Dissolvable Conducting Polymers for Electrochemical Energy Storage
Authors: Diaz Virginia , Warren Roseanne 
Published: February 2020

Conducting polymers have attracted great interest as energy storage materials in supercapacitors and Li-ion batteries. Polypyrrole in particular is well known for its reversible capacitive charge storage, ease of synthesis, and potential application as a robust, conductive coating for Si-based anodes.1,2 While much research on conducting polymer energy storage has focused on improving energy density and cycle lifetime, conducting polymers are also promising materials for environmentally-friendly, transient, and/or biocompatible energy storage applications.3 An important first step in the advancement of environmentally-friendly, biodegradable energy storage devices is developing materials that break down under mild conditions. Previous work on conducting polymer derivatives indicates that side-chain moieties can reduce polymer crosslinking, enabling conducting polymers to dissolve easily under physiological conditions.4 Here we present for the first time a repeating polymer of methyl 1H-pyrrole-3-carboxylate monomer (“MPC polymer”) as a supercapacitor energy storage material, and its ability to dissolve in a mild aqueous environment. This work is, to the best of our knowledge, the first demonstration of electrochemical energy storage using a dissolvable conducting polymer derivative and thus represents an important contribution to the field of transient, biocompatible energy storage materials.
Figure 1 provides a conceptual illustration of supercapacitor energy storage using MPC polymer vs. polypyrrole. Both polymers were electrodeposited onto a Si substrate with Cr/Au coating from a 0.1 M solution of the corresponding monomer in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile. Cyclic voltammetry measurements conducted in 0.1 M TBAPF6 electrolyte in acetonitrile indicate comparable charge storage abilities for MPC polymer and polypyrrole, with capacitances of 0.98 mF/cm2 and 1.75 mF/cm2, respectively, for the planar electrodes. Figure 3 compares the solubility of MPC polymer and polypyrrole in Tris acetate-EDTA buffer solution (pH 8.2) at 37 oC. Within one hour, most of the MPC polymer film had dissolved, compared to the polypyrrole electrode which remained unchanged after four hours in the buffer solution. These results demonstrate that conducting polymer derivatives can function as dissolvable energy storage materials – an important first step in realizing fully biodegradable electrochemical energy storage materials. Detailed characterization of MPC polymer and its capacitive energy storage performance will be presented, including FT-IR spectroscopy characterization, capacitive charge-discharge cycle life measurements, and impedance characterization.

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Figure 1