Characterization of fuel cell behavior at low-pressure operation

Kurzfassung

Fuel cells may help meet new aircraft to cope with increased electrical demands while mitigating environmental and safety concerns. For an optimized integration of fuel cells into this new application area it is necessary to characterize fuel cells under unusual operation conditions, in particular low-pressure ambient conditions and a wide temperature of operation including -55 °C. This contribution presents investigation of fuel cells under low-pressure operation and the influence of low-pressure on the performance of the cell itself and on the fuel cell system. Especially the reduced supply of oxygen as well as the reduced humidification of the cell has an important influence on performance, related current density distribution of the cell and on fuel cell system operation. At the German Aerospace Center (DLR) several test facilities for low-pressure operation (down to 0.2 bar at the fuel cell outlet) of fuel cells have been built-up. For the investigation of the inherent electrochemical cell behavior a standard small cell with 5 x 5 cm2 and single meander flow field was investigated at various operating pressures in a test facilities were low-pressure operation was realized by evacuating the fuel cell exhaust by pumps. The channel in the flow field was 1 mm wide and has a depth of 1 mm on the cathode side and 0.7 mm on the anode side. The rib width was 1 mm. A significant pressure drop results from the flow field design (single channel with a total length of approx. 1.3 m). Consequently the local operating conditions and the related local current density in the test cell vary significantly. The main advantage of the single meander flow field is that the local gas flows in the cell are well defined. In order to yield a detailed understanding of the problems of low-pressure fuel cell operation and correlate the pressure variation along the flow path with the local performance the current density distribution was measured under the different operating conditions. Of course, a strong dependence of the current density distribution on the operating pressure was observed. The most significant alteration in the current density distribution was observed at a operating pressure of 600 mbar at the cell outlet which corresponds to the saturated water vapor pressure at 80°C (pH20, saturation ≈ 450 mbar). The local variations of the current density distribution as a function of the operating pressure can yield relevant information for the optimization of fuel cells and their components for aircraft applications. To correlate the electrochemical behavior of the cell with the influence of the low-pressure operation on the overall fuel cell system, a portable fuel cell system with 300 W power developed at DLR was tested in a vacuum chamber at reduced pressures. In this case all fuel cell systems components including the air supply system had to operate at reduced pressures down to 200 mbar. The observed changes in performance can be analyzed taking into account the single cell behavior which is representative of the electrochemical processes under low-pressure. From this information, an optimized system design becomes possible and the implications will be discussed in this contribution.