Nature, Stability and Distribution of Subsurface Oxygen in Copper Electrodes during the Electrochemical CO2 Reduction
Authors: Cavalca Filippo , Ferragut Rafael , Aghion Stefano , Eilert André , Diaz Morales Oscar , Liu Chang , Koh Ai Leen , Hansen Thomas Willum , G. M. Petterson Lars , Nilsson Anders 
Published: February 2020

The electrochemical carbon dioxide reduction reaction (CO2RR) allows the storage of energy in readily available chemicals such as ethylene and other hydrocarbons, while contributing to the abatement of CO2. Copper is the only pure metal able to perform CO2RR with appreciable activity and selectivity towards multi-carbon products, due to optimal binding of the key intermediate CO,1 especially when the electrode is nanostructured and derived from an oxide.2 Oxide-derived copper (OD-Cu) electrodes exhibit activity and ethylene selectivity higher than pristine copper during the carbon dioxide reduction reaction (CO2RR), and the presence of residual subsurface oxygen in OD-Cu is associated with such improvement.3 In order to establish a structure-activity relationship for the catalyst during reaction, operando investigation is necessary.
Using in situ x-ray photoelectron spectroscopy (XPS), quasi in situ electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) and quasi in situ positron annihilation spectroscopy (PAS), we show that oxygen is primarily concentrated in an amorphous 1-2 nm thick layer on the OD-Cu surface (see Figure), it is stable during CO2RR for up to 1 hour at -1.15 V vs RHE and is associated with a high density of defects in the OD-Cu structure.4

Corroborated with density functional theory (DFT) calculations on copper nanoclusters we show that both the low-coordination of the amorphous OD-Cu surface and the presence of subsurface oxygen that withdraws charge from the copper d-band selectively enhance the binding energy of CO without altering that of other reaction intermediates, therefore breaking the scaling relation between d-band center vs CO binding energy.4,5

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(4) Cavalca, F.; Ferragut, R.; Aghion, S.; Eilert, A.; Diaz-Morales, O.; Liu, C.; Koh, A. L.; Hansen, T. W.; Pettersson, L. G. M.; Nilsson, A. Nature and Distribution of Stable Subsurface Oxygen in Copper Electrodes During Electrochemical CO 2 Reduction. J. Phys. Chem. C
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2017, acs.jpcc.7b08269.

Figure 1