Neutron Radiographic Investigations on the Effect of Hydrophobicity Gradients within MPL and MEA on Liquid Water Distribution and Transport in PEMFCs

Neutron radiography imaging is employed to investigate the effect of hydrophobicity gradients between the membrane electrode assembly (MEA) and microporous layer (MPL) of a low-temperature polymer electrolyte membrane fuel cell (PEMFC) on the water distribution and transport in operating cells. Single cells specially developed for neutron imaging experiments with an active area of 8 cm² are equipped with in-house made MEAs and MPLs featuring different degrees of wettability. Imaging is performed with a pixel size of 13 µm and the cells are operated at two different current densities of 1 and 0.75 A cm-2 and a relative humidity of RH=70% and RH=120% at a temperature of 55°C. A first quantitative analysis of the water thickness in electrodes and gas diffusion layers (GDLs) of 3 cells at RH=120% and i=1 A cm-2 as well as an exemplary radiogram for the water distribution inside one of the cells is shown in figure 1. It is observed that the implementation of both a more hydrophobic MEA and MPL results in an increase of overall water content inside the cell. Nevertheless, the effect of a more hydrophobic MPL on the increase of the water content of the cell, under the operating conditions investigated here, is apparently more significant compared to the effect of a more hydrophobic MEA. As expected, the amount of water found on the cathode side is higher than on the anode side for all components of the cell. The sudden accumulation of water on anode side which is appearing in periodic peaks with equal time intervals is associated to an increased rate of back diffusion mechanisms. The cell with a more hydrophobic MEA has decreased the magnitude of these fluctuation peaks by 30%, whereas this reduction ratio is 60% for the cell with more hydrophobic MPL, resulting to a more balanced water profile across the membrane. In this presentation, we will show further morphological and wetting property characterization results of various MEAs and MPLs and analyze the effect of hydrophobicity on the dynamical changes of water distribution and transport at different operating conditions. Figure 1