Ion movement in thin Nafion films under an applied electric field

The electromechanical response of Nafion films with and without an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) ([emim]Tf₂N) additive were characterized under an applied electric field in situ using neutron reflectometry (NR) and voltage modulated atomic force microscopy (VM-AFM). NR showed that pure Nafion films exhibited no response under field strengths of ∼0.18 V/μm, while VM-AFM measurements showed a minimal response at higher field strengths (∼200 V/μm), which is ascribed to the residual water presence in the films. The addition of ionic liquid resulted in clear electroresponsiveness seen in both NR and VM-AFM. NR results indicated mass migration away from the cathodic interface driven by the [emim]⁺ movement in the direction of the electric field. The lack of ionic liquid accumulation at the electrode interfaces contrasts the bulk electromechanical behavior of similar systems reported in the literature. VM-AFM measurements were able to resolve the relative contributions of the [emim]⁺ cation and Tf₂N⁻ anion to film deformation by alternating the direction of the applied field and support the [emim]⁺ dominant migration seen in the NR results. The findings presented here emphasize the need for the nanoscale analysis of material properties of electroresponsive thin film systems and demonstrate the potential for probing electric field effects using in situ techniques.