Influence of ionic speciation on electrocatalytic performance of polyaniline coated platinum electrode: Fe(III)/Fe(II) redox reaction

Porous-polyaniline coated Pt electrode (PANI/Pt) was electro-synthesized potentiodynamically in 0.1 M aniline + 0.5 M H2SO4 and morphologically characterized by scanning electron microscopy (SEM). Nature of predominant Fe-species in HCl and H2SO4 was checked by UV–vis spectrophotometry. Electrocatalysis of Fe(III)/Fe(II) reaction was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for three different solution compositions viz. (i) FeCl3/FeCl2 in 1 M HCl, (ii) FeCl3/FeCl2 in 0.5 M H2SO4 and (iii) Fe2(SO4)3/FeSO4 in 0.5 M H2SO4. For different thicknesses of PANI, the peak current increased irrespective of the nature of the Fe-species, but the polarity of the charge on the Fe-species showed great influence on reversibility of electrocatalysis by PANI/Pt. The Donnan interaction of the polyaniline modified electrode for the three compositions was investigated with respect to [Fe(CN)6]3−/H2[Fe(CN)6]2− which are believed to be the predominant species present in K3[Fe(CN)6]/K4[Fe(CN)6] solution in 0.5 M H2SO4. The electrocatalytic performance of PANI/Pt for Fe(III)/Fe(II) redox reaction was found superior in HCl compared to that in H2SO4.