The states of methanol within Nafion and sulfonated poly(phenylene ether ether sulfone) membranes

The states of methanol in proton exchange membranes are considered to have a significant influence on the methanol permeability of membranes for direct methanol fuel cells, however, they have been rarely studied in the literature. Herein, we use high-resolution 1H solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to identify the microenvironment of methanol molecules in sulfonated poly(phenylene ether ether sulfone) (sPEES) and Nafion117 membranes. Our results show that the majority of methanol molecules are bound with the backbone of sPEES, while almost all of the methanol molecules in Nafion are in the “free” state. In particular, both the –OH and –CH3 of the bound methanol molecules interact with the polymer within sPEES. It demonstrates the existence of a larger amount of bound methanol molecules and stronger interaction between methanol molecules and the background of polymer within sPEES membranes compared to that in Nafion. Such results allow us to understand the mechanism of the low methanol permeability associated with sPEES as opposed to Nafion.