Cu2O catalyst with ultrahigh

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Article

2

Metal-organic frameworks derived Cu/CuO catalyst with ultrahigh current density for continuous-flow CO electroreduction 2

Junyu Liu, Luwei Peng, Yue Zhou, Li Lv, Jing Fu, Jia Lin, Daniel Guay, and Jinli Qiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03892 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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ACS Sustainable Chemistry & Engineering

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Metal-organic frameworks derived Cu/Cu2O

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catalyst with ultrahigh current density for

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continuous-flow CO2 electroreduction

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Junyu Liua, Luwei Penga, Yue Zhoua, Li Lvc*, Jing Fua,d*, Jia Line, Daniel Guayf, Jinli

5

Qiaoa,b*

6

aState

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College of Environmental Science and Engineering, Donghua University, 2999 Ren’min

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North Road, Shanghai 201620, China. E-mail: [email protected]

9

bShanghai

Key Laboratory for Modification of Chemical Fibers and Polymer Materials,

Institute of Pollution Control and Ecological Security, Shanghai 200092,

10

China

11

c

12

dCollege

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China

14

e

15

f INRS

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Varennes, QC J3X 1S2, Canada

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KEYWORDS: Carbon dioxide, Electrochemical reduction, Copper metal organic

18

frameworks,

Research Institute of Chemical Defense, Beijing 100191, China of Materials Science and Engineering, Donghua University, Shanghai, 201620,

College of Mathematics and Physics, Shanghai University of Electric Power, 200090

Énergie Matériaux Télécommunication, 1650 boul. Lionel Boulet, CP 1020

Flow

MEA

ACS Paragon Plus Environment

reactor

ACS Sustainable Chemistry & Engineering 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

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Abstract: The electrochemical reduction of carbon dioxide (ECR-CO2) to produce low

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carbon fuels and high-value industrial chemicals has been proven to be a viable solution to

3

energy sustainability. However, the energy efficiency of electrocatalytic CO2 reduction is

4

seriously limited by both the poor electrocatalyst with insufficient activity, selectivity and

5

stability, and ineffective electrochemical reactors. In this work, the electroreduction of CO2

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to CO is highly improved by the design of copper metal organic frameworks-derived

7

nanoparticle (Cu-MOF/NP) catalysts, in which Cu/Cu2O particles form a porous octahedral

8

structure containing tunable Cu0 and Cu+ catalytic active sites. The ECR-CO2 can be

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realized with a high current density of 25.15 mA cm-2 at a very low applied potential of

10

merely 0.79VRHE even in H-type cell, owing to the high-surface-area porous structure with

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optimal surface chemistry of exposed Cu cations. Notably, a new flow electrochemical

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reactor integrating with a membrane electrode assembly (MEA) is designed to not only

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largely reduce the applied potential ( 200mV), but also prompt the sensitivity of the reactor

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for identifying and quantifying reaction products. Accordingly, the Cu-MOF/NP catalyst

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enables an ultrahigh current density beyond 230 mA cm-2 at a low applied potential of -

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0.86VRHE in the flow MEA reactor, and the ethanol product (often undetectable in the

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traditional H-type cell) can be harvested.

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INTRODUCTION

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The greenhouse effect and energy crisis are getting worse with the burning of fossil fuels.

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In order to protect our earth, controlling the content of CO2 in the atmosphere has become

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the spotlight. Developing new clean energy to replace the original fossil energy is

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undoubtedly the key to solving the problem, and how to convert CO2 into valuable clean

7

energy is not only the focus of today's scientific research but also the necessary

8

requirements for improving the environment1-3. There are several ways to meet this

9

challenge, including chemical conversion of CO2, photocatalytic reduction of CO2,

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photoelectrocatalysis reduction of CO2 and electrochemical reduction of CO2 (ECR-CO2)

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4-5.

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energy efficiency and milder reaction conditions6. However, CO2 is one of a

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thermodynamically stable molecule that requires a large amounts of energy to activate it to

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form the CO2 radical anion (ERHE = -1.90 V)7, which is a core involved in the whole

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reaction processes. Moreover, the energy required for the hydrogen evolution reaction

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(ERHE = 0 V) is much less than the energy required to activate CO2, resulting in low

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selectivity to the desired product7. While there is considerable effort made in the quest for

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new electrocatalysts, there are still numerous challenges to create efficient and economical

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CO2 reduction catalysts including improving their often low energy efficiency and

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selectivity, reducing cost, and increasing catalyst durability.

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Among them, the ECR-CO2 is one of the most promising strategies owing to its higher

Typically, the ECR-CO2 is proceeded through multiple electron reduction reactions R5)6. Firstly, CO2•- is produced

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in mildly acidic solutions (referred to as the reactions R1

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by an one electron transfer, and then various kinds of reduction products are formed 3



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according to different adsorption capacities8-9. The main products of ECR-CO2 are small

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organic molecules such as ethylene (C2H4), methanol (CH3OH), ethanol (C2H6O) and

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inorganic carbon monoxide (CO)6. CO2+ e-

CO2•-

ERHE = - 1.9V

R1

CO2+ 2H+ + 2e-

CO (g) + H2O

ERHE = - 0.11V

R2

CO2+ 6H+ + 6e-

CH3OH (l) + H2O

ERHE = + 0.02V

R3

CO2+ 8H+ + 8e-

CH4 (g) + 2H2O

ERHE = + 0.17V

R4

ERHE = + 0.08V

R5

2CO2+ 12H+ + 12e-

CH3CH2OH + 3H2O

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In order to produce these high value-added chemicals, the most critical step is to

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convert CO2 to CO10-14. In addition, the as-produced CO can be used with H2 as synthesis

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gas for Fischer-Tropsch reactions and methanol synthesis15-17. Therefore, many efforts

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have been devoted to the development of electrocatalysts for CO2 conversion to CO

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through the engineering of their chemical composition18-20, morphology21, and structures22-

9

23.

However, despite of the progress in catalysts’ exploring, the energy efficiency of the

10

CO2 conversion to CO is still much low because large overpotentials (e.g.,

-1.0 mA

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cmL at -0.6 VRHE)20 are often required during the reduction reaction, which cannot meet

12

the needs for practical applications.

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Compared to the acidic electrolyte system, recent studies seeking on ECR-CO2

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catalysts have been targeted to the alkaline aqueous electrolyte system due to higher

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solubility of CO2 and suppression of hydrogen evolution24-25. In this regard, metals such as

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Co26, Pb27, Sn28, Bi29, In8 and Cu30-32 have shown capability to reduce CO2 for the 4


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production of formate or formic acid. Among them, Cu is particular promising in reducing

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CO2 to CO in alkaline solutions owing to its earth abundance and high electrical

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conductivity. It is worth to mention that Cu is the only metal catalyst that can further reduce

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CO to C2, C3 and multiple carbons32-33. Nevertheless, both the activity and selectivity of

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the Cu catalyst needs to be further improved18-20.

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Metal organic frameworks (MOF) and their derivatives with open porous structures

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have been demonstrated to increase the metal active sites for improved electrocatalytic

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performance. Zheng et al34 reported the MOF-derived nitrogen-doped nanoporous carbon

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(NC) as a highly efficient catalyst for the conversion of CO2 to CO. The NC catalyst with

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the best performance achieves high selectivity with 95.4% CO faradaic efficiency (FE) at

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L+.9 VRHE. Kim et al35 have recently developed MOF-derived Cu nanoparticles (NPs)

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catalyst by electrochemical reduction of Cu-based MOF-74, which endows high faradaic

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efficiency (>50%) for the CH4 production. In addition, Albo et al36 synthesized the

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HKUST-1 MOF catalyst by temperature-assisted solvent-free route, which shows unique

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advantages in the ECR-CO2 for producing methanol and ethanol. Despite of work applying

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the MOF-templated method to prepare ECR-CO2 catalyst, the use of this approach to

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improve the catalytic activity for CO2 electroreduction to CO has rarely been concerned in

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the literature.

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In addition to the catalyst development, the design and the engineering of the

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electrochemical reactor are crucial in the ECR-CO2 performance improvement. In this

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regard, Lee et al37 reported a H-type cell consisting of a cathode chamber and an anode

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chamber that are separated by an ion-exchange membrane. The H-type cell is widely used

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because of its simplicity in fabrication and effectively protected from the oxidation of the 5



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reduced product38. However, limitations of this traditional H-type cell, such as the large

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electrode distance, low accumulation of liquid products, and large pH gradient near

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electrode surfaces need to be spontaneously addressed for realizing the electrochemical

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CO2 reduction technology.

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Other types of ECR-CO2 reactors such as the membrane electrode assembly (MEA)-

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integrated stationary reactor39, where CO2 gas flows directly to the catalyst (instead of

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purging into the electrolyte) and is reduced to form CO, water and H2, have been reported

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to shorten the electrode distances, thereby reducing the internal resistance and ohmic

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polarization. Unfortunately, as the product gas (e.g., CO, H2, O2) generation increases at

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high currents in both the cathode and anode, a thin gas film is formed on the surface of the

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electrodes, which hinders the accessibility of the electrolyte to the catalyst and prevents the

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reduction reaction from further proceeding. Moreover, the identification and quantification

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of gas products has always been an issue in these electrochemical reactors, since the gas

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products always enters the gas chromatography along with the reaction gas (i.e., CO2), and

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the exact content of the gas products is difficult to be detected when the gas production rate

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is low40-42. Therefore, it is necessary to design more advanced reaction devices to enable

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wide integration of ECR-CO2 for practical applications.

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Inspired by the above conceptions, in this work, a Cu-MOF-derived nanoparticle

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catalysts system and a flow MEA reactor with separation of reaction CO2 gas and gas/liquid

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products have been innovatively established for the electroreduction of CO2 to CO. Such

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reactor is designed to not only reduce the electrode distance and therefore the overall ohmic

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polarization of the system, but also increase the sensitivity of identification and

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quantification of the gas products by increasing the product yield. The as-prepared Cu6


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MOF/NP catalysts reveal an outperformed electrochemical performance in the flow MEA

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reactor, demonstrated by a high current density of 34.97 mA cm-2 at a very low applied

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potential of -0.64 VRHE, which is 2.8 times greater than that obtained in the traditional H-

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type cell. An ultrohigh current density beyond 230 mA cm-2 was realized at a low applied

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potential of -0.87VRHE. Specifically, in the gas/liquid separatied flow MEA reactor, the

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interference of the CO2 gas is further eliminated by separating the gas/liquid products and

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the reaction gas (CO2), which stimulates the measurement accuracy of the ECR-CO2 gas

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product largely enhanced.

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RESULT AND DISCUSSION

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Physical characterization of Cu-MOF20/300

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The schematic of the synthesis procedure of the Cu-MOF-derived nanoparticle (Cu-

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MOF/NP) catalysts is illustrated in Figure 1a. Cu-MOF octahedral nanoparticles were

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firstly synthesized by mixing the Cu(NO3)2 3H2O, 1,3,5-benzenetricarboxylic acid (H3BTC)

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and polyvinylpyrrolidone (PVP) in methonal at room temperature for 5 h. Then Cu-MOF

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octahedrons were pyrolyzed in N2 gas at 300 oC with a heating rate of 20 oC/min. During

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this process, Cu-MOF octahedrons were reduced to a heterogeneous hybrid of Cu and

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Cu2O, and the relative proportions of Cu0/Cu+ of the pyrolyzed samples is tunable by

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different holding temperatures and heating rates. The Cu-MOF octahedrons treated with

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different temperatures (referred to as y oC) and heating rates (referred to as xoC/min) during

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the pyrolysis were thus prepared and denoted as Cu-MOFx/y. The octahedral morphology

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of the Cu-MOF is confirmed by the scanning electron microscope (SEM) (Figure 1b). 7



1

After pyrolyzing the Cu-MOF octahedrons at 300 oC with a heating rate of 20 oC/min, their

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octahedral structures are still largely maintained by forming nanoparticles on the surfaces

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(Figure 1c). The crystal structure of the obtained Cu-MOF20/300 was examined by X-ray

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diffraction (XRD). As shown in Figure 1d, the diffraction peaks of the Cu-MOF20/300 at

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43.30o, 50.45o and 74.12o can be indexed to the (111), (200) and (220) facets of Cu (JCPDS,

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99-0034), and the diffraction peaks at 36.42o, 42.28o and 61.40o can be indexed to the (111),

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(200) and (220) facets of Cu2O (JCPDS, 99-0041), respectively. When observed under

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transimission electron microscope (TEM), these nanoparticles are anchored and distributed

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on the carbon structure that is derived from the MOF carbonization (Figure 1e). The high-

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resolution TEM image (Figure 1f) shows regions of two crystal lattice fringes (111) of Cu

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and (220) of Cu2O, which are consisted with the XRD results. The energy dispersive X-

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ray spectroscopy (EDX) analysis of the Cu-MOF20/300 catalyst (Figure 1g) reveals that the

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central skeleton is dominated by Cu, while the outer surface is composed mainly of carbon

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and oxidized Cu fine particles.

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Further investigation into the chemical composition of the Cu-MOF20/300

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nanoparticles was conducted by using the X-ray photoelectron spectroscopy (XPS). The

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high-resolution Cu 2p XPS spectrum of the Cu-MOF20/300 is shown in Figure 2a. The peaks

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of the Cu 2p at 932.20 and 951.90 eV are assigned to the binding energies of Cu 2p3/2 and

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Cu 2p1/243. The peaks of Cu 2p1/2 can be deconvolued into two peaks of 951.43 and 952.15

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eV, corresponding to Cu0 and Cu+, respectively. The peaks of Cu 2p3/2 can be deconvolued

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into two peaks of 931.54 and 932.26 eV, corresponding to Cu0 and Cu+, respectively44-45.

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The O 1s peak at 530.76 eV, as shown in Figure 2b, can be indexed to the lattice O of the

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Cu2O46, while the peak at 532.46 eV is related to the C-O-H from the residue oxidized 8


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carbon47. Moreover, Cu 2p XPS spectra for Cu-MOF/NP treated at different heating rates

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and temperatures are shown in Figure S1 and S2. When holding at the same temperature,

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the relative ratio of Cu0/Cu+ of the Cu-MOF/NP samples decreases as increased heating

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rate. When the heating rate keeps the same, the relative ratio of Cu0/Cu+ of the Cu-MOF/NP

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samples increases as the holding temperatures increase. From Figure S1 and S2, it can be

6

clearly seen that Cu-MOF10/300, Cu-MOF20/300 and Cu-MOF20/350 catalyst exhibit the

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relative ratio Cu0/Cu+ of 0.92, 0.52 and 0.99, respectively. This result suggests that the

8

increase of heating rate favours for the formation of Cu+ chemcial state, whereas Cu+

9

decreases as the temperature increased. XRD pattern (Figure S3) further confirms the

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trends in the above materials. When the heating rate is low of 1 oC/min, the XRD patterns

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of the Cu-MOF1/300 and Cu-MOF5/300 only show the diffraction peak of Cu. Intriguingly,

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the diffraction peak of Cu2O appears as the heating rate increased, which is in a well

13

agreement with the Cu compsotion trend from the XPS analysis. It is confirmed that a high

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heating rate does promote the formation of Cu+, as the XRD of the Cu-MOF20/300 reveals

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more defined Cu2O crystal structure compared to Cu-MOF10/300. When increasing the

16

temperatures, the relative ratio of Cu0/Cu+ of the Cu-MOF/NP samples declines and the

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originally existed Cu2O diffraction peaks disappear accordingly, indicating that high

18

temeprature condition is unfavourable for the formation of Cu2O.

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Figure 2c shows the nitrogen adsorption-desorption isotherm of the Cu-MOF20/300,

20

which is similar to isotherm of type II and IV 48 and dense peaks in the range of 0-40 nm.

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This suggests that the catalysts have a graded porous structure. The specific surface area

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of the Cu-MOF20/300 is calculated to be 129.11 m2g-1 based on the Brunauer–Emmett–Teller

23

(BET) analysis. The pore size distribution plot (Figure 2d) demonstrates the dominated 9



1

mesopores in the Cu-MOF20/300, which is favour for ion transport. Moreover, we found that

2

as the heating rate increased, the specific surface area of the samples increases also

3

gradually. As can be seen in Figure S4 and S5, the specific surface area of the Cu-

4

MOF10/300 catalyst was 123.54 m2g-1, which is smaller than that of the Cu-MOF20/300.

5

Further from the SEM images of the Cu-MOFx/300 (Figure S6), it was observed that the

6

porous octahedral structure is gradually covered with more nanoparticles as the heating

7

rate increases. When changing the temperatures, the specific surface area of the Cu-

8

MOF20/350 catalyst was reduced to 61.10 m2g-1, and further reduced to 52.87 m2g-1 at 400

9

oC.

SEM images (Figure S7) indicates that the increase in temperature tends to stimulate

10

the agglomeration of the fine particles and subsequent collapse of the octahedral structure.

11

Evidently, the high heating temperature would cause damage to the structure of the catalyst,

12

which in turn results in a reduction in the specific surface area of the catalyst.

13 14

Electrocatalytic activity of Cu-MOF20/300 catalyst for ECR-CO2

15

To investigate the electrocatalytic activity for ECR-CO2, three different Cu-MOFx/y

16

electrodes were fabricated in both the traditional H-type cell(Figure S8) and the homemade

17

flow MEA reactor. Figure S9 illutrates the design of the homemade flow MEA reactor,

18

where the cathode and the anode are separated by a Nafion membrane, similar to the

19

traditional H-type cell. It should be noted that the working electrode and the counter

20

electrode are in close contact with the Nafion membrane, hence the distance between the

21

electrodes is significantly shortened. Besides, the flow structure promotes efficiently

22

contact of the electrodes with the electrolyte. Figure 3 (a and b) provides linear sweep

23

voltammetry (LSV) curves for the Cu-MOF/NP catalysts which were synthesized at 10


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1

different temperature and different heating rate in the H-type cell. As the temperature

2

changed, the electrodes activity exhibites a trend of Cu-MOF20/300 > Cu-MOF20/350 > Cu-

3

MOF20/400 > Cu-MOF20/500, whereas for the different heating rate, the electrodes activity

4

increases as the heating rate increased. Among these electrodes, the Cu-MOF20/300 electrode

5

manifests the most positive onset potential of -0.29 VRHE (inset in Figure 3a), indicating

6

its improved ECR-CO2 kinetics compared to those of the Cu-MOF10/300 and Cu-MOF20/350

7

electrodes, respectively. Additionly, the Cu-MOF20/300 electrode provides a significant

8

higher current density than that of the Cu-MOF10/300 and Cu-MOF20/350 electrodes within

9

the tested potential range. Compared to the previous report18, the current density of the Cu-

10

MOF20/300 is profoundly strengthened by ~20 times with the same faradaic efficiency of 45%

11

for CO production. For a clear understanding of the improved performance of the Cu-

12

MOF20/300 catalyst, specific ECR-CO2 activities of the Cu-MOF10/300, Cu-MOF20/300 and

13

Cu-MOF20/350 catalyst normalized by the ECSA were further evaluated. As shown in

14

Figure 3c, a similar performance improvement trend was observed to the previeous result

15

in Figure 3b, where the Cu-MOF20/300 catalyst exchibits a higher catalytic activity than the

16

Cu-MOF10/300 one. Because of a similar ECSA of these two catalysts as indicated in Figure

17

S10, the better performance of the Cu-MOF20/300 catalyst can be attributed to its relatively

18

higher content of Cu+ on the catalyst surface (Figure 2a). Moreover, XPS analysis in

19

Figure S1 reveals that the relative ratio of Cu0/Cu+ in Cu-MOF10/300 (1:1) is greater than

20

that of the Cu-MOF20/300 (0.5:1). Therefore, it is reasonably to infer that the Cu+ plays an

21

important role in enhancing the intrinsic activity of the Cu-MOF20/300 catalyst. Figure 3d

22

shows a similar trend of the improved activity when comparing the Cu-MOF20/300 and Cu-

23

MOF20/350 catalysts. Given a similar relative ratio of Cu0/Cu+ in these two catalysts (Figure 11



1

S2), the higher performance of the Cu-MOF20/300 catalyst is related to its larger ECSA

2

(Figure S11) and BET surface area (Figure S5), further confirming that the highest

3

performance of the Cu-MOF20/300 catalyst is benefit from its more Cu+ active sites.

4

Figure 4a shows the homemade flow MEA reactor where the Cu-MOF20/300 catalyst

5

was furhter examined. The CO2 gas is purged into the KHCO3 solution through the fillable

6

gas-liquid separation device and then the saturated KHCO3 solution flows to the MEA

7

reactor by using the pump. After the electrochemical CO2 reaction, the KHCO3 solution

8

with liquid product and gas product are input together to the fillable gas-liquid separation

9

device, then the gas products are detected by gas chromatography through gas outlets.

10

Instead, the liquid product is mixed with CO2 saturated KHCO3 and recycled into the flow

11

MEA reactor through the pump to realize the circulating flow (details are shown in Figure

12

S9). The LSV curves were performed under CO2 saturated KHCO3 solution, as shown in

13

Figure 4b. It can be seen that with the flow of the electrolyte, the current density of the

14

ECR-CO2 is remarkably improved within the tested potential range. Compared with the

15

traditional static H-type cell (Figure S8), the current density of the flow MEA reactor was

16

enhanced by 22.5 mA cm-2 at a very low applied potential of merely -0.66V, about ~3 times

17

greater than that of the H-type cell. Notably, the onset potential of the flow MEA reactor

18

is positively shifted by approximately 200 mV when compared to the H-type cell. The main

19

reason can be explained by the fact that the gas product generated on the surface of working

20

electrode flows out of the reactor as the electrolyte flows into MEA reactor. Therefore, the

21

gas diffusion resistance is largely reduced, which in turn enhances the activity of the

22

catalyst.

12


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1

To further explain the principle of the flow MEA reactor, the electrochemical

2

impedance spectroscopy (EIS) of the Cu-MOF20/300 catalyst at -0.17 VRHE was analyzed.

3

As shown in Figure 4c, the ohimic resistance of the flow MEA reactor (0.49 ohms) is much

4

lower than that of the H-type cell (1.75 ohms), demonstrating its superiority of reducing

5

the electrode distance compared to the H-type cell. Moreover, the charge transfer resistance

6

of the flow MEA reactor decreases remarkably as the flow rate of the electrolyte increases

7

from 0 to 16 mL min-1. However, as the flow rate of the electrolyte increases to 20 mL

8

min-1, the charge transfer resistance also increases. This may be caused by the fact that the

9

high flow rate of the electrolyte inhibits the accessibility of reaction species to the electrode

10

surface49. On the contrary, the flow MEA reactor realizes the highly increased current

11

density of the electrochemistry reaction, and reduces the energy loss during the

12

electrochemistry reaction.

13 14

Product analysis

15

In order to further illustrate the effect of the flow MEA design on the electrochemical

16

CO2 reaction, the faradaic efficiency of CO (FECO) was measured at controlled various

17

potentials (-0.36, -0.46, -0.56 and -0.66 VRHE), as shown in Figure 4d. For a comparison,

18

in the traditional H-type cell, only the FECO can be measured at controlled various

19

potentials (-0.56, -0.66, -0.76 and -0.86 VRHE). Under two different measuring conditions,

20

both CO and H2 were detected, along with very small amount of CH4 (Figure 4d). The

21

FECO obtained from the H-type cell is 6.3% at -0.56V and increases to 43.8% at -0.76 VRHE

22

(Figure S12a). In contrast, the flow MEA reactor shows FECO of 6.1% at -0.36 VRHE,

23

which increases to 39.6% at -0.56 VRHE (Figure S12b), with an applied potential positively 13



Page 14 of 28

1

shifted by 200 mV when compared to the traditional H-type cell. The energy efficiency is

2

largely improvd by 6.3 times (30%) therein by Eqn 150. Surprisingly, the FECH4 (1.2%) and

3

FEethanol (4.1%) are also harvested at a relative high current density at -0.66 VRHE.

4

According to the literature, the current density of CO2 conversion to CO is very low, and

5

it is often unfavourable for achieving conversion at high currents18-20, 37, 51 (Table S1).

6

Nevertheless, the flow MEA reactor can improve the electrochemical activity and enhance

7

sensitivity of the quantification of ethanol products. The current density can reach to 230

8

mA cm-2 at -0.86 VRHE (Figure 4e), superior to most recently reported in literatures52-54.

9

To the best of our knowledge, it is the highest current density realized based on MOF-Cu

10

catalyst for ECR-CO2. Additonly, it was found that over the wide potential ragnge from

11

the -0.36 to -0.66 VRHE, the ratio of CO and H2 could be readily tuined from 0.06 to 0.80

12

(Figure 4d), implying that the as-produced CO can be used with H2 as synthesis gas for

13

Fischer-Tropsch reactions and methanol synthesis15-16. Evidently, the flow MEA reactor

14

can effectively reduce the energy loss of the electrochemical reaction, meantime, improve

15

the identification and quantification of gas and trace liquid products. These results highlight

16

strongly comparable catalytic activity of MOF-Cu/NP and the flow MEA reactor with

17

separation of reaction CO2 gas and gas/liquid products for realizing efficient ECR-CO2,

18

and provides a more efficient product collection and detection in future industrial

19

production systems. 0

20

E1

E1: EE(%) =

21 22

CONCLUSIONS 14


Page 15 of 28 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


1

In this work, Cu-MOF/NP catalysts were prepared by effective morphological and

2

compositional control through the MOF-templated approach. The Cu-MOF20/300

3

nanoparticles, with optimal Cu/Cu2O heterogeneous structure and high active surface area

4

with active Cu+ sites, stand out as the efficient electrocatalyst for electrochemical CO

5

production. As a result, the Cu-MOF20/300 catalyst manifests a high FEco of 43.8%. Notably,

6

the flow MEA reactor with the fillable gas-liquid separation component has been

7

successfully fabricated and exhibits superior electrochemical reaction performance to the

8

traditional H-type configuration, when conducting the same electrochemical tests. A

9

current density in exceeding 230 mA cm-2 at a low applied potential (-0.86VRHE) is

10

achieved owing to a more compact reactor structure with lower ohimic and charge transfer

11

resistances. Overall, the flow MEA reactor greatly shortens the distance between the

12

electrodes and not only significantly improve the current density of the CO2

13

electrocatalysis, but also the energy efficiency has increased nearly 6 times. The fillable

14

gas-liquid separation component increases the sensitivity of the reactor and can identify

15

more subtle reaction products, therefore higher sensitivity in identifying and quantifying

16

trace reduction products.

17 18

ASSOCIATED CONTENT

19 20

Supporting Information: Experimental Section, Table S1 and Figure S1–S12.

21

AUTHOR INFORMATION

22

Corresponding Author

15



+86-21-67792379.

Fax:

+86-21-67792159.

1

Tel:

2

[email protected](JF); [email protected] (JLQ)

3

ACKNOWLEDGMENT

4

The authors thank the financial support from the National Natural Science Foundation of

5

China (91645110) and the Fundamental Research Funds for the Central Universities

6

(2232018A3-06).

16


E-mail:

Page 16 of 28

[email protected];

Page 17 of 28

a

Cu2+ COOH

COOH N

Self-assembly

Pyrolysis 300oC

5h

O Cu-MOF

n

c

d

1 m

500 nm

(111) (220)

40

60

80

c

100 nm

Cu

f

(220)

2 theta / degree

g

100 nm

(200)

—Cu PDF # 99-0034 —Cu2O PDF # 99-0041

20

e

Cu Cu2O

(111)

Intensity / a.u.

b

Cu-MOF20/300

(200)

COOH

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


O

10 nm

Figure 1 (a) Illustration of the synthesis procedure for the Cu-MOF/NP catalysts. (b) SEM image of the Cu-MOF catalyst. (c) SEM image of the Cu-MOF20/300 catalyst. (d) XRD patterns of the CuMOF20/300 catalyst. (e) TEM and (f) High-resolution TEM images of the Cu-MOF20/300 catalyst. (g) EDX mapping of the Cu-MOF20/300 catalyst.

17



a

b

O 1s

0

Cu 2p 3/2

Cu 2p 1/2 Cu 2p 1/2

satellite peak

935

940

945

950

955

960

Binding Energy (eV)

d

200

SBET=129.112m2 g-1 160 120 80 40 0

0.0

0.2

0.4

0.6

Cu-O

Intensity / a.u.

0

0.8

1.0

525

Cumulative Pore Volume / cm-3 g-1

Intensity / a.u.

c

+

Cu+ 2p 3/2

930

Quantity Adsorbed (cm-3 g-1 STP)

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

Page 18 of 28

C-O-H

530

535

540

Binding Energy (eV) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

10

20

30

40

50

60

Pore width / nm

Relative Pressure (p/p ) 0

Figure 2 XPS spectra of (a) Cu 2p and (b) O1s for the Cu-MOF20/300 catalyst. (c) N2 adsorption-desorption isotherm and (d) the pore diameter distribution of the Cu-MOF20/300 catalyst.

18


Page 19 of 28

b

Current density / mA cm-2

5 0 0

-5

-1 -2

-10

-3 -4

-15

-5

-20

c

-0.2

300 400

-25 -0.8

-0.4

-0.6

-0.4

-0.2

350 500

0.0

0.2

0 -5

-10 -15 1 /min 5 /min 10 /min 20 /min

-20 -25

0.4

-0.8

-0.6

Potential / V vs. RHE

-0.2

0.0

0.2

0.4

d0

0 -2 -4 -6 -8 10oC/min 20oC/min

-10 -12 -0.8

-0.4




a


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


-0.6

-0.4

-0.2

0.0

0.2

-2 -4 -6 -8 -10 -12 -0.8

0.4

300oC 350oC

-0.6


-0.4

-0.2

0.0

0.2


Figure 3 (a) LSV curves of the Cu-MOF/NP catalysts treated at different temperatures with the same heating rate of 20 oC/min.

(b) LSV curves of the Cu-MOF/NP catalysts treated at 300 oC with different heating rates. (c) LSV curves of the

Cu-MOF10/300 and Cu-MOF20/300 catalyst with currents normalized by electrochemical active surface area. (d) LSV curves of Cu-MOF20/350 and Cu-MOF20/300 catalyst with currents normalized by electrochemical active surface area. Electrolyte: CO2-saturated 0.5 M KHCO3 (pH=7.4).

19




Page 20 of 28

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For Table of Contents Use Only

Adjustable Cu-MOF/NP structures for CO2 reduction to CO through a homemade MEA flow reactor for realizing ultrahigh current density and detectable ethanol product.


Page 28 of 28

Cu2O catalyst with ultrahigh

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