What are the best organic redox active materials for flow battery applications?

1 ORAMs classification

ORAMs can be classified into three main categories, based on their molecular structures and properties: quinones, viologens, and nitroxides. Quinones are aromatic compounds that have two oxygen atoms attached to the ring, and can undergo two-electron or four-electron redox reactions. Viologens are derivatives of 4,4'-bipyridine, and can undergo one-electron or two-electron redox reactions. Nitroxides are organic radicals that have a nitrogen atom with an unpaired electron, and can undergo one-electron redox reactions.

2 ORAMs advantages

ORAMs have several advantages over metal-based electrolytes for flow battery applications. First, they are abundant, cheap, and renewable, which can reduce the cost and environmental impact of the batteries. Second, they have high solubility and stability in water or organic solvents, which can increase the energy density and lifetime of the batteries. Third, they have tunable redox potentials and molecular structures, which can enable the design of customized and optimized electrolytes for different applications.

3 ORAMs limitations

ORAMs also have some limitations that need to be addressed before they can be widely adopted for flow battery applications. One of the main limitations is the crossover of the electrolytes through the membrane that separates the positive and negative compartments of the battery. Crossover can cause capacity loss, self-discharge, and degradation of the electrolytes and the membrane. Another limitation is the low conductivity and viscosity of some ORAMs, which can affect the performance and efficiency of the batteries. Moreover, some ORAMs may have toxicity, flammability, or corrosion issues that need to be carefully evaluated and mitigated.

4 ORAMs selection criteria

To select the best ORAMs for flow battery applications, several criteria need to be considered, such as the redox potential, solubility, stability, conductivity, viscosity, toxicity, flammability, and corrosion of the electrolytes. The redox potential determines the voltage and power output of the battery, and should be as high as possible. The solubility and stability determine the energy density and lifetime of the battery, and should be as high as possible. The conductivity and viscosity determine the resistance and flow rate of the electrolytes, and should be as low as possible. The toxicity, flammability, and corrosion determine the safety and environmental impact of the battery, and should be as low as possible.

5 ORAMs examples

Organic redox active molecules (ORAMs) have been successfully used or tested for flow battery applications, such as anthraquinone-2,6-disulfonic acid (AQDS), methyl viologen (MV), and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). AQDS has high solubility and stability in water, and can reach a voltage of 1.2 V and an energy density of 20 Wh/L. MV has high solubility and stability in organic solvents, and can reach a voltage of 1.5 V and an energy density of 40 Wh/L. TEMPO has high solubility and stability in water, and can achieve a voltage of 1.1 V and an energy density of 25 Wh/L. These ORAMs offer great potential for flow battery applications.

6 ORAMs future prospects

ORAMs are a promising class of materials for flow battery applications, but further research and development is needed to overcome their limitations and improve their performance. Future prospects include developing new ORAMs with better properties, improving membrane materials and configurations to prevent crossover, exploring new combinations of ORAMs with other materials, and scaling up the ORAMs-based flow batteries from lab-scale to pilot-scale and commercial-scale. Demonstrating their feasibility and competitiveness in real-world scenarios is also essential.

7 Here’s what else to consider

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