Auxiliary Linker-Enabled Local Protonation Boosts the Ammonia Electrosynthesis in Heterometallic Metal–Organic Frameworks

Controlling the local proton environment is a powerful strategy for accelerating proton-coupled electron transfer reactions, but realizing this within stable heterogeneous catalysts remains challenging. Herein, we report a series of metal-organic frameworks (MOFs), PCN-800M-L, featuring precisely placed proton relays adjacent to catalytic sites for boosting electrocatalytic nitrate reduction to ammonia (NO₃RR). PCN-800M-L series was constructed by sequentially installing catalytically active metal sites (M = Ni²⁺ or Co²⁺) and auxiliary nitrogen-donor linkers (L = L1-L5) into a robust Zr⁴⁺-based framework. Auxiliary linkers reversibly dissociate upon protonation, transiently exposing active metal sites while functioning as localized proton relays. By systematically varying the metals and linkers, we uncover a volcano-type dependence of NO₃RR activity on linker p<i>K</i>_a, with the PCN-800Co-L3 achieving the highest turnover frequency (1863 h^-1) and Faradaic efficiency (97.9%). This work establishes a molecular-level design principle that harnesses auxiliary linkers as programmable proton sources in MOFs, enabling efficient multielectron electrocatalysis.