Alternative cathode catalysts for PEM fuel cells

Proton exchange membrane fuel cells, PEMFC’s, offer a clean, flexible mode of energy generation, though efficiency improvements are required before they can become commercially viable. While platinum is currently the most commonly used cathode catalyst, a large overpotential exists for the oxygen reduction reaction, ORR, limiting the effective potential of a fuel cell to approximately 0.9 V. Efforts have been made within the literature to develop a low cost, efficient and durable alternative cathode catalyst for use within a PEMFC though at present no viable alternative has been found. However, M-Nx/C active sites, consisting of a central metal atom co-ordinated to two or four nitrogen atoms embedded within a carbon support, show great promise as possible replacements to platinum. While advances have been made in the identification of possible active sites, no study yet exists that examines the influence of each active site component on the overall activity. In this thesis the influence of each component is examined, and in doing so a highly active site is predicted. The efficacy of the model is first proven by a simultaneous computational and experimental study of the activity of both platinum and Fe/Coporphyrins, which are commonly used as precursors in the development of highly active M-Nx/C catalysts. 16 active sites are modelled using graphene and amorphous carbonlike ligands embedded with two or four nitrogen atoms and co-ordinated to either a cobalt or iron centre. The activity of each active site towards the ORR is assessed by the calculation of redox potentials, and by modelling 20 different elementary reactions which collectively form a comprehensive reduction mechanism. By direct comparison between each active site, utilising natural population, bond orbital and localised molecular orbital analysis along with electrostatic potential maps, the influence of each constituent part is quantified. Finally, it is predicted that active sites consisting of a disrupted graphene ligand, embedded with four nitrogen atoms coordinated to either an iron or cobalt centre, would demonstrate the highest activity towards the ORR, and that such an active site is responsible for the activities reported within the literature.