Gas Permeation in SPE Method: I . Oxygen Permeation Through Nafion and NEOSEPTA

Abstract

The permeation of oxygen at atmospheric pressure through Nation | 120 and NEOSEPTA | ACH-45T ion-exchange membranes was investigated by an electrochemical monitoring technique, which utilizes SPE composite electrodes prepared by an electroless plating method. The oxygen diffusion coefficients were almost the same (-10 7cm 2 9 s-') for each material, but the oxygen solubility was much higher in Nation than in NEOSEPTA. The oxygen solubility in NEOSEPTA could be explained in terms of dissolution in the aqueous component of the membrane, but the oxygen solubility in Nation was too high for such an explanation, and was postulated to involve the role of the polytetrafluoroethylene backbone. Recently, electrolyzers employing solid polymer elec-trolyte (SPE) have undergone extensive investigation for applications such as water electrolysis (1), brine electrolysis (2), and electro-organic synthesis (3-5). With these SPE methods, the permeation of electro-chemically reactive species through the SPE material results in decreases in current efficiency and un~canted side reactions. This has been observed, for example, in electro-organic syntheses using "both-sides" SPE methods (5), in which anodic and cathodic reactions take place on electrodes deposited onboth sides of the SPE membrane. Permeation by electrically neutral gases is easier than by ionic species bearing the same charge as the fixed ion of the SPE material because of the smaller size and absence of appreciable electrostatic repulsion in the case of the former. The permeation of diffusant depends strongly on the properties of the ion exchange membrane and on the conditions encountered. Recent solid-polymer electrolyte fuel cells have received considerable interest as electrochemically re-generative energy storage systems (6). As encountered in .electrolyzers, gas permeation through the solid-polymer materials (usually ion-exchange membranes) decreases efficiency of fuel cells. Because of its high chemical and thermal stability, a perfluorinated sulfonic acid membrane such as Nation@ usually is employed in a solid-polymer electrolyte fuel cell. Perfluorinated ion-exchange membranes also are desirable for use with SPE electrolyzers, although conventional ion-exchange membranes based on a styrene-divinylbenzene copolymer may be applicable to some systems (3). Naton is based on a polytetrafluoroethylene backbone with pendant side chains of the form * Electrochemical Society Ac.tive Member.