Redox flow batteries: Pushing the cell voltage limits for sustainable energy storage

Electrode kinetics of zinc at the anode in an alkaline medium holds a great prospective for energy storage systems due to low redox potential of Zn(OH)42−/Zn redox couple (−1.26 V vs SHE), high capacity, good stability, involves two electron transfer, high reversibility, eco-friendly and low cost. Undoubtedly, enlarging the voltage of the flow cell is the ultimate goal for enhancing the energy density of the system. Here, we demonstrate the increase in the operating cell voltage of Zinc-Polyiodide (ZnI2) flow battery by meticulously switching the anolyte from an acidic/neutral to an alkaline medium. Very interestingly, swapping the electrolyte from neutral to an alkaline medium shows drastic increase in the voltage window from 1.37 V (neutral) to 1.89 V (alkaline) for ZnI2 redox flow battery (RFB). Furthermore, the developed advanced hybrid ZnI2 electrolyte highly improved the rate capability of the RFB, even at 100 mA cm−2, exhibiting a round trip efficiency of 98 % and the corresponding energy efficiency is about 46 %. On the other hand, the ZnI2 flow cell with the given electrolyte condition delivered a maximum coulombic (96 %), voltage (83 %) and energy efficiency (77 %) at 20 mA cm−2. Moreover, the alkaline based ZnI2 RFB performed stably over 90 cycles at 30 mA cm−2. At the same time, 50 cycles at 50 mA cm−2 have also been tested. This reported cycle life is ever maximum cyclability achieved for alkaline based metal halides RFBs. Thus, a creative protocol of switching the anolyte pH from acid/neutral to alkaline medium offers an efficient route to attain high cell voltage and high energy density device when paired with high positive redox potential species.