Nitrogen containing carbon nanofibers as non-noble metal cathode catalysts in PEM and direct methanol fuel cells.

摘要

PEM and direct methanol fuel cells (DMFC) have great potential for use as alternative fuel energy conversion devices. Before this potential can be realized, however, performance improvements must be made and material costs need to be reduced. The limiting reaction in the PEMFCs and DMFCs is the oxygen reduction reaction (ORR), which occurs at the cathode. In an attempt to improve the reaction kinetics, substantial loadings of Pt catalysts are required on the cathode. This significantly increases the overall cost of the fuel cell. Also, in DMFCs, methanol crossover from the anode allows for competing reactions at the cathode catalyst to occur, reducing the power output of the fuel cell further. As the demand for fuel cells increase, the demand for Pt will far outpace the supply of Pt.;Replacements studied for Pt cathode catalysts include Pt alloys, other noble metals, chalcogenides and nitrogen-containing carbons. Nitrogen-containing carbons made from simple precursors can provide an economical replacement to Pt catalysts. Before this can be realized, improvements in the activity and selectivity of the nitrogen-containing carbons need to occur.;The work presented here involves the study of nitrogen-containing carbon nanostructures (CNx) as ORR catalysts for PEMFCs and DMFCs. Improving the ORR catalytic performance in both activity and selectivity for CN x catalysts, while gaining a better understanding of the catalyst materials and the way they are evaluated were the major driving forces behind this research.;CNx catalyst performance was studied by incorporating heteroatoms beyond nitrogen and surface functional groups into the catalyst. Boron and sulfur heteroatoms were studied along with oxygen functional groups. It was found that the methods to introduce boron into the nanostructure had a large impact on the ORR performance. Sulfur did not have an effect on the ORR performance, but was successfully used as a CNx growth promoter in the form of thiophene during acetonitrile pyrolysis. An increase in oxygen functional groups on the surface of CNx catalysts improved the ORR selectivity to water formation.;The role CNx catalyst nanostructure plays in ORR activity was studied using model nanofiber systems with both high levels of graphitic edge plane exposure and low levels of graphitic edge plane exposure. Experiments showed that un-doped graphitic edge planes were not the ORR active site. Incorporation of nitrogen into the graphitic edge planes significantly improved ORR activity compared to the nitrogen-free nanofibers.;The use of electrochemical half cell methods have been evaluated and reported here. Rotating Ring Disk Electrode (RRDE) testing is commonly used to measure the ORR activity and selectivity of a catalyst. The factors affecting catalyst selectivity reporting including catalyst loading and RRDE catalyst ink aging were studied.;In addition to these studies, the performance of CNx catalysts developed in the laboratory for use as cathode catalysts in DMFCs were evaluated. It was found that CNx catalysts are both methanol tolerant and inactive towards the methanol oxidation reaction, making them favorable potential DMFC catalysts.;Throughout the studies, materials developed and evaluated were characterized using classic heterogeneous catalysis techniques to gain a better understanding of the systems being analyzed. These techniques include, X-Ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Temperature Programmed Oxidation (TPO) and Desorption (TPD) studies and Thermogravimetric Analysis, among others.