# Autonomous electrochemical system for ammonia oxidation reaction measurements at the International Space Station

Ammonia (NH3) is a small and uncharged molecule typically used as a fertilizer, refrigerant, a fuel, and it is generated in wastewater processes1. Since ammonia is a fuel with a high energy density, it is possible to take this advantage by converting ammonia to nitrogen and electrical energy via the ammonia oxidation reaction (AOR). This reaction requires a catalyst to decrease the energy barrier that prevents the molecule from reacting and transforming into nitrogen. The AOR has been taken to the International Space Station (ISS) using an autonomous potentiostat system with electrode arrays, fluid pumps, and liquid reservoirs, and an autonomous potentiostat.

The anodic electrochemical oxidation of ammonia was done on platinum nanocubes2 catalyst on screen-printed carbon electrodes (SPE). The cited literature suggests that under standard conditions, the products of the AOR on monocrystalline platinum (i.e., Pt{100}) is molecular nitrogen at an applied bias of 0.65 V vs. NHE. Nevertheless, other oxides of nitrogen may form at more positive potentials3,4. The gas molecules produced by the electro-oxidation of ammonia can detach from the catalyst interface due to the buoyancy effects that are exerted when in the presence of gravity. Below you may find an AOR mechanism developed by Gericher-Mauerer mechanism5.

$$NH_{3(aq)} to NH_{3ads}$$

(1)

$$NH_{3ads} + OH^ – to NH_{2ads} + H_2O + e^ -$$

(2)

$$NH_{2ads} + OH^ – to NH_{ads} + H_2O + e^ -$$

(3)

$$NH_{xads} + NH_{y;ads} to N_2H_{x + y;ads}$$

(4)

$$N_2H_{x + y;ads} + left( {x + y} right)OH^ – to N_2 + left( {{{{mathrm{x}}}} + {{{mathrm{y}}}}} right)H_2O + left( {x + y} right)e^ -$$

(5)

$$NH_{ads} + OH^ – to N_{ads} + H_2O + e^ -$$

(6)

Under microgravity conditions the AOR has shown to have a lower current density because of the lack of buoyancy which allows the gaseous molecules to remain/stay near or at the electrode catalyst interface6,7,8. The lack of buoyancy for mass transfer convection affects the efficiency of the AOR at the platinum surface6. In a parabolic flight where a direct ammonia alkaline fuel cell (DAAFC) was used, the performance decreases up to 27% when using platinum nanocubes supported on Vulcan (Pt-V)8. This catalyst was selected for the ISS AOR study since it is robust and provides the means to achieve reproducibility in our experiments. In addition, it showed the highest AOR current densities9.

The purpose of the Ammonia Electrooxidation Lab at the ISS (AELISS)10 project was to develop an autonomous electrochemical systems for studies at the ISS and to validate the previous results under parabolic flights6,7,8 and elucidate the factors affecting the ammonia oxidation reaction during long-term μG conditions at the ISS. There is an interest on electrochemical processes in space for Environmental Control and Life Support System11,12.

For the AELISS experiment an autonomous potentiostat needed to be developed for the ISS, a plug-and-play device. Autonomous potentiostats have been developed for wearable technologies13 and smartphones14. For the ISS, a 2-U Nanorack (Nanode)15 (4” x 4” x 8”) was connected to the ISS station equipment rack through a USB-b port. Inside the Nanode the AELISS was placed, which consisted of an autonomous potentiostat, two screen-printed electrode (SPE) Channel Flow-Cells (Metrohm DropSens), two Dolomite Microfluidics peristaltic micropumps, two liquid plastic containers, and a USB flash data storage drive. The autonomous potentiostat, designed and produced by NuVant Systems Inc., controlled all the AELISS components. The AELISS was launched to the ISS on a cargo resupply mission CRS-14/NG-14, in the vehicle Antares, at 9:38 p.m. EDT on October 1, 2020. The data acquisition followed is shown in Fig. 1.

The aim of this research work is to create an autonomous electrochemical device able to improve the time and reproduction of multiple cyclic voltammetry and chronoamperometry experiments at the International Space Station. This will provide a better insight into the selected platinum nanocube catalyst performance for the ammonia oxidation reaction (AOR) and compare results with those generated on Earth gravity.

Source: Resources - nature.com