Morphological characterization of grafted polymer electrolyte membranes at a surface layer for fuel cell application

Abstract

The morphology and elemental compositions at the surface of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes (ETFE‐PEMs) in a grafting degree (GD) range of 0%–127% are investigated by using FE‐SEM and XPS analyses. The concentrations of elemental components at the surface are not a linear function of GDs, resulting from different grafting speeds at the surface and bulk. In addition, low accumulation of PSSA grafts on the surface of ETFE‐PEMs in the low GDs of 0%–19%, as well as their homogeneous distribution through the membranes at least at a GD of 19% are observed. At higher GDs of 19%–127%, the PSSA accumulation on the surface is relatively limited, indicating the presence of more PSSA grafts inside the bulk than on the surface. The surface signatures of ETFE‐PEMs over the entire GD range can be determined based on the grafting process. These interesting observations suggest the significant advantages of interfacial properties of ETFE‐PEMs for fuel cell applications.

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

“…For the ETFE‐PEMs, C o decreases dramatically from 34.87% to 10.66% in the GD range of 0%–127% and is in accordance with that reported previously 5,29 . A much lower crystallinity in the ETFE‐PEMs in comparison to the grafted‐ETFE films is expected because SO 3 − groups are inherently amorphous moreover they can further result in the loss ordering of partial inherent crystallites due to the dilution effects as mentioned above 5,6,27,28 . Note that evolution of C o versus GD is quite different from those for C 120 and C 200 depicted in Figure 3.…”

“…In the meantime, the sulfonation results in an increase of FWHM for r 3 (0.046) and V 3 (0.027) but a significant decrease for r 4 (0.008) and V 4 (0.016). The much lower FWHM values of r 4 and V 4 for the ETFE‐PEM indicate that the sulfonation induced a strong microphase separation between the hydrophilic containing PSSA groups and the hydrophobic of ETFE backbones, leading to the reorganization of subnano holes to be more well‐defined structures, which could not be observed at the graft polymerization step 6,27,28 …”

“…In addition to relating the lifetime and intensity of o ‐Ps to free volume, many works have further investigated relating the o ‐Ps parameters to different characteristics of PEMs such as molecular weight or ion exchange capacity (IEC), 18,21 water, methanol, and gas permeability, 17,19–22 proton conductivity, 17,21–23 crystallinity, 21 mechanical property, 8 and so forth. However, there have been no reports on the above relations for the graft‐type PEMs.Recently, the ETFE‐PEMs have been investigated extensively by our group 5,6,24–28 and others 29–32 . Within these studies, the electrochemical and mechanical properties and the hierarchical structures of ETFE‐PEMs have been investigated through the preparation procedures and as functions of grafting degree (GD) or IEC.…”

Positron annihilation lifetime spectroscopic analysis of Nafion and graft‐type polymer electrolyte membranes for fuel cell application

“…In terms of the annihilation intensities, I 3 (3c) dramatically decreases from 9.5% to 4.3% and then increases and keeps stably around 6%-7% in the GDs of 0-56 and 79%-127%, respectively. This result should be associated with the phase transition [8,11,12] and the strong inhibition process as reported previously. [27] I 4 (4c) and (I 3 + I 4 ) (4c) are lower and higher than those of I 3 (3c), respectively, and show a similar trend with grafting degree.…”

“…[5,6] Recently, poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene-co-tetrafluoroethylene) PEMs (ETFE-PEMs) have been widely studied because of their superior properties for fuel cell operation. [7][8][9][10][11][12][13] Quite similar to Nafion, ETFE-PEM is a synthesized random copolymer composed of the semicrystalline ETFE backbone and a random styrene side chain with a terminal of sulfonic acid group (Scheme 1B). The higher-order structures of the membrane revealed by small-and ultrasmall-angle x-ray scattering (SAXS/USAXS) measurements indicate that ETFE-PEM possesses the lamellar structure of the original ETFE-based film, which develops into the lamellar grain and the crystallite network structure after graft polymerization.…”