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

Abstract

The subnano free‐volume hole features of Nafion‐212 and poly(styrene sulfonic acid) grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes are investigated by using the positron annihilation lifetime spectroscopic analyses with three‐ and four‐component models (i.e., one‐ and two‐ortho‐positronium [o‐Ps] components). The four‐component model provides a more adequate description of free‐volume hole features for both membranes, in which the longer o‐Ps lifetime is assigned to the larger free volume hole sizes in the mobile side chains, while the shorter o‐Ps lifetime is associated with the smaller free volume hole sizes within the backbones (the rigid amorphous fractions and the crystalline‐amorphous interfaces) and the interfaces between the main chains and the side chains. The o‐Ps annihilation is found to occur primarily in the side chains. The subnano volume hole features revealed by the positron annihilation lifetime (PAL) spectra suggest the primary water uptake and conductance in the side chains and the possible presence of water molecules in the rigid amorphous fractions and the interfaces. Note that the three‐component model as usually reported in the literature may underestimate the lifetime and intensity of ortho‐positronium annihilation.

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 this study, the subnanometer holes in all the samples are assumed to be spherical. This assumption is used for many PAL spectroscopic analyses of polymers 13 . The relationship between the hole radius and the lifetime of ortho-positronium trapped in the holes is shown by the Tao-Eldrup (TE) model, which is represented by the following formula 10,25 :…”

“…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.…”

Investigation of KOH‐doped PVA‐based membrane electrolytes cross‐linked with tartaric acid for DMAFCs

“…13 In recent decades, there has been growing interest in fuel cell technology, with a particular emphasis on the adoption of PEMFCs. [14][15][16][17][18] They can be classified according to electrolyte type and operating temperature. PEM fuel cells are typically used in portable and transportation systems owing to their advantages over conventional technologies, including high energy efficiency, silence, low pollutant emissions, ease of installation, and quick start-up.…”

“…Among the different types of fuel cells, five main categories are noteworthy: (i) polymer electrolyte membrane (PEM), (ii) molten carbonate, (iii) solid oxide, (iv) phosphoric acid, and (v) alkaline‐type fuel cells 13 . In recent decades, there has been growing interest in fuel cell technology, with a particular emphasis on the adoption of PEMFCs 14–18 . They can be classified according to electrolyte type and operating temperature.…”