Fabrication And Characterization Of Composite Cathodes For Proton Conducting SOFCs

Protons conducting solid oxide fuel cells (H-SOFC) have attracted much attention because of their unique characters, such as their great efficiency in fuel utilization, low activation energies for proton conduction, high ionic transferring numbers and high electromotive force (EMF). Compared with oxygen-ion conducting SOFCs(O-SOFCs), the theories on H-SOFCs are just inchoate. Lots of efforts are made to search for suitable proton conductors with high proton conductivity and long-term stability, and few studies on electrode materials and reaction mechanisms have been reported. However, water is formed at cathode in H-SOFCs which makes the cathode reaction more complex compared with those of O-SOFCs. Such distinguished characteristic of cathode reactions calls for intensive study on their reaction mechanisms and might lead to some special demands on the cathode materials, as well as microstructure.This p.h. D thesis aims to lower the cathode polarization resistances, improve the performances of cells, and explore cathode reaction mechanisms for H-SOFCs. The main contents are summarized as follows:1) developing novel cathodes materials for H-SOFCs to reduce polarization resistances and to improve cell performance; 2) applying ion impregnation technique to optimize the electrode microstructures; 3) exploring cathode reaction mechanisms both by theoretical models and by experiments, analyzing the proton transfer processes and the rate limiting reactions, discussing the relationship between the cathode polarization resistances and the components, as well as the electrode microstructures. The purpose of all this work is to find how to improve the performance of H-SOFC cathodes.Chapter 1:the background and significance of H-SOFCs, key component materials, and development, are generally introduced. The effect factors of H-SOFCs' performance are also briefly reviewed. Since cathode performance restrict the electrode performance of H-SOFCs with thin-film electrolyte, this thesis aims to develop appropriate cathode materials, optimize cathode microstructure, and study the cathode reaction mechanisms.Chapter 2:the specific steps of cathode reactions are briefly introduced to understand the oxygen reduction process during cathodes of H-SOFCs, with respect to different conducting species in cathode materials, such as electron conductor, electron-oxygen ion mixed conductor and electron-proton mixed conductor, respectively. Since the electron-oxygen ion mixed conductors are the most potential cathodes, rate limiting reactions of such cathodes in H-SOFCs has been studied with Sm0.5Sr0.5CoO3-δ-BaCe0.8Sm0.2O2.9 composite cathodes.In chapters 3 and 4, electrochemical performance of composite cathodes consisted of Sm0.5Sr0.5CoO3-δ(SSC) and BaCe0.8Sm0.2O3-δ(BCS) are investigated for H-SOFCs by symmetrical cells and single cells, and the systematic study on the effect of its composition and microstructures on its performance has been carried out.Chapter 3:SSC-BCS cathodes are fabricated by screen printing process with BCS as the electrolytes and Ni-BCS as the anodes. The polarization resistances of such cathodes are investigated as function of SSC content in composite cathodes. Details are as follows:1) Interfacial polarization resistances of the composite decrease first and then increase with SSC content. The minimum polarization resistance,0.67Ωcm2 at 600℃, is reached with 60 wt.% SSC-BCS cathode, which corresponds to 50 Vol.% SSC.2) With electrolyte 70μm in thickness, maximum powder density of 0.24 W/cm2 is achieved with 60 wt.% SSC-BCS cathode using humidified hydrogen as the fuel and ambient air as the oxidant. The polarization resistance is 0.21Ωcm2 at 700℃, which was about 25% of the total cell resistance.The low interfacial polarization resistance suggests that SSC-BCS composite cathodes are suitable cathode for H-SOFCs. and that the composition of SSC-BCS cathode has a great effect on its electrode performance due to its impact on TPBs length. Therefore, enlarging the length of TPBs by microstructure optimizations could help to improve the electrode performance.Chapter 4, ion impregnation technique is adopted to fabricate composite cathodes with nano microstructure, by which nano-sized Sm0.5Sr0.5CoO3-δ(SSC) particles are deposited onto the inner face of porous BaCe0.8Sm0.2O2.9 (BCS) backbone. The main achievements are summarized as follows:1) The electro-performance of the composite cathodes is investigated as function of fabricating conditions, such as the SSC-loading, the SSC firing temperatures and the backbone sintering temperatures. The lowest polarization resistance, about 0.21 Qcm2 at 600℃, is achieved with BCS backbone sintered at 1100℃, SSC layer fired at 800℃, and SSC loading of 55 wt.%, which is only 1/3 of that prepared by screen-printing. 2) Impedance spectra of the composite cathodes consisted of two depressed arcs with peak frequency of 1 kHz and 30 Hz, respectively, which might correspond to the reaction of proton and the dissociative adsorption and diffusion of oxygen, respectively.3) There is an additional arc peaking at 1Hz in the Nyquist plots of a single cell, which should correspond to the anode reactions.4) With anode, cathode and electrolyte about 500μm,100μm and 70μm in thickness, the simulated anode, cathode and bulk resistances of cells are 0.021,0.055 and 0.68Ωcm2 at 700℃, respectively, and the maximum power density is 307mWcm-2 at 700℃, about 30% improved compared with the cell with printed cathode.Chapter 5, Ag-BaCe0.8Sm0.2O2.9 (BCS) composite cathodes are fabricated by ion impregnation technique in this work to intensively reduce the polarization resistance. The performances of H-SOFCs with electron conductor as cathode are studied. The main achievements are summarized as follows:1) The polarization resistances are greatly affected by firing temperature of impregnated Ag particles. The minimum polarization resistance of symmetric cells reaches 0.11Ωcm2 at 600℃with Ag loading of 0.40mgcm-2 and tired at 400℃.2) With the same volume ratio of Ag and SSC (57Vol.%), the polarization resistance reaches 0.13Ωcm2 at 600℃with Ag based composite cathode, half of that with the SSC impregnated cathode.3) The simulated high and low frequency resistances are 0.08 and 0.16Ωcm2 for SSC impregnated cathodes, and 0.06 and 0.07 Qcm2 for Ag impregnated cathodes, respectively, suggesting that the reduction of low frequency resistances is the main reason for the decrease of polarization resistances in Ag impregnated cathode, in consistence with the high oxygen diffusion coefficient of Ag.4) With 0.46 mgcm-2 Ag impregnated cathode fired at 400℃, the maximum power densities of single cells is 283 mWcm-2 at 600℃with humidified hydrogen as the fuel and ambient air as the oxidant, about 17% improved compared with SSC impregnated cathode.Consider the long-term test of Ag-impregnated cathodes, Ag impregnated cathode is a promising cathode for fuel cells operating at temperature lower than 600℃.