I think I've used this subject line before, but it's evergreen, right???
Carbon-eating robot, eh? Made me think of carbon-shitting microbes or bugs: they'd take in atmospheric CO2, excrete oxygen gas (hey, plants do it) and excrete carbon as a solid, like the disgusting little pellets that roaches leave behind, but pure carbon. Collect it, bind it into blocks -- or squeeze it into diamonds! :-) -- and (as they say in Montreal) voila! ...Sequestration!