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The advance of efficient and economical energy carrier technology is an important challenge in terms of storage and transport of hydrogen fuels produced from renewable energy. Ammonia is a promising candidate of energy carrier because of high energy density and easy liquefaction as well as a carbon-free fuel.1 Electrochemical synthesis has a potential for an efficient ammonia production in comparison with the industrial Haber–Bosch process. In our previous study, we observed the improvement of electrochemical synthesis of ammonia using iron-based electrode catalyst such as K-Al-Fe-BaCe0.9Y0.1O3 (BCY).2 In the study, basically, H2 decomposition occurs to form protons in the anode side, whereas produced protons diffuses through electrolyte and electrons react with N2 to form NH3 in the cathode side (anode: H2 → 2H+ + 2e-; cathode: N2 + 6H+ + 6e- → 2NH3). However, when H2-N2 gaseous mixture was fed in the cathode side, the ammonia formation rate was increased significantly in comparison with feeding pure N2 in the cathode side. Also, we found that that the electrochemical synthesis of ammonia was significantly promoted with an increase in cathodic polarization.2 However, the promotion mechanism is still unclear, and its understanding is important to design practical electrolysis cells. In this study, the electrochemical synthesis of ammonia was studied with Fe-based BCY cathodes to clarify the reaction mechanism of ammonia formation. We used proton-conducting ceramic fuel cells (PCFCs) with a BCY electrolyte membrane, which has high proton conductivity. We investigated ammonia formation rate using the PCFCs and also performed deuterium isotope analysis for ammonia formation using FTIR spectroscopy to observe interfacial reaction of ammonia formation between the electrode and the electrolyte.
Single cell fabrication: Firstly, BCY powders were synthesized by the coprecipitation method. Then, a porous BCY electrode on BCY electrolyte were fabricated by the doctor-blade method and calcined at 1573 K to obtain a cermet electrode of Fe-BCY. Finally, a Pt counter electrode was attached on the electrolyte. The cell configuration was Pt|BCY|Fe-BCY. To discuss the ammonia formation mechanism further, we prepared Pt|BCY|Fe cell with pure iron electrode. Also, we prepared a single cell using an oxide ion conductor as a electrolyte membrane. The cell configuration was Pt|(ZrO2)0.9(Y2O3)0.1(YSZ)| Fe-YSZ.
Electrochemical synthesis of ammonia: the ammonia formation reaction was investigated using Pt, H2 H2O-Ar |BCY|Fe-BCY, H2-N2 with a potentiostat. Typical operation temperature was 873 K. Ammonia formed in the cathode was captured by H2SO4 solution and then the solution was analyzed by ion chromatography. The ammonia formation was also observed using an FTIR spectrometer with a multiple refraction cell to conduct deuterium isotope analysis.
Results and discussion
The results in Pt|BCY|Fe-BCY showed that electrochemical promotion of ammonia formation was observed, i.e., ammonia formation rate was increased with an increase in cathodic polarization. Besides, when K or Cs was added into Fe-BCY electrode as co-catalyst, ammonia formation rate was improved. The donation of electron from the co-catalysts to nitrogen molecules may contribute to improve ammonia formation rate. On the other hand, when we use a cell of Pt|BCY|Fe, the electrochemical promotion was not observed, which suggested that a triple phase boundary in the Fe-BCY cathode might play an important role in ammonia formation reaction. Also, in the oxide ion conducting cell, Pt|(ZrO2)0.9(Y2O3)0.1(YSZ)| Fe-YSZ, the ammonia formation rate was not improved by cathodic polarization. This suggests that proton or hydrogen atom can contribute to promote ammonia formation reaction. Next, deuterium isotope analysis was conducted with Pt, D2-H2O-Ar|BCY|Fe-BCY, H2-N2. FTIR spectra suggested the formation of NH3 and NH2D under the condition. Although the absorbance of NH2D was very weak, the result indicated that the ammonia formation involved charge transfer reaction but the most of hydrogen in the ammonia molecule originated from H2 fed in the cathode side. Considering the results, the electrochemical promotion of ammonia would be induced by enhancement of N2 dissociation, which is a rate determining step of ammonia formation, accelerated by proton or hydrogen at the triple phase boundary of Fe-BCY electrode. Thus, this study provides an important strategy for designing efficient ammonia formation and contribute to further improve of ammonia electrochemical synthesis with PCFCs.
Electrochemical synthesis of ammonia was investigated with PCFCs at intermediate temperature, and the electrochemical promotion of ammonia formation was observed. Electrochemical measurements and relevant deuterium isotope analysis using FTIR spectroscopy suggests that proton and/or hydrogen atom at around triple phase boundary of Fe-BCY can play an important role in electrochemical promotion of ammonia formation reaction.
This work was supported by CREST, Japan Science and Technology Agency (JPMJCR1441).
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