Electroactive ion exchange membranes based on conducting polymers

Ion exchange membranes are indispensable for the separation of ionic species. They can discriminate between anions and cations depending on the type of fixed ionic group present in the membrane. These conventional ion exchange membranes (CIX) have exceptional ionic conductivity, which is advantageous in various electromembrane separation processes such as electrodialysis, electrodeionisation and electrochemical ion exchange. The main disadvantage of CIX membranes is their high electrical resistance owing to the fact that the membranes are electronically non conductive. An alternative can be electroactive ion exchange membranes, which are ionically and electronically conducting. Polypyrrole (PPy) is a type of electroactive ion exchange material as well as a commonly known conducting polymer. When PPy membranes are repeatedly reduced and oxidised, ions are pumped through the membrane. The main aim of this thesis was to develop electroactive cation transport membranes based on PPy for the selective transport of divalent cations. Membranes developed composed of PPy films deposited on commercially available support materials. To carry out this study, cation exchange membranes based on PPy doped with immobile anions were prepared. Two types of dopant anions known to interact with divalent metal ions were considered, namely 4-sulphonic calix[6]arene (C6S) and carboxylated multiwalled carbon nanotubes (CNT). The transport of ions across membranes containing PPy doped with polystyrene sulphonate (PSS) and PPy doped with para-toluene sulphonate (pTS) was also studied in order to understand the nature of ion transport and permeability across PPy(CNT) and PPy(C6S) membranes. In the course of these studies, membrane characterisation was performed using electrochemical quartz crystal microbalance (EQCM) and scanning electron microscopy (SEM). Permeability of the membranes towards divalent cations was explored using a two compartment transport cell. EQCM results demonstrated that the ion exchange behaviour of polypyrrole is dependent on a number of factors including the type of dopant anion present, the type of ions present in the surrounding medium, the scan rate used during the experiment and the previous history of the polymer film. The morphology of PPy films was found to change when the dopant anion was varied and even when the thickness of the film was altered in some cases. In nearly all cases the permeability of the membranes towards metal ions followed the order K+ > Ca2+ > Mn2+. The one exception was PPy(C6S), for which the permeability followed the order Ca2+ ≥ K+ > Mn2+ > Co2+ > Cr3+. The above permeability sequences show a strong dependence on the size of the metal ions with metal ions having the smallest hydrated radii exhibiting the highest flux. Another factor that affected the permeability towards metal ions was the thickness of the PPy films. Films with the least thickness showed higher metal ion fluxes. Electrochemical control over ion transport across PPy(CNT) membrane was obtained when films composed of the latter were deposited on track-etched Nucleopore® membranes as support material. In contrast, the flux of ions across the same film was concentration gradient dependent when the polymer was deposited on polyvinylidene difluoride membranes as support material. However, electrochemical control over metal ion transport was achieved with a bilayer type of PPy film consisting of PPy(pTS)/PPy(CNT), irrespective of the type of support material. In the course of studying macroscopic charge balance during transport experiments performed using a two compartment transport cell, it was observed that PPy films were non-permselective. A clear correlation between the change in pH in the receiving solution and the ions transported across the membrane was observed. A decrease in solution pH was detected when the polymer membrane acted primarily as an anion exchanger, while an increase in pH occurred when it functioned as a cation exchanger. When there was an approximately equal flux of anions and cations across the polymer membrane, the pH in the receiving solution was in the range 6 - 8. These observations suggest that macroscopic charge balance during the transport of cations and anions across polypyrrole membranes was maintained by introduction of anions (OH-) and cations (H+) produced via electrolysis of water

Transport of metal ions across an electrically switchable cation exchange membrane based on polypyrrole doped with a sulfonated calix[6]arene

The conducting polymer polypyrrole, PPy, was used as the active component of a cation exchange membrane for transferring a range of metal ions between two solutions by electrical modulation of the polymer between its conducting and non-conducting states. The cation exchange membranes consisted of platinum sputter-coated polyvinylidene difluoride (PVDF) which had been coated with polypyrrole doped with sulfonated calix[6]arene (PPy(C6S)). It was shown that applying a constant potential in the range 0.4 to +0.6 V did not result in any metal ion flux across the PVDF/Pt-PPy(C6S) membrane. However, a gradual but steady increase in metal ion concentration was detected in the receiving cell when -0.8 V was applied to the membrane. To investigate the effect of film thickness on the permeability of metal ions, transport experiments were performed with composite membranes containing 2.0, 3.3, and 5.6 mu m thick PPy(C6S) films. The permeability of the metals across the membrane increased as the PPy(C6S) film thickness decreased and was found to decrease in the following order: Ca2+ \u3e K+ \u3e Mn2+ \u3e\u3e Co2+, when the receiving cell contained deionised water. The PVDF/Pt-PPy(C6S) composite membrane showed significant permeability towards metal ions such as Ca2+, K+ and Mn2+, with the flux for Ca2+ higher than that seen for this metal ion with any previously studied PPy films containing other complexing dopants. (C) 2010 Elsevier B.V. All rights reserved

Electrochemically controlled transport of anions across polypyrrole-based membranes

Electrosynthesized polypyrrole (PPy) films on PVDF/Pt support membranes were used to study electrochemically controlled transport of monovalent and divalent anions across the PPy-based membranes. The PVDF/Pt/PPy membrane separated two compartments in a transport cell and served simultaneously as the working electrode allowing electrochemical switching of PPy between its oxidized and reduced forms during the transport experiments. PPy was electrosynthesized in presence of p-toluene sulfonate, camphorsulfonate and hexafluorofosfate (doping ions) and the ion transport properties of the resulting PPy-based membranes were investigated. The morphology and elemental composition of PPy before and after the anion transport studies were studied by SEM and EDXA. The anion fluxes obtained during transport of a mixture of Cl−, NO3−, SO42− and HAsO42− across the PPy-based membranes were found to decrease in the following order: NO3− > Cl− » SO42− > HAsO42−. PPy films with the same composition as for the transport studies were also electrodeposited on glassy carbon electrodes (GC/PPy) and studied by potentiometry. The potentiometric selectivity of the GC/PPy electrodes towards Cl−, NO3− and SO42− was found to correlate with the ion transport characteristics of the corresponding PPy membranes used in the transport cell. The PPy-based membranes allowed separation of monovalent from divalent anions.Accepted versio

Ion exchange behaviour and charge compensation mechanism of polypyrrole in electrolytes containing mono-, di- and trivalent metal ions

The electrochemical quartz crystal microbalance technique was used to examine the ion exchange behaviour and charge compensation mechanism of polypyrrole (PPy) doped with polystyrene sulfonate (PSS) immersed in electrolytes containing singly, doubly and triply charged metal ions. New insights were obtained concerning the mechanism of charge compensation and ion exchange behaviour of PPy/PSS films in electrolytes with triply and doubly charged transition metal ions. Understanding the ion exchange behaviour of conducting polymers immersed in these types of aqueous media could play a pivotal part in the development of methods for removing toxic metals from water supplies. Charge compensation occurred predominantly by means of cation movement for PPy/PSS films when the electrolyte was KNO3, Ca(NO3)(2), Mn(NO3)(2) or Co(NO3)(2). After prolonged redox cycling the electroactivity, of the polymer decreases slightly, and the charge compensation mechanism becomes complex owing to movement of ions and neutral species in opposite directions. The charge compensation mechanism for PPy/PSS films immersed in aqueous Cr(NO3)(3) and Al(NO3)(3) solutions was also complex, with the ion exchange behaviour shifting towards anion movement to and from the polymer. (C) 2009 Elsevier B.V. All rights reserved

Simultaneous monitoring of the transport of anions and cations across polypyrrole based composite membranes

A mechanism for the macroscopic charge balance during the transport of anions and cations across polypyrrole based composite membranes is proposed. For the mechanism to be studied, anions and cations were monitored simultaneously across PPy based composite membranes, which are known to have cation exchange (PPy(PSS)), anion exchange (PPy(ClO 4)) and mixed ion exchange properties (PPy(pTS)). Even though none of the membranes were found to be completely permselective, the flux of cations was higher than that of anions across the PPy(PSS) composite membrane, while the flux of anions was higher than that of cations across the PPy(ClO 4) composite membrane. Distinct changes in pH of the receiving solution were also observed. These were a decrease in pH when a predominantly anion exchanging polypyrrole composite membrane was used, and an increase in pH when a membrane that maintains charge balance principally by cation exchange was used. When membranes which display approximately equal permeability towards anions and cations were used the pH of the receiving solution was ca. 6-8. There was only a negligible flux of Ca 2+ across the PPy(PSS) membrane in the transport experiments carried out with the source solution consisting of either Ca(NO 3) 2 or an equimolar mixture of KNO 3 and Ca(NO 3) 2. The PPy(PSS) composite membrane was impermeable towards NO 3 - ions when the source solution was Ca(NO 3) 2 but permeability towards NO 3 - was observed when the source solution was either KNO 3 or an equimolar mixture of KNO 3 and Ca(NO 3) 2. Copyright 2011 Elsevier Ltd. All rights reserved

Electrochemically controlled ion transport across polypyrrole/multi-walled carbon nanotube composite membranes

Polypyrrole (PPy) films doped with acid-treated multi-walled carbon nanotubes (CNTs) were prepared by galvanostatic polymerisation of pyrrole (Py) dissolved in aqueous dispersions of CNTs, which served as the background electrolyte. Morphological characterization of PPy(CNT) films showed a random deposition of the polymer on the surface of the support material, with the former creating a porous, interconnecting three-dimensional network structure. The porous nature of PPy(CNT) films favours movement of large amounts of water in and out of the films during redox cycling, which also causes extensive volume changes. Raman spectroscopy, the electrochemical quartz crystal microbalance method and scanning electron microscopy were used to explore the properties of PPy(CNT) films. In addition, the factors influencing the electrochemically controlled transport of metal ions across composite membranes composed of polypyrrole doped with acid-treated multi-walled carbon nanotubes (PPy(CNT)) were studied. The factors examined were: The polymerization conditions (current density and time), the type of support material on which the polypyrrole films were deposited, and the use of single layer or bilayer type polypyrrole films. The flux of metal ions across the composite membranes was able to be electrochemically controlled when NucleoporeR track-etched membranes were used as the support material, while the flux was controlled by the concentration gradient present when polyvinylidene difluoride (PVDF) was used as the support material for PPy(CNT) films. However, electrochemically controlled movement of metal ions across PVDF-supported membranes was achieved when a bilayer type PPy film (PPy(pTS)/PPy(CNT)) was used (pTS = toluene-4-sulfonic acid). Increasing the thickness of the PPy(CNT) layer in the composite membrane was found to enhance the membranes permeability towards K+. Copyright 2011 Elsevier B.V. All rights reserved