Synthesis And Electrical Properties Of La₂(Mo_1-xAl_x)₂O_9-α As New Type Of Oxide-ion Conductors

Oxygen-ion conductors keep on attracting considerable interest because of their important filds of application,including solid oxygen fuel cell (SOFC),oxygen sensor,oxygen pump, etc. La₂Mo₂O₉ was reported as a new oxide-ion conductor by Lacorre in 2000. It has quite a few oxygen vacancies in crystal lattice without doping low-valence metal cation, and the oxide-ion conductivity of La₂Mo₂O₉ is higher than YSZ in middle temperature range of 600 800℃.A series of La₂(Mo1-xAlx)2O_9-α (x = 0.025, 0.005, 0.075, 0.10) samples were prepared by high-temperature solid-state reaction and sol-gel method. Using the samples as solid electrolytes and porous platinum as electrodes, the ion conduction properties and oxygen ionic transference number of these samples were investigated by means of ac impedance spectroscopy, oxygen concentration cell and electrochemical oxygen permeation (oxygen pump) at 550 1000℃in various atmospheres(dry oxygen, nitrogen, air and wet air). In addition, the oxygen-hydrogen fuel cell has been studied using the sample, La₂Mo_1.9Al_0.1O_9-α, as the solid electrolyte and porous platinum as electrodes. We attained some important results as follows:(1) The samples are of single cubic phase of La₂Mo₂O₉.(2) The phase transition in La₂Mo₂O₉ is completely suppressed with x≥5 % Al³⁺ at Mo⁶⁺ sites and keep theβ-phase at the room temperature.(3) The observed EMF values of oxygen concentration cell coincided with theoretical values perfectly, indicating that the ceramics were purely oxide-ionic conductors with oxygen ion transference number of 1 under oxygen atmosphere over the whole temperature range.(4) The conduction of the samples is different under the different oxygen partial pressure range.At high oxygen partial pressure range (pO₂ = 10^-13～1atm) at 800℃and 900℃, the conduction isn't ralated with the oxygen partial pressure indicating that they were almost pure ionic conductors, which was consistent with the result by means of oxygen concentration cell. Whereas at low oxygen partial pressure range (pO₂ = 10-14～10-20 atm) at 800℃and 900℃, the samples exhibited mixed ionic and electronic conduction.(5) The electrochemical oxygen permeation rates determined were in accordance with the theoretical rates calculated from Faraday's law when the current density is lower than 30 mA·cm^-2 with oxygen ion transference number of 1, which was consistent with the result by means of oxygen concentration cell and influence pO2 to conductivities, verifying directly experimentally that the oxide-ionic nature of the conduction under oxygen atmosphere.(6) The ionic conductivities of these ceramics are higher than 10^-2 S·cm^-1, which are obviously higher than that of La₂Mo₂O₉, over the temperature range of 600～1000℃because the oxygen vacancy of doped-La₂Mo₂O₉ is higher than that of parent compound;The conductivity of La₂Mo_1.9Al_0.1O_9-α with bigger free volume is higher than that of La₂Mo_1.9Ga0.1O_9-α .(7) The level of dopant distinctly affects the conductivities of sample. The conductivities of samples increase and then reduce with the increasing the level of dopant, La₂Mo_1.9Al0.1O_9-α is shown to have the highest oxide ionic conductivity of 0.12 S·cm^-1 at 1000℃.(8) The oxygen-hydrogen fuel cell has been studied using the sample, La₂Mo_1.9Al_0.1O_9-α, as the solid electrolyte, with the highest power density 145.4 mW·cm^-2, which indicate that the sample may be used as electrolyte material of fuel cell.