Ammonia Synthesis at Intermediate Temperatures from N2 and Steam in Solid-State Electrochemical Cells Using Cesium Hydrogen Phosphate Based Electrolytes
Authors: Kikuchi Ryuji , Kishira Shota , Qing Geletu , Takagaki Atsushi , Oyama Shigeo Ted 
Published: February 2020

Solid-state electrochemical synthesis of ammonia is promising to overcome the limited conversion of the conventional catalytic reactors [1] and to enable ammonia synthesis in more mild conditions. Several electrochemical reactors based on the following electrolytes: polymers, perovskites, fluorites, pyrochlores, and composite electrolytes, were investigated to synthesize ammonia from N2 and H2. The highest NH3 formation rate reported so far for an electrochemical system was found to be 1.13×10-8 mol cm-2 s-1 using a Nafion membrane, wet H2 and dry N2 at 80 °C and atmospheric pressure [2]. For N2-H2O ammonia synthesis Amer et al. reported 4.0×10-10 mol cm-2 s-1 at375ºC [3]. In this research ammonia synthesis was performed using solid-state electrochemical cells, for the first time, at an intermediate temperature of 220 °C and atmospheric pressure. Composites composed of CsH2PO4 and SiP2O7 were used as the electrolyte and various noble metals as the anode and cathode catalysts. The effects of the applied voltage and electrode catalysts on the NH3 formation rate and Faradaic efficiency were investigated.
CsH2PO4 was prepared by dissolving stoichiometric quantities of Cs2CO3 and H3PO4 in distilled water and drying overnight at 120 °C. SiP2O7 was prepared from mesoporous SiO2 and H3PO4 as reported previously [4]. As electrode catalysts Pt/C-loaded carbon paper (Pt loading 1 mg cm-2, ElectroChem Inc.), Ru-loaded carbon paper, and Ag/Pd paste (TR-4865, Tanaka Kikinzoku Kogyo K.K.) were used. The Ru-loaded carbon paper was prepared by liquid phase reduction of RuCl3 by NaBH4, followed by filtration of the metallic Ru particles using a carbon paper. The membrane electrode assembly (MEA) was prepared by uniaxial pressing. The molar ratio of CsH2PO4 and SiP2O7 in the mixture was 2:1. The anode, electrolyte, and cathode were placed in a die, layer by layer, and then uniaxially pressed at pressure of 250 MPa for 10 min. When the Ag/Pd paste was used as the electrode catalyst, a thin layer (~0.1 mm in thickness) of the catalyst paste was pasted onto the surface of the electrolyte pellet. In ammonia synthesis, humidified N2 was supplied to the cathode chamber, and humidified H2 or humidified Ar to the anode chamber. The feed gas flow rates were 50 cm3 min-1. A water concentration of 30 % was obtained by bubbling the gas flow through water at 70 °C.
Figure 1 summarizes ammonia synthesis rate from N2 and H2 for different applied voltages. The largest ammonia synthesis rate was 2.4×10-10 mol cm-2 s-1 at 0.3 V when the Pt/C was used as the cathode. Over the Ru and Ag-Pd cathodes the ammonia synthesis rate decreased monotonically for the increase in the applied voltages. A similar trend in the Faradaic efficiency, which designates the ratio of the current used for ammonia synthesis, was observed for the increase in the applied voltage. This means that H2 is preferentially formed at the cathode for the high applied voltages. The current density increased at the high applied voltages and it is probable that the coverage of the cathode surface by protons becomes high, leading to suppression of N2 adsorption to the cathode surface and preferential formation of H2. This is partially evidenced by the low NH3 formation rate over the Ag/Pd cathode which is less active for dissociation of N2.
N2-H2O ammonia synthesis was carried out by feeding water vapor to the anode. The ammonia synthesis rate at 1.2 V was decreased to the order of 10-11 mol cm-2 s-1, compared with the N2-H2 synthesis. The anode might be inefficient for water electrolysis. The ammonia synthesis rate normalized to electrochemically active surface area of each cathode was compared. It was found that the Ru cathode is most active for ammonia synthesis. This result indicates that the catalyst active for dissociation of N-N bond is also requisite in electrochemical ammonia synthesis when the adsorption sites at the cathode surface are available for N2 adsorption.
[1] I.A. Amar, R. Lan, C.T.G. Petit, S.W. Tao, J. Solid State Electrochem., 15 (2011) 1845-1860.
[2] G.C. Xu, R.Q. Liu, J. Wang, Science in China Series B-Chemistry, 52 (2009) 1171-1175.
[3] I.A. Amar, R. Lan, S.W. Tao, RSC Adv., 5 (2015) 38977-38983.
[4] T. Matsui, T. Kukino, R. Kikuchi, K. Eguchi, Electrochem. Solid State Lett. 8 (2005) A256-A258.

Figure 1