Electrocatalytic water splitting powered by renewable energy is a sustainable approach for hydrogen production. However, conventional water electrolysis may suffer from gas mixing, and the different kinetics between hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) will limit the direct use of unstable renewable energies, leading to increased cost of H 2 production. Herein, a novel phenazine-based compound is synthesized to develop the solid-state redox mediator associated water splititng process, and thus decoupling the H 2 and O 2 production in acid solution without the use of membrane. Excitingly, this organic redox mediator exhibits high specific capacity (290 mAh g À 1 at 0.5 A g À 1 ), excellent rate performance (186 mAh g À 1 at 30 A g À 1 ) and long cycle life (3000 cycles) due to its π-conjugated aromatic structure and the fast kinetics of H + storage/release process. Furthermore, a membrane-free decoupled water electrolysis architecture driven by solar energy is achieved, demonstrating high-purity H 2 production at different times.

Water splitting driven by renewable energy is considered to be a promising approach for sustainable H2 production. However, the simultaneous generation of H2 and O2 requires the use of a membrane to prevent the gas mixing, which significantly limits the flexibility of H2/O2 separation and H2 transportation, impeding the direct use of renewable sources. In recent years, the concept of decoupled water electrolysis has been proposed along with the utilization of solid‐state redox mediators, by which the H2 and O2 generated at different times and spaces could be separated in absence of membrane. Here, a comprehensive overview of the decoupled water electrolysis using solid‐state redox mediators (SRMs) is presented. After a brief introduction, the SRMs are systematically summarized, including the structural features, the reaction mechanisms, and the properties of decoupled systems. In addition, the coupling of decoupled hydrogen production with other valuable reactions is also involved. Finally, this work discusses the challenges and perspectives for the decoupled water splitting system for commercialization and sustainable H2 production.

Water electrolysis has been considered a promising technology for the conversion of renewables to hydrogen. However, preventing mixing of products (H2 and O2) and exploring cost‐efficient electrolysis components remains challenging for conventional water electrolyzers. Herein, we design a membrane‐free decoupled water electrolysis system using graphite felt supported nickel‐cobalt phosphate (GF@NixCoy‐P) material as a tri‐functional (redox mediator, hydrogen evolution reaction (HER), oxygen evolution reaction (OER)) electrode. The versatile GF@Ni1Co1‐P electrode obtained by one‐step electrodeposition not only exhibits high specific capacity (176 mAh g‐1 at 0.5 A g‐1) and long cycle life (80% capacity retention after 3000 cycles) as a redox mediator, but also has relatively outstanding catalytic activities for HER and OER. The excellent properties of the GF@NixCoy‐P electrode endow this decoupled system with more flexibility for H2 production by fluctuating renewable energies. This work provides guidance for multifunctional applications of transition metal compounds between energy storage and electrocatalysis.