Continuous-flow electrosynthesis of ammonia by nitrogen reduction and hydrogen oxidation

Abstract

Ammonia is a critical component in fertilizers, pharmaceuticals, and fine chemicals and is an ideal, carbon-free fuel. Recently, lithium-mediated nitrogen reduction has proven to be a promising route for electrochemical ammonia synthesis at ambient conditions. In this work, we report a continuous-flow electrolyzer equipped with 25–square centimeter–effective area gas diffusion electrodes wherein nitrogen reduction is coupled with hydrogen oxidation. We show that the classical catalyst platinum is not stable for hydrogen oxidation in the organic electrolyte, but a platinum-gold alloy lowers the anode potential and avoids the decremental decomposition of the organic electrolyte. At optimal operating conditions, we achieve, at 1 bar, a faradaic efficiency for ammonia production of up to 61 ± 1% and an energy efficiency of 13 ± 1% at a current density of −6 milliamperes per square centimeter.

Lithium-mediated electrochemical dinitrogen reduction reaction

“…However, some other serious challenges such as high overpotential, unsustainable proton source from sacrificial solvent, a large ohmic resistance, and organic solvent recovery have arisen. 22,64,[69][70][71][72] Fig. 15 (a) Expanded view of the continuous-flow electrolyzer configuration; (b) Schematic process of the Li-NRR in a continuous-flow electrolyzer.…”

Phenol as Proton Shuttle and Buffer for Lithium-mediated Ammonia Electrosynthesis

“…Our group proposed a rigorous procedure with gas purification and quantitative isotope measurements to avoid false positives, which validated that the produced ammonia during the Li-NRR process was from N2 reduction 9 . Since then, many strategies have been proposed to improve the performance of the Li-NRR system 4, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] . Recently, high current density (-1 A cmgeo -2 ) and faradaic efficiency (FE) towards ammonia (close to 100%) have been achieved in a pressurized batch-type reactor (15 bar or 20 bar), but the solvent was oxidized to generate the necessary protons 26,27 .…”

“…Recently, high current density (-1 A cmgeo -2 ) and faradaic efficiency (FE) towards ammonia (close to 100%) have been achieved in a pressurized batch-type reactor (15 bar or 20 bar), but the solvent was oxidized to generate the necessary protons 26,27 . In general, most Li-NRR investigations published so far use a sacrificial solvent as a proton source 28 . In order to achieve practical viability, however, the protons must come from hydrogen (or water), so the hydrogen oxidation reaction (HOR) has been proposed as the anode reaction of the Li-NRR system to provide a sustainable hydrogen source 15,28 .…”

“…In order to achieve practical viability, however, the protons must come from hydrogen (or water), so the hydrogen oxidation reaction (HOR) has been proposed as the anode reaction of the Li-NRR system to provide a sustainable hydrogen source 15,28 . Very recently, we achieved an ammonia FE of 61% in a continuous-flow reactor by employing nitrogen reduction coupled with HOR at ambient pressure and temperature 28 . Isotope-labeling studies using operando mass spectrometry revealed the hydrogen implemented in the ammonia is indeed coming from the anode via H2 oxidation 28 .…”

“…Very recently, we achieved an ammonia FE of 61% in a continuous-flow reactor by employing nitrogen reduction coupled with HOR at ambient pressure and temperature 28 . Isotope-labeling studies using operando mass spectrometry revealed the hydrogen implemented in the ammonia is indeed coming from the anode via H2 oxidation 28 .…”