Research Article pubs.acs.org/journal/ascecg
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*,†,‡ †
Clean Energy Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ‡ Division of Energy and Environmental Technology, KIST School, Korea University of Science and Technology (UST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea § Department of Chemical and Biological Engineering and ∥Green School, Korea University, 145 Anam-ro, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea S Supporting Information *
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 a 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 Znbased catalyst in CO2RR later. The RE-Zn activated in CO2-bubbled electrolyte condition achieves a Faradaic efficiency of CO production (FECO) of 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 10-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 using the CO2-pretreated catalyst in KCl electrolyte. The higher amount of oxidized zinc states has been found in the high performing Zn electrode surface by high-resolution Xray photoelectron spectroscopy studies, which suggest that oxidized zinc states induce the active sites for electrochemical CO2RR. Additionally, in pre- and post-CO2RR performance tests, the carbon deposition is also significantly suppressed on RE-Zn surfaces having a higher ratio of oxidized Zn state. KEYWORDS: CO2 reduction reaction, Zinc catalyst, CO production, Electrocatalysis, Pretreatment
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INTRODUCTION Fossil fuels have been used globally as energy sources for economic growth and industrialization over the last century. Nevertheless, heavy dependence on fossil fuels causes unparalleled high atmospheric concentrations of carbon dioxide, a major greenhouse gas contributing to global warming and climate change.1 The climate change issues have motivated the development of sustainable technologies to mitigate the impact of pollution on the environment and human life.2 Electrochemical CO2 reduction is considered as one of the promising processes which utilize and convert CO2 into valueadded carbon chemicals3−5 by combination with renewable energy sources. Several bulk metallic catalysts have been identified to catalyze the CO2 reduction reaction (CO2RR), but low energy efficiency, product selectivity, and catalytic stability are still limitations.6,7 The initiation of CO2RR is known to require a large negative overpotential due 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. © 2017 American Chemical Society
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 past 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 an inexpensive, abundant, and non-noble metal. Remarkably, bulk Zn metal was historically reported to catalyze for CO2-to-CO conversion, so Zn can be a promising metal to replace precious metals in CO2RR. A few reported Received: July 21, 2017 Revised: October 11, 2017 Published: October 23, 2017 11377
DOI: 10.1021/acssuschemeng.7b02460 ACS Sustainable Chem. Eng. 2017, 5, 11377−11386
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Zn-Based Electrocatalyst Preparation
nanostructured Zn-based catalysts including dendritic Zn,19 nanoscale 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 state variation is emphasized as an important activation factor as well. For example, the Cu+ state remaining on the Cu surface under a 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 elements can be controlled by manipulating the oxidation−reduction process which can influence CO2 reduction activity, but systematic works have not been studied. Besides, no work specifically studied the importance of reduction conditions from its metal oxide, which may have a strong influence on the final activity, selectivity, and stability of the catalyst caused by changing not only the surface morphology of the 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 CO2RR to enhance catalytic activity and selectivity for CO. Remarkably, the electrolyte environment during the reduction pretreatment step plays a role for the catalyst activity. The purging CO2 gas in the electrolyte stimulated the CO2RR condition during the activation step; this 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 greater 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 deposits after CO2RR is suppressed more in the case of Zn
electrocatalysts having a higher oxidized zinc ratio and high CO Faradaic efficiency.
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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 EZnO are zinc nitrate and hydrogen peroxide, and the bath temperature was maintained at 60 °C 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 3 M KCl solution; these 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. After 2 h of growth time, the working electrode was immediately rinsed with DI water and dried under vacuum at room temperature. E-ZnO can be synthesized by initially generating OH− ions 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 °C.24,27 The chemical reactions are described as follows: NO3− + H 2O + 2e− → NO2− + 2OH−
(1)
H 2O2 + 2e− → 2OH−
(2)
Zn 2 + + 2OH− → Zn(OH)2 ↔ ZnO + H 2O
(3)
Zn-Based Electrocatalyst Preparation. A polycrystalline Zn foil is mechanically polished to be used as bare Zn electrode. For other Znbased electrodes, electrochemical pretreatment was applied to the EZnO 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 to −1.27 V vs RHE at scan rate of 50 mV/s for 15 min, and the pretreatment conditions such as the types of electrolyte and bubbling gas were varied. The 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 the RE-Zn-CO2/KCl electrode, 0.5 M KCl electrolyte purged with CO2 gas bubbling was used as the replacement for the 0.5 M KHCO3 electrolyte during the 11378
DOI: 10.1021/acssuschemeng.7b02460 ACS Sustainable Chem. Eng. 2017, 5, 11377−11386
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. SEM images of (a) Zn foil, (b) as-prepared E-ZnO, (c) RE-Zn-Ar, and (d) RE-Zn-CO2. where VH2 orCO is the volume concentration of H2 or CO based on calibration of the GC, Q is the flow rate measured by a universal flow meter (Agilent Technologies, ADM 2000) at the exit of the electrochemical cell, itotal is the total current density, F is the Faradaic constant, p0 is pressure, R is the ideal gas constant, and T 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) using the following equation.
pretreatment step. Each preparation condition is summarized in Scheme 1. CO2 Reduction Reaction Measurement. The electrochemical CO2RR measurements were conducted in a gastight 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 RE-Zn-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 h 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-ZnCO2/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 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 (i H2 orCO) and Faradaic efficiency (FE H2 orCO) were calculated as follows: i H2 orCO = VH2 orCOQ
FE H2 orCO =
i H2 orCO i total
E(vs RHE) = E(vs Ag/AgCl) + 0.197V + (0.0591pH)
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 80-300, 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 Au 4f peak at 84.0 eV as a reference.
2Fp0 RT
× 100
(6)
(4)
(5) 11379
DOI: 10.1021/acssuschemeng.7b02460 ACS Sustainable Chem. Eng. 2017, 5, 11377−11386
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ACS Sustainable Chemistry & Engineering
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RESULTS AND DISCUSSION Figure 1 shows high-resolution scanning electron microscope (SEM) images of Zn foil, E-ZnO, 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 shapes 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 20° to 80° in Figure 2. All the diffraction peaks of the Zn
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 rodlike shape whose SAED patterns matched with the hexagonal wurtzite ZnO phase (Figures S2a−c), while those of RE-Zn-Ar and RE-ZnCO2 are smaller and have hexagonal crystal structures of metallic Zn (Figures S2d-i and g-i, respectively). The electrocatalytic reduction of CO2 on bulk Zn foil, REZn-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 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) were 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-ZnAr 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 RE-Zn-Ar and RE-ZnCO2 had more than a 10-fold increase over 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 5-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
Figure 2. XRD patterns of Zn foil, as-prepared E-ZnO, RE-Zn-Ar, and RE-Zn-CO2 samples.
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. 80-0075) 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.
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. 11380
DOI: 10.1021/acssuschemeng.7b02460 ACS Sustainable Chem. Eng. 2017, 5, 11377−11386
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ACS Sustainable Chemistry & Engineering
Figure 4. (a) SEM images, (b) XRD patterns, and (c) CO2RR performance of RE-Zn-CO2/KCl. (d) Faradaic efficiency (green-dense) comparison with Zn foil (black-filled), RE-Zn-Ar (red-filled), RE-Zn-CO2 (blue-filled), (e) CO current density, and (f) H2 production partial current density of all samples.
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), is composed of the porous nanoparticles but has smaller particle 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 REZn-Ar or RE-Zn-CO2, the surfaces of RE-Zn-CO2/KCl catalyst have a 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 RE-Zn-CO2/KCl, whose surface area was increased by 23-fold over 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
found that RE-Zn catalysts have inherently enhanced CO production rates at low overpotential regions (
