Abstract:

Redox flow batteries (RFBs) have been widely recognized as the primary choice for large-scale energy storage due to their high energy efficiency, low cost, and versatile design of decoupled energy storage and power output. However, the broad deployment of RFBs in the power grid has long been plagued by the high cost and low energy density of existing inorganic metal-based electrodes. Redox-active organic molecules (ROMs), on the other hand, have recently been extensively explored as the potentials electrodes in RFBs for their potential low cost, abundant resources, and highly tunable structure. The energy density of RFBs is proportional to the number of electrons transferred per unit reaction, the concentration of active materials, and the cell voltage. Therefore, strategies to improve the energy density of RFBs could be categorized into three classes: (1) expanding the cell voltage; (2) maximizing the practical concentration of active materials; (3) realizing multi-redox process. Benefited by the highly tunable structure and properties of ROMs, the cell voltage of RFBs could be realized by lowering the redox potentials of anolytes or/and increasing the redox potentials of catholytes. To fully exploit the low-potential anolytes and high-potential catholytes, non-aqueous electrolytes with wider electrochemical potential windows (EPWs) are preferred over the aqueous systems. However, the solubility of most ROMs in commonly used non-aqueous electrolytes is very limited. Several effective strategies to improve the practical concentrations of ROMs have been proposed: (1) the solubility of ROMs could be easily tailored by adjusting the intermolecular interactions between ROMs and the interactions between ROMs and electrolytes via molecular engineering; (2) the redox-active eutectic systems remain liquid at or near room temperature, allowing us to reduce or completely remove the inactive solvent used in the conventional electrolyte of RFBs, which leads to an enhanced practical concentration of the redox-active components; (3) the semi-solid suspension achieves a high practical concentration of ROMs by combining the advantages of solid ROMs with high energy density and liquid electrolytes with flowability; (4) the redox-targeting approach breaks the solubility limitation by realizing remote charge exchange between the solid active materials deposited in the tanks and the current collectors of the electrochemical stacks via ROMs dissolved in electrolytes. The first three strategies employ a homogeneous flowing redox-active fluid which suffers from deteriorated physical and electrochemical properties as the practical concentration of ROMs increase, e.g., high viscosity, phase separation, and salt precipitation. The redox-targeting approach uses a hybrid flowing liquid/static solid system, which avoids the aforementioned unfavorable changes in electrolyte properties, however, this design introduces additional chemical reactions between the ROMs and the solid active materials, which may reduce the power output. Another efficient method to improve the energy density of RFBs without affecting the properties of the electrolyte is achieved by realizing the multi-redox process of ROMs, however, the generated high valence state ROMs are highly reactive. Further optimization of the structure of these ROMs is required to improve their lifetime at high valence states. In this perspective, we summarize the general working principle of the RFBs, highlight the recent developments of the ROMs in non-aqueous redox flow batteries (NRFBs), with an emphasis on the strategies to improve the energy density of NRFBs, and discuss the remaining challenges and future directions of the non-aqueous organic redox flow batteries (NORFBs).

Key words: Redox-active organic molecule, Flow battery, Energy density, Power output