Electrochemical reduction of CO2 into tunable syngas production by

Subscriber access provided by Kaohsiung Medical University

Energy, Environmental, and Catalysis Applications

Electrochemical reduction of CO2 into tunable syngas production by regulating crystal facets of earth-abundant Zn catalyst Binhao Qin, Yuhang Li, Hongquan Fu, HongJuan Wang, Shengzhou Chen, Zili Liu, and Feng Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04809 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 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

ACS Applied Materials & Interfaces

Electrochemical reduction of CO2 into tunable syngas production by regulating crystal facets of earth-abundant Zn catalyst Binhao Qina,b, Yuhang Lib, Hongquan Fub, Hongjuan Wangb, Shengzhou Chena, Zili Liua, Feng Penga,b,* a

Guangzhou Key Laboratory for New Energy and Green Catalysis, School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, 510006, China. b

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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 electrochemical reduction of CO2 to syngas with tunable CO/H2 ratio is regarded as more economical and promising method in the future. Herein, a series of earth-abundant Zn catalysts with different crystal facet ratios of Zn (002) to Zn (101) in bulk phase have been prepared on electrochemically polished Cu foam by electrochemical deposition method. The Zn catalyst with more (101) crystal facet shows a good electrochemical activity for CO2 reduction reaction (CO2RR) to CO, and with more (002) crystal facet favors to hydrogen evolution reaction (HER). The linear relationship between the crystal facet ratio of Zn (101) to Zn (002) and the faradaic efficiency of CO2RR to CO has been revealed for the first time. The prepared catalyst with more (101) facet shows greater than 85% faradaic efficiency to syngas at -0.9 V (vs RHE) in aqueous electrolyte with tunable CO/H2 ratios ranging from 0.2 to 2.31 that can be used in existing industrial systems. Meanwhile, the mechanism of electroreduction of CO2 on Zn electrode has been studied by in-situ infrared absorption spectroscopy. The highly selective role of Zn (101) crystal facet in CO2RR to CO has been evidenced by density functional theory calculations. KEYWORDS: CO2 electroreduction; zinc catalyst; syngas; crystal facet; catalytic mechanism 1. INTRODUCTION Excessive emission of carbon dioxide (CO2) is the main cause of environmental concerns, such as greenhouse effect and ocean acidification. Therefore, CO2 capture and recycling (CCR) become more prevalent for reducing the concentration of CO2 in the atmosphere and cutting down the artificial carbon cycle. Among varieties of CCR methods (e.g., chemical reduction, electrochemical reduction, photo-reduction, biological reduction and inorganic transformations), the electrochemical CO2 reduction reaction (CO2RR) of has caught the 2


Page 2 of 30

Page 3 of 30 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


universal attention because it is a controllable and eco-friendly process with reproducible energy at atmospheric conditions1-3. Recently, the solar-driven electrochemical CO2RR to fuels has provided a promising approach for the integrated utilization of CO24. Nevertheless, the activity, selectivity and overall efficiency for CO2RR are still far from the requirements for practical industrial applications. The key challenge of electrochemical CO2RR remains to search energetic, efficient, stable and cost-effective catalysts, which has also been one of the “holy grails” of energy and environmental catalysis for present and future. As is well known, a high over potential is required to activate CO2RR because of the thermodynamic stability of carbon dioxide molecules. However, hydrogen evolution reaction (HER) is unavoidable in the aqueous solution under a high potential at the same time and the faradaic efficiency (FE) of CO2RR is discontented. In previous studies, most of the researchers have always tried to suppress HER to synthesize the liquid chemicals5-10, which can directly be applied to the existing industrial systems. For example, Albo et al.5 used Cu2O as an electrocatalyst for CO2RR to methanol and showed the FE of 45.7%. Zhang et al.6 reported the FE of alcohols (methanol and ethanol) on Cu63.9Au36.1/NCF reached 28% which was 5.8 times that on bulk Cu. Wu et al.7 have reported a metal-free electrocatalyst (nitrogen doped graphene quantum dots) for CO2RR, and the results showed the total FE of 90%. However, there are too many products in the CO2RR (e.g., CO, formic acid, methane, ethylene, ethanol, acetic acid and n-propanol) and the selectivity of liquid products is not ideal with a total FE less than 30%. In conclusion, despite tremendous efforts have been devoted to CO2RR, it remains a challenging problem to increase the selectivity of liquid chemicals. Most recently, researchers presented the electrochemical reduction CO2 to produce the syngas, a mixture of carbon monoxide (CO) and hydrogen (H2), which could be used as 3



important chemical raw materials for methanol synthesis (CO/H2 ratio of 0.5), syngas fermentation (CO/H2 ratio of 1 to 3.33) and Fischer-Tropsch synthesis (CO/H2 ratio of 1.67) 11, 12

. Comparing with the mixture of pure CO and pure H2 directly, CO2RR to directly produce

the syngas can effectively reduce the cost in transportation and storage syngas of H2. Therefore, many researchers have presented that electrochemical reduction CO2 to produce the syngas is a good alternative route in the future. Using syngas to synthesize chemicals by existing industrial processes is more economical than using CO2RR to make liquid products of low concentration with a low efficiency of one-step method that required expensive separation cost5, 6. The effective and mature technique of syngas conversion provides more product selections by controlling CO/H2 ratio. Hence, the tunable CO/H2 ratio is particularly important in the process of CO2RR. From various catalysts reported for CO2RR to syngas, gold (Au) and silver (Ag) are known to be unexceptionable catalysts for CO2RR to CO13-21. Thus, derivatives of them are also favorable materials for CO2 to syngas, for example, a Cu-enriched Au surface catalyst was reported to adjust the CO/H2 ratio by controlling the Cu enrichment level12. And the CO/H2 ratio could be tuned from 0.01 to 0.5 on Ag/g-C3N4 by changing Ag loadings22. In addition, Pd/C11, Cu-In23 and Ru (Ⅱ) polypyridyl carbene complex24 were also used for CO2RR to syngas. For economic consideration, non-precious metals or their compounds were also widely used for CO2RR to syngas. Kumar et al.25 have reported that copper (Cu) foil showed 100% selectivity to yield syngas, and the CO/H2 ratios could be controlled from ~0.03 to 1.78 with different preparation methods of Cu foil. Furthermore, MoS2 , MoSe2 and MoSeS alloy monolayers have been reported for CO2 electrochemical reduction into syngas, among which, MoSeS alloy monolayers showed the highest CO/H2 ratio (about 0.83) and the largest current density (about -43 mA cm-2) at the 4


Page 4 of 30

Page 5 of 30 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


lower potential26. The metal-free material (g-C3N4/MWCNTs) also exhibited good CO/H2 ratio (1.54)27, while the current density (-0.93 mA cm-2) was too low, indicating its weak catalytic activity. As an abundant, inexpensive and non-toxic metal, zinc (Zn) or its correlative catalysts have attracted more attention for electrochemical reduction of CO2 to CO and formic acid28-31. Urbain et al.32 have reported a Zn catalyst for solar-driven CO2RR to syngas with CO/H2 ratio of 2 and solar-to-syngas conversion efficiency of 4.3%. However, there are few reports on the effect of crystal structure of Zn catalyst on CO2RR, especially adjusting CO/H2 ratio. Remarkably, Won et al.30 have reported that a hexagonal Zn catalyst that was electrodeposited on Zn foil had good selectivity of CO. They considered that Zn (101) facet in bulk phase was favorable for CO2RR to CO through X-ray diffraction analysis. However, Zn foil as the substrate prejudiced the accurate analysis for the diffraction intensity of Zn facets. Herein, a series of Zn catalysts with different crystal facet ratios of Zn (002) to Zn (101) have been prepared on electrochemically polished Cu foam by electrochemical deposition method to eliminate the effect of Zn substrate for X-ray diffraction analysis. The electrochemical reduction performances of these catalysts have been investigated carefully and the results reveal the dependence of CO/H2 ratio on the crystal facet ratio of Zn and the electrochemical reduction conditions. The mechanism of electroreduction of CO2 on Zn electrode has been studied by in-situ infrared absorption spectroscopy. The highly selective role of Zn (101) crystal facet in CO2RR to CO has been evidenced by density functional theory calculations. 2. EXPERIMENTAL SECTION 2.1. Fabrication of Zn catalysts on Cu foam Refer to our previous work33, Cu foam, as a square substrate (working area is 11 cm2), 5



was cleaned under 2 M hydrochloric acid and absolute ethanol for five minutes and dried at room temperature with nitrogen purging. The substrate was electrochemically polished for 150 seconds in phosphoric acid (85%) at 1 V vs. Ag/AgCl with an electrochemical workstation (CHI 630E, Shanghai) in three-electrode system with Cu foam as working electrode, KCl saturated Ag/AgCl electrode as reference electrode and Pt plate as counter electrode, and then cleaned with absolute ethanol and deionized water (Milli-Q®, ~18 MΩ cm) under ultrasonication. The pretreated Cu foam is called p-Cu. Zn catalysts were fabricated in three-electrode system with p-Cu as working electrode at two potentials of -2 and -2.5 V vs. Ag/AgCl, each potential kept three seconds with a total of 120 repetitions. The operating electrolyte was zinc acetate dihydrate (C4H10O6Zn·2H2O) aqueous solution of 100 g L-1 with different concentrations of cetrimonium bromide (CTAB). The prepared Zn catalyst samples on Cu foam are denoted as Zn-1 (without CTAB), Zn-2 (0.016 mM CTAB) and Zn-3 (1 mM CTAB). 2.2. Electrocatalytic test for CO2RR performances Electrochemical reduction of CO2 was conducted in an airtight H-type cell (polytetrafluoroethylene) with an electrochemical workstation (CHI 630E, Shanghai) for 1 hour with the stirring rate of 400 r min-1 in the cathodic compartment. The H-type cell was separated by Nafion membrane (Nafion 117, DuPont) and filled with 50 mL 0.1 M KHCO3 aqueous solution in each side. Zn catalyst was used for the working electrode, an Ag/AgCl electrode was used for the reference electrode and a Pt plate was used as counter electrode. Before the test, the electrolyte was saturated with CO2 or N2 flow of 14.74 mL min-1 at least 30 min and kept the flow in the process of reaction. In order to facilitate comparison of the results, the experimental potential against an Ag/AgCl electrode was transformed into the 6


Page 6 of 30

Page 7 of 30 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


potential versus the reversible hydrogen electrode (RHE) by the Nernst equation (1). V(𝑣𝑠. RHE) = V(𝑣𝑠. Ag/AgCl) + 0.197 + 0.0591 × pH

(1)

Where, the pH of 0.1 M KHCO3 is 6.8 and the pH of 0.5 M KHCO3 is 7.2. The main reduced product, syngas, was measured by a gas chromatograph (GC, Fuli 9790) that was connected with the cathode section of the H-type cell. The GC was equipped with a thermal conductivity detector for analyzing H2 and a flame ionization detector for analyzing CO and with a packed column. And a GC run was initiated every 30 minutes during the reaction. Liquid products from CO2RR were analyzed with high performance liquid chromatograph (HPLC, Angilent 1260) equipped with an ultraviolet detector. The FEs of CO2RR are calculated by equation (2). FE% =

𝑛𝑒 𝑛𝐹 𝑄

× 100%

(2)

Where, ne (number of electrons exchanged) is 2 for CO2RR to CO or formic acid and HER, n is the mole number of product, F is 96485 C mol-1 (Faraday’s constant) and Q is the total charge passed. 2.3. Catalyst characterizations and in-situ spectroscopic study The morphology of Zn catalysts with Cu foam substrate was observed with a field emission scanning electron microscope (FESEM, ZEISS Merlin) with an accelerating voltage of 5 kV and elemental analysis was characterized by the same FESEM equipped with an energy dispersive spectrometer (EDS, Oxford X-MaxN20) with an accelerating voltage of 20 kV. High resolution transmission electron microscopy (HRTEM) was carried out on a JEM 2100F electron microscope (JEOL) and elemental mappings was obtained from the same microscope equipped with an EDS (Bruker, XFlash 5030T) at 200 kV. X-ray photoelectron spectroscopy (XPS) was obtained with a Krato Axis Ulrta DLD (15 kV, 150 W) and C 1s peak 7



Page 8 of 30

at 284.8 eV as standard. X-ray diffraction (XRD) measurements were recorded on a Bruker D8-advance diffractometer (samples were pressed at 30 MPa before the measurement). In order to compare the crystal facet ratio of different Zn catalysts, texture coefficients were calculated by equation (3). texture coefficient (ℎ𝑘𝑙) = ∑𝑛

𝐼(ℎ𝑘𝑙) /𝐼0(ℎ𝑘𝑙)

𝑖=1 𝐼(ℎ𝑘𝑙)𝑛 /𝐼0(ℎ𝑘𝑙)𝑛

× 100%

(3)

Where I(hkl) is the diffraction intensity of the (hkl) facet and automatically calculated by the XRD analysis software, I0(hkl) is the standard intensity of the (hkl) facet and taken from PDF#65-5973, n is the number of diffraction peaks of Zn (n = 6, in here). In situ spectroscopic study was carried out by a Thermo Nicolet 6700 FT-IR spectrometer, the technique of attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) was used to monitor the intermediate species stabilized on the surface of Zn catalyst. The design of the electrolytic cell is shown in the supporting information (Figure S1). The parameter settings of the spectra were 64 co-added, 6 cm-1 resolution and 400 cm-1 to 4000 cm-1. Before the measurement, catalysts were pressed at 30 MPa with tablet press. Experimental results for the ATR-SEIRAS test were obtained under a potential of -0.9 V (vs RHE) with 0.1 M KHCO3 (CO2 saturated, 5 mL min-1) at room temperature for 1 hour. 2.4. Density functional theory (DFT) calculations Density functional theory (DFT) calculations were performed by Quantum ESPRESSO 6.2

with

ultrasoft

pseudopotential

based

on

Perdew-Burke-Ernzerhof

(PBE)

exchange-correlation functional34, 35. Global kinetic energy cutoffs were set to 35 Ry for the involved elements of H, C, O and Zn, which referred to the convergence accuracy benchmark36. Meanwhile, it meets the balance between the credible error and the accessible 8


Page 9 of 30 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


time consuming. 280 Ry were set for charge density and potential. Grimme’s DFT-D2 was adopted as the long-range dispersion correction method37, 38. The electronic smearing value was 0.25 eV. The appropriate k-points setting was 4*4*1 for both systems related to Zn (002) facet and (101) facet. For Zn (002), the primitive zinc cell was cleaved and the surface was extended to a (3*3) supercell including four-layer atoms totally. Therein, 36 atoms locate in the cell for calculation. Likewise, for Zn (101), the primitive cell was cleaved, followed by extending to a (2*4) supercell including three-layer atoms. There are 48 atoms in the calculated cell. Accordingly, the configurations with specific exposed facets of Zn (002) and Zn (101) were established with the vacuum slab of 15 Å. In both cases, the first layer of the structure could be relaxed and the other layers were fixed. The initially established Zn (002) and Zn (101) structures were optimized for the following adsorption calculations. We calculated potential energies of the possible reaction intermediates during the reduction of carbon dioxide. For comparison, the initial configuration, corresponding to the zinc substrate with a CO2 molecule, was set as the zero point in potential energy surface in order to get the relative energy variation (ΔE). 3. RESULTS AND DISCUSSION 3.1 Material structure analyses According to the FESEM photos of Zn catalysts on Cu foam, as shown in Figure 1, Zn-1 presents a pile of compact structure with a small amount of nanoflake. Zn-2 also emerges a massive structure. However, there is more wrinkles on the surface of Zn-2 than on Zn-1. Zn-3 shows a large amount of ginkgo biloba-like two-dimensional Zn nanoflake structure with the thicknesses of 30 nm covered on its surface. 9



Figure 1. FESEM images: (a-c) Zn-1, (d-f) Zn-2 and (g-i) Zn-3. These Zn nanoflakes on Zn-3 were further characterized by TEM, as shown in Figure 2a. The element mapping and line scan images of the Zn nanoflake indicate that Zn and O distribute homogeneously on the surface of Zn-3 (Figure 2b-d). The selected area electron diffraction (SAED, Figure 2e) pattern of Zn-3 shows a series of rings with different sizes that can be allocated to different lattice planes of Zn and ZnO crystallites. The HRTEM of Zn-3 shows the characteristic spacings of 0.21 nm and 0.26 nm for Zn (101) and ZnO (002) lattice planes, as shown in Figure 2f. Based on these analyses, it can be concluded that the surface of Zn-3 is composed of Zn and ZnO. This is in agreement with the observation from FESEM-EDS (Figure S2a-c).

10


Page 10 of 30

Page 11 of 30 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


Figure 2. TEM images of Zn-3: (a-c) element mapping, (d) line scan, (e) SAED pattern and (f) a typical image. Figure 3a and b show the XPS spectra of Zn2p and O1s. Comparing with the Zn2p3/2 binding energy (1021.7 eV) of Zn-1 and Zn-2, the Zn2p3/2 binding energy of Zn-3 (1021.5 eV) is negatively shifted, which is attributed to the increase of Zn-O bond31. The typical O1s peak in the surface can be consistently fitted to three peaks. The peak of low binding energy at 530.05 eV is attributed to the lattice oxygen of ZnO. The peak of medium binding energy at 531.50 eV is associated with the oxygen vacancy defects within the matrix of ZnO. And the peak of high binding energy at 532.30 eV is usually attributed to the loosely bound oxygen, 39 such as CO23 , adsorbed O2 or absorbed H2O on the surface of the sample . Inevitably, the

surfaces of Zn catalysts are oxidized during the preparation process. And it can be also found that a very slight amount of Cu is exposed on the surface of Zn-1 and Zn-2, as shown in Figure S3.

11



Zn-1

1021.7 eV

Zn-2

1021.7 eV

O 1s Zn-1

Zn-2

1021.5 eV

Zn-3

1060

(b)

Intensity / a.u.

2p1/2

Intensity / a.u.

2p3/2

Zn 2p

(a)

Zn-3

1050

1040

1030

1020

1010

538

536

Binding energy / eV

534

532

530

528

80 (d)

Texture coefficient / %

Zn-1 Zn-2 Zn-3 200

002 100

35

40

Zn(PDF#65-5973) Cu(PDF#89-2838)

111

45

50

220 103 110

102

55

60

524

Zn-1 Zn-2 Zn-3

70

101

526

Binding energy / eV

(c)

Intensity / a.u.

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

Page 12 of 30

65

70

60 50 40 30 20 10 0

75

(002)

(100)

2 Theta / 

(101)

(102)

(103)

(110)

Crystal plane

Figure 3. (a, b) XPS spectrums, (c) XRD patterns and (d) texture coefficients of Zn catalysts. XRD patterns of Zn catalysts on Cu foam are shown in Figure 3c, there are six peaks at 36.3°, 39.0°, 43.2°, 54.3°, 70.1°and 70.6°, corresponding to (002), (100), (101), (102), (103) and (110) lattice planes of Zn (PDF#65-5973), respectively. And there are also three peaks at 43.3°, 50.4°and 74.1°, which are characteristic of metal Cu (PDF#89-2838). After adding CTAB during the preparation process, the intensity of Zn (002) peak is obviously decreased with the intensity of Zn (101) peak increasing. To gain more insight, we calculated texture coefficient of each crystal facet for the Zn catalysts, as shown in Figure 3d. The texture coefficients of Zn (002) plane are 56.45%, 21.93% and 9.13%, corresponding to Zn-1, Zn-2 and Zn-3, respectively, and the texture coefficients of Zn (101) plane are 24.12%, 49.62% and 72.31%. Therefore, the crystal facets of Zn catalyst can be regulated by a simple method of adding different amount of CTAB during preparation process. Zn-1 catalyst has more (002) 12


Page 13 of 30 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


facet and Zn-3 catalyst has more (101) facet in bulk phase. 3.2 Electrochemical performances for CO2RR Before testing the activity of Zn catalysts for CO2RR, we evaluated the performance of p-Cu for CO2RR firstly (Figure S4). One unanticipated finding is that p-Cu only has poor activity at all potentials in CO2RR with the FEs of CO between 0.29% and 2.01% and almost all the electricity is employed to HER with H2 FEs of more than 90%. The possible explanations are that p-Cu is too smooth to catalyze CO2RR after electrochemical polishing40 and the reaction temperature of 40 oC is better for HER than room temperature. To evaluate the electrochemical activity of Zn catalysts for CO2RR, linear sweep voltammetry (LSV) is conducted in CO2-saturated and CO2-free 0.1 M KHCO3 solutions. As shown in Figure 4, all Zn catalysts perform higher current densities in CO2-saturated environment than in CO2-free environment, implying that the Zn catalysts have electrochemical activity for CO2RR. The original LSV curves of Zn catalysts show obvious cathodic peaks at -0.6 to -0.7 V (vs RHE) in CO2-free 0.1 M KHCO3 solution due to the reduction of ZnO to metallic Zn (Figure S5). After the electrochemical reduction at -0.6 V (vs RHE) for 300 s in CO2-free environment, the reduction peak of ZnO disappeared in the same CO2-free environment. It is concluded that the oxide layer on the surface of the Zn catalysts has been reduced30. Therefore, we will reduce the ZnO by this way to avoid its effects before every electrochemical investigation. More than that, we also evaluated the performance of Zn-3 sample with different electro-deposition time during the preparation process, i.e. 60, 90, 120 and 150 repetitions. With the electro-deposition time increasing, the FE of H2 decreases and the FE of CO increases, and after 120 repetitions of electro-deposition, the FEs of gas products are basically constant, as shown in Figure S6. It is demonstrated that the coverage of Zn on p-Cu is gradually increased 13



with the increase of electro-deposition time until it is basically stable. And this result is also proved by FESEM-EDS, as shown in Figure S2c-f. 0

(a)

-10 -15 -20 CO2-saturated

-25

CO2-free

-30 -35 -1.2

-1.1

-1.0

-0.9

-0.8

Potential / V vs. RHE

-0.7

-0.6

(c)

-5 -10 -15 -20

CO2-saturated

-25

CO2-free

-30 -35 -40 -1.3

-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

Current density / mA cm-2

-5

-40 -1.3

0

(b) Current density / mA cm-2

0


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

Page 14 of 30

-5 -10 -15 -20 CO2-saturated

-25

CO2-free

-30 -35 -40 -1.3


-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6


Figure 4. LSV of (a) Zn-1, (b) Zn-2 and (c) Zn-3 in 0.1 M KHCO3 at 40 oC under CO2-saturated and CO2-free environment. For comparing the electrochemical CO2 reduction performance on the different Zn catalysts, CO2RRs are evaluated at potential ranging from -0.7 to -1.3 V (vs RHE) in CO2-saturated 0.1 M KHCO3 aqueous solution at 40 oC. As shown in Figure 5, syngas is the primary product from CO2RR with net total FE of 64.1% to 90.6%. In addition, a small amount of formic acid with FEs less than 5% is also detected by HPLC. Figure 5a shows the highest CO/H2 ratio on Zn-1 is 0.77 at -1.2 V with the highest CO FE of 38.12% and lowest H2 FE of 49.19%. The CO/H2 ratio on Zn-2 reaches to the maximum value of 1.10 with 47.55% CO FE and 43.13% H2 FE at -1.1 V (Figure 5c). As shown in Figure 5e and f, Zn-3 reveals the highest CO/H2 ratio of 1.14 at -0.9 V with a high current density of -11.36 mA cm-2, while Zn-1 and Zn-2 show lower CO/H2 ratios of 0.43 and 0.79, respectively, at the same potential. Undisputed, the lower electrolytic potential is conductive to the utilization efficiency of electric energy. Therefore, Zn-3 is more suitable for CO2 electrochemical reduction. Further, we also analyzed the origin of CO by N2 bubbling. As shown in Figure S7, it shows the average current density of -11.42 mA cm-2 in CO2-saturated environment, however, the average current density is only -4.10 mA cm-2 in N2-saturated environment. The analysis of 14


Page 15 of 30

gas chromatogram also shows that obvious CO was detected in CO2-saturated environment, while CO was barely detectable in N2-saturated environment. The result can clearly prove that CO comes from CO2 instead of KHCO3.

1.0

60

0.8 40

0.6 H2 FE CO FE

20

0.4

Syngas FE 0.2 CO/H2 ratio

0 -0.7 100

CO / H2 ratio

FE / %

1.2

-0.8

-0.9 -1.0 -1.1 -1.2 Potential / V vs. RHE

100

CO FE

0.4

Syngas FE

0.2

CO/H2 ratio


0.8 40

0.6 H2 FE CO FE

0.4

Syngas FE

0.2

CO/H2 ratio

-0.7

-0.8

Production rate of H2

400 300

-20

200

-10

100 0 -0.8


-1.3 500

(f) Average current density

-40

400

Production rate of H2 Production rate of CO

-30

300

-20

200

-10

100

0.0


500

-30

-50

CO / H2 ratio

1.0

-1.3

(d)

-0.7

1.2

60


0

1.4 80

0 -0.8

Production rate of CO

0.0

1.6

0

100

-40

-1.3

(e)

20

-10

Average current density


0.6 H2 FE


200

-0.7

CO / H2 ratio

FE / %

40

-0.8

-20

-50

0.8

-0.7

300

1.6

1.0

60

0

-30

0

1.2

400


0.0

1.4 80

Production rate of H2

-40

-1.3

(c)

20

Average current density

0

-1.3

-0.7

-0.8

-0.9

-1.0

-1.1

-1.2

-1.3

Production rate / mol h-1 cm-2

1.4 80

500

(b)


-50


1.6

(a)


100

FE / %

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


0


Figure 5. Faradaic efficiencies of CO, H2 and syngas, and CO/H2 ratios for CO2RR on Zn catalysts in CO2-saturated 0.1 M KHCO3 at 40 oC with different potentials. Average current densities and production rates of CO and H2 in the same conditions. (a, b) Zn-1, (c, d) Zn-2, (e, f) Zn-3.

15



We think the difference of crystal facet ratios in Zn bulk phase changes the active site of the catalyst surface, and then it affects the catalytic activity of CO2RR. To a certain extent, growth of crystal facet in bulk phase will increase the chance of this crystal facet exposed on the surface. In order to directly compare the relationship between CO2 reduction activity and crystal facet ratio of Zn (002) and Zn (101), the FEs of CO and H2 at -0.9 V are associated with texture coefficients of Zn (101) and Zn (002), respectively, as show in Figure 6a. Zn (101) crystal facet in bulk phase shows a good electrochemical activity for CO2RR to CO and Zn (002) crystal facet favors to HER. The linear relationship between the Zn (101) crystal facet ratio and the faradaic efficiency of CO2RR to CO is revealed for the first time. The long-term stability of Zn-3 is evaluated at -0.9 V in CO2-saturated 0.1 M KHCO3 solution. Texture coefficient of (002) / % 50

40

30

20

10

50

60

0

55

40

50

35

45

30

40 Zn-1 Zn-2 Zn-3

25

35


45

FE of H2 / %

(a)

40

50

60

80

-20

60

40

-30 Current density H2 FE

-40

20

CO FE

70

0

-50

30 30

-10

Syngas FE

20 20

100

(b)

FE / %

60

FE of CO / %

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

Page 16 of 30

0

80

1

2

3

4

5

6

7

8

9

Time /h

Texture coefficient of (101) / %

Figure 6. (a) The relationship of FE of CO and H2 and the texture coefficient of Zn (101) and Zn (002). (b) Long-term stability of Zn-3. The experimental conditions: CO2-saturated 0.1 M KHCO3, 40 oC, -0.9 V (vs RHE). As shown in Figure 6b, it shows the average current density of -11.54 mA cm-2 and there is almost no obvious significant deterioration, and the FE of syngas is 81.39% in the first half hour, and then the FE gradually stabilizes to 88.13%, 90.33% and 89.75% at 1 h, 1.5 h and 2 h, respectively. Nevertheless, the FE of syngas slightly decreases to 84.28% at 9.5 h. In a 16


Page 17 of 30

separate analysis of CO and H2, the FE of CO remains considerably stable between 45.29% and 47.84%, while the FE of H2 shows a slight fluctuation between 44.6% and 36.10%. Therefore, the decrease of H2 FE is the main reason for the slight decrease of syngas FE. A change in the surface environment of the electrode is a major factor for the reduced FE of H2, which we will discuss through the ATR-SEIRAS below. Moreover, the XRD patterns of Zn-3 were analyzed before and after electrochemical reaction, as shown in Figure S8. The result also shows the crystal facet ratio of Zn (101) and Zn (002) of Zn-3 was stable after electrochemical reaction. On the whole, the CO/H2 ratios of syngas keep in the range from 1.03 to 1.25 during the long-term operation, showing a good stability of Zn-3.

2.0 60

1.5 40

1.0

H2 FE CO FE

20

0.5

Syngas FE CO/H2 ratio

0 10

30

20

Average current density Production rate of H2

-15

80

-5

40

0

0.0

0 10

20 30 40 Reaction temperature / C

Reaction temperature / C

-35 Current density / mA cm-2

1.5

(c)

1.2

80 60

0.9

40

0.6

CO/H2 ratio

100

H2 FE CO FE

20

0.3

Syngas FE CO/H2 ratio

0 0.1

120


-10

50

40

160

(b)

0.0

50

(d) Average current density

-30

400

Production rate of H2 Production rate of CO

-25

300

-20 200

-15 -10

100 -5 0

0.5 0.4 0.3 0.2 Concentration of KHCO3 / M



2.5

80

FE / %

-20


3.0

(a)

CO/H2 ratio

100

FE / %

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


0 0.1

0.2 0.3 0.4 0.5 Concentration of KHCO3 / M

Figure 7. Faradaic efficiencies, CO/H2 ratios, average current densities and production rates of CO and H2 for CO2RR on Zn-3 at -0.9 V (vs RHE) under different conditions: (a, b) 0.1 M KHCO3 with different temperatures; (c, d) 40 oC with different concentrations of CO2-saturated KHCO3. 17



In order to meet the demand of syngas with different CO/H2 ratio in industrial production, we try to get a different proportion of syngas by adjusting the electrolytic temperature and concentration of KHCO3 with Zn-3 catalyst (Figure 7). With the temperature decreasing, the FE of CO increases and FE of H2 decreases gradually and it shows a highest CO/H2 ratio of 2.31 at 10 oC with the highest CO FE of 62.56% and the lowest H2 FE of 27.08%. As shown in Figure 7b, with the temperature decreasing, the current density decreases from -15.59 mA cm-2 at 50 oC to -6.50 mA cm-2 at 10 oC. The result shows that lowering the temperature can greatly inhibit the HER, while moderately suppress the CO2RR to CO. As shown in Figure 7c and d, the CO/H2 ratio reaches the lowest value of just 0.20, however, the average current density reaches the highest value of -29.35 mA cm-2 in 0.5 M KHCO3. Clearly, with the concentration of KHCO3 increasing, the rate of H2 production rises sharply and the rate of CO production obviously decreases after a slight increase, which means that high concentration of KHCO3 is not suitable for CO2RR. The comparison of CO/H2 ratios and current densities of Zn-3 with those of reported electrocatalysts for CO2RR to syngas in different conditions is shown in Figure S9. Although Zn-3 is not superior to precious metals (e.g., Au, Pd, Ru) in potential, it shows better performance of CO/H2 ratios than most of the catalysts and a larger range of CO/H2 ratios (from 0.2 to 2.31) for different requirements by simple control. At the same time, current densities of Zn-3 are also better than most of the catalysts, indicating its good catalytic activity. 3.3 Electrochemical mechanism for CO2RR ATR-SEIRAS analyses are further conducted to verify the changes on the surface of the electrode in the process of CO2RR, as shown in Figure 8a. When the electrode was just placed 18


Page 18 of 30

Page 19 of 30 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


in the CO2 saturated 0.1 M KHCO3, the originally obtained signal appears as background. The downward peak at 2358 cm-1 could be attributed to CO2. The intensity of the peak increases gradually in the early stages, illustrating the gradual adsorption of CO2 on the surface of the electrode, and its intensity almost constant after 20 min due to adsorption saturation of CO2. The downward peaks at 1436 cm-1 and 1596 cm-1 are associated with carboxylate (-COO-)41-44, and the downward peak at 1723 cm-1 is associated with C=O of formate (COOH-). Further, as shown in Figure S10, the downward peaks at 3545 cm-1 and 3629 cm-1 are associated with -OH of COOH- and free -OH, respectively43, 44. The intensities of these groups increase along with the electrolysis, which indicates that these groups increase on the surface of the electrode and change the electrolytic environment. Two extremely weak peaks are observed at 2014 cm-1 and 2040 cm-1 that are attributed to the adsorbed CO on Zn45, 46 and it can also be proven in Figure S11. A possible reaction mechanism of CO2RR is illustrated in Figure 8b based on ATR-SEIRAS results and the reported references23, 45. At the beginning of the reaction, CO2 and HCO-3 are adsorbed on the surface of the electrode and convert to each other. Subsequently, (1) the reversible conversion of HCO-3 and CO23 radical occurs on the electrode surface, playing the role of providing proton; (2) CO2 is reduced to CO∗2 by one electron from the electrochemical process; (3) the CO∗2 radical is electrochemically reduced to reaction intermediate of COOH* by a proton from step (1); (4) few COOH* radical is reduced via an electron transfer step to COOH- that desorbs from the catalysts to form HCOOH; (5) most of the COOH* radical is reduced with an electron and a proton to CO* that desorbs from Zn to produce the main product (CO). Therefore, the pathway for CO2RR to CO * * on Zn catalysts might be CO2 → CO∗2 → COOH → CO → CO.

19



Figure 8. (a) ATR-SEIRAS analyses on Zn-3. (b) A possible reaction mechanism of CO2RR. (c-e) Relative potential energy diagrams for CO2RR on Zn (002) facet and Zn (101) facet with different potentials. DFT calculations are used to simulate the relative potential energy of reaction intermediates of COOH* and CO* in the process of CO2RR to CO (Figure 8c-e). Obviously, CO2 reduced to COOH* is the limiting step of the whole reaction17-19, 30. CO2RR cannot occur because the relative energy of CO2 to COOH* is 0.543 eV on Zn (002) facet and 0.185 eV on Zn (101) facet, which is too high to overcome at -η V. With the potential decreasing, CO2RR occurs first on Zn (101) facet at -(η+0.2) V, because of the relative energy of CO2 to COOH* is -0.015 eV on this facet. However, on Zn (002) facet, the relative energy of CO2 to COOH* is 0.343 eV which is too high to conquer. And then CO2RR occurs on Zn (002) facet at -(η+0.6) V when the relative energy of CO2 to COOH* is -0.015 eV. The DFT calculations 20


Page 20 of 30

Page 21 of 30 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


indicate that Zn (101) facet is easier than Zn (002) facet for CO2RR at lower reduction potential, because of the relative energy of CO2 to COOH* on Zn (101) facet is 0.358 eV less than the energy on Zn (002) facet. Identical conclusion is obtained in the experiments of Zn-1, Zn-2 and Zn-3 for CO2RR as shown in Figure 5. When the FEs of CO reach the highest, the potentials of Zn-1 and Zn-2 are -1.2 V and -1.1 V, respectively, for Zn-3 with more Zn (101) facet, the potential is only -0.9 V. Based on the DFT calculations and the experimental results, it is clear that the performance of CO2RR to CO can be further improved with more Zn (101) facet. 4. CONCLUSIONS In summary, a series of earth-abundant Zn catalysts with different crystal facet ratios of Zn (002) to Zn (101) have been controllably prepared on electrochemically polished Cu foam by a simple electrochemical deposition method. Zn-1 catalyst has more (002) facet in bulk phase that favors to HER. Zn-3 catalyst has more (101) facet in bulk phase that shows a good electrochemical activity for CO2RR to CO. The linear relationship between Zn (101) crystal facet ratio and the faradaic efficiency of CO2RR to CO is revealed for the first time. Zn-3 catalyst exhibits that its faradaic efficiency of syngas is greater than 85% at -0.9 V (vs RHE) in aqueous electrolyte with tunable CO/H2 ratios ranging from 0.2 to 2.31, which is superior to most of the electrocatalysts reported to date. Meanwhile, the mechanism of electroreduction of CO2 on Zn electrode has been presented based on in-situ infrared absorption spectroscopy. The highly selective role of Zn (101) crystal facet in CO2RR to CO has been evidenced by density functional theory calculations. ASSOCIATED CONTENT Supporting Information 21



The electrolytic cell for ATR-SEIRAS; FESEM-EDS and XPS spectrums of Zn catalysts; catalytic activity for CO2RR of p-Cu; the original LSV curves and the LSV after electrochemical reduction at -0.6 V; catalytic activity for CO2RR of Zn-3 with different electrodeposition repetitions during the preparation process; i-t curves of Zn-3 under N2-saturated and CO2-saturated environment, and corresponding gas chromatogram analysis; XRD patterns of Zn-3 before and after electrochemical reaction; comparison of CO/H2 ratios and current densities of Zn-3 with other electro catalysts; ATR-SEIRAS analyses of -OH of carboxylic acid and free -OH; ATR-SEIRAS analyses of Zn-3 in N2 and CO without potential. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (F. Peng). ACKNOWLEDGEMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21373091, 21476089), the Provincial Science and Technology Project of Guangdong (No. 2014A030312007) and the Fundamental Research Funds for the Central Universities (No. 2015ZP021). REFERENCES (1) Wang, Y.; Liu, J.; Wang, Y.; Al-Enizi, A. M.; Zheng, G. Tuning of CO2 Reduction Selectivity on Metal Electrocatalysts. Small 2017, 13, 1701809. (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, 19-30. (3) ElMekawy, A.; Hegab, H. M.; Mohanakrishna, G.; Elbaz, A. F.; Bulut, M.; Pant, D. 22


Page 22 of 30

Page 23 of 30 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


Technological Advances in CO2 Conversion Electro-biorefinery: A Step toward Commercialization. Bioresour. Technol. 2016, 215, 357-370. (4) Schreier, M.; Héroguel, F.; Steier, L.; Ahmad, S.; Luterbacher, J. S.; Mayer, M. T.; Luo, J.; Grätzel, M. Solar Conversion of CO2 to CO Using Earth-abundant Electrocatalysts Prepared by Atomic Layer Modification of CuO. Nat. Energy 2017, 2, 17087. (5) Albo, J.; Sáez, A.; Solla-Gullón, J.; Montiel, V.; Irabien, A. Production of Methanol from CO2 Electroreduction at Cu2O and Cu2O/ZnO-based Electrodes in Aqueous Solution. Appl.Catal. B: Environ. 2015, 176-177, 709-717. (6) Jia, F.; Yu, X.; Zhang, L. Enhanced Selectivity for the Electrochemical Reduction of CO2 to Alcohols in Aqueous Solution with Nanostructured Cu–Au Alloy as Catalyst. J. Power Sources 2014, 252, 85-89. (7) Wu, J.; Ma, S.; Sun, J.; Gold, J. I.; Tiwary, C.; Kim, B.; Zhu, L.; Chopra, N.; Odeh, I. N.; Vajtai, R.; Yu, A. Z.; Luo, R.; Lou, J.; Ding, G.; Kenis, P. J.; Ajayan, P. M. A Metal-free Electrocatalyst for Carbon Dioxide Reduction to Multi-carbon Hydrocarbons and Oxygenates. Nat. Commun. 2016, 7, 13869. (8) Song, Y.; Peng, R.; Hensley, D. K.; Bonnesen, P. V.; Liang, L.; Wu, Z.; Meyer, H. M.; Chi, M.; Ma, C.; Sumpter, B. G.; Rondinone, A. J. High-selectivity Electrochemical Conversion of CO2 to Ethanol Using a Copper Nanoparticle/N-doped Graphene Electrode. ChemistrySelect 2016, 1, 6055-6061. (9) Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New Insights into the Electrochemical Reduction of Carbon Dioxide on Metallic Copper Surfaces. Energy Environ. Sci. 2012, 5, 7050-7059. (10) Raciti, D.; Livi, K. J.; Wang, C. Highly Dense Cu Nanowires for Low-overpotential CO2 23



Reduction. Nano lett. 2015, 15, 6829-6835. (11) Sheng, W.; Kattel, S.; Yao, S.; Yan, B.; Liang, Z.; Hawxhurst, C. J.; Wu, Q.; Chen, J. G. Electrochemical Reduction of CO2 to Synthesis Gas with Controlled CO/H2 Ratios. Energy Environ. Sci. 2017, 10, 1180-1185. (12) Ross, M. B.; Dinh, C. T.; Li, Y.; Kim, D.; De Luna, P.; Sargent, E. H.; Yang, P. Tunable Cu Enrichment Enables Designer Syngas Electrosynthesis from CO2. J. Am. Chem. Soc. 2017, 139, 9359-9363. (13) 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. (14) 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. (15) Ma, M.; Trześniewski, B. J.; Xie, J.; Smith, W. A. Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide-derived Nanostructured Silver Electrocatalysts. Angew. Chem. Int. Ed. 2016, 55, 9748-9752. (16) Kim, C.; Jeon, H. S.; Eom, T.; Jee, M. S.; Kim, H.; Friend, C. M.; Min, B. K.; Hwang, Y. J. Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles. J. Am. Chem. Soc. 2015, 137, 13844-13850. (17) Sun, K.; Cheng, T.; Wu, L.; Hu, Y.; Zhou, J.; Maclennan, A.; Jiang, Z.; Gao, Y.; Goddard, W. A., Ⅲ; Wang, Z. Ultrahigh Mass Activity for Carbon Dioxide Reduction Enabled by Gold-iron Core-shell Nanoparticles. J. Am. Chem. Soc. 2017, 139, 15608-15611. (18) Gao, D.; Zhang, Y.; Zhou, Z.; Cai, F.; Zhao, X.; Huang, W.; Li, Y.; Zhu, J.; Liu, P.; Yang, F.; Wang, G.; Bao, X. Enhancing CO2 Electroreduction with the Metal-oxide Interface. J. 24


Page 24 of 30

Page 25 of 30 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


Am. Chem. Soc. 2017, 139, 5652-5655. (19) Liu, S.; Tao, H.; Zeng, L.; Liu, Q.; Xu, Z.; Liu, Q.; Luo, J. L. Shape-dependent Electrocatalytic Reduction of CO2 to CO on Triangular Silver Nanoplates. J. Am. Chem. Soc. 2017, 139, 2160-2163. (20) Choi, J.; Kim, M. J.; Ahn, S. H.; Choi, I.; Jang, J. H.; Ham, Y. S.; Kim, J. J.; Kim, S.-K. Electrochemical CO2 Reduction to CO on Dendritic Ag-Cu Electrocatalysts Prepared by Electrodeposition. Chem. Eng. J. 2016, 299, 37-44. (21) Peng, X.; Karakalos, S. G.; Mustain, W. E. Preferentially Oriented Ag Nanocrystals with Extremely High Activity and Faradaic Efficiency for CO2 Electrochemical Reduction to CO. ACS Appl. Mater. Inter. 2018, 10, 1734-1742. (22) Sastre, F.; Muñoz-Batista, M. J.; Kubacka, A.; Fernández-García, M.; Smith, W. A.; Kapteijn, F.; Makkee, M.; Gascon, J. Efficient Electrochemical Production of Syngas from CO2 and H2O by Using a Nanostructured Ag/g-C3N4 Catalyst. ChemElectroChem 2016, 3, 1497-1502. (23) Hoffman, Z. B.; Gray, T. S.; Moraveck, K. B.; Gunnoe, T. B.; Zangari, G. Electrochemical Reduction of Carbon Dioxide to Syngas and Formate at Dendritic Copper-indium Electrocatalysts. ACS Catal. 2017, 7, 5381-5390. (24) Kang, P.; Chen, Z.; Nayak, A.; Zhang, S.; Meyer, T. J. Single Catalyst Electrocatalytic Reduction of CO2 in Water to H2+CO Syngas Mixtures with Water Oxidation to O2. Energy Environ. Sci. 2014, 7, 4007-4012. (25) Kumar, B.; Brian, J. P.; Atla, V.; Kumari, S.; Bertram, K. A.; White, R. T.; Spurgeon, J. M. Controlling the Product Syngas H2:CO Ratio through Pulsed-bias Electrochemical Reduction of CO2 on Copper. ACS Catal. 2016, 6, 4739-4745. 25



(26) Xu, J.; Li, X.; Liu, W.; Sun, Y.; Ju, Z.; Yao, T.; Wang, C.; Ju, H.; Zhu, J.; Wei, S.; Xie, Y. Carbon Dioxide Electroreduction into Syngas Boosted by a Partially Delocalized Charge in Molybdenum Sulfide Selenide Alloy Monolayers. Angew. Chem. Int. Ed. 2017, 56, 9121-9125. (27) Lu, X.; Tan, T. H.; Ng, Y. H.; Amal, R. Highly Selective and Stable Reduction of CO2 to CO by a Graphitic Carbon Nitride/Carbon Nanotube Composite Electrocatalyst. Chem. Eur. J. 2016, 22, 11991-11996. (28) Quan, F.; Zhong, D.; Song, H.; Jia, F.; Zhang, L. A Highly Efficient Zinc Catalyst for Selective Electroreduction of Carbon Dioxide in Aqueous NaCl Solution. J. Mater. Chem. A 2015, 3, 16409-16413. (29) Rosen, J.; Hutchings, G. S.; Lu, Q.; Forest, R. V.; Moore, A.; Jiao, F. Electrodeposited Zn Dendrites with Enhanced CO Selectivity for Electrocatalytic CO2 Reduction. ACS Catal. 2015, 5, 4586-4591. (30) Won D. H.; Shin, H.; Koh, J.; Chung, J.; Lee, H. S.; Kim, H.; Woo, S. I. Highly Efficient, Selective, and Stable CO2 Electroreduction on a Hexagonal Zn Catalyst. Angew. Chem. Int. Ed. 2016, 55, 9297-9300. (31) Zhang, T.; Zhong, H.; Qiu, Y.; Li, X.; Zhang, H. Zn Electrode with a Layer of Nanoparticles for Selective Electroreduction of CO2 to Formate in Aqueous Solutions. J. Mater. Chem. A 2016, 4, 16670-16676. (32) Urbain, F.; Tang, P.; Carretero, N. M.; Andreu, T.; Gerling, L. G.; Voz, C.; Arbiol, J.; Morante, J. R. A Prototype Reactor for Highly Selective Solar-driven CO2 Reduction to Synthesis Gas Using Nanosized Earth-abundant Catalysts and Silicon Photovoltaics. Energy Environ. Sci. 2017, 10, 2256-2266. 26


Page 26 of 30

Page 27 of 30 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


(33) Qin, B.; Wang, H.; Peng, F.; Yu, H.; Cao, Y. Effect of the Surface Roughness of Copper Substrate on Three-dimensional Tin Electrode for Electrochemical Reduction of CO2 into HCOOH. J. CO2 Util. 2017, 21, 219-223. (34) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. Quantum Espresso: a Modular and Open-source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Mat. 2009, 21, 395502. (35) Giannozzi, P.; Andreussi, O.; Brumme, T.; Bunau, O.; Buongiorno Nardelli, M.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Cococcioni, M.; Colonna, N.; Carnimeo, I.; Dal Corso, A.; de Gironcoli, S.; Delugas, P.; DiStasio, R. A.; Ferretti, A.; Floris, A.; Fratesi, G.; Fugallo, G.; Gebauer, R.; Gerstmann, U.; Giustino, F.; Gorni, T.; Jia, J.; Kawamura, M.; Ko, H. Y.; Kokalj, A.; Küçükbenli, E.; Lazzeri, M.; Marsili, M.; Marzari, N.; Mauri, F.; Nguyen, N. L.; Nguyen, H. V.; Otero-de-la-Roza, A.; Paulatto, L.; Poncé, S.; Rocca, D.; Sabatini, R.; Santra, B.; Schlipf, M.; Seitsonen, A. P.; Smogunov, A.; Timrov, I.; Thonhauser, T.; Umari, P.; Vast, N.; Wu, X.; Baroni, S. Advanced Capabilities for Materials Modelling with Quantum Espresso. J. Phys.: Condens. Mat. 2017, 29, 465901. (36) Lejaeghere, K.; Van Speybroeck, V.; Van Oost, G.; Cottenier, S. Error Estimates for Solid-state Density-functional Theory Predictions: an Overview by Means of the Ground-state Elemental Crystals. Crit. Rev. Solid State 2014, 39, 1-24. 27



(37) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long-range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787-1799. (38) Barone, V.; Casarin, M.; Forrer, D.; Pavone, M.; Sambi, M.; Vittadini, A. Role and Effective Treatment of Dispersive Forces in Materials: Polyethylene and Graphite Crystals as Test Cases. J. Comput. Chem. 2009, 30, 934-939. (39) Chen, M.; Wang, X.; Yu, Y. H.; Pei, Z. L.; Bai, X. D.; Sun, C.; Huang, R. F.; Wen, L. S. X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Studies of Al-doped ZnO Films. Appl. Surf. Sci. 2000, 158, 134-140. (40) Tang, W.; Peterson, A. A.; Varela, A. S.; Jovanov, Z. P.; Bech, L.; Durand, W. J.; Dahl, S.; Norskov, J. K.; Chorkendorff, I. The Importance of Surface Morphology in Controlling the Selectivity of Polycrystalline Copper for CO2 Electroreduction. Phys. Chem. Chem. Phys. 2012, 14, 76-81. (41) Liu, Y.; Chen, S.; Quan, X.; Yu, H. Efficient Electrochemical Reduction of Carbon Dioxide to Acetate on Nitrogen-doped Nanodiamond. J. Am. Chem. Soc. 2015, 137, 11631-11636. (42) Neatu, Ş.; Maciá-Agulló, J. A.; Concepción, P.; Garcia, H. Gold-copper Nanoalloys Supported on TiO2 as Photocatalysts for CO2 Reduction by Water. J. Am. Chem. Soc. 2014, 136, 15969-15976. (43) Socrates G. Infrared and Raman Characteristic Group Frequencies; John Wiley & Sons, Ltd: Chichester, 2001. (44) Hesse M, Meier H, Zeeh B. Spektroskopische Methoden in der organischen Chemie. Georg Thieme Verlag, 2005; Stuttgart, Germany. (45) Dunwell, M.; Lu, Q.; Heyes, J. M.; Rosen, J.; Chen, J. G.; Yan, Y.; Jiao, F.; Xu, B. The 28


Page 28 of 30

Page 29 of 30 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


Central Role of Bicarbonate in the Electrochemical Reduction of Carbon Dioxide on Gold. J. Am. Chem. Soc. 2017, 139, 3774-3783. (46) Zhu, S.; Jiang, B.; Cai, W. B.; Shao, M. Direct Observation on Reaction Intermediates and the Role of Bicarbonate Anions in CO2 Electrochemical Reduction Reaction on Cu Surfaces. J. Am. Chem. Soc. 2017, 139, 15664-15667.

29



TOC

30


Page 30 of 30

Electrochemical reduction of CO2 into tunable syngas production by

Recommend Documents