TY - JOUR
T1 - Halogen enabled aqueous flow cells for large-scale energy storage: Current status and perspectives
AU - Li, Jiayi
AU - Xu, Zeyu
AU - Wu, Maochun
N1 - Funding Information:
Direct surface treatment or modification of positive electrodes is another strategy to improve electrochemical activity. Suresh et al. [110] introduced oxygen functional groups onto the CF surface to increase the electrochemical activity and hydrophilicity through acid treatment. The ZBFB assembled with the modified electrode delivered an improved VE of 86% at the current density of 20 mA cm−2. Moreover, the GF could be modified by thermal treatment and plasma treatment under oxygen and nitrogen atmospheres to simultaneously increase surface area and O- and N-containing functional groups. The ZBFB with the modified positive electrode showed an enhanced EE of 77% at 10 mA cm−2 [111]. To further boost the surface area, cobalt-assisted thermal treatment was used to modify GF surface. The catalytic etching in the presence of Co resulted in aligned carbon nanostructures and abundant oxygen functional groups on the fiber surface, which can facilitate charge and mass transfer. Consequently, the EE of ZBFB was increased from 68% to 84% at 60 mA cm−2 [112]. Furthermore, Lu et al. [113] proposed a multifunctional CF-based electrode (NTCF) with N-rich defects, which can enhance the absorption of bromine and facilitate the Br2/Br− reactions (Fig. 4e). Impressively, the NTCF enabled ZBFB to operate at a current density of as high as 180 mA cm−2 with an EE of 63.07%. In addition, a composite electrode based on a CF supported TiN nanorod array with a 3D hierarchical structure (CTN) was proposed. Compared with the smooth surface of pristine CF (Fig. 4f), the nanorod array alignment and abundant pores in CTN (Fig. 4g) allow faster ion transfer and provide much more active sites for Br2/Br− redox reactions. Therefore, the ZBFB with this CTN electrode achieves an EE of 66% at an ultrahigh current density of 160 mA cm−2 [114]. This work offers a new strategy for the development of advanced electrodes for high-power density bromine-based flow batteries.Another significant role played by halogen species in RFBs is complexing with the active species to increase the energy density and electrochemical performance. In fact, in halogen-based RFBs, the halogens generated during the charging process are usually complexed with halide ions to form polyhalide ions in aqueous electrolytes, such as I3−, I2Br−, I2Cl− and Br2Cl− [194,219,220]. These polyhalide ions have higher solubility than the diatomic molecules, which enables the halogen-based flow batteries to deliver higher energy densities, particularly in I2-based flow batteries [221]. Moreover, halide ions can also be applied in VRFBs to improve the energy density over the current sulfate electrolyte system. Li et al. [222] proposed a VRFB with sulfate and chloride mixed electrolyte containing 2.5 M SO42− and 6 M Cl−, in which four valence states of vanadium (V2+, V3+, V4+ and V5+) were stable up to 2.5 M. They also found that this mixed electrolyte remained stable over a wide temperature range of −5 ∼ 50 °C due to the reduction of SO42− concentrations and the formation of soluble neutral complex VO2Cl(H2O)2 above 20 °C, breaking the limitation of the low solubility of VOSO4 at −5 °C and the precipitation of V2O5 at 40 °C. Therefore, a high EE of 87% was achieved during 20 days with a higher energy density of about 40 Wh L−1. To further understand the mechanism of improved stability of V5+ in mixed acid supporting electrolyte, the Pacific Northwest National Laboratory conducted a series of nuclear magnetic resonance (NMR) spectroscopies and DFT calculations [20,223]. It was found that the formation of [V2O3Cl2‧6H2O]2+ complex was more favorable at a higher vanadium concentration (≥1.75 M). In particular, the ligand exchange process occurred between the above complex and nearby solvent chlorine molecule to form [V2O3Cl‧6H2O]2+ compound, which was less prone to de-protonation, thus prohibiting the precipitation of V2O5 [223]. In addition, they also found that in pure hydrochloride acid electrolyte, V5+ tends to form vanadium dinuclear ([V2O3‧4H2O]4+) or dinuclear-chloro complexes ([V2O3Cl‧4H2O]3+) with good thermal stability in the temperature range of 0–50 °C. Moreover, the viscosity of this chloride solution was 30–40% lower than that of the sulfate electrolyte, thus reducing pumping energy loss [224]. Based on a previous study, Yang et al. [225] systematically optimized the composition of mixed acid electrolyte (2.4 M vanadium, 6.2 M chloride ion and 2.5 M sulfate), which enabled the VRFB to achieve a high EE and highest energy density at 40–80 mA cm−2 for more than 100 cycles over a wide temperature range from −20 to 50 °C.The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 16205721).
Funding Information:
The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 16205721 ).
Publisher Copyright:
© 2023 Elsevier B.V.
PY - 2023/10/15
Y1 - 2023/10/15
N2 - Aqueous flow cells, including redox flow batteries and regenerative fuel cells, are promising technologies for grid-scale energy storage due to their intrinsic safety, high scalability, and flexibility in decoupling power and energy. Redox active species are critical components of aqueous flow cells as they largely determine the energy density, cell performance, and system cost. Halogens have attracted enormous interest as redox active species for aqueous flow cells due to their low cost, high natural abundance, and desirable electrochemical properties, such as high redox potential and high reversibility. Moreover, halogen species have been widely used as redox mediators and complexing agents to improve the reversibility and solubility of active materials, enabling aqueous flow cells to achieve high energy density and performance. This review provides a comprehensive summary of various types of aqueous flow cells that use halogens as active materials, redox mediators, and complexing agents. The working principles, critical issues, and recent progress are systematically discussed based on the roles and types of halogen species. Finally, existing challenges and future perspectives on halogen-based flow cells are highlighted.
AB - Aqueous flow cells, including redox flow batteries and regenerative fuel cells, are promising technologies for grid-scale energy storage due to their intrinsic safety, high scalability, and flexibility in decoupling power and energy. Redox active species are critical components of aqueous flow cells as they largely determine the energy density, cell performance, and system cost. Halogens have attracted enormous interest as redox active species for aqueous flow cells due to their low cost, high natural abundance, and desirable electrochemical properties, such as high redox potential and high reversibility. Moreover, halogen species have been widely used as redox mediators and complexing agents to improve the reversibility and solubility of active materials, enabling aqueous flow cells to achieve high energy density and performance. This review provides a comprehensive summary of various types of aqueous flow cells that use halogens as active materials, redox mediators, and complexing agents. The working principles, critical issues, and recent progress are systematically discussed based on the roles and types of halogen species. Finally, existing challenges and future perspectives on halogen-based flow cells are highlighted.
KW - Energy storage
KW - Halogens
KW - Redox flow batteries
KW - Redox mediator
KW - Regenerative fuel cells
UR - http://www.scopus.com/inward/record.url?scp=85169829912&partnerID=8YFLogxK
U2 - 10.1016/j.jpowsour.2023.233477
DO - 10.1016/j.jpowsour.2023.233477
M3 - Review article
AN - SCOPUS:85169829912
SN - 0378-7753
VL - 581
JO - Journal of Power Sources
JF - Journal of Power Sources
M1 - 233477
ER -