Decoupling Electrolytic Water Splitting by an Oxygen-Mediating Process

Decoupled water electrolysis systems, incorporating a reversible redox mediator that allows for the construction of membrane-free electrolyzers, have emerged as a promising approach to produce high-purity hydrogen with remarkable flexibility. The key factor crucial for practical applications lies in the development of mediator electrodes that possess suitable redox potential, high redox capacity, excellent cycling reversibility and stability. Herein, we introduce a novel concept of oxygen-mediating redox mediators (ORMs) employing Bi₂O₃ as an example material, which are capable of sequestering oxygen during the hydrogen evolution reaction and subsequently releasing it to generate oxygen gas under alkaline conditions. Thanks to its remarkable reversible redox activity and specific capacity, the Bi₂O₃ electrode boasts an impressive reversible specific capacity of 300.8 mA h g^-1 and delivers outstanding cycling performance for >1000 cycles at a current density of 2.0 A g^-1. Furthermore, the implementation of such a decoupled alkaline water electrolysis system can be integrated with a Bi₂O₃-Zn battery, enabling both power-to-fuel (hydrogen production) and chemical-to-power (rechargeable Bi₂O₃-Zn battery) conversion. With many oxygen-carrier materials readily available and the potential integration with rechargeable alkaline batteries, this study provides an alternative competitive route for membrane-free decoupled water splitting through the oxygen-mediating mechanism with combined energy transformation and storage.