Electrochemical Ammonia Synthesis Using Mixed Protonic-Electronic Conducting Cathodes with Exsolved Ru-Nanoparticles in Proton Conducting Electrolysis Cells

Abstract

Electrochemical synthesis of ammonia was performed at 500 • C using a mixed protonic-electronic conducting cathode, Ru-doped BaCe 0.9 Y 0.1 O 3 (BCYR) in a proton-conducting electrolysis cell (PCEC) using a BaCe 0.9 Y 0.1 O 3 (BCY) electrolyte. Ru nanoparticles were formed in situ on the surface of the BCYR particles after a heat-treatment in a reducing atmosphere, as determined by TEM and XPS measurements. The BCYR cathode exhibited activity toward electrochemical ammonia formation, which indicates that the Ru nanoparticles were active sites for the electrochemical synthesis of ammonia. We found that altering the reduction temperature could be used to control the exsolved Ru-nanoparticle size; decreasing the Ru particle size contributes to an improvement in electrochemical ammonia formation due to an increased triple-phase-boundary active length. The ammonia formation rate per amount of exsolved Ru nanoparticles achieved with BCYR was higher than that achieved with a previously reported Ru-doped La 1-x Sr x TiO 3 (LSTR) cathode. Our results suggest that the mixed protonic-electronic conduction in the BCYR cathode may contribute to the increased Ru-nanoparticles activity toward electrochemical ammonia synthesis. The dependence of the ammonia formation rate on the applied voltage and the reaction mechanism were discussed based on kinetic analysis.

Kinetic and deuterium isotope analyses of ammonia electrochemical synthesis

“…13 Further, several studies have reported electrochemical ammonia formation rates of approximately 10 À11 to 10 À8 mol cm À2 s À1 and current efficiencies of 0.7-56% under a pure N 2 atmosphere at temperatures below 100 C. [14][15][16][17][18][19][20][21] On the other hand, many researchers have achieved the electrochemical reduction of N 2 by introducing pure N 2 into the cathode at high temperatures (>500 C) using a variety of catalysts. [22][23][24][25][26][27][28][29][30][31][32][33] Low ammonia formation rates of 10 À11 to 10 À9 mol s À1 cm À2 have been reported under these conditions. [22][23][24][25][28][29][30][31][32][33] Wang et al proposed that the ammonia formation rate is affected by several factors, including the electrode area, the conductivity of the proton-conducting solid oxide electrolyte, and the N 2 ow rate.…”

“…[22][23][24][25][26][27][28][29][30][31][32][33] Low ammonia formation rates of 10 À11 to 10 À9 mol s À1 cm À2 have been reported under these conditions. [22][23][24][25][28][29][30][31][32][33] Wang et al proposed that the ammonia formation rate is affected by several factors, including the electrode area, the conductivity of the proton-conducting solid oxide electrolyte, and the N 2 ow rate. 22,29,30,32 At present, the hypothesis that the enhancement of the ammonia formation rate with cathodic polarization is caused by a faradaic reaction (i.e., charge-transfer reaction), in which N 2 reacts with the H + supplied from the anode to form ammonia, is generally accepted.…”

Electrochemical promotion of ammonia synthesis with proton-conducting solid oxide fuel cells

“…In the past decade, there have been many studies on the direct electrochemical synthesis of ammonia at low temperature [3][4][5][6][7][8][9][10][11][12], intermediate temperature [13][14][15][16][17][18] and high temperature [19][20][21][22][23][24][25][26][27][28][29]. The reaction at the anode is water or hydrogen decomposition to form protons and oxygen and emitted electron (Eq.…”

“…At temperatures higher than 500C, proton conductors based on perovskite materials such as BaCe1-x-yZryYxO3-δ (BCZY) were dominant because of significant proton conductivity. Many papers have reported ammonia electrochemical synthesis above 500C with a variety of electrode catalysts such as Ni [20], Fe [22,27], Ru [24][25][26], Ag [29] and Pt [29]. However, the mechanism of the effect of cathodic polarization on ammonia formation rate has not been clarified.…”

“…Recently, we reported the electrochemical promotion of ammonia synthesis using protonconducting solid oxide fuel cells using Fe-based and Rubased electrode catalysts [22,25]. It was observed that Ru nanoparticles 1-10 nm in diameter formed on BCY and La0.3Sr0.6TiO3 (LST) surfaces after appropriate treatment for reduction of the electrodes [25]. The results revealed that the electrochemical synthesis of ammonia using Ru-doped BCY was better than that using Rudoped (LST) under pure N2 atmosphere.…”