In‐tandem Electrochemical Reduction of Nitrate to Ammonia on Ultrathin‐Sheet‐Assembled Iron‐Nickel Alloy Nanoflowers

The development of alternative routes for ammonia (NH₃) synthesis with high Faradaic efficiency (FE) is crucial for energy conservation and to achieve zero carbon emissions. Electrocatalytic nitrate (NO₃ ^-) reduction to NH₃ (e-NO₃RRA) is a promising alternative to the energy-intensive, fossil-fuel-driven Haber-Bosch process. The implementation of this innovative NH₃ synthesis technique requires an efficient electrocatalyst and in-depth mechanistic understanding of e-NO₃RRA. In this study, we developed an ultrathin sheet (μm) iron-nickel nanoflower alloy through electrodeposition and used it for e-NO₃RRA under alkaline conditions. The prepared Fe-Ni alloy exhibited an FE of 97.28±1.36 % at -238 mV_RHE and an NH₃ yield rate up to 3999.1±242.59 μg h^-1 cm^-2. Experimental electrolysis, in situ Raman spectroscopy, and density functional theory calculations showed that the adsorption and reduction of NO₃ ^- to NO₂ ^- occurred on the Fe surface, whereas subsequent hydrogenation of NO₂ ^- to NH₃ occurred preferentially on the Ni surface. The catalysts exhibited comparable FE for at least 10 cycles, with a long-term stability of 216 h. Electron paramagnetic resonance results confirmed that adsorbed hydrogen was consumed during e-NO₃RRA. This work introduces a sustainable, robust, and efficient Fe-Ni alloy electrocatalyst, offering an environmentally friendly approach for synthesizing NH₃ from NO₃ ^--contaminated water.