Ionoelastomers at Electrified Interfaces: Differential Electric Double Layer Capacitances of Cross-Linked Polymeric Ions and Mobile Counterions

Ionoelastomers (IEs), consisting solely of crosslinked networks of polymerized ionic liquids, have gained attention for use in various electrochemical applications due to their unique combination of the electrochemical properties of ionic liquids and the solid-state elastic properties of the polymer networks. However, the structure of the electric double layer (EDL) of IEs at the electrified interfaces is poorly understood, especially considering that one of the ionic species is covalently bound to the crosslinked polymer network. In this work, we investigate the differential EDL capacitances of crosslinked polymeric ions and free counterions in IEs. A pair of IEs with opposite polarity are prepared; the polycationic IE with cross-linked imidazolium cations and free bis(trifluoromethyl sulfonyl)imide (TFSI) anions and the polyanionic IE with cross-linked sulfonimide anions and free 1-ethyl-3-methylimidazolium (EMIM) cations. By applying two electrodes with a large contrast in surface area, the EDL capacitances of the crosslinked polymeric ions and the mobile counterions can be decoupled. We find that the EDL capacitances of the fixed ions are lower than that of the counterions in both the polyanion and polycation IEs, resulting in a highly asymmetric capacitance profile depending on the sign of the applied voltage. In contrast, we observe the symmetric EDL capacitance results for polymeric ions and counterions without cross-linking. It suggests that the elastic energy of the crosslinked networks in IEs restricts the freedom of polymeric ions to rearrange at the electrified interfaces, thereby reducing the EDL capacitance of crosslinked ions.