Surface features of polymer electrolyte membranes for fuel cell applications: An approach using S2p XPS analysis

Abstract

Introduction: Hydrogen fuel cell is a promising renewable energy technology. Proton exchange membranes (PEMs) are an essential component of the fuel cell. Although the top surface nature of PEMs directly relates to the durability and efficiency of PEM fuel cells (PEMFC), there has been very limited attention on the chemical compositions and their distribution at the membrane surface. Thus, this study reports the surface characterization of poly(styrene sulfonic acid) (PSSA)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) for fuel cell applications using S2p X-ray photoelectron spectroscopic (XPS) analysis. Methods: The ETFE-PEMs were prepared by radiation-induced grafting and subsequent sulfonation. The surface features of ETFE-PEMs with grafting degrees (GDs) of 55-101% were characterized using the survey-wide and narrow scans of S2p XPS. Results: The surface concentration of sulfur slightly decreased with increasing GDs. This interesting result has not been reported in the other graft-type PEMs. The four-component model of narrow scan deconvolution exhibited more reasonable results than the two-component model, in which the sulfonic acid groups (SO-3) slightly increased with increasing GD , not as the case observed by wide scan spectra due to the presence of by-products. Conclusion: The PSSA grafts were mainly generated inside the bulk rather than at the surface possibly due to the morphological changes. Moreover, the less presence of SO-3 group on the membrane surface despite the large GD values (GD = 55-101%) suggests the advanced electrolyte properties and interfacial stability for fuel cell applications.