Development of Large-Scale Proton-Conducting Fuel Cells for Next Generation SOFC

Abstract

Solid oxide fuel cells (SOFCs) are known to exhibit an exceptional energy conversion efficiency over 80 % of the various fuel cell types and have environmentally friendly systems with fuel flexibility. Unfortunately, the SOFC based on yttria-stabilized zirconia (YSZ) electrolytes has a major obstacle to commercialization due to its high cost and low durability at high operating temperature range (800~1000 oC). To overcome this issue, many researchers have made efforts in studying materials with performance comparable to conventional SOFCs at intermediate temperature range (500~700 oC). Among these efforts, to use a proton conductive material with a perovskite structure (ABO3) is regarded as an attractive solution. A lot of previous literatures have already proved the potential of proton-conducting oxide fuel cells (PCFCs) as a next generation of SOFCs. However, most of the results are based on laboratory-scale, so the reliability and feasibility have not been confirmed yet. In the attempt of entering the market with the PCFCs, several issues that must be faced are related to large-scale production. Firstly, when it comes to large-scale cell, the difference in the thermal expansion coefficient (TEC) between the electrode and the electrolyte dominantly affects the overall mechanical property of cells. So, the shrinkage rate after sintering should be elaborately controlled to inhibit several problems such as cracking, lifting and warping. Secondly, the low sinterability of BaCeO3 based proton conductors requires a high sintering temperature, which is more serious problem particularly in large-scale cells. That is, the possibility of Ba evaporation during the heat treatment increase, involving in formation of a secondary phase, which eventually causes performance deterioration. To our knowledge, few literatures are reported on this topic, large-scale PCFCs fabrication. In this work, we attempt to establish large-scale anode-supported PCFCs with BaCe0.6Zr0.2Y0.1Yb0.1 produced by citric-nitrate synthesis method. The electrostatic slurry spray deposition (ESSD) technology was used for each layer fabrication of large-scale cells. An anode functional layer (AFL) was adopted to adjust the TEC and shrinkage rate between the anode and the electrolyte. The quantitative shrinkage rates depending on temperature were analyzed by Dilatometer (Linseis Co.). A scanning electron microscope (SEM, JEOL Co.) was used for microstructural analysis. To examine Ba evaporation and interlayer diffusion, energy dispersive X-Ray spectroscopy (EDX) was used, and the phase development of the synthesized BCZYYb was analyzed by X-ray diffractometer (XRD, Rigaku Co.). The I-V curves and impedance spectra of the single cell were measured in the range of 500 ℃~700 ℃ with humidified hydrogen (~3 % water vapor). Finally, a short-term stability test (100 hours) was performed to confirm the electrochemical durability.