Performance Investigation On Sr_2-xA_xMnO₄(A=La, Pr) As Cathode Of Solid Oxide Fuel Cells

Solid Oxide fuel cell (SOFC), as a high efficiency, environmentally friendly energy conversion device, is attracting widely attentions. Nowadays, it becomes a main topic that operating SOFCs at an intermediate temperature so as to reduce the fabricating cost and elongate the cell life. As SOFC working, polarization loss in cathode, which is a major component of SOFC, takes up over half of all loss of the cell's polarization. So searching new cathode materials and improving cathode properties is important for enhancing the cell performance.K2NiF4-type structure oxide becomes a hot researching field. It possesses mixed ionic-electronic conducting properties, better thermal stability, desirable thermal expansion coefficients, higher surface oxygen-exchange coefficients and oxide-ion diffusivity. It is a potential IT-SOFC cathode material. We prepared Sr_2-xA_xMnO₄ (A=La, Pr) and studied its basic physics property and cathodic electrochemistry behavior.Sr_2-xA_xMnO₄ was prepared using sol-gel process. SPM1703, SPM1505, SLM1703 and SLM1505 exhibited single-phase structure. Lattice volumes and theoretical densities of the Sr_2-xA_xMnO₄ was calculated. The result demonstrated that the lattice volumes of Sr2-xPrxMnO4 were less than that of Sr2-xLaxMnO4, but its theoretical densities were larger than that of the latter. This was because the Pr3+ radius was smaller than La3+, but the mass was bigger than the latter. The Mn4+ ion changed into Mn3+ above 400℃wherein the TG results,which resulted in oxygen wastage. Raising sintering temperature would improve electrical conductivity of samples. SPM1505 had largest conductivity, which is 4.67 S/cm at 800℃with density from 88% to 92%. It was two orders of magnitude lower than of traditional LSM cathode.As a cathode of SOFC, Sr_2-xA_xMnO₄ and GDC electrolyte had a good compatibility. The TEC of Sr_2-xA_xMnO₄ is closed to GDC, which were 18.1～19.5×10-6 K^-1 and 17.8×10-6 K^-1, respectively. There was no reaction between Sr_2-xA_xMnO₄ and GDC after sintered 1200℃for 2 h.Using the AC impedance spectra and polarization, we founded that the single-phases SPM1703, SPM1505, and SLM1505 were better than others. SPM1505 was the best one among all samples. Its AC impedance was 0.10 ?cm² and polarization current density was 0.56 A/cm² under overpotential of 0.18 V. Its performance could compare favorably with the traditional LSM cathode. We investigated the electrochemical performance of SPM1505 at different testing temperature from 600℃to 800℃and at different sintering temperature range of 1000℃-1100℃. The result showed that higher testing temperature enhanced electrochemical catalytic activity and improved cathodic performance. From SEM and overpotential results, it can be concluded that the appropriate temperature for cathode preparation is 1050℃. Along with the DC potential on cathode enlargement, AC impedance of the SLM1505 and SPM1505 cathode increased.The AC impedance and I-V curves results of the single cells with SLM1505 and SPM1505 cathode indicated that the maximum power density of the SLM1505 cell and the SPM1505 cell were 69 mW/cm² and 147 mW/cm² at 650℃and 700℃, respectively. Their AC impedance all of cathode and anode were 1.66 cm² and 0.35 ?cm² separately.The test results above indicate that Sr_2-xA_xMnO₄(A=La , Pr) can be a candidate for cathode material of ITSOFC.