Chronoamperometric Study of Ammonia Oxidation in a Direct Ammonia Alkaline Fuel Cell under the Influence of Microgravity

Abstract

This is a study of the chronoamperometry performance of the electrochemical oxidation of ammonia in an alkaline fuel cell for space applications. Under microgravity the performance of a fuel cell is diminished by the absence of buoyancy since nitrogen gas is produced. The following catalysts were studied: platinum nanocubes of ca. 10nm, platinum nanocubes on a Vulcan carbon support and platinum on carbon nanoonion support of ca. 10nm. These nanomaterials were studied in order to search for catalysts that may reduce or counter the loss of ammonia oxidation current densities performance under microgravity conditions. Chronoamperometries at potential values ranging from 0.2 V to 1.2V vs. cathode potential (Breathing Air/300ml/min/12psi) in 1.0M NH4OH (30ml/min in anode) were done during over 30 parabolas. The current densities at 15s in the chronomaperometry experiments showed diminishing current Pt mass densities under microgravity and in some cases to showed improvement of up to 92%, for Pt-carbon nanoonions, and over 70% for the three catalysts versus ground at potentials ranging from 0.2 to 0.4V after 5 minutes of chronoamperometry conditions. At higher potentials, 1.0V or higher, Pt nanocubes and Pt-carbon nanoonions showed enhancements of up to 32% and 24%, respectively. We attribute this behavior to the sizes of the catalyst materials and the time needed for the N2 bubble detachment from the Pt surface under microgravity.

Determination of Specific Electrocatalytic Sites in the Oxidation of Small Molecules on Crystalline Metal Surfaces

“…Then, the development of electrochemical sensors and selective catalysts for the degradation of ammonia to a harmless molecule such as the N2 gas, are a subject of intense research in electrochemistry [75]. Other interest on the ammonia electro-oxidation is because, from the electrochemical energy conversion point of view, ammonia is a potential candidate to be used as fuel in direct "ammonia" fuel cells [76,77]. In extreme alkaline media, the standard potential for the ammonia oxidation reaction NH3 (g) + 3OH -(aq) ⇄ ½N2 (g) + 3H2O (l) + 3eis E 0 ≃ -0.770 VSHE (or ~0.055 VRHE) [76].…”

Temperature-Dependent Kinetics and Reaction Mechanism of Ammonia Oxidation on Pt, Ir, and PtIr Alloy Catalysts

“…Pt has been studied mostly since it yields the highest AOR peak current and Ir is known for its lowest onset potential. [11][12][13][14][15]16 Thus, Pt, Ir, and PtIr alloy nanocatalysts are the baseline materials for AOR studies. 17,18 Quantitative comparison of AOR catalysts remains elusive because the data accumulated so far were obtained using various potential sweep rates, ammonia concentrations, pH values, and were not normalized to PGM loadings.…”

Fundamentals and future applications of electrochemical energy conversion in space

“…Nicolau et al 29 reported that the oxidation of ammonia with different Pt-based nanocatalysts resulted in a decreased performance of 20-65% in microgravity environments in comparison to terrestrial control experiments. Similarly, Acevedo et al 31 showed that the performance of ammonia-based alkaline fuel cells decreased in microgravity environment. Nicolau et al (2012) attributed the current decrease to the lack of buoyancy-driven mixing in the near-absence of gravitation (Fig.…”

“…Besides hydrogen and methanol, fuel cells have been proposed utilizing ammonia as a fuel [29][30][31] . Ammonia has recently been considered as the main substitution for hydrogen and the next generation fuel 32 due to its high energy density (12.6 MJ L −1 ) and the easiness of storage and transportation 29 .…”