Operating Proton Exchange Membrane Fuel Cells at a Constant Relative Humidity

This study investigates the possibility of operating a proton exchange membrane fuel cell at a constant humidity of 100% from cathode inlet to cathode outlet. Employing the Engineering Equation Solver (EES), a model has been developed where the cathode channel flow is calculated via the discretized Hagen-Poiseuille equation. The stoichiometric flow ratio such that the decrease in the RH caused by the pressure drop is balanced by the addition of water due to the electrochemical reaction is calculated. Results show that in many cases the calculated stoichiometry is too low to be viable for a straight channel flow field. However, reducing the temperature yields acceptable stoichiometric flow ratios. The channel geometry plays a critical role, and shorter and deeper channels are preferable. In a refined version of the model, the limiting current density is calculated to avoid concerns about the low stoichiometric flow ratios.