Redox-active small molecules, used traditionally in redox flow batteries (RFBs), are susceptible to parasitic crossover of electroactive species through a porous separator, and require expensive ion exchange membranes (IEMs) to achieve long lifetimes. Redox-active polymer (RAP) solutions show great promise as candidate electrolytes to mitigate crossover through size-exclusion, enabling the use of relatively inexpensive porous separators in place of IEMs. This study holistically evaluates poly(vinylbenzyl ethyl viologen) RAPs as potential active species for RFBs, based on trends in electrolyte transport properties, electrochemical performance, and reactor cost. The ionic conductivity of these solutions is found to be of the same order of magnitude as typical Li-ion battery electrolytes, indicating that RAP macromolecular design does not limit the mobility of conducting ions in solution. The electrochemical performance of a RAP-based RFB is predicted by accounting for capacity losses due to electrolyte mixing, and polarization within its reactor. Techno-economic analysis evaluates the impact of electrolyte transport properties and operating conditions on RFB reactor cost. Minimum reactor cost lies between 11-17 dollars kWh-1 across the entire range of active species concentrations studied, which is comparable to the estimated target mean RFB reactor cost of $13.8 kWh-1. The achievement of low cost reactors is enabled by the deviation in transport properties of RAP solutions from the prediction based on the Stokes-Einstein equation. The methodology used here could potentially serve as an approach for examining other candidate active species for RFBs.
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Low cost reactors for redox-active polymer flow batteries
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