Tuning the Selectivity of Liquid Products of CO2RR by Cu-Ag Alloying

Revealing the structural evolution of CuAg composites during electrochemical carbon monoxide reduction

Comprehending the catalyst structural evolution during the electrocatalytic process is crucial for establishing robust structure/performance correlations for future catalysts design. Herein, we interrogate the structural evolution of a promising Cu-Ag oxide catalyst precursor during electrochemical carbon monoxide reduction. By using extensive in situ and ex situ characterization techniques, we reveal that the homogenous oxide precursors undergo a transformation to a bimetallic composite consisting of small Ag nanoparticles enveloped by thin layers of amorphous Cu. We believe that the amorphous Cu layer with undercoordinated nature is responsible for the enhanced catalytic performance of the current catalyst composite. By tuning the Cu/Ag ratio in the oxide precursor, we find that increasing the Ag concentration greatly promotes liquid products formation while suppressing the byproduct hydrogen. CO2/CO co-feeding electrolysis and isotopic labelling experiments suggest that high CO concentrations in the feed favor the formation of multi-carbon products. Overall, we anticipate the insights obtained for Cu-Ag bimetallic systems for CO electroreduction in this study may guide future catalyst design with improved performance.

Ag1+ incorporation via a Zr4+-anchored metalloligand: fine-tuning catalytic Ag sites in Zr/Ag bimetallic clusters for enhanced eCO2RR-to-CO activity

Attaining meticulous dominion over the binding milieu of catalytic metal sites remains an indispensable pursuit to tailor product selectivity and elevate catalytic activity. By harnessing the distinctive attributes of a Zr4+-anchored thiacalix[4]arene (TC4A) metalloligand, we have pioneered a methodology for incorporating catalytic Ag1+ sites, resulting in the first Zr-Ag bimetallic cluster, Zr2Ag7, which unveils a dualistic configuration embodying twin {ZrAg3(TC4A)2} substructures linked by an {AgSal} moiety. This cluster unveils a trinity of discrete Ag sites: a pair ensconced within {ZrAg3(TC4A)2} subunits and one located between two units. Expanding the purview, we have also crafted ZrAg3 and Zr2Ag2 clusters, meticulously mimicking the two Ag site environment inherent in the {ZrAg3(TC4A)2} monomer. The distinct structural profiles of Zr2Ag7, ZrAg3, and Zr2Ag provide an exquisite foundation for a precise comparative appraisal of catalytic prowess across three Ag sites intrinsic to Zr2Ag7. Remarkably, Zr2Ag7 eclipses its counterparts in the electroreduction of CO2, culminating in a CO faradaic efficiency (FECO) of 90.23% at -0.9 V. This achievement markedly surpasses the performance metrics of ZrAg3 (FECO: 55.45% at -1.0 V) and Zr2Ag2 (FECO: 13.09% at -1.0 V). Utilizing in situ ATR-FTIR, we can observe reaction intermediates on the Ag sites. To unveil underlying mechanisms, we employ density functional theory (DFT) calculations to determine changes in free energy accompanying each elementary step throughout the conversion of CO2 to CO. Our findings reveal the exceptional proficiency of the bridged-Ag site that interconnects paired {ZrAg3(TC4A)2} units, skillfully stabilizing *COOH intermediates, surpassing the stabilization efficacy of the other Ag sites located elsewhere. The invaluable insights gleaned from this pioneering endeavor lay a novel course for the design of exceptionally efficient catalysts tailored for CO2 reduction reactions, emphatically underscoring novel vistas this research unshrouds.

Tuning Carbon Dioxide Reduction Reaction Selectivity of Bi Single-Atom Electrocatalysts with Controlled Coordination Environments

Control over product selectivity of the electrocatalytic CO2 reduction reaction (CO2RR) is a crucial challenge for the sustainable production of carbon-based chemical feedstocks. In this regard, single-atom catalysts (SACs) are promising materials due to their tunable coordination environments, which could enable tailored catalytic activities and selectivities, as well as new insights into structure-activity relationships. However, direct evidence for selectivity control via systematic tuning of the SAC coordination environment is scarce. In this work, we have synthesized two differently coordinated Bi SACs anchored to the same host material (carbon black) and characterized their CO2RR activities and selectivities. We find that oxophilic, oxygen-coordinated Bi atoms produce HCOOH, while nitrogen-coordinated Bi atoms generate CO. Importantly, use of the same support material assured that alternation of the coordination environment is the dominant factor for controlling the CO2RR product selectivity. Overall, this work demonstrates the structure-activity relationship of Bi SACs, which can be utilized to establish control over CO2RR product distributions, and highlights the promise for engineering atomic coordination environments of SACs to tune reaction pathways.

High selective electrocatalytic reduction of carbon dioxide to ethylene enabled by regulating the microenvironment over Cu-Ag nanowires

Copper-based tandem catalysts are effective candidates for yielding multi-carbon (C2+) products in electrochemical reduction of carbon dioxide (CO2RR). However, these catalysts still face a significant challenge regarding in the low selectivity for the production of a specific product. In this study, we report a high selectivity of 77.8 %±2 % at -1.0 V (vs RHE) for the production of C2H4 by using a Cu88Ag12NW catalyst which is primarily prepared through a combined Cu-Ag co-deposition and wet chemical method, employing an attractive strategy focused on regulating the microenvironment over Cu-Ag nanowires. The experimental and computational studies show that the higher *CO coverage and lower intermediate adsorption energy are important reasons for achieving the high C2H4 selectivity of Cu88Ag12NW catalyst. Comsol simulation results indicate that dense nanowires exhibit a nano-limiting effect on OH- ions, thereby leading to an increase in local pH and promoting coupling reactions. The catalyst demonstrates no noticeable decrease in current density or selectivity even after 12 h of continuous operation. The Cu-Ag nanowire composite exhibits remarkable catalytic activity, superior faradaic efficiency, excellent stability, and easy synthesis, which highlights its significant potential for electro-reducing carbon dioxide into valuable products.

Regulation Strategy of Nanostructured Engineering on Indium-Based Materials for Electrocatalytic Conversion of CO2

Electrochemical carbon dioxide reduction (CO2 RR), as an emerging technology, can combine with sustainable energies to convert CO2 into high value-added products, providing an effective pathway to realize carbon neutrality. However, the high activation energy of CO2 , low mass transfer, and competitive hydrogen evolution reaction (HER) leads to the unsatisfied catalytic activity. Recently, Indium (In)-based materials have attracted significant attention in CO2 RR and a series of regulation strategies of nanostructured engineering are exploited to rationally design various advanced In-based electrocatalysts, which forces the necessary of a comprehensive and fundamental summary, but there is still a scarcity. Herein, this review provides a systematic discussion of the nanostructure engineering of In-based materials for the efficient electrocatalytic conversion of CO2 to fuels. These efficient regulation strategies including morphology, size, composition, defects, surface modification, interfacial structure, alloying, and single-atom structure, are summarized for exploring the internal relationship between the CO2 RR performance and the physicochemical properties of In-based catalysts. The correlation of electronic structure and adsorption behavior of reaction intermediates are highlighted to gain in-depth understanding of catalytic reaction kinetics for CO2 RR. Moreover, the challenges and opportunities of In-based materials are proposed, which is expected to inspire the development of other effective catalysts for CO2 RR.

Pulsed Electrolysis Promotes CO2 Reduction to Ethanol on Heterostructured Cu2O/Ag Catalysts

The electrochemical conversion of carbon dioxide (CO2) into ethanol with high added value has attracted increasing attention. Here, an efficient catalyst with abundant Cu2O/Ag interfaces for ethanol production under pulsed CO2 electrolysis is reported, which is composed of Cu2O hollow nanospheres loaded with Ag nanoparticles (named as se-Cu2O/Ag). The CO2-to-ethanol Faradaic efficiency is prominently improved to 46.3% at a partial current density up to 417 mA cm-2 under pulsed electrolysis conditions in a neutral flow cell, notably outperforming conventional Cu catalysts during static electrolysis. In situ spectroscopy reveals the stabilized Cu+ species of se-Cu2O/Ag during pulsed electrolysis and the enhanced adsorbed CO intermediate (*CO)coverage on the heterostructured catalyst. Density functional theory (DFT) calculations further confirm that the Cu2O/Ag heterostructure stabilizes the *CO intermediate and promotes the coupling of *CO and adsorbed CH intermediate (*CH). Meanwhile, the stable Cu+ species under pulsed electrolysis favor the hydrogenation of adsorbed HCCOH intermediate (*HCCOH) to adsorbed HCCHOH intermediate (*HCCHOH) on the pathway to ethanol. The synergistic effect between the enhanced generation of *CO on Cu2O/Ag and regenerated Cu+ species under pulsed electrolysis steers the reaction pathway toward ethanol. This work provides some insights into selective ethanol production from CO2 electroreduction via combined catalyst design and non-steady state electrolysis.

Engineering the Inhomogeneity of Metal-Insulator-Semiconductor Junctions for Photoelectrochemical Methanol Oxidation

Si-based inhomogeneous metal-insulator-semiconductor (MIS) junctions with a discontinuous metal nanostructure on the Si/insulator layer are expected to be efficient photoelectrodes for solar energy conversion. However, the formation of a metal nanostructure with an optimized arrangement on semiconductors for efficient charge carrier collection is still a big challenge. Herein, we report a method for the in situ formation of an n-Si inhomogeneous MIS junction with well-dispersed metal nanocontacts through a self-assembly process during photoelectrochemical (PEC) methanol oxidation. The photovoltage shows a strong dependence on the inhomogeneity of the n-Si MIS junction, which can be precisely tuned by the applied electrode potential and operation time. The appropriate inhomogeneity of the Schottky junction as well as the high barrier regions induced by the metal oxide/(oxy)hydroxide layer synergistically produces a large photovoltage of 500 mV for the n-Si inhomogeneous MIS junction. Finally, the n-Si-based photoanode is coupled with a CO2-to-formate reaction to realize the production of formate at both electrodes, resulting in a high faradic efficiency (FE) of 86 and 93% for anode and cathode reactions at an operational current of 30 mA/cm2, respectively. These findings provide important insights into the design of highly efficient inhomogeneous MIS junctions through an in situ self-assembly route for solar energy conversion and storage.

Active-site stabilized Bi metal-organic framework-based catalyst for highly active and selective electroreduction of CO2 to formate over a wide potential window

Bismuth-based materials have been validated to be a kind of effective electrocatalyst for electrocatalytic CO2 reduction (ECR) to formate (HCOO-). However, the established studies still encounter the problems of low current density, low selectivity, narrow potential window, and poor catalyst stability. Herein, a bismuth-terephthalate framework (Bi-BDC MOF) material was successfully synthesized. The optimized Bi-BDC-120 °C exhibited excellent activity, selectivity, and durability for formate production. At an operating potential of -1.1 V vs. RHE in 0.1 mol L-1 KHCO3 electrolyte, the ECR catalyzed by Bi-BDC-120 °C achieved a Faraday efficiency (FE) of 97.2% towards formate generation, and the total current density reached about 30 mA cm-2. The operating potential window with FEformate values > 95% ranged in -0.9 to -1.5 V vs. RHE. The density-functional theory (DFT) calculation demonstrated that the (001) crystalline planes of Bi-BDC are preferable for the adsorption of CO2 and the conversion of *OCHO intermediates, thus ultimately promoting the electrocatalytic production of formate. Although the MOF structure of Bi-BDC-120 °C was insufficiently stabilized, the FEformate could be maintained at around 90% after 36 h of ECR operation. The long-term durability for formate production was attributed to the fact that the in situ reconstructed Bi2O2CO3 could retain the Bi-O active sites in the structure. These results offer an opportunity to design CO2 reduction electrocatalysts with high activity and selectivity for potential applications.

Controlling the C1/C2+ product selectivity of electrochemical CO2 reduction upon tuning bimetallic CuIn electrocatalyst composition and operating conditions

Electrochemical carbon dioxide (CO2) reduction (eCO2R) over Cu-based bimetallic catalysts is a promising technique for converting CO2 into value-added multi-carbon products, such as fuels, chemicals, and materials. For improving the process efficiency, electrocatalyst development for the eCO2R must be integrated with tuning of operating conditions. For example, CuIn-based materials typically lead to preferential C1 product selectivity, which delivers the desired C2+ products upon varying the In/Cu ratio and operating conditions (i.e., in 0.1 M KHCO3 electrolytes using an H-type cell with a cation exchange membrane vs. in 1 M KOH electrolytes using a flow cell with an anion exchange membrane). At lower Cu-loading (i.e., InCu5Ox material), the maximum faradaic efficiency of HCOOH (FEHCOOH) of 70% was achieved at -1 V versus the reversible hydrogen electrode (vs. RHE) in an H-type cell. However, upon increasing the Cu loading, the preferential product selectivity could be altered: the InCu73Ox material led to a high CO selectivity (maximum FE of 51%) in the H-type cell at -0.8 V vs. RHE and delivered a current density of 100 mA cm-2 with a FEC2+ of up to 37% at -0.8 V vs. RHE in the flow cell configuration. Various characterization tools were also employed to probe the catalytic materials to rationalize the electrocatalytic performance.

Photoelectrocatalytic hydrogen evolution reaction stimulated by the surface plasmon resonance effect of copper and silver surrounded with MoS2

Developing plasmonic metal-based photocatalysts can improve the photoelectrocatalytic hydrogen evolution reaction (HER) and overcome the limitations of semiconductor-based photocatalysts. A MoS2@Ag-Cu foam composite electrode was fabricated on a copper foam and employed for photoelectrocatalytic HER. The optical behavior and photoelectrocatalytic HER test results indicate that the surface plasmon resonance effect of copper and silver is the primary source of light absorption. Additionally, molybdenum sulfide was employed as a hot-electron trap to capture the energetic electrons generated from copper and silver, thereby promoting the hydrogen evolution reaction. Its binder-free electrode exhibits the better HER performance and excellent stability. Low-cost plasmonic metals, copper and silver, were used as the source of photocatalysis, providing a novel perspective for enhancing photoelectrocatalytic HER performance.

Computational screening of effective g-C3N4 based single atom electrocatalysts for the selective conversion of CO2

Two-dimensional (2D) material-based single-atom catalysts (SACs) have demonstrated their potential in electrochemical reduction reactions but exploring suitable 2D material-based SACs for the CO₂ reduction reaction (CO₂RR) by experiments is still a formidable task. In this study, theoretical screening of transition metal (TM)-doped graphitic carbon nitride (g-C₃N₄) materials as catalysts for the CO₂RR was systematically performed based on density functional theory (DFT) calculations. An indicator for the selective formation of one carbon (C1) products was developed to screen catalysts that are active and selective in the CO₂RR. The results indicated that Ti- and Ag-g-C₃N₄ demonstrate excellent catalytic activity and selectivity for the formation of CO and HCOOH, with limiting potentials of -0.330 and -0.096 V, respectively, while Cr-g-C₃N₄ exhibits the highest catalytic activity for yielding CH₃OH and CH₄ (-0.355 and -0.420 V, respectively), but none of the screened catalysts have been identified as ideal candidates for the selective production of CH₃OH and CH₄. Furthermore, Bader charge analysis suggested that excessive electron transfer from TM leads to stronger adsorption of intermediates and high limiting potentials, which subsequently result in lower catalytic activity. This work provides theoretical insights into the effective screening of active and selective 2D material-based SACs which has the potential to significantly reduce the time and resources required for the discovery of novel electrocatalysts for the controlled formation of various products.

Copper-based catalysts for the electrochemical reduction of carbon dioxide: progress and future prospects

There is an urgent need for the development of high performance electrocatalysts for the CO₂ reduction reaction (CO₂RR) to address environmental issues such as global warming and achieve carbon neutral energy systems. In recent years, Cu-based electrocatalysts have attracted significant attention in this regard. The present review introduces fundamental aspects of the electrocatalytic CO₂RR process together with a systematic examination of recent developments in Cu-based electrocatalysts for the electroreduction of CO₂ to various high-value multicarbon products. Current challenges and future trends in the development of advanced Cu-based CO₂RR electrocatalysts providing high activity and selectivity are also discussed.

Theoretical Investigation of Cu-Au Alloy for Carbon Dioxide Electroreduction: Cu/Au Ratio Determining C1/C2 Selectivity

Copper-gold alloy exhibits excellent catalytic performance for the carbon dioxide electroreduction (CO₂ER) reaction, but the mechanism of the effect of the Cu/Au ratio on the selectivity of C₁/C₂ products has not been carefully investigated. In this work, (100) and (111) surfaces of three CuAu alloys with different Cu/Au (3:1, 1:1, 1:3) ratios are constructed. The properties of CuAu surfaces like density of states, Bader charge, and the whole CO₂ER to C₂H₄ and C₂H₅OH mechanisms are investigated. Our calculation reveals that the adsorption capacity of the catalyst surface for the intermediates *COOH and *CO is enhanced with the increase of the Au ratio. The calculation results show that the Cu1Au1(100) surface has the highest activity for CO₂ER to CO (U_L = -0.32 V). Furthermore, the Cu3Au1(100) surface exhibits the best coupling performance, and ethanol is the dominant product for CO₂ER to C₂ products. Our work provides a useful guideline for further developing CO₂ER electrocatalysts.