Enabling enhanced energy efficiency for decoupled water splitting by a hierarchical hybrid redox mediator with exceptional supercapacitive performance

The surplus renewable energy can be converted into H₂ fuel through decoupled water splitting without the formation of explosive H₂/O₂ mixtures. The supercapacitor electrode materials effectively function as solid redox mediators in decoupled water splitting; however, their limited supercapacitive performance impedes the efficiency and durability. To address this issue, we have developed a hierarchical Ni/Co hydroxides/chalcogenides hybrid as the electrode material with a high specific capacitance of 1527.60 F g⁻¹ at 2 A g⁻¹ and stability over 10,000 cycles at 10 A g⁻¹. When used as a redox mediator in decoupled water splitting, the NiFe LDH-NiFe alloy hybrid bifunctional electrode achieves low onset voltages of 1.458 V for H₂ evolution and 0.162 V for O₂ evolution at 100 mA cm⁻², with an energy efficiency exceeding 95% over an extended duration of 104.19 h encompassing 640 decoupled cycles. This study highlights the crucial role of depolarization effect from battery-type supercapacitor electrode materials in achieving enhanced energy efficiency for decoupled water splitting.