Electrochemical membrane cell for NH3 synthesis from N2 and H2O by electrolysis at 200 to 250 °C using a Ru catalyst, hydrogen-permeable Pd membrane and phosphate-based electrolyte

Abstract

Ammonia (NH₃) is an energy carrier that can be synthesized from nitrogen and water using electricity generated from renewable sources. The present work investigated NH₃ synthesis using an electrochemical system with the structure of Ru/Cs⁺/MgO|Pd–Ag|CsH₂PO₄/SiP₂O₇|Pt and operating at 200 to 250 °C. In this system, the NH₃ being generated is isolated from the CsH₂PO₄/SiP₂O₇ electrolyte by the hydrogen-permeable Pd–Ag membrane, resulting in the production of dry NH₃. A maximum NH₃ synthesis rate of 0.90 nmol s⁻¹ cm⁻² was obtained from the cathode at the Ru/Cs⁺/MgO|Pd–Ag side at a current density of 10 mA cm⁻², temperature of 250 °C and N₂ flow rate of 3 cm³ min⁻¹ as converted to standard temperature and pressure. Deviations in the NH₃ production rate from the theoretical amounts predicted by Arrhenius plots were observed at temperatures approaching 250 °C, possibly because the reaction approached equilibrium. A current efficiency for NH₃ production of 2.6% was obtained, with the remainder of the current consumed during H₂ generation. The apparent activation energies for the NH₃ synthesis were 69 and 93 kJ mol⁻¹ at 3.2 and 10 mA cm⁻², respectively. These values are significantly lower than that reported for conventional NH₃ synthesis from nitrogen and hydrogen (139 kJ mol⁻¹) over the same catalyst in this cell. The optimum N₂ flow rates were approximately 0.5 and 5 cm³ min⁻¹ at 3.2 and 10 mA cm⁻², respectively, representing nitrogen amounts approximately 42 times the stoichiometric quantities. The provision of an excess of N₂ is believed to reduce the suppression of N₂ activation by so-called hydrogen poisoning of the catalyst.