Electrochemical Synthesis of Ammonia Using Proton Conducting Solid Electrolyte and Ru-Doped BaCe0.8Y0.1Ru0.1O3-
d Electrode Catalyst
Authors: Kobayashi Yusuke , Kimura Yutaka , Shimoda Naohiro , Satokawa Shigeo 
Published: February 2020

Introduction
Recently, the demands for the renewable energy such as water power, wind power, and solar power have increased around the world. These energy production is affected by the climate, and thus, stable power supply is difficult. In addition, the development of technologies of energy storage and transportation is important for effective utilization of the surplus power. Ammonia is a chemical material produced massively and is used in fertilizer and industrial process such as organic synthesis and nitrate acid. In the recent years, ammonia has been paid attention as an energy carrier substance because its hydrogen content and energy density are significantly high. The current process of ammonia production is a Haber-Bosch process. This process, however, requires the high pressure and temperature condition. Thus, a new synthesis process need to be developed. The electrochemical synthesis of ammonia as shown in Fig. 1 is considered to be an effective process of ammonia synthesis from N2 and H2O and/or H2. However, the formation rate of ammonia for the electrochemical synthesis has not been achieved at the demanded level; thus, there are needed to improve ammonia synthesis performance of electrode catalyst and ion conductivity of electrolyte. Therefore, in the present work, to improve the electrochemical synthesis process of ammonia, we have studied the effect of Ru-doping into yttrium-doped barium cerate as an electrode catalyst.
Experimental
The proton conducting BaCe0.9Y0.1O3-
d (BCY) solid electrolyte was synthesized by a co-precipitation method. The BCY powder was mixed with a small amount of NiO as a sintering aid. The resultant powder was uniaxially pressed at 2 MPa into a pellet, followed by cold isostatic pressing at 200 MPa. The pellet was calcined at 1300 ºC for 12 h to obtain BCY pellet. The BaCe0.8Y0.1Ru0.1O3-
d (BCYR) was also prepared by the co-precipitation method. The BCY and BCYR powders were mixed with isometric NiO powders by a planetary ball milling, followed by the calcination at 1200 ºC for 5 h. The NiO-BCYR slurry mixed with polyethylene glycol was pasted on one side of the BCY pellet as the cathode, and then, the sample pellet calcined at 1300 ºC for 5 h. After that, The NiO-BCY slurry mixed with polyethylene glycol was pasted on the other side of the pellet as the anode, followed by the calcination at 1300 ºC for 5 h. The powders and pellet were analyzed using X-ray diffraction (XRD), X-ray fluorescence (XRF) and transmission electron microscopy (TEM). Electrochemical synthesis of ammonia was conducted using the single cell fabricated at 500ºC. A gaseous mixture of N2 and Ar was fed into the working and a gaseous mixture of H2 and Ar was fed into the counter. Before the measurement, the Ni-BCY and Ni-BCYR electrodes were well reduced at 600 ºC for 2 h to activate the electrode.
Results and discussion
According to XRD analysis, both BCY and BCYR samples were assigned to perovskite-type oxides. In addition, Ru component is considered to be doped into BCY, evidenced from XRF and XRD analyses. As a result of electrolysis experiment, ammonia was synthesized electrochemically on the Ni-BCYR electrode. The maximum rate of ammonia formation was ca. 2.0 × 10-10 mol s-1 cm-2 at -600 mV of applied voltage. TEM images indicate that Ru particles existed on the Ni-BCYR surface after the reduction. Therefore, the reason of improvement is probably that the Ru particles on the electrode catalyzes N-N bond breaking. Faradaic efficiency of the ammonia formation was achieved ca. 3.0%, which means that proton on the cathode is mainly converted to hydrogen molecule.
Conclusion
Electrochemical synthesis of ammonia was carried out using proton conducting solid electrolyte from N2 and H2. Ammonia formation rate was improved by using Ni/BCYR electrode, because doping of Ru component enhances ammonia formation reaction at the cathode. The maximum rate of ammonia formation was ca. 2.0 × 10-10 mol s-1 cm-2 at 500 ºC.
Acknowledgments
This work was supported by CREST, Japan Science and Technology Agency.
Reference
[1] S. Giddey, S.P.S Badwal, A Kulkarni, Int. J. Hydrogen Energy 38 (2013) 14576-14594.
[2] A. Skodra, M. Stoukides, Solid State Ionics 180 (2009) 1332-1336.
[3] H. Iwahara, Solid State Ionics 77 (1995) 289-298.

Figure 1