Molecular‐Level Modulation of Mass Transfer Kinetics in Trinuclear Copper Cluster–Based COFs Enables Efficient Electrocatalytic Nitrate Reduction

Electrocatalytic nitrate reduction (NO 3 RR) provides a sustainable route for ammonia synthesis while mitigating nitrate pollution, yet catalyst design has largely overlooked mass transfer kinetics. Herein, we report a series of vinylene‐linked trinuclear copper cluster–based covalent organic frameworks (COFs; CuDB‐TMT, CuDA‐TMT, and CuDA‐TMB) synthesized via Knoevenagel condensation, enabling precise modulation of the catalytic microenvironment. Systematic structural variation reveals that steric hindrance and pore architecture critically govern substrate accessibility and interfacial kinetics. CuDB‐TMT, bearing methyl‐substituted copper clusters, exhibits suppressed activity due to hindered mass transfer, whereas CuDA‐TMB, featuring enlarged pores and an unobstructed active site environment, achieves a high ammonia Faradaic efficiency of 95.36% and a yield rate of 10.26 mg h −1 cm −2 in 50 mM nitrate, outperforming most reported NO 3 RR electrocatalysts. Combined experimental and theoretical studies identify mass transfer regulation as a key determinant of catalytic performance. Moreover, CuDA‐TMB functions effectively as a cathode for Zn‐nitrate batteries. This work highlights molecular‐level kinetic control as a viable strategy for designing high‐performance porous electrocatalysts.