Dipole moment regulation of a small-molecule quinone mediator boosts long-term cycling stability for decoupled water electrolysis†

Decoupled water electrolysis with the aid of small-molecule quinone as a redox mediator, which separates the hydrogen evolution reaction and oxygen evolution reaction in both time and space, offers an efficient strategy for green hydrogen production with high purity and flexibility. However, the impact of molecular structure, particularly structural symmetry, on the properties of quinone-based materials is still unclear. Herein, we discuss the design of a series of small-molecule quinone compounds with different dipole moments and investigate their electrochemical performance in acidic aqueous electrolytes. Among them, the nonpolar tetramethylquinone (TMBQ) shows exceptional cycling stability (30 000 cycles), outperforming the majority of reported solid-state redox mediators. The remarkable stability of TMBQ is attributed to its low dipole moment, which significantly reduces its solubility in polar solvents. In addition, the TMBQ electrode also delivers excellent rate performance, enabled by rapid H⁺ diffusion (10⁻⁶ cm² s⁻¹) and favorable electronic conductivity. Using the environmentally friendly TMBQ as a solid-state redox mediator, a membrane-free decoupled water electrolysis system is constructed, achieving flexible and green hydrogen production.