Synthesis and properties of Y-doped SrTiO3 as an anode material for SOFCs

Y-doped SrTiO₃ was synthesized via solid-state reaction. The effects of Y-doping on the sinterability and the electrical conductivity of Y_xSr_1−xTiO₃ were investigated. Y-doping can increase the sintering activity and the electrical conductivity of SrTiO₃ when yttrium amount is less than 0.09 in Y_xSr_1−xTiO₃. Excessive yttrium will cause the generation of an insulating phase Y₂Ti₂O₇, which impedes the densification process and decreases the electrical conductivity of Y_xSr_1−xTiO₃ material. With the increased temperature, the electrical conductivity of Y-doped SrTiO₃ increases first and then decreases gradually, showing a mixed conduction behavior of semi-conductors and metals. The optimized Y_0.09Sr_0.91TiO₃ possesses an electrical conductivity on the order of 32.5–195.8 S cm⁻¹ in the temperature range of 25–1000 °C and being 73.7 S cm⁻¹ at 800 °C in forming gas. The thermal cycling in air does not remarkably affect the electrical conductivity and the conduction behavior of Y_0.09Sr_0.91TiO₃ at high temperatures. Y_0.09Sr_0.91TiO₃ displays a relatively stable electrical conductivity at different oxygen partial pressures and excellent chemical compatibility with YSZ at temperatures lower than 1300 °C.     

Dy doped SrTiO3: A promising anodic material in solid oxide fuel cells

The perovskite-type oxides, having a general formula ABO₃, are promising candidates for anode materials in solid oxide fuel cells. In particular, doped SrTiO₃ based perovskites are potential mixed ionic-electronic conductors and they are known to have excellent thermal and chemical stability along with carbon and sulfur tolerance. In this work, Dy_xSr_1-xTiO_3-δ system with x = 0.03, 0.05, 0.08 and 0.10 is studied to understand the influence of Dy content on its structural and electrical behavior. Electrochemical properties are measured, both in air and hydrogen atmosphere, and structural characterizations are performed before and after electrochemical tests and compared each other to study the stability. Results show that Dy_xSr_1-xTiO_3-δ powders with x ≤ 0.05, are single phase, while for x ≥ 0.08 a small amount of secondary phases is formed. In air, the conductivity is predominantly mixed ionic-electronic type for x ≤ 0.05, becoming ionic for x ≥ 0.08. It is observed that conductivity, for each composition, increases passing from air to hydrogen and activation energy decreases. Dy_0.05Sr_0.95TiO_3-δ shows the highest conductivity in air whereas Dy_0.08Sr_0.92TiO_3-δ in H₂ atmosphere. Degradation observed by XRD is negligible for x ≤ 0.05 but increases with higher Dy content.     

Structural and electrical properties of Cr-doped SrTiO3 porous materials

Series of single-phase materials with assumed formula SrTi_1?xCr_xO₃ (where x = 0, 1, 4, 6 mol.%) were obtained by sol-gel method. The structure and microstructure of materials were characterised by X-ray diffraction and scanning electron microscopy methods. Moreover, the study of electrical properties and evaluation of chemical stability in CO₂/H₂O atmosphere was performed by electrochemical impedance spectroscopy and thermogravimery methods, respectively. The possibility of participation of Cr-doped strontium titanate in oxidation–reduction processes was analysed by temperature-programed reduction (TPR) and temperature-programed oxidation (TPOx) measurements. The changes of lattice parameters together with XPS analysis, the Seebeck coefficient measurements results and TPR profiles obtained for SrTi_1?xCr_xO₃ materials prove the presence of chromium on +3 and +6 oxidation stages. Thus, chromium can be treated as both acceptor- and donor-type dopant in the SrTiO₃ structure. The Cr³⁺/Cr⁶⁺ ratio strongly affects the electrical properties, as the change of conduction mechanism was observed. The results of performed stability test clearly indicate that incorporation of chromium into SrTiO₃ structure results with decrease of chemical stability in CO₂ atmosphere.     

Investigation of the protonic conduction in Sm doped BaCeO3

In the present work the structural and electrical properties of samarium-doped barium cerate perovskites of BaCe_1−xSm_xO_3−δ formula (with x = 0–0.2), prepared by following the solid state reaction method, are investigated. The crystal structure and microstructure of the samples is determined by employing the techniques of X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). According to the XRD analysis at 0 ≤ x ≤ 0.2 the formed continuous series of BaCe_1−xSm_xO_3−δ solid solutions have the structure of cubic perovskite with orthorhombic distortions. It was found that the relative density of the samples is ∼87% for 0.02 < x < 0.05 and ∼94% for 0.05 < x < 0.25. It was also found that the highest conductivity is observed for x = 0.15. Finally, the thermal expansion of BaCe_1−xSm_xO_3−δ (x = 0–0.2) is studied and the thermal expansion coefficients for the high temperature region are calculated.     

Use of strontium titanate (SrTiO3) as an anode material for lithium-ion batteries

Strontium titanate nanoparticles have been synthesized using a combination of sol-precipitation and hydrothermal techniques for subsequent testing as an anode material for lithium-ion batteries. The potentials associated with lithiation are 0.105 V and 0.070 V vs. Li/Li⁺ and 0.095 V and 0.142 V vs. Li/Li⁺ during de-lithiation. These potentials are significantly lower than the 1.0 V to 1.5 V vs. Li/Li⁺ typically reported in the literature for titanates. In an attempt to improve the lithiation and de-lithiation kinetics, as well as capacity retention, SrTiO₃ nanoparticles were platinized using a photoinduced reduction of chloroplatinic acid. No significant changes in the morphology or crystal structure of the platinized nanoparticles were observed as a result of the reduction reaction. The voltage profile, charge and discharge kinetics, and cyclability of the platinized SrTiO₃ nanoparticles are compared to that of the non-platinized SrTiO₃ nanoparticles.     

Y-substituted SrTiO3-YSZ composites as anode materials for solid oxide fuel cells: Interaction between SYT and YSZ

Donor-substituted SrTiO₃ ceramic materials were investigated as the anodes of solid oxide fuel cells (SOFCs). Sr_0.89Y_0.07TiO_3−δ (SYT) samples with good electrical conductivity and redox stability were prepared. The thermal and chemical expansions of SYT are both compatible with YSZ electrolyte. Half cells consisting of a flat anode substrate and an electrolyte layer with outer dimensions of 5 cm × 5 cm were fabricated, Ni particles were infiltrated on the pore walls within the ceramic anode framework, and the redox stability of the half cell was tested by He leakage tests after redox cycling. Ti diffusion but no Sr migration was found between the anode and electrolyte layers. Sr_0.895Y_0.07Ti_xO_3±δ (x = 1.00-1.20)-YSZ composites with a volume ratio 2:1 were prepared to investigate the influences of this interdiffusion between YSZ and SYT materials. The results indicate that the conductivity of SYT decreases because of the Ti diffusion, and a small amount of Ti excess can solve this problem.     

Solution combustion synthesis of Ce0.6Mn0.3Fe0.1O2 for anode of SOFC using LaGaO3-based oxide electrolyte

This paper describes the potential of solution combustion synthesis (SCS) method for preparing Ce_0.6Mn_0.3Fe_0.1O₂ (CMF) as the anode material for solid oxide fuel cells (SOFC). The stability, crystallinity, morphology, and surface area of the products were depended on the fuel ratio used in SCS as investigated by TGA, XRD, SEM, and BET, which correspondingly influenced their electrochemical properties. The SCS-derived products were directly used for preparing anodes by sintering the screen-printed powders on the electrolyte membrane, and were evaluated from power generation performance, which were compared with the conventional solid-state-reaction (SSR) sample. Significantly, under configuration of the cell of CMF/La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃/Sm_0.5Sr_0.5CoO₃ using humidified hydrogen gas as a fuel and O₂ as an oxidizing agent, the maximum power densities obtained were 1.23 W/cm² at 1000 °C for the SCS product (CMF1) obtained at ? = 0.5. This value was higher than 1.09 W/cm² for the SSR-derived sample under the same evaluation conditions. The results appealed benefits of SCS method for preparing CMF as the anode material with high power generation performance for SOFC, due to its large surface area and nanosized grains, in which fuel ratio was a key parameter for its synthesis.     

Electrical conduction behavior of La,Co co-doped SrTiO3 perovskite as anode material for solid oxide fuel cells

The effects of La- and Co-doping into SrTiO₃ perovskite oxides on their phase structure, electrical conductivity, ionic conductivity and oxygen vacancy concentration have been investigated. The solid solution limits of La in La_xSr_1 − xTiO_3 − δ and Co in La_0.3Sr_0.7Co_yTi_1 − yO_3 − δ are about 40 mol% and 7 mol%, respectively, at 1500 °C. The incorporation of La decreases the band gap and thus increases the electrical conductivity of SrTiO₃ remarkably. La_0.3Sr_0.7TiO_3 − δ shows an electrical conductivity of 247 S/cm at 700 °C. Co-doping into La_0.3Sr_0.7TiO_3 − δ increases the oxygen vacancy concentration and decreases the migration energy for oxygen ions, leading to a significant increase in ionic conductivity but at the expense of some electrical conductivity. The electrical and ionic conductivities of La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ are 63 S/cm and 6 × 10⁻³ S/cm, respectively, at 700 °C. Both La_0.3Sr_0.7TiO_3 − δ and La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ show relatively stable electrical conductivities under oxygen partial pressure of 10⁻¹⁴–10⁻¹⁹ atm at 800 °C. These properties make La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ a promising anode candidate for solid oxide fuel cells.     

Preparation,electrical and thermal properties of new anode material based on Ca-doped perovskite CeAlO3

Tetragonal perovskite phase Ce_0.9Ca_0.1AlO_2.95 + x was obtained for the first time. Such phase, containing cerium in the oxidation state of 3+, can be promising anode materials for a solid oxide fuel cells (SOFCs). Ce_0.9Ca_0.1AlO_2.95 + х (space group I4/mcm) was synthesized by the solid-phase method at 1400°С in a nitrogen flow with using ammonium oxalate (NH₄)₂C₂O₄ to create a reducing atmosphere. Thermogravimetry results showed that Ce_0.9Ca_0.1AlO_2.95 + x was stable to oxidation up to 500°С in air and up to 700°С in argon (partial pressure of oxygen рО₂ = 10⁻⁴ bar). The thermal expansion coefficient measured by dilatometry was equal to 11.16·10⁻⁶ К⁻¹. The temperature dependences of the electrical conductivity (for undoped phase CeAlO₃ σ ≈ 1·10⁻³ S/cm and for doped Ce_0.9Ca_0.1AlO_2.95 + x σ ≈ 3·10⁻² S/cm at 500°С in air) were obtained by the electrochemical impedance spectroscopy measurements). The electrical conductivity of these samples at the temperatures range of 350–500°С was almost independent of the partial pressure of oxygen рО₂ from 10⁻¹⁸ to 0.21 bar, however, there was a slight negative slope at T > 500 °C (рО₂). The total ionic transport numbers measured by the EMF method were close to 1·10⁻³, which indicated the dominance of electronic conductivity. The measurement of the sign of the thermal-EMF showed that positive charge carriers (holes) were dominant charge carriers.     

Electrochemical performances of solid oxide fuel cells based on Y-substituted SrTiO3 ceramic anode materials

Yttrium-substituted SrTiO₃ has been considered as anode material of solid oxide fuel cells (SOFCs) substituting of the state-of-the-art Ni cermet anodes. Sr_0.895Y_0.07TiO_3−δ (SYT) shows good electrical conductivity, compatible thermal expansion with yttria-stabilized ZrO₂ (YSZ) electrolyte and reliable stability during reduction and oxidation (redox) cycles. Single cells based on SYT anode substrates were fabricated in the dimension of 50 mm × 50 mm. The cell performances were over 1.0 A cm⁻² at 0.7 V and 800 °C, which already reached the practical application level. Although Ti diffusion from SYT substrates to YSZ electrolytes was observed, it did not show apparent disadvantage to the cell performance. The cells survived 200 redox cycles without obvious OCV decrease and macroscopic damage, but performance decreased due to the electronic properties of the SYT material. The influence of water partial pressure on cell performance and coking tolerance of the cells are also discussed in this study.     

Nanocrystalline Ti2/3Sn1/3O2 as anode material for Li-ion batteries

We prepared nanocrystalline Ti_2/3Sn_1/3O₂ by a coprecipitation method starting from Ti(isopropoxide)₄ and SnCl₄·5H₂O followed by calcination at 600 °C. TEM and XRD measurements reveal crystallite sizes of about 5 nm and a crystal structure equivalent to those of TiO₂ rutile and SnO₂ cassiterite. The local structure was investigated with ¹¹⁹Sn NMR and Sn Mössbauer spectroscopy. The material was cycled with C/20 at voltages between 3.0 and 0.02 V against Li metal. Specific capacities of 300 mAh g⁻¹ were obtained for 100 cycles with voltage profiles very similar to those of pure SnO₂. Faster cycling leads to strong decrease of the capacities but after returning to C/20 the initial values are obtained.     

Tape casting fabrication,co-sintering and optimisation of anode/electrolyte assemblies for SOFC based on BIT07-Ni/BIT07

BaIn_0.3Ti_0.7O_2.85 (BIT07) is an electrolyte material for SOFC due to its high ionic conductivity level and its compatibility with mixed ionic and electronic conductor (MIEC) cathode materials, such as LSCF and Nd₂NiO_4+δ. BIT07 is also compatible with NiO to form a cermet as anode material. In this study, the electrolyte material has been prepared by tape casting and characterised. The coupled influences of the powder grain size and the firing temperature have been investigated and optimised to yield a dense electrolyte at 1300 °C. Then anode-supported half-cells (electrolyte/anode), based on BIT07/BIT07-Ni have been prepared by tape casting and co-sintered. The composition and the microstructure of the cermet anode (BIT07 grain size, amount and nature of pore-forming agent) have been optimised to achieve an area surface resistance (ASR) value of about 0.15 Ω cm² at 700 °C under wet reducing atmosphere. The stability of the most performing anode has been followed during 500 h and the observed degradation seems to be due a loss of nickel percolation.     

Microstructural effects on the electrical and mechanical properties of Ni–YSZ cermet for SOFC anode

The electrical and mechanical properties of Ni–YSZ cermet as the anode support of solid oxide fuel cell (SOFC) are determined by the metallic and ceramic components, respectively. We used YSZ and NiO commercial powders of the average particle size from 1 to 10 μm to fabricate Ni–YSZ cermets with different microstructures. The porosity of the cermets was also modified by the amount of carbon black addition. The distribution of each phase of cermets was analyzed with scanning electron microscopy combined with energy dispersive spectroscopy. The electrical conductivity and fracture strength of the Ni–YSZ cermets were investigated and interpreted in a view of percolation phenomena. The finer particles, either NiO or YSZ, were interlinked well by sintering and the electrical and mechanical properties of Ni–YSZ cermets were enhanced by the percolation of Ni and YSZ, respectively.     

Lithium-storage and cycleability of nano-CdSnO3 as an anode material for lithium-ion batteries

Nano-CdSnO₃ is prepared by thermal decomposition of the precursor, CdSn(OH)₆ at 600 °C for 6 h in air. The material is characterized physically by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and selected-area electron diffraction (SAED) techniques. Nano-CdSnO₃ exhibits a reversible and stable capacity of 475(±5) mAh g⁻¹ (∼5 mol of cycleable Li per mole of CdSnO₃) for at least 40 cycles between 0.005 and 1.0 V at a current rate of 0.13 C. Extensive capacity fading is found when cycling in the range 0.005-1.3 V. Cyclic voltammetry studies complement galvanostatic cycling data and reveal average discharge and charge potentials of 0.2 and 0.4 V, respectively. The proposed reaction mechanism is supported by ex situ XRD, TEM and SAED studies. The electrochemical impedance spectra taken during 1st and 10th cycle are fitted with an equivalent circuit to evaluate impedance parameters and the apparent chemical diffusion coefficient (DLi⁺) of Li. The bulk impedance, R_b, dominates at low voltages (≤0.25 V), whereas the combined surface film and charge-transfer impedance (R_(sf+ct)) and the Warburg impedance dominate at higher voltages, ≥0.25 V. The DLi⁺ is in the range of (0.5-0.9) × 10⁻¹³ cm² s⁻¹ at V = 0.5-1.0 V during the 10th cycle.     

Li3V2(PO4)3/C composite as an intercalation-type anode material for lithium-ion batteries

The carbon coated monoclinic Li₃V₂(PO₄)₃ (LVP/C) powder is successfully synthesized by a carbothermal reduction method using crystal sugar as the carbon source. Its structure and physicochemical properties are investigated using X-ray diffraction (XRD), scanning electron microscopy, high-resolution transmission electron microscopy and electrochemical methods. The LVP/C electrode exhibits stable reversible capacities of 203 and 102 mAh g⁻¹ in the potential ranges of 3.0-0.0 V and 3.0-1.0 V versus Li⁺/Li, respectively. It is identified that the insertion/extraction of Li⁺ undergoes a series of two-phase transition processes between 3.0 and 1.6 V and a single phase process between 1.6 and 0.0 V. The ex situ XRD patterns of the electrodes at various lithiated states indicate that the monoclinic structure can still be retained during charge-discharge process and the insertion/deinsertion of lithium ions occur reversibly, which provides an excellent cycling stability with high energy efficiency.     

Impact of Cr-poisoning on the conductivity of LaNi0.6Fe0.4O3

This study demonstrates the significant impact of Cr on the electronic conductivity of a LaNi_0.6Fe_0.4O₃ (LNF) porous cathode layer at 800 °C. Vapor transport of Cr-species, originating from a porous metallic foam, and subsequent reaction with LNF, results in a decrease of the electronic conductivity of the LNF-layer. Cr has been detected throughout the entire cross-section of a 16 μm thick LNF layer, while Ni, besides its compositional distribution in the LNF layer, has also been found in enriched spots forming Ni-rich metal oxide crystals. Transmission electron microscopy revealed that Cr is gradually incorporated into the LNF-grains, while Ni is proportionally expelled. Electron diffraction performed in the center of a sliced grain showed the initial rhombohedral crystal structure of LNF, whereas diffraction performed close to the edge of the grain revealed the orthorhombic perovskite crystal structure, indicating a Cr-enriched perovskite phase. Progressive Cr deposition and penetration into the LNF grains and necks explains the electronic conductivity deterioration. The impact of Cr-poisoning on the electronic conductivity of the LNF porous layer is considerably smaller at 600 °C than at 800 °C.     

Characteristics of Ni/YSZ ceramic anode prepared using carbon microspheres as a pore former

Microstructural features and physical properties of the anodes crucially affect the electrochemical performance of anode-supported solid oxide fuel cells (SOFCs). This paper evaluated the microstructural characteristics and properties including porosity, pore size distribution, sintering shrinkage, mechanical strength, and electrical conductivity of the SOFC anode using carbon microspheres (CMSs) as the pore-former in the fabrication of Ni/YSZ ceramic anode. CMSs with different average particle sizes (CMS1: 11.54 μm, CMS2: 4.39 μm, and CMS3: 0.27 μm) were synthesized, and then incorporated into NiO/YSZ at various volumetric blend ratios ranging from 4.4 to 44.6 vol.%. SOFC anode cermets with a desirable range of porosity (30–40%), shrinkage (15.9–17.3%), flexural strength (75.4–157.8 N), and electrical conductivity (253.5–510.7 S/cm) were obtained using approximately 4–10 vol% of CMS1, 4–20 vol.% of CMS2, and 10–34 vol.% of CMS3. In addition, the use of CMS as the pore former reduced the amount of closed pores in the anode disks from 2.05% to <1%.     

Effect of substitution with Cr and addition of Ni on the physical and electrochemical properties of Ce0.9Sr0.1VO3 as a H2S-active anode for solid oxide fuel cells

Whereas Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ is an active fuel cell anode catalyst for conversion of only the H₂S content of 0.5% H₂S-CH₄ at 850 °C, inclusion of 5 wt% NiO to form a composite catalyst enabled concurrent electrochemical conversion of CH₄. A fuel cell with a 0.3 mm thick YSZ membrane and Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ as anode catalyst had a maximum power density of 85 mW cm⁻² in 0.5% H₂S-CH₄ at 850 °C, arising only from the electro-oxidation of H₂S. Using a same thick membrane, promotion of the anode with 5 wt% NiO increased the total anode electro-oxidation activity to afford maximum power density of 100 mW cm⁻² in 0.5% H₂S-CH₄. The same membrane provided 30 mW cm⁻² in pure CH₄, showing that the incremental improvement arose substantially from CH₄ conversion. Performance of each anode was stable for over 12 h at maximum power output. XPS and XRD analyses showed that an increase in conductivity of Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ in H₂S-containing environments resulted from a change in composition and structure from the tetragonal oxide to monoclinic Ce_0.9Sr_0.1Cr_0.5V_0.5(O,S)₃.     

Performance of La0.75Sr0.25Cr0.5Mn0.5O3−δ perovskite-structure anode material at lanthanum gallate electrolyte for IT-SOFC running on ethanol fuel

Perovskite-structure La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) powders were prepared using a simple combustion process. Thermal analysis was carried out on the perovskite precursor to investigate the oxide-phase formation. The structural phase of the powders was determined by X-ray diffraction. These results showed that the decomposition of the precursors occurs in a two-step reaction and temperatures higher than 1100 °C are required for these decomposition reactions. For the electrochemical characterization, LSCM anode materials and (Pr_0.7Ca_0.3)_0.9MnO₃ (PCM) cathode materials were screen-printed on two sides of dense La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte layers prepared by tape casting with a thickness of about 600 μm, respectively. The morphology of the screen-printed La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ perovskite thick films (65 μm) was investigated by field emission scanning electron microscope and showed a porous microstructure. In addition, fuel cell tests were carried out using humidified hydrogen or ethanol stream as fuel and oxygen as oxidant. The performance of the conventional electrolyte-supported cell LSCM/LSGM/PCM while operating on humidified hydrogen was modest with a maximum power density of 165, 99 and 62 mW cm⁻² at 850, 800 and 750 °C, respectively, the corresponding values for the cell while operating on ethanol stream was 160, 101 and 58 mW cm⁻², respectively. Cell stability tests indicate no significant degradation in performance has been observed after 60 h of cell testing when LSCM anode was exposed to ethanol steam at 750 °C, suggesting that carbon deposition was limited during cell operation.     

Sr2Fe4/3Mo2/3O6 as anodes for solid oxide fuel cells

Sr₂Fe_4/3Mo_2/3O₆ has been synthesized by a combustion method in air. It shows a single cubic perovskite structure after being reduced in wet H₂ at 800 °C and demonstrates a metallic conducting behavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 °C in wet H₂ (3% H₂O) is about 16 S cm⁻¹. This material exhibits remarkable electrochemical activity and stability in H₂. Without a ceria interlayer, maximum power density (P_max) of 547 mW cm⁻² is achieved at 800 °C with wet H₂ (3% H₂O) as fuel and ambient air as oxidant in the single cell with the configuration of Sr₂Fe_4/3Mo_2/3O₆|La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM)| La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). The P_max even increases to 595 mW cm⁻² when the cell is operated at a constant current load at 800 °C for additional 15 h. This anode material also shows carbon resistance and sulfur tolerance. The P_max is about 130 mW cm⁻² in wet CH₄ (3% H₂O) and 472 mW cm⁻² in H₂ with 100 ppm H₂S. The cell performance can be effectively recovered after changing the fuel gas back to H₂.