Rechargeable Zn-H₂O hydrolysis battery for hydrogen storage and production

Reactive metals hydrolysis offers significant advantages for hydrogen storage and production. However, the regeneration of common reactive metals (e.g., Mg, Al, etc.) is energy-intensive and produces unwanted byproducts such as CO₂ and Cl₂. Herein, we employ Zn as a reactive mediator that can be easily regenerated by electrolysis of ZnO in an alkaline solution with a Faradaic efficiency of >99.9 %. H₂ is produced in the same electrolyte by constructing a Zn-H₂O hydrolysis battery consisting of a Zn anode and a Raney-Ni cathode to unlock the Zn-H₂O reaction. The entire two-step water splitting reaction with a net energy efficiency of 70.4 % at 80 °C and 50 mA cm^-2. Additionally, the Zn-H₂O system can be charged using renewable energy to produce H₂ on demand and runs for 600 cycles only sacrificing 3.76 % energy efficiency. DFT calculations reveal that the desorption of H* on Raney-Ni (-0.30 eV) is closer to zero compared with that on Zn (-0.87 eV), indicating a faster desorption of H* at low overpotential. Further, a 24 Ah electrolyzer is demonstrated to produce H₂ with a net energy efficiency of 65.5 %, which holds promise for its real application.