Roles of Interface and Surface of Electrode Catalysts in Ammonia Electrochemical Synthesis with Proton-Conducting Ceramic Fuel Cells
Authors: Otomo Junichiro , Li Chien-I , Matsuo Hiroki 
Published: December 2020

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).

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Figure 1