Efficient CO2 Electroreduction Research Articles

Shear Field-Controlled Synthesis of Nitrogen-Doped Carbon Nanochains Forest with High-Density sp3 Defects for Efficient CO2 Electroreduction Reaction.

Defect engineering and nitrogen doping being effective strategies for modulating the surface chemical state of the carbon matrix have been widely explored to promote the catalytic activity in the territory of electrochemical energy storage and conversion devices. However, the controllable synthesis of carbon material with high-density specific defects and high nitrogen doping is still full of challenges. Here, we first synthesize one-dimensional necklace-like nitrogen-doped carbon nanochains (N-CNCs) with abundant defects on carbon fiber paper (CFP) by chemical vapor deposition (CVD) method. The resultant nanostructures are a bunch of interconnected carbon spheres with a hollow structure at the internode and present the complete one-dimensional nanochain configuration. Specifically, the N-CNCs with a corrugated surface possesses high content of sp3 defects (31.2%) and nitrogen (23.6 at %). Combining finite element analysis and experimental results, it reveals that the robust shear field generated by etching gas releasing from thermal decomposition of melamine in situ modulates the CVD process via changing the size and force environment of the metal catalyst droplets for formation of N-CNCs. Benefiting from the high ratio of sp3/sp2 and nitrogen doped on the surface, the N-CNCs@CFP displays a superior electrocatalytic performance for CO2RR, delivering CO Faradaic efficiency of 95.9% and a current density of 23.2 mA cm-2 at -0.86 V vs RHE. This work provides promising synthesis strategy and some inspirations for construction of ultradense and specific defects coupling with nitrogen doping sites into carbon materials to achieve high-efficiency electrocatalysis applications.

Theoretical study on CO production mechanism from CO2 reduction on Cu–catalyst surface with different oxidation states

The rational design of copper (Cu)-based catalysts with different oxidation states is essential to achieve highly selective and efficient CO2 electroreduction. However, the effect of different oxide states of Cu-based catalysts has rarely been studied. Herein, the reaction mechanism of CO2 hydrogenation into CO on Cu-based catalysts with different oxidation states, such as Cu(111), Cu2O(111) and CuO(111), was studied using the density functional theory. The most favored pathway for CO production and the rate-controlling step on the three catalyst models were determined. The results show that, Cu2O (111) shows the best catalytic activity among the three catalysts due to the lowest activation barrier. The electronic structure analysis shows that Cu2O has the proper electronic structure to activate CO2 which is further reduced to CO. This work provides an important insight on the effect of oxidation state of Cu-based catalysts on the reduction of CO2 to CO.

Mesoporous Electrocatalysts with p-n Heterojunctions for Efficient Electroreduction of CO2 and N2 to Urea.

The electrocatalytic synthesis of high-value-added urea by activating N2 and CO2 is a green synthesis technology that has achieved carbon neutrality. However, the chemical adsorption and C-N coupling ability of N2 and CO2 on the surface of the catalyst are generally poor, greatly limiting the improvement of electrocatalytic activity and selectivity in electrocatalytic urea synthesis. Herein, novel hierarchical mesoporous CeO2/Co3O4 heterostructures are fabricated, and at an ultralow applied voltage of -0.2 V, the urea yield rate reaches 5.81 mmol g-1 h-1, with a corresponding Faraday efficiency of 30.05%. The hierarchical mesoporous material effectively reduces the mass transfer resistance of reactants and intermediates, making it easier for them to access active centers. The emerging space-charge regions at the heterointerface generate local electrophilic and nucleophilic regions, facilitating CO2 targeted adsorption in the electrophilic region and activation to produce *CO intermediates and N2 targeted adsorption in the nucleophilic region and activation to generate *N ═ N* intermediates. Then, the electrons in the σ orbitals of *N ═ N* intermediates can be easily accepted by the empty eg orbitals of Co3+ in CeO2/Co3O4, which presents a low-spin state (LS: t2g6eg0). Subsequently, *CO couples with *N ═ N* to produce the key intermediate *NCON*. Interestingly, it was discovered through in situ Raman spectroscopy that the CeO2/Co3O4 catalyst has a reversible spinel structure before and after the electrocatalytic reaction, which is due to the surface reconstruction of the catalyst during the electrocatalytic reaction process, producing amorphous active cobalt oxides, which is beneficial for improving electrocatalytic activity.

A novel Cu-based covalent organic framework with Cu-N2O2 single sites for efficient CO2 electroreduction to methane

Single-site catalysts (SSCs) are useful for CO2 electroreduction (CO2ER) to methane (CH4), but their selectivities for CH4 are still unsatisfactory due to the lack of efficient active sites. Herein, a novel Cu-based covalent organic framework (CuN2O2-COF) with Cu-N2O2 single sites was synthesized by the Schiff base reaction and used in CO2ER to improve CH4 generation for the first time. Compared with typical SSCs containing Cu-N4 or Cu-O4 sites, CuN2O2-COF exhibited superior CH4 production efficiency in CO2ER: the Faradaic efficiency of CH4 was as high as 85.4%, and the CH4 partial current density reached 313 mA·cm−2 at −1.3 V, which exceeded those of most electrocatalysts reported thus far. Theoretical calculations indicated that compared with the Cu-N4 or Cu-O4 sites in typical SSCs, the new Cu-N2O2 configuration in CuN2O2-COF increased the d‐band center of the Cu sites and enhanced the adsorption of key intermediates (*COOH and *CHO) required for CO2ER to CH4.

2+1] Cycloadditions Modulate the Hydrophobicity of Ni‐N4 Single‐Atom Catalysts for Efficient CO2 Electroreduction

Microenvironment regulation of M‐N4 single‐atom catalysts (SACs) is a promising way to tune their catalytic properties toward the electrochemical CO2 reduction reaction. However, strategies that can effectively introduce functional groups around the M‐N4 sites through strong covalent bonding and under mild reaction conditions are highly desired. Taking the hydrophilic Ni‐N4 SAC as a representative, we report herein a [2+1] cycloaddition reaction between Ni‐N4 and in‐situ generated difluorocarbene (F2C:), and enable the surface fluorocarbonation of Ni‐N4, resulting in the formation of a super‐hydrophobic Ni‐N4‐CF2 catalyst. Meanwhile, the mild reaction conditions allow Ni‐N4‐CF2 to inherit both the electronic and structural configuration of the Ni‐N4 sites from Ni‐N4. Enhanced electrochemical CO2‐to‐CO Faradaic efficiency above 98% is achieved in a wide operating potential window from −0.7 V to −1.3 V over Ni‐N4‐CF2. In‐situ spectroelectrochemical studies reveal that a highly hydrophobic microenvironment formed by the −CF2− group repels asymmetric H‐bonded water at the electrified interface, inhibiting the hydrogen evolution reaction and promoting CO production. This work highlights the advantages of [2+1] cycloaddition reactions on the covalent modification of N‐doped carbon‐supported catalysts.

2+1] Cycloadditions Modulate the Hydrophobicity of Ni-N4 Single-Atom Catalysts for Efficient CO2 Electroreduction.

Microenvironment regulation of M-N4 single-atom catalysts (SACs) is a promising way to tune their catalytic properties toward the electrochemical CO2 reduction reaction. However, strategies that can effectively introduce functional groups around the M-N4 sites through strong covalent bonding and under mild reaction conditions are highly desired. Taking the hydrophilic Ni-N4 SAC as a representative, we report herein a [2+1] cycloaddition reaction between Ni-N4 and in-situ generated difluorocarbene (F2C:), and enable the surface fluorocarbonation of Ni-N4, resulting in the formation of a super-hydrophobic Ni-N4-CF2 catalyst. Meanwhile, the mild reaction conditions allow Ni-N4-CF2 to inherit both the electronic and structural configuration of the Ni-N4 sites from Ni-N4. Enhanced electrochemical CO2-to-CO Faradaic efficiency above 98% is achieved in a wide operating potential window from -0.7 V to -1.3 V over Ni-N4-CF2. In-situ spectroelectrochemical studies reveal that a highly hydrophobic microenvironment formed by the -CF2- group repels asymmetric H-bonded water at the electrified interface, inhibiting the hydrogen evolution reaction and promoting CO production. This work highlights the advantages of [2+1] cycloaddition reactions on the covalent modification of N-doped carbon-supported catalysts.

Boosting *CO coverage on Cu octahedra enclosed by Cu(1 1 1) for efficient CO2 electroreduction to C2H5OH

Copper (Cu) electrocatalysts for carbon dioxide reduction reaction (CO2RR) have attracted much attention due to their ability to produce high value-added C2+ products (e.g., C2H4, C2H5OH, and n-propanol). However, achieving high selectivity of Cu electrocatalysts for C2H5OH in aqueous-based electrolytes remains a significant challenge. In this paper, we report Cu octahedra modified with low-coordinated Cu nanoclusters (Cu oct @Cu_NCs) as an efficient electrocatalyst for CO2RR to C2H5OH. Notably, the defect-site-rich Cu nanoclusters can promote the adsorption of *CO and increase the *CO coverage on Cu octahedra enclosed by Cu(111), thus promoting CO2RR to C2H5OH. We achieved a high Faradaic efficiency(FE) of 48.16 % for C2H5OH, and that of the C2+ products is 83.31 % at −1.17 V vs. RHE. This study provides a surface nanostructured route for rationalizing efficient and selective copper electrocatalysts for CO2RR.

Bi2S3 nanofiber bunch for highly efficient CO2 electroreduction to formate at low overpotential

Bi2S3 nanofiber bunch for highly efficient CO2 electroreduction to formate at low overpotential

Efficient acidic CO2 electroreduction to formic acid by modulating electrode structure at industrial-level current

Acidic CO2 electroreduction has the potential to synthesize low carbon footprint chemicals using renewable electricity. However, proton-rich environments can lead to strong hydrogen evolution reaction (HER) and severe degradation of electrochemical CO2 reduction reaction (CO2RR) performance. Modulating electrode structure plays a critical role in improving the local microenvironment of CO2RR electrolytes. Here, by covering the catalyst layer with a hydrophobic silica aerosol layer, the surface of the catalyst is protected from corrosion, thereby increasing the efficiency and stability of the system. In the flow cell, the Sn-C/SiO2-3 catalyst has high formic acid (HCOOH) selectivity at high current density (∼90 % at −400 mA cm−2). Importantly, the Sn-C/SiO2-3 catalyst can operate stably for 45 h at −400 mA cm−2. Combined in situ infrared analysis reveals that hydrophobic SiO2 aerosol layer can engineer the local microenvironment over the Sn-C catalyst surface by increasing the coverage of *OCHO to improve the CO2RR, which ultimately promotes high-efficiency CO2 conversion to HCOOH in strong acidic media. This surface coating strategy for adjusting the interface microenvironment can also be used for other electrocatalytic reactions.

A Unique Amorphous Porous BiSbOx Nanotube with Abundant Unsaturated Sb‐Stabilized BiO8−x Sites for Efficient CO2 Electroreduction in a Wide Potential Window

AbstractThe CO2 electroreduction reaction using sustainable electricity emerges as a viable strategy to produce high‐value‐added and profitable chemicals. The achievement of superior activity at a lower overpotential and higher selectivity in a wide potential window is vitally important for large‐scale industrial applications. Herein, a carbon‐composite amorphous porous BiSbOx nanotube with abundant unsaturated sites is reported to boost the conversion of CO2 to formate, exhibiting a formate selectivity of >90% in an extremely broad range of potentials from −0.5 to −1.4 V versus reversible hydrogen electrode (RHE) and a maximal energy conversion efficiency of 77.1%. Importantly, pure formic acid solution is directly obtained in a solid‐electrolyte cell for industrial‐scale applications. The porous tubular structure can expose more catalytic active sites, accelerate the mass transfer, and show fast surface charge transfer. Moreover, the unique coordination‐unsaturated Sb‐stabilized BiO8−x site can not only enhance the adsorption and activation of CO2 but also reasonably balance the stabilization and hydrogenation of *OCHO intermediate, thus leading to its obviously higher catalytic performance. As a result, a novel amorphous porous BiSbOx nanotube is successfully designed for efficient CO2 electroreduction, which may shed new light on developing many more amorphous composite metal oxide catalysts for conversion of inert small molecules.

Built-in Electric Field Promotes Interfacial Adsorption and Activation of CO2 for C1 Products over a Wide Potential Window.

The unsatisfactory adsorption and activation of CO2 suppress electrochemical reduction over a wide potential window. Herein, the built-in electric field (BIEF) at the CeO2/In2O3 n-n heterostructure realizes the C1 (CO and HCOO-) selectivity over 90.0% in a broad range of potentials from -0.7 to -1.1 V with a maximum value of 98.7 ± 0.3% at -0.8 V. In addition, the C1 current density (-1.1 V) of the CeO2/In2O3 heterostructure with a BIEF is about 2.0- and 3.2-fold that of In2O3 and a physically mixed sample, respectively. The experimental and theoretical calculation results indicate that the introduction of CeO2 triggered the charge redistribution and formed the BIEF at the interfaces, which enhanced the interfacial adsorption and activation of CO2 at low overpotentials. Furthermore, the promoting effect was also extended to CeO2/In2S3. This work gives a deep understanding of BIEF engineering for highly efficient CO2 electroreduction over a wide potential window.

Efficient CO2 Electroreduction to CO Facilitated by Porous Ag(111)‐dominant Ag Nanofoams and Cooperative Ionic Liquid Electrolytes

AbstractThe application of electrochemical CO2 reduction reaction (CO2RR) to generate value‐added products, including carbon monoxide (CO), represents a sustainable strategy for addressing the global carbon balance. Silver (Ag) has gained significant attention as an attractive and cost‐effective electrocatalyst for CO2RR‐to‐CO due to high activity. Here, the porous Ag nanofoam catalysts with Ag(111)‐dominant were prepared by in‐situ electrolysis‐deposition method in the ionic liquid (IL) electrolyte. The Ag nanofoam catalysts exhibited exceptional activity in converting CO2 to CO, with a high Faradaic efficiency (>95 %) in a wide range of −1.9 ~ −2.4 V vs. Ag/Ag+ in the 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([Bmim][BF4]) electrolyte. The maximum CO partial current density of −125.40 mA cm−2 was obtained on this Ag nanofoam catalyst, representing 62 % improvement over Ag(110)‐dominant Ag electrode (−77.35 mA cm−2) at −2.4 V vs. Ag/Ag+ in the [Bmim][BF4] electrolyte. Density functional theory calculations demonstrated that the Ag(111) crystal facet formed by in‐situ electrolysis‐deposition method prefers to adsorb [Bmim][BF4] which can stabilize the reaction intermediate, thereby weakening the reaction free energy and promoting CO2 electroreduction.

Efficient electroreduction of CO2 to CO on silver single-atom catalysts: Activity enhancement through coordinated modulation of polyaniline

The electrochemical reduction of CO2 to CO is considered to be one of the most mature routes that could be commercialized for efficient utilization of large amount of CO2. Silver (Ag) has shown high selectivity towards CO but usually requires high reduction potential. Herein, a novel Ag single atom-polyaniline (PANI) is constructed directly on gas diffusion electrode (GDE). The amino group with strong electron-donating ability in PANI can effectively promote the adsorption and activation of CO2. In addition, the adsorption of *COOH is significantly enhanced on N-coordinated Ag. The proposed catalyst electrode shows an outstanding performance with the Faraday efficiency of CO close to 100% at −0.75 V (vs. RHE), the energy efficiency is 12.8% and the output of CO is 108.7 mL cm−2 h−1, which exceeds most state-in-art Ag-based catalysts, highlighting the importance for the rational design and controllable fabrication of active sites and their favorable chemical environments.

Coordination Environment Engineering of Metal Centers in Coordination Polymers for Selective Carbon Dioxide Electroreduction toward Multicarbon Products.

Electrocatalytic carbon dioxide reduction reaction (CO2RR) toward value-added chemicals/fuels has offered a sustainable strategy to achieve a carbon-neutral energy cycle. However, it remains a great challenge to controllably and precisely regulate the coordination environment of active sites in catalysts for efficient generation of targeted products, especially the multicarbon (C2+) products. Herein we report the coordination environment engineering of metal centers in coordination polymers for efficient electroreduction of CO2 to C2+ products under neutral conditions. Significantly, the Cu coordination polymer with Cu-N2S2 coordination configuration (Cu-N-S) demonstrates superior Faradaic efficiencies of 61.2% and 82.2% for ethylene and C2+ products, respectively, compared to the selective formic acid generation on an analogous polymer with the Cu-I2S2 coordination mode (Cu-I-S). In situ studies reveal the balanced formation of atop and bridge *CO intermediates on Cu-N-S, promoting C-C coupling for C2+ production. Theoretical calculations suggest that coordination environment engineering can induce electronic modulations in Cu active sites, where the d-band center of Cu is upshifted in Cu-N-S with stronger selectivity to the C2+ products. Consequently, Cu-N-S displays a stronger reaction trend toward the generation of C2+ products, while Cu-I-S favors the formation of formic acid due to the suppression of C-C couplings for C2+ pathways with large energy barriers.

Dynamic Restructuring of Cu7S4/Cu for Efficient CO2 Electro-reduction to Formate.

Optimized catalytic properties and reactant adsorption energy played a crucial role in promoting CO2 electrocatalysis. Herein, Cu7S4/Cu underwent in situ dynamic restructuring to generate S-Cu2O/Cu hybrid catalyst for effective electrochemical CO2 reduction to formate that outperformed Cu2O/Cu and Cu7S4. Thermodynamic and in situ Raman spectra revealed that the optimized adsorption of the HCOO* intermediate on S-Cu2O/Cu was regulated and the H2 pathway (surface H) was suppressed by S-doping. Meanwhile, Cu7S4/Cu nanoflowers created abundant boundaries for ECR and strengthened the CO2 adsorption by inducing Cu. These findings provide a new perspective on synthetic methods for various electrocatalytic reduction processes.

Oxygen-tolerant CO2 electroreduction over covalent organic frameworks via photoswitching control oxygen passivation strategy

The direct use of flue gas for the electrochemical CO2 reduction reaction is desirable but severely limited by the thermodynamically favorable oxygen reduction reaction. Herein, a photonicswitching unit 1,2-Bis(5’-formyl-2’-methylthien-3’-yl)cyclopentene (DAE) is integrated into a cobalt porphyrin-based covalent organic framework for highly efficient CO2 electrocatalysis under aerobic environment. The DAE moiety in the material can reversibly modulate the O2 activation capacity and electronic conductivity by the framework ring-closing/opening reactions under UV/Vis irradiation. The DAE-based covalent organic framework with ring-closing type shows a high CO Faradaic efficiency of 90.5% with CO partial current density of −20.1 mA cm−2 at −1.0 V vs. reversible hydrogen electrode by co-feeding CO2 and 5% O2. This work presents an oxygen passivation strategy to realize efficient CO2 electroreduction performance by co-feeding of CO2 and O2, which would inspire to design electrocatalysts for the practical CO2 source such as flue gas from power plants or air.

Crystalline-Amorphous Interfaces Engineering of CoO-InOx for Highly Efficient CO2 Electroreduction to CO.

As a fundamental product of CO2 conversion through two-electron transfer, CO is used to produce numerous chemicals and fuels with high efficiency, which has broad application prospects. In this work, it has successfully optimized catalytic activity by fabricating an electrocatalyst featuring crystalline-amorphous CoO-InOx interfaces, thereby significantly expediting CO production. The 1.21%CoO-InOx consists of randomly dispersed CoO crystalline particles among amorphous InOx nanoribbons. In contrast to the same-phase structure, the unique CoO-InOx heterostructure provides plentiful reactive crystalline-amorphous interfacial sites. The Faradaic efficiency of CO (FECO ) can reach up to 95.67% with a current density of 61.72mA cm-2 in a typical H-cell using MeCN containing 0.5M 1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6 ) as the electrolyte. Comprehensive experiments indicate that CoO-InOx interfaces with optimization of charge transfer enhance the double-layer capacitance and CO2 adsorption capacity. Theoretical calculations further reveal that the regulating of the electronic structure at interfacial sites not only optimizes the Gibbs free energy of *COOH intermediate formation but also inhibits HER, resulting in high selectivity toward CO.

Electronic Regulation on Ni‐based Catalysts for Efficient CO2 Electroreduction

AbstractGreen power‐driven CO2 electroreduction using earth‐abundant Ni‐based catalysts is a highly promising approach to fulfill renewable energy storage and a negative carbon cycle. However, metallic Ni nanoparticles and clusters always show severe hydrogen evolution during CO2 electroreduction. Single atomic Ni catalysts demonstrate promising performance for CO2 electroreduction, whereas the very weak binding to intermediates and the very low metal loadings severely limit their activity, efficiency, and practical application. In this concept, we discuss how to regulate the electronic structures of Ni‐based catalysts to enhance the activity and selectivity of CO2 electroreduction. This work will offer instructive guidance in elaborating transition metal catalysts for highly efficient CO2 electroreduction, which may become suitable for future industrial applications.

Selective and Efficient CO2 Electroreduction to Formate on Copper Electrodes Modified by Cationic Gemini Surfactants

AbstractElectroreduction of CO2 into valuable chemicals and fuels is a promising strategy to mitigate energy and environmental problems. However, it usually suffers from unsatisfactory selectivity for a single product and inadequate electrochemical stability. Herein, we report the first work to use cationic Gemini surfactants as modifiers to boost CO2 electroreduction to formate. The selectivity, activity and stability of the catalysts can be all significantly enhanced by Gemini surfactant modification. The Faradaic efficiency (FE) of formate could reach up to 96 %, and the energy efficiency (EE) could achieve 71 % over the Gemini surfactants modified Cu electrode. In addition, the Gemini surfactants modified commercial Bi2O3 nanosheets also showed an excellent catalytic performance, and the FE of formate reached 91 % with a current density of 510 mA cm−2 using the flow cell. Detailed studies demonstrated that the double quaternary ammonium cations and alkyl chains of the Gemini surfactants played a crucial role in boosting electroreduction CO2, which can not only stabilize the key intermediate HCOO* but also provide an easy access for CO2. These observations could shine light on the rational design of organic modifiers for promoted CO2 electroreduction.

Selective and Efficient CO2 Electroreduction to Formate on Copper Electrodes Modified by Cationic Gemini Surfactants.

Electroreduction of CO2 into valuable chemicals and fuels is a promising strategy to mitigate energy and environmental problems. However, it usually suffers from unsatisfactory selectivity for a single product and inadequate electrochemical stability. Herein, we report the first work to use cationic Gemini surfactants as modifiers to boost CO2 electroreduction to formate. The selectivity, activity and stability of the catalysts can be all significantly enhanced by Gemini surfactant modification. The Faradaic efficiency (FE) of formate could reach up to 96 %, and the energy efficiency (EE) could achieve 71 % over the Gemini surfactants modified Cu electrode. In addition, the Gemini surfactants modified commercial Bi2 O3 nanosheets also showed an excellent catalytic performance, and the FE of formate reached 91 % with a current density of 510 mA cm-2 using the flow cell. Detailed studies demonstrated that the double quaternary ammonium cations and alkyl chains of the Gemini surfactants played a crucial role in boosting electroreduction CO2 , which can not only stabilize the key intermediate HCOO* but also provide an easy access for CO2 . These observations could shine light on the rational design of organic modifiers for promoted CO2 electroreduction.