Balancing pH and\nPressure Allows Boosting Voltage\nand Power Density for a H<sub>2</sub>–I<sub>2</sub> Redox Flow\nBattery

The decoupled power and energy output of a redox flow\nbattery (RFB)\noffers a key advantage in long-duration energy storage, crucial for\na successful energy transition. Iodide/iodine and hydrogen/water,\nowing to their fast reaction kinetics, benign nature, and high solubility,\nprovide promising battery chemistry. However, H<sub>2</sub>–I<sub>2</sub> RFBs suffer from low open circuit potentials, iodine crossover,\nand their multiphase nature. We demonstrate a H<sub>2</sub>–I<sub>2</sub> operation with a combined neutral-pH catholyte (I<sub>3</sub><sup>–</sup>/I<sup>–</sup>) and an alkaline anolyte\n(KOH), producing an open circuit cell voltage of 1.28 V. Additionally,\nwe incorporate a pressure-balanced gas diffusion electrode (GDE) to\nmitigate mass transport limitations at the anode. These improvements\nresult in a maximum power density of 230 W/m<sup>2</sup> when allowing\na mild breakthrough of H<sub>2</sub> through the GDE. While minimal\ncrossover occurs, side reactions of permeating active species were\nfound reversible, enabling long-term operation. Future work should\naddress the stability of the GDE and optimization of the electrolyte\nthickness and concentration to fully leverage the potential unlocked\nby balancing the pressure and pH in the H<sub>2</sub>–I<sub>2</sub> RFB.