Design And Fabrication Study Of Intermediate-to-low Temperature Protonic Ceramic Membrane Fuel Cells

Energy and environmental issues have become major challenges that all country is now facing for the sustainable development. Fuel cells, which are electrochemical devices that convert chemical energy directly to electrical energy, have received enormous attention for their high energy conversion efficiency and low impact to environment. Solid oxide fuel cell (SOFC), also called high temperature (500℃—1000℃) fuel cell system, is the third-generation fuel cell system followed the first molten carbonate fuel cell (MCFC) and the second phosphate acid fuel cell (PAFC). SOFCs provide many advantages over traditional energy conversion systems including high efficiency, reliability, modularity, fuel adaptability, and very low levels of NOx and SOx emissions. Now SOFC is on the way to practical application and commercialization, and the challenge in successfully commercializing SOFCs offering high power densities and long term durability requires reduction of costs associated the cells and the balance-of-plant.Facing the challenges of key materials and preparation techniques in successfully industrializing SOFCs, we focus on development concepts of "intermediate-to-low temperature+simple", consider comprehensively key materials (electrolyte, cathode, anode and interconnect) and preparation techniques, and develope pro tonic ceramic membrane fuel cells (PCMFCs) and simple solid oxide fuel cells (SSOFCs). The thesis investigates cathode-supported SOFCs, intermediate-to-low temperature (400℃—700℃) SOFCs, PCMFCs, SSOFCs and the simple & cost-effective preparation techniques.Chapter 1 reviews the working principle, key materials, stack designs and the thin-membrane preparation techniques for SOFCs. In addition, we also propose the thesis work aimed at successfully industrializing SOFCs.Chapter 2 investigates counter-mainstream cathode-supported SOFCs based on dense Ceo.8Sm0.2O1.9 (SDC) thin electrolyte. In order to develop a simple & cost-effective route for low-temperature co-fired preparation of cathode-supported SOFCs, we consider comprehensively high active SDC powders and suspension spray process which is developed in our laboratory. The uniform nano-sized SDC1 powders were synthesized by co-precipitation from metal nitrates with ammonia as the precipitant. Crystalline solid solutions with cubic fluorite structure were formed directly during precipitation, and that was mainly for the strong complexation ability of OH-, which made the oxidation of Ce (III) much easier. The solid solution nanoparticles were slightly hydrated and major dehydration could occur at 400℃, without significant crystallite growth. Besides, even if the powders are calcined at 800℃for 2 hours, no significant hard-aggregates are formed, with a bit crystallite growth, and the average particle size is about 20 nm. A 10-μm-thick Ceo.8Sm0.2O1.9 electrolyte membrane was successfully fabricated on the porous cathode substrates by a suspension spray technology. Co-firing of the bi-layers of cathode/electrolyte and a subsequent Ni-cermet forming were successful at 1250℃and at 1000℃, respectively. A laboratory-sized tri-layer cell, not yet optimized for performance, was operated at 600℃fed with humidified H2 (3wt.%H2O), and the maximum power density obtained was 67.2 mWcm-2. These results illustrate that a suspension spray technology is a simple and potentially commercial prospective method for preparing the key components of SOFCs.Chapter 3 designs and investigates low-temperature (400—600℃) solid oxide fuel cells with novel La0.6Sr0.4Coo.8Cuo.2O3-δ(LSCCu) perovskite cathode and functional graded anode. The perovskite LSCCu exhibits excellent ionic and electronic conductivities in the intermediate-to-low temperature range (400-800℃). Thin Sm0.2Ce0.8O1.9 (SDC) electrolyte and NiO-SDC anode functional layer are prepared over macroporous anode substrates composed of NiO-SDC by a one-step dry-pressing/co-firing process. A single cell with 20μm thick SDC electrolyte on a porous anode support and LSCCu-SDC cathode shows peak power densities of only 583.2 mWcm-2 at 650℃and 309.4 mWcm-2 for 550℃. While a cell with 20μm thick SDC electrolyte and a anode functional layer on the macroporous anode substrate shows peak power densities of 867.3 and 490.3 mWcm-2 at 650 and 550℃, respectively. The dramatic improvement of cell performance is attributed to the much improved anode microstructure that is confirmed by both SEM observation and impedance spectroscopy. The results indicate that LSCCu is a very promising cathode material for LT-SOFCs and the one-step dry-pressing/co-firing process is a suitable technique to fabricate high performance SOFCs.Chapter 4 investigates stable SrCoo.9Sb0.1O3-δ(SCS) cubic perovskite cathode based low-temperature solid oxide fuel cells. The polarization resistances of symmetrical SCS cathode in air were as low as 0.09 and 0.24Ωcm2 at 700℃and 650℃, respectively. A cell of NiO-SDC/SDC/SCS was operated from 500℃to 650℃ fed with humidified H2 (～3% H20). A high OCV of 0.86 V and a maximum power density of 354 mWcm-2 was achieved at 650℃. The polarization resistance of the electrodes was as low as 0.13Ωcm2 at 650℃. A cell of NiO-BZCY/BZCY/SCS was operated from 500℃to 700℃. A high OCV of 1.004V, a maximum power density of 259 mWcm-2, and a low polarization resistance of the electrodes of 0.14Ωcm2 was achieved at 700℃. The results indicate that the SCS cubic perovskite cathode is a good candidate for operation at or below 700℃, and that dip co(?)ting is a simple and potentially commercial prospective route for preparing the key components of SOFCs.Considering comprehensively stable perovskite proton-conducting electrolyte and high-performance cobalt-free cathode, chapter 5 investigates intermediate-to-low temperature proton-conducting solid oxide fuel cells with Ba0.5Sr0.5Zn0.2Fe0.8O3-δ(BSZF) perovskite cathode. The stable BaCe0.5Zr0.3Y0.16Zn0.04O3-δ(BCZYZn) perovskite electrolyte synthesized by a modified Pechini method exhibited higher sinterability and reached 97.4% relative density at 1200℃for 5 h in air, which is about 200℃lower than that without Zn dopant. A laboratory-sized tri-layer cell of NiO-BCZYZn/BCZYZn(30μm)/BSZF, not yet optimized for performance, was operated from 550 to 700℃fed with humidified H2 (～3%H2O). An open-circuit potential of 1.00 V and a maximum power density of 236 mWcm-2 were achieved at 700℃. The polarization resistance of the electrodes was as low as 0.17Ωcm2 at 700℃. For optimized BSZF-BZCY composite cathode based quad-layer NiO-BZCY/ NiO-BZCY (-50μm)/BZCY (-20μm)/BSZF-BZCY cell, an open-circuit potential of 1.015 V, a maximum power density of 486 mWcm-2 and a low polarization resistance of the electrodes of 0.08Ωcm2were achieved at 700℃, which are comparable with Ba0.5Sr0.5Co0.8Fe0.2O3-δ-BaZr0.1Ce0.7Y0.203-δ(BSCF-BZCY) composite cathode based quad-layer NiO-BZCY/NiO-BZCY (-50μm)/BZCY (-20μm)/BSCF-BZCY cell. Obviously, cobalt-free BSZF-BZCY composite cathode is the better choice in view of chemical stability. The results indicated that proton-conducting electrolyte with BSZF perovskite cathode is a promising material system for the next generation SOFCs.Chapter 6 proposes and develops a new research stage of solid oixde fuel cells: protonic ceramic membrane fuel cell (PCMFC). Considering comprehensively new key materials compatibility and optimization of cell preparation techniques, layered GdBaCo2O5+x/SmBaCo2O5+x (GBCO/SBCO) perovskite cathode based PCMFCs were prepared by all solid-state in-situ reaction based gel-casting and suspension spray/screen printing process. A laboratory-sized tri-layer cell of Ni-BZCY/BZCY (～10μm)/GBCO, not yet optimized for performance, was operated from 550 to 700℃fed with humidified H2 (～3%H2O). An open-circuit potential of 0.98 V and a maximum power density of 266 mWcm-2 were achieved at 700℃. The polarization resistance of the electrodes was as low as 0.16Ωcm2 at 700℃. For optimized Ni-BZCY/BZCY(～25μm)/SBCO cell, an open-circuit potential of 1.01 V, a maximum power density of 382 mWcm-2 and a low polarization resistance of the electrodes of 0.15Ωcm2 were achieved at 700℃. The results indicated that proton-conducting electrolyte with layered GBCO/SBCO perovskite cathode is a promising material system for PCMFCs. Using this process, we also successfully realized the preparation of a dense Lao.99Ca0.01Nb04 (LCN) thin membrane. The results indicate that all solid-state in-situ reaction based gel-casting and suspension spray/screen printing process is a simple and commercial prospective route for preparing SOFCs.Chapter 7 proposes and develops a new concept of solid oixde fuel cells:simple solid oxide fuel cells (SSOFCs). SSOFC, that is simply composed of an electronic conductor and an ionic conductor, was proved to be a valid alternative to the traditional SOFC configurations. High-performance interconnect material Lao.7Ca0.3Cro.9703_s (LCC97) perovskite oxide was examined as both anode and cathode materials for SOFCs based on YSZ electrolytes. XRD results indicate that LCC97 chemically compatible with YSZ/SDC. The LCC97 sintered at 1250℃had the conductivity of 62.0 S.cm-1 at 850℃in air, which is about 2.6 times as high as that of La0.7Cao.3Cr03-δwith no chromium deficiency. The TEC of the material was about 11.4×10-6 K-1, which is very compatible to that of other common used SOFC components. The interfacial polarization resistances of the LCC97-SDC (1:1) composite cathode were much lower than the LCC97-YSZ (1:1) composite cathode. At 800℃, the ASRs of composite cathodes were only 0.15 Q.cm2 and 0.30Ω.cm2 for LCC97-SDC (1:1) and LCC97-YSZ (1:1), respectively. This investigation indicated that chromium deficient lanthanum calcium chromites LCC97 perovskite oxides are promising redox-stable interconnect and eletrode materials for solid oxide fuel cells.In chapter 8, the researches presented in this dissertation are evaluated and future work concerning industrialization of SOFCs is discussed.