Enhancing electrochemical carbon dioxide reduction efficiency through heat-induced metamorphosis of copper nanowires into copper oxide/copper nanotubes with tunable surface

Electrochemical CO2 reduction reaction (CO2RR) presents a unique opportunity to convert carbon dioxide (CO2) to value-added products while simultaneously storing renewable energy in the form of chemical energy. However, particle applications of this technology are limited due to the poor efficiency and product selectivity of the existing catalyst. In this study, we demonstrate a facile method for the heat-induced transformation of copper nanowires into CuOx/Cu nanotubes with defect-enriched surfaces. During this transformation, the outward migration of copper results in the formation of tubular structures encased within nanosized oxide grains. Notably, the hydrogen faradaic efficiency (FE) decreases with extended heat treatment, while carbon monoxide (CO) FE increases. As compared to Cu NWs, Cu NTs exhibit lower selectivity towards H2 and single-carbon (C1) products and favor the formation of multi-carbon (C2+) products. Consequently, a 2-fold increase in the single pass CO2 conversion (SPCC) and C2+ half-cell energy efficiency (EEhalf cell) was noted after heat treatment. The Cu NT-4 variant, synthesized under optimized conditions, exhibits the highest FE of 72.1 % for C2+ products at an operating current density (ID) of 500 mA cm-2.

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.

Single-cluster anchored on PC6 monolayer as high-performance electrocatalyst for carbon dioxide reduction reaction: First principles study

Tremendous challenges remain to develop high-efficient catalysts for carbon dioxide reduction reaction (CO2RR) owing to the poor activity and low selectivity. However, the activity of catalyst with single active site is limited by the linear scaling relationship between the adsorption energy of intermediates. Motivated by the idea of multiple activity centers, triple metal clusters (M = Cr, Mn, Fe, Co, Ni, Cu, Pd, and Rh) doped PC6 monolayer (M3@PC6) were constructed in this study to investigate the CO2RR catalytic performance via density functional theory calculations. Results shows Mn3@PC6, Fe3@PC6, and Co3@PC6 exhibit high activity and selectivity for the reduction of CO2 to CH4 with limiting potentials of -0.32, -0.28, and -0.31 V, respectively. Analysis on the high-performance origin shows the more binding sites in M3@PC6 render the triple-atom anchored catalysts (TACs) high ability in regulating the binding strength with intermediates by self-adjusting the charges and conformation, leading to the improved performance of M3@PC6 than dual-atom doped PC6. This work manifests the huge application of PC6 based TACs in CO2RR, which hope to prove valuable guidance for the application of TACs in a broader range of electrochemical reactions.

Highly dispersed Cu-Cu2O-CeOx interfaces on reduced graphene oxide for CO2 electroreduction to C2+ products

The Cu0-Cu+ interfaces play a key role in the electrochemical CO2 reduction reaction (CO2RR) to produce multi-carbon products (C2+), however, it is difficult for Cu+ to exist stably under reducing conditions. Herein, we construct highly dispersed and stable Cu-Cu2O-CeOx interface on reduced graphene oxide (rGO) for CO2 electroreduction to C2+ products. During the synthesis process, utilizing strong electrostatic interactions, the complex ions of Cu2+ and Ce3+ are uniformly adsorbed on the surface of graphene oxide. Then, under the solvothermal reaction of ethylene glycol and thiourea, the two metal complex ions are converted into highly dispersed and ultrafine Cu2S-CeOx nanocomposites on rGO. Interestingly, CeOx and thiourea synergistically regulate the generation of only Cu+. Under the CO2RR process, the reconstruction of Cu2S promotes the formation of Cu0 and Cu2O species. CeOx stabilizes partial Cu+ species and promotes the formation of Cu-Cu2O-CeOx composite interface. With the help of synergistic effect of Cu0, Cu+ and CeOx, the optimized reaction interface achieves the Faradaic efficiency (FE) of 74.5 % for C2+ products with the current density of 230 mA cm-2 at -0.9 V versus the reversible hydrogen electrode. In situ attenuate total reflectance-infrared absorption spectroscopy (ATR-IRAS) spectra show that the composite interfaces promote the adsorption and activation of H2O and CO2, improve the surface coverage of CO intermediates (*CO), and thus accelerate the CC coupling process.

Granular protruded irregular Cu2O catalysts for efficient CO2 reduction to C2 products

The electrochemical reduction of carbon dioxide (CO2) to high-energy multi-carbon compounds is a significant challenge. Efforts have been made to design efficient catalysts for high selectivity toward multi-carbon products. In this study, granular protruded irregular Cuprous oxide (Cu2O) nanoparticles were synthesized using a simple water bath wet chemical reduction method. Polyethylene glycol (PEG) was utilized as a directing agent to control the morphology of Cu2O in the process. The optimized irregular Cu2O (ir-Cu2O) catalyst exhibits a remarkable faraday efficiency of 69.3% (±3.3%) for double-carbon compounds (C2), which is significantly higher than that of polyhedral Cu2O (p-Cu2O) (50.4%±1.1%) synthesized without adding PEG. Cu2O nanoparticles with irregular shape featuring randomly distributed spherical protrusions offer more active sites for CO2 adsorption than p-Cu2O catalysts, which is beneficial for the conversion of CO2 to C2. In addition, in situ infrared spectra reveal that ir-Cu2O reduces CO2 to C2 mainly through the coupling of the CO* and CHO*, thereby promoting the formation of C2. These findings provide valuable insights for the design of high-efficiency electrocatalysts for CO2 electroreduction to C2.

Elucidating the origin of catalytic activity of nitrogen-doped carbon coated nickel toward electrochemical reduction of CO2

Converting CO2 into valuable chemicals and fuels through clean and renewable energy electricity provides a way to achieve sustainable development for human societies. In this study, carbon coated nickel catalysts (Ni@NCT) were prepared by solvothermal and high-temperature pyrolysis methods. A series of Ni@NC-X catalysts were obtained by pickling with different kinds of acids for electrochemical CO2 reduction reaction (ECRR). The results show that Ni@NC-N treated with nitric acid has the highest selectivity but lower activity, Ni@NC-S treated with sulfuric acid has the lowest selectivity, and Ni@NC-Cl treated with hydrochloric acid shows the best activity and good selectivity. At -1.16 V, Ni@NC-Cl has a considerable CO yield of 472.9 μmol h-1 cm-2, which is significantly superior to Ni@NC-N (327.5), Ni@NC-S (295.6) and Ni@NC (270.8). The controlled experiments show that there is a synergistic effect between Ni and N. The chlorine adsorbed on the surface can promote the performance of ECRR. The poisoning experiments indicate that the contribution of surface Ni atoms to the ECRR is very small, and the increase of activity is mainly due to the nitrogen doped carbon coated Ni particles. The relationship between activity and selectivity of ECRR on different acid-washed catalysts was correlated by theoretical calculations for the first time, which is also in good agreement with the experimental results.

New reaction path for long-chain hydrocarbons by electrochemical CO2 and CO reduction over Au/stainless steel

The Fischer-Tropsch (F-T) synthesis is recognized for its ability to produce long-chain hydrocarbons. In this study, we aimed to replicate F-T synthesis using electrochemical CO2 reduction and CO reduction reactions on a stainless steel (SS) support with a gold (Au) overlayer. Under CO2-saturated conditions, the presence of Au on the SS surface led to the formation of CH4 and a range of hydrocarbons (CnH2n and CnH2n+2, n = 2-7), while bare SS primarily produced hydrogen. The Au(10 nm)/SS exhibited the highest hydrocarbon production in CO2-saturated phosphate, indicating a synergistic effect at the Au-SS interface. In CO-saturated conditions, bare SS also produced long-chain hydrocarbons, but increasing Au thickness resulted in decreased production due to poor CO adsorption. Hydrocarbons were formed through both direct and indirect CO adsorption pathways. Anderson-Schulz-Flory analysis confirmed surface CO hydrogenation and C-C coupling polymerization following conventional F-T synthesis. The C2 hydrocarbons exhibited distinct behavior compared to C3-5 hydrocarbons, suggesting different reaction pathways. Despite low reduction product levels, our EC method successfully replicated F-T synthesis using the Au/SS electrode, providing valuable insights into C-C coupling mechanisms and electrochemical production of long-chain hydrocarbons. Depth-profiling X-ray photoelectron spectroscopy revealed significant changes in surface elemental compositions before and after EC reduction.

Engineering iron carbide catalyst with aerophilic and electron-rich surface for improved electrochemical CO2 reduction

Highly efficient electrocatalyst for carbon dioxide reduction (CO₂RR) is desirable for converting CO₂ into carbon-based chemicals and reducing anthropogenic carbon emission. Regulating catalyst surface to improve the affinity for CO₂ and the capability of CO₂ activation is the key to high-efficiency CO₂RR. In this work, we develop an iron carbide catalyst encapsulated in nitrogenated carbon (SeN-Fe₃C) with an aerophilic and electron-rich surface by inducing preferential formation of pyridinic-N species and engineering more negatively charged Fe sites. The SeN-Fe₃C exhibits an excellent CO selectivity with a CO Faradaic efficiency (FE) of 92 % at -0.5 V (vs. RHE) and remarkably enhanced CO partial current density as compared to the N-Fe₃C catalyst. Our results demonstrate that Se doping reduces the Fe₃C particle size and improves the dispersion of Fe₃C on nitrogenated carbon. More importantly, the preferential formation of pyridinic-N species induced by Se doping endows the SeN-Fe₃C with an aerophilic surface and improves the affinity of the SeN-Fe₃C for CO₂. Density functional theory (DFT) calculations reveal that the electron-rich surface, which is caused by pyridinic N species and much more negatively charged Fe sites, leads to a high degree of polarization and activation of CO₂ molecule, thus conferring a remarkably improved CO₂RR activity on the SeN-Fe₃C catalyst.

Regulating the coordination environment of single-atom catalysts for electrocatalytic CO2 reduction

Electrochemical CO₂ reduction (ECR) through single-atom catalysts (SACs) consisting of transition metals (TMs) anchored on nitrogenated carbon (TM-N-C) has shown promise for carbon neutralization. However, high overpotentials and low selectivity are still issues. Regulating the coordination environment of anchored TM atom is important to address these problems. In this study, we evaluated nonmetal atom (NM = B, O, F, Si, P, S, Cl, As and Se) modified TM (TM = Fe, Co, Ni, Cu and Zn)@N₄-C catalysts for their ECR to CO performance using density functional theory (DFT) calculations. NM dopants can induce active center distortion and tune electron structure, promoting intermediate formation. Doping heteroatoms can improve ECR to CO activity on Ni and Cu@N₄ but worsen it on Co@N₄ catalysts. Fe@N₄-F₁(I), Ni@N₃-B₁, Cu@N₄-O₁(III), and Zn@N₄-Cl₁(II) have excellent activity for ECR to CO, with overpotentials of 0.75, 0.49, 0.43, and 0.15 V, respectively, and improved selectivity. The catalytic performance is related to the intermediate binding strength, as evidenced by d band center, charge density difference, crystal orbital Hamilton population (COHP), and integrated COHP (ICOHP). It is expected that our work can be used as the design principle to guide the synthesis of the high-performance heteroatoms modified SACs for ECR to CO.

Highly selective electrocatalytic reduction of CO2 to HCOOH over an in situ derived hydrocerussite thin film on a Pb substrate

A metal oxide electrode has been developed for the electrochemical CO₂ reduction reaction (eCO₂RR). It exhibits superior activity and product selectivity towards eCO₂RR by circumventing the previously encountered problem of self-reduction with high-valence metals. Specifically, a hydrocerussite [Pb₃(CO₃)₂(OH)₂] thin film has been synthesized in situ on a Pb substrate (denoted as ER-HC) by an electroreduction method using a lead-based metal-organic framework (Pb-MOF) as a precursor. The ER-HC electrode exhibits a high selectivity of 96.8% towards HCOOH production with a partial current density of 1.9 mA cm^-2 at -0.88 V vs. the reversible hydrogen electrode (RHE). A higher HCOOH partial current density of 7.3 mA cm^-2 has been achieved at -0.98 V vs. RHE. Physicochemical and electrochemical characterization results demonstrate that the defective hydrocerussite surface exhibits appropriate adsorption free energy of formate (HCOO^-) and a lower reaction free energy for HCOOH production from CO₂, which greatly boosts the eCO₂RR activity and HCOOH production selectivity. The structure and eCO₂RR performance of the hydrocerussite thin film remain stable in 0.1 M KHCO₃ as electrolyte, ensuring its durability. Overall, this work not only provides a metal oxide electrode (metal hydroxide, to be more precise) with excellent eCO₂RR performance, but also expands the in situ electrochemical derivatization strategy for the fabrication of metal oxide electrodes.

HKUST-1-derived highly ordered Cu nanosheets with enriched edge sites, stepped (211) surfaces and (200) facets for effective electrochemical CO2 reduction

A novel electrode composed of Cu nanosheets constructed from nanoparticles was synthesized by in situ electrochemical derivation from the metal-organic framework (MOF) HKUST-1. The prepared derivative electrode (HE-Cu) exhibited higher Faradaic efficiency (FE, 56.0%) of electrochemical CO₂ reduction (CO₂R) compared with that of pristine Cu foil (p-Cu, 32.3%) at an overpotential of -1.03 V vs. a reversible hydrogen electrode (RHE). HE-Cu also exhibited lower onset potential of CO₂R as well as inhibiting the H₂ evolution reaction. Electrochemical measurements revealed that HE-Cu exhibited higher CO₂ adsorption (1.58-fold) and a larger electrochemical active surface area (1.24-fold) compared with p-Cu. Physicochemical characterization and Tafel analysis showed that stepped Cu (211) surfaces, (200) facets and Cu edge atoms on HE-Cu contributed significantly to the enhanced CO₂R activity and/or HCOOH and/or C2 product selectivity. The FEs of HCOOH and C2 products for HE-Cu increased 1.57-fold and 10.6-fold at an overpotential of -1.19 V vs. RHE compared with p-Cu. Although CH₄ was produced on p-Cu, its formation was totally suppressed on HE-Cu due to the increase of edge sites and (200) facets. Our study demonstrates that electroreduction of MOFs is a promising method to prepare novel and stable electrochemical catalysts with unique surface structures. The fabricated derivative electrode not only promoted electrochemical CO₂R activity but also exhibited high C2 product selectivity.

Ultrasound-boosted selectivity of CO in CO2 electrochemical reduction

Among the possible products of CO₂ electrochemical reduction, CO plays a unique and vital role, which can be an ideal feedstock for further reduction to C₂₊ products, and also the important component of syngas that can be used as feedstock for value-added chemicals and fuels. However, it is still a challenge to tune the CO selectivity on Cu electrode. Here we newly construct an ultrasound-assisted electrochemical method for CO₂ reduction, which can tune the selectivity of CO₂ to CO from less than 10% to >80% at -1.18 V versus (vs.) reversible hydrogen electrode (RHE). The partial current density of CO production is significantly improved by 15 times. By in-situ Raman study, the dominating factor for the improved CO production is attributed to the accelerated desorption of *CO intermediate. This work provides a facile method to tune the product selectivity in CO₂ electrochemical reduction.

Controlled assembly of nitrogen-doped iron carbide nanoparticles on reduced graphene oxide for electrochemical reduction of carbon dioxide to syngas

The electrocatalytic CO₂ reduction reaction (CO₂RR) decreases the amount of greenhouse gas in the atmosphere while enabling a closed carbon cycle. Herein, iron oleate was used as a precursor to produce oleic acid-coated triiron tetraoxide nanoparticles (Fe₃O₄@OA NPs) by pyrolysis, which was then assembled with reduced graphene oxide (rGO) and doped with dicyandiamide as a nitrogen source to obtain nitrogen-doped iron carbide nanoparticles assembled on rGO (N-Fe₃C/rGO NPs). The catalyst prepared by nitrogen doping at 800 °C with an Fe₃O₄@OA NPs to rGO weight ratio of 20:1 showed good activity and stability for the CO₂RR. At -0.3 to -0.4 V, the H₂/CO ratio of the product from the catalyzed CO₂RR was close to 2; thus, the product can be used for Fischer-Tropsch synthesis. The results of a series of experiments and X-ray photoelectron spectroscopy analysis showed that the synergy between the CN and FeN groups in the catalyst can promote the reduction of CO₂ to CO. This work demonstrates a facile method for improving the catalytic reduction of CO₂.

A bio-inspired O2-tolerant catalytic CO2 reduction electrode

The electrochemical reduction of CO₂ to give CO in the presence of O₂ would allow the direct valorization of flue gases from fossil fuel combustion and of CO₂ captured from air. However, it is a challenging task because O₂ reduction is thermodynamically favored over that of CO₂. 5% O₂ in CO₂ near catalyst surface is sufficient to completely inhibit the CO₂ reduction reaction. Here we report an O₂-tolerant catalytic CO₂ reduction electrode inspired by part of the natural photosynthesis unit. The electrode comprises of heterogenized cobalt phthalocyanine molecules serving as the cathode catalyst with >95% Faradaic efficiency (FE) for CO₂ reduction to CO coated with a polymer of intrinsic microporosity that works as a CO₂-selective layer with a CO₂/O₂ selectivity of ∼20. Integrated into a flow electrolytic cell, the hybrid electrode operating with a CO₂ feed gas containing 5% O₂ exhibits a FE_CO of 75.9% with a total current density of 27.3 mA/cm² at a cell voltage of 3.1 V. A FE_CO of 49.7% can be retained when the O₂ fraction increases to 20%. Stable operation for 18 h is demonstrated. The electrochemical performance and O₂ tolerance can be further enhanced by introducing cyano and nitro substituents to the phthalocyanine ligand.