Ion transport competition in ionomer filmsloaded with Fe(CN) 3−/4−6

Abstract

Anodic to cathodic peak current ratios in cyclic voltammograms of the Fe(CN) 3−/4− couplein poly{[3-methylthiophene]- co-[1-methyl-3-(pyrrol-1-ylmethyl) pyridinium]} ionomer films were found to vary from ca 0.75 to 1.0 as the scan speed was increased. X-ray emission analysis indicated that this variation was due to competition between Fe(CN) 3−/4− 6 and HPO 2− 4 (or H 2PO − 4) during the redox process. HPO 2− 4 insertion is kinetically favoured, while Fe(CN) 3−/4− 6 is thermodynamically favoured by the copolymer. At high scan rates, the copolymer contains a constant quantity of Fe(CN) 3−/4− 6 because the excess positive charge in the film produced by the oxidation of Fe(CN) 4− 6 is balanced by HPO 2− 4 insertion. The cathodic and anodic peaks are therefore of equal height. At low scan rates, the excess positive charge is balanced by Fe(CN) 3− 6 insertion and so the cathodic peak is 33% larger than the anodic peak. The copolymer shows excellent permselectivity to anions.