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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.

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Microgravity effect on the electrochemical oxidation of ammonia at platinum nanoparticles modified mesoporous carbon (MPC) substrates, with three different pore diameters, e.g. 64, 100, and 137 Å, have been studied in a parabolic flight between 24,000 and 32,000 ft. Microgravity effects on the chronoamperometric ammonia oxidation current density was a function of the mesoporous carbon support used. This support may be described as graphitic multilayer hollow globules, even though the carbon globules do not present a specific shape as seen in the HRTEM images.  Platinum nanoparticles were successfully chemically deposited, with a high dispersion, throughout the support’s topography.  An onboard accelerometer was the trigger in order to start each chronoamperomtric ammonia oxidation experiments when the microgravity condition of less than 0.02g (i.e. gravitational force) was achieved. Pt/MPC64 sustained the current density between terrestrial and microgravity conditions by a slight margin over Pt/MPC100. However, Pt/MPC137 resulted with the smallest current density decrease under microgravity conditions versus ground based experiments.  Pt/MPC137 has the largest pore diameter and shows a better capacity to sustain the oxidation current, which involves N2 formation.  This effect can be ascribed to an easier diffusional process obtained by the larger pore diameter, which improves the access for the mobility of the ammonia molecules towards the electroactive sites and the corresponding detachment of the gaseous N2molecules (bubbles) from the MPC cavities. Pt/MPC137 catalyst resulted with the smallest current density decrease under microgravity conditions versus ground based experiments. This MPC support has the largest average pore diameter (137 Å) and shows a better capacity to sustain the oxidation current. This effect can be ascribed to an easier diffusional process facilitated by the larger pore diameter, which improves the access for the mobility of the ammonia molecules towards the electroactive sites and the corresponding detachment of the gaseous N2 molecules (bubbles) away from them. In conclusion, it was demonstrated that under microgravity environment a porous infrastructure for a catalyst support has an impact on the mass transfer process of electroactive species, and a current density decreasing factor of ca. 50 - 67% must be taken into account. The lowest being the MCP average pore diameter 137 Å. In order to improve the current densities for ammonia oxidation, (100) faceted Pt nanoparticles need to be achieved.

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