Grain-Boundary Engineering Boosted Undercoordinated Active Sites for Scalable Conversion of CO 2 to Ethylene.

The development of highly selective and energy efficient technologies for electrochemical CO 2 reduction combined with renewable energy sources holds great promise for advancing the field of sustainable chemistry. The engineering of copper-based electrodes facilitates the conversion of CO 2 into high-value multicarbon products (C 2+ ). However, the ambiguous determination of the intrinsic CO 2 activity and the maximization of the density of exposed active sites have severely limited the use of Cu for the realization of practical electrocatalytic devices. Here, we report a scalable strategy to obtain a high density of undercoordinated sites by maximizing the exposure of grain-boundary active sites using a direct chronoamperometric pulse method. Our numerical investigations predicted that grain boundaries modulate the adsorption behavior of *CO on the Cu surface, which acts as a key intermediate species associated with the production of multicarbon species. We investigated the consequence of grain-boundary density on dendric Cu catalysts (GB-Cu) by combining transmission electron microscopy, in situ Raman spectroscopy, and X-ray photoelectron spectroscopy with electrochemical measurements. A linear relationship between the Faradaic efficiency of the C 2+ product and the presence of undercoordinated sites was observed, which allowed to directly quantify the contribution of the grain boundary in Cu-based catalysts on the CO 2 RR properties and the formation of multicarbon products. Using a membrane electrode assembly electrolyzer, the high grain-boundary density Cu electrodes achieved a maximum Faradaic efficiency of 73.2% for C 2+ product formation and a full-cell energy efficiency of 20.2% at a specific current density of 303.6 mA cm -2 . The GB-Cu was implemented in a 25 cm 2 MEA electrolyzer and demonstrated selectivity of over 62% for 70 h together with current retention of 88.4% at the applied potential of -3.80 V. The catalysts and electrolyzer were further coupled to an InGaP/GaAs/Ge triple-junction solar cell to demonstrate a solar-to-fuel (STF) conversion efficiency of 8.33%. This work designed an undercoordinated Cu-based catalyst for the realization of solar-driven fuel production.