Cation effect has emerged as a promising strategy for modulating the product distribution during the electrocatalytic CO₂ reduction reaction (CO₂RR). However, the strategy of solely increasing bulk cation concentration in the electrolyte to intensify cation effect at the electrode interface exacerbates carbonate formation issue. Therefore, it is crucial to achieve local cation enrichment at the electrolyte interface without increasing bulk cation concentration. Herein, we propose a "surface charge density modulation" strategy to strengthen interfacial electric field, intensifying the local cation effect at the electrode interface in a low-concentration electrolyte. We implement this strategy using leaf-like CuO nanosheets, introducing a high-curvature morphology into the catalysts. As a result, the CuO nanosheets display 3.4-fold enhancement in Faradic efficiency (FE) of multi-carbon products (C₂₊) compared to CuO nanospheres with low-curvature. In-situ Raman spectroscopy and control experiment varying concentration of K⁺ reveal the mechanism on how the cation effect and interfacial electric field influence CO₂RR performance.