Synthesis, Conduction And Application Of ABO₃ (A=La, Ba; B=Ga, Ce) Ceramics

Solid state electrolytes in which charge carriers are proton are important functional materials, and they have valuable and promising applications in solid oxide fuel cell, hydrogen sensor, preparation, separation and purification of hydrogen, hydrogenation and dehydrogenation of some organic compounds, recycling of hydrogen isotopes from exhaust gas of nuclear reactor and ammonia synthesis at atmospheric pressure. So the investigation on preparation, proton conduction and application of proton conducting ceramics has important significancy.It is well known that SrO- and MgO- doped LaGaO₃ ceramics possess excellent oxide ion conduction, and the ceramics are pure oxide ionic conductors over a wide range of oxygen partial pressures from 1 to 10-20 atm, and their oxide ionic conductivity at 800℃is comparable to that of YSZ at 1000℃. Therefore, they are regarded as the promising candidate electrolytes for intermediate temperature fuel cells. But their proton conduction is neglected by researchers in a long time. Recently our group discovered that La0.9Sr0.1Ga0.8Mg0.2O₃–αpossesses excellent proton conduction in hydrogen atmosphere at 600-1000℃. Moreover, Zn2+ doped LaGaO₃ ceramics, La(Sr)Ga(Zn)O₃, in which Zn2+ has almost same ionic radius as Mg2+ but different electron configuration from Mg2+, also exhibit good proton conductionBaO- and MgO-doped LaGaO₃ is another important solid electrolyte system in LaGaO₃-based family, whether they have pronton conduction is still not clear up to now. Moreover, the proton conduction in BaCeO₃-based oxides at high temperatures of 600-1000℃has been widely reported, but there are only few reports on the proton conduction in the materials at intermediate temperatures of 300-600℃. Especially, the intermediate temperature proton conduction in Gd doped BaCeO₃ ceramics hasn't been still reported until now. For this reason, in this paper, ionic conduction, especially proton conduction in BaO- and MgO-doped LaGaO₃ was investigated. Moreover, proton conduction in Gd doped BaCeO₃ also was investigated at intermediate temperature of 300-600℃.Main works and results in this paper are as follows:(1) A series of La0.90Ba0.10Ga1–xMgxO₃–α(x = 0.20, 0.25, 0.30) ceramics were prepared by a conventional solid-state reaction. Their conduction was studied in wet hydrogen, wet air and dry air atmospheres by various electrochemical methods including AC impedance spectroscopy, isotope effect, electrochemical hydrogen pumping, steam concentration cell, hydrogen concentration cell and oxygen concentration cell at 600-1000℃. It was found that the samples were almost pure protonic conductors in wet hydrogen, mixed (proton + oxide ion + hole) conductors in wet air atmosphere, and mixed (oxide ion + hole) conductors in dry air atmosphere. These results were different from the reports that BaO- and MgO-doped LaGaO₃ ceramics were mixed conductors of oxide ion and electron hole in O2 and air; whereas they were oxide ion conductors in N2 and H2 atmospheres.(2) La0.9Ba0.1Ga1-xMgxO₃–α(0≤x≤0.25) was prepared usually by a conventional solid-state reaction at high sintering temperature up to 1500℃, and some impure phases existed in the samples of x<0.2. In order to decrease sintering temperature and impure phase, a wet chemical method, microemulsion method, was employed to prepare sample powders. A single phase of LaGaO₃ perovskite structure was obtained when sample powders with x≥0.15 were sintered at 1410-1430℃for 10 h. Electrochemical hydrogen pumping proved that La0.9Ba0.1Ga1-xMgxO₃–αh?ad proton conduction, and protonic conductivity was measured from 400-800℃in hydrogen atmospheres. Among these samples, the sample of x=0.2 has the highest protonic conductivity with the values of 0.01 S cm-1 at 600℃. Ammonia was synthesized from nitrogen and hydrogen at atmospheric pressure in an electrolytic cell using La_0.9Ba_0.1Ga_0.8Mg_0.2O_{3–α} as electrolyte. The rate of NH3 formation was 1.89×10-9 mol s-1cm-2 at 520℃upon imposing a current of 1 mA through the cell.(3) In order to compare the effects of different alkaline-earth cations doped for La-site on proton conduction in LaGaO₃, the precursors of La_0.9M_0.1Ga_0.8Mg_0.2O_{3–α} (M = Ca²⁺, Sr2+, Ba2+) were prepared by the hydrothermal precipitation method in which the precipitant was from hydrolysis of urea (CO(NH2)2) in water solution under hydrothermal conditions. The ceramic samples were obtained at lower sintering temperature. XRD showed the ceramic samples are of a single orthorhombic phase of LaGaO₃ perovskite structure. The proton conduction in La_0.9M_0.1Ga_0.8Mg_0.2O_{3–α} w?as confirmed by an isotope effect and a hydrogen pumping experiment. Protonic conductivity was measured from 300-600℃in a hydrogen atmosphere, and depended on the alkaline earth cations doped for the La site and increased in the following order:σ(M = Sr2+) >σ(M = Ba2+) >σ(M = Ca²⁺). Ammonia synthesis at atmospheric pressure was carried out using La_0.9M_0.1Ga_0.8Mg_0.2O_{3–α} as electrolyte. The order of the rate of NH3 formation is same as that of protonic conductivity.(4) Gd doped BaCeO₃ are conventional ceramics, and their proton conduction was mainly investigated at high temperatures of 600-1000℃. However there are hardly any studies on proton conduction in Gd-doped BaCeO₃ at intermediate temperatures of 300-600℃. Moreover, BaCeO₃ based ceramics were prepared generally through a solid state reaction at high sintering temperature up to 1650℃In this paper, the ceramic sample powders of BaCe1?xGdxO₃–α?(0.05≤x≤0.20) were prepared by a microemulsion method. A single phase of BaCeO₃ orthorhombic perovskite structure was obtained by sintering the powders at the lower temperature of 1500℃. Proton conduction in the samples was investigated from 300-600℃in hydrogen atmosphere. It was verified that the ceramic samples were almost pure protonic conductors at 300-600℃in hydrogen atmosphere. The highest protonic conductivity was observed for x = 0.15 at 600℃with the value of 0.014 S cm-1. Ammonia was synthesized successfully in an electrolytic cell based on BaCe0.85Gd0.15O₃–αfilm (about 90μm in thickness). It was found that the rate of NH3 formation was 4.63×10?9 mol s?1 cm?2 at 480℃and 1.5 mA.(5) Synthesis of ammonia was carried out successfully in an electrolytic cell based on Ni-BaCe0.9Gd0.1O₃–αas anode, Sm0.5Sr0.5CoO₃–αas cathode and PIL proton conducting membrane as electrolyte. The rate of NH3 formation was 3.2×10?9 mol s?1 cm?2 at 100℃and 1.5 mA...