Electrochemical CO2 Reduction with High Selectivity of HCOOH

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Zinc Modified Copper Catalyst for Efficient (Photo-)Electrochemical CO Reduction with High Selectivity of HCOOH Production 2

Saira Ajmal, Yang Yang, Kejian Li, Muhammad Ali Tahir, Yangyang Liu, Tao Wang, Aziz-Ur-Rahim Bacha, Yiqing Feng, Yue Deng, and Liwu Zhang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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The Journal of Physical Chemistry

Zinc Modified Copper Catalyst for Efficient (Photo-)electrochemical CO2 Reduction with High Selectivity of HCOOH Production Saira Ajmal,† Yang Yang,† Kejian Li,† Muhammad Ali Tahir,† Yangyang Liu,† Tao Wang,† AzizUr-Rahim Bacha,† Yiqing Feng,† Yue Deng,† and Liwu Zhang*†,‡ †Shanghai

Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of

Environmental Science & Engineering, Fudan University, Shanghai, 200433, Peoples’ Republic of China ‡Shanghai

Institute of Pollution Control and Ecological Security, Shanghai, 200092, Peoples’

Republic of China Corresponding Author *Liwu Zhang Email: [email protected]

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ABSTRACT: The (photo-)electrochemical carbon dioxide (CO2) reduction has gained

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significant importance as it has the potential to yield valuable organic products by utilizing

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renewable energy. Here CuZn alloy catalysts fabricated by electrodeposition method were

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evaluated as an efficient catalyst for selective CO2 reduction to HCOOH. By varying the

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concentration of zinc (Zn) in CuZn bimetallic catalysts, the faradaic efficiency and partial current

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density of HCOOH on CuZn-0.5 was enhanced nearly 4 and 5 times respectively as compared to

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Cu foil. Furthermore, we developed a photoelectrochemical cell system for CO2 reduction by using

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CuZn-0.5 catalyst as cathode and BiVO4 as photoanode. HCOOH formation was maximized with

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approximately 60% faradaic efficiency under simulated sunlight, which is among the highest for

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copper-based cathodes. The higher selectivity of CuZn originates due to synergistic effect of Cu

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and Zn. The reaction process is further studied by in-situ Raman and density functional theory

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calculations. This work can provide a detail understanding of developing a catalyst with highly

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selective HCOOH formation.


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1. INTRODUCTION

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To meet the energy demands, the fossil fuel consumption is increasing rapidly that leads to various

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issues worldwide due to the carbon dioxide (CO2) emission into the atmosphere.1 Electrochemical

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CO2 reduction into carbon monoxide (CO), methane (CH4), formic acid (HCOOH) ethylene

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(C2H4) and other fuels is a promising approach to deal with an excess amount of CO2 emission for

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reducing the greenhouse gas concentration and lessen the fossil energy shortage.2 HCOOH is

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considered to be an important product of CO2 reduction and has attracted much attraction as

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storage source of H2. 3 There are several other aspects that make HCOOH is considerably important

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for the synthesis of organic agents and its use as silage additive in animal husbandry.4, 5 Therefore,

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the demand of high performance and low-cost catalysts is critically important for an efficient CO2

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conversion to HCOOH. However, the production of liquid hydrocarbon fuels from CO2 by

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utilizing renewable energies through electrocatalysis, photo-electrocatalysis or photocatalysis is

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still challenging.6, 7 For electrochemical CO2 reduction, various metals have been proposed such as Cu,8-16 Au,17,

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Ag,19, 20 Zn,21, 22 etc. to achieve low cost and easy handling.23-33 These metals have the ability in

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catalyzing CO2 reduction reaction due to the availability of d-electron and weak surface CO2

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adsorption strength.4, 34 Cu is the most effective electrocatalyst for CO2 conversion into various

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products. At the same time, there are several challenges including poor selectivity and low faradaic

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efficiency (FE) were found in Cu catalyst. By exploring the synergistic effect during catalysis,

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various Cu alloys have been reported such as Ag-Cu,35, 36 Ni-Cu,37, 38 Pd-Cu,39, 40 Au-Cu,41-43 In-

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Cu,44 and Sn-Cu,45, 46 which result in changing their electrocatalytic activity and product selectivity

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toward CO2 reduction. Synergistic effect plays its role in sustainability of energy and carbon

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neutrality through electroreduction of CO2 to a reusable form of carbon by developing an efficient 3 ACS Paragon Plus Environment


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catalyst.47-49 Several electrons/protons are involved in transfer during electroreduction process and

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also leads to product distribution at large scale. Therefore, it is a dire need to develop such a cost-

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effective catalyst having low hydrogen evolution, optimal CO binding and high selectivity for an

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efficient CO2 reduction reaction that could make it possible to yield valuable organic products.50-

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52 However, Zn have been screened as promising metal for CO reduction due to its lower hydrogen 2

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evolution in several studies.53 Among all metal catalysts, both Cu and Zn are found to be cost-

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efficient and eco-friendly for CO2 reduction. As the process of hydrogen evolution is found to be

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slower on Zn with respect to Cu catalyst, thus the electrocatalytic activity of Cu is enhanced

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towards CO2 reduction by forming CuZn catalyst.21 Several previous works conducted on CuZn

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alloy showed comparatively higher efficiency for CO2 reduction as compared to pure Cu or pure

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Zn.54 For instance, Hu et. al. reported that, CO2 could be reduced to HCOOH with a FE of up to

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27 % on CuZn in 0.1 M KHCO3 solution.55 In another study, Keerthiga and co-workers studies the

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electrochemical reduction of CO2 using CuZn electrode that results in higher product formation

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due to protonation reaction after the deposition of Zn on Cu.56 Briefly functionalization of Cu with

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Zn cannot only dramatically enhance the product selectivity, but also increase FE for (photo-

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)electrochemical reduction of CO2.54,

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platforms for enhancing solar-driven CO2 conversion to CO.58 There is still great need to further

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improve the reaction rate, and FE of CO2 reduction products.

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Moreover Zn-containing photocatalysts are promising

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For the sake of improving the selectivity and efficiency of CO2 reduction towards HCOOH,

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herein we report Zn modified Cu catalysts with varying amount of Zn precursors (CuZn-1, CuZn-

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0.5 and CuZn-0.1) for the (photo-) electrochemical CO2 conversion. Among all, the CuZn-0.5

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showed higher selectivity towards HCOOH with 60% FE. Furthermore, a photoelectrochemical

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cell (PEC) was established by using CuZn-0.5 catalyst as cathode and BiVO4 as photoanode for


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selective CO2 conversion to HCOOH under simulated sunlight irradiations. To the best of our

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knowledge we first time report the CuZn alloy catalyst as cathode in combination with BiVO4 as

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photoanode, achieved higher selectivity of approximately 60% FE for HCOOH in PEC system.

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2. METHODS

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2.1. Materials. All the chemicals of analytical grade were either purchased from SigmaAldrich. Ultrapure water was used for washing and solution preparation.

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2.2. Catalysts Preparation. The CuZn catalysts were prepared by an electrodeposition method

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reported by Sattayasamitsathit et al. with few modifications.59 Initially Cu foils (99.995%) were

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electro polished in 85% phosphoric acid at 4 V vs. platinum electrode for 5 min, followed by

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thoroughly rinsed with ultrapure water and dried. For the preparation of Cu plating solution, 10 g

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CuSO4.5H2O (99.97%, Sigma-Aldrich), 4 g H3BO3 (99.97%, Sigma-Aldrich) were dissolved in

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100 mL H2O. Similarly, 6 g ZnCl2, 2.5 g H3BO3 and 20 g KCl were dissolved in 100 mL H2O for

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Zn plating solution. To fabricate CuZn catalysts, a constant voltage was applied at -1.05 V for a

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total charge of 1.0 C to deposit Zn on polished Cu foil followed by deposition of CuZn at -0.3 V

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for 15 C in a three-electrode setup. The design strategy of CuZn catalysts is shown in Figure 1.

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Figure 1. Schematic diagram illustrating the preparation of CuZn catalysts.



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2.3. Catalysts Characterization. The morphology and elemental composition of prepared

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catalysts were studied with field emission scanning electron microscope (FESEM-Phenom ProX)

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equipped with transmission electron detector energy dispersive X-ray spectroscopy (EDX). To

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investigate the crystallographic phases of the prepared samples, X-ray diffractometer (XRD) study

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was conducted using monochromatic CuKα radiation (λ=1.5406 Ǻ, 40 kV and 40 mA). The X-ray

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photoelectron spectroscopy (XPS) was used to investigate the surface chemical properties and

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performance of samples. The in-situ Raman spectra were collected on XploRA confocal

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spectrometer (Jobin Yvon, Horiba Gr, France) with a charge coupled device (CCD) detector. The

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Raman spectra were obtained using the 785 nm laser excitation with 25% energy and coupled with

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10x Olympus microscopic objective lens. A holographic grating (1200gr/mm) was employed and

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holographic notch filter was equipped to filter the excitation line. A CCD with 1024 × 256 pixels

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and a resolution of 1.2 cm-1 was used to collect the spectra with 2 accumulations at 2 s acquisition

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time.

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2.4. Electrochemical CO2 Reduction Performance Tests. The catalysts were tested in an

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IVIUM electrochemical workstation which is a conventional three-electrode setup for

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electrochemical CO2 reduction. A platinum electrode and an Ag/AgCl electrode were used as the

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counter and reference electrodes, respectively. The anodic and cathodic compartment of

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electrochemical cell were separated by a Nafion 117 membrane. The CO2 was bubbled into 50 mL

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0.1 M KHCO3 electrolyte before each reaction. The resulting pH of the electrolyte was 6.8. All the

20

reported potentials were converted into RHE.60

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2.5. Photoelectrochemical Measurements. For photoelectrochemical CO2 reduction, the

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CuZn-0.5 catalyst was used as cathode in H-type cell. While the prepared BiVO4 electrode was

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employed as photoanode. More details regarding the preparation and characterization of BiVO4


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can be found in supporting information. The electrolyte on the cathodic side was 0.1 M KHCO3

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CO2-saturated solution and on anodic side was 0.1 M KHCO3 containing 0.05 M Na2SO3. During

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the reaction, the anodic side was irradiated with simulated sunlight.

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2.6. Products Analyses. For the analyses and quantification of gas products H2, CO, CH4, C2H4

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and C2H6, the gas chromatograph (SHIMADZU) equipped with TDX-1 and HT-POLT Al2O3/S

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columns was employed. A thermal conductivity detector (TCD) was mainly used to detect and

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quantify H2. Similarly, a flame ionization detector (FID) with methanizer was used to analyze and

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quantify CO by TDX-1 column, and another FID was used for separating and quantifying CH4 and

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other alkane contents with a HT-POLT Al2O3/S column. In this study, Ar (99.999%) was used as

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carrier gas. In order to quantify the liquid phase product, the solution was diluted 10 times with

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ultrapure water, then injected into ion chromatograph (Metrohm).

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3. RESULTS AND DISCUSSION

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3.1. Characterization of the Catalysts. Three bimetallic CuZn catalysts with varying

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concentrations of Zn, as well as bare Zn catalyst, were fabricated by electrodeposition. The

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morphologies of these catalysts were recorded by FESEM-Phenom ProX equipped with

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transmission electron detector and EDX (Figure 2a-e). As it can be seen that commercial Cu foil

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has uniform smooth surface (Figure 2a). However, the Zn catalyst possess whisker like crystals

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morphology can be observed in Figure 2b. Moreover, the CuZn catalysts showed rough

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morphology (Figure 2 c-e), and their adhesion indicates the strong interaction between Cu and Zn,

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which facilitates the rapid electron flow during the reaction. The elemental composition of CuZn

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catalysts were being confirmed by EDX. (Figure S2).

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Figure 2. SEM characterization of the catalysts (a) Cu, (b) Zn, (c) CuZn-1, (d) CuZn-0.5 and (e)

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CuZn-0.1. Scale bars: 5 µm for (a) to (e).

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The electrodeposition of Zn on Cu foil was measured by XRD in the range of 2θ from 30ᴼ

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to 80ᴼ. Figure 3a illustrates the XRD patterns of CuZn-1, CuZn-0.5 and CuZn-0.1 catalysts, which

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confirm the formation of CuZn (Cu5Zn8) alloys PDF#25-1228 with prominent peak of Cu foil

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substrate. Further, Cu foil and bare Zn showed similar pattern with previously reported diffraction

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peaks of Cu: JCPDS 04-0836 and Zn: JCPDS: 04-0831 respectively (Figure 3a).54


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Figure 3. (a) XRD patterns and XPS spectra of (b) Cu 2p, (c) Zn 2p and (d) O 1s the peaks of Cu,

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Zn, CuZn-1, CuZn-0.5 and CuZn-0.1 catalysts.

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In order to investigate the chemical states of Cu to Zn, XPS was conducted on CuZn-1,

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CuZn-0.5 and CuZn-0.1 catalysts. The high resolution XPS spectra of Cu 2p, Zn 2p and O 1s are

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shown in Figure 3b-d. It can be observed from the Figure 3b that peaks at about 933.5 eV and

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954.5 eV corresponded to binding energies of Cu 2p3/2 and Cu 2p1/2 respectively. The peak at about

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945 eV corresponded to the CuO satellite peak which confirmed the presence of very trace amount

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of CuO.61 Figure 3c shows the Zn 2p spectra, the peaks at 1021.5 eV and 1045.5 eV are

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corresponded to Zn 2p3/2 and Zn 2p1/2, respectively. Further, the peaks at 533 eV in Figure 3d can 9 ACS Paragon Plus Environment


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be attributed to surface oxygen.62 The position of Cu 2p and Zn 2p characteristic peaks were

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slightly different for CuZn catalysts (CuZn-1, CuZn-0.5 and CuZn-0.1) than those for Cu and Zn,

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indicating that CuZn catalysts are neither pure Cu nor Zn (Figure 3b-d). There is also slight

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variation among three CuZn catalysts depending on their Zn concentration, which can be seen

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from the deconvolution. All the above peaks values were in good agreement with previous study.63-

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65

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3.2. Electrochemistry of Cu, Zn and CuZn Catalysts. To characterize the performance of the

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catalysts as CO2 reduction, linear sweep voltammetry (LSV) was conducted using CO2–saturated

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0.1 M KHCO3 solution in a three-electrode set-up. The LSV curves of Cu, Zn, CuZn-1, CuZn-0.5

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and CuZn-0.1 catalysts are shown in Figure S3. It was observed that CuZn catalysts showed high

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current density as compared to other catalysts. Hence the combination of CuZn significantly

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enhanced the current density. The Cyclic voltammetry (CV) scans were conducted for the

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roughness measurement of Cu, Zn, and CuZn (CuZn-1, CuZn-0.5 and CuZn-0.1) catalysts at

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different scan rates with voltage range from 0.2 to 0.4 V vs. RHE in 0.1 M KHCO3 electrolyte

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purged with N2. The double layer capacitance was obtained from the slope of the current vs. scan

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rate plot. The roughness factor was calculated from the corresponding value of capacitance.

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Assuming that Cu-foil has surface roughness of 1, and by normalizing the capacitance values with

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Cu-foil the roughness factor of other catalysts was calculated (Table 1, Figure S4). Among all,

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CuZn-0.5 catalyst showed highest roughness factor and has about 40-fold increase in surface area

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than Cu foil. Thus, the CuZn alloy catalysts could provide a larger surface area for CO2 reduction

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as compared to Cu foil and Zn.

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Table 1. Measurements of double layer capacitance and roughness factor of catalysts. Sample Roughness Factor Cu 1 CuZn-1 6.09 CuZn-0.5 6.30 CuZn-0.1 4.1 Zn 1.1 *The roughness factors are obtained by normalizing the capacitance values of Cu sample.

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The electrochemical performance of Cu, Zn, CuZn-1, CuZn-0.5 and CuZn-0.1 catalysts

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towards CO2 electroreduction was assessed in 0.1 M KHCO3 at -1.1 V vs. RHE. Figure 4a presents

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the current densities of all catalysts. The current density of CuZn alloy catalysts are relatively

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higher than Cu foil and Zn. The increase in current density illustrates the high reaction rate on the

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catalyst. Hence in the combination of CuZn catalysts, morphology change enhance the surface

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area that contributed to higher current density. H2, CO and HCOOH were detected as major

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products. It is significant that all the CuZn (CuZn-1, CuZn-0.5 and CuZn-0.1) catalysts showed

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high selectivity towards HCOOH. The highest FE of 60% was observed on CuZn-0.5, which is

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almost 4 times higher than Cu foil. However, as the amount of Zn increased in CuZn-1 catalyst, a

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noticeable amount of C2 products (C2H4 and C2H6) were detected. Furthermore, to optimize the

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highest selectivity of CuZn-0.5 catalysts we performed CO2 reduction on CuZn-0.25, and CuZn-

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0.75, the FE% of the obtained products are shown in Figure S5. The CuZn-0.25, and CuZn-0.75

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also showed selectivity towards HCOOH. However, CuZn-0.5 catalyst possess the higher activity

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and selectivity for CO2 reduction among all the catalysts having different concentration of Zn.

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Figure 4. Electrochemical reduction of CO2 on the catalysts: (a) Current density and faradaic

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efficiency ratio of H2, CO, C2H4, C2H6 and HCOOH on different catalysts in 0.1 M KHCO3 at -1.1

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V vs. RHE, (b-d) faradaic efficiency ratios of H2, CO, CH4 and HCOOH on Cu, Zn and CuZn-0.5

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in 0.1 M KHCO3 respectively.

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The electrochemical CO2 reduction of Cu, Zn and CuZn-0.5 catalysts were evaluated at

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different voltage range from -0.9 to -1.3 V. It was observed that current density of CuZn-0.5 was

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highest among all the catalysts as shown in Figure S6. On Cu foil, it was observed that the FE of

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H2 was relatively higher than HCOOH (Figure 4b). The Zn catalyst reduced CO2 to H2, CO and

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HCOOH with increasing FE of CO to a maximum of 55 % at -1.1 V. It is noteworthy that Zn


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showed a high selectivity for CO, in accordance with report that Zn is capable of reducing CO2 to

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CO (Figure 4c).21 On comparison, the FE of HCOOH was found to be greater on CuZn-0.5 catalyst

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at -1.1 V vs. RHE. Thus, the FE was almost 60% which is found to be greater than Cu-foil and Zn,

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but the amount of HCOOH formation displayed a strong and nonlinear dependence on applied

5

potential for CuZn-0.5 catalyst. It can be observed that as the applied potential increases from -0.9

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to -1.1 V, the FE for HCOOH is also increased. However, there is a little decrease in the FE of

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HCOOH at the applied potential of -1.2 and -1.3 V, which may lead to formation of few

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unidentified products (Figure 4d). A comparison of FE of CO, H2 and HCOOH on Cu, Zn and

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CuZn-0.5 catalysts at different applied potential in 0.1 M KHCO3 is shown in Figure S7. The

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preferred formation of HCOOH on CuZn-0.5 catalyst indicated the synergistic effect of Cu and Zn

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that plays the key role towards products selectivity and H2 evolution. As Zn has larger

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overpotential for H2 and weaker adsorption ability for CO, resulting in CO as the main product. 66

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Here, Zn in CuZn catalyst can modify the surface structure and change the binding energy of

14

intermediates, which will further alter the products selectivity. Therefore, the synergistic effect of

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Cu and Zn, enhanced surface area in CuZn catalyst contributed to higher selectivity. By analyzing

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the FE of H2 and CO on Cu and Zn catalysts, the synergistic effect could be demonstrated. The FE

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of H2 and CO on CuZn-0.5 is evidently lower than Cu foil and Zn. On the basis of these

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observations, we postulated that CO formed on the Zn may further react to give HCOOH in CuZn-

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0.5. The overall FE of all the products including (H2, CO, HCOOH) on CuZn-0.5 lies between 86

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to 95%. The products (C2H4, C2H6) with less than 2% FE are negligible.

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From the Figure 5, partial current density of jHCOOH presents the efficacy of the Cu foil, Zn

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and CuZn-0.5 catalyst for producing valuable liquid fuel. The similar trend with respect to FE of

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HCOOH was observed on all these catalysts. CuZn-0.5 showed a high current density of -4.5 mA



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cm-2 for HCOOH at -1.1 V vs. RHE, which is 5 times higher than Cu foil. At atomic scale, the

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combination of Cu and Zn favored the selective CO2 reduction to HCOOH. Further, the stability

3

of CuZn-0.5 was tested at constant voltage (-1.1 V) for 5 cycles (1 h for each cycle) in 0.1 M

4

KHCO3 solution. Figure S8 shows that the FE of HCOOH is slightly increased in the first 3 cycles,

5

and then dropped. However, the FE of HCOOH was still higher than 50 % after 5 cycles, indicating

6

that CuZn-0.5 electrode has a good stability for CO2 reduction.

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Figure 5. Partial current density vs. applied potential for HCOOH on Cu, CuZn-0.5 and Zn

9

catalysts.

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Furthermore, a PEC cell was developed to assess the (photo-)electrochemical performance

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of CuZn-0.5 as cathode towards CO2 reduction, while BiVO4 was employed as photoanode. When

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the photoanode BiVO4 was under irradiation by simulated sunlight, the electrons being excited 14 ACS Paragon Plus Environment

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were shifted towards CuZn-0.5 catalyst for CO2 reduction. The reaction products obtained after

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CO2 reduction are shown in Figure 6a. The HCOOH generation at five different voltages ranging

3

from 0.7 to 0.9 vs. RHE were observed, and noticeable amount of C2 products formation occurred

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at high voltage. Figure 6b shows the FE of CO and HCOOH at different applied potentials. The

5

highest FE of HCOOH is obtained at 1.3 V, which is approximately 60% and in consistence with

6

the performance in the electrochemical cell. At 1.3 V, the current density is measured at 2.5 mA

7

cm-2, indicating a high efficiency of the PEC cell. Therefore, the fabricated CuZn-0.5 catalyst can

8

act as an efficient cathode in a PEC cell for CO2 reduction to HCOOH.

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Figure 6. (a) Products generated by photoelectrochemical cell and (b) faradaic efficiency ratios of

12

CuZn-0.5 catalyst as the cathode in CO2-saturated 0.1 M KHCO3 solution.

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To understand the mechanism of CO2 reduction, the reaction process was investigated by

14

Raman spectroscopy during electrochemical CO2 reduction. In-situ Raman spectroscopy was

15

performed on prepared catalysts in CO2-saturated 0.1 M KHCO3 electrolyte at -1.1 vs. RHE in an

16

electrochemical cell. The Raman spectra of Cu-foil hardly showed any distinct peak during the



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CO2 reduction (Figure 7a). Compared with Cu foil, the Raman spectra of CuZn-0.5 showed distinct

2

peaks (Figure 7b). The CuZn-0.5 catalyst have a roughness surface which may contribute to

3

enhance the surface Raman scattering. 67 At 0 second before applying voltage, CuxO peaks were

4

observed at 526 cm-1 and 630 cm-1.68, 69 These peak disappeared within 60 s when external voltage

5

was applied, indicating that CuxO was reduced to the metallic state. However when voltage was

6

applied, distinct peak appeared at 1026 cm-1, which corresponds to adsorbed carbonate/bicarbonate

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in the interfacial region.68 Moreover after voltage removal for 2 to 20 min, the CO32- peaks still

8

remained. Here we consider that, detected carbonate/bicarbonates are one of the products formed

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during the reduction process of CO2 to CO .71, 72 Specifically, as CO2 molecule accept the electron,

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CO2-(carbon dioxide radical anion) is formed, which will couple with another CO2 to form the

11

C2O4- (carbon dioxide dimer radical anion). Then, the C2O4- intermediate will be attacked by

12

another electron resulting in CO and CO32- formation. The CO32- can also convert to HCO3- after

13

accepting proton. This observation is in agreement with a recently proposed mechanism for

14

electrocatalytic CO2 reduction at Cu metal.73 Additionally, it will provide a new strategy to obtain

15

the intermediates signals during CO2 reduction by in-situ Raman spectroscopy.


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Figure 7. In-situ Raman spectra of (a) Cu-foil and (b) CuZn during CO2 reduction at -1.1 V vs.

3

RHE in 0.1 M KHCO3.

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For better understanding regarding the performance of CuZn alloy catalyst during CO2

5

reduction, first-principle density functional theory (DFT) calculations were conducted. The Cu

6

(100) facet was used for DFT calculations. According to the study, hydrogenation of CO2 to

7

*COOH is primarily involved in CO2 activation, followed by the addition of H* to generate

8

HCOOH.74 The Cu (100) facet shows higher selectivity towards C2 products as compared to Cu

9

(111).75-77 The CuZn alloy catalyst was generated after reducing the Zn2+ to metallic Zn atoms,

10

which further replaced the surface Cu atoms. This leads to CuZn (100) facet which is highly stable

11

and efficient for CO2 hydrogenation as compared to Cu (111). The DFT studies further confirmed

12

the hypothesis that Gibbs free energy for all the products (CO, HCOOH, CH4 and C2H4) obtained

13

on the Cu (111) and CuZn (100) facet at 298.15 k, 1 atm and 0 V vs. RHE, indicating that CuZn

14

lower the energy barrier for all the products (Figure 8). Thus, presence of Zn on the Cu surface is

15

beneficial for intermediate stabilization and lowering energy barrier.



Page 18 of 26

1 2

Figure 8. Possible reaction route of CO2 reduction into CO, HCOOH, CH4 and C2H4 on Cu and

3

CuZn catalysts. The Gibbs free energy of electroreduction of CO2 on Cu (111) and CuZn (100).

4

Based on the in-situ Raman study and DFT calculations, we have proposed that surface

5

modification of Cu with Zn results in bimetallic CuZn interface which permits the CO2 diffusion

6

for reduction and maintain high catalytic activity. Therefore, synergistic effect and active sites of

7

CuZn interface involve in increasing the product selectivity which favors the HCOOH production.

8

Thus, the product selectivity is basically influenced by surface morphology and binding energy of 18 ACS Paragon Plus Environment

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1

the key intermediates. It is suggested that *COOH formation on the catalyst surface initiates the

2

HCOOH generation during CO2 reduction pathway. Tafel plot for Cu and CuZn-0.5 catalysts are

3

shown in Figure S9. For both Cu and CuZn-0.5 catalysts, the slopes are found to be linear and

4

greater than 120 mV dec-1, indicating the rate determining steps are initial single electron flow to

5

form CO2 radical. For the product selectivity, surface adsorption and desorption play an important

6

role. The Zn has very weak surface adsorption for CO*. Therefore, during reaction, the *COOH

7

radical converted to CO* that is desorbed by the metal surface, results in the formation of CO.

8

Thus, at the interface between Cu and Zn on CuZn catalysts surface, adsorption for CO* decreased

9

that results in HCOOH formation as a CO2 reduction product.78 73 In CuZn-0.5 catalyst, surface

10

adsorption and desorption play a key role in enhancing the HCOOH formation and lowering the

11

hydrogen evolution. This proposal is favored by DFT calculations on the free binding energies of

12

hydrogen and *COOH intermediate of CuZn catalyst.79

13

4. CONCLUSIONS

14

The CuZn catalysts reported in this study were prepared with varying amount of Zn to enhance

15

CO2 reduction to various products particularly HCOOH. Further we used this catalyst as electrode

16

in electrochemical CO2 reduction and as cathode in combination with BiVO4 for PEC reactions.

17

The CuZn-0.5 catalyst was capable to achieve high 60% FE as well as 5 times higher current

18

density than Cu-foil. The enhancement in selectivity for HCOOH during CO2 reduction was due

19

to synergistic effect between Cu and Zn sites. The presence of Zn on the surface of Cu is beneficial

20

for intermediate stabilization and lowering energy barrier. Our work provides a detail

21

understanding of fabricating efficient catalysts with high selectivity for HCOOH.



Page 20 of 26

1

COMPETING INTERESTS

2

The authors declare no competing financial interests.

3

ACKNOWLEDGEMENTS

4

This work was jointly supported by National Natural Science Foundation of China (No. 21507011

5

and No. 21677037) and Ministry of Science and Technology of the People’s Republic of China

6

(2016YFE0112200 and 2016YFC0202700).


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TOC Graphic

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We developed a zinc modified copper catalyst by electrodeposition method for efficient and

4

selective CO2 (photo-)electrochemical reduction into HCOOH.


Electrochemical CO2 Reduction with High Selectivity of HCOOH

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