Stabilization effects in binary colloidal Cu and Ag nanoparticle electrodes under electrochemical CO2 reduction conditions

Reaction Environment Regulation for Electrocatalytic CO2 Reduction in Acids

The electrocatalytic CO2 reduction reaction (CO2RR) is a sustainable route for converting CO2 into value-added fuels and feedstocks, advancing a carbon-neutral economy. The electrolyte critically influences CO2 utilization, reaction rate and product selectivity. While typically conducted in neutral/alkaline aqueous electrolytes, the CO2RR faces challenges due to (bi)carbonate formation and its crossover to the anolyte, reducing efficiency and stability. Acidic media offer promise by suppressing these processes, but the low Faradaic efficiency, especially for multicarbon (C2+) products, and poor electrocatalyst stability persist. The effective regulation of the reaction environment at the cathode is essential to favor the CO2RR over the competitive hydrogen evolution reaction (HER) and improve long-term stability. This review examines progress in the acidic CO2RR, focusing on reaction environment regulation strategies such as electrocatalyst design, electrode modification and electrolyte engineering to promote the CO2RR. Insights into the reaction mechanisms via in situ/operando techniques and theoretical calculations are discussed, along with critical challenges and future directions in acidic CO2RR technology, offering guidance for developing practical systems for the carbon-neutral community.

Tuning the CO2 Reduction Selectivity of an Immobilized Molecular Ag Complex beyond CO

The electrochemical reduction of carbon dioxide (CO2) to produce fuels and chemicals has garnered significant attention. However, achieving control over the selectivity of the resulting products remains a challenging task, particularly within molecular systems. In this study, we employed a molecular silver complex immobilized on graphitized mesoporous carbon (GMC) as a catalyst for converting CO2 into CO, achieving an impressive selectivity of over 90% at -1.05 V vs RHE. Notably, the newly formed silver nanoparticles emerged as the active sites responsible for this high CO selectivity rather than the molecular system. Intriguingly, the introduction of copper ions into the restructured Ag-nanoparticle-decorated carbon altered the product selectivity. At -1.1 V vs RHE in 0.1 M KCl, we achieved a high C2 selectivity of 75%. Furthermore, not only the Ag-Cu bimetallic nanoparticle but also the small-sized Ag-Cu nanocluster decorated over GMC was proposed as active sites during catalytic reactions. Our straightforward approach offers valuable insights for fine-tuning the product selectivity of immobilized molecular systems, extending beyond C1 products.

Pd-Decorated Cu2O-Ag Catalyst Promoting CO2 Electroreduction to C2H4 by Optimizing CO Intermediate Adsorption and Hydrogenation

Electrocatalytic CO2 reduction reaction (CO2RR) to high value-added products, such as ethylene (C2H4), offers a promising approach to achieve carbon neutrality. Although recent studies have reported that a tandem catalyst (for example, Cu-Ag systems) exhibits advantage in C2H4 production, its practical application is largely inhibited by the following: (1) a traditional tandem catalyst cannot effectively stabilize the *CO intermediate, resulting in sluggish C-C coupling, and (2) inadequate H2O activation ability hinders the hydrogenation of intermediates. To break through the above bottleneck, herein, palladium (Pd) was introduced into Cu2O-Ag, a typical conventional tandem catalyst, to construct a Cu2O-Pd-Ag ternary catalyst. Extensive experiment and density functional theory calculation prove that Pd can efficiently stabilize the *CO intermediate and promote the H2O activation, which contributes to the C-C coupling and intermediate hydrogenation, the key steps in the conversion of CO2 to C2H4. Beneficial to the efficient synergy of Cu2O, Pd, and Ag, the optimal Cu2O-Pd-Ag ternary catalyst achieves CO2RR toward C2H4 with a faradaic efficiency of 63.2% at -1.2 VRHE, which is higher than that achieved by Cu2O-Ag and most of other reported catalysts. This work is a fruitful exploration of a rare ternary catalyst, providing a new route for constructing an efficient CO2RR electrocatalyst.

Unraveling the synergistic effects of Cu-Ag tandem catalysts during electrochemical CO2 reduction using nanofocused X-ray probes

Controlling the selectivity of the electrocatalytic reduction of carbon dioxide into value-added chemicals continues to be a major challenge. Bulk and surface lattice strain in nanostructured electrocatalysts affect catalytic activity and selectivity. Here, we unravel the complex dynamics of synergistic lattice strain and stability effects of Cu-Ag tandem catalysts through a previously unexplored combination of in situ nanofocused X-ray absorption spectroscopy and Bragg coherent diffraction imaging. Three-dimensional strain maps reveal the lattice dynamics inside individual nanoparticles as a function of applied potential and product yields. Dynamic relations between strain, redox state, catalytic activity and selectivity are derived. Moderate Ag contents effectively reduce the competing evolution of H2 and, concomitantly, lead to an enhanced corrosion stability. Findings from this study evidence the power of advanced nanofocused spectroscopy techniques to provide new insights into the chemistry and structure of nanostructured catalysts.

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.

In situ probing the dynamic reconstruction of copper-zinc electrocatalysts for CO2 reduction

Unravelling the dynamic characterization of electrocatalysts during the electrochemical CO₂ reduction reaction (CO₂RR) is a critical factor to improve the production efficiency and selectivity, since most pre-electrocatalysts undergo structural reconstruction and surface rearrangement under working conditions. Herein, a series of pre-electrocatalysts including CuO, ZnO and two different ratios of CuO/ZnO were systematically designed by a sputtering process to clarify the correlation of the dynamic characterization of Cu sites in the presence of Zn/ZnO and the product profile. The evidence provided by in situ X-ray absorption spectroscopy (XAS) indicated that appropriate Zn/ZnO levels could induce a variation in the coordination number of Cu sites via reversing Ostwald ripening. Specifically, the recrystallized Cu site with a lower coordination number exhibited a preferential production of methane (CH₄). More importantly, our findings provide a promising approach for the efficient production of CH₄ by in situ reconstructing Cu-based binary electrocatalysts.

Sonoelectrosynthesis of monodisperse metal nanoparticles

Traditional colloidal syntheses of metal nanoparticles (NPs) are highly sensitive to the selection of and quality of chemical reducing agents and metal precursors. To address these challenges, we demonstrate the complete sonoelectrochemical synthesis of monodisperse metal NPs starting from bulk metal, using Cu as a model system. Electrochemical syntheses of NPs are of great interest as the oxidation and reduction processes that account for product formation can occur directly at the anode and cathode, respectively. This ability has the potential to improve reproducibility by simplifying the chemical pathway to NPs, with electrosyntheses often also providing unique kinetic pathways toward green product formation. Herein, ultrasound is coupled with electrosynthesis to clean the electrode surface, dispersing the NPs produced at the electrode into solution. We were able to shift the size distribution to form monodispersed metal NPs through control of applied potential (V_applied) and ultrasonic pulses. The synthesis begins with electrooxidation of bulk Cu metal to directly dissolve metal ions into a microemulsion system. This step is followed by sonoelectroreduction of the ions, which facilitates the formation of dispersible, monodisperse Cu NPs with diameters <10 nm. The size distribution can be controlled by adjusting the V_applied, pulse intensity, and pulse sequence implemented during sonoelectroreduction. We view this technique as a scalable method to synthesize metal NPs from bulk metal without chemical reducing agents.

Recent Advances in Bimetallic Cu-Based Nanocrystals for Electrocatalytic CO2 Conversion

An elevated level of anthropogenic CO₂ has been the major cause of global warming, and significant efforts are being made around the world towards the development of CO₂ capture, storage and reuse technologies. Among various CO₂ conversion technologies, electrochemical CO₂ reduction (CO₂ RR) by nanocrystals is one of the most promising strategies as it is facile, quick, and can be integrated with other renewable energy techniques. Judiciously designed catalytic nanomaterials promise to be the next generation of electrochemical electrodes that offer cutting-edge performance, low energy consumption and aid in reducing overall carbon footprint. In this minireview, we highlight the recent developments related to the bimetallic Cu-based nanocatalysts and discuss their structure-property relationships. We focus on the design principles and parameters required for the enhancement of CO₂ conversion efficiency, selectivity, and stability.

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 .