A gas-phase H 2 O 2 exposure method in which a membrane was exposed to gaseous hydrogen peroxide to simulate the polymer electrolyte fuel cells ͑PEFC͒ environment was introduced to accelerate and assess membrane degradation. Gaseous hydrogen peroxide is able to degrade a membrane in the same manner as an actual fuel cell operation. This method is suitable for clarifying the membrane degradation mechanism because a membrane is degraded uniformly without contamination and mechanical degradation. The degradation mechanism of perfluorosulfonated membrane in a PEFC environment was investigated using the gasphase H 2 O 2 exposure method. An increase in the number of carboxyl groups and a rapid drop of molecular weight in degraded membrane with time exposed to gaseous hydrogen peroxide was observed. We concluded that degradation of the perfluorosulfonated membrane was composed of the following two modes: ͑i͒ unzipping reaction at unstable polymer end groups and ͑ii͒ scission of main chains and forming new unstable polymer end groups at severed points. It is very likely that membranes can be degraded by hydrogen peroxide alone; ferrous ions are not necessary for membrane degradation.

The degradation mechanism of the perfluorosulfonic acid (PFSA) membrane was investigated by a new accelerated test method, in which the membrane was exposed to H2O2 gas. It was clarified that the degradation of the PFSA polymer proceeds via a novel degradation mechanism. Subsequently, the influence of Pt particles, which deposited in the membrane during accelerated tests, on the degradation of the membrane was investigated. It was concluded that the Pt particles will not contribute to the degradation of the membrane.

Perfluorosulfonic acid (PFSA) ionomer is one of the key materials used in polymer electrolyte fuel cell (PEFC). Most of the membranes used in high performance and high durability PEFC are made from PFSA ionomers. The ionomer for the membrane is required to perform well at high temperatures up to 120 °C at low relative humidity (RH). The ionomer for the electrode, besides transporting the protons to the membrane, its function in cathode of delivering the oxygen to the Pt surface is highlighted in the recent progress of high activity catalyst enabling low Pt loading. Asahi Glass Co., Ltd. (AGC) is also proposing the PFSA ionomer to be used for creating hydrophilic microporous layer (MPL) in between the gas diffusion media (GDM) and the catalyst layer to achieve better performance in low RH conditions. The monomer structures and their copolymer properties as well as their cell performance examined are presented in this paper.

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