Operando Insights Uncover Important Role of Hydroxide and Oxide Species in Pulsed CO2 Electroreduction Toward Ethanol
Authors: Herzog Antonia , Bergmann Arno , Roldan Cuenya Beatriz 
Published: December 2024

The electrochemical reduction of CO2 (CO2RR) represents a compelling approach to address the energy crisis by closing the carbon cycle and converting renewable energy into valuable multicarbon fuels and chemicals [1]. Cu2O nanocubes have demonstrated effective catalytic activity in producing various C2+ hydrocarbons, although they exhibit broad product selectivity [2]. To enhance selectivity for energy-rich products like ethanol, strategies such as modifying the catalyst's structure and composition through oxide-derived and bimetallic approaches have been explored [3]. Recently, pulsed CO2RR has been employed as an effective method to adjust product selectivity toward ethanol in situ, eliminating the need for extensive redesigns of the catalytic system [4]. Nonetheless, mechanistic insights are essential and require comprehensive operando studies to identify the dynamic interplay of the electrocatalytic interface.

This study aims to deepen our understanding of the catalyst-adsorbate interactions specific to ethanol production by employing sub-second time-resolved operando spectroscopy and diffraction techniques during CO2RR. We synthesized bare and ZnOx-decorated Cu2O nanocubes with 30 nm edge lengths using a simple ligand-free method. During pulsed CO2RR on monometallic Cu cubes, we monitored the evolution of key adsorbates using time-resolved operando surface-enhanced Raman spectroscopy (SERS). Our experiments across various pulse length conditions revealed the pivotal roles of co-adsorbed hydroxide and CO on the Cu surface and the oxidative formation of Cu-Oad or CuOx/(OH)y species, which significantly affect CO adsorption kinetics and enhance ethanol selectivity. Notably, insufficient hydroxide coverage leads to increased C1 product selectivity due to the formation of bulk-like Cu2O, while excessive hydroxide coverage inhibits C-C coupling [5].

These findings were further validated by studying a bimetallic Cu-ZnO nanocatalyst under pulsed CO2RR conditions, where the selectivity could be modulated by adjusting the anodic potential while the pulse length remained constant. Time-resolved operando techniques including X-ray absorption spectroscopy (XAS), SERS, and X-ray diffraction (XRD) revealed the dynamic interactions among Cu, Zn, and the CuZn alloy, which formed under CO2RR conditions, as well as the adsorption behaviors of hydroxide and CO. These insights highlight the critical role of oxides and hydroxide coverage in enhancing ethanol selectivity, which can be finely tuned through the oxidative modification of Cu- or Cu-Zn-based catalytic materials using potential pulses. This enhances our fundamental understanding of the mechanisms underlying CO2RR, significantly advancing the field.

References

[1] Y. Hori, K. Kikuchi, S. Suzuki, Chem. Lett. 47 (1985) 1695.

[2] D. Gao, I. Sinev, F. Scholten, R. M. Aran-Ais, N. J. Divins, J. Timoshenko, B. Roldan Cuenya, Angew. Chem. Int. Ed. 58 (2019) 17047.

[3] A. Herzog, A. Bergmann, H. S. Jeon, J. Timoshenko, S. Kühl, C. Rettenmaier, M. Lopez Luna, F. T. Haase, B. Roldan Cuenya, Angew. Chem. Int. Ed. 60 (2021) 74264.

[4] J. Timoshenko, A. Bergmann, C. Rettenmaier, A. Herzog, R. Arán-Ais, H. S. Jeon, F. T. Haase, U. Hejral, P.Grosse, S. Kühl, E. M. Davis, J. Tian, O. M. Magnussen, B. Roldan Cuenya, Nat. Catal. 2022, 5 (4), 259–267.

[5] A. Herzog, M. Lopez Luna, H. S. Jeon, C. Rettenmaier, P. Grosse, A. Bergmann, B. Roldan Cuenya, Nat. Comm. 2024, accepted.