Sandwich-structure PI/SPEEK/PI proton exchange membrane developed for achieving the high durability on excellent proton conductivity and stability

The proton exchange membrane (PEM) is the core unit in fuel cells, in which the balance among proton conductivity, dimensional stability and durability is a long-term pursuit. Nanofiber composite membrane as the most facile and practical strategy, however, commonly suffers from the obviously reduced proton conductivity although the dimensional stability and durability can be improved distinctly. Herein this work, we proposed a sandwich-structure nanofiber composite PEM, in which the inexpensive proton-transport matrix of sulfonated poly (ether ether ketone) (SPEEK) is sandwiched between polyimide (PI) nanofibers mats via simple steps. Thanks to the support for the matrix and resistance to the tough working condition provided by the PI nanofiber mats, significantly improved stability and durability in contrast to the referenced PEM of SPEEK. More importantly, with respect to the proton conductivity, a numerically equivalent initial proton conductivity and significantly improved proton conductivity stability, in comparing to the SPEEK membrane, has been achieved; where the conductivity only decreased by 21% in our sandwich-structure PEM after 6 weeks, much less than that of commercial Nafion 117 of 30% and pure SPEEK of 55%. Such results can be attributed to the hydrogen bond interaction between PI nanofibers and SPEEK matrix and thus the formation of the sulfonic acid-rich layer in our developed PI/SPEEK/PI membrane, confirmed by in situ Fourier Transform Infrared Spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). This combination of high proton conductivity, long-term stability, low cost, simple preparation suggests the sandwich-like PEM will provide a promising commercial application for future PEMs.