Over the past decade, there has been significant research in electrochemical reduction of CO2, but it has been difficult to develop catalysts capable of C-C bond formation. Here, we report bioelectrocatalysis of vanadium nitrogenase from Azotobacter vinelandii, where cobaltocenium derivatives transfer electrons to the catalytic VFe protein, independent of ATP-hydrolysis. In this bioelectrochemical system, CO2 is reduced to ethylene (C2H4) and propene (C3H6), by a single metalloenzyme.