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Surface Morphology Dependent Electrolyte Effects on Gold-Catalyzed Electrochemical CO Reduction 2

Haeri Kim, Hyun S Park, Yun Jeong Hwang, and Byoung Koun Min J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06286 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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The Journal of Physical Chemistry

Surface Morphology Dependent Electrolyte Effects on Gold-Catalyzed Electrochemical CO2 Reduction Haeri Kim1,†, Hyun Seo Park2, Yun Jeong Hwang1,3*, and Byoung Koun Min1,4* 1

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792,

Republic of Korea 2

Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic

of Korea 3

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

and Technology, Seoul 02792, Republic of Korea 4

Green School, Korea University, Seoul 02841, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT

The electrocatalytic property of a flat or an oxide-derived nanostructure Au electrode was investigated using surface sensitive analysis methods such as impedance spectroscopy and Kelvin probe force microscopy (KPFM) when electrochemical conversion of carbon dioxide (CO2) to carbon monoxide (CO) was performed with either KHCO3 or NaHCO3 based neutral electrolyte. A strong dependence on the cation of the electrolyte was exhibited on the flat Au electrode surface. CO selectivity and capacitance dispersion are significantly higher with the KHCO3 electrolyte. On the other hand, the nanostructured Au electrodes, having much more improved activity and durability of CO2 reduction, showed much less electrolyte-dependent catalytic activity. The difference in CO selectivity with KHCO3 and NaHCO3 electrolytes can be explained by the difference in hydration level and consequent adsorption strength of the cations on the flat Au metal electrodes, implying that ion-pairing interactions between the metal, cations, CO2, and its intermediate play an important role in the reduction reaction. The local electric field fluctuation caused by the nanostructured rough Au surface can affect the electric double layer near the electrode surface and suppress the electrolyte-dependency of the reduction. Furthermore, according to X-ray spectroscopy analysis of the electrode after electrolysis, the nanostructured Au electrode is less prone to surface cation deposition. These results provide a basic understanding of the role of electrolyte cations in the CO2 reduction reaction.


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INTRODUCTION

Carbon dioxide (CO2) emissions from cars, factories, houses, and power plants are thickening the blanket of greehouse gases around the Earth, which raises a concerns about global climate change.1, 2 To resolve this environment issue, researchers are trying to develop efficient and durable carbon capture, storage, and utilization technologies. Electro-reduction of CO2 is one of the promising technologies utilizing CO2 to produce value-added carbon chemicals which can be an alternative to petrochemicals. In addition, it can also combine with renewable energy resources in a cleaner way to produce chemicals.3-5 Through various surface modifications, Au-based electrocatalysts have been developed to exhibit outstanding activities in the electrochemical reduction of CO2 to CO,3, 6 compared to other metal electrodes.7-8 Several research groups have successfully improved the product selectivity, the required overpotential, and the durability of CO2 reduction with nanostructure Au electrocatalysts compared to their bulk flat metal surfaces. For example, pores of the mesostructured Au electrode can generate diffusional gradients, inducing a pH gradient near the electrode, which effectively suppress the catalytic activity of H2 evolution reaction, one of the most competitive reactions, by increased alkalinity within the porous network.9, 10 On the other hand, supported by the positive correlation between grain boundary density and CO2 reduction activity/durability, grain boundary defect of the nanoparticle is proposed as an active center which can stabilize the intermediate state of CO2 reduction reaction.11, 12 Moreover, environmental variations such as electrolyte type or salt concentration are suggested to influence the product selectivity and reduction current density (i.e., the production rate) of electro-catalysts.13-15 For electrochemical CO2 reduction, carbonate buffered electrolyte (HCO3- / CO2) has often been utilized to provide a steady pH near neutral. The high conductivity


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of a concentrated electrolyte contributes to the rise of the total current density. It has also been reported that the Faradaic efficiency (FE) of CO2 reduction increases with lower electrolyte concentration, while the current density decreases.16 Particularly, relevant for this paper is the finding that alkali cations are suggested to promote CO2 reduction activity by raising the local concentration of CO2 surrounding electro-catalysts because of stronger C-metal bonds in the presence of cations adsorbed on the metal electrode.17-19 The specific alkali cation of the carbonate electrolyte can change the product distribution of the CO2 reduction by altering anion stability or local pH near the cathode surface. For example, due to changes of cation, Hg electrodes showed different product ratio between formic acid and hydrogen, and the Cu electrode also has a different ratio between methane and ethylene.20, 21 This is because the degree of electrostatic adsorption of cations can differ depending on their sizes and hydration powers, and this can play a vital role in determining the catalytic activity of the metal electrodes. Therefore, the effect of the electrolyte, specifically the cation species, is an important variable in the electrochemical reduction of CO2. However, few systematic studies have reported the effects of electrolyte on the most significant CO2 reduction catalyst, Au. The properties of the electrical double layer (EDL), where chemical redox reactions take place, is a function of the dielectric constant of the aqueous solution, which in terms of HubbardOnsager continuum dielectric friction theory is closely related to the ionic size of the salts.14 The surface morphology of an electrode can also change EDL properties because electric field variations along with the shape of the electrode surface can strongly influence local capacitance.22 Impedance spectroscopy is a useful technique in the field of interfacial electrochemistry for both kinetic studies and interfacial capacitance determination. The impedance also functionally varies depending on sample situations (surface morphology state:


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smooth, rough, or porous) and experimental conditions (electrolyte type and concentration), related to frequency dependent double layer capacitance, ‘capacitance dispersion’.23 Therefore, electrochemical impedance analysis can be an important alternative to understanding the surface properties of electro-catalysts by measuring their current density and electrochemical reaction product selectivity. Furthermore, atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM), AFM-based electrostatic probe techniques, are helpful tools for resolving surface morphology and work function at nanometer resolutions.24, 25 The work function is a particularly important physical quantity associated with activation energy in electrochemical reactions.26, 27 The work function also contains the morphological information and local electrical property; therefore, it is expected to give structure-activity insights for the efficient reduction of CO2 and may clarify the key reaction pathways in mediating electron transfer from active sites to CO2. This study systematically investigates the electrochemistry of the CO2 reduction reaction within the EDL to address the effects of electrolyte on the catalytic activities of Au electrodes with different morphologies, flat or nanostructured, including changes in the electrostatic adsorption of alkali cations near the Au electrodes. A combination of electrochemical catalytic activity measurement, impedance spectroscopy, KPFM, and element analyses showed the influence of alkali cations on the electrochemical reduction with regard to surface characteristics. The flat surface was revealed to be more sensitive to changes in cations than the rough nanostructured surface. Our results promisingly point to a catalyst shape effect, and suggest a more efficient surface structure for the CO2 reduction reaction that is less prone to performance degradation.


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EXPERIMENT

A flat Au cathode was prepared by cleaning polycrystalline Au foil (99.99%, 0.2 mm thickness, Dasom RMS) with an aqua regia solution. The nanostructured Au electrode was electrochemically prepared from the clean polycrystalline Au foil slightly modified from the previous study.3 Amorphous Au oxide was fabricated by pulsed anodization in a 0.5 M H2SO4 (99%, Aldrich) electrolyte, and subsequently reduced to a metallic Au state at -1.0 V vs. an Ag/AgCl reference electrode (BAS model MF-2079) for 15 min prior to CO2 reduction in a 0.25 M NaHCO3 (99.99% purity, Aldrich) solution saturated with CO2 gas. All electrochemical CO2 reduction measurements were conducted in a two-compartment electrochemical cell in which the Au cathode and Ag/AgCl reference electrode were separated from a Pt counter anode by a proton exchange membrane (Nafion®, 117). An aqueous 0.5 M KHCO3 (99.99% purity, Aldrich) or NaHCO3 (99.99% purity, Aldrich) electrolyte was saturated by flowing CO2 gas, resulting in a pH of 7.0. The current density was measured during the CO2 reduction reaction using a chronoamperometry technique at a fixed potential, -1.4 V vs. Ag/AgCl, for 5 h. It has already been reported that nanostructured Au electrodes maintain a CO FE of around 90-94% from -1.0 V to -1.45 V vs. Ag/AgCl.28 Gaseous products were quantified by a gas chromatographer (GC, Younglin 6500 GC) equipped with a capillary column (Restek, RTMsieve 5A) and a pulsed discharge detector (PDD). Detailed experimental conditions can be found in the supporting information S3 modified from the previously reported measurement.7, 8, 28


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The capacitance dispersion of flat and nanostructured electrodes was estimated by electrochemical impedance spectroscopy (EIS). EIS measurements were performed using the electrochemical interface and impedance analyzer CompactStat, by Ivium Technologies (Eindhoven, The Netherlands) controlled by IviumStat Control software. The frequency range between 105 Hz and 10-2 Hz was covered with 10 points per decade. During EIS measurement, the same electrolyte saturated with CO2 gas was used. A potential of -0.19 VRHE (= -0.60 VNHE) was chosen for all measurements in order to avoid the occurrence of any Faradaic reactions on the Au electrodes and ensure only double-layer charging adjacent to the electrode surface. The AFM and the KPFM measurements were carried out using an atomic force microscope system (XE-100, Park Systems Co.). The topography was obtained using AC mode with a drive frequency of ~160 kHz, and KPFM images of the surface were simultaneously acquired by applying an AC modulation voltage of amplitude (Vac) 2 V and a frequency (ω) of 17 kHz to the tip with a scan speed of 500 nm/s. Conductive Pt-coated Si cantilevers (NSG14/CrAu, resonance frequency: ~160 kHz, NT-MDT) were used for both work function and topography measurements in the noncontact mode. Just before each measurement, the work function of the tip was calibrated with the HOPG (highly ordered pyrolytic graphite, SPI Supplies) reference sample. Prior to measurement, the samples were stored in a desiccator for at least one day in air whose relative humidity was less than 10%. X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe, Ulvac-PHI) with monochromatic aluminum Al Kα (1486.6 eV) anode (24.5W, 15kV) radiation was carried out to analyze elemental composition. The binding energies were calibrated based on the C 1s peak at 284.5 eV as a reference. The XPS peaks were analyzed using a linear fitting of the experimental data based on a mixed Gaussian/Lorentzian peak shape.


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RESULTS AND DISCUSSION

Two Au catalyst electrodes with different morphologies were prepared: flat and nanostructured. The flat Au electrode was Au foil untreated except for cleaning and the nanostructured Au electrode was Au foil with a surface nanostructure achieved by an anodization and subsequent reduction process (see Experimental for more details). Figure 1 (a) shows CO Faradaic efficiency (FE) on the flat and the nanostructured electrodes over the course of 5 h CO2 reduction reactions. Higher and more stable FEs were achieved on nanostructured Au cathodes than on flat Au cathodes regardless of electrolyte type. The enhanced electrochemical reduction activity apparent on nanostructured Au has already been examined in previous studies3,6 which proposed defect structures such as edge sites or grain boundary have higher catalytic activity or nanostructure induced local pH gradient near the electrode surface contribute to selective CO production over H2. In addition, in our previous study, low the work functions and contracted Au-Au bond lengths was suggested to play important roles in improving reduction catalytic activity of the nanostructure Au derived from electro-oxidation/reduction by facilitating electron transfer and adsorbate stabilization on the surface.6 High resistance to impurity deposition was also observed in the nanostructure having more stable FEs.6


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Figure 1. (a) Comparison of the CO FE values in two different electrolytes depending on the reaction time with a flat Au (○) and a nanostructured Au (●) electrode. CO2 reduction reaction was carried out in 0.5 M KHCO3 (navy circles) and 0.5 M NaHCO3 (pink circles) electrolyte saturated with continuous CO2 gas flow. (b) The total current density vs. reaction time in each condition. More importantly, it was found that Au electrodes had a higher CO FE with KHCO3 electrolyte than with NaHCO3 electrolyte. While the current densities with NaHCO3 electrolyte were very slightly lower than those with KHCO3, the deteriorations of this parameter over time were almost identical, as shown in Fig. 1(b). Considering that the total current densities obtained with the electrolytes were similar, the CO FE differences arose from cation-induced relative


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changes in H2 and CO production activity (i.e., the suppression of H2 production and the enhancement of CO production by the KHCO3 electrolyte). Our experimental results correspond closely with earlier electrochemical studies of the effect of cations on the catalytic activity of the metal surface.13,15 Under the high electrochemical CO2 to CO conversion rate, depletion of the H+ ions can increase the pH near the electrode surface. Such local pH gradient up to basic condition can decrease the concentration of the dissolved CO2 molecules near surface, and thus decrease the CO2 reduction activity and selectivity as well. However, the amount of the local pH gradient strongly depends on the types of the cation because the difference hydrolysis capacity of the hydrated cation. In other words, the hydrated K+ ions can undergo hydrolysis more easily than Na+ ones, which can suppress the local pH increase. Our results were consistent with the previous study reporting higher CO partial current density and CO Faradaic efficiency on the flat Ag surface with KHCO3 electrolyte compared with NaHCO3. 15 Moreover, because of its larger size, K+ has a lower hydration power than Na+, and thus can be more adsorbed near the cathode surface. Ions adsorbed on the electrode surface can influence the reaction activity in terms of product selectivity, current density, and energetic efficiency in several ways. Firstly, consider how the cations affect hydrogen evolution reaction (HER) or CO2 reduction reaction activity. Ion adsorption can modify the potential of the Helmholtz layer near the electrode surface. As mentioned, the larger K+ is more likely to be adsorbed, and this causes a more positive potential near the electrode surface which can decrease HER activity.14 Second, adsorbed cations can influence the CO2 reduction reaction activity by stabilizing the reaction intermediate ion on the cathode surface.13, 16 It has been suggested that the rate determining step (RDS) of the CO2 reduction pathway involves an initial transfer of one


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electron to the intermediate species, CO2 → COOH or CO2•- , because the intermediate species are less thermodynamically stable than the CO2 molecule.3 The CO2 reduction pathway could be facilitated through this intermediate species being stabilized by its binding to the cathode surface, and the adsorbed cations may favor this process through ion pairing with the intermediate. An increase in anion adsorption on a cathode in the presence of larger electrolyte cations, due to more cation-anion interactions in the double layer adjacent to the surface, has been corroborated.13 It has been suggested that cations adsorbed on the electrode could favor a reaction step from an unstable anion intermediate to a neutral species13 and that a more stable anion intermediate would be formed in the presence of larger cations in electrolyte solutions.29

Figure 2. Schematic diagram of a double layer structure near the Au cathode surface representing a K+ ion interacting with a CO2 molecule in the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP).


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If a charged electrode is immersed in an electrolyte solution, charge imbalances are neutralized by the attraction of counter-ions to its surface. The two-close layer formed at the electrode/electrolyte interface termed the electrical double layer (EDL) is shown in Fig. 2.22 A notable feature shown in Fig. 1 (a) is that with the flat Au electrode CO FE differs depending on whether KHCO3 or NaHCO3 electrolyte is used, but with the nanostructured surface Au electrode there is little difference. To interpret the influence of cations with regards to surface morphology, the capacitance dispersions of the two Au electrode surfaces were compared using electrochemical impedance spectroscopy (Fig. 3). A discernible difference between the impedance measurements with KHCO3 and NaHCO3 electrolytes was only observed for the flat Au cathode (Fig. 3). According to basic electrochemistry, electrodes should exhibit ideal capacitive impedance in the absence of Faradaic processes and charge transfer, and their Nyquist plots should be a vertical line of infinite slope. However, impedance is not purely capacitive but shows capacitance frequency dispersion. Generally, this dispersion can be seen through the microstructural complexity of rough or porous electrodes in that increasing roughness may broaden the time constant distribution of adsorption kinetics.30 Similar phenomena can even occur on flat surfaces when adsorption and/or reaction processes have broadly distributed the relaxation time.23, 31


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Figure 3. Frequency dependence of the electrochemical impedance measured in 0.5M KHCO3 or NaHCO3 electrolyte at a polarization potential of 0.05 V vs. Ag/AgCl. Nyquist plots of (a) the flat Au cathode and (b) the nanostructured Au cathode, and Bode plots of (c) the flat Au cathode and (d) the nanostructured Au cathode.

To characterize capacitance dispersion, the EDL is approximated by a constant phase element, the power-law function of frequency, C(ω)∝(i ω)α-1.32 All the electrodes showed a Nyquist plot approaching a straight vertical line (Figs. 3 (a) and (b)). However, Fig. 3(a) shows that the flat Au electrode presents a significant variation of finite slopes in KHCO3 and NaHCO3. The dependence of the imaginary part of the impedance (-Im (Z)) on frequency of the same


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samples is presented in Figs. 3 (c) and (d) and the Bode plots of the flat Au cathode can also distinguish the cation of the electrolyte (Fig. 3(c)) through slope changes. As mentioned earlier, cation adsorption takes place near the surface, and the capacitance dispersion of the flat Au electrode adequately confirms the different adsorption features of the different cations. That is, the water molecules around the larger sized K+ ion are able to more easily detach leading to a more stable interaction between K+ and the flat Au electrode surface in the EDL, resulting in larger capacitance dispersion in KHCO3 electrolytes than NaHCO3 electrolytes. In addition, previous studies have proposed the bifunctional complexation of CO2 on the metal electrodes.1719

CO2 might be placed in the EDL in such a way that the electrophilic carbon atom of the CO2

molecule is bonded to a metal center and the nucleophilic oxygen atom of the CO2 is bonded to a Lewis acid, such as a K+ cation, assisting the formation of the intermediate state. Furthermore, recently the Gibbs free energy diagrams obtained from density functional theory (DFT) calculations have shown that K+ ions adsorbed on the Au surface lower the thermodynamic energy barrier for CO2 to CO conversion.16 In contrast to the flat Au cathode, the nanostructured Au electrode Nyquist plots for KHCO3 and NaHCO3 electrolytes (Fig. 3(b)) exhibited very similar behavior. Likewise, the Bode plots of the nanostructured electrode with the different electrolytes (Fig. 3(d)) did not differ noticeably in slope. Whichever electrolyte is used, the high frequency region shows a distorted response, which is a typical characteristic of rough electrodes at the high frequency limit (≥104Hz).33 This small difference in capacitance dispersion on the nanostructured Au surface is consistent with the CO2 reduction activity variation, which showed only the small improvement of the CO FE with the KHCO3 electrolyte (Fig. 1(a)).


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These electro-catalytic and electrochemical impedance measurements suggest that the cationic effects of two different electrolytes are substantially attenuated by the rougher Au surface morphology, although the resulting larger surface area is also expected to lead to more interfaces between the electrode and the electrolytes. Therefore, an alternative explanation is required for the reduced electrolyte dependency associated with the enlarged interfacial region.34 One effect may be the modification of the potential distribution near the surface. Recently, Liu et al reported that catalysts with sharp tip-shapes provide greater CO2 reduction activity due to the strong electric fields surrounding them and the associated strong field-induced reagent concentration.16 To predict the electric potential variation at the local region of the rough surface, we did a simple electric field simulation with a capacitor formed by two metals of different shape separated by a dielectric (Fig. S1). The region near the rough electrode is divided into a relatively strong electric field region (the hump area) and a weak electric field region (the dinged area). It can affect local effect that strong field at the hump area induce increasing surface K+ ion concentration while weak field at the dinged area. The inhomogeneous electric field can diverge in various directions as well due to random nanostructuring. In this study, electric field fluctuations along the surface of the nanostructured surface are postulated to cause the cancellation of the electrolyte composition effect. The kinetic behaviors of the ions in the solution could be easily influenced at the interfacial region by their finite size and their orientation behavior.35 Therefore, it is understandable that the local field variation caused by the nanoscale morphology can reduce the effect of the experimental environmental conditions, such as electrolytes.


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Figure 4. AFM and KPFM images of the flat Au and the nanostructured Au cathodes pre- and post- the CO2 reduction reaction in KHCO3 or NaHCO3 electrolyte. Gray color images, (a)-(f), are topography maps and orange color images, (g)-(l), are work function maps of the same positions in the same order. (a)-(c) and (g)-(i) present the surface states of the flat Au and (d)-(f) and (j)-(l) of the nanostructured Au electrode. Work function maps have different scale bars according to the work function. The voltage range from black to white contrast is ∆=100 meV.


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Figure 5. Work function values of the flat and the nanostructured Au electrodes pre- and postCO2 reduction reactions, respectively.

To recognize the physical morphology and electrical properties of the Au electrodes before and after the CO2 reduction reaction, we compared the surface topography and work function images of the two Au samples using AFM (Fig. 4 (a)-(f)) and KPFM techniques (Fig. 4 (g)-(l)). The nanostructured Au electrode clearly presents a clustered structure with several spheres of 20-30 nm in diameter in Fig. 4 (d), and low work function values in Fig. 4 (j). The work function value is a physical quantity related to the activation energy characterizing electron transfers in electrochemical reactions. The smaller surface work function of the nanostructured Au electrode indicates the presence of electrically different surface states accompanying the geometrical changes caused by the electrode sample preparation process of chemical oxidation and reduction.6


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Figure 6. High resolution X-ray photoemission spectra (XPS) of the flat Au and nanostructured Au cathodes. (a) and (b) show K 2p and Na 1s spectra measured after a 5h CO2 reduction reaction in KHCO3 and NaHCO3 electrolyte conditions, respectively. (c) Comparison of K and Na atomic concentrations (%) of the nanostructured Au (filled circle) and the flat Au (empty circle) electrodes after the CO2 reduction reaction. Next, the work function properties of the Au electrodes pre and post CO2 reduction are discussed in detail. Fig. 5 shows the average work function values of the Au electrodes graphed and tabulated from the images in Fig. 4 (g)-(l). CO2 reduction reaction processes also bring about a change in the work function of both the flat and the nanostructured Au electrodes. The


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deposition of exotic elements on the surface accounts for the decrease in surface work function during the CO2 reduction reaction. It has been proposed that the deposition of metal impurities, carbon, and potassium cause the deactivation of the catalyst electode during the reduction.6, 36-38 In practice, the deposition of impurities on the flat Au surface can be estimated using AFM and KPFM images. Fig. 4 (b) and (c) show bright spots (higher heights) and Fig. 4 (h) and (i) present dark spots (low work functions), and the EDAX analysis confirms that these spots consist of carbon, potassium (or sodium), and oxygen elements (Fig. S2). Reduced work function values are expected from the deposition of carbon and potassium (or sodium) due to the smaller work functions of potassium (~2.3 eV), sodium (~2.36 eV) and carbon (~4.81 eV) than that of gold (5.1-5.47 eV). In addition, cation adsorption on the electrode surface can form a higher electric dipole moment indicating downward band bending, resulting in further deterioration of the work function.25 Comparing the electrodes pre- and post-CO2 reduction reaction, less work function loss was measured for the nanostructured Au electrode than for the flat electrode under both KHCO3 and NaHCO3 electrolyte scenarios (Fig. 4 (e), (f), (k), and (l)). To investigate the elemental composition and chemical properties of the flat and nanostructured Au electrodes, a highresolution XPS study was carried out. We ascertained that nanostructured Au was completely reduced to metallic Au since the Au 4f and O 1s spectra for the nanostructured and flat electrodes were almost the same (not shown here) [6]. The K 2p and Na 1s spectra of the flat and nanostructured Au electrodes post-CO2 reduction are presented in Fig. 6 (a) and (b). The atomic concentrations estimated by XPS showed that potassium and sodium were deposited on the Au electrode, but higher concentrations of potassium were observed on both of the Au electrodes, as would be expected from the higher adsorption tendency of K+ than Na+ on the cathode surfaces.


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Moreover, the nanostructured Au surface had much lower concentrations than the flat one; the potassium concentration was 1% rather than 8%, and the sodium concentration was 0% rather than 4% shown in Fig. 6 (c). This XPS result of lower concentrations on the nanostructured Au surface corresponds clearly with the KPFM result of smaller variations in the work function of the nanostructured Au indicating that it is less prone to surface cation deposition. The relationship between these chemical deposits and the catalytic activity/stability of the Au electrode surface is consistent with the XPS and KPFM experiment results.

CONCLUSIONS

We showed how the electrode catalyst surface structure and the electrolyte composition influence the efficiency and stability of the CO2 reduction reaction, by comparing the results obtained with flat Au and nanostructured Au electrodes conducted in two different electrolytes, KHCO3 and NaHCO3. Although with both flat and nanostructured Au catalysts, CO2 reduction to CO production efficiencies were better in the KHCO3 than NaHCO3 electrolyte, the flat Au surfaces show signs of higher improvement in the initial period. The higher CO FE in the KHCO3 electrolyte was obtained because larger cations have a higher propensity for adsorption on the Au surface due to their low hydration powers. Impedance measurements showed that with KHCO3 electrolyte the flat Au surface had varied double layer capacitance near the electrode surface through increased capacitance dispersion. In addition, the nanostructured Au electrode showed less dependency on the electrolyte cation, that is, the local field variation caused by the rough surface morphology suppressed the effect of the size and orientation behavior of the ions. After the CO2 reduction reaction, we again observed smaller changes in work functions and cation concentrations for the nanostructured Au electrode by AFM/KPFM and XPS analysis. Our


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study helps improve understanding of not only the chemical properties associated with CO2 reduction catalysts but also the fundamental and important solid/liquid interface interactions involved in chemical phenomena.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Electric field map of a rough surface electrode and simulation script (file type, i.e., PDF) SEM images and elemental maps from energy dispersive analysis of X-rays (EDAX) of a flat Au cathode after a 5h CO2 reduction reaction (file type, i.e., PDF) Detailed explanation about the calculation of Faradaic efficiency in our experiment condition (file type, i.e., PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] and *[email protected] Present Addresses † Center for Correlated Electron Systems, Institute for Basic Science, Department of Physics and Astronomy, Seoul National University, Seoul 08823, Republic of Korea Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This research was supported by the program of the Korea Institute of Science and Technology (KIST). This work was also supported by the Young Scientists Fellowship through the National Research Council of Science & Technology (NST) of Korea.

ABBREVIATIONS CO2, CO, AFM, KPFM, FE, EDL, GC, PDD, EIS, HOPG, XPS, HER, RDS, IHP, OHP, and DFT


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