Metallurgical Alloying Enables Supersaturated Cu Solid Solution for Efficient CO₂-to-ethanol Electroconversion

Converting CO₂ into value-added chemicals through electrocatalysis not only utilizes the CO₂ to achieve net negative carbon emission, but also enables long-term storage of intermittent renewable electricity. There has been a great interest in electrochemical CO₂ reduction reaction (CO₂RR) to produce liquid fuels which has high energy density. In light of the progress, energy efficiency and reaction rate for C₂₊ products have been largely improved, especially for ethylene (C₂H₄) production. However, the development of electrocatalysts that efficiently produce ethanol (C₂H₅OH), another C₂ product which is one of the most essential organic chemical, has been retarded. To enhance C₂H₅OH productivity, heteroatom alloying and defective sites have been known to be effective by promoting preferential binding with reaction intermediates and lower an energy barrier of rate-determining step.<br/>Herein, we developed alloy catalysts in which Cu sites were evenly dispersed in Ag lattice over a thermodynamic immiscibility for efficient C₂H₅OH production. For precise microstructural control, alloying and chemical de-alloying strategies were adopted which were scalable for porous electrode. Removing sacrificial metal through the de-alloying formed defect sites such as vacancy in the catalysts which were effective to enhance the affinity to the reaction intermediate. Microstructural optimization was judiciously carried out based on metallurgical considerations in the perspective of composition and undercooling. Cu catalysts from eutectic composition and rapid quenching acquired high porosity and the undercoordinated sites after the de-alloying, enabling robust CO₂-to-C₂₊ products electroconversion at the Faradaic efficiency (FE) of 80.1±1.6% and partial current density of -127.7±30.3 mA/cm² using membrane electrode assembly (MEA) reactor in a neutral electrolyte. Main products of the de-alloyed Cu was C₂H₄. Thereafter, introduction of the heteroatom (Ag) to the Cu catalysts was conducted to steer the catalysis toward C₂H₅OH production. Elemental distribution of Ag conformed to the immiscibility at initial addition of heteroatoms. However, it was noteworthy that non-equilibrium alloying exploiting large undercooling enabled dissolution of Cu in Ag₂Al phase overcoming the immiscibility. After removing Al, supersaturated solid solution of Ag-Cu alloy was fabricated. Well-defined crystal structure and even distribution of Cu sites significantly facilitated C₂H₅OH formation rather than C₂H₄, achieving the maximum selectivity of 40.4±2.4% and full cell energy efficiency of 14.4%. The relative productivity of C₂H₅OH to C₂H₄ was widely controlled within the range of 0.1 to 1.9. Our metallurgical alloying strategy not only extended the range of alloy catalysts beyond thermodynamic immiscibility, but also highlighted the modifications of microstructure and atomic configuration for efficient production of liquid chemicals from CO₂.