Probing water structure and transport in proton exchange membranes

Proton exchange membrane fuel cells (PEMFCs) have attracted tremendous attention as alternative energy sources because of their high energy density and practically zero greenhouse gas emission - water is their only direct by-product. Critical to the function of PEMFCs is fast proton and water transport in the proton exchange membrane (PEM); fast water transport facilitates optimized water management and optimal PEMFC operation. In order to design a PEM for outstanding PEMFC performance, we need to know how the membrane properties: chemistry and structure, affect the water transport. My PhD work focused on studying the water structure and transport in PEMs.
I developed and applied in situ nonlinear spectroscopy to study fundamental physical chemical properties of water in PEMs. Specifically, I experimentally demonstrated the existence of two types of water subspecies in PEMs: 1) bulk water (bulkW) interacting only with other water molecules; 2) non-bulk water (nonbulkW) interacting with the membrane structure. Interestingly, the amount of these different water subspecies is tunable by simply changes the processing conditions of the polymer material that constitutes the PEM. Moreover, I further found that the different subspecies exhibit distinct diffusivities in the PEM and that overall water transport in PEMs can be explained as a linear combination of the diffusivity of each water subspecies, weighted by the respective fractional contributions. Based on these results, we suggest a general design target as maximizing the fractional contribution of nonbulkW, while maintaining large water retention in the membrane, to accelerate water transport while maintaining fast proton transport.