Up to date copper is the only electrocatalyst with relevant activity for the reduction of CO₂ and CO to value added hydrocarbons and alcohols¹. However, CO reduction studies over nanostructured copper catalysts, which are believed to have a high abundancy of active sites, were hindered by coppers instability in alkaline conditions. This instability makes Cu-based catalysts prone to dissolution during immersion into the electrolyte. Recently, we reported on an experimental methodology for immersing catalysts under potential control in reactors generally used for CO₂ and CO reduction². Compared to experiments without electrocatalyst immersion under potential control our method increases the CO reduction activity by four orders of magnitude, showing that small, mass-selected Cu nanoparticles are active catalysts for electrochemical CO reduction. This improvement in activity is attributed to the inhibition of Cu dissolution during immersion into the electrolyte as demonstrated by subsequent Cu stripping experiments.

We now utilize the above described methodology to study the pulsed electrochemical CO reduction on 5.2 nm mass-selected Cu nanoparticles. Pulsed electrolysis has shown promise to improve CO₍₂₎ reduction activity and steer product selectivity by potential oscillations^3–5. Nevertheless, detailed mechanistic understanding of the dynamic reactivity upon potential pulsing is still lacking. Using highly sensitive electrochemical mass-spectrometry we demonstrate a highly active transient activity over mass-selected Cu nanoparticles. By conducting pulsed electrolysis in different electrolytes we ascribe the high transient activity to an initial presence and local depletion of proton donors. Our results highlight the importance of proton donor nature and its local concentration to guide activity and selectivity. We believe that similar strategies can be of importance for the selective conversion of more complex biomass molecules and electrosynthesis.

References

(1) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendorff, I. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Chemical reviews 2019, 119 (12), 7610–7672. DOI: 10.1021/acs.chemrev.8b00705. Published Online: May. 22, 2019.

(2) Hochfilzer, D.; Sørensen, J. E.; Clark, E. L.; Scott, S. B.; Chorkendorff, I.; Kibsgaard, J. The Importance of Potential Control for Accurate Studies of Electrochemical CO Reduction. ACS Energy Lett. 2021, 6 (5), 1879–1885. DOI: 10.1021/acsenergylett.1c00496.

(3) Kimura, K. W.; Fritz, K. E.; Kim, J.; Suntivich, J.; Abruña, H. D.; Hanrath, T. Controlled Selectivity of CO2 Reduction on Copper by Pulsing the Electrochemical Potential. ChemSusChem 2018, 11 (11), 1781–1786. DOI: 10.1002/cssc.201800318. Published Online: May. 22, 2018.

(4) Bui, J. C.; Kim, C.; Weber, A. Z.; Bell, A. T. Dynamic Boundary Layer Simulation of Pulsed CO 2 Electrolysis on a Copper Catalyst. ACS Energy Lett. 2021, 1181–1188. DOI: 10.1021/acsenergylett.1c00364.

(5) Arán-Ais, R. M.; Scholten, F.; Kunze, S.; Rizo, R.; Roldan Cuenya, B. The role of in situ generated morphological motifs and Cu(i) species in C2+ product selectivity during CO2 pulsed electroreduction. Nat Energy 2020, 5 (4), 317–325. DOI: 10.1038/s41560-020-0594-9.