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

(1) Peterson, A. A.; Nørskov, J. K. Activity Descriptors for CO₂ Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3 (2), 251–258.

(2) Roberts, F. S.; Kuhl, K. P.; Nilsson, A. High Selectivity for Ethylene from Carbon Dioxide Reduction over Copper Nanocube Electrocatalysts. Angew. Chemie 2015, 127, 5268–5271.

(3) Eilert, A.; Cavalca, F.; Roberts, F. S.; Osterwalder, J.; Liu, C.; Favaro, M.; Crumlin, E. J.; Ogasawara, H.; Friebel, D.; Pettersson, L. G. M.; et al. Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction. J. Phys. Chem. Lett. 2017, 8 (1), 285–290.

(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 2017, acs.jpcc.7b08278.

(5) Liu, C.; Lourenço, M. P.; Hedström, S.; Cavalca, F.; Diaz-Morales, O.; Duarte, H. A.; Nilsson, A.; Pettersson, L. G. M. Stability and Effects of Subsurface Oxygen in Oxide-Derived Cu Catalyst for CO 2 Reduction. J. Phys. Chem. C 2017, acs.jpcc.7b08269.