Significance A hierarchical structure with multiscale pore networks has been explored to enhance electrocatalytic performance in CO ₂ reduction. Here, we thoroughly analyze the effect of each pore network on catalytic performances using nanostructures composed of nanopores and periodic macroporous channels. The tough mass transport within complex nanostructures has attracted much attention as increasing selectivity. However, the productivity improvements of hierarchical nanostructure by efficient mass transport have been rarely investigated in CO ₂ reduction. This paper systematically reports the structural effect on both selectivity and productivity by controlling pore sizes and distributions. Therefore, the proposed hierarchically nanostructures show improved mass activity and can further be applied to other electrochemical systems in the same way.