Selective CO₂ electroreduction toward multicarbons (C₂₊) is hampered by the competing pathways at ampere-level current densities. Here, theoretical calculations reveal that the binding strength and protonation of the ∗CO intermediate are a pair of key descriptors in governing the selectivity-determining bifurcation pathway on copper (Cu) catalyst. Hence, we propose an intermediate-modulator strategy with a nonmetallic phosphorus (P)-modified Cu (P-Cu) hetero-site catalyst for ideal C₂₊ formation. The P site enhances charge accumulation at the neighboring Cu site, which strengthens ∗CO adsorption and active ∗H supply from H₂O activation, favoring a rich-∗H-assisted-protonation (RHP) pathway toward ∗CHO formation. Subsequently, the lowest-energy-barrier ∗CO-∗CHO coupling pathway switches the predominant reaction pathway away from undesired CO and H₂ to higher-value ethylene and ethanol. We report a C₂₊ partial current density of 1.05 A cm⁻² and a Faradaic efficiency of 87.7%. Utilizing cheaper nonmetallic elements, this catalyst design principle outperforms reported outcomes with precious metal dopants.