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

Nonaqueous redox
flow batteries (NRFBs) use energized organic fluids
that contain redox active organic molecules (ROMs) and supporting
electrolyte. Such all-organic 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.

非水氧化还原液流电池（Nonaqueous redox flow batteries, NRFBs）采用包含氧化还原活性有机分子（redox active organic molecules, ROMs）与支撑电解质的活化有机流体。此类全有机电解质相较广为应用的水相电解质，拥有更宽的电化学稳定窗口，有望在带电ROMs溶液中实现更高的能量密度。由于能量密度随载流子浓度线性增长，处于充放电两种荷电状态下的高浓度ROM溶液的物理化学性质，均具备重要的实际研究价值。若要使NRFBs具备与其他类型液流电池竞争的实力，当前的技术经济分析更倾向于采用离子电导率大于5 mS/cm的高浓度溶液（浓度高于1 M）。目前尚不清楚此类溶液是否能够满足所需的动态性能要求。本研究表明，含ROM电解质的离子扩散系数与电导率在浓度约为0.5 M时达到峰值，在更高浓度下会出现显著下降；本研究还确定了这些参数变化的实际限值。此外，通过采用开壳层带电ROMs的闭壳层类似物开展对照实验，本研究证实，电化学充电过程中高浓度电解液的结构重构会对其性能产生显著负面影响，具体包括粘度升高、电导率与离子扩散系数降低。结合本研究结果来看，NRFB电解液的目标浓度需要结合浓度依赖的电导率与粘度特性进行重新评估。