Investigating drying conditions of screen-printed platinum-free fuel cell cathode layers

Abstract

Renewable energy sources are becoming more significant in a society where sustainable development is becoming more crucial. Efficient ways to store and convert energy are therefore essential for the success implementation of the new sources. One part of the puzzle is hydrogen fuel cell technology. A challenge, however, is to develop a hydrogen fuel cell that is both cost-effective and high performing. The anode and cathode of the fuel cell are separated by a membrane, and together they are the heart of the fuel cell. The cathode side has been the focus of this study, specifically how to create a suitable coating with as minimal resistance as feasible. There are various alternative deposition procedures that may be employed; however, screen printing was used in this dissertation. The aim of this thesis is to investigating drying conditions of screen-printed platinum-free fuel cell cathode layers and minimize the ohmic resistance caused by the defects in the deposited layer. In addition, a valid resistance measurement strategy needs to be implemented and integrated during the characterization step without negatively affecting the eventual use of the membrane in fuel cell testing. The work process was divided into broad testing, reproducibility, and characterization. The samples were analyzed by microscopy techniques, profilometry and resistance measurements. From the broad testing experiments, it could be concluded that the drying environment and time had an influence on the formation of cracks in the plane. The best possible settings found in this study were 24 hours in ethanol followed by 24 hours in slow drying air. In addition, a decreased in-plane electrical resistance could be obtained by increasing the thickness of the deposition layer. The reproducibility experiments concluded the properties of the ink also had a significant impact on resistance. Higher mass-% of dry matter resulted in increased viscosity, which aided the screen-printing process by enabling more ink to remain on the sample, resulting in a thicker deposit layer and fewer fractures. In the characterization and fuel cell experiments, tests were performed to evaluate the substrate selection closer. In general, all samples dried in air performed slightly better compared to the ones dried in a saturated environment. Possible explanations for achieving these results can be the structure of the substrate. Printing on a porous material, a portion of the ink may penetrate the substrate rather than staying on the surface as a coating. In these circumstances, the drying process benefits from a faster drying period to reduce the loss of active catalyst. Another possibility is to use a microporous sublayer as a membrane to prevent ink from entering the substrate. The third substrate evaluated, Nafion membrane, proved to be extremely fragile. Fuel cell experiments suggest that Nafion dried in air performs better, but since several samples collapsed, a controlled drying technique would be preferable. Future prospects include further optimizing the correct ink in relation to the right environment and substrate choice, increase the knowledge about Nafion membrane and how to increase the adhesion strength and investigate how the mesh marks impact the substrates.