Engineering the cathode catalyst layer architecture for fuel cells
File(s)
Author(s)
Kellner, Simon
Type
Thesis
Abstract
Approaches to improve the performance and durability of fuel cells range from system level engineering down to the atomic level engineering of the catalyst’s active sites. In the length scales, catalyst design focuses on the sub-nanometer and nanometer range, whereas the integration of the catalyst into the catalyst layer concerns agglomerates within length scales between 50 nm and 10 μm. This thesis focuses on the interface between catalyst design and cathode catalyst layer structure of platinum-group-metal (PGM) free catalysts. The intrinsic lower volumetric activity of PGM-free catalysts necessitates increased loading of the cathode. The structure of thick conventional PGM-free catalyst layers lacks a defined order of pores, ionomer paths and catalyst agglomerates, and therefore faces the risk of mass-transport limitation at high current densities. In the thesis, two freestanding carbon film supports with precisely defined pores within the length scale between 5 and 500 nm are proposed with the goal to facilitate mass transport and to increase the active site utilization. The first concept is based on vertically aligned carbon tubes, which are grown by chemical vapor deposition of a nitrogen-rich organic precursor 2-methylimidazole on anodic aluminium oxide (AAO). The second concept relies on polystyrene (PS) nanospheres molding the carbon precursors, mesophase pitch and polyvinyl alcohol, to create a conductive inverse opal macroporous carbon film support. The performances of the cathode film architectures with anchored Fe-Nx sites are characterized at high current densities in a half-cell gas-diffusion-electrode (GDE) setup. Furthermore, three commercial alkaline exchange ionomers are combined with a commercial PGM-free catalyst to establish a benchmark alkaline ionomer. The investigation also evaluated the performance of commercial alkaline exchange membranes. In addition, an oxygen limiting current measurement protocol is introduced to characterize the mass transport of cathode catalyst layers in the GDE half-cell.
Version
Open Access
Date Issued
2024-04-15
Date Awarded
01/07/2024
License URL
Advisor
Titirici, Magda
Stephens, Ifan
Heutz, Sandrine
Sponsor
Imperial College London
Publisher Department
Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)