Publication

Abstract : 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.