Room-temperature ionic liquids (RTILs) have been regarded as potential electrolytic media for electropolymerization processes.¹ Many kinds of RTILs have been investigated for the electropolymerizations; however, RTILs based on tetracyanoborate (B(CN)₄^-, TCB) anion have been rarely employed. On the other hand, we have designed quaternary phosphonium RTILs exhibiting high transport property and high stability.² In this work, we report the physicochemical and electrochemical characterization of the phosphonium RTILs based on the TCB anion (Fig. 1) as electrolytes for electropolymerization of pyrrole (Py).

The preparation of the TCB-based phosphonium RTILs was performed by aqueous ion exchange reaction of the precursor quaternary phosphonium halides with sodium tetracyanoborate (Na-TCB). Residual metal and halide ions in the clude RTILs were removed by a silica gel column. The combination of the asymmetric phosphonium cations with TCB anion successfully gave liquids at ambient temperature. The TCB-based phosphonium RTILs showed thermal decomposition temperatures at more than approximately 400°C.

The electropolymerization of Py in the TCB-based phosphonium RTILs was carried out by cyclic voltammetry (CV) method. Fig. 2 showed the cyclic voltammograms for the electropolymerization of Py in P₂₂₂₅-TCB. It was found that an irreversible oxidation peak was observed and maintained when the cycle was repeated, giving the PPy films on the electrode surface. As shown in Fig. 3, the SEM images of the PPy films exhibited the relatively uniform surface when compared to the PPy films grown in the corresponding bis(fluorosulfonyl)amide-based phosphonium RTILs. The thermal stability of the PPy film grown in the TCB-based phosphonium RTILs will be also discussed.

References

1  K. Sekiguchi, et al, J. Electroanal. Chem., 557, 1 (2003).

2  K. Tsunashima, et al, Electrochem. Commun., 9, 2353 (2007).