Advances in Understanding the Mechanism and Improved Stability of the Synthesis of Ammonia from Air and Water in Hydroxide Suspensions of Nanoscale Fe_2O_3

摘要

We report a mechanism of electrochemical ammonia (NH_3) production via an iron intermediate in which H_2 and NH_3 are cogenerated by different electrontransfer pathways. Solar thermal can contribute to the energy to drive this synthesis, resulting in a STEP, solar thermal electrochemical process, for NH_3. Enhancements are presented to this carbon dioxide (CO_2)-free synthesis, which uses suspensions of nano-Fe_2O_3 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 NH_3, ηNH_3, increases with decreasing current density, and at j_(electrolysis) = 200, 25, 2, and 0.7 mA cm~(-2), η_(NH_3) = 1%, 7%, 37%, and 71%, respectively. At 200 mA cm~(-2), over 90% of applied current drives H_2, rather than NH_3, formation. Lower temperature supports greater electrolyte hydration. At 105 °C in the hydrated Na_(0.5)K_(0.5)OH electrolyte, η_(NH_3) increases and then is observed to be highly stable at η_(NH_3) = 24(+2)%.