Electrochemistry and Transport of Redox Active Polymers Across a Porous Separator: Towards a Size-Selective Strategy for Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries (NRFBs) are a potentially viable alternative to their aqueous counterparts. NRFBs offer a wider range of redox active species and electrolytes available for their design, thus increasing the chance that highly soluble and stable redox couples can be used. However, the lower ionic conductivity observed in NRFBs due to the use of organic electrolytes in combination with ion-exchange membranes, which are often not designed for these environments, have slowed down their wide-scale implementation. Separating the redox active species in the electrolyte compartments by size-exclusion without greatly impeding the transport of supporting electrolyte is a potentially powerful alternative to the use of poorly-performing ion-exchange membranes. We will present our advances towards implementing such strategy using viologen-based redox active polymers (RAPs) together with a commercial off-the-shelf porous separator such as Celgard™ [1]. These RAPs, with molecular weight between 21 and 318 kDa exhibited size-dependent transport across the porous separator, while retaining many of the attractive properties of a non-aqueous redox couple: high solubility (up to 2.7 M in acetonitrile), facile electron transfer even at high concentration, 94-99% charge/discharge efficiency, and cyclability without evidence of decomposition. Preliminary testing using a stirred cell showed the possibility of integrating RAPs and porous separators into a working device that achieved close to nominal open circuit voltage, stable charge/discharge over several hours and crossover on the order of only 2%. These characteristics, as well as size-dependent transport measurements strongly suggest that porous separators can be used to allow an unimpeded transfer of supporting electrolyte while discriminating the RAP component. Advances in the synthesis of RAPs with other functionalities and their evaluation in flow conditions will be presented.   

[1] Gavvalapalli, N.; Hui, J.; Cheng, K.; Lichtenstein, T.; Shen, M.; Moore, J.S.; Rodríguez-López, J. J. Am. Chem. Soc. 2014, 136, 16309-16316