Electrocatalysts for CO₂ electroreduction require not only high-performance active materials to control the series reaction but also conductive and durable supports to ensure long-term stability under harsh operating conditions. Instead of conventional heterogeneous catalysts made by attaching metal on supports, we manufactured a self-formed tandem catalyst designed for a cascade electroreduction of CO₂ to C₂H₄. Using oxygen partial pressure-controlled calcination, electrospun copper acetate/polyacrylonitrile nanofibers were successfully transformed into porous carbon nanofibers consisting of doped N and metallic Cu particles. Doped nitrogen atoms adjacent to Cu atoms trigger the reaction by increasing the amount of CO* on the Cu surfaces, which lowers the energy required for CO dimerization that is used for C₂H₄ production. The Cu-embedded N-doped carbon nanofibers exhibit a C₂H₄ faradaic efficiency of 62% at a potential of −0.57 V vs. RHE with high current density of 600 mA cm⁻² and excellent long-term stability. DFT calculations suggest that the lowered overpotential originates from the decreased CO dimerization energy barrier due to the doped N triggering CO production around the Cu particles.