Operating Micro Fuel Cells at a Constant Relative Humidity

Abstract

Proton exchange membrane fuel cells are currently being commercialized for numerous applications ranging from electric vehicles to stationary applications to miniature type of fuel applications. For the miniature fuel cells, it is desirable to operate at very high current densities in order to maximize the power density. Because at high current densities, the waste heat generation and removal becomes a substantial practical problem, gas diffusion layers (GDL) made out of perforated metals have recently become of interest. Here, a chemical etching process is applied to create uniform holes of equal size (around 80 microns) in a metal sheet. While this type of GDL has superior heat transfer properties and has been found to keep the fuel cell at an almost constant temperature, the uniformly sized holes must be kept from flooding. On the other hand, it is known that the fuel cell membrane must be kept hydrated in order to have a satisfactory proton conductivity.The purpose of this project is to investigate the possibility of maintaining a relative humidity of 100 % across the entire length of the flow channel. This is to keep the membrane perfectly hydrated while at the same time preventing flooding. The height and width of the flow channels in these fuel cells are in the range of 200 microns, causing a high pressure drop, and as the pressure decreases along the flow channel, the relative humidity would decrease. However, the relative humidity can then be maintained by adding water as a result of the electro-chemical chemical reaction in the fuel cell. Thereby, conditions for which the relative humidity stays constant at 100% over the entire channel length can be calculated, assuming the current density is uniform and that for simplicity all the product water exits the cell at the cathode side.In this work, a model will be constructed and solved in the Engineering Equation Solver (EES). The output of the model will be the required stoichiometric flow rate to maintain a constant relative humidity for different channel geometries, pressure gradients and temperatures.