Synthesis of Redox Polymer Nanoparticles Based on Poly(vinylcatechols) and Their Electroactivity

Organic materials are being investigated as an alternative to inorganic cathodes in lithium batteries with the promise of higher sustainability as well as increased theoretical capacity. Among organic materials, redox polymers based on poly(vinyl catechol) presenting the catechol/o-benzoquinone redox pair show high energy storage potential. Our main motivation in this work was to prepare polymer nanoparticles of different sizes and study their redox properties. As linear polymers are usually soluble/mechanically unstable, we prepared cross-linked polymers to maintain the size of the nanoparticles in different electrolytes. (Mini)emulsion polymerization of two dimethoxy monomers was used to synthesize spherical polymer nanoparticles cross-linked with divinylbenzene (DVB) where the size was controlled by changing the concentration of the surfactant. Deprotection yielded poly(4-vinyl catechol) and poly(3-vinyl catechol) redox active nanoparticles (RPNs) of sizes ranging between 40 and 330 nm as characterized by scanning electron microscopy. The electrochemical properties of the RPN were tested in aqueous- and acetonitrile-based electrolytes using cyclic voltammetry. First the effect of the nanoparticle size and the cross-linker content in the electrochemical properties was investigated. Particle size did not have a crucial effect on the electrochemistry and only the biggest 330 nm RPN showed slightly diminished electrochemical properties. The cross-linker had a negative effect on the maximum reduction current (iC) but improved cycling stability. Based on these results we decided to use the smallest 40 nm nanoparticles with the lowest (1% DVB) cross-linker content for further electrochemical testing. Both RPN isomers showed reversible behavior in aqueous acidic-, neutral-, as well as in acetonitrile-based electrolyte. Poly(4-vinyl catechol)-based RPN had slightly higher reduction potential at 0.45 V versus Ag/AgCl (0.1 M HClO4) compared to another isomer with 0.40 V versus Ag/AgCl. When switching from an acidic aqueous electrolyte to neutral, the redox potential was shifted 440-475 mV to lower values. Because of their high reduction potential and theoretical capacity at 394 mA h/g these synthetic RPNs show promising properties to be used as cathodes in a variety of batteries. However, exact capacity determination, long term cycling, testing in nonaqueous electrolytes, and redox flow batteries needs to be performed in the future.This work was financially supported by the European Research Council by Starting Grant Innovative Polymers for Energy Storage (iPes) 30625 and Slovenian research agency (ARRS): research project J2-8167 and public call MS-ERC-FS/2017-002. N.B. acknowledges the financial support obtained through the Post-Doctoral fellowship Juan de la Cierva-Incorporacion (IJCI-2016-28442), from the Ministry of Economy and Competitiveness of Spain. L.P. has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no 797295