Contactless actuation of perfluorinated ionomer membranes in salt solution: an experimental investigation

Abstract

A variety of modeling frameworks have been proposed for ionic polymer metal composites (IPMCs), but the physical underpinnings of their actuation remain elusive. A critical step toward the validation of existing theories and transition to engineering practice entails the design of new experimental paradigms that could support hypothesis-driven research. While several factors exacerbate the complexity of experimenting with IPMCs, the presence of the electrodes plays a major role by hindering the repeatability of the results and bringing a number of difficult-to-measure parameters into the picture. Here, we seek to address these experimental confounds by investigating contactless actuation of perfluorinated ionomer membranes in salt solution. In contrast to IPMCs that bend toward the anode in response to an applied voltage, ionomer membranes display a consistent deflection toward the cathode. Through hypothesis-driven experiments where the membrane width, solution concentration, and voltage applied across the electrodes are systematically varied, we elucidate electrochemistry and mechanics of contactless actuation. The applied voltage and solution concentration have a dominant role on the electrochemistry, while mechanics is mainly affected by the applied voltage and membrane width. Our results depict a complex scenario, which is expected to inform future theoretical inquiries about IPMC actuation.

Solvation‐Driven Actuation of Anion‐Exchange Membranes

“…Even though individual membranes exhibited absolute peak tip displacements on the same order of Nafion membranes, [ 33,34 ] the median of the absolute actuation magnitude was three times smaller. Accounting for differences in the Young's modulus and the smaller moment of inertia, the strength of the actuation is considerably lower than in cation‐exchange membranes (a comparison between actuation in anion‐ and cation‐exchange membranes can be found in the Supporting Information).…”

“…[22,33], pointing at the involvement of other actuation mechanisms. Further supporting this claim, the time-scale of the actuation of ionexchange membranes is in the order of seconds, [33,34] compared to minutes for polyelectrolyte gels. [31] Building on the work of Kim et al, [22] we proposed a series of experimental and theoretical endeavors to elucidate the inner workings of cation-exchange membrane actuation.…”

“…The voltage signal was controlled by an electrical circuit similar to our previous works. [33,34] A data acquisition system was employed to record the voltage across the external electrode, the current passing through the electrodes, and the membrane tip displacement.…”

“…[22,33], pointing at the involvement of other actuation mechanisms. Further supporting this claim, the time‐scale of the actuation of ion‐exchange membranes is in the order of seconds, [ 33,34 ] compared to minutes for polyelectrolyte gels. [ 31 ]…”

Modeling Actuation of Ionomer Cilia in Salt Solution Under an External Electric Field

“…Inspired by cilia and flagella in cells, Kim's group from the University of Nevada, Las Vegas, recently demonstrated the possibility of actuating ionomer fibers without plated electrodes [8]. (Equivalent predictions have been later confirmed by our group [9].) When placed between two external electrodes in a salt solution, these actuators exhibit consistent motion toward the cathode in response to the applied electric field.…”