CO2 Reduction Reaction Selectivity Research Articles

Morphology and composition dependence of multicomponent Cu-based nanoreactor for tandem electrocatalysis CO2 reduction

Multicomponent heterogeneous catalysts present excellent catalytic performance attributed to the synergistic effect of multi-sites. Nevertheless, identifying the composition of the multi-sites and exploring the synergistic catalytic mechanisms for the multiple active sites in electrocatalytic CO2 reduction reaction (CO2RR) still lack intensive study. This work regulates the chemical composition of Cu-based nanoreactors readily by adjusting the geometrical morphology of metal-organic frameworks precursor. The obtained cuboctahedron nanoreactor containing Cu-N4/Cu2O/Cu multiple active sites exhibits excellent CO2RR selectivity towards deep reduction product (80%) with high current density. Moreover, the tandem catalytic mechanism of multicomponent active sites has been studied intensively. The CO2 molecule is firstly reduced in Cu-N4 sites to form CO and then the CO is transfered to Cu2O/Cu sites for further deep reduction. The high concentration of CO provided by Cu-N4 sites decreases the free energy of rate-determining step for CH4 products in Cu2O sites. This work provides a promising direction for designing and synthesizing multicomponent Cu-based tandem catalysts to access high efficiency and selectivity in the electrocatalytic CO2 reduction reaction.

Well-Defined Copper-Based Nanocatalysts for Selective Electrochemical Reduction of CO2 to C2 Products

The electrochemical CO2 reduction reaction (CO2RR) holds the promise to revolutionize the chemical industry by producing value-added chemicals and fuels from CO2 and water while storing renewable energy. However, catalyst design still remains one of the major hurdles in the field. This Perspective discusses the current and future contributions of well-defined nanocatalysts to improving the CO2RR selectivity toward C2 products, particularly ethylene and ethanol. In this regard, the shape and size selectivity dependence of single metal copper nanocrystals is briefly reviewed and linked to single-crystal studies. Representative studies on Cu-based bimetallic nanocrystals are discussed to highlight the importance of composition and distribution of the metals for selectivity. Finally, a vision on design strategies in terms of shape and composition for the next generation of CO2RR nanocatalysts is proposed.

Understanding the effect of transition metals and vacancy boron nitride catalysts on activity and selectivity for CO2 reduction reaction to valuable products: A DFT-D3 study

Single-atom catalysts have recently emerged as a promising approach for catalyzing the electrochemical CO2 reduction reaction (CRR). Transition metal (TM) atom doping to 2-dimensional layer material has been studied for CRR, but compared to studies on TM doped single vacancy (TM-SV) sites, those on double vacancies (TM-DV) sites are minor. In this research, we investigated the doping of 26 (3d-, 4d-, and 5d-groups) TM atoms to the DV of boron nitride nanosheets (BN) using the dispersion-corrected density functional theory method for the complete CRR mechanism. We analyzed the limiting potential of the reactions of different TM-DVBN using the integrated crystal orbital Hamiltonian partition (ICOHP) of TM–O binding, universal descriptor, charge, and the number of valence electrons. We found the volcano plot model which suggests that a moderate OH binding energy of around −0.50 eV, the universal descriptor value around 9.40, and the ICOHP descriptor around −0.20 will provide the lowest limiting potential for CRR. From these studies, we find Ni-DVBN is the most reactive and can produce CH3OH at −0.48 V. This is much better than Ni-SVBN, which requires −1.0 V to produce HCOOH also lower than Fe-SVBN (−0.52 V), which was the best catalyst in the previous study of TM-SVBN. This shows that Ni doping to DVBN is more effective for CRR compared to doping to SVBN.

Copper(II) Frameworks with Varied Active Site Distribution for Modulating Selectivity of Carbon Dioxide Electroreduction.

Metal-organic frameworks (MOFs) can be utilized as electrocatalysts for CO2 reduction reaction (CO2RR) due to their well dispersed metal centers. However, the influence of metal node distribution on electrochemical CO2RR was rarely explored. Here, three Cu-MOFs with different copper(II) site distribution were employed for CO2 electroreduction. The Cu-MOFs [Cu(L)SO4]·H2O (Cu1), [Cu(L)2(H2O)2](CH3COO)2·H2O (Cu2), and [Cu(L)2(H2O)2](ClO4)2 (Cu3) were achieved by using the same ligand 1,3,5-tris(1-imidazolyl)benzene (L) but different Cu(II) salts. The results show that the Faraday efficiency of CO (FECO) for Cu1 is 4 times that of the FEH2, while the FECO of Cu2 is twice that of the FEH2. As for Cu3, there is not much difference between FECO and FEH2. Such difference may arise from the distinct electrochemical active surface area and charge transfer kinetics caused by different copper site distribution. Furthermore, the different framework structures also affect the activity of the copper sites, which was supported by the theoretically calculated Gibbs free energy and electron density, contributing to the selectivity of CO2RR. This study provides a strategy for modulating the selectivity of CO2RR by tuning the distribution of the active centers in MOFs.

Effect of nickel-based electrocatalyst size on electrochemical carbon dioxide reduction: A density functional theory study

The electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) used for converting higher-value chemicals is a promising solution to mitigate CO2 emissions. Nickel (Ni)-based catalysts have been identified as a potential candidate for CO2 activation and conversion. However, in the CO2RR, the size effect of the Ni-based electrocatalysts has not been well explored. Herein, the single Ni atom and the Ni4 cluster doped nitrogen-doped carbon nanotube (Ni@CNT and Ni4@CNT), and the Ni (110) facet were designed to explore the size effect in the CO2RR by using density functional theory (DFT) calculations. The results show that carbon monoxide (CO) is produced on the Ni@CNT with a free energy barrier of 0.51 eV. The reduction product of CO2 on the Ni4@CNT and Ni(110) facet is methane (CH4) in both cases, via different reaction pathways, and the Ni(110) facet is a more efficient electrocatalyst with a low overpotential of 0.27 V when compared to Ni4@CNT (0.50 V). The rate-determining step (RDS) is the formation of *CHO on the Ni4@CNT (The “*” represents the catalytic surface), while the *COH formation is the RDS on the Ni(110) facet. Meanwhile, the Ni(110) facet also has the highest selectivity of CH4 among the three catalysts. The CO2 reduction product changes from CO to CH4 with the increasing size of the Ni-based catalysts. These results demonstrate that the catalytic activity and selectivity of CO2RR highly depend on the size of the Ni-based catalysts.

Guiding CO2RR Selectivity by Compositional Tuning in the Electrochemical Double Layer.

The electrochemical conversion of carbon dioxide to value-added chemicals provides an environmentally benign alternative to current industrial practices. However, current electrocatalytic systems for the CO2 reduction reaction (CO2RR) are not practical for industrialization, owing to poor specific product selectivity and/or limited activity. Interfacial engineering presents a versatile and effective method to direct CO2RR selectivity by fine-tuning the local chemical dynamics. This Account describes interfacial design strategies developed in our laboratory that use electrolyte engineering and porous carbon materials to modify the local composition at the electrode-electrolyte interface.Our first strategy for influencing surface reactivity is to perturb the electrochemical double layer by tuning the electrolyte composition. We approached this investigation by considering how charged molecular additives can organize at the electrode surface and impact CO2 activation. Using a combination of advanced electrochemical techniques and in situ vibrational spectroscopy, we show that the surfactant properties (the identity of the headgroup, alkyl chain length, and concentration) as well as the electrolyte cation identity can affect how surfactant molecules assemble at a biased electrode. The interplay between the electrolyte cations and the surfactant additives can be regulated to favor specific carbon products, such as HCOO-, and suppress the parasitic hydrogen evolution reaction (HER). Together, our findings highlight how molecular assemblies can be used to design selective electrocatalytic systems.In addition to the electrolyte design, the local spatial confinement of reaction intermediates presents another strategy to direct CO2RR selectivity. We were interested in uncovering the role of porous carbon-supported catalysts toward selective carbon product formation. In our initial study, we show that carbon porosity can be optimized to enhance C2H4 and CO selectivity in a series of Cu catalysts embedded in a tunable carbon aerogel matrix. These results suggested that local confinement of the active surface plays a role in CO2 activation and motivated an investigation into probing how this phenomenon can be translated to a planar Cu electrode. Our findings show that carbon modifiers facilitated surface reconstruction and regulated CO2 diffusion to suppress HER and improve the C2-3 product selectivity. Given the ubiquity of carbon materials in catalysis, this work demonstrates that carbon plays an active role in regulating selectivity by restricting the diffusion of substrate and reaction intermediates. Our work in tuning the composition of the electrochemical double layer for increased CO2RR selectivity demonstrates the potential versatility in boosting catalytic performance across an array of catalytic systems.

Spatially separated oxygen vacancies and nickel sites for ensemble promotion of selective CO2 photoreduction to CO

The CO2 reduction reaction (CRR), which is key to addressing global environmental and energy issues, must overcome the strong C = O bond and is hindered by low CRR selectivity and activity. Here, we unravel the ensemble effect of catalytically active sites for promotion of CRR while suppressing hydrogen evolution reactions (HERs). We design a 2D/2D hybrid consisting of an ordered assembly of nickel-rich Ni-Mn layered double hydroxide (LDH) nanosheets (NSs) and oxygen vacancies (OVs)-rich Mn3O4 (OVs-Mn3O4) NSs. On its own, Ni-Mn LDH demonstrates high CRR selectivity but mediocre CRR efficiency, whereas OVs-Mn3O4 gives a higher efficiency but poorer selectivity. The 2D/2D hybrid inherits advantages from both components, attaining high CRR selectivity and markedly higher efficiency than Ni-Mn LDH and OVs-Mn3O4. Calculations and in situ Fourier transform infrared spectroscopy (FT-IR) measurements reveal that nickel sites in the 2D/2D hybrid activate CO2 to generate CO2•– via a one-electron injection, whereas OVs in OVs-Mn3O4 promote light harvesting and charge transport without activating unwanted HERs.

Selective bimetallic sites supported on graphene as a promising catalyst for CO2 Reduction: A first-principles study

Developing efficient and inexpensive electrocatalysts for CO2 reduction reaction (CRR) has been a key scientific issue. Several factors limit the electrocatalyst efficiency of materials, including the relatively high overpotential, low stability and low selectivity for CRR. The use of bimetallic catalyst systems is an efficient approach to improve the catalytic performance. In this study, by using the density functional theory (DFT) calculations, we explore the CRR processes of three different bimetal doped graphenes (M1M2/DG (M1, M2 = Cu, Fe, Ni)). Various reduction reaction pathways of CO2 lead to different products, including CH4, CH3OH, HCOOH and CO. The Eads of different intermediates on different M1M2/DG, the free energy variation and the overpotential of the different M1M2/DG were analyzed. The obtained results confirm the CO2 capture ability of all the studied M1M2/DG systems. The low overpotential of 0.49 V for Cu_Ni/DG is even lower than that of the most outstanding metallic electrocatalyst (Cu (211)). This work provides useful information for the development of efficient CRR electrocatalysts.

Selectivity in Electrochemical CO2 Reduction.

The electrocatalytic CO2 reduction reaction (CO2RR) to generate fixed forms of carbons that have commercial value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO2 emissions as well as to generate carbon-based feed chemicals, which are generally obtained from the petrochemical industry. The area of electrochemical CO2RR has seen substantial activity in the past decade, and several good catalysts have been reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward the more chemically challenging issue of selectivity. CO2 can be partially reduced to produce several C1 products like CO, HCOOH, CH3OH, etc. before its complete 8e-/8H+ reduction to CH4. In addition to that, the low-valent electron-rich metal centers deployed to activate CO2, a Lewis acid, are prone to reduce protons, which are a substrate for CO2RR, leading to competing hydrogen evolution reaction (HER). Similarly, the low-valent metal is prone to oxidation by atmospheric O2 (i.e., it can catalyze the oxygen reduction reaction, ORR), necessitating strictly anaerobic conditions for CO2RR. Not only is the requirement of O2-free reaction conditions impractical, but it also leads to the release of partially reduced O2 species such as O2-, H2O2, etc., which are reactive and result in oxidative degradation of the catalyst.In this Account, mechanistic investigations of CO2RR by detecting and, often, chemically trapping and characterizing reaction intermediates are used to understand the factors that determine the selectivity in CO2RR. The spectroscopic data obtained from different intermediates have been identified in different CO2RR catalysts to develop an electronic structure selectivity relationship that is deemed to be important for deciding the selectivity of 2e-/2H+ CO2RR. The roles played by the spin state, hydrogen bonding, and heterogenization in determining the rate and selectivity of CO2RR (producing only CO, only HCOOH, or only CH4) are discussed using examples of both iron porphyrin and non-heme bioinspired artificial mimics. In addition, strategies are demonstrated where the competition between CO2RR and HER as well as CO2RR and ORR could be skewed overwhelmingly in favor of CO2RR in both cases.

Activity switching of Sn and In species in Heusler alloys for electrochemical CO2 reduction.

The electrochemical CO2 reduction reaction (CO2RR) activity of Ni2MnIn and Ni2MnSn Heusler alloys was investigated. Although pure In, Sn and Ni2MnIn generated formate as the major product, Ni2MnSn generated H2 as the major product. The CO2RR selectivity could be controlled by selecting the constituent elements of the intermetallic catalysts.

Optimizing the Oxygen Vacancies Concentration of Thin NiO Nanosheets for Efficient Selective CO2 Photoreduction

In designing highly efficient CO2 reduction reaction (CRR) photocatalysts with excellent selectivity and efficiency, a key limitation is the poor understanding on the mechanism response of the active sites of catalysts to CRR selectivity and activity. Herein, it is revealed how the concentration of point defect affects the CRR selectivity and activity of catalysts. Quasi‐2D NiO nanosheets (NSs) that are composed of NiO nanoparticles (NPs), and finely tuned the oxygen vacancies (OVs) concentration of the NSs by regulating the grain size (≈5–25 nm) of NPs are created. The NiO with moderate OVs concentration has the highest photocatalytic CRR efficiency and selectivity (V CO = 17.2 μmol h−1, 98.3%), outperforming other prepared NiO catalysts and reported Ni‐based photocatalysts. Density functional theory calculation associated with the CO2 temperature‐programmed desorption confirms that the moderate OVs concentration enables strong CO2 binding to promote CO2 adsorption and activation and allows efficient charge transfer. In contrast, excessive OVs reduce the CO2 binding affinity and restrain charge mobility, both detrimental to the CRR performance.

Single Nickel Atom-Modified Phosphorene Nanosheets for Electrocatalytic CO2 Reduction

Nowadays, carbon dioxide (CO2) produced by global energy consumption far exceeds what the environment can absorb. So, the world is seeking a way to control and reduce CO2 emissions. The electrocatalytic CO2 reduction reaction (CRR) can effectively convert this greenhouse gas into energy sources, thus providing a method to solve CO2 emission and energy crisis issues. However, only quite limited catalysts are capable of converting CO2 into high-value C1 products. Herein, four structures of single Ni atom-modified phosphorene, as an electrocatalyst for the CRR, have been studied by first-principles calculations based on density functional theory (DFT). The results show that a single Ni atom adsorbed on monoatomic defective phosphorene (Ni-D-BP) has higher long-term activity and stability, and better CRR selectivity against the hydrogen evolution reaction (HER). In particular, Ni-D-BP shows good selectivity for HCOOH with a limiting potential of −0.31 V. The production of CH3OH and CH4 has the same limiting potential of −0.98 V, indicating that Ni-D-BP also has good catalytic properties for CH3OH or CH4 production. This study can reveal the mechanism of the CRR for single Ni atom-modified phosphorene-based catalysts and provide a way to design electrocatalysts for the CRR on the atomic scale.

Molecular-Scale Insights into Electrochemical Reduction of CO2 on Hydrophobically Modified Cu Surfaces.

Depressing the competitive hydrogen evolution reaction (HER) to promote current efficiency toward carbon-based chemicals in the electrocatalytic CO2 reduction reaction (CO2RR) is desirable. A strategy is to apply the hydrophobically molecular-modified electrodes. However, the molecular-scale catalytic process remains poorly understood. Using alkanethiol-modified hydrophobic Cu as an electrode and CO2-saturated KHCO3 as an electrolyte, we reveal that H2O, rather than HCO3-, is the major H+ source for the HER, determined by differential electrochemical mass spectrometry with isotopic labeling. As a result, using in situ Raman, we find that the hydrophobic molecules screen the cathodic electric field effect on the reorientation of interfacial H2O to a "H-down" configuration toward Cu surfaces that corresponds to the decreased content of H-bonding-free water, leading to unfavorable H2O dissociation and thus decreased H+ source for the HER. Further, density functional theory calculations suggest that the absorbed alkanethiol molecules alter the electronic structure of Cu sites, thus decreasing the formation energy barrier of CO2RR intermediates, which consequently increases the CO2RR selectivity. This work provides a molecular-level understanding of improved CO2RR on hydrophobically molecule-modified catalysts and presents general references for catalytic systems having H2O-involved competitive HER.

Lewis‐Basic EDTA as a Highly Active Molecular Electrocatalyst for CO2 Reduction to CH4

AbstractThe most active catalysts so far successful in hydrogenation reduction of CO2 are mainly heterogeneous Cu‐based catalysts. The complex coordination environments and multiple active sites in heterogeneous catalysts result in low selectivity of target product, while molecular catalysts with well‐defined active sites and tailorable structures allow mechanism‐based performance optimization. Herein, we firstly report a single ethylenediaminetetraacetic acid (EDTA) molecular‐level immobilized on the surface of carbon nanotube as a catalyst for transferring CO2 to CH4 with an excellent performance. This catalyst exhibits a high Faradaic efficiency of 61.6 % toward CH4, a partial current density of −16.5 mA cm−2 at a potential of −1.3 V versus reversible hydrogen electrode. Density functional theory calculations reveal that the Lewis basic COO− groups in EDTA molecule are the active sites for CO2 reduction reaction (CO2RR). The energy barrier for the generation of CO from *CO intermediate is as high as 0.52 eV, while the further protonation of *CO to *CHO follows an energetic downhill path (−1.57 eV), resulting in the high selectivity of CH4. This work makes it possible to control the product selectivity for CO2RR according to the relationship between the energy barrier of *CO intermediate and molecular structures in the future.

Efficient electrochemical CO2 reduction on C2N monolayer supported transition metals trimer catalysts: A DFT study

The capture and catalytic conversion of CO2 into value-added chemicals is a promising and sustainable approach to resolve the increasingly severe global warming and energy crisis. However, the catalytic efficiency for CO2 reduction is seriously restricted by the free energy changes of the intermediate reaction. Herein, using density functional theory (DFT), the electrocatalytic CO2 reduction reaction (CRR) was applied towards making C1 products (CO, HCOOH, CH3OH, HCHO, and CH4) on monolayer C2N frameworks with M3 (M = Fe, Co, Ni, Cu) metal clusters introduction. Analyses of the adsorption configurations and electronic structures suggested that CO2 could be chemically adsorbed on Fe3@C2N, Co3@C2N, Ni3@C2N and Cu3@C2N. The H2 evolution reaction (HER), as a suppression of CRR, was investigated, and results showed that the CRR selectivity of Fe3@C2N, Co3@C2N, Ni3@C2N and Cu3@C2N is higher than that of HER. The CRR reaction produces different C1 products in different reaction paths due to proton transfer on Fe3@C2N, Co3@C2N, Ni3@C2N and Cu3@C2N. Free energy profiles demonstrate that Cu3@C2N and Co3@C2N were identified to be effective in catalyzing CRR with lowered overpotentials (0.51 V-0.67 V).

Dual 2D CuSe/g-C3N4 heterostructure for boosting electrocatalytic reduction of CO2

CO2 Reduction Reaction (CO2RR) is an effective route to convert CO2 to value-added products in energy and environment. In this work, a series of CuSe/g-C3N4 electrodes were synthesized by anchoring hexagonal CuSe nanoplates on g-C3N4 nanosheets and used for electrochemical CO2RR. Among the as-prepared electrodes, 50wt% CuSe on g-C3N4 performed the highest Faradaic efficiency of 85.28% to achieve 1.47 times higher than that of the pure CuSe NPs at -1.2 V vs. RHE due to the larger specific area provided by the introduction of planar-structure g-C3N4 NSs and faster electron transfer speed on the interface of CuSe NPs and g-C3N4 NSs. Meanwhile the calculations of density functional theory confirmed the electronic coupling on the electrodes to form an internal electric field between two materials and electron direction from g-C3N4 NSs to CuSe NPs. This work created a novel opportunity for optimizing design of electrode materials to enhance the CO selectivity of CO2RR.

Effect of 3d-transition metals doped in ZnO monolayers on the CO2 electrochemical reduction to valuable products: first principles study

CO2 conversion to valuable products on ZnO (0001) monolayer doped by transition metals (TM-ZnO where TM is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) was investigated by density functional theory calculation. The results show that doping TMs can reduce the overpotential for CO2 reduction reaction (CRR) compared to pristine ZnO. Significantly, the oxidation state of TMs by different d-orbital occupancy results in a change of the electronic properties of the catalysts, leading to a difference in reactivity, reaction pathway, and selectivity of the final products. Early TMs (Sc to Cr) showing oxidation state 3+ prefer CH4 as a product while late TMs (Mn to Cu) showing oxidation state 2+ can make HCOOH. Remarkably, Co-ZnO can produce HCOOH with ultra-low overpotential at 0.02 V and can further produce CH3OH with an overpotential of only 0.45 V. Therefore, Co-ZnO monolayer is suggested as a promising CRR catalyst for experimental research. This work sheds light on the rational design of low-cost metal oxides with high stability, activity, and product selectivity for CRR and other reactions.

Boosting CO2 Electrochemical Reduction with Atomically Precise Surface Modification on Gold Nanoclusters

AbstractThiolate‐protected gold nanoclusters (NCs) are promising catalytic materials for the electrochemical CO2 reduction reaction (CO2RR). In this work an atomic level modification of a Au23 NC is made by substituting two surface Au atoms with two Cd atoms, and it enhances the CO2RR selectivity to 90–95 % at the applied potential between −0.5 to −0.9 V, which is doubled compared to that of the undoped Au23. Additionally, the Cd‐doped Au19Cd2 exhibits the highest CO2RR activity (2200 mA mg−1 at −1.0 V vs. RHE) among the reported NCs. This synergetic effect between Au and Cd is remarkable. Density‐functional theory calculations reveal that the exposure of a sulfur active site upon partial ligand removal provides an energetically feasible CO2RR pathway. The thermodynamic energy barrier for CO formation is 0.74 eV lower on Au19Cd2 than on Au23. These results reveal that Cd doping can boost the CO2RR performance of Au NCs by modifying the surface geometry and electronic structure, which further changes the intermediate binding energy. This work offers insights into the surface doping mechanism of the CO2RR and bimetallic synergism.

Graphitic Carbon Nitride Nanosheets-Immobilized Single-Atom Zn Towards Efficient Electroreduction of CO2

Electrocatalytic reduction of CO2 into carbon-containing chemical product is greatly desired for mitigating the air pollution and energy crisis. However, there is great challenge to design highly active and stable electrocatalyst for the CO2 reduction reaction (CO2RR). Here, we develop a facile strategy of graphitic carbon nitride nanosheets-immobilized single-atom Zn (ZnSA–CN–G) to control the mitigation of electrocatalytic CO2RR. The as-synthesized ZnSA–CN–G nanosheets not only possess abundant single-atom Zn as active sites, but also show unique architectures with high surface area and high electrical conductivity. Consequently, the ZnSA–CN–G nanosheets exhibit high electrocatalytic selectivity for CO2RR, including a high Faradaic efficiency value of 87.1% for the production of CO at [Formula: see text]0.5[Formula: see text]V versus RHE. Moreover, such simple approach can be further applied to synthesize a variety of other single-atom catalysts.

Design of bimetallic atomic catalysts for CO2 reduction based on an effective descriptor

Our descriptor based on the valence and electronegativity of atoms in active centers can effectively describe the activity and selectivity of CO2 reduction reaction on bimetallic atomic catalysts and thus can be used to screen advanced catalysts.