Novel technologies with limited earth support are required to enable energy-efficient maintenance of closed air, water, and waste systems in microgravity spacecraft habitats such as International space station (ISS). One area of need for the interstellar travel community is cleaning/sanitizing supplies to meet personal hygiene requirements, which is currently accomplished through the use of pre-packaged, disposable, wetted wipes, or ion exchange cartridges. These items represent an appreciable carry-along mass and disposal/replacement burden requiring earth based support. Therefore, if a system could be developed to use onboard utilities to create cleaning and disinfecting solutions it would reduce the astronaut’s dependence on earth based supplies.

We are addressing this challenge by demonstrating a technology that will use utilities on-board the spacecraft habitat to create sanitizing solutions and eliminate the need for regular delivery of disinfecting wipes. This concept is founded on the electrochemical reduction of oxygen to hydrogen peroxide using readily available on-board supplies of O₂ and water. Peroxide is well-established disinfectant with non-toxic decomposition products (viz., O₂ and H₂O), that is safe enough for human contact to be sold commercially as a 3% w/w solution. Thus, it is an ideal disinfecting solution for closed space environments.

The electrochemical reduction of oxygen to hydrogen peroxide occurs by two electron transfer reactions which proceeds as [O₂ + 2H⁺ + 2e^- → H₂O₂ or O₂ + 2H₂O + 2e^- → H₂O₂ + 2OH^-]^[1]. We designed and constructed an electrochemical reactor for generating peroxide that is compatible with the available low conductivity water stream based on a patented concept of one of our commercialization partners (De Nora Technology, Inc. U.S. patent number 6,254,762). The peroxide generation unit (PGU) system consists of oxygen chamber, catholyte chamber (with catalyzed GDE cathode[2]), whose volume can be modified as desired by changing the catholyte plate (width/length) and size/density of the NAFION bead packing material, and anolyte chamber with commercially-available mixed-metal oxide anode pressed against a cation exchange membrane, as shown in the conceptual schematic (Figure - left) and the modified PGU (Figure - right).

Initial trials confirmed that the PGU can generate peroxide utilizing high conductivity sodium sulfate as catholyte. Further, the PGU was used to demonstrate the potential of generating peroxide in very low conductivity electrolytes such as distilled water (DI) and reverse osmosis water (RO) by enabling transfer conductivity through the use of NAFION ionomer “beads”. These results are promising and an early indication that the PGU will be able to generate peroxide with high performance efficiency. Faraday is continuing the technology development efforts by: (1) demonstrating the potential of the device design by testing under zero gravity; (2) optimizing the gas diffusion electrode (GDE) catalytic structure, and wettability, (3) demonstrating the potential to use onboard International Space Station (ISS) water utilities, (4) developing an electrochemical peroxide detector, and (5) designing and building an α-scale reactor to produce 1 L/day of 2 w/w% H₂O₂. Some of the results aimed at evaluating the system in zero-gravity environments will be presented at the conference.

In addition to being an integral component of long term life support on NASA manned space missions; this technology has the potential to be an alternative method for synthesis of commercial hydrogen peroxide for on-demand onsite disinfection of process waste streams and/or field remediation systems.

Acknowledgements: The financial support of NASA Contract No. NNX16CA43P and NNX17CJ12C is acknowledged. We would also like to thank Dr. Jeffrey J. Sweterlitsch from NASA Johnson Space Center (Houston, TX), who provided insight and expertise that greatly assisted the research

References:

[1] Vilambi and Taylor, “CPRM: a new method for detection of peroxide formation during

oxygen reduction using the rotating ring-disc electrode technique” J. Electroanal. Chem.,

270 61-77 (1989).

[2] E.J. Taylor et al “Preparation of High-Utilization Gas Diffusion Electrodes for Proton-

Exchange-Membrane Fuel Cells” J. Electrochem. Soc. 139(5) L45-6 (1992). {US Pat

Nos. 5,084,144 / 6,080,504}