The Role of the Copper Oxidation State in the Electrocatalytic Reduction of CO₂ into Valuable Hydrocarbons

Abstract

Redox-active copper catalysts with accurately prepared oxidation states (Cu 0 , Cu + and Cu 2+) and high selectivity to C2 hydrocarbon formation, from electrocatalytic cathodic reduction of CO2, were fabricated and characterized. The electrochemically prepared copper-redox electro-cathodes yield higher activity for the production of hydrocarbons at lower oxidation state. By combining advanced X-ray spectroscopy and in situ micro-reactors it was possible to unambiguously reveal the variation in the complex electronic structure that the catalysts undergo at different stages (i.e. during fabrication and electrocatalytic reactions). It was found that the surface, sub-surface and bulk properties of the electrochemically prepared catalysts are dominated by the formation of copper carbonates on the surface of cupric-like oxides, which prompts catalyst deactivation by restraining effective charge transport. Furthermore, the formation of reduced or partially-reduced copper catalysts yields the key dissociative protonconsuming reactive adsorption of CO2 to produce CO; allowing the subsequent hydrogenation into C2 and C1 products by dimerization and protonation. These results yield valuable information on the variations in the electronic structure that redox-active copper catalysts undergo in the course of the electrochemical reaction, which, under extreme conditions are mediated by thermodynamics but, critically, kinetics dominate near the oxide/metal phase transitions.

Electrochemical Real‐Time Mass Spectrometry (EC‐RTMS): Monitoring Electrochemical Reaction Products in Real Time

“…On the pristine electrode, metallic Cu 0 and Cu 2 Os pecies were predominantly present (blue curve in Figure 3). [18] Thepresence of Cu 2+ species for the anodized sample was confirmed by as atellite feature at binding energies of about 940-945 eV (red curve in Figure 3). [18] Thepresence of Cu 2+ species for the anodized sample was confirmed by as atellite feature at binding energies of about 940-945 eV (red curve in Figure 3).…”

“…[18] Thepresence of Cu 2+ species for the anodized sample was confirmed by as atellite feature at binding energies of about 940-945 eV (red curve in Figure 3). [18] Theenhanced formation of C 2 and C 3 products here may be related to different anodization conditions compared to those used by Velasco-VØlez et al [18] Further systematic work involving in situ surface characterization is needed to determine the relationship between the anodization conditions,t he resulting in operando chemical state of the electrode and the impact on reaction activity and selectivity. Copper carbonate was not present after the potential scan in the CO 2 RR region, which implies that the thin surface layer of CuCO 3 ·Cu(OH) 2 -like species was reduced within seconds.I nterestingly,arecent in situ X-ray spectroscopy study showed that ab ulk Cu 2+ -like electrode terminated by ap assivating layer of practically insoluble carbonates,a lso prepared by anodization, suppresses the formation of CO 2 RR products owing to charge transport limitations.…”

Electrochemical Real‐Time Mass Spectrometry (EC‐RTMS): Monitoring Electrochemical Reaction Products in Real Time

“…After anodization (red curve in Figure 3), the signal was substantially shifted towards copper in more positive oxidation states such as CuO,orCuCO 3 ·Cu-(OH) 2 ,p art of which was reversed after the follow-up reductive cyclic voltammetry scan in the CO 2 RR region (green curve in Figure 3). [18] Thepresence of Cu 2+ species for the anodized sample was confirmed by as atellite feature at binding energies of about 940-945 eV (red curve in Figure 3). Theq uantitative analysis of respective Cu L 3 M 4,5 M 4,5 Auger spectra (Supporting Information, Figure S2) further revealed the presence of alkaline copper carbonate species on the electrode after anodization (Supporting Information, Table S2).…”

“…Copper carbonate was not present after the potential scan in the CO 2 RR region, which implies that the thin surface layer of CuCO 3 ·Cu(OH) 2 -like species was reduced within seconds.I nterestingly,arecent in situ X-ray spectroscopy study showed that ab ulk Cu 2+ -like electrode terminated by ap assivating layer of practically insoluble carbonates,a lso prepared by anodization, suppresses the formation of CO 2 RR products owing to charge transport limitations. [18] Theenhanced formation of C 2 and C 3 products here may be related to different anodization conditions compared to those used by Velasco-VØlez et al [18] Further systematic work involving in situ surface characterization is needed to determine the relationship between the anodization conditions,t he resulting in operando chemical state of the electrode and the impact on reaction activity and selectivity.…”

In-situ Spectroscopic Techniques as Critical Evaluation Tools for Electrochemical Carbon dioxide Reduction: A Mini Review

“…It was identified that the surface and bulk properties of the copper oxide catalysts are subjugated by the development of copper carbonates on the cupric oxide surfaces. This led to passivation of the catalyst by impeding the charge transport resulting in formation of CO, and subsequently followed by hydrogenation into C1 and C2 products (Velasco-Vélez et al, 2019). Grosse et al, studied the chemical state and the catalytic selectivity of Cu nanocubes during electrochemical CO 2 RR and using XAFS coupled with wave-let transform and found that no Cu 2 O species remain after 1 h of CO 2 RR either at the surface or in sub-surface regions (Grosse et al, 2018).…”