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Title: Electrocatalytic and catalytic oxygen reduction utilising transition metal and heteroatom doped carbon materials
Author: Malko, Daniel
ISNI:       0000 0004 6496 1492
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The commercialisation of polymer electrolyte fuel cells (PEFCs) is partly delayed due to the use of expensive and scarce precious metal catalysts for the oxygen reduction reaction (ORR). Transition metal and nitrogen containing carbon materials (M-N/C) could potentially replace Pt. However, the activity and stability is still too low. This is due to a lack in the understanding of the active site, a difficulty in determining intrinsic catalyst parameters and the challenges posed by the higher catalyst loading in fuel cells. A new precursor was identified. It can be doped with different metal centres and readily forms self-supporting ORR active carbon catalysts upon pyrolysis. Physicochemical characterisations of the Fe-N/C material suggests atomic metal centres as active sites. A proton coupled electron transfer is presumably the rate determining step. The catalyst is exceptionally poison tolerant against a wide range of compounds that affect Pt based materials. It was found that nitrite and nitric oxide interact with the active site(s). Fundamental insight was gained and it seems that two different types of metal centred active sites are present within Fe-N/C catalysts. A methodology was developed to electrochemically count one type of those active sites by means of reductive nitrite stripping in a conventional rotating disk electrode (RDE) setup. It is possible to estimate the turnover frequency and active site density. The material also catalyses the epoxidation of alkenes at room temperature and ambient pressure, suggesting a similar working principle as transition metal macrocycles. A study of M-N/C catalysts in operating PEFCs has been conducted. The catalyst layer was investigated by means of impedance spectroscopy. The peculiar 45 degree feature and its deviation in the impedance spectrum can be used to determine the optimal ionomer content in the catalyst layer and therefore speed up the investigation in single cells.
Supervisor: Kucernak, Anthony Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral