Abstract
This thesis investigates primarily the oxygen reduction reaction (ORR) taking place on cathode catalysts applied for fuel cells. The study deals with the broader scope of hydrogen fuel cells and the chemistry involved. Two types of cathode catalysts are investigated. Platinum based catalysts are part of studies that deal both with important fuel cell parameters as well as surface modification to prevent poisoning. Non-precious metal catalysts (NPMCs), are investigated as part of studies that look into the behaviour of this more recent catalyst type in varied chemical environments.
Electrochemistry is often applied to investigate specific reactions at catalytic surfaces. We show that it is also a viable tool, when measuring other parameters of interest for fuel cell investigations. A series of measurements verify that electrochemical methods can be used to find reasonable values of oxygen solubility and diffusion in acidic media.
The acidic electrolytes (proton conductors) used in fuel cells have been the target of many studies. In spite of well-known poisoning effects, phosphoric acid is still considered an important electrolyte for modern high temperature fuel cells. Our investigations show that chemical modification of a platinum model catalyst to mitigate poisoning have implications that question the use of such an approach.
A central part of the thesis is devoted to the NPMC, which in the long term can become an alternative to the advanced but expensive platinum-based catalysts. Being based on cheaper materials, the NPMC can be applied in larger amounts, resulting in decreased activity targets. Contrary to platinum-based catalysts, which are 3D materials, we demonstrate that NPMCs can be regarded as 2D materials. This strategy involves a different view on the catalytic site/electrolyte interactions taking place during the ORR. In one study it is shown that the poisoning effects observed on platinum surfaces become more or less insignificant for NPMCs, and that it can even promote the ORR in some cases.
Based on recent findings in the literature, a study was made using acetic acid buffer as electrolyte. In line with the 2D surface study above, we find that the buffer media greatly enhance the ORR activity for the NPMC. The studies involving NPMCs are based on both electrochemical measurements and DFT calculations. The nature of NPMCs is intriguingly different than what is known for platinum and should be studied even closer in the future.
Electrochemistry is often applied to investigate specific reactions at catalytic surfaces. We show that it is also a viable tool, when measuring other parameters of interest for fuel cell investigations. A series of measurements verify that electrochemical methods can be used to find reasonable values of oxygen solubility and diffusion in acidic media.
The acidic electrolytes (proton conductors) used in fuel cells have been the target of many studies. In spite of well-known poisoning effects, phosphoric acid is still considered an important electrolyte for modern high temperature fuel cells. Our investigations show that chemical modification of a platinum model catalyst to mitigate poisoning have implications that question the use of such an approach.
A central part of the thesis is devoted to the NPMC, which in the long term can become an alternative to the advanced but expensive platinum-based catalysts. Being based on cheaper materials, the NPMC can be applied in larger amounts, resulting in decreased activity targets. Contrary to platinum-based catalysts, which are 3D materials, we demonstrate that NPMCs can be regarded as 2D materials. This strategy involves a different view on the catalytic site/electrolyte interactions taking place during the ORR. In one study it is shown that the poisoning effects observed on platinum surfaces become more or less insignificant for NPMCs, and that it can even promote the ORR in some cases.
Based on recent findings in the literature, a study was made using acetic acid buffer as electrolyte. In line with the 2D surface study above, we find that the buffer media greatly enhance the ORR activity for the NPMC. The studies involving NPMCs are based on both electrochemical measurements and DFT calculations. The nature of NPMCs is intriguingly different than what is known for platinum and should be studied even closer in the future.
| 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 |