Autogenous Oxidation/Reduction of Polyaniline in Aqueous Sulfuric Acid

Amrita Singh,Asfiya Contractor,Ravindra Kale,Vinay Juvekar,Arindam Sarkar

doi:10.3390/pr10030443

Amrita Singh, Asfiya Contractor + Show 3 more

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https://doi.org/10.3390/pr10030443

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Abstract

In this work, we have shown through open circuit potential experiments that in aqueous sulfuric acid solutions, a thick polyaniline film undergoes autogenous oxidation when reduced below a threshold potential and autogenous reduction when oxidized above the threshold potential. This phenomenon is associated with the high resonance stability of polarons in long polyaniline chains present in thicker films. We have determined the rates of these reactions using a linear sweep chronopotentiometry technique. We propose that the oxidation reaction of polyaniline produces polarons with a concomitant reduction of hydrogen ions to hydrogen radicals, which further combine with each other to produce the hydrogen molecule in the absence of dissolved oxygen. On the other hand, at high potentials polarons are reduced with the concomitant oxidation of water to hydroxyl radicals. Both the radicals are stabilized due to the interaction of their unpaired electrons with pi-electrons of the aromatic rings of the polymer backbone. At the equilibrium value of the open circuit potential, both the hydrogen radicals and hydroxyl radicals are generated at equal rates and react with each other to form water.

Highlights

Conducting polymers possess dopable sites which can be doped/dedoped either by using oxidizing/reducing agents or by the application of the external electric potential
Since the ability of polyaniline to undergo autogenous oxidation/reduction is subSince the ability of polyaniline to undergo autogenous oxidation/reduction is substanstantial at high surface mass density of the polymer, we have synthesized the polyaniline tial at high surface mass density of the polymer, we have synthesized the polyaniline films films with high surface mass density
We have observed that polyaniline film undergoes both autogenous oxidation and reduction reactions

Summary

Introduction

Conducting polymers possess dopable sites which can be doped/dedoped either by using oxidizing/reducing agents or by the application of the external electric potential. The oxidized sites of the polymer exist as radical cations (polarons and bipolarons). The extent of conjugation depends on the number of energy states available for the charges associated with polarons. When conjugation is sufficiently high, the polymer gains the ability to perform autogenous reactions in which the polymer can reduce (oxidize) a reactive species present in the solution at the expense of its own oxidation (reduction). This would cause an increase (decrease) in the potential of the polymer when it is kept under the open circuit condition in a solution containing the reactant. We observed that when a polyaniline film, electrochemically grown on the platinum electrode, is electrochemically reduced to a low potential (−0.2 VSCE ) and immersed in an aqueous solution of sulfuric acid, its open circuit potential increases with time until it attains a constant value (equilibrium potential)

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