Ammonia synthesis from nitrogen and water at intermediate temperatures and elevated pressures by using an electrochemical hydrogen-membrane reactor with supported Ru catalysts and phosphate electrolytes

Abstract

Electrochemical synthesis of NH₃ from N₂ and H₂O with electrical power is a promising technology to convert redundant electricity to a chemical fuel. NH₃ synthesis from N₂ and H₂O using a combination of Cs-promoted Ru catalysts, a Pd-based hydrogen-membrane cathode, a CsH₂PO₄/SiP₂O₇ electrolyte, and a Pt anode was investigated in the temperature range of 200–250 °C and pressure range of 0.1–0.7 MPa. A maximum NH₃ formation rate of 12.4 nmol s⁻¹ cm⁻² (759 μg h⁻¹ cm⁻²) was obtained at 250 °C and 0.7 MPa using 5 wt%-Ru/Cs⁺/MgO (Cs/Ru = 1 mol) for a current density of 30 mA cm⁻². The corresponding current efficiency for NH₃ formation was estimated to be 12%, and the remaining part of the current was confirmed to be used for H₂ production. The NH₃ formation rate increased upon increasing the total pressure, following thermodynamic equilibrium. 5 wt%-Ru/Cs⁺/CeO₂ (Cs/Ru = 1 atom) was found to yield the highest formation rate of NH₃ at 200–220 °C and 0.1 MPa, while Ru/Cs⁺/MgO yielded a higher NH₃ formation rate than Ru/Cs⁺/CeO₂ under elevated pressure conditions. Because the dissociation of linearly adsorbed molecular N₂ on Ru is known to be the rate-determining step in NH₃ synthesis, infrared spectroscopy was utilized to examine the linearly adsorbed N₂ on the Ru sites at the adsorption equilibrium at 30 °C.