SYNTHESIS AND CHARACTERIZATION OF NEW ELECTROCATALYTIC MATERIALS TO IMPROVE THE PERFORMANCE, OPERABILITY AND LIFE-TIME OF PEMFCS
Low-temperature fuel cells have been becoming important as an environment- friendly technology and are acquiring considerable interest in both research and commercial purposes. However, several limitations such as lifetime, reliability and cost are hindering the commercialization of these systems. The enhancement of catalytic activity and durability of catalysts represents one of the most important issues in fuel cell technology. To increase the performace and decrease the load of the catalysts, materials with high surface area and pore volume, commonly carbons, are widely used as catalyst supports in low-temperature fuel cells. The support materials can enhance the dispersion of metal particles. In addition, an interaction effect between the support material and the metal exists. It could change the activity of catalytic sites on the metal surface and modify the number of active sites by both electronic effect and geometric effect. The supports such as tungsten trioxide (WO3) may bring its own (electro)chemical function for oxygen reduction reaction (ORR), methanol or CO oxidation. What’s more, the support materials can also influence the durability of the catalysts. The development of promising and new catalyst surports, therefore, is significantly vital for improving the electrochemical activities and stabilities of the catalysts. The present work focuses on the investigation of new catalyst supports in order to improve the performace of the catalysts. In addition, the durability issue in freezing condition is also studied. In the introductory section, two chapters are included. In chapter 1, the history and different fuel cell types are reviewed. The basic principles of the fuel cell reactions, both the thermodynamic considerations as well as the kinetic aspects, are discussed. Chapter 2 focuses on proton exchange membrane fuel cells (PEMFCs), including the oxygen reduction and hydrogen oxidation reactions, the various materials and components in the PEMFCs, the durability issues of the material and PEMFCs, the applications of PEMFCs and the challenges in the further development. Thereinto, the catalyst supports, especially the carbon supports, and the durability of carbon supported catalysts are discussed in details. In the experimental section, the methods of synthesizing the materials are summarized, including mesoporous silica, mesoporous carbon as well as carbon supported catalysts. The synthesis details of the samples prepared and the experimental methods employed in the work are also described. The structural and morphological characterization tools include: scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), Nitrogen physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The electrochemical methods are: cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements. The inductively coupled plasma (ICP) used for quantitative elemental analysis is also briefly described. In addition, the preparation of membrane electrode assembly (MEA) and the operation of a single fuel cell are also presented. The third section presents the results and discussion of the work. This thesis deals with three kinds of mesoporous carbons as catalyst supports. One kind is ordered mesoporous carbons (OMC) which is discussed in chapter 5. Another kind is a disordered mesoporous carbon, namely hollow core mesoporous shell carbon (HCMSC), which is presented in chapter 6. The chapter 7 describes the WO3 modified ordered mesoporous carbon (OMC: CMK-3) as catalyst supports for oxygen reduction and methanol oxidation reactions. In addition, the experimental studies on Nafion® 112 single PEMFCs exposed to freezing conditions are specified in chapter 8. In chapter 5, three ordered mesoporous carbons supported Pt catalysts (10 wt. % Pt/ OMCs) are synthesized and characterized. The electrochemical performances and durability properties of the Pt/ OMCs are measured in acidic solution at room temperature by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The catalytic activities of the Pt/ OMCs toward oxygen reduction reaction are also tested at the cathode of a complete polymer electrolyte fuel cell setup. Subsequently, the polarization curves obtained in the fuel cell are analyzed by means of a simplified model and important parameters about the catalytic layer are obtained. A comparative discussion of these results indicated the electrochemical performances of the Pt/ OMCs strongly depend on the pore structure of the OMCs. Chapter 6 deals with a mesoporous carbon synthesized with a novel procedure. The Pt catalyst supported on this carbon (20 wt. % Pt/ HCMSC) is physically and electrochemically characterized. The CV method is employed to evaluate the electrochemical active surface area (EAS) and durability of the Pt/ HCMSC catalyst. In CV and LSV tests, the electrocatalytic activities of Pt/ HCMSC toward methanol oxidation and oxygen reduction reaction are tested and compared with commercial 20 wt. % Pt/ C catalyst. Chapter 7 deals with carbon-WO3 composite supported Pt catalysts. WO3 was deposited on ordered mesoporous carbon (OMC: CMK-3) by impregnation of phosphotungstic acid (PWA) and subsequent annealed in N2 atmosphere. The WO3 modified ordered mesoporous carbons (MC-WO3) retained the ordered mesostructure, high BET specific surface area and large pore volume. The MC-WO3 supported Pt catalysts (Pt/ MC-WO3) were prepared by wet impregnation. According to the results of electrochemical tests of CV and LSV, the catalytic activity of the supported catalysts was dependent on the nature of the supporting materials, including the structure and particle size of WO3 particles. Chapter 8 presents the phenomena of PEMFCs when exposed to sub-freezing conditions. In particular, the analysis is focused on the effects of suitable dehydration processes (purging procedures) on MEAs prepared with Nafion® 112, before exposing them to various freezing/ thawing (F/ T) cycles: both electrochemical performance and mechanical stresses are considered, in order to verify the effectiveness of the dehydration processes in avoiding cracks and maintaining thus a satisfactory FC performance (limited decay of the polarization curve). Purging procedures with N2, air and H2 are considered to investigate the effect of the purging gases to the MEAs performance.
Settore ING-IND/22 - Scienza e Tecnologia dei Materiali
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