A Characterization Study on Catalyst Layers in Proton Exchange Membrane Fuel Cells

This thesis describes the work for the catalyst layer (CL) characterization study of proton exchange membranes (PEM) for fuel cells. In particular, both the structure and performance of catalyst layers with alternative ionomers were studied. Structure wise, the morphology, surface area and pore size distribution studies were accomplished with scanning electron microscopy (SEM), transmission electron microscope (TEM) and nitrogen adsorption processed through Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda (BJH) theory. Water uptake isotherms of the CLs have been developed under well controlled relative humidity (RH) levels. The performance characterization focuses on polarization study, catalyst layer proton conductivity measurement and estimation of the proton conduction tortuosity. Also, a thermal investigation between various components of the catalyst layer was performed.

Two different sets of CLs were examined, the in-house fabricated 3M ionomer CLs and free-standing 3M CLs directly provided from the 3M Company. A characteristic comparison of the structure and electrochemical performance have been carried out, along with further discussion on the formation of CLs containing 3M polytetrafluoroethylene (PTFE) ionomer.

Our data revealed that higher ionomer to carbon (I/C) ratio reduced the amount of micro- , meso- and macropores. This allowed the construction of a more completely developed ionic transport network, however, could potentially hinder the mass transfer. Also, our study showed that higher Pt:C ratio lead to a more intense Pt agglomeration. The CL’s porosity was strongly affected by such Pt clustering. Furthermore, energy dispersive X-ray analysis (EDS) revealed that the 3M ionomer preferred attaching to the carbon surface over the Pt particles.

According to our polarization study, in contrast of tradition Nafion fuel cells, the 3M fuel cells reached its optimal performance at 60%-70% RH and suffered dramatic mass transfer losses at saturated humidity level. Therefore, the 3M fuel cells are able to function fully under drier conditions than the Nafion units. However, the high sensitivity on the cells’ water content requires efficient water management during operation, especially at higher current density. Polarization study also showed an optimal 3M ionomer loading of 36 wt.% at 30:70 Pt:C ratio, which is similar to traditional Nafion fuel cells.