Synthesis of Hyperbranched Polymers with Fluorine Groups at the Periphery and Their Application to High Temperature PEM Fuel Cell

Proton exchange membrane (PEM) fuel cell has attracted attention due to its wide range of potential applications. Supply of hydrogen and oxygen gases is one of the most important factors to determine the PEM fuel cell efficiency. Loading of pure oxygen has a spatial limitation in the device, which complicates the structure of the PEM fuel cells, as well as the risk of explosion. Introduction of fluorinated compound in this study was expected to improve oxygen permeability in high temperature PEM fuel cells. Fluorine-functionalized hyperbranched poly(phenylene oxide) (HPEF) was synthesized by Suzuki coupling reaction between hyperbranched poly(phenylene oxide) (HPE) and the corresponding fluorinated compounds. HPE was prepared by self-condensation of 3,5-difluorophenol in the presence of K₂CO₃, which is an environment-friendly and relatively low cost catalyst. The enhanced density of highly electronegative fluorine groups at the periphery of the hyperbranched polymers led to the improvement of oxygen permeability. In thermogravimetric analysis, HPEF displayed thermal stability with the degradation temperature over 500 C. Enhanced oxygen permeability resulted in improved cell performance of the electrode containing HPEF and reduced Pt loading amount down to 31%, compared to the standard electrode without any additives in the electrode. HPEF as an additive for both electrode and membrane led to the reduced activation time to the highest performance, improved cell performance, increased thermal stability, and reduced Pt loading contents.