the model F structure. In the high porosity range above 50%, the measured κ values approached the κ value curve for the model G structure with increases in porosity.

Formation and Properties of Porous ZrO2-8wt%Y2O3 Coatings

The efficiency of a gas turbine engine is highly dependent upon the clearance between blades and engine case. To control the over-tip leakage, an abradable coating system is sprayed on the case surface. In the high temperature section, a ZrO2-8 wt%Y2O3 (YSZ) coating is usually applied. Pores in the coating enhance its abradability. In this study, poly-p-hydroxybenzoate (PHB) powder mixed in the YSZ powder was used as pore-forming material. To avoid burning loss of PHB during the spraying process, a sol-gel method was used to clad a TiO2 shell on the PHB powder surface. The porous YSZ coating was deposited by plasma spraying. The characteristics of the coatings were investigated in this work, such as morphology, porosity, hardness, corrosion resistance and abradability. The results show that the coated PHB increases the porosity of the YSZ coating, and then decreases the coating hardness. Finally, a rotated invading test indicates the incursion depth increases with the increase of coating porosity, suggesting the coating abradability is improved.

Evolution of porous YSZ surface morphology in YSZ‐MnOx system

AbstractA unique porous YSZ surface morphology is observed and reported, for the first time, during the sintering of YSZ‐MnO2 composite. The porous YSZ morphology largely depends on YSZ/MnO2 ratio and sintering temperature, time, and atmosphere. The structural evolution is hypothesized to be due to the presence of Mn especially related to Mn diffusion, dissolution, precipitation, and evaporation. A series of experiments were designed and conducted to investigate the porous morphology formation with emphasis on performing quench experiments that showed the initiation of porous surface evolution during cooling. Computational thermodynamics has been applied to investigate the role of Mn on the surface porosity formation.

Modeling of hydrogen separation through porous YSZ hollow fiber‐supported graphene oxide membrane

In this work, hydrogen (H2) permeation fluxes through 230 nm‐thick graphene oxide (GO) membrane deposited on porous YSZ hollow fiber were measured and correlated to an explicit H2 permeation model. H2 fluxes through such GO‐YSZ hollow fiber membrane increased from 4.83 × 10−8 mol cm−2 s−1 to 2.11 × 10−7 mol cm−2 s−1 with temperature rise from 20 to 100 °C. The activation energy of H2 permeation was determined by the linear regression of the experimental data and was applied in the theoretical calculations. The model predictions fit well the temperature dependent and the argon sweep gas flow rate dependent H2 fluxes data. Using the derived permeation model, the effects of vacuum pressure at lumen side and H2 partial pressure at shell side, membrane area, and GO membrane film thickness on the membrane performance were simulated and discussed to provide insights for practical applications. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2711–2720, 2018

Protonic surface conduction controlled by space charge of intersecting grain boundaries in porous ceramics

Two different time constants of surface protonic conduction in porous YSZ are assigned to transport along grain surfaces and across intersects of grain boundaries.

Hollow ceramic microspheres prepared by combining electro-spraying with non-solvent induced phase separation method: A promising feedstock for thermal barrier coatings

Hollow microspheres were prepared by a combination method of electro-spraying and non-solvent induced phase separation, using a slurry of polymer with Y2O3-stabilized ZrO2 powder (YSZ). Subsequent sintering was carried out to obtain porous hollow YSZ microspheres with proper strength aiming to be thermal spraying feedstock. The ESNP particle size and distribution strongly depended on the applied voltage of the electro-spraying process and the jet nozzle diameter. The YSZ microspheres always consisted of a thin spongy shell and highly porous interior structure. The microsphere size decreased from 1000 to 100μm with increasing voltage (10–20kV). The cone-jet mode of electrospray reached steady as the nozzle diameter varied from 0.5 to 0.8mm. Compared with the coatings made from commercial hollow spherical powder, the coatings prepared by ESNP powder had enhanced resistance to sintering and exhibited a longer life than commercial spray powder. It has demonstrated that the ESNP method provides a versatile way to manipulate the microstructure of ceramic microspheres, and could been used to fabricate high performance thermal barrier coatings by plasma spraying.

Dense LaSrMnO3 composite electrodes for NOx sensing

Impedancemetric NOx sensors composed of dense La0.8Sr0.2MnO3 (LSM) and LSM composite electrodes were evaluated based on the electrical response, sensitivity, and selectivity to NOx at operating temperatures ranging from 575 - 675°C under dry and humidified gas environments. The composite electrodes investigated containing 10 wt% Au or 30 wt% Y2O3-ZrO2 (YSZ) were accompanied by a porous YSZ electrolyte. The LSM-Au electrodes demonstrated a higher response to NOx with limited cross-sensitivity to water, in comparison to LSM and LSM-YSZ sensing electrodes. However, the response and recovery time of LSM and LSM-YSZ based sensors was more rapid than that of LSM-Au based sensors. The rate limiting mechanism effecting sensor behavior appeared to be oxygen reduction regardless of the sensor electrode. Transport of atomic oxygen was an additional rate limiting step for the LSM and LSM-YSZ based sensors. Cross-sensitivity analysis indicated the LSM-Au based sensors were more selective to NOx, in comparison to CO, CO2 and CH4. Overall, the 10 wt% Au addition to LSM appeared to enhance sensor sensitivity to NOx by enabling NOx reactions to proceed more readily.

Durability of symmetric-structured metal-supported solid oxide fuel cells

Symmetric-structure metal-supported solid oxide fuel cells (MS-SOFC) with YSZ electrolyte are fabricated with porous YSZ backbone electrodes, stainless steel supports, and infiltrated catalysts on both anode and cathode side. Durability towards aggressive thermal and redox cycling, and long-term operation is assessed. Many sealing material candidates are screened for compatibility with the cell materials and operating conditions, and a commercial sealing glass, GM31107, is selected. LSM/SDCN cells are then subjected to 200 very fast thermal cycles and 20 complete redox cycles, with minimal impact to cell performance. LSM/SDCN and SDCN/SDCN cells are operated for more than 1200 h at 700 °C. The seal and cell hermeticity is maintained, and cell ohmic impedance does not change significantly during operation. Electrode polarization increases during operation, leading to significant degradation of the cell performance. In-operando EIS and post-mortem SEM/EDS analysis suggest that catalyst coarsening and cathode Cr deposition are the dominant degradation modes.

(Invited) Decoding Multi-Gas Mixtures Using a Mixed-Potential Sensor Array for Vehicular Emission Monitoring

Electrochemical gas sensors with the ability to detect the concentration of individual gases in a multi-gas environment are in huge demand in many areas of technology, such as threat detection, industrial quality control and, importantly, vehicular emission monitoring. While oxygen l sensors are used for on-board diagnostics in spark-ignition vehicle emission systems, a similar sensor is not available for NOx, NH3 and non-methane hydrocarbon (NMHC) gases found in diesel engine exhaust. LANL has developed patented pre-commercial prototype mixed-potential sensors with strong sensitivity and selectivity towards these target gases, but a single device with absolute selectivity is still elusive (1, 2). To address this pressing problem, we have tested and successfully demonstrated the utility of employing a sensor array rather than individual sensors, in conjunction with a first-principles data analysis approach. Our computational framework was employed to predict the composition of gas mixtures containing two individual gases, both of which affect sensor readings, with less than 2% average error (3). Here we present the results of our model trained on three and four-gas mixtures involving NO, NO2, NH3 and C3H6. The three sensors used in the array comprise a dense La0.8Sr0.2CrO3 or AuPd sensing electrode and dense Pt electrode with porous YSZ electrolyte. A physically motivated model is used to analyze the data and find the composition of the gas mixture. This work aims to provide robust and accurate estimates of NOx, NH3 and unburnt non-methane hydrocarbon content in diesel vehicle exhaust, laying a foundation for on board, real-time sensing applications. Acknowledgments The research was funded by the Los Alamos National Laboratory Directed Research Development Exploratory Research (LDRD-ER) program. References R. Mukundan, E. L. Brosha, F. H. Garzon, US 7,575,709 (2009).R. Mukundan, E. L. Brosha, F. H. Garzon, Journal of The Electrochemical Society 150, H279 (2003).K. P. Ramaiyan, C. R. Kreller, E. L. Brosha, R. Mukundan, U. Javed, A. V. Morozov, ECS Transactions (2016) 75, 107-111. LA-UR-17-23016

In-Situ Formation of Nano-Structured Cathode on Metal Supported SOFC

Metal supported solid oxide fuel cells (MS-SOFC) possess several advantages like good mechanical property, easy assembling, fast start-up and capability to maintain redox stable due to fuel leakage. However, it has been a critical challenge to deposit cathode on the metal supported cell in the oxidizing atmosphere, which is mainly limited by the low calcination temperature yielding vulnerable electrolyte-electrode interface. The cathodes prepared in the protecting atmosphere are also not satisfied, instead it will increase the complexity of the cell fabrication process. Lower down the cathode deposition temperature using aqueous infiltration has become one of the optimized methods for MS-SOFC. Also, we have proposed an alternative way for the fabrication of metal supported single cell with robust cathode through whole cell reduction. Here, we present results of both above two approaches for the fabrication of nano-structured cathodes in MS-SOFC. The cells in this study are based on Fe-Ni (1:1 in mol ratio) support, Fe-Ni/YSZ anode and YSZ electrolyte prepared through tape-casting, thermal isostatic pressing and co-firing. As illustrated in the following schematic configuration, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes were deposited in the porous YSZ layer of the pre-reduced Ni-Fe|Ni-Fe-YSZ||YSZ||P-YSZ substrate by infiltration and fired at the cell operation temperature around 700 oC. A-site deficiency of ~4-8% was introduced into LSCF cathode and was expected to enhance the sintering activity of LSCF nano particles at low calcination temperature and to enhance the cathode performance by the excess cobalt and iron oxide at LSCF surface. As in the second approach, Sr0.95Ti0.3Fe0.63Ni0.07O3-δ – Gd0.1Ce0.9O1.95 (STFNi-GDC) graded cathode will be screen printed on the NiO-Fe2O3|NiO-Fe2O3-YSZ||YSZ|GDC substrate and fired at high temperature in air as normal cathode preparation procedure. Then the whole cell will be reduced in H2-Ar to obtain Fe-Ni alloy support and reduced STFNi cathode with in-situ exsolved (Fe,Ni) alloy nano particles anchored in the oxide surface. During the cell test, the reduced STFNi-(Fe,Ni) will be re-oxidized in air, forming the surface decorated iron and cobalt oxide particles, which were presumably to be expected to increase the cathode performance through improvement of oxygen surface exchange coefficient. Figure 1

Infiltration of La0·6Sr0·4FeO3-δ nanoparticles into YSZ scaffold for solid oxide fuel cell and solid oxide electrolysis cell

La0·6Sr0·4FeO3-δ (LSF) nanoparticles were infiltrated into porous YSZ scaffold to fabricate composite LSF-YSZ oxygen electrode for both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC). X-ray diffraction (XRD) pattern and scanning electron microscopy (SEM) images have demonstrated that the formation of LSF perovskite phase and uniform distribution of nano-structural LSF particles onto inner surface of the YSZ scaffold, respectively. Polarization curves and electrochemical impedance spectra (EIS) were employed for the characterization of electrochemical properties of the cell for both power generation and high-temperature steam electrolysis. Furthermore, the long-term durability of the LSF-infiltrated cell was galvanostatically carried out under both SOFC and SOEC operations. For SOFC operation, the cell experienced a significant degradation in the initial 50 h and subsequently was sustained at a stable operation, with overall ohmic and polarization resistance variation from 0.20 to 0.25 Ω cm2 and from 0.55 to 0.60 Ω cm2, respectively. For SOEC operation, the cell showed high long-term durability operation, with almost unchangeable ohmic resistance and slightly varied polarization resistance from 0.41 to 0.43 Ω cm2.

Mechanisms of Protonic Surface Transport in Porous Oxides: Example of YSZ

The electrical properties and the charge carrier mechanism of porous 8 mol % yttria-stabilized zirconia (8YSZ) ceramic samples have been investigated over wide ranges of relative humidity (RH) and temperature (25–400 °C). The presence of humidity introduces protonic surface conduction, and porous YSZ shows pure protonic conduction below ∼150 °C. H/D isotope effect studies combined with transport number measurements reveal a change in transport mechanism from structural diffusion (Grotthuss type) to vehicular transport when the relative humidity exceeds ∼60% RH, coinciding with a change in the adsorbed water layer from an “ice-like” to a “water-like” structure. Similarly, the activation energy for proton transport decreases from 0.43 to 0.28 eV when the relative humidity increases from 20% RH to 84% RH, reflecting changes in the enthalpies of formation and migration of charged species with increasing water layer thickness.

A porous yttria-stabilized zirconia layer to eliminate the delamination of air electrode in solid oxide electrolysis cells

Abstract Delamination of La 0.8 Sr 0.2 MnO 3-δ (LSM) in solid oxide electrolysis cells (SOECs) is usually associated with the high oxygen partial pressure build-up at the LSM-YSZ (yttria-stabilized zirconia) interface. Here we sandwich a porous YSZ layer between the LSM electrode and YSZ electrolyte to release this oxygen pressure. Symmetric cells with and without the porous YSZ layers are prepared and tested in air at 800 °C under the current densities of 0.5 and 1 A cm −2 for 100 h. Voltage change is continuously monitored, and impedance spectrum studies have been carried out before and after testing. No delamination has been observed for the samples with the porous YSZ layer even after 100 h. The improved performance for these samples is due to the shift of oxygen evolution reaction from the dense YSZ-LSM interface to a porous YSZ-LSM interface. This shift also helps the oxygen to be easily released instead of going into the pores or grain boundaries of the electrolyte. On the other hand, for the sample without the porous YSZ layer, the LSM is totally delaminated from the electrolyte just after 70 h.

Facile hydrogen/nitrogen separation through graphene oxide membranes supported on YSZ ceramic hollow fibers

From the early of 1990s, intensive research has been started to seek a possible more efficient gas separation using inorganic membranes with more hopes on zeolite membrane. Although prohibitive difficulties have been encountered, our enthusiasm or imagination has never been quenched out, particular with the modern era of “magic” material-graphene. In this work, we explored the possibility of using supported graphene oxide (GO) membrane for gas separation. For this purpose, the porous YSZ hollow fiber ceramic support and GO flakes with size up to 5µm were separately prepared; subsequently, vacuum-suction impregnation was applied to assemble the GO laminates on the fiber external surface with the thickness of 230nm. The 2D nano-channels formed by the two neighboring one-atom-thick GO layers endow the membrane with molecular-sieving function. Gas permeation behavior was investigated by the measurement of gas permeances from the single gas components and the gas mixture (H2/N2). The ideal selectivity of H2/N2 at 20oC was up to 76, mirroring the molecular sieving function via molecular size limited diffusion and preferable adsorption. Compared to the ideal selectivity, the H2/N2 separation factor is lower due to the blocking effect of the other components. At higher temperature, N2 permeance was increased by a percentage more than that of H2 thus decreasing the separation factor from 68 (20oC) to 37 (100oC). The H2 separation test for 240h highlights stable performance in maintaining the separation factor and the permeance. H2 permeation using GO/YSZ hollow fiber was also theoretically probed. Simulation implies that in achieving the overall H2 permeance, the front part (near the inlet) of the hollow fiber makes much more contribution than the rear part. To more effectively use the GO hollow fiber, the feed gas pressure can be increased or the vacuum pressure can be applied in the permeate side.

Impedancemetric NOx Sensors Based on LSM and LSM-Au Sensing Electrodes

An ideal NOx sensing electrode for diesel exhaust gas systems should possess high sensitivity and selectivity to NOx (i.e., NO and NO2) and low cross-sensitivity to interfering exhaust gases. In addition, it should be possible to fabricate the electrode readily at low cost. Gold (Au) has been rigorously studied as a sensing electrode for Y2O3-doped ZrO2 – YSZ electrolyte based sensors and found to demonstrate high NOx sensitivity and low water cross-sensitivity. However, the low melting temperature of Au and thermal expansion mismatch with YSZ limits the feasibility of Au as a sensing electrode. The perovskite, strontium-doped lanthanum manganite, La0.8Sr0.2MnO3 – LSM is an attractive alternative to Au from a manufacturing perspective as tolerates the high firing processing steps for accompanying sensor components and its thermal expansion coefficient is more compatible with YSZ. In addition, LSM has demonstrated low cross-sensitivity to interfering gases, such as NH3. Yet managing water cross-sensitivity remains challenging for LSM based sensors. In this work a detailed study was carried out on LSM and LSM-Au composite sensing electrodes in order to explore the combined benefits of LSM and Au. The NOx response of LSM and LSM-Au supported sensors was evaluated with respect to sensitivity, selectivity, accuracy, and response rate using the impedancemetric method. In addition, the frequency dependence of the sensors was studied to better understand the role of the operating frequency on the electrochemical behavior of the sensors. LSM and LSM-Au composite sensing electrodes were fabricated using standard ceramic processing methods. Electrode supports containing LSM-Au were made using powder mixtures of 90 wt% LSM (Inframat Advanced Materials) and 10 wt% Au (Alfa Aesar) that was uniaxially pressed at 200 MPa and fired at 1400 °C for 5 hours to achieve a density of ~ 95%. The electrolyte was 8 mol% YSZ (Tosoh Corp.) that was made into a slurry. A portion of the LSM-Au pellet was dip coated with the YSZ slurry. Several LSM-Au pellets with YSZ coating were co-fired at 1000 °C for 1 hour. The counter electrode was coated onto the porous YSZ electrolyte using a slurry made from LSM-Au powder, and the pellets were fired again at 1000 °C for 1 hour resulting in LSM-Audense/YSZ/LSM-Auporous cells to serve as NOx sensors. The sensors with pure LSM SE were made following the same procedure as LSM-Au based sensors. Frequency dependent impedance measurements were collected using a Gamry Reference 600 for sensors exposed to various gases at a flow rate of 500 sccm and operated at 575 °C under dry and humidified conditions. The oxygen concentration in the test gas environment was varied from 5 - 18%. The microstructural properties were analyzed based on scanning electron microscopy (SEM), X-ray powder diffraction (XRD), Mercury Intrusion Porosimetry (MIP) and Archimedes method. The electrochemical behavior of the sensors in terms of frequency dependence, response time (τ90), exhaust gas cross-sensitivity, and oxygen partial pressure dependence was determined from impedance spectroscopy data. The impedance studies indicated the LSM-Au based NOx sensors demonstrated about 25% greater sensitivity to NOx for concentrations between 0–50 ppm, in comparison to LSM based sensors. As the gas concentration increased, NOx sensitivity for the LSM based sensors became more similar to the response for LSM-Au based sensors. Sensor sensitivity was based on the angular phase component of the impedance that varies with frequency. It was found that NOx sensors with an LSM-Au sensing electrode were sensitive to NOx over a wider frequency range from 1 - 250 Hz, in comparison to LSM based sensors where the frequency range was 1 - 125 Hz for NOx sensitivity. Cross-sensitivity studies indicated that sensors composed of LSM and LSM-Au electrodes were mostly impacted by interference of H2O and O2. However, the addition of Au in LSM significantly reduced the impact of water cross-sensitivity. Data indicated water cross-sensitivity was approximately 58% lower for sensors with LSM-Au electrodes, in comparison to those with LSM electrodes. However, increasing O2 in the gas stream caused water sensitivity to become more similar for both electrodes. This indicated oxygen cross-sensitivity was slightly higher for sensors with LSM-Au electrodes versus LSM electrodes. Measurements for the response time (τ90) indicated LSM-Au based sensors were slower than those supported with LSM, possibly due to oxygen inference. Overall, sensors containing the LSM-Au composite electrode demonstrated improved NOx sensing capabilities, particularly at lower NOx concentrations, and lower water cross-sensitivity in comparison to those with an LSM electrode. Improving response time and reducing oxygen dependence may be possible by varying the amount of Au and LSM within the sensing electrode.

Effect of porosity on the electrical conductivity of LAMOX materials

Porous La2Mo1.5W0.5O9 (LMW05) samples with different levels of densification were obtained by varying the sintering temperature in the range of 750 to 1100°C. Impedance spectroscopy was used to evaluate the effect of porosity on the electrical response of LMW05-type compounds. The bulk conductivity was found to depend on the microstructure and the pore volume fraction. Effective medium theories (Bruggeman and Maxwell–Garnett approximations) were used to predict the macroscopic properties of the heterogeneous medium. The Bruggeman approximation proved to be better suited to predicting the bulk electrical response of porous LMW05 samples in the pore volume fraction range of 0 to 0.22. Compared with other ceramic materials, LMW05 was found to have a low blocking factor (αR). This characteristic of LMW05 could be related to the microstructure of ceramic materials. Two porous YSZ model samples with the same porosity but different pore morphologies were used to explain why microstructural defects (MD) have a small impact on the electrical properties of LMW05.

Fabrication and electrochemical performance of La0.5Sr0.5CoO3 impregnated porous YSZ cathode

La0.5Sr0.5CoO3-yttria-stabilized zirconia (LSCO-YSZ) composite cathode for solid oxide fuel cell (SOFC) has been fabricated by wet impregnation method. Nitrate precursors of La, Sr, and Co have been impregnated into the pre-sintered porous YSZ matrix, which is converted into LSCO phase after calcination at 850 °C in the presence of glycine as confirmed from X-ray diffraction. LSCO of 5, 7, and 10 wt% impregnated porous YSZ have been electrochemically characterized using 2-probe AC conductivity method. Maximum ionic conductivity of 0.27 S/cm at 800 °C and activation energy of 0.15 eV between 600 and 800 °C have been observed for 10 wt% LSCO-YSZ cathode. Area-specific resistance of 1.01 Ω cm2 at 800 °C is estimated for the electrolyte-supported half-cell (10 wt% LSCO-YSZ/YSZ). After testing the LSCO-YSZ cathode matrix, the electrolyte-supported full cell (10 wt% LSCO-YSZ/YSZ/NiO-YSZ) has been tested and produced maximum power density 51.12 mW/cm2 (109.38 mA/cm2) at 800 °C. The electrolyte-supported full cell exhibited 6 Ω cm2 electrode polarization at 800 °C in H2, which is in higher side leading to low performance. LSCO-YSZ/YSZ/NiO-YSZ SOFC found to give stable performance up to 2 h and scanning electron microscopy analysis has been carried out before and after cell testing to assess the morphological changes.

Morphology and Structure of Ceramic Thin Films Deposited by Spray Pyrolysis

The method of Spray Pyrolysis (SP) was applied to prepare thin films of ceramic or ceramic-metal (cermets) composites based on Cu starting from aqueous solutions of soluble metal salts. Specifically, air pressurized spray pyrolysis was used in order to prepare thin film cermets of Cu-CeO2, Cu-La0.75Sr0.25Cr0.5Mn0.5O3-δ (Cu-LSCM) and the perovskite La0.75Sr0.25MnO3 (LSM) onto dense and porous YSZ substrates. These films have the potential for application in ceramic fuel cells, ceramic membranes, batteries, catalysis and oxygen sensors. Emphasis was given on tuning process parameters such as substrate temperature, solution concentration, nozzle to substrate distance, solution flowrate and deposition time to obtain films with the desired morphology, thickness and crystal structure. Film microstructure was analyzed by scanning electron microscopy (SEM) and it was found that thin defect-free films with excellent adhesion to the substrate could be produced on dense and porous ceramic substrates. X-ray diffraction (XRD) analysis showed that the films were crystalline with only the desired functional phases at relatively low (i.e. 700°C) post-deposition sintering temperatures. It was found that for a given established set of other operational parameters, substrate deposition temperature in conjunction with precursor solution concentration constitute the most significant factors that affect film morphology, adhesion and thickness.

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.

Validation and Electrochemical Characterization of LSCF Cathode Deposition on Metal Supported SOFC

The cathode fabrication for metal supported solid oxide fuel cell (MS-SOFC) has been considered as a severe problem. In this study tri-layer substrate with structure of Ni-Fe|Ni-Fe-YSZ||YSZ||porous-YSZ is designed for cathode infiltration, with the purpose to avoid the metal substrate being seriously oxidized. The above substrates are prepared through tape-casting, thermal isostatic pressing, high temperature co-sintering and reducing. Then, (La0.6Sr0.4)1-xCo0.2Fe0.8O3-δ (LSCF; x = 0, 0.08) cathodes are infiltrated into the porous YSZ layer with multiple calcinations at 430°C in air. The pre-reduced Ni/Fe substrates show decent oxidation tolerance during the repeated firing processes. Moreover, the infiltrated LSCF cathodes present good perovskite crystalline structure and comparable conductivity of ∼2 S/cm after calcined in air at 750°C, which enables the real industrial application. The A-site deficient (La0.6Sr0.4)0.92Co0.2Fe0.8O3-δ exhibits better polarization resistance performance and stability than the stoichiometric La0.6Sr0.4Co0.2Fe0.8O3-δ which is presumably due to the in-situ formed heterostructure. Furthermore, the evolution process of the polarization resistance, ohmic resistance and the conductivity for the infiltrated LSCF cathodes during long-term annealing at 750°C are discussed in detail.