Selective CO2 Electromethanation on Surface-Modified Cu Catalyst by Local Microenvironment Modulation

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.

Local reaction environment in electrocatalysis

Beyond conventional electrocatalyst engineering, recent studies have unveiled the effectiveness of manipulating the local reaction environment in enhancing the performance of electrocatalytic reactions. The general principles and strategies of local environmental engineering for different electrocatalytic processes have been extensively investigated. This review provides a critical appraisal of the recent advancements in local reaction environment engineering, aiming to comprehensively assess this emerging field. It presents the interactions among surface structure, ions distribution and local electric field in relation to the local reaction environment. Useful protocols such as the interfacial reactant concentration, mass transport rate, adsorption/desorption behaviors, and binding energy are in-depth discussed toward modifying the local reaction environment. Meanwhile, electrode physical structures and reaction cell configurations are viable optimization methods in engineering local reaction environments. In combination with operando investigation techniques, we conclude that rational modifications of the local reaction environment can significantly enhance various electrocatalytic processes by optimizing the thermodynamic and kinetic properties of the reaction interface. We also outline future research directions to attain a comprehensive understanding and effective modulation of the local reaction environment.

Favoring CO Intermediate Stabilization and Protonation by Crown Ether for CO2 Electromethanation in Acidic Media

The large-scale deployment of CO2 electroreduction is hampered by deficient carbon utilization in neutral and alkaline electrolytes due to CO2 loss into (bi)carbonates. Switching to acidic media mitigates carbonation, but suffers from low product selectivity because of hydrogen evolution. Here we report a crown ether decoration strategy on a Cu catalyst to enhance carbon utilization and selectivity of CO2 methanation under acidic conditions. Macrocyclic 18-Crown-6 is found to enrich potassium cations near the Cu electrode surface, simultaneously enhancing the interfacial electric field to stabilize the *CO intermediate and accelerate water dissociation to boost *CO protonation. Remarkably, the mixture of 18-Crown-6 and Cu nanoparticles affords a CH4 Faradaic efficiency of 51.2 % and a single pass carbon efficiency of 43.0 % toward CO2 electroreduction in electrolyte with pH=2. This study provides a facile strategy to promote CH4 selectivity and carbon utilization by modifying Cu catalysts with supramolecules.

Highly Stable Layered Coordination Polymer Electrocatalyst toward Efficient CO2 -to-CH4 Conversion

Cu2+ -based materials, a class of promising catalysts for the electrocatalytic carbon dioxide reduction reaction (CO2 RR) to value-added chemicals, usually undergo inevitable and uncontrollable reorganization processes during the reaction, resulting in catalyst deactivation or the new active sites formation and bringing great challenges to exploring their structure-performance relationships. Herein, a facile strategy is reported for constructing Cu2+ and 3, 4-ethylenedioxythiophene (EDOT) coordination to stabilize Cu2+ ions to prepare a novel layered coordination polymer (CuPEDOT). CuPEDOT enables selective reduction of CO2 to CH4 with 62.7% Faradaic efficiency at the current density of 354 mA cm-2 in a flow cell, and the catalyst is stable for at least 15 h. In situ spectroscopic characterization and theoretical calculations reveal that CuPEDOT catalyst can maintain the Cu2+ -EDOT coordination structurally stable in CO2 RR and significantly promote the further hydrogenation of *CO intermediates, favoring the formation of CH4 instead of dimerization to C2 products. The strong coordination between EDOT and Cu2+ prevents the reduction of Cu2+ ions during CO2 RR. The finding of this work provides a new perspective on designing molecularly stable, highly active catalysts for CO2 RR.

Gaining Deep Understanding of Electrochemical CO2 RR with In Situ/Operando Techniques

Electrocatalysis for CO2 conversion has been extensively studied to mitigate the energy shortage and environmental issues, which are gaining ever-increasing attention. However, the complicated CO2 reduction process and the dynamic evolution occurring on electrocatalyst surface make it hard to understand the catalytic mechanism. The development of advanced in situ/operando techniques intelligently coupled with electrochemical cells sheds light on the related study via capturing surface atomic rearrangement, tracing chemical state change of catalysts, monitoring the behavior of intermediates and products, and depicting microenvironment near the electrode surface. In this review, fundamentals of the state-of-the-art in situ/operando techniques are clarified first. Case studies on the in situ/operando techniques performed to probe the CO2 reduction reaction processes are then discussed in detail. Finally, conclusions and outlook on this field are presented.

Organic-moiety-engineering on copper surface for carbon dioxide reduction

Electrochemical conversion of carbon dioxide (CO₂) into value-added products powered by sustainable electricity is considered as one of the most promising strategies for carbon neutrality. Among the products, hydrocarbons, especially ethylene and ethanol are the most desired species due to their wide industrial applications. Copper-based catalysts are currently the very limited option available for catalyzing the reduction of CO₂ to multi-carbon products. How to enhance the selectivity and current density is the focus in both academia and industry. In recent years, some organic molecules, oligomers and polymers with well-defined structures have been applied and demonstrated to be effective on enhancing electrocatalytic activity of copper catalysts. However, the molecular/copper interaction and CO₂ molecules' behavior at the hetero-interface remain unclear. In this review, we classify the different organic materials which have been applied in the field of electrochemical CO₂ reduction. We focus on the regulation of local microenvironment on the copper surface by organic compounds, including surface hydrophobicity, local electric field, local pH, and coverage of intermediates etc. The relationship between local microenvironment and catalytic activity is specifically discussed. This review could provide guidance for the development of more organic/inorganic hybrid catalysts for further promoting CO₂ reduction reaction.

Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu

Self-assembled monolayers (SAMs) of thiolates have increasingly been used for modification of metal surfaces in electrochemical applications including selective catalysis (e.g., CO₂ reduction, nitrogen reduction) and chemical sensing. Here, the stable electrochemical potential window of thiolate SAMs on Au, Pt, and Cu electrodes is systematically studied for a variety of thiols in aqueous electrolyte systems. For fixed tail-group functionality, the reductive stability of thiolate SAMs is found to follow the trend Au < Pt < Cu; this can be understood by considering the combined influences of the binding strength of sulfur and competitive adsorption of hydrogen. The oxidative stability of thiolate SAMs is found to follow the order: Cu < Pt < Au, consistent with each surface's propensity toward surface oxide formation. The stable reductive and oxidative potential limits are both found to vary linearly with pH, except for reduction above pH ∼10, which is independent of pH for most thiol compositions. The electrochemical stability across different functionalized thiols is then revealed to depend on many different factors including SAM defects (accessible surface metal atom sites decrease stability), intermolecular interactions (hydrophilic groups reduce the stability), and SAM thickness (stability increases with alkanethiol carbon chain length) as well as factors such as SAM-induced surface reconstruction and the ability to directly oxidize or reduce the non-sulfur part of the SAM molecule.

Design of molecular MNC dual-atom catalysts for nitrogen reduction starting from surface state analysis

Under electrocatalytic conditions, the state of a catalyst surface (e.g., adsorbate coverage) can be very different from a pristine form due to the existing conversion equilibrium between water and H- and O-containing adsorbates. Dismissing the analysis of the catalyst surface state under operating conditionsmay lead to misleading guidelines for experiments. Given that confirming the actual active site of the catalyst under operating conditions is indispensable to providing practical guidance for experiments, herein, we analyzed the relations between the Gibbs free energy and the potential of a new type of molecular metal-nitrogen-carbon (MNC) dual-atom catalysts (DACs) with a unique 5 N-coordination environment, by spin-polarized density functional theory (DFT) and surface Pourbaix diagram calculations. Analyzing the derived surface Pourbaix diagrams, we screened out three catalysts, N₃-Ni-Ni-N₂, N₃-Co-Ni-N₂, and N₃-Ni-Co-N₂, to further study the activity of nitrogen reduction reaction (NRR). The results display that N₃-Co-Ni-N₂ is a promising NRR catalyst with a relatively low ΔG of 0.49 eV and slow kinetics of the competing hydrogen evolution. This work proposes a new strategy to guide DAC experiments more precisely: the analysis of the surface occupancy state of the catalysts under electrochemical conditions should be performed before activity analysis.

Asymmetric Low-Frequency Pulsed Strategy Enables Ultralong CO2 Reduction Stability and Controllable Product Selectivity

Copper-based catalysts are widely explored in electrochemical CO₂ reduction (CO₂RR) because of their ability to convert CO₂ into high-value-added multicarbon products. However, the poor stability and low selectivity limit the practical applications of these catalysts. Here, we proposed a simple and efficient asymmetric low-frequency pulsed strategy (ALPS) to significantly enhance the stability and the selectivity of the Cu-dimethylpyrazole complex Cu₃(DMPz)₃ catalyst in CO₂RR. Under traditional potentiostatic conditions, Cu₃(DMPz)₃ exhibited poor CO₂RR performance with the Faradaic efficiency (FE) of 34.5% for C₂H₄ and FE of 5.9% for CH₄ as well as the low stability for less than 1 h. We optimized two distinguished ALPS methods toward CH₄ and C₂H₄, correspondingly. The high selectivities of catalytic product CH₄ (FE_CH4 = 80.3% and above 76.6% within 24 h) and C₂H₄ (FE_C2H4 = 70.7% and above 66.8% within 24 h) can be obtained, respectively. The ultralong stability for 300 h (FE_CH4 > 60%) and 145 h (FE_C2H4 > 50%) was also recorded with the ALPS method. Microscopy (HRTEM, SAED, and HAADF) measurements revealed that the ALPS method in situ generated and stabilized extremely dispersive and active Cu-based clusters (∼2.7 nm) from Cu₃(DMPz)₃. Meanwhile, ex situ spectroscopies (XPS, AES, and XANES) and in situ XANES indicated that this ALPS method modulated the Cu oxidation states, such as Cu(0 and I) with C₂H₄ selectivity and Cu(I and II) with CH₄ selectivity. The mechanism under the ALPS methods was explored by in situ ATR-FTIR, in situ Raman, and DFT computation. The ALPS methods provide a new opportunity to boost the selectivity and stability of CO₂RR.

Highly selective CO2 electroreduction to CO by the synergy between Ni-N-C and encapsulated Ni nanoparticles

Efficient catalysts are highly desirable for the selective electrochemical CO₂ reduction reaction (CO₂RR). Ni single-atom catalysts are known as promising CO₂RR catalysts, while Ni NPs are expected to catalyze the competing HER. In this work, we have modified the Ni NPs by encapsulating them into porous Ni-N-C nanosheets (Ni@Ni-N-C), to boost the synergy between Ni NPs and dispersed Ni-N species towards CO₂RR. The CO faradaic efficiency (FE_CO) reached 96.4% at -0.9 V and retained over 90% in a wide potential window. More importantly, FE_CO values of over 94% have been obtained from -50 to -170 mA cm^-2 with a peak FE_CO of 99% in a flow cell. Our work demonstrates that the surface modification of Ni NPs can inhibit the unexpected HER and activate the surface sites, offering a practical design strategy for CO₂RR catalysts.