Electrochemical Soft Actuator: Deciphering the Difference in the Characteristics of Polaronic and Bipolaronic Forms of Polyaniline

Abstract

Polyaniline (PANI) has been projected as an efficient electrochemical actuator due to its ease of synthesis, lightweight, biocompatibility, low cost, and possible low operating potential and high stress generation. However, challenges such as low inherent ionic and electronic conductivity of the polymer lead to small accumulation of ions and high ionic diffusion path length inside the polymer remain. In the present study, a highly conjugated, planar, conducting polaronic form of PANI with a nanofiber morphology is synthesized using in situ electrochemical polymerization on a reduced graphene oxide (rGO) electrode. The polymerization is carried out in the Schaefer mode at the air–water interface under controlled surface pressure in a Langmuir trough. Electrochemical, UV–visible, XPS, and Raman spectroscopic studies confirm the formation of the planar polaronic PANI form. Polymerization without surface pressure leads to the bipolaronic form of PANI. The two forms are subsequently used to understand their contributions toward electrochemical actuation in a bilayer configuration. The conducting polaronic PANI/EGO (exfoliated graphene oxide) exhibits a remarkably larger total angular displacement of 220° in aqueous 1 M NaClO4 during a potential scan in the range ±0.9 V than the bipolaronic counterpart which exhibits a total angular displacement of 125°. Current imaging in the scanning electrochemical microscopy mode confirms a high volumetric expansion in the case of the polaronic form as compared to its bipolaronic counterpart. Raman spectroscopy reveals the oxidation to the emeraldine form in the polaronic PANI and to the pernigraniline form in the bipolaronic form during actuation. Electrochemical impedance spectroscopy study evidences the existence of a small charge transfer resistance with high bulk capacitance for the polaronic structure.

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“…Up to this day, solution processing methods have relied on interactions between layers of rGO sheets, such as induced self-assembly, , coating, , spraying, ice templates, and vacuum filtration. − These methods have enabled the preparation of large-area patterned rGO structures for a wide range of applications in biosensors, − actuators, − and energy devices. − A flexible electro-actuator is a device that utilizes an electrodeformation effect to realize motion, which consists of elastic materials and electrodes and deforms the material by applying a voltage to produce functions such as environment sensing, target capture, and actuation. Researchers have successfully used hydrogels, − polymers, − carbon-based materials, , and other materials to create soft actuators. Conventional actuators rely on the reaction of the material with a solvent or the environment to achieve deformation; it is difficult to realize complex deformation through simple patterns.…”