[theothermkn]

I think that you're thinking of the cost of turning the CO2 (and water) back into fuel, which can be argued along thermodynamic and irreversibility lines to show that unscrambling that egg always requires more energy than obtained from combusting. However, I don't think there is a similar thermodynamic argument on the energy costs of mere capture. It's not a priori impossible that a fractional distillation column with a clever configuration of heat exchangers, for example, could capture all the carbon released by the fossil-fueled plant that powered the column in the first place (and more).

Arguing that mere capture would require more energy than is released would be akin to arguing that cleaning the fireplace would require more energy than was released by the fire, on the mistaken grounds that ash removal took as much energy as regrowing the wood that was burned. Both the ash and the CO2 are in a lower energy state, so there's thermodynamic room to maneuver.