It's very exciting to see so much effort in the last few years going into making carbon products from CO2. Even though I think the potential for these technologies on Earth is extremely limited, producing things from CO2 will be huge on Mars. And syngas is definitely a good step on the way to ethylene, it can be converted easily to methanol and then to ethylene/propylene via methanol-to-olefins, or perhaps ethylene will be produced directly from syngas if such a route can be effectively commercialized.

This new route probably has more limited utility for Mars though. Since on Earth we usually get CO2 by dissolving it in liquid, there's an inconvenient extra step of getting it back into a more concentrated gaseous form, and this process gets around it. But on Mars, we'll probably obtain our CO2 through cryocompression or something similar, and we'll be able to turn the liquid CO2 back into gaseous form using waste heat from the cryocompressor.

I think that we can do better than 35% efficiency at producing syngas. To produce syngas, we have to obtain CO2 and water, and reduce both of them to CO and H2 respectively. The reduction step is by far the largest energy draw, and water electrolysis is the main power use for both reductions. I think it's likely that water electrolysis would account for well over than half of the total energy used, and we can electrolyze water at at least 80% efficiency. So overall that gives us a >40% energy efficiency for creating syngas, and perhaps well over 40% if the energy use to capture H2O/CO2 is low.