Conduction Behaviors And Application Of ABO₃ （A = La, Ba; B = Ga, Ce） Perovskite-type Solid Electrolytes At Intermediate Temperature

Solid electrolyte materials are a kind of important functional materials and have attracted much attention because of their potential application values and wide application prospects in solid oxide fuel cells （SOFCs）, gas sensors, steam electrolyzes, separation and purification of hydrogen, hydrogenation and dehydrogenation of some organic compounds and ammonia synthesis at atmospheric pressure, etc.The traditional electrochemical devices using Y₂O₃ stabilized ZrO₂ （YSZ） as electrolytes, due to its good chemical stability, high-mechanical stability and pure ionic conduction, require operating temperatures around 1000℃. However, relatively high operating temperature can result in not only high fabrication cost for electrochemical device systems, but also complex material problems including electrode sintering, interfacial diffusion between electrolyte and electrodes, and mechanical stress due to the different thermal expansion coefficients. Therefore, more recently, ABO₃ perovskite- type intermediate-temperature solid electrolyte materials such as doped SrCeO₃、BaCeO₃、BaZrO₃, LaGaO₃, etc. have attracted much attention because of their well conduction behaviors. Compared to low- or high-temperature electrochemical devices, intermediate-temperature electrochemical devices have many advantages: （1） faster speed of electrode reactions; （2） higher CO and other impurities’tolerance performance; （3） more easily electivity for sealing materials and connection materials; （4） simple structure of the electrochemical device; （5） more easily management in water, thermal cycle. Therefore, studying the details of intermediate-temperature conduction behaviors of ABO₃ perovskite- type electrolytes themselves and their application may be of significant for the commercial development of electrochemical devices.Therefore, in the thesis we focus on conduction behaviors and some applications of ABO₃ （A = La, Ba; B = Ga, Ce） perovskite-type solid electrolytes at intermediate temperature. Main works and results in this paper are as follows:（1） In chapter 1, we review the definition of solid electrolyte materials and their application prospects, and introduce briefly the typical ABO₃ perovskite- type electrolyte materials. In addition, we also propose our main research work.（2） It is generally believed that doped BaCeO₃-based ceramic materials show the highest proton conductivities. However, to the best of our knowledge, related studies on proton conduction in Dy-doped BaCeO₃ at the intermediate temperature range of 300 - 600℃have not been reported until now. In chapter 3, ionic conduction at the intermediate temperature range, especially ionic conduction under reducing atmosphere in Dy₂O₃ doped BaCeO₃ ceramics was investigated. The precursor powders of BaCe_1-xDy_xO_3-α（x = 0.05, 0.10, 0.15, 0.20） ceramics were prepared via a microemulsion process. It was shown from SEM images that all the ceramic samples were quite dense. Structure investigation on the powder X-ray diffraction patterns showed that the ceramic samples have single perovskite structures. Rietveld analysis suggested that there was a possible orthorhombic distortion of structure with increasing Dy concentration. The proton conduction in these ceramics at intermediate temperature range of 300 - 600℃was investigated. It was found from the isotope effect on conductivity that the samples had obvious proton conduction in hydrogen-containing atmosphere. The highest ionic conductivity 0.93×10^-2 S·cm^-1 was observed for x = 0.15 in wet hydrogen atmosphere （pH₂O = 0.023 atm） at 600℃. Ammonia was synthesized successfully in an electrolytic cell using BaCe_0.85Dy_0.15O_3-α ceramic as an electrolyte at atmospheric pressure and the maximum rate of NH₃ formation was 3.5×10^-9 mol/s·cm² at 530℃with an imposed direct current of 1.2 mA, which confirmed that Dy - doped BaCeO₃ ceramic electrolyte has excellent proton conduction at intermediate temperature.（3） In 1998, ammonia synthesis at atmospheric pressure has been realized successfully in an electrolytic cell reactor using a high temperature proton conductor SrCe_0.95Yb_0.05O_3-α. The process provides an alternative route that permits operation at atmospheric pressure and avoids the problem of thermodynamic restrictions imposed on the conventional Haber catalytic reactors. But how to further improve the ammonia formation rate and hydrogen conversion rate are two important research topics. Therefore, from a practical application point of view, developing thin solid electrolyte membrane for various electrochemical devices is a trend. A electrolytic cell using electrolyte membrane （fewμm to tens ofμm） for ammonia formation will be help to reduce the electrolyte resistance, improve ammonia formation and hydrogen conversion rates. The chapter 4 proposes a new research direction of ammonia synthesis reactor at atmospheric pressure with a thin solid electrolyte BaCe_0.85Y_0.15O_3-α membrane. A tri-layer membrane reactor for ammonia synthesis at atmospheric pressure comprised with a thin proton conduction electrolyte （BCY15） membrane and two functional electrode layers （Ni-BCY15 and BSCF） was assembled successfully. A dense, uniform and crack-free thin proton conduction electrolyte BCY15 membrane prepared by a modified spin coating was obtained after sintering at 1400℃for 5h. AC impedance spectra showed that the membrane reactor had a relatively small resistance. The polarization resistance and the electrolyte resistance increased with the decrease of temperature. The present study demonstrated that higher rate of ammonia rate can be achieved in a reactor with thin solid electrolyte BCY15 membrane. Compared with our prior result, the ammonia formation rate was improved obviously and reached 4.1×10^-9 mol s^-1 cm^-2 with an applied current 1mA at 530℃. The current efficiency of the thin membrane reactor had also been improved obviously.（4） The BaCeO₃-base perovskite materials exhibit fairly high ionic conductivity at intermediate temperature in humid reducing atmosphere. However, these oxides display rather poor chemical stability in atmosphere containing CO₂ and H₂O. On the orther hand, BaZrO₃-base perovskite materials have rather good chemical stability in atmosphere containing CO₂ and H₂O. Therefore preparing the solid solution of BaCeO₃ and BaZrO₃ combined the high ionic conduction of barium cerate with the good chemical stability of barium zirconate has been a research foucs. In order to develop a cost-effective and simple process to fabricate intermediate temperature solid oxide fuel cell, a simple and cost-effective dense pin-coating method was developed to prepare BaCe_0.75Zr_0.1Y_0.15O_3-α electrolyte membrane （25μm in thickness） on porous anode support. A tri-layer NiO- BaCe_0.75Zr_0.1Y_0.15O_3-α / BaCe_0.75Zr_0.1Y_0.15O_3-α /BSCF single cell was assembled and tested from 650-800℃with humidified H₂ as the fuel and ambient air as the oxidant. The open-circuit voltage （OCV ）values reached 0.94V at 800℃, and 0. 99V at 650℃,respectively. The maximum power densities were 349 mWcm^-2 at 800℃and 206 mWcm^-2 at 700℃, respectively. Microstructure analysis showed that the anode support without pore former had low porosity, which was responsible for the low fuel cell performance. Further optimizing process, especially the technical of preparing anode supports, can further improve the performance of intermediate-temperature SOFC.（5） Strontiumm- and mmagnesium-doped lanthanum gallate （LSGM） perovskite-type material is often regarded as a promising family of alternative electrolyte for IT-SOFCs due to their superior oxygen ionic （almost one order of magnitude larger than that of YSZ） and protonic conduction, good chemical stability and especially, high ionic transference number in the intermediate temperature range. Recently our research group discovered that La_0.9Sr_0.1Ga_0.8Mg_0.2O₃–α? ? possesses excellent proton conduction in hydrogen atmosphere at 600-1000℃. Recently, considerable research work has been devoted to developing thin LSGM membrane fuel cell with high output power density capable of operating in the intermediate temperature range. However, to date there is little information available on LSGM membrane prepared via spin-coating process. In chapter 6, a simple and cost-effective spin coating process was applied to deposit LSGM membrane on porous NiO-GDC anode support with a GDC barrier layer. After co-sintering at relatively low temperature 1300℃for 5 h, a dense and uniform electrolyte membrane with the thickness of ca. 50μm was obtained. An anode-support LSGM membrane fuel cell with BSCF cathode was assembled and tested from 600℃to 700℃using humidified hydrogen as the fuel and ambient air as the oxidant. The OCV values reached 1.00V at 700℃, and 1.02 V at 600℃,respectively. The maximum power densities were 760 mW cm^-2 at 700℃and 257 mW cm^-2 at 600℃, respectively. In addition, the AC impedance analysis of the single cell demonstrated desirable low electrode polarizations. In conclusion, the spin coating process has been confirmed to be an effective method to prepare thin LSGM electrolyte membrane for IT-SOFCs.（6） In chapter 7, we summarized the work and highlights in the dissertation. We also briefly analyzed the lack of work and further work in the future.