Study On Lithium Oxide As Sintering Aid For Doped Ceria-based Material

Solid Oxide Fuel Cells （SOFCS） have attracted researchers’ attention for itshigh efficience, environment-friendly and fuel-diversity. However, several problemshave emerged due to the high operating temperature of SOFCs, such as chemicaldiffusion between electrolyte and electrodes, slowly deteriorating of other cellcomponents as well as the corrosion of metal connector. Therefore, the reduction ofoperating temperature is a critical issue in SOFCs, and the majority of currentresearch is concentrated on the development of intermediate temperature solid oxidefuel cells （IT-SOFCs） that can be operated in the range of500–700℃.Because of the high ionic conductivity, low active energy and stable fluoritestructure of samarium doped ceria（SDC）, SDC is becoming a new promisingelectrolyte material. However, the sintering temperature of SDC is so high to be ableto co-sinter with electrode materials. Thus, the key point of application of the SDCis to improve the sintering property of it and lower the sintering temperature.In this paper, we used co-precipitation method to synthetize Sm_0.2Ce_0.8O_1.9powders, then we studied the sintering, conductivity and electrochemical property ofSDC after adding differednt amounts of Li₂O and evaluate the feasibility of Li₂O assintering add. What’s more, we discussed the sintering mechanism of Li₂O.The results indicated that the adding of Li₂O can really reduce the sinteringtemperature of SDC, the sintering temperature of SDC with2mol%Li₂O （noted asSDC2）was900℃，it was much lower than SDC without Li₂O （SDC0） which was ashigh as1500℃. After being sintered at900℃for6h, the relative density ofSDC2was up to99%, while measured at650℃under operating condition ofSOFCs, the conductivity of SDC with3mol%Li₂O （SDC3） was0.026S·cm^-1,which was almost the same as SDC0sintered at1500℃for24h abtainning aconductivity of0.027S·cm^-1.The grain boundary diffusion dominated the densification processes of SDC2and the grain boundary mobility was4.2×10^-17m³·N^-1·s^-1at900℃, while it wasonly9.8×10^-19m³·N^-1·s^-1for SDC0at the same temperature. This was because theliquid phase in the grain boundary area greatly increased the flux of atoms along thegrain boundary. The activation energy of SDC2for densification was5.5±0.5eVhigher than4.70±0.32eV of SDC0which can be contributed to the formation ofliquid phase in the grain boundary. The OCV of the cell using SDC2as electrolyteunder the working conditions of SOFC was kept stable at0.78V at600℃, whichwas a litter lower than OCV of SDC0due to the electronic conductivity of the reduction of lithium oxide to lithium at high temperature. When we fabricatedinterlayer using SDC2by screen print, during the sintering process, the SDC2shrinked much quickly than YSZ electrolyte and LSFC cathode. As a result, bothsides of SDC2may accumulate stress, thereby leading to tiny cracks.