Synthesis, Structure, Ionic Conduction And Fuel Cell Performance Of Ba_xCe_0.8RE_0.2O_3-α Ceramics

High temperature protonic conductors have a vast range of application perspective aselectrolytes in fuel cells, gas sensors, membrane separators and membrane reactors, etc..Protonic conduction in ABO₃ perovskites was first discovered by Iwahara et al., and hasaroused great attention. Among the ABO3 perovskites studied, doped BaCeO₃,BaCe_1-yM_yO_3-α (M is some rare earth element; y is less than its upper limit of solid solutionformation range, usually less than 0.20; a is the oxygen deficiency per unit formula whichdepends on the concentration of dopant M and surrounding atmosphere), exhibits excellentprotonic conduction under hydrogen-containing atmosphere at high temperature. Inaddition, it also exhibits considerable oxide ionic conduction under certain condition, thusattracts increasing interest world-wide. Until now, many studies have focused onconducting mechanism, structure and electrical property of the high temperature protonicconductors with stoichiometric composition BaCe_1-yM_yO_3-α. Few reports are available onthe research of nonstoichiometric Ba_xCe_1-yM_yO_3-α(x < 1, x > 1). Author synthesized boththe nonstoichiometric and stoichiometric Ba_xCe_0.8M_0.2O_3-α (M = Y, Sm, etc.; x = 1.03, 1,0.98) and studied their defect structure, ionic conduction as well as their chemical stability,discovering that a proper change in the content of Ba²⁺ can change their ionic conductionproperties while keeping pervoskite-type structure. For example, a proper increase in thecontent of Ba²⁺ (x > 1) can improve protonic conduction and fuel cell performance of thematerials; a proper decrease in the content of Ba²⁺ (x < 1) can better the electrical propertyand chemical stability of the materials. Thus, it is of a great importance to synthesizeBa_xCe_1-yM_yO_3-α (x < 1, x > 1) and study the relationship between the nonstoichiometry andelectrical property.In this thesis, BaxCeo.8REo.203-a (RE = Eu, Er, Ho, Tb; x > 1, x = 1, x < 1) solid electrolytes were selected as the studying objects and synthesized by high temperature solid-stated reaction. Among these materials, nonstoichiometric materials (x < 1, x > 1) and parts of the stoichiometric materials (x = 1) were synthesized for the first time. For comparison, BaCeo.8Hoo.203-a was also synthesized by sol-gel method. The research works can be summarized as the following: (1) The crystal phase and lattice parameters of the sinters were determined by XRD method; (2) Protonic and oxide ionic transport numbers of the materials were measured in the temperature range of 500-1000 °C by means of gas concentration cells, steam concentration cells and electrochemical hydrogen permeation (hydrogen pump); (3) The electrical conductivities of the materials under the different atmospheres and temperatures were measured by ac impedance spectroscopy; (4) The performance of the hydrogen-air fuel cells using the materials as solid electrolytes and Pt or Pt-Rh alloy as electrode materials were measured in the temperature range of 500-1000 °C; (5) The effects of nonstoichiometric composition, RE3+ dopant, temperature and experimental atmosphere on electrical conduction behavior of the materials and the performance of the hydrogen-air fuel cells were investigated, which provides some useful information for obtaining novel pervoskite-type protonic conductors with high performance and stability and enlarging g the research range of protonic conductor.In summery, the following conclusions were made: (1) All of the ceramic materials show pervoskite-type BaCeO3 orthorhombic structure. (2) Under high temperature and hydrogen-containing atmosphere, they exhibit good protonic conduction except BaxCeo.8Tbo.203.a (x = 1, 0.98). (3) Under high temperature and oxygen-containing atmosphere, the four series all show mixed oxide ionic and electronic hole conduction, and electronic hole conduction is predominant. (4) The hydrogen-air fuel cells using BaxCeo.gREo 2O3.a (RE = Eu, Er, Ho) as solid electrolytes can work stably, but their discharging performances are difference. At 1000°C, the sequence of the maximum short-circuit current density and the maximum power output density is BaxCeBaxCeo.8Ero.203-a > BaxCeo.8Euo.203-a. BaxCeo.8Tbo.203.a materials don't fit for the solid electrolytes of the fuel cells, because a steady and stable current can't be drawn from the cells. (5) In the same series, the material with x = 1.03 shows the highest protonic conduction and the fuel cell performance; the electrical conductivities of the nonstoichiometric materials are higher those of the stoichiometric one. (6) The protonic and oxide ionic transport numbers of BaCeosHoo^C^.,, synthesized by sol-gel method are close to those of BaCeo.8Hoo.203.a synthesized by traditional high temperature solid-stated reaction method, but the fuel cell performance of the latter is high than that of the former.