Electrocatalytic CO2 Reduction Reaction Research Articles

Boosting Electroreduction of CO2over Cationic Covalent Organic Frameworks: Hydrogen Bonding Effects of Halogen Ions

AbstractWe present the first example of charged imidazolium functionalized porphyrin‐based covalent organic framework (Co‐iBFBim‐COF‐X) for electrocatalytic CO2reduction reaction, where the free anions (e.g., F−, Cl−, Br−, and I−) of imidazolium ions nearby the active Co sites can stabilize the key intermediate *COOH and inhibit hydrogen evolution reaction. Thus, Co‐iBFBim‐COF‐X exhibits higher activity than the neutral Co‐BFBim‐COF, following the trend of F−<Cl−<Br−<I−. Particularly, the Co‐iBFBim‐COF‐I−showed nearly 100 % CO2selectivity at a low full‐cell voltage of 2.3 V, and achieved a high CO2partial current density of 52 mA cm−2with a turnover frequency of 3018 h−1at 2.4 V in the anion membrane electrode assembly, which is 3.57 times larger than that of neutral Co‐BFBim‐COF. This work provides new insight into the importance of free anions in the stabilization of intermediates and decreasing the local binding energy of H2O with active moiety to enhance CO2reduction reaction.

Electric Double Layer Structure in Electrocatalytic Carbon Dioxide Reduction

Environmental degradation and climate change caused by excessive CO2 emissions have become the most serious challenges facing humanity. The electrocatalytic CO2 reduction reaction (CO2RR) is an ideal way to produce high‐value chemicals and solve environmental pollution at the same time. The electric double layer (EDL) structure at the electrochemical interface can greatly influence the local catalytic environment, but it is not fully understood. Herein, recent advances in EDL structure for CO2RR are focused. The main components of the EDL, including adsorbates on the electrode surface, metal/water interface, cations, anions, and differential capacitance are described. The latest experimental and theoretical understanding of EDL and their impact on CO2RR are presented. Finally, the current challenges encountered and the outlook for future development are discussed. This review describes the influence of EDL structure on CO2 reduction, which is of guiding significance for the design of new and efficient CO2 electrocatalysts.

Selective CO2 Reduction to Ethylene Over a Wide Potential Window by Copper Nanowires with High Density of Defects.

Electrochemical reduction of CO2 to ethylene using renewable electricity is an attractive approach for sustainable carbon recycling. In situ generation of defects in catalysts is found to be a promising method to guarantee high ethylene production from CO2 with high stability. In this study, copper nanowires are prepared in situ with a high density of defects for electrocatalytic CO2 reduction. These defects effectively improve C-C coupling, thus realizing a remarkable performance toward CO2 reduction to C2 products. The obtained copper nanowires showed a high selectivity of ∼79% for C2 products and >58% for C2H4. More importantly, a significantly wide potential window of 500 mV was realized for the selective production of C2H4 with FE(C2H4) >55%. Finally, in situ Raman spectroscopy revealed that Cu0 is the real reactive site for the electrocatalytic CO2 reduction reaction.

Surface and Interface Engineering for the Catalysts of Electrocatalytic CO2 Reduction.

The massive use of fossil fuels releases a great amount of CO2 , which substantially contributes to the global warming. For the global goal of putting CO2 emission under control, effective utilization of CO2 is particularly meaningful. Electrocatalytic CO2 reduction reaction (eCO2 RR) has great potential in CO2 utilization, because it can convert CO2 into valuable carbon-containing chemicals and feedstock using renewable electricity. The catalyst design for eCO2 RR is a key challenge to achieving efficient conversion of CO2 to fuels and useful chemicals. For a typical heterogeneous catalyst, surface and interface engineering is an effective approach to enhance reaction activity. Herein, the development and research progress in CO2 catalysts with focus on surface and interface engineering are reviewed. First, the fundaments of eCO2 RR is briefly discussed from the reaction mechanism to performance evaluation methods, introducing the role of the surface and interface engineering of electrocatalyst in eCO2 RR. Then, several routes to optimize the surface and interface of CO2 electrocatalysts, including morphology, dopants, atomic vacancies, grain boundaries, surface modification, etc., are reviewed and representative examples are given. At the end of this review, we share our personal views in future research of eCO2 RR.

Near-Unity Electrochemical CO2 to CO Conversion over Sn-Doped Copper Oxide Nanoparticles.

Bimetallic electrocatalysts have emerged as a viable strategy to tune the electrocatalytic CO2 reduction reaction (eCO2RR) for the selective production of valuable base chemicals and fuels. However, obtaining high product selectivity and catalyst stability remain challenging, which hinders the practical application of eCO2RR. In this work, it was found that a small doping concentration of tin (Sn) in copper oxide (CuO) has profound influence on the catalytic performance, boosting the Faradaic efficiency (FE) up to 98% for carbon monoxide (CO) at -0.75 V versus RHE, with prolonged stable performance (FE > 90%) for up to 15 h. Through a combination of ex situ and in situ characterization techniques, the in situ activation and reaction mechanism of the electrocatalyst at work was elucidated. In situ Raman spectroscopy measurements revealed that the binding energy of the crucial adsorbed *CO intermediate was lowered through Sn doping, thereby favoring gaseous CO desorption. This observation was confirmed by density functional theory, which further indicated that hydrogen adsorption and subsequent hydrogen evolution were hampered on the Sn-doped electrocatalysts, resulting in boosted CO formation. It was found that the pristine electrocatalysts consisted of CuO nanoparticles decorated with SnO2 domains, as characterized by ex situ high-resolution scanning transmission electron microscopy and X-ray photoelectron spectroscopy measurements. These pristine nanoparticles were subsequently in situ converted into a catalytically active bimetallic Sn-doped Cu phase. Our work sheds light on the intimate relationship between the bimetallic structure and catalytic behavior, resulting in stable and selective oxide-derived Sn-doped Cu electrocatalysts.

Recent Advances of the Confinement Effects Boosting Electrochemical CO2 Reduction.

Powered by clean and renewable energy, electrocatalytic CO2 reduction reaction (CO2 RR) to chemical feedstocks is an effective way to mitigate the greenhouse effect and artificially close the carbon cycle. However, the performance of electrocatalytic CO2 RR was impeded by the strong thermodynamic stability of CO2 molecules and the high susceptibility to hydrogen evolution reaction (HER) in aqueous phase systems. Moreover, the numerous reaction intermediates formed at very near potentials lead to poor selectivity of reaction products, further preventing the industrialization of CO2 RR. Catalysis in confined space can enrich the reaction intermediates to improve their coverage at the active site, increase local pH to inhibit HER, and accelerate the mass transfer rate of reactants/products and subsequently facilitate CO2 RR performance. Therefore, we summarize the research progress on the application of the confinement effects in the direction of CO2 RR in theoretical and experimental directions. We first analyzed the mechanism of the confinement effect. Subsequently, the confinement effect was discussed in various forms, which can be characterized as an abnormal catalytic phenomenon due to the relative limitation of the reaction region. In specific, based on the physical structure of the catalyst, the confinement effect was divided in four categories: pore structure confinement, cavity structure confinement, active center confinement, and other confinement methods. Based on these discussions, we also have summarized the prospects and challenges in this field. This review aims to stimulate greater interests for the development of more efficient confined strategy for CO2 RR in the future.

Ag-organic coordination polymers with multi-dimensional electron transfer channels for enhancing CO2 electroreduction

Although, electrons transfer plays an important role in achieving the electrocatalytic CO2 reduction reaction (CO2RR), it is scarce to regulate the electron transfer channels for enhancing the performance of CO2RR via rational design. Here, based on different solvothermal conditions, two metal–organic coordination polymers (MOCPs, Ag-MOCP-1 and Ag-MOCP-2) electrocatalyst for CO2RR were obtained by the same Ag ions and 5-phenyltetrazolium ligands. Single crystal X-ray diffraction showed Ag-MOCP-2 with intra-&inter-layer CH···π and π···π interactions, while Ag-MOCP-1 with intra-layer π···π interactions. The intra-&inter-layer π interactions in Ag-MOCP-2 makes it show multi-dimensional electron transfer channels, which enable Ag-MOCP-2 with higher electron conductivities and lower band gap than Ag-MOCP-1. And the Faradaic efficiency of CO for Ag-MOCP-2 with 78.3 % is higher than that for Ag-MOCP-1 with 69.1 % at −1.08 V vs RHE, and the maximum difference can reach 22 % at −1.48 V vs RHE. Based on the same central metal and organic ligands of the two MOCPs, we believe that multi-dimensional electron transfer channels make a predominant contribution for improving CO2RR performance in this work.

Enhancing CO2 electroreduction to syngas by active protons of imidazolium ionic liquids: From performance to mechanism

High current density at specific CO/H2 molar ratios is the key to the industrial application of heterogeneous electrocatalytic CO2 reduction reaction (CO2RR) to syngas, which is still very challenging. Here, we report a Ni-foam-supported nano-Ag catalyst for CO2RR in ionic liquid (IL)-acetonitrile electrolyte originally designed for CO2RR-to-CO. It is surprising to find that this is a highly efficient catalytic system for CO2RR-to-syngas, the CO/H2 molar ratio in the syngas can be tuned from 1:5–26:1, and the current density is as high as 363.6, 458.2, and 644.7 mA cm−2 at the CO/H2 molar ratios of 1:1, 1:2, and 1:3, respectively. The high efficiency of CO2RR-to-syngas is attributed to the synergistic stabilization action of nano-Ag and IL cationic C4/5-H for intermediate *COO- to produce CO, as well as cationic C2-H contribution to hydrogen evolution. This work opens a new way to improve catalytic performance of CO2RR-to-syngas.

Electrocatalytic conversion of CO2 into CO over dual-metal M-N-C catalyst in a flow cell

Exploring efficient metal-nitrogen-doped carbon (M-N-C) catalyst is urgent to improve the performance of electrocatalytic CO2 reduction reaction (CO2RR). In this study, a series of dual-metal M-N-C electrocatalysts (Ni, Fe-N-C) under different calcination temperatures (at 950 °C, 1000 °C and 1050 °C) was prepared by a heat-treatment process. The electrochemical results show that, in a flow cell, as-prepared optimal Ni, Fe-N-C (1000 °C) indicates the highest selectivity for CO (at −1.0 V), with a FECO of 92.37 % and CO partial current density of − 13.76 mA cm−2. Moreover, in 24 h stability test, the current densities of Ni, Fe-N-C (1000 °C) always remains around − 10 mA cm−2, and the FECO remains above 86 %, indicating it has high efficiency and good long-term stability. This study is a promising and cost-effective exploration of the dual-metal M-N-C catalysts for CO2RR, would provide a valuable case for exploring the other M-N-C in future.

Ultrathin ZnIn2S4 nanosheet arrays activated by nitrogen-doped carbon for electrocatalytic CO2 reduction reaction toward ethanol

Ternary ZnIn2S4 (ZIS) is a kind of potential electrocatalyst for CO2 reduction reaction (CO2RR). However, the reported reduction products are usual C1 species. Considering that N doped carbon can increase adsorption of CO2 molecules, facilitate electron transfer, and activate active sites, here ultrathin ZIS nanosheet arrays (ZIS NSAs) grown on N doped carbon cloth (NDCC) were synthesized through a facile hydrothermal route. The as-synthesized ZIS NSAs/NDCC enabled electrocatalytic CO2RR toward ethanol with the highest 42 % faradaic efficiency (FE) at an applied potential of − 0.7 V versus the reversible hydrogen electrode (RHE) in CO2-saturated 0.5 M KHCO3 aqueous solution. This is first report on ZIS-based electrocatalyst for electrocatalytic CO2RR toward ethanol production. The experimental results and density functional theory (DFT) calculations showed that NDCC could enhance the CO2RR activity and promote CO-CO coupling process. Our work not only provides a new electrocatalyst for CO2RR to ethanol but also stresses the influence of N doped carbon substrate and bimetallic effect on electrocatalytic CO2RR.

Modulating the Local Electronic Property and Intermediate Binding Energy of ZnO by Heteroatom Doping for Efficient Electrocatalytic CO2 Reduction

AbstractEngineering the electronic properties of catalysts by element doping to obtain appropriate intermediate binding energy is a promising strategy to design efficient catalysts for electrocatalytic CO2 reduction reaction (CO2RR). Herein, we dope ZnO with heteroatoms (Bi, Mn, Co) of different external electronic activities and electronegativities, which lead to different CO2RR performances. The introduction of Bi (p‐block metal element with stable external electrons and relatively high electronegativity) into ZnO results in enhanced CO2RR performance. Density functional theory (DFT) calculations demonstrate that Bi−ZnO could regulate the local charge distribution of ZnO, as well as weak the binding energy of *H and *COOH while increase the binding energy of *OCHO, thus significantly inhibit hydrogen evolution reaction (HER) and accelerate CO2RR. This work demonstrates the feasibility of modulating electronic property of electrocatalysts in optimizing CO2RR selectivity and activity.

Bismuth nanosheets with rich grain boundaries for efficient electroreduction of CO2 to formate under high pressures

AbstractElectrochemical CO2 reduction reaction (CO2RR) driven by sustainable energy has emerged as an attractive route to achieve the target of carbon neutral. Formate is one of the most economically viable products, and electrocatalytic CO2RR to formate is a promising technology. High-pressure H-cell electrolyzer is easy to operate and allows high CO2 solubility for realizing high current density, but the design of highly efficient catalysts for working under high CO2 pressures remains challenging. Bismuth-based catalysts exhibit high formate selectivity, but suffer from limited activity. Here, we report a high-performance catalyst, which is derived from BiPO4 nanopolyhedrons during electrocatalytic CO2RR to formate in neutral solution under high CO2 pressures. A high partial current density of formate (534 mA cm−2) and formate formation rate (9.9 mmol h−1 cm−2) with a formate Faradaic efficiency of 90% have been achieved over BiPO4-derived catalyst at an applied potential of −0.81 V vs. RHE under 3.0 MPa CO2 pressure. We discover that BiPO4 nanopolyhedrons evolve into metallic Bi nanosheets with rich grain boundaries in electrocatalytic CO2RR under high CO2 pressures, and the grain boundaries of the BiPO4-derived catalyst play a vital role in promoting electrocatalytic CO2RR to formate. Our theoretical studies reveal that the charge redistribution occurs at the grain boundaries of Bi surface, and this promotes CO2 activation and increases HCOO* intermediate stability, thus making the pathway for CO2RR to formate more selective and energy-favorable. This work not only demonstrates a highly efficient catalyst for CO2RR to formate but also discovers a unique feature of catalyst evolution under high CO2 pressures.

Iron porphyrin with appended guanidyl group for significantly improved electrocatalytic carbon dioxide reduction activity and selectivity in aqueous solutions

AbstractIron porphyrins have high activity and selectivity for electrocatalytic CO2 reduction reaction (CO2RR) in nonaqueous solutions, but they usually display poor or moderate selectivity for CO2RR in aqueous solutions because of the competitive hydrogen evolution reaction. Using water as the electrocatalytic reaction solvent is more favored because not only it is cheap, green and abundant but also it can sufficiently provide protons required for CO2RR. Therefore, developing Fe porphyrins as electrocatalysts for efficient and selective CO2RR in aqueous solutions is of both fundamental and practical significance. Herein, we report the design and synthesis of Fe porphyrin 1 with an appended guanidyl group and its electrocatalytic features for CO2RR in both nonaqueous and aqueous solutions. In acetonitrile, Fe porphyrin 1 and its guanidyl-free analogue, tetrakis(3,4,5-trimethoxyphenyl)porphyrin 2, are both efficient for electrocatalytic CO2-to-CO conversion, but the turnover frequency with 1 (3.9 × 105 s−1) is one order of magnitude larger than that with 2 (1.7 × 104 s−1), showing the critical role of the appended guanidyl group in improving electrocatalytic CO2RR activity. More importantly, in 0.1 mol L−1 KHCO3 aqueous solutions, 1 showed very high selectivity for electrocatalytic CO2-to-CO conversion with a Faradaic efficiency of 96%, while 2 displayed a Faradaic efficiency of 65% for the CO2-to-CO conversion. This work is of significance to show the effect of appended guanidyl group on improving both activity and selectivity of Fe porphyrins for CO2RR electrocatalysis.

Electrocatalytic Carbon Dioxide Reduction to Ethylene over Copper-based Catalytic Systems.

The electrocatalytic carbon dioxide (CO2 ) conversion to ethylene (C2 H4 ) has attracted significant attention in recent years. Copper-based catalytic systems have been proven to be the most efficient for producing C2 H4 from electrocatalytic CO2 reduction reaction. In this review, we present the recent progress on the electrocatalytic CO2 reduction to C2 H4 over copper-based catalytic systems, mainly focusing on reaction mechanism, design of catalysts and influences of electrolyte, CO2 supplement and electrolyzer on activity, selectivity and stability.

Hybrid Catalyst Coupling Single-Atom Ni and Nanoscale Cu for Efficient CO2 Electroreduction to Ethylene.

A hybrid catalyst with integrated single-atom Ni and nanoscale Cu catalytic components is reported to enhance the C-C coupling and ethylene (C2H4) production efficiency in the electrocatalytic CO2 reduction reaction (eCO2RR). The single-atom Ni anchored on high-surface-area ordered mesoporous carbon enables high-rate and selective conversion of CO2 to CO in a wide potential range, which complements the subsequent CO enrichment on Cu nanowires (NWs) for the C-C coupling to C2H4. In situ surface-enhanced infrared absorption spectroscopy (SEIRAS) confirms the substantially improved CO enrichment on Cu, once the incorporation of single-atom Ni occurs. Also, in situ X-ray absorption near-edge structure (XANES) demonstrates the structural stability of the hybrid catalyst during eCO2RR. By modulating hybrid compositions, the optimized catalyst shows 66% Faradaic efficiency (FE) in an alkaline flow cell with over 100 mA·cm-2 at -0.5 V versus reversible hydrogen electrode, leading to a five-order enhancement in C2H4 selectivity compared with single-component Cu NWs.

Highly Dispersed NiO Clusters Induced Electron Delocalization of NiNC Catalysts for Enhanced CO2 Electroreduction

AbstractOxygen‐regulated Ni‐based single‐atom catalysts (SACs) show great potential in accelerating the kinetics of electrocatalytic CO2 reduction reaction (CO2RR). However, it remains a challenge to precisely control the coordination environment of NiO moieties and achieve high activity at high overpotentials. Herein, a facile carbonization coupled oxidation strategy is developed to mass produce NiO clusters‐decorated NiNC SACs that exhibit a high Faradaic efficiency of CO (maximum of 96.5%) over a wide potential range (−0.9 to −1.3 V versus reversible hydrogen electrode) and a high turnover frequency for CO production of 10 120 h−1 even at the high overpotential of 1.19 V. Density functional theory calculations reveal that the highly dispersed NiO clusters induce electron delocalization of active sites and reduce the energy barriers for *COOH intermediates formation from CO2, leading to an enhanced reaction kinetics for CO production. This study opens a new universal pathway for the construction of oxygen‐regulated metal‐based SACs for various catalytic applications.

Across the Board: Rui Cao on Electrocatalytic CO2 Reduction.

In this series of articles, the Board Members of ChemSusChem review recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. Rui Cao, who discusses how tuning the second-sphere environments of Fe porphyrins can improve the activity and selectivity for CO2 reduction. Substituents with proton relay capability, hydrogen-bonding, and electrostatic features have significant impact on the efficiency of Fe porphyrins for the electrocatalytic CO2 reduction reaction.

A Honeycomb-Like Porous Crystalline Hetero-Electrocatalyst for Efficient Electrocatalytic CO2 Reduction.

Porous heterostructured electrocatalysts with multifunctionality and synergistic effect have much benefit for efficient electrocatalytic CO2 reduction reaction (CO2 RR), yet it still remains a daunting challenge to explore heterostructures based on covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) in this field. Here, a series of honeycomb-like porous crystalline hetero-electrocatalysts (MCH-X, X= 1-4, X stands for the numbered sample obtained from different MOF doses in the synthesis of the MCH) are synthesized, and these are successfully applied in electrocatalytic CO2 RR. The specially designed heterostructures with integrated porous MOF-template and ultrathin COF-coating enable efficient CO2 adsorption/activation and conversion into CH4 . The best of them, MCH-3, shows greatly inhibited H2 evolution, excellent current density (-398.1mA cm-2 ), and superior (76.7%) to the physical mixture (38.0%), the MOF@COF without the honeycomb-like morphology (47.7%), and the bare COF (37.5%) and MOF (15.9%) at -1.0V. Based on the density functional theory calculations and various characterizations, the vital roles of the MOF in facilitating CO2 adsorption/activation, stabilizing intermediates, and conquering the energy barrier of rate-determining step are intensively studied.

Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

Thermo-, Electro-, and Photocatalytic CO2 Conversion to Value-Added Products over Porous Metal/Covalent Organic Frameworks.

ConspectusThe continuing increase of the concentration of atmospheric CO2 has caused many environmental issues including climate change. Catalytic conversion of CO2 using thermochemical, electrochemical, and photochemical methods is a potential technique to decrease the CO2 concentration and simultaneously obtain value-added chemicals. Due to the high energy barrier of CO2 however, this method is still far from large-scale applications which requires high activity, selectivity, and stability. Therefore, development of efficient catalysts to convert CO2 to different products is urgent. With their well-engineered pores and chemical compositions, high surface area, elevated CO2 adsorption capability, and adjustable active sites, porous crystalline frameworks including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are potential materials for catalytic CO2 conversion. Here, we summarize our recent work on MOFs and COFs for thermocatalytic, electrocatalytic, and photocatalytic CO2 conversion and describe the structure-activity relationships that could guide the design of effective catalysts.The first section of this paper describes imidazolium-functionalized porous MOFs, including porous liquid and cationic MOFs with nucleophilic halogen ions, which can promote thermocatalytically CO2 cycloaddition reaction with epoxides toward cyclic carbonates at one bar pressure. A porous liquid MOF takes on the role of a CO2 reservoir to tackle the low local CO2 concentrations in gas-liquid-solid heterogeneous reactions. Imidazolium-functionalized MOFs with halogen ions for CO2 cycloaddition could avoid the use of cocatalysts, and this leads to milder and more facile experimental conditions and separation processes.In a section dealing with the electrocatalytic CO2 reduction reaction (CO2RR), we developed a series of conductive porous framework materials with fast electron transmission capabilities, which afford high current densities and outperform the traditional MOF and COF catalysts that have been reported. The intrinsically conductive two-dimensional 2D MOFs and COFs nanosheets based on the fully π-conjugated phthalocyanine motif with excellent electron transport capability were prepared, and strong electron transporters were also integrated into metalloporphyrin-based COFs for CO2RR. Cu2O quantum dots and Cu nanoparticles (NPs) can be uniformly dispersed on porous conductive MOFs/COFs to afford synergistic and/or tandem electrocatalysts, which can achieve highly selective production of CH4 or C2H4 in CO2RR.A third section describes our efforts to facilitate electron-hole separation in CO2 photocatalysis. Our focus is on regulation of coordination spheres in MOFs, fabrication of the architecture of MOF heterojunctions, and engineering MOF films to facilitate photocatalytic CO2 reduction.Finally, we discuss several problems associated with the studies of MOFs and COFs for CO2 conversion and consider some prospects of the fabrication of effective porous frameworks for CO2 adsorption and conversion.