It appears that three or four types of chemical reactions will be employed in order to conduct a Mars mission. They are 1) the electrolysis of water, 2) the Sabatier reaction, which produces methane, 3) the electrolysis of carbon dioxide, yielding carbon monoxide and oxygen, and 4) the Bosch reaction to recover water, and consume the carbon monoxide produced by the third reaction. Each reaction has its strengths and weaknesses, which can be turned to advantage for an in situ resourcing strategy for keeping the crew alive on a Mars mission.
Let's examine these reactions in more detail. The first reaction is employed to obtain hydrogen and oxygen from the water. It is expected that there's plenty of water on Mars, so that's one way of obtaining resources from the red planet. If this isn't done, everything has to be brought from Earth, which would make missions much more challenging. The oxygen will be used to make the oxidizer part of rocket fuel. Oddly enough, it isn't going to be much good for production of oxygen for breathing. There will be a need to produce lots and lots of fuel for the trip back to Earth. The Sabatier reaction will efficiently utilize the Martian resources. The two resources are the water mentioned, and the carbon dioxide in the atmosphere. The burning of the rocket fuel will return the carbon dioxide and water to the environment. The reaction is the favored one to produce rocket fuel. The rocket fuel would be the methane and the oxygen produced from the Sabatier reaction.
The preferred reaction to obtain oxygen for breathing would come from the moxie device. It will electrolyze the carbon dioxide, thus obtaining some oxygen. A waste product would be the carbon monoxide. What to do with that? It could be used to obtain water and elemental carbon through the Bosch reaction. Perhaps the carbon can be used for making other things. The Bosch reaction is slow, but it doesn't have to produce a whole lot of water. In fact, it will simply get back the water from the electrolysis of water that is needed to sustain the reaction. Thus, a small amount of water will be needed for this purpose. Since it is a slower reaction than the Sabatier reaction, it is not preferred for making rocket fuel. Oxygen can always be used for breathing. So the water can help get the most from the carbon dioxide being used in the electrolysis of carbon dioxide. The oxygen can be obtained from that water, which was obtained to get hydrogen so as to grab that single oxygen atom from the carbon monoxide waste product. Why waste the carbon monoxide? The amount of oxygen from each carbon dioxide molecule could be doubled. Such can be used for breathing.
The making of rocket fuel will always use up a lot of water. This is unavoidable. Presumably, Mars has plenty of water, so that loss can be made good. In time, it may be better to use some other method of producing thrust, but that is what is available now.
So the Mars environment will provide the carbon dioxide and water that will be used in in situ resourcing. These reactions will be how these resources can be utilized in producing what would be needed to support life on the red planet. Without these resources on Mars, it would be necessary to bring them from Earth. Therefore, there is a big advantage to using what's available on Mars.
To recap, it is best to use the Sabatier reaction for the production of rocket fuel, and the moxie electrolysis device to obtain the oxygen. The Bosch reaction can get more oxygen for breathing and close the resource loop for water used in that reaction. The waste products will be elemental carbon, but that may be usable for other things. The water produced by the burning of methane in rocket fuel will most likely be lost.
The discussion of energy requirements for these reactions are another thing. To put it simply, a nuclear reactor is the best bet. The energy requirements will be considerable. Solar power won't be reliable enough, nor is the energy density sufficient. Solar panels can work on the ISS, but it's not as intense on Mars in comparison to low Earth orbit. Plus there are dust storms. Mars rovers have died from lack from lack of sunlight because of dust storms getting on the panels.
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