Interrogation of Oxidative Pulsed Methods for the Stabilization of Copper Electrodes for CO2 Electrolysis.

Using copper (Cu) as an electrocatalyst uniquely produces multicarbon products (C2+-products) during the CO2 reduction reaction (CO2RR). However, the CO2RR stability of Cu is presently 3 orders of magnitude shorter than required for commercial operation. One means of substantially increasing Cu catalyst lifetimes is through periodic oxidative processes, such as cathodic-anodic pulsing. Despite 100-fold improvements, these oxidative methods only delay, but do not circumvent, degradation. Here, we provide an interrogation of chemical and electrochemical Cu oxidative processes to identify the mechanistic processes leading to stable CO2RR through electrochemical and in situ Raman spectroscopy measurements. We first examine chemical oxidation using an open-circuit potential (OCP), identifying that copper oxidation is regulated by the transient behavior of the OCP curve and limited by the rate of the oxygen reduction reaction (ORR). Increasing O2 flux to the cathode subsequently increased ORR rates, both extending lifetimes and reducing "off" times by 3-fold. In a separate approach, the formation of Cu2O is achieved through electrochemical oxidation. Here, we establish the minimum electrode potentials required for fast Cu oxidation (-0.28 V vs Ag/AgCl, 1 M KHCO3) by accounting for transient local pH changes and tracking oxidation charge transfer. Lastly, we performed a stability test resulting in a 20-fold increase in stable ethylene production versus the continuous case, finding that spatial copper migration is slowed but not mitigated by oxidative pulsing approaches alone.

Spatiotemporal Mapping of Local Heterogeneities during Electrochemical Carbon Dioxide Reduction.

The activity and selectivity of a copper electrocatalyst during the electrochemical CO2 reduction reaction (eCO2RR) are largely dominated by the interplay between local reaction environment, the catalyst surface, and the adsorbed intermediates. In situ characterization studies have revealed many aspects of this intimate relationship between surface reactivity and adsorbed species, but these investigations are often limited by the spatial and temporal resolution of the analytical technique of choice. Here, Raman spectroscopy with both space and time resolution was used to reveal the distribution of adsorbed species and potential reaction intermediates on a copper electrode during eCO2RR. Principal component analysis (PCA) of the in situ Raman spectra revealed that a working electrocatalyst exhibits spatial heterogeneities in adsorbed species, and that the electrode surface can be divided into CO-dominant (mainly located at dendrite structures) and C-C dominant regions (mainly located at the roughened electrode surface). Our spectral evaluation further showed that in the CO-dominant regions, linear CO was observed (as characterized by a band at ∼2090 cm-1), accompanied by the more classical Cu-CO bending and stretching vibrations located at ∼280 and ∼360 cm-1, respectively. In contrast, in the C-C directing region, these three Raman bands are suppressed, while at the same time a band at ∼495 cm-1 and a broad Cu-CO band at ∼2050 cm-1 dominate the Raman spectra. Furthermore, PCA revealed that anodization creates more C-C dominant regions, and labeling experiments confirmed that the 495 cm-1 band originates from the presence of a Cu-C intermediate. These results indicate that a copper electrode at work is very dynamic, thereby clearly displaying spatiotemporal heterogeneities, and that in situ micro-spectroscopic techniques are crucial for understanding the eCO2RR mechanism of working electrocatalyst materials.

Near-Unity Electrochemical CO2 to CO Conversion over Sn-Doped Copper Oxide Nanoparticles.

Bimetallic electrocatalysts have emerged as a viable strategy to tune the electrocatalytic CO2 reduction reaction (eCO2RR) for the selective production of valuable base chemicals and fuels. However, obtaining high product selectivity and catalyst stability remain challenging, which hinders the practical application of eCO2RR. In this work, it was found that a small doping concentration of tin (Sn) in copper oxide (CuO) has profound influence on the catalytic performance, boosting the Faradaic efficiency (FE) up to 98% for carbon monoxide (CO) at -0.75 V versus RHE, with prolonged stable performance (FE > 90%) for up to 15 h. Through a combination of ex situ and in situ characterization techniques, the in situ activation and reaction mechanism of the electrocatalyst at work was elucidated. In situ Raman spectroscopy measurements revealed that the binding energy of the crucial adsorbed *CO intermediate was lowered through Sn doping, thereby favoring gaseous CO desorption. This observation was confirmed by density functional theory, which further indicated that hydrogen adsorption and subsequent hydrogen evolution were hampered on the Sn-doped electrocatalysts, resulting in boosted CO formation. It was found that the pristine electrocatalysts consisted of CuO nanoparticles decorated with SnO2 domains, as characterized by ex situ high-resolution scanning transmission electron microscopy and X-ray photoelectron spectroscopy measurements. These pristine nanoparticles were subsequently in situ converted into a catalytically active bimetallic Sn-doped Cu phase. Our work sheds light on the intimate relationship between the bimetallic structure and catalytic behavior, resulting in stable and selective oxide-derived Sn-doped Cu electrocatalysts.

Probing the Dynamics of Low-Overpotential CO2-to-CO Activation on Copper Electrodes with Time-Resolved Raman Spectroscopy.

Oxide-derived copper electrodes have displayed a boost in activity and selectivity toward valuable base chemicals in the electrochemical carbon dioxide reduction reaction (CO2RR), but the exact interplay between the dynamic restructuring of copper oxide electrodes and their activity and selectivity is not fully understood. In this work, we have utilized time-resolved surface-enhanced Raman spectroscopy (TR-SERS) to study the dynamic restructuring of the copper (oxide) electrode surface and the adsorption of reaction intermediates during cyclic voltammetry (CV) and pulsed electrolysis (PE). By coupling the electrochemical data to the spectral features in TR-SERS, we study the dynamic activation of and reactions on the electrode surface and find that CO2 is already activated to carbon monoxide (CO) during PE (10% Faradaic efficiency, 1% under static applied potential) at low overpotentials (-0.35 VRHE). PE at varying cathodic bias on different timescales revealed that stochastic CO is dominant directly after the cathodic bias onset, whereas no CO intermediates were observed after prolonged application of low overpotentials. An increase in cathodic bias (-0.55 VRHE) resulted in the formation of static adsorbed CO intermediates, while the overall contribution of stochastic CO decreased. We attribute the low-overpotential CO2-to-CO activation to a combination of selective Cu(111) facet exposure, partially oxidized surfaces during PE, and the formation of copper-carbonate-hydroxide complex intermediates during the anodic pulses. This work sheds light on the restructuring of oxide-derived copper electrodes and low-overpotential CO formation and highlights the power of the combination of electrochemistry and time-resolved vibrational spectroscopy to elucidate CO2RR mechanisms.

Waste-Derived Copper-Lead Electrocatalysts for CO2 Reduction.

It remains a real challenge to control the selectivity of the electrocatalytic CO2 reduction (eCO2R) reaction to valuable chemicals and fuels. Most of the electrocatalysts are made of non-renewable metal resources, which hampers their large-scale implementation. Here, we report the preparation of bimetallic copper-lead (CuPb) electrocatalysts from industrial metallurgical waste. The metal ions were extracted from the metallurgical waste through simple chemical treatment with ammonium chloride, and CuxPby electrocatalysts with tunable compositions were fabricated through electrodeposition at varying cathodic potentials. X-ray spectroscopy techniques showed that the pristine electrocatalysts consist of Cu0, Cu1+ and Pb2+ domains, and no evidence for alloy formation was found. We found a volcano-shape relationship between eCO2R selectivity toward two electron products, such as CO, and the elemental ratio of Cu and Pb. A maximum Faradaic efficiency towards CO was found for Cu9.00Pb1.00, which was four times higher than that of pure Cu, under the same electrocatalytic conditions. In situ Raman spectroscopy revealed that the optimal amount of Pb effectively improved the reducibility of the pristine Cu1+ and Pb2+ domains to metallic Cu and Pb, which boosted the selectivity towards CO by synergistic effects. This work provides a framework of thinking to design and tune the selectivity of bimetallic electrocatalysts for CO2 reduction through valorization of metallurgical waste.

Sub-Second Time-Resolved Surface-Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO2 Reduction on Copper.

The electrocatalytic carbon dioxide (CO2 ) reduction reaction (CO2 RR) into hydrocarbons is a promising approach for greenhouse gas mitigation, but many details of this dynamic reaction remain elusive. Here, time-resolved surface-enhanced Raman spectroscopy (TR-SERS) is employed to successfully monitor the dynamics of CO2 RR intermediates and Cu surfaces with sub-second time resolution. Anodic treatment at 1.55 V vs. RHE and subsequent surface oxide reduction (below -0.4 V vs. RHE) induced roughening of the Cu electrode surface, which resulted in hotspots for TR-SERS, enhanced time resolution (down to ≈0.7 s) and fourfold improved CO2 RR efficiency toward ethylene. With TR-SERS, the initial restructuring of the Cu surface was followed (<7 s), after which a stable surface surrounded by increased local alkalinity was formed. Our measurements revealed that a highly dynamic CO intermediate, with a characteristic vibration below 2060 cm-1 , is related to C-C coupling and ethylene production (-0.9 V vs. RHE), whereas lower cathodic bias (-0.7 V vs. RHE) resulted in gaseous CO production from isolated and static CO surface species with a distinct vibration at 2092 cm-1 .