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The electrochemical reduction of CO₂ to value-added chemicals renewable electricity is a promising and ecofriendly strategy to achieve the national strategic goal of “carbon peak and carbon neutrality” and solve the greenhouse effect. Due to the variety of products in CO₂ electroreduction (CO₂ER), catalytic selectivity has become a key factor in the design of electrode structure. Herein, a systematic investigation of CO₂ER on the nanoporous gold films with different thicknesses prepared by the self-deposition method developed by ourselves. Mass transfer effects are found to play an important role in determining product selectivity and activity. The specific activity for CO evolution (<i>j_CO</i>) with exponential declination has more dramatic tendency than the specific activity for hydrogen evolution (<i>j_H2</i>) with linear decay with increasing nanoporous gold film thickness. Different from the behaviors within the mesoporous structures in previous studies, the retarded transport of HCO₃⁻ ions within the nanoscale pores is more sensitive than that of protons. This phenomenon implies the necessity of considering mass transfer effects in the design of outstanding electrocatalysts for CO₂ER as well as for understanding the geometrical infrastructure-performance relationships.