  We report the development of redox-active conjugated polymers with potential
application to electrochemical energy storage. Side chain engineering enables
processing of the polymer electrodes from solution, stability in aqueous
electrolytes and efficient transport of ionic and electronic charge carriers.
We synthesized a 3,3' dialkoxybithiophene homo polymer (p type polymer) with
glycol side chains and prepared naphthalene
1,4,5,8-tetracarboxylic-diimide-dialkoxybithiophene (NDI gT2) copolymers (n
type polymer) with either a glycol or zwitterionic side chain on the NDI unit.
For the latter, we developed a post-functionalization synthesis to attach the
polar zwitterion side chains to the polymer backbone to avoid challenges of
purifying polar intermediates. We demonstrate fast and reversible charging of
solution processed electrodes for both the p- and n type polymers in aqueous
electrolytes, without using additives or porous scaffolds and for films up to
micrometers thick. We apply spectroelectrochemistry as an in operando technique
to probe the state of charge of the electrodes. This reveals that thin films of
the p-type polymer and zwitterion n-type polymer can be charged reversibly with
up to two electronic charges per repeat unit (bipolaron formation). We combine
thin films of these polymers in a two-electrode cell and demonstrate output
voltages of up to 1.4 V with high redox stability. Our findings demonstrate the
potential of functionalizing conjugated polymers with appropriate polar side
chains to improve specific capacity, reversibility and rate capabilities of
polymer electrodes in aqueous electrolytes.
