Hydrophobicity Optimization of Cathode Catalyst Layer for Proton Exchange Membrane Fuel Cell

Hydrophobicity of the cathode catalyst layers (CCLs) crucially determines the performance of proton exchange membrane fuel cells (PEMFCs) by affecting the transports of oxygen and liquid water. In this regard, polytetrafluoroethylene (PTFE) is usually used as a hydrophobic additive to facilitate the oxygen and water transports in CCLs. So far, there remains lacking systematic effort to optimize the addition methods of PTFE in CCLs and the mechanisms behind. In this work, the effects of the approaches for PTFE addition and the distribution of PTFE on the mass transport of oxygen and the proton conduction in CCLs were studied by using a number of electrochemical characterization methods and contact angle tests. It was found that direct adding PTFE molecules is a better way than adding the PTFE-modified carbons to improve the electrochemical properties of CCLs, since the latter causes an increase in the proton transport resistance, whereas the direct molecule addition results in the obviously improved oxygen transport without affecting the proton conduction. In addition, the gradient distribution of PTFE in CCLs, more specifically, adding PTFE near the interface between CCL and gas diffusion layer (GDL), yielded higher catalyst utilization than the homogeneous distribution of PTFE due to the lower oxygen transport resistance.