Effects of microstructure on the retention of proton conductivity of Nafion/ SiO2 composite membranes at elevated temperatures:An in situ SAXS study

The operating temperature of modified Nafion membranes remains too low to satisfy the requirements for hightemperature proton exchange membrane fuel cells (HT-PEMFCs) because most can only operate well below 120 degrees C. In this article, we prepare a series of Nafion/SiO2 composite membranes with different contents of doped silica nanoparticles by in situ growth of SiO2 nanoparticles near the -SO3H group. The water dynamic change process and the functional properties of Nafion/SiO2 composite membranes are studied and found to correlate well with structural parameters, such as crystallinity and average chain spacing. Also, microstructure information including the phase separation structure changes, structural complexity, and ion cluster fraction of Nafion/ SiO2 composite membranes at different temperatures are investigated using synchrotron-based in situ small angle X-ray scattering (SAXS). Differing from expectations, the amount of water in the Nafion/SiO2 composite membranes may not be the decisive factor to maintain proton conductivity at relatively high temperatures. Instead, the complexity or connectivity of the proton transport channel may play a more important role in the effective proton transfer inside the composite membranes, which also provides valuable guidance and insight for the development of novel HT-PEM materials with well-designed ion transport pathways in the future.