SOFC Anode Materials Research Articles

Thermodynamics of gaseous strontium and calcium cerates studied by Knudsen effusion mass spectrometry and estimation of relative electron ionization cross-section for CeO2(g).

Ceria-based systems are of great interest because of their unique properties. Such systems may be used as anode materials for SOFCs or in oxygen sensors. Theexploitation of these materials often requires high temperatures. In such conditions, the partial or complete evaporation of materials is possible. Therefore, knowledge of the values of partial pressures and thermodynamic properties is essential to predict and/or prevent possible consequences and accidents. Knudsen effusion mass spectrometry was used to determine partial pressures of vapor species over the SrO-CeO2 and CaO-CeO2 systems. Measurements of partial pressures were performed with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum or tungsten effusion cells. A theoretical study of gaseous strontium and calcium cerates was performed by several quantum chemical methods: density functional theory (DFT) M06, DFT PBE0, and MP2. The minimum value of relative electron ionization cross-section for CeO2 was estimated. In the temperature range of 2218-2249 K above the SrO-CeO2 system and of 2128-2208 K above the CaO-CeO2 system, gaseous M, MO, MO, CeO2, O, O2 and MCeO3 (M = Sr, Ca) were found. Energetically favorable structures of gaseous SrCeO3 and CaCeO3 were found, and vibrational frequencies were evaluated in the "rigid rotor-harmonic oscillator" approximation. On the basis of the equilibrium constants of gaseous reaction MO + CeO2 = MCeO3, the standard formation enthalpy of gaseous SrCeO3 (-913 ± 26 kJ/mol) and of gaseous CaCeO3 (-917 ± 26 kJ/mol) at 298 K were determined. The estimated value of relative electron ionization cross-section for CeO2 is in agreement with the general rule applicable for dioxides. The stability of SrCeO3 and CaCeO3 gaseous species was confirmed by KEMS. Gas-phase reactions involving gaseous SrO (CaO) and CeO2 with gaseous SrCeO3(CaCeO3) were studied. Enthalpy of formation reaction of gaseous SrCeO3(CaCeO3) from gaseous SrO (CaO) were evaluated theoretically, and the obtained value is in agreement with the experimental one.

Influence of A-Site Modifications on the Electrical and Crystallographic Properties of Ln0.2Sr0.7-XCaxTi0.95Fe0.05O3-δ (Ln=La, Nd) Based Fuel Electrode for Solid Oxide Cell

Perovskite oxides, in particular SrTiO3–based compounds have attracted particular attention over the past years due to their decent conductivity, good dimensional stability and high tolerance and resistance towards sulfur poisoning and carbon depositions [1, 2]. Optimal doping and stoichiometry levels on A- (La, Nd etc.) and B-sites (Fe, Ni etc.) of SrTiO3 based compounds allow to improve catalytic activity, electrical conductivity and chemical stability [2]. In addition, doping of A-site with calcium is predicted to enhance the electronic conductivity because it should decrease the unit cell volume bringing the conduction orbitals of titanium closer to each other [3].In this research influence of chemical composition of A-site on electrical and electrochemical performance of Ln0.2Sr0.7-xCaxTi0.95Fe0.05O3-δ (Ln=La, Nd) (x=0.35, 0.45) (LSCTF-x, NSCTF-x) hydrogen electrode has been investigated. The crystal structure and microstructure have been studied using X-ray diffraction and SEM to confirm the phase purity and visualize the microstructure of studied electrode layers. To understand the influence of MIEC material composition on electrical conductivities of LSCFT and NSCTF, the DC four-probe conductivity measurements of porous electrode layers has been performed. Conductivities have been measured at two different atmospheres: 1% H2 + 3% H2O + 96% Ar and 97% H2 + 3% H2O. It has been shown that the both LSCTF and NSCTF materials behave like semiconductor and the conductivity is significantly dependent on composition. The maximal total electrical conductivity of porous electrode layers were 5.5 S cm− 1 and 4.8 S cm− 1 at 850 °C characteristic for the La0.2Sr0.25Ca0.45Ti0.95Fe0.05O3-δ (LSCTF-45) and Nd0.2Sr0.35Ca0.35Ti0.95Fe0.05O3-δ (NSCTF-35) materials, respectively, in 97% H2 + 3% H2O atmosphere. Keywords: solid oxide fuel cell; perovskite; fuel electrode; MIEC; LST. [1] X. Li, H. Zhao, X. Zhou, N. Xu, Z. Xie, N. Chen, Electrical conductivity and structural stability of La-doped SrTiO3 with A-site deficiency as anode materials for solid oxide fuel cells, international journal of hydrogen energy 35 (2010) 7913-7918.[2] A. Gondolini, E. Mercadelli, G. Constantin, L. Dessemond, V. Yurkiv, R. Costa, A. Sanson, On the manufacturing of low temperature activated Sr0.9La0.1TiO3-δ-Ce1-xGdxO2-δ anodes for solid oxide fuel cell, Journal of the European Ceramic Society, 38 (2018) 153–161.[3] A.D. Aljaberi, J.T.S. Irvine, Ca-substituted, A-site deficient perovskite La0.2Sr0.7TiO3 as a potential anode material for SOFCs. J. Mater. Chem. A 2013, 1, 5868-5874.

Improved cell performance and sulphur tolerance using A-site substituted Sr2Fe1.4Ni0.1Mo0.5O6–δ anodes for solid-oxide fuel cells

AbstractSolid-oxide fuel cells (SOFCs) offer great promise for producing electricity using a wide variety of fuels such as natural gas, coal gas and gasified carbonaceous solids; however, conventional nickel-based anodes face great challenges due to contaminants in readily available fuels, especially sulphur-containing compounds. Thus, the development of new anode materials that can suppress sulphur poisoning is crucial to the realization of fuel-flexible and cost-effective SOFCs. In this work, La0.1Sr1.9Fe1.4Ni0.1Mo0.5O6–δ (LSFNM) and Pr0.1Sr1.9Fe1.4Ni0.1Mo0.5O6–δ (PSFNM) materials have been synthesized using a sol-gel method in air and investigated as anode materials for SOFCs. Metallic nanoparticle-decorated ceramic anodes were obtained by the reduction of LSFNM and PSFNM in H2 at 850°C, forming a Ruddlesden–Popper oxide with exsolved FeNi3 bimetallic nanoparticles. The electrochemical performance of the Sr2Fe1.4Ni0.1Mo0.5O6–δ ceramic anode was greatly enhanced by La doping of A-sites, resulting in a 44% decrease in the polarization resistance in reducing atmosphere. The maximum power densities of Sr- and Mg-doped LaGaO3 (LSGM) (300 μm) electrolyte-supported single cells with LSFNM as the anode reached 1.371 W cm −2 in H2 and 1.306 W cm–2 in 50 ppm H2S–H2 at 850°C. Meanwhile, PSFNM showed improved sulphur tolerance, which could be fully recovered after six cycles from H2 to 50 ppm H2S–H2 operation. This study indicates that LSFNM and PSFNM are promising high-performance anodes for SOFCs.

Tailoring the Stability of Ti-Doped Sr2Fe1.4TixMo0.6-xO6-δ Electrode Materials for Solid Oxide Fuel Cells.

In this work, the stability of Sr2(FeMo)O6-δ-type perovskites was tailored by the substitution of Mo with Ti. Redox stable Sr2Fe1.4TixMo0.6-xO6-δ (x = 0.1, 0.2 and 0.3) perovskites were successfully obtained and evaluated as potential electrode materials for SOFCs. The crystal structure as a function of temperature, microstructure, redox stability, and thermal expansion properties in reducing and oxidizing atmospheres, oxygen content change, and transport properties in air and reducing conditions, as well as chemical stability and compatibility towards typical electrolytes have been systematically studied. All Sr2Fe1.4TixMo0.6-xO6-δ compounds exhibit a regular crystal structure with Pm-3m space group, showing excellent stability in oxidizing and reducing conditions. The increase of Ti-doping content in materials increases the thermal expansion coefficient (TEC), oxygen content change, and electrical conductivity in air, while it decreases the conductivity in reducing condition. All three materials are stable and compatible with studied electrolytes. Interestingly, redox stable Sr2Fe1.4Ti0.1Mo0.5O6-δ, possessing 1 μm grain size, low TEC (15.3 × 10-6 K-1), large oxygen content change of 0.72 mol·mol-1 between 30 and 900 °C, satisfactory conductivity of 4.1-7.3 S·cm-1 in 5% H2 at 600-800 °C, and good transport coefficients D and k, could be considered as a potential anode material for SOFCs, and are thus of great interest for further studies.

Characteristics of (Sr0.92Y0.08)0.85Ti1-XNixO3-

Perovskite with A-site deficiency(A1-xBO3) is promising candidate due to introduction of oxygen vacancy for ionic conductivity and nano-sized exsolution of B component with metal phase for catalytic activity. In this study, Ni-doped Sr0.92Y0.08TiO3 perovskite has been investigated to improve the catalytic activity for CO2 dry internal reforming of methane in solid oxide fuel cells. (Sr0.92Y0.08)0.85Ti1-xNixO3- \U0001d6ff (x=0.05, 0.10, 0.15, 0.20) perovskite with A-site deficiency is prepared by pechini method and compared to stoichiometric Sr0.92Y0.08Ti1-xNixO3- \U0001d6ff (SYTN15) perovskite. The physical and physicochemical characteristics of the synthesized powders are analyzed by GC, XRD, BET, TEM, XPS, and TPO. (Sr0.92Y0.08)0.85Ti0.85Ni0.15O3- \U0001d6ff (SYTN(+)15) shows the highest methane conversion with 93% and carbon dioxide conversion with 86%. In addition, the SYTN(+)15 results to the lowest decreasing rate of the methane conversion with 1.86% drop for 50h. The SYTN(+)15 shows better catalytic performance by 10% for dry reforming of methane (DRM) comparing to the SYTN15. The cell performances of the SYTN(+)15 and the SYTN15 are 60.05mW/cm2 and 28.50mW/cm2, respectively. Consequently, A-site deficiency catalyst, SYTN(+)15 exhibits excellent catalytic activity for DRM and sufficient electrochemical property for SOFC anode materials.

Towards integrated gasification and fuel cell operation with carbon capture: Impact of fuel gas on anode materials

Integrated gasification fuel cell technology is a promising option for processing solid fuels, which would enable high efficiencies to be reached in small-scale power generation. Among the different fuel cell types, solid oxide fuel cells present a good temperature match with fluidised bed gasification as well as greater versatility in terms of the fuel gas composition they can handle. However, their resistance to impurities in the gas needs to be addressed. The main objective of this work is to assess the impact on typical materials used in fuel cell anodes of the gases produced from a gasifier operating with a N2-free gasification agent, which would make the system carbon-capture ready. A laboratory scale continuous pressurised fluidised bed reactor has been modified to study CO2 and steam (concentration up to 40 mol%.) gasification of lignite at 850 °C. A second stage fixed bed reactor has been specially designed and constructed to study degradation of two SOFC anode materials (nickel/yttrium–stabilised zirconium oxide (Ni/YSZ) and nickel/gadolinium-doped ceria (Ni/CGO)) after exposure to real fuel gas at 765 °C. Under these conditions, which did not involve any gas cleaning/conditioning between stages, carbon deposition on the surface of anode materials was much smaller than in previous studies that used model tar compounds as feeds. Fuel gas from CO2/H2O gasification tended to deposit less carbon and sulphur on tested anode materials, particularly on Ni/CGO, than that from CO2 gasification. The anode materials converted a significant fraction of the fed tar to gas.

Zirconia-based materials in alternative energy devices - A strategy for improving material properties by optimizing the characteristics of initial powders

This work is devoted to the demonstration of a real multifunctional material – zirconia based nanopowders, which can be used in the manufacture of various types of devices that convert natural energy sources into electricity (electrolyte and anode for SOFC, moisture-to-electricity converter). It was shown that such characteristics of nanopowders as particle size, specific surface area, and type of surface centers, which depend on the temperature of nanopowder synthesis, could play both a positive and a negative role in the formation of the functional properties of a device. It was found that the synthesis of 8YSZ nanopowders at 700 °C is optimal for the manufacture of a dense electrolyte material and a porous SOFC anode material during sintering at 1400–1450 °C. The addition of a small amount of Al2O3 at the synthesis stage accelerates the sintering of the SOFC electrolyte material by 120 °C and leads to increasing the density to 97% of the theoretical value. The strength of the sintered anode material practically does not change after reduction in an atmosphere of N2–10% H2–5% CO2 and the value is 100 MPa.The synthesis temperature of 8YSZ nanopowder at 700 °C is also optimal for the manufacture of porous converters of ambient humidity into electricity. The formation of particles with a given size and type of surface centers allows generating an electric potential (100–200 mV) for hundreds of hours, which is 2–3 times higher than that of larger and smaller powders. The result of the work show that there are optimal conditions for the synthesis of powders (synthesis method, additives, synthesis temperature), as a result of which nanopowders are formed with characteristics (particle size, specific surface area, morphology) necessary and sufficient for the manufacture of several types of materials for devices of alternative energy.

Sr2Ni0.7Mg0.3MoO6-δ: Correlation between synthesis conditions and functional properties as anode material for intermediate-temperature SOFCs

The Sr2Ni0.7Mg0.3MoO6-δ double perovskites synthesis has been performed by a pyrolysis of organic-nitrate compositions with various ratios of ϕ (glycerol- or glycine content) and R (ammonium nitrate content). The solution combustion study revealed that the redox mixture composition for the Sr2Ni0.7Mg0.3MoO6-δ synthesis influenced the pyrolysis temperature, thermochemical charge generation, particles size, phase composition and physicochemical properties of the final oxide material. Investigations of thermodynamic stability, thermal expansion and electrical conductivity demonstrated that the Sr2Ni0.7Mg0.3MoO6-δ sample, obtained via pyrolysis synthesis with glycine triple excess without exothermic additive, may be considered as a prospective anode material for intermediate-temperature solid oxide fuel cells. It was confirmed by the sample stability in oxidative and reducing atmospheres, optimal values of linear coefficient of thermal expansion (14.4·10−6 K−1 in air; 13.1·10−6 K−1 in 50% H2/Ar) and the highest electrical conductivity value (0.53 S cm−1 at 800 °C in air) among glycine-samples.

Electrochemical Characteristics of Ni-Co Alloy Cermet Anodes for SOFCs

Introduction Nickel (Ni) is widely used as the anode material for SOFCs. However, metallic Ni is oxidized to nickel oxide (NiO) in an oxidizing atmosphere at SOFC operational temperatures. Ni anode degradation occurs when the hydrogen-containing fuel supply is reduced in high fuel utilization operation or interrupted during the shutdown [1]. Due to the redox cycles, Ni volume expansion and shrinkage cause irreversible degradation, by breaking electrically-conducting pathways. Highly durable SOFC anodes using e.g. Ni-based alloy are therefore under development [2]. Here, in this study, we focus on the Ni-Co alloy, by preparing a new Ni-Co-GDC cermet anode using the Ni-Co alloy, and evaluate the effects of Cobalt addition on their electrode performance and durability. Experimental In this study, we prepared the Ni-alloy cermet by mixing the powder and the binder, preparing and printing electrode paste on the electrolyte plate, followed by heat treatment. Scandia-Stabilized Zirconia (ScSZ, 10mol% Sc2O3-1mol% CeO2-89mol% ZrO2) was used as the solid electrolyte. The powder prepared by mixing Ni-Co-based oxide powder prepared by ammonia co-precipitation with Ce0.9Gd0.1O2 (GDC) in a specific ratio by weight, was used as the anode material. La1-xSrxMnO3 (LSM) powder was used as the cathode material. The electrochemical characteristics of the cells, with the Ni-alloy cermet anodes, were evaluated by I-V measurements and the current interruption method under the condition at 800oC, and 3%-humidified hydrogen fuel was supplied to the anode. Furthermore, the redox stability was evaluated using the durability test protocol simulating the fuel interruption. Anode microstructural changes before and after the redox cycling tests were observed by a focused-ion beam system coupled with a scanning electron microscope (FIB-SEM). Results and discussion Ni-alloy GDC anode with x mol% of Co added to Ni, is denoted as Ni(100 – x)Cox -GDC anode. The anodes of x = 0, 5, 10, 20, and 30 were fabricated. The I-V characteristics and overvoltages, under the condition where the operating temperature was 800oC and 3%-humidified hydrogen fuel was supplied, are shown in Fig. 1 for different Co contents in the alloy. With increasing the Co content, the electrochemical performance decreases due to a gradual increase in overvoltages. However, when the Co content is as low as 5 mol%, performance decrease is negligible. The microstructure of the Ni95Co5-GDC anode shown in Fig. 2 also shows that the distribution of Ni and Co is overlapped, confirming that a Ni-Co alloy is actually formed. Electrochemical properties and durability of the cells with the Ni-Co alloy anodes with various Co concentrations will be reported and discussed.

High-performance SrFe0.1Mo0.9O3-δ -based composites for the anode application in solid oxide fuel cells

The present work reports the electrochemical performance of the composites formed by SrFe0.1Mo0.9O3-δ and Gd0.1Ce0.9O2-δ for their possible anode application in solid-oxide fuel cells. Phase analysis reveals the absence of impurity phase on thermally treating the mixture at 950 °C in both oxidising and reducing conditions. The DC-electrical conductivity of the dense composite sample is metallic in nature with the magnitude of ~42.2 S cm−1 at 850 °C under reducing conditions. The electrode area-specific resistance is measured by performing two-probe impedance spectroscopy on symmetrical cells with Gd0.1Ce0.9O2-δ coated YSZ as an electrolyte from 700 °C to 850 °C. Porous electrodes with the compositions of xGd0.1Ce0.9O2-δ/SrFe0.1Mo0.9O3-δ (where x is a wt. fraction of Gd0.1Ce0.9O2-δ with the value of 0, 0.5, 0.6, 0.7 and 0.8) are fabricated by blade-coating the ceramic slurries and subsequently sintering at 950 °C for 1 h in air. The symmetrical cells with the composite electrode of 0.7Gd0.1Ce0.9O2-δ/SrFe0.1Mo0.9O3-δ exhibit an extremely low electrode area-specific resistance of ~0.086 Ω cm−2, which is nearly one-third of the magnitude shown by cells with pure SrFe0.1Mo0.9O3-δ as electrodes at 850 °C in 3% H2O/H2. Introducing higher porosity-level in the optimised composite electrode further reduces this value to ~0.06 Ω cm−2. However, not much effect has been observed on increasing the pore-former amount above 30 wt% of the electrode material. The overall activation energy for the polarisation resistance in 0.7Gd0.1Ce0.9O2-δ/SrFe0.1Mo0.9O3-δ is in the range of 0.4–0.5 eV. The impedance analysis indicates that adding the electrolyte phase of Gd0.1Ce0.9O2-δ in SrFe0.1Mo0.9O3-δ significantly enhances the kinetics of processes relating to the hydrogen-oxidation reaction occurring in the electrode. The exceptional performance of the Ni-free ceramic composite electrodes developed in the present work without employing expensive and labour-intensive infiltration technique for fabrication highlights the exciting prospects of Gd0.1Ce0.9O2-δ/SrFe0.1Mo0.9O3-δ composite as an anode material for SOFCs.

In-situ exsolved FeRu alloy nanoparticles on Ruddlesden-Popper oxides for direct hydrocarbon fuel solid oxide fuel cells

FeRu alloy (FRA) nanoparticles surface decorated Ruddlesden-Popper type layer perovskite PrSrFe1-xRuxO4+δ (RP-PSFeRu) was prepared by in-situ reduction of the cubic (Pr0.5Sr0.5)0.9Fe0.9Ru0.1O3-δ (P–PSFeRu) in H2 at 800 °C. When used as the SOFC anode material, it has excellent catalytic activity for H2 and hydrocarbon fuels. The La0.8Sr0.2Ga0.83Mg0.17O3-δ electrolyte supported SOFC single cell with RP-PSFeRu-FRA-GDC composite anode can deliver a maximum power density of 0.75 and 0.50 W cm−2 in wet H2 and C3H8 at 800 °C, respectively. Furthermore, the single cell shows a stability outputs at a constant current load of 0.5 A cm−2 in wet H2 and 0.15 A cm−2 in wet C3H8 fuels, indicating an exceptional stability and cooking resistance.

Effect of Arsenic and Selenium on Nickel in SOFC Anode Materials

Effect of Arsenic and Selenium on Nickel in SOFC Anode Materials

Designing experiments for the preparation of Ni-GDC cermets with controlled porosity as SOFC anode materials: effects on the electrical properties

NiO-Ce0.9Gd0.1O1.95 (NiO-GDC) composites (30, 40 and 50 wt% NiO) were obtained by a “one-step” synthesis method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses were applied to the calcined compositions, revealing powders in the nanometer range. NiO-GDC composites were sintered at 1450–1550 °C and the respective Ni-GDC cermets were produced in a 10%H2-N2 atmosphere at 800 °C. A factorial design tool allowed to establish a regression equation that correlates the effects of Ni content and sintering temperature on the porosity of Ni-GDC cermets. Impedance spectroscopy was used to analyse the electrical properties of the solid oxide composites in air (at 500–650 °C) and of the Ni-GDC cermets in 10%H2-N2 (at 650 °C). The impedance analysis in reducing conditions revealed an improved in the electrical properties for the Ni-GDC cermet derived from the composite with 50 wt% NiO and sintered at 1450 °C. The electrical resistivity values are among the best ones reported for Ni-based cermets produced by different conditions.

Preparation and Characterization of Alternative Ni-Based Anode Materials for SOFCs

Introduction Ni is widely used as the anode material of SOFCs as a cermet containing ionic conductor and electronic conductor. In the SOFC power generation, the oxygen partial pressure in the downstream region of the SOFC systems increases, the metallic Ni can be oxidized to NiO, and thus volume expansion occurs1. Aggregation of adjacent Ni particles can degrade the electrode structure, which makes it difficult to fully utilize the fuel supplied. We have succeeded in fabricating an SOFC anode which is resistant to redox cycling using oxide conductors, La-doped SrTiO3 (LST) and Gd-doped CeO2 (GDC). In this study, we aim to develop (i) a novel electrode in which a metallic Ni catalyst is dissolved from the electrode material and precipitated on the surface of the anode in a reducing atmosphere (Ni-LST-GDC anode), and (ii) a novel electrode using a Ni-based alloy in the electrode framework (Ni-alloy-GDC anode)2,3. Experimental Alternative anode materials were prepared via wet process2,3. The cells were prepared by screen-printing the anode and the cathode followed by each sintering process in air. A platinum mesh was used for the current collector, and a Pt-based reference electrode was deposited on the cathode side for overvoltage separation between the anode and the cathode. The electrochemical performance of the fuel cells was evaluated from the I-V characteristics at the operating temperature of 800°C, using 3%-humidified hydrogen fuel. In order to evaluate the electrode activity, the electrode overvoltage was measured. The current interruption method was applied for evaluating the electrode overvoltages. Results and discussion Figure 1 shows the I-V characteristics of the cells with the newly prepared Ni-LST-GDC anode, the Ni-impregnated anode, and the LST-GDC anode without adding Ni, at 800°C, in 3%-humidified hydrogen fuel. Regarding the anode voltage and the anodic overvoltage, the Ni-LST-GDC anode exhibited better performance than the anode composed only of LST and GDC. In comparison with the Ni-impregnated anode, however, the cell performance was still lower both in anode voltage, overvoltage, and IR loss. Figure 2 shows STEM micrographs indicating that the Ni catalyst particle precipitated could locate on the GDC. Anode materials design principles of alternative Ni-based anodes will be discussed. But most of Ni particles located on the electron-conductive LST. In contrast, the cell with Ni-alloy anode exhibited comparable performance compared to that with the Ni-SSZ cermet.

Methane Pulse and in-Situ Raman Study of Impregnated Ni/CGO As Anodes of SOFC

Nickel/gadolinium doped ceria (Ni/CGO) has been a popular SOFC anode material for decades for its superior mix conductivity and excellent oxygen storage capacity. However, the study of Ni/CGO in a realistic working condition is still lacking. Also, not much research has linked the chemical properties of CGO with the carbon resistance which is essential when using hydrocarbon as fuels. In this research, Ni/CGO powder was prepared via impregnation-calcination method. A novel pulse experiment has been conducted on impregnated powder. We did certain pulses of a fixed amount of methane into the sample at 600℃.The outlet gas was monitored by a quadrupole mass spectrometer. The product gas was composed of several species including hydrogen, steam, carbon monoxide, carbon dioxide, and methane that was not participated to the reaction. The major reaction was the partial oxidation of methane. With the increase of pulsing number, the product and shape of product peak changed a lot. Also, we observed a “tail” after each carbon monoxide production peak which was deduced to be the self-oxidation of carbon by the bulk migrated oxygen. After detailed quantitative analysis of the shape and intensity of the peaks and tails. We came up with a model in which the mechanism of methane partial oxidation was confirmed and so was the relation between carbon self-oxidation rate, surface oxygen and bulk oxygen migration rate of CGO. In-situ Raman spectroscopy was performed during the pulse experiment to monitor the change of CGO surface oxygen stoichiometry and deposited carbon. Thus we can confirm the conclusion deduced by pulse experiments and directly relate the carbon self-oxidation rate with bulk oxygen migration of CGO. The results would provide an insight into the design and modification of SOFC anode materials.

Development and Testing of Impregnated La0.20Sr0.25Ca0.45TiO3 Anode Microstructures for Solid Oxide Fuel Cells

The Ni-based cermet SOFC anode material exhibits a number of undesirable properties. It is often redox unstable 1, Ni-particles agglomerate over time 1, ultimately reducing the catalytic activity of the anode and, in unprocessed natural gas feeds, this material exhibits severe coking intolerance 1 as well as sulphur poisoning 1. Therefore, there is a requirement to produce a novel anode material which can offer better durability under the aforementioned conditions. An ideal replacement material comprises a ‘backbone’ microstructure of the A-site deficient perovskite: La0.20Sr0.25Ca0.45TiO3 (LSCTA-), impregnated with an oxide ion conducting material and a metallic electrocatalyst. This ‘backbone’ material, impregnated with Ce0.8Gd0.2O1.9 (CGO) and Ni, has previously been employed as a SOFC anode material on an industrially relevant scale at HEXIS AG, with promising results 2. Current research focusses on optimisation of the LSCTA- ‘backbone’, to improve current distribution through the anode, and development of impregnated catalyst systems that provide enhanced durability and tolerance to natural gas feeds containing sulphur. Thick-film ceramic processing techniques, such as ink formulation, screen printing and control of sintering protocol, have been used as the primary method in controlling the anode microstructure. Rheological analysis of a variety of LSCTA- inks showed that a formulation with 75 wt. % solids loading possessed ideal (pseudoplastic) properties for screen printing. Extensive investigation of screen printing parameters and screen mesh counts, as well as sintering temperatures and dwell times, allowed determination of the optimal conditions required to produce a LSCTA- anode ‘backbone’ microstructure with an advantageous combination of porosity and grain connectivity. Screen printing of the 75 wt. % solids loading ink with a 230 mesh count (per inch) screen and sintering at 1350 °C for 2 hours facilitated production of the required anode microstructure, ensuring sufficient lateral electronic conductivity through the anode to prevent generation of localised temperature ‘hotspots’. Four-point DC conductivity analysis of several LSCTA- ‘backbone’ microstructures showed that ‘effective’ conductivities of up to 21 S cm-1 could be achieved (in 5% H2/Ar), with the highest values pertaining to the most advantageous microstructure. Electrolyte-supported fuel cells employing this ‘backbone’ microstructure, impregnated with 12-16 wt. % (of the ‘backbone’) of CGO and 2-5 wt. % of either Ni, Ru, Rh, Pt or Pd, showed very promising performances during short-term electrochemical testing in humidified hydrogen. Fuel cells with anodes containing Rh/CGO and Pd/CGO catalyst systems were particularly promising, achieving Area Specific Resistances (ASR) of 0.41 Ω cm2 and 0.39 Ω cm2 (figure 1), respectively.

Methane Pulse Study on Nickel Impregnated Gadolinium Doped Ceria

Nickel/gadolinium doped ceria (Ni/CGO) has been a popular SOFC anode material for decades for its superior mix conductivity and excellent oxygen storage capacity. However, the study of Ni/CGO in a realistic working condition is still lacking. Also, not much research has linked the chemical properties of CGO with the carbon resistance which is essential when using hydrocarbon as fuels. In this research, Ni/CGO powder was prepared via impregnation-calcination method. A novel pulse experiment has been conducted on impregnated powder. We did certain pulses of a fixed amount of methane into the sample at 600℃.The outlet gas composition was monitored by a quadrupole mass spectrometer. The product gas was composed of several species including hydrogen, steam, carbon monoxide, carbon dioxide, and methane that was not participated to the reaction. The major reaction was the partial oxidation of methane that produced carbon monoxide and hydrogen. With the increase of pulsing numbers, the product and shape of product peaks changed a lot. Also, we observed a “tail” after each carbon monoxide production peak which extend with the number of pulsing. This was deduced to be caused by the self-oxidation of carbon by the bulk migrated oxygen. These changes revealed the changing of CGO during methane oxidation. After detailed quantitative analysis of the shape and integrated intensity of the peaks and tails. We confirmed that the majority of methane partial oxidation reaction on Ni/CGO follows a deposition-oxidation route rather than direct oxidation of methane. Also we were able to evaluate the ability of carbon self-oxidation with different ceria oxidation state. Consequently, a model was proposed to elucidate the relation between carbon self-oxidation rate, surface oxygen concentration and bulk oxygen migration rate of CGO, in forms of partial differential equation. In-situ Raman spectroscopy was performed during the pulse experiment to give a direct parallel evidence. We observed the change of CGO surface oxygen stoichiometry and deposited carbon. Thus we can confirm the conclusion deduced by pulse experiments to prove its validation, and directly relate the carbon self-oxidation rate with bulk oxygen migration rate of CGO. The results provided a novel yet facile method to evaluate the oxygen storage properties of materials and gave an insight into the design and modification of ceria based SOFC anode materials.

A Reactive Force Field (ReaxFF) for Molecular Dynamics Simulations of NiO Reduction in H2 Environments

Ni/YSZ cermet is a widely used SOFC anode material, particularly due to its combination of good electric conductivity, high catalytic activity and low cost. Nonetheless, Ni/YSZ cermet anodes are facing unique challenges such as nickel agglomeration and reoxidation, eventually resulting in anode degradation. A solution to these problems might be obtained via a better comprehension of the underlying manufacturing processes, particularly the final H2-induced reduction reaction of NiO to Ni. Experimental approaches have contributed immensely toward a better understanding of these phenomena, yet are generally limited in terms of spatial and temporal resolution, thus motivating us to investigate the problem from a multiscale simulation viewpoint. In this study, we developed a reactive force field (ReaxFF) for nickel/oxygen interactions, which was specifically tailored toward molecular dynamics (MD) simulations of NiO reduction in H2 environments. The force field parameters were derived by fitting the ReaxFF energies and forces of various nickel/oxygen structures to the respective values of first-principles calculations. The structures considered here include bulk NiO, Ni2O3 and Ni3O4 at various densities, NiO under shear, the NiO(100) surface, oxygen vacancies in bulk and surface NiO as well as oxygen adsorbed onto the NiO(100) surface. The developed force field was subsequently used to predict the reduction kinetics of nano-scale NiO crystals in a H2 atmosphere at different temperatures and hydrogen partial pressures. Here, we observed a significantly improved structural stability of bulk NiO at temperatures over 700 K, compared to our previous ReaxFF nickel/oxygen description [1] and similar ReaxFF parameter sets published by other authors [2]. Besides quantifying the reduction rate, further core phenomena such as the induction period before the onset of a steady state reduction process, the pore formation in NiO at lower temperatures, as well as the importance of oxygen vacancies during the reduction are discussed and compared with experimental results. Fig. 1 NiO nanocrystal after 0.2 ns of MD reduction simulation at a global temperature of 850 K and hydrogen partial pressure of 10 bars, using our previous ReaxFF parameter set (left) and the parameter set introduced in this publication (right).

Ni-SDC Based Cermets for Direct Dry Reforming of Methane on SOFC Anode

In this study, Ni-SDC (samarium-doped ceria) and CaO-modified Ni-SDC were examined as anode materials for SOFC with direct internal dry reforming of methane. The CaO addition is aimed to enhance the coking resistance in the methane direct internal dry reforming operation. Current-voltage characteristics were measured by feeding directly CH4-CO2 mixture to the anodes, and overvoltage in the power generation was separated from alternative current impedance measurements. In addition, stability tests under a constant current loading were carried out to evaluate the durability. From these results, it is demonstrated that Ni-SDC based materials are promising due to their high performance, coking resistance and stability.

Methane Pulse Study on Nickel Impregnated Gadolinium Doped Ceria

Ni/CGO was prepared via impregnation. A facile methane pulse experiment was performed on Ni/CGO under SOFC operating condition. The outlet gas was monitored by a quadrupole mass spectrometer. A Van-Mars Krevelen type mechanism was proposed for methane reaction on Ni/CGO and analyzed in detail. The method showed promising prospect in analyzing mechanism of hydrocarbon oxidation and the results provide a new insight in the strategy of preventing carbon deposition on nickel catalyst when operating as a SOFC anode materials.