Abstract
This thesis systematically investigates the influence of the physical properties of high surface area Pt/C catalysts on their oxygen reduction reaction (ORR) activity. It is crucial to understand the relation between the properties such as particle size or metal loading and the ORR activity for an efficient utilization of platinum group metals (PGMs) as catalysts in polymer electrolyte membrane fuel cells (PEMFCs). In previous studies, the so-called particle size effect and particle proximity effect on the ORR activity have been reported separately. However, even for the same commercial catalysts discrepancies have been observed in the reported experimental results for these effects, probably due to insufficient activity determination procedures. Moreover, in none of the earlier studies it was achieved to vary only a single property, e.g. the platinum particle size, without changing others. This was mainly related to limitations in catalyst preparation. In this thesis, several crucial advancements have been achieved. The thin filmrotating disk electrode (TF-RDE) technique has been refined for the ORR activity determination to enable more reproducible results. Especially, it is demonstrated that the pH of catalyst inks, which has not been discussed in previous TF-RDE studies, is an important parameter that needs to be carefully controlled to determine the intrinsic ORR activity of high surface area catalysts. In addition, the “toolbox” approach is applied for the Pt/C catalyst preparation and developed further. A crucial yet missing piece of the “toolbox” approach was a reliable size control. Control over this parameter has been achieved by carefully varying the NaOH/Pt ratio in the precursor solutions. The improved synthesis method and advanced measurement techniques presented in this thesis are used to investigate for the first time the particle size effect and the particle proximity effect independently.
It is demonstrated that Pt nanoparticles exhibit more bulk-like properties with increasing particle size and/or decreasing inter-particle distance. Interestingly, the proximity effect becomes more significant at small Pt particle size than at larger ones. Therefore, the highest ORR mass activity is achieved by small Pt particles at high Pt loading. The last part of this thesis is dedicated to the development of a novel half-cell platform, the gas diffusion electrode (GDE) cell, for simple and straightforward catalyst testing at high mass transport conditions. It is demonstrated that performance data obtained using the GDE cell can be directly compared to membrane electrode assembly (MEA) tests. Therefore, the developed platform bridges the gap between conventional TF-RDE tests and MEA tests. It therefore enables systematic studies of Pt/C catalysts under more realistic PEMFC operating conditions (i.e. at elevated temperature with high mass transport) as compared to the RDE technique. It is thus expected that the GDE methodology will be widely adapted in future work.
It is demonstrated that Pt nanoparticles exhibit more bulk-like properties with increasing particle size and/or decreasing inter-particle distance. Interestingly, the proximity effect becomes more significant at small Pt particle size than at larger ones. Therefore, the highest ORR mass activity is achieved by small Pt particles at high Pt loading. The last part of this thesis is dedicated to the development of a novel half-cell platform, the gas diffusion electrode (GDE) cell, for simple and straightforward catalyst testing at high mass transport conditions. It is demonstrated that performance data obtained using the GDE cell can be directly compared to membrane electrode assembly (MEA) tests. Therefore, the developed platform bridges the gap between conventional TF-RDE tests and MEA tests. It therefore enables systematic studies of Pt/C catalysts under more realistic PEMFC operating conditions (i.e. at elevated temperature with high mass transport) as compared to the RDE technique. It is thus expected that the GDE methodology will be widely adapted in future work.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2018 |