LAUSR

Despite the advantage of high carbon utilization, CO2 electroreduction (CO2ER) in acid is challenged by the competitive hydrogen evolution reaction (HER). Designing confined catalysts is a promising strategy to suppress HER and boost CO2ER, yet the relationship between the confined structure and catalytic performance remains unclear, limiting rational design. Herein, using Cu2O@mesoporous SiO2 core‐shell catalysts as a well‐defined platform, a volcano‐shaped relationship is found between the thickness of mesoporous SiO2 layer and productivity of multicarbon (C2+) products in CO2 electroreduction. The optimal shell thickness of 15 nm is identified, with in situ spectroscopies and theoretical simulations attributing this to the trade‐off between the local alkalinity and CO2 concentration, arising from the nanoconfinement effect. At this optimal thickness, the Cu2O@ mesoporous SiO2 catalyst achieves a C2+ Faradaic efficiency of 83.1% ± 2.5% and partial current density of 687.8 mA cm−2 in acidic electrolytes, exceeding most reported catalysts. This work provides valuable insights for the rational design of confined catalysts for electrocatalysis.