Metal Phosphates as Proton Conducting Materials for Intermediate Temperature Fuel Cell and Electrolyser Applications

Abstract

The present thesis presents the results achieved during my ph.d. project on a subject of intermediate temperature proton conducting metal phosphates as electrolyte materials for fuel cells and electrolysers. Fuel cells and electrolysers are electrochemical devices with high energy conversion efficiency and have been proposed as the future energy technology in association with renewable power sources. The currently available technologies are either operating at lower or higher temperatures. To achieve an intermediate temperature operation between 200 and 400 °C, a key material is the electrolyte with a proton conductivity of above 10-2S cm-1. Chapter 1 of the thesis is an introduction to basics of fuel cell and electrolyser technologies as well as proton conducting materials. Extended discussion on the proton conducting materials, a particularly phosphates is made in Chapter 2. Three major types of phosphates were systematically reviewed including solid acids or alkali hydrogen phosphates, pyrophosphates, and rare earth metal phosphates. Demonstration of the fuel cell technology based on solid acid proton conductor CsH2PO4 has inspired the active research in the area. Based on the literature survey, the thesis work is defined and outlined. Chapter 3 describes in details the theoretical background of techniques used in the present study. A significant part of the present study involves conductivity measurements with electrochemical impedance technique, which are discussed in Chapter 4. The research work starts with synthesis and investigation of three rare earth metal phosphate hydrates, which is first presented in Chapter 5. Structural and surface water as well as its stability has been investigated using thermogravimetric and differential thermal analyses combined with structural modeling calculations. Conductivity results of the phosphates are presented. The rare earth metal phosphates are further explored by preparing composites with cesium dihydrogen phosphate. The properties of the composites are characterized using SEM/EDX, XRD, TGA/DTA and conductivity measurements in slightly humidified atmosphere. The results are presented in Chapter 6. In Chapter 7 the structure and conductivity mechanism of indium doped tin pyrophosphates are reported. For this purpose three synthetic techniques are employed to prepare a variety of the pyrophosphates with or without post treatments with phosphoric acid. The conductivity and its stability are studied and correlated with the phosphate morphologies. The additional solid state NMR studies have been performed in collaboration with Southern Denmark University (SDU). Chapter 8 presents the result obtained for a novel proton conductor based on cerium ultraphosphate. The ultraphosphate is found to be in form of the orthorhombic phase, whose stability is confirmed by heat treatments combined with XRD measurements. Initial measurements of the conductivity and its stability for the cerium ultraphosphate are also presented. Chapter 9 summarizes conclusions of the thesis work. Further information concerning experimental measurements is given in Appendixes.