An efficient 3D ordered mesoporous Cu sphere array electrocatalyst for carbon dioxide electrochemical reduction

Journal Pre-proof An efficient 3D ordered mesoporous Cu sphere array electrocatalyst for carbon dioxide electrochemical reduction Jun-Tao Luo, Guo-Long Zang, Chuang Hu

PII:

S1005-0302(19)30448-7

DOI:

https://doi.org/10.1016/j.jmst.2019.08.059

Reference:

JMST 1845

To appear in:

Journal of Materials Science & Technology

Received Date:

12 June 2019

Revised Date:

9 August 2019

Accepted Date:

27 August 2019

Please cite this article as: Luo J-Tao, Zang G-Long, Hu C, An efficient 3D ordered mesoporous Cu sphere array electrocatalyst for carbon dioxide electrochemical reduction, Journal of Materials Science and amp; Technology (2019), doi: https://doi.org/10.1016/j.jmst.2019.08.059

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An efficient 3D ordered mesoporous Cu sphere array electrocatalyst for carbon dioxide electrochemical reduction

Jun-Tao Luo, Guo-Long Zang*, Chuang Hu

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Tianjin Key Lab of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, No. 92 Weijin Road,

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Nankai District, Tianjin 300072, China

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*Corresponding author:

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E-mail: [email protected] (Guo-Long Zang).

Abstract

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The electrochemical reduction of CO2 is a promising solution for sustainable energy research and carbon emissions. However, this solution has been challenged by the

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lack of active and selective catalysts. Here, we report a two-step synthesis of 3D

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ordered mesoporous Cu sphere arrays, which is fabricated by a dual template method using a poly methyl methacrylate (PMMA) inverse opal and the nonionic surfactant Brij 58 to template the mesostructure within the regular voids of a colloidal crystal. Therefore, the well-ordered 3D interconnected bi-continuous mesopores structure has advantages of abundant exposed catalytically active sites, efficient mass transport, and high electrical conductivity, which result in excellent electrocatalytic CO2RR 0

performance. The prepared 3D ordered mesoporous Cu sphere array (3D-OMCuSA) exhibits a low onset potential of -0.4 V at a 1 mA cm−2 electrode current density, a low Tafel slope of 109.6 mV per decade and a long-term durability in 0.1 M potassium bicarbonate. These distinct features of 3D-OMCuSA render it a promising method for the further development of advanced electrocatalytic materials for CO2

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reduction.

Keywords: 3D ordered mesoporous Cu sphere array electrocatalyst; copper

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nanoparticles; dual-template method; CO2 electrochemical reduction reaction

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1. Introduction

The ever-increasing consumption of fossil fuels has accelerated the overproduction of

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carbon dioxide, which is generally accepted to have a significant impact on climate [1-7]

. To minimize carbon emissions and mitigate this

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and environmental conditions

impact, different measures, such as capturing and geologically sequestering CO2 or

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converting CO2 into useful low-carbon fuels, have been proposed during the past few decades

[7,8]

. Among these technologies, the electrochemical reduction of CO2 to high

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energy density hydrocarbons (such as CH4, C2H4, HCOOH and CH3OH) using renewable energy (such as wind and solar) has attracted considerable attention and is considered a promising method for closing the anthropogenic carbon cycle

[9,10]

.

However, due to the fully oxidized and thermodynamically stable CO2, the impediments of the slow kinetics of CO2 electroreduction, low energy efficiency of 1

the process and high energy consumption have hindered the widespread adoption of this technology

[11]

. Researchers have recognized that a promising catalyst for CO2

electroreduction is an urgent requirement to achieve high energy efficiency, high current density and high selectivity [12]. Metal materials are well known to be capable of catalytic function and have been universally investigated for their potential application in the catalysis field

[13]

. For

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example, Cao et al. have shown that at an overpotential of approximately 0.46 V versus the reversible hydrogen electrode (RHE), the N-heterocyclic (NHC) carbene-functionalized Au NP can exhibit improved faradaic efficiency (FE = 83%) [14]

. A nanoporous silver electrocatalyst,

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towards carbon monoxide (CO) production

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which is able to electrochemically reduce carbon dioxide to carbon monoxide with an approximately 92% selectivity under moderate overpotentials of <0.50 V was [15]

. A highly efficient, low-cost and stable Cu-CDots nanocoral

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reported

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electrocatalyst for CO2 reduction was reported by Guo et al., achieving an inconceivably low overpotential of 0.13 V and a high Faraday efficiency of 79% at a [16]

. Among the currently identified

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moderate potential of −0.7 V vs. RHE

electrocatalyst materials, Cu is the most unique candidate that can reduce CO2 to

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products ranging from C1 compounds (e.g., CO, HCOO−, and CH4) to C2 compounds (e.g., C2H4, C2H5OH), which is in sharp contrast to other metals that produce mostly one major C1 (CO, CH4 or HCOO−) product.

[17,18]

To use its attractive properties, a

number of measures have been applied to tune and promote the catalytic ability of Cu. By controlling the structure (e.g., specific surface area 2

[19]

, morphology [20], surface

crystal facet overlayers

[21,22]

[26,27]

and grain boundary

[23]

), surface modification

[24,25]

, Cu metal

and local electrolysis environments (e.g., electric field

electrolytes [29], temperature

[30]

, CO2 pressure and local pH evolution

[31]

[28]

,

), a series of

copper-based catalysts with high energy efficiency and product selectivity have been reported. Among these studies, the morphological effects of copper catalysts have a significant effect on the selectivity and energy efficiency. Using boron to tune the

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ratio of the Cuδ+ to Cu0 active sites of copper, Zhou et al. found that not only were

both the stability and C2-product generation improved but also CO adsorption and dimerization was controlled, which makes it possible to implement a preference for [32]

. As the reverse process of crystal growth,

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the electrosynthesis of C2 products

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crystal etching was used by Wang et al. to etch Cu nanocubes to obtain enriched high-energy {110} facets with significantly enhanced activity towards CO2 reduction . A class of core–shell vacancy engineering catalysts designed with a structure of

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[33]

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sulfur atoms in the nanoparticle (NP) core and copper vacancies in the shell was produced by Wang et al., which achieved a C2+ alcohol production rate of 126 ± 5 [34]

. Using Cu NP

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mA cm−2 with a selectivity of a 32 ± 1% faradaic efficiency

catalysts with a mean size range from 2−15 nm, Reske et al. (2014) revealed the

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presented activity−selectivity−size relations, which provide novel insight into the CO2 electroreduction reaction on nanoscale surfaces

[35]

. However, although remarkable

progress has been achieved for the morphological effects of copper nanocatalysts, the durability, catalytic selectivity and selectivity for diversiform products should be substantially improved before meeting the requirements of practical applications 3

[36,37]

.

Herein, we demonstrated a 3D ordered mesoporous Cu sphere array (3D-OMCuSA) electrocatalyst with a precisely controlled morphology via a reduction process using poly methyl methacrylate (PMMA) inverse opal as a hard template. Our 3D-OMCuSA electrocatalyst exhibited a significantly enhanced CO2RR performance due to the increased surface area, porous structure and excellent mass transport

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properties. The 3D-OMCuSA catalyst exhibited a high activity for CO2 reduction with an unprecedentedly low average onset potential of -0.4 V (vs. RHE). Moreover, this

catalyst displayed a 17-fold enhancement in the current densities obtained at -0.60 V

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(versus RHE) compared to copper nanoparticles, along with a 9.4-fold increase in the

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electrochemical surface area. Through a specific activity analysis and an electro-hydro dynamics study, the enhanced activity and improved product selectivity

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could be mainly attributed to massive active sites resulting from the increased surface

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area. Additionally, these materials are robust enough to maintain long-term stability without significant loss in activity, even when exposed to external conditions and

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reintroduced into fresh electrolyte for a full round of CO2 electroreduction. Apart from providing insight into the reaction mechanisms, this study also provides a major

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step forward towards the realization of the industrial-scale reduction of CO2. 2. Experimental 2.1. Fabrication of the Cu inverse opal structures 2.1.1 Preparation of the silica colloidal crystal (opal) Monodispersed silica spheres were prepared by published techniques 4

[38]

. TEOS

hydrolysis methods were used to produce monodispersed silica spheres with diameters of 200-300 nm. The spheres were ultrasonically dispersed in absolute ethanol for 30 min and centrifuged at 3500 rpm for 10 min to remove large particles. The remaining suspension was adjusted with ethanol to a 0.5% weight fraction of SiO2. ITO glass slides cut to 10 mm × 50 mm were pretreated by immersion in freshly

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prepared piranha (preparation: 70 mL concentrated sulfuric acid was slowly added to 30 mL hydrogen peroxide; caution: piranha solutions are potentially explosive if the hydrogen peroxide volume rises above 50 mL) without additional heating for one day.

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After ultrasonically rinsing with copious amounts of ultra-pure water and ethanol,

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ITO glass slides were vertically inserted into a 10 ml vial filling with 10 ml of the above-described silica-ethanol dispersion. Placed in a dry, dust-free oven at 45 °C, the

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was evaporated.

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glass slides grow with the SiO2 photonic crystal template when the absolute ethanol

The SiO2 photonic crystal template obtained in the self-assembly process usually has

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loose binding between the colloidal microspheres and is easily dispersed. Therefore, heat treatment at a certain temperature, which aims to slightly melt and connect the

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contact portions of the colloidal microspheres, is usually required to fix the colloidal microspheres. In this way, not only the mechanical strength of the microsphere template is increased but also the hole can be directly connected. The silica template was treated in a muffle furnace at 550 °C for 3 h at a heating rate of 5 °C min-1. 2.1.2 Preparation of PMMA inverse opal 5

A piece of silica opal template was horizontally immersed on the bottom of the weighing bottle containing a certain amount of the purified methyl methacrylate (MMA) monomer with an initiator of 1 wt% BPO. Polymerization took place at 40 °C for 12 h and then increased to 60 °C for 6 h. After polymerization, the liquid MMA was converted to solid PMMA to complete the filling SiO2 colloidal crystal template, and excess PMMA on the surface of opal was wiped with CH2Cl2. The

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silica opal spheres were etched for 8 h in a 10 wt% HF solution to obtain a freestanding PMMA inverse opal from the substrate, which floated on the HF liquid surface.

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2.1.3 Preparation of the 3D-OMCuSA

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To form a homogeneous templating precursor solution, 8.524 g CuCl2·2H2O was dissolved in 10 mL deionized water and ultrasonically dispersed for 30 min at room

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temperature to adjust the concentration of Cu2+ 5 mol/L. Then, 12.22 g nonionic

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surfactant Brij 58 (C16H33(OCH2CH2)20OH) was added into the solution to adjust the concentration of Brij 58 to 60 wt% (Brij 58/ (water + Brij 58)). The mixture was

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stirred at 80 °C for 12 h to obtain a homogeneous templating precursor solution. The PMMA inverse opal template was then immersed into the precursor solution at 80 °C

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for 12 h so that the mixed solution was immersed with the spherical pores of the inverse opal template. After filling the pores with Cu2+, the inverse opal template was carefully removed into a closed container with the reducing agent DMAB (1.0 g) in a small dish. While being maintained at 25 °C for 1 day, the closed container was full of DMAB vapor, which infiltrated into the pores of the inverse opal template to reduce 6

the Cu2+. Immersed with copious amounts of tetrahydrofuran (THF), the PMMA inverse opal was dissolved, and Cu deposition occurred. After rinsing and centrifuging with ample amounts of ultra-pure water and ethanol, the resulting precipitate was dried at 25 °C in vacuum overnight to obtain the 3D-OMCuSA powder. 2.1.4 Preparation of Cu nanoparticles (Cu NPs)

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To form a homogeneous solution, 4.262 g CuCl2·2H2O was dissolved in 5 mL deionized water and ultrasonically dispersed for 30 min at room temperature. Then, 0.6 g DMAB was added into the homogeneous Cu solution to chemically reduce Cu2+

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to Cu nanoparticles. Once rinsed and centrifuged with copious amounts of ultra-pure

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water and ethanol, the resulting precipitate was dried at 25 °C in vacuum overnight to obtain the Cu NPs. To be as accurate as possible for the CO2 electroreduction

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experiments, each sample was prepared and stored under the same conditions.

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2.2 Physicochemical characterization

The particle morphology and size were observed using field emission scanning

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electron microscopy (SEM) and field emission transmission electron microscopy (TEM). SEM images were obtained on a Hitachi S-4000 scanning electron

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microscope with an acceleration voltage of 15 keV. TEM was carried out on a JEOL JEM-2100F transmission electron microscope with an operating voltage of 200 kV. Powder X-ray diffraction (XRD) data were collected on a Bruker D8-Focus diffractometer with Cu-Kα radiation (λ = 1.5418 Å) working at 40 mA and 40 kV. XRD patterns were obtained in the 2ϴ range of 20 to 80 degrees with degree steps of 7

0.02 and acquisition times of 0.1 s/step. X-ray photoelectron spectroscopy (XPS) (Catalysis Surface Science Laboratory, University of Calgary) was carried out on an ESCALAB 250 X-ray photoelectron spectrometer with a monochromatic Al Kα radiation source. A survey scan of 0−1350 eV was performed, and high sensitivity scans were collected in the Cu and O regions. All spectra analyses were performed

Quadrasorb SI surface area and pore size analyzer. 2.3 Electrochemical analysis of products 2.3.1. Electrochemical conditions

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using Avantage software. N2 adsorption−desorption isotherms were carried out by a

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All the electrochemical measurements were conducted in an airtight H-type

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three-electrode electrochemical glass cell system, where the working and counter electrode compartments were separated with a Nafion 117 anion exchange membrane.

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Before each experiment, cells were immersed in a piranha solution for one hour and

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rinsed with copious amounts of deionized water. Each compartment was injected into 12 mL of electrolyte with an 8 mL headspace. The electrolyte was prepared with 0.1

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M potassium bicarbonate (KHCO3) dissolved with 18.2 MΩ deionized water and sparged with 1 atm of CO2 (Air liquid 4.5) (30 mL/min) from the bottom of the

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cathode chamber to equilibrate the solution with a final stable pH of 6.8. A glass gas frit bubbler was inserted into the cathode chamber to sparge CO2 bubbles into the cathode electrolyte. A leak-free Ag/AgCl (saturated KCl) and a platinum mesh (2 cm × 1 cm × 0.1 mm) were used as the reference and counter electrodes, respectively. Reference electrode should always be stored in the same electrolyte that is inside the 8

electrode. Prior to each experiment, the Ag/AgCl reference electrode was checked relative to pristine reference electrodes in case of a potential drift and was stored in 3mol L-1 KCl. between measurements. The potentials measured against the reference electrode were calibrated against a reversible hydrogen electrode (RHE) reference (all potentials here are referred to this reference). The working electrode is a glassy carbon electrode with a diameter of 8 mm. Alpha

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alumina (300 nm) and gamma alumina (50 nm) were employed to polish the glassy carbon plates as smooth as a mirror surface. Then, the glassy carbon electrode was rinsed with copious amounts of ultra-pure water, sonicated, and blown dry with

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nitrogen. The catalyst ink mixture solution was fabricated by the ultrasonic dispersion

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of 10 mg of the catalyst sample with 20 μl Nafion solution (5 wt%) in 1 ml methanol and 1 ml water for 30 min. Five microliters of the as-prepared ink was deposited onto

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the substrate and were slowly dried under air at room temperature for the subsequent

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electrochemical testing experiments. 2.3.2. Electrochemical setup

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Before the start of each experiment, the electrolyte was bubbled with high-purity Ar to degas oxygen and air dissolved, and then research grade CO2 was sparged for 30

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min at a rate of 20 mL/min to reach a stable pH of 6.8. During the experiments, the catholyte and the anolyte were saturated constantly with CO2 (99.999%) at a flow rate of 20 mL/min controlled with a mass flow controller (Alicat Scientific) to avoid depletion of CO2, and the evolved gaseous products were carried by CO2 and directly injected into an online gas chromatograph. Electrochemical analysis was controlled 9

using a potentiostat system (CHI 630E, CH Instruments, Inc.) and performed at constant IR-corrected potential. Prior to each experiment, ohmic loss (iRu) was compensated using the current interrupt mode. The electrode potentials were corrected using the automatic iR compensation function of the potentiostat system by following equation (Vcorrected=Vmeasured–iR). iRu compensation degrees were set to 85% of the measured Ru values in situ, and the last 15% voltage loss was mathematically

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corrected during the data workup. All linear sweep voltammetry (LSV) and cyclic

voltammetry (CV) measurements were carried out at ambient temperature. The

durability test of the catalyst was performed at −0.6 V for 72 h. All current density

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values, which were obtained by normalizing to the electrochemically active surface

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area (ECSA) of the working electrode, were reported instead of the current values. After each experiment, the three-electrode electrochemical glass cell system was

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thoroughly rinsed with fresh 0.1 M pre-electrolyzed KHCO3 solution and filled for the

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next test.

The cathode side was electrochemically reduced using the CV method, which ranged

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from − 0.5 to − 1.0 V (versus RHE) at a rate of 0.1 V s–1 for 5 cycles to completely reduce the possible oxidized species.

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2.3.3. Product analysis

Each gas and liquid product distribution converted into Faradaic efficiency were quantitatively analyzed using gas chromatography and high-performance liquid chromatography (HPLC), respectively. Upon the completion of electrolysis, a 60 μL gas sample was removed through a septum port via a gastight syringe and was 10

injected into a gas chromatograph (GC) instrument (GC, SRI Instrument) to quantify the amount of CO and C2H4. For all experiments, electrolysis was allowed to proceed for 60 min with gas analysis done at 10, 30, and 50 min. The liquid samples were collected from the cathode and anode chambers after electrolysis and analyzed by high-performance liquid chromatography (HPLC) (1260 Infinity II, Agilent Technologies) with a SUGAR SH1101 column (Shodex). Vials

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filled with the collected samples were placed in an autosampler holder, and then 50 μL of the sample was automatically injected into the column using an autosampler.

The temperature of the column was maintained at 60 °C in a column oven, and a

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refractive index detector (RID) was used to detect the separated products. A standard

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calibration curve of the expected products of the CO2RR were analyzed by HPLC in the same condition.

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3. Calculation

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3.1 Conversion of Evs. Ag/AgCl to Evs. RHE

The potential measured against the reference electrode Ag/AgCl was converted to the

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potential vs. reversible hydrogen electrode (RHE) using the following equation[39]: Evs. RHE = Evs Ag/AgCl + Eø Ag/AgCl + 0.059 pH

(1)

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The potential vs. RHE was used throughout this work. 3.2 Calculation of the electrochemically active surface area (ECSA) of the working electrode To determine the electrochemically active surface areas (ECSAs) of the catalysts, the double layer capacitance method in the presence of a known concentration of a 11

redox-active molecule, ferri-/ferrocyanide ([Fe(CN)6]3−/4−), was employed and measured. Cyclic voltammetry of the double layer capacitance regions was conducted in a nitrogen-purged 5 mM K3Fe (CN)6/0.1 M KCl solution with scan rates ranging from 20 to 200 mV/s. With a selected certain potential, the current density was then plotted against the scan rate to obtain a linear plot. For the electropolished Cu surface, the slope of the linear layer, which also represents the double layer capacitance, was

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29 μF/cm2. Thus, the ratio of the slope/29 gives the roughness factor (RF) of the catalyst. The ECSA of the catalyst was calculated using the following equation [40]:

（2）

where Ageo is the geometric area of the catalyst.

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3.3 Calculation of the current density

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ECSA= RF × Ageo

The current density of each product was calculated using the following equation:

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ji = FEi × I / ECSA

（3）

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where ji is the current density of product i, I is the recorded current (A), FEi is the faradaic efficiency of product i, and ECSA is the electrochemically active surface area

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of the catalysts.

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3.4 The loading of the catalyst on the electrode M=v×m/V

（4）

where M is the loading of the catalyst on the electrode (mg), m is the catalyst sample dispersed in mixture solution (mg), V is the volume of the catalyst ink mixture solution (mL), and v is the volume of deposited onto the electrode. 3.5 Calculation of the Faradaic efficiency 12

The Faradaic efficiency (FE) was calculated as follows: Calculation of faradaic efficiencies (%) of CO2RR FECO2RR=100 × FECO2RR / (FECO2RR + FEHER)

(5)

where FECO2RR is the sum of the FEs of all the C-products. Calculation of faradaic efficiencies (%) of liquid products FE = e × n × F /Q

(6)

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where e is the number of electrons required to obtain 1 molecule of product, F is the Faraday constant, 96485 C mol-1, Q is the measured charge (coulomb), and n is the recorded total amount of product (moles).

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Calculation of faradaic efficiencies (%) of gaseous products

Therefore

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Q = I × t= I × V/r/60

FE = e × n × F × r / I × V ×60

（7） (8)

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recorded flow rate (sccm).

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where I is the recorded current (A), V is the Volume of sampling loop (cm3), and r is

4. Results and discussion

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4.1 Physical and chemical performance A typical SEM image of SiO2 opal shows a uniform structure with a SiO2 size of

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approximately 200 nm (Fig. 2(a)). Because of the existence of pores between orderly SiO2, liquid MMA was converted to PMMA to complete the filling of the SiO2 opal to form a PMMA-SiO2 opal component (Fig. 2(b)). After the etching of the MMA-SiO2 opal component (Fig. 2(b)) with HF, a precious microporous structure of a PMMA inverse opal monolith (Fig. 2(c)) consisting of highly ordered macropores with similar 13

sizes to the original SiO2 was obtained. Therefore, due to the highly ordered macropores, the precursor solution can easily diffuse through the porous network and fill the pores in the PMMA inverse opal. Fig. 2(d-e) suggests that 3D-OMCuSA replicated the original silica opal structure. The replicate structure with a spherical shape and ordered 3D-interconnected pores is a highly ordered close-packed face-centred cubic (fcc) structure. The sizes of the macropores are approximately 290

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nm, which is effectively consistent with the size of the original PMMA inverse opal.

This consistency suggests that the PMMA inverse opal pores and the 3D-OMCu

spheres do not contract during removal of the PMMA template. The typical TEM

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images provide further insight into the structural characterizations of the

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3D-OMCuSA (Fig. 2(f-h)). A well-defined mesoporous structure can be seen in Fig. 2(f), and Fig. 2(h) with the higher magnification TEM image shows that the average

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diameter of Cu particles is approximately 10 nm. Compared with the Cu nanoparticles,

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the structure is much smaller, which results in a larger surface area for the catalyst. In order to distinguish the differences of Cu nanoparticles with the as-prepared

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3DOMCuSA clearly, the SEM images of Cu nanoparticles (Fig. 3(e-f)) was prepared. As can be seen from the comparison of the SEM images (Fig. 3(a-f)), the

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3DOMCuSA has a smaller structural size with approximately 200 nm, and its structural arrangement is more orderly and regular than the copper particles. The copper particles show a tight arrangement of blocks, while the 3DOMCuSA has obvious regular internal voids. This different morphology and structures have a great effect on activity and selectivity for Cu nanoparticles and the 3D-OMCuSA. 14

The structural information and insightful chemical state of 3DOMCuSA before and after the experiment were investigated by XRD and XPS analyses. As shown in Fig. 4(a), the crystal structure of the catalysts was determined from XRD, which were obtained in the 2ϴ range of 20 to 80 degrees. Well-defined characteristic peaks corresponding to (111), (200) and (220) crystal planes are indexed to the Cu (PDF #04-0836) and Cu2O, (PDF #05-0667), respectively, in agreement with the XPS [41]

. According to the XPS results, the elements of Cu, O, and B

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analysis (Fig. 4(b))

were detected on the surface of 3D-OMCuSA (Fig. 4(b)). Table 1 summarizes the surface composition percentages (at%) derived by XPS before and after the

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experiments. Before electrolysis, the atomic ratio of Cu:O was 23.83:43.01,

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suggesting that the copper−oxygen species existed on the surface of 3D-OMCuSA. After CO2RR catalysis for 2 h, the atomic ratio of Cu:O was observed to be

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36.83:29.63, indicating that the thickness of the surface oxide atom layer was reduced

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after the electrochemical reduction. The high-resolution Cu 2p spectra (Fig. 4(c)) of 3D-OMCuSA are consistent with the presence of Cu 2p3/2 (932.6 eV) and Cu 2p1/2

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(952.2 eV), and two intense shakeup satellite lines in the range of 940−945 eV are predominantly ascribed to Cu2+ cations. The spectra of Cu 2p show two main signals

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corresponding to Cu 2p3/2 (933.6 eV) and Cu 2p1/2 (953.6 eV), respectively, which are ascribed to Cu2+, while those at 932.6 and 952.2 eV are produced by the CuO species. For the Cu and Cu2O standard samples, the Cu 2p3/2 binding energies are nearly identical, i.e., 932.6 and 932.8 eV, resulting in the fact that these two compounds cannot be precisely distinguished by their Cu 2p peak position 15

[42]

.

Therefore, from the XRD analysis (Fig. 4(a)), it can be deduced that the main chemical states of 3DOMCuSA are Cu and Cu2O. After CO2RR catalysis for 2 h, the peak located at 933.6 eV for Cu2+ disappeared, indicating that the surface Cu2+ was reduced after CO2RR. The high-resolution O1s region of the 3D-OMCuSA catalyst shows a strong Cu-O peak at approximately 530.3 eV, indicating the presence of Cu2O (Fig. 4(d)). The two peaks located at approximately 531.5 and 533.0 eV are

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ascribed to hydroxyl O atoms and surfacing bridging O atoms, respectively, revealing that oxygenated groups were absorbed on the surface of the 3D-OMCuSA catalyst

during the synthesis process. Notably, the surfacing bridging O atom peak

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disappeared after testing for 2 h, which could be associated with a reduction on the

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3D-OMCuSA surface in the CO2RR. Approximately 6 wt% boron was contained, originating from the DMAB reducing agent used to produce Cu. The result of XPS

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analysis presents a possible core−shell structure for 3D-OMCuSA, where an

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amorphous Cu core is coated by a hydrated Cu oxide shell. Brunauer–Emmett–Teller (BET) characterization was employed to explore the pore

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structure of 3DOMCuSA. The nitrogen adsorption-desorption isotherm of the 3DOMCuSA shown in Fig. 5(a) exhibits a noticeable hysteresis loop in the relative

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pressure range of 0.65-1.0, which indicates the existence of a layered structure. And classified as type IV according to the IUPAC classification, the isotherm is typical for mesoporous materials. The BET surface area of the 3D-OMCuSA were measured with 105.2 m2 g-1. Applying the desorption branch of the isotherm, the Barrett– Joyner–Halenda (BJH) model was carried out to calculate the pore size distribution of 16

the 3D-OMCuSA (Fig. 5(b)). The average pore diameter is 4.2 nm, and a broad pore distribution (2–20 nm), with most of the pores being in the mesopore range of 2.5–6 nm, implying substantial homogeneity of the mesopores for the 3D-OMCuSA. 4.2 Electrochemical performance The ECSA, measured with double layer capacitance methods, was estimated to elucidate the origin of the high activity of 3D-OMCuSA. Fig. 6(a) suggests that the

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ECSA of 3D-OMCuSA is (normalized to electrode area) approximately 9.4 times larger than that of copper nanoparticles. The current densities obtained at -0.60 V

(versus RHE) for 3D-OMCuSA are 17 times higher than those of the copper

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nanoparticles; therefore, the 2-fold differences suggest that the surface area effect is

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not the only factor that explains the catalytic activity (Fig. 6(c)). This result could indicate that the intrinsic activity of the catalytic sites on the 3D-OMCuSA curved

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internal surface is much higher than that on a flat surface. The greatly enhanced

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electrochemical surface area, the highly active curved internal surface and the monolithic self-supported structure with convenient electronic transportation

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contribute to the high performance. These factors partially account for the fact that the 3D-OMCuSA catalysts in the CO2RR work exhibit significant advantages over their

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copper nanoparticle counterparts in aqueous environments in both overall and per surface site activity. A Tafel analysis obtained by plotting the overpotential versus the log (cathodic current density) was performed to obtain insight into the mechanistic pathway(s) for CO2 reduction with the 3D-OMCuSA catalyst surface. The Tafel plot fit well with the 17

Tafel equation h= b×log(j) + a, where j is the current density and b is the Tafel slope (Fig. 6(b)). The Tafel slope represents an inherent property of the catalyst, which can be used to further analyse the intrinsic activity of 3D-OMCuSA during the process of CO2 electrocatalytic reduction, and the smaller slope indicates a faster increase in the CO2RR rate with increasing potential. The plot for 3D-OMCuSA and the copper nanoparticles is linear over the range of overpotentials from -1.5 to 1.5 V with slopes

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of 109.6 mVdecade-1 and 338.24 mV decade-1(Tafle slope of copper nanoparticles is similar to the literature [35]), respectively. Compared with the copper nanoparticles, the Tafel slope of 3D-OMCuSA exhibits a sharp decrease of 3 times, which further proves

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the intrinsically better performance of the 3D-OMCuSA surface. The 3DOMCuSA

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shows a Tafel slope of ∼109.6 mV decade−1, which is not only lower than the copper nanoparticles (∼338.24 mV decade−1) but also smaller than or comparable to those

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reported for the other highly active CO2RR catalysts in the literature, such as [15]

annealed Cu (∼116 mV decade−1)

, Au NP/C (138 mV decade−1)

na

polycrystalline Ag (∼132 mV decade−1)

[40]

, CdSe nanorods (∼135 mV decade−1) [43], [14]

, and [44]

,

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hydroxide-mediated copper catalysts at an abrupt interface (∼135 mV decade−1) demonstrating the outstanding CO2RR kinetics of the 3D-OMCuSA.

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This Tafel slope of 3D-OMCuSA is consistent with a rate-determining initial electron transferred to CO2 to form a surface adsorbed CO2•− intermediate, which indicates that the 3D-OMCuSA formed by the macro/mesoporous structure enables the formation of the CO2•− intermediate while suppressing hydrogen reduction. A previous study on different copper crystal facets showed that enriched high-energy 18

Cu(110) facets can significantly enhance the catalytic activity towards CO2 reduction, while Cu(100) and Cu(111) catalyst surfaces are less active in dissociating CO2 for their “self-poisoning” mechanism [33]. This may explain why the further improvement of the 3D-OMCuSA catalyst may be the result of its higher density of Cu (110) facets with possibly higher active sites supported by the highly curved surface. Notably, the 3D-OMCuSA catalyst achieved the onset potential at a much lower overpotential than

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that in a single crystal (110) face (-0.4V versus RHE). Additionally, the Tafel slope

has a sharp increase along with higher overpotentials, indicating that the other factor, which is most likely mass transport issues such as the diffusion of products and

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reactants out of/into the nanopores, became the rate-limiting step. It is widely

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accepted that the presence of a high density of atomic steps, ledges, nanocrystal clusters and kinks, which usually act as active sites for breaking chemical bonds, can

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efficiently enhance chemical activity [45]. The schematic shown in Fig. 9 describes the

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formation process of nanocrystal clusters of 3D-OMCuSA and proposes a mechanism to explain their high CO2RR electrocatalytic activities. The copper crystallites

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anchored on the inner surface of mesoporous gradually grow and aggregate to form a nanocrystal cluster on 3D-OMCuSA.

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Linear sweep voltammetry (LSV) was carried out to evaluate the catalytic performance of the 3D-OMCuSA and copper nanoparticle catalysts (Fig. 6(c)). The total and partial CO2RR current density potential curves of 3D-OMCuSA and copper nanoparticles in CO2-saturated 0.1 M KHCO3 were presented, respectively. The total current density includes CO2RR and the hydrogen evolution reaction (HER), while 19

the CO2RR current density only means CO2 conversion. The current density was normalized by the ECSA, and the onset potential was defined as the potential when the current density reached 1 mA cm-2

[36]

. As clearly shown in Fig. 6(c), the

3D-OMCuSA and copper nanoparticles showed average onset potentials at -0.4 V and -0.85 V (vs. RHE), respectively. This outstanding onset potential of 3D-OMCuSA is not only superior to that of noble

ro of

metal electrocatalysts, such as Au NPs [14] but is also superior to that of C-based electrocatalyst catalysts [46] and most of the same class Cu-based electrocatalysts, such as highly dense Cu nanowires [32]

, high-energy facets on copper nanocrystals

[33]

,

, demonstrating the promising catalytic behaviour of

-p

boron-doped copper

[6]

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3D-OMCuSA.

A high surface area of 3D-OMCuSA significantly improves the electrocatalytic

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performance for both HER and CO2RR at low overpotentials. The observed current

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density at -1.0 V was 22.8 mAcm-2 for 3D-OMCuSA, while copper nanoparticles have a sharp decrease of 1.4 mAcm-2 at -1.0 V (vs. RHE).

ur

The partial current density of CO is plotted against the applied potential. (Fig. 7). 3D-OMCuSA can electrochemically reduce CO2 to CO at a potential of -0.53 V

Jo

versus RHE with the current density of 0.97mA cm-2, which is 5 to 50 times higher rate than polycrystalline Cu

[40]

, Cu nanoparticles, polycrystalline Au [14] respectively.

Although outcompeting these currently best-performing copper-based electrodes, such as ECR Cu nanowire [6] and Cu foil annealed at 500°C for 12h [40], 3D-OMCuSA also show comparable performance at low potentials (-0.2 to -0.6 V versus RHE). 20

To further characterize the CO2 reduction activity of 3D-OMCuSA, faradaic efficiencies and the partial current densities for the reduction products were measured at a variety of potentials between -0.3 and -1.0 V. The faradaic efficiencies and partial current densities for the major products (CO, HCOOH, C2H4, C2H5OH) are shown in Fig. 8(a-d). Compared with copper nanoparticles, the 3D-OMCuSA exhibits high faradaic efficiencies for CO2 reduction at a remarkably low potential range. HCOOH

ro of

was the major product with the FE being above 20% at -0.4 to -0.7 V, reaching the

peak faradaic efficiency of 27.47% at -0.61 V. Another significant product of CO2 reduction in the low-overpotential region was CO, attaining a peak faradaic efficiency

-p

of 12.26% at potentials ranging from -0.5 to -0.7 V, whereas copper particles require

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-0.75 to -0.9 V to attain a comparable faradaic efficiency (Fig. 8(a)). And at the low potential region (-0.4~-0.7V), the overall FE for CO and HCOOH was as high as

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30-40%. At relatively negative potentials (<-0.7 V), C2H4 and C2H6O have become

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the dominant productions. For C2H4 and C2H5OH production of 3D-OMCuSA, a peak faradaic efficiency of 29.79% and 10.78% were obtained at -0.83 V, respectively. The

ur

sum of FEs for the C2 species was about 20%-35% between -0.7 and -0.9 V. Compared to the previously reported Cu-based catalysts, the 3D-OMCuSA are much

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more active and selective. By comparison, at a remarkably low potential range, HCOOH was produced with 20.64% FE on copper nanoparticles. And small amounts of C2H4 and C2H5OH were also detected below -0.8 V on the copper nanoparticles, with a peak faradaic efficiency of were 27.3% and 6.81% at -0.87, respectively. but the total FEs of C2 species were generally below 30% (Fig. 8(a-b)). We also note that 21

no methane or methanol was observed down to -1.0 V among the reduction products, which maybe owing to the suppression of the local pH. Fig. 8(c-d) summarized the comparison of the catalytic performance for different production among the 3D-OMCuSA and copper nanoparticles. Both 3D-OMCuSA and copper nanoparticles are much more selective for producing CO and HCOOH in the low-overpotential region, while the main productions are C2H4 and C2H5OH in the

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high-overpotential region. However, whether C1 productions or C2 productions, the potential required for 3D-OMCuSA to reach the maximum current density and

Faraday efficiency is less than that of copper nanoparticles. Here are two aspects to

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explain the observed drop in FE and current density toward CO and HCOOH in the

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high-overpotential region (E < -0.7 V). One of reason was that CO was further reduced and C-C bond formed, confirmed by the production of C2 species below -0.7

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V. Another was likely due to the mass transport limitation at high current densities.

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With the more negative potential, the hydrogen evolution reaction was more efficiently hindered. The extensive coverage of the Cu surface with CO, impeding the

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hydrogen evolution reaction, was proposed to explain the lower current density recorded during the process of CO2RR on 3D-OMCuSA and the copper nanoparticle

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catalysts. Compared with copper nanoparticles, the percentage of partial CO2RR current density exhibits significant advantages for the 3D-OMCuSA catalysts. Hence, the higher current density achieved in the cathode at the lower potential is already an indication of the distinctive performance of 3D-OMCuSA towards CO2 reduction. Except for the improved exposure of active sites originating from the high specific 22

area and high chemical energy facet, the outstanding active electrocatalytic performance of the 3D-OMCuSA for CO2RR may originate from another aspects. For one part, the exposure efficiency of active sites in the case of 3D-OMCuSA is promoted in different ways; that is, the stacking of 3D-OMCuSA nanocrystals on the inner larger surface of the mesoporous structure provides a higher number of exposed sites. The broader space resulting from the mesoporous structure ensures sufficient

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CO2 diffusion through 3D-OMCuSA and rapid electron transfer along the integrated

network, consequently enhancing the contact between CO2 and 3D-OMCuSA. In

addition, the highly porous structure limited the mass transfer of CO2RR byproducts

-p

(OH−) and prolonged the retention time of intermediates in 3D-OMCuSA, resulting in

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increased alkalinity at/near porous structures. Therefore, the effect of local pH at the electrode/electrolyte interface suppressed the hydrogen evolution reaction and

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enhanced CO2 reduction towards C2 species (e.g., C2H4/ CH4) efficiently [47]

and Au

[48]

. Similar

mesostructure electrodes.

na

phenomena were observed from Ag-IO

[6]

Therefore, 3D-OMCuSA is reasonable to present excellent CO2RR electrocatalytic

ur

activity.

The stability of the 3D-OMCuSA was detected at an applied potential of -0.38~-1.05

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V versus RHE (Fig. 6(d)). Continuous electrolysis of 2h was performed to test the drop of current density. During a 2 h potentiostatic test at -0.38~-1.05 V, all the 3D-OMCuSA samples retained long-term stability without a significant loss in current density, in agreement with the physical and chemical performance. XRD and XPS images of the morphology of the 3D-OMCuSA before or after extensive electrolysis 23

(Fig. 4(a) and 4(b)) did not show any differences. And to explore the impact of external conditions, 3D-OMCuSA was exposed to the air for one hour and reintroduced to fresh electrolyte for a full round of CO2 electroreduction. There was no obvious activity difference between the exposed and original catalysts. That is means it has an excellent stability. Herein, we introduce a two-step facile synthesis of 3D-OMCuSA via a dual-template

ro of

method. 3D-OMCuSA displays a large surface area furnished with more catalytically

active sites, resulting in a high current density. The sharp decrease in the Tafel slope and durability test further proves the intrinsically better catalytic performance and

-p

stability compared with copper nanoparticles. The XRD, TEM and SEM results

re

demonstrate that the 3D-OMCuSA has a porous structure with a precisely controlled morphology, combining the good characteristics of both mesoporous and

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macroporous structures. XRD and TEM data indicate that the high surface area

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crystallites anchored on the inner surface of the mesoporous structure bring about a large number of active sites, which is one of the main reasons that explain the high

ur

electrocatalytic performance of 3D-OMCuSA. The mesoporous distribution in 3D-OMCuSA can facilitate CO2 diffusion through the catalyst and enhance an access

Jo

of reactant molecules to the active sites. In addition, the hierarchical macroporous framework can partly counterbalance the high resistance and accelerate the electron transfer to advance the conductivity and electrochemical kinetics. Our results provide qualitative insight into the effect of porous structure on reactivity and activity as well as promote understanding of the electroreduction of CO2 on 24

existing materials. However, it falls short of capturing the more detailed behaviour of highly curved surfaces for 3D-OMCuSA. Future research directions may include the exploration of different Cu crystal facets, the effect of local ion concentration, the inclusion of local pH effects and the effect of surface coverage as well as the expansion of the study to metals other than copper.

5. Conclusion

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In summary, as a CO2RR catalyst with potentially efficient, high catalytic performance and long-term stability, a 3D-OMCuSA catalyst can be easily synthesized by a two-step replication approach. During the process, PMMA inverse

opal and nonionic surfactant Brij58 were employed as the macropore template and

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mesopore template, respectively. The unique 3D interconnected network structure composed of ordered close-packed mesoporous spheres contributes to the enlarged

re

accessible surface area and massive active sites, efficient mass and charge transport, improved conductivity and high structural stability, resulting in the outstanding

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electrocatalytic CO2RR performance of 3D-OMCuSA. The prepared 3D-OMCuSA requires an onset potential of only -0.4 V and a low Tafel slope of 109.6 mV per

na

decade, a high faradaic efficiency as well as a good durability with a negligible activity loss. For C2H4 and C2H5OH production of 3D-OMCuSA, a peak faradaic efficiency can reach 29.79% and 10.78% at -0.83V, respectively. And for HCOOH,

ur

the peak faradaic efficiency was 27.47% obtained at -0.61 V. The distinct features of 3D-OMCuSA is likely owing to the stacking of 3D-OMCuSA nanocrystals on the

Jo

inner larger surface of the mesoporous structure and highly integrated network which can limit the mass transfer of CO2RR byproducts and prolong the retention time of intermediates. This novel 3D ordered mesoporous spherical structure may potentially be extended to other metals or alloys, which can be technological used in other applications. This new understanding of the structure-property effects of copper nanocatalysts could provide further insight into electrocatalytic materials for CO2 25

reduction.

Acknowledgements This work was supported by the NSFC [grant number 21607113]; and the Natural

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na

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ro of

Science Foundation of Tianjin [grant number 17JCQNJC07700].

26

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Figure 1. Schematic of the preparation of the 3D-OMCuSA

31

ro of -p re lP na ur Jo Figure 2. SEM images of (a) SiO2 opal Inset: at a higher magnification. and (b) SiO2 opal-MMA. (c) PMMA inverse opal. (d-e) SEM image of 3DOMCuSA. (f-h) Higher magnification TEM image of one

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of the mesoporous Cu spheres.

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Figure 3. SEM images of (a-d) 3DOMCuSA and (e-f) copper nanoparticles.

33

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Figure 4. (a) Cu catalyst XRD spectra; (b) Cu catalyst XPS spectra; (c) Cu 2p and (d) O 1s XPS

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spectra of 3D-OMCuSA before and after the experiment.

34

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Figure 5. Nitrogen adsorption/desorption isotherm and pore size distribution curve for 3D-OMCuSA.

35

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Figure 6. (a) Cu catalyst ECSA figure and (b) Tafel plot of the Cu catalyst in CO 2-saturated KHCO3

re

electrolyte. (c) LSV plot of the Cu catalyst in the CO2-saturated KHCO3 electrolyte. and (d) I-t plot of

Jo

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the Cu catalyst in a CO2-saturated KHCO3 electrolyte.

36

Figure 7. Activity of various electrodes in water. Comparison of the performance of different

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electrodes on the basis of the partial current density with CO at variable potentials. [6,14,15,40]

37

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Figure 8. Comparison of the electrocatalytic activities of 3DOMCuSA and Cu nanoparticles. (a)

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Faradaic efficiencies for CO and HCOOH vs potential and (b) Faradaic efficiencies for C 2H4 and C2H5OH vs potential and (c) current density for CO and HCOOH vs potential and (d) current density

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for C2H4 and C2H5OH vs potential.

38

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Figure 9. Schematic graph of the formation process of nanocrystal clusters of 3D-OMCuSA.

39

Table 1. XPS surface composition percentages (at%) of 3D-OMCuSA before and after the

Cu

O

B

C

23.83

43.01

6.89

26.26

36.83

29.63

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experiments.

Before the experiment After the

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re

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experiment

5.96

40

27.57