Investigation of chemical degradation and water states in the graft‐type polymer electrolyte membranes

Abstract

Chemical degradation in the presence of H2O2 aqueous solution and water states of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes (ETFE‐PEMs) with grafting degree (GD) of 8.8%–21.1% are investigated using Fourier‐transform infrared spectroscopy (FT‐IR), thermogravimetric analysis (TGA), and electrochemical impedance spectroscopy (EIS). The ETFE‐PEMs show the complete detachment at the α‐hydrogen atoms of the whole graft PSSA chains from the ETFE backbones, while their precursor polystyrene‐grafted ETFE films do not. This chemical degradation exhibits independently with GDs and takes place even at very low GD (<10%). Water molecules of the membranes dried at 40°C are not completely de‐absorbed. The remains are partial non‐bonded water molecules, which are assumed to locate within the lamellar amorphous domains (constrained water molecules) of the crystalline phases. These constrained water molecules exist independently with the PSSA groups and do not detach together with them during the degradation. Note that the water molecules remained at 40°C show the additional contribution of 18%–19% to the total water uptake but do not contribute clearly to the conductance.

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 .…”

“…In this method, styrene and/or its derivatives are usually grafted onto fully or partly fluorinated polymers such as poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA), (poly(tetrafluoroethyleneco-hexafluoropropylene) (FEP), poly(ethylene) (PE), poly(ethylene-co-tetrafluoroethylene) (ETFE), poly(vinylidene fluoride) (PVDF), and poly(tetrafluoroethylene) (PTFE) and subsequently by sulfonation to obtain graft-type PEMs 8 . Among these graft-type PEMs, polystyrene sulfonic acid (PSSA)-grafted poly(ethylene-cotetrafluoroethylene) (ETFE) polymer electrolyte membranes (ETFE-PEMs) have been investigated extensively as alternative membranes for PEMs [8][9][10][11][12][13][14][15][16] . The membrane exhibits good proton conductivity, mechanical integrity, and thermal stability compared to Nafion at a similar water uptake (40%).…”

“…The membrane exhibits good proton conductivity, mechanical integrity, and thermal stability compared to Nafion at a similar water uptake (40%). Moreover, ETFE-PEM shows the unique higher-order structures of lamellar, lamellar grains, and crystallite network structures, which are quite stable under immersed conditions 9,13,[17][18][19][20][21][22] . 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 .…”

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.…”