Keep in mind that the energy expended in C02 sequestration is concentration. It's easily soluble in water when injected and then is reactive to produce mineral

Keep in mind that the energy expended in C02 sequestration is concentration. It's easily soluble in water when injected and then is reactive to produce mineral precipitates. And that's only the geologic sequestration. There are multiple approaches, both biological, engineered, and geologic. Either way, the hard part is going from dispersed to concentrated in most cases (or for the biologic cases, actually keeping the result in the biosphere and not decaying into the atmosphere). You don't need to reverse the combustion reaction. You don't have to spend anywhere near as much energy as you gained from combustion to sequester it. You're not turning it back into bare carbon or into a hydrocarbon. You're breaking hydrogen - carbon bonds to gain energy during combustion. You're then sequestering C02 -- you don't need to chemically alter the C02. You do need to concentrate it, but in terms of thermodynamics, you get a large amount of energy from combustion and it takes less than that to concentrate the combustion products. In principle (but often not reality to the practicalities of concentrating a dispersed gas) combustion and then sequestration can be energy positive overall. The thermodynamics isn't quite what folks often assume.