Investigation of the Lamellar Grains of Graft-type Polymer Electrolyte Membranes for Hydrogen Fuel Cell Application using Ultrasmall-angle X-ray Scattering

Abstract

The extensive ultrasmall-angle X-ray scattering measurements are performed in order to investigate the changes of lamellar grains of poly(styrenesulfonic acid)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) that occur during the alteration of grafting degree (GD) under dry and immersed conditions. The lamellar grains of three series of the samples (polystyrene-grafted ETFE films and dry and hydrated ETFE-PEMs) are formed during the grafting process and develop independently with the change of the lamellar stacks. Interestingly, three series of samples exhibit a very similar trend of lamellar grain at any GD and a significant amount of graft chains is observed directly in the region between the grains (GD £ 59%) and outside of the grain network structures (GD > 59%). This observation indicates: i) The formation of the lamellar grains; ii) The rapid changes in characteristic sizes of the lamellar grains compared with the lamellar stacks; and iii) The newly generated phases consisting of only the graft materials. These findings explain why the lamellar grains and the graft chains play an important role in the higher proton conductivity and compatible tensile strengths of the membranes, compared with Nafion, at the immersed and severe operating conditions.

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

“…The membranes were finally hydrolyzed by pure water at 50 • C for 24 hours to obtain the ETFE-PEMs. Note that in our previous works, the grafting was already confirmed using Fourier transform infrared spectroscopy (FT-IR) 12,29 and X-ray photoelectron spectroscopy (XPS) 30 . Moreover, the presence of polystyrene grafts in the grafted-ETFE films and final membranes (ETFE-PEMs) was also affirmed using ultrasmall and small-angle X-ray scattering (USAXS/SAXS) 14,21,25,27,28 and positron annihilation lifetime spectroscopy (PALS) 24,27 .…”

“…However, the DTG curve exhibits an additional thermal decomposition step at 242 1). 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] .…”

“…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 .…”

From Academia to Industry: Criteria for Upscaling Ionic Liquid-Based Thermo-Electrochemical Cells for Large-Scale Applications

“…The XPS spectra show mainly the presence of graphitic carbon, fluorine, oxide, nitrogen, and sulfur. The latter two varieties come from the interaction with the ionic liquid; in fact, they reflect the binding energy associated, respectively, with organics containing nitrogen at around 400.4 eV [32] and sulfur compounds at around 167.2 eV [33,34]. F1s has a dominant peak at 687.2 eV associated with halogenated compounds [35] related to the ionomer used for electrode preparation, which contains fluorine.…”

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

“…Recently, poly(styrene sulfonic acid)-grafted poly(ethyleneco-tetrafluoroethylene) (EP) electrolyte membranes (ETFE-PEMs) have been studied intensively to replace Nafion because of their competitive price, performance, and durability. Compared with Nafion, the ETFE-PEM shows greater mechanical strength, conductance, and microstructural stability under immersed conditions [6][7][8][9] . Positron annihilation lifetime spectroscopy (PALS) is a potential method that provides molecular and nanoscale information on the free-volume hole features of materials through the lifetime and annihilation intensity of positron and positronium (Ps) ions.…”