Unexpected Interaction between PEDOT and Phosphonium Ionic Liquids

In-situ-polymerized films of poly(3,4-ethylenedioxythiophene) (PEDOT) are known to be relatively ordered materials and maintain this order under changing chemical and electrochemical conditions. It is therefore surprising that certain ionic liquids (ILs) were found to interact with PEDOT and thereby to a large extent disrupt the ordered structure. The current work demonstrates the expansion of the interlayer distance (d(100)) of PEDOT and the composite of PEDOT with poly(ethyleneglycol) (PEDOT(PTS):PEG) in the presence of IL mixtures containing triisobutylmethylphosphonium tosylate (P1444PTS) and water. In presence of the mixtures, the PEDOT(PTS):PEG film expands up to similar to 100% while the PEDOT(PTS) film expanded similar to 50%. The expansion did not increase the electrical resistance but increased the absorption in the pi-pi* range, which can be explained by increased shielding of the PEDOT chains by the IL. The incorporation of P1444PTS increased the capacitance by 350%, compared to the theoretical capacitance of PEDOT(PTS), due to the formation of additional double-layer capacitance.