This segment discusses the application of VRFBs in the electrochemical treatment of wastewater. It also includes the repurposing of VRFBS to capacitive deionization module for desalination.
APPLICATIONS OF VRFB FOR WASTEWATER TREATMENT
Studies have shown the use of RFB for desalination and examples include the use of a redox catholyte for the development of a RFB desalination system (Kim et al., 2024). However, VRFBs have also been adopted for use in desalination including for electrocoagulation of wastewater.
Electrocoagulation of wastewater
Electrocoagulation uses a flow battery where electricity is generated using metal electrodes and water or wastewater is used as the electrolyte to concentrate certain emulsified pollutants including organic materials into flocs (or coagulants) that can then be removed. The floc (or coagulants) are produced due to the formation of hydroxides.
While solar-powered electrocoagulation of wastewater has been applied successfully to remove turbidity and organic materials in synthetic wastewater (Sharma et al., 2011), solar-powered electrocoagulation of wastewater has also been applied using VRFBs by Millan et al. (2018). This study tested the use of this technology to remove oxyfluorfen from wastewater.
There were two modes of operation. During the sunlight hours while the solar panels were exposed to sunlight, an external power supply was used for the operation of the VRFB and for the electrocoagulation process for wastewater. This is known as the photovoltaic operation mode. The stored energy from the solar panels was used to continue the process at nighttime. The VRFB consisted of VO2+ in the positive electrolyte and V3+ in the negative electrolyte in sulfuric acid.
A carbon soft felt was used as the electrode material at both the cathode and the anode while a cationic exchange membrane was used to separate the anodic and cathodic compartments in the cell. To deliver the electrolyte to the RFB at a constant rate, a peristaltic pump was used. Electrocoagulation was carried out using a continuous stirred tank reactor containing water with oxyfluorfen.
Electrocoagulation was able to remove only up to 25% of oxyfluorfen into the floc and not completely remove the oxyfluorfen. Notably the solar profile was adjusted to avoid damage to the materials in the electrocoagulation reactor and in the VRFB. Additionally, the study noted that high voltage can damage the bipolar plates in the VRFB.
The current discharge profile showed that the stored solar electricity in the VRFB was not sufficient to ensure the continuous electrical supply for the electrocoagulation process for the entire night because of the fluctuation in the solar energy during the days. To achieve the same treatment efficiency at nighttime as during the daytime, almost one third more energy storage is required during the daytime sunlight period to power the nighttime treatment. Nevertheless, the study showed that electrocoagulation powered by the VRFB and solar energy can remove some of the oxyfluorfen from the wastewater.
Repurposing VRFBs to capacitive deionization (CDI) module
The CDI technology removes charged species in water via the functioning of electrodes which are usually made of different carbon configurations such as nanotubes and graphene. Progress in CDI technology includes the improvements in electrode functioning and also in membrane use. Membrane CDI stack has also been used for the removal of nitrite ions from brackish water (Broseus et al., 2009). However challenges remain including for the stability in performance for the membrane CDI stack and the durability of the electrodes and the membranes.
The high efficiency, long-life cycle of up to 5,000 to 20,200 charge/discharge cycles, scalability which includes the ease of production of its repetitive components are features of VRFB that makes it a good candidate for use in scale-up systems that use similar components such as a CDI (capacitive deionization) system for water desalination (Garcia-Quismondo et al., 2019). Lado et al. (2021) repurposed a VRFB stack into a CDI module for desalination.
Although the Nafion ion exchange membrane was discarded, this study recycled the graphite felt electrodes and other components including the flow frame distributors and the expanded graphite bipolar current collectors from the VRFB stack. After cleaning, the components were assembled for use in a CDI stack module and the functioning was optimized. In summary, a 5kWh-VRFB 40-cell system was refurbished into a CDI stack consisting of five cells connected in series and equipped with 1250 cm2 electrodes. This set-up was tested using 0.21 M NaCl solution, equivalent to a highly concentrated brackish water. The system, in addition to removing NaCl, showed a robust performance between 45% and 50%. It also showed a significant potential to reduce the energy consumption from 0.84 kW.h/m3 to 0.21 kW.h/m3, and these values are higher than established technologies such as reverse osmosis for desalination.
While the above section discussed a case study that repurposed a VRFB stack into a CDI module for desalination, the basic operational principles in a VRFB can also be used in desalination as shown by Wang et al. (2021). In this study, a flow-electrode capacitive deionization (FDCI) technology was used which consisted of carbon nanotubes and the application of a constant current.
The electrochemical desalination mechanism involved the oxidation of the V2+ ions to V3+ ions at the negative chamber and the reduction of V3+ ions to V2+ ions at the positive chamber. It also involved the removal of the feed NaCl without any change in the electrolyte. The study reported electrochemical desalination, specifically the salt removal rate of up to 0.253 μg/seconds/cm2 while the energy consumption was decreased to 72.62 kJ/mol. The study attributed this performance due to the vanadium ion redox reactions and fast electron transport through the carbon nanotubes.
VRFBs have been used in the electrocoagulation of wastewater and have also been repurposed to CDIs for desalination. These technologies, although proven to work as shown in this segment, have come a long way with limitations and progress.
In the next segment
What were the earliest VRFBs like? And how did they advance to the VRFBs we have today? These questions will be answered in the next segment on the history of VRFBs.
References
Broseus, R., Cigana, J., Barbeau, B., Daines-Martinez, C., & Suty, H. (2009) Removal of total dissolved solids, nitrates and ammonium ions from drinking water using charge-barrier capacitive deionization, Desalination, 249(1), 217-223. https://doi.org/10.1016/j.desal.2008.12.048
Garcia-Quismondo, E., Almonacid, I., Martinez, M. A. C., Miroslavov, V., Serrano, E., Palma, J., & Salmoeron, P. A. (2019) Operational experience of 5 kw/5 kwh all-vanadium flow batteries in photovoltaic grid applications, Batteries, 5(3), 52. https://doi.org/10.3390/batteries5030052
Kim, S., Kim, N., Kim, Y., Park, S., & Cho, K. H. (2024) Optimization of a redox flow battery desalination system: Experiment and modeling, Journal of Water Processing Engineering, 64, 105597. https://doi.org/10.1016/j.jwpe.2024.105597
Lado, J. L., Garcia-Quismando, E., Almonacid, I., Garcia, G., Castro, G., & Palma, J. (2021) A successful transition from a vanadium redox flow battery stack to an energy efficient electrochemical desalination module, Journal of Environmental Chemical Engineering, 9(6), 106875. https://doi.org/10.1016/j.jece.2021.106875
Millan, M., Rodrigo, M. A., Fernandez-Marchante, C. M., Diaz-Abad, S., Pelaez, M. C., Canizares, P., & Lobato, J. (2018) Towards the sustainable powering of the electrocoagulation of wastewater through the use of solar-vanadium redox flow battery: a first approach, Electrochimica Acta, 270, 14-21. https://doi.org/10.1016/j.electacta.2018.03.055
Sharma, G., Coi, J., Shon, H. K., & Phuntsho, S. (2011) Solar-powered electrocoagulation system for water and wastewater treatment, Desalination and Water Treatment, 32(1-3), 381-388. https://doi.org/10.5004/dwt.2011.2756
Wang, Z., Hu, Y., Wei, Q., Li, W., Liu, X., & Chen, F. (2021) Enhanced Desalination Performance of a Flow-Electrode Capacitive Deionization System by Adding Vanadium Redox Couples and Carbon Nanotubes, The Journal of Physica; Chemistry C, 125(2), 1234-1239. https://doi.org/10.1021/acs.jpcc.0c09058
