Sub-freezing endurance of PEM fuel cells with different catalyst-coated membranes

The sub-freezing endurance of proton exchange membrane (PEM) fuel cells with hydrophobic and hydrophilic catalyst-coated membranes (CCMs) was investigated. The polarization curves, electrochemical characteristics and physical structures of the CCMs were measured. The cells were frozen at −20 °C with saturated residual water after operating at 60 °C. After eight freeze/thaw cycles, no evident negative effect on the performance of the cell with a hydrophobic CCM was observed, while the cell with a hydrophilic CCM degraded severely. By analyzing the polarization curves, it was concluded that the mass transport limitation was the main reason for the performance loss of the hydrophilic cell. The electrochemical active surface area (ECA) results suggest that the hydrophobicity of the catalyst layer (CL) has an apparent impact on the residual water distribution of the membrane electrode assembly (MEA). A larger water content in the hydrophilic CL has a negative effect on the subzero endurance. From the polarization resistance obtained from electrochemical impedance spectroscopy (EIS) the origin of degradation was further clarified. Mercury intrusion porosimetry showed that the pore size of the hydrophilic catalyst layer changed significantly after freezing; the mean pore size increased from 5.68 to 6.71 nm. However, with a water removal method, namely, gas purging, it was possible to prevent degradation effectively.