Oxidative and Photochemical Stability of Ionomers for Fuel‐Cell Membranes

Abstract

In memoriam Professor Hanns FischerTo predict hydroxyl-radical-initiated degradation of new proton-conducting polymer membranes based on sulfonated polyetherketones (PEK) and polysulfones (PSU), three nonfluorinated aromatics are chosen as model compounds for EPR experiments, aiming at the identification of products of HOC-radical reactions with these monomers. Photolysis of H 2 O 2 was chosen as the source of HOC radicals. To distinguish HOC-radical attack from direct photolysis of the monomers, experiments were carried out in the presence and absence of H 2 O 2 . A detailed investigation of the pH dependence was performed for 4,4'-sulfonylbis[phenol] (SBP), bisphenol A (= 4,4'-isopropylidenebis [phenol]; BPA), and [1,1'-biphenyl]-4,4'-diol (BPD). At pH ! pK A of HOC and H 2 O 2 , reactions between the model compounds and O 2 C À or 1 O 2 are the most probable ways to the phenoxy and semiquinone radicals observed in this pH range in our EPR spectra. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings. Investigations at low pH are particularly relevant for understanding degradation in polymer-electrolyte fuel cells (PEFCs). However, the chemistry depends strongly on pH, a fact that is highly significant in view of possible pH inhomogeneities in fuel cells at high currents. It is shown that also direct photolysis of the monomers leads to semiquinone-type radicals. For SBP and BPA, this involves cleavage of a CÀC bond.Introduction. -In the past years, there has been a tremendous acceleration in research devoted to nonfluorinated polymer membranes, both as competitive alternatives to commercial perfluorosulfonic acid membranes operating in the same temperature range and with the objective of extending the range of operation of polymer fuel cells toward those more generally occupied by phosphoric acid fuel cells [1]. Consequently, much effort has focused on the development of alternative proton-exchange membranes for PEFCs (polymer-electrolyte fuel cells) and DMFCs (direct methanol fuel cells), in particular with the aim of increasing their temperature of operation. Important advantages in terms of water management, increased proton conductivity, and CO tolerance in fuel cells can be gained by operating at higher temperature, typically at ca. 1208. This operating temperature imposes additional stringent requirements in terms of membrane stability in a highly oxidative environment. There are few nonfluorinated membrane materials appropriate for fuel cell applications at temperatures above 808, and they are made up of polyaromatic or polyheterocyclic repeat units.

Degradation of a sulfonated aryl ether ketone model compound in oxidative media (sPAEK)

“…This is in good agreement with the work of Hü bner and Roduner [37], as the presence of electron-withdrawing groups is reported to disfavour the addition of HO . As witnessed by the LC-MS analysis (Fig.…”

“…While mechanical aging processes such as successive swellingdeswelling cycles can induce some membrane fragility, chemical degradations are suspected to be the major factors limiting the durability of these alternative polymers [1,[29][30][31][32][33][34][35][36][37][38][39][40]. As H 2 O 2 has already been detected in the water produced during a fuel cell test performed with Nafion membrane [36], hydroxyl radicals (HO ) are suspected to be responsible for oxidative degradation reactions.…”

Partially Fluorinated Arylene Polyethers and Their Ternary Blend Membranes with PBI and H3PO4. Part I. Synthesis and Characterisation of Polymers and Binary Blend Membranes

“…For testing the membrane degradation by radicals (for membrane degradation mainly OH radicals are responsible as could be ascertained by Roduner et al [28][29][30]), two different methods were used: l a) The membranes were immersed in a 5 wt.-% H 2 O 2 solution, and the solution was kept at a temperature of 60 C for the duration of immersion of up to 96 h. After different H 2 O 2 treatment times, the membrane samples were removed from the solution, rinsed with water, dried and weighed to determine the weight loss for the respective immersion times. For some of the pure PBI and baseexcess (70% PBI and 30% 1b) membranes the M w degradation was also determined via GPC.…”

Preparation and Characterization of Stable Ionomers and Ionomer Membranes for Fuel Cells

“…In the present study, attention is focused on 4,4′-diphenol (DHBP) and the sulfonated 4,4′-dichlorodiphenylsulfone (sDCPS), which are monomeric building blocks of typical alternative polysulfone ionomers, for fuel cell membrane applications [15]. Results of analogous experiments with 4,4'-sulfonyldiphenol and with 4,4'-isopropylidenediphenol (bisphenol A) have been reported elsewhere [16].…”