Sm-doped Ceria Research Articles

Additional Voltage Loss to Explain an Equilibration Process in Response to a Change in the Anode Gas Using Sm-Doped Ceria Electrolytes

The electronic conductivity of Sm-doped ceria is very low in air and increases substantially under hydrogen gas. We measured the equilibration process using a very thick (6.6 mm) samarium-doped ceria electrolyte in response to a change in the anode gas. The open circuit voltage gradually increased to an equilibrium voltage of 0.80 V within 5 minutes. The time constant was only 1 minute. However, according to Weppner’s approach, the equilibrium time should be much longer than 5 minutes. It is impossible to calculate the open current voltage during equilibration in response to a change in the oxygen activity of the anode gas. With the assumption of a current-independent constant anode voltage loss, the calculation results provided a significantly improved explanation of the experimental results, both quantitatively and qualitatively. Consequently, both Weppner’s approach and Wagner’s equation can coexist with this method. This proposed voltage loss does not contradict the model for very thin films but is necessary for improvement to explain the experimental results. Furthermore, a current-independent constant anode voltage loss is a general topic in electrochemistry and important to explain physical phenomena not only in the area of solid oxide fuel cells.

Influence of Ag on Chemical and Thermal Compatibility of LSCF-SDCC for LT-SOFC

In addition to the good electrochemical performance criteria in solid oxide fuel cell (SOFC) applications, cathode material must match thermal expansion with other SOFC components. Thus, effects of Ag on thermal mismatch, chemical reactions, and microstructure are investigated. Ag (1 wt% to 5 wt. %) was mixed with La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF6428) and Sm-doped ceria carbonate (SDCC) composite cathode powder. LSCF6428-SDCC-Ag samples were sintered at 600 °C for 2 h. The thermal expansion coefficients (TECs), which were determined using a dilatometer, indicated relatively less TEC mismatch between LSCF-SDCC-Ag cathodes composite and SDCC electrolyte. The average TEC value obtained from 20 °C to 600 °C implied that LSCF-SDCC-A5 (5 wt. % Ag) showed better thermal matching (13.18×10−6 K−1) with SDCC electrolyte (12.84×10−6 K−1) and achieved better compatibility. The X-ray diffraction patterns indicated that the LSCF6428-SDCC-Ag peak increased with the increase in the amount of Ag. Scanning electron microscopy analysis showed that Ag was capable of maintaining the porosity that is required for cathodes (20%–40%). Results showed that Ag exhibited desirable thermal and chemical compatibility with LSCF-SDCC. Thus, LSCF6428-SDCC-Ag can be used as a composite cathode for low-temperature SOFCs.

Characterization of Sm-Doped Ceria Ceramics Synthesized by Two Different Methods

Ceria-based materials have attracted much attention as electrolyte materials for low and intermediate temperature solid oxide fuel cells (SOFCs). In this study, we examined the effect of synthesis method on the microstructure and the ionic conductivity of ceria-based electrolytes. Sm0.2Ce0.8O1.9 (SDC) electrolytes for SOFCs were prepared using the Pechini and cellulose templating (CT) methods. Microstructures of the calcined and sintered samples were characterized by XRD and SEM techniques. The XRD results indicate that a single-phase fluorite structure formed at the relatively low calcination temperature of 500 °C. The relative densities of the sintered pellets were higher than 90%, which was proved by the SEM images. Calcined powders were characterized by FTIR technique. The electrical properties of the samarium-doped ceria electrolytes were analyzed by electrochemical impedance spectroscopy. The total ionic conductivities are 3.02 × 10−2 and 3.42 × 10−2 S/cm at 750 °C for the SDC electrolytes prepared by the Pechini method and the CT method, respectively.

Enhanced ionic conductivity of Sm0.2Ce0.8O2−δ electrolyte for solid oxide fuel cells through doping transition metals

Ceria-based electrolytes are considered as candidate electrolyte materials for intermediate temperature solid oxide fuel cells due to the high oxygen-ion conductivity. Lowing sintering temperature for ceria-based electrolyte is favorable for co-firing process as well as the reduction of fabrication cost. Transition metals (Fe, Co, Ni) were added to Sm-doped ceria (SDC) and the crystal structure, sintering performance and ionic conductivity were investigated by X-ray diffraction, SEM and AC impedance spectroscopy. The transition metal elements added to SDC as sintering aids reduced the sintering temperature of SDC electrolyte by 100–150 °C and the grain size decreased with increased doping amount of transition metals. The ionic conductivity, especially the grain-boundary conductivity was significantly improved by a small addition of transition metals. Among Fe, Co, Ni doped SDC electrolytes, the 0.01 mol Fe3+ doped SDC sintered at 1350 °C exhibited the highest ionic conductivity about 0.063 S cm−1 at 800 °C and the lowest activation energy of 0.73 eV.

Electrochemical behavior of thin-film Sm-doped ceria: insights from the point-contact configuration

The electrochemical behavior of chemical vapor deposition (CVD) grown porous films of Sm-doped ceria (SDC) for hydrogen oxidation has been evaluated by impedance spectroscopy using a point contact geometry at a temperature of 650 °C. Porous SDC films, 950 nm in thickness, were deposited on both sides of single-crystal YSZ(100). Pt paste was applied over the surface of one SDC layer to create a high-activity counter electrode. Ni wire was contacted to the surface of the other SDC layer to create a limited contact-area working electrode. The active area of contact at the working electrode was determined using the Newman equation and the electrolyte constriction impedance. The radius of this area varied from 5 to 18 μm, depending on gas composition and bias. The area-normalized electrode impedance (where the area was that determined as described above) varied from 0.03 to 0.17 Ω cm(2) and generally decreased with cathodic bias and decreasing oxygen partial pressure. From an analysis of the dimensions of the active area with bias, it was found that the majority of the overpotential occurred at the SDC|gas interface rather than the SDC|YSZ interface. Overall, the anode overpotential is found to be extremely small, competitive with the best oxide anodes reported in the literature. Nevertheless, the impedance falls in line with expected values based on extrapolations of the properties of dense, flat SDC model electrodes grown by pulsed laser deposition (Chueh et al., Nat. Mater., 2012). The results demonstrate that, with suitable fabrication approaches, exceptional activity can be achieved with SDC for hydrogen electrooxidation even in the absence of metal-oxide-gas triple phase boundaries.

A DFT+UStudy of Strain-Dependent Ionic Migration in Sm-Doped Ceria

Recent experiments on thin films and hetero-structures of ionically conducting oxides have indicated enhancement of oxygen ion conduction typically ascribed to strain-related effects. We performed density functional theory (DFT) calculations with Hubbard U correction to investigate the effects of biaxial lattice strain on the oxygen anion diffusivity in Sm-doped ceria (SDC). Here we found that the migration barriers are strongly affected by the applied strain. The changes in the migration barriers can almost be linearly correlated with cation–anion bond-lengths between the migrating oxygen and its nearest neighbor cations. In this work, we reported various possible dopant configurations for oxygen ion migrations at the vicinity of the oxygen vacancy. Our results indicate that the ionic migration barriers can be lowered by both compressive and tensile strains. The insight gained from this study may enable us to engineer and to improve oxygen ion migration in ceria-based solid electrolyte materials by compression or tension of the lattice parameters.

A Current-Independent Constant Anode Voltage Loss Using Sm-Doped Ceria Electrolytes in SOFCs

Numerous approaches have been used to solve the transport equation that describes a solid-oxide fuel cell with mixed ionic and electronic conductors, such as Sm-doped ceria. Typically, the open circuit voltage is calculated using Wagner’s equation. By accounting for the polarization voltage losses, the open circuit voltage can be calculated. However, electrodes degrade over time, and the open circuit voltage cannot be calculated during electrode degradation because the open circuit voltage remains unchanged. The current work proposes a solution to this problem using a current-independent constant anode voltage loss. The denial of large leakage currents does not deny the Wagner-Schmalzried method when a large current-independent constant anode voltage loss occurs. Furthermore, the importance of this voltage loss is discussed.

Interfacial Phenomena of Alkalimetal Carbonate on Sm-Doped Ceria for Composite Electrolytes

High temperature Raman spectroscopy of Sm doped CeO2/carbonate composite was measured in order to discuss the interaction between carbonate ion and solid oxide electrolyte at an interface in Sm doped CeO2/carbonate LT-SOFC. In Raman spectra with temperature of carbonate breathing mode of NK in initial heating process, whereas separated ν1(CO3 2-) bands assigned to Na2CO3 and K2CO3 are observed clearly at lower temperature, unifying of these bands was observed due to solid state migration of carbonates at temperature lower than eutectic point (983 K). Degree of migration is affected by apparent average thickness (AAT) and Sm amount in oxide. Solid state diffusion at temperature lower than eutectic point of carbonate mixture is assisted by coexistence of solid oxides.

Interfacial Phenomena of Alkalimetal Carbonate on Sm-Doped Ceria for Composite Electrolytes

It is well-known that high temperature fuel cells as MCFCs and SOFCs operated at ca. 923-1273 K,generate electricity with high power density and fuel efficiency. Since component materials and cell configuration that sustainable at high operating temperature under oxidation/ reduction atmosphere are undefined, comprehensive improvement for included electrodes and electrolyte is needed. Composite electrolyte materials including solid oxide and carbonate indicate high conductivity at relative low temperature and are expected that various materials can be utilized for the operation of low temperature SOFCs etc. Wang et al. consider that there is an interaction at the interface of solid oxide particles and sodium carbonate phase, leading to the superior proton conduction [1].In case of heterophase system such as carbonate/ceria based oxide composite system, it is important to consider the interfacial properties and behavior of ionic species at interface. In this study, to clarify the properties of solid oxide/carbonate composite system, behavior of carbonate ion at ca. 300—800 K was investigated by spectroscopic method.CeO2 and Sm-doped CeO2 samples(SDCs) were synthesized by co-precipitation method[2]. Obtained precursor was separated, dried, milled and calcined at 1073 K for 5 hours under air atmosphere. Samples are indicated as SDC10(10 mol%), SDC20(20 mol%), SDC30(30 mol%), and SDC50(50mol%), according to amount of dopant Sm.For eutectic mixture defined as NK. Na2CO3 and K2CO3 was dried at 473 K for 48 hours under CO2 atmosphere, and mixed at Na:K = 59:41 and milled throughly. The carbonate mixture is added to CeO2 or SDCs powder indicated as CeO2-NK or SDCs-NK, respectively. Raman spectroscopy and TG-DTA were used for characterization of the samples as described elsewhere[3].According to XRD patterns of solid oxide samples, CeO2, SDC10, 20 and 30 have fluorite structure specific to pure CeO2. On the other hand, SDC50 has both patterns of CeO2 fluorite structure and pure Sm2O3. This result indicates there are two coexisting phases of CeO2-Sm2O3 solid solution and segregated pure Sm2O3phase in SDC50.Surface state and defects were observed by Raman spectroscopy. The band at about 460 cm-1is called as F2g band, which is assigned to Ce-O8 breathing mode [3]. F2g band in samples CeO2, SDC20 and SDC30 shifted to lower wavenumber in associate with dope amount increasing, but in SDC50, F2g band shifted to higher wavenumber. Quantitative analysis by XPS and XRF, deviation of Sm amount was observed. Change of surface composition and structure associated with solid solution formation and segregation of Sm2O3makes a profound difference on surface state. From these results, it is projected that properties of composite materials do not follow Sm content change linearly.In Raman spectra with temperature of carbonate breathing mode of NK in initial heating process, whereas separated n1(CO3 2-) bands assigned to Na2CO3 and K2CO3are observed clearly at lower temperature, unifying of these bands was observed due to solid state migration of carbonates at temperature lower than eutectic point (983 K). Degree of migration is affected by apparent average thickness (AAT) and Sm amount in oxide (Fig. 1).[1] X. Wang et al., J. Power Sources, 196, 2754 (2011).[2] Z. Ning, Int. J. Hydrog. Energy, 37, 797(2012).[2] M. Mizuhata et al. Int. J. Hydrogen Energy, 37, 24, 19407(2012).

Synthesis and electric properties of perovskite Pr0.6Ca0.4Fe0.8Co0.2O3 for SOFC applications

In this study, polycrystalline powder Pr0.6Ca0.4Fe0.8Co0.2O3 (PCFC) was synthesized by a sol–gel process. This oxide was analyzed by X-ray powder diffraction. Synthesized Pr0.6Ca0.4Fe0.8Co0.2O3 showed up to be single phase and belongs to the orthorhombic crystalline system with a Pbnm space group. The microstructural features of the synthesized products display particles having an irregular morphology and a size in the range of 50–100 nm. X-ray diffraction (XRD) analysis shows the chemical compatibility between the PCFC cathode and the electrolyte Sm-doped ceria since no reaction products were honored when the material was mixed and co-fired at 1,000 °C for 168 h. The thermal expansion coefficient of PCFC 16.9 × 10−6 °C−1 is slightly higher than that of Ce0.8Sm0.2O1.9 (SDC) over the studied temperature range. The greater contribution to the total resistance of the electrode is the electrochemical resistance associated with oxygen exchange in the cathode surface (0.96 Ωcm2). The dc four-probe measurement indicated that PCFC exhibits fairly high electrical conductivity, over 100 S cm−1 at T ≥ 500 °C, making this material promising as a cathode material for intermediate temperature solid oxide fuel cells.

Chemical compatibility and properties of suspension plasma-sprayed SrTiO3-based anodes for intermediate-temperature solid oxide fuel cells

La-doped strontium titanate (LST) is a promising, redox-stable perovskite material for direct hydrocarbon oxidation anodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this study, nano-sized LST and Sm-doped ceria (SDC) powders are produced by the sol–gel and glycine–nitrate processes, respectively. The chemical compatibility between LST and electrolyte materials is studied. A LST–SDC composite anode is prepared by suspension plasma spraying (SPS). The effects of annealing conditions on the phase structure, microstructure, and chemical stability of the LST–SDC composite anode are investigated. The results indicate that the suspension plasma-sprayed LST–SDC anode has the same phase structure as the original powders. LST exhibits a good chemical compatibility with SDC and Mg/Sr-doped lanthanum gallate (LSGM). The anode has a porosity of ∼40% with a finely porous structure that provides high gas permeability and a long three-phase boundary for the anode reaction. Single cells assembled with the LST–SDC anode, La0.8Sr0.2Ga0.8Mg0.2O3 electrolyte, and La0.8Sr0.2CoO3–SDC cathode show a good performance at 650–800 °C. The annealing reduces the impedances due to the enhancement in the bonding between the particles in the anode and interface of anode and LSGM electrolyte, thus improving the output performance of the cell.

A Current-Independent Constant Anode Voltage Loss Using Sm-Doped Ceria Electrolytes in SOFCs

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MoO3 nanorods/Fe2(MoO4)3 nanoparticles composite anode for solid oxide fuel cells

MoO3 nanorods/Fe2(MoO4)3 nanoparticles composite has been prepared by a hydrothermal method combined with an in situ diffusion growth process. Single cells based on 300 μm LSGM electrolyte have been fabricated with the MoO3 nanorods/Fe2(MoO4)3 nanoparticles composite anode and a composite cathode consisting of Sr0.9Ce0.1CoO3−δ and Sm-doped ceria (SDC). The peak power densities reach 225, 50, 75 mW cm−2 at 900 °C in H2, CH4 and C3H8, respectively. The cell shows excellent long-term stability at 850 °C. The preliminary results demonstrate that the MoO3 nanorods/Fe2(MoO4)3 nanoparticles composite is a promising alternative anode for solid oxide fuel cells.

Ce1−xSmxO1.9−δ nanoparticles obtained by microwave-assisted hydrothermal processing: an efficient application for catalytic oxidation of α-bisabolol

Heterogeneous catalysts based on Sm-doped ceria were employed for the first time in the liquid-phase oxidation of α-bisabolol. Nanometer-sized catalysts were obtained by microwave-hydrothermal synthesis and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), Raman spectroscopy and N2-physisorption. The influence of Sm doping, temperature and the solvent used on the catalytic behavior was investigated. Conversions up to 84% and a combined selectivity for the products up to 77% were obtained for Ce0.9Sm0.15O1.85−δ catalysts. The reactions were highly selective for the epoxidation products (only bisabolol oxides A and B were obtained) and shown to be strongly dependent on the temperature and solvent employed. Best results were achieved for higher Sm concentrations, which indicate that changes in the textural properties due to doping produced a significantly more active catalyst.

First-Principles Study of the Lattice Strain Effects on the Ionic Migration Barrier of Sm-Doped Ceria

Recent experiments on thin films and hetero-structures of ionically conducting oxides have indicated to an enhancement of oxygen ion conduction typically ascribed to strain-related effects. We performed first-principles calculations to investigate the effects of biaxial lattice strain on the oxygen anion diffusivity in Sm2O3 doped CeO2 (SDC). The oxygen ion migration paths and barriers in SDC as functions of lattice strain were assessed computationally using density functional theory (DFT) with U correction and climbing image nudged elastic band (CI-NEB) method. Our current results indicate that both expansive and compressive strains lead to decreases in the ionic migration barriers.

Electrical Properties of Sm‐doped Ceria (SDC) and SDC Carbonate Composite

AbstractThis study prepared a dense Sm‐doped ceria (SDC) and an SDC carbonate composite (abbreviated as SDC‐C). The latter was prepared by immersing porous SDC with a formula of (Ce0.8Sm0.2)O1.9 and a relative density of approximately 65‐70% into a molten mixture of carbonates containing 1:1 molar ratio of Li2CO3 and Na2CO3 at 500 °C. The relative density of the SDC‐C was close to 100%. In addition, SDC oxide without carbonates, which also has a relative density of close to 100%, was heat treated at 1600 °C. At 500 °C, the electrical conductivity and ionic transference number (ti) of the SDC oxide were 1.79(5) × 10−3 S·cm−1 and 0.99(2), respectively, such that electronic conduction could be disregarded. Increasing the temperature caused a gradual decrease in the ti of SDC. Following the addition of carbonates to SDC, the electrical conductivity reached 1.23(9) × 10−1 S·cm−1 at 500 °C. After 14 days (340 h), the electrical conductivity of the SDC‐C at 490 °C, leveled off at about 6 × 10−2 S·cm−1. SDC‐C could be used as a potential electrolyte in solid oxide fuel cells (SOFCs) at temperatures below 500 °C.

Effect of porous interlayer on the long-term stability of (La0.8Sr0.2)0.97MnO3 cathode for solid oxide fuel cells

In this work, the effect of porous interlayer on the microstructural change of LSM cathodes and yttria-stabilized zirconia (YSZ) electrolyte was investigated under a current-accelerated condition at 1000 °C. The polarization resistance of LSM cathode on bare yttria-stabilized zirconia (YSZ) electrolyte decreased significantly after discharge at 1.2 A cm−2 for 5 h, and then started to increase gradually. However, when a porous YSZ interlayer was employed between LSM cathode and YSZ electrolyte, the polarization resistance did not increased even after discharge at the same condition for 40 h. The performance changes of the two kinds of cathodes were related with the microstructural change of the interface between LSM cathode and YSZ interlayer. Quantitative analysis for the microstructural change of LSM cathode on YSZ interlayer revealed that even after discharge at 1.2 A cm−2 for 40 h, 90% of the triple phase boundary of LSM, YSZ and gas phase remained active for oxygen reduction reaction. On the other hand, the LSM cathode on YSZ electrolyte with a porous Sm-doped ceria interlayer showed remarkable long-term stability even after discharge at 1.2 A cm−2 for 160 h.

Effect of Y on the Properties of Sm-Doped Ceria for IT-SOFC Applications

The co-doped ceria-based materials with general composition formula Ce0.8-xYxSm0.2O2-δ (x = 0, 0.02, 0.04, 0.06) were prepared through the sol-gel method. The single phase of the prepared materials was confirmed by X-ray diffraction (XRD). The lattice parameters were determined by least square fitting of UNIT CELL programme. The linear variation of lattice parameter with concentration of Y into the samarium doped ceria (SDC) indicates the validity of Vegard’s law. The crystallite size of the samples obtained by using of Scherrer formula is in the range from 34 nm to 49 nm. The thermal expansion studies were carried out by using dilatometric technique in the temperature range from room temperature to 1000°C. It was observed that the thermal expansion increased linearly with increasing temperature for all the samples. The electrical conductivity was studied using impedance spectroscopy. It was observed that the composition Ce0.74Y0.06Sm0.2O2-δ showed higher electrical conductivity and lower activation energy (≈1.05 eV) than other compositions in the present investigation. This composition is thus a possible candidate for use the electrolyte in intermediate temperature solid oxide fuel cells (IT-SOFC).

New buffer layer material La(Pr)CrO3for intermediate temperature solid oxide fuel cell using LaGaO3-based electrolyte film

Abstract

Properties of Al2O3–Sm2O3–CeO2 electrolyte

Ce1-xSmxO2 (x=0, 0.1, 0.2 and 0.3) and Sm-doped ceria+(2%–8%)Al2O3 were synthesized through sol-gel process followed by low temperature combustion. The synthesis, structure, densification, conductivity and thermal expansion were studied, and the structure and phase were confirmed by XRD. Dense ceramics were obtained through sintering the pellets at 1300 °C. 2% and 4% Al2O3 were added into Ce0.8Sm0.2O2 to promote the densification at 1250 °C. The surface morphology of the sintered pellets was analyzed using SEM. A two-probe AC impedance spectroscopy was used to study the total ionic conductivity.