Reaction Analysis of Electrochemical Synthesis of Ammonia with Proton Conducting BaCe0.9Y0.1O3 Solid Electrolyte
Authors: Kosaka Fumihiko , Nakamura Takehisa , Otomo Junichiro 
Published: February 2020

Ammonia is one of candidates as an energy carrier for renewable energies because of its high energy density and easy liquefaction. In this study, we focused on electrochemical synthesis of ammonia using proton conducting solid electrolyte[1]. Ammonia is produced from nitrogen and water with a proton conductor as follows.

N2 + 3H2O → 2NH3 + 3/2O2 (1)

Water dissociates on an anode to form proton, and then ammonia forms on a cathode by reactions among proton, electron and nitrogen as follows.

H2O → 1/2O2 + 2H+ + 2e- (2)

N2 + 6H+ + 6e- → 2NH3 (3)

Therefore, appropriate reaction among nitrogen, proton and electron at the triple-phase boundary in the cathode is important for the efficient electrochemical production of ammonia. So far, AgPd cathode has been mainly used because Pd can permeate hydrogen[2]. However, to accelerate the ammonia formation rate and to improve Faradaic efficiency for the ammonia formation, the control of cathode microstructure and the comprehension of electrochemical ammonia formation mechanism are required. In this study, Ru-doped La
x
Sr1-x
TiO3 (LSTR) and Ru-doped BaCe1-x
Y
x
O3 (BCYR) were used as cathodes. Ru nanoparticles was expected to be exsoluted from B-site in perovskite lattice in reduction atmosphere and to act as active sites for ammonia formtion[1]. Addiotionally, these perovskite materials have different protonic and electrical conductivities. LaxSr1-xTiO3 (LST) has high electrical conductivity such as 102-103 S cm-1[3]. On the other hand, although the electrical conductivity of BCYR is lower than that of LaxSr1-xTiO3, BCYR is a mixed protonic and electronic conductor[4]. To realize efficient electrochemical ammonia production, the role of the transport properties of these cathode materials was investigated. A kinetic analysis of electrochemical ammonia synthesis was also performed.

Proton conducting solid electrolyte, BaCe0.9Y0.1O3 (BCY), was synthesized by the co-precipitation method. Ru-doped La0.3Sr0.6TiO3 and Ru-doped BaCe0.9Y0.1O3 were prepared by the solid state reaction method. BCY electrolyte, LSTR and BCYR cathodes were characterized by XRD, XPS, SEM, TEM and TG. Electrolyte-supported LSTR-BCY|BCY|Pt and BCYR|BCY|Pt cells were fabricated for the electrochemical synthesis of ammonia. The electrochemical synthesis was performed at 500℃ by supplying a gas mixture of 2.1%H2O/98%N2 to the cathode side and 2.1%H2O/10%Ar/88%Ar to the anode side. AC impedance measurements with a frequency range of 10-2-105 Hz and cyclic voltammetry with a sweep rate of 2 mV s-1were conducted. Synthesized ammonia was detected with an ion-exchange chromatograph.

0, 10, 20 and 40% Ru-doped La0.3Sr0.6TiO3 and BaCe0.9Y0.1O3 were synthesized. x% Ru-doped LST and BCY are described as LSTRx and BCYRx hereafter (e.g. 40% Ru doped LST is LSTR40). 40% Ru was successfully doped in LST. On the other hand, impurities such as RuO2were observed for BCYR, especially for BCYR20 and BCYR40. BCYR10 was used for the electrochemical ammonia production because the peak shifts to the higher angle was observed in XRD patterns by 10% Ru doping and peak intensities of the impurities were small in BCYR10. From the observation using TEM, Ru nanoparticles of 2-3 nm were observed on the surface of LSTR and BCYR particles, which were exsoluted from B-site in the perovskite lattice. The amount of the exsoluted Ru nanoparticles was highest in LSTR40. Exsolution of Ru nanoparticles on LSTR and BCYR was also confirmed using CO pulse adsorption measurement and XPS.

Electrochemical measurements were conducted with LSTRx (x = 0, 20 and 40)-BCY|BCY|Pt cells and a BCYR10|BCY|Pt cell. Cyclic voltammetry measurements showed that current density was improved with an increase in the amount of Ru-doping. Formation of ammonia was detected for both the LSTR and BCYR cells. The maximum ammonia formation rate was 1.1×10-11 mol s-1 cm-2with the LSTR40 cathode at the applied voltage of -500 mV, as shown in Fig. 1. Although the ammonia formation rate with the mixed protonic and electronic conducting BCYR cathode was relatively low because of the low current density, Faradaic efficiency for the ammonia formation with the BCYR cathode was improved in comparison with the LSTR40 cathode. This result suggests that proton conduction in a cathode may be an important factor for the efficient electrochemical production of ammonia because proton can be effectively transported to active Ru nanoparticles. For further high ammonia formation rate, improvement of the electrical conductivity in mixed protonic and electronic conducting cathode is important.

Acknowledments: This work was supported by CREST, JST.

[1] J. Otomo et al., ECS Transaction, 68, 2663-2670 (2015).

[2] I. Garagounis et al., Frontiers in Energy Research, 2, 1 (2014).

[3] X. Zhou et al., RSC Advances,
4, 118 (2014).

[4] H. Matsumoto et al., J. Electrochem. Soc., 152, A488 (2005).

Figure 1