Effect of Molecular Weight on Hydrated Morphologies of the Short-Side-Chain Perfluorosulfonic Acid Membrane

Abstract

We have carried out dissipative particle dynamics (DPD) simulations in an attempt to better understand how molecular weight (MW) affects the hydrated morphology of the short-side-chain (SSC) perfluorosulfonic acid (PFSA) fuel cell membrane. Previously, we demonstrated that such coarse-grained simulations are capable of revealing differences in the morphology of PFSA membranes when either the length of the side chain or equivalent weight (EW) of the ionomer is changed [Wu et al. Energy EnViron. Sci. 2008, 1, 284-293]. In the present investigation, the SSC ionomer was modeled using macromolecules of the ionomer with EWs of 753, 798 and 849, each at three distinct MWs. The morphological structures were then investigated as a function of EW, MW and degree of hydration (with water contents corresponding to λ ) 5, 7, 9, 11, and 16 H 2 Os/SO 3 H). Water contour plots reveal that the isolated water clusters present at lower water contents increase in size with increasing levels of hydration, and eventually form continuous water domains. The increase of MW induces aggregation of the fluorocarbon backbone in order to minimize chain bending forces while maintaining a phaseseparated structure, and results in larger, more elongated water domains, especially at high EWs. Furthermore, the Bragg spacing corresponding to periodicity of water domains, computed from radial distribution functions (RDFs), shows that the spacing between water domains increases with increasing hydration levels. This occurs especially for higher MW polymers at high hydration (16 H 2 Os/SO 3 H), whereas there is little difference at lower hydration levels between polymers with different MW.

Atomistic and mesoscale simulation of polymer electrolyte membranes based on sulfonated poly(ether ether ketone)

“…Averaging over the configurations generated for the CG model revealed that connectivity of the water domains occurs at relatively low water content, viz., at k* % 9. This value is noticeably lower than that estimated in the DDFT [20] and DPD [32] simulations of Nafion (k* % 16). It should be noted that percolating cluster at and just beyond the percolation point coexists with a small fraction of isolated (smaller) aggregates.…”

Variational Models of Network Formation and Ion Transport: Applications to Perfluorosulfonate Ionomer Membranes

“…There has been controversy regarding the morphology of the water network as numerical and experimental investigation of the microstructure of Nafion and related PFSA membanes have lead to the proposition of bi-layer morphologies [42,43], cylindrical pores [44], an inverted pore morphology with solvent groups surrounding cylinders of crystallized backbone [6,45], spherical clusters [39], as well as more complex morphologies suggested by atomistic simulations [3,35,46,47]. Regarding the crystalline domains, proposed models include lamellar models [48], the rod-like model [45], a fringed-micelle model [40], and bi-modal networks [49].…”

“…By virtue of their tethered charges, these networks are selectively conductive, inhibiting the transport of co-ions while facilitating the transport of counter-ions. The broad class of ionomer membranes have important applications in efficient energy conversion devices such as polymer electrolyte membranes for fuel cells [3][4][5][6], dye sensitized solar cells [7], bulk-heterojunction solar cells [8,9], and lithium ion batteries [1,10,11]. Considerable computational and experimental effort has been invested in the design and optimization of membranes with strongly charge-selective transport characteristics.…”

Fuel cell catalyst layers containing short-side-chain perfluorosulfonic acid ionomers

“…A simpler route for the synthesis of SSC ionomer was developed and marketed under the trade name Aquivion ® (previously Hyflon ® ) by Solvay Solexis [12]. SSC-PFSA ionomer membranes (PEMs) have been previously characterised ex situ and in fuel cells [10,[13][14][15], as have theoretical studies aiming to explain the morphology of SSC-PFSA as a function of IEC [16,17]. The development of new ionomers to replace Nafion as a membrane has led to the incorporation of new ionomers in the catalyst layer.…”