Electrochemical Cells for Novel Routes to Ammonia
Authors: Hendriksen Peter Vang , Sun Xiufu , Mondi Francesco , Monich Patricia Rabelo , Pirou Stéven , Frandsen Henrik Lund 
Published: September 2023

Ammonia is an extremely important chemical compound. Annual production exceeds 180 million tons of which ~80% is used for fertilizer production. The conventional production route is CO2 intensive and almost 2% of global anthropogenic CO2 emissions stems from ammonia synthesis. This will have to change over the coming decades to ensure the politically targeted emission reductions.
The synthesis gas a mixture of N2 and H2 will have to be produced in a green and sustainable manner eventually only relying on green power and sustainable synthesis routes as discussed in [1]. High temperature electrochemical cells can play a role in this, not only for providing a green stream of hydrogen, but also in realizing the required N2 as discussed in [2]. Here, a special mode of operating an SOEC was proposed including feeding some air together with the steam to the SOEC cathode to produce directly a N2/H2 mixture suitable for down-stream ammonia synthesis. This can be done with excellent efficiency [2].
Here, we shall present a different and in several aspects advantageous route for providing N2 for green ammonia synthesis. It is suggested to produce the N2 by electrochemical pumping the oxygen out of an air stream. The overall energy efficiency of the concept will be discussed in detail. However, the motivation to pursue this route relies on that it would be applicable on “small” scale. Considering the scale of renewable electricity sources it would be advantageous if also ammonia synthesis could be scaled down to match the small scale and distributed nature of renewable electricity generation. Here, a technology that scales just like the electrochemical routes to be applied for the Hydrogen production would be advantageous. Whereas cryogenic distillation is an energy efficient route to provide N2 it is a technology, that would not scale down well.
A solid oxide cell for electrochemical production of N2 by oxygen extraction from air was developed. 5*5 cm pieces were prepared and characterized both with respect to electrochemical performance and long term durability as tracked over 2000 hr. The cells are based on a ~10 micron CGO (Ce0.9Gd0.1O2) electrolyte sandwiched between composite supports (electrodes) made in either LSCF/CGO, or LSF/CGO. (LSCF=La0.6Sr0.4Co0.2Fe0.8O3-
δ
, LSF= La0.6Sr0.4FeO3-
δ). To boost performance at low temperature the electrodes were infiltrated with Pr-oxide [3]. The performance with the different supports is compared at a range of temperatures and current densities with and without infiltration. Both materials were found to perform well and enable current densities ~0.8 A/cm2 at ~ 150 mV polarization, when tested at 700 oC. A 90% pure N2 stream can be produced with at a cell voltage below 150mV at temperatures from 700-800 ºC. As illustrated in Fig. 1, a purer N2 stream with 96% N2 could be produced at a cell voltage below 200 mV. Also a 98% pure N2 stream was demonstrated. Moreover, the cells were demonstrated to operate without degradation over a 2000 hr. period.
The results are quite promising in terms of the excellent durability demonstrated and in terms of absolute energy consumption. For comparison with literature, the energy consumption is related to the removed oxygen rather than the produced N2. At 150 mV the energy consumption is ca. 0.7 kWh/m3 of oxygen extracted. This compares well with performance reported by Wang et al. [4] describing a similar concept. The energy consumption is ca. twice what it takes to produce the oxygen via cryogenic distillation. Here, it should be noted, however, that all the produced heat (0.7 kWh/m3) can be directly utilized in synthesizing the H2 needed for the ammonia due to the endothermic nature of steam splitting. Hence, the energy invested in producing the N2 is not really wasted in this case, as also discussed in [2].
The results of detailed electrochemical testing will be presented and the factors limiting cell performance will be discussed. Finally, an analysis of what dictates the optimal point of operation in terms of energy consumption and component volume and reliability is presented.

Acknowledgment

The work was supported by the EU commission via project; ARENHA, H2020 GRANT AGREEMENT: 862482.

References

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Hendriksen, P. V., Khoshkalam, M., Tong, X., Tripkovic, D., Faghihi-Sani, M. A., & Chen, M. 2019 ECS Transactions, 91, 1413-1424.

Mei Wang, Kamil Maciej Nowicki, and John Thomas Sirr Irvine, Journal of The Electrochemical Society, 2022, 169, 064509

Figure 1