Our air cleaner serves multiple purposes: keeping people warm, eliminating pathogens from the air, and getting rid of multiple toxic gases.
The device uses an electrolytic cell powered by a battery to produce hydrogen and chlorine gas from a saline solution containing hydrochloric acid. The hydrogen and chlorine gases are kept separate. For the reduction half-reactions, the possible reactions are:
Since the reduction of hydrogen ions has the larges standard reduction potential, it will be the most favored reduction half-reaction, so a large amount of H2 gas will be produced. For the oxidation half-reaction, the following reactions are possible:
Since the oxidation of water has a larger E_ox value, it will be more favored, and more O2 gas will be produced than Cl2 gas. However, there will still be a fair amount of Cl2 produced during the reaction, as demonstrated by prototype tests.
After the gases are separated, air from the surroundings is brought into a chamber with chlorine gas (using a one-way valve), neutralizing pathogens such as COVID-19. Additionally, air from the surroundings is brought into a chamber containing hydrogen gas and a transition metal catalyst on an alumina substrate. Through a process called catalytic hydrogenation, many toxic organic gases, including formaldehyde, acetaldehyde, benzene, and acrolein are reduced into mostly alkanes and alcohols. It is important to capture toxic gases such as formaldehyde, acetaldehyde, benzene, and acrolein in space since they are often formed after the combustion of lithium-ion batteries that malfunction .
Then, the H2 and Cl2 react to form HCl gas in an exothermic reaction while in the presence of other species formed earlier. HCl reacts with hydrazine, a possible contaminant on the ISS (from bi-propellant tanks), to form hydrazine chloride. Hydrazine chlorine, along with the species formed earlier, is then dissolved into an aqueous solution. Any excess HCl gas dissolves to form hydrochloric acid.
Any heat produced is dispersed through a radiator, providing warmth to the surroundings. The HCl is cycled back into the electrolytic cell, while the cleaned air passes through dry NaOH, which acts as a gas trap, eliminating HCl and Cl2 as well as CO2. Clean air is then released from the device.
Our team chose the Purify the Air challenge because we felt that it had the most direct impact on people’s lives. Most people are currently quarantined at home because of the pandemic, so researching and designing an air purification system is relevant to our individual lives. Air purification seemed like the most expansive issue since it is an issue that both people on Earth and in space have to deal with. As such, our team decided that responding to this challenge would be a great way to help people by ensuring clean, virus-free air. Our first step in developing the project was brainstorming. What exactly would our device be able to do?
We thought that, given the nature of the challenge, our device should be able to minimize airborne pathogens as well as toxic compounds that are especially dangerous in spacecraft cabins. With these goals in mind, we designated tasks to each group member. Some team members researched. They used the NASA open data portal and Google Scholar to look for articles and sort through information about both the measurements and treatments used to minimize the presence of toxic compounds in spacecraft cabins. The other team members used the information from the articles to synthesize potential designs. We would periodically stop our work to review our current plan and make sure we were all on the same page.
We had to overcome multiple problems with our design. When trying to make sure the proper reactions would occur, we discovered flaws in the chemical reactions we were basing our design off of. Our design required that chlorine ions would be oxidized to form chlorine gas. However the oxidation of water to form oxygen gas was more favorable. To resolve this problem, our team did some research and found that even if the oxygen reduction half-reaction was more favorable, chlorine would still be formed. We tested it, and we were able to detect the smell of chlorine.
Another problem involved chloramines. When hydrazine reacts with chlorine, it can form chloramines. Theoretically, chloramine can then either react with ammonia to form hydrazine or hydrolyze in water. If it reacted with ammonia to hydrazine, that process would essentially negate the entire purpose of our purification system, which was to use salt water to remove hydrazine and sanitize pathogens in the air. Fortunately, our group found that in a low pH, most of the chloramines wouldn't re-form hydrazine.
We only used hardware to complete our project. Our design was made of copper wires, garden tubing, a 6-volt lantern battery, graphite electrodes, a yellow lego brick, and a water bottle. Although we had few chemicals at our disposal, the salt and citric acid we did have were sufficient to make the project work. We passed a current through the saline solution using the electrodes powered by the battery to form hydrogen and chlorine gas. We were able to detect chlorine gas by smell, which confirmed that the chlorine ions which came from the dissolved NaCl were oxidized. The hydrogen and chlorine gases produced were separated by the lego brick into tubing from garden hoses. The gases and possible hydrazine contaminants were then bubbled into the water bottle which would dissolve any leftover contaminants.
https://www.epa.gov/indoor-air-quality-iaq
https://techport.nasa.gov/view/93546
https://techport.nasa.gov/view/16793
https://pubs.rsc.org/en/content/articlelanding/2019/dt/c9dt03789f#!divAbstract