Highly selective and efficient ammonia synthesis from N2 and H2O via an iron-based electrolytic-chemical cycle

Abstract

Ammonia production via electroreduction of N2 and water under mild conditions is emerging as a promising alternative to the fossil fuels-reliance and CO2 emitting Haber-Bosch process. However, the achievement of high Faradaic efficiency and high ammonia formation rate is still challenging. Here, we demonstrate how ammonia can be selectively produced from N2 and H2O via a two-step iron-based cyclic process using a molten hydroxide electrolyte. The first step is the production of Fe by electrochemical reduction of Fe2O3. The second step is the steam-hydrolysis of Fe with bubbling N2 to produce NH3 and reform Fe2O3. Both reaction steps proceed isothermally at 250 °C in a molten salt electrolytic cell without switching of temperature and needing separation of the mediator, resulting in more easily putting into industrial practice. The cycle achieves an ultrahigh Faradaic efficiency of 79.8% at 1.15 V and a high ammonia formation rate of 1.34 × 10−8 mol s−1 cm−2 at 1.75 V. This is a critical advance in breaking the domination of hydrogen evolution reaction (HER) competition to achieve highly selective and efficient NH3 synthesis from N2 and H2O beyond reliance of fossil fuels.