(Invited) Ammonia Oxidation Reaction Under Microgravity Conditions
Authors: Cabrera Carlos R 
Published: February 2020

Ammonia oxidation is an important electrochemical reaction needed in urine purification processes for water reclamation in the International Space Station (ISS). The objective of the microgravity experiment was to compare the ammonia oxidation capabilities of different catalysts at a half cell and in an alkaline fuel cell. Different Pt catalysts were studied, from nanoparticles on carbon supports to platinum nanocubes with preferential crystalline (100) sites. Under microgravity conditions the performance of a fuel cell was diminished by the absence of buoyancy since nitrogen gas was produced, blocking the catalytic sites. The following catalysts were studied: platinum nanocubes (ca. 10nm in diameter), platinum nanocubes on carbon Vulcan and platinum on carbon nanoonion support prepared by the RoDSE method [1].
This research has shown the difference between the performance of the catalysts in an ammonia fuel cell under microgravity and ground experimental conditions, in terms of normalized current densities [1]. In the case of a fuel cell with platinum black at the anode there is a reduction in the produced mass-normalized current densities of around 7.5% between the hypergravity and microgravity experimental episodes. This decrease was shown to be reversed when gravity effects were once again applied to the fuel cell, confirming the direct influence from microgravity on the electrochemical oxidation of ammonia.
For the half cell, a decrease in performance of 20-65% over ground-based controls for the electrochemical oxidation of ammonia was found [2,3]. This decrease in current is theorized to be due to the lack of buoyancy driven mixing that occurs in microgravity. In microgravity only diffusional mixing occurs and, therefore, molecules may be unable to reach the nanocatalyst interface at the same rate as on the ground. During the first oxidation peak at ca. 0.7 V vs. Ag QRE, the formation of N2 molecule, during microgravity, conditions creates a stagnant gas-surface interaction that avoids ammonia molecules to reach the Pt surface. This situation leads to a significant peak current decrease compared to the ground experiments. On the other hand, during the second oxidation peak an oxynitride layer is formed at the interface of the electrode without the formation of N2 molecule. This latter peak does not have the effect of microgravity due to the surface chemistry reaction and the oxide adsorbates are no readily desorbed. Therefore, no major differences between the microgravity and ground conditions are observed in such an oxynitride layer region.
These findings indicate that significant reductions in electrode performance for ammonia electrooxidation may occur if catalyst design measures are not taken in consideration to mitigate these microgravity effects. This conclusion is applicable specifically to electrochemical processes, but also may be relevant to all process constrained by nanoscale mixing of diffusion limited structures such as thermal catalysts, separation membranes, microbial growth and human cellular functions. For these microgravity experiments, a tailored electrochemical lab was designed and constructed that would handle 2g and 0g gravitational conditions (see Figure 1) [2,3].
References

Poventud-Estrada, C. M.; Acevedo, R.; Morales, C.; Betancourt, L.; Diaz, D. C.; Rodriguez III, M. A.; Larios, E.; José-Yacaman, M.; Nicolau, E.; Flynn, M.; and Cabrera, C. R., “Microgravity Effect on Chronoamperometric Ammonia Oxidation at Platinum Nanoparticles Modified Mesoporous Carbon Supports”, Microgravity Science and Technology
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Acevedo, R.; Poventud-Estrada, C.M; Morales-Navas, C.; Martinez-Rodriguez, R.A.; Ortiz-Quiles, E.; Vidal-Iglesias, F.; Sollá-Gullón, J.; Nicolau, E.; Feliu, J.M.; Echegoyen, L.; Cabrera, C.R., “Chronoamperometric Study of Ammonia Oxidation in a Direct Ammonia Alkaline Fuel Cell under the Influence of Microgravity”, Microgravity Science and Technology
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29(4),253–261. http://dx.doi.org/10.1007/s12217-017-9543-z

Nicolau, E., Poventud-Estrada, C.M., Arroyo, L., Fonseca, J., Flynn, M., Cabrera, C.R.: Microgravity effects on the electrochemical oxidation of ammonia: A parabolic flight experiment. Electrochim. Acta 2012, 75, 88-93. doi:10.1016/j.electacta.2012.04.079

Figure 1