A High-Voltage Aqueous Zinc-Vanadium Redox Flow Battery with Bi-Modal Tin and Copper Clusters by a Continuous-Flow Electrometallic Synthesis

Aqueous zinc-based redox flow batteries are promising large-scale energy storage applications due to its low cost, high safety, environmental friendliness and low redox potential of the zincate couple of Zn/[Zn(OH)4]²⁻ in the alkaline media. However, there have still tremendous obstacles towards practical use, such as zinc dendritic growth during the charging process, resulting in internal short circuit. These can create the dead zinc metal, which accumulate on the porous electrode or membrane and increase the cell resistance. In addition, the formation of the zincates has a problem of a low reversibility in the strong alkaline electrolytes, which can degrade the cycle efficiency and performance. Currently, several methods have been proposed to reduce the zinc dendritic growth in the zinc-based redox flow batteries, such as the surface modification of the carbon felt electrode and membrane developments. However, there were still limitations in terms of the complicated modification process. Also, the water splitting limits the voltage enhancement of the aqueous RFBs, allowing the narrow potential window under 1.23 V. Therefore, it is necessary to develop a low-cost and high energy density aqueous RFB system with the high voltage.<br/>In this work, we report a bi-modal sized tin and copper (Sn, Cu) incorporated carbon felt (SCCF) via a novel multi-step, continuous-flow electrometallic synthesis method to solve the zinc dendrite issue in the zinc-based redox flow batteries. The SCCF electrode lowers the zinc nucleation overvoltage in the zinc symmetric flow battery and zinc-based hybrid redox flow battery, inducing the homogenous zinc plating and suppressing the zinc dendritic growth. A combined electrochemical and computational investigation reveals that the bi-modal tin and copper particles could suppress unwanted alloy formation, such as Cu-Zn alloy, strongly adsorbing zinc without the dendrite formation. The SCCF electrode exhibits 75% improved cycling stability than the pristine carbon felt electrode in the zinc symmetric flow battery. The pH differential hybrid Zn-V redox flow battery with the double membrane, pairing the VO²⁺/VO₂⁺ catholyte with the Zn/[Zn(OH)₄]²⁻ anolyte, affords a high average cell voltage over 2.31 V at 40 mA cm⁻². The SCCF electrode in the hybrid Zn-V redox flow battery led to high coulombic efficiency (&gt;99.9%), average energy and voltage efficiency 92.6% and 92.3% over 100 cycles, respectively.<br/>We anticipate that this facile preparation method assisted by the continuous-flow electrometallic synthesis and high voltage hybrid Zn-V redox flow battery can pave the way to be large-scale applied in ZFBs, and even expanded into a wide range of energy storage system.<br/><br/>Acknowledgments<br/>This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2021R1C1C1008349, NRF-2021R1A4A1022198). This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE)(20214000000140, Graduate School of Convergence for Clean Energy Integrated Power Generation). This work was supported by BK21 FOUR Program by Pusan National University Research Grant, 2021.