# Can a machine process and run on $CO_2$ and $H_2O$ for an $O_2$ by product?

On Mars studies have shown that carbon dioxide solidifies to dry ice at the poles. During the winter months can electrolysis be applied to collected and then contained liquid $$CO_2$$ to replenish oxygen reserves by creating an engine that uses $$CO_2$$ as the oxidizer?

Could solar powered electrolysis of liquid $$CO_2$$ and $$H_2O$$ during the summer provide liquid $$O_2$$ and Hydrogen to power a generator in place of using batteries for electrical storage for generator during winter?

What can be added to liquid CO2 to allow better electrolysis to crack $$CO_2$$ or to used as an oxidizer for $$CO_2$$ combustion engine?

The main component of the atmosphere of Mars is carbon dioxide (CO2) at 95.9%. Each pole is in continual darkness during its hemisphere's winter, and the surface gets so cold that as much as 25% of the atmospheric CO2 condenses at the polar caps into solid CO2 ice (dry ice). When the pole is again exposed to sunlight during summer, the CO2 ice sublimes back into the atmosphere. This process leads to a significant annual variation in the atmospheric pressure and atmospheric composition around the Martian poles.

$$CO_2$$ sublimes but it can be self collected in vast containers or an underground cave that can be capped open during winter, closed and kept as a super liquid during summer to power generators similar to steam engines?

https://chemistry.stackexchange.com/questions/66265/can-co2-be-broken-into-its-component-elements-by-electrolysis

Martian Carbon Dioxide Turbine Oxygen Generator

https://chemistry.stackexchange.com/questions/112136/what-is-the-ideal-temperature-to-crack-water

• Can you show the study that liquid CO2 forms naturally on Mars? – uhoh Jan 12 '17 at 3:40
• Looks better! I think the word "liquifies" in the first sentence still needs to go. It goes straight from gas to solid, just like frost forming on windows in the winter (at least in places where it's cold enough). – uhoh Jan 12 '17 at 6:29
• If the energy source is sunlight, or nuclear fission, why not just store it in batteries instead of engaging in chemical fuel manufacturing? Batteries are simple, reliable, storable, partitionable and mobile. And electric engines and other uses of electricity are efficient and versatile. Batteries in space are like gold is good as money. – LocalFluff Jan 14 '17 at 20:51
• @localFluff Batteries are expensive if you want to store gigawatt years of power. Pumping gas into a large cavern under pressure, or making methane and oxygen from CO2 and water (or, more difficult but doesn't need water, making carbon monoxide and oxygen from CO2) are more scalable. – Steve Linton Dec 18 '18 at 21:22
• @SteveLinton - creating underground fuel stores worked on Earth because plants came long before people. Do you think we will put the storage tools on Mars that long before people show up to spend it? – amI Mar 9 '19 at 5:02

Can electrolysis of water provide fuel for the generator where both water and liquid carbon dioxide is utilized in one self sustaining machine?

NO. A 'self sustaining machine' would be a perpetual motion machine, and those are impossible. You have to provide energy from an external source.

Electrolysis of water costs energy, about 50 kWh per kg of hydrogen produced, the process is about 80% efficient. So when you burn that hydrogen, you can recover 80% of the energy you used to electrolyze it. And part of that 80% is lost converting the heat (from burning the hydrogen) into usable forms of energy (motion, electricity). Fuel cells are about 80% efficient. You can also burn the hydrogen in an internal combustion engine, but those are usually less than 50% efficient.

Similarly, cracking CO2 costs a lot of energy.

Current processes for cracking CO2 use H2 to react with CO2 to reduce the CO2 to CO plus H2O.

NASA is investigating ways to reduce CO2 directly to carbon and O2.

Could we not add something to liquid CO2 to allow better electrolysis?

Possibly, yes. That's the subject of NASA's research: to find an ionic liquid that can serve as a catalyst for CO2 reduction.

• If a ionic liquid is found to split CO2 by electrolysis, electric energy is still necessary. The ionic liquid does not serve as a catalyst to split CO2, it serves as an electrolyte in order to let an electric current flow. – Uwe Sep 18 '17 at 10:32
• From the linked article: "Electrochemically reducing CO2 is normally a very energy intensive process. However, ILs have properties that let them act as both CO2 absorbers and reduction catalysts, effectively decreasing the required energy to recycle CO2. " – Hobbes Sep 18 '17 at 11:03
• Electricity supplied externally could be used on Mars will crack CO2 to make O2 for fuel. I was not talking about a self-perpetuating machine. – Muze Mar 9 '19 at 3:19
• I think the OP is asking about using summertime sunlight to electrolize water and/or carbon dioxide into storable fuels for consumption during polar winter when there is no sunlight, not build a perpetual motion machine. – Anthony X Mar 10 '19 at 22:36

note: the question has been significantly modified (potentially improved) - I'll leave this slightly-outdated answer here as-is for reference.

There probably are no studies that have shown that carbon dioxide liquifies naturally on Mars. Below Carbon Dioxide's triple point of 5.1 atmospheres it can not exist in liquid phase at the surface.

I should mention that a few papers several years ago discussed the possibility of frozen CO2 in the soil creating "pockets" where the pressure might possibly rise above the triple point, creating small, temporary pockets of CO2 as liquid, but I don't think that concept panned out.

Electrolysis of CO2 is definitely a way to make oxygen, but the method being looked at with the greatest interest is high temperature (800 - 1000C) gaseous electrolysis with conductive oxide solid electrodes.

This answer to the question How to produce oxygen out of Venusian atmosphere's carbon dioxide? does a good job of outlining some options for obtaining oxygen from atmospheric CO2. It points to this very nice, long answer about MOXIE (Mars OXygen In situ resource utilization Experiment) which is a solid electrode, high temperature gaseous CO2 electrolytic cell which reduces CO2 to oxygen and carbon monoxide. It is one of the experiments planned for the Mars 2020 Rover. These answers are very well written and the answer linked for MOXIE contains extensive links for further reading, so I won't discuss it any more here.

Now your question about liquid CO2. While I could find some discussion of CO2 dissolved in other ionic liquids, so far I haven't found anything about trying to do electrolysis on pure liquid CO2. Since CO2 is a stable, covalently bonded molecule, I think we can assume that it will have extremely poor electrical conductivity as a liquid. The reason water works so well is that it dissociates readily - pH 7 means (oh my gosh I'm going to say something about high school chemistry) one in every roughly 1E-07 of the water molecules is split all of the time, resulting in H+ (or H3O+) and OH- Add some salt, or other ionic compounds and the conductivity goes up even farther. Liquid CO2? possibly no useful conductivity.

It looks like gaseous high temperature electrolysis has been identified as the best way to go on Mars.

Above: Phase diagram of CO2 from here.

• While this does address the fact that liquid CO$_2$ does not naturally occur, it does not actually answer the question of whether or not you can use electrolysis to separate the carbon and the oxygen, which is the main point of the question. – Phiteros Jan 12 '17 at 4:12
• @Phiteros done. If the question improves, I can adjust the answer. There are other methods besides electrolysis to get oxygen from CO2, and there are other several existing answers here about that already. I'm trying to address the part of the question that is unique and new here. – uhoh Jan 12 '17 at 4:20
• You should probably put links in your answer to these other questions then. – Phiteros Jan 12 '17 at 4:20
• @Phiteros OK I'm 'out the door' at the moment, I'll be back here in a few hours. Thanks! – uhoh Jan 12 '17 at 4:22
• Thank you for helping me so much – Muze Apr 4 '19 at 18:39