A comparative study of the hydrated morphologies of perfluorosulfonic acid fuel cell membranes with mesoscopic simulations

Abstract

The hydrated morphology of Nafion, the short-side-chain (SSC), and 3M perfluorosulfonic acid (PFSA) fuel cell membranes have been investigated through dissipative particle dynamics (DPD) simulations as a function of ionomer equivalent weight (EW) and degree of hydration. Coarse-grained mesoscale models were constructed by dividing each hydrated ionomer into components consisting of: a common polytetrafluoroethylene backbone bead, ionomer specific backbone beads, a terminal side chain bead, and a bead consisting of a cluster of six water molecules. Flory–Huggins χ-parameters describing the interactions between the various DPD particles were calculated. Equilibrated morphologies were determined for the SSC and 3M PFSA membranes both at EW's of 678 and 978, and Nafion with an EW of 1244. The hydration level was varied in each system with water contents corresponding to 5, 7, 9, 11, and 16 H2O/SO3H. The high EW ionomers exhibit significantly greater dispersion of the water regions than the low EW membranes. Water contour plots reveal that as the hydration level is increased, the isolated water clusters present at the lower water contents increase in size eventually forming continuous regions resembling channels or pores particularly at a hydration of 16 H2O/SO3H. The DPD simulations reveal differences in the hydrated morphology when only the side chain length was altered and indicate that the 3MPFSA ionomer exhibits much larger clusters of water when compared to the SSC ionomer at the same EW and water content above 9 H2O/SO3H. The average size of the clusters were estimated from the water–water particles’ RDFs and vary from about 2 nm to nearly 13 nm for hydration levels from λ = 5 to λ = 16. Finally, computed Bragg spacing in each of the hydrated membranes indicate separation of the domains containing the water from 2 to 6 nm, exhibiting an approximately linear relationship with hydration.