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1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 H2/air, Ambient H2/air, 30 psig and th e flow set t Pressure Reformate (50 ppm CO) / Air, Ambient Pressure These 3 polarization curves were generated using a 70oC cell, saturated anode and cathode, o a const ant stoich of 1.5 ano de / 2.5 ca thode. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Amps / cm2 0.8 0.9 1.0 1.1 Figure 3-3 Polarization Curves for 3M 7 Layer MEA (12) The electrochemical reactions of the PEFC are similar to those of the PAFC10: molecular hydrogen at the anode is oxidized to provide protons, while at the same time freeing two electrons that pass through an external “electrical” circuit to reach the cathode. The voltages at each electrode, due to the hydrogen oxidation potential and the oxygen reduction potential, form a voltage gradient of approximately 1 volt (depending on conditions) at open circuit, i.e., zero current draw. It is this potential that drives the proton through the membrane. As the proton is “pulled” through the membrane, it drags with it a certain number of water molecules. The proton reacts with oxygen to form water at the catalyst sites on the cathode. Because of the intrinsic nature of the materials used, the PEFC operates at temperatures between 0 °C to 90 °C, typically in the 60 °C to 80 °C range. When compared to other fuel cells, PEFC technology has been capable of very high current densities: while most technologies can operate up to approximately 1 amp/cm2, polymer electrolyte membrane fuel cells have operated at up to 4 amps/cm2 (13). Stack level power densities under pra 2 ctical operating conditions (cathode stoichiometry less than 3, anode utilization more than 85%, pressure less than 3 bar, and catalyst loadings less than 1 mg/cm2) with reformate of around 50 mW/cm2 at 0.7 V and of around 400 – 600 mW/cm2 when operating with hydrogen are feasible (14, 15, 16, 17). This performance is due primarily to the impressive ionic conductivity of PEFC membranes and the high electrical conductivity of the materials used in the gas diffusion layers and bipolar plates (mostly carbon or metals). Other desirable attributes include fast start capability and rapid response to load changes. Because of the high power density capability, smaller, lighter-weight stacks are possible (18). Other beneficial attributes of the cell include no corrosive fluid hazard and lower sensitivity to orientation. As a result, the PEFC is thought to be best suited for vehicular power applications. The low operating temperature of a PEFC has both advantages and disadvantages. Low temperature operation is advantageous because the cell can start from ambient conditions 10. Equations 5-1, 5-2, and 5-3 for the PAFC apply as well to the PEFC. 3-7 VoltagePDF Image | Fuel Cell Handbook (Seventh Edition)
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