(Keynote) Redox Active Polymers: A Size Selective Solution for Nonaqueous Redox Flow Batteries

Non-aqueous redox flow batteries (NRFBs) are a potentially viable alternative to their aqueous counterparts as they offer a wider range of redox active species and electrolytes available for their design.[1] For instance, the larger electrochemical windows of organic solvents can be used to search for redox molecules with higher energy densities. However, NRFBs suffer from the general lack of suitable membranes. The low ionic conductivity observed when non-dedicated ion-exchange membranes are used in organic solvents has slowed down their wide-scale implementation. We have hypothesized that separating the redox active species in the electrolyte compartments by size-exclusion while allowing free flow of the supporting electrolyte presents a potentially powerful alternative to the use of poorly-performing ionexchange membranes. In our case, this approach is made possible by the careful design and evaluation of soluble Redox Active Polymers (RAPs), together with porous commercial off-the-shelf separators, as exemplified by the strategy depicted in the Figure below. RAPs consist of a polymeric unconjugated backbone that is decorated with high energy density and highly stable redox active pendants that display facile electron transfer. In addition to an attractive reactivity, RAPs must have solubilities in the M range and display low cross-over characteristics across porous separators. 

We will present our advances towards implementing such strategy using RAPs with different chemical functionalities, for both catholytes and anolytes with the aim of creating an “ALL RAP” non-aqueous redox flow battery. Our first studied system, based on viologen RAPs with molecular weight between 21 and 318 kDa exhibited size dependent transport across the porous separator.[2] This was achieved with RAPs that displayed a high solubility of up to 2.7 M, reversible electron transfer, and 94-99% charge/discharge efficiency. In the study of RAPs we have introduced new measurement techniques, including micro- and nano-electrodes for the steady state characterization of this species. Microelectrode voltammetry allowed to prove facile electron transfer even in highly concentrated solutions (+1.0 M). Preliminary testing using a stirred cell showed the possibility of integrating RAPs and porous separators into a working device that achieved stable charge/discharge and only 2% crossover over several hours.

We are working towards the development of guidelines that aid our understanding of RAP electrochemistry.The application of new electrochemical and ionic imaging methods based on scanning electrochemical microscopy using micro- and nano- electrodes developed in our group [3] has allowed us to better understand the careful balance between RAP size and ion and electron mobility. We will discuss how these guidelines have significantly aided our molecular design and how they impact the performance of this new type of flow batteries.

Acknowledgement.  Work supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. 

[1] R.M. Darling, K.G. Gallagher, J.A. Kowalski, S. Ha, F.R. Brushett. Pathways to Low-Cost Electrochemical Energy Storage: a Comparison of Aqueous and Nonaqueous Flow Batteries. Energy Environ. Sci. 7 (2014) 3459.
[2] N. Gavvalapalli, J. Hui, K. Cheng, T. Lichtenstein, M. Shen, J.S. Moore and J. Rodríguez-López. J. Impact of Redox Active Polymer Molecular Weight on the Electrochemical Properties and Transport Across Porous Separators in Non-Aqueous Solvents. J. Am. Chem. Soc. 136 (2014) 16309.
[3] Z. J.Barton, and J. Rodríguez-López. Lithium Ion Quantification Using Mercury Amalgams as In Situ Electrochemical Probes in Nonaqueous Media. Anal. Chem. 86 (2014) 10660.