Cyclic voltammetry and morphology of polyaniline-coated electrodes containing [Fe(CN) ₆ ] 3–/4– ions

Three strategies for incorporation of [Fe(CN)6]3–/4– into polyaniline (PANI)-coated electrodes have been investigated: A, ion-exchange of [Fe(CN)6]3–/4– into pre-formed PANI-Cl- or PANI-SO4-coated electrodes; B, electropolymerisation of aniline from acidified K4[Fe(CN)6] or K3[Fe(CN)6]; and C, electropolymerisation of aniline from H4[Fe(CN)6]. Method A led to observable ion exchange into PANI-Cl or PANI-SO4 with acidified K4[Fe(CN)6] or K3[Fe(CN)6] solutions. The amount of film-confined [Fe(CN)6]3– corresponds to protonation of ca. 19% of the N atoms in PANI, but the response is not long-lived in hexacyanoferrate(III)-free electrolyte; a partition equilibrium constant of 220 ± 50 is measured. Method B effected [Fe(CN)6]3–/4– incorporation, but only when high concentrations of redox anion were present during PANI growth. The potential of the [Fe(CN)6]3–/4– couple in the polymer is shifted to +0.32 V (ΔEp= 40 mV) for PANI-[Fe(CN)6]3– compared with the standard potential +0.36 V vs. SCE in solution (at a freshly polished glassy carbon electrode). The oxidized form of the redox couple PANI-[Fe(CN)6]3– has a time-dependent voltammogram which collapses to a single broad wave at +0.32 V (ΔEp= 30 mV). The results also show that [Fe(CN)6]4– is exchanged more rapidly when the electrode potential of the film is cycled in HCl rather than in H2SO4, owing to ion-size considerations. Scanning electron microscopy (SEM) analysis on the PANI-[Fe(CN)6] films revealed a fibrous morphology indicative of a slower rate of growth than PANI-Cl. Method C also led to successful incorporation of [Fe(CN)6]3–/4–, whose presence was confirmed by fourier transform infrared (FTIR) spectroscopy.