Fibrous Mixed Conducting Cathodes for Solid Oxide Fuel Cells

Abstract

Current research in the field of Solid Oxide Fuel Cells (SOFCs) aims at lowering the operating temperature below 800 oC to overcome the problems caused by high temperature operation. However, it inevitably decreases the cathodic reaction, which deteriorates the performance of a single cell. It hasn’t been long since the study on the application of one-dimensional (1D) structure, which was started with activating the SOFCs for intermediate temperature range. Despite a short-term research period, many groups have reported the excellent performance caused by 1D structure using various cathode materials. When a fiber structure is applied to the cathode, it is expected to not only maximize the number of reaction sites but also minimize the interfacial resistance by improving the phase connectivity which ensures much faster transport of both electrons and oxygen ions. We propose a simple and easy approach that makes it possible to embed the ionic conducting particles into the MIEC fiber by electrospinning method. To our knowledge, there is no attempt to prepare the ceramic fiber with ceramic particles for SOFC application. This fiber structure used in this thesis is designed to strengthen the advantages of fibrous cathode; (i) suppressing the grain growth of the MIEC matrix by dispersed ionic conducting particles, (ii) relieving residual thermal stress within the fibers, (iii) maintaining the overall porosity without filling the network pores, and (iv) simplifying the composite processing compare to other process such as infiltration. First, in Chapter 2, to achieve the synthesis of composite fiber, the basic works for the process optimization were conducted based on the single phase fiber. Polycrystalline SSC fibers (diameter: about 150 nm) were obtained from the precursor gel of 0.4 M after calcinations. As an improved approach, we propose a simple and easy technique to fabricate the composite fibers in Chapter 3. The produced fiber contained the ionic conducting particles within the MIEC backbone, which effectively suppressed the grain growth of MIEC. This cathode exhibits a much lower resistance, which is sufficient evidence for the structural excellence of this 1D cathode, i.e. promoting charge transfer reactions by continuous path and maximizing the TPB sites by dispersed ionic conducting particles. In Chapter 4, to prove the feasibility of this fibrous composite cathode a practical cathode, we conducted the stability test using a half-cell under cathodic polarization to ensure the degradation phenomenon from the cathode only. After the measurement, the resistance related to charge transport and surface catalytic reaction were increased by 159 and 77 %, respectively. The cause of this degradation can be considered common problems of perovskites cathode materials such as Sr-enrichment, grain growth. Fortunately the fibers retain their shape, porosity and interfacial adhesion. This fiber structure was introduced to proton conducting ceramic fuel cells in Chapter 5. This highly porous cathode will provide maximum air flow through the whole cathode layer since the cathode for PCFCs must have sufficient porosity to permit water vapor which has a harmful effect. The maximum power densities with LSCF-BCZY fiber cathode were slightly low, but the polarization resistances were considerably low. The BCZY electrolyte layer was fairly thick, increasing the ohmic resistance proportionally. Therefore, we could expect that the performance of this single cell is enhanced with a decrease of the electrolyte thickness.