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Besides, hydrazine oxidation reaction (HzOR) can be combined with a reduction reaction with more positive potential to form a hydrazine battery/cell for simultaneous generation of electricity 29, 30, 31. Among available options, hydrazine, as a liquid proton carrier, can be oxidized to inert nitrogen and release protons at the low voltage, effectively decreasing the input electrical energy 27, 28. However, both the capacity and cycle performance of currently developed redox mediators cannot meet the high energy/power requirement and the research is still in the preliminary stage, which greatly limits the practical application of decoupled systems 18.Ĭonsidering the sluggish kinetics of OER, the anodic OER can be replaced by other anodic oxidation reactions that are kinetically and thermodynamically favorable for energy-saving H 2 production, such as the methanol oxidation reaction 19, 20, 21, the formate oxidation reaction 22, 23, and the isopropanol oxidation reaction 24, 25, 26. The CuFe TBA electrode shows high rate performance and good cycling performance due to the Grotthuss proton conduction 17. introduced a Prussian blue analogs of Cu 2/3♳.4H 2O (CuFe TBA) as a solid-state redox mediator to decouple the HER and OER in acid water electrolysis. The decoupled device was driven by one single perovskite solar cell with a solar-to-hydrogen efficiency of 14.4% 16. For instance, our previous work presented a sodium nickelhexacyanoferrate mediator to decouple acid water electrolysis and amphoteric water electrolysis. Recently, several solid-state redox mediators have been developed for decoupled water electrolysis systems 14, 15. Compared with the soluble mediator, decoupled electrolysis system with solid-state redox mediators can avoid using membranes, which shows considerable prospects 14. This “decoupled” water electrolysis strategy with the aid of a mediator electrode enables the production of H 2 and O 2 at different rates, time, and spaces, which greatly increases the flexibility to harness the intermittent renewable energy. proposed a new electrolysis architecture, in which a soluble redox mediator of phosphomolybdic acid (H 3PMo 12O 40) was employed as electron-coupled proton buffer to decouple the one-step water splitting process into two steps 13. Thirdly, the pressure differences between the sides of the separator and reactive oxygen species caused by the coexistence of H 2, O 2, and catalysts will accelerate the degradation of membrane, thus increasing safety issues 10, 11, 12. Secondly, water electrolysis rate is limited by the sluggish kinetics of the oxygen evolution reaction (OER) because the hydrogen evolution reaction (HER) and OER are tightly coupled and the reaction kinetics of two half-reactions are interdependent to each other. Firstly, using membranes will increase the system costs and restrict the direct use of the fluctuating renewable energy. Conventional one step water electrolysis with diaphragm or membrane as the separator typically faces several critical challenges 8, 9. Among various methods for hydrogen production, water electrolysis is a sustainable and environmentally friendly technology that has been receiving lots of attention 4, 5, 6, 7. Hydrogen (H 2) is considered one of the most promising alternatives to traditional fossil fuels due to its zero carbon emissions and high energy density (120 MJ kg −1) 1, 2, 3. Therefore, this work enables the flexible energy conversion and storage with hydrogen production driven by solar cell at day-time and electricity output at night-time. More importantly, a decoupled electrolysis system for hydrogen production and hydrazine oxidation is constructed, which realizes not only separate hydrogen generation but electricity generation through the p-VHCF-N 2H 4 liquid battery. By using this mediator, a membrane-free water electrolytic cell is built to achieve decoupled hydrogen and oxygen production. It offers a high reversible specific capacity up to 128 mAh g −1 and long cycling performance of 6000 cycles with capacity retention about 100% at a current density of 10 A g −1 due to the enhanced hydrogen bonding network. Herein, a pre-protonated vanadium hexacyanoferrate (p-VHCF) redox mediator is synthesized. Decoupled electrolysis for hydrogen production with the aid of a redox mediator enables two half-reactions operating at different rates, time, and spaces, which offers great flexibility in operation.
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