Year

2010

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

The main goal of this study was to design and develop novel electrocatalysts that exhibit high catalytic activity whilst maintaining 100 % selectivity for a desired product. Such catalytic materials will improve the performance of fuel cells by increasing cell efficiency and by removing by-products, thus eliminating waste in fuel cells electrocatalytic reactions.

A cobalt porphyrin electrocatalyst embedded in a conducting polypyrrole matrix was prepared using either a vapour phase polymerization or in-situ chemical polymerization method. The composite material showed enhanced catalytic activity and stability toward the electrochemical reduction of oxygen when compared with the original cobalt complex. The performance of this material was investigated using various electrochemical methods including rotating disk electrode (RDE), rotating disk electrode (RRDE) measurements and (half) single-fuel cell tests. In addition, the effect of the concentration of both oxidant and cobalt porphyrin on the structure and electrocatalytic activity of the composite electrode was investigated.

Carbon materials such as carbon nanotubes supported platinum particles are emerging as exciting and very important catalytic materials for use in fuel cells. In the present study, a modified microwave-polyol process has been developed to synthesize platinum nanoparticles on single-walled carbon nanotubes (SWNTs) and carbon black. The effect of pH, irradiation time and concentration of Pt salt solutions on controlling the size of nanoparticles, as well as the electrocatalytic activity of the resulted catalysts for oxygen reduction, has been fully investigated. In order to increase the platinum loading on single-walled carbon nanotubes, a sonochemical method was employed to active the side walls of SWNT in order to introduce functionalities such as carboxyl (-COOH), hydroxyl (-OH) and carbonyl (>C=0) groups. Such species are demonstrated to be suitable for anchoring platinum metal ions to the SWNT support. The electrocatalytic advantage of the platinum decorated SWNT as a catalyst for fuel cell catalysis over conventional carbon black substrate was confirmed using electrochemical characterizations and fuel cell evaluations.

A pseudo Pt-Pd core-shell bimetallic catalyst for proton exchange membrane electrode fuel cells was prepared. Palladium (Pd) nanoparticles supported on multiwalled carbon nanotubes (MWNTs) scaffold was used as template with the subsequent surface Pd atoms replacement with platinum being achieved by a microwave assisted galvanic method. The electrocatalysts were characterized using thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), cyclic voltammetry (CV), rotating disk electrode (RDE), rotating ring disk electrode and fuel cell techniques. The best catalyst for oxygen reduction and hence fuel cell operation was shown to be that with a Pt/ (Pt + Pd) atomic ratio of 30/100 which was confirmed by electrochemical characterizations and fuel cell evaluations.

In addition, a high performance membrane electrode assembly (MEA) comprised of platinum nanoparticles loaded onto an aligned carbon nanotube (ACNT) array was fabricated using a hot transfer and pressing method. This nanostructured MEA was characterized using TGA, XRD, SEM, in-situ CV, in-situ electrochemical impedance spectroscopy (EIS) and fuel cell measurements. The results indicated that the nano-Pt-loaded ACNT/Nafion/ACNT MEA had a higher Pt utilization efficiency than the conventional Pt/C MEA.

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