Structure-Tailored Surface Oxide on Cu-Ga Intermetallics Enhances CO 2 Reduction Selectivity to Methanol at Ultralow Potential.

Electrochemical CO 2 reduction reaction (eCO 2 RR) is performed on two intermetallic compounds formed by copper and gallium metals (CuGa 2 and Cu 9 Ga 4 ). Among them, CuGa 2 selectively converts CO 2 to methanol with remarkable Faradaic efficiency of 77.26% at an extremely low potential of -0.3 V vs RHE. The high performance of CuGa 2 compared to Cu 9 Ga 4 is driven by its unique 2D structure, which retains surface and subsurface oxide species (Ga 2 O 3 ) even in the reduction atmosphere. The Ga 2 O 3 species is mapped by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) techniques and electrochemical measurements. The eCO 2 RR selectivity to methanol are decreased at higher potential due to the lattice expansion caused by the reduction of the Ga 2 O 3 , which is probed by in situ XAFS, quasi in situ powder X-ray diffraction, and ex situ XPS measurements. The mechanism of the formation of methanol is visualized by in situ infrared (IR) spectroscopy and the source of the carbon of methanol at the molecular level is confirmed from the isotope-labeling experiments in presence of 13 CO 2 . Finally, to minimize the mass transport limitations and improve the overall eCO 2 RR performance, a poly(tetrafluoroethylene)-based gas diffusion electrode is used in the flow cell configuration.