(50d) What Causes Fuel Cell Membranes to Dry out?
The requirements for feed humidification in Polymer Electrolyte Membrane (PEM) fuel cells was examined with one and two dimensional PEM fuel cell (PEMFC) reactors. Fuel cells were built with flow patterns that operated as 1D, stirred tank reactors or 2D, plug flow reactors. The fuel cell were operated as autohumidified fuel cells with bone dry feeds. Autohumidified fuel cell operation is sustained in 1D at temperatures of 40-80°C provided the production of water (1/2 current) exceeds the convection of water vapor exiting the fuel cell (saturation water vapor mole fraction × vapor molar flow rate). Detailed water balances for the one dimensional reactor demonstrate that in autohumidified fuel cell operation water crosses from the cathode to the anode such that the ratio of the water activity at the anode to that at the cathode is approximately 0.8. The water activity difference between the anode and cathode sets an upper limit of electro-osmotic drag < 0.1. Membrane dehydration occurs when the convective water vapor flow from the anode and cathode exceeds water produced by reaction. Membrane dehydration is more significant with higher temperature, higher gas feed rates. Membrane dehydration is more severe in 2-dimensional fuel cells with long thin gas flow channels because of greater gas flow velocity convecting more water vapor. Autohumidified operation with moderate current density (~ 0.5 A.cm-2) was sustained in a 2D H2/O2 fuel cell with small stoichiometric excess. Sustained autohumidified operation in a 2D H2/air was only possible with low current densities (<0.1 A.cm-2).