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.