Spectroscopic Studies of Low Temperature Polymer Electrolyte Membrane Fuel Cells

Licentiate thesis, 2014

Fuel cell technology is one of the many competing technologies in the future of energy conversion and transport. With a growing demand for efficient, versatile and environmental friendly alternatives to the internal combustion engine, fuel cells show promising potential where batteries and grid-electricity are not available. For portable and transport applications, the low temperature Polymer Electrolyte Membrane Fuel Cell (PEMFC) is the most promising candidate. With a simple design, high power density and fast start-up, this technology have advanced quickly during the last decade with numerous field trials of several big automotive companies. However, two important challenges still remain. The current PEMFC system is still several times more expensive than competing technologies, even in serial production. Alternative cheaper materials, as well as improved design and manufacturing techniques can ultimately lower the cost. The durability of the PEMFC system must also be improved to compete with current technologies. The US Department of Energy estimates that the durability of a fuel cell stack need to reach no less than 5000 operating hours to be a competitive option. In the fuel cell stack, the membrane electrode assembly is known to be the limiting factor in durability tests. In this thesis, Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) are used to detect and identify degradation mechanisms in membranes and electrodes used in PEMFC. The two main degradation mechanisms detected in poly-fluorosulfonic acid-based (PFSA) membranes are loss of active end groups and degradation of the polymer backbone (cutting of the polymer chain). Both these mechanisms are quantified by Raman spectroscopy. Degradation of the backbone, and carbon migration from the electrodes into the membrane are certain signs of an upcoming failure of the membrane, this degradation is measured in detail with micro-Raman spectroscopy. Finally, XPS is used to measure the oxidation state and particle distribution in the interface between the electrodes and the membrane. A lowered concentration of active catalyst in electrodes lowers the efficiency of the fuel cell and leads to rapid degradation.