Nanostructured graphene electrodes for energy applications

Thesis

Polymer electrolyte (or proton exchange) membrane fuel cells (PEMFCs) are promising for green energy applications, particularly for automobiles. However, the cost and durability issues surrounding the platinum/carbon (Pt/C) electrode materials are barriers to their widespread use. The development of electrodes which have better utilisation of Pt crystals, and which retain their activity for longer periods of time, are of great interest to the fuel cell community. Graphene is a promising candidate for catalyst support applications because of its advantageous properties. For this reason the production of graphene and graphene hybrid catalyst supports, and their interactions with Pt particles, are active areas of research. This project involved the synthesis of 3D graphene and graphene hybrid materials, their integration with Pt nanocrystals, and investigations of their electrochemical stability.

Firstly, 3D multilayer graphene foam was synthesised using a chemical vapour deposition (CVD) method. This was then used as a substrate in order to investigate the role of oxygen-plasma induced defects on the graphene surface on the synthesis of Pt nanocrystals formed through thermal annealing. It was found that the induction of defects could increase the nucleation density, and decrease and narrow the size distribution of Pt nanocrystals. In addition, results suggest that Pt can prevent certain types of defects from healing during heat treatment.

A novel method was then explored to produce 3D graphene and 3D graphene/carbon nanotube (CNT) powders using the thermal annealing of nickel acetate. The synthesis can be varied to produce a variety of materials with different crystallinities, and it was found that rapid heating was a key factor for the formation of the CNTs. These materials were explored as supports for thermally produced Pt nanocrystals and compared to commercially available carbon black, and it was found that good dispersion could not be achieved on the supports with the poorest crystallinity. Some of the materials produced performed similarly to HiSPEC 3000, a commercially available catalyst, in cyclic voltammetry (CV) experiments.

Within the past few years, a popular strategy to increase the durability of Pt/C catalysts has been to cover the Pt crystals with a protective layer. A CVD approach was used to modify a commercially available Pt/C catalyst with graphene layers. These were studied using HRTEM, and their electrochemically active surface areas (ECSAs) and durabilities were examined using CV experiments. Only a detrimental effect of the graphene layers was found in this case, and the reasons for why this might be the case are discussed.

Authors: Samuels, T

• DOI: 10.5287/ora-2nebnbbkw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford