| Energy crisis and environmental pollutions are the problems which the whole world is now facing for the sustainable development. Solid oxide fuel cells (SOFC), which have been regarded as keystone for the future energy economy, have received considerable attention for their high energy conversion efficiency and low impact to environment as a mean of generating electricity. Proton conducting solid oxide fuel cells (H-SOFC) have attracted much attention for their unique characters, such as great efficiency in fuel utilization, high electromotive force (EMF), high ionic transferring numbers and low activation energies for proton conduction.However, compared with oxygen-ion conducting SOFC (O-SOFC), the materials and theories on H-SOFC are just inchoate, especially, for cathodes of H-SOFC. In these H-SOFC, hydrogen is oxidized at the anode to form protons, which migrate through the electrolyte to the cathode, and undergo a half-cell reaction with oxygen to produce water, which makes the cathode reactions more complex compared with those of O-SOFC. Such distinguished characteristic of cathode reactions calls for intensive consideration on reaction mechanism and might lead to some special demands on the cathode materials. In order to lower the cathode polarization resistances and improve the performances of cells, this p.h.D thesis investigates the materials, electrochemical performance and reaction model for cathodes of H-SOFC.Chapter1describes the background, significance, working principle as well as research progress of H-SOFC. The key component materials, such as anodes and electrolytes, and their development, are also generally introduced. The development of cathode materials and different types of cathode reaction models are intensively discussed.In Chapter2:The cathodes of H-SOFC are devided into several types, such as: cathodes cooperated with proton conductor and cathodes cooperated with oxygen-ion conductor. Proton conductor cooperation to the cathode can thus increase the H-TPB length while oxygen-ion conductor could not. However, previous reports show that oxygen-ion conductor can also significantly increase the cathode performance, suggesting different cathodic mechanism. In this work, SSC (Sm0.5Sr0.5CoO3-δ)-SDC (Ce0.8Sm0.2O2-δ), typical cathodes cooperated with oxygen-ion conductor are fabricated and investigated as cathodes for SOFC with BCS (BaCe0.8Sm0.2O3-δ) electrolytes. The interfacial polarization resistance is characterized by A.C. impedance as a function of oxygen and water vapor partial pressures to reveal the rate-limiting steps. The results show that the diffusion of Oad-and V0.. might both be the rate-determining steps for SSC-SDC cathodes, while the water formation reaction is not a rate-limiting step, suggesting the H-TPB at the interface of cathode and electrolyte might be enough for SSC-SDC. The three-electrode cells tests suggest that the SSC-SDC cathodes might be beneficial for H-SOFC during actual operation.In Chapter3:Iron doped to PrBaCo2O5+δ to form a serial of layered perovskites, PrBaCo2-xFexO5+δ (x=0,0.5,1.0,1.5and2.0), are evaluated as cathode materials for proton conducting solid oxide fuel cells (H-SOFC). The lattice parameter and oxygen nonstoichiometry content,δ, at room temperature increase, whereas the conductivity and thermal expansion coefficient decrease with increasing iron content, x. PrBaCo2-xFexO5+δ exhibit excellent stability at700℃in atmosphere consisting of3%CO2and97%air. The symmetrical cell experiment demonstrates oxygen catalytic activity of PrBaCo2-xFexO5+δ decrease with increasing iron content, which is a good agreement with density functional theory (DFT). For high oxygen catalytic activity of PrBaCo2O5+δ (PBCO), the maximum power density of the PBCO/BaZr0.1Ce0.7Y0.2O3-δ (BZCY)/NiO-BaZr0.1Ce0.7Y0.2O3-δ (BZCY) cell is545mW cm-2at700℃, suggesting that the PBCO could be a good candidates as H-SOFC cathode material.Chapter4, Micro tubular SOFC are fabricated and investigated. In the first section, hollow fiber NiO-BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) substrates have been successfully fabricated by the phase inversion methode. The fibers possess an asymmetric structure comprising of a microporous layer integrated with a finger-like porous structure. The inside finger-like porous layer may greatly accelerate mass transfer of the fuel, while the outside sponge-like porous layer could provide much TPBs. The fiber’s mechanical strength and electrical conductivity increase while its porosity decreases with sintering temperature. This makes an optimized sintering temperature of1350℃for the anode-electrolyte bi-layers. A micro-tubular proton-conducting single cell consisting of a Ni-BZCYYb anode, a BZCYYb electrolyte, and a SSC-BZCYYb cathode generates a peak power density of254mW cm-2at650℃. In the second section, BaZr0.1Ce0.7Y0.2O3-δ (BZCY), an intermediate temperature proton conductor, has been applied as an electronic blocking material for micro tubular solid oxide fuel cells (SOFC) using doped ceria electrolyte. Bi-layer electrolyte consisting of3μm thick BZCY and10μm thick Sm0.2Ce0.8O1.9(SDC) are successfully deposited on anode substrates with1.0mm diameter using phase inversion, suspension-coating and co-firing techniques. At700℃, open circuit voltage increases from0.72V for the cells with single SDC electrolyte to0.97V for those with bi-layer electrolyte, demonstrating that BZCY can effectively prevent the internal shorting in doped ceria electrolyte. |
