Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc

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Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment Dang Le Tri Nguyen, Michael Shincheon Jee, Da Hye Won, Hyejin Jung, Hyung-Suk Oh, Byoung Koun Min, and Yun Jeong Hwang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02460 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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ACS Sustainable Chemistry & Engineering

Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment

Dang Le Tri Nguyena, b, Michael Shincheon Jeea, c, Da Hye Wona, Hyejin Junga, b, HyungSuk Oh a, Byoung Koun Min a, d, and Yun Jeong Hwanga, b, *

a

Clean Energy Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarang-

ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea b

Division of Energy and Environmental Technology, KIST School, Korea University of Science

and Technology (UST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea c

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Anam-

dong, Seongbuk-gu, Seoul 136-713, Republic of Korea d

Green School, Korea University, 145 Anam-ro, Anam-dong, Seongbuk-gu, Seoul 136-713,

Republic of Korea

Corresponding Author *Email: [email protected] (Dr. Yun Jeong Hwang), Phone: +82-2-958-5227.

ACS Paragon Plus Environment


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Abstract Here, we have developed porous nanostructured Zn electrocatalysts for CO2 reduction reaction (CO2RR), fabricated by reducing electrodeposited ZnO (RE-Zn) to activate the CO2RR electrocatalytic performance. We discovered that the electrochemical activation environment using CO2-bubbled electrolyte during reducing ZnO in pretreatment step is important for highly selective CO production over H2 production, while using Ar gas bubbling instead can lead to less CO product of the Zn-based catalyst in CO2RR later. RE-Zn activated in CO2-bubbled electrolyte condition achieves the Faradaic efficiency of CO production (FECO) to be 78.5%, which is about 10% higher than that of RE-Zn activated in Ar-bubbled electrolyte. The partial current density of CO product had more ten-fold increase with RE-Zn electrodes than that of bulk Zn foil at - 0.95 V vs. RHE in KHCO3. In addition, a very high FECO of 95.3% can be reached by using the CO2pretreated catalyst in KCl electrolyte. The higher amount of oxidized zinc states has been found in the high performing Zn electrode surfaces by high-resolution X-ray photoelectron spectroscopy studies, which suggest that oxidized zinc states induce the active sites for electrochemical CO2RR. In addition, pre- and post-CO2RR performance test, the carbon deposition is also significantly suppressed on RE-Zn surfaces having higher ratio of oxidized Zn state.

Keywords: CO2 reduction reaction, zinc catalyst, CO production, electrocatalysis, pretreatment

Introduction Fossil fuels have been used globally as the energy sources for the economic growth and industrialization over the last century. Nevertheless, heavy dependence on the fossil fuels causes unparalleled high atmospheric concentration of carbon dioxide, a major greenhouse gas contributing to global warming and climate change.1 The climate change issues motivate to develop a sustainable technology to mitigate the impact on environment and human life.2 Electrochemical CO2 reduction is considered as one of the promising processes which utilize and convert CO2 into value-added carbon chemicals3-5 by combining with renewable energy sources. Several bulk metallic catalysts have been identified to be able to catalyze CO2 reduction reaction (CO2RR) but low energy efficiency, product selectivity, and catalytic stability are still the limitations.6-7 The initiation of CO2RR is known to require a large negative overpotential due


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to the unstable intermediate state. Also, hydrogen evolution reaction (HER) is a prevalent competitive reaction that decreases CO2RR activity in an aqueous electrolyte due to their similar equilibrium potentials. Recently, nanostructured metallic catalysts, which possess distinct structural properties over bulk catalysts, have been developed to have enhanced performance for CO2RR.8 For instance, Kanan et al. discovered that the grain-boundaries of oxide-derived Au or Cu nanoparticles contribute to the improved performance for selective CO2RR.9-10 In addition to the defect structure, high index facets, edge sites, or surfaces are suggested to be the active sites for CO2RR.11-14 During the last few years, superior activities over 90% of CO Faradaic efficiency have been realized with special nanostructures of Au or Ag electrocatalysts.15-18 Meanwhile, Zn is another still promising metallic catalyst for selective CO production7 that is a cost-effective alternative to Au and Ag. Zinc is a non-noble metal with low price and in abundant reserves. Remarkably, bulk Zn metal was historically reported to catalyze for CO2-toCO conversion, so Zn can be a promising metal to replace precious metals in CO2RR. A few reported nanostructured Zn-based catalysts including dendritic Zn,19 nano scale Zn,20 and hexagonal Zn12 have demonstrated. High efficiency and product selectivity in electrochemically reducing CO2 to CO can be achieved by changing surface orientation or the morphology of Zn electrocatalysts. The previous studies mostly found that the morphological or structural changes of metal nanostructures are the main factor to the improved CO2RR activities. Lately, the increase of activity and selectivity induced by the chemical states variation is emphasized as an important activation factor as well. For example, Cu+ state remaining on the Cu surface under reducing environment is suggested to be active for selective ethylene production from CO2 reduction.21 Also, we have recently reported the presence of the residual stable surface oxygen species on nanostructured Ag electrocatalysts after the reduction from the oxidized Ag electrodes which have efficient CO2 reduction activity and selectivity to CO product.18, 22 The surface of Zn metal is facilely oxidized even when exposed to air or immersed in aqueous electrolytes. Therefore, it is expected that the oxidized Zn or some interaction between Zn and O element can be controlled by manipulating the oxidation-reduction process which can influence CO2 reduction activity, but systematic works has not been studied. Besides, no work specifically studied on the importance of reduction condition from its metal oxide, which may have a strong influence on the final activity, selectivity and stability of catalyst



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caused by changing not only the surface morphology of obtained catalysts but also the chemical states of some significant components. Here, we demonstrate that porous nanostructured Zn based catalysts reduced from ZnO can enhance electrochemical activity of CO2 to CO conversion with high efficiency and selectivity. ZnO is prepared by a cathodic electrodeposition method, and porous surfaces are generated by applying cyclic voltammetry as an activation step before using in CO2RR to enhance catalytic activity and selectivity for CO. Remarkably, electrolyte environment during reduction pretreatment step plays a role for the catalyst activity. The purging CO2 gas in the electrolyte simulated the CO2RR condition during the activation step is expected to induce surface reconstruction possessing more active sites for CO2RR compared to the electrolyte purged with Ar gas, a condition for hydrogen evolution reaction. In addition, the more oxidized zinc proportion relates to the enhancement of selective CO production and is obtained when bubbling CO2 gas in reduction pretreatment. Furthermore, the increase of carbon deposit post CO2RR is more suppressed in case of Zn electrocatalysts having higher oxidized zinc ratio and high CO Faradaic efficiency.

Experimental Section Electrodeposited ZnO preparation The electrodeposited ZnO (E-ZnO) thin film was grown on a Zn foil by cathodic electrochemical deposition from 50 mL electrolyte containing 0.01 M Zn(NO3)2 (Sigma-Aldrich, ≥99.0%), 0.01 M H2O2 (Yunsei, 30% solution), 0.01 M hexamethylenetetramine (HMTA; C6H12N4) (Sigma-Aldrich, ≥99.0%), and 0.1 M ammonium chloride (Sigma-Aldrich, ≥99.5%). The sources of zinc and oxygen for the electrochemical growth of E-ZnO are zinc nitrate and hydrogen peroxide, and the bath temperature was maintained at 60 oC during the electrodeposition of E-ZnO.23-25 The presence of HMTA and ammonium chloride assists to grow hexagonal E-ZnO nanowire arrays.23, 26 Clean Zn foil (Alfa Aesar, 99.98%, 0.25 mm) electrodes were prepared by mechanical polishing using sand paper (Daesung, CC-80CW) and cleaned by sonication in acetone, methanol, and finally deionized (DI) water, sequentially. For all the electrochemical preparation and measurements were carried out in a three-electrode system with a platinum sheet and an Ag/AgCl electrode in 3M KCl solution were used as a counter electrode and a reference electrode, respectively. A negative potential at 0.8 V relative to the reference electrode was applied to the cleaned Zn foil working electrode.


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After 2 hours of growth time, the working electrode was immediately rinsed with DI water, dried under vacuum at room temperature. E-ZnO can be synthesized by initially generating of OHions by electrochemical reduction of precursors such as O2, NO3-, and H2O2 in aqueous solution.24 These OH- ions then react with the zinc ion present in the solution to form zinc oxide via dehydration of zinc hydroxide at 60 oC.24, 27 The chemical reactions are described as follows: NO3- + H2O + 2e- → NO2- + 2OH-

(1)

H2O2 + 2e- → 2OH-

(2)

Zn2+ + 2OH- ↔ Zn(OH)2 ↔ ZnO + H2O

(3)

Zn based electrocatalyst preparation A polycrystalline Zn foil is mechanically polished to be used as bare Zn electrode. For other Zn-based electrodes, electrochemical pretreatment was applied to the E-ZnO before using for an elecrocatalyst to obtain a reduced E-ZnO electrode (denoted as RE-Zn). The cyclic voltammetry (CV) was carried out from -0.77 V to -1.27 V vs. RHE at scan rate of 50 mV/s for 15 mins, and the pretreatment conditions such as the types of electrolyte and bubbling gas were varied. RE-Zn-Ar electrode was prepared as the result of the reduction pretreatment in the aqueous 0.5 M KHCO3 electrolyte with Ar gas flow. The reduction pretreatment was carried out in the same manner, but Ar gas was replaced with CO2 gas to obtain RE-Zn-CO2. Meanwhile, for RE-Zn-CO2/KCl electrode, 0.5 M KCl electrolyte purged with CO2 gas bubbling was used as the replacement for 0.5 M KHCO3 electrolyte during pretreatment step. Each preparation condition is summarized in the Scheme 1. CO2 reduction reaction measurement The electrochemical CO2RR measurements were conducted in a gas-tight polyether ether ketone (PEEK) cell separated into two compartments by a piece of a proton exchange membrane (Nafion®, 117). Both compartments contained 38 mL aqueous 0.5 M KHCO3 (Sigma-Aldrich, ≥99.99%) electrolyte when Zn foil, RE-Zn-Ar, and REZn-CO2 catalysts were tested, while 0.5 M KCl (Sigma-Aldrich, ≥99.99%) instead of KHCO3 was used for the RE-Zn-CO2/KCl electrocatalyst. The solution was purged with 20 sccm CO2 until saturated (pH 7.2) for 1 hour before the CO2RR and the CO2 flow was continuously flowed during the CO2RR. An Ag/AgCl reference electrode and the working electrode (i.e., mechanically polished Zn foil, RE-Zn-Ar, RE-Zn-CO2, or RE-Zn-CO2/KCl) were positioned in the catholyte while the Pt sheet played a role as a counter electrode in the anolyte. The catholyte was stirred at 350 rpm with a magnetic stir bar. Constant potential was applied using a



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potentiostat (CHI Instruments), and all the measured current was normalized by the geometric electrode area unless noted as normalized by the measured electrochemical surface area. The gaseous products were analyzed by gas chromatography (GC, Younglin 6500) equipped with a capillary column (Restek, RT-Msieve 5A) and pulsed discharge ionization detector (PDD) using ultra high purity (UHP, 99.9999%) He as the carrier gas. The partial current density of products ( ) and Faradaic efficiency (FE ) were calculated as the following: = × × FE =

× 100

(4) (5)

where is the volume concentration of H2 or CO based on calibration of the GC, is the flow rate measured by a universal flow meter (Agilent Technologies, ADM 2000) at the exit of the electrochemical cell, !" is the total current density, # is the Faradaic constant, $% is pressure, & is the ideal gas constant, and ' is the temperature. Liquid products were also analyzed by ion chromatography (IC, DIONEX IC25A). The procedure was repeated for various potentials. Solution resistance was obtained by measuring the electrochemical impedance spectroscopy (EIS) at various potentials. The measured potentials for CO2RR were compensated for iR loss, and were reported versus the reversible hydrogen electrode (RHE) by using the following equation. ((*+. RHE) = ((*+. Ag/AgCl) + 0.197V + (0.0591 × pH)

(6)

Material Characterization The field emission gun scanning electron microscopy (FEG-SEM, Inspect F, FEI) was used to observe the morphology of the samples. The morphology image and crystal structure information from selected area diffraction (SAED) patterns were also observed by high resolution transmission electron microscopy (HRTEM, Titan 80-300TM, FEI). The crystal structure analysis was confirmed by using X-ray diffraction (XRD, Rigaku, D/max 2500) operating at 40 kV and 200 mA with Cu-Kα radiation (λ = 0.154 nm). High-resolution X-ray photoelectron spectroscopy (XPS) experiments were performed to analyze the surface element composition. XPS with monochromated aluminum Al Kα (1486.4 eV) anode (72 W, 15 kV) radiation was conducted. Binding energies were calibrated based on the the Au 4f peak at 84.0 eV as a reference.


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Results and Discussion Figure 1 shows high-resolution scanning electron microscope (SEM) images of Zn foil, EZnO, RE-Zn-Ar, and RE-Zn-CO2. The mechanically polished Zn foil electrode showed a flat and rock-like surface while arrays of vertically aligned hexagonal nanowires on the electrode surface were obtained by electrodeposition for as-prepared E-ZnO (Figure 1a, b). At a few positions on the surface at low-resolution SEM (Figure S1), the presence of some octahedral shape was observed (Figure S1b), possibly due to the incomplete decomposition of Zn(OH)2 to ZnO in the preparation procedure.23 After electrochemical activation step, the surface nanostructure and morphology of RE-Zn-Ar and RE-Zn-CO2 (Figures 1c, d) changed into smaller particles in comparison with E-ZnO. The crystal structures of E-ZnO, RE-Zn-Ar, and RE-Zn-CO2 were analyzed by X-ray diffraction (XRD) in the range of 2θ from 20o to 80o in Figure 2. All the diffraction peaks of the Zn foil are precisely matched with the hexagonal metallic Zn pattern (JCPDS No. 87-0713). XRD patterns of E-ZnO indicates that the as-prepared one has the wurtzite ZnO (JCPDS No. 800075) and the weaker intensity of Zn(OH)2 (JCPDS No. 76-1778) patterns as well. Meanwhile, RE-Zn-Ar and RE-Zn-CO2 have the similar XRD patterns to those of metallic Zn foil as the result of the electrochemical reduction. However, the diffraction peak for the (002) is dominant for Zn foil while the (101) reflection becomes the dominant phase in RE-Zn-Ar and RE-Zn-CO2, similar to the powder Zn pattern (JCPDS No. 87-0713). Although the zinc (101) facet is demonstrated to contribute to the good activity of electroreduction catalyst on CO2 reduction to CO,12 in this study, the facet effect of Zn crystal structure cannot explain the differences between the performance of RE-Zn-Ar and RE-Zn-CO2, because of the almost identical XRD patterns of these two samples. The changes of the morphology and crystal structure are also consistent with HRTEM measurement results (Figure S2). A single particle of E-ZnO had a rod-like shape whose SAED patterns matched with the hexagonal wurtzite ZnO phase (Figures S2a-c), while those of RE-Zn-Ar and RE-Zn-CO2 are smaller and have hexagonal crystal structures of metallic Zn (Figures S2d-i and S2g-i, respectively). The electrocatalytic reduction of CO2 on bulk Zn foil, RE-Zn-Ar, and RE-Zn-CO2 were performed at selected potential range from -0.75 to -1.15 V (vs. RHE) in CO2-saturated 0.5 M



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KHCO3 electrolyte (pH 7.2) at ambient temperature and pressure. The main products obtained during the electroreduction of CO2 process were only CO and H2 measured by gas chromatography. Liquid products (such as formic acid) was not detectable by the ion chromatography measurement, similar to the previous studies.12, 19 The comparison of CO2RR activity of bulk Zn foil, RE-Zn-Ar, and RE-Zn-CO2 is presented in Figure 3. The FEs of CO were significantly improved on the RE-Zn-Ar and RE-Zn-CO2 compared to the Zn foil for all potential regions, and especially lower potential region showed much enhanced selectivity. The FECO reached 71.3% at -0.75 V (vs. RHE) on the RE-Zn-CO2, which had almost 15% higher FECO than RE-Zn-Ar, as well. In addition, the CO partial current densities (jCO) of RE-Zn-Ar and RE-Zn-CO2 catalysts were also highly improved compared with those of Zn foil. These current densities were obtained by normalized by the geometric electrode area. To be specific, at -0.95 V (vs. RHE), jCO of REZn-Ar and RE-Zn-CO2 had more ten-fold increase than that of bulk Zn foil. To understand whether the current density was enhanced due to the increased surface area of RE-Zn, we measured the electrochemical surface area (ECSA, seen in Figure S3) or surface roughness factors (Table S1), which showed that both nanostructured RE-Zn-Ar and RE-Zn-CO2 catalysts have about five-fold increased surface areas compared to bulk Zn foil surface. The CO partial current densities normalized by the roughness factors (jCO/ECSA) are also compared as shown in Figure S4. It is found that RE-Zn catalysts have inherently enhanced CO production rates at low overpotential regions (< -1.0 V vs. RHE) while they become rather smaller than bulk Zn foil under large bias potentials. Remarkably, the RE-Zn-CO2 catalyst exhibited a higher FECO and suppressed HER than the corresponding values for the RE-Zn-Ar catalyst (Figure 3a) although the only difference between these two catalysts is the bubbled gas during the pretreatment step to reduce E-ZnO to metallic catalyst. It is believed that there was an influence on the activity of zinc surface electrode which favors CO more than H2 when using CO2-bubble in replacement of Ar gas during the pretreatment step. Thus, more analyses on the zinc surface are required to explain the improvement of prepared catalysts and the different activities between RE-Zn-Ar and RE-Zn-CO2. Some previous studies have discussed the enhancement of FE for CO2RR to CO promoted by the adsorbed chlorine ions on the metal catalyst surfaces.12, 20, 28-29 Despite a highly enhanced performance of RE-Zn-CO2 catalyst, an expectation to further optimize the higher FECO still was


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suspected by manipulating the role of chlorine anion in this research. Therefore, RE-Zn-CO2/KCl catalyst was prepared by reducing E-ZnO and testing its electrocatalytic ability in CO2-saturated 0.5 M KCl electrolyte. Interestingly, RE-Zn-CO2/KCl surface catalyst, as seen in Figure 4a and TEM images (Figure S2), compose of the porous nanoparticles but having smaller particles size than the other RE-Zn catalysts. The interaction between chlorine ion and the Zn electrode surface is proposed to have an effect in forming smaller nanoparticles. Compared with RE-Zn-Ar or REZn-CO2, the surfaces of RE-Zn-CO2/KCl catalyst have porous and rougher morphology (Figures S2j-l), which can enhance the surface areas associated with more active sites for CO2RR. The direct comparison in the particle sizes of the RE-Zn-CO2 and RE-Zn-CO2/KCl catalysts by SEM is presented in Figure S5. In addition, a dramatic increase of the ECSA was obtained with REZn-CO2/KCl, whose surface area was increased by 23-fold than that of Zn foil. This value is four times larger than that of RE-Zn-CO2 as well. In other words, the surface morphology and area can be changed presenting the smaller size of particles and a huge increase of surface area depending on the types of the electrolyte, which can be another factor to affect the selectivity of the CO2RR as porous structures are proposed to be selective for CO production over HER in the previous studies.14, 30 The XRD patterns of RE-Zn-CO2/KCl catalyst is also confirmed the conversion to the metallic Zn by the reducing pretreatment step, and the relative peak intensity between (002) and (101) facets is also similar with other nanostructured RE-Zn catalysts (Figure 4b). Although the total current density decreased, the FECO on the RE-Zn-CO2/KCl catalyst had an extremely high value over 90% (-0.95 V ~ -1.25 V vs. RHE as seen in Figure 4c). The FECO on the RE-ZnCO2/KCl catalyst in comparison with Zn foil, RE-Zn-Ar, and RE-Zn-CO2 was also illustrated in Figure 4d. An increase of FECO on the RE-Zn-CO2/KCl catalyst was up to 94.7% at -0.95 V (compared to the maximum FE of 78.5% in 0.5 M KHCO3 electrolyte) obtaining a maximum value of 95.3% at -1.1 V. Obviously, the FECO of the RE-Zn-CO2/KCl was enhanced to be superior than other catalysts at potentials from -0.85 V to -1.15 V. Partial current densities of CO and H2 were also compared in Figure 4e and 4f, respectively, and it is clear that RE-Zn-CO2 and RE-Zn-CO2/KCl have enhanced selectivity for CO product compared to Zn foil by the different reasons. In other words, highly enhanced CO partial current density contribute to high FECO with RE-Zn-Ar and RE-Zn-CO2. However, the effective suppression of the hydrogen evolution reaction is recognized as the major origin of high CO selectivity by RE-Zn-CO2/KCl catalysts. A



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converse decrease of CO partial current was obtained as a result of the presence of chloride ions in the electrolyte. The stability performance profile of the RE-Zn-CO2/KCl was examined by collecting the current density and FECO at -1.1 V vs. RHE over 20 hours (Figure S6) of CO2RR measurement. The RE-Zn-CO2/KCl exhibited somewhat stable operation of electrolysis with slight increase of the total current density but degradation of 15% of FECO after 20 hours. In addition, the CO2RR performances of our RE-Zn-CO2 and RE-Zn-CO2/KCl catalysts were compared with other reported Zn-based catalysts (Table 1). Although our Zn-based catalysts did not mark some records, their activities on CO2RR are still at good level when comparing with other studies. To further examine the differences in the electronic states of the zinc electrodes, analysis with high resolution XPS was conducted (Figure 5). Zn 2p photoelectron spectrum of bare Zn foil electrode exhibits two main peaks around 1045.4 and 1022.2 eV (Figure S7), which are assigned to Zn 2p1/2 and Zn 2p3/2, respectively. Zn 2p spectra of RE-Zn-Ar, RE-Zn-CO2, and REZn-CO2/KCl samples are shifted slightly toward higher binding energies showing the peak positions of Zn 2p3/2 at 1022.7 eV, 1022.8 eV, and 1022.9 eV, respectively (Figure 5a). Although all RE-Zn samples had similar XRD patterns with that of Zn foil, indicating E-ZnO film was mostly reduced to the metallic Zn of the same crystal structure by the electrochemical pretreatment step, the shifts of their XPS Zn 2p peak to higher binding energies suggest the presence of oxidized zinc species (i.e. Zn2+) on the surface.19, 31 Figure 5 reveals that both Zn 2p3/2 and O 1s XPS spectra of Zn foil are clearly distinguished from the other RE-Zn electrodes, and there were slight variations of the zinc states among the three RE-Zn electrodes depending on the pretreatment manner, which can be observable from the deconvolution. The deconvoluted signals indicates XPS Zn 2p3/2 spectra of the zinc electrodes are composed of the three different peaks centered at 1022.1, 1022.7 and 1023.1 eV (Figure 5a). They are associated with the metallic zinc (Zn0 state) (1022.1 eV),32 and two oxidized zinc species, i.e. Zn–O bonding (1022. 7 eV)32-33 or hydroxyl form with Zn-OH bonding (1023.1 eV),34-36 respectively. Zn-foil sample contained the highest proportion of metallic Zn0 state and smaller amounts of the oxidized states plausibly induced by the naturally oxidized zinc surface. However, the proportion of Zn2+ signal profoundly increased in the three RE-Zn electrodes, (RE-Zn-Ar, RE-Zn-CO2, and RE-ZnCO2/KCl). Notably, RE-Zn-CO2 has smaller ratio of the metallic Zn0 state than RE-Zn-Ar although only the bubbling gas was replaced during the reduction pretreatment step between the


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two samples. In addition, the largest ratio of oxidized zinc signal was observed on RE-ZnCO2/KCl where impurity chlorine element was detected (Figure S8). In detail, the amounts of every component in XPS spectra and the proportion of oxidized Zn peak area were compared (Table 2, and Table S2). Some previous studies reported the presence of oxidized zinc species in catalyst according to the higher binding energy of Zn 2p spectra but did not specifically discuss on its role on CO2RR.12, 19 A correlation between the oxidized zinc species on the Zn electrodes and FE of CO2RR to CO is proposed in this research. According to the XPS analysis, the ratio of oxidized Zn peak area and total peak area in Zn foil, RE-Zn-Ar, RE-Zn-CO2, RE-Zn-CO2/KCl are 0.36, 0.65, 0.75, and 0.83, respectively (Table S2). Interestingly, higher FECO is obtained with the Zn catalyst containing higher proportion of oxidized Zn states (Figure 6a). The catalytic enhancement of catalysts contained high oxidized Zn proportion suggests that Zn2+ species involve the descriptor for selective CO production. Note this XPS analysis is ex-situ procedure which has the limitation to assign the active species, but our hypothesis attempts to explain the high performance of catalyst with possessing high oxidized Zn species. One possible explanation of this correlation is that the electrodeposition of E-ZnO followed by the electrochemical reduction leads to a higher surface area and highly roughened surfaces in a form of nanostructured porous with under-coordinated sites, which can be a storing oxygen reservoir or favorably bond more strongly to have higher proportion of oxidized zinc. These sites may facilitate the adsorption of reactant or COOH* intermediates to activate CO2RR to CO production. Strong stabilization energy of COOH* intermediates on the oxide-derived metal electrocatalyst surface was observed in the previous study.15 It is also consistent with the recent study proposing the critical contribution of the residual subsurface oxide for CO2RR on Cu surface by affecting the binding strength of the reaction intermediates as well.37 Also, the cooperative interaction of adsorbed CO2 and H2O on Cu (111) composed of subsurface oxide as a result of the existence of subsurface oxygen is responsible for activating and stabilizing the first intermediate for CO2RR.38 In addition, we have reported in our previous studies the existence of stable surface oxygen on some nanostructured silver electrodes generated by electrochemical oxidation-reduction process for efficient CO2RR to CO product.18, 22 We observed the chemical states of the oxygen substantially changed on the RE-Zn electrodes compared to the bare Zn foil but the similar oxygen binding energies among the three



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RE-Zn electrodes. The O 1s XPS peaks can be deconvoluted into three peaks (Figure 5b). A lower binding energy peak is centered at 531.05 eV corresponding to O2- ions in metal-O bonding,23,

39-40

while another peak is located at 532.45 eV, typically attributed to hydroxyl

groups on the surface.41-42 The high binding energy peak at 533.2 eV can be associated with adsorbed water molecules.43-45 The proportion of O 1s peaks at 532.4 and 533.2 eV, associated with surface hydroxyl group or water molecule, was low in Zn foil compared to all RE-Zn samples which have shown relatively similar O states. Zn foil could possess a native oxide layer only by spontaneous contact to the air and moisture while all RE-Zn samples can be oxidized as a result of the electrodeposition of E-ZnO followed by the reduction pretreatment conducted in the aqueous electrolyte conditions. In addition, the atomic percentages of O on the Zn electrode surface slightly increased in the order of Zn foil, RE-Zn-Ar, RE-Zn-CO2, to RE-Zn-CO2/KCl, which is the same trend of the oxidized zinc states on these electrodes. Next, the surface morphological properties of the electrodes post CO2RR were characterized by SEM and XRD. Figure S9 shows the SEM images of Zn foil, RE-Zn-Ar, RE-Zn-CO2, and RE-Zn-CO2/KCl after 5 hours of CO2RR, respectively. As can be seen, the nanostructured surface was kept with no significant changes in morphology comparing to pre-CO2RR SEM images. Furthermore, XRD patterns of Zn foil, RE-Zn-Ar, RE-Zn-CO2, and RE-Zn-CO2/KCl post-CO2RR exhibited same as the respective pre-CO2RR ones and matched with the metallic Zn (Figure S10). On the other hand, XPS C 1s spectra are also compared pre- and post-CO2RR on the Zn based electrode showing noticeable increase of the carbon percentage after CO2RR only on the Zn electrocatalysts exhibiting poorer CO2RR activities (i.e. Zn foil and Zn-RE-Ar) (Figure 6b). However, the carbon amount remained similar between pre- and post-CO2RR for RE-Zn-CO2 or RE-Zn-CO2/KCl electrode which showed high selectivity for CO production as the result of CO2RR in the aqueous condition. The detail C 1s spectra can be fit to peaks at 285.9 ± 0.3, 286.9 ± 0.2, and 290.5 ± 0.5 eV (Figure S11), and thus assigned to sp3-hybridized carbon atoms (C-C bonding), C-O bonding, and O-C=O bonding, respectively.46-48 It can be seen that the peak area of C-C bonds tends to especially increase during CO2RR with Zn foil and RE-Zn-Ar samples. The poisoning of catalysts due to an increase of carbon deposit have been considered as a deactivation factor for CO2RR which decreases CO production and increases HER.49 An assumption was supposed to explain the high catalytic performance of CO2-bubbled catalysts in


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reduction pretreatment that CO2-bubble gas can activate the low-coordinated sites for CO2RR by changing the surface structure with stable oxygen species and high oxidized Zn active sites leading to the favorable adsorption of CO2 reduction intermediates. Assume that carbon would bind to the uncoordinated sites as well, the oxidized Zn species might have another unexpected benefit of hindering carbon deposition that poisons the catalysts. Overall, according to CO2RR catalytic activity and XPS analysis of Zn foil, RE-Zn-Ar, REZn-CO2, and RE-Zn-CO2/KCl pre- and post-CO2RR, we propose that the reducing environment during the reduction pretreatment could have an important role for the catalytic activity in CO2RR. Due to the CO2 gas feeding during pretreatment activation step at negative potential windows, the similar conditions of the real CO2RR is created. Therefore, the reduction reaction of bubbled CO2 gas to CO can be simultaneous with the conversion of E-ZnO to reduced metallic Zn catalysts (RE-Zn-CO2 or RE-Zn-CO2/KCl) during the activation step using CV, and then might rearrange the RE-Zn-CO2 surface structures to favor CO2RR intermediates, which exposes more active sites for CO2RR. Meanwhile, bubbling Ar, an inert gas providing more reducing environment, results in less proportion of oxidized Zn state, and simulating the analogous environment as HER, during reduction pretreatment, may be unfavorable. As a result of that, the achieved catalyst surface might favor more metallic Zn proportion, which can be adsorbed by carbon impurity on the surface associated with lower CO2RR activity. This hypothesis is proposed based on the analysis of ex-situ XPS data, but density functional theory (DFT) or another in-situ method is required to understand better the role of oxidized Zn species in the catalysts and the mechanism behind the catalytic improvement for CO2RR.

Conclusions We have synthesized the porous nanostructured Zn catalysts by the electrochemical deposition of ZnO followed by reduction in various electrochemical activation treatment conditions and have discovered a critical role of the bubbled gas in the treatment environment. Simulating CO2 reduction reaction during the activation step by bubbling CO2 gas in the electrolyte generates catalysts showing higher CO production selectivity compared to the Zn electrode treated in Ar-bubbled electrolyte instead. Highly improved selectivity (95.3% of CO Faradaic efficiency) was also demonstrated when Zn based electrocatalyst was prepared in KCl electrolyte which successfully suppressed the competitive hydrogen evolution reaction.



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Significantly, we found that the high performance of CO2RR towards CO product is related with the presence of high proportion of oxidized zinc species on the nanostructure Zn electrode. The Zn2+ rich surface is proposed as an indicator to have enhanced catalytic activity and selectivity for CO2RR to CO. In addition, the Zn electrocatalysts containing high Zn2+ proportion also have resistance to the carbon deposition after CO2RR. The results obtained in this study with a lowcost and simple synthesis of Zn catalyst but high efficiency and selectivity to CO production in CO2RR, competitive to precious metal such as Au and Ag, are promising development to provide a more practical option for industrial application.

Associated Content Supporting information The supporting information is available free of charge on the ACS Publication website at DOI: . SEM and HR-TEM images, electrochemical surface area (ECSA) measurements, current density (jCO/ECSA) normalized by the ECSA roughness factors, XPS spectra analysis, and stability test of catalysts

Author Information Corresponding Author *Email: [email protected], Phone: +82-2-958-5227. Notes The authors declare no competing financial interest.

Acknowledgments


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The authors acknowledge the support from the Korea Institute of Science and Technology (KIST) institutional program and partly by the KU-KIST program by the Ministry of Science, ICT and Future Planning.

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Scheme 1. Zn-based electrocatalyst preparation.



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Figure 1. SEM images of (a) Zn foil, (b) as-prepared E-ZnO, (c) RE-Zn-Ar, and (d) RE-ZnCO2.


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Figure 2. XRD patterns of Zn foil, as-prepared E-ZnO, RE-Zn-Ar, and RE-Zn-CO2 samples.



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Figure 3. Electrochemcial CO2 reduction reaction activities of Zn-based electrocatalysts (Zn foil, RE-Zn-Ar, and RE-Zn-CO2): (a) CO Faradaic efficiency, and (b) CO partial current density.


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Figure 4. (a) SEM images, (b) XRD patterns, and (c) CO2RR performance of RE-ZnCO2/KCl. (d) Faradaic efficiency (green-dense) comparison with Zn foil (black-filled), RE-ZnAr (red-filled), RE-Zn-CO2 (blue-filled), and (e) CO current density, and (f) H2 production partial current density of all samples.



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Figure 5. XPS spectra of (a) Zn 2p3/2 and (b) O 1s peaks of Zn foil, RE-Zn-Ar, RE-Zn-CO2, RE-Zn-CO2/KCl pre-CO2RR.


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Figure 6. (a) Faradaic efficiency for CO production vs. area ratio of oxidized Zn states showing high selectivity with more oxidized Zn electrode, and (b) carbon atomic percentage of all samples pre- and post-CO2RR.



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Table 1. Comparison of CO2RR performance on various Zn-based catalysts.

a

This value is converted to RHE scale based on the information in the article.

b

This value is not mentioned in the article but derived from the graphical results.

Table 2. Comparison of CO production selectivity and XPS analysis.

1

Faradaic efficiency values of CO product measured at -0.95 V vs. RHE.


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For Table of Contents Use Only TOC & Synopsis

In electrochemical CO2 reduction catalysis, the relation between oxidized zinc states and selective CO production is demonstrated on the Zn electrodes reduced from oxide.


Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc

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