Decoupled alkaline water electrolysis by a K 0.5 MnO 2 -Ti mediator via K-ion insertion/extraction.

Conventional one-step water electrolyzers generate H 2 accompanied by O 2 evolution, and may cause gas mixing and high cell voltage inputs. Herein, using the potassium ion battery material of K 0.5 MnO 2 -Ti as a mediator, we effectively decoupled the H 2 and O 2 evolution of alkaline water electrolysis temporally, thereby achieving a membrane-free pathway for H 2 production. As a mediator electrode for charge storage, the K 0.5 MnO 2 -Ti exhibited a stable capacity of 100 mA h g -1 at 0.1 A g -1 owing to the reversible K-ion insertion/extraction mechanism. The decoupled water electrolysis device exhibited the step voltages for hydrogen and oxygen production of 1.02 and 0.57 V at 5 mA, respectively. A nearly unity Faradaic efficiency and sustained production of pure H 2 has been demonstrated at a constant current density. We anticipate that this mediator demonstrated here may provide a route for the practical application of the decoupling strategy.