Copper‐Based Electrocatalysts for CO₂Reduction

In Situ Infrared Spectroscopic Evidence of Enhanced Electrochemical CO2 Reduction and C–C Coupling on Oxide-Derived Copper

“…The unique electrocatalytic properties of OD-Cu have been attributed to multiple factors, such as the presence of subsurface oxygen, ,− grain boundaries, ,, and high-index planes, in the resulting “oxide-derived” nanostructures. However, the exact electrochemically active site of OD-Cu has been the subject of debate in various papers, with studies yielding contradictory results on whether the copper active site of OD-Cu is fully reduced or partially oxidized. ,, According to the Pourbaix diagram of copper, bulk copper oxide phases should be unstable under the cathodic potentials usually applied during CO 2 RR. In agreement with this, copper oxide-containing electrocatalysts have been observed to completely convert to metallic copper under the common reducing conditions of CO 2 RR/CORR when probed using Raman spectroscopy ( in situ ), − X-ray diffraction (XRD, ex situ ,, and in situ , ), electron energy loss spectroscopy (EELS, ex situ ), , energy-dispersive X-ray spectroscopy (EDS, ex situ ), and X-ray absorption spectroscopy (XAS, in situ ). − Residual oxygen sites were also demonstrated to be unstable when analyzed using 18 O isotope labeling and using density functional theory (DFT) calculations to simulate the removal of oxygen from the subsurface layers of the copper lattice. ,, …”

“…However, the exact electrochemically active site of OD-Cu has been the subject of debate in various papers, with studies yielding contradictory results on whether the copper active site of OD-Cu is fully reduced or partially oxidized. 1,23,24 According to the Pourbaix diagram of copper, 25 bulk copper oxide phases should be unstable under the cathodic potentials usually applied during CO 2 RR. In agreement with this, copper oxide-containing electrocatalysts have been observed to completely convert to metallic copper under the common reducing conditions of CO 2 RR/CORR when probed using Raman spectroscopy (in situ), 26−28 X-ray diffraction (XRD, ex situ 22,26,27 and in situ 29,30 ), electron energy loss spectroscopy (EELS, ex situ), 6,22 energy-dispersive X-ray spectroscopy (EDS, ex situ), 6 and X-ray absorption spectroscopy (XAS, in situ).…”

“…28,34,35 Contrary to these findings, however, several papers have argued that a small amount of oxygen atoms can remain on the surface/subsurface of OD-Cu under strongly reducing conditions. 1,23,24,36 Because of the rapid oxidation rate of copper under ambient conditions, 26,30,33,37 meticulously designed experiments are needed to detect oxygen during or after CO 2 RR. Nevertheless, the elusive, metastable oxygen that persists on the copper surface/subsurface layers during or after CO 2 RR/CORR has been experimentally observed using EDS (ex situ), 3,13,38 X-ray photoelectron spectroscopy (XPS, under ex situ, 13 quasi in situ, 3 and in situ ambient pressure 18 conditions), Auger electron spectroscopy (AES, quasi in situ), 39 EELS (quasi in situ), 18,40 photoresponse measurements (in situ), 13 Raman spectroscopy (in situ), 41 XAS (ex situ 42 and in situ 38,42 ), and positron annihilation spectroscopy (PAS, ex situ) 40 experiments.…”