Vicious cycle during chemical degradation of sulfonated aromatic proton exchange membranes in the fuel cell application (original) (raw)
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Journal of The Electrochemical Society, 2006
Nonfluorinated sulfonic acid membranes are a group of promising candidate materials for the commercialization of proton exchange membrane fuel cell ͑PEMFC͒ technology. However, one of the main obstacles is that the harsh fuel cell environment may originate different modes of degradation and aging processes that result in either chemical or morphological alteration in these membranes. The effect of peroxide radicals on PEM durability is of particular interest because a common feature of many hydrocarbon-based membranes is that the building block consists of sulfonic acid-substituted aromatic rings, which are much more sensitive to radical attack than the Teflon-like backbone in perfluorinated sulfonic acid type materials. In this work, we attempt to provide answers to the hydroxyl radical initiated durability issues at the PEM and electrocatalyst interface by analyzing the performance of two novel membranes, sulfonated poly͑arylene ether sulfone͒ and sulfonated poly͑ether ether͒ ketone, using a newly designed durability evaluation method under fuel cell-like conditions. This method is able to separate the membrane evaluation process into cathode and anode aspects. Under experimental conditions in this work, degradations in SPES-40 samples were found happening at the cathode ͑oxygen͒ side of the PEMFC.
Physical Chemistry Chemical Physics, 2003
The lifetime behavior of a H 2 /O 2 proton exchange membrane (PEM) fuel cell with polystyrene sulfonic acid (PSSA) membrane have been investigated in order to give an insight into the degradation mechanism of the PSSA membrane. The distribution of sulfur concentration in the cross section of the PSSA membrane was measured by energy dispersive analysis of X-ray, and the chemical composition of the PSSA membrane was characterized by infrared spectroscopy before and after the lifetime experiment. The degradation mechanism of the PSSA membrane is postulated as: the oxygen reduction at the cathode proceeds through some peroxide intermediates during the fuel cell operation, and these intermediates have strong oxidative ability and may chemically attack the tertiary hydrogen at the a-carbon of the PSSA; the degradation of the PSSA membrane mainly takes place at the cathode side of the cell, and the loss of the aromatic rings and the SO 3 À groups simultaneously occurs from the PSSA membrane. A new kind of the PSSA-Nafion composite membrane, where the Nafion membrane is bonded with the PSSA membrane and located at the cathode of the cell, was designed to prevent oxidation degradation of the PSSA membrane in fuel cells. The performances of fuel cells with PSSA-Nafion101 and PSSA-recast Nafion composite membranes are demonstrated to be stable after 835 h and 240 h, respectively.
Chemistry of Materials, 2019
The perceived poor durability of non-fluorous, hydrocarbon solid polymer electrolyte membranes in the presence of reactive hydroxyl radicals remains a significant hurdle for their integration into electrochemical systems such as fuel cells. However, recent reports point to sulfonated phenylated polyphenylenes (sPPP) being considerably stable in accelerated fuel cell tests. In order to investigate the possible reaction of hydroxyl radicals with this promising class of hydrocarbon polymer electrolytes, a structurallyanalogous oligophenylene model compound was synthesized and its degradation route was studied in the presence of hydroxyl radicals. Using NMR spectroscopy and mass spectroscopy, all significant degradation products are characterized and based on their chemical structures, along with changes in concentration over time, a degradation route is proposed. Hydroxyl-radical degradation was observed and found to be initiated by the oxidation of pendant phenyl rings to form fluorenone sub-structures which, upon further oxidation, lead to ring-opening of a main chain phenyl ring which, if occurring in sPPP, leads to chain-scission of the polymer backbone. In keeping with this hypothesis, molecular weights of sPPP were found to decrease when subject to hydroxyl radicals. Although degraded polymer NMR spectra remain unchanged, resonances consistent with the elimination of sulfobenzoic acid emerge. The results outlined in this work point towards a promising future for sPPP membranes and suggest a simple modification which should enhance their lifetime within fuel cell systems.
A review of molecular-level mechanism of membrane degradation in the polymer electrolyte fuel cell
Membranes, 2012
Chemical degradation of perfluorosulfonic acid (PFSA) membrane is one of the most serious problems for stable and long-term operations of the polymer electrolyte fuel cell (PEFC). The chemical degradation is caused by the chemical reaction between the PFSA membrane and chemical species such as free radicals. Although chemical degradation of the PFSA membrane has been studied by various experimental techniques, the mechanism of chemical degradation relies much on speculations from ex-situ observations. Recent activities applying theoretical methods such as density functional theory, in situ experimental observation, and mechanistic study by using simplified model compound systems have led to gradual clarification of the atomistic details of the chemical degradation mechanism. In this review paper, we summarize recent reports on the chemical degradation mechanism of the PFSA membrane from an atomistic point of view.
Journal of Membrane Science, 2004
Proton exchange membrane fuel cells (PEMFC) are promising new power sources for vehicles and portable devices. Membranes currently used in PEMFC are perfluorinated polymers such as Nafion ® . Even though such membranes have demonstrated good performances and long-term stability, their high cost and methanol crossover makes them unpractical for large-scale production. Sulfonated aromatic poly(ether ether ketones) (S-PEEKs) based membranes have been studied due to their good mechanical properties, thermal stability and conductivity. In this study, PEEK membranes directly prepared from the sulfonated monomer were evaluated for possible fuel cell applications by determining the degree of sulfonation, water swelling, proton conductivity, methanol diffusivity and thermal stability. As synthesized S-PEEK membranes exhibit conductivities (25 • C) from 0.02 to 0.07 S/cm, water swelling from 13 to 54%, ion-exchange capacities (IEC) from 0.7 to 1.5 meq/g and methanol diffusion coefficients from 3 × 10 −7 to 5 × 10 −8 cm 2 /s at 25 • C. These diffusion coefficients are much lower than that of Nafion ® (2 × 10 −6 cm 2 /s), making S-PEEK membranes a good alternative to reduce problems associated with high methanol crossover in direct methanol fuel cells.