N. Dukhan
University of Detroit Mercy,
United States
Keywords: metal foam, bipolar plate, PEM fuel cells, flow field, experiment
Summary:
Bipolar plates in proton exchange membrane fuel cells contain mini-channels (called flow fields) in order to uniformly distribute hydrogen and oxygen (or air) over the reactive area of the membrane electrode assembly. Current flow fields are parallel or serpentine channels machined into bipolar plates which are made from graphite. Graphite is rather heavy, brittle and costly. The low flexural strength of graphite has prevented reduction in the thickness of bipolar plates, limiting them to about 4 to 6 mm. This has resulted in heavy and large fuel cell stacks with low power density. The fabrication of the channels includes inaccuracy and comes at an added cost. A new design of flow fields is described here. These new flow fields are made from open-cell aluminum-alloy foam inserted inside solid aluminum plate. Open-cell aluminum foam is a highly-porous material with a web-like internal structure. The porosity of the flow fields at the anode is 72% and at the cathode is 63%. The performance of a commercial PEM fuel cell with the newly-designed flow fields is assessed by direct experiment, and direct comparison to the state-of-the-art bipolar plates, at the same operational conditions. Each experimental run was conducted according to ASME PTC 50-2002 test standard. The polarization curves for the new and commercial bipolar plate designs are compared at the same initial (maximum) voltage Results show that the cell current and voltage, and hence the power density, are improved and temperature and pressure distribution on the membrane are more even in the case of aluminum-foam bipolar plates. The even temperature lies within the allowable safe limit of the fuel cell. An increase of 8.3 % in power density and an increase in the conversion efficiency of 2.55% are observed for the metal-foam case compared to the commercial serpentine flow field. It should be noted that the above enhancements were achieved at a lower hydrogen (4.0%) and air (3.1%) consumption for the metal-foam flow fields. In addition, the weight of the new bipolar plates is reduced significantly (27.8%). Certainly, this weight reduction for each bipolar plate multiplies for fuel-cell stacks having hundreds of unit cells, e.g. as in fuel-cell cars. This paper touches the impact of the new design on future fuel-cell vehicles and transportation regulations concerning burning hydrogen as appose to fossil fuels.