Electrode permeability and flow-field configuration: influence on the performance of a PEMFC

摘要

The objective of this work was to investigate the effect of both the permeability of the electrodes and the configuration of the gas flow distributor on the performance of a proton exchange membrane fuel cell (PEMFC). For that purpose, MEAs including electrodes of two types, carbon paper and carbon cloth, have been characterised electrochemically by measuring the polarization curves for a wide range of operational conditions with H_2 and O_2/air as reactants. MEAs with surface active areas of 50 and 290 cm~2 have been characterised in single cells with two flow-field configurations: a grooved plate with parallel gas channels and solid ribs, and a solid plate. The latter is a novel gas flow distributor that has been designed and tested in our laboratory. A subsequent series of experiments were carried out in order to measure the gas permeability of the electrodes of the MEAs characterised previously. The permeability of the electrodes was measured separately for O_2, N_2 and H_2 in the absence of water vapour. The fuel cell performance strongly depends on both the gas permeability of the electrodes and the type of gas flow distributor. The effect of the electrode permeability is not meaningful in the case of the grooved plates, but it is rather important in the case of the solid plates. With the grooved plates, the differences in the fuel cell performance observed with the various MEAs must be attributed to factors mostly related to the catalyst layer (platinum and Nafion content, dispersion of the catalyst, etc.). With solid plates, however, the MEAs of both short and large sizes performed consistently with the gas permeability values of the electrodes measured in this work. In general, the performance of the fuel cell with solid plates declines when the permeability of the electrodes decreases. In the range of current densities covered here, below 300 mA/ cm~2, the MEAs with the more permeable electrodes performed comparably with either grooved or solid plates. The less permeable electrodes were the ones in the MEAs provided by E-TEK, which are made of carbon cloth with 40 wt.% of hydrophobic material. In this case, noticeably higher gas transport losses were observed with the solid plates than with the grooved plates, particularly when air was used as oxidant. The influence of water vapour on the gas electrode permeability and the effect of the electrode hydraulic permeability on the fuel cell performance will be investigated in a future work.