Controlling Hydronium Diffusivity in Model Proton Exchange Membranes.

Fuel-cell-based proton exchange membranes (PEMs) show great potential as cost-effective and clean energy conversion devices. In our recent work, we found that for the low-hydrated model PEMs with a inhomogeneous water distribution and a sulfonate anionic functional end group (SO 3 - ), the H 3 O + reacts with SO 3 - according to SO 3 - + H 3 O + ↔ SO 3 H + H 2 O, indicating that the anions in PEMs become active participants in the hydronium diffusion. In this work, we use fully atomistic ab initio molecular dynamics simulations to elucidate the optimal conditions that would promote the participation of SO 3 - in the hydronium diffusion mechanism by increasing the H 3 O + /SO 3 - reactivity, thus increasing the hydronium diffusivity along the cell. The results presented in this work allow us to suggest a set of design rules for creating novel, highly conductive PEMs operating at high temperatures under a nonuniform water distribution using a linker/anion with a relatively high p K a such as (CH 2 ) 2 SO 3 . We expect that the discovery of these key design principles will play an important role in the synthesis of high-performing materials for emerging PEM-based fuel cell technologies.