Interrogating the copper

Angew. Chem. Int. Ed. https://doi.org/10.1002/anie.202011137 (2020)

Electrochemical carbon dioxide reduction has received a great deal of attention in recent years because it offers a pathway to transform a greenhouse gas into fuels and chemicals by using renewable electricity. Of particular interest are copper catalysts, which are able to reduce CO₂ to multi-carbon products, such as ethylene, ethanol or propanol. Copper catalysts have been found to undergo reconstruction and restructuring during start-up and under reaction conditions, but the specific nature of the process remains elusive.

Now, Raffaella Buonsanti, Vasiliki Tileli and colleagues unveil the structural dynamics of copper nanoparticles during the initial stages of CO₂ electroreduction by combining in situ/operando microscopy and spectroscopy. To this end, the researchers have designed and built a microelectrochemical cell with a glassy carbon working electrode, on which as-prepared metallic copper nanoparticles with spherical morphology are deposited. In situ transmission electron micrographs are obtained in CO₂-saturated 0.1 M KHCO₃ during a potential sweep from the open circuit voltage (OCV, ~0.5 V_RHE) to cathodic potentials (up to –0.8 V_RHE), as well as from OCV to –0.25 V_RHE followed by a chronoamperometric measurement at this potential. Time-resolved in situ selected area electron diffraction and operando X-ray absorption spectroscopy are also performed during a potential sweep in the aforementioned CO₂-saturated electrolyte.

The researchers found that the copper nanoparticles are oxidized to Cu₂O at OCV after being exposed to air and the electrolyte, with concomitant dissolution of Cu ions. During start-up, while sweeping towards more cathodic potentials and with an onset at 0.1 V_RHE, the dissolved ions redeposit onto the working electrode, forming metallic Cu secondary nanoparticles. The Cu₂O primary nanoparticles are reduced back to metallic Cu and further Cu ions are likely to dissolve. Finally, during cell operation at constant potential the researchers observed a process similar to Ostwald ripening, where a subpopulation of secondary particles increases in size while the primary particles decrease and eventually disappear. This process involves transient Cu species whose nature is still unclear.