Abstract Understanding the CO2 transformation mechanism on materials is essential for the design of efficient electrocatalysts for CO2 reduction. In aconventional adsorbate evolution mechanism (AEM), the catalysts encounter multiple high‐energy barrier steps, especially CO2 activation, limiting the activity and selectivity. Here, lattice carbonate from Cu2(OH)2CO3 is revealed to be a mediator between CO2 molecules and catalyst during CO2 electroreduction by a 13C isotope labeling method, which can bypass the high energy barrier of CO2 activation and strongly enhance the performance. With the lattice carbonate mediated mechanism (LCMM), the Cu2(OH)2CO3 electrode exhibited ten‐fold faradaic efficiency and 15‐fold current density for ethylene production than the Cu2O electrode with AEM at a low overpotential. Theoretical calculations and in situ Raman spectroscopy results show that symmetric vibration of carbonate is precisely enhanced on the catalyst surface with LCMM, leading to faster electron transfer, and lower energy barriers of CO2 activation and carbon–carbon coupling. This work provides a route to develop efficient electrocatalysts for CO2 reduction based on lattice‐mediated mechanism.
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