Electrochemical nitrate (NO3−) reduction reaction (NO3−RR) represents an ideal alternative for ammonia (NH3) generation. Despite recent success on the synthesis of Cu-based electrocatalysts, the kinetics of Cu-catalyzed NO3−RR is still greatly limited by the slow proton transfer rate since the large energy barrier for water dissociation. Here, we report the construction of a yolk-shell structure, comprising a Cu core and Cu2Se shell that functions like the tandem nanoreactor. Specific, the

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... shell with strong water dissociation ability can easily produce protons and then transfer to the Cu core for driving the reduction of NO3−. Intriguingly, the proton flux arriving to the Cu core can be well tuned by altering the void size of the yolk-structure, thereby enabling rapid proton transfer yet hindering the competitive hydrogen evolution. More importantly, operando Raman spectra reveal that the rapid proton transfer significantly promotes the hydrogenation of key intermediates for reducing the overall energy barrier of the NO3−RR. Consequently, the optimized yolk-shell structure enables highly selective and efficient NO3−RR with a large NH3 yield rate of 0.94 mmol cm−2 h−1. This work offers a fresh concept to boost the NO3−RR by tuning proton transfer rate.