Electrochemical fragmentation of Cu2O nanoparticles enhancing

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Electrochemical fragmentation of Cu2O nanoparticles enhancing selective C-C coupling from CO2 reduction reaction Hyejin Jung, Si Young Lee, Chan Woo Lee, Min Kyung Cho, Da Hye Won, Cheonghee Kim, Hyung-Suk Oh, Byoung Koun Min, and Yun Jeong Hwang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11237 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Journal of the American Chemical Society

Electrochemical Fragmentation of Cu2O Nanoparticles Enhancing Selective C-C Coupling from CO2 Reduction Reaction Hyejin Jung,†,‡ Si Young Lee,†,‡ Chan Woo Lee,†,∇ Min Kyung Cho,§ Da Hye Won,† Cheonghee Kim,∥ Hyung-Suk Oh,† Byoung Koun Min*,†,⊥ and Yun Jeong Hwang*†,‡ †Clean

Energy Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ‡Division

of Energy and Environmental Technology, KIST school, Korea University of Science and Technology, Seoul 02792, Republic of Korea ∇Department §Advanced

Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea

∥Department

⊥Green

of Applied Chemistry, Kookmin University, Seoul 02707, Republic of Korea

of Chemistry, Chemical Engineering Division, Technical University of Berlin, 10623 Berlin, Germany

School, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea

ABSTRACT: In this study, we demonstrate the initial morphology of nanoparticles can be transformed into small fragmented nanoparticles, which were densely contacted to each other, during electrochemical CO2 reduction reaction (CO2RR). Cu-based nanoparticles were directly grown on a carbon support by using cysteamine immobilization agent, and the synthesized nanoparticle catalyst showed increasing activity during initial CO2RR, doubling Faradaic efficiency of C2H4 production from 27 % to 57.3 %. The increased C2H4 production activity was related with the morphological transformation over reaction time. 20 nm cubic Cu2O crystalline particles gradually experienced in-situ electrochemical fragmentation into 2~4 nm small particles under the negative potential, and the fragmentation was found to be initiated from the surface of the nanocrystal. Compared to Cu@CuO nanoparticle/C or bulk Cu foil, the fragmented Cu-based NP/C catalyst achieved enhanced C2+ production selectivity, accounting 87 % of the total CO2RR products, and suppressed H2 production. In-situ X-ray absorption near edge structure studies showed metallic Cu0 state was observed under CO2RR, but the fragmented nanoparticles were more readily re-oxidized at open circuit potential inside of the electrolyte, allowing labile Cu states. The unique morphology, small nanoparticles stacked each other, is proposed to promote C-C coupling reaction selectivity from CO2RR by suppressing HER.

1.

INTRODUCTION

Nanoparticle catalysts have often demonstrated enhanced activities compared to bulk surfaces due to high surface to volume ratio and newly exposed sites contributing to high intrinsic activity. In addition, nanoparticles with wellcontrolled shape and size are useful to understand catalytic activity trends and design efficient catalysts. However, nanoparticles have been always suspected whether their initial morphologies remain intact during catalytic reactions. Small sized particles can agglomerate together to larger ones,1 or high index facets can change to lower ones by a thermodynamically-derived spontaneous process during thermal reactions,2 resulting in deactivated performance. Similarly to thermal catalytic reactions, electrochemical catalytic reactions can induce morphological change of the surface by applied potentials

or adsorbates.3-5 It would be ideal if this weakness is exploited to improve the catalytic performance by an insitu electrochemical activation which can avoid particle agglomeration and rather induces transformation of nanoparticles into more active ones. Among various electrochemical reactions, electrocatalytic CO2 reduction reaction (CO2RR) has been reported to be sensitive to the size and morphology of the nanoparticle catalysts which affect the product selectivity among CO2RRs and hydrogen evolution reaction (HER) from an aqueous condition.6 Meanwhile, the electrochemical conversion of CO2 to valuable products has been proposed as a promising carbon utilizing technology for a carbon cycle in a sustainable manner by integrating with renewable electricity.7-9 Despite intensive studies, Cu is an almost unique metal catalyst, capable of

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producing considerable amounts of hydrocarbons and alcohols beyond the two-electron reduced products such as CO and formate. Multi-carbon (C2+) products are highly targeted due to their commercial values. For instance, C2H4 is a widely used industrial raw chemical employed in the manufacture of plastics, and its selective production over CH4 is important.10 Suitably high CO binding energy on Cu has been proposed as the origin of the special activity because CO intermediate can be subjected to further reduction steps and C-C coupling before desorption. A bulk flat Cu generally favors CH4 production and also generates significant amounts of H2 with ~ 40 % Faradaic efficiency (F.E.)11 even at an optimal potential, while Cu nanostructures can provide advantages for selective C2H4 production. Cu nanoparticles loaded on the carbon support were demonstrated to have 40 % of F.E. for C2H4 (F.E.C2H4),12 and Loiudice et al.13 also achieved similarly high selectivity with optimally sized Cu cubes of 44 nm by varying size and shape. Moreover, Cu nanostructures prepared from copper oxides can have improved selectivity for C2H4 production because when copper oxides undergo electrochemically reductive reactions, it can bring out a low-coordinated Cu surface atom or residual oxygen, which were experimentally and theoretically suggested to increase strong CO binding sites and thus lead to C-C coupling.14-16 For example, an electrochemically synthesized Cu2O/Cu electrode can have F.E.C2H4 ~40 %,17 and O2 plasma-treated Cu2O/Cu achieved highly selective production of C2H4 which occupying ~ 60 % of CO2RR products.18 However, the origin of the enhanced activity of Cu-based nano-catalysts is not easily identified, and the changes of copper states can accompany morphological and crystal structural changes as well.19, 20 C-C coupling activities can be also modified by electrochemical treatment processes. Moreover, various approaches but no unified solution are proposed for CO2RR towards C2+ chemicals production since the reaction pathway is complicated and still under investigation. To date, many factors have been successfully demonstrated, i.e. high density of grain boundaries,21-22 under-coordinated sites,23 residual subsurface oxygen,24 Cu+ copper states,16 local pH gradient,25-26 halide ion,27 cation,28 ensemble effect,19 and local field effect.20 Based on these strategies, recently, a number of Cu-based electrocatalysts are reported to have F.E.C2H4 in the ranges of 30~40 %, but higher than 50 % is still challenging.18-19, 29 Therefore, breakthroughs are still required to develop catalysts for highly selective C2H4 production. In this study, we synthesized copper oxide nanoparticles which were directly grown on a carbon black by using cysteamine and applied for CO2RR electrocatalysts. Interestingly, it was revealed that the initial 20 nm Cu2O NP/C was gradually fragmented into 2~4 nm particles, which were densely contacted with each other, over few hours of CO2RR. Cu-based NP/C catalyst exhibited increasing C2H4 production highly related with the morphological deformation, and reached up to 57.3 % of F.E.C2H4, one of the highest values for C2H4. H2 production

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was also highly suppressed compared with those of nonfragmented nanoparticle or bulk Cu foil. In-situ X-ray absorption near edge structure (XANES) analysis and transmission electron microscopy (TEM) analysis during the long term CO2RR supports the electrochemical fragmentation of the nanoparticle induces high selectivity for CO2 to C2+ conversion, which can be applied for the design of the efficient catalysts. 2.

EXPERIMENTAL SECTION

2.1 Synthesis of Cu-based nanoparticles (NPs). A simple one-pot wet-chemical synthesis is developed to prepare Cu-based NPs directly grown on carbon supports. In addition to a Cu metal precursor and carbon powder support, cysteamine molecules were added as immobilization agents which assisted nucleation of NPs on the carbon support.30 Two types of NPs were prepared by controlling the growth temperature and the retention time which affect the size of the particle as well as the shape. The size of the nanoparticle has been reported to be important to determine the catalytic activity of CO2RR. To synthesize Cu NPs around 20 nm size, 60 mg of copper (II) nitrate hydrate (Cu(NO3)2·xH2O; Sigma-Aldrich, 99.999 %) was dissolved in 10 mL of ethyleneglycol (EG; SigmaAldrich, 99.8 %) with vigorous stirring and this solution was heated to 120 ℃ . Meanwhile, 20 mg of carbon (Ketjenblack, EC-300J) and 1 mg of cysteamine (SigmaAldrich, 95 %) were dispersed in 10 mL of EG by ultrasonication for 30 min. This carbon solution was injected to the copper precursor solution at 120 ℃ and the mixed solution was kept stirring for 20 min before rising temperature up to the growth temperature. When the growth temperature was raised to 180 ℃ and kept for 7 hours (denoted as Cu2O NP/C), cubic morphology was obtained. On the other hand, sphere morphology was grown at 220 ℃ as the growth temperature, maintaining for 4 hours (denoted as Cu@CuO NP/C). In addition, the 2 ~ 4 nm sized Cu-based NP/C were synthesized at 200 ℃ for 1 hour of growth time, and copper precursor amount was 20 mg. The ramping rate of the temperature was 3 ℃/min for all the time. The product solution was cooled down naturally to room temperature and washed with isopropyl alcohol, filtered and dried. 2.2 Material Characterizations. Transmission electron microscopy (TEM) was performed with TitanTM 80-300 operated at 300 kV to observe the morphology of asprepared catalysts and their morphological changes after CO2RR. High resolution (HR) TEM images were also obtained to assign d-spacing of Cu NPs to understand the crystal structure. In addition, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) mapping images of the post CO2 RR catalysts were obtained through Talos F200X 80-200 microscope operated at 200 kV. The crystal structure of as prepared catalysts was also analyzed through X-ray diffraction (XRD; Bruker D8 Advance). X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-alpha) was used to investigate


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the elemental composition of the surfaces and the chemical states of individual elements, especially the oxidation state of Cu. To investigate electronic structure of the Cu-based NP/C catalysts further, X-ray absorption near edge structure (XANES) Cu L-edge spectra were obtained at the Pohang Light Source (PLS; 10D beam line) using monochromatic soft X-rays of the synchrotron source, and Cu K-edge spectra were taken through 1D beam line in PLS using hard X-ray source. The atomic content of copper versus carbon support was measured by inductively coupled plasma optical emission spectrometry (ICP-OES; Thermo Scientific iCAP 7000) to calculate loaded Cu. amounts. 2.3 Working Electrode Preparation. To prepare catalyst electrode, the synthesized Cu2O NP/C and Cu@CuO NP/C catalysts were loaded on the glassy carbon electrode by spraying catalyst ink. The catalyst ink was prepared by dispersing 1.0 mg of catalysts into a mixture solution of 1.0 mL isopropyl alcohol and 30 μL Nafion (Aldrich, 5 wt%) with sonication for 30 min. Glassy carbon plate (Alfa Aesar, 1 mm thick) was used as the substrate electrode after mechanical polishing and complete cleaning with deionized water. The loaded amount of the catalyst on carbon support was 0.4 mg/cm2, and ICP-OES results were used to calculate the amount of Cu. To compare the catalytic activity of the Cu-based NP/C catalysts with bulk Cu, a polycrystalline Cu foil (Alfa Aesar, 99.9999 %, 0.1 mm) was used. The Cu foil was electrochemically polished in phosphoric acid (SigmaAldrich, ≥85 wt. % in H2O) at 4.0 V vs. Pt counter electrode for 300 s. The geometric area of the catalyst electrode was masked to be 0.50 cm2. Electrochemical surface area (ECSA) of Cu-based NP/C were measured over reaction time by Pb underpotential deposition (UPD) method following the previous studies12, 31-32. The electrolyte of 0.1 M HClO and 1 mM PbCl aqueous 4 2 solution (pH 1.54) was purged with Ar gas at least 1 hour. before. After each hour of CO2RR at -1.1 V vs. RHE in 0.1 M KHCO3, the electrodes were transferred and cyclic voltammetry was measured from -0.1 V to -0.5 V vs. Ag/AgCl reference electrode in a scan rate of 10 mV/s. The area of a Pb monolayer stripping peak around -0.3 V vs. Ag/AgCl was integrated to calculate the transferred charge. The ECSAs of the Cu-based electrodes were estimated with the conversion factor of 310 μ C/cm2 adatoms on Cu and 2e- Pb oxidation.

32

assuming Pb

2.4 Electrochemical Measurement. All of the electrochemical measurements were conducted with a potentiostat (Iviumstat) in a two-compartment electrochemical cell, and an anion exchange membrane, Selemion AMV, was used to separate the catholyte and the anolyte. The electrocatalytic activity was measured in a three-electrode configuration with a Pt foil as a counter and an Ag/AgCl (3 M NaCl) as a reference electrode. The Cu-based electrocatalyst was used as a working electrode placed in the catholyte with the reference electrode, while the Pt counter electrode was placed in the anolyte side. 0.1

M KHCO3 (Sigma-Aldrich, ≥ 99.95 %) electrolyte was purged with high purity CO2 gas (99.999 %) with an average flow rate of 20 sccm measured by a universal flow meter (Agilent technologies, ADM 2000) for more than 1 hour before CO2RR, and the pH of the CO2 saturated electrolyte was measured as 6.8. CO2RR was performed with chrono-amperometry measurement at each fixed potentials for 30 min when gaseous products (H2, CO, CH4 and C2H4) were quantified by a gas chromatography (GC, Agilent 6890) equipped with a carboxen 1000 column (Supelco). The GC was on-line connected with the closed two-compartment electrochemical cell for in-situ analysis of gas products. The GC is installed with a thermal conductivity detector (TCD) to detect H2 and flame ionization detector (FID) to detect hydrocarbons. Before the FID, a methanizer (Agilent) is equipped for the detection of CO. High purity of Ar gas (99.999 %) was used as the carrier gas for all compartments of the GC. All potentials were corrected for iR-loss compensation by an electrochemical impedance spectroscopy (EIS) and converted to potentials versus the reversible hydrogen electrode (RHE) by using the following equation.

E (vs. RHE) = E (vs. Ag/AgCl) + 0.209 V + 0.0591 V × pH 𝑖𝐻2 𝑜𝑟 𝐶𝑂,𝐶𝐻4,𝐶2𝐻4 𝐹.𝐸. 𝐻2 𝑜𝑟 𝐶𝑂, 𝐶𝐻4, 𝐶2𝐻4 (%) = × 100 𝑖𝑡𝑜𝑡𝑎𝑙 2𝐹𝑝𝑜 𝑉𝐻2 𝑜𝑟 𝐶𝑂,𝐶𝐻4,𝐶2𝐻4 × 𝑄 × 𝑅𝑇 = × 100 𝑖𝑡𝑜𝑡𝑎𝑙 Faradaic efficiency of H2, CO, CH4 and C2H4 production were calculated following the equation. The total current was measured by the potentiostat, and the partial currents (𝑖𝐻2 𝑜𝑟 𝐶𝑂, 𝐶𝐻4, 𝐶2𝐻4) were obtained from the areas of GC chromatogram peaks where 𝑉𝐻2 𝑜𝑟 𝐶𝑂, 𝐶𝐻4, 𝐶2𝐻4 is the volume concentration of H2 or CO, CH4, and C2H4, respectively, based on the calibration of GC. 𝑄 is flow rate of CO2, 𝐹 is Faradaic constant, 𝑝𝑜 is pressure, 𝑇 is room temperature and 𝑅 is ideal gas constant, 8.314 J•mol/K. 2.5 Liquid products identification and quantification. After each bulk electrolysis at constant potential, 700 μL of catholyte was syringed out from the electrochemical cell and mixed with 35 μL of an internal standard, composed of 10 mM dimethyl sulfoxide (DMSO, Sigma-Aldrich, 99.9 %) and 50 mM phenol (Simga-Aldrich, 99.0~100.5 %) in D2O (Sigma-Aldrich, 99.9 atom % D). This mixture was transferred to a nuclear magnetic resonance (NMR) sample tube and analyzed by 1H NMR spectroscopy with 600 MHz spectrometer (DD2, Agilent Technologies) to identify liquid products. The peak of water was suppressed by pre-saturation sequence, and the ratio of peak area for formate to phenol, the ratio of peak areas for other products to DMSO were compared to the standard curves to quantify the concentrations of each liquid product. The coulombs required to produce the concentration of each product was calculated and divided by the total coulombs passed during the chrono-



amperometry to determine the current efficiency (i.e. Faradaic efficiency). 2.6 In-situ and operando X-ray Absorption Spectroscopy. XANES measurement of the Cu2O NP/C cathode was carried out simultaneously with CO2RR at -1.1 V vs. RHE. The oxidation state of Cu during the CO2RR was analyzed with a synchrotron radiation beam at Pohang Accelerator Laboratory (PAL) 10 C beam line, and a twocomponent electrochemical cell was installed, similarly to the CO2RR activity measurement as mentioned above. The working electrode was prepared with Cu2O NP/C spraying on the glassy carbon plate (Alfa Aesar, 0.3 mm thickness) and was masked with the Kapton polyimide tape by 0.50 cm2 of active area. The back-side of the electrode was affixed to the Kapton polyimide film of the cell to avoid loss of X-ray by electrolyte and X-ray absorption spectroscopy was measured in fluorescence mode. An Ag/AgCl reference electrode and a Pt coil counter electrode were used. The potential was applied by a potentiostat (Ivium Vertex) and high purity (99.999 %) CO2 gas was continuously supplied to the electrolyte, 0.1 M KHCO3. Firstly, XANES of Cu2O NP/C was measured in the electrolyte of the electrochemical cell prior to CO2RR. Then, XANES measurements were carried out in the electrochemical cell without exposure to the air, after 1, 2, 3, 6, 8, and 10 hours of CO2RR at -1.1 V vs. RHE, respectively, and the chronoamperometry was paused. Each XANES measurement was taken about 30 min. In addition, insitu/operando XAS was obtained during CO2RR under the applied potential of -1.1 V vs. RHE.

Page 4 of 13

different from that of the polycrystalline Cu foil having both metallic Cu0 (568.0 eV) and natively oxidized Cu+ features (569.9 eV). In addition, XPS S 2p and N 1s spectra showed sulfur and nitrogen element were present on the nanoparticle catalyst, related to cysteamine molecules (Figure S1).30 Lastly, XANES spectra of Cu K-edge (Figure 1f) pointed out the nanoparticle catalyst had well matched spectra with the Cu2O reference. Therefore, we confirm the synthesized nanoparticles are crystalline Cu2O loaded on the carbon support. 3.2 Electrochemical fragmentation of nanoparticles during CO2RR. First, CO2RR activity for C2H4 production was characterized using the Cu2O NP/C (denoted as Cubased NP/C), and sharp increase of C2H4 selectivity was especially noted during the first few hours (Figure 2a). The prepared sample was sprayed on a glassy carbon substrate, and 0.1 M KHCO3 aqueous solution saturated with CO2 was used as the electrolyte. Figure 2a plots Faradaic efficiency for C2H4 production (F.E.C2H4) and total current density during 10 hours at -1.1 V vs. RHE, a favorable bias potential of Cu electrocatalysts for CO2RR. The Cu-based NP/C catalyst achieved an average 57.3 % F.E.C2H4 after 6 hours, one of the highest F.E.s reported for C2H4 production (Table 1). Cu-based NP/C catalyst exhibited more than five times higher C2H4 production selectivity than polycrystalline Cu foil (F.E.C2H4 = 11.2 %) (Figure S2). In addition, the improvement of C2H4 production is directly related to the suppression of H2 production (Figure S3).

3. RESULTS & DISCUSSIONS 3.1 Cu2O nanoparticles on carbon support prepared from one-pot synthesis. We successfully synthesized nanoparticles directly grown on the carbon black (Figure 1) by a facile one-pot wet chemical method, using cysteamine as an immobilizing agent.30 TEM images showed that the nanoparticles were well dispersed on the carbon support and had cubic shape with about 20 nm size (Figure 1a). The high resolution TEM (HRTEM) image of the representative particle showed a d-spacing of 0.302 nm, (110) plane of cubic copper (I) oxide, and all of the observed selected area electron diffraction (SAED) patterns represent the crystal structure of Cu2O (Figure 1b). The X-ray diffraction (XRD) patterns were consistent with a face-centered cubic Cu2O crystalline structure (JCPDS #78-2076) having (111), (200) and (220) as the main diffraction peaks (Figure 1c). Next, spectroscopic characterization showed oxidized copper state, consistent with the crystal structure analysis results. X-ray photoelectron spectroscopy (XPS) had a main Cu 2p3/2 feature at 932.4 eV which can be assigned to either Cu2O (Cu+) or Cu (Cu0) but is not distinguishable from the metallic polycrystalline Cu foil because the binding energies between Cu+ and Cu0 differ by only 0.1 eV (Figure 1d).33 Therefore, we additionally obtained an Auger electron spectroscopy (AES) of Cu LMM signal (Figure 1e), clearly showing the nanoparticle catalyst mainly consisted of a Cu+ (Cu2O) oxidation state near 569.9 eV, which is

Figure 1. Characterization of synthesized Cu2O NP/C : a) TEM image (inset figure is SAED pattern with 5 1/nm of scale bar), b) HRTEM image of representative nanoparticle, c) XRD spectrum indexed with crystalline phase, d) XPS Cu 2p spectra, e) Auger spectra of Cu LMM, and f) XANES Cu K-edge spectra compared with standard materials of CuO, Cu, and Cu2O.


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Figure 2. Increase of electrocatalytcic activity for selective C2H4 production during 10 h CO2RR and characterization after the reaction: a) Faradaic efficiency for C2H4 production depending on the time, with error bar and corresponding total current density at right y-axis, b) HRTEM image (corresponding FFT image, e)), c) TEM image, d) STEM image, and f) STEM EDS elemental mapping for Cu after 10 h of CO2RR with the Cu-based NP/C.

Table 1. Comparison of optimized C2+ products from various Cu-based catalysts in neutral pH aqueous media. Preparation method

Sample

Substrate

Electrolyte

E vs. RHE

One-pot wet-chemical

Cu2O NP/C

Glassy carbon

0.1 M KHCO3

”

Cu@CuO NP/C

”

-

Cu foil

Electro-redeposition

Redeposited Cu from Cu2(OH)3Cl sol-gel

Electrodeposition

Faradaic Efficiency (%)

Ref.

C2H4

H2

CH4

C2+C3

-1.1 V

57.3 %

15.0 %

1.9 %

74.0 %

this study

”

-1.1 V

33.2 %

43.5 %

7.6 %

44.5 %

this study

-

0.1 M KHCO3

-1.07 V

18.3 %

36.4 %

25.0 %

-

this study

Carbon paper

0.1 M KHCO3

-1.2 V

38.5 %

31.0 %

0.03 %

52.9 %

20

Cu2O-derived Cu NPs

Cu plate

0.1 M KHCO3

- 1.1 V

33.5 %

32.0 %

4.0 %

33.5 %

43

32.1 %

48.1 %

44

”

”

Cu disc

”

-1.0 V

Preparation method ”

Sample Cu2O (Cl induced)

Substrate ”

Electrolyte 0.1 M KCl

E vs. RHE -1.8 V

23.0 C2H%4

38.0 H2%

1.2 % CH 4

55.1 % C2+C 3

Ref. 45

” One-pot wet-chemical

Cu2O (1.7 μm) Cufilm 2O NP/C

Glassy”carbon

0.1 0.1 M M KHCO KHCO33

-0.99 -1.1 VV

38.8 57.3 % %

39.0 15.0 % %

0.7 1.9 % %

47.9 74.0% %

this46 study

”

Mesoporous Cu2O Cu@CuO NP/C

Cu foam on ” Cu wafer

0.5 M ” 3 NaHCO

-0.8 -1.1 V

20.0 33.2 % %

16.0 43.5% %

- % 7.6

55.0 44.5% %

this47study

”

Cu foil Mesocrystal Cu from CuCl film Redeposited Cu from Agglomerated Cu Cu 2(OH)3Cl sol-gel nanocrystal (oxideCu2O-derived Cu NPs reduced)

Cu disc

0.1 M KHCO3 0.1 M KHCO3

-1.07 V -0.99 V

18.3 % 26.9 %

36.4 % 66.0 %

25.0 % 1.7 %

27.2 %

this study 48

Carbon paper

0.1 M KHCO3

-1.2 V

38.5 %

31.0 %

0.03 %

52.9 %

16

” Cu plate

” 0.1 M KHCO3

-0.95 V - 1.1 V

35.8 % 33.5 %

26.0 % 32.0 %

1.5 % 4.0 %

60.3 % 33.5 %

49 37

” Colloidal synthesis ”

” Cu NP Transformed Ensemble Cu2O (Cl induced)

Cu disc Carbon paper ”

-1.0 V -0.85 V -1.8 V

32.1 % 33.2 % 23.0 %

35.0 % 38.0 % 38.0 %

0.5 % 3.0 % 1.2 %

48.1 % 55.2 % 55.1 %

38 39 19

””

” (1.7 μm) Cu2O film

””

” ” 0.1 M KCl 0.1 M 0.1CsHCO M KHCO 3 3

-0.75 -0.99VV

31.9 38.8%%

43.0 39.0% %

0.1 0.7%%

49.3 47.9% %

40

”

40 wt% Cu Mesoporous Cu2O NP/Vulcan C

Cu foam on Glassy carbon Cu wafer

0.5 M 0.1 M KHCO3 NaHCO3

-2.0 V (vs. -0.8 V Ag/AgCl)

40.5 20.0% %

29.5 16.0 % %

9.5-%

- % 55.0

12 41

””

Mesocrystal Cu nanocube Cu (44from nm) CuCl film

Cu ”disc

0.1 M ”KHCO3

-1.1 V V -0.99

41.1 26.9%%

21.0 66.0%%

20. 1.70% %

50.1 27.2%%

13 42

Cu NPs (7 nm)Cu Agglomerated nanocrystal (oxidereduced) oxide Nanostructured layer* Cu NP Transformed

” ”

0.1 M NaHCO3 ”

-1.15 V -0.95 V

5.0 % 35.8 %

15.0 % 26.0 %

64.0 % 1.5 %

5.0 % 60.3 %

50 43

Cu foil

0.1 M KHCO3

-0.9 V

60 %*

35 %*

-

-

18

Ensemble CuOzCly nanocube

Carbon paper ”

33.2 % 39.7 %

38.0 % 30.0 %

3.0 % 4.0 %

55.2 % 64.6 %

47 15

”

”

”

” ” 0.1 M CsHCO3

-0.85 V -1.05 V -0.75 V

31.9 %

43.0 %

0.1 %

49.3 %

”

40 wt% Cu NP/Vulcan C

Glassy carbon

0.1 M KHCO3

-2.0 V (vs. Ag/AgCl)

40.5 %

29.5 %

9.5 %

-

8

”

Cu nanocube (44 nm)

”

”

-1.1 V

41.1 %

21.0 %

20. 0%

50.1 %

9

”

0.1 M NaHCO3

-1.15 V

5.0 %

15.0 %

64.0 %

5.0 %

44

Electro-redeposition ” Electrodeposition

” ” O2 Plasma treatment Colloidal synthesis ”

35.0 % 0.5 % Faradaic Efficiency (%)

”

Cu NPs (7 nm)

O2 Plasma treatment

Nanostructured oxide layer*

Cu foil

0.1 M KHCO3

-0.9 V

60 %*

35 %*

-

-

14

”

CuO Cl nanocube

”

”

-1.05 V

39.7 %

30.0 %

4.0 %

64.6 %

45



To understand the interesting activity change, the morphology of the Cu-based NP/C catalyst was characterized during 10 hours of CO2RR. TEM images were taken over the elapsed time, at 1, 2, 4, 6, 8, and 10 hours of CO2RR, respectively (Figure S4). We observed that the size and morphology of the nanoparticles dramatically changes during the initial few hours, and aggregation and destruction coincided. Notably, enlarged HRTEM image of post-2h CO2RR (Figure S5) clearly showed that the distortion of the crystal planes started from the outside of particles and formed distinct domains with different orientation indicating the destruction of the nanoparticle was induced at its surface. It is also interesting that the increase of the F.E.C2H4 is greatest between one and two hours when the fragmentation begins to be observed. As the reaction time passed by 6 hours to 10 hours, the overall morphology was similar showing fragmented small nanoparticles, and F.E.C2H4 reached a plateau around 55~60 %. These TEM analysis showed high correlation of the morphological changes to the CO2RR activity. After 10 hours of CO2RR, scanning transmission electron microscopy (STEM) and HRTEM images clearly pointed out that the initial 20 nm Cu2O NPs (Figure 1a) was completely collapsed and fragmented into smaller 2~4 nm sized crystalline Cu-based nanoparticles with compact arrangement (Figures 2b and S6, especially marked by the arrows). Cu-based nanoparticles stacked on each other were more noticeable in an enlarged TEM image. The fragmented nanoparticles were difficult to distinguish on the carbon support, because of the small size and low contrast differences (Figure 2c). However, noticeable bright spots in the STEM image indicate even dispersion of copper element over the carbon support (Figure 2d), and Cu2O structure was confirmed according to the patterns (Figure 2e). Energy-dispersive X-ray spectroscopy (EDS) element mapping results also showed that the Cu elements are well dispersed with small domains over the carbon support (Figure 2f). To investigate whether simple negative potential induces the fragmentation or CO2RR is required, we performed the electrochemical reaction without CO2 gas flow, but Ar gas was flew in 0.1 M KHCO3 electrolyte, at which condition HER occurred (Figure S7). After 10 hours of HER, initial cubic morphology was strongly deformed, but the size and shape of the nanoparticle were not uniform. Also, the above-mentioned fragmentation of the nanoparticle was not obtained. This experimental result suggests that CO2RR is crucial possibly due to the interaction between the surface of the nanoparticle catalyst and the chemical species such as intermediates (i.e. *CO or its coupled OCCO* intermediates). We also found that the applied potentials (Figure S8), which affect the catalytic activity and selectivity of the products, are important to cause optimal fragmentation of the Cu2O nanoparticle. In addition, when CO2RR was conducted in the 0.1 M KCl acidic electrolyte (pH 3.9), agglomeration of the particles dominantly occurred (Figure S9). pH or the electrolyte type can influence on the crystal deformation at

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the surface. It is proposed that the adsorbate species (i.e. CO* or H*) or the negative repulsive electrostatic force on the surface can influence the thermodynamic stability of the copper surface leading to reconstruction in the opposite direction of the bulk expectation.4-5 Based on the TEM morphological analysis (Figure S4) and control CO2RR activity results, we can propose the morphological transformation of the Cu-based NP/C proceeds following three steps. Due to a low cohesive energy of the surface Cu atom, electrochemically applied potential can induce surface reconstruction of the Cu crystalline. First, the initial Cu-based nanoparticles are agglomerated to bigger crystal size induced by the negatively applied potential to decrease the surface energy, and at this stage, it has low CO2RR activity. Second, the crystal plane started to be distorted from the surface to form defects gradually through the interaction with the intermediates of CO2RR, cysteamine, and the surface copper atoms. The defective structures are expected to have high selectivity for C-C coupling reaction from CO2RR. Next, during the next few hours, the deformation of the crystal structure can be accelerated as the coverage of intermediates increases with enhanced ethylene production from CO2RR. It would reach a steady stage to have few nanometer sized particles with compact boundaries. We could found that negative potential, CO2RR, and electrolyte are important to induce fragmentation of Cu-based nanoparticles. Partial current density for each product was normalized with geometric area or ECSAs to understand activity more. The measured ECSAs of Cu-based NP/C increased as the nanoparticle was fragmented into small particles over the reaction time (Figure S10). The current density normalized by the measured ECSAs (jECSA) suggest that selectivity of CC coupling reaction increased mainly due to by the suppression of H2 production, while C2H4 production were more or less similar after 2 hours of CO2RR (Figure S11). Meanwhile, jgeometric, for C2H4 production increased during the initial CO2RR, which was mainly due to the increase of ECSA. It was also found that the decrease in H2 production current was the main reason to cause decrease in the total current density normalized by ECSAs over time.


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Journal of the American Chemical Society In addition, near the onset potential region (~ -0.9 V vs. RHE) our fragmented Cu-based NP/C catalyst can convert CO2 to CO reaching 18.5 % of F.E.CO, enhanced value compared with the Cu foil (only 3.7 % of F.E.CO) at the same applied potential, implying fragmented Cu-based NP/C can also produce CO but C-C coupling became favorable as the applied potential was more negative (Figure 3c). The potential dependent product change can be explained because adsorbed CO is considered to be the intermediate state of C2H4 generation.34-35 The partial current densities of all gas products also demonstrated that the fragmented Cu-based NP/C catalyst had the highest C2H4 producing current as well as the highest F.E. (Figure 3d). Because the

Figure 3. Electrochemical CO2RR performances of fragmented Cu-based NP/C: a) Faradaic efficiencies at various potentials for total products classified by H2, C1, C2+C3, b) accumulated bar graph for quantification of products at -1.1 V vs. RHE compared with Cu foil, c) Faradaic efficiencies for gas products at various potentials and d) corresponding partial current densities with total current density.

3.3 CO2RR activity for selective C2+ production and effective HER suppression. Fragmented Cu-based NP/C catalyst was further studied to characterize the CO2RR catalytic activity depending on the applied potentials, and both gaseous and liquid (i.e. formate, ethanol, acetate, acetone, and n-propanol) products were analyzed (Figure 3). The detailed Faradaic efficiencies for all products at various potentials list in Supplementary Table S1 showing the total Faradaic efficiency is close to 100 %. The total Faradaic efficiency of gas and liquid products for C2+ chemicals reached up to 74 %, which is 87 % selectivity over all CO2RR products. Compared with previously reported catalysts, our fragmented Cu-based NP/C shows high selectivity for C2+ products (Table 1). When total CO2 conversion rate is calculated from the partial current density of each product, it is found that CO2 conversion rate is closed to be saturated below -1.1 V vs. RHE. The saturation of the CO2 conversion rate has been reported due to the mass transport limitation of the dissolved CO2 gas in the electrolyte (Figure S12a). The impact of the mass transport is studied by varying CO2 flow rate. The product distribution especially between H2 and C2H4 was affected at -1.1 V vs. RHE, when CO2 flow rate was decreased from 20 sccm to 10 sccm, while similar production selectivity was achieved at -0.93 V vs. RHE (Figure S12b). Overall, the fragmented Cu-based NP/C exhibited the highly enhanced CO2RR catalytic activity with suppressed CH4 production and HER, compared to the Cu foil (Figure 3a-b, and S2). To be specific, the fragmented Cu-based NP/C achieved 57.3 % F.E.C2H4 with less than 2 % of F.E.CH4, resulting in a highly selective product ratio of C2H4 over CH4, while Cu foil produced more CH4 (F.E.CH4= 17.5 %) than C2H4 (F.E.C2H4= 11.2 %) at the same potential, -1.1 V vs. RHE.

Figure 4. Characterization of Cu@CuO NP/C before and after CO2RR: a) TEM image with corresponding SAED patterns, b) HR TEM image, c) XPS spectra assigned with metallic foil and d) XANES spectra of Cu K-edge compared with standard materials of Cu, Cu2O, CuO and e) Faradaic efficiency for C2H4 at -1.1 V (vs. RHE) during 10 hours, f) Faradaic efficiencies for all products at various potentials, g) TEM image, h) STEM EDS elemental mapping image after 10 hours reaction.


Figure 4. Characterization of Cu@CuO NP/C before and after CO2RR: a) TEM image with corresponding SAED patterns, b) HR TEM image, c) XPS spectra assigned with metallic foil and


Cu-based NPs were immobilized on the carbon support, the real weight percent of the loaded Cu had to be calculated. We used inductively coupled plasma-optical emission spectrometry (ICP-OES) to obtain the Cu content of the initial Cu2O NP/C catalyst. Then, the mass activity was obtained by normalizing the current density by the amount of Cu used. The mass activity of the fragmented Cu-based NP/C for C2H4 production was about -60 mA/mgCu at maximum current (Figure S13). To exam whether other nanoparticle catalysts can undergo the same in-situ electrochemical fragmentation during CO2RR, we also prepared another nanoparticles on the carbon support by the one-pot wet chemical synthesis. The same precursor solution was used, but the growth temperature and time were modified to obtain the similar size particles. The synthesized particle had a sphere-like shape (Figure 4a), and both of SAED (inset of Figure 4a) and XRD (Figure S14a) patterns correspond to the crystalline Cu. The HRTEM image exhibits the crystalline copper core with the d-spacing of 0.208 nm corresponding to Cu (111), but amorphous shell are also observed (Figure 4b). To identify the shell, XPS and XANES spectra were obtained. Surface sensitive XPS Cu 2p3/2 peak at 933.7 eV (Figure 4c) and Auger spectra (Figure S14b) indicated CuO (Cu2+) states, and XANES spectrum of Cu L-edge was consistent with that of the CuO reference (Figure S14c). Also, hard X-ray Cu K-edge spectra indicated combination of Cu and CuO (Figure 4d). Therefore, we can confirm that the metallic copper exists as a core and amorphous copper (II) oxide envelopes the outer side of the nanoparticle attached on the carbon support (Cu@CuO NP/C). CO2RR performance of Cu@CuO NP/C was measured (Figure 4ef), and only 33.2 % of F.E.C2H4 was obtained after 10 hours CO2RR at -1.1 V vs. RHE. Contrary to the fragmented Cubased NP/C, the morphology of Cu@CuO NP/C was changed but no fragmentation or compact boundary was evolved after CO2RR (Figures 4g and S15). STEM analysis showed the particles consisted of copper-rich core and oxygen-rich shell, and the average size of the nanoparticles became slightly bigger (Figure 4h). The Cu@CuO NP/C shows higher selectivity for CH4 production compared with fragmented Cu-based NP/C, and maximum F.E. for C2+ products was only 50 %. In addition, it is noted that H2 is the most predominant product in the case of Cu@CuO NP/C or Cu foil (i.e. 63.0 % of F.E.H2 at -1.1 V vs. RHE; Figure S3c-d). Compared with fragmented Cu-based NP/C, Cu@CuO NP/C has higher mass activity for C2H4 production but lower F.E.C2H4 due to high HER activity. To investigate the importance of the initial morphology, we also prepared spherical shaped Cu2O nanoparticles on the carbon black by the one-pot synthesis, and they had different morphology changes after CO2RR indicating the initial morphology of the particle is important to have the ideally fragmented nanoparticles and their arrangements (Figure S16). It is hypothesized that the special morphology of the fragmented Cu-based NP/C induces the enhancement of C-C coupling selectivity from CO2RR. When size controlled

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nanoparticles have been studied as electrocatalysts for CO2RR, electrochemical morphological transformations were sometimes observed, accompanied by changes in catalytic activity at the initial stage of the electrocatalytic reaction.19, 3637 In most cases, agglomeration of nanoparticles occurred toward larger size particles. However, our Cu-based NP/C catalyst exhibited interesting fragmentation during CO2RR, in the range from 20 nm into 2~4 nm nanoparticles stacked on each other. Our one-pot synthesis grew nanoparticles directly on the carbon support by adding the cysteamine molecules as the immobilizing agents, and sparsely dispersed Cu2O nanoparticles were obtained. In our previous study,30 cysteamine molecules were found to assist nucleation and uniform growth of nanoparticle on the carbon black powder as the immobilizing agents. XPS S 2p analysis (Figure S17) indicated the presence of the cysteamine on the catalyst surface before and after the electrochemical fragmentation. To understand the importance of the cysteamine, we also prepared Cu2O nanoparticles on the carbon black in the absence of cysteamine, but fragmented nanoparticles along with compact contact were not found after the 10 hours of CO2RR (Figure S18), implying the Cu2O NP synthesized by the one-pot synthesis has special fragmentation with the assistance of the cysteamine.

Our fragmented Cu-based NP/C catalyst exhibited selective C2+ production, and its low HER activity provided an excellence as well. We propose the improvement in CO2RR activity is associated with combined two factors, the small sized nanoparticles and their compact arrangement. We also synthesized 2 nm sized nanoparticles on the carbon support as a control and found that the simple small nanoparticles had mainly HER activity (F.E.H2= 55~75 %) and almost negligible amount of C2H4 production, and their initial morphology lost during 10 hours of CO2RR (Figure S19) which confirms that the small particle size alone do not contribute to CO2RR activity. The defective structures having strain or undercoordinated surface sites at boundaries have been demonstrated to be active sites for promoting C-C coupling.15, 24 Many of the shaped controlled nanoparticle studies were focused on Cu not copper oxide, for CO2RR, although Cu2O-derived catalysts have been reported to be active for selective C2H4 production (Table 1). Here, we prepared size and shape controlled Cu2O nanoparticles and studied its activity. In the case of the Cu2O-derived catalysts, the subsurface oxygen or Cu metal embedded in the oxidized matrix are proposed to cause high activity.16, 38. However, the stability of the residual oxygen has been questioned due to the electro-reductive environment during CO2RR.39 For instance, calculations show the subsurface oxygen is thermodynamically unstable because oxygen migration toward the surface is more favorable than remaining in the bulk Cu position.38 In contrast, Pettersson et al.38 pointed out that small nanocubes (with ca. 1.7 nm side) can have highly stable subsurface oxygen due to disordered structures and greater flexibility by calculation. Experimentally, oxygen-rich 1~2 nm surface layer on the oxide-derived Cu catalyst was reported acting as active


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Journal of the American Chemical Society lattice and be re-oxidized even inside of the electrolyte, as opposed to the as-prepared bigger Cu2O NP/C. It supports the assumption that the fragmented nanoparticles with compact arrangement are favorable to have facile oxygen access on the surface. To sum up, we demonstrate that Cu2O nanoparticles on the carbon support are electrochemically fragmented into 2~4 nm small nanoparticles in contact with each other. As the result of the special morphological changes, high CO2RR activity for C2+ products and effective suppression of HER are achieved. This can be useful for designing efficient electrocatalysts for CO2RR for valuable C2+ products.

Figure 5. In-situ XANES analysis of Cu K-edge for identifying fragmentation of Cu-based NP/C: (a) under CO2RR at -1.1 V vs. RHE condition, and (b) open circuit voltage condition in the electrolyte.

sites.40 Also, fast oxygen diffusion has been reported along the grain boundaries, which may provide oxygen access to the small Cu nanoparticles and contributing to high C-C coupling selectivity.39 According to the previous studies, small Cu2O nanoparticles of a few nanometers are expected to be especially preferable to maintain oxygen rich active sites. Therefore, the combination of the small size nanoparticle and compact arrangement may be advantageous to have selective C2+ production due to high flexibility of the nanoparticle as well as fast oxygen diffusion along the dense boundaries between particles. We conducted the XANES analyses of Cu-based NP/C during 10 hour CO2RR at -1.1 V vs. RHE (Figure 5 and Figure S20). In-situ/operando XANES spectra measured at -1.1 V vs. RHE clearly showed that metallic Cu0 is the active state under CO2RR (Figure 5a). We observed that the asprepared electrode had an oxidation state of Cu2O in the electrolyte saturated with CO2 gas, denoted as “0 h” (Figure 5b). The oxidation state (Cu2O) of the catalyst reduces towards Cu0 during CO2RR, consistent with the other previous study reporting oxide-derived Cu catalysts.18, 39, 4142 XANES was also measured under open circuit voltage (OCV) condition at each time, right after the applied potential was paused, while the electrode was keep in situ electrochemical cell (Figure 5a). During the initial period of CO2RR, metallic Cu0 state was remained during 20 min of XANES measurement (denoted as “1 h”, and “3 h”). However, notably, after the 6 hour of CO2RR, corresponding to the time when F.E.C2H4 was saturated to the maximum value and nanoparticles were fragmented into small one, the catalyst was easily re-oxidized to Cu2O at open circuit voltage condition in the electrolyte without exposing air (Figure 5b, denoted as “6 h”, “8 h”, and “10 h”). The reduced metallic Cu0 was still observed as the active state of CO2RR at - 1.1 V vs. RHE (Figure 5a) when CO2RR had conducted for 6~10 h, but it restored Cu2O state quickly when the applied potential was paused. After the end of reaction, the catalyst was exposed to the air atmosphere, and the ex-situ XANES was measured showing the fragmented catalyst had the same Cu2O state (Figure S21). These results indicate that fragmented small nanoparticle catalyst can have labile oxygen more in the

4. CONCLUSIONS Here, we demonstrated size controlled Cu2O nanoparticle catalysts have a unique morphological evolution during the initial state of CO2RR, and this in-situ electrochemical fragmentation can be utilized to develop high selectivity for C2+ chemical production. We synthesized 20 nm Cu2O nanoparticles directly grown on the carbon support by a one-pot synthesis method, and their catalytic activity was highly improved for selective C2H4 production during the first few hours of CO2RR. The initial 20 nm of Cu2O NP/C catalyst experienced morphological changes induced by the negative potential of CO2RR. The deformation of the crystal plane started from the surface of the aggregated nanoparticles which were fragmented into 2~4 nm Cu-based particles on the carbon support, and they contacted each other forming a large number of boundaries. The fragmented Cu-based NP/C catalysts reached especially high selectivity for C2H4 (F.E.C2H4=57.3 %) and C2+C3 chemicals (F.E.C2+C3 =74 %) compared to those of bare Cu foil as well as another types of Cu-based nanoparticle on the carbon support (Cu@CuO NP/C). Meanwhile, higher HER and CH4 production activity were achieved using Cu@CuO NP/C catalyst which did not evolve morphological transformation toward fragmentation. TEM and XANES analysis of Cu2O NP/C during CO2RR indicate that there were high relation among the fragmentation of the nanoparticle, Cu status, and catalytic activity. Therefore, it is proposed that the overall high performance of the fragmented Cu-based NP/C for CO2RR can be attributed to the unique morphology, combination of small sized Cu-based nanoparticles and compact arrangement, which selectively catalyzes C-C coupling and suppress HER.

ASSOCIATED CONTENT Supporting Information. Detail characterization of catalysts and their CO2RR performance, TEM morphology analysis studies, CO2RR activity studies with various control conditions are available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * [email protected] (B.K. Min) * [email protected] (Y.J. Hwang)



9.

Lee, S. Y.; Jung, H.; Kim, N. K.; Oh, H. S.; Min, B. K.;

Hwang, Y. J., Mixed copper states in anodized Cu

Notes

The authors declare no competing financial interests.

electrocatalyst for stable and selective ethylene production from CO2 reduction. J. Am. Chem. Soc. 2018, 140, 8681-8689.

ACKNOWLEDGMENT This study was supported by the program of the Korea Institute of Science and Technology (KIST) and Young Fellow program. Also, this study was supported by Next Generation Carbon Upcycling Project (No. 2017M1A2A2046713) through the National Research Foundation funded by Ministry of Science and ICT, Republic of Korea.

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