The electrochemical promotion of ammonia formation on Fe-based electrode catalysts is investigated using proton-conducting-electrolyte-supported cells of H2–Ar, Pt |BaCe0.9Y0.1O3 (BCY)| Fe-based catalysts, H2–N2 at temperatures between 550˚C and 600˚C, and...

Electrochemical promotion of ammonia formation is mainly governed by surface reaction with N2 and H2 in the cathode.

In this study, the deuterium isotope analysis was conducted to investigate the electrochemical promotion mechanism of ammonia synthesis at 650˚C using H2-N2, 1%W-10%Fe-BaCe0.9Y0.1O3 (BCY)|BCY|Pt, 3%H2O-10%D2-87%Ar. The performance of ammonia electrochemical synthesis was examined using high pressure liquid chromatography (HPLC) and Fourier-transform infrared spectroscopy (FTIR). The ammonia formation rate increased by around 20 times with an increase in cathodic polarization. The electrochemical synthesized ammonia of NH3 and NH2D were detected by FTIR. Furthermore, a positive correlation between the ammonia formation rate and hydrogen partial pressure in the cathode was observed. Considering the results, the electrochemical promotion mechanism of ammonia formation was probably caused by accelerated N2 dissociation via supplying D+, subsequently N* and ND* were formed from N2D*, and then ND* and N* reacted with H* which originated from H2 fed in the cathode to form NH2D and NH3.

Investigation of energy carriers is of critical importance for chemical energy storage and transport technologies in combination with the large-scale utilization of renewable energy. Ammonia is a promising energy carrier in terms of carbon-free fuel, high energy density, and easy liquefaction (1).Electrochemical approaches can provide a more efficient reaction process and allow more flexible operations for ammonia production, compared with the industrial Haber–Bosch process. In this study, we investigated direct electrochemical synthesis of ammonia using proton-conducting ceramic fuel cells (PCFCs) operating at high temperatures around 773 K. A ceramic proton conductor, BaCe1-xYxO3 (BCY), was used for an electrolyte membrane. In our previous studies, metal catalysts of Ru and Fe were investigated with a variety of electrode structures such as metal nanoparticle (2, 3) and cermet electrode (4). In terms of metal nanoparticle catalyst, Ru-doped BCY (BCYR) was examined (2,3). The ammonia formation rate was not so high with BCYR (ca. 10-11 mol/s cm2). To improve the ammonia formation rate, Ru-BCY and Fe-BCY (or K-Al-Fe-BCY) cermet electrodes were investigated. When pure N2 was supplied to the cathode, low ammonia formation rate was observed (ca. 10-11 mol/s cm2). When a N2/H2 gaseous mixture was supplied to the cathode, higher ammonia formation rate was observed; ammonia formation rate with Fe-BCY electrode was increased up to ca. 10-10 mol/s cm2 with an increase in cathodic polarization. In addition, our recent study showed a significant improvement of ammonia formation rate with pure Fe electrode (ca. 10-8 mol/s cm2) (5). Considering the above, the roles of interface and surface of electrode catalysts on ammonia electrochemical synthesis should be investigated for clarifying the reaction mechanism and developing effective cell design. In this study, we conducted the deuterium isotope analysis for ammonia electrochemical synthesis to investigate the contributions of interface between electrode and electrolyte, i.e., triple-phase boundary, and electrode surface for ammonia formation processes as well as relevant kinetic measurements of ammonia formation on Fe catalyst (Fig. 1a). The cell configuration was Pt|BCY|Fe-BCY for a cermet electrode. Also, Pt|BCY|Fe cell with pure iron electrode was fabricated to investigate the ammonia formation mechanism. Ammonia formation rate was measured with the configuration of Pt, H2 H2O-Ar |BCY|Fe, H2-N2. Ammonia formed in the cathode was trapped in an H2SO4 solution. Then, the obtained solution was analyzed by ion chromatography. The ammonia formation was also observed using an FTIR spectrometer with a multiple refraction cell to perform the deuterium isotope analysis. Significantly high ammonia formation rate was observed using the Pt|BCY|Fe cell: 1~6x10-8 mol/s cm2 at -1.5 V and 623 K with changing a flow rate in the cathode, which is the best performance for ammonia electrochemical formation under the H2 flow condition. This suggests that Fe surface may play a dominant role in ammonia formation rather than the triple-phase boundary between Fe and BCY. Deuterium isotope analysis was carried out with Pt, D2-H2O-Ar|BCY|Fe, H2-N2. Observed FTIR spectra showed the formation of NH3 and NH2D under the condition (Fig. 1b). The absorbance of NH2D was very weak, which suggested that most of hydrogen atoms in the ammonia molecule originated from H2 in the cathode. In the case of Pt, H2-H2O-Ar|BCY|Fe, D2-N2, ND3 was observed as a main product under the D2 flow condition in the cathode. Considering the above, the electrochemical promotion of catalyst (EPOC) for ammonia formation will be induced via promotion of N2 dissociation on Fe. N2 dissociation is the rate determining step in the ammonia formation reaction. Possible mechanism for the promotion of N2 dissociation is electron back donation with an increase in cathodic polarization. This mechanism gives a hint for designing an electrochemical reactor to promote efficient ammonia formation. Acknowledgements This work was supported by CREST, Japan Science and Technology Agency (JPMJCR1441). References D. Miura, T. Tezuka, Energy, 68(15), 428-436 (2014). J. Otomo, N. Noda, F. Kosaka ECS Trans., 68, 2663-2670 (2015). F. Kosaka, T. Nakamura, J. Otomo, J. Electrochem. Soc., 164, F1323-F1330 (2017). F. Kosaka, T. Nakamura, A. Oikawa, J. Otomo. ACS Sustainable Chem. Eng., 5(11), 10439–10446 (2017). J. Otomo, et al., 435c, 2019 AIChE Annual Meeting, Orlando, USA. Figure 1