Evaluation of novel fuel cell catalysts with ultra-low platinum loading as well as platinum-free catalysts

Kurzfassung

Electrochemical active and nonaging catalysts are an essential component for fuel cells such as the proton exchange membrane fuel cell (PEMFC) which represent a promising, alternative energy technology for stationary and mobile applications. The objective of this master thesis is to investigate and evaluate the electrochemical activity and stability of newly developed catalysts for the oxygen reduction reaction (ORR) with intent to find a potential alternative to the commonly used catalyst components platinum on carbon. At first, the growing importance and the capability of electrochemical energy converter is pointed out. Afterwards, fuel cell systems and the functionality of hydrogen-oxygen-fuel cells are shown. Then, the function of catalysts in fuel cells is elucidated and methods are presented to characterize these catalysts. The electrochemical characterization has been realized using the widely-used rotating disc electrode (RDE) method. This enables the comparison of the measuring systems and released results. According to this, the experimental procedure and the approach to the determination of the kinetic parameters are specified. Carbon supported Platinum shows a high activity and is usually used as catalyst in fuel cells at present. For this reason, it serves as a reference for the evaluation of the measuring results of three platinum-based and five platinum-free catalysts. Finally, the measuring results of the reviewed catalysts are presented. Possible explanatory statements for the determined activity and the long-term behavior are given and further approaches to the investigation of several catalysts are proposed. On the basis of the evaluation of newly developed catalysts regarding to its electrochemical performance the chance of utilization in fuel cells can be reviewed and advantages respectively disadvantages over the commonly used catalysts can be rated. Thereby, continuative arrangements for investigations and improvements of the catalysts can be commenced if necessary.   Two out of three examined catalysts with ultra-low platinum loading showed a good capability to be used in fuel cells. One catalyst (Pt/ATO) offered an activity that was almost similar to Pt/C and furthermore a very good start-stop-behavior. The other catalyst (PtCoMoON/C) showed a higher efficiency compared to Pt/C and exhibited obvious improvements regarding to stability. To achieve further improvements physical analyses are necessary to study the chemical structure and to understand the chemical and physical properties in detail. In favor of this, samples of the material before and after the long term tests have been retained for analyzing structural changes. All of the platinum-free catalysts showed an electrochemical activity for the ORR. However, the activity of four out of five catalysts is not sufficient for an application in fuel cells. The catalyst NPC-2000 on its own is electrochemical active enough. This catalyst is produced by the sole commercial manufacturer of platinum-free metal catalysts named Pajarito Powder and is already inserted in fuel cells. Nevertheless, there is still room for improvement.