Selectivity Of CO2 Electroreduction Research Articles

Size and Morphology Dependent Activity of Cu Clusters for CO2 Activation and Reduction: A First Principles Investigation.

Various Cu-based materials in diverse forms have been investigated as efficient catalysts for electrochemical reduction of CO2; however, they suffer from issues such as higher over potential and poor selectivity. The activity and selectivity of CO2 electro reduction have been shown to change significantly when the surface morphology (steps, kinks, and edges) of these catalysts is altered. In light of this, size and morphology dependent activity of selected copper clusters, Cun (n=2-20) have been evaluated for the activation and reduction of CO2 molecule. The phase-space of these copper clusters is rich in conformations of distinct morphologies starting from planar, 2D geometries to prolate-shaped geometries and also high-symmetry structures. The binding efficiency and the activation of CO2 are highest for medium sized clusters (n=9-17) with prolate-morphologies as compared to small or larger sized CunCO2 clusters that are existing mainly as planar (triangular, tetragonal etc.) or highly-symmetric geometries (icosahedron, capped-icosahedron etc.), respectively. The best performing (prolate-shaped) CunCO2 conformations are quite fluxional and also they are thermally stable, as demonstrated by the molecular dynamics simulations. Furthermore, on these CunCO2 conformations, the step-by-step hydrogenation pathways of CO2 to produce value-added products like methanol, formic acid, and methane are exceptionally favorable and energy-efficient.

First-Principles Insights into the Selectivity of CO2 Electroreduction over Heterogeneous Single-Atom Catalysts.

Heterogeneous metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have garnered considerable attention in the two-electron CO2 reduction reaction (2e-CO2RR). Interestingly, almost M-N-C SACs mainly produce CO, while Sb is one of the few SACs reported so far that can produce HCOOH. Nevertheless, the underlying factors for different selectivities on Sb-N-C SAC remain controversial, and the lack of in-depth understanding of limiting factors hampers further regulations. Here, by using constant-potential first-principles calculations, we revealed that the high HCOOH selectivity of Sb-N-C SAC is mainly attributed to their weak charge accumulation ability. Remarkably, considering the highly tunable geometric structure of M-N-C SACs, we provide that Sb-N-C SAC with the SbN3S1 center is a promising candidate for CO production. Our work provides the mechanism insight into 2e-CO2RR selectivity and further paves the way toward electrocatalyst regulation and design.

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.

C2+ Selectivity for CO2 Electroreduction on Oxidized Cu-Based Catalysts.

Design for highly selective catalysts for CO2 electroreduction to multicarbon (C2+) fuels is pressing and important. There is, however, presently a poor understanding of selectivity toward C2+ species. Here we report for the first time a method of judiciously combined quantum chemical computations, artificial-intelligence (AI) clustering, and experiment for development of a model for the relationship between C2+ product selectivity and composition of oxidized Cu-based catalysts. We 1) evidence that the oxidized Cu surface more significantly facilitates C-C coupling, 2) confirm the critical potential condition(s) for this oxidation state under different metal doping components via ab initio thermodynamics computation, 3) establish an inverted-volcano relationship between experimental Faradaic efficiency and critical potential using multidimensional scaling (MDS) results based on physical properties of dopant elements, and 4) demonstrate design for electrocatalysts to selectively generate C2+ product(s) through a co-doping strategy of early and late transition metals. We conclude that a combination of theoretical computation, AI clustering, and experiment can be used to practically establish relationships between descriptors and selectivity for complex reactions. Findings will benefit researchers in designing electroreduction conversions of CO2 to multicarbon C2+ products.

Local Weak Hydrogen Bonds Significantly Enhance CO 2 Electroreduction Performances of a Metal–Organic Framework

Local Weak Hydrogen Bonds Significantly Enhance CO ₂ Electroreduction Performances of a Metal–Organic Framework

Ultrafine Bi Nanoparticles Confined in Hydrothermal Carbon-Modified Carbon Nanotubes for Highly Efficient CO2 Electroreduction to Formate

Electroreduction of carbon dioxide (CO2) into formate is a sustainable and promising approach to solve the greenhouse effect. Herein, ultrafine Bi nanoparticles (Bi NPs) with a diameter of around 3.4 nm on hydrothermal carbons-modified carbon nanotubes were successfully obtained via dip-coating method, and could be used as an effective catalyst for electroreduction of CO2 into formate. As a result, the as-prepared catalyst exhibited an outstanding CO2 reduction performance with a faradaic efficiency of 94.8% and a higher current density of 17.8 mA cm−2. The high selectivity could maintain over a wide potential range of 400 mV, together with satisfactory durability for more than 24 h. Electrochemical results and density functional theory (DFT) calculation showed that the rich intrinsic active sites of Bi could effectively adsorb CO2 molecules and promote the CO2(ads) intermediate to form the intermediate, and resulting in the higher intrinsic activity of the CO2 reduction. Thus, our work demonstrated a synergistic effect between carbon supports and metal nanoparticles could broaden the activity and selectivity for CO2 electroreduction and offer useful perspectives to design efficient and low-cost electrocatalysts.

Strategy for improving the activity and selectivity of CO2 electroreduction on flexible carbon materials for carbon neutral

Selective electrochemical reduction of CO2 is of interest in relieving the accumulation of CO2 in the atmosphere and the global energy crisis. This study investigates active sites and reaction mechanisms for CO2 electroreduction over metal N-doped flexible carbon materials using Phthalocyanine (Pc) and Iron Phthalocyanine (FePc) molecules as model catalysts. Both the FePc molecule and Pc molecule show catalytic activity for CO production and H2 evolution based on experimental results and DFT calculations. The presence of Fe atom results in a significantly higher activity for CO production and a little lower activity for H2 evolution. Calculated pKa and redox potentials (Ure) are introduced for the multi-step proton-coupled electron transfer (PCET) steps of two competitive reactions. Specifically, a comparison between them is conducted to explain the competition between CO2 electroreduction and H2 evolution reaction with the support of energy diagram. The rate-determining steps for CO2 reduction and H2 evolution are also investigated. Both the CO and H2 formation on Pc molecule depend on electron transfer process. Besides, the Ure of two rate-determining steps are close, which is related to the closed Eonset for CO and H2 formation in the experiment. This study demonstrates that the mechanism for CO2 reduction from DFT calculation is in agreement with the experimental results. The technological approach and reaction mechanism can be applied to other types of CO2 electroreduction on carbon materials. Thus, this work provides a guidance in the rational design of the green catalyst and the implementation of carbon neutral.

Bi@Sn Core-Shell Structure with Compressive Strain Boosts the Electroreduction of CO2 into Formic Acid.

As a profitable product from CO2 electroreduction, HCOOH holds economic viability only when the selectivity is higher than 90% with current density (j) over −200.0 mA cm−2. Herein, Bi@Sn core–shell nanoparticles (Bi core and Sn shell, denoted as Bi@Sn NPs) are developed to boost the activity and selectivity of CO2 electroreduction into HCOOH. In an H‐cell system with 0.5 m KHCO3 as electrolyte, Bi@Sn NPs exhibit a Faradaic efficiency for HCOOH (FEHCOOH) of 91% with partial j for HCOOH (j HCOOH) of −31.0 mA cm−2 at −1.1 V versus reversible hydrogen electrode. The potential application of Bi@Sn NPs is testified via chronopotentiometric measurements in the flow‐cell system with 2.0 m KHCO3 electrolyte. Under this circumstance, Bi@Sn NPs achieve an FEHCOOH of 92% with an energy efficiency of 56% at steady‐state j of −250.0 mA cm−2. Theoretical studies indicate that the energy barrier of the potential‐limiting step for the formation of HCOOH is decreased owing to the compressive strain in the Sn shell, resulting in the enhanced catalytic performance.

PH dependence of CO2 electroreduction selectivity over size-selected Au nanoparticles

Electrocatalytic CO2 reduction to liquid fuels and chemical feedstock, powered by renewable electricity, is an important approach for storing renewable energy and closing carbon cycle. Here, we study the pH effect on CO2 electroreduction selectivity over size-selected Au nanoparticles (NPs) in citrate buffer solutions with pH 3.7–6.0. A maximum CO Faradaic efficiency of 96.3% is achieved over Au NPs at pH 6.0, and synthesis gas with tunable H2/CO ratios (0.04–22) can be produced in the studied pH range. By careful kinetic analysis, the observed pH dependence reveals a most likely mixed mechanism of concerted proton–electron transfer and sequential electron–proton transfer in the initial step of CO2 activation. The pH-dependent selectivity changes over Au NPs can be rationalized by the difference in the pH dependence of CO2 electroreduction and concurrent hydrogen evolution reaction. This work provides new guidelines into controlling activity and selectivity of CO2 electroreduction via efficiently tuning electrolyte pH. Selectivity of CO2 electroreduction over size-selected Au nanoparticles is tuned via electrolyte pH.

The critical role of hydride (H−) ligands in electrocatalytic CO2 reduction to HCOOH by [Cu25H22(PH3)12]Cl nanocluster

Copper is the prototype metal to produce hydrocarbons from CO2 electroreduction, however, rationale of the catalytic mechanism by copper nanoparticles has been severely impeded due to structural polydispersity. The recent efforts in the synthesis of nanosized copper clusters open up new opportunities for atomic-level understanding of the catalytic mechanism. Herein, we considered [Cu25H22(PH3)12]Cl cluster, bearing the highest ratio of hydride ligands, as a test model to examine its activity for CO2 electroreduction. Our DFT calculations showed that both the surface Cu and the hydride ligands are actively participated in the hydrogenation reactions and selectively reduce CO2 to HCOOH via the proton-reduction channel and the hydride-proton channel, as opposed to the commonly produced CO on copper surface/nanoparticles. We also evaluated the competitive HER activity, and found that H2 evolution requires higher barrier than HCOOH. Our studies highlight the important role of hydrides in directing the activity and selectivity for CO2 electroreduction.

Electrodeposited AgCu Foam Catalysts for Enhanced Reduction of CO2 to CO

Selective electrochemical reduction of CO2 is an emerging field which needs more active and stable catalysts for its practicability. In this work, we have studied the influence of Ag metal incorporation into Cu dendritic structures on the product distribution and selectivity of CO2 electroreduction. Bimetallic AgCu foams prepared by hydrogen bubble templated electrodeposition shift the potentials of CO production to more positive values compared to bulk silver. The presence of Ag during the electrodeposition significantly changed the size and the shape of the dendrites in the pore walls of AgCu foams compared to Cu foam. The CO adsorption characteristics are studied by operando Raman spectroscopy. In the presence of Ag, the maximum CO adsorption is observed at a more positive potential. As a result, an improved selectivity for CO is obtained for AgCu foam catalysts at lower overpotentials compared to Cu foam catalyst, evidencing a synergistic effect between the bimetallic components. We were successful in increasing the CO mass activity with respect to the total Ag amount. AgCu foams are found to retain the CO selectivity during long-term operation, and with their easily scalable electrodeposition synthesis they possess high potential for industrial application.

The Effect of Organic Additives on the Activity and Selectivity of CO2 Electroreduction: The Role of Functional Groups.

Electrochemical reduction of CO2 (ERC) to useful chemicals is an environmentally and technologically significant process. The process is confronted with significant challenges to simultaneously enhance the catalyst activity and product selectivity. In this paper, the effects of organic additives on the ERC process were systematically investigated by using DFT to screen additives with different functional groups for enhanced activity and selectivity. In particular, the additives with -NH3 + and -SO3 H groups had a remarkably positive effect on the ERC activity and hydrocarbon selectivity, which were predicted to impart a positive shift on onset potential of approximately 162 and 108 mV, respectively. Importantly, the additive can accelerate the electron transfer of the intermediate and tune the electronic structure of the catalyst surface, resulting in a clear deviation from transition-metal scaling lines. Combining bonding energy of crucial intermediates with partial atomic charge analysis, we rationalized the negative effect of high concentration additives and confirmed the proposed electron transfer model. Furthermore, additive molecules containing functional groups with positive charges and maximizing the deviation from transition-metal scaling lines are meaningful strategies to design and choose organic additives to enhance activity and selectivity of ERC.

Spectroscopic Evidence of Size-Dependent Buffering of Interfacial pH By Cation Hydrolysis during CO2 Electroreduction

The nature of the electrolyte cation is known to affect the Faradaic efficiency and selectivity of CO2 electroreduction. Singh et al. (J. Am. Chem. Soc. 2016, 138, 13006–13012) recently attributed this effect to the buffering ability of cation hydrolysis at the electrical double layer. According to them, the pKa of hydrolysis decreases close to the cathode due to the polarization of the solvation water molecules between the cation’s positive charge and the negative charge on the electrode surface. We have tested this hypothesis experimentally, by probing the pH at the gold-electrolyte interface in situ using ATR-SEIRAS. The ratio between the integrated intensity of the CO2 and HCO3 - bands, which has to be inversely proportional to the pH, provided a means to determining the pH change in-situ during the electroreduction of CO2. Our results confirm that the magnitude of the pH increase at the interface follows the trend Li+ > Na+ > K+ > Cs+, adding strong experimental support to Singh’s et al.’s hypothesis. We show, however, that the pH buffering effect was overestimated by Singh et al., their overestimation being larger the larger the cation. Figure 1

Catalysts for CO2 Electroreduction to Hydrocarbons and Oxygenates

Rising atmospheric carbon dioxide (CO2) levels and the associated rise in global temperatures have been brought to light as major global concerns. In fact, annual atmospheric CO2 emissions have risen from about 2 Gt to 10 Gt in the last 5 decades.1 Electricity production (via fossil fuel, etc.) is a large contributor, accounting for 37% of total CO2 emissions, and therefore scientists have turned to renewable energy as a mitigation strategy.2,3 Another widely-studied approach has been to reduce CO2 to value-added chemicals and fuels (carbon monoxide, ethanol, formic acid, etc.). Previous works have aimed at increasing the feasibility of the CO2 electroreduction (CO2RR). Being able to improve and control the selectivity of CO2RR would greatly contribute to its industrial application. Developing thermodynamically efficient and kinetically active catalysts is a key part of CO2RR. Depending on the catalyst or operating conditions, a variety of products can be formed.4 Hydrocarbons and oxygenates, such as ethanol and methanol, are attractive products because of their high-energy density and variability in application.5 For example, methanol is a feedstock for the industrially mature methanol to gasoline (MTG) process, which has a 95% thermal energy conversion efficiency.6 Although some state of the art electrocatalysts for CO2 electroreduction can produce CO and HCOOH at high selectivity (>80%) and energy efficiency (>60%), the selective production of higher hydrocarbons, especially oxygenates such as ethanol and methanol, is still lacking.7 In this talk, we will present examples of various catalysts for the selective electroreduction of CO2 to oxygenates and hydrocarbons using an alkaline flow electrolyzer at ambient conditions. These catalysts include copper supported on metal oxides (Cu/MxOy), copper-based alloys, etc. Electroanalysis results of the Cu/MxOy catalysts include selective production of C1 products (CO and methane) with traces of methanol when compared to copper nanoparticles. The copper-based alloys depict Faradaic efficiencies of up to 75% for C2 products (ethylene and ethanol). Acknowledgement We gratefully acknowledge financial support from I2CNER and the SURGE Fellowship for UN. References Hansen J., Kharecha, P., et al., PLoS One, 2013.Oerlemans, J., Science, 2005, 308(5722), 675-677.https://www.eia.gov Accessed 05/29/2017.R.M. Jhong, S. Ma, P.J.A. Kenis, Curr. Opin. Chem. Eng., 2013, 2, 191-199.Ma, M. Sadakiyo, R. Luo, M. Heima, M. Yamauchi, P.J.A. Kenis, J. Power Sources, 2016, 301, 219-228.L. Spath, D.C. Dayton, NREL/TP-510-34929, 2003.J. Martin, G.O. Larrazabál, J. Pérez-Ramírez. Green Chem., 2015.

Spectroscopic Evidence of Size-Dependent Buffering of Interfacial pH by Cation Hydrolysis during CO2 Electroreduction.

The nature of the electrolyte cation is known to affect the Faradaic efficiency and selectivity of CO2 electroreduction. Singh et al. (J. Am. Chem. Soc. 2016, 138, 13006-13012) recently attributed this effect to the buffering ability of cation hydrolysis at the electrical double layer. According to them, the pKa of hydrolysis decreases close to the cathode due to the polarization of the solvation water molecules sandwiched between the cation's positive charge and the negative charge on the electrode surface. We have tested this hypothesis experimentally, by probing the pH at the gold-electrolyte interface in situ using ATR-SEIRAS. The ratio between the integrated intensity of the CO2 and HCO3- bands, which has to be inversely proportional to the concentration of H+, provided a means to determining the pH change at the electrode-electrolyte interface in situ during the electroreduction of CO2. Our results confirm that the magnitude of the pH increase at the interface follows the trend Li+ > Na+ > K+ > Cs+, adding strong experimental support to Singh's et al.'s hypothesis. We show, however, that the pH buffering effect was overestimated by Singh et al., their overestimation being larger the larger the cation. Moreover, our results show that the activity trend of the alkali-metal cations can be inverted in the presence of impurities that alter the buffering effect of the electrolyte, although the electrolyte with maximum activity is always that for which the increase in the interfacial pH is smaller.

Enhanced CO2 electroreduction on armchair graphene nanoribbons edge-decorated with copper

Inspired by the recent experimental synthesis of graphene nanoribbons (GNRs) and theoretical research on their edge-decoration, we comprehensively studied the electrocatalytic performance of GNRs edge-decorated with Cu for CO2 reduction. Compared to zigzag GNRs, the Cu-terminated armchair GNRs with a width of n = 3p + 2 were more efficient catalysts for producing methanol from CO2 with a free energy barrier of less than 0.5 eV, offering the advantages of a lower overpotential and higher selectivity than bulk Cu and other graphene-supported Cu structures. On the other hand, the competing hydrogen evolution reaction could be effectively suppressed by Cu-terminated armchair GNRs. Hence, the edge-decorated GNRs offer great flexibility for tuning the catalytic efficiency and selectivity for CO2 electroreduction.