NH₃ synthesis is one of the essential chemical processes to maintain a human life, and most of them is used as fertilizers for agricultural production. NH₃ is currently synthesized from N₂ in air and H₂ from fossil fuels by Haber-Bosh process. Quite recently, NH₃ is attracting attention as an energy carrier since NH₃ can be easily liquefied at room temperature with weak compression, and NH₃ has high hydrogen density. It is also possible to storage and transport NH₃ using existing infrastructure. Habor Bosch process is an exothermic reaction, thus NH₃ is synthesized from H₂ with releasing large heat. NH₃ synthesis from H₂O and electricity from a renewable energy is a promising technology in the future society, since it reduces the regional inhomogeneous distribution and temporal fluctuation of renewable energies. Electrochemical NH₃ synthesis from H₂O is a one-step process because reduced H atoms from the electrolytes on the catalyst surfaces can be reacted with N atoms without a large exothermic reaction between N₂ and H₂.

In this work, NH₃ synthesis from H₂O and N₂ by electricity using an electrochemical cell, which are consists of Ru catalysts, H₂ permeable membranes, phosphate electrolytes, and Pt anode has been investigated. It is possible to avoid the absorption of NH₃ into the electrolyte and the contamination of produced NH₃ by H₂O because of the isolation using the hydrogen permeable membrane. This devise is illustrated in Fig. 1A. The phosphate electrolytes, CsH₂PO₄/SiP₂O₇, can be used at 200-250°C, and it has an advantage over the solid-oxide electrolyte systems at high temperatures because of the limitation of chemical equilibrium in NH₃ synthesis.

Formation of NH₃ from N₂ and H₂O by the Ru/Cs⁺/MgO|Pd-Ag|CsH₂PO₄/SiP₂O₇|Pt cell at 250°C and 10 mA cm^-2 with changing N₂ flow rate is shown in Fig. 1B. The flow rate of the N₂ gas in the cathode side was varied from 1 to 7 cm³-STP min^-1, and the mixture of Ar and H₂O at 10 cm³-STP min^-1 and 16 μL liquid-H₂O min^-1, respectively, was flowed to the anode side. The maximum rate of NH₃ synthesis was obtained at the N₂ flow rate of 5 cm³-STP min^-1, and it was 9×10^-10 mol s^-1 cm^-2. The current efficiency was estimated as 2.6% for NH₃ synthesis, and the remaining part of current was used for the production of H₂ as detected by a gas chromatograph. The theoretical amount of H₂ penetrated to the Pd-Ag membrane can be calculated as 0.22 cm³‑STP min^-1 from 10 mA cm^-2. Therefore, the quantity of H₂ at optimum N₂ flow rate was much smaller than stoichiometry. For typical Ru catalysts, it is known that they are easily poisoned by the presence of H₂,so that the reaction order for H₂ is usually negative. Thus, the optimum N₂ flow rate for NH₃ production was much larger than the stoichiometry against amount of H₂ which was obtained from electrolysis. This suggests that the development of catalysts with resistance against H₂-poisoning leads further improvement of this system.

NH₃ synthesis from H₂O and N₂ by electricity at 250°C using an electrochemical cell, that consists of a Ru/Cs⁺/MgO catalyst, Pd-Ag membrane cathode, a CsH₂PO₄/SiP₂O₇ electrolyte, and Pt anode was examined. Significant NH₃ formation was observed under the various conditions of temperature and flow rates. In this presentation, we would like to discuss the kinetic properties of this system in the apparent activation barriers and chemical equilibrium in addition to the H₂ poisoning.

This work was supported by JST/CREST, “Creation of core technologies for innovative energy carrier utilization aimed at the transport, storage, and use of renewable energy”.