Ammonia synthesis from water and nitrogen using electricity with a hydrogen-permeable membrane electrochemical cell with Ru catalysts and molten hydroxide electrolyte: integration with ammonia separation and unreacted gas recirculation

Abstract

There is considerable interest in synthesizing NH₃ directly from abundant H₂O and N₂ using electricity from renewable energy sources, for applications such as synthetic fuels, artificial fertilizers, and raw materials for plastics. NH₃ synthesis from N₂ and H₂O was investigated using an electrochemical setup featuring Ru/Cs⁺/C catalysts, Pd alloy membrane cathodes, NaOH–KOH molten electrolytes, and Ni anodes operated at 250 °C and 1.0 MPa (absolute). This electrochemical setup was integrated with a refrigerated gas/liquid separator at −75 °C to concentrate NH₃ and a recirculation pump for unreacted H₂ and N₂. As a single-pass reactor, if NH₃ separation and unreacted gas recirculation were not used, this electrochemical device produced NH₃ at 1.0 MPa and 250 °C, with an apparent current efficiency of 32–20% at 10–100 mA cm⁻². This efficiency was limited by the chemical equilibrium, which is calculated to be 36%. The study achieved a 90% apparent current efficiency, with a 320 nmol s⁻¹ cm⁻² production rate of NH₃ at 100 mA cm⁻², 250 °C, and 1.0 MPa with NH₃ separation and unreacted gas recirculation. The remaining 10% of the apparent current efficiency was used for H₂ production. The reaction kinetic properties and scalability of the present system were discussed.