UC Berkeley

Ionic conductivity in phase-separated polymer-electrolyte-fuel-cell membranes is limited by the multiple scales via which protons travel. The interactions of protons in the conducting pores determine the mobility at the nanoscale, while the tortuosity of the conductive phase dictates the macroscopic conductivity. Previous efforts to model transport in this multiscale network have relied on assumptions of the morphology. In this paper, direct imaging of a hydrated membrane is used to construct a realistic network through which transport occurs. Results indicate that pore-level proton conductivity is lower than the corresponding bulk-solution conductivity due to hydronium solvation at negative moieties of the polymer sidechains. Free protons in the pores diffuse at nearly bulklike rates. The distribution of sizes of the hydrophilic transport domains leads to inhomogeneous transport pathways across the network.