Counterion Effects of Protic Ionic Liquids on the Conductivity and Stretchability of Free-Standing PEDOT:PSS Films

Influenced by the hard-soft acid-base (HSAB) theory, different hard-soft cations and anions are used to investigate the counterion effects of ionic liquids (ILs) on electrical and mechanical characteristics of the poly-(3,4-ethylenedioxythiophene):poly-(styrenesulfonic acid) (PEDOT:PSS) and IL free-standing films. Two protic ILs, i.e., methylimidazolium dicyanamide ([p-MIM]-[DCA]) and imidazolium dicyanamide ([p-IM]-[DCA]), are synthesized and characterized for the first time. Six ions ranging from hard to soft (cations: [p-IM]⁺, [p-MIM]⁺, and 1-ethyl-3-methylimidazolium ([EMIM]⁺), anions: tetrafluoroborate ([BF₄]^-), tetracyanoborate ([TCB]^-), and [DCA]^-) are studied to investigate their impact on advancing conductive flexible films. Herein, PEDOT:PSS with protic ILs [p-MIM]-[DCA] and [p-IM]-[DCA] exhibited tremendous increase in electrical conductivity of 2730 S/cm and 3547 S/cm, respectively. Both protic ILs improved external strain of up to 11%, surpassing aprotic IL [EMIM]-[DCA] (∼7%) and pristine PEDOT:PSS (<4%). This study revealed that ILs containing hard cation and soft anion exhibited superior conductivity and flexibility in the composite films.