CO Electroreduction: Current Development and Understanding of Cu

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CO Electroreduction: Current Development and Understanding of Cu-Based Catalyst Haochen Zhang, Jing Li, Mu-Jeng Cheng, and Qi Lu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03780 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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CO Electroreduction: Current Development and Understanding of Cu-Based Catalyst Haochen Zhang,†,§ Jing Li,†,§ Mu-Jeng Cheng*,‡ and Qi Lu*,† †State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China. ‡Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan. Abstract Electrochemical synthesis of value-added chemicals from CO and H2O powered by renewable energy is a promising technique to supplement, if not eventually replace, the Fischer-Tropsch process, which requires high energy input (i.e., typical temperatures of 200-400 °C and pressures of 10-200 atm) and large-scale reactors that are difficult to match to the disperse renewable energy sources. Moreover, this technique is also ideal for implementing the electroreduction of CO2 using a tandem strategy because many catalysts can convert CO2 to CO with high efficiency. However, the direct reduction of CO2 to more valuable hydrocarbons and oxygenates is hindered by the lack of efficient catalysts. This perspective highlights the current understanding and progress of CO electroreduction from both experimental and computational approaches. The challenges that must be overcome for further development are also identified and discussed. Keywords: electroreduction, carbon monoxide, copper-based material, catalysis, hydrocarbon formation 1. Introduction The electroreduction of CO to value-added chemicals in aqueous media is an attractive technique to supplement the current Fischer-Tropsch synthesis because the electrolysis processes can be conveniently powered using electricity generated from solar or wind sources. In addition, the high-energy input large-scale Fischer-Tropsch reactors are unlikely to be compatible with these diffuse renewable energy sources.1-3 Moreover, electrolyzers can be easily designed in much smaller sizes than those used in the Fischer-Tropsch process, which requires a large infrastructure.4-6 They can be operated at ambient conditions and do not require hydrogen gas. Therefore, these electrolyzers would be ideal for small natural gas reservoirs that would not be profitable connected to pipeline infrastructures and for the storage of the intermittent rewardable energy.7-8 In addition, an efficient CO electroreduction technique will allow for the electrochemical reduction of CO2 to fuels and commodity chemicals through a tandem strategy because many catalysts can convert CO2 to CO with both high activity and selectivity.9-18 However, the direct CO2 reduction to higher-value products is hindered by the lack of efficient catalysts.9, 19-21 To study CO electroreduction will also benefit 1 ACS Paragon Plus Environment

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the development of CO2 electroreduction. This is because CO2 electroreduction is believed to share similar reaction pathways with CO electroreduction after CO2 was converted to the CO adsorbate.22-23 Moreover, the CO dimerization or CO hydrogenation is currently regarded as the key step for the formation of C2+ products.2428 A direct feed of CO as reactant to the catalyst surface may promote the production of C2+ species due to the increased CO concentration. Furthermore, CO electroreduction can be conducted in alkaline condition that can suppress the competing hydrogen evolution reaction. However, it is not feasible to operate CO2 electroreduction in alkaline electrolyte due to its chemical reaction with hydroxide ion. Also, because CO does not participate in any electrolyte reaction, it is more convenient to conduct mechanistic studies of C2+ products formation in CO electroreduction due to its much less complex reaction interface. Cu is the only known element that exhibits appreciable CO electroreduction activities in a conventional batch-type electrochemical cell.23, 29-30 The specific CO reduction density is approximately 1 mA·cm-2, and its reaction mechanism remains unclear.23, 29, 31-34 All current research efforts are devoted to improving the performance and fundamental understanding of the CO electroreduction reaction on Cu surfaces. This perspective aims to provide an overview of the current achievements in the design and development of catalysts, electrodes and reactors as well as current understanding of the influence of the electrolyte and the reaction mechanism. The challenges that must be overcome and future opportunities are also discussed. 2. Catalyst, electrode and reactor for CO electroreduction 2.1 Catalyst surface design and development for high performance. In 1987, Hori et al. reported that a polycrystalline Cu surface was able to electrochemically convert CO to various hydrocarbons and oxygenates with an appreciable current density of approximately 1.1 mA·cm-2 in 0.1 M KOH at -0.7 V (vs. the reversible hydrogen electrode, RHE; the same reference electrode is used in the following discussion unless otherwise specified).23 However, the electrolysis was dominated by the competing hydrogen evolution reaction, and the Faradaic efficiency for hydrocarbon and oxygenate production was less than 23%. This result agreed well with modern examinations by Verdaguer-Casadevall et al. and Bertheussen et al., respectively.32, 34 More recently, Wang et al. conducted a more systematic investigation of CO electroreduction on a polycrystalline Cu surface over a much wider potential range.30 By using a custom-designed electrochemical cell that provided improved overall product detection sensitivity, the authors were able to measure activities that were two orders of magnitude smaller than those in previous studies. The data obtained in this study demonstrated that (1) the CO electroreduction activities of the polycrystalline Cu surface were lower than 1.0 mA·cm-2 over the entire potential range due to the low CO solubility in the electrolyte (ca. 0.1 mM). (2) The formation of C2+ products was favored at lower overpotentials than that of the methane C1 product, indicating that the C2+ and C1 pathways most likely diverged at an early stage in CO reduction. (3) Ethylene and ethanol were the major products at potentials more negative than -0.55 V. Their partial 2 ACS Paragon Plus Environment

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current densities plateaued at -0.63 V, which was mostly likely due to the mass transport limitation of CO. (4) The aldehydes exhibited increased partial current densities at more negative potentials. However, their production disappeared at approximately -0.55 V when alcohols started to form, suggesting that the aldehydes were likely the intermediates for further reduction to alcohols (Figure 1). This work benchmarked the performance of CO electroreduction on a polycrystalline Cu surface in an aqueous medium. In addition, these results indicate that both the selectivity and activity of polycrystalline Cu are much too poor for applicable implementation. The improvement of the selectivity requires the discovery of advanced catalytic sites, and the enhancement of the activity relies on overcoming the poor mass transport of CO, which is due to its low solubility.

Figure 1. (a) Faradaic efficiencies and (b) partial current densities of polycrystalline Cu in CO electrolysis as a function of the applied potential. Reproduced with permission from Ref. 30. Li et al. reported an oxide-derived Cu surface that greatly improved the selectivity of CO electroreduction.31 By electrochemical reduction or thermal H2 reduction of an oxidized Cu foil (denoted as OD-Cu 1 and OD-Cu 2, respectively), the authors achieved coarsened Cu surfaces (Figures 2a, b) that reduced CO with a more than 50% Faradaic efficiency to liquid products and significantly lower overpotentials (Figure 2c). The 3 ACS Paragon Plus Environment

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authors attributed this improvement to the appearance of grain boundary terminations on the OD Cu surfaces, which were proposed as novel catalytic sites. In addition, the authors evaluated Cu nanoparticle electrodes (Figure 2c), which did not exhibit improved performance compared to polycrystalline Cu. Therefore, surface nanostructuring (Figure 2a) was not key to achieving the high selectivity.

Figure 2. SEM images of (a) OD-Cu 1 and (b) OD-Cu 2. (c) Faradaic efficiencies of OD-Cu 1, OD-Cu 2 and Cu nanoparticles for CO electroreduction at different potentials. Reproduced with permission from Ref. 31. To establish a correlation between the CO electroreduction activity of OD-Cu and the metastable grain boundary (GB) termination surface feature, Verdaguer-Casadevall et al. examined the microstructure and the adsorbate binding thermodynamics of OD-Cu prepared by thermal H2 reduction of oxidized Cu precursors.32 A TEM nanodiffraction study revealed that OD-Cu was composed of irregularly shaped, 100-300 nm grains that were joined by grain boundaries in a dense polycrystalline network (Figure 3a). The GBs consisted of 37% low-energy twin boundaries, and the remaining GBs were randomly oriented or non-twin coincident site lattice boundaries (Figure 3b). In a temperature-programmed desorption (TPD) study of CO (Figures 3c and d), the ODCu that was prepared at an oxidation temperature of 500 °C (OD-Cu-500) exhibited a high-temperature feature centered at 275 K, which was absent from the profile for regular polycrystalline Cu foil. This feature was correlated to a unique surface site with strong CO binding using the microkinetic model. The strong-binding sites were believed to be introduced by the GB surface terminations and responsible for the improved selectivity in CO electroreduction. To confirm this hypothesis, the OD-Cu4 ACS Paragon Plus Environment

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500 electrodes were further annealed in N2 at elevated temperatures to relax the microstructures and eliminate the strong-binding sites (Figure 3e) by reducing the GBs. Upon annealing, the CO electroreduction selectivity significantly decreased from approximately 48% to less than 5% (Figure 3f). Therefore, the high selectivity of ODCu was more intrinsically correlated to the surface sites, which could bind CO more strongly than the low-index and stepped Cu facets.

Figure 3. (a) Image quality map of OD-Cu 500 constructed from TEM electron diffraction data. (b) TEM orientation mapping data for OD-Cu 500. Yellow line: randomly oriented boundaries. Redline: twin boundaries. TPD files for (c) Polycrystalline Cu; (d) OD-Cu-500. Experimental data (solid black lines) and fit resulting from microkinetic simulations (dashed red lines). The dotted green lines indicate the central position of the low-index facets and stepped sites in polycrystalline Cu. (e) Surface-area corrected jCORedn at -0.4 V vs. the percentage of strong binding sites estimated from microkinetic modeling. (f) Production distributions of OD-Cu electrodes prepared at different oxidation temperatures and OD-Cu-500 electrodes annealed at elevated temperatures. Reproduced with permission from Ref. 32. The role of GBs in improving the CO reduction selectivity was also demonstrated 5 ACS Paragon Plus Environment

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with polycrystalline Cu nanoparticles. Feng et al. synthesized carbon nanotube (CNT)supported Cu nanoparticles (Cu NPs) composed of multiple Cu crystallites connected by GBs using e-beam evaporation (Figure 4a).33 For comparison, a series of Cu/CNTs with lower GB surface densities were also prepared by introducing a post thermal annealing process with minimum geometry change (Figures 4b-e). The evaluation of CO electroreduction indicated that the Faradaic efficiencies for the CO reduction products on the five Cu/CNT electrodes decreased monotonically as the annealing pretreatment temperature increased (Figure 4f). More interestingly, the surface areanormalized CO reduction current densities (specific jCOredn) were linearly proportional to the density of the GBs across all samples (Figure 4g). This quantitative correlation as well as the fact that a near-zero activity was observed on Cu/CNT no GBs suggested that the GBs were responsible for the vast majority of active sites in CO electroreduction.

Figure 4. High-resolution TEM images of the (a) as-deposited Cu/CNT electrode and electrodes annealed under N2 at (b) 200 °C, (c) 300 °C, (d) 400 °C, and (e) 500 °C. (f) Faradaic efficiencies and (g) correlation between the GB surface density and specific activity for CO reduction on the five Cu/CNT electrodes at -0.4 V. Reproduced with permission from Ref. 33. To gain additional fundamental insights into the GB-improved CO reduction selectivity, Cheng et al. conducted theoretical investigations on simulated Cu/CNT materials.35 The authors computationally “synthesized” a Cu NP (158,555 atoms) deposited on a CNT by simulating a chemical vapor deposition process. The simulated particle exhibited a nominal thickness of approximately 10 nm (Figure 5a) and predicted an XRD pattern consistent with an FCC structure (Figure 5b), which was consistent with the Cu NPs in the experimental work by Feng et al. (Figure 4).33 The GBs on this Cu NP were clearly observed in the predicted TEM images and dislocation analysis (Figure 5c). 5‰ of the surface sites were selected for CO binding energy 6 ACS Paragon Plus Environment

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calculations using density functional theory (DFT). The results indicated that 9% of the surface sites exhibited binding energies larger than those of the (111), (100), and (211) facets, which is consistent with a high-temperature TPD desorption peak at 275 K.32 Subsequently, the authors considered the formation energy of *OCCOH as a descriptor to characterize the C2 production activity of these surface sites because *OCCOH has been reported as the key intermediate in C2 product formation in a CO electroreduction study that employed Fourier Transform infrared spectroscopy (FTIR).36 Notably, not all strong CO binding sites were active for C2 formation. Only the strong CO binding sites with at least one under-coordinated neighbor square site were able to promote C-C coupling. Therefore, the authors proposed that a periodic structure consisting of metallic Cu with continuously stepped square sites (Figures 5d, e) may afford the high performance for C-C coupling.

Figure 5. (a) Atomic structure of the Cu NP “synthesized” computationally by 7 ACS Paragon Plus Environment

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simulating a chemical vapor deposition process. (b) Predicted XRD diffraction pattern of the Cu-NP in a, which was compared to the diffraction peaks of Cu FCC metal. (c) Predicted TEM images of the predicted Cu NP. The inset enlarged the selected area to show some of the GBs. Atomic structure of Cu surface site with (d) two surface-bonded CO molecules, and (e) the surface bonded *OCCOH intermediate that led to the formation of C2 products. The colors indicate Cu in orange, C in silver, O in red, and H in white. The closest Cu binding sites are shown in blue for viewer convenience. Reproduced with permission from Ref. 35. Oxide-derived Cu nanowires were employed as another experimental model system to investigate the active sites of GBs. Raciti et al. synthesized highly dense Cu nanowires via thermal reduction of CuO nanowires using H2 at elevated temperatures of 150, 200, and 300 °C (denoted as HR-150, 200, and 300, respectively). HR-150 exhibited higher geometric current densities and Faradaic efficiencies than other counterparts (Figures 6a, b).37 The scanning precession electron diffraction (SPED) analysis indicated that the GB density of these electrodes decreased as the reduction temperature increased, which was consistent with observations for previously reported OD-Cu electrodes. Based on CO-TPD and cyclic voltammetry studies of *OH (Figures 6c, d), the authors determined that the (110) facets decreased much more dramatically than the (111) and (100) facets. In addition, the (211)* (referred to low-index facets) facet increased by a factor of approximately two as the reduction temperature increased (Figure 6e). Therefore, Cu(110) rather than lower index number facets may be responsible for the high performance achieved on the HR-150 nanowires, which possessed the highest GB density. This conclusion was further supported by free energy diagrams that were calculated using DFT.

Figure 6. (a) Faradaic efficiencies and (b) geometric current densities of CO electroreduction on various Cu nanowires. (c) Voltammograms collected in 1 M KOH 8 ACS Paragon Plus Environment

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showing the *OH peaks. (d) CO-TPD patterns collected for the HR-150 Cu nanowires. (e) Fractions of facets for the Cu nanowires derived from the CO-TPD patterns. Reproduced with permission from Ref. 37. The current efforts in developing efficient CO electroreduction catalysts are primarily focused on the increase of surface site with more optimal CO binding. Except for the “oxide-derived” method, many other techniques demonstrated to be effective in adjusting the CO binding strength in CO2 electroreduction may also be explored in future CO electroreduction study, for example, plasma treatment38-39 and electroredeposition40-41 for introducing stable Cu+ species on surface, morphology control for exposing high density low-coordinated surface sites42 and tuning local electrolyte pH43. Alloying Cu with metals that bind CO strongly, such as Pt, Pd, Ni etc., can be another approach to improve the CO binding on the catalyst surface.44-45 The metal compositions and mixing patterns can be critical in determining the activity and product distribution thus detailed investigations are necessary. This work can be ideally facilitated using high-throughput computational search for promising candidate surfaces. In taking a step forward, the identification of further reduced reaction intermediates and the optimization of their bindings are highly desired to improve the selectivity of CO electroreduction. 2.2 Electrode structural design for improving activity. To implement the strong CO binding sites on a high surface area catalyst electrode with better CO accessibility, Wang et al. developed a technique to produce nanostructured oxide-derived Cu surfaces on three-dimensional (3D) commercial Cu foams.46 The commercial Cu foams were annealed in air at different annealing temperatures of 500, 700 and 1000 °C, which afforded CuO nanowires or porous structures (denoted as CuO-NS-500C, CuO-NS700C and CuO-NS-1000C, respectively) (Figures 7a-c). Then, the 3D structured Cu electrodes with distinct surface morphologies were prepared via the electrochemical reduction of CuO at -0.4 V in 0.1 M KOH (denoted as Cu-NS-500C, Cu-NS-700C and Cu-NS-1000C, respectively) (Figures 7d-f). All electrodes exhibited improved specific current density and selectivity towards CO reduction compared to a counterpart electrode that was prepared from a planar Cu mesh (denoted as ECR-Cu-mesh) (Figures 7g, h). A large specific current density of 9.22 μA cm-2 was achieved on the Cu-NS700C electrode at -0.35 V. This current density was 6-fold higher than that of ECR-Cumesh (1.45 μA cm-2). In addition, a high Faradaic efficiency of 76% was achieved on the Cu-NS-500C electrode at -0.30 V, which set a new benchmark for CO electroreduction in a batch-type reactor with a three-electrode setup. The authors attributed this enhancement to the framework inherited from the Cu foam, which facilitated the supply of CO to the active sites on the electrode surface. However, due to the low CO solubility limitation, the highest geometric current density achieved on these electrodes was only few milliamperes per square centimeter.

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Figure 7. SEM images of (a) CuO-NS-500C, (b) CuO-NS-700C, (c) CuO-NS-1000C, (d) Cu-NS-500C, (e) Cu-NS-700C, and (f) Cu-NS-1000C. (g) Specific current densities and (h) Faradaic efficiencies (FEs) for overall CO electroreduction. Reproduced with permission from Ref. 46. To improve the CO electroreduction current density that was limited by the low solubility of CO, Han et al. developed Cu nanoparticle-based gas diffusion electrodes (GDEs) to improve the CO transport to the catalyst surface.47 The GDEs were fabricated by casting a mixture of Cu nanoparticles, a carbon powder conducting additive and a Teflon binder onto a Cu gaze substrate. The thickness of the mixture layer was 0.1 mm, yielding a Cu loading of 7 mg cm-2. Two types of electrode configurations were characterized for CO electroreduction (Figures 8a, b). A flow-through configuration allowed CO to be directly introduced onto the electrode surface in the gas phase, and a flow-by configuration required the CO molecules to diffuse from an aqueous solution to the electrode surface for comparison. In the flow-through GDE configuration, a high partial current density of 50.8 mA·cm-2 for CO reduction to C2H4 was achieved at 15 °C and -0.85 V in 10 M KOH. In contrast, the partial current density for C2H4 production in the flow-by configuration using identical electrodes was limited to < 1 mA·cm-2 (Figure 8d). These results demonstrated that the geometric partial current densities for CO electroreduction were effectively improved by employing GDEs with direct CO feed to eliminate the mass transport limitations. However, more sophisticated design of the GDE as well as its reactor is required to achieve optimal activity and selectivity for CO electroreduction.

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Figure 8. Schematic illustration of the GDE materials (a) in the flow-through configuration and (b) flow-by configuration. (c) Faradaic efficiencies and (d) total operating current density (dotted curves, left y-axis) and partial current density for CO reduction to C2H4 (solid curves, right y-axis) as a function of the applied potential for the flow-through configuration (red) and flow-by configuration (blue). Reproduced with permission from Ref. 47. To simultaneously improve the electrode surface area and mass transport has been proven to be a very effective strategy to enhance the electrode performance. Taking the CO2 electroreduction as an example, the 3D porous hollow Cu fiber electrode designed and synthesized by Kas et al. exhibited a significantly improved reaction activity and Faradaic efficiency due to the direct introduction of CO2 molecules onto the muchincreased electrode surface.48 Due to the lower solubility of CO than CO2, it can be expected that such electrode design strategy may benefit CO electroreduction with an even larger performance improvement. 2.3 Reactor design for improving the overall performance. An appropriate reaction design to accommodate a GDE can further eliminate the mass transport problems associated with the low solubility of CO. Recently, Jouny et al. reported an electrochemical flow-cell reactor that achieved hundreds of milliamperes per square centimeter activities for CO electroreduction.49 The studied catalysts consisted of commercial Cu particles (denoted as micrometer copper) with an average particle size of 0.5-1.5 µm and OD-Cu derived from oxidized micrometer copper. In their reactor design, the catalysts were loaded onto a hydrophobic porous carbon support and positioned between the gas and liquid chambers in which the CO stream and electrolyte flowed, respectively (Figure 9a). This configuration allowed for direct feed of the CO reactant to the electrode-electrolyte interface. Therefore, the reaction rate was substantially enhanced and surpassed the best previously reported batch-type cell results by two order of magnitude.23, 29, 31-34, 37, 46 The competing hydrogen reduction reaction was also suppressed due to the much-improved surface CO occupation. Faradaic efficiencies of more than 80% for C2+ products were achieved at high overpotentials (Figures 9b, c). 11 ACS Paragon Plus Environment

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Figure 9. (a) Schematic representation of the three-compartment microfluidic CO flow electrolyzer. (b) Geometric partial current density for C2+ products as a function of the applied potential for CO reduction in 1 M KOH on OD-Cu and micrometer copper. (c) Corresponding Faradaic efficiency profiles for OD-Cu. Error bars represent the standard deviation from at least three independent measurements. Reproduced with permission from Ref. 49. 3 Influence of electrolyte for CO electroreduction 3.1 Cation effect. The electrolytic cation plays an important role in CO electroreduction as a key component in the reaction interface due to the negatively biased electrode. In 1991, Murata et al. reported that alkali cations in a bicarbonatebased electrolyte influenced the selectivity of CO reduction on polycrystalline copper.50 The authors found that larger cations promoted the formation of C2 products over C1 product (Table 1). At a constant cathodic current density of 1.5 mA·cm-2, the C2/C1 ratio increased from 0.515 to 7.95 as the cation size increased. In addition, the overpotential required to drive the current density exhibited a decreasing trend, indicating that larger cations may also improve the reaction activity. 12 ACS Paragon Plus Environment

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Table 1. Faradaic efficiencies of the products in the electrochemical reduction of CO in 0.1 M alkali bicarbonate solutions. Potential

Faradaic Efficiencies / %

Cation

C2/C1 V vs. SHE

CH4

C2H4

EtOH

PrOH

H2

Total

Li+

-1.40

26.0

13.4

0.4

Tr

61.1

100.9

0.515

Na+

-1.43

21.6

15.8

1.4

1.1

59.9

99.6

0.731

K+

-1.37

8.9

25.7

3.1

2.2

54.7

94.6

2.89

Cs+

-1.31

2.0

15.9

1.7

1.3

76.8

97.7

7.95

Similar observations have also been reported by Pérez-Gallent et al. in 2017. In this study, the larger cations promoted the formation of C2+ products in an alkali hydroxide solution based on online electrochemical mass spectrometry (OLEMS) analysis (Figure 10).51 In addition, the authors reported that the cation effect differed for different reduction products and sometimes depended on the surface orientations. Although the onset potential of C2H4 on a polycrystalline surface decreased as the cation size increased, this potential was not influenced by single crystalline surfaces (Figures 10a, d, g). In contrast, the onset potential for CH4 (ca. -0.65 V) was independent of both the cation size and the surface orientations (Figures 10b, e, h). The C2H4 production was improved by larger cations on all surfaces but CH4 production only improved on single crystal surfaces. The enhancement that was observed for C2H4 was more significant at lower overpotentials compared to that of CH4 and was the most significant on the Cu(100) surface (Figures 10a-h). Therefore, the C2H4/CH4 ratio decreased as the potential became more negative. However, the decrease in this ratio was less rapid when a larger cation was used (Figures 10c, f, i).

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Figure 10. OLEMS mass fragments measured during CO reduction associated with the formation of C2H4 (top panel, a, d, and g) and CH4 (middle panel, b, e, and h) on (a) Cu(100), (b) Cu(111), and (c) polycrystalline Cu for different 0.1 M alkali hydroxide solutions. Bottom panel (c, f, and i) shows the potential-dependent ratio (m/z = 26)/(m/z = 15) of the OLEMS mass fragments associated with the formation of C2H4 and CH4 during CO reduction. Reproduced with permission from Ref. 51. It is important to note that the enhanced production of C2+ products by larger cations is also typically observed in CO2 electroreduction.50, 52-53 Because CO2 electroreduction is believed to share common reaction pathways with CO electroreduction after *CO formation on the catalyst surface, several investigations of the cation effects in CO2 electroreduction are included for discussion to provide more comprehensive mechanistic insights. Hori et al. attributed the change in the C1/C2 product ratio during CO2 electroreduction to the variation in the potential of the outer Helmholtz plane (OHP) of the cathode (Figure 11).50 Because larger cations tend to be less hydrated, they would be more easily adsorbed on the electrode, which would lead to an increase in the electrode potential (Figure 11b). In general, C2H4 starts to form at less negative potentials than CH4. Therefore, the authors believed that C2H4 production was favored over CH4 due to the electrode potential being shifted towards in a positive direction by the adsorption of larger cations.

Figure 11. (a) Variation of the C1/C2 ratio in the electroreduction of CO2 in a bicarbonate electrolyte at different concentrations. (b) Schematic illustration of the potential distribution in the neighborhood of the electrode. Reproduced with permission from Ref. 50. To gain a deeper understanding of C2 product promotion by larger cations, Resasco et al. proposed that some important reaction intermediates of the C2 products, such as *OCCO and *OCCHO, could be better stabilized by a localized electric field of 14 ACS Paragon Plus Environment

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approximately 1 V Å-1 that was induced by cations adsorbed at the electrode surface. The correlation between the cation size and the promotion effect may originate from the different electric fields of different cations.54 The difference in the electric field may be due to two main factors as follows: (1) intrinsic disparities in the magnitude of the electrostatic field induced by cations of different size and (2) an increase in the concentration of the solvated cations at the outer Helmholtz plane as a function of size. The first factor can be quickly ruled out because the constrained minima hopping (CMH) calculation of the field in the vicinity of *CO did not exhibit a systematic variation with the cation sizes. The second factor was supported by the results from DFT calculations where the driving force for each cation to migrate from the bulk electrolyte to the OHP increased with the cation size. In addition, C2 product formation during CO electroreduction in a 0.1 M LiHCO3 electrolyte was experimentally determined to be substantially improved when only 10% of Li+ cations were replaced by larger cations (i.e., Na+, K+ or Cs+) (Figure 12).54 Therefore, the authors believed that C2 product promotion by larger cations was due to their higher concentrations near the electrode surface, which induced a stronger electric field that stabilized the key intermediates for C2 product formation.

Figure 12. Relative effects of alkali metal cations. Partial currents for ethylene (a) and ethanol (b) formation as a function of the electrolyte composition on Cu(100). Data are presented for a potential of −1.0 V. A mixture consisting of LiHCO3 and XHCO3 electrolytes was used, where X was Na, K, Cs (second cation). A fixed total salinity of 0.1 M was used in all experiments. Reproduced with permission from Ref. 54. This theory was further supported by Pérez-Gallent et al. using DFT investigations of a cation chemisorbed Cu(100) as a model surface.51 The authors determined that *OCCO and *OCCOH were typically stabilized by approximately 1.2 eV and 1.16 eV in the presence of alkali cations. Although the reaction energies for *CO hydrogenation without cations were calculated to be 0.73 and 0.87 eV for the C1 and C2 pathways, the energies drastically decreased to 0.54 and 0.18 eV in the presence of cations (Figure 13). The onset potential of an electrochemical reaction was believed to be determined by the largest uphill reaction step energy. The DFT results suggested that the largest 15 ACS Paragon Plus Environment

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uphill reaction step energy became significantly smaller for reactions that led to C2 products compared to C1 products in the presence of cations (Figure 13), indicating that C2 product formation is favored by the introduction of cations onto the electrode surface.

Figure 13. Energetics of the first electrochemical steps of CO reduction for the C1 and C2 pathways on Cu(100) at 0 V. (a) In a vacuum and (b) with cations, averaging the energies for Li, Na, and Cs. However, in a vacuum, both pathways are highly endothermic, and the C2 pathway is substantially promoted by alkaline cations. Reproduced with permission from Ref. 51. Theoretical investigations can provide molecular-level insights into the influence of cations on CO electroreduction. However, the accurate description of those ions in aqueous media is very challenging. In the study conducted by Pérez-Gallent et al., the cation atoms were directly placed at the Cu surface to mimic the presence the cations during the actual electrolysis.51 This is an over simplified model because the alkali metal ions cannot be reduced to atoms in the potential range of CO electrolysis and their hydrations are not considered. To describe the cations at electrochemical reaction interface, hydrated alkaline metal atoms with explicit water molecules were adapted in many computational studies.54 However, the deficiency for those models is that the alkaline metal atoms are not positively charged. Advanced calculation models with better description of alkali metal ions are highly desired for conducting more insightful computational studies. 16 ACS Paragon Plus Environment

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3.2 pH effect. The electrolyte pH is also essential for CO electroreduction because hydrogenation processes are considered key reaction steps for the conversion of CO to hydrocarbons and oxygenates. The local pH near an electrode surface is typically higher than the pH of the electrolyte bulk due to the consumption of protons and the consequent generation of OH-. Hori et al. conducted CO electroreduction studies on Cu in a series of electrolytes with different buffer strengths by tuning the HCO3concentration (Table 2).29 The electrolysis results indicated that although C1 product (i.e., CH4) formation was promoted at high HCO3- concentrations, C2 product (i.e., C2H4 and C2H5OH) formation was promoted at low HCO3- concentrations. The authors believed that the increase in the local electrode pH could be better suppressed at high HCO3- concentrations due to a stronger buffer capability. Therefore, CH4 formation was favored by a low pH environment, and C2 product formation was favored by a high pH environment. To further probe the influence of the electrolyte pH on CH4 and C2H4 formation, the authors evaluated the CO electroreduction in a series of electrolytes with pH values ranging from 6.0-12.2, which were prepared from phosphate buffer, borate buffer, or K2HPO4-KOH with a total electrolyte concentration of 0.2 M. The logarithmic plot of the C2H4 partial current density as a function of the potentials (Figure 14a) exhibited good correlations regardless of the pH value, indicating that the formation of C2H4 was likely independent of the electrolyte pH. However, the data acquired for the CH4 partial current density did not result in a Tafel relationship unless corrected for the pH (Figure 14b). The transfer coefficient for C2H4 formation was determined to be 0.35, which indicated that the rate determining step was the first electron transfer. The transfer coefficient for CH4 formation was determined to be 1.33, which indicated that an electron transfer process was in equilibrium prior to the rate determining step. These results suggested that the formation of CH4 and C2H4 followed completely different mechanisms. The pH independence of the C2H4 partial current density shown in Figure 14 appears to contradict the results shown in Table 2, where the C2H4 formation was promoted in a high pH environment. In this case, an increase in the local electrode pH may have shifted the electrode potential on the RHE scale to more positive values that favored C2 product formation according to the authors’ previous work (Figure 11).50 Table 2: Faradaic Efficiencies of Various Products Affected by KHCO3 Concentration.a Faradaic efficiencies / %

KHCO3/mol/L

Potential/V vs SHE

CH4

C2H4

EtOH

n-PrOH

MeCHO

EtCHO

H2

total

C2H4/CH4

0.03

-1.38

16.2

28.1

13.1

3.1

2.1

2.7

40.1

105.4

1.73

0.05

-1.36

18.5

19.1

7.6

1.9

1.6

1.3

49.4

99.4

1.03

0.1

-1.36

22.3

21.7

7.1

2.2

1.4

0.5

41.7

96.9

0.973

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0.2

-1.35

23.2

15.8

7.1

2.2

0.7

0.6

51.5

101.1

0.681

0.3

-1.35

32.4

15.3

6.3

3.5

0.4

0.8

44.8

103.5

0.472

a

Total ionic concentration was maintained at 0.3 mol/L by adding KCl. Temperature: 19 °C. Current density: 2.5 mA·cm-2.

Figure 14. (a) Partial current densities of C2H4 formation and (b) log [i(CH4) + pH] in electrochemical reduction of CO correlated with the electrode potential in various electrolyte solutions: pH 6.0-6.3 (○), pH 7.1-7.7 (△), pH 8.0-8.6 (□), pH 8.7-8.9 (×), pH 9.0-9.3 (●), pH 10.5-11.3 (▲), and pH 12.2 (■). Reproduced with permission from Ref. 29. The investigation of pH influence on CO electroreduction can shed a light on its elusive reaction mechanism. For example, the pH dependence of product formations can in part suggest whether their rate determining steps involve proton transfer coupled with electron transfer. However, the local pH near electrode surface is typically different than the pH of bulk electrolyte due to the consumption of proton by the proceeding reaction and the limited mass transport. Accurate pH dependence study could be challenging due to the lack of efficient techniques to measure the pH near electrode surface. Dunwell et al. currently developed a spectroscopic method to determine the electrode local pH in CO2 electroreduction reaction.55 The authors found that the overpotential can be overestimated for as large as 150 mV at -1.0 VRHE in 0.25 M NaHCO3 solution due to the deviation of the pH near electrode surface from bulk electrolyte. Therefore, to gain concrete mechanistic sights from pH dependence study of CO electroreduction, an accurate measurement or estimation of electrode local pH is of great importance. 4. Reaction mechanism investigations

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Understanding the mechanism of CO electroreduction is critical for achieving more optimal reaction activities and selectivities. Systematic studies of the reaction kinetics are currently not possible due to the lack of model catalysts that exhibit sufficient activities over a reasonably large potential window.23, 29-30 CO2 electroreduction could be alternatively used for mechanistic probing because it is believed to share the same reaction pathways as CO electroreduction after the formation of *CO.56-58 However, recent study results have been controversial due to the chemical reaction of CO2 with aqueous electrolytes complicating the reaction interface.55, 59-63 As a result, computational investigations were employed as major tools in predicting the key reaction intermediates, potential determining step and pathways to provide molecularlevel insights. Spectroscopy studies to identify reaction intermediates were conducted to facilitate the computational predictions. The initial efforts were focused on understanding the reaction pathways leading to C1 product (i.e., CH4) formation. With the development of more advanced computational models, more complex C2+ product formation mechanisms have also been proposed and discussed. The potential determining step of CO electroreduction to CH4 is widely accepted to involve *CO hydrogenation. However, the product of *CO hydrogenation remains unclear. In 2010, Peterson et al. made the first attempt to calculate the pathway from *CO to CH4 on the stepped Cu(211) surface using a computational hydrogen electrode (CHE) model.64 The authors proposed that *CHO was the energetically favored product of *CO hydrogenation, and the complete reaction mechanism was as follows: *CO → *CHO → *CH2O → *OCH3 → *O + CH4 → *OH + CH4 → H2O + CH4. The rate limiting step (i.e., *CO → *CHO) required the largest uphill reaction energy of 0.74 eV, which indicated that the calculated onset potential for CH4 production was 0.74 V. Despite insightful probes for mechanistic understanding of CH4 formation, all discussions in this study were based solely on thermodynamics. The energy barriers of the reaction steps were not studied. Moreover, because CH2O was identified as an intermediate towards CH4 formation, experimental investigations of CH2O electroreduction were conducted to confirm this proposed reaction mechanism.65 However, the reduction products were primarily CH3OH rather than CH4, suggesting that *CH2O and its subsequent adsorbate *OCH3 may not be intermediates in CH4 formation. In 2013, Nie et al. included kinetics in their DFT studies of CH4 formation on Cu(111) for the first time.66 Based on their results, a more negative potential of approximately -1.15 V was required to make all activation barriers surmountable at room temperature. At this potential, the product of *CO hydrogenation was proposed to be *COH rather than *CHO because the activation barrier for *COH was only 0.21 eV, which was smaller than that for CHO (i.e., 0.39 eV). The overall reaction mechanism was considered to be *CO → *COH → *C + H2O → *CH + H2O → *CH2 + H2O → *CH3 + H2O → CH4 + H2O. Moreover, the hydrogenation of *OCH3 to form CH4, which was key in the previous mechanism reported by Peterson et al., was determined to be kinetically prohibited. The predominant product of this process was 19 ACS Paragon Plus Environment

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predicted to be CH3OH, which is consistent with experimental observations. To better correlate theoretical and experimental results, the calculations must be performed over a surface that is convenient for experimental studies. Polycrystalline Cu is the most common model catalyst employed in experimental studies.23, 29-30, 50, 56 However, recent efforts based on quasi-operando electrochemical tunneling microscopy suggest that the polycrystalline Cu surface is reconstructed to Cu(111) and ultimately Cu(100) under CO electroreduction conditions. 67 This result was supported by the fact that single crystal Cu(100) exhibited a product distribution that was similar to that of polycrystalline Cu. Therefore, Cu(100) can be regarded as a more suitable model surface for obtaining theoretical insights into experimental studies. Cheng et al. performed DFT calculations of CH4 formation on Cu(100).68 Moreover, the description of water layers was found to be critical in better simulating the electrolysis environment.69-72 Therefore, the authors adopted a computational model that included explicit solvent to better describe the electrolyte/electrode interface. 49 H2O molecules (~5 layers, one molecule was protonated) were added to the Cu(100) surface for quantum molecular dynamics calculations (Figure 15). The reaction free energies were obtained using enhanced sampling methodology. The resulting CH4 formation pathway was reported as *CO → *CHO → *CHOH → *CH + H2O → *CH2 + H2O → *CH3 + H2O → CH4 + H2O.

Figure 15. Water-Cu(100) interface (side view, a) and first layer water on Cu(100) (top view, b). Reproduced with permission from Ref. 68. The mechanism of C2 product formation was probed by Schouten et al. by performing CO electroreduction experiments on two single-crystal copper surfaces (i.e., Cu(100) and Cu(111)) using online electrochemical mass spectrometry (OLEMS).73 On Cu(111), the potential dependence of the m/z 15 curve for CH4 was very similar to that of the m/z 26 curve for C2H4 (Figure 16a). However, on Cu(100), these hydrocarbon formations are very different. C2H4 was formed much earlier at potentials of -0.4 V at a pH of 7 and -0.3 V at a pH of 13 and peaked at -0.6 V and -0.4 V, respectively. Although C2H4 production decreased at more negative potentials at a pH of 13, the C2H4 20 ACS Paragon Plus Environment

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production increased again at a pH of 7 and a potential of approximately -0.8 V, which was also accompanied by the formation of CH4 (Figure 16b). These results strongly suggest that two distinct reaction pathways lead to the formation of C2H4. One pathway shares an intermediate with the pathway to CH4, as observed on Cu(111) and below 0.8 V on Cu(100) at pH 7. The other pathway occurred exclusively on Cu(100), as observed in the middle of the potential window. Therefore, many research efforts have been devoted to understanding these two pathways.

Figure 16. Top: corresponding facet of the copper fcc crystal, middle: cyclic voltammograms for the reduction of a saturated solution of CO in phosphate buffer (pH 7) and NaOH solution (pH 13), and bottom: associated mass fragments of volatile products measured with OLEMS. Data for pH 7 are shown with blue dotted lines and plotted against the left axis, and data for pH 13 are shown with green solid lines and plotted against the right axis on (a) Cu(111) and (b) Cu(100), respectively. A very small amount of C2H4 was observed at approximately −0.45 V on Cu(111) due to the (100) defects on the Cu(111) surface. Reproduced with permission from Ref. 73. Schouten et al. compared C2H4 and CH4 formation on Cu(100) and Cu(111) at different pH values. In this study, although the onset potentials (in SHE scale) for C2H4 formation were independent of pH on Cu(100), the onset potentials (in SHE scale) for both C2H4 and CH4 formation were pH dependent on Cu(111).74 This result indicates that the potential determining step for C2H4 formation on Cu(100) did not involve proton transfer. However, the proton transfer process is required for both C2H4 and CH4 formation on Cu(111). Based on both experimental and computational results, the authors proposed the existence of different reaction pathways on Cu(100) and Cu(111), as shown in Figure 17. The potential determining step for C2H4 formation was 21 ACS Paragon Plus Environment

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considered to be *CO dimerization on Cu(100) and *CO hydrogenation to *CHO on Cu(111), which is also believed to be the intermediate that is further converted to CH4.

Figure 17. Proposed mechanism for C2H4 and CH4 formation on Cu(100) and Cu(111) by Schouten et al. Reproduced with permission from Ref. 74. Montoya et al. further explored *CO dimerization computationally by including electric field and solvation effects.26 The energy barrier for *CO dimerization was determined to be 0.33 eV on Cu(100), which was significantly lower than that of 0.68 eV on Cu(111) (Figure 18). This result is consistent with the proposed mechanism reported by Schouten et al. where *CO dimerization was favored on Cu(100) over Cu(111).75 The authors also noted the importance of modeling the electrochemical environment. The introduction of an electric field can change the final state energy of the CO dimer by 0.2 eV, and the solvent can stabilize the dimer by 0.6 eV compared to the energy barriers obtained in vacuum.

Figure 18. Kinetic barriers for the formation of a CO dimer from two adsorbed CO species on Cu(111) (left) and Cu(100) (right). The barriers of 0.68 eV (111) and 0.33 22 ACS Paragon Plus Environment

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eV (100) demonstrated that Cu(100) should form CO dimers at significantly higher rates than Cu(111). Schematic representations of the initial, transition, and final states for both calculations are also shown. Reproduced with permission from Ref. 26. To improve the computational investigations by using an improved model of the electrolyte/electrode interface, Cheng et al. modeled 5 layers of explicit solvent on a Cu(100) surface and studied the reaction pathways of C2H4 and CH4 formation based on kinetics by applying ab initio molecular metadynamics simulations (AIMμD).76 Their results suggested that *CO dimerization was the potential determining step in C2H4 formation, and this step was followed by hydrogenation to *COCOH, which is the next intermediate (Figure 19). Then, two possible reaction pathways were proposed to lead to the formation of C2H4. Using AIMμD, the authors re-examined the reaction pathway of CH4 formation and determined that it was consistent with their previous work.68

Figure 19. Lowest kinetic pathways for the eight-electron reduction of CO to C2H4. Both Eley–Rideal (ER) (in black) and Langmuir–Hinshelwood (LH) mechanisms (in blue) were considered. In ER, H2O + e– (producing OH–) were the reactants, and in LH, H* was the reactant. The reaction free-energy barriers (ΔG‡) were provided. The broken line shows the minor pathway for C2H4 formation through *C=C=O. Reproduced with permission from Ref. 76. The electric potential must be carefully considered to better represent the electrochemical environment during CO electroreduction. The calculations that employed an electric potential in previous studies were based on the CHE model, which does not control the electrode potential but only considers the influence of the potential on the energy of the electrons transferred in each reduction step. Therefore, the CHE model typically introduces variations up to 1 V in the electrode potential during modeling of the adsorbed intermediates.26, 77 However, the CO electroreduction experiments proceed at constant potentials. To solve this problem, researchers recently 23 ACS Paragon Plus Environment

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developed a more advanced constant electrode potential (CEP) model in which the electrode potential can be controlled by adjusting the number of electrons in the system.24-25, 27, 78-80 In 2016, Xiao et al. reported the first computational study of CO electroreduction on Cu(111) using the CEP model.24 Their results indicated that (1) at acidic pH, C2H4 formation was kinetically blocked and CH4 formation proceeded through *CO → *COH → *CHOH→ *CH2 → *CH3 → CH4. (2) At neutral pH, C2H4 and CH4 formation shared a common reaction step (i.e., *CO → *COH), and the C-C bond was formed due to coupling of *COH and *CO. (3) Under basic pH, CO hydrogenation was kinetically blocked, and CO reduction proceeded via CO dimerization. In a more detailed follow-up study, the authors calculated the energy barriers of all possible pathways under constant potentials and reported the complete reaction pathways for C2H4 and CH4 formation on Cu(111) at different pH values (Figure 20).25

Figure 20. Predicted reaction pathways for C2H4 and CH4 formation at different pH values. Reproduced with permission from Ref. 25. To obtain experimental evidence of the reaction intermediates for more reliable computational studies, Pérez-Gallent et al. conducted in situ Fourier transform infrared spectroscopy (FTIR) studies of the CO electroreduction over Cu(100) in a potential window of +0.05-0.2 V.36 In Figures 21a and b, the characteristic adsorption bands that were observed at 1584 and 1191 cm-1 corresponded to C=O and C-OH stretching, respectively. Because CH4 typically forms at a much more negative potential, the 24 ACS Paragon Plus Environment

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authors excluded the possibility that those adsorption bands were the fingerprints of *CHO or *COH. The infrared active vibrational frequencies of the CO dimer and their possible hydrogenated products were calculated, and *COCOH was determined to be consistent with two observed bands (Figure 21c). However, a later study that was reported by Goodpaster et al. indicated that *COCHO with a different adsorption configuration than that shown in Figure 21c could exhibit characteristic adsorption bands similar to those of *COCOH.27 The reaction intermediate after *CO dimerization may be *COCOH and/or *COCHO.

Figure 21. (a) Potential-dependent absorbance spectra for Cu(100) in the presence of CO in a 0.1 M LiOH solution. Reference spectrum was recorded at 0.1 V. Highlighted bands and their corresponding frequencies were indicated with a vertical line at 1191 cm-1 for 12C-OH stretching, 1677 cm-1 for 12C=O stretching CO adsorbate and 1600 cm1 for O-H bending in water. The 1584 cm-1 band overlapped with the absorption of water. (b) 1584 cm-1 band for 12C=O stretching of a C=O containing intermediates. D2O was used as the solvent to prevent the strong water absorption signal nearby. (c) Schematic structures of possible adsorbed intermediates on Cu(100) and their calculated infrared 25 ACS Paragon Plus Environment

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active vibrational frequencies in the 1100-1600 cm-1 region. Cu, Li, C, O, and H atoms are depicted as orange, yellow, gray, red and white spheres. Reproduced with permission from Ref. 36. Several recent studies have indicated that *CO dimerization may not be the only pathway that can lead to the formation of *COCHO. Using the CEP model, Goodpaster et al. determined that the pathway involving *CO → *CHO followed by *CHO + *CO → *COCHO was favored over *CO dimerization on Cu(100) at a potential of approximately -1 V (typical electrolysis potential for experiment).27 Garza et al. expanded on the work reported by Goodpaster et al. and found that the formation of *COCHO via a CO dimer on Cu(100) was only thermodynamically favored in a less negative potential range of -0.37 to -0.65 V.28 As the potential became more negative than this range, the CO dimer became highly unstable, leaving *CHO as the only possible intermediate leading to both C1 and C2 products. The complete reaction pathway for the formation of C2 products after C-C bonding was calculated and is shown in Figure 22.

Figure 22. Proposed mechanism for the reduction of CO to C2 products at high overpotentials on Cu(100) by Garza et al. ΔG values (eV) at 0 V using the CHE model are shown in standard font (steps involving H+ + e- can be corrected to -1 V by subtracting 1 eV). ΔG values at -1 V using the CEP model (at pH = 7) are shown in bold 26 ACS Paragon Plus Environment

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font. The seven C2 products from CO reduction on copper are highlighted in green. Calculated free energy barriers (eV) are provided for the critical reductive step (i.e., from intermediate 3 to either 4a or 4b) that determines the selectivity between the pathways to ethylene (purple) and ethanol (blue). Reproduced with permission from Ref. 28. The current major obstacle preventing the development of CO electrolysis is the lack of a fundamental understanding of the underlying reaction mechanism. However, due to the complexity of this reaction as well as electrode-electrolyte interface, and the dynamic nature of electrolysis environment, experimental investigations are extremely challenging. Currently, very limited proposed adsorbates have been widely accepted as reaction intermediates. The development of in situ and operando spectroscopy techniques that can identify reaction intermediates at electrode surface are highly desired. The computational investigation has been proven to be a powerful way to probe the reaction mechanism. However, to accurately simulate the reaction interface including solvated cations, protons and hydroxide ion that are critical in determining the activity and selectivity of CO electroreduction remains to be difficult for the currently available models. The development of more advanced computational models capable of describing the electrode-electrolyte interface is also highly desired. 5. Summary and perspectives Significant progress in CO electroreduction has been achieved based on the highlighted efforts from recent years. Novel reaction sites have been discovered and studied. The optimization of electrode and reactor configurations is an effective method for improving the overall performance. The mechanistic investigations have undoubtedly narrowed the possible potential determining steps, and based on these steps, the activity of CO electroreduction could be enhanced by improving *CO hydrogenation or the C-C bond formation process. However, to improve the reaction selectivity requires conclusive identification of all reaction intermediates as well as a comprehensive understanding of the reaction pathways after the potential determining steps. The development of techniques that can identify these reaction intermediates under in situ and operando conditions is highly desired. Reliable reaction kinetics must be obtained by developing model catalysts with sufficiently large activity over reasonably large potential windows. Computational models that can more precisely describe the electrode-electrolyte interface are also needed to provide more accurate predictions and design guidelines to advance future CO electroreduction techniques. Acknowledgements This work was supported by the National Basic Research of China (grant number 2017YFA0208200) and the National Natural Science Foundation of China (grant number 21872079, 21606142). Author Information 27 ACS Paragon Plus Environment

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Corresponding Author * E-mail for M.C.: [email protected] * E-mail for Q.L.: [email protected] Author Contributions These authors contributed equally to this work

§

References (1) Liserre, M.; Sauter, T.; Hung, J., Future Energy Systems: Integrating Renewable Energy Sources into the Smart Power Grid Through Industrial Electronics. IEEE Ind. Electron. Mag. 2010, 4, 18-37. (2) Baños, R.; Manzano-Agugliaro, F.; Montoya, F. G.; Gil, C.; Alcayde, A.; Gómez, J., Optimization methods applied to renewable and sustainable energy: A review. Renew. Sust. Energ. Rev. 2011, 15, 1753-1766. (3) Mathiesen, B. V.; Lund, H.; Karlsson, K., 100% Renewable energy systems, climate mitigation and economic growth. Appl. Energy 2011, 88, 488-501. (4) Storch, H. H.; Golumbic, N.; Anderson, R. A., The Fischer-Tropsch and related syntheses. Wiley New York: 1951; Vol. 6. (5) Schulz, H., Short History and Present Trends of Fischer–Tropsch Synthesis. Appl. Catal., A: Gen. 1999, 186, 3-12. (6) Dry, M. E., The Fischer–Tropsch Process: 1950–2000. Catal. Today 2002, 71, 227241. (7) Van Bibber, L.; Shuster, E.; Haslbeck, J.; Rutkowski, M.; Olson, S.; Kramer, S., Technical and economic assessment of small-scale Fischer-Tropsch liquids facilities. 2007. (8) Goellner, J.; Shah, V.; Turner, M.; Kuehn, N.; Littlefield, G.; Marriott, J., Analysis of Natural Gas-to-Liquid Transportation Fuels via Fischer-Tropsch. National Energy Technology Laboratory. Office of Strategic Energy Analysis and Planning, DOE/NETL2013/1597, USA 2013. (9) Hori, Y.; Kikuchi, K.; Suzuki, S., Production of CO and CH4 in electrochemical reduction of CO2 at metal electrodes in aqueous hydrogencarbonate solution. Chem. Lett. 1985, 14, 1695-1698. (10) Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O., Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueous Media. Electrochim. Acta 1994, 39, 1833-1839. (11) Hori, Y., Electrochemical CO2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry, Vayenas, C. G.; White, R. E.; Gamboa-Aldeco, M. E., Eds. Springer New York: New York, NY, 2008; pp 89-189. (12) Chen, Y.; Li, C. W.; Kanan, M. W., Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles. J. Am. Chem. Soc. 2012, 134, 1996919972. (13) Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G.; Jiao, F., A selective and efficient electrocatalyst for carbon dioxide reduction. Nat. Commun. 2014, 5, 3242. 28 ACS Paragon Plus Environment

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