Carbon nanofiber electrodes for PEM fuel cells

Abstract

Increasing energy demand in the world and the environmental concerns due to air pollution and the greenhouse effect of fossil fuel resources stimulate the research into renewable alternative energy sources. Among these alternatives, hydrogen has a life changing opportunity for the human being for the future. The chemical energy stored in hydrogen can be converted into electricity via various types of fuel cells. The proton exchange membrane fuel cell is the most promising type providing high power densities at operating temperatures typically less than 100 °C. The necessity of increasing the platinum utilization in PEMFCs, due to its cost and availability, requires an optimization of the catalyst support and the gas diffusion layers. In many long term fuel cell tests, a decrease in water removal capacity leads to a gradual decrease of the PEMFC performance. The use of more graphitic and highly oriented carbon as an alternative material for use in the gas diffusion media combines a high electronic conductivity with an acceptable surface area. One approach, introduced in this thesis, is the direct growth of carbon nanofibers (CNFs) on a gas diffusion layer providing a strong network with a high surface area. Two methods are developed for the direct growth of CNFs on a carbon based gas diffusion layer: CNF growth via nickel complex particles (chapter 2) and via homogeneous deposition precipitation of nickel (chapter 3). The production of CNF grown carbon paper via the nickel complex particles was rather fast compared to the homogenous deposition precipitation of nickel, since deposition of nickel complex particles on a carbon paper took less than one hour whereas it took more than two days for the homogeneous deposition precipitation of nickel. The time of the deposition of nickel hydroxide in the deposition precipitation depends upon the decomposition rate of urea at the deposition temperature. Since the deposition was carried out near the boiling point of water, the decomposition rate of urea was nearly constant. It was attempted to increase the total amount of nickel by working with highly concentrated nickel and urea solutions. However, this caused a non-uniform deposition of nickel hydroxide which resulted in the detachment of CNFs during ultrasonic treatments. The loading of the nickel catalyst was controlled precisely during homogeneous deposition precipitation of nickel on carbon paper in this slow deposition process. In addition, the growth of nickel hydroxide layers covered the entire surface of the carbon paper and thereby closed the pores bigger than 1 µm. In our experiments, it was observed that open pores on the CNF grown carbon paper surface caused instabilities in operation due to water accumulation at these locations of the cathode compartment of the PEMFCs. Considering the two main factors described here, homogeneous deposition precipitation of nickel was utilized to obtain a highly controlled nickel deposition and thereby a highly controlled CNF loading on the carbon papers. The decoration of carbon paper with CNFs was investigated as a water management layer (chapters 4 and 5) and as a direct catalyst layer for platinum (chapter 6) in order to give more accessibility for the gas transport. The CNF grown carbon paper provided effective gas diffusion to the catalyst layer when they were used as a water management layer, especially under high gas flows. It suffered from performance losses at the mid-current density region (400-800 mA.cm-2), which can be improved by loading of a hydrophobic polymer. The CNF grown carbon paper can be a good alternative as a catalyst support, however, further optimization of the Nafion-platinum contact is required to get the benefits of using CNF grown carbon paper as a direct catalyst layer.