Gas Permeation in SPE Method: II . Oxygen and Hydrogen Permeation Through Nafion

Abstract

The permeation of hydrogen and oxygen through Nation | of different water contents was investigated by an elec-trochemical monitoring technique utilizing SPE composite electrodes. For both gases, the solubility, c ~ was very high compared with the value in aqueous solutions and was on the order of 10-2 tool-din-3, The diffusion constant, D, was on the order of 10-7 cm~-s-'. While D increased with increasing exchange capacity, c o was independent of the capacity. D and c o were almost independent of the water content of the Nation. Hydrophobic cations, R4N § decreased D. It was concluded that the permeation of gases through Nation takes place in the intermediate region, which consists of the flexible amorphous part 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 syntheses (3-5). In gas evolving electrolyzer systems, gas permeation through SPE materials results in decreases in current efficiencies. This has been observed in Kolbe-type reactions using a "both-sides" SPE method as well as in water electrolysis. Similar decreases in efficiencies probably are also observed in SPE fuel cells (6), Nation pertiuorinated cation exchange membranes are used widely in many electrochemical systems (7-9), including SPE technologies, because of their chemical and thermal stabilities and their high ionic conductivities. The structure of Nation has attracted the interest of many workers and has been the subject of considerable investigation. An ionic cluster model by Gierke et al. (10-12) has been basically accepted widely. This model satisfactorily explains Nation's mechanical properties (I0) and transport behavior (I 1-15). Yeager and Steck have modified the spherical ionic cluster model and have proposed a model based on irregular ionic clusters, composed of three regions, viz., a hydrophobic backbone, aqueous ionic clusters, and intermediate regions (16). The present authors have investigated oxygen permea-tion through Nation 120 using an electrochemical monitoring technique and have reported previously the diffusion coefficient and the solubility of oxygen (17). The very high solubility of oxygen was briefly discussed, and the influence of the polytetrafluoroethylene backbone was inferred to explain the high solubility. In the present work, solubilities and diffusion coefficients of hydrogen and oxygen have been examined in detail for different eounterions and water contents. Based on the results obtained, the permeation of these gases through Nation is discussed with reference to the model by Yeager and Steck. * Electrochemical Society Active Member. Experimental Chemicals .-All chemicals, with the exception of hydra-zinc, were reagent grade. Hydrazine was chemical pure grade. All chemicals were used without further purification. Hydrogen, nitrogen, and argon were of purity higher than 99.9%, oxygen of 99.8%. Membranes and deposition of platinum.-Nafion 125 (nominal exchange capacity of 0.83 meq/g-dry polymer and thickness of 0.125 mm), Nation 120 (0.83 meq/g-dry polymer, 0.250 turn), and Nation 117 (0.91 meq/g-dry polymer , 0.175 ram) were chosen for this study. Nation is known to change its water content depending on the treatment conditions, such as boiling time and counterions, and the electrolyte concentration of the soaking solutions (18). The membranes were pretreated in the manner of Yeager and Steck (19). Platinum was deposited on one side of the membranes after boiling membranes in the H + form for 3h. The prepared SPE composite electrode, referred to as Pt-Nation, was soaked in a solution containing a given M + ion for more than 36h at 25~ for equilibration. These composites are referred to as M-type membranes. Solutions containing M ~ were prepared from M2SO 4 and distilled water at 0.5 mol-dm-3 for alkali metal ion solutions and from R4NC1 at 1 mol-dm-3 for tetraalkylammonium ion solutions. Some of the Nation membranes were treated according to a different procedure in which they were boiled in the Na ~ form in water for 0.5 or 3h, followed by deposition with platinum, The prepared Pt-Nafion then was soaked in M + ion solutions as described above. Composites prepared thus are referred to as Ma-type (0.5h boiling) and Mb-type (3h boiling) membranes. Platinum deposition was carried out by utilizing a previously described eleetroless plating method using hydrazine as a reduetant (3, 17). The resis-tivity of the prepared Pt-Nafion was less than 4~ at 2 cm distance on the surface arid it did not increase beyond 6~1 after repeated permeation measurements.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 155.97.178.73 Downloaded on 2014-12-02 to IP