Positron annihilation lifetime study of subnano level free volume features of grafted polymer electrolyte membranes for hydrogen fuel cell applications

Abstract

The subnano level free‐volume hole features of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes (ETFE‐PEMs) are investigated using positron annihilation lifetime spectroscopy (PALS). The hole sizes are formed mainly at the graft‐polymerization step and are not altered dramatically at the sulfonation. With increasing grafting degree, τ4, standing for the annihilation of ortho‐positronium (o‐Ps) in the larger free‐volume holes of the mobile amorphous layers and the PSSA grafts outside of the lamellae, decreases monotonously, whereas τ3, representing for that in the smaller free‐volume holes of the lamellar amorphous regions, the PSSA grafts, and the interface zones inside the lamellae, only shows a slight decrease. From the viewpoint of free‐volume hole sizes, gas molecules are predicted to pass dominantly through the mobile amorphous phases and the PSSA grafts outside of the lamellae. Specifically, the limited hole concentration induced by confining lamellae at the interfacial zones is observed. Thus, the obtained results are associated with a significant reduction of gas passing and the moderate or higher tensile strength for the ETFE‐PEMs in comparison to Nafion‐212.

Characterization of graft-type polymer electrolyte membranes at low grafting degrees for fuel cells

“…As shown in the DTG curves, the decomposition temperature at 242 • C of the original ETFE and grafted film relates to the intrinsic crystallinity of the ETFE base film 30 . Namely, ETFE possesses a lamellar crystal with a crystallinity of 35%, as determined by small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) 13,17,20,[24][25][26][27] . Steps 2 and 3 are attributed to the decomposition of the polystyrene graft and the pristine ETFE backbone, respectively 25 .…”

“…The membrane exhibits good proton conductivity, mechanical integrity, and thermal stability compared to Nafion at a similar water uptake (40%). Moreover, ETFE-PEM shows the unique higher-order structures of lamellar, lamellar grains, and crystallite network structures, which are quite stable under immersed conditions 9,13,[17][18][19][20][21][22] . These hierarchical microstructural features strongly relate to the mechanical integrity of the membranes under severe operation conditions (high temperature and high relative humidity) of fuel cells 14 .…”

“…These hierarchical microstructural features strongly relate to the mechanical integrity of the membranes under severe operation conditions (high temperature and high relative humidity) of fuel cells 14 . However, previous reports mainly represented the results of ETFE-PEMs having a grafting degree (GD) higher than 19% [14][15][16][17][18][20][21][22][23][24][25][26][27][28][29][30][31] . The higher the GD is, the higher the conductance, but the lower the mechanical integrity and the higher the swelling [7][8][9]11,17,25,29 .…”

Effects of source correction on positron annihilation lifetime spectroscopic analysis of graft-type polymer electrolyte membranes

“…Moreover, the FWHM(r 4 ) is much lower than the FWHM(r 3 ). The result can be elucidated from the fact that the amorphous lamellar region is stiffer and less mobile than mobile amorphous phases and polystyrene layers outside of the lamellar structure, which can be rearranged, resulting in a smaller FWHM(r 4 ) 12,13 . Table 3 presents the V 3 , V 4 , FWHM(V 3 ), and FWHM(V 4 ) values.…”

“…In essence, the lifetime of o-Ps (142 ns) is usually shortened to a few nanoseconds (1-10 ns) by picking up a molecular electron with an opposite spin to that of the positron (the "pick-off " mechanism) 10,11 . Recently, free-volume hole features of ETFE-PEM have been probed by PALS [12][13][14] . The PALS of ETFE-PEM was analyzed using 4 lifetime components.…”