Bicarbonate Rebalances the *COOH/*OCO- Dual Pathways in CO2 Electrocatalytic Reduction: In Situ Surface-Enhanced Raman Spectroscopic Evidence

Electrochemical valorization of captured CO2: recent advances and future perspectives

The excessive emission of CO2 has led to severe climate change, prompting global concern. Capturing CO2 and converting it through electrochemistry into value-added products represent promising approaches to mitigating CO2 emissions and closing the carbon cycle. Traditionally, these two processes have been performed independently, involving multiple steps, high energy consumption, and low efficiency. Recently, the electrochemical conversion of captured CO2, which integrates the capture and conversion processes (also referred to as electrochemically reactive CO2 capture), has garnered increasing attention. This integrated approach bypasses the energy-intensive steps involved in the traditional independent process, including CO2 release, purification, compression, transportation, and storage. In this review, we discuss recent advances in the electrochemical conversion of captured CO2, focusing on four key aspects. First, we introduce various capture media, emphasizing the thermodynamic aspects of carbon capture and their implications for integration with electrochemical conversion. Second, we discuss product control mediated by the selection of different catalysts, highlighting the connections between the conversion of captured CO2 and gas-fed CO2. Third, we examine the effect of reactor systems and operational conditions on the electrochemical conversion of captured CO2, shedding light on performance optimization. Finally, we explore real integration systems for CO2 capture and electrochemical conversion, revealing the potential of this new technology for practical applications. Overall, we provide insights into the existing challenges, potential solutions, and thoughts on opportunities and future directions in the emerging field of electrochemical conversion of captured CO2.

GC-DFT-Based Dynamic Product Distribution Reveals Enhanced CO2-to-Methanol Electrocatalysis Durability by Heterogeneous CoPc

Heterogeneous cobalt phthalocyanine has emerged as a promising molecular catalyst for electrochemical reduction of CO2 to methanol. Boosting both electrocatalytic durability and selectivity remains a great challenge, which is more difficult with unknown regulation factors for the HER side reaction. Herein, to discover the key to balancing the durability and selectivity, as well as HER regulation, we carried out GC-DFT calculations, based on which dynamic product distribution modeling was conducted to visually present the variation of the product distribution within the applied voltage range. The strongly electron-donating NMe2-substituted CoPc is found to be an excellent candidate. The dynamic product distribution reveals that the key to selectivity and durability balance is to regulate both the potential of the highest methanol Faradaic efficiency and the corresponding energy barrier of the selectivity-determining step for hydrogenated CoPc. The pivotal factor in HER regulation stems from hindered H adsorption due to ligand hydrogenation, arising from the decreased Co-to-H charge transfer. The dynamic product distribution analysis provides intuitive theoretical guidance for highly selective and durable CO2 electroreduction.

Anion Effect in Electrochemical CO2 Reduction: From Spectators to Orchestrators

The electrochemical CO2 reduction reaction (eCO2RR) offers a pathway to produce valuable chemical fuels from CO2. However, its efficiency in aqueous electrolytes is hindered by the concurrent H2 evolution reaction (HER), which takes place at similar potentials. While the influence of cations on this process has been extensively studied, the influence of anions remains largely unexplored. In this work, we study how eCO2RR selectivity and activity on a gold catalyst are affected by a wide range of inorganic and carboxylate anions. We utilize in situ differential electrochemical mass spectrometry (DEMS) for real-time product monitoring coupled with molecular dynamics (MD) simulations. We show that anions significantly impact eCO2RR kinetics and eCO2RR selectivity. MD simulations reveal a new descriptor─free energy of anion physisorption─where weakly adsorbing anions enable favorable CO2 reduction kinetics, despite the negative charge carried by the electrode surface. By leveraging these fundamental insights, we identify propionate as the most promising anion, achieving nearly 100% Faradaic efficiency while showing high CO production rates that are comparable to those in bicarbonate. These insights underscore the vital role of anion selection in achieving a highly efficient eCO2RR in aqueous electrolytes.

Surfactant-Free Method to Prevent Gold Nanoparticle Aggregation and Its Surface-Enhanced Raman Scattering Applications

The introduction of surfactants to stabilize colloidal citrate-reduced gold nanoparticles (prevent aggregation) is usually used in surface-enhanced Raman scattering (SERS) applications. However, the surfactants have many drawbacks for SERS applications, such as increasing the SERS background and blocking surface active sites. Here, we develop a surfactant-free method to stabilize colloidal cit-AuNPs based on alkali regulation, and this method can prevent gold nanoparticle aggregation under different harsh treatments, including ligand modification, centrifugation-based washing/enrichment, and salt addition. The SERS spectra, density functional theory simulation, and ζ potentials of cit-AuNPs indicate that the stability of enhanced cit-AuNPs under alkaline conditions is attributed to both the increased negative charge density (by ∼6 times from pH 7 to 12) and the molecular configuration on the metal surface. Compared with surfactant-based methods, this method can well maintain the inherent optical and interface properties of nanoparticles, avoid the SERS background, and avoid blocking of the surface active site due to the presence of surfactants. This method will enable AuNPs to have a wide range of applications in areas such as highly sensitive SERS sensors.

Spatially Confined Radical Addition Reaction for Electrochemical Synthesis of Carboxylated Graphene and its Applications in Water Desalination and Splitting

Due to the chemical stability of graphene, synthesis of carboxylated graphene still remains challenging during the electrochemical exfoliation of graphite. In this work, a spatially confined radical addition reaction which occurs in the sub-nanometer scaled interlayers of the expanded graphene sheets for the electrochemical synthesis of highly stable carboxylated graphene is reported. Here, formate anions act as both intercalation ions and co-reactant acid for the confinement of electro-generated carboxylic radical (●COOH) in the sub-nanometer scaled interlayers, which facilitates the radical addition reaction on graphene sheets. The controllable carboxylation of graphene is realized by tuning the concentration of formate anions in the electrolyte solution. The high crystallinity of the obtained product indicates the occurrence of spatially confined ●COOH addition reaction between the sub-nanometer interlayers of expanded graphite. In addition, the carboxylated graphene have been used for water desalination and hydrogen/oxygen reduction reaction. Therefore, this work provides a new method for the in situ preparation of functionalized graphene through the electrolysis and its applications in water desalination and hydrogen/oxygen reduction reactions.

Recent advances in dynamic reconstruction of electrocatalysts for carbon dioxide reduction

Electrocatalysts undergo structural evolution under operating electrochemical CO2 reduction reaction (CO2RR) conditions. This dynamic reconstruction correlates with variations in CO2RR activity, selectivity, and stability, posing challenges in catalyst design for electrochemical CO2RR. Despite increased research on the reconstruction behavior of CO2RR electrocatalysts, a comprehensive understanding of their dynamic structural evolution under reaction conditions is lacking. This review summarizes recent developments in the dynamic reconstruction of catalysts during the CO2RR process, covering fundamental principles, modulation strategies, and in situ/operando characterizations. It aims to enhance understanding of electrocatalyst dynamic reconstruction, offering guidelines for the rational design of CO2RR electrocatalysts.

Insights into Electrochemical CO2 Reduction on Metallic and Oxidized Tin Using Grand-Canonical DFT and In Situ ATR-SEIRA Spectroscopy

Electrochemical CO2 reduction (CO2R) to formate is an attractive carbon emissions mitigation strategy due to the existing market and attractive price for formic acid. Tin is an effective electrocatalyst for CO2R to formate, but the underlying reaction mechanism and whether the active phase of tin is metallic or oxidized during reduction is openly debated. In this report, we used grand-canonical density functional theory and attenuated total reflection surface-enhanced infrared absorption spectroscopy to identify differences in the vibrational signatures of surface species during CO2R on fully metallic and oxidized tin surfaces. Our results show that CO2R is feasible on both metallic and oxidized tin. We propose that the key difference between each surface termination is that CO2R catalyzed by metallic tin surfaces is limited by the electrochemical activation of CO2, whereas CO2R catalyzed by oxidized tin surfaces is limited by the slow reductive desorption of formate. While the exact degree of oxidation of tin surfaces during CO2R is unlikely to be either fully metallic or fully oxidized, this study highlights the limiting behavior of these two surfaces and lays out the key features of each that our results predict will promote rapid CO2R catalysis. Additionally, we highlight the power of integrating high-fidelity quantum mechanical modeling and spectroscopic measurements to elucidate intricate electrocatalytic reaction pathways.

Dynamics of bulk and surface oxide evolution in copper foams for electrochemical CO2 reduction

Oxide-derived copper (OD-Cu) materials exhibit extraordinary catalytic activities in the electrochemical carbon dioxide reduction reaction (CO2RR), which likely relates to non-metallic material constituents formed in transitions between the oxidized and the reduced material. In time-resolved operando experiment, we track the structural dynamics of copper oxide reduction and its re-formation separately in the bulk of the catalyst material and at its surface using X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy. Surface-species transformations progress within seconds whereas the subsurface (bulk) processes unfold within minutes. Evidence is presented that electroreduction of OD-Cu foams results in kinetic trapping of subsurface (bulk) oxide species, especially for cycling between strongly oxidizing and reducing potentials. Specific reduction-oxidation protocols may optimize formation of bulk-oxide species and thereby catalytic properties. Together with the Raman-detected surface-adsorbed *OH and C-containing species, the oxide species could collectively facilitate *CO adsorption, resulting an enhanced selectivity towards valuable C2+ products during CO2RR.

In situ scanning tunneling microscopy studies of carbonate-induced restructuring of Ag-decorated Cu(100) electrodes

Ag-decorated Cu electrocatalysts are of great interest for electrochemical CO2 reduction, because of an increased yield of multi-carbon products. Here, we present studies of well-defined AgCu electrodes by in situ scanning tunneling microscopy. These bimetallic model electrocatalysts are prepared by electrodepositing submonolayer Ag coverages on Cu(100) in 0.1 M H2SO4, resulting in monolayer islands with a hexagonal quasi-Ag(111) atomic lattice. Upon exchanging the solution at potentials in the double layer range to 0.1 M KHCO3, pronounced Ag island restructuring towards anisotropic shapes, the nucleation and growth of new islands, and a strong reduction in surface mobility are observed. In addition, high-resolution images reveal a highly disordered molecular adlayer, contrary to the case of Ag-free Cu(100) electrodes. These observations can be explained by interactions of metal adatoms with adsorbed (bi)carbonate and show that Ag redispersion on Cu electrocatalysts may occur even in the absence of CO2 reduction.

From Traditional to New Benchmark Catalysts for CO2 Electroreduction

In the last century, conventional strategies pursued to reduce or convert CO₂ have shown limitations and, consequently, have been pushing the development of innovative routes. Among them, great efforts have been made in the field of heterogeneous electrochemical CO₂ conversion, which boasts the use of mild operative conditions, compatibility with renewable energy sources, and high versatility from an industrial point of view. Indeed, since the pioneering studies of Hori and co-workers, a wide range of electrocatalysts have been designed. Starting from the performances achieved using traditional bulk metal electrodes, advanced nanostructured and multi-phase materials are currently being studied with the main goal of overcoming the high overpotentials usually required for the obtainment of reduction products in substantial amounts. This review reports the most relevant examples of metal-based, nanostructured electrocatalysts proposed in the literature during the last 40 years. Moreover, the benchmark materials are identified and the most promising strategies towards the selective conversion to high-added-value chemicals with superior productivities are highlighted.