Writing in the journal Energy Storage Materials, researchers from the University of Science and Technology of China have reviewed recent progress and perspectives on the incorporation of organic species into redox flow batteries.
Study: Recent Progress in Organic Species for Redox Flow Batteries. Image Credit: Black_Kira/Shutterstock.com
Improving Energy Storage
Renewable energy has become a key research area due to anthropogenic climate change and the environmental impact of utilizing greenhouse gas-emitting fossil fuels. Renewable technologies are quickly becoming a cornerstone of energy production in many countries across the world.
Several renewable technologies have been developed in recent decades, including photovoltaic solar cells, wind turbines, and geothermal energy. However, solar and wind power, two of the main renewable energy generation strategies currently, are hindered by the intermittent nature of the resources they exploit. Reliable, safe, low-cost, and efficient energy storage systems for backup purposes during periods of low power generation are needed.
Pumped storage hydropower plants have existed since the 20th century and provide significant storage potential, with efficiencies of over 75% and 95% storage capacities reported. However, these power plants are limited by cost, long construction time, low energy density, geographical dependence, and hydrological restrictions. Incorporating low-cost, geographically independent, and compact battery systems into these power plants could address these shortcomings.
Several battery technologies have been developed in recent decades, which provide opportunities for energy storage but can be limited by issues such as low energy density, safety, limited lifecycle, and cost, depending on the type of technology employed. Redox flow batteries, on the other hand, are a flexible, long-life, and safe energy storage solution.
Redox Flow Batteries
In a redox flow battery, the reaction and storage areas are physically separated. This ensures that capacity and power are independent. Power is determined by the electrode area in the cell stack, whereas capacity is determined by the volume of the storage tank and the concentration of the electrolyte.
Another key feature of a redox flow battery is the flow of electrolytes, which dissipates generated heat generated through charge and discharge cycles, avoiding self-discharge, combustion, and subsequent damage to the battery. Pioneering work into redox flow batteries was conducted in the 1970s and over the next two decades.
Vanadium redox flow batteries are the most commercialized devices currently in use. If uncontaminated, they can achieve high energy efficiency and open-circuit voltage, and theoretically, these devices can possess an infinite lifetime. However, there are some limitations to vanadium redox flow batteries.
Vanadium and the ion exchange membranes are expensive, V2O5 can precipitate at temperatures over 45oC, and they have problems with toxicity and corrosion.
To overcome these limitations, several studies have investigated organic compounds for use in redox flow batteries. The two main classes of devices are aqueous organic and non-aqueous organic redox flow batteries. Organic devices have enormous potential for commercialization due to the abundance of materials, low cost, safety, and non-toxicity. Research into aqueous organic flow cell batteries is more mature than non-aqueous devices.
The Study
The new paper has provided a comprehensive review of current research perspectives to guide future research in this field. The authors have noted that several promising review articles have been published in recent years on redox flow batteries, but there is a lack of research on the latest developments in organic species in organic redox flow battery research.
Several recent advances have been explored and discussed in this review paper. Key properties such as redox potential, solubility, voltage, and stability of species have been highlighted by the authors, alongside their effect on device performance. Aqueous devices which use redox-active molecules such as phenazine, active polymers, ferrocene, and anthraquinone and their derivatives have been summarized, as well as non-aqueous devices based on nitronyl nitroxide and phenothiazine radicals.
There are some limitations to currently used organic species. For instance, aqueous devices exhibit lower calendar stability and energy density than vanadium redox flow cells. This is due to lower potential and solubility. Improvements in energy density and current density are a key research opportunity to enhance the commercial viability of organic redox flow batteries. Furthermore, non-aqueous batteries need further research into enhancing the stability of redox species.
Despite their limitations, incorporating organic species into redox flow batteries has expanded the design of novel devices which can overcome the challenges associated with conventional devices. Utilizing these materials can provide cost and sustainability benefits for this field of renewable energy research.
Overcoming current design and material limitations in organic redox flow batteries will help to drive these devices further toward large-scale industrial production and commercialization. Future perspectives highlighted in the study include designing new materials, developing robust electrolytes, and developing new cell configurations and high-performance membranes. Overall, the paper has provided an important contribution to this emerging field of energy storage research.
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Further Reading
Li, Z et al. (2022) Recent Progress in Organic Species for Redox Flow Batteries Energy Storage Materials 50 pp. 105-138 [online] sciencedirect.com. Available at: https://www.sciencedirect.com/science/article/pii/S2405829722002331?via%3Dihub
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