Ammonia as an SOFC Fuel
Authors: Hendriksen Peter Vang , Mondi Francesco , Sun Xiufu , Caldogno Riccardo , Frandsen Henrik Lund , Babaie Rizvandi Omid , Høgh Jens Valdemar Thorvald , Hansen John 
Published: September 2023

The conventional production route of ammonia relies on natural gas as a source of hydrogen and is thus very CO2 intensive; almost 2% of global anthropogenic CO2 emissions stems from ammonia synthesis. This will have to change over the next two to three decades, where a “green route” to ammonia must be deployed to ensure required emission reductions.
The annual production of ammonia is currently around 180 million tons of which ~80% is used for fertilizer production. This will likely increase due to increased fertilizer demand, but also because ammonia is a promising energy vector and a potential green fuel for the shipping sector. The expected energy demand for global shipping by 2050 is in the order of ~15 EJ [1]. It will be difficult to cover such a large demand by bio-mass derived fuels (e.g. methanol) considering global limitations in sustainable biomass resources and the needs of other sectors like aviation, heavy transport and chemical industry [1]. Hence, for the green transition efficient and sustainable routes to green ammonia are important, as will be efficient technologies to convert ammonia to electricity.
Ammonia is an excellent fuel for SOFCs as previously reported in several studies [2], [3]. We have in the framework of the project “SOC4NH3” [3] investigated the impacts of firing an SOEC with ammonia in terms of its impact on overall losses, the cell degradation mechanisms as well as potential benefits.
SOEC durability, when operating on ammonia, was followed over extended test periods between 2000 and 6000 hours at single cell level at different temperatures and current densities. The cells were Ni/YSZ-supported thin electrolyte cells of a 5*5 cm2 foot-print. The degradation rates observed when feeding with ammonia were very similar to degradation rates observed for the same cells when operated in a 1:3 gas mixture of N2/H2.
Also the detailed electrochemical performance was explored at different temperatures using impedance spectroscopy. An example of impedance spectra obtained when feeding ammonia to the cell or a N2/H2 gas mixture at ca. 850 oC and 675 oC is presented in Figure 1. When feeding ammonia a cooling is observed due to the endothermic nature of the cracking reaction. This leads to a slight increase in both series resistance and polarization resistance. Therefore, the impedance responses in the ammonia experiments is in the Figure compared to responses obtained in N2/H2 at a slightly lower furnace set-point temperature (5-10 oC lower). Evidently, at around 850 oC, the only effect on the impedance response of feeding ammonia directly is the slight reduction in temperature; the N2/H2 and NH3 responses are practically identical when comparing characteristics at furnace set-point temperatures of 845 oC and 850 oC for N2/H2 and NH3, respectively. In contrast, if the same experiment is conducted at around 675 oC a difference between the response in ammonia and the response in N2/H2 is observed. The difference in behavior cannot be compensated by just lowering the furnace temperature a bit to simulate the impact of a cooling. The impedance at high frequency in ammonia is clearly much larger than for the N2/H2 feed. Note here, that the Rs in the two different feeds is practically identical. Hence, we have to conclude that there is a direct negative effect of the presence of ammonia at the electrode on the hydrogen oxidation kinetics. The effect is stronger than what can be ascribed to cooling and dilution of the H2. The details will be discussed in the paper.
SOFC operation on ammonia and its impact at stack level was also explored. Potential advantages of ensuring that a part of the ammonia cracks inside the stack, as opposed to up-stream the stack, was addressed by modelling. Finally, a TSP1 SOFC stack from Topsoe A/S was operated with some internal ammonia cracking. The results will be presented and discussed to elucidate benefits and limitations in operating SOFC stacks with internal cracking of ammonia.

Acknowledgment

The work was supported by the EUDP-project SOC4NH3; Grant Agreement 64018-0546.

References

MarEfuel Project report; “Electro-fuels for long range maritime transport”, 2021. https://orbit.dtu.dk/en/projects/electro-fuels-for-long-range-maritime-transport, available from Orbit DTU.

J.B. Hansen, J. Madsen, J.U. Nielsen, N. Christiansen, Proceedings European SOFC Fuel Cell Forum, Luzern, Switzerland (2010), and A. Hagen. Risø International Energy Conference 2007, Risø-R-1608(EN)

3. John Bøgild Hansen and Peter Vang Hendriksen 2019 ECS Trans.
91 2455.

Figure 1