Investigation of platinum utilization and morphology in catalyst layer of polymer electrolyte fuel cells

Platinum utilization in the gas-diffusion catalyst layer and thin-film catalyst layer is investigated. The morphology of PTFE and Nafion in a simulated catalyst layer is examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the platinum utilization of the thin-film catalyst layer containing only Pt/C and Nafion is 45.4%. The low utilization is attributed to the fact that the electron conduction of many catalyst particles is impaired by some thick Nafion layers or clumps. For the gas-diffusion (E-TEK) electrode, the platinum utilization is mainly affected by the proton conduction provided by Nafion. The blocking effect of PTFE on the active sites is not serious. When the electrode is sufficiently impregnated with Nafion by an immersion method, the platinum utilization can reach 77.8%. Transmission electron micrographs reveal that although some thick Nafion layers and clumps are observed in the Pt/C+Nafion layer, the distribution of Nafion in the catalyst layer is basically uniform. The melted PTFE disperses in the catalyst layer very uniformly. No large PTFE clumps or wide net-like structure is observed. The reactant gas may have to diffuse evenly in the catalyst layer.