Preparation and characterisation of Pt/C and Ni/C modified electrocatalysts for use towards the oxygen reduction reaction for proton exchange membrane fuel cells

Abstract

The aim of this thesis is to develop more active catalysts for the oxygen reduction reaction whilst decreasing the metal content to drive forward the emergence of the fuel cell technology on the market. Chapter 3 presents the preparation of Ni modified Pt/C catalysts (Ni(acac)2 and Ni(Cp)2) using the controlled surface modification technique. The resulting catalysts were heat treated at 200, 500, 750 and 900 °C and the catalysts were characterised by ICP-OES, TEM, EDX, CV, RDE, EXAFS and XPS. The catalysts exhibited up to 8-fold increase in specific activity and up to 9-fold increase in mass activity. The increase in activity was assigned to (a) the synergistic effect of Ni on Pt and (b) the degree of alloying which has two consequences: (a) decrease of the Pt d-band centre and (b) change of the arrangement of the Pt and Ni atoms at the surface of the particles. The decrease in the Pt d-band centre resulted in the lowering of the adsorption strength of the oxide species which in turn led to a lower Pt-O coverage. This was supported by the decrease of the reduction potential of the oxide reduction peak as the heat treatment temperature increased. In addition, as the heat treatment temperature increased, the Pt surface concentration increased due to the diffusion of the Ni atoms inwards and the diffusion of the Pt atoms towards the outside of the particle. This led to larger and more well-defined Pt crystal planes. The presence of more well-defined Pt crystal planes seemed to provide more suitable adsorption site for the dual-site adsorption of the oxygen, thus increasing the activity. Last but not least, the highest increase in catalytic activity was exhibited by the catalysts heat treated at 500 °C. This demonstrated the importance of the choice of the secondary metal and the importance of the arrangement of the atoms at the surface of the particles. Chapter 4 presents what is believed to be the first attempt to prepare Pt modified Ni/C catalysts (Pt(acac)2) using the controlled surface modification technique. The catalysts were characterised by TEM, CV and RDE. The deposition of the Pt precursor was shown to be incomplete; however, the catalysts still had a Pt content of ~ 4 wt%. Despite the low Pt content, the catalysts exhibited up to 8-fold increase in specific activity. The increase in activity was assigned to the synergistic effect between Ni and Pt which was shown by the decrease in the lattice parameter and the decrease of the overpotential of the oxide reduction peak. Chapter 5 offers a summary of the thesis as well as a list of the strategies employed to date to increase the catalytic activity of the cathode catalysts. It also includes some suggestion for future work including underpotential deposition (UPD), MEA testing and stability testing

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ORCID for A.E. Russell: orcid.org/0000-0002-8382-6443

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