Electrocatalytic Conversion of Carbon Dioxide to Formic Acid over Nanosized Cu6Sn5 Intermetallic Compounds with a SnO2 Shell Layer

A novel ordered intermetallic compound of carbon-black-supported Cu6Sn5 nanoparticles (Cu6Sn5 NP/CB) in which Cu6Sn5 has a NiAs-type structure was successfully prepared through a wet chemical method using lithium triethylborohydride as a reducing agent. The prepared ordered intermetallic compound was characterized using X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray absorption fine structure spectroscopy (XAFS). The XRD measurements confirm the formation of the NiAs-type ordered intermetallic Cu6Sn5. XPS and STEM-X-ray energy dispersive spectroscopy measurements allowed us to confirm the Cu6Sn5 structure. The surface of the intermetallic Cu6Sn5 was found to be covered by SnO2, indicating that a core-shell structured intermetallic compound (i.e., Cu6Sn5 core/SnO2 shell) had formed. The Cu6Sn5 NP/CB material exhibited a faradaic efficiency of 65.3% at -0.6 V for HCOO- formation via electrochemical CO2 reduction, which is superior to those of the non-intermetallic Cu NP/CB and Sn NP/CB samples. From the XAFS measurements, we determined the Sn-Sn distance in the SnO2 on the surface of the Cu6Sn5 NPs, and the key factor affecting the high selectivity was found to be the 4.9% compressive strain of the SnO2 shell layers on the Cu6Sn5 compared to that of the Sn NP/CB sample.