Twin-boundary engineering boosted undercoordinated active sites for scalable conversion of CO2 to ethylene

Abstract 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 provides a pathway for 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 twin-boundary active sites using a direct chronoamperometric pulse method. Our numerical investigations predicted that twin-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 twin-boundary density on dendric Cu catalysts (TB-Cu) by combining transmission electron microscopy, in-situ Raman and X-ray photoelectron spectroscopy with detailed electrochemical measurements. A linear relationship between the Faradaic efficiency of the C 2+ product and the presence of under-coordinated sites was observed, which allowed to directly quantify the contribution of the twin-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 twin-boundary density Cu electrodes achieve 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 TB-Cu was implemented in a 25 cm 2 MEA electrolyzer and demonstrated selectivity of over 62 % for 70 hours 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 opens new avenues for the design of an undercoordinated Cu-based catalyst for the realization of solar-driven fuel production.