Nitrogen-doped carbon nanotubes are selective and robust electrocatalysts for CO2 reduction to formate in aqueous media without the use of a metal catalyst. Polyethylenimine (PEI) functions as a co-catalyst by significantly reducing catalytic overpotential and increasing current density and efficiency. The co-catalysis appears to help in stabilizing the singly reduced intermediate CO2 center dot- and concentrating CO2 in the PEI overlayer.