The news reporting on this article has been a little misleading. First, the main goal of this is not to reduce CO2 into carbon, but simply to sequester CO2. The "carbonaceous sheets" that are produced are "graphene oxide", which is less oxidized than CO2 but still highly oxygenated. So the energy required is much less than what would be needed to reduce CO2 completely. But yeah, you won't be able to burn the end product.