Effects of the Carbon Support on Heterogeneous Molecular Catalysts for Carbon Dioxide Reduction.

Molecular catalysts stabilized on a support material, also called heterogeneous molecular catalysts, exhibit excellent performance in carbon dioxide reduction reaction (CO2 RR). Different support in these electrocatalysts can have a substantial influence on the activity, making support control one tool to enhance the CO2 RR performance. However, a systematic understanding of the support effects is lacking. Taking cobalt phthalocyanine (CoPc) immobilized onto different carbon materials as examples, we demonstrate that the surface area, pore structure and the morphology of the as-prepared heterogeneous molecular catalysts can influence the CO2 transfer and adsorption, and then change the CO2 RR activity. In contrast to the other four materials, CoPc/mesoporous carbon (MC) can efficiently convert carbon dioxide to carbon monoxide at minimal overpotential (-0.8 V vs. RHE) due to its special nanostructure and pore distribution. The results of this study suggest that the performance of electrocatalytic reduction of carbon dioxide can be improved by changing different substrates.

InBi Bimetallic Sites for Efficient Electrochemical Reduction of CO2 to HCOOH.

Formic acid is receiving intensive attention as being one of the most progressive chemical fuels for the electrochemical reduction of carbon dioxide. However, the majority of catalysts suffer from low current density and Faraday efficiency. To this end, an efficient catalyst of In/Bi-750 with InOx nanodots load is prepared on a two-dimensional nanoflake Bi2 O2 CO3 substrate, which increases the adsorption of * CO2 due to the synergistic interaction between the bimetals and the exposure of sufficient active sites. In the H-type electrolytic cell, the formate Faraday efficiency (FE) reaches 97.17% at -1.0 V (vs reversible hydrogen electrode (RHE)) with no significant decay over 48 h. A formate Faraday efficiency of 90.83% is also obtained in the flow cell at a higher current density of 200 mA cm-2 . Both in-situ Fourier transform infrared spectroscopy (FT-IR) and theoretical calculations show that the BiIn bimetallic site can deliver superior binding energy to the * OCHO intermediate, thereby fundamentally accelerating the conversion of CO2 to HCOOH. Furthermore, assembled Zn-CO2 cell exhibits a maximum power of 6.97 mW cm-1 and a stability of 60 h.

Multifunctional Charge and Hydrogen-Bond Effects of Second-Sphere Imidazolium Pendants Promote Capture and Electrochemical Reduction of CO2 in Water Catalyzed by Iron Porphyrins.

Microenvironments tailored by multifunctional secondary coordination sphere groups can enhance catalytic performance at primary metal active sites in natural systems. Here, we capture this biological concept in synthetic systems by developing a family of iron porphyrins decorated with imidazolium (im) pendants for the electrochemical CO2 reduction reaction (CO2 RR), which promotes multiple synergistic effects to enhance CO2 RR and enables the disentangling of second-sphere contributions that stem from each type of interaction. Fe-ortho-im(H), which poises imidazolium units featuring both positive charge and hydrogen-bond capabilities proximal to the active iron center, increases CO2 binding affinity by 25-fold and CO2 RR activity by 2000-fold relative to the parent Fe tetraphenylporphyrin (Fe-TPP). Comparison with monofunctional analogs reveals that through-space charge effects have a greater impact on catalytic CO2 RR performance compared to hydrogen bonding in this context.

Grain Boundary-Rich Copper Nanocatalysts Generated from Metal-Organic Framework Nanoparticles for CO2 -to-C2+ Electroconversion.

Due to severe contemporary energy issues, generating C2+ products from electrochemical carbon dioxide reduction reactions (eCO2 RRs) gains much interest. It is known that the catalyst morphology and active surface structures are critical for product distributions and current densities. Herein, a synthetic protocol of nanoparticle morphology on copper metal-organic frameworks (n-Cu MOFs) is developed by adjusting growth kinetics with termination ligands. Nanoscale copper oxide aggregates composed of small particulates are yielded via calcining the Cu-MOF nanoparticles at a specific temperature. The resulting nanosized MOF-derived catalyst (n-MDC) exhibits Faradaic efficiencies toward ethylene and C2+ products of 63% and 81% at -1.01 V versus reversible hydrogen electrode (RHE) in neutral electrolytes. The catalyst also shows prolonged stability for up to 10 h. A partial current density toward C2+ products is significantly boosted to -255 mA cm-2 in an alkaline flow cell system. Comprehensive analyses reveal that the nanoparticle morphology of pristine Cu MOFs induces homogeneous decomposition of organic frameworks at a lower calcination temperature. It leads to evolving grain boundaries in a high density and preventing severe agglomeration of copper domains, the primary factors for improving eCO2 RR activity toward C2+ production.

Zn-Sb Bimetallic Electrocatalyst Enhances the Conversion of CO2 to Formate.

It was discovered the bimetallic Zn-Sb nanoparticles supported on carbon nanotubes can electrocatalyze CO2 into formate efficiently, comprising the best performance to date for Sb-based catalysts under moderate overpotential. This project was accomplished by a versatile two-step alcoholysis precipitation strategy with tunable Zn : Sb ratios, and the performance of optimized Zn2.33 Sb0.67 O4 was locked in the electrochemical CO2 reduction reaction. During the subsequent electrolysis, the mixed phases of metallic Zn and Sb served as active centers. The Zn-Sb heterostructure and the electron relocation were confirmed. By means of interactions and possible additional binding sites for reaction intermediate *OCHO, the material displayed different catalytic properties from either Zn or Sb, and was selective for formate up to 92%, which was ca. 6.1 times or 4.6 times than that of each single component. The encouraging results highlight the power of the interaction between binary metallic components to synergistically electrocatalyze CO2 conversion.

Indium doped bismuth subcarbonate nanosheets for efficient electrochemical reduction of carbon dioxide to formate in a wide potential window.

Electrochemical carbon dioxide (CO2) reduction reaction (E-CO2RR) to formate with high selectivity driven by renewable electricity is one of the most promising routes to carbon neutrality. Herein, we developed a novel indium (In)-doped bismuth subcarbonate (BOC) nanosheets (BOC-In-x NSs) through transformation of In-doped bismuth (Bi) nanoblocks (Bi-In-x NBs). The BOC-In-0.1 NSs achieved a maximum Faraday efficiency of formate (FEformate) nearly 100% with high stability (22 h) and an appreciable average FEformate of 93.5% in a wide potential window of 450 mV. The experimental and theoretical calculations indicate that the incorporation of In into BOC nanosheets enhanced the adsorption of CO2 and the intermediates during the process of E-CO2RR, and reduced the energy barrier for the formation of formate.

Bimetallic Gold-Silver Nanostructures Drive Low Overpotentials for Electrochemical Carbon Dioxide Reduction.

Alloy formation is an advanced approach to improve desired properties that the monoelements cannot achieve. Alloys are usually designed to tailor intrinsic natures or induce synergistic effects by combining materials with distinct properties. Indeed, unprecedented properties have emerged in many cases, superior to a simple sum of pure elements. Here, we present Au-Ag alloy nanostructures with prominent catalytic properties in an electrochemical carbon dioxide reduction reaction (eCO2RR). The Au-Ag hollow nanocubes are prepared by galvanic replacement of Au on Ag nanocubes. When the Au-to-Ag ratio is 1:1 (Au1Ag1), the alloy hollow nanocubes exhibit maximum Faradaic efficiencies of CO production in a wide potential range and high mass activity and CO current density superior to those of the bare metals. In particular, overpotentials are estimated to be similar to or lower than that of the Au catalyst under various standard metrics. Density functional theory calculations, machine learning, and a statistical consideration demonstrate that the optimal configuration of the *COOH intermediate is a bidentate coordination structure where C binds to Au and O binds to Ag. This active Au-Ag neighboring configuration has a maximum population and enhanced intrinsic catalytic activity on the Au1Ag1 surface among other Au-to-Ag compositions, in good agreement with the experimental results. Further application of Au1Ag1 to a membrane electrode assembly cell at neutral conditions shows enhanced CO Faradaic efficiency and current densities compared to Au or Ag nanocubes, indicating the possible extension of Au-Ag alloys to larger electrochemical systems. These results give a new insight into the synergistic roles of Au and Ag in the eCO2RR and offer a fresh direction toward a rational design of bimetallic catalysts at a practical scale.

Low-Crystalline AuCuIn Catalyst for Gaseous CO2 Electrolyzer.

Despite its importance for the establishment of a carbon-neutral society, the electrochemical reduction of CO2  to value-added products has not been commercialized yet because of its sluggish kinetics and low selectivity. The present work reports the fabrication of a low-crystalline trimetallic (AuCuIn) CO2  electroreduction catalyst and demonstrates its high performance in a gaseous CO2  electrolyzer. The high Faradaic efficiency (FE) of CO formation observed at a low overpotential in a half-cell test is ascribed to the controlled crystallinity and composition of this catalyst as well as to its faster charge transfer, downshifted d-band center, and low oxophilicity. The gaseous CO2  electrolyzer with the optimal catalyst as the cathode exhibits superior cell performance with a high CO FE and production rate, outperforming state-of-the-art analogs. Thus, the obtained results pave the way to the commercialization of CO2  electrolyzers and promote the establishment of a greener society.

Silk fibroin-derived carbon aerogels embedded with copper nanoparticles for efficient electrocatalytic CO2-to-CO conversion.

Metal-carbon matrix catalyst has attracted a great deal of interest in electrochemical carbon dioxide reduction reaction (CO2RR) due to its excellent electrocatalytic performance. However, the design of highly active metal-carbon matrix catalyst towards CO2RR using natural biomass and cheap chemical precursors is still under challenge. Herein, a self-assembly strategy, along with CO2 gas as acidifying agent, to fabricate silk fibroin (SF) derived carbon aerogels (CA) combining trace copper nanoparticles (SF-Cu/CA) is developed. Zinc nitrate was introduced as a pore-forming agent to further optimize the pore structure of the as-prepared catalysts to form SF-Cu/CA-1. The rich mesoporous structure and unique constitute of SF-Cu/CA-1 is conducive to exposed numerous active sites, fast electron transfer rate, and the desorption of *CO intermediate, thus leading to the electrocatalytic CO2RR of SF-Cu/CA-1 catalyst with an excellent current density of 29.4 mA cm-2, Faraday efficiency of 83.06% towards carbon monoxide (CO), high the ratio value of CO/H2 (19.58), and a long-term stability over a 10-hour period. This performance is superior to that of SF-Cu/CA catalyst (13.0 mA cm-2, FECO=58.43%, CO/H2 = 2.16). This work not only offers a novel strategy using natural biomass and cheap chemicals to build metal-carbon matrix catalyst for electrocatalytic CO2-to-CO conversion, but also is expected to promote the industrial-scale implementations of CO2 electroreduction.

Synergy effects on Sn-Cu alloy catalyst for efficient CO2 electroreduction to formate with high mass activity.

To acquire the synergy effects between Sn and Cu for the jointly high Faradaic efficiency and current density, we develop a novel strategy to design the Sn-Cu alloy catalyst via a decorated co-electrodeposition method for CO2 electroreduction to formate. The Sn-Cu alloy shows high formate Faradaic efficiency of 82.3% ± 2.1% and total C1 products Faradaic efficiency of 90.0% ± 2.7% at -1.14 V vs. reversible hydrogen electrode (RHE). The current density and mass activity of formate reach as high as (79.0 ± 0.4) mA cm-2 and (1490.6 ± 7.5) mA mg-1 at -1.14 V vs. RHE. Theoretical calculations suggest that Sn-Cu alloy can obtain high Faradaic efficiency for CO2 electroreduction by suppressing the competitive hydrogen evolution reaction and that the formate formation follows the path of CO2 â†’ HCOO* â†’ HCOOH. The stepped (2 1 1) surface of Sn-Cu alloy is beneficial towards selective formate production.

Electronic Effects Determine the Selectivity of Planar Au-Cu Bimetallic Thin Films for Electrochemical CO2 Reduction.

Au-Cu bimetallic thin films with controlled composition were fabricated by magnetron sputtering co-deposition, and their performance for the electrocatalytic reduction of CO2 was investigated. The uniform planar morphology served as a platform to evaluate the electronic effect isolated from morphological effects while minimizing geometric contributions. The catalytic selectivity and activity of Au-Cu alloys was found to be correlated with the variation of electronic structure that was varied with tunable composition. Notably, the d-band center gradually shifted away from the Fermi level with increasing Au atomic ratio, leading to a weakened binding energy of *CO, which is consistent with low CO coverage observed in CO stripping experiments. The decrease in the *CO binding strength results in the enhanced catalytic activity for CO formation with the increase in Au content. In addition, it was observed that copper oxide/hydroxide species are less stable on Au-Cu surfaces compared to those on the pure Cu surface, where the surface oxophilicity could be critical to tuning the binding strength of *OCHO. These results imply that the altered electronic structure could explain the decreased formation of HCOO- on the Au-Cu alloys. In general, the formation of CO and HCOO- as main CO2 reduction products on planar Au-Cu alloys followed the shift of the d-band center, which indicates that the electronic effect is the major governing factor for the electrocatalytic activity of CO2 reduction on Au-Cu bimetallic thin films.