Simplification and investigation of polymer electrolyte fuel cells using micro-patterned glassy carbon flow fields

Abstract

This work deals with the development, optimization and application in determination of electrochemical effects of a miniaturized and simplified polymer electrolyte fuel cell (PEFC). It can be regarded as a model system for PEFCs of various scales. The simplification of the micro fuel cell is based on the possibility to produce micro-patterned flow fields from glassy carbon. The fuel cell consists of a catalyst-coated membrane sandwiched between two of these micro-patterned flow fields. The micro flow fields are produced in a three step process. An aluminum film is sputtered on glassy carbon substrates. The aluminum film is selectively removed by laser ablation in areas where microchannels are supposed to be. The uncovered carbon is removed by oxygen reactive ion etching to form micro-channels, while aluminumprotected areas persist and form ribs between the channels. The patterning process had been developed already when this work started, but it needed further optimization to be adapted to the needs of micro flow field production. It was important to understand the influence of particular parameters like the width of the laser spot used to ablate the aluminum area or the etching time to yield predictable dimensions and aspect ratios of the micro-channels. ns-shadowgraphy was applied to investigate the particular processes occurring during single pulse laser ablation of the aluminum mask. On the one hand, it is important to completely remove the aluminum from irradiated areas, on the other hand an aluminum redeposition must be avoided as a consequence of a too high laser fluence. With the well understood and adapted process we can obtain various well defined micro-structures that can be used as flow fields in miniaturized polymer electrolyte fuel cells. The miniaturized PEFC is simplified by assembling it without gas diffusion layers (GDL). Their functions need to be fulfilled by other parts of the PEFC. The micropatterned flow fields ensure a fine gas distribution, while the catalyst layers exhibit a sufficient electronic conductivity that 100 μm wide microchannels can be employed without distinct ohmic losses. Such a miniaturized fuel cell can deliver power densities up to 700 mW/cm, an outstanding value for miniaturized PEFC. More important than the high power density is the knowledge gained during the optimization process of a PEFC without gas diffusion layers. Water, which is introduced into the fuel cell by gas humidification and produced by reaction, plays a significant role. On the one hand it is necessary to ensure proton conductivity in the polymer electrolyte membrane, on the other hand it can create mass transport limitations, in particular if water condensation occurs. Neutron radio-