Water Balance in Non-Humidified PEM Fuel Cell with Different Membrane Thicknesses

Abstract

It is possible to design and operate a fuel cell with non-humidified reactant gases, by relying on water produced by electrochemical reaction inside the fuel cell. However, in order to do so there must be adequate water and heat management applied. For example, temperature distribution along the cathode channel must be such that relative humidity is maintained close to 100%. As water is produced on the cathode side there is a water concentration gradient across the mebrane, and in order to humidify hydrogen on the other side there must be back-diffusion flux greater than or at least equal to the electroosmotic drag. Membrane thickness, thus, plays an important role in realization of this concept. A segmented fuel cell is designed such that each segment can be maintained at different pre-determined temperature. The temperatures of the segments along the cathode channel are selected by the help of Molliere h-x diagram such that relative humidity is maintained close to 100%. Operating conditions are selected such that the product water is sufficient to saturate the cathode exhaust without appearance of liquid water. Humidity measurements are taken between each segment on both anode and cathode sides. The anode and cathode exhaust streams are condensed so that water balance may be established. The entire setup is modeled with FLUENT CFD package, and the experiments are conducted with two different mebrane thickness. Good agreement is achieved between the modeling and experimental results. The results indicate that it is possible to establish the required temperature profile along the cathode channel and keep the cathode stream relatively well humidified without external humidification. Performance of such variable temperature fuel cell is better than the uniform temperature fuel cell. The thinner membrane results in better humidification of the hydrogen side, which is important especially at higher current densities, i.e., at higher electro-osmotic drags. Figure 1