Fachbeirat 2022

Cu2O Catalyst Restructuring during Carbon Dioxide and Nitrate Electroreduction

Not scheduled

20m

Speaker

Aram Yoon (ISC)

Description

In electrocatalytic reactions, redox reactions taking place on the surface of catalysts can also change the catalysts’ structural characteristics. Such structural transformations can be especially drastic for oxidized pre-catalysts in electroreduction reactions leading to the loss of initial morphology or to the formation of new architectures. These long-range catalytic changes that take place within the electrolyte and under applied potential directly impact the properties of the as-synthesized catalysts but can be challenging to observe using conventional microscopy due to the difficulty in preserving this dynamical morphology. Moreover, the stability of metallic and oxidized species may not follow our conventional predictions based on Pourbaix diagrams and thermodynamic considerations because these transformations may be constrained by kinetics or be altered by the local chemistry that is clearly distinct from the bulk counterpart.
Recently, we demonstrated that we can use electrochemical cell transmission electron microscopy (EC-TEM) to directly observe and rationalize these transformations. In the carbon dioxide electrocatalytic reduction reaction (CO₂RR), a reaction that can valorize CO₂ into useful chemicals, we reported observations of highly restructured surfaces forming from Cu(I) oxide cubes^1,2 and Cu iodide pyramids³. We further extended this approach to track the restructuring of Cu₂O pre-catalysts for the nitrate reduction reaction (NO₃RR). In NO₃RR, the desired reaction product is ammonia, but whether metallic Cu, Cu oxide or Cu₂O/Cu interfaces are responsible for ammonia selectivity remains an open question⁴.
We found that the oxide stability and the restructured morphologies of Cu2O cubes differ significantly between CO₂RR and NO₃RR. In CO₂-saturated 0.1 M KHCO₃ electrolyte for CO₂RR, Cu oxide restructuring happens abruptly at -0.7 V_RHE, leading to fragmented Cu frames and re-deposited Cu nanoparticles. The changes were completed in a few minutes of potential application and the morphologies of Cu catalysts did not show further drastic structural changes below -0.7 V_RHE. In 0.1 M Na₂SO₄ with 8mM NaNO₃, the electrolyte for NO₃RR, the Cu2O catalysts dissolved gradually with continual re-deposition at moderately negative potentials (-0.2 V_RHE to -0.6 V_RHE). The dissolution kinetics of the Cu₂O catalysts was much slower than that observed for CO₂RR. Our data serve to rationalize the impact of the stability of Cu electrocatalysts and its role in electrocatalysis.

References

1. R,A. Arán-Ais et al., Nat. Comm. 11 (2020), 3489.
2. P Grosse et al., Nat. Comm. 12 (2021), 6736.
3. A Yoon et al., J. Mat. Chem. A. 10 (2022), 14041.
4. Y Wang et al., Angew. Chem. 59 (2020), 5350.