High Sensitivity of Electron Transfer Rates within Nafion Coatings Saturated with Os ( bpy ) 3 2 +  to the Extent of Hydration of the Coating

Abstract

The sensitivity of the electrochemical responses obtained from electroactive counterions incorporated in Nation coatings to the extent of hydration of the coatings 2748 is shown to be enhanced significantly when the electroactive ion is the only counterion present in the coating. Nation coatings fully saturated with Os(bpy)~ + exhibit unusually narrow and sharply peaked anodic currents in first-scan oxidative voltammograms. This voltammetric feature is accompanied by the expulsion of one-third of the Os complex from the coating. The counterion actually ejected is primarily Os(bpy)] § As the Os(bpy)~ § counterions are ejected, H20 molecules enter the coatings. The quantity of H20 molecules incorporated is so large that the mass of the coating (monitored with a quartz crystal microbalance) increases despite the ejection of the heavy Os(bpy)~ § cations. After several voltammetric cycles, normally shaped voltammograms are obtained that are relatively insensitive to the initial state of hydration of the coatings. The contrasting behaviors of hydrated and unhydrated coatings are compared, and possible explanations are offered for the differences observed.Nation deposited on electrode surfaces is one of the most extensively investigated polyelectrolyte coatings that have been employed to modify the behavior of electrodes. The facile incorporation and retention by Nation coatings of cationic complexes with hydrophobic ligands such as 2,2 '-bipyridine have induced many previous workers to examine the electrochemical responses of electroactive complexes of this type within Nation coatings. Representative early examples can be found in Ref. 1-13. Most previous studies have involved coatings in which only a portion of the initially present, electroinactive counterions (e.g., H § Na § were replaced by electroactive ions, but in one case experiments were conducted with Nation coatings that had been exposed to solutions of Os(bpy)~ § (bpy = 2,2'-bipyridine) for extended periods so that Os(bpy)~ § cations became the only counterions within the coating. 14 Electro-oxidation of these counterions to Os(bpy)~ § required that some of them be ejected from the permselective, polyanionic Nation to maintain electroneutrality within the coating. Such ejection was monitored by means of microprobe electrodes positioned just above the coatings, but only qualitative measurements were possible because of variability in the sensitivity of the microprobe electrodes at Nafion coated electrodes. 14 The present study constitutes an extension and elaboration of this previous work. The use of rotating ring-disk electrodes to monitor the ejection of counterions from Nation coatings allowed quantitative determinations to be obtained. In addition, we observed a high sensitivity of the voltammetric responses obtained from coatings saturated with Os(bpy)~ § to the water content of the coatings. Procedures for varying the extent of hydration during the preparation of coatings were developed to demonstrate the dramatically different types of behavior that c...

Electrochemical quartz crystal microbalance study of mass transport during electrochemical oxidation of [Os(bpy)3]2+ in clay-modified electrodes

“…Here the MPE increased when the charge passing in the initial part of the anodic scan was omitted. It is also similar to early reports by Anson and coworkers [24,25] that [Os(bpy) 3 ] 3+ ions were ejected from Nafion films saturated with [Os(bpy) 3 ] 2+ during the initial anodic scan. Subsequent EQCM measurements on this system however, revealed that the expulsion of the Os(III) cations resulted in an increase in the mass of the films, attributed to a large inflow of water molecules and of electrolyte counter ions into the films during ejection of the Os(III) ions [26,27].…”

“…They proposed that one [Ru(bpy) 3 ] 3+ ion was ejected out of the films for each three [Ru(bpy) 3 ] 2+ ions oxidized. This is similar to a report that charge neutralization during the electrochemical oxidation [Os(bpy) 3 ] 2+ in Nafion films occurred by ejection of some of the oxidized ions [24,25]. However, the ejection of [Os(bpy) 3 ] 3+ ions from hydrated Nafion films has been reported to result in a mass increase during the initial anodic scan.…”

An infrared study of the effects of hydration on cation-loaded nafion thin films

“…These cast films of Nafion retain many of the properties (e.g., water insolubility and ionexchange ability) that characterize Nafion membranes. 22 Shi and Anson 23,24 showed that the ionomer clustering that occurs in membranes also is obtained in spin-coated thin films as evidenced by AC impedance and voltammetric measurements. The spectroscopic data obtained in this study also follow trends seen in IR data obtained with Nafion membranes.…”

“…After a very rapid initial uptake, there appears to be a regime of slower incorporation that continues past 75 h of immersion. Shi and Anson 24 reported that Os(bpy) 3 2ϩ loads from a 1 mM solution into hydrated Nafion within 2 h but that in an unhydrated film immersions ranging from 24 -48 h were required to obtain full loading. The initial rate behavior we obtained appears to follow the former profile most closely.…”

“…This model might reflect a varying polarization of the OSO 3 Ϫ by an associated cation 13 or even the localization of that cation in portions of the membrane with varying hydrophobic character, as has been suggested by others. 7,24 These arguments as to the microscopic nature of the mechanism are essentially a static description of the electrostatic interactions of the cationanion pair (and possibly, closely related hydrogen-bonding interactions as well). From data such as those shown in Figure 3, we know that such models are overly simplistic and fail to account for the quantitative functional form of the optical anisotropies seen as the water content is changed.…”