Binder-Free Nickel Borate/Nickel Hydroxide Bifunctional Catalyst for Coupled Nitrate Reduction and Glycerol Oxidation Toward Sustainable Ammonia Production

The accumulation of nitrate (NO3-) from agricultural runoff poses a growing threat to ecosystems and public health. Converting nitrate into ammonia (NH3) through the electrochemical nitrate reduction reaction (NO3RR) offers a promising strategy to mitigate environmental contamination while creating a sustainable circular route to fertilizer production. However, achieving high NH3 production and energy efficiency remains challenging. Here, we report a binder-free mixed-phase nickel borate/nickel hydroxide bifunctional electrocatalyst directly grown on Ni foam that enables efficient NO3RR without using an external Ni precursor. The optimized catalyst achieves a high NH3 yield rate of 1.49 ± 0.04 mmol h-1 cm-2 and a Faradaic efficiency for ammonia (FENH3) of 81 ± 4.8% at -0.55 V vs RHE. To further reduce the energy input, we couple NO3RR with glycerol oxidation reaction at the anode, replacing the energy-intensive oxygen evolution reaction. The integrated NO3RR||GOR system operates at a low cell voltage of 1.47 V (10 mA cm-2), producing a NH3 yield rate of ∼0.029 mmol h-1 cm-2 (∼63% FENH3), and ∼0.101 mmol h-1 cm-2 of HCOO- (∼73% Faradaic efficiency of formate). The hybrid electrolyzer enables a potential energy-saving of ∼13% compared to NO3RR||OER. This work advances the development of electrochemical platforms for sustainable ammonia synthesis by highlighting the cost-effective, energy-saving, and environmental benefits of coupling reaction.