Electroreduction of CO₂ to valuable multicarbon (C₂₊) products is a highly attractive way to utilize and divert emitted CO₂. However, a major fraction of C₂₊ selectivity is confined to less than 90% by the difficulty of coupling C-C bonds efficiently. Herein, we identify the stable Cu⁰/Cu²⁺ interfaces derived from copper phosphate-based (CuPO) electrocatalysts, which can facilitate C₂₊ production with a low-energy pathway of OC-CHO coupling verified by in situ spectra studies and theoretical calculations. The CuPO precatalyst shows a high Faradaic efficiency (FE) of 69.7% towards C₂H₄ in an H-cell, and exhibits a significant FE_C2+ of 90.9% under industrially relevant current density (j = −350 mA cm⁻²) in a flow cell configuration. The stable Cu⁰/Cu²⁺ interface breaks new ground for the structural design of electrocatalysts and the construction of synergistic active sites to improve the activity and selectivity of valuable C₂₊ products.