Developing high site density non-precious M-N/C catalysts for oxygen reduction reaction in proton exchange membrane fuel cells
File(s)
Author(s)
Gong, Mengjun
Type
Thesis
Abstract
Proton exchange membrane fuel cells (PEMFCs) are zero-emission power generation systems
that have the potential to address the environmental issues associated with combustion engines,
particularly in vehicles. However, the high cost of precious metal catalysts has hindered the
commercialisation of PEMFCs. Non-precious metal-based single atom catalysts (M-N/Cs)
offer a promising solution to replace precious cathode catalysts, which primarily consist of
platinum group metals. Nevertheless, there are several challenges that need to be addressed to
enhance the activity and stability of M-N/Cs. This thesis mainly focuses on understanding the
active sites in M-N/Cs, synthesising M-N/Cs and improving the site density of M-N/Cs to
achieve high-performance oxygen reduction reaction (ORR) in PEMFCs.
Seven different transition metal based M-N/Cs were synthesised using commercial zeolitic
imidazolate framework-8 (ZIF-8) as a precursor. Physicochemical characterisations confirmed
the formation of single atom sites. The ORR performance under different pH conditions
indicated the presence of multiple types of active sites, and nitrite striping was employed to
assess the electrochemical accessible sites and the activity of different sites.
Considering that the metal-based active sites in Fe-N/C demonstrated the highest activity,
efforts were made to improve the site density of Fe-N/C. Rotating ring-disk electrode (RRDE)
and single cell PEMFC were performed to evaluate the ORR activity. Significantly enhancing
the metal loading (around 7 wt%) and increasing the number of electrochemical accessible sites
Page | 5
(4.67 x 1019 site g-1
) greatly improved the ORR activity of Fe-N/C. Physicochemical
characterisations not only confirmed the production of single atomic sites, but also indicated
the role of second pyrolysis in the synthesis process.
Finally, Fe-N/Cs exhibited excellent catalytic performance on the Mukaiyama epoxidation and
Baeyer-Villiger oxidation at ambient temperature and pressure. Kinetic and mechanistic studies
provided evidence for the in-situ generation of peroxy acid and acylperoxy radicals.
Furthermore, these organic oxidation processes demonstrated the presence of two distinct
active sites within Fe-N/C.
that have the potential to address the environmental issues associated with combustion engines,
particularly in vehicles. However, the high cost of precious metal catalysts has hindered the
commercialisation of PEMFCs. Non-precious metal-based single atom catalysts (M-N/Cs)
offer a promising solution to replace precious cathode catalysts, which primarily consist of
platinum group metals. Nevertheless, there are several challenges that need to be addressed to
enhance the activity and stability of M-N/Cs. This thesis mainly focuses on understanding the
active sites in M-N/Cs, synthesising M-N/Cs and improving the site density of M-N/Cs to
achieve high-performance oxygen reduction reaction (ORR) in PEMFCs.
Seven different transition metal based M-N/Cs were synthesised using commercial zeolitic
imidazolate framework-8 (ZIF-8) as a precursor. Physicochemical characterisations confirmed
the formation of single atom sites. The ORR performance under different pH conditions
indicated the presence of multiple types of active sites, and nitrite striping was employed to
assess the electrochemical accessible sites and the activity of different sites.
Considering that the metal-based active sites in Fe-N/C demonstrated the highest activity,
efforts were made to improve the site density of Fe-N/C. Rotating ring-disk electrode (RRDE)
and single cell PEMFC were performed to evaluate the ORR activity. Significantly enhancing
the metal loading (around 7 wt%) and increasing the number of electrochemical accessible sites
Page | 5
(4.67 x 1019 site g-1
) greatly improved the ORR activity of Fe-N/C. Physicochemical
characterisations not only confirmed the production of single atomic sites, but also indicated
the role of second pyrolysis in the synthesis process.
Finally, Fe-N/Cs exhibited excellent catalytic performance on the Mukaiyama epoxidation and
Baeyer-Villiger oxidation at ambient temperature and pressure. Kinetic and mechanistic studies
provided evidence for the in-situ generation of peroxy acid and acylperoxy radicals.
Furthermore, these organic oxidation processes demonstrated the presence of two distinct
active sites within Fe-N/C.
Version
Open Access
Date Issued
2023-08-03
Date Awarded
01/12/2023
License URL
Advisor
Kucernak, Anthony R.J.
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)