Experimental and Theoretical Elucidation of Electrochemical CO2

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Experimental and Theoretical Elucidation of Electrochemical CO Reduction on Electrodeposited CuSn Alloy 2

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Masayuki Morimoto, Yoshiyuki Takatsuji, Satoshi Iikubo, Shoya Kawano, Tatsuya Sakakura, and Tetsuya Haruyama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11431 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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The Journal of Physical Chemistry

Experimental and Theoretical Elucidation of Electrochemical CO2 Reduction on Electrodeposited Cu3Sn Alloy Masayuki Morimoto†, Yoshiyuki Takatsuji†*, Satoshi Iikubo§, Shoya Kawano§, Tatsuya Sakakura†, and Tetsuya Haruyama†* †Division

of Functional Interface Engineering, Department of Biological Functions Engineering,

Kyushu Institute of Technology, Kitakyushu Science and Research Park, Fukuoka 808-0196, Japan §Division

of Environmental Materials Design, Department of Life and Systems Engineering,

Kyushu Institute of Technology, Kitakyushu Science and Research Park, Fukuoka 808-0196, Japan

KEYWORDS CO2 reduction, Cu–Sn alloy, Cu3Sn, DFT

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The reaction selectivity of an electrode catalyst can be modulated by regulating its crystal structure, and the modified electrode may show different CO2 reduction selectivity from that of its constituent metal. In this study, we investigated the mechanisms of the electrochemical CO2 reduction on electrodeposited Cu3Sn alloy by experimental and theoretical analyses. The electrodeposited Cu3Sn alloy electrode showed selectivity for CO production at all the applied potentials, and HCOOH production increased with an increase in applied potential. In particular, hydrocarbon generation was well suppressed on Cu3Sn(002). To understand this selectivity change in electrochemical CO2 reduction, we conducted density functional theory calculations for the reaction on the Cu3Sn(002) surface. According to the theoretical analysis, the Cu sites in Cu3Sn(002) contributed more to the stabilization of H*, COOH*, and CO* as compared with the Sn sites. Furthermore, the results indicated that Cu3Sn(002) decreased the surface coverage of reaction intermediates such as H*, COOH*, and CO*. We believe that these effects promoted CO* desorption while suppressing H2 generation, CO* protonation, and C–C bond formation. The results also suggested that the surface Sn concentration significantly affected the reaction selectivity for HCOOH production from CO2.


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INTRODUCTION The reaction selectivity in the electrochemical reduction of CO2 has attracted the interest of many researchers because it depends on the metal species and their surface state1,2. Hori et al. demonstrated the fundamental catalytic properties of various bulk metals for CO2 reduction3. In aqueous solutions, CO2 reduction competes with the hydrogen evolution reaction (HER). The binding energy between the reaction intermediate and the metal is a critical factor for understanding the CO2 reduction, which progresses via the adsorption of the reaction intermediate onto the metal surface. For example, the difference in reaction selectivity between Cu and Ag, Au, and Zn depends on the strength of CO adsorption onto the respective surface (CO*)4. Cu binds CO* more strongly than do Au, Ag, and Zn; CHO* (surface-adsorbed CHO) and COH* (surface-adsorbed COH) are generated from further reduction of CO*. Hydrocarbons such as CH4 and C2H4 are formed via the multielectron reduction of CHO* and COH*4–7. Since Au, Ag, and Zn bind CO* more weakly than dose Cu, CO* desorption occurs in preference to CO* reduction4,8. The binding strength of the reaction intermediate to the metal is related to the limiting potential, and the requisite overpotential decreases with an increase in the binding strength. However, very strong binding of the intermediate results in surface poisoning, which decreases the catalytic activity4,9,10. It has been reported that the selectivity of CO2 reduction depends to a greater extent on the surface morphology11–13, crystal structure14–16, and oxidation state17–19 than on the metal species. Because of these physicochemical properties, alloying with metals can make the catalytic activity different from the CO2 reduction activity of single metals. To guide the design of alloy catalysts for CO2 reduction, density functional theory (DFT) calculations are conducted for screening various combinations of metals20–23. Cheng et al. carried out screening for selective



CH4 production from CO2 using a single-atom bimetallic alloy, and substituted Au and Ag with various metals (Cu, Ni, Pd, Pt, Co, Rh, and Ir)22. They investigated the intermediates after CO* formation by supposing that insertion metals produce CH4 from CO generated by the Au or Ag surface. Their calculations revealed that Rh substitution resulted in the lowest limiting potential for CH4 production22. Hirunsit et al. fabricated copper-based alloys, Cu3X (X = Co, Ni, Rh, Pd, Ag, Ir, Pt, and Au), and evaluated the reaction pathway of CO2 reduction on these alloys by DFT calculations23. They clarified that the physical properties of Cu change depending on the metal combination employed. However, they did not perform the CO2 reduction using an experimentally prepared Cu3X and single-atom alloy. There are a number of studies concerning CO2 reduction on alloy electrodes24–28. Kortlever et al. reported that the selectivity for HCOOH production depends on the composition ratio of Pd and Pt in PdxPt(100−x)/C particles24. They suggested that the change in selectivity of HCOOH production was a result of a shift in the d-band center and distortion of the lattice due to alloy formation. With regard to Cu-based alloys, Ren et al. reported that electrodeposited oxidederived CuxZn showed 30% Faradaic efficiency (FE) for ethanol production25. The results, which revealed an increase in the selectivity for C2 products, suggested that the Zn sites in the Cu–Zn alloy provide the neighboring Cu sites with CO molecules, and that CO* and CHO* on the Cu sites react with these CO molecules. Takatsuji et al. reported that an electrodeposited Cu–Co alloy promoted CH4 production by inhibiting the C–C coupling reaction26. They suggested that the surface Co atoms play a critical role in the protonation of intermediates such as CO*, CHO*, and COH*. In our previous work, we reported an electrodeposited Cu–Sn alloy that shows good selectivity for CO production27. Based on these studies, it can be deduced that the selectivity in CO2 reduction can be controlled by using the appropriate combination of metals. Understanding


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the relation between the selectivity in CO2 reduction and the alloy configuration can be a useful guideline to design the alloy catalyst. In this study, we investigated in detail the mechanisms underlying the electrochemical CO2 reduction on an electrodeposited Cu3Sn alloy by both experimental and theoretical analyses. A Cu3Sn electrode was prepared using the electrodeposition method, and its surface properties were analyzed by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The Cu3Sn electrode showed distinctive selectivity in the electrochemical CO2 reduction, i.e., selective CO generation at a low applied potential and an increase in HCOOH production at a high applied potential. To understand the change in the reaction selectivity of the electrodeposited Cu–Sn alloy electrode, we investigated thermodynamic stability of the alloy surface and reaction intermediates by DFT calculations.

EXPERIMENTAL SECTION Experimental Details Cu3Sn alloy was prepared via electrodeposition on a Cu substrate in an electroplating bath consisting of 0.5 M K4P2O7, 0.05 M C6H14N2O7, 0.16 M CuSO4∙5H2O, and 0.04 M SnSO4. The electrodeposition was conducted using a two-electrode system, in which a Pt plate was used as the counter electrode (CE) and reference electrode (RE). The deposition current and time were set at −3.2 mA cm−2 and 5 min, respectively. The electrode surface of the prepared alloy was analyzed by AES, SEM, and XRD. The detailed measurement conditions have been described in an earlier report27. The results of the surface analyses are indicated in the Supporting Information.



Electrochemical CO2 reduction was conducted by chronoamperometry for 30 min, and the IRdrop was corrected. A Pt plate and a Ag/AgCl electrode were used as the CE and RE, respectively. The electrolyte used was CO2-saturated 0.1 M KHCO3 solution, and the CE and working electrode were separated by a cation-exchange membrane. During the electroreduction, CO2 flowed into the electrocatalysis cell at ~5 mL min−1 and the exhaust gas from the electrocatalysis cell was collected in a gas bag. The gaseous and aqueous products were analyzed by gas chromatography and anion chromatography, respectively. The detailed measurement conditions have been described elsewhere27. Computational Details The DFT calculations on metal surfaces were conducted on the Vienna Ab initio Simulation Package (VASP)29,30, and the projector augmented wave (PAW)31,32 method was applied. The exchange-correlation functionals used were the generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE)33,34. The surface slab models of Cu(111), Sn(200), and Cu3Sn(002) were constructed as 3×3×3, 2×2×4, and 4×4×3, respectively, and the vacuum layer was inserted at about 10 Å (Figure 1). The cutoff energy was set at 553 eV, and the k-point at 9×9×1 (Cu3Sn used 12×14×1). The Methfessel–Paxton method was employed for the structure optimizations, with the sigma set at 0.1 eV. After the optimizations, the energies were accurately calculated using the Blöchl corrected tetrahedron method. The convergence value for ion calculations was set at 10−5 eV, and spin correction was not applied. In the structure optimizations, the surface two layers had a relaxed and other layers were fixed. The structure optimizations of H2, CO2, CO, HCOOH, and H2O molecules were conducted within a 7×7×12 Å space using Gaussian smearing (sigma: 0.1 eV).


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The thermodynamic quantities of surface adsorbates, i.e., zero-point energy, specific heat capacity, and entropy, were calculated using Phonopy35 and VASP code. For the phonon dispersion, the finite displacement method (FDM) was employed36,37. The cut-off energy and kpoint in first-principles calculations were set at 553 eV and 9×9×1, respectively, and the Methfessel–Paxton method was used for the smearing with a 0.1 eV sigma. The convergence value for ion calculations was set at 10−8 eV. The supercell used for phonon calculation was 1×1×1, and the displacement for the FDM was set at 0.01 Å. The calculational procedures for the binding energy and Gibbs free energy are described in the Supporting Information.



Figure 1. Surface Slab Models of Cu(111), Sn(200), and Cu3Sn(002) (Binding Reaction Intermediate Models Calculated for Various Positions A to H).

RESULTS AND DISCUSSION CO2 reduction was performed on electrodeposited Cu, Sn, and Cu76Sn24 electrodes (Figure 2 a–c). On the electrodeposited Cu electrode (Cu100), the HER preferentially progressed at a low applied potential. At a high applied potential, the FE of H2 decreased and that of hydrocarbons (CH4 and C2H4) increased. The maximum FE for hydrocarbons on the Cu100 electrode was 36.1%. The FEs of CO and HCOOH, the two-electron reduction products, increased with the potential up to −0.89 V vs. RHE, and then decreased. On the electrodeposited Sn electrode (Sn100), HCOOH production dominated, with suppression of the HER, at all the applied potentials; the maximum FE for HCOOH production was 87.5% at −1.09 V. CO production on the Sn100 electrode decreased with an increase in the applied potential. The Cu76Sn24 electrode demonstrated a different distribution of CO2 reduction products from that of the other electrodes. This electrode consisted of Cu3Sn, as revealed by XRD analysis, and the change in CO2 reduction selectivity was believed to be due to the crystal structure. The Cu76Sn24 electrode showed a higher FE for CO production as compared with those of the Cu100 and Sn100 electrodes, and alloying with Sn drastically suppressed the HER and hydrocarbon production. For the HCOOH production on the Cu76Sn24 electrode, the FE increased with an increase in the applied potential. The relationship between the applied potential and the partial current density for H2, CO, and HCOOH on each electrode is shown in Figure 2 d–f. The partial current density for each product showed the same trend as that of the FE. These results implied that alloying of Sn with Cu


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decreased the CO* binding energy by changing the lattice parameter and electron density of Cu. In addition, the Cu and Sn sites on the Cu3Sn surface probably catalyzed different CO2 reduction reactions, i.e., CO and HCOOH were produced at the Cu and Sn sites, respectively.

Figure 2. Faradaic Efficiency (a, b, and c) and Partial Current Density (d, e, and f) for Electrochemical CO2 Reduction on Cu, Cu3Sn (Cu76Sn24), and Sn Electrodes.

To investigate the change in reaction selectivity on Cu3Sn, DFT calculations were performed on the prepared surface slab models of Cu(111), Sn(200), and Cu3Sn(002) (Figure 1). The orientation of the surface slab models was decided by XRD analysis. H2, CO, and HCOOH production was assumed to progress through the surface binding states indicated in Figure S3 in



the Supporting Information5,10,38. The reaction intermediate during H2 evolution was H*, and it competed with CO2 reduction. In the first step of CO2 reduction, COOH* (adsorbed carboxyl) and HCOO* (adsorbed formate) were formed via a proton and electron transfer. CO was generated from the COOH* intermediate, whereas HCOOH was generated from the COOH* and HCOO* intermediates. Calculation of the intermediates related to CH4 and C2H4 formation was performed up to the level of CHO* because the Cu3Sn electrode prevented hydrocarbon formation. The binding energies of H*, COOH*, and CO* to Cu, Sn, and Cu3Sn at each position are indicated in Table 1. In Cu(111), H* was stabilized at the hollow sites and not at the atop and bridge sites, whereas COOH* was bound most strongly at the atop sites. CO* indicated a stronger binding state than that of the other intermediates, at all the positions, and was most stabilized at the hollow sites. All the intermediates on Sn(200) were more weakly adsorbed than on the Cu(111) and Cu3Sn(002) surfaces. The atop sites on Sn(200) stabilized surface adsorbates better as compared with the bridge and hollow sites. The results of DFT calculations revealed that on Cu3Sn(002), all the intermediates were more strongly adsorbed at the Cu sites than at the Sn sites. The results of binding energy calculations for the intermediates suggested that the Sn sites suppressed the HER and CO2 reduction. Additionally, the number of reaction sites for the HER and CO2 reduction on Cu3Sn(002) decreased owing to alloying with Sn. The binding energies of HCOO* at the most stable sites are shown in Figure S7 in the Supporting Information. The Cu3Sn(002) surface weakened the bond with CO* relative to that for Cu(111), thus increasing the CO selectivity. These results suggested that alloying with Sn destabilized the binding state of C with Cu. All the structures indicated a similar binding energy for HCOO*.


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Table 1. Binding Energy (eV) of H*, CO*, and COOH* at Each Reaction Site on Cu(111), Sn(200), and Cu3Sn(002) Surfaces.

Cu(111)

H*

COOH*

CO*

A

0.3879

0.4622

−0.6841

B

−0.0511

n.d.a

−0.7722

C

−0.1982

n.d.a

−0.8297

Sn(200)

H*

COOH*

CO*

A

0.432

0.725

0.1701

B

0.4985

0.7145

0.2083

C

0.8048

1.3184

0.3546

D

0.4221

0.6993

0.1617

E

0.8867

1.38

0.1418

Cu3Sn(002)

H*

COOH*

CO*

A

0.5355

0.8372

0.2594

B

0.6164

0.6513

−0.6155

C

0.5005

0.9847

−0.2534

D

0.5826

n.d.a

n.d.a

E

n.d.a

n.d.a

−0.4972

F

0.4051

1.9224

0.2199

G

−0.2626

n.d.a

−0.6837



H aThe

−0.2065

0.8446

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−0.6682

adsorbed molecule moved to another position after structure optimization.

The free energy diagram for the HER on each electrode is shown in Figure 3a. It is seen that Sn(200) requires a high overpotential for H2 evolution because H* is weakly adsorbed on the Sn surface. The overpotential required for H2 evolution on the Cu(111) and Cu3Sn(002) surfaces is low because of the strong binding of H* on these surfaces. The difference in free energy between Cu(111) and Sn(200) is in accordance with the H2 partial current density obtained by CO2 reduction. The H2 partial current density on Sn(200) increased only slightly even when the applied potential was increased, owing to the unstable adsorption of H* (Figure 3b). On the Cu(111) surface, H2 evolution increased with an increase in the applied potential, which is in agreement with the results of earlier studies showing that H2 evolution is closely related to the binding energy of H*9. Cu3Sn(002) showed the same binding energy for H* as that in the case of Cu(111). This result is expected to be high selectivity for H2 production, based on a report that the H2 partial current density shows a volcano relation with the binding energy of H*. However, Cu3Sn(002) showed a low H2 partial current density. These results suggested that the suppression of the HER on Cu3Sn(002) can be attributed to the decrease in the number of reaction sites for H2 evolution. According to the results in Table 1, Cu3Sn(002) weakened the adsorption of H* at the Sn sites; the stabilized sites of bound H* were the bridge and hollow sites of Cu. In other words, alloying with Sn suppresses the HER due to a decrease in the number of reaction sites for H2 evolution.


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Figure 3. Calculated Free Energy Diagram for the HER (a) and Relationship between Binding Energy of H* and H2 Partial Current Density (b).

Based on the energy diagram of CO formation (Figure 4a), the rate-determining step for the reactions on all the electrodes was identified as COOH* generation. Further reduction of CO* on Sn(200) does not occur because CO desorption is a more energetically favorable reaction than CO* formation. Generation of hydrocarbons on Cu(111) required a high overpotential, and it progressed in preference to CO formation at a high applied potential (Figure 2d). Additionally, the CO partial current density did not increase with an increase in the applied potential (Figure 4b). These results suggested that CO* reduction is prioritized over CO* desorption. In earlier studies, it was found that the adsorbed amounts of CO* and H* play an important role in hydrocarbon production15,39,40. The results suggested efficient C–C coupling for the stabilization of CHO* via hydrogen bond formation with surface CO*15,39. Additionally, in CH4 production, the high surface coverage of H* facilitates the protonation of the surface adsorbate40. However, Cu3Sn(002) weakened the bond of CO* and CHO* relative to that of Cu(111). The CO partial



current density increased with an increase in the applied potential (Figure 4b). From a standpoint of free energy, hydrocarbons should be generated to some degree on the Cu3Sn(002) surface; however, this did not occur in reality. These results suggest three possibilities. First, CO* desorption and CO formation are prioritized because of the weak adsorption of CO*. This mechanism is similar to that on Au electrodes, which exhibit high selectivity for CO production despite the slightly weaker binding state of CO* than that on Cu4. The second possibility is that only CO production on Cu3Sn(002) is attributable to a decrease in the number of stable adsorption sites for CO* as compared with that on Cu(111), because of Sn insertion (Table 1). Alloying with Sn decreased the number of reaction sites for CO* formation, hindering C–C coupling and stabilization of CHO* via hydrogen bonding. The third possibility is that Cu3Sn(002) suppresses the protonation of CO* due to the low surface coverage of H*15,40. Owing to one of these effects, CO* desorption became an energetically favorable reaction.

Figure 4. Calculated Free Energy Diagram for CO and CHO* Formation (a) and Relationship between the Binding Energy of CO* and CO Partial Current Density (b).


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HCOOH is generated through the reaction pathway of COOH* and HCOO*5,10. Based on the energy diagram for HCOOH production (Figure 5), the pathway via the HCOO* intermediate was the energetically favorable reaction at all the electrodes. However, with regard to the reduction mechanism on Cu, Shi et al. reported that the HCOO* pathway was possibly more difficult than the COOH* pathway, considering the transition states41. This is because Cu indicates low selectivity for HCOOH production in CO2 reduction. For the Sn electrode, Kwon and Lee proposed a pathway via the surface H*, i.e., CO2 reacts with the surface H* to form HCOO* on Pb42. HCOOH formation on Sn(200) probably progressed through this reaction pathway because a steady current density was observed for H2 evolution despite an increase in the applied potential. A comparison of H2 (ΔG = −0.687 eV) and HCOO* (ΔG = −0.724 eV) generation from H* indicated that the latter reaction was slightly more energetically favorable. The most stable adsorption sites for HCOO* on Sn(200) were the atop positions of two Sn atoms (Figure S7 in the Supporting Information). On the Cu3Sn electrode, CO was selectively generated at a low applied potential, and HCOOH generation increased with an increase in the applied potential. The most stable sites for HCOO* adsorption on Cu3Sn(002) were the atop positions of both the Cu and Sn sites (Figure S7). The results of a study on the reaction barrier for CO2 reduction41 suggested that Cu sites hinder HCOOH production because they favor COOH* adsorption over HCOO* adsorption. Thus, at a low applied potential, the Sn sites in Cu3Sn(002) showed low binding strength for adsorbed HCOO* due to an increase in the atomic distance between the Sn atoms, and COOH* adsorption became a thermodynamically favorable reaction. At a high applied potential, HCOO* adsorption intermediated by one or two Sn sites was probably stabilized, resulting in increased HCOOH production. These results suggested that the Sn sites of Cu3Sn(002) play an important role in HCOOH production. This consideration



agrees with earlier results showing that Cu87Sn13 exhibited high selectivity for CO production and suppressed HCOOH formation27.

Figure 5. Calculated Free Energy Diagram for HCOOH Formation.

According to the DFT calculation results, the CO2 reaction pathway on Cu3Sn(002) is proposed, as shown in Figure 6. At a low applied potential, Cu3Sn(002) suppresses the HER, and CO production preferentially progresses at the Cu sites in Cu3Sn. At a high applied potential, HCOOH production intermediated by one or two Sn sites increases. Alloying with Sn suppresses the HER and hydrocarbon formation; the CO and HCOOH selectivity can be expected to change according to the surface Sn concentration. These results suggest that alloying with metals that produce HCOOH results in a similar selectivity as that for CO2 reduction on Cu3Sn.


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Figure 6. Proposed Reaction Pathways for Electrochemical CO2 Reduction on Cu3Sn(002).

CONCLUSIONS Electrodeposited Cu76Sn24 alloy consisting of Cu3Sn suppressed H2 and hydrocarbon production during CO2 reduction, relative to an electrodeposited Cu electrode. In addition, the alloys showed selectivity for CO production at all the applied potentials, and HCOOH production increased with an increase in the applied potential. To understand this change in selectivity, we investigated the free energies of the intermediates of the CO2 reduction by DFT calculations. Surface slab models of Cu(111), Sn(200), and Cu3Sn(002) were constructed based on the main orientation obtained by XRD analysis. A comparison of the binding energies, indicated that the binding energy of H* on Cu3Sn(002) was the same as that on Cu(111), and the binding of CO* on Cu3Sn(002) was weaker than on Cu(111). The binding energy of the



intermediates at each position showed that the Cu sites in the alloy make a greater contribution to the stabilization of H*, COOH*, and CO* as compared with the Sn sites. Moreover, this effect led to a decrease in the surface coverage of the intermediates on Cu3Sn. Consequently, the electrodeposited Cu3Sn electrode suppressed H2 evolution, unlike electrodeposited Cu. The increase in CO selectivity was because the binding of CO* was weaker than on Cu(111), and CO desorption was a more energetically favorable reaction than CO* reduction. Additionally, a decrease in the amount of aurface-adsorbed CO* and H* suppressed CO* dimerization and protonation of the intermediates. The results also suggested that the surface Sn concentration significantly affected the reaction selectivity for HCOOH production from CO2. The experimental and theoretical results revealed that alloying of Cu with Sn can be a useful strategy to control the selectivity for two-electron reduction products.

ASSOCIATED CONTENT SEM observations of electrodeposited Cu3Sn; XRD patterns of Cu3Sn; calculation details of binding energy and Gibbs free energy; the structure after optimization.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Phone: +81-(0)93-695-6082 (Y. T.) *E-mail: [email protected], Phone: +81-(0)93-695-6065 (T. H.)


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Author Contributions M. M., Y. T., and T. H. designed the experiments. M. M., Y. T., T. S, and T. H. were involved in the preparations and electrochemical measurements, as well as the evaluation of the electrode properties. M. M., S. I., and S. K. performed the DFT calculations. All the authors discussed the results and contributed to writing the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Early-Career Scientist (Grant Number 18K14324).

ABBREVIATIONS HER, hydrogen evolution reaction; DFT, density functional theory; FE, Faradaic efficiency; AES, Auger electron spectroscopy; SEM, scanning electron microscopy; XRD, X-ray diffraction; CE, counter electrode; RE, reference electrode; VASP, Vienna Ab initio Simulation Package; PAW, projector augmented wave; GGA; generalized gradient approximation; PBE, Perdew–Burke– Ernzerhof; FDM, finite displacement method



REFERENCES (1)

Wang, Y.; Niu, C.; Wang, D. Metallic Nanocatalysts for Electrochemical CO2 Reduction in Aqueous Solutions. J. Colloid Interface Sci. 2018, 527, 95–106.

(2)

Kumar, B.; Brian, J. P.; Atla, V.; Kumari, S.; Bertram, K. A.; White, R. T.; Spurgeon, J. M. New Trends in the Development of Heterogeneous Catalysts for Electrochemical CO2 Reduction. Catal. Today 2016, 270, 19–38.

(3)

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.

(4)

Kuhl, K. P.; Hatsukade, T.; Cave, E. R.; Abram, D. N.; Kibsgaard, J.; Jaramillo, T. F. Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces. J. Am. Chem. Soc. 2014, 136, 14107–14113.

(5)

Durand, W. J.; Peterson, A. A.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K. Structure Effects on the Energetics of the Electrochemical Reduction of CO2 by Copper Surfaces. Surf. Sci. 2011, 605, 1354–1359.

(6)

Xiao, H.; Cheng, T.; Goddard, W. A.; Sundararaman, R. Mechanistic Explanation of the PH Dependence and Onset Potentials for Hydrocarbon Products from Electrochemical Reduction of CO on Cu (111). J. Am. Chem. Soc. 2016, 138, 483–486.

(7)

Nie, X.; Luo, W.; Janik, M. J.; Asthagiri, A. Reaction Mechanisms of CO2 Electrochemical Reduction on Cu(111) Determined with Density Functional Theory. J. Catal. 2014, 312, 108–122.

(8)

Lim, H.-K.; Shin, H.; Goddard, W. A.; Hwang, Y. J.; Min, B. K.; Kim, H. Embedding Covalency into Metal Catalysts for Efficient Electrochemical Conversion of CO2. J. Am. Chem. Soc. 2014, 136, 11355–11361.

(9)

Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23.

(10)

Feaster, J. T.; Shi, C.; Cave, E. R.; Hatsukade, T.; Abram, D. N.; Kuhl, K. P.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F. Understanding Selectivity for the Electrochemical Reduction of Carbon Dioxide to Formic Acid and Carbon Monoxide on Metal Electrodes. ACS Catal. 2017, 7, 4822–4827.

(11)

Dutta, A.; Rahaman, M.; Luedi, N. C.; Mohos, M.; Broekmann, P. Morphology Matters: Tuning the Product Distribution of CO2 Electroreduction on Oxide-Derived Cu Foam Catalysts. ACS Catal. 2016, 6, 3804–3814.

(12)

Hsieh, Y.-C.; Senanayake, S. D.; Zhang, Y.; Xu, W.; Polyansky, D. E. Effect of Chloride Anions on the Synthesis and Enhanced Catalytic Activity of Silver Nanocoral Electrodes for CO2 Electroreduction. ACS Catal. 2015, 5, 5349–5356. 20 Environment ACS Paragon Plus

Page 20 of 24

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13)

Morimoto, M.; Takatsuji, Y.; Hirata, K.; Fukuma, T.; Ohno, T.; Sakakura, T.; Haruyama, T. Visualization of Catalytic Edge Reactivity in Electrochemical CO2 Reduction on Porous Zn Electrode. Electrochim. Acta 2018, 290, 255–261.

(14)

Hori, Y.; Takahashi, I.; Koga, O.; Hoshi, N. Selective Formation of C2 Compounds from Electrochemical Reduction of CO2 at a Series of Copper Single Crystal Electrodes. J. Phys. Chem. B 2002, 106, 15–17.

(15)

Huang, Y.; Handoko, A. D.; Hirunsit, P.; Yeo, B. S. Electrochemical Reduction of CO2 Using Copper Single-Crystal Surfaces: Effects of CO∗ Coverage on the Selective Formation of Ethylene. ACS Catal. 2017, 7, 1749–1756.

(16)

Hoshi, N.; Kato, M.; Hori, Y. Electrochemical Reduction of CO2 on Single Crystal Electrodes of Silver. J. Electroanal. Chem. 1997, 440, 283–286.

(17)

Lum, Y.; Yue, B.; Lobaccaro, P.; Bell, A. T.; Ager, J. W. Optimizing C–C Coupling on Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction. J. Phys. Chem. C 2017, 121, 14191–14203.

(18)

Kas, R.; Kortlever, R.; Milbrat, A.; Koper, M. T. M.; Mul, G.; Baltrusaitis, J. Electrochemical CO2 Reduction on Cu2O-Derived Copper Nanoparticles: Controlling the Catalytic Selectivity of Hydrocarbons. Phys. Chem. Chem. Phys. 2014, 16, 12194–12201.

(19)

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, 19969–19972.

(20)

Hirunsit, P. Electroreduction of Carbon Dioxide to Methane on Copper, Copper-Silver, and Copper-Gold Catalysts: A DFT Study. J. Phys. Chem. C 2013, 117, 8262–8268.

(21)

M., K.; Tripkovic, V.; Rossmeisl, J. Intermetallic Alloys as CO Electroreduction Catalysts – The Role of Isolated Active Sites. ACS Catal. 2014, 4, 2268–2273.

(22)

Cheng, M. J.; Clark, E. L.; Pham, H. H.; Bell, A. T.; Head-Gordon, M. Quantum Mechanical Screening of Single-Atom Bimetallic Alloys for the Selective Reduction of CO2 to C1Hydrocarbons. ACS Catal. 2016, 6, 7769–7777.

(23)

Hirunsit, P.; Soodsawang, W.; Limtrakul, J. CO2 Electrochemical Reduction to Methane and Methanol on Copper-Based Alloys: Theoretical Insight. J. Phys. Chem. C 2015, 119, 8238–8249.

(24)

Kortlever, R.; Peters, I.; Koper, S.; Koper, M. T. M. Electrochemical CO2 Reduction to Formic Acid at Low Overpotential and with High Faradaic Efficiency on CarbonSupported Bimetallic Pd-Pt Nanoparticles. ACS Catal. 2015, 5, 3916–3923.

(25)

Ren, D.; Ang, B. S.-H.; Yeo, B. S. Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived CuxZn Catalysts. ACS Catal. 2016, 6, 8239–8247.



(26)

Takatsuji, Y.; Nakata, I.; Morimoto, M.; Sakakura, T.; Yamasaki, R.; Haruyama, T. Highly Selective Methane Production Through Electrochemical CO2 Reduction by Electrolytically Plated Cu-Co Electrode. Electrocatalysis 2019, 10, 29-34.

(27)

Morimoto, M.; Takatsuji, Y.; Yamasaki, R.; Hashimoto, H.; Nakata, I.; Sakakura, T.; Haruyama, T. Electrodeposited Cu-Sn Alloy for Electrochemical CO2 Reduction to CO/HCOO−. Electrocatalysis 2018, 9, 323–332.

(28)

Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. Synergistic Geometric and Electronic Effects for Electrochemical Reduction of Carbon Dioxide Using Gold-Copper Bimetallic Nanoparticles. Nat. Commun. 2014, 5, 4948.

(29)

Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186.

(30)

Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50.

(31)

Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979.

(32)

Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Phys. Rev. B 1999, 59, 1758–1775.

(33)

Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868.

(34)

Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396–1396.

(35)

Togo, A.; Tanaka, I. First Principles Phonon Calculations in Materials Science. Scr. Mater. 2015, 108, 1–5.

(36)

Kresse, G.; Furthmüller, J.; Hafner, J. Ab Initio Force Constant Approach to Phonon Dispersion Relations of Diamond and Graphite. Europhys. Lett. 1995, 32, 729–734.

(37)

Parlinski, K.; Li, Z. Q.; Kawazoe, Y. First-Principles Determination of the Soft Mode in Cubic ZrO2. Phys. Rev. Lett. 1997, 78, 4063–4066.

(38)

Klinkova, A.; De Luna, P.; Dinh, C. T.; Voznyy, O.; Larin, E. M.; Kumacheva, E.; Sargent, E. H. Rational Design of Efficient Palladium Catalysts for Electroreduction of Carbon Dioxide to Formate. ACS Catal. 2016, 6, 8115–8120.

(39)

Luo, W.; Nie, X.; Janik, M. J.; Asthagiri, A. Facet Dependence of CO2 Reduction Paths on Cu Electrodes. ACS Catal. 2016, 6, 219–229.

(40)

Nie, X.; Esopi, M. R.; Janik, M. J.; Asthagiri, A. Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps. Angew. Chem. Int. Ed. 2013, 52, 2459–2462.

(41)

Shi, C.; Chan, K.; Yoo, J. S.; Nørskov, J. K. Barriers of Electrochemical CO2 Reduction on Transition Metals. Org. Process Res. Dev. 2016, 20, 1424–1430. 22 Environment ACS Paragon Plus

Page 22 of 24

Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42)

Kwon, Y.; Lee, J. Formic Acid from Carbon Dioxide on Nanolayered Electrocatalyst. Electrocatalysis 2010, 1, 108–115.



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Experimental and Theoretical Elucidation of Electrochemical CO2

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