Electrochemical N 2 reduction at ambient condition - Overcoming the selectivity issue via control of reactants' availabilities

Ammonia production via the electrochemical N<sub>2</sub> 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 N<sub>2</sub>) on NRR using a FeO<sub>x</sub>-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 N<sub>2</sub> supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 +/- 19 pmolNH<sub>3</sub> 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.