It takes intensive efforts to convert CO2 into valuable materials. Perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies. So far, however, there is not any attempt to relate CO2 conversion to perovskite solar cells. In this study, we successfully synthesized mesochannel carbon nanowalls directly from CO2 via its exothermic reaction with liquid Na at 550 degrees C. Furthermore, the mesochannel carbon nanowalls exhibited excellent performance for a hole-conductor-free perovskite solar cell, achieving a high power conversion efficiency of 10.43%. This efficiency is much larger than that of a perovskite solar cell (4.74%) with a chemical-exfoliated-graphene counter electrode. This work provides not only a promising method to synthesize efficient electrode materials but also a novel approach to utilize CO2.