Electrocatalytic conversion of carbon dioxide to formic acid over nanosized Cu6Sn5 intermetallic compounds with a SnO2 shell layer

Abstract

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.