Electrode-Supported Protonic Ceramic Electrolysis Cells for Ammonia Electrosynthesis at Intermediate Temperatures
Authors: Okazaki Moe , Otomo Junichiro 
Published: September 2023

Ammonia electrosynthesis, the electrochemical process of converting water and nitrogen into ammonia using renewable energy, has been the focus of attention in the search for a Haber-Bosch alternative. Ammonia produced using renewable energy can not only be used as a fertilizer, but also in its liquefied form as an energy-dense fuel (“energy carrier”). Protonic ceramic electrolysis cells (PCECs) are suitable for ammonia electrosynthesis at high temperatures due to their superior chemical and thermal stability and their ability to keep ammonia and water separate throughout the process, enabling efficient separation of pure ammonia after synthesis. However, on the system scale, high temperature conditions are less compatible with the fluctuating output of renewable energy generation. Additionally, as the temperature dependence of ammonia synthesis faces a trade-off between thermodynamic and kinetic favorability, intermediate temperatures, defined here as between 200-400˚C, are considered promising for ammonia electrosynthesis [1].
We previously demonstrated that under a mixed cathodic gas feed of N2/H2, ammonia electrosynthesis using PCECs can be accelerated by cathodic polarization [2-3]. Non-Faradaic effects were observed; that is, the ammonia formation rate was not proportional to the current density. Our results demonstrated similar characteristics as those observed under the electrochemical promotion of catalysis (EPOC), first proposed by Stoukides et. al [4]. Recently, using electrolyte-supported PCECs operated at 600˚C, we showed that high ammonia formation rates on the order of 10-8 mol s-1 cm-2 can be attained in a single-chamber co-fed with N2 and H2 [5], which can eliminate gas sealing requirements.
In this work, we aim to understand the factors needed to lower the operating temperature of ammonia electrosynthesis using PCECs co-fed with N2 and H2, from high temperatures between 500-600˚C to intermediate temperatures of 400˚C and below. Electrolyte-supported and electrode-supported cells were compared for their ammonia electrosynthesis rates and cell performances. Electrochemical testing was conducted in a single-chamber reactor co-fed with 50% H2/50% N2 at 300-600˚C. Ammonia formation rates were evaluated by bubbling the outlet gas into a dilute H2SO4 solution and measuring the resulting ammonium ion concentrations via high-performance liquid chromatography. We conducted electrochemical control tests in H2/Ar to validate that ammonia has been produced electrochemically from dinitrogen, rather than from potential traces of ammonia or NOx species (which are easier to reduce) found in the experimental environment.
Preliminary experiments using electrode-supported cells with the configuration Fe|BaCe0.9Y0.1O3-δ (BCY)|Ni-BaZr0.8Y0.2O3-δ (Ni-BZY) have resulted in ammonia formation rates over 10-8 mol s-1 cm-2 at an applied voltage of -1 V, at just 400˚C, as shown in Fig. 1. This corresponds to a 150~200˚C reduction in temperature compared to that needed to obtain similar ammonia formation rates using electrolyte-supported cells of the configuration Pt|BCY|Fe. We further evaluate both types of cells in terms of their current density and electrochemical impedance, to understand how the effect of applied voltage varies with the different cell structures. The comparison suggests that Ohmic and anodic overpotentials are effectively limited in our electrode-supported cells, leading to an increased cathodic polarization that aids in accelerating ammonia formation even at intermediate temperatures.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers JP21H04938 and JP22J11546.

References

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Figure 1