Prof. Dr. Jean - François Gohy

Abstract

Redox-active polymer based nano-objects via polymerization induced self-assembly

Fadoi Boujioui and Jean-François Gohy*

Institute of Condensed Matter and Nanosciences (IMCN),
Université catholique de Louvain,
Place L. Pasteur 1,
1348 Louvain-la-Neuve, Belgium.
Email: jean-francois.gohy@uclouvain.be

Flow-assisted electrochemical systems (FAES) are receiving increasing interest because of their application in various systems including redox flow battery (RFB), electrochemical flow capacitor (EFC) and semi-solid flow battery (SSFB). For this type of electrochemical systems, the active species are stored in solution in external tanks and pumped through the main cell to fuel it continuously, thereby generating the energy. While the energy density is related to the size of the tanks, the power density is determined by the composition of electrodes. The suspension electrodes are called catholytes or anolytes and are composed between 5 and 20 wt. % of active storing material, contrary to the electrode films used in batteries for which the composition is greater than 80 wt. %. Among the different materials used for the preparation of suspension electrodes, redox-active polymers are receiving special interest because their properties can be easily tailored by macromolecular engineering. For example, their solubility can be tuned in various solvents by co-polymerizing with the adequate monomers. Their molar masses, molar masses distribution as well as their architectures (i.e. linear, branched, statistical, block copolymers, etc.) can be easily tuned provided that controlled or living polymerization techniques are used for their synthesis. Poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl-methacrylate) (PTMA) is indeed considered as an excellent redox-active polymer for application in FAES.¹

One of the main challenges related to redox-active polymer-based electrodes remains the relatively low concentration of redox-active material that can be dissolved while keeping a viscosity sufficiently low to allow the pumping of the liquid electrode in the main electrochemical cell. Decreasing the molar mass of the polymer is not a valid option since the electroactive polymer should not pass through the semi-permeable membrane located in the main electrochemical cell. One way to allow the dispersion of a high amount of electroactive polymer while not reaching a too high viscosity consists in tethering the redox-active polymer chains in a micellar design. Basically, the redox-active polymer chains could be incorporated into two different compartment of the micellar cargo: the micellar core or the micellar corona.

In this respect, we have designed micellar nano-objects comprising PTMA coronal chains tethered onto a polystyrene (PS) core. Those objects were synthesized from PTMA-b-PS diblock copolymers containing a major PTMA block dissolved in carbonate solvents that are selective solvents for the PTMA blocks.² The accordingly obtained micelles were successfully tested as catholytes in RFB.³ Such a design presents the advantage to allow a good accessibility of the electroactive PTMA chains for redox reaction since they are located in the micellar corona but the disadvantage to be limited to organic solvents for the preparation of the liquid electrode since both PTMA and PS are hydrophobic groups.

Aqueous micellar catholytes could however be obtained from amphiphilic block copolymer containing a water-soluble hydrophilic block and a hydrophobic PTMA block. In aqueous medium, such a copolymer is expected to form micellar nano-objects containing a PTMA core and a hydrophilic polymer corona. In order to maximize the amount of PTMA in the system, the composition of our copolymers will be adjusted to target cylindrical micelles rather than spherical ones. Practically, our attention is focused on the so-called polymerization induced self-assembly (PISA) method. This method has indeed emerged as a promising alternative to classical water-based polymerization techniques since it is surfactants-free (the formed copolymer plays indeed the role of surfactant) and it leads to the formation of the micellar nano-objects directly during the synthetic process of the copolymer. The strategy developed in the present contribution is to synthesize directly PTMA based nano-objects in suspension in aqueous medium by copolymerizing it with a hydrophilic polymer block using a PISA process. Herein, we report the RAFT polymerization induced self-assembly of the 2,2,6,6-tetramethylpiperidin-4-yl methacrylate (TMPM) monomer which is the precursor of the PTMA. Two types of water-soluble macro-CTAs are synthesized, i.e. the thermo-responsive poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) and the cationic poly[(4-methacryloyloxy)-2,2,6,6-tetramethylpiperidinium chloride] (PTMPM⁺Cl^-). This PISA process leads to the formation of micellar nano-objects with a PTMPM core and either a POEGMA or a PTMPM⁺Cl^- corona. After the oxidation step to transform PTMPM into PTMA, the redox properties of redox-active polymer based nano-objects are confirmed through cyclic voltammetry measurements.

1. (a) Winsberg, T. Hagemann, T. Janoschka, M. D. Hager and U. S. Schubert, Angew. Chem. Int. Ed., 2017, 56, 686–711. (b) J. Winsberg, T. Janoschka, S. Morgenstern, T. Hagemann, S. Muench, G. Hauffman, J. F. Gohy, M. D. Hager and U. S. Schubert, Adv. Mater., 2016, 28, 2238–2243. (c) T. Janoschka, N. Martin, U. Martin, C. Friebe, S. Morgenstern, H. Hiller, M. D. Hager and U. S. Schubert, Nature, 2015, 527, 78–81.
2. Hauffman, Q. Maguin, J. P. Bourgeois, A. Vlad and J. F. Gohy, Macromol. Rapid Commun., 2014, 35, 228–233.
3. Winsberg, S. Muench, T. Hagemann, S. Morgenstern, T. Janoschka, M. Billing, F. H. Schacher, G. Hauffman, J. F. Gohy, S. Hoeppener, M. D. Hager and U. S. Schubert, Polym. Chem., 2016, 7, 1711–1718.

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