Abstract
Ammonia production via the electrochemical N2 reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N2) on NRR using a FeOx-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton- and high N2 availabilities, which are achieved using 0.12 vol% water in LiClO4-ethyl acetate electrolyte and gaseous N2 supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 ± 19 pmolNH3 s−1 cm−2 at a Faradaic efficiency of 70 ± 24% at the applied potential of −0.1 V vs. NHE. Other conditions (high proton-, or low N2-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.
Highlights
Renewable energy, in storable forms, is critically needed to address the global challenge of meeting our ever-growing energy demand at minimal cost of carbon emission and global warming
The practical electrochemical potentials required for reductively breaking the N2 bond are often higher than for proton reduction. This causes H2 to form in a wasteful side-reaction (i.e. H2 evolution reaction; HER) instead of ammonia formation
This is a notorious reason behind the low Faradaic efficiency for N2 reduction reaction (NRR), which may be alleviated by using H2 instead of water
Summary
In storable forms, is critically needed to address the global challenge of meeting our ever-growing energy demand at minimal cost of carbon emission and global warming In this regard, ammonia (NH3), which is a conventional precursor of fertilizers, is considered a promising candidate to replace the fossil fuels, due to its comparably high gravitational and volumetric energy densities, and the ease of transportation and storage with the existing facilities. The practical electrochemical potentials required for reductively breaking the N2 bond are often higher than for proton reduction This causes H2 to form in a wasteful side-reaction (i.e. H2 evolution reaction; HER) instead of ammonia formation. Aqueous and nonaqueous solutions commonly employed as the electrolyte in the cathodic compartment have an extremely low N2 availability due to the low solubility of N2 in these liquid media, gravely impeding NRR
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