Tailoring Electrolytes for Redox-Active Polymer Flow Batteries with Low-Cost Reactors

Abstract

Redox flow-batteries (RFBs) utilize electroactive species dissolved in electrolyte to store energy, instead of the solid-state intercalation compounds utilized in Li-ion batteries. Traditionally, redox-active small molecules (e.g., vanadium ions) have been used in RFBs. The flowability of redox-active species enables the independent scalability of power and energy achievable with a tank/reactor device architecture. Redox-active polymers (RAPs) have shown great promise as candidate electrolytes to mitigate the parasitic crossover of electroactive species in RFBs. Previous work [G. Nagarjuna et al., J. Am. Chem. Soc., 136, 16309 (2014)] has shown that RAP solutions exhibit nearly ten-fold lower trans-separator permeability than their monomer counterparts. These results suggest that size-exclusion may be a viable means of reducing the crossover of electroactive species and concomitant capacity/efficiency losses in redox flow batteries (RFBs) when a separator is used in lieu of an ion-selective membrane. Though low trans-separator permeability can be achieved with large RAP molecules, the viscosity of concentrated RAP solutions increases substantially with large RAPs. It has been observed that RAPs exceeding 21 kg/mol have produced ten-fold higher viscosity than their monomer counterparts at the same concentration [G. Nagarjuna et al., J. Am. Chem. Soc., 136, 16309 (2014)]. Aqueous RAPs have also exhibited non-Newtonian rheology that is sensitive to the concentration of supporting salt [T. Janoschka et al., Nature, 527, 78 (2015); T. Janoschka et al., Polym. Chem., 6, 7801 (2015)]. In addition to decreasing the flowability of RAP solutions, maintaining the mobility of the conductive ions needed for RFB power capability is likely to be a challenge for highly viscous electrolytes. To assess the limits of RFB performance for concentrated RAP solutions, we are investigating ionic/RAP mobility and viscosity as a function of solution composition and RAP molecular weight (MW). Initial experiments have focused upon determining the ionic conductivity of viologen2+ RAPs (associated with PF6 - anions) dissolved in acetonitrile with LiBF4 as the supporting electrolyte. The observed conductivity is a result of the mobility, concentration, and valence of charged species in solution (charged RAPs, PF6 -, Li+, and BF4 -). Fig. 1(a) shows the ionic conductivity (measured via electrochemical impedance spectroscopy) as a function of LiBF4 concentration. 0.64 mol/kg RAP solutions exhibit higher ionic conductivity than plain LiBF4 solutions at all salt concentrations investigated. This result suggests that the increased concentration of conductive species (charged RAPs and PF6 -) has a more substantial effect on conductivity than any reduction in the mobility due to increased solution viscosity. Fig. 1(b) shows the variation of ionic conductivity with RAP concentration at a fixed salt concentration. Solutions of monomers show higher ionic conductivity than high MW RAP solutions (318kDa) in both plots. This result is likely due to the decreased mobility of charged RAPs with high MW. Both of the RAP solutions investigated (with high and low MW, respectively) show ionic conductivity that is relatively insensitive to LiBF4 concentration. This result is important because it suggests that minimal amounts of supporting electrolyte are needed for a viologen-based cell to function. Also, once the RAP concentration is increased beyond 0.32 mol/kg, the ionic conductivity varies little with RAP concentration. This enables identification of an upper limit for active species concentration that optimizes cell performance and minimizes RAP viscosity. The currently measured samples contain viologen2+, and, consequently, ionic conductivity is likely to decrease when reduced to viologen1+. On-going work therefore targets measurement of ionic conductivity across a range of state-of-charge for viologen-based RAPs. Concurrently, techno-economic analysis is being used to assess the impact of salt and actives concentration on the price of RAP flow batteries. Preliminary analysis has predicted that RAP flow-battery reactors can be engineered with low area-specific resistance and low cost. In particular, concentrated RAP solutions show lowest area-specific resistance due to their facile redox-reaction kinetics and high ionic conductivity. We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). Figure 1