Ammonia Synthesis from Water and Nitrogen By Electricity Using Ru Catalysts, Hydrogen-Permeable Membranes, and Phosphate Electrolytes at 250ºC
Authors: Imamura Kanako , Kubota Jun 
Published: February 2020

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”.

Figure 1