Molecular-Level Modulation of Mass Transfer Kinetics in Trinuclear Copper Cluster-Based COFs Enables Efficient...

Electrocatalytic nitrate reduction (NO₃RR) 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 NO₃RR 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.