Mechanical ionomer membrane

ESR spectroscopy, used in the direct detection or spin trapping modes, is a sensitive method for the detection of polymer fragments and for determining the degradation mechanism. Recent applications for the study of stability in ionomer membranes used as proton exchange membranes in fuel cells demonstrate the capability of ESR to detect details that cannot be obtained by other methods. 

Early research of ionomer membrane degradation was conducted in the context of PEM electrolyzers. The detection of fluoride and other chain fragments in the condensed effluent water indicates the decomposition of PFSA ionomer and has long been noticed. Baldwin15 reported the detection of fluoride in the effluent of PEM electrolyzer and believed that it is the result of membrane mechanical failure. Extensive research has been conducted to elucidate the reaction pathways for membrane decomposition. Many controversial results and mechanisms have been reported in the literature, demonstrating the complex nature and the current inadequate understanding of the membrane degradation mechanisms. 

Yamaguchi T, Koval CA, Noble RD, and Bowman CN. Transport mechanism of carbon dioxide through perfluorosulfonic ionomer membranes containing an amine carrier. Chem. Eng. Sci. 1996 51 4781 789. 

Ionomer membranes based on perfluorocarbon polymers became available In the late 196O s. These materials have excellent chemical resistance, thermal stability, mechanical strength and strong acid strength, A number of functionalities have been studied. Including carboxylate, sulfonate and sulfonamide, but only the first two are available as commercial materials. Ferfluorlnated lonomers have been evaluated as membranes In a variety of applications, such as water electrolysis, fuel cells, air driers, Donnan dialysis In waste metal recovery, and acid catalysts, but the primary interest in these materials is for the permselective membrane In electrochemical processes such as In the production of chlorine and caustic (58). 

A good measure of past and continuing interest in ionomer membranes issued from the development of perfluorinated ionomers, the first-announced being Nafion(44). These materials are characterized by remarkable chemical resistance, thermal stability and mechanical strength, and they have a very strong acid strength, even in the carboxylic acid form. The functionalities that have been considered include carboxylate, sulfonate, and sulfonamide, the latter resulting from the reactions of amines with the sulfonyl fluoride precursor. 

The electrochemical (1) and mechanical properties(2,3) of the perfluorinated ionomer membranes as an ion-exchange membrane are obviously influenced by their internal structure of the membranes, especially spatial organization of the ionic sites. In this paper we attempted to carry out very basic studies on the structure of the perfluorinated ionomer membranes in the absence of applied external electric field. Although for practical applications of the membranes it is extremely important to study the structure under... 

Table III. Mechanical Properties of Perfluoro Ionomer Membrane. (Ion Exchange Capacity 0.03 meq/g Dry Resin).

There are unresolved issues in device models. For example sorption isotherms, the waters per acid group measured at steady state under zero flux conditions as a function of ambient relative humidity and temperature, play a central role in modeling hydration levels in ionomer membranes. However, the sorption isotherms depend sensitively upon pretreatment of the membrane, see [3], and moreover there is no reliable data for hydration levels under the conditions typical of fuel cell operation in which significant water and ion flux pass through the membrane which is under mechanical constraints which impact its ability to swell. 

M. Saito, S. Ikesaka, J. Kuwano, J. Qiao, S. Tsuzuki, K. Hayamizu, T. Okada, Mechanisms of proton transport in alcohol-penetrated perfluorosulfonated ionomer membranes for fuel cells. Solid State Ionics 178 (2007) 539—545. 

Mechanical stabilizing membranes in this way can allow significant stabilization of low EW ionomers. However, since many applications of PEM fuel cells require not only hotter and/or drier operation conditions but also require that the membrane to be insoluble in liquid water (often hot ) at times during operation, there is a limit to how low this method will allow one to go. In order to allow very low EW ionomers to be feasible, a change in the polymer chemistry will also probably be required. 

One approach is to modify the PFSA side-chains such that they cany more than one acid site since the ciystallinify and morphological properties arise essentially from the ratio of non-substituted TFE to functionalised I FE of the backbone polymer repeat unit, multi-acid side-chain ionomer membranes have the potential to demonstrate the mechanical properties of a higher EW polymer (characteristic of a single acid site per side-chain), and the proton conduction properties of a lower EW material (conferred by the presence of multi-acid sites per side-chain). This direction is being followed at 3M, where introduction of... 

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