Development of palladium nanoparticle-based catalysts for next-generation direct alcohol fuel cell systems

Fossil fuels are in limited supply and are the predominant contributor of CO2 emissions leading to climate change. Direct alcohol fuel cells (DAFC’s) are a clean and renewable source of energy making them an ideal candidate as a replacement for fossil fuels. Alcohol is an attractive fuel due to being a stable liquid at room temperature, making it easier to store and transport. Extensive research has been performed on methanol which has demonstrated it can be successfully used to power small devices. Recently, glycerol has also shown potential to be used as a fuel, which like methanol can be made via a renewable production process and has a high energy output.

A key challenge to overcome in this technology is to find an efficient and cost-effective catalyst to oxidize these alcohols at low temperatures. Currently platinum is the main catalyst used which increases the cost of these cells. Palladium-based catalysts have been proposed over platinum as a promising and cheaper alternative for oxidizing alcohols under basic conditions. However, palladium alone does not perform well enough to warrant its replacement of platinum and so a variety of cocatalysts to work along with the palladium are under investigation. The aim is to gain a higher activity and long-term stability for DAFC’s under basic conditions.

In this thesis palladium nanoparticle’s electrocatalytic activity towards alcohol electrooxidation in standard fuel cell conditions was characterized. Palladium nanoparticles are ideal compared to bulk palladium because the distribution of the particles means more active sites are available for the reaction to take place. The kinetics of these alcohol oxidation reactions were investigated by calculating the activation energy from Arrhenius plots. These were created using data obtained from cyclic voltammetry at various temperatures. Chronoamperometry analysis was used at each temperature to determine long term stability of the various bimetallic catalysts. All data was collected in a temperature controlled; custom made 3-electrode set up. In the comparison of the various electrooxidation reactions, the peak current density values (J) of the various alcohols at 60oC were from highest to lowest: Jethanol> Jethylene glycol> J¬glycerol> Jbutanol> Jmethanol The activations energies were shown to be from highest to lowest: Eamethanol > Eaethanol > Eabutanol > Eaethylene glycol > Eaglycerol.

It was decided from these results that methanol and glycerol would be the alcohols further investigated. A series of palladium nanoparticle based bimetallic electrocatalysts were then developed using copper, antimony and niobium supported on glassy carbon to improve the activity and stability of these alcohol reactions. The cocatalysts were deposited onto the palladium via a titration of their solutions.

The extensive data collected demonstrated that the best coverage of copper as a cocatalyst was Cu=0.37 for methanol oxidation and Cu=0.32 for glycerol oxidation. In the case of antimony Sb=0.15 was shown to be the best coverage for methanol oxidation and Sb=0.30 for glycerol oxidation. For the niobium study, it was shown that the best coverage for methanol oxidation was Nb=0.51 and Nb=0.54 for glycerol oxidation. Niobium presented an opportunity for a particularly interesting investigation because only limited research into its use had been carried out.

All cocatalysts studied demonstrated an improvement in catalytic activity and stability but it was found that niobium showed the best current density and activation energy. The development of palladium nanoparticle based bimetallic catalysts has shown to significantly enhance the electrooxidation of both methanol and glycerol and demonstrated their potential for use in direct alcohol fuel cells.