Energy-efficient electrochemical ammonia production from dilute nitrate solution

Abstract

Highly efficient electrochemical nitrate reduction could become a key process for sustainable ammonia production overcoming many limitations of the Haber–Bosch process. Current state-of-the-art electrocatalysts have severe drawbacks regarding yield, selectivity and energy efficiency when dealing with dilute nitrate solutions. Herein, we report a layered double hydroxide (LDH)/Cu foam hybrid electrocatalyst that offers a potential solution to this challenge. The [Ni_0.75Fe_0.25(OH)₂](CO₃)_0.125 (Ni₃Fe–CO₃ LDH) exhibits an appropriate kinetic energy barrier for the Volmer step generating hydrogen radicals as well as suppressing H–H bond formation by inhibition of the Heyrovsky step. The electrochemically generated hydrogen radicals transfer to a Cu surface enabling NO₃⁻ reduction to NH₃. The Ni₃Fe–CO₃ LDH/Cu foam hybrid electrode exhibits an 8.5-fold higher NH₃ yield compared to a pristine Cu surface, while exhibiting an NH₃ selectivity of 95.8% at 98.5% NO₃⁻ conversion. The best half-cell energy efficiency (36.6%) was recorded while achieving 96.8% faradaic efficiency at −0.2 V in 5 mM NO₃⁻_(aq).