Ammonia synthesis from nitrogen and steam using electrochemical cells with a hydrogen-permeable membrane and Ru/Cs+/C catalysts

Abstract

The ammonia (NH₃) synthesis from nitrogen (N₂) and steam (H₂O) using electrochemical cells with a hydrogen-permeable membrane, Ru/Cs⁺/C catalysts, and CsH₂PO₄/SiP₂O₇ electrolytes at 250 °C was investigated. The highest formation rate was achieved in the hydrogen-permeable membrane cell with 30 wt%-Ru/Cs⁺/VXC72R catalysts, where VXC72R represents a carbon black powder of Cabot Vulcan XC-72R, reaching 32 nmol s⁻¹ cm⁻² at 60 mA cm⁻², 1.0 MPa, and 250 °C, corresponding to 15% current efficiency for NH₃ formation. This rate can be converted to 1.8 mmol h⁻¹ g_cat⁻¹ as a catalyst-based unit. At 10 mA cm⁻², 1.0 MPa, and 250 °C, a rate of NH₃ formation of 9.8 nmol s⁻¹ cm⁻² was obtained with a corresponding current efficiency of 28%. The rate of NH₃ formation under this condition was close to the maximum current efficiency of 36% determined by the theoretical chemical equilibrium on thermodynamics. Three-electrode measurements with a reference electrode were conducted on the present cell. It was observed that the anode exhibited significant overpotential, especially at higher current density.