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^•– and concentrating CO₂ in the PEI overlayer.