A sustainable two-step electrochemical conversion of N2 to ammonia using rhodium nanoparticles on carbon nanosheets

Abstract

A unique approach combining the electrochemical nitrogen oxidation reaction (eNOR) and electrochemical nitrate reduction reaction (eNO₃RR) for green ammonia synthesis addresses the kinetic and selectivity challenges associated with direct nitrogen reduction. Rhodium (Rh) supported on carbon nanosheets (Rh/C) was synthesized via a one-step calcination method and optimized for eNOR performance by varying the metal to carbon ratio, applied potential, and atmosphere. Detailed analysis showed that 5 wt% Rh/C catalysts achieved a NO₃⁻ formation rate of 94.9 ± 4.9 μg h⁻¹ mg_Rh⁻¹ (21.3 ± 1.6 μg h⁻¹ cm⁻²) with a faradaic efficiency (FE) of 21.2 ± 1.5% at 1.7 V vs. RHE in N₂ saturated 0.1 M KOH. In addition, the eNOR kinetics of 5 wt% Rh/C exhibited 50% higher yields in air compared to N₂, with stable NO₃⁻ formation over 5 hours. Temperature-programmed desorption analysis exhibited a shift in the NOR indicating the formation of activated oxygen sites leading to N₂ oxidation. The eNOR mechanism based on the hydroxyl pathway, involving water activation, is also promoted to the atomic oxygen pathway driven by both water activation and oxygen from the air. Also, the electrochemically generated NO₃⁻ was reduced to NH₃ via eNO₃RR using a commercial RuO₂ catalyst at −0.0 V yielding 45.4 μg h⁻¹ mg_Ru⁻¹ ammonia with 5.8% FE. The integrated eNOR–eNO₃RR approach enables onsite NO₃⁻ formation without relying on NO_x sources from wastewater and achieves significantly higher performance values than conventional N₂ reduction on precious metal catalysts, presenting a promising pathway for green ammonia synthesis.