Molten-salt Synthesis And Characterization Of Apatite La_9.33Si₆O₂₆ Electrolyte

Solid Oxide Fuel Cells (SOFCs) draw tremendous attention recently,because they offer a clean, environment-friend and energy-saving technology withhigh efficiencies. As compared to traditional energy-conversion systems, SOFCs holdmore advantages including fuel diversity, low noise, low pollution, high reliability,modularity and fuel adaptability etc. Howerever, the very high operating temperatureof SOFCs results in longer start-up times and mechanical and chemical compatibilityissues that severelly hinders industrilization. In order to conque these drawbacks,much attention and many effort have been focused on the intermediate temperaturesolid oxide fuel cells (IT-SOFCs). Novel apatite-type lanthanum silicates havedemonstrated promising applicability as electrolytes in IT-SOFCs. Sinteringproperties, crystal phase transformation, and electrical conductivity of the La9.33Si6O26systems were investigated. The unique interstitial oxygen conductive mechanism ofapatite-type La9.33Si6O26makes its conductivity significantly improved in addition tothe above advantages. Under such circumstance, in this paper, the apatite-typeLa9.33Si6O26electrolyte has been prepared as well as the conrresponding propertieshave been systematically investigated. Traditionally, the La9.33Si6O26powder issynthesized via solid state method, however, the solid sintering temperature is high,mostly above the1300°C, which results in poor homogeneity of the resultant productand consequently highly costs. Thus, in the present work, molten salt method is usedto prepare La9.33Si6O26powder to reduce the synthesis temperature, cost price,meanwhile guarantee high conductivity. NaCl and LiCl with mass ratio of1:1,2:1and1:2as mixtures of molten salt have been selected to synthesize La9.33Si6O26. Theproperites concerning the structure, mthriuorphology and electrical features have beenthoroughly studied. It is found that when NaCl or LiCl solely as molten salt,900°Cand550°C are the lowest temperatures to synthsize La9.33Si6O26, which are400°Cand750°C lower than traditional solid state. When NaCl/LiCl (mass ratio of1:1,2:1,1:2) mixtures are used as molten salts, La9.33Si6O26has been successfully prepared at600°C,600°C and550°C respectively, which is700°C,700°C and750°C lowerthan traditional solid state. By characterizations, the pure La9.33Si6O26powder can beobtained through molten salt method with good crystallinity, uniform particles and nosignificant agglomeration. The La9.33Si6O26powders have been ground and tableted aspieces, followed by molding-sintering at1300°C for4h and1300°C for8h respectively. It is suggested that prolonging the sintering time will improve thesample density as observed by the scanning electron microscopy (SEM)measurements. AC impedance measurements have been carried out for the densedLa9.33Si6O26samples, it is found that the electrical conductivity is3.13×10-1S/cm at800°C, higher than YSZ electrolyte material, thus demonstrating excellentconductivity.