The organic redox flow battery possess a separate structure, which is completely different from the traditional cell structure. The monomer organic redox flow battery consists of end plate, insulating plate, collector plate, flow frame, electrode, electrolyte, diaphragm, clamping device, peristaltic pump, liquid storage tank and pipeline lines. Among them, electrode, electrolyte and diaphragm are the three most important core components. According to the system, organic redox flow battery can be divided into aqueous organic redox flow battery and non-aqueous organic redox flow battery. The most obvious characteristic of aqueous organic redox flow battery is water as the solvent of the active substance. The obvious advantage of non-aqueous organic redox flow battery is the high energy density. Organic redox flow battery has the advantages of safe operation, friendly environment, adjustable power, controllable capacity and long cycle life, which is very suitable for large-scale energy storage.
Figure 1. Schematic diagram of the organic redox flow battery.
- Electrode materials: In the organic redox flow battery, the electrode itself is inert and does not participate in the redox process, and the energy storage only depends on the redox reaction of the active substance itself on the electrode surface. Electrode is the site of redox reaction of active substances in the battery. The characteristics of electrode material have a direct impact on electrochemical reaction. Therefore, the electrode of organic redox flow battery should have high conductivity, which can effectively reduce the influence of ohmic polarization. Furthermore, the electrode material should have high activity in response to specific active substances, and the active substances in the flow state can react on the electrode. Electrode materials should be chemically stable in common highly corrosive environments. In addition, given the cost of batteries and the feasibility of applications, the materials used to make electrodes should be cheap, readily available and have a long life.
- Electrolyte active material: Electrolyte solution with active material is the storage medium of organic redox flow battery, which has significant influence on the performance of the battery. The redox reversibility of the electrolyte active material determines the charge and discharge efficiency of the battery. The solubility of electrolyte active materials in the supporting electrolyte determines whether the battery can have large capacity and high energy density. The redox potential of positive and negative active materials determines the voltage window range of the battery.
- Diaphragm material: Diaphragm plays an important role in the whole battery operation process, and the performance data of the organic redox flow battery are closely related to diaphragm. The diaphragm material completely separates the redox active electrical pair to prevent short circuits. The selectivity of the diaphragm is critical because cross-contamination of the positive and negative electrodes leads to a loss of efficiency. Furthermore, organic redox flow battery need to be assembled and sealed either by a single cell or a stack. Stress is generated during this process, so the diaphragm material is required to have good mechanical properties.
- Liu B, Tang C W, Zhang C, et al. Cost-Effective, High-energy-density, nonaqueous nitrobenzene organic redox flow battery[J]. Chemistry of Materials, 2021, 33(3).