Formation of Copper Catalysts for CO₂ Reduction with High Ethylene/Methane Product Ratio Investigated with In Situ X-ray Absorption Spectroscopy

Abstract

Nanostructured copper cathodes are among the most efficient and selective catalysts to date for making multicarbon products from the electrochemical carbon dioxide reduction reaction (CO2RR). We report an in situ X-ray absorption spectroscopy investigation of the formation of a copper nanocube CO2RR catalyst with high activity that highly favors ethylene over methane production. The results show that the precursor for the copper nanocube formation is copper(I)-oxide, not copper(I)-chloride as previously assumed. A second route to an electrochemically similar material via a copper(II)-carbonate/hydroxide is also reported. This study highlights the importance of using oxidized copper precursors for constructing selective CO2 reduction catalysts and shows the precursor oxidation state does not affect the electrocatalyst selectivity toward ethylene formation.

Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction

“…Both spectra (b) and (c) in Figure 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 Recently, Lee et al 23 and Mistry et al 24 proposed residual Cu 2 O after reduction that would explain the enhanced selectivity of oxide-derived Cu. Their in situ XANES data are consistent with a similar study by Eilert et al 9 and an in situ Raman spectroscopy study by Ren et al, 11 that showed a pure metallic phase after holding the sample at reductive potentials, but their ex situ TEM work that occurred after long exposure to air and post mortem sample preparation showed the presence of an oxide phase (a reference sample reduced with H 2 indicated that the oxide phase was not solely caused by exposure to air). However, in the present study our APXPS experiment was carried out in situ and our TEM experiment in a quasi in situ environment …”

“…6,7 However, the explanation becomes more complicated when dealing with nanostructured copper materials derived from oxidized precursors, which show both higher activity and greatly enhanced selectivity towards multicarbon products. [8][9][10][11][12][13][14][15][16] It has been hypothesized that this could be related for instance to an increase of local pH, [17][18][19] grain boundaries, 20 undercoordinated sites, 21 or residual oxides. [22][23][24] Several groups have shown the existence of sites with higher CO binding energy in oxidederived copper.…”

Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO

“…Furthermore, after electrolysis Sn is found to be uniformly distributed and is present mainly in the +2 oxidation state with the appearance of some Cu(I) in contrast to the initial state where Sn is mainly found as Sn(IV) at the surface. The question of whether residual oxides play a role in CO 2 reduction on oxide-derived catalysts is still under debate 15,[28][29][30] . Furthermore, previous literature suggested that SnO 2 led to a good performance as a CO 2 reduction catalyst only when in a metastable oxide state beyond its reduction potential 31 .…”

Catalyst electro-redeposition controls morphology and oxidation state for selective carbon dioxide reduction

“…In situ hard X-ray absorption spectroscopy (hXAS) experiments have suggested stable Cu + species exist at highly negative CO 2 RR potentials of ~− 1.0 versus RHE 9 . However, the presence of Cu + species during CO 2 RR is still the subject of debate; 7,19 and in situ tracking of the copper oxidation state with time resolution during CO 2 RR has remained elusive.Morphological effects of copper nanostructures have a significant effect on the selectivity of CO 2 RR to multi-carbon products [20][21][22][23][24] . Copper catalysts with different morphologies have been synthesized through annealing, chemical treatments on thin films, colloidal synthesis and electrodeposition from solution 6,17,25,26 .…”

“…In situ hard X-ray absorption spectroscopy (hXAS) experiments have suggested stable Cu + species exist at highly negative CO 2 RR potentials of ~− 1.0 versus RHE 9 . However, the presence of Cu + species during CO 2 RR is still the subject of debate; 7,19 and in situ tracking of the copper oxidation state with time resolution during CO 2 RR has remained elusive.…”