Y-doped BaZrO3 Research Articles

Preparation and proton conductivity of ultrafine Y-doped BaZrO<sub>3</sub> ceramics

Preparation and proton conductivity of ultrafine Y-doped BaZrO<sub>3</sub> ceramics

Proton segregation and space-charge at the BaZrO3 (0 0 1)/MgO (0 0 1) heterointerface

Y-doped BaZrO3 (BZY) can be deposited epitaxially on MgO (0 0 1) and the considered interface serves as a model system for studying heterointerface properties of protonic conductors. In this study, the defect chemistry of the interface between ZrO2-terminated BaZrO3 (0 0 1) and MgO (0 0 1) was investigated by first-principles calculations and space-charge theory. Segregation energies from the BZY bulk to the interface ZrO2 and MgO layers were calculated for effectively charged protons, oxygen vacancies, Y-acceptors as well as cation vacancies. Protons were found to exhibit a strong tendency for segregating to the interface, particularly to an oxide ion in the MgO layer, rendering a net positive charge of the interface. According to the applied thermodynamic space-charge models, the interface potential could reach more than 1V under the Mott-Schottky approximation, with depletion regions extending up to 2nm into BZY. With fully equilibrated Y-segregation profiles, the interface potential was significantly diminished to about 0.2V at 573K and 0.025bar H2O. While the interface was found to be close to saturated by protons under most condition, it was concluded that proton conduction along the interface could not contribute significantly to the in-plane conductivity of BZY films deposited on MgO substrate.

Exploration of Novel Oxide Cathode Materials for Proton-Conducting Solid Oxide Fuel Cell: Phase Relation in the Ba(Zr, Y)O3-Ba(Co, Y)O3-Ba(Fe, Y)O3 System

1. Introduction Proton-conducting solid oxide fuel cell (pc-SOFC) is expected to be one of the most efficient, highly stable, and cost-effective SOFC. As a cathode material for pc-SOFC, a composite cathode system, which comprises of both electron- and proton-conducting materials, has been examined and developed. In such cases, reaction area is limited to the triple phase boundary (TPB), and resultant cathode performance is poor in most cases. On the other hand, upon successful development of mixed proton-hole conductor (MphC) for the oxide cathode, the reaction area will be expanded to the whole surface of cathode grains, because the reaction area turned into two-dimensional boundary from one-dimensional TPB, and the efficiency of the cell will be significantly enhanced[1]. Our primary strategy to develop protonic/electronic mixed conductor for a pc-SOFC cathode is to realize a double percolation system in a single phase of solid solution, that is, Y-doped BaZrO3 (BZO) as a proton conducting host matrix, doped with Co and Fe for the formation of percolation network of electronic carriers, since BaZrO3 is reported as chemically stable perovskite with the best proton conductivity at the target temperature between 400 to 600 °C[2]. In addition, the electronic conductivity is reported in Co-[3] or Fe-doped[4] BaZrO3, while a possible low solubility of Co and its poor chemical stability has to be resolved. Fe is known to dissolve to form solid solution over the entire composition range in the BZO-BaFeO3 quasi binary system at elevated temperatures, providing both chemical stability and the hole conductivity to BZO. As the first step of the development of the MphC cathode materials, phase relation in the Ba(Zr, Y)O3-Ba(Co, Y)O3-Ba(Fe, Y)O3 system at elevated temperatures is examined to understand the feasibility of the present system to the double percolation scheme. In this study, the composition is expressed as BCFZYxyz1 for the formula, BaCox/10Fey/10Zrz/10Y0.1O3-δ, and BCFZY4411, corresponding to the composition of novel cathode for pc-SOFC reported by Duan et al.[5], is included in this compositional contour of the present study. Also further analyses including electrical conductivity and so on will be reported. 2. Experimental Powders of BaCox/10Fey/10Zr0.9-x/10-y/10Y0.1O3-δ with x and y ranging from zero to nine were synthesized via the Pechini method from nitrate aqueous solutions. Concentrations of cations in the nitrate aqueous solutions were determined by both chemical titration and an inductively coupled plasma atomic emission spectroscopy (ICP-AES). The powders were calcined at 1000 °C for 2 h, pressed into pellets using a uniaxial pressing at 1000 kgf/cm2 with an 11.3 mm diameter mold, and subsequently sintered for 10 h at either 1200, 1300, or 1400 °C. The pellets were ground into powders and X-ray diffraction (XRD) measurements were performed with the obtained powders. A field emission scanning electron microscopy and (FESEM) and an energy dispersive X-ray spectroscopy (EDX) were performed on the polished fresh surfaces of the pellets and the annealed surfaces of the pellets after thermal etching. 3. Results and Discussion From the results of XRD measurements, the most of the peaks observed in the XRD patterns were attributed to the primary cubic perovskite structure in reference to the ICDD data for cubic BaZrO3. In BCFZY3061, 3151, 3241, 3331, 2251, and 1351, peaks from the secondary perovskite phase were observed, while those from secondary and tertiary perovskite phases were observed in BCFZY4411 and 4141. The secondary and tertiary phases, however, were not clearly identified by EDX. In most of the samples, the peaks indexed with BaCO3 were observed because of the BaO-rich composition of the precursors identified by ICP-AES analyses. The other peaks were attributed to the impurity phases such as BaCoO3 and YBaCo4O7. In BCFZY3061, 3151, 3241, 3331, 4411, and 4141, partially melted area and thin plate-/needle-like crystals were observed with FESEM. These crystals were identified as YBaCo4O7 or its analog from the EDX results. In addition to the EDX analyses, from the relation between lattice constants of cubic BaZrO3-like phases and Fe/(Co+Fe+Zr) ratio, the phase relation in the BaZr0.9Y0.1O3-δ-BaFe0.9Y0.1O3-δ-BaCo0.9Y0.1O3-δ quasi-ternary system is estimated and proposed that there exist four separate solid solutions of BaZrO3-, BaFeO3-, BaCoO3- based phases and an intermediate phase between Ba(Zr, Co)Y0.1O3 and Ba(Zr, Fe)Y0.1O3.

(Invited) Multi-Element-Doped Ceria-Based Metal Oxides for Advanced Proton Conducting SOFCs

The ability of solid oxide fuel cells (SOFC) to convert the chemical energy of various kinds of fuels into electricity at high efficiency, and with reduced environmental impacts makes them an attractive technology for current and future plans for clean power generation. State-of-the-art yttria-stabilised ZrO2 (YSZ) electrolyte shows high conductivity (10-2 S/cm) at about 1000 °C [1]. However, the high operating temperature leads to durability and cost issues and hinders its full market implementation. Operating temperature of SOFCs can be lowered by employing ceramic proton conducting electrolytes based on doped- BaCeO3 which exhibits high proton conductivity (10-2 S/cm) in the intermediate temperature (IT, 400-700 °C) range [2]. However, the poor chemical stability of doped-BaCeO3 under SOFC by-products CO2, and H2O limits its use as stable electrolyte [2]. Here we report, perovskite–type Ba0.5Sr0.5Ce1-x-y-z Zr x Gd y Y z O3-δ as proton conductors for IT-SOFCs [3]. A-and B-site of BaCeO3 were doped by more electronegative elements to improve its chemical stability under H2O and CO2 at elevated temperature. In terms of chemical stability and conductivity, Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ seems to be the optimal composition with conductivity of 10-3 S/cm at 700 °C in 3% H2O/H2. Open circuit voltage of 1.15 V at 700 °C for H2-air cell suggests pure ionic (proton) conduction in Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ [3]. The effect of sintering temperature on bulk and grain boundary conductivity of these oxides was investigated using dielectric loss spectroscopy [4]. Moreover, the difference in the relaxation times in the current study suggests that short-range and long-range proton dynamics seems to be differing to previous studies on Y-doped BaZrO3 systems [5]. Additionally, layered perovskite-type Gd0.5Pr0.5BaCo2O5+δ were characterised as cathode for H-SOFCs [6]. Symmetrical cell measurements under air and wet air gave an area specific resistance of 2.4 Ω cm2 and 1.9 Ω cm2 for oxygen reduction reaction at 700 °C [6]. The effect of phase purity and synthesis methods on the electrochemical performance of Ni+Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3 -δ anode composites was investigated through symmetrical cell measurements in 3% H2O/H2. References 1) J. W. Fergus, R. Hui, X. Li, D. P. Wilkinson, J. Zhang, Solid Oxide Fuel Cells: Materials Properties and Performance, CRC Press, New York (2009). 2) K. D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. 3) R. Kannan, K. Singh, S. Gill, T. Fürstenhaupt, V. Thangadurai, Sci. Reports. 3 (2013) 2138. 4) K. Singh, A. Baral, V. Thangadurai, J. Am. Ceram. Soc. 99 (2016) 866. 5) Y. Yamazaki, F. Blanc, Y. Okuyama, L. Buannic L, J. C. Lucio-Vega, C. P. Grey, S. M. Haile, Nat. Mater. 12 (2013) 647. 6) K. Singh, A. K. Baral, V. Thangadurai, Solid State Ionics (2016) DOI:10.1016/j.ssi.2015.12.010.

Polaronic contributions to oxidation and hole conductivity in acceptor-dopedBaZrO3

Acceptor-doped perovskite oxides like BaZrO$_3$ are showing great potential as materials for renewable energy technologies where hydrogen acts an energy carrier, such as solid oxide fuel cells and hydrogen separation membranes. While ionic transport in these materials has been investigated intensively, the electronic counterpart has received much less attention and further exploration in this field is required. Here, we use density functional theory (DFT) to study hole polarons and their impact on hole conductivity in Y-doped BaZrO$_3$. Three different approaches have been used to remedy the self-interaction error of local and semi-local exchange-correlation functionals: DFT$+U$, pSIC-DFT and hybrid functionals. Self-trapped holes are found to be energetically favorable by about $-$0.1 eV and the presence of yttrium results in further stabilization. Polaron migration is predicted to occur through intraoctahedral transfer and polaron rotational processes, which are associated with adiabatic barriers of about 0.1 eV. However, the rather small energies associated with polaron formation and migration suggest that the hole becomes delocalized and band-like at elevated temperatures. These results together with an endothermic oxidation reaction [A. Lindman et al., Phys. Rev. B 91, 245114 (2015)] yield a picture that is consistent with experimental data for the hole conductivity. The results we present here provide new insight into hole transport in acceptor-doped BaZrO$_3$ and similar materials, which will be of value in the future development of sustainable technologies.

Microstructure and Compositional Defects Affects Proton Conductivity and Reactions in Y-doped BaZrO3 Thin Films

In this paper, we report on BaZr0.8Y0.2O3-x (BZY) thin films grown on highly mismatched NdGaO3 (110) substrates by RHEED assisted pulsed laser deposition. The conduction and electrochemical performances are studied by Electrochemical Impedance Spectroscopy and Electrochemical Strain Microscopy respectively. Conductivity, as well as electrochemical response improves while decreasing the thickness, indicating that the proton movement is prompt at the defective interface region. Structural defects at the interface between film and substrate are clearly displayed by x-ray diffraction, RHEED patterns and transmission electron microscopy. The role of chemical defects on BZY film properties is elucidated by Hard X-ray Photoelectron Spectroscopy. Our results demonstrate that both structural dislocations and chemical defects influence the proton conduction and reaction process in BZY thin films.

A-Site Cation Substitutions in Strained Y-Doped BaZrO3 Multilayer Films Leading to Fast Proton Transport Pathways

Proton-conducting perovskite oxides form a class of solid electrolytes for novel electrochemical devices operating at moderate temperatures. Here, we use hard X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory calculations to investigate the structure and elucidate the origin of the fast proton transport properties of strained ultrathin films of Y-doped BaZrO3 grown by pulsed lased deposition on NdGaO3. Our study shows that our BaZr0.8Y0.2O3 films incorporate a significant amount of Y dopants, and to a lesser extent also Zr ions, substituting for Ba2+, and that these substitutional defects agglomerate forming columnar regions crossing vertically from the surface to the interface the entire film. Our calculations also show that, in regions rich in Y substitutions for both Zr and Ba, the proton transfer process involves nearly zero-energy barriers, indicating that A-site cation substitutions by Y lead to fast transport pathways and hence are responsibl...

(Invited) Non-Classical Electrostriction in Materials with a Large Concentration of Point Defects

We have recently demonstrated anomalously large electrostriction, a nonlinear induction of strain by an electric field, in three well-studied ionic conductors: Gd-doped ceria, (Nb, Y)-stabilized cubic bismuth oxide and Y-doped hydrated barium zirconate. At room temperature, all three materials exhibit a strain electrostriction coefficient (M11) exceeding 10-17 m2/V2 and can generate stress of tens of MPa without apparent signs of saturation. According to the theory of classical electrostriction, presented two decades ago by Prof. R. Newnham (Penn State), the electrostriction polarization coefficient (Q) scales with the ratio of elastic compliance to dielectric constant. This theory successfully describes most classes of materials from polymers, generating large strain and small stress, to relaxor ferroelectrics that generate small strain and large stress. However, the three ionic conductors mentioned above have a large elastic modulus (> 80 GPa) and moderately low dielectric constant (<100), which places their Q coefficients at least two orders of magnitude above the values predicted by the classical theory. Of these three materials, Gd-doped ceria has been the subject of the most detailed investigation, including in-situ high-resolution, room temperature XANES and EXAFS. According to these data, oxygen vacancies giving rise to elastic dipoles play a central role in the appearance of electrostriction. CeCe-oxygen vacancy (Vo) repulsion increases the CeCe-VO distance and forms six anomalously short CeCe-oxygen (OO) bonds. The resulting 7OO-CeCe-VO complex behaves as a strong elastic dipole with uniaxial symmetry. The components of the elastic tensor estimated from the differential EXAFS data are -4.7% and +11%. Application of an external electric field reduces the CeCe-VO repulsion, causing reversal of the local distortions. Because of the large strains involved, very large macroscopic stresses, which can reach hundreds of MPa, are generated, in spite of the fact that only a few percent of anion-cation complexes are involved. However, the presence of oxygen vacancies is not a sufficient condition for non-classical electrostriction. Oxygen deficient ceria (Ce(III)-doped) and Lu-doped ceria with the same concentration of vacancies as Gd-doped ceria exhibit much lower electrostriction, while Eu-doped ceria displays electrostriction similar to that of Gd doped ceria. Since Gd and Eu have almost identical ionic radii while Ce(III) is much larger and Lu(III) is much smaller, the radius of the dopant seems to be a critical parameter. Similar to Gd-doped ceria, (Nb, Y)-stabilized cubic Bismuth oxide has a fluorite structure and exhibits large non-classical electrostriction, which monotonically increases wihin the range of 16-23% of the oxygen vacancies. It is interesting to note that increase in the vacancy concentration from 16 to 23 % causes almost 50% decrease in the Young’s modulus, in the cubic Bismuth oxide, whereas in Gd-doped ceria increase in the vacancy concentration decreases the Young’s modulus by less than 10%. Similar differences between cubic Bismuth oxide and Gd-doped ceria are observed for Poisson’s ratio: it decreases two fold for the cubic Bismuth oxide and remains unchanged for Gd-doped ceria. These differences indicate that the influence of the large concentration of vacancies on electrostrictive and mechanical properties differ considerably from material to material. This is also supported by the fact that the electrostriction coefficient of 14.5 mol% Y-doped zirconia falls very close to the expected from the classical electrostriction. Hydrated Y-doped BaZrO3 is a well-studied protonic conductor in which OH groups can be present at »7% of the oxygen sites. Preliminary investigations indicate that this material also exhibits large non-classical electrostriction; yet hydrated Y-doped BaZrO3 at room temperature in fact does not contain any vacancies. On this basis, one can only conclude that a primary condition for large non-classical electrostriction is the presence of a large concentration of point defects, which can be of any type, but which must form elastic dipoles with a particular degree of freedom at room temperature. We suggest that a search for new electromechanically active materials must include those with a large concentration of such point defects in general and, ionic conductors, in particular.

Microstructure and Compositional Defects Affects Proton Conductivity and Reactions in Y-doped BaZrO3 Thin Films

Doped perovskite oxides have been widely studied in recent years as proton conducting solid electrolytes for different electrochemical devices, e.g. Solid Oxide Fuel Cells (SOFCs). By using the proton conductor as the electrolyte in a SOFC, the fuel dilution issue can be avoided due to the water formation at the cathode side instead of the anode. Among all the alkaline-earth cerates and zirconates, Y doped BaZrO3 (BZY) has been considered one of the most promising electrolyte for the SOFCs applications for its significant proton conductivity as well as chemical stability.[1] However, most reported fuel cell performance values for this material is much lower than oxide ion conducting ceramics such as YSZ owing to an unavoidable microstructure change, i.e. high grain boundary density and low degree of the crystallinity. In order to correlate the microstructure with the conductivity, we prepared BZY thin films by Pulsed Laser Deposition and discussed the effect of the microstructure on the proton conductivity. In this presentation I will show the conductivity behavior as a function of temperature and thickness by Electrochemical Impedance Spectroscopy (EIS).[2] I will focus on the interface between BZY and substrate NdGaO3 and show the cross-sectional electrochemical activity change with nanoscale resolution by Electrochemical Strain Microscopy (ESM). [3] I will highlight the interplay between the interface defects and the electrochemical performance by comparing the EIS, ESM results with Scanning Transmission Electron microscopy (STEM). I will further show the complementary study by bulk sensitive hard x-ray photoemission spectroscopy measurements which, together with Density Functional Theory calculations, demonstrate that Y and, to some extent, also Zr ions, substitute for Ba near the interface with the substrate. [1] A. D’Epifanio, E. Fabbri, E. Di Bartolomeo, S. Licoccia, E. Traversa, Fuel Cells, 1,69-762008 (2008). [2] V. Foglietti, Nan Yang, A. Tebano, C. Aruta, E. Di Bartolomeo, S. Licoccia, C. Cantoni, G. Balestrino, Applied Physics Letters 104 , 081612 (2014). [3] Nan Yang, C. Cantoni, V. Foglietti, A. Tebano, A. Belianinov, E. Strelcov, S. Jesse, D. Di Castro, E. Di Bartolomeo, S. Licoccia, S. V. Kalinin, G. Balestrino and C. Aruta, Nano Letters, 10.1021/acs.nanolett.5b00698, (2015).

Size-controlled growth of spherical nanoparticles of Y-doped BaZrO3 perovskite

Yttrium-doped BaZrO3 (BZY) was processed by a simple yet effective chemical synthesis route that yielded perfectly spherical particles. The particle size was found to decrease with increasing molar concentration of cations in the solution. Thus, the process offered a great ease of controlling the particle size as well. The close control over morphology and size can provide an opportunity of tuning the properties of such particles. The average particle size as obtained from high-resolution scanning electron microscopy (HR-SEM) was 243, 206, 149, and 79 nm at 0.2, 0.5, 0.8, and 1 M concentration, respectively. At a processing temperature of 60 °C, the undesirable phases, BaCO3 and Zr1−xYxO2, were present along with BZY even after calcination at 600 °C, and it took a very long processing time (160 min) for complete phase formation. The BZY phase formed within 45 min at a reaction temperature of 90 °C and yielded phase pure powders on calcination at the same temperature (600 °C). The d-spacing (2.98 A) obtained from the lattice fringes in high-resolution transmission electron microscopy (HR-TEM) confirmed the cubic perovskite phase of BaZrO3. The average crystallite size calculated from XRD analysis coupled with the TEM observations revealed that each spherical particle consists of only few crystallites.

Enhanced interfacial proton migration on BaZr(Y)O3 by molten carbonate: A first principles study

Enhanced proton conductivity of Y-doped BaZrO3 (BZY) by molten carbonate (MC) has recently been reported. To understand the phenomenon, the present work investigates the pathways and energetics of proton migration along ZrO2-terminated (100) surface of BaZr(Y)O3 without and with the presence of MC from a density functional theory (DFT) perspective. The calculations explicitly show that the proton migration on BaZrO3 prefers a curved pathway with an energy barrier of 0.567eV. In the presence of carbonate-ion, the proton migration barrier is remarkably reduced to 0.332eV, which is in excellent agreement with 0.33eV of the experimental activation energy for conductivity in the BZY-MC electrolyte. Overall, the enhanced proton conductivity in the BZY-MC composite electrolyte is originated from facile interfacial proton migration between BZY and MC phase.

Influence of defect states on proton conductivity of Y-doped BaZrO3

Nuclear energy is a promising new energy to solve energy crisis. Separation and purification of hydrogen isotopes play an important role in the developing and utilizing of nuclear energy. BaZrO3-based oxide is an effective material for the separation and purification of hydrogen isotopes. In this paper, a series of BaZr1-xYxO3- (0 x 0.3) are synthesized by high-temperature solid state reaction method. The raw materials are calcined at 1200 ℃ for 5 h in air. Then the calcined powder is consolidated by an isostatic press and sintered at 1500 ℃ for 48 h in air, using a furnace equipped with aluminum oxide heater. Phase purity and phase structure of the obtained BaZr1-xYxO3- are analyzed by XRD. Results show that the structures of the BaZr1-xYxO3- are consistent with the BaZrO3 diffraction pattern (JCPDS 06-399). The Y ions are already incorporated into the lattice of BaZrO3, and the maximum doping concentration of Y rangs from 0.24 to 0.26. Besides, the proton conductivity of Y-doped BaZrO3 is determined under hydrogen atmosphere by the electrochemical impedance spectroscopy (EIS). Experiments show that the BaZr1-xYxO3- with 20 mol% Y has the highest conductivity of 0.0015 S/cm at 600 ℃ which is higher than that of the BaZrO3 matrix material by two orders of magnitude. As the concentration of Y increases, the strain in the crystal structure of BaZrO3 increases, which may be created by the defect of Y-doped BaZrO3. In order to reveal the mechanism of proton conduction in Y-doped BaZrO3, the influence of defect types on proton conduction is also investigated via photoluminescence (PL) and thermoluminescence (TL). For the BaZrO3 matrix, an asymmetrical broad emission peak at 350 to 650 nm occurs in PL with an excitation light of 334 nm. Analysis of Gaussian decomposition shows that the asymmetrical broad emission peak is created by two kinds of different oxygen vacancies (Vo..), which are beneficial to proton conduction. Interestingly, when BaZrO3 is doped with Y, a new emission peak P1 at 388 nm appears owing to the negatively charged YZr' of proton-trapping-type defects, which is harmful to the proton conduction in general. TL analysis shows that the number of YZr' increases and the depth of the trap reduce, as the Y concentration increases in BaZr1-xYxO3- (x=0, 0.05, 0.1, 0.2). Although the YZr' is noxious for the proton conduction, the proton conductivity of BaZr1-xYxO3- (x=0, 0.05, 0.1, 0.2) can be improved via the increase of the release ability of proton trapping as the depth of trap is reduced.

Solubility of transition metal interstitials in proton conducting BaZrO3and similar perovskite oxides

The defect chemistry of foreign transition metals in perovskite oxides was investigated by first-principles calculations in combination with experiments with focus on Ni and Zn in Y-doped BaZrO3.

Protonic Conduction of Nanostructured Y-Doped BaZrO3

Nanostructured ionic conductors have recently attracted our attention due to the expectation that they may lead to new functionalities absent in microcrystalline conductors. In this study, nanostructured barium zirconate with perovskite crystal structure was prepared and its grain and grain boundary protonic conduction was investigated using ac impedance measurements as a function of temperature (RT ~ 400°C) and water partial pressure. The grain was highly conductive of protons, which is governed by the concentration of protonic defects at all temperatures. On the other hand, the grain boundary was not the preferred route for protonic conduction due to high resistance. However, enhanced protonic transport was observed at certain temperatures (&lt;100°C). The protonic resistivity below that temperature decreased with decreasing temperature, showing positive activation energy in relation to temperature. The conduction route for the enhanced transport was the serial grain boundary. In addition, the route was compared with that of nanostructured zirconia with fluorite crystal-structure.

Alkali metals as efficient A-site acceptor dopants in proton conducting BaZrO3

The present contribution demonstrates that A-site acceptor doping of BaZrO3 by alkali metals may significantly enhance its bulk proton conductivity compared to that of Y-doped BaZrO3.

Design of New Power Source Based on Micro-SOFC for Mobile Electronic Devices

Low operating temperature micro solid oxide fuel cell (micro-SOFC) systems which utilize liquid fuels are attractive for high energy efficiency and density. Long time operation is possible for micro-SOFCs by refueling and it is not restricted by battery charge. Hence, it is expected to alternative power source for compact mobile electronic devices which consume a large amount of power. In previous, diverse types of micro-SOFCs focused on its performance was reported and a maximum power density of 1 W/cm2 at 500oC was achieved. However, they have many problems of durability and repeatability for practical use. Additionally, it is necessary to consider how to heat up micro-SOFC cells to operating temperature, compatibility with other electronic circuits and thermally self-sustained condition. In this study, micro-SOFC system was proposed in order to adopt the micro-SOFCs into a power source of compact mobile electric devices. We designed the micro-SOFC package with thermal insulation and heat exchanger. Its insulation performance was experimentally confirmed and power generation test with micro heater in the micro-SOFC package was performed. Micro-SOFC packages require thermal insulation to keep the surface temperature below about 85oC, which is the maximum operating temperature of other electronic circuits. We applied vacuum insulation widely used for thermal insulation in MEMS devices to the package. The heat of the exhaust gas was recovered by the liquid fuel through heat exchanger. From the calculation of temperature distribution by the finite element method, surface temperature of the package was kept from 57 to 88oC when operating temperature is 500oC. As long as the liquid is evaporated, temperature of the heat exchanger is kept constant. Therefore, heat conduction from high temperature glass flow channels to vacuum insulator was prevented and low surface temperature was achieved. Applying micro-SOFC system as independent power sources, thermally self-sustained condition is required, i.e. micro-SOFC packages should keep operating temperature more than 300oC only by the generated heat from reaction of fuel cells during power generation. For thermally self-sustained condition, the amount of heat generated during power generation needs to exceed that of heat loss from surface of the package and transferred heat by exhaust gas. We calculated thermally self-sustained condition of a micro-SOFC system with 4 W output fueled by methanol and water that has reformer and heat exchanger from exhaust gas. From the result, generated heat from reaction of fuel cells was 4.8W, heat loss from surface was 1.5 W and transferred heat by exhaust gas was 4.6 W at 500oC. The result confirmed that transferred heat by exhaust gas was dominated in this system. Hence, recovering the heat of exhaust gas with the gas-liquid heat exchanger was essential to achieve thermally self-sustained condition. Micro-SOFC cells and micro heater were fabricated in general MEMS technology. Power generation test and electrochemical impedance spectroscopy of micro-SOFC (Pt-Pd | 15 mol% Y-doped BaZrO3 | Pt-Pd) by micro heater were measured in the temperature range of 200-330oC. A maximum power density of 1.12 mW/cm2 and open circuit voltage of 1.09 V at 330oC were obtained using wet H2 as fuel and air as the oxidant. As a result of impedance measurement, electrode reaction resistance was very high in cell total resistance. Therefore, the improvement of electrode will reduce cell resistance to a level for actual utility. We actually fabricated the micro-SOFC package installed vacuum structure and heat exchanger to evaluate cooling performance of the insulation package and performed power generation test in the package. In the case of heat exchanger filled with 50 vol% ethanol solution, surface temperature was 89.6oC when micro heater temperature was 500oC. In accordance with the result, it was possible to keep surface temperature of the package sufficiently low with heat exchanger. As a result of power generation test of micro-SOFC inside the package, a maximum power density of 1.27 mW/cm2 and open circuit voltage of 0.94 V at 300oC were obtained using wet 25% or 50% H2 as fuel and air as the oxidant. Additionally, surface temperature of the package was kept under 80oC. Therefore I achieved power generation in compact micro-SOFC system with thermal insulation. The micro-SOFC package was designed for applying micro-SOFCs to power source of mobile electronic devices in this study. As a result of calculation, the package installed vacuum insulation and heat exchanger could achieve thermally self-sustained condition and low surface temperature. Power generation test of micro-SOFC inside the prototype package was achieved and the package kept low surface temperature. Hence, this study presented that applicability of micro-SOFC system to power source for compact mobile electronic devices.

Yttrium and Nickel Co-Doped BaZrO3 as a Proton-Conducting Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

Conventional solid oxide fuel cells (SOFCs) based on yttria-stabilized zirconia (YSZ) require high operating temperatures in the 800-1000 °C range. Such high operating temperatures hinder the commercialization of SOFCs due to degradation and high cost issues. On the other hand high temperature proton conductors (HTPCs), due to their lower activation energy for proton conduction, can achieve high conductivity at relatively lower temperatures compared to their oxygen ion conducting counterparts. BaZrO3 related HTPC electrolyte materials possess good chemical stability and high bulk conductivity. However, due to its high refractory nature, fabrication of fully dense electrolyte films is very challenging. The poor sinterability of BaZrO3based materials leads to a large number of resistive grain boundaries, thus lowering the total proton conductivity. Previous reports have shown that NiO can function as an effective additive for enhancing the densification of BaZrO3 based materials [1, 2]. These studies did not focus on controlling the stoichiometry so as to accommodate Ni at a specific site in the perovskite structure. Contrary to the previous NiO sintering aid studies, the present work focuses on substitution of Zr with Ni in the perovskite B-site in a targeted fashion. Y-doped BaZrO3 (BZY) is chemically stable at fuel cell operating conditions and shows the best protonic conductivity when Y-content is 20 mol% [3]. In this work, the sinterability of 20 mol% Y-doped BaZrO3 (BZY20) was significantly improved by partial substitution of Zr with Ni in the perovskite B-site. The effect of Ni-doping on the shrinkage behavior of BZY has been identified through dilatometric measurements. The SEM analysis showed a pore-free dense microstructure for 4 mol% Ni-doped BZY (BaZr0.76Y0.2Ni0.04O3-δ, BZYNi04) and therefore electrochemical characterization studies were carried out on this material. The total conductivity of BZYNi04 in wet air was observed to be 0.0037 Scm-1 at 600 °C. The open circuit voltage (OCV) values recorded for BZYNi04 pellet fuel cell were 0.96, 0.99, 1.01, 1.03 and 1.034 V at 700, 650, 600, 550 and 500 °C respectively indicating that there is not much electronic conductivity. Ni-doped BZY electrolyte films were fabricated on an optimized NiO-BZY anode using co-pressing and co-firing process. Subsequently, a full fuel cell was fabricated with PrBaCo2O5+δ (PBCO) and BZY composite cathode. The ease of film electrolyte fabrication of low thickness assisted by the good sinterability of BZYNi04 coupled with employing an anode functional layer aided in attaining a superior power density output. Peak power density values of 428 and 340 mWcm-2 were obtained at 700 and 650 °C respectively when humidified hydrogen (≈ 3% H2O) and static air were used as the fuel and oxidant respectively. To the best of our knowledge, the power performance obtained in this work is the best reported yet for BaZrO3based proton conducting SOFCs. References P. Babilo, S. M. Haile, J. Am. Ceram. Soc. 88 (2005) 2362.J. Tong, D. Clark, M. Hoban, R. O’Hayre, Solid State Ionics 181 (2010) 496.K. D. Kreuer, S. Adams, W. Munch, A. Fuchs, U. Klock, J. Maier, Solid State Ionics 145 (2001) 295.

Preparation of Y-Doped BaZrO&lt;sub&gt;3&lt;/sub&gt; Proton Conducting Solid Electrolyte via Modified Low Temperature Pechini Method

One of the promising material for proton-conducting solid electrolyte operating at intermediate temperature range (400-600 °C) is the Yttrium-doped BaZrO3 (BZY) due to its high conductivity and chemical stability. In this study, a modified citrate-nitrate combustion method (Pechini method) has been employed for BZY powder preparation. A stoichiometric amounts of starting nitrates and oxide raw materials with nitric acid, citric acid and ethylene glycol for the synthesis of 20 mol% Y-doped BaZrO3 (BZY20) were prepared, then calcined and sintered at 1000 °C for two heat treatment durations of 24 hours and 48 hours. The obtained BZY20 powder samples have been fully characterized for its structure, morphology, and thermal properties. From the X-ray diffraction (XRD) results, the sample sintered for 48 hours showed a cubic phase of BZY20 which can be indexed to a Pm3m cubic structure which is also supported by Raman analysis. The calculated lattice parameter is 4.2067 Å which is higher than the reported lattice parameter of a pure BaZrO3 (BZ) of 4.1930 Å which indicates a successful doping due to higher ionic radius of Y3+ dopant as compared to Zr4+ in the B-site ABO3 perovskite sub-lattice. In addition, SEM-EDX analyses of the sintered pellet revealed a uniform distribution of Yttrium dopant in the BZY20 prepared solid electrolyte.

Synthesis of Yttrium-Doped Barium Zirconate/Barium Cerate (BZY20/BCY20) Core-Shell Structured Proton-Conducting Solid Electrolyte via Modified Pechini Method

Intermediate temperature solid oxide fuel cells (IT-SOFCs) operating at 400-700°C utilizes proton conducting electrolytes and are now being one of the focus of many research studies with regards to efficient, clean power sources and energy conversion. Potential electrolyte materials include acceptor-doped barium zirconates and barium cerates. In this study, preparation of a core-shell structured proton-conducting solid electrolytes of 20 vol% BaZr0.8Y0.2O3-δ (20BZY20) for the core and 80 vol% BaCe0.8Y0.2O3-δ (80BCY20) for the shell is done by wet chemistry route. The synthesized core-shell structured material (20BZY20/80BCY20) is developed to possibly address the problem of high grain boundary impedance of Y-doped BaZrO3 and low chemical stability of Y-doped BaCeO3. The obtained samples were characterized for its structure, thermal stability, morphology and elemental distribution of the material. At a lower sintering temperature of 1150°C, a densified pellet was obtained as observed by SEM analysis. The diffraction pattern of 20BZY20/80BCY20 powder shows two distinct phases corresponding to BZY20 and BCY20 suggesting a successful synthesis of the core-shell solid electrolyte.

First-Principles Insight into the Hydration Ability and Proton Conduction of the Solid State Proton Conductor, Y and Sn Co-Doped BaZrO3

Y and Sn co-doped BaZrO3 (BZSY) has recently been shown to exhibit superior hydration ability and improved power output performance compared to those of the traditional solid oxide fuel cell (SOFC) electrolyte, Y-doped BaZrO3. The fact that BZSY is also chemically stable in both H2O and CO2 atmospheres further illustrates the great potential for BZSY as a future electrolyte material in proton-conducting SOFCs. In this work, we conducted comprehensive density functional theory calculations to analyze the energetics of hydration and proton migration in BZSY. The energy of hydration is calculated at four different locations in the cell to better assess the specific contributions of each defect type. For all locations tested, hydration is found to be strongly exothermic and is most favorable when Y ions are in the vicinity. The calculated energies of hydration (−1.72 to −1.11 eV) are significantly lower than any previously calculated for acceptor-doped BaZrO3-based electrolytes. Nudged elastic band calculations confirm low proton reorientation energies (0.08–0.24 eV) and low intraoctahedral hopping energies (0.21–0.41 eV) and that these diffusion barriers are at minimum when the proton is migrating to an oxygen ion in a YO6 octahedron. Our results show how the synergy of the Sn and Y dopant ions produces the excellent hydration and conduction performance of BZSY and fully support its potential application in next-generation SOFCs.