Boosting Tunable Syngas Formation via Electrochemical CO2

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Energy, Environmental, and Catalysis Applications

Boosting Tunable Syngas Formation via Electrochemical CO2 Reduction on Cu/In2O3 Core/Shell Nanoparticles Huan Xie, Shaoqing Chen, Feng Ma, Jiashun Liang, Zhengpei Miao, Tanyuan Wang, Hsing-Lin Wang, Yunhui Huang, and Qing Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12747 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Applied Materials & Interfaces

Boosting Tunable Syngas Formation via Electrochemical CO2 Reduction on Cu/In2O3 Core/Shell Nanoparticles Huan Xie,a Shaoqing Chen,b Feng Ma,a Jiashun Liang,a Zhengpei Miao,a Tanyuan Wang,a Hsing-Lin Wang,b,* Yunhui Huang,a Qing Lia,* aState

Key Laboratory of Material Processing and Die & Mould Technology, School of

Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China bDepartment

of Materials Science and Engineering, Southern University of Science and

Technology, Shenzhen, Guangdong 518055, China

*Corresponding Author. E-mails: [email protected] (Q. Li); [email protected] (H. Wang).

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ABSTRACT: In this work, monodisperse core/shell Cu/In2O3 nanoparticles (NPs) were developed to boost efficient and tunable syngas formation via electrochemical CO2 reduction for the first time. The efficiency and composition of syngas production on the developed carbon supported Cu/In2O3 catalysts is highly dependent on the In2O3 shell thickness (0.4 to 1.5 nm). As a result, a wide H2/CO ratio (4/1 to 0.4/1) was achieved on the Cu/In2O3 catalysts by controlling the shell thickness and the applied potential (from -0.4 to -0.9 V vs. RHE), with Faraday efficiency (FE) of syngas formation larger than 90%. Specifically, the best-performing Cu/In2O3 catalyst demonstrates remarkably large current densities under low overpotentials (4.6 and 12.7 mA/cm2 at -0.6 and -0.9 V, respectively), which are competitive with most of the reported systems for syngas formation. Mechanistic discussion implicates that the synergistic effect between lattice compression and Cu doping in In2O3 shell may enhance the binding of *COOH on Cu/In2O3 NP surface, leading to the enhanced CO generation relative to Cu and In2O3 catalysts. This report demonstrates a new strategy to realize efficient and tunable syngas formation via rationally designed core/shell catalyst configuration. KEYWORDS: CO2 reduction, electrocatalysis, syngas, copper, indium oxide.

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1. INTRODUCTION Reducing CO2 electrochemically under mild conditions provides an alternative protocol to producing industrial feedstock chemicals with intermittent renewable electricity sources (electricity generated by solar, wind, tide, etc.), holding great promise compared to conventional chemical processes which are often environmentally hazardous and costly.1-7 So far the products obtained from CO2 electroreduction in an aqueous solution mainly include CO, HCOOH, CH4, and C2H4.4-5,

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Besides, H2 generation

originated from the competitive hydrogen evolution reaction (HER) is inevitable on most of the studied catalysts. Such scenarios have stimulated research on electrochemical co-production of CO and H2 (i.e., syngas) via CO2 reduction,1-3, 7, 10 as it can be directly used for large-scale synthesis of bulk chemicals (methanol, dimethyl ether, and acetic acid, etc.) and synthetic fuels through well-established industrial processes.1, 3, 7, 11-12 For the synthesis of different downstream products, syngases with varied H2/CO ratios are required.10, 12 Specifically, aldehyde and dimethyl ether productions require H2/CO ratio of 1/1,1, 10, 12 Fischer-Tropsch (F-T) process needs H2/CO ratio of 2/1,1, 7, 10, 12 while the H2/CO ratio for syngas fermentation should be ca. 1/3.7 Presently, syngas is produced by either coal gasification or natural gas reforming and the H2/CO ratios are manipulated by the reverse water-gas shift reaction, which highly rely on the consumption of fossil fuels.1, 10, 12

Producing syngas via CO2 reduction by renewable electricity sources with easily

controlled syngas composition (e.g., by adjusting the applied electrode potential) would obviously offer an alternative and eco-friendly approach for large-scale production of syngas mixtures. 3



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Several catalysts including carbon supported Pd nanoparticles (NPs)3, Cu-enriched Au surface7, Co3O4-CDots-C3N4 ternary composite1, and Se-doped CdS nanowires10 have demonstrated syngas formation from CO2 electroreduction with tunable H2/CO ratios. Nevertheless, to make this technology commercially available, electrocatalysts should be capable of achieving larger current density under lower overpotential to reach acceptable energy efficiency (the performance of representative electrocatalysts for syngas formation are summarized in Table S1). Compared to traditional bulk metal electrodes, NP electrocatalysts possess larger surface-to-volume/mass ratios13 and allow for the elaborate control of the dimension, component, atomic structure, and morphology,5,

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which usually lead to higher performance for electrochemical CO2 reduction.5, 13 Cu is an earth-abundant metal and capable of generating CO, HCOOH, and hydrocarbons (CH4, C2H4, etc.) via aqueous CO2 electroreduction.2, 5 However, Cu itself usually generates a wide product distribution with poor selectivity.15 To boost CO generation while suppress H2 evolution on Cu during CO2 electroreduction, it has been proposed that coupling Cu with an oxyphilic metal (e.g., Sn and In) would enhance the adsorption of *COOH and *HCOO (* indicates an adsorbed species) at mild overpotentials.16-18 Previous studies demonstrate that binary Cu-Sn catalysts could favor CO formation (Faraday efficiency (FE) > 90%), no matter on Cu-Sn alloy19 or Cu/SnO2 core/shell structure.20 Electrodeposited Cu-In alloy dendritic foils have been reported to convert CO2 to syngas and formate, but the FE for syngas formation still needs further improvement and the partial current density for syngas is insufficient for practical use due to the low surface area of the foil catalysts (< 1 mA/cm2 at -1.0 V vs. RHE, Table S1).16

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In this work, monodisperse 5.5/0.4-1.5 nm core/shell Cu/In2O3 NPs were successfully prepared for the first time and demonstrated to be capable of boosting efficient and tunable syngas production via CO2 electroreduction with respectable current density under low overpotential. The thickness of In2O3 shell can be easily controlled from 0.4 to 1.5 nm which significantly affects the performance of syngas formation. Importantly, the developed Cu/In2O3 NP electrocatalysts enable wide and tunable H2/CO ratios from 4/1 to 0.4/1 with high current densities under low overpotentials (4.6 and 12.7 mA/cm2 at -0.6 and -0.9 V vs. RHE, respectively), which is competitive with most of the reported electrocatalysts for syngas production. Mechanistic discussion suggests that the synergistic effect between lattice compression and Cu doping in In2O3 shell may stabilize *COOH rather than *HCOO on Cu/In2O3 NP surface, leading to the enhanced activity for CO formation. 2. EXPERIMENTAL METHODS 2.1 Materials Copper(I) acetate (93%) and trioctylamine (TOA, 97%) were purchased from Tokyo Chemical Industry Corporation. Tetradecylphosphonic acid (TDPA, 97%), In(acac)3 (99%) and In foil (0.5 mm, 99.998% trace metals basis) were from Sigma Aldrich. N-butylamine (99%), N-methyl-2-pyrrolidone (NMP, 99%), polyvinylidene fluoride (PVDF) and potassium bicarbonate (99%) were from Sinopharm Chemical Reagent Corporation. Carbon Paper (TGP-H-60) was from Toray Corporation. All reagents were used as received without further treatments.

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2.2 Synthesis of Cu and Cu/In2O3 core/shell NPs 5.5 nm Cu NPs were synthesized based on previous reports with modifications.20-22 Briefly, 94 mg copper(I) acetate and 139 mg TDPA were added into 10 mL TOA in a four-neck flask. The mixture was stirred and degassed at 130 °C for 30 min under Ar atmosphere. Then it was heated to 180 °C and held for 30 min. After that, the solution was quickly heated to 270 °C and maintained for 30 min to obtain 5.5 nm Cu NPs. To prepare Cu/In core/shell NPs, after the solution was cooled down to room temperature, certain amounts of In(acac)3 (165, 297, and 593 mg) dissolved in 4 mL TOA were injected. Then the solution was reheated to 270 °C (5 °C/min) for 1 h under Ar atmosphere. The obtained NPs were precipitated by ethanol and collected by centrifugation at 9000 r/min for 10 min, then dispersed in hexane for further use. Cu/In2O3 core/shell NPs were formed after Cu/In NPs were collected and exposed in air for several hours. 2.3 Working electrode preparation To prepare carbon supported Cu/In2O3 catalysts (denoted as C-Cu/In2O3), 20 mg carbon black (Ketjen EC-300J) was dispersed in the mixture of isopropanol and hexane (volume ratio of 1:2) under ultrasonication. Then 20 mg Cu or Cu/In2O3 NPs dispersed in hexane was added dropwise into the mixture and sonicated for 1 h. The catalysts were collected by centrifugation at 9000 r/min for 10 min and washed with ethanol and DI water. In order to remove the surfactant on NPs, the catalysts were dispersed in 50 mL n-butylamine and stirred for 24 h under ambient conditions. Afterwards, they were centrifuged and washed with ethanol for several times and dried in a vacuum oven at 6


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60 °C for 12 h. The mass loadings of metals on carbon support for C-Cn/In2O3 samples with In2O3 shell thicknesses of 0.4, 0.8, and 1.5 nm are measured to be ca. 14%, 13%, and 25%, respectively (Table S2). For the preparation of working electrode, 3 mg catalysts, 2 mg PVDF, and several drops of NMP were grounded to produce pastes, which were then painted onto both sides of carbon paper (~1.0 cm × 1.0 cm) and dried in a vacuum oven at 60 °C overnight. For In foil electrode, the as-purchased In foil was soaked in 0.5 M H2SO4 for 60 seconds to remove surface contaminants, followed by washing with abundant deionized water. 2.4 Electrochemical CO2 reduction Electrochemical CO2 reduction tests were performed in a gas tight two chamber reaction vessel with a total volume of 30 mL on an electrochemical workstation (CHI760e, Chenhua). The chambers were separated by a proton exchange membrane (Nafion®212, Ion Power). An aqueous 0.5 M KHCO3 solution was used as the electrolyte. A 99.9% platinum gauze and an Ag/AgCl electrode (4.0 M KCl) were used as the counter and reference electrodes, respectively. All potentials were converted to the reversible hydrogen electrode (RHE) scale by measuring the potential difference between the reference electrode and RHE. In the cathodic compartment, the electrolyte was purged with Ar for 10 min to remove air, and then bubbled with CO2 for 30 min to reach saturation (pH = 7.3) before electrochemical tests. The flow rate of CO2 during electroreduction was maintained at 20 mL/min. The gas products were directly introduced into the sample loop of a Shimadzu GC-2014 gas chromatograph for gas composition analysis. Liquid products were characterized by a Bruker AscendTM 600 7



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MHz 1H NMR spectrometer. 0.5 mL electrolyte was mixed with 0.15 mL D2O and 0.1 mL 0.5 uL/mL DMSO aqueous solution as internal standard. The FE of gas and liquid products were calculated as follows:23

FE (gas) =

𝑛𝐺𝐶g𝐹

FE (liquid) =

𝐼

× 100%

𝑛𝑉𝐶l𝐹 𝐼𝑡

× 100%

(1)

(2)

where n is the number of electron transfer to form products, G is the gas flow rate of CO2 during electroreduction, Cg designates the concentration of gas products, F is Faraday constant (96485 C/mol), I is current density for CO2 reduction, V is the volume of electrolyte, Cl denotes the concentration of liquid product, and t is the time for CO2 electroreduction. 2.5 Electrochemically accessible surface areas of C-Cu/In2O3 electrocatalysts 2 mg C-Cu/In2O3 electrocatalysts were suspended in 1 mL ethanol and 10 μL Nafion solution (5 wt% in water and propyl alcohol, Alfa Aesar) and sonicated for 1 h. Then 20 μL of the catalyst ink was deposited on glassy carbon (GC) electrode (5 mm diameter) and dried at ambient conditions. The electrochemically accessible surface area (Sa) of the C-Cu/In2O3 electrocatalysts were tested through the double-layer charging method. Sa is directly related to the gravimetric double layer capacitance C (F/g) of sample which can be calculated according to the following equation: C = I/m

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(3)

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where I is the current,  is scan rate, and m is the electrode mass. Then, Sa can be determined through dividing the gravimetric capacitance C by the double layer capacitance (F/m2) of glassy carbon electrode CGC (0.2 F/m2): Sa = C/CGC

(4)

2.6 Characterizations Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were from FEI Tecnai G2 20 and FEI Tecnai G2 F30 microscopies operated at 200 kV. High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image, energy dispersive spectroscopy (EDS) line scan and elemental mapping were recorded with a Thermo Fisher Talos F200X transmission electron microscopy operated at 200 kV. The surficial In/Cu atomic ratios and valence band structure of samples were collected by X-ray photoelectron spectroscopy (XPS) performed on a Shimadzu-Kratos XIS-ULTRA DLD-600W spectrometer using Mg Kα X-ray source. The binding energy of collected spectra were calibrated by referencing the C 1s binding energy (284.6 eV). Inductively coupled atomic mass spectroscopy (ICP-MS) analyses were performed on a PerkinElmer ELAN DRC-e spectrometer. X-ray diffraction (XRD) patterns were from a Rigaku Miniflex600 instrument with Cu Kα radiation. UV-vis spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer.

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3. RESULTS AND DISCUSSION 3.1 Structural characterizations of NPs Monodisperse core/shell Cu/In2O3 NPs were synthesized via a seed-mediated method via the decomposition of In(acac)3 in the presence of 5.5 nm Cu NPs (details see experimental section). TEM images of Cu and Cu/In2O3 NPs are shown in Figure 1a-d. As demonstrated in Figure 1a, the average size of pristine Cu NPs is 5.5 ± 0.5 nm. In the synthesis, the In2O3 shell thickness was controlled by the amount of In(acac)3 added. Specifically, by controlling the In/Cu precursor ratios from 0.5:1, 0.8:1, to 1.8:1, the NP sizes consistently increase to 6.3 ± 0.2 nm (Figure 1b), 7.3 ± 0.3 nm (Figure 1c), and 8.5± 0.6 nm (Figure 1d), corresponding to In2O3 shell thicknesses of 0.4, 0.8, and 1.5 nm, respectively (denoted as Cu/In2O3-0.4, Cu/In2O3-0.8, and Cu/In2O3-1.5). The NP compositions measured by ICP-MS indicate that the In to Cu molar ratios for Cu/In2O3-0.4, Cu/In2O3-0.8, and Cu/In2O3-1.5 NPs are 0.31:1, 0.45:1, and 0.68:1 (Table S2), respectively. HRTEM image of a representative Cu/In2O3-1.5 NP in Figure 1e exhibits the lattice fringes with average spacing of ~0.209 nm in the core, corresponding to the (111) plane of metallic Cu. In the shell area, the crystalline domain reveals the lattice fringe distance of ~0.291 nm, which can be ascribed to the (222) plane of cubic In2O3. For electrochemical measurements, the obtained Cu and three Cu/In2O3 NPs were loaded onto carbon support (denoted as C-Cu, C-Cu/In2O3-0.4, C-Cu/In2O3-0.8, and C-Cu/In2O3-1.5, respectively) and the TEM images of the C-Cu/In2O3-0.4 (Figure 1f), C-Cu/In2O3-0.8 and C-Cu/In2O3-1.5 (Figure S1) samples demonstrated that the NPs were well dispersed on carbon without NP agglomeration. 10


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Figure 1. TEM images of (a) 5.5 nm Cu, (b) Cu/In2O3-0.4, (c) Cu/In2O3-0.8, and (d) Cu/In2O3-1.5 NPs; (e) HRTEM image of a representative Cu/In2O3-1.5 NP; (f) TEM image of the C-Cu/In2O3-0.4 sample. The core/shell nanostructure of Cu/In2O3 NPs can be further proved by the EDS elemental mapping analysis. The EDS elemental mappings of Cu and In for a representative Cu/In2O3-1.5 NP clearly show an In (red) rich shell surrounding around the Cu (green) core (Figure 2a-c). The EDS line scan across a Cu/In2O3-1.5 NP (Figure 2d, inset shows the HAADF-STEM image) further proves that the thickness of In rich shell is ca. 1.5 nm which is in accordance with the TEM measurements. Additionally, the EDS elemental mapping images for Cu/In2O3-0.4 and Cu/In2O3-0.8 NPs are demonstrated in

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Figures S2 and S3, respectively, which also clearly illustrate their core/shell configurations.

Figure 2. EDS elemental mappings of (a) Cu (green), (b) In (red), (c) Cu and In for a representative Cu/In2O3-1.5 NP. (d) EDS line scan across a Cu/In2O3-1.5 NP, inset is the HAADF-STEM image. Figure 3a presents the XRD patterns of C-Cu and different C-Cu/In2O3 samples. It is obvious that C-Cu and C-Cu/In2O3-0.4 exhibit the diffraction peaks of cubic Cu2O (JCPDF No. 05-0667), suggesting that the 0.4 nm In2O3 shell could not protect Cu core 12


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from being oxidized by the atmospheric oxygen. In contrast, metallic Cu features (JCPDF No. 04-0836) instead of Cu2O dominate the C-Cu/In2O3-0.8 and C-Cu/In2O3-1.5 samples, indicating that the formation of a thicker In2O3 shell could protect Cu core from being oxidized. Notably, diffraction peaks of cubic In2O3 (JCPDF No. 06-0416) appear in the XRD pattern of the C-Cu/In2O3-1.5 sample, which should be originated from the crystalline In2O3 shell.

Figure 3. (a) XRD patterns of C-Cu and C-Cu/In2O3 samples. (b) UV-vis absorption spectra of Cu and Cu/In2O3-0.4 NPs. High resolution (c) In 3d and (d) Cu 2p XPS spectra of C-Cu and C-Cu/In2O3 samples.

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The formation of In2O3 shell on Cu NPs remarkably changes its surface plasmon resonance induced UV-vis absorption property. As shown in Figure 3b, the absorption peak of 5.5 nm Cu NPs is located at ca. 656 nm while that for Cu/In2O3-0.4 NPs blue shifts to 586 nm. XPS survey spectra of C-Cu and C-Cu/In2O3 samples (Figure S4) clearly reveal the presence of Cu, In, and C elements. The valence states of Cu and In in C-Cu and C-Cu/In2O3 samples are also investigated by XPS (Figure 3c-d). Specifically, both C-Cu/In2O3-0.4 and C-Cu/In2O3-0.8 samples show In 3d doublets at 444.9 and 452.5 eV (Figure 3c), which correspond to the 3d5/2 and 3d3/2 electrons of In(III) in In2O3, respectively (the binding energies of the 3d5/2 and 3d3/2 electrons for metallic In are 444.0 and 451.5 eV, respectively).24-25 These values are positively shifted by ~0.5 eV relative to those observed from pure In2O3 (444.4 and 451.9 eV),24 and can be ascribed to the compressive strain triggered electronic structure change of In which is induced by the lattice constant mismatch between In2O3 shell and Cu core.26 The valence band spectra of C-Cu and three C-Cu/In2O3 catalysts by XPS have also been determined (Figure S5). The valence band top of C-Cu/In2O3-0.4 and C-Cu/In2O3-0.8 catalysts are obviously shifted to higher binding energies compared to C-Cu/In2O3-1.5 sample, suggesting the existence of compressive strain on In2O3 shell for C-Cu/In2O3-0.4 and C-Cu/In2O3-0.8 catalysts.26-27 For the C-Cu/In2O3-1.5 sample, the binding energies of the 3d5/2 and 3d3/2 electrons of In(III) are 444.7 and 452.3 eV, respectively, which are only ~0.3 eV higher than pure In2O3 (Figure 3c).24 It is probably caused by the electron transfer between Cu core and In2O3 shell, as evidenced by its negative shift of valence band top compared to that of C-Cu (Figure S5).

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The high resolution Cu 2p XPS spectrum of the C-Cu sample in Figure 3d indicates that both Cu(II) (954.8 and 934.8 eV) and Cu(I)/Cu(0) species (952.8 and 932.8 eV) exist on the catalyst surface, suggesting the partial oxidation of Cu NPs by air.20 Other small peaks can be ascribed to the satellite peaks of Cu.28 Noteworthy, the intensities of Cu peaks are consistently decreased with the increase of In2O3 shell thickness. In addition, the binding energies of Cu (II) and Cu(I)/Cu(0) peaks for C-Cu/In2O3-0.4 and C-Cu/In2O3-0.8 samples are both negatively shifted by ~0.3 eV compared to that of pure Cu NPs, which may also be attributed to the compressive strain induced electronic structure change. Besides, the atomic ratios of surface In/Cu for C-Cu/In2O3-0.4, C-Cu/In2O3-0.8, and C-Cu/In2O3-1.5 samples collected by XPS are 0.61:1, 1.5:1, and 2.4:1 (Table S3), respectively, further testifying the core/shell nanostructure and Cu doping in In2O3 shells compared to the bulk In/Cu ratios measured by ICP-MS (Table S2). 3.2 Electrochemical characterizations The cyclic voltammetry (CV) curves of C-Cu and C-Cu/In2O3 catalysts in Ar-saturated 0.5 M KHCO3 solution are measured to investigate the redox potentials of surface Cu and In (Figure 4a). The two anodic peaks of the C-Cu catalyst at ~0.60 and 0.95 V can be ascribed to the oxidation of Cu to Cu2O and CuO and the two cathodic peaks correspond to the reduction of CuO to Cu2O (~0.45 V) and Cu2O to Cu (~0.15 V),20 respectively. For C-Cu/In2O3 samples, the redox peaks for Cu are significantly suppressed compared to that of C-Cu with the increase of In2O3 shell thickness. However, the redox peaks of Cu are still observable even on C-Cu/In2O3-1.5 NPs, pointing to the 15



exposure of Cu around In2O3 shell. On the other hand, the anodic peaks appear at -0.1 and -0.01 V of C-Cu/In2O3 samples can be assigned to the step-wise oxidation of In(0) to In(III), while the cathodic peaks at -0.45-0.60 V are due to the reduction of In(III) to In(0).29 Furthermore, the redox features of In enhance steadily with the increase of In2O3 shell thickness. Noteworthy, the reductive potentials of In(III)/In(0) for C-Cu/In2O3-0.4 and C-Cu/In2O3-0.8 catalysts are negatively shifted compared to the value for In2O3 particles (by ~0.15 V),30 while the reductive peaks of Cu(II)/Cu(I) and Cu(I)/Cu(0) shift to larger potentials compared to the C-Cu catalyst (by ~0.05 V), which may also be ascribed to the existence of compressive strain as reflected by valence band spectra (Figure S5).

Figure 4. (a) CV curves of C-Cu, C-Cu/In2O3-0.4, C-Cu/In2O3-0.8, and C-Cu/In2O3-1.5 catalysts in Ar-saturated 0.5 M KHCO3 solution with a scan rate of 20 mV/s; (b) linear sweep voltammetry (LSV) curves of C-Cu, C-Cu/In2O3-0.4, and In foil catalysts in Arand CO2-saturated 0.5 M KHCO3 solution with a scan rate of 20 mV/s.

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Linear sweep voltammetry (LSV) curves of C-Cu, C-Cu/In2O3, and In foil catalysts in Ar- and CO2-saturated 0.5 M KHCO3 solutions are shown in Figure 4b and Figure S6. The C-Cu catalyst demonstrates very similar curves in both Ar- and CO2- saturated electrolytes from -0.3 to -1.0 V, indicating its insignificant activity for CO2 reduction in tested potential ranges. Contrastingly, all studied C-Cu/In2O3 catalysts exhibit remarkably enhanced current densities in CO2-saturated electrolyte than those in Ar-saturated one, demonstrating much higher intrinsic activity for CO2 electroreduction than pure C-Cu catalyst. In addition, acid-treated In foil also demonstrates increased current density in CO2-saturated electrolyte compared to Ar-saturated one, but its current density is significantly lower than that of C-Cu and C-Cu/In2O3 catalysts, likely due to the less active sites of bulk electrode.

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Figure 5. Reduction potential dependent FEs of various products for CO2 electroreduction measured on (a) C-Cu, (b) C-Cu/In2O3-0.4, (c) C-Cu/In2O3-0.8, (d) C-Cu/In2O3-1.5, and (e) acid-treated In foil. (f) H2/CO ratios generated on C-Cu/In2O3 catalysts at different potentials. The FEs of various products for CO2 electroreduction on C-Cu and C-Cu/In2O3 catalysts at different potentials are calculated and illustrated in Figure 5a-d. For the C-Cu catalyst, the major product is H2 (FE > 60%) and the FE of CO generation is less than 10% 18


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from -0.4 to -0.9 V (Figure 5a), indicating its poor activity for CO2 electroreduction to CO. Besides, other minor products such as C2H4, C2H6, and HCOOH are also detected on C-Cu catalyst (Figure 5a and Figure S7). Obviously, the activity for CO formation on the developed C-Cu/In2O3 catalysts are dramatically enhanced compared to that of the C-Cu counterpart, and their total FEs for syngas formation and resulting H2/CO ratios are found to be dependent on the thickness of In2O3 shell (Figure 5b-d). Specifically, the FEs for syngas formation on the C-Cu/In2O3-0.4 catalyst (Figure 5b) are all more than 90% from -0.4 to -0.9 V, and the maximum FE reaches 95% at -0.4 V. While H2 is predominant at -0.4 V, CO becomes more favored at lower potentials (-0.5 to -0.9 V) with the highest FE of 68% at -0.7 V, and the H2/CO ratios are tunable from 1.5/1 to 0.4/1 (Figure 5f). If increasing the In2O3 shell from 0.4 to 0.8 nm (Figure 5c), the activity for syngas formation is slightly reduced. FEs for syngas formation on C-Cu/In2O3-0.8 from -0.4 to -0.9 V are > 82% and H2/CO ratios are from 1.5/1 to 0.5/1 (Figure 5f). Similarly, CO is preferably produced at relatively negative potentials (-0.5 to -0.8 V) on C-Cu/In2O3-0.8 with the maximum FE of 61% at -0.5 V. Further increasing In2O3 shell thickness to 1.5 nm would lead to considerable HCOOH formation starting from E < -0.5 V (Figure 5d and Figure S8), which is similar with that measured on anodized In2O3 electrode.31 For acid-treated In foil (Figure 5e and Figure S9), no CO can be detected at investigated potentials and HCOOH can be only detected at lower potentials (-0.7 to -0.9 V), with the highest FE of 27% at -0.7 V, which is similar to the previous reports.16, 29 These results illustrate that C-Cu/In2O3 core/shell NP catalysts are more active for syngas formation than C-Cu and In foil counterparts, and H2/CO ratios

19



are adjustable in a wide range of 4/1 to 0.4/1 with respectable FE of more than 90% by adjusting In2O3 shell thickness as well as applied potential for CO2 electroreduction.

Figure 6. (a) Total current density of electrocatalysts at different potentials for CO2 electroreduction. (b) Partial current density of syngas formation on C-Cu/In2O3 catalysts. (c) Chronoamperometry test of the C-Cu/In2O3-0.4 catalyst at -0.7 V for CO2 electroreduction. (d) XRD patterns of the C-Cu/In2O3-0.4 catalyst before and after CO2 electroreduction stability test. Another key parameter to evaluate the performance of electrocatalysts for CO2 reduction is the partial current density of products, which is closely related to productive rate. Reducing the overpotential for syngas formation while maintaining a high level of current density is crucial to achieve high energy efficiency. The total current densities 20


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(normalized by the geometric area of the electrode) of all studied catalysts (Figure 6a) demonstrate a quasi linear relationship from -0.4 to -0.9 V. More specifically, the partial current density of C-Cu/In2O3-0.4 revealed in Figure 6b is generally larger than that of C-Cu/In2O3-0.8 and C-Cu/In2O3-1.5 within studied potentials. Importantly, the current densities for syngas formation achieved on the C-Cu/In2O3-0.4 catalyst at -0.6 and -0.9 V are 4.6 and 12.7 mA/cm2, respectively, which are competitive with most of previously reported catalysts (Table S1). The enhanced performance of C-Cu/In2O3 electrocatalysts for syngas formation at lower overpotentials can be ascribed to the highly active In2O3 surface induced by the unique Cu/In2O3 core/shell structure (details are discussed in the next section) and the relatively large surface areas resulted from the small particle sizes (less than 10 nm). The CV curves of C-Cu/In2O3 catalysts from 0.2 to 0.4 V at different scan rates are shown in Figure S10. The measured electrochemically accessible surface areas of C-Cu/In2O3-0.4, C-Cu/In2O3-0.8, and C-Cu/In2O3-1.5 electrocatalysts through calculating the double-layer charging currents are 35.1, 53.6, and 92.4 m2/g, respectively. The relatively large surface area of the C-Cu/In2O3 electrocatalysts provides more active sites for syngas formation during electrochemical CO2 reduction, leading to enhanced current densities at lower overpotentials. The stability of the C-Cu/In2O3-0.4 catalyst for syngas formation at -0.7 V has been investigated (Figure 6c). The total current density is almost unchanged after 5 h and the total FE for syngas formation during stability test is consistently more than 90%, indicating its high stability for syngas formation. Meanwhile, the structure and morphology of the C-Cu/In2O3-0.4 catalyst after the stability test are well maintained, as 21



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characterized by XRD (Figure 6d) and TEM (Figure S11). Additionally, the surficial In to Cu atomic ratios of three C-Cu/In2O3 catalysts before and after CO2 electroreduction test are almost unchanged, suggesting the robustness of catalyst structures during the tests (Table S3). 3.3 Mechanism discussion The production of HCOOH and CO during electrochemical CO2 reduction can be generally described along the following reaction pathways:9, 16, 21, 32 CO2 + e- + * → *CO2-

(5)

*CO2- + H+ → HCOO*

(6)

HCOOH:

HCOO* + H+ + e- → HCOOH + *

(7)

CO: *CO2- + H+ → *COOH

(8)

*COOH + H++ e- → CO + H2O + *

(9)

where * represents the surface adsorption site. Besides, HER is the competing reaction during electrochemical CO2 reduction and the competition of HER and CO2 electroreduction at different potentials is responsible for the varied H2/CO ratios on C-Cu/In2O3 catalysts.33

22


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Figure 7. Illustrations of possible reaction pathways on (a) In foil, (b) Cu NPs, and (c) Cu/In2O3 NPs during electrochemical CO2 reduction. Theoretical studies have pointed out that the weak adsorption of *CO2- on pure In electrode leads to the preferential formation of HCOOH through Eq.(5-7) (as illustrated in Figure 7a).5,

9, 16

In comparison, Cu electrode has moderate binding energies with

*CO2-, *COOH, and *CO, thus the varied products including HCOOH, CO, CH4, and C2H4 can be produced (Figure 7b).5,

32

In our system, the enhanced CO formation on

Cu/In2O3 NPs relative to In and Cu electrodes for CO2 electroreduction is considered to be closely related to its unique core/shell structure and the doping of Cu atoms in In2O3 23



shell. Former studies have demonstrated that the core/shell structure with different components would normally induce the surface strain effect at the core/shell interface due to the lattice mismatch of core and shell materials.20, 34-36 Recently, such surface strain effect has been widely employed to tune the activity of electrocatalysts towards electrochemical CO2 reduction. For instance, AuCu/Au core/shell NPs with AuCu intermetallic core and 3 atomic layered Au shell was found to impose ~6% compressive strain on the Au shell by density functional theory (DFT) studies, which was believed to be the reason for its enhanced activity for CO formation during CO2 electroreduction.37 Au/Cu core/shell nanocubes with 7-8 atomic Cu layers were found to prefer C2H4 formation while the ones with 14 Cu layers favored CH4 generation during CO2 electroreduction, which was ascribed to the strain effect of core/shell structure and the cubic morphology.34 Previous study on Cu/SnO2 core/shell NPs through DFT calculations demonstrated that there was a large compression stress (~10%) on the SnO2 shell. The lattice compression together with the Cu doping at the surface are responsible for the switch of reduction product from formate to CO compared to SnO2 catalyst. 20 In the case of our Cu/In2O3 NPs, such compression stress on the shell should also exist as evidenced by the valence band spectra (Figure S5). The synergistic effect of compression stress and Cu doping in In2O3 shell (as evidenced by XPS and CV tests) are speculated to strengthen the binding of *COOH rather than HCOO* on the surface of Cu/In2O3-0.4 NPs, leading to the favored formation of CO instead of HCOOH (Figure 7c).

4. CONCLUSIONS 24


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In this work, efficient syngas formation from electrochemical CO2 reduction has been achieved on monodisperse core/shell Cu/In2O3 NPs for the first time. By controlling In2O3 shell thickness (0.4 to 1.5 nm) and the applied potential (-0.4 to -0.9 V), the H2/CO ratios in syngas on C-Cu/In2O3 NP electrocatalysts are tunable within a wide range from 4/1 to 0.4/1 and the FE for syngas production are larger than 90%. Importantly, the best-performing C-Cu/In2O3-0.4 catalyst demonstrates high current densities under low overpotentials (4.6 and 12.7 mA/cm2 and at -0.6 and -0.9 V, respectively), which are competitive with most of the reported electrocatalysts for syngas production. Mechanistic discussion implicates that the coexistence of compression strain and Cu doping in In2O3 shell may stabilize *COOH on Cu/In2O3 surface, leading to the enhanced activity for CO formation and resulting the wider H2/CO ratios. This report demonstrates a new approach to efficient and tunable syngas production from the electrochemical reduction of CO2 via the rationally designed core/shell configuration. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXXX. Additional information including the performance comparison of the reported electrocatalysts for syngas formation, the surficial composition of electrocatalysts before and after electrochemical CO2 reduction test, TEM images, EDS elemental mapping

25



images, valence band spectra, LSV curves, NMR spectra, and double-layer charging tests. AUTHOR INFORMATION Corresponding Author *E-mail for Q. Li: [email protected]. *E-mail for H. Wang: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by National 1000 Young Talents Program of China, National Nature Science Foundation of China (21603078, 51602223), National Materials Genome Project (2016YFB0700600). The authors thank the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for carrying out the XPS and XRD measurements. REFERENCES 1.

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Boosting Tunable Syngas Formation via Electrochemical CO2

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