Carbon Dioxide Electroreduction Research Articles

Electrocatalytic reduction of carbon dioxide in confined microspace utilizing single nickel atom decorated nitrogen-doped carbon nanospheres

Carbon dioxide electroreduction reaction (CO2RR), as a rational regulation of CO2 resource utilization, demands effectively selective catalysts for converting CO2 into high-value-added chemicals. Carbon-based nanoreactors featuring rationally designed porous framework structures might provide a unique chemical environment for confining and stabilizing the active metal species, consequently improving the CO2RR activity. Herein, nitrogen-doped porous carbon nanospheres decorated by single Ni atom (Ni-NCN) featuring a Ni-N4 structure were synthesized using the modified sol-gel method for the reduction of CO2 to CO. The synergistic effect of the Ni-N4 active sites homogenously distributed in the interconnected pore structure and the favorable chemical confined microspace of carbon nanospheres endows it with excellent CO2RR activity. In the H- type cell, Ni-NCN displays a CO Faradaic efficiency up to 96.6 % and a CO current density of 9.8 mA cm−2 at − 0.83 V (vs. RHE), as well as a high turnover frequency (TOF) of 10658 h−1 at − 1.33 V (vs. RHE). In the flow cell, the mass transfer can be further facilitated by the formation of three-phase interface. The Faradaic efficiency and current density of CO2RR catalyzed by Ni-NCN is enhanced to 97.9 % and 102.4 mA cm−2 at − 1.13 V (vs. RHE), and the wide potential window ranges from − 0.53 V to − 1.33 V (vs. RHE) with the Faradaic efficiency more than 95 %. Density functional theory (DFT) calculations reveal that the high selectivity of Ni-N4 sites is mainly ascribed to the high energy barrier that restrains the hydrogen evolution reaction (HER). Meanwhile, the lower CO binding energy on Ni-N4 site helps the escape of CO to increase the TOF of active sites. The in-situ Fourier transform infrared (FTIR) spectroscopy verifies that the intermediate *COOH can be more stable in the confined environment of Ni-NCN to promote the selectivity of CO2RR. The strategy of constructing confined microspace paves a new path for the rational design of high-efficient single atom catalysts for CO2 reduction.

Efficient Electrocatalytic Reduction of CO2 to Ethanol Enhanced by Spacing Effect of CuCu in Cu2‐xSe Nanosheets

AbstractIt is highly desired yet challenging to strategically steer carbon dioxide (CO2) electroreduction reaction (CO2ER) toward ethanol (EtOH) with high activity, which provides a promising way for intermittent renewable energy reservation. Controlling spatial distance between the adjoining active centers and promoting the CC coupling progress are crucial to realize this purpose. Herein, ultrathin 2D Cu2‐xSe is prepared with abundant Se vacancies, where the spatial distance between the CuCu around the Se vacancies is effectively shortened because of the lattice stress. Besides, the moderate spatial distance induced by Se vacancies can significantly decrease the Gibbs free energy of asymmetric *CO*CHO coupling progress, effectively change the local charge distribution, decrease the valence state of Cu atoms and increase the electron‐donating capacity of the dual active sites. Combining experimental observations and density functional theory simulations, the CuCu dual sites with spatial distance of 2.51 Å in VSe‐Cu2‐xSe sample can catalyze CO2ER to EtOH with high selectivity in a potential range from −0.4 to −1.6 V, and reach the highest faradaic efficiency of 68.1% at −0.8 V. This work reveals the influence of spacing effect on ethanol selectivity, and provides a new idea for future design of catalysts with chain elongation reaction, which can bring extensive attention.

Steering CO2 electroreduction pathway toward ethanol via surface-bounded hydroxyl species-induced noncovalent interaction

Selective electroreduction of carbon dioxide (CO2RR) into ethanol at an industrially relevant current density is highly desired. However, it is challenging because the competing ethylene production pathway is generally more thermodynamically favored. Herein, we achieve a selective and productive ethanol production over a porous CuO catalyst that presents a high ethanol Faradaic efficiency (FE) of 44.1 ± 1.0% and an ethanol-to-ethylene ratio of 1.2 at a large ethanol partial current density of 501.0 ± 15.0 mA cm-2, in addition to an extraordinary FE of 90.6 ± 3.4% for multicarbon products. Intriguingly, we found a volcano-shaped relationship between ethanol selectivity and nanocavity size of porous CuO catalyst in the range of 0 to 20 nm. Mechanistic studies indicate that the increased coverage of surface-bounded hydroxyl species (*OH) associated with the nanocavity size-dependent confinement effect contributes to the remarkable ethanol selectivity, which preferentially favors the *CHCOH hydrogenation to *CHCHOH (ethanol pathway) via yielding the noncovalent interaction. Our findings provide insights in favoring the ethanol formation pathway, which paves the path toward rational design of ethanol-oriented catalysts.

Chlorine-induced mixed valence of CuOx/C to promote the electroreduction of carbon dioxide to ethylene

Chlorine-induced mixed valence of CuOx/C to promote the electroreduction of carbon dioxide to ethylene

Honeycomb-like CuO@C for electroreduction of carbon dioxide to ethylene

The electrochemical CO2 reduction (ECR) of high-value multicarbon products is an urgent challenge for catalysis and energy resources. Herein, we reported a simple polymer thermal treatment strategy for preparing honeycomb-like CuO@C catalysts for ECR with remarkable C2H4 activity and selectivity. The honeycomb-like structure favored the enrichment of more CO2 molecules to improve the CO2-to-C2H4 conversion. Further experimental results indicate that the CuO loaded on amorphous carbon with a calcination temperature of 600 °C (CuO@C-600) has a Faradaic efficiency (FE) as high as 60.2% towards C2H4 formation, significantly outperforming pure CuO-600 (18.3%), CuO@C-500 (45.1%) and CuO@C-700 (41.4%), respectively. The interaction between the CuO nanoparticles and amorphous carbon improves the electron transfer and accelerates the ECR process. Furthermore, in situ Raman spectra demonstrated that CuO@C-600 can adsorb more adsorbed *CO intermediates, which enriches the CC coupling kinetics and promotes C2H4 production. This finding may offer a paradigm to design high-efficiency electrocatalysts, which can be beneficial to achieve the “double carbon goal.”

An exploration on CO faradaic efficiency with catalyst surface wettability ranging from hydrophilic to superhydrophobic

Hydrophobic electrode can facilitate CO2 mass transfer for carbon dioxide electroreduction reaction (CO2ER) in the microchannel reactor. However, ‘trade-off’ exists because the reaction cannot proceed in the absence of electrolyte. Therefore, effective control of wettability on the surface of the catalyst is significant to achieve efficient CO2ER performance. In this paper, a nanocone-shaped Zn@Ag has been fabricated through a simple two-step preparation process involving the hydrothermal treatment and the galvanic replacement. Further modification of the myristic acid (MA)/stearic acid (SA) layer on the nanocone by the immersion method can extend the interfacial wettability to the superhydrophobic scale. The experimental parameters including the time of three synthesis procedures were adjusted to regulate the dimensions and surface free energy of the nanocone. Consequently, the nanoncone catalyst itself possessed a maximum intrinsic water contact angle (CA) of ∼114°, while the SA modified catalyst demonstrated an outstanding water repellency with CA of ∼152°. The relationship between the CO Faradaic efficiency (FECO) and the surface wettability ranging from hydrophilic to superhydrophobic was ultimately explored. The optimal Zn@Ag@MA catalyst with appropriate hydrophobicity can achieve a FECO of 91%, which is nearly 20% higher than the hydrophilic/superhydrophobic counterpart. In situ attenuated total reflectance infrared (ATR-IR) tests verified that the hydrophobic catalyst can enhance CO2 concentration near the surface and promote the selectivity to CO.

Continuous carbon dioxide electroreduction to formate coupled with the single-pass glycerol oxidation to high value-added products

CO2 electroreduction has been considered a promising alternative to simultaneously reduce CO2 emissions and produce value-added products. Among others, the production of formic acid/formate is particularly attractive. Although promising results have already been obtained in the literature, one of the recent approaches to improve the process deals with the use of an alternative reaction at the anode instead of the traditional oxygen evolution reaction (OER). In this context, this work reports, for the first time, the study of the CO2 electroreduction to formate coupled with the electrooxidation of glycerol to high-added value products where both half-reactions operate in a continuous mode with a single pass of the reactants through the electrochemical cell. Interestingly, at the cathode, similar results to those previously reported were obtained, reaching formate concentrations of about 18 g·L-1 at a 200 mA·cm-2. In addition, at the anode, promising dihydroxyacetone productions of 196 µmol·m-2·s-1 were simultaneously achieved in the output stream of the anodic compartment. These findings represent a significant step forward for the development and application of the technology.

Highly Selective CO2 Electroreduction to C2+ Products over Cu2O-Decorated 2D Metal-Organic Frameworks with Rich Heterogeneous Interfaces.

The electroreduction of carbon dioxide into high-value-added products is an effective approach to alleviating the energy crisis and pollution issues. However, there are still significant challenges for multicarbon (C2+) product production due to the lack of efficient catalysts with high selectivity. Herein, a Cu-rich electrocatalyst, where Cu2O nanoparticles are decorated on two-dimensional (2D) Cu-BDC metal-organic frameworks (MOFs) with abundant heterogeneous interfaces, is synthesized for highly selective CO2 electroreduction into C2+ products. A high C2+ Faradaic efficiency of 72.1% in an H-type cell and 58.2% in a flow cell are obtained, respectively. The heterogeneous interfaces of Cu2O/Cu-BDC can optimize the adsorption energy of reaction intermediates during CO2 electroreduction. An in situ infrared spectroscopy study indicates that the constructed interfaces can maintain the particular distribution of Cu valence states, where the C-C coupling is promoted to efficiently produce C2+ products owing to the stabilization of *CHO and *COH intermediates.

A Technoeconomic Model Coupled with HYSYS to Analyze the Electroreduction of Carbon Dioxide to Ethanol

AbstractThis research develops a technoeconomic analysis to study the profitability of ethanol production from CO2 electroreduction. A HYSYS simulation is used to calculate the separation costs, a challenge in previous models available in literature. The profitability of a 10 000 kg per day CO2 electroreduction plant to produce ethanol is studied. An optimization of the pressure swing adsorber and distillation tower, which greatly influence the total cost of the plant (≈20% of total cost), is carried out, obtaining a total cost of separation of £1.94106. The study demonstrates that reducing the voltage applied to values around 0.5 V, i.e., by increasing the pH up to 12 makes the process economically feasible with a current density over 15 mA cm−2. It also shows that the process is economically feasible using a current density of 5 mA cm−2 if the electricity cost from renewable sources drops to 2.010−2 £ kWh−1. Finally, it is proved that if catalyst stability is not considered, some catalysts currently available in the literature can be used with a positive economic income. The results of this research show that the industrial electroreduction of ethanol can be feasible and can attract interest in the industrial adoption of the technology.

Operando studies reveal active Cu nanograins for CO2 electroreduction.

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals1. Although Cu enables CO2-to-multicarbon product (C2+) conversion, the nature of the active sites under operating conditions remains elusive2. Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques3-5. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu2O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO2 reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C-C coupling. Quantitative structure-activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C2+ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C2+ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.

Interfacial engineering of Copper(I) oxide nanocubes wrapped with functionalized carbon nanotubes toward carbon dioxide electroreduction to ethylene

Copper(I) oxide (Cu2O)-based nanomaterial is considered to be a highly attractive electrocatalyst for the carbon dioxide (CO2) reduction to ethylene (C2H4), while the selectivity of C2H4 still faces challenges. Herein, a unique nanocomposite consisted of Cu2O nanocubes trapped with oxygen-containing groups functionalized carbon nanotubes (O-CNTs) through a facile wet chemical method is reported. Benefiting from the novel structure and robust interface, the nanocomposite (Cu2O/O-CNTs) electrocatalyst exhibits an enhanced C2H4 selectivity (Faradic efficiency = 40.3%) at –1.3 V vs. RHE, a current density of 13.6 mA/cm2, and long-term durability (over 8 h) in KHCO3 electrolyte. The O-CNTs trapping around Cu2O nanocubes with an intimately interface ensures highly conductive network and uniform dispersion of active sites in the electrochemical CO2 reduction reaction (ECO2RR). The improved performance of Cu2O/O-CNTs can be attributed to the rapid mass transport, increased local concentration of *CO intermediates, and stable heterointerface. This study presents a new route to increase the performance and cyclic lifetime of Cu-based catalyst for CO2 electroreduction to C2H4 by the trapping interfacial engineering.

Electroreduction of Carbon Dioxide to Acetate using Heterogenized Hydrophilic Manganese Porphyrins.

The electrochemical reduction of carbon dioxide (CO2 ) to value-added chemicals is a promising strategy to mitigate climate change. Metalloporphyrins have been used as a promising class of stable and tunable catalysts for the electrochemical reduction reaction of CO2 (CO2 RR) but have been primarily restricted to single-carbon reduction products. Here, we utilize functionalized earth-abundant manganese tetraphenylporphyrin-based (Mn-TPP) molecular electrocatalysts that have been immobilized via electrografting onto a glassy carbon electrode (GCE) to convert CO2 with overall 94 % Faradaic efficiencies, with 62 % being converted to acetate. Tuning of Mn-TPP with electron-withdrawing sulfonate groups (Mn-TPPS) introduced mechanistic changes arising from the electrostatic interaction between the sulfonate groups and water molecules, resulting in better surface coverage, which facilitated higher conversion rates than the non-functionalized Mn-TPP. For Mn-TPP only carbon monoxide and formate were detected as CO2 reduction products. Density-functional theory (DFT) calculations confirm that the additional sulfonate groups could alter the C-C coupling pathway from *CO→*COH→*COH-CO to *CO→*CO-CO→*COH-CO, reducing the free energy barrier of C-C coupling in the case of Mn-TPPS. This opens a new approach to designing metalloporphyrin catalysts for two carbon products in CO2 RR.

Molecular Modulation of Sequestered Copper Sites for Efficient Electroreduction of Carbon Dioxide to Methane

AbstractThe sustainable production of methane (CH4) via the electrochemical conversion of carbon dioxide (CO2) is an appealing approach to simultaneously mitigating carbon emissions and achieving energy storage in chemical bonds. Copper (Cu) is a unique material to produce hydrocarbons and oxygenates. However, selective methane generation on Cu remains a great challenge due to the preferential *CO dimerization pathway toward multi‐carbon (C2+) products at neighboring catalytic sites. Herein, a conjugated copper phthalocyanine polymer (CuPPc) is designed by a facile solid‐state method for highly selective CO2‐to‐CH4 conversion. The spatially isolated CuN4 sites in CuPPc favor the *CO protonation to generate the key *CHO intermediate, thus significantly promoting the formation of CH4. As a result, the CuPPc catalyst exhibits a high CH4 Faradaic efficiency of 55% and a partial current density of 18 mA cm−2 at −1.25 V versus the reversible hydrogen electrode. It also stably operates for 12 h. This study may offer a new solution to regulating the chemical environment of the active sites for the development of highly efficient copper‐based catalysts for electrochemical CO2 reduction.

Stabilizing copper sites in coordination polymers toward efficient electrochemical C-C coupling

Electroreduction of carbon dioxide with renewable electricity holds promise for achieving net-zero carbon emissions. Single-site catalysts have been reported to catalyze carbon-carbon (C-C) coupling—the indispensable step for more valuable multi-carbon (C2+) products—but were proven to be transformed in situ to metallic agglomerations under working conditions. Here, we report a stable single-site copper coordination polymer (Cu(OH)BTA) with periodic neighboring coppers and it exhibits 1.5 times increase of C2H4 selectivity compared to its metallic counterpart at 500 mA cm−2. In-situ/operando X-ray absorption, Raman, and infrared spectroscopies reveal that the catalyst remains structurally stable and does not undergo a dynamic transformation during reaction. Electrochemical and kinetic isotope effect analyses together with computational calculations show that neighboring Cu in the polymer provides suitably-distanced dual sites that enable the energetically favorable formation of an *OCCHO intermediate post a rate-determining step of CO hydrogenation. Accommodation of this intermediate imposes little changes of conformational energy to the catalyst structure during the C-C coupling. We stably operate full-device CO2 electrolysis at an industry-relevant current of one ampere for 67 h in a membrane electrode assembly. The coordination polymers provide a perspective on designing molecularly stable, single-site catalysts for electrochemical CO2 conversion.

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics-boosted Ethylene Electrosynthesis.

Cu-based catalysts have been widely applied in electroreduction of carbon dioxide (CO2ER) to produce multicarbon (C2+) feedstocks (e.g., C2H4). However, the high energy barriers for CO2 activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in-situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N-enriched carbon support with a sodalite (SOD) topology (Ni-SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C2H4 selectivity of 62.5% and an industrial-level current density of 160 mA cm-2 at a low potential of -0.72 V were achieved. Our studies revealed that the isolated NiN3 active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni-SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO2ER to C2H4.

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics‐boosted Ethylene Electrosynthesis

AbstractCu‐based catalysts have been widely applied in electroreduction of carbon dioxide (CO2ER) to produce multicarbon (C2+) feedstocks (e.g., C2H4). However, the high energy barriers for CO2activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N‐enriched carbon support with a sodalite (SOD) topology (Ni‐SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C2H4selectivity of 62.5 % and an industrial‐level current density of 160 mA cm−2at a low potential of −0.72 V were achieved. Our studies revealed that the isolated NiN3active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni‐SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO2ER to C2H4.

Cu–Pd Bimetallic Gas Diffusion Electrodes for Electrochemical Reduction of CO2 to C2+ Products

Carbon dioxide electroreduction driven by renewable electricity into high‐value chemicals enables energy storage while contributing to climate change mitigation. Herein, a CuPd bimetallic catalyst is developed by electrodeposition for efficient CO2 electroreduction, achieving a C2+ Faradaic efficiency as high as 75.6% with a current density of −200 mA cm−2 at −1.15 V versus reversible hydrogen electrode. Upon incorporation of Pd, the average d‐band center of the CuPd bimetallic catalyst downward shifts relative to the Fermi level, making the adsorbed CO intermediates active for further C–C coupling. Theoretical calculations confirm that CO generated on the Pd domain can spill over to the CuPd interface for C–C coupling with a lower energy barrier, further promoting the formation of C2+ compounds. This study provides insights into the rational design of Cu‐based bimetallic catalysts for highly efficient CO2 electroreduction to multicarbon products.

Selectivity of CO2, carbonic acid and bicarbonate electroreduction over Iron-porphyrin catalyst: A DFT study

Carbon dioxide reduction is a promising approach to convert CO2 to value-added products. Carbon dioxide electroreduction is commonly performed at neutral pH where CO2, H2CO3, HCO3–, and CO3−2 are in equilibrium. Despite this, only CO2 is commonly considered as the active species while H2CO3, HCO3–, and CO3−2 (carbonate species) are neglected. Using density functional theory (DFT), we investigate the contribution of carbonate species in the CO2 electroreduction reaction over a Fe-porphyrin (Fe(ppy)) model system. We find that the hydrogen evolution reaction (HER) is blocked by a high activation barrier of 1.54 eV associated with the Fe("I")(ppy)-H formation. This is opposed to the reduction of H2CO3 and HCO3– whose rate-limiting step requires significantly lower activation energies of approximately 0.4 eV. Direct CO2 reduction on the other hand requires to overcome a rate-limiting barrier of 0.95 eV for the CO2 adsorption step. Based on these results we suggest that H2CO3 and HCO3– are the true reactants for the electrochemical "CO2"RR over Fe(ppy).

Directly‐Deposited Ultrathin Solid Polymer Electrolyte for Enhanced CO2 Electrolysis

AbstractThe economic viability of carbon dioxide electroreduction (CO2R) relies on improved performance accompanied by scalable system design. Membranes are commonly used for the separation of reduction and oxidation products as well as to provide a suitable micro‐environment for CO2R. Commercial membranes often address only one of the key challenges in CO2R: either they offer a suitable micro‐environment for CO2R (e.g., anion exchange membrane) or suppress carbonate cross‐over (e.g., cation exchange membrane and bipolar membrane). This work presents a cation‐infused ultrathin (≈3 µm) solid polymer electrolyte (CISPE) that concomitantly addresses both challenges via a bidirectional ion transport mechanism and suppressed cathode flooding. This directly‐deposited CISPE (that substitutes the commonly used pre‐made membrane) enables record high full‐cell energy efficiency of 28% at 100 mA cm−2 for one‐step CO2 electrolysis to ethylene (C2H4) with ≈110 h of stable operation. This translates into a record low energy cost of 290 GJ per ton C2H4 for the end‐to‐end process (i.e., CO2 capture and electroreduction, carbonate regeneration, CO2 separation from anode and cathode streams) in a membrane electrode assembly CO2R. The present work offers a versatile design paradigm for functional polymer electrolytes, opening the door to stable, and efficient electrolysis of high‐value feedstock chemicals and fuels using low‐cost catalysts.

Hydrosilylation and electroreduction of CO2using a zirconocene hydride catalyst

The use of commercially available [Cp2ZrHCl] (Schwartz reagent) for the hydrosilylation and electroreduction of carbon dioxide is presented.