3D graphene, which was synthesized directly from CO2 via its exothermic reaction with liquid K, exhibited excellent performance as a counter electrode for a dye-sensitized solar cell (DSSC). The DSSC has achieved a high power conversion efficiency of 8.25%, which is 10 times larger than that (0.74%) of a DSSC with a counter electrode of the regular graphene synthesized via chemical exfoliation of graphite. The efficiency is even higher than that (7.73%) of a dye-sensitized solar cell with an expensive standard Pt counter electrode. This work provides a novel approach to utilize a greenhouse gas for DSSCs.