Preparation And Electrochemical Characterization Of Electrolytes For Intermediate-temperature Solid Fuel Cells

The widespread interest in fuel cells stems from their high efficient in energy conversion. Although various types of high temperature (above 400°C) and low temperature (below 150°C) fuel cells have been developed, there still exist lots of material issues to be solved for commercialization. Intermediate temperature solid state fuel cells, which are operated in the range of 150-400°C, are promising to solve these problems and offer many advantages such as suppression of CO poisoning of Pt catalyst, higher efficiency of energy conversion than that of polymer electrolyte membrane fuel cells, utilization of metals and plastics, and lower materials degradation than that of solid oxide fuel cells. Therefore, it can not overstate the importance on developing intermediate temperature fuel cells. One of the key points to develop intermediate temperature fuel cells is to develop electrolytes which can be used in the temperature range of 150-400°CThis dissertation aims to study electrochemical characterizations and proton conduction mechanism of ammonium polyphosphate composite (NH₄PO₃-SiO₂) and pyrophosphate Tin (SnP₂O₇), which is one of the proton conductors that exhibit high conductivity and can be used as potential electrolytes in intermediate temperature fuel cells.In chapter 1 of this dissertation, the theory and significance of intermediate temperature fuel cells was introduced at first. The latest progress in the study of intermediate temperature electrolyte materials and electrode materials was also reviewed. The development in electrolyte materials for intermediate temperature fuel cells was especially highlighted.The chapter 2 gives detail information about sol-gel technique and solid state reaction method, characterization technique such as Ac impedance, X-ray diffraction (XRD), scanning electron microscope (SEM) technique etc. In chapter 3, Proton-conductive composite electrolytes consisting of xNH₄PO₃-SiO₂(x=1, 2, 4) were synthesized by a sol-gel method. X-ray diffraction investigation showed that NH4PO3 was chemically stable in the process of sol-gel preparation. The conductivity of the resulted composites was measured with impedance spectroscopy. The conductive behavior was improved by increasing the molar ratio of NH₄PO₃. Meanwhile the activation energy for conductivity decreased with NH₄PO₃ content. This indicates that NH₄PO₃ is responsible for the high conductivity and SiO₂ serves as a supporting matrix, although the conductivity was slightly affected by the size of SiO₂ particles. The proton conductivity was dramatically enhanced by increasing the water content, indicating that the effect of water is significant.In chapter 4, SnP₂O₇ proton conductors of xP/Sn(x=2.0, 2.2, 2.4, 2.6, 2.8, 3.0) was synthesized and characterized as a potential electrolyte for intermediate temperature that operated range of 120°C-260°C. X-ray diffraction investigation (XRD) shows that SnP₂O₇ was cubic structure and Thermal gravimetric analysis (TG) shows that SnP₂O₇ is stable when it was operated at intermediate temperature. The conductivity, measured using impedance spectroscopy, was improved with increasing the molar remains of HPO3 in SnP₂O₇, This indicates that HPO₃ is responsible for the high conductivity and SnP₂O₇ serves as a supporting matrix. For 3.0P/Sn, the highest proton conductivity is 5.1×10^-2 Scm^-1 under dry atmosphere and 6.6×10^-2 Scm^-1 under wet atmosphere at 200°C, respectively.Chapter 5 summarizes conclusions of this work and suggests some issues to be researched in the future.