The performance of fluidized beds, packed beds, and screens as fuel cell electrodes

Abstract

At present, most fuel cells employ porous gas diffusion (PGD) electrodes. Although much effort has been spent on their development, the performance and cost of PGD electrodes are still major obstacles to the successful commercialization of fuel cells. As a means to bypass the drawbacks associated with PGD electrodes, several researchers have taken an alternative approach to electrode design by considering fluidized bed electrodes (FBEs), a type of flooded electrode that relies on convective rather than diffusive mass transport. Several reviews and past studies claim that FBEs have the potential for reaching high power density at a low cost due to several inherent advantages. However, the results so far of fluidized bed electrodes applied to fuel cells have been poor, and the past studies have not offered effective explanations for the discrepancy between expected and actual performance. A fluidized bed electrode model has been developed and applied to the data obtained by previous researchers in order to explain the poor performance of these past designs. As points of comparison, models have also been developed to predict the performance of packed bed electrodes and screen electrodes (two other flooded electrode designs).

(cont.) Separate models have been developed to consider both ionic and mass transport. Upper bounds on the performance of all three electrodes have been established, and then compared to the performance of PGD electrodes. The results of the models indicate that the PGD electrodes perform better than the packed bed or screen electrodes by at least a factor of two, unless the flooded electrodes are very short (on the order of millimeters). Both mass transfer and the saturation concentration of oxygen in the electrolyte serve as limitations in the flooded designs. The models also indicate that the two-phase and three-phase FBEs are inferior to the other flooded electrodes. The paper concludes with several recommendations for further work, including methods to boost performance.