Hierarchical Co/MoNi heterostructure grown on monocrystalline CoNiMoOx nanorods with robust bifunctionality for hydrazine oxidation-assisted energy-saving hydrogen evolution

Replacing thermodynamically unfavorable water oxidation by hydrazine oxidation reaction to accomplish energy-saving hydrogen evolution while efficiently disposing toxic hydrazine-rich wastewater is generally considered as an advantageous strategy. However, the unsatisfactory high voltage of the cell system owing to the lack of the active bifunctional catalysts and insufficient mechanistic understanding of hydrazine oxidation severely limit its development. Hence, we demonstrate the bifunctional metallic hierarchical Co/MoNi heterostructure grown on oxygen vacancy-modified monocrystalline CoNiMoOx nanorods for accelerating both hydrazine oxidation (-23 mV at 100 mA cm(-2)) and seawater reduction (-79 mV at 100 mA cm(-2)). Impressively, such catalyst-assembled hybrid seawater electrolyzer demands an electricity consumption of only 0.143 kW h m(-3) H-2 at 100 mA cm(-2) and cuts 90% power expense compared to traditional alkaline water splitting electrolyzer. DFT calculations reveal that the boosted bifunctional activity is attributed to the construction of Co/MoNi heterostructure that promotes the reaction kinetics of water dissociation, hydrogen adsorption, and stepwise dehydrogenation. These findings help to fundamentally explore the catalytic mechanism of hierarchical metallic heterostructure and highlight the rational design of fast-kinetic bifunctional catalysts for realizing large-scale energy-saving hydrogen evolution and simultaneous fast disposal of hydrazine-rich sewage.

Automated summary

Replacing the thermodynamically unfavorable water oxidation with hydrazine oxidation is considered advantageous for energy-saving hydrogen evolution and efficient disposal of hydrazine-rich wastewater. However, the lack of active bifunctional catalysts and limited understanding of hydrazine oxidation severely hinder its development. In this study, a metallic hierarchical Co/MoNi heterostructure was used to accelerate hydrazine oxidation and seawater reduction, resulting in significantly reduced electricity consumption and power expense. DFT calculations revealed the key role of the Co/MoNi heterostructure in promoting reaction kinetics. These findings contribute to the understanding of catalytic mechanisms and the design of fast-kinetic bifunctional catalysts for large-scale energy-saving hydrogen evolution and hydrazine-rich wastewater disposal.