Astronauts could travel back to Earth from Mars using rocket fuel converted directly from the carbon dioxide in the Red Planet’s atmosphere, using a technique known as electrochemical reduction, a recent study reports.
It’s an old method involving electrolysis, which is the application of an electrical current to drive a chemical reaction, but the new trick produces methane pure enough to be used as liquid propellant. (A number of rocket engines these days burn liquid oxygen and liquid methane — including the Raptors that power Starship, the rocket SpaceX is developing to help humanity get to Mars.)
“The work we’re doing is basically taking CO2 and using electricity to convert it into carbon-containing fuels and chemicals,” said Carter Racine in a statement. Racine is a Ph.D. student in mechanical engineering at Texas A&M University and a member of the study team, which was led by University of Mississippi chemical engineer Ahmed Badreldin.
“You can’t bring up everything you need [on Mars] from Earth, because every additional kilogram adds enormous cost and complexity to launch and escape Earth’s gravity,” added Badreldin. “So the question becomes, How do we make the fuels and chemicals needed for space exploration from the resources already available at the destination?”
One answer is electrochemical reduction, which takes place in an electrolyzer, with electrons flowing from the negatively charged anode to the positively charged cathode. Water at the anode is oxidized, meaning it loses an electron, which is transferred to the carbon dioxide at the cathode, where one of its atoms gains an electron and becomes what chemists refer to as “reduced” (meaning its oxidation state is lowered). It can then react with the hydrogen and oxygen atoms that have split from the oxidized water molecules.
The anode contains a catalyst, and the choice of material for the catalyst dictates what carbon-based compounds are produced when the reduced carbon reacts with the hydrogen and oxygen. For example, a gold catalyst leads to the production of carbon monoxide (CO), while copper produces more complex compounds including alcohols, ethanol and methane (CH4).
What makes the work of Badreldin’s team stand apart from previous experiments is that they have developed a nanometer-scale copper catalyst doped in nitrogen that is able to produce nearly pure methane.
Electrochemical reduction of carbon dioxide has been used to produce methane before, in applications on Earth. The problem is, those methods have produced a lot of byproducts alongside methane. On Earth we have complex equipment that separates the methane from the other unwanted compounds, but astronauts are not going to be able to take such unwieldy devices to Mars when every kilogram counts. That’s a problem, because for methane to be used as a liquid propellant in rocket fuel it needs to be pure.
However, the specific copper catalyst used by Badreldin’s team is able to preferentially produce methane over other compounds, thereby creating it pure without having to refine it. And there’s plenty of carbon dioxide available on Mars to use: While Mars’ atmosphere is thin, it is nonetheless composed of 96% carbon dioxide.
Even if astronauts never go to Mars, the usefulness of this technique on Earth is clear. Rather than drilling for this natural gas, we can create methane from the carbon dioxide already in our atmosphere.
“We want to do that because right now many of these carbon-based products ultimately come from virgin fossil resources,” said Racine. “If we can make them using captured CO2 and renewable electricity, it could reduce reliance on virgin fossil carbon and help close the carbon cycle.”
Consequently, electrochemical reduction has also been suggested for use on Earth as a carbon sequestration technique, drawing carbon dioxide out of our atmosphere where it acts as an unwanted greenhouse gas.
The problem with that is, converting carbon dioxide into methane to be burned as a fuel simply releases that carbon dioxide straight back into the atmosphere in a net-zero cycle. Ideally, we want to lock that carbon dioxide away from the atmosphere for a long period of time, but Badreldin doesn’t think the technology is ready yet.
“What we are ultimately interested in is seeing whether we can move beyond simply recycling CO2 and toward pathways that are net-negative,” he said. “That means using CO2 as a carbon source while converting it into products where the carbon is stored or used long enough that it is not immediately released back into the atmosphere.”
It should be noted that the best way to alleviate climate change is to cease burning all fossil fuels, including methane. However, using methane created from electrochemical carbon dioxide reduction in a net-zero process rather than drilling for it could help during the transition from fossil fuels to renewable resources.
The study reporting this research was published in April in the journal ACS Catalysis.
