Achieving Exceptional Cell Voltage (2.34 V) through Tailoring pH of Aqueous Zn-Br2 Redox Flow Battery for Potential Large-Scale Energy Storage

Environmentally benign, affordable alkaline Zn(OH)42-/Zn redox flow battery (RFB) remain a promising energy storage system for stationary applications owing to it's two electron transfer abilities having redox potential (-1.26 V vs SHE) with 820 Ahkg- 1 theoretical capacity. It is hypothesized that meticulously tailoring the pH of the anolyte from neutral to alkaline medium could significantly lower the redox potential of zinc from-0.76 V vs standard hydrogen potential (SHE) to-1.26 V adding 500 mV to the conventional RFB systems significantly improving overall energy density. Herein, a new hybrid alkaline based ZnBr2 redox flow battery (AZBB) is demonstrated by pairing Zn(OH)42-/Zn redox couple as anolyte with 2Br- /Br2 redox couple as catholyte with exceptionally high (2.34 V) cell voltage. Very interestingly, AZBB shows a higher charging voltage of 2.47 V and discharge voltage of 2.1 V at current density of 40 mA cm-2 with 88 % coulombic efficiency, 79 % voltage efficiency and 70 % energy efficiency. Even at high current density of 120 mA cm-2, the AZBB shows a coulombic efficiency of 90 %, voltage efficiency of 52% and energy efficiency 46 % with the high discharge voltage of 1.75 V. More importantly, the new hybrid alkaline based ZnBr2 redox flow battery stably runs 500 cycles at 20 mA cm-2 with the coulombic efficiency of 73 %, voltage efficiency of 80 % and energy efficiency of 58 %. Thus, the new strategy of fine tuning the pH of electrolytes leads to an efficient route to achieve higher cell voltage with 27 % high energy density RFB systems.

Automated summary

The environmentally friendly and affordable alkaline Zn(OH)42-/Zn redox flow battery shows promising potential for energy storage applications. By carefully adjusting the pH of the electrolyte, the redox potential of zinc can be lowered, resulting in significantly improved energy density of the system.