Understanding of Ionic Structure Change Under Hydration Levels by Using CSAFM

Abstract

Proton exchange membranes (PEMs) are a key component of proton exchange membrane fuel cells. They work as gas separator, insulator, and proton conductor. Thus, characterization of the membrane is a crucial process for developing PEM. However, this work is not easy because its complicated phase separated morphological structure and widely varying ionic structure under hydration. Current sensing atomic force microscopy (CSAFM) measures nano current flow between a tip and the sample surface and can map the nano current flow distribution of scanned area. In this study, the ionic structure of Nafion under different hydration condition is studied by using CSAFM. Specifically, the area variation of ionic domain on the membrane surface under different relative humidity is characterized by quantitative method based on CSAFM measurement. The study is done by several steps. First, the Nafion 212 is scanned in the environmental chamber by varying 15% to 65% relative humidity. In this process, the current distribution and the morphology are simultaneously mapped at each RH. Second, the local net charge value of protons, which is connected with area of ionic domain on the membrane surface, are calculated based on current distribution. In this process, the area of ionic domain on the membrane surface under different RH is estimated. The study shows several remarkable results. First of all, the non-uniform ionic domain increasing is observed from sequentially mapped current distribution on the surface under different RH. Second, the reliable estimation method of ionic domain variation is introduced by this study.