Influence of Ionic and Electronic Transport Properties on Ammonia Electrochemical Synthesis in Protonic Ceramic Fuel Cells
Authors: Otomo Junichiro , Okazaki Moe , Ortiz Corrales Julián Andrés 
Published: December 2024

Energy carriers play an important role in chemical energy storage and transport technology for effective hydrogen utilization. Ammonia is a carbon-free fuel and a promising energy carrier in terms of high energy density and easy liquefaction. Electrochemical synthesis provides an efficient method, and protonic ceramic fuel cells (PCFCs) can be applied to ammonia production (N2 + 3H2O → 2NH3 + 1.5O2). We reported that Fe-based electrode catalysts with a proton-conducting solid electrolyte improved the ammonia formation rate by the electrochemical promotion of catalysis (EPOC) (1-3). Ionic and electronic transports in PCFCs can affect the ammonia formation reaction. In this study, we investigate the correlation between the electrochemical synthesis of ammonia and the transport properties of proton, oxide ion, and hole in the PCFCs to clarify the ammonia formation reaction.

BaCe0.9Y0.1O3−δ (BCY10) electrolyte-supported cells were used for all experiments. A Fe cathode was deposited as the ammonia-forming electrode. For hydrogen pumping mode, Pt was used as the anode, while for water electrolysis mode, an electrode based on BaCo0.4Fe0.4Zr0.1Y0.1O3−δ (BCFZY) was used instead due to its triple-conducting properties (4). Electrochemical testing was conducted at 600 °C in a double-chamber reactor with the cathodic atmosphere fixed as dry 50% H2/N2. The ammonia produced bubbled into a dilute sulfuric acid solution and was quantified using liquid chromatography.

With all cells, both the ammonia formation rates and current densities were shown to increase with applied voltage (Fig.1a). Under the dry H2 condition in the anode, the ammonia formation rate was shown to exhibit a strong dependence on the applied voltage, reaching 10−8 mol s−1 cm−2. Meanwhile, the ammonia formation rates were comparatively lower for hydrated conditions, i.e., wet H2 and wet air with 3%H2O. On the contrary, the current density was found to be lowest for dry H2 and highest for wet air, with a six-fold difference between the two conditions.

The Nernst–Planck–Poisson (NPP) model was used to investigate the defect transport across the electrolyte and elucidate the experimental results. The NPP model was discretized and numerically calculated by improving our previous work (5). However, due to the unavailability of required thermodynamic and transport parameters for BCY10, a set of parameters reported for BaZr0.8Y0.2O3−δ (BZY20) was used instead (6). The fluxes of protons, oxygen vacancies, and holes were calculated at 600 °C with an applied voltage of 1 V under varying atmospheres (Fig. 1b). The cathodic atmosphere was fixed as dry 50% H2/N2, while three anodic atmospheres were considered, dry H2, wet H2, and wet air, considering the experimental conditions in this study. It was observed that under wet H2 conditions, proton flux significantly dominated the defect transport, showing an increase with rising voltage. In contrast, both hole and oxygen vacancy fluxes remained relatively insignificant compared to the proton flux regardless of applied voltage. However, under dry conditions, the oxygen vacancy flux became notable in relation to the total current and increased with higher voltages. Furthermore, higher current densities were observed under wet H2 compared to those under dry H2, indicating that total current might not be the most important factor in ammonia production. The NPP-model predictions imply that a higher oxygen vacancy flux toward the cathode could promote ammonia formation as well as proton flux. Under wet air, the proton transport number considerably decreases on the anode side due to the formation of holes. Even though the current density is significantly high, most of it consists of leakage current, as holes are the dominant charge carriers.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Numbers JP21H04938.

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Figure 1