Advances in understanding the mechanism and improved stability of the synthesis of ammonia from air and water in hydroxide suspensions of nanoscale Fe₂O₃.

Abstract

We report a mechanism of electrochemical ammonia (NH3) production via an iron intermediate in which H2 and NH3 are cogenerated by different electron-transfer pathways. Solar thermal can contribute to the energy to drive this synthesis, resulting in a STEP, solar thermal electrochemical process, for NH3. Enhancements are presented to this carbon dioxide (CO2)-free synthesis, which uses suspensions of nano-Fe2O3 in high-temperature hydroxide electrolytes at nickel and Monel electrodes. In a 200 °C molten eutectic Na(0.5)K(0.5)OH electrolyte, the 3 Faraday efficiency per mole of synthesized NH3, η(NH3), increases with decreasing current density, and at j(electrolysis) = 200, 25, 2, and 0.7 mA cm(-2), η(NH3) = 1%, 7%, 37%, and 71%, respectively. At 200 mA cm(-2), over 90% of applied current drives H2, rather than NH3, formation. Lower temperature supports greater electrolyte hydration. At 105 °C in the hydrated Na(0.5)K(0.5)OH electrolyte, η(NH3) increases and then is observed to be highly stable at η(NH3) = 24(+2)%.