Junichiro Mizusaki

Nonstoichiometry, diffusion, and electrical properties of perovskite-type oxide electrode materials

Based on the study of the present author and coinvestigators, the relationships between the nonstoichiometry, electrical properties and diffusion of the perovskite-type oxides, ABO3 (B = Al, Zr, Bi, Cr, Mn, Fe, Co), were reviewed to elucidate the characteristics of perovskite-type electrode materials, La1-xSrxBO3 (B = Mn, Fe, Co). For the oxides with the fixed valence B-site ions, A13+ and Zr1+, the change in nonstoichiometry, δ, with PO2 is small and oxide-ion conduction is predominant with small p-type electronic conduction. In a solid solution of the (SrZrO3)y(La0.75Sr0.25FeO3)1−y system, a continuous conductivity change was observed from semiconductor type to small polaron hopping-type with decrease in y. La1−xSrxBO3−δ (B = Cr and Fe) shows hopping-type conduction: δ changes with PO2 in the range between 0 and a value around x2. With an increase in δ, [B·B] decreases and [B′B] increases. The conductivity, σ, is proportional to [B·B] and [B′B]. σ shows a minimum at the electonic stoichiometry of [B·B]=[B·B]. La1−xSrxCoO4, BaBiO3, and La1−xSrxMnO3 (x 0.2) and La1−xSrxCoO3−δ. From the measurement of ionic conductivity, chemical diffusion, and isotope diffusion, the oxide vacancy diffusion coefficient, DV, was obtained. In spite of the large difference in the electronic properties, the perovskite oxides show similar DV values.

Kinetic studies of the reaction at the nickel pattern electrode on YSZ in H2H2O atmospheres

In order to elucidate the reaction mechanism at the anode of solid oxide fuel cells (SOFC) in H2H2O atmospheres, instead of conventional nickel-zirconia cermet anodes, we employed nickel stripe patten electrodes prepared on the surface of 8m/o Y2O3- doped ZrO2 (YSZ), which have well-defined length and morphology of the gas/nickel/YSZ triplet phase boundary (TPB). On the surface of single crystalline YSZ-plates, compact nickel layer was prepared by ICB (ionized cluster beam) method. By photolithography, the stripe patterns of alternative nickel and YSZ lines were prepared. Electrochemical measurements were made on the electrode impedence and steady-state polarization current using a terminal method in the H2H2OAr gas mixtures with hydrogen partial pressure, PH2 = 102−104 Pa and water vapor pressure, pH20=3×10su2−2×103 Pa at 700°C. It was shown that the rate of electrode reaction at 700°C can be approximately expressed by I=kPH2aO−k′PH2O12aO−12 (k, k′: rate constant), where aO is the oxygen activity at the TPB which is related to the electrode potential E versus 1.013×105 Pa O2 (g) by RTO=2EF. The rate of anodic reaction was found to be essentially determined by the reaction of H2(g) and the adsorbed oxygen on the nickel surface, while the rate of cathodic reaction seems to be determined by the first order reaction or surface diffusion process of adsorbed hydrogen, Had, which may take place after the reaction of H2O(g)→H2Oad→Had+OHad.

Nonstoichiometry and thermochemical stability of the perovskite-type La1−xSrxMnO3−δ

In order to make clear the extent of nonstoichiometry and decomposition P(O2) of the perovskite-type La1-xSrxM nO3−δ, measurements are made by high temperature gravimetry, coulometric titration and iodometry on the oxides with x=0.2 and 0.4. The decomposition P(O2) was found to increase with increasing x. The decomposition products for x=0.4 were (La0.6Sr0.4 )2MnO4 and MnO, and mixtures of (La1−xSrx)2MnO4, MnO, and La2O3 for x=0.2. For x=0.4, the plot of 3−δ versus log P(O2) showed plateaus at °=0 and 0.0225, while no plateau was observed corresponding to the Mn3+ state at °=x2(=0.2). By calculation for oxygen partial molar enthalpy and entropy, it was found that oxygen vacancies distribute randomly on the oxygen sublattice and electronic state is essentially metallic for La0.6Sr0.4MnO3−° with °>0.0225. In the region of °< 0.0225, oxygen vacancies seemed to distribute on limited sites. However, details of the defect equilibrium for this region was not clarified.

Kinetic studies on the reaction at the La0.6Ca0.4MnO3/YSZ interface, as an SOFC air electrode

To determine the rate equation for the reaction at the SOFC (Solid Oxide Fuel Cell) air electrode of the type O2(g)/porous La0.6Ca0.4MnO3/YSZ, measurements were made on the impedance due to the electrode reaction and the steady-state polarization in the oxygen partial pressures, PO2, between 1 and 10−4 atm at temperatures, T, between 700 and 1000°C. At T > 900°C and in PO2<10−3 atm, the electrode conductivity (reciprocal of the electrode impedance) was proportional to PO2. The rate of reaction for this condition was found to be controlled by the oxygen gas diffusion in the electrode pores. At higher PO2 or at the lower temperatures, the steady-state current, i, was found to obey the rate equation, i=k[aO−PO2a−1O], where k is the rate constant and aO is the oxygen activity in YSZ at the La0.6Ca0.4MnO/YSZ interface. Here, aO is related to the electrode potential, E , by 2FE = RT 1n aO. The electrode conductivity for this condition was proportional to P12O2.

High temperature electrical properties of the perovskite-type oxide La1 − xSrxMnO3 − d

Abstract In order to elucidate the conduction mechanism of the perovskite-type oxide La1 − xSrxMnO3 − d (0

High temperature gravimetric study on nonstoichiometry and oxygen adsorption of SnO2

= x

Kinetics of the electrode reaction at the CO-CO2, porous Pt/stabilized zirconia interface

= 0.4) , the electrical conductivity, σ, and Seebeck coefficient, Q, were measured as a function of temperature, T, up to 1100 °C in l atm O2 gas and as a function of the oxygen partial pressure, P(O2), at 800–1100 dgC. At T ⪢ ~200 °C, σt increased with T, indicating that thermally activated type conduction may predominate irrespective of x. The Q values were generally positive, indicating that p-type conduction was predominant. The quantity of Q was relatively small in comparison with other perovskite-type oxides, implying that these materials are metallic, irrespective of whether the conduction is thermally activated or itinerant. The relationship between σ and Q for x

Lithium carbonate as a solid electrolyte

= 0.2 can be interpreted in terms of a multi-level hopping conduction model by calculating the distribution of electrons in high-spin and low-spin levels of Mn3+ and Mn4+ using statistical dynamics. For x = 0.3 and 0.4, a calculation based on this model revealed that the energy difference between high-spin and low-spin states was smaller than kB T, and the calculated hopping mobility gave either a negative activation energy or a negative pre-exponential factor, suggesting that the hopping model is not applicable and that the electrons are itinerant. The electronic nature of the oxides therefore changes from localized to itinerant between x =0.2 and x = 0.3.

Oxygen nonstoichiometry and phase instability of Bi2Sr2CaCu2O8+δ

Abstract Gravimetric studies on porous SnO2 between 200 and 1150°C and for oxygen partial pressures, P(O2), between 1 and 10−6 atm showed that the mass change of below 900°C is essentially due to oxygen adsorption on the SnO2 surface. The oxygen adsorption was interpreted in terms of three Langmuir-type equations with different heats of adsorption. Mass changes due to nonstoichiometry were observed above 1000°C. The degree of nonstoichiometry of SnO2 was found to be very small: For example, d in SnO2−d at 0.1 atm of P(O2) was 7 = 10−5 at 1000°C and 2 × 10−4 at 1150°C. The nonstoichiometry arises from oxygen deficiencies present as oxygen vacancies, VO··; d in SnO2−d above 1000°C is proportional to P( O 2 ) − 1 6 .

Oxygen chemical potential profile in a solid oxide fuel cell and simulation of electrochemical performance

To elucidate the reaction kinetics of CO gas at the electrodes of zirconia sensors and solid oxide fuel cells, measurements were made on the electrochemical impedance spectra and steady-state polarization current at the porous Pt/stabilized zirconia (SZ) electrodes in the COCO2 atmosphere at 600–1000°C. The impedance arcs were depressed semicircles, the centers of which were 45° below the abscissa. The eletrode impedance was expressed by a parallel circuit of resistance, RE, and Warburg impedance, ZE=A(1−i)ω−12. Here, A is related to the diffusion constant, D, of the diffusion process in the electrode reaction by A=kD−12, where k is constant. The PO2 and temperature dependences of D agreed with those of the electronic conductivity of SZ. It was concluded that ZE is determined by the oxygen chemical diffusion in SZ in the relaxation at the closely contacted interface of the Pt particles/SZ. From the steady-state current-potential relationships, the rate equation in the CO2 rich atmosphere was determined as i=kOP12COa120−k0P12CO2. It was shown that the rate determining reaction is COad(Pt)+Oad(SZ)→CO2ad(Pt) at the triple phase boundary. Possible mechanism in the CO rich atmosphere was also discussed.