Water-Based Electrolytes
Water-based electrolytes are the primary type of substance that is employed in redox flow batteries (RFBs). As Zhang et al. (2017) describe, the most common type of RFB uses vanadium that is dissolved in sulfuric acid as the electrolyte. It is contained in two different oxidation states that are separated by a proton exchange membrane, suspended between two carbon-based electrodes. When the battery is being charged, additional electrons are introduced to the ions, converting them into more energetically potent but less stable versions. As a result, when energy is being drawn from the battery, the vanadium ions lose electrons and return to their former states, releasing the power that is produced. Another option for recharging the battery is to refill it with fresh electrolytes, which is a significant advantage over traditional batteries.
The most significant advantage of water-based RFBs over their organic counterparts is the lack of toxicity and overall danger. As Khattak (2020) notes, they are not subjected to the dangers of dangerous leakages, fire, or contamination through oxygen or moisture exposure. Additionally, this type of battery can operate for extended periods without deteriorating due to the nature of its internal components. However, the electrolyte type also has numerous significant disadvantages, most importantly, the low capacity. Khattak (2020) explains that, due to the effects of water decomposition, the potential window that defines the battery’s power output is less than 2 V, which is substantially lower than the alternatives. Water-based RFBs also struggle with stability, as they can typically only operate within a narrow temperature range, which limits usability. Research into resolving these problems is ongoing, but so far, the most popular models are those that suffer from them.
Organic Electrolytes
Organic electrolytes that are not necessarily based on water are a newer development than aqueous ones. They are still being researched, and the technology is in an emerging state rather than one of widespread adoption. The principle of operation is the same as for water-based electrolytes, though no single popular type can be named due to the ongoing experimentation and innovation in the field. Zhang et al. (2017) provide several examples of active materials that have been proposed and tested: 2,5-di-tert-butyl-1,4-bis(2methoxyethoxy)benzene, 2,2,6,6-tetramethyl-1piperidinyloxyl, N-methylphtalimide, and (5-bis(2-(2-(2methoxyethoxy)ethoxy)ethoxy)anthracene-9,10-dione. They are suspended in a variety of different materials, both organic and otherwise, and demonstrate promising results. The electrodes remain the same as in the water-based versions due to the overall low number of options in their choice mandated by the intrinsic qualities of RFBs.
The primary reason for the development of organic electrolytes for this battery type is that they address many of the problems mentioned in the section above. Zhang et al. (2017) mention that, compared to 1.23 V of water-based solvents, organic variations have reached up to 6.1V while retaining the capability to operate at a much broader temperature range without compromising effectiveness. With that said, the issues mentioned above also need to be taken into consideration. Importantly, if the battery is damaged, it can leak toxic materials into the environment, and its contents may be highly flammable, depending on the substance chosen. Deterioration over time is also a concern due to the increased complexity of the battery and the possibility of it being affected by various factors. With that said, these problems are mostly theoretical and may be possible to address through continued research, which would make organic electrolytes the superior alternative to water-based versions.
References
Khattak, R. (Ed.). (2020). Redox. IntechOpen.
Zhang, H., Li, X., & Zhang, J. (Eds.). (2017). Redox flow batteries: Fundamentals and applications. CRC Press.