Study on Bismuth-based Oxide Ion Conductors with High Performance and Durability for Lower Temperature Solid Oxide Fuel Cells

Abstract

Solid Oxide Fuel Cells (SOFCs) have great potential since they have not only the highest fuel to electricity conversion efficiency but also fuel flexibility through a series of electrochemical reactions due to high operating temperature (≧ 800 oC), allowing the use of various types of hydrocarbon such as methane, propane, methanol and biomass, and hydrogen, thereby offering a critical energy solution. However, such a high operating temperature also limits their practical use because of expensive system cost, performance degradation, and sealing issues. Therefore, lowering the operating temperature has been important issue for decades to secure lower system cost, greater reliability and faster start-up time. At reduced temperature, however, thermally activated nature of oxide ion transport and oxygen reduction reaction (ORR) exponentially decreases. The major portion of performance reduction comes from sluggish kinetics of the ORR at the cathode side. Thus, to achieve high performance of SOFCs at intermediate temperature (IT, ≦ 750 oC), the ORR process at the cathode should be improved. Perovskite oxide such as La0.8Sr0.2MnO3 (LSM), Ba1-xSrxCoyFe1-yO3-𝛿 (BSCF), La1-xSrxCoO3-𝛿 (LSC), La1-xSrxCo1-yFeyO3-𝛿 (LSCF), and Sm1-xSrxCoO3-𝛿 (SSC) are usually used as SOFC cathodes. Since cobalt-containing perovskite materials show high performance at the expense of poor chemical stability, they are not reliable from the point of view of durability. On the other hand, LSM is known to exhibit great chemical and mechanical stability. However, at IT regime, LSM shows poor cathode performance due to its negligible ionic conductivity, decreasing effective triple phase boundary (TPB) length. To increase TPB length, LSM is usually utilized with oxide ion conductors as composites. LSM-yttria-stabilized zirconia (LSM-YSZ) composite and LSM-gadolinia-doped ceria (LSM-GDC) composite succeeded in enhancing cathode performance. However, the performance of aforementioned composite cathodes is still unsuitable for practical use of SOFCs operated at reduced temperature, demanding necessity of development of novel oxide ion conductors. Bi2O3-based oxide ion conductors are promising materials with potential for replacing conventional oxide ion conductors such as YSZ and GDC because of their exceptionally high ionic conductivity. Particularly, Dy and Y co-doped Bi2O3 (DYSB) with low total dopant concentration (12 mol%) required for cubic phase stabilization showed extremely high conductivity at IT regime (1.1 S·cm-1 at 700 oC). Moreover, LSM-DYSB composite cathode showed the enhanced oxygen incorporation and surface diffusion rate. A conventional YSZ electrolyte-based SOFC with the novel LSM-DYSB cathode exhibited record-high performance (2.23 W·cm-2 at 700 oC) with absence of noticeable performance degradation during long-term operation at 700 oC for 100 h. However, because DYSB is not durable at IT regime, Hf and Er co-doped Bi2O3 was designed. 1 mol% of Hf doped ESB (1HESB) maintained its superior conductivity (0.14 S·cm-1 at 600 oC) with no degradation for 1200 h at 600 oC. The Boltzmann-Matano analysis showed that interdiffusion coefficients of Bi3+ in 1HESB decreased by 53% compared to that of Bi3+ in ESB. By taking advantage of high durability of HESB, LSM-1HESB composite cathode was prepared and it showed excellent durability during over 200 h at 600 oC and comparable performance to LSM-ESB composite cathode. Also, YSZ electrolyte-based SOFC to which LSM-1HESB composite was incorporated showed high performance (570 mW·cm-2 at 600 oC) without marked degradation during 100 h of operation at 600 oC. Since diffusion coefficient can be also controlled by microstructure, to confirm effect of microstructure on diffusion coefficient of Bi3+, grain boundary was controlled by pulsed laser deposition (PLD). As a result, absence of grain boundary decreased Bi3+ diffusion coefficient compared to the case for presence of grain boundary. After durability test at 600 oC, ESB thin film without grain boundary retained 91.4% of conductivity compared to initial value, showing negligible portion of rhombohedral phase. On the other hand, ESB thin film with grain boundary retained only 5.4% of initial conductivity, attributable relatively severe phase transformation from cubic to rhombohedral phase. This result suggests that Bi3+ diffusion coefficient is a dominant factor governing durability of fluorite-type Bi2O3. In order to further reduce operating temperature down to near 500 oC, metallic dopant-doped Bi2VO5.5 (BiMEVOx.) materials were synthesized. 5 mol% of Ti and 5 mol% of Cu co-doped Bi2VO5.5 (BiTiCuVOx.55) particularly showed the conductivity (0.09 S·cm-1 at 500 oC) higher than ESB by a factor of 3 and great durability at 500 oC for 550 h. BPRO7-BiCuVOx.10, composite of Bi1.8Pr0.2Ru2O7 (BPRO7) and 10 mol% of Cu-doped Bi2VO5.5 (BiCuVOx.10), cathode indicated comparable electrode polarization resistance to conventional LSCF-GDC composite cathode as well as remarkable durability at 500 oC for around 270 h. By structuredly selecting dopants for advanced bismuth oxides, it was possible to manipulate fundamental properties such as conductivity, crystal phase and diffusion coefficient of bismuth oxide-based materials in a useful manner, thereby reducing operating temperature of SOFCs down to LT regime. This systematic study suggests how to rationally design bismuth oxide-based oxide ion conductors with advantageous properties for application to LT-SOFCs.