Charge-redistribution in bimetallic oxides buried in microporous curled carbon for efficient nitrate electroreduction to ammonia

Electrochemical reduction from nitrate into ammonia is a chance for nitrate removal from drinking water, while at higher concentrations, this 8-electron reduction process could even become relevant for energy storage, high conversions and low onset potentials assumed. Herein, we report the synthesis and analysis of a NiFe₂O₄/C-MS hybrid system made by a molten-salt strategy where the Ni-Fe oxide spinel nanoparticles act as the active center for electrochemical nitrate (NO₃⁻) reduction reaction, while the microporous carbon serves as a conductive support to form a cohesive electrode material. The NiFe₂O₄/C-MS catalyst achieves a maximum NH₃ yield rate of 5.4 mg mg_cat⁻¹ h⁻¹ and Faradaic efficiency of 98% at −0.6 V versus reversible hydrogen electrode. With NiFe₂O₄ nanoparticles buried into microporous carbon, the onset potential decreases dramatically. We propose that this reduction originates from charge redistribution in NiFe₂O₄ in the electronic heterojunction with carbon, while enhanced electrolyte diffusion in microporous carbon facilitates high conversion rates. Density functional theory calculations clarify the low energy barrier on NiFe₂O₄, highlighting the essential role of Ni in activating Fe species. The COMSOL Multiphysics simulations demonstrate that the microporous curled carbon accelerates NO₃⁻ transport and enhances adsorption on the reactive sites. This work offers insights for designing carbon-based nanocomposites for efficient nitrate reduction electrocatalysis.