Preparation And Properties Of Electrolyte And Air Electrode Materials For Intermediate Temperature Reversible Solid Oxide Cell

One unavoidable difficulty in direct using solar cells and windmills as power generators is that both sunlight and wind are neither continuous nor steady, which may cause fluctuations harmful to standard electrical grids. Reversible solid oxide cells (RSOCs) are a kind of green energy devices that can as work as the solid oxide electrolysis cells (SOECs) to store excess electrical energy as chemical energy (as hydrogen) and as the solid oxide fuel cells (SOFCs) to release electrical energy when necessary. Another great potential in RSOCs application is for the peak load regulation of the electric grid by working in SOECs to store the electricity in fuels in the off-peak period of electricity and working in SOFCs to discharge in the peak period. Therefore, developing RSOCs with good performance is very important to alleviate the energy crisis.Based on the conducting species in electrolyte, RSOCs can be divided into oxide ion conducting RSOCs (O-RSOCs) and proton conducting RSOCs (H-RSOCs). It has been suggested that the main obstacle of O-RSOCs is the electronic conduction in doped ceria electrolyte, which lowers the conversion efficiency; while the main challenge of H-RSOCs is the low proton conductivity in air electrodes. In this work, several studies have been conducted to improve the electro-performance of O-RSOCs and H-RSOCs, including (1) Fabrication of an electronic blocking layer for O-RSOCs using a simple in-situ technology;(2) Exploring novel single phase air electrode with high proton conduction;(3) Exploring novel non-perovskite air electrode materials against Cr deposition; and (4) Internal reforming layer for CH4fuel again carbon deposition. And the main results are listed as follows:Chapter1:A brief introduction on the background and reaction mechanism RSOCs was given. An analysis on the limiting factors of RSOCs performance was made and the theme of this thesis was made based on this analysis as:(1) To hamper the internal short of doped ceria electrolyte materials; and (2) To explore suitable air electrodes to improve the electro-performance of RSOCs.Chapter2:Ni-Ba1+xCe0.5Zr0.3Y0.2O3-δ hydrogen electrodes were used to prevent the internal short in SDC electrolyte. A dense reaction layer of BaCeO3based oxide formed at the inner face of electrolyte for Ba partially transferred from hydrogen electrode to electrolyte during the co-sintering process for cell fabrication. This formed layer successfully blocked the electronic conduction in doped ceria electrolyte as the open circuit voltage rising from0.82V to1.0V. Intensive studies suggested that Ba content in Ni-Ba1+xCe0.5Zr0.3Y0.2O3-δ has a great effect on the electro-performance of RSOCs. Best performance was achieved with Ba content of0.96. Using Ni-Ba0.96Ce0.5Zr0.3Y0.2O3-δ as hydrogen electrode, a current density of0.7Acm-1under1.3V bias voltages was achieved.Chapter3:Polarization resistances from air electrode were reported to be the main resistance contributor of RSOCs, which restrict their electro-performance. Proton transferring was suggested as the rate-limiting steps in air electrode reaction. Therefore exploring novel air electrode with high proton conductivity was important to the development of RSOCs. In this work, BaZr1-xCoxO3-δ was designed to achieve high proton conduction and suggested as a single-phase electrode for H-RSOCs. The crystal structure, ionic and electronic conductivity and the electro-performance of BaZr1-xCoxO3-δ were investigated as function of Co cotent. The results are listed as follows:(1) Single-phase BaZr0.6Co0.4O3-δ was acquired when Co content less than0.4;(2) Within the tested Co content, BaZr0.6Co0.403-δ shows highest ionic and electronic conductivity, about1.2x10-3and5.24Scm-1, respectively, measured at700℃,(3) Low polarization resistances with the value of0.19Ωcm2at700℃, was achieved when using BaZr0.6Co0.403-δ single phase air electrode. This value was approximately65%lower than that using a traditional Sm0.5Sr0.5Co03-δ/BaCe0.5Zr0.3Y0.2O3-δ composite air electrode;(4) Good electro-performance of cell based on BaZr0.6Co0.4O3-δ single electrode was achieved, about299mAcm-2(at0.7V) and-935mAcm-2(at1.5V), respectively, measured at700℃. These performances are better than those with a traditional Sm0.5Sr0.5Co03-δ/BaCe0.5Zr0.3Y0.2O3-δ composite air electrode(4) Polarization resistance of BaZr0.6Co0.4O3-δ as single-phase electrode appied in cells was0.19Ωcm2, approximately65%lower than that using.The results show that BaZr0.6Co0.4O3-δ is a good proton and electron mixed conductor and suitable for H-RSOCs air electrode.Chapter4:Cr evaporating from metal interconnects tends to deposit at the surface of air electrode with perovskite or perovskite-like structure. This Cr deposition results in low electro-performance of air electrodes. In this work, spinel structured NiFe2-xCoxO4was used as air electrode again Cr deposition. The conductivity, thermal expansion coefficient and the polarization resistance of NiFe2-xCoxO4were characterized as function of Co cotent. The main results are listed as follows:(1) NiFe2-xCox04have good chemical compatibility with SDC electrolyte.(2) NiFe2-xCoxO4have a relatively high ionic conductivity, approxmiately10-3Scm-1, three to four orders of magnitude higher than that of LSM perovskite materials.(3) With NiFe1.5Co0.5O4air electrode, maximum power density about360mWcm-2at650℃was achieved with the polarization resistance of0.19Ω cm2.Chapter5Carbon deposition on Ni surface leads to the catalyst deactivation. In this work, NiFe2O4was applied as an internal reforming layer to monitor the catalysis effect of Ni anode on the cracking of carbon hydrogen fuels. Our investigation suggests that the NiFe2O4internal reforming layer could greatly improve the long term stability of cells in hydrocarbon fuels.