Fabrication Study Of Proton Conductor Ceramic Membrane Fuel Cells

Solid oxide fuel cell (SOFC) is a high efficiency and clean energy conversion device. Unfortunately, the expensive SOFC system limits the commercialization for the high operating temperature. In order to widen the materials chosen, decrease material degradation, prolong the lifetime and reduce the cost, the reduction of the working temperature of SOFCs becomes the urgent demand. There is now considerable interest in proton-conducting oxide electrolytes for solid oxide fuel cells. The proton-conducting SOFCs exhibit more advantages than typical SOFCs based on oxygen-ion-conducting electrolyte (SOFC-O2-), such as low activation energy and high energy efficiency. The development of proper cathode materials for proton-conducting solid oxide fuel cells in order to improve materials compatibility and reduce costs remains a challenge. In this work, some cathode materials were investigated to be possible materials for proton-conducting membrane fuel cells (PCMFCs).In chapter 1, investigating and analyzing of SOFC's development and status, the key materials and structure types of SOFC were summarized. Reducing fabrication and operation costs while maintaining high performance is a major consideration for SOFC. In chapter 2, BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY7) perovskite oxide was prepared by gel-casting and solid-state reaction techniques as proton-conducting electrolyte for low-temperature solid oxide fuel cells (LT-SOFCs). The two kinds of different powders were compared. In chapter 3, The SrCo_0.9Sb_0.1O_3-δ (SCS) composite oxide with cubic perovskite structure was synthesized by a modified Pechini method and examined as a novel cathode for protonic ceramic membrane fuel cells (PCMFCs). In chapter 4, BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY7) electrolyte based PCMFCs with stable, cobalt-free Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3-δ (BSZF) perovskite cathode were investigated. The main achievements and innovations in this paper are summarized as follows:1. BaZr_0.1Ce_0.7Y_0.2O_3-δ proton-conducting electrolyte prepared by gel-castingAt present, for the preparation of barium cerate-based oxides, there are various synthesis techniques such as Pechini method, pulse laser deposition (PLD), and glycine-nitrate process (GNP). However, these methods cannot be applied in industry due to the complex process and high cost. The gel-casting process has proven to be an attractive synthetic route for the preparation of multi-component oxides, since a homogeneous mixture of several components can be easily reached for the formation of a polymer network. Therefore, by using the simple and cost-effective method, it is easy to lower phase formation temperature and enhance the sintering activity. Meanwhile, there are no other elements introduced in this process. BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY7) perovskite oxide was prepared by gel-casting and solid-state reaction techniques as proton-conducting electrolyte for low-temperature solid oxide fuel cells (LT-SOFCs). X-ray diffraction analysis indicated that the phase formation temperature of BZCY7 powder synthesized by gel-casting was 100-150℃lower than that by solid-state reaction method. The BZCY7 electrolytes prepared by gel-casting exhibited better sintering activity and higher electrical conductivity. After sintering at 1550℃for 5 h, the relative density and electrical conductivity of the gel-casting derived BZCY7 reached 95 % and 5×10-3 S/cm in H2 (～3% H2O) at 500℃, respectively. However, for the solid-state reaction method, the sample had a relative density of 86 % and electrical conductivity of 2×10-3 S/cm under the same condition.2. SrCo_0.9Sb_0.1O_3-δ cubic perovskite cathodeMany simple perovskite-type mixed ionic–electronic conductors such as doped LaCoO3, BaCoO3 or LaFeO3 have been extensively studied as possible cathodes, however not much attention has been paid to the perovskite structures such as doped SrCoO3. Recently, Aguadero et al. have prepared a novel SrCo0.9Sb0.1O3–δ(SCS) perovskite, and shown its potential use as a MIEC in SOFCs.3. High performance protonic ceramic membrane fuel cells (PCMFCs) with Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3-δ perovskite cathodeMany cobalt-containing perovskite-type mixed ionic-electronic conductors such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ, Ba0.5Pr0.5CoO3-δ, La0.5Sr0.5CoO3-δ, La0.6Ba0.4CoO3-δ, Sm0.5Sr0.5CoO3-δor GdBaCo2O5+x have been extensively studied as possible PCMFC cathodes, however not much attention has been paid to the cobalt-free perovskite-type materials. These cobalt-based cathodes in practical long-term applications often suffer some problems like poor chemical stability in CO2, high thermal expansion coefficients (TECs), ease of evaporation and high cost of cobalt element. Several cobalt-free perovskite oxides, such as BaCe0.4Pr0.4Y0.2O3-δ, La0.7Sr0.3FeO3-δ, etc., have been reported as PCMFC cathodes. Recently, a Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3-δ (BSZF) perovskite oxide was developed by Wang et al. as a novel cobalt-free oxygen-permeable membrane, which showed high permeation behavior and good chemical stability at high temperatures. Wei et al. have shown the potential of cobalt-free BSZF for cathode application and evaluated the performance of this material working as a cathode in SOFCs based on oxide ion conductors (SDC). In this work, the cobalt-free BSZF synthesized by a modified Pechini method was employed as a new PCMFC cathode.