Solution Properties and Practical Limits of Concentrated Electrolytes for Nonaqueous Redox Flow Batteries

Abstract

Nonaqueous redox flow batteries (NRFBs) use energized organic fluids that contain redox active organic molecules (ROMs) and supporting electrolyte. Such allorganic electrolytes have wider electrochemical stability windows than the more familiar aqueous electrolytes, potentially allowing a higher energy density in the solutions of charged ROMs. As this energy density increases linearly with the concentration of the charge carriers, physicochemical properties of concentrated ROM solutions in both states of charge present considerable practical interest. For NRFBs to become competitive with other types of flow cells, the current techno-economic analyses favor highly concentrated solutions (>1 M) with high ionic conductivity (>5 mS/cm). It is not presently clear that such solutions can have the required dynamic properties. In this study, we show that ion diffusivities and conductivities of ROM-containing electrolytes reach maxima around 0.5 M and decrease significantly at higher concentrations; realistic limits are established for variations of these parameters. Furthermore, using closed-shell analogues for open-shell charged ROMs, we show that reconstitution of highly concentrated fluids during electrochemical charging will have strong adverse effects on their properties, including an increase in viscosity and decrease in conductivity and ion diffusivity. Given our results, it appears that the target concentrations of NRFB fluids need to be reconsidered in terms of concentration-dependent conductivity and viscosity.

Dual function organic active materials for nonaqueous redox flow batteries

“…The dynamic viscosities of electrolyte solutions (in ACN) were measured at 25 °C using a microfluidic pressure-driven flow viscometer (m-VROC, RheoSense, Inc.). 44,45 Each fluid sample was pushed through a microfluidic channel at a known volume flow rate, and viscosity was determined from the generated steady-state pressure drop resulting from the flow resistance. 46 However, from moderate to high concentrations (0.50 M to 1.00 M), the viscosity of MEEV-(TFSI) 2 and EPRT-TFSI increased rapidly and non-linearly compared to the viscosity of TEATFSI, which implies stronger solute-solute interactions in these systems, e.g.…”

Bio-inspired Molecular Redesign of a Multi-redox Catholyte for High-Energy Non-aqueous Organic Redox Flow Batteries

“…12,25,31,32 Moreover, p-type ROMs capable of multi-electron redox reactions are even rarer because their oxidized states, such as dications, are often highly unstable in a solution. 13 Considering a realistic limit of the ROM concentration in non-aqueous media, 15,33 exploiting multi-electron redox from a single p-type ROM is indispensable to achieve high energy density at a given concentration. 34 Several attempts have very recently been made to utilize multi-electron redox p-type ROMs as catholytes.…”

Bipolar Redox‐Active Molecules in Non‐Aqueous Organic Redox Flow Batteries: Status and Challenges

“…[12,16] However, at ROM concentrations above 0.5 M, OÀ NRFBs experience high viscosity (> 10 cP) and low solution conductivity (< 5 mS/cm). [18] Therefore, battery cycling is often limited to low concentrations of active species (� 0.5 M), which inhibits energy density. Another significant challenge to OÀ NRFB performance is the compatibility of ion-exchange membranes (IEMs) in nonaqueous solvents.…”

“…While O−NRFBs enable the development of energy‐dense systems, the energy density is often limited by the solubilities of ROMs, which must exceed 1 M in the non‐aqueous solvent to compete with organic aqueous RFBs (O−ARFBs) [12,16] . However, at ROM concentrations above 0.5 M, O−NRFBs experience high viscosity (>10 cP) and low solution conductivity (<5 mS/cm) [18] . Therefore, battery cycling is often limited to low concentrations of active species (≤0.5 M), which inhibits energy density.…”

“…These ROMs are further tunable using a molecular engineering strategy, which can improve properties such as solubility [15] or redox potential [16] . However, organic NRFBs (O−NRFBs) still face limitations including poor stability (often ≤200 cycles) [17] and low solubility (mostly <1 M) [18] for all charge states of ROMs [14] . In particular, crossover is a challenge preventing the widespread use of NRFBs [19] .…”