Perseverance, the Mars rover, includes an experiment called MOXIE. This is a technology demonstration to show that Mars' atmospheric carbon dioxide can be broken into carbon monoxide and oxygen. Future missions would use a much scaled up version of MOXIE to generate oxygen for life support and particularly for rocket propulsion for the return trip. The referenced article talks about taking a 1 tonne oxygen generator and 7 tonnes of rocket fuel (RP1?) to Mars.
My question is why not just take the 1 tonne generator and accumulate and store both the carbon monoxide and the oxygen locally and burn them together to propel the rocket. I assume there is a big disadvantage compared with more conventional fuels, but does this really outweigh having to convey 7 tonnes of rocket fuel to Mars?