Polymer electrolyte fuel cells (PEFCs) are promising for automobiles and various applications because of their high power density and low operating temperature. The cost must be reduced without sacrificing performance of cells by decreasing Pt loading. However, at lower Pt loading, the efficiency decreases because mass transport losses largely increase at higher current density. Proton transport resistance is one of the causes of the losses. It is considered that proton transport property in ionomer thin films is improved with increasing the thickness of the films, whereas oxygen transport resistance increases with increasing the thickness. Ionomer thin films with high proton conductivity are required even though the thickness is thin. For this reason, we obtained the thickness dependence of proton transport property and structure property in ionomer thin films using a molecular dynamics method.

As the ionomer thin film, Nafion was employed. We constructed a system of the ionomer thin film on carbon surface. The contact angle of carbon surface was set at 90 degrees. To analyze the thickness dependence of proton transport property and structure property, the films with the thickness of 4-10 nm were made by changing the number of the Nafion chains. The analyses have carried out by changing water content to 3 and 14. To analyze a correlation between the dynamics and structural properties of Nafion thin films, dynamics properties were evaluated by self-diffusion coefficient obtained from mean square displacement of protons, and structural properties were evaluated by mass density distributions and maximum water cluster length. We found that at high water content, self-diffusion coefficient of protons has hardly changed by the change of ionomer thickness, whereas at low water content, diffusion coefficient has a peak when the ionomer thickness is around seven nm. At this surface wettability, water cluster connectivity was highest when the thickness is around seven nm, and liquid molecule was concentrated at near ionomer/carbon interface and near ionomer/gas interface in the case of low water content. However, in the case of high water content, water cluster connectivity has hardly changed by the change of thickness. We find the thickness dependence of water cluster connectivity. Self-diffusion coefficient increases with increasing maximum cluster length.

Acknowledgment

This research is supported by New Energy and Industrial Technology Development Organization (NEDO). We use Integrated Supercomputation System at Institute of Fluid Science. I appreciate all the support we received from everyone.