Gas Concentration Cell Research Articles

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 La1-xSrxScO3-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 La1-xSrxScO3-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 La0·9Sr0.1ScO3-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 La0·9Sr0.1ScO3-δ. Thus, La0·9Sr0.1ScO3-δ should be considered as a promising proton-conducting electrolyte for various electrochemical devices, such as protonic ceramic fuel cells.

Intermediate temperature fuel cell durability of Eu-doped SrCeO3-SrZrO3 solid solution/ NaCl-KCl composite electrolyte

A SrCe0.7Zr0.2Eu0.1O3-α (SCZE-SG) electrolyte was successfully fabricated by a sol-gel method using Eu2O3, Sr(C2H3O2)22O, (NH4)2Ce(NO3)6 and Zr(NO3)4·5H2O as raw materials and was subsequently reacted with NaCl/KCl. The obtained materials were characterized by XRD and SEM techniques. XRD results showed that SCZE-SG and NaCl/KCl did not chemically react. The SEM images revealed that the particle size of SCZE-SG is uniform, and NaCl/KCl is coated on the surface of SCZE-SG, acting as a binder in SrCe0.7Zr0.2Eu0.1O3-α-NaCl-KCl (SCZE-SG-NK). The oxide ion and protonic conduction of SCZE-SG was studied using gas concentration cells, and the results indicate that the durability of the fuel cell based on SCZE-SG-NK exhibits excellent cell performance with a maximum power output density of approximately 0.58Wcm−2 during a 40-h operation at 700°C.

A Study of Transition Metal Doping on the Electrical Properties of Perovskite Type Proton Conductor

Proton-conducting oxides have been studied as intermediate-temperature electrolyte materials for fuel cells and steam electrolysis. These materials can operate at intermediate temperatures because the activation energy for proton conductivity is lower than that of oxide ion conductivity. When proton conducting oxides are used as the electrolyte for fuel cells and steam electrolysis, nickel or its composite with electrolyte materials, so-called Ni cermet, are used as the fuel-side electrode, and transition-metal-containing perovskite such as (La,Sr)(Co,Fe)O3 (LSCF) and (La,Sr)MnO3 (LSM), are used for the air-side electrodes. The electrode materials contain Ni, Fe, Co, Mn, and so forth, which possibly react with the electrolyte oxides during processing and operation of the fuel cells and electrolysers. A possible concern is that the diffusion of these transition metals will degrade the electrical conductivity of the electrolyte. Shimura et al. reported a significant decrease in the electrical conductivity of BaCe0.9Y0.1O3-δ-based (BCY) proton conductors on partially substituting Fe, Mn and Co for Ce [1]. In this study, transition metal doping of perovskite type oxide have been performed, i.e., BaCe0.85Y0.1 M 0.05O3-δ, BaZr0.85Y0.1 M 0.05O3-δ, SrCe0.85Y0.1 M 0.05O3-δ and SrZr0.85Y0.1 M 0.05O3-δ which are referred to hereafter as BCYM, BZYM, SCYM and BZYM, respectively (M = Co, Fe, Mn and Ni), and their electrical conduction properties investigated to understand the effect of introducing transition metals to the proton conductor oxides. (Ba,Sr)(Ce,Zr)0.9-x Y0.1 Mx O3-δ were prepared by a solid-state reaction method. BaCO3, SrCO3, CeO2, ZrO2, Y2O3 and transition metal oxides were appropriately weighed, mixed, and calcined at 1200-1300 °C for 10 h in air, ball-milled, pressed into pellets, and sintered at 1400-1700°C for 10 h in air. Phase identification was carried out by X-ray diffraction. The conductivity was measured by a four-terminal AC impedance method. The electromotive force of gas concentration cells was measured in the temperature range from 873 to 1073 K. The electrical conductivity change was observed by introducing transition metals. These results suggest that the introduction of transition metals causes the change in the conductivity of the proton conducting electrolytes in most cases, and that the change in the conductivity depends on the sort of transition metals. This means that the impact of introducing transition metals to the electrical conduction properties depends on the oxide. The above two cases suggests that either A-site (Sr or Ba) or B-site (Ce or Zr) possibly governs for the impact of conductivity is sensitive to the transition metals. Acknowledgement This work was supported by the Cross-ministerial Strategic Innovation Promotion Program (SIP), International Institute for Carbon Neutral Research (I2CNER) and World Premium International Research Center Initiative (WPI), Japan. [1] T. Shimura, H. Tanaka, H. Matsumoto, T. Yogo, Solid State Ionics, 176 (2005) 2945–2950

Transport properties of acceptor-doped barium zirconate by electromotive force measurements

In this work, a systematic work was performed to investigate the electrochemical transport properties of acceptor-doped BaZrO3 by measuring electromotive force on various gas concentration cells. For the measurements in the wet oxidizing atmosphere, where significant hole conduction occurs, the transport numbers of the ionic conduction were corrected by taking the effect of electrode polarization into consideration. The results revealed that regardless of whether Sc, Y, In, Ho, Er, Tm or Yb was doped, proton conduction predominated in the reducing atmosphere with the transport number close to unit. However, the contribution of ionic conduction weakens, and the contribution of hole conduction enhances, when the samples are exposed to the moist oxidizing atmosphere. In addition, introducing Ba-deficiency results in degraded electrochemical conductivity, but the transport number in either the moist reducing or the moist oxidizing atmosphere does not change obviously.

Deposition and Characterization of Y-doped CaZrO3 Electrolyte Film on a Porous SrTi0.8Fe0.2O3-δ Substrate

The fabrication, the characterization and the electrochemical performance of a solid electrolyte film of Y-doped CaZrO3 deposited on a porous SrTi0.8Fe0.2O3-δ substrate are studied in the present work. A dense 6-μm-thick film of the electrolyte is prepared by multi-step chemical solution deposition. Phase purity, concentrations of the elements and microstructure of the film are characterized by means of X-ray diffraction, energy-dispersive X-ray spectroscopy and scanning electron microscopy. The results indicate interdiffusion between the film and the substrate. For the electrochemical characterization of the electrolyte film a gas concentration cell Pt/STF/CZY-film/Pt is fabricated. Open circuit voltage (OCV), impedance spectra (EIS) and current-voltage characteristics are measured at the oxygen concentration cell in the temperature range from 500 to 600°C. The ionic transport numbers in the electrolyte film determined from the impedance and OCV data are found to vary within 0.93–0.97 interval. This is significantly higher than the ratio Em/EN between the measured OCV value (Em) and the theoretical EMF value (EN), which determines the transport numbers in the case of non-polarizable electrodes. It is concluded that this difference is caused mainly by the high polarization resistance of the cell.

Incorporation and conduction of proton in Sr-doped LaMO3 (M=Al, Sc, In, Yb, Y)

In order to clarify the effect of the B site species in ABO3 perovskite oxides on the proton transport properties, the proton incorporation into a series of La0.9Sr0.1MO3-δ, (M=Al, Sc, In, Yb, Y) was studied by measuring the electrical conductivity and electromotive forces of the gas concentration cells, and by a thermogravimetric analysis. The proton concentration and electrical conductivity increased in the order of the B site species, Al<Y=Yb<Sc<In and Al<Y=Yb<In<Sc, respectively. La0.9Sr0.1AlO3-δ showed an oxide ion conductivity, while La0.9Sr0.1YbO3-δ and La0.9Sr0.1YO3-δ exhibited a protonic conductivity in the temperature range of 573–1173K. La0.9Sr0.1ScO3-δ and La0.9Sr0.1InO3-δ showed a protonic conductivity under 873K, and a mixed proton and oxide ion conductivity at 1073K.

Proton transport properties of La0.9M0.1YbO3−δ (M = Ba, Sr, Ca, Mg)

The proton transport properties of La0.9M0.1YbO3−δ (M=Ba, Sr, Ca, Mg) were studied by the electrical conductivity measurement technique in the temperature range of 673–1173K. The proton concentration was estimated by the weight changes due to the hydration reaction. The electrical conductivity and proton concentration of La0.9Ba0.1YbO3−δ were the highest in La0.9M0.1YbO3−δ. The electromotive force of a hydrogen gas concentration cell using La0.9Ba0.1YbO3−δ as the electrolyte was observed at 673–1073K, and was in a good agreement with the theoretical values assuming that the proton transport number was unity.

Ionic Conduction in In3+‐doped ZrP2O7 at Intermediate Temperatures

AbstractA new series of Zr1−xInxP2O7 (x=0.03, 0.06, 0.09, 0.12) samples were prepared by a solid state reaction method. XRD patterns indicated that the samples of x=0.03–0.09 exhibited a single cubic phase structure, and the doping limit of In3+ in ZrP2O7 was x=0.09. The conduction behavior was investigated in wet hydrogen using various electrochemical methods including AC impedance spectroscopy, isotope effect, gas concentration cells at intermediate temperatures (373–573 K). The conductivities were affected by the doping levels, and increased in the order: σ (x=0.03)&lt;σ (x=0.12)&lt;σ (x=0.06)&lt;σ (x=0.09). The highest conductivity was observed for the sample Zr0.91In0.09P2O7 to be 1.59×10−2 S·cm−1 in wet hydrogen at 573 K. The isotope effect also confirmed the proton conduction of the sample under water vapor‐containing atmosphere. It was found that in wet hydrogen atmosphere Zr0.91In0.09P2O7 was almost pure ionic conductor, the ionic conduction was contributed mainly to proton and partially to oxide ionic. The H2/air fuel cell using x=0.09 sample as electrolyte (thickness: 1.73 mm) generated a maximum power density of 13.5 mW·cm−2 at 423 K and 16.9 mW·cm−2 at 448 K, respectively.

Ionic conduction in Zn2+-doped ZrP2O7 ceramics at intermediate temperatures

A novel series of Zr1−xZnxP2O7 (x=0.00, 0.03, 0.06, 0.1) was prepared by a solid state reaction method. XRD patterns indicated that all the samples exhibited a single cubic phase structure except Zr0.9Zn0.1P2O7. The conduction behavior was investigated using various electrochemical methods including ac impedance spectroscopy, isotope effect, gas concentration cells etc. in the temperature range of 300–600°C. The conductivities were affected by the doping levels, and increased in the order: σ (x=0.00)<σ (x=0.03)<σ (x=0.1)<σ (x=0.06). Zr0.94Zn0.06P2O7 exhibited the highest conductivities of 1.85×10−4S·cm−1 in wet hydrogen and 1.02×10−4S·cm−1 in dry air at 600°C. Isotope effect confirmed the proton conduction of the samples under water vapor-containing atmosphere. It was found that the ceramic samples were mixed conductors of oxide ionic and electron hole in dry oxygen-containing atmosphere. Whereas the ceramic samples were almost pure ionic conductors, and the ionic conduction was mainly contributed to proton and partially to oxide ionic in wet hydrogen atmosphere.

Proton and oxide-ion conduction in ZnO doped SnP2O7 ceramics

A series of Sn1−xZnxP2O7 (x=0, 0.03, 0.06, 0.09, 0.12) ceramic samples have been prepared by a conventional solid state reaction method. The ceramic samples of x=0.00–0.09 exhibited a single phase structure, and the doping limit of Zn2+ in SnP2O7 was at least x=0.09. The conduction behaviors of the ceramic samples were investigated by means of electrochemical methods including AC impedance spectroscopy, isotope effect, gas concentration cells etc. in the temperature range of 300–650°C. Among the ceramic samples studied, the highest conductivities were observed for the Sn0.91Zn0.09P2O7 sample to be 1.69×10−4Scm−1 in wet air and 1.91×10−4Scm−1 in wet hydrogen at 600°C. Isotope effect confirmed the ceramic samples possessed proton conduction under water vapor-containing atmosphere. It was found that the sintering temperature of 1200°C in present investigation could result in not only appropriate densities, but also high conductivities of ceramic samples. It was also found that in wet hydrogen atmosphere Sn0.91Zn0.09P2O7 was almost a pure ionic conductor, the ionic conduction was contributed mainly to proton and partially to oxide-ion, whereas in wet air atmosphere, the ceramic sample exhibited a mixed ion and electron hole conduction.

Electromotive Force of the High-Temperature Concentration Cell Using Al-Doped CaZrO&lt;sub&gt;3&lt;/sub&gt; as the Electrolyte

In order to clarify the electrochemical properties of the Al-doped CaZrO3 system, a gas concentration cell was assembled adopting 0.4mol%Al-doped CaZrO3 polycrystalline sintered material as the electrolyte and its electromotive force (emf) was measured for various oxygen and hydrogen chemical potential gradients. The measurements were performed in a hydrogen-rich atmosphere for the temperature range from 973 to 1473K. For almost all the conditions in the experiment, the measured emf’s were well explained by regarding that the substantial predominant charge carrier is the proton. Under the conditions that the transport number of proton is less than unity, the agreement was examined between the measured emf and the theoretical one estimated based on the conduction parameters determined by the conductivity measurement reported in the previous work. It was confirmed that they well coincide with each other in all experimental conditions. This fact shows that the conduction parameters of CaZr0.996Al0.004O3¹i and also the model of defect structure reported in the previous work were reasonable. The proton conduction domain of Al-doped CaZrO3 in the oxygen-hydrogen chemical potentials plane was examined based on these conduction parameters and it was found to be a little wider than that of In-doped CaZrO3 system. [doi:10.2320/matertrans.M2012020]

Chemical stability and electrical property of Ba 1.03Ce 0.6Zr 0.2Yb 0.2O 3– α ceramic

Ba 1.03Ce 0.6Zr 0.2Yb 0.2O 3– α ceramic was prepared by solid state reaction. Phase composition, surface and cross-section morphologies of the material were characterized by using X-ray diffractometer (XRD) and scanning electron microscopy (SEM), respectively. Its chemical stability against carbon dioxide and water steam at high temperature was tested. Ionic conduction of the material was investigated by ac impedance spectroscopy and gas concentration cell methods under different gas atmospheres in the temperature range of 500–900 °C. Using the ceramic as solid electrolyte and porous platinum as electrodes, the hydrogen-air fuel cell was constructed, and the cell performance at the temperature from 500 to 900 °C was examined. The results indicated that Ba 1.03Ce 0.6Zr 0.2Yb 0.2O 3– α was a single-phase perovskite-type orthorhombic system, with high density and good chemical stability under carbon dioxide and water steam atmospheres at high temperature. In wet hydrogen, the material was a pure protonic conductor with the protonic transport number of 1 from 500 to 700 °C, a mixed conductor of proton and electron with the protonic transport numbers of 0.945–0.916 from 800 to 900 °C. In wet air, the material was a mixed conductor of proton, oxide ion and electronic hole. The protonic transport numbers were 0.013–0.003, and the oxide ionic transport numbers were 0.346–0.265. Under hydrogen-air fuel cell conditions, the material was a mixed conductor of proton, oxide ion and electron, the ionic transport numbers were 0.945–0.848. The fuel cell could work stably, and at 900 °C, the maximum power output density was 36.5 mW/cm 2.

Chemical Stability, Ionic Conductivity of BaCe0.9−xZrxSm0.10O3−αand Its Application to Ammonia Synthesis at Atmospheric Pressure

Dense ceramic samples BaCe0.9−xZrxSm0.10O3−α (x=0.10, 0.15, 0.20, 0.30) were obtained by heat-treating the precursors prepared from a coprecipitation route. The phase structure, chemical stability and conduction behaviors of the ceramic samples have been investigated by X-ray powder diffraction and alternating current impedance spectroscopy methods. All the ceramic samples displayed a single phase of orthorhombic perovskite. The samples with x≧0.20 were relatively stable after exposed to the flowing mixed gases: CO2 +H2O+N2 at 873 K for 12 h. Among the samples tested, the sample with x=0.20 exhibited both adequate conductivity and better chemical stability. The contribution of different charged species for x=0.20 sample to the conduction in wet hydrogen atmosphere was investigated by means of gas concentration cells. It was found that the sample of x=0.20 was almost a pure ionic conductor, and the ionic conduction was contributed mainly by proton and partially by oxide ion in wet hydrogen atmosphere at 773–1073 K. The ammonia synthesis at atmospheric pressure in an electrolytic cell based on the sample of x=0.20 was successfully conducted and the peak ammonia formation rate achieved 2.67×10−9 mol·s−1·cm−2 with direct current of 0.80 mA at 773 K.

Ionic Conduction and Application of Ba1.03Ce0.8Tm0.2O3−α Ceramic

Ba1.03Ce0.8Tm0.2O3−α ceramic with orthorhombic perovskite structure was prepared by conventional solid-state reaction. The conductivity and ionic transport number of Ba1.03Ce0.8Tm0.2O3−α were measured by ac impedance spectroscopy and gas concentration cell methods in the temperature range of 500–900°C in wet hydrogen and wet air. Using the ceramic as solid electrolyte and porous platinum as electrodes, the hydrogen-air fuel cell was constructed, and the cell performance was examined at 500–900°C. The results indicate that the specimen is a pure ionic conductor with the ionic transport number of 1 at 500–900°C in wet hydrogen. In wet air, the specimen is a mixed conductor of proton, oxide ion and electron hole. The protonic transport numbers are 0.071–0.018, and the oxide ionic transport numbers are 0.273–0.365. The conductivities of Ba1.03Ce0.8Tm0.2O3−α under wet hydrogen, wet air or fuel cell atmosphere are higher than those of Ba1.03Ce0.8RE0.2O3−α (REY, Eu, Ho) reported previously by us. The fuel cell can work stably. At 900°C, the maximum power output density is 122.7 mW·cm−2, which is higher than that of our previous cell using Ba1.03Ce0.8RE0.2O3−α (REY, Eu, Ho) as electrolyte.

Electrical conduction in (La 0.97Sr 0.03) 2(Mo 1 − x Ga x) 2O 9 − α ceramics

A novel series of ceramic samples (La 0.97Sr 0.03) 2(Mo 1 − x Ga x) 2O 9 − α (x = 0, 0.01, 0.03, 0.05, 0.08) were prepared by a solid-state reaction method. All the doubly doped ceramics have a single cubic structure except (La 0.97Sr 0.03) 2(Mo 0.92Ga 0.08) 2O 9 − α . The conduction behaviors of the samples with x ≤ 0.05 were investigated using various electrochemical methods including gas concentration cells, oxygen electrochemical permeation and AC impedance spectroscopy in the temperature range of 823–1273 K. In dry oxygen-containing atmosphere, the ceramic samples were almost pure oxide-ionic conductors. In wet oxygen-containing atmosphere, the ceramic samples were also almost pure oxide-ionic conductors, but didn't possess the protonic conduction. The electrochemical oxygen permeation experiment further verified the oxide-ionic conduction nature under oxygen-containing atmosphere. The phase transition has been completely suppressed in the doubly doped samples in both dry and wet air atmospheres. The oxide-ionic conductivities were affected by the doping levels, and increased in the order: σ (x = 0.05) < σ (x = 0) < σ (x = 0.03) < σ (x = 0.01). (La 0.97Sr 0.03) 2(Mo 0.99Ga 0.01) 2O 9 − α exhibited the highest oxide-ionic conductivity of 0.103 S cm −1 at 1273 K.

Synthesis and characterization of dense SnP2O7–SnO2 composite ceramics as intermediate-temperature proton conductors

Dense SnP2O7–SnO2 composite ceramics were prepared by reacting a porous SnO2 substrate with an 85% H3PO4 solution at elevated temperatures. At 300 °C and higher, SnO2 reacted with H3PO4 to form an SnP2O7 layer on exterior and interior surfaces in the substrate, the growth rate of which increased with increasing reaction temperature. Finally, at 600 °C, the pores of this composite ceramic were perfectly closed and its electrical conductivity became several orders of magnitude higher than that of the SnO2 substrate alone. Proton conduction was demonstrated in this composite ceramic using electrochemical measurements and various analytical techniques. Comparison of the observed electromotive force with the theoretical value in two gas concentration cells demonstrated that this composite ceramic is a pure ion conductor, wherein the predominant ion species are protons. Fourier transform infrared (FT-IR) and proton magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses revealed that the protons interacted with lattice oxide ions in the SnP2O7 layer to form hydrogen bonds. An H/D isotope effect suggested that proton conduction in this composite ceramic was based on a proton-hopping mechanism. The proton conductivity in this material reached ∼10−2 S cm−1 in the temperature range of 250–600 °C.

Ionic Conduction and Fuel Cell Performance of Ba0.98Ce0.8Tm0.2O3−α Ceramic

The perovskite-type oxide solid solution Ba0.98Ce0.8Tm0.2O3−α was prepared by high temperature solid-state reaction and its single phase character was confirmed by X-ray diffraction. The conduction property of the sample was investigated by alternating current impedance spectroscopy and gas concentration cell methods under different gases atmospheres in the temperature range of 500–900°C. The performance of the hydrogen-air fuel cell using the sample as solid electrolyte was measured. In wet hydrogen, the sample is a pure protonic conductor with the protonic transport number of 1 in the range of 500–600 °C, a mixed conductor of proton and electron with the protonic transport number of 0.945-0.933 above 600 °C. In wet air, the sample is a mixed conductor of proton, oxide ion, and electronic hole. The protonic transport numbers are 0.010–0.021, and the oxide ionic transport numbers are 0.471-0.382. In hydrogen-air fuel cell, the sample is a mixed conductor of proton, oxide ion and electron, the ionic transport numbers are 0.942-0.885. The fuel cell using Ba0.98Ce0.8Tm0.2O3−α as solid electrolyte can work stably. At 900 °C, the maximum power output density is 110.2 mW/cm2, which is higher than that of our previous cell using BaxCe0.8RE0.2O3−α (x≤1, RE=Y, Eu, Ho) as solid electrolyte.

Ionic conduction in Sn 1− xSc xP 2O 7 for intermediate temperature fuel cells

A novel series of mixed ion conductors, Sn 1− x Sc x P 2O 7 ( x = 0.03, 0.06, 0.09, 0.12), were synthesized by a solid-state reaction method. The conduction behaviors of the ion conductors in wet hydrogen atmosphere were investigated by some electrochemical methods including AC impedance spectroscopy, gas concentration cells in the temperature range of 323–523 K. It was found that the doping limit of Sc 3+ in SnP 2O 7 was between 9 mol% and 12 mol%. The highest conductivity was observed to be 2.76 × 10 −2 S cm −1 for the sample of x = 0.06 under wet H 2 atmosphere at 473 K. The ionic conduction was contributed mainly to proton and partially to oxide ion in wet hydrogen atmosphere from 373 K to 523 K. The H 2/air fuel cells using Sn 1− x Sc x P 2O 7 ( x = 0.03, 0.06, 0.09) as electrolytes (1.7 mm in thickness) generated the maximum power densities of 11.16 mW cm −2 for x = 0.03, 25.02 mW cm −2 for x = 0.06 and 14.34 mW cm −2 for x = 0.09 at 423 K, respectively. The results indicated that Sn 1− x Sc x P 2O 7 is a promising solid electrolyte system for intermediate temperature fuel cells.

Chemical Stability and Electrical Property of BaCe0.7Zr0.2Nd0.1O3−α Ceramic

Abstract BaCe0.7Zr0.2Nd0.1O3−α ceramic was prepared by solid state reaction. Phase composition, surface and fracture morphologies of the material were characterized by using XRD and SEM, respectively. Chemical stability against carbon dioxide and water steam at the high temperature was tested. The conductivity and ionic transport number of the material were measured by ac impedance spectroscopy and gas concentration cell methods in the temperature range of 500–900°C in wet hydrogen and wet air, respectively. Using the ceramic as solid electrolyte and porous platinum as electrodes, the hydrogen‐air fuel cell was constructed, and the cell performance at the temperature from 500 to 900°C was examined. The results indicate that BaCe0.7Zr0.2Nd0.1O3−α was a single phase perovskite‐type orthorhombic system, with high density and good chemical stability in carbon dioxide and water steam atmospheres at the high temperature. The conductivity of the material in wet hydrogen and wet air was increased as the temperature rises. In wet hydrogen, the material was a pure protonic conductor with the protonic transport number of 1 from 500 to 600°C, a mixed conductor of proton and electron with the protonic transport number of 0.973–0.955 from 700 to 900°C. In wet air, the material was a mixed conductor of proton, oxide ion and electron hole. The protonic transport numbers were 0.002–0.003, and the oxide ionic transport numbers were 0.124–0.179. The fuel cell could work stably. At 900°C, the maximum short‐circuit current density and power output density were 156 mA·cm−2 and 40 mW·cm−2, respectively.

Defects and oxide ion transport properties in the substituted Zn2TiO4

Electric conductivities of Zn2−x/2Ti1−xTaxO4 have been measured at 900°C in the oxygen partial pressure range of 1 to 10−13 atm, in order to estimate the ionic conduction separately from the electronic one. The obtained ionic conductivities were in good agreement with the products of electric conductivities and tion by oxygen gas concentration cells, which grew exponentially with the cation vacancy concentration in this system. Dielectric relaxation experiments on Zn2−(x−y)/2Ti1−(x+y)TaxAlyO4 system revealed that ionic diffusion process varied gradually from the oxide ion vacancy mechanism to the interstitialcy diffusion with increasing extrinsic cation vacancy concentration.