Efficiently enhance the proton conductivity of YSZ-based electrolyte for low temperature solid oxide fuel cell

Yttrium stabilized zirconia (YSZ) as a typical oxygen ionic conductor has been widely used as the electrolyte for solid oxide fuel cell (SOFC) at the temperature higher than 1000 °C, but its poor ionic conductivity at lower temperature (500–800 °C) limits SOFC commercialization. Compared with oxide ionic transport, protons conduction are more transportable at low temperatures due to lower activation energy, which delivered enormous potential in the low-temperature SOFC application. In order to increase the proton conductivity of YSZ-based electrolyte, we introduced semiconductor ZnO into YSZ electrolyte layer to construct heterointerface between semiconductor and ionic conductor. Study results revealed that the heterointerface between ZnO and YSZ provided a large number of oxygen vacancies. When the mass ratio of YSZ to ZnO was 5:5, the fuel cell achieved the best performance. The maximum power density (P_max) of this fuel cell achieved 721 mW cm^?2 at 550 °C, whereas the P_max of the fuel cell with pure YSZ electrolyte was only 290 mW cm^?2. Further investigation revealed that this composite electrolyte possessed poor O^2? conductivity but good proton conductivity of 0.047 S cm^?1 at 550 °C. The ionic conduction activation energy of 5YSZ-5ZnO composite in fuel cell atmosphere was only 0.62 eV. This work provides an alternative way to improve the ionic conductivity of YSZ-based electrolytes at low operating temperatures.     

Direct evidence of high oxygen ion conduction of perovskite-type ceramic membrane under hydrogen atmosphere in solid oxide fuel cell

The ionic conduction of perovskite-type oxides remains a fundamental and important issue in the research of solid oxide fuel cells (SOFCs). In this research, a thin perovskite-type ceramic membrane was fabricated in situ at anode side attached to the surface of Gd_0.2Ce_0·8O_1.9 (GDC20) electrolyte membrane. The single cell working between H₂ and static air showed good stability (over 50 h), high open circuit voltages (above 1.0 V) as well as high peak power densities (749-264 mW cm^?2) from 600 to 500 °C. Detailed analyses of current research demonstrated that the thin perovskite film mainly possessed the oxygen ion conductivity under reducing atmosphere, while the proton conductivity was severely suppressed, showing the high flexibility in ionic conductivity of perovskite oxide. This work also implies that the oxygen ion and proton conduction may be in high correlation with each other, which provides important information to unveil the nature of the ionic conduction of perovskite-type oxides.     

Oxygen Ion Conduction in Bulk and Grain Boundaries of Nominally Donor‐Doped Lead Zirconate Titanate (PZT): A Combined Impedance and Tracer Diffusion Study

Oxygen ion conduction in Nd³⁺‐doped Pb(Zr_xTi_1?x)O₃ (PZT) was investigated by impedance spectroscopy and ¹⁸O‐tracer diffusion with subsequent secondary ion mass spectrometry (SIMS) analysis. Ion blocking electrodes lead to a second relaxation feature in impedance spectra at temperatures above 600°C. This allowed analysis of ionic and electronic partial conductivities. Between 600°C and 700°C those are in the same order of magnitude (10^?5–10^?4 S/cm) though very differently activated (2.4 eV vs. 1.2 eV for ions and electron holes, respectively). Oxygen tracer experiments showed that ion transport mainly takes place along grain boundaries with partly very high local ionic conductivities. Numerical analysis of the tracer profiles, including a near‐surface space charge zone, revealed bulk and grain‐boundary diffusion coefficients. Calculation of an effective ionic conductivity from these diffusion coefficients showed good agreement with conductivity values determined from impedance measurements. Based on these data oxygen vacancy concentrations in grain boundary and bulk could be estimated. Annealing at high temperatures caused a decrease in the grain‐boundary ionic conductivity and onset of additional defect chemical processes near the surface, most probably due to cation diffusion.     

Determinations of Ce solid solution mechanism and limit thermal conductivity of YTaO4 ceramics

x mol% CeO₂-YTaO₄ (x = 0, 3, 6, 9, 12) ceramics have been synthesized by the spark plasma sintering (SPS) technique. We focus on the changes in lattice distortion, bonding length, thermal conductivity, thermal expansion, and phase stability of the prepared samples. XRD, Raman, and XPS are used to determine the chemical valence and solid solution mechanism of Ce in the lattice of YTaO₄, while its effects on thermal/mechanical properties are elucidated from microstructures. Y³⁺ is substituted via Ce³⁺, and all samples maintain a monoclinic phase. The limit thermal conductivity (1.2 W?m^?1?K^?1, 900 °C) is realized in 9 mol% CeO₂-YTaO₄, and the thermal expansion coefficients are increased to 10.2 × 10^?6 K^?1 at 1200 °C. Furthermore, the exceptional phase stability and mechanical properties of all samples indicate that they can provide good thermal insulation at high temperatures, and have higher working temperatures than the current YSZ thermal barrier coatings.     

Temperature-dependent electrical transport behavior and structural evolution in hollandite-type titanium-based oxide

Electrical transport behavior and structural characteristics directly determine the use of functional ceramic materials in electronic information storage, catalytic conversion, and energy field applications. However, these properties are poorly understood because most of the relevant experiments were performed in a rather narrow temperature range. Herein, we used hollandite-type K_xTi₈O₁₆ as an example to systematically study the temperature-dependent structure and electrical transport properties in a wide temperature range from 25 to 900°C. The electrical transport involves both potassium ionic conduction and electronic conduction. With increasing temperature, the ionic conductivity increases below 800°C and decreases above 800°C. The electronic conductivity displays two maxima at 0.15 S/cm at 400°C and 5.2 × 10⁻⁴ S/cm at 800°C. These interesting variations in the conductivities are related to the presence of Ti³⁺ and the structural transformation from hollandite to a mixture of rutile and jeppeite. The findings reported herein support the potential application of titanium-based hollandites and provide an understanding of the electrical transport properties of functional ceramic materials.     

Bio-template synthesis of CeO2 ultrathin nanosheets for highly selective and sensitive detection of ppb-level p-xylene vapor

Highly selective of carcinogenic and flammable p-xylene vapor and its sensing detection through metal oxides-based sensors has recently attracted much attention. In this work, mesoporous CeO₂ nanosheets were synthesized by simple cerium nitrate impregnation and air calcination using rose petals as bio-template. The effect of calcination temperature on its microstructure, Ce³⁺/Ce⁴⁺ mole ratio, as well as sensing performance was investigated. The CeO₂-650 ultrathin nanosheets calcined at 650 °C are assembled by cross-linking nanoparticles with small size, which possess homogeneous mesoporous distribution and relatively large specific surface area. At 217 °C, the sensor fabricated from CeO₂-650 ultrathin nanosheets shows short response time (T_res = 5 s), high selectivity and response (S = 22.1) towards 100 ppm p-xylene vapor, and its limit of detection (30 ppb) is the lowest among reported sensors based on pure metal oxides. The good sensing performance mainly originate from the synergistic effect of intrinsic features of mesoporous CeO₂-650 ultrathin nanosheets, surface adsorbed oxygen control, oxygen vacancy defects induced by Ce³⁺ and biotemplate imprinting. Therefore, mesoporous CeO₂-650 ultrathin nanosheets could be utilized as candidate for the detection of trace p-xylene vapor.     

Ceria (La3+, Sr2+)/Carbonates Nanocomposite Electrolytes with High Electrical Conductivity for Low‐Temperature SOFCs

A series of ceria‐based nanocomposites consisting of lanthanum and strontium codoped ceria with composition Ce_0.89La_0.07Sr_0.04O_1.925 (CL7S4) and eutectic mixture of carbonates Li₂CO₃‐Na₂CO₃ (LNCO) have been prepared by mixing nanosize powders of CL7S4 and LNCO. Samples have been characterized using differential thermal analysis, X‐ray diffraction, scanning electron microscopy combined with energy‐dispersive spectroscopy, thermal expansion, and impedance spectroscopy. A sharp increase in ionic conductivity is observed in all the composite specimens corresponding to superionic transition. Sample containing 35 wt% of carbonate shows the maximum conductivity (2.56 × 10^?1 S/cm at 500°C) with activation energy of conduction, E_a 0.23 eV.     

Structural and ionic conductivity of Cu-doped titania (Ti0.95Cu0.05O2−δ) for high temperature energy devices

The Cu-doped titania (Ti_0.95Cu_0.05O_2-δ) is studied here as a solid-state ionic conductor for its possible application in high temperature energy devices such as an electrolyte for SOFC. The sample in the powder form was obtained by solid state method using TiO₂ and copper acetate by heating up to 1200 °C for 10 h. It was characterized by XRD, FT-IR, Raman, SEM/EDS, DRS-UV-Visible, photoluminescence, BET and ac-impedance techniques. The oxide ion conductivity (σ_t) values obtained from ac-impedance measurements showed a linear increase with temperature from 300 ? 700 °C. The σ_t values are similar to that of Ln-doped ceria, and the highest conductivity of 1.41 × 10^?4 Scm^?1 was recorded at 700°C. The activation energy for total conductivity was found to be 0.82 eV. The ionic and electronic transport numbers are 0.79 and 0.21, respectively. This study suggests the plausible use of rutile TiO₂ based (low-cost and structurally stable) materials as electrolytes in SOFC.     

The influence of chelating agents on cerium oxide decorated on graphite synthesized by the hydrothermal route

Morphology features of cerium oxide nanoparticles, such as size and agglomeration, are important as a coating that improves corrosion resistance and as reinforcement in mechanical applications. In this work, the influence of two heat treatments (160° and 190 °C) in combination with three different chelating agents in the preparation of CeO₂ and CeO₂ decorated on graphite (CeO₂_Gr) nanoparticles is studied. The novelty of this work is that CeO₂_Gr was successfully prepared using the hydrothermal method. All the samples evaluated by X-ray diffraction exhibit a single fluorite-type structure in the cubic phase and Fm3m space group. The spherical harmonics method using the Fullprof Suite program was used to determine the average crystallite sizes, which were 9 nm for CeO₂ and 7 nm for CeO₂_Gr. Transmission electron micrographs for the prepared samples with citric acid showed non-agglomerate particles with homogeneous particle sizes and a quasi-spherical shape distribution. Raman spectra show a band centre at 600 cm^-1 associated with the presence of Frenkel-type oxygen vacancies that induced the reduction of Ce⁴⁺ to Ce³⁺. The analysis of X-ray photoelectron spectra corroborates the coexistence of Ce³⁺ and Ce⁴⁺ species for CeO₂ and CeO₂_Gr nanoparticles. This work forms new perspectives in the development of CeO₂ decorated on graphite prepared by the hydrothermal method to obtain composites not only for sensing applications and wastewater treatment but also for corrosion resistance and reinforcement materials.     

Ionic conductivity across the disorder–order phase transition in the SmO1.5–CeO2 system

Samples of Sm_xCe_1 ? xO_2 ? δ (0.05 ≤ x ≤ 0.55) were prepared by solid-state reactions and the disorder–order phase transition and grain ionic conductivity were investigated using XRD and ac impedance spectroscopy technique, respectively. For 0 ≤ x ≤ 0.35 the material has a fluorite structure and gradually stabilizes into a C-type rare-earth structure at 0.40 ≤ x ≤ 0.55 because of oxygen-vacancy ordering. The highest grain ionic conductivity observed is 0.0565(37) S cm^?1 at 700 °C for Sm_0.20Ce_0.80O_2 ? δ with an associated activation energy (E_A) of 0.791(7) eV. The slopes for E_A and pre-exponential factor change during phase transition and the conductivity decreases monotonically. Upon comparison of the E_A between the SmO_1.5–CeO₂ and NdO_1.5–CeO₂ systems, it is seen E_A for the SmO_1.5–CeO₂ system is lower than NdO_1.5–CeO₂ system at compositions with less than 25% trivalent rare earth element while higher E_A is observed for the SmO_1.5–CeO₂ system at Nd/Sm concentrations above 25%.     

Structural modifications of nanostructured ceria CeO2,xH2O during dehydration process

Hydrated nanostructured cerium dioxide CeO₂, xH₂O (hydrated nanoceria) has been synthesized in room conditions via a precipitation route. This hydrated nanoceria phase has been subjected to thermal decomposition in the temperature range from 25 °C to 800 °C. At least three decomposition steps have been observed in thermal and thermogravimetric analyses. Three different samples of cubic nanoceria respectively obtained at room temperature (RT-nanoceria), 80 °C (80-nanoceria) and 600 °C (600-nanoceria) have been studied by X-Ray diffraction, Raman spectroscopy and scanning electron microscopy analyses. The analyses of X-ray diffraction profiles and Raman vibrational bands have clearly shown that dehydration is accompanied by increasing crystallite size, lattice parameter contraction. The cubic structure of hydrated RT-nanoceria might be associated with a complex chemical formula unit involving Ce⁴⁺, Ce³⁺ mixed valences, oxygen vacancies, lattice and surface water and OH^? proton species.     

Preparation and conductive properties of double perovskite oxides Ba3Ba1+xTa2-xO9-δ

In this work, Ba₃Ba_1+xTa_2-xO_9-δ (x = 0, 0.1, 0.3, and 0.5) double perovskite proton conductors were prepared by solid-state reaction process. Phase compositions and microstructures were characterized by X-ray diffraction and field emission scanning electron microscope techniques. valence and semi-quantitative composition of components were identified by X-ray photoelectron spectroscopy. Conductivities of Ba₃Ba_1+xTa_2-xO_9-δ were then measured under various vapor and oxygen pressures by AC impedance spectroscopy technique. Results revealed linear increase in total conductivities of Ba₃Ba_1+xTa_2-xO_9-δ oxides as a function of temperature. Ba₃Ba_1.3Ta_1.7O_8.55 exhibited the highest total conductivity of 8.41 × 10^?4 S cm^?1 under humidity and 800 °C. The transport numbers calculated by defect equilibria model revealed. Ba₃Ba_1+xTa_2-xO_9-δ oxides as pure proton conductors at 400–800 °C. Also, transport numbers of oxide ions and holes both increased with temperature. Ba₃Ba_1.3Ta_1.7O_8.55 illustrated the highest protonic transport number of 0.60 at 800 °C. In sum, these results suggest that Ba₃Ba_1+xTa_2-xO_9-δ oxides display excellent proton conductivity.     

Facile synthesis of doped ceria-based oxide by co-precipitation technique and performance evaluation in solid oxide fuel cell

Co-precipitated nanocrystalline gadolinium (Gd)-doped ceria (CeO₂) has been synthesized with varying mole ratios of Gd in CeO₂ from 0.1-0.8 with intricate maintenance of solution pH. Phase pure Gd-doped CeO₂ (CGO) is obtained after calcination at 750°C and depending on the mole % dopant concentration, the typical crystallite size is found to vary in the range 9-27 nm. Careful observation reveals that, calcined particle size decreases with the decrease in Gd content and an optimization of the particle size with exposed (111) [stable] and (100) plane [reactive] happens to occur for Ce_0.8Gd_0.2O_2−δ (CG_0.8). High resolution transmission electron micrograph of CG_0.8 reveals highly interpenetrating lattice planes corresponding to cubic fluorite structure. The effectivity of such CG_0.8 is further supported by its high electrical conductivity of 0.02 and 0.105 S/cm @ 600 and 700°C respectively. The bulk impedances exhibit ohmic (R₀) and interfacial (R_p) polarizations to be 146 and 29.96 Ω cm² at a temperature of 600 and 700°C with activation energies 0.36 and 0.8 eV respectively. The application of CG_0.8 is further established as an interlayer in nickel oxide-yittria stabilized zirconia (NiO-YSZ)/YSZ/CGO/La-Sr-Co-Fe-based single cell with a current density of 1.2 A/cm² @ 0.5 V and 700°C using hydrogen as the fuel and oxygen as the oxidant.     

Proton conductivity and mobility in Sr-doped LaScO3 perovskites

In this work, we set out to investigate the electrical conductivity of single-phase and high-density La_1-xSr_xScO_3-δ (x = 0.05; 0.1) ceramics depending on temperature and рО₂ and рН₂О. The crystal structure of materials was characterized by XRD method. The samples show the structure of an orthorhombic perovskite with a Pnma space group. The unit cell volume increases along with the Sr concentration. The microstructure features of samples were investigated by SEM analysis. The transference numbers of protons and oxygen-ions were determined by the EMF (electromotive force) measurements in a gas concentration cell. In addition, the proton, oxygen-ion and hole conductivities were evaluated from the рО₂-dependencies of electrical conductivity at different humidity. The results obtained using both methods showed a good level of agreement. It is found that the partial conductivity of each charge carrier in La_1-xSr_xScO_3-δ increases along with an increase in the concentration of the Sr dopant from x = 0.05 to x = 0.1. The highest proton conductivity about 3 × 10^?2 S cm^?1 is achieved for La_0·9Sr_0.1ScO_3-δ at 800 °C. The mobility of proton defects increases with Sr concentration and reaches 2.5 × 10^?4 cm² V^?1 s^?1 at 800 °C for La_0·9Sr_0.1ScO_3-δ. Thus, La_0·9Sr_0.1ScO_3-δ should be considered as a promising proton-conducting electrolyte for various electrochemical devices, such as protonic ceramic fuel cells.     

Synthesis of Yb and Sc stabilized zirconia electrolyte (Yb0.12Sc0.08Zr0.8O2–δ) for intermediate temperature SOFCs: Microstructural and electrical properties

Ceramic electrolytes based on Yb and Sc stabilized zirconia enable efficient heat transfer and effective ionic conductivity. Here, the design and synthesis of Yb and Sc stabilized zirconia electrolyte is presented for intermediate temperature solid oxide fuel cells (SOFCs). Yb_0.12Sc_0.08Zr_0.8O_2–δ was synthesized using the sol-gel method, and a thorough characterization of the electrolyte properties was conducted including structural and electrical properties. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) confirmed the composition of the electrolyte. A single-phase cubic structure with a density of 6.7041 ± 0.0008 g cm⁻³ was obtained. The thermal expansion coefficient in the temperature range from 25 °C to 800 °C is equal to 1.17 × 10⁻⁶ K⁻¹. The activation energy of 1.06 eV and 1.15 eV was obtained for the bulk and grain boundary conductivity, respectively. The ionic conductivity of approx. 2.10 S m⁻¹ was achieved at 667 °C, thus it is suitable for efficient ionic conduction at intermediate temperatures.     

Thermal and mechanical properties of Sc2O3–CeO2 co-stabilized zirconia ceramic as promising thermal barrier coating material

CeO₂ and Sc₂O₃ co-stabilized ZrO₂ ceramics have attracted much attention as potential thermal barrier coatings (TBCs) materials for applications above 1300 °C. In this study, a series of Sc_0.04Ce_xZr_0.96-xO_1.98 (SCZ, x = 0.08, 0.10, 0.12, 0.16) ceramic materials were synthesized with the solid-state method and their phase stability, microstructures and thermo-physical properties were systemically investigated by x-ray diffraction (XRD), Raman spectra, field emission scanning electron microscopy (SEM), thermal dilatometer, laser flash apparatus (LFA), and Vickers hardness tester. The results showed that Sc_0.04Ce_0.12Zr_0.84O_1.98 (4S12CZ) and Sc_0.04Ce_0.16Zr_0.80O_1.98 (4S16CZ) ceramic materials still maintained stable tetragonal phase structure after 100 h high temperature treatment at 1500 °C. SCZ had a high thermal expansion coefficient (TEC), low thermal conductivity, and high fracture toughness. The TEC of the ceramics increased with CeO₂ addition because lattice energy reduced with increasing substitution of Zr⁴⁺ by bigger Ce⁴⁺ while thermal conductivity decreased due to the increase of lattice distortion. Compared with 4S12CZ, 4S16CZ exhibited a higher fracture toughness of 6.48 ± 0.04 MPa m^1/2 and showed the better anti-sintering property. Besides, the thermal conductivity, TEC and thermal cycling lifetime of 4S16CZ were optimal. The comprehensive performance of 4S16CZ suggested it could be explored as a promising TBC material for high-temperature application.     

Investigation on Proton Conductivity of La2Ce2O7 in Wet Atmosphere: Dependence on Water Vapor Partial Pressure

The AC conductivity of fluorite‐structured La₂Ce₂O₇ ceramic was measured under air and argon with different humidity between 250 and 550 °C. It was observed that the total conductivity in wet air and argon was higher than that under dry atmospheres. The effect of water vapor partial pressure ( ) on the conductivity of La₂Ce₂O₇ in air was investigated in detail. The total conductivity increased remarkably with the water vapor partial pressure, and this phenomenon became more notable at lower temperatures. The enhancement of the conductivity was attributed to the proton conduction behavior of La₂Ce₂O₇ in wet atmospheres, and the proton conductivity reached 6.68 × 10^–5 S cm^–1 in wet air (3% H₂O) at 550 °C. The relationship between the proton conductivity (σ_H) and in wet air could be fitted to . The estimated proton transport number increased with increasing water vapor partial pressure and decreasing temperature, and varied between 0.05 and 0.89 in this study.     

Effect of preparation route on the microstructure and electrical conductivity of co-doped ceria

Dense Ce_0.8Sm_0.1Gd_0.1O_2?δ electrolytes were fabricated by sintering of CeO₂ solid solutions which were prepared from metal nitrates and NaOH using self propagating room temperature synthesis (SPRT). Three different routes were employed to obtain CeO₂ solid solution powders: (I) hand mixing of reactants, (II) ball milling of reactants and (III) ball milling of Ce_0.8Sm_0.2O_2?δ and Ce_0.8Gd_0.2O_2?δ solid solutions previously prepared by ball milling of corresponding nitrates and NaOH. Density measurements showed that ball milling, which is more convenient than hand mixing, is an effective way to obtain almost full dense samples after presureless sintering at 1550 °C for 1 h. These samples had larger grain size and consequently higher conductivity than the samples obtained by hand mixing. The highest conductivity of 2.704×10^?2 (Ω cm)^?1was measured at 700 °C in a sample prepared by route II. It was found that reduced grain size in samples obtained by hand mixing leads to a decrease in grain boundary conductivity and therefore decrease in the total conductivity. The results showed that mixing of single doped ceria solid solutions improved densification and inhibited grain growth.     

Ionic Transport Properties in Nanocrystalline Ce0.8A0.2O2-δ (with A = Eu,Gd, Dy,and Ho) Materials

The ionic transport properties of nanocrystalline 20 mol% Eu, Gd, Dy, and Ho doped cerias, with average grain size of around 14 nm were studied by correlating electrical, dielectric properties, and various dynamic parameters. Gd-doped nanocrystalline ceria shows higher value of conductivity (i.e., 1.8 × 10⁻⁴ S cm⁻¹ at 550°C) and a lower value of association energy of oxygen vacancies with trivalent dopants Gd³⁺ (i.e., 0.1 eV), compared to others. Mainly the lattice parameters and dielectric constants (ε_∞) are found to control the association energy of oxygen vacancies in these nanomaterials, which in turn resulted in the presence of grain and grain boundary conductivity in Gd- and Eu-doped cerias and only significant grain interior conductivity in Dy- and Ho-doped cerias.     

Synthesis and characterization of co-doped ceria-based electrolyte material for low temperature solid oxide fuel cell

Co-doped CeO₂ (Ba_0.10Ga_0.10Ce_0.80O_3–δ) was synthesized via a cost-effective co-precipitation technique, and the electrochemical properties of the solid oxide fuel cell were studied. The microstructural and surface morphological properties were investigated by XRD and SEM, respectively. The structure of the prepared material was found to be cubic fluorite with an average crystallite size of 36?nm. The ionic conductivity of the prepared BGC (Ba_0.10Ga_0.10Ce_0.80O_3–δ) electrolyte material was measured as 0.071?S?cm^?1. The activation energy was found to be 0.46?eV using an Arrhenius plot. The maximum power density and current density achieved were 375?mW?cm^?2 and 893?mA?cm^?2, respectively, at 650?°C with hydrogen as a fuel. This study shows that the prepared co-doped electrolyte material could be used as a potential electrolyte to lower the operating temperature of solid oxide fuel cells.