Surface features of graft-type polymer electrolyte membranes using tapping mode atomic force microscopy analysis

Abstract

Introduction: Polymer electrolyte membrane fuel cell (PEMFC) is a promising renewable energy technology, which is increasing commercial use in transportation, stationaries, and portable devices. In the PEMFC, the surface nature of PEMs plays a key role in interfacial degradation of membrane-electrolyte assembly and proton conductance and thus, highly impacting on the performance and stability of PEMFC. Consequently, the objective of this work is to investigate the surface features of graft-type poly(styrenesulfonic acid) (PSSA)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) using tapping mode atomic force microscopy (TM-AFM). Methods: The ETFE-PEMs are prepared using the pre-irradiation-induced grafting of styrene onto the ETFE film and subsequent sulfonation. All the ETFE-PEMs and their precursor films (original ETFE and polystyrene (PS) grafted-ETFE films) are characterized by the TM-AFM. Results: Upon the grafting and sulfonation process, partially grafted PS and PSSA chains are found to be accumulated on the sample surface at a low grafting degree (GD) of 21% and more graft chains are diffused into the bulk of membranes at higher GDs of 36 and 57%. The polystyrene grafts are immiscible mostly with the amorphous phase of the pristine ETFE film, leading to the formation of new amorphous phase containing of only PS grafts. The sulfonation resulted in the microphase separation between the backbone of ETFE and side chain of PSSA but the final surface morphology of membranes can be determined at the grafting step. Conclusions: The surface morphology characteristics are more or less ascertained at the grafting process but not the sulfonation one. The PSSA grafts of ETFE-PEMs are dispersed on the membrane surface, favoring for the connection of ionic domains, leading to the increase in proton conductance on the membrane surface. It is suggested that the ETFE-PEMs exhibit the advantages in the membrane-electrode interfacial properties for PEMFC application due to its low accumulated surface in the low GD.