Redox-mediated flow battery for spatially decoupled and charge-free hydrogen production

Hydrogen production from water electrolysis is crucial for low-carbon strategies. However, the slow oxygen evolution reaction (OER) often limits the efficiency of current electrolysis systems, leading to significant energy losses. In this work, we introduce a novel charge-free flow battery that decouples the hydrogen evolution reaction (HER) from the OER, providing a more efficient path to hydrogen production. This system utilizes 7,8-dihydroxy-2-phenazinesulfonic acid (DHPS) as the catholyte and Zn plate as the sacrificial anode. DHPS acts as a redox mediator, transferring protons and electrons from the cathode to the H-NiFe-LDH catalyst in the catholyte tank to drive hydrogen evolution. Electrochemical and in-situ UV-visible spectroscopy confirm the balanced consumption and regeneration of DHPS at both the electrode and catalyst interfaces. Operating with a 0.5 V potential difference between DHPS redox couple and Zn²⁺/Zn couple, the system generates a 0.4 V voltage plateau and produces high purity hydrogen at 20 mA cm^-2. Furthermore, modifying the Zn anode with copper foam boosts the faraday efficiency of hydrogen production from 55.12 % to 75.73 %. This innovative charge-free flow battery offers a promising alternative to conventional water electrolysis, advancing sustainable hydrogen production for low-carbon energy systems.