Current distribution measurements and modeling of mass transfer in polymer electrolyte fuel cells

The polymer electrolyte fuel cell (PEFC) is considered as an attractive option to produce electric power in many applications ranging from a few watt portable up to several kilowatt automotive applications. The advantage of the PEFC in these applications stems from its high efficiency, low emissions, silent operation and possible low production costs in the future. However, the main factor hindering the market penetration of PEFC applications is the present high production cost of the cell. To allow lower costs for the PEFC, the cell area has to be used efficiently in order to minimize the material usage. This requires the maximization of the cell performance by enhancing the current production at low potential losses. At high current densities, mass transfer losses become the dominating loss mechanism. The mass transfer losses usually produce uneven current production throughout the active area of the cell. The local current production can be studied by experimental and computational methods. For the experimental characterization of the local current production, two different measurement system based on segmented current collectors have been constructed. The other is for a small PEFC operating with natural convection and the other is for a large PEFC operating with forced convection. In addition to the experimental methods, two different theoretical PEFC models have been developed, the other for the free-breathing PEFC and the other for the forced convection PEFC. The current distribution studies were conducted for the free-breathing PEFC in order to determine the feasibility of using natural convection as an air supply method for the cathode reaction at different cell temperatures and ambient conditions. It was observed that the cell performance is highly dependent on the operating conditions and that the current distribution is uneven in the most cases. The current distribution measurements conducted with the large PEFC were used mainly for the model validation purposes. It was shown that under certain operating conditions the current distribution was uniform and thus a one-dimensional PEFC model could be used. The results showed that two-phase and non-isothermal conditions are likely to exist when a PEFC is operated at high current densities and with well humidified gases.reviewe