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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Monolithic Nanoporous In−Sn Alloy for Electrochemical Reduction of Carbon Dioxide Wan Jae Dong,† Chul Jong Yoo,‡ and Jong-Lam Lee*,†,‡ †

Department of Materials Science and Engineering, and ‡Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea S Supporting Information *

ABSTRACT: Nanostructured metal catalysts to convert CO2 to formate, which have been extensively studied over decades, have many problems such as durability, lifetime, high process temperature, and difficulty in controlling the morphology of nanostructures. Here, we report a facile method to fabricate monolithic nanoporous In−Sn alloy, a network of nanopores, induced by electroreduction of indium tin oxide nanobranches (ITO BRs). The electroreduction process concentrated a local electric field at the tip of the nanostructure, leading to current-assisted joule-heating to form a nanoporous In−Sn alloy. Scanning electron microscopy images showed that the nanopore size of In−Sn alloy could be controlled from 1176 to 65 nm by tuning the electroreduction condition: the applied potential and the time. As a result, formate Faradaic efficiency could be improved from 42.4% to 78.6%. Also, current density was increased from −6.6 to −9.6 mA/cm2 at −1.2 VRHE, thereby resulting in the highest HCOO− production rate of 75.9 μmol/(h cm2). Detachment of catalysts from the substrate was not observed even after a long-term (12 h) electrochemical measurement at high potential (−1.2 VRHE). This work provides a design rule to fabricate highly efficient and stable oxidederived electrocatalysts. KEYWORDS: nanoporous, electroreduction, In−Sn alloy, carbon dioxide, formate

1. INTRODUCTION Converting CO2 into chemical fuel is a promising technology to mitigate the global climate crisis and exhaustion of energy resources.1−3 Electrochemical reduction of CO2 at a metal surface in aqueous solution has attracted great attention because the system is simple, and the product can be selectively controlled by changing the metal electrode.4−8 Among the various products, formic acid is a good candidate as an anode fuel for fuel cells and a promising material as a hydrogen carrier. Therefore, electrochemical reduction of CO2 into formate (HCOO−) was investigated on various metal foils such as Sn, In, Pb, Tl, and Cd.9−14 Although selective HCOO− production has been demonstrated with the bulk foils, these metal foils required a high overpotential. Thus, the main challenge of converting CO2 into HCOO− is decreasing the overpotential. For a decrease in the overpotential, a number of studies on metal-based nanoparticles (NPs) such as Pd−Pt alloy,15 Sn−Pb alloy,16 and tin oxide (SnOx) nanocrystals17 have been developed. The Pd−Pt NPs demonstrated HCOO− formation at potentials close to 0 VRHE. However, Pd-based electrodes quickly degraded catalytic activity under high potential.15,18 For an enhancement of the stability, Sn−Pb alloy NPs were coated on a carbon paper. This led to high Faradaic efficiency (FE = 79.8%) and to large current density = 45.7 mA/cm2.16 Also, SnOx nanocrystals on carbon support showed maximum FE of 93% and large current density (>10 mA/cm2).17 However, the process of synthesizing metal NPs on carbon paper has several © XXXX American Chemical Society

problems: Metal NPs must be attached on a carbon support using a binder.15−20 However, the insulating binder covers the catalytically active sites and lowers the electrical conductivity, thereby resulting in deterioration of catalytic performance.21 Also, the stability is relatively poor because the catalysts tend to be exfoliated from the substrate under vigorous gas evolution conditions. There were a number of reports on electrocatalysts not using the binder. Nanostructured monolithic catalysts such as a hierarchical Sn dendrite,22 Zn dendrite,23 porous Cu,24 oxidederived Cu (OD-Cu),25 and OD-Pb26 were integrated with the conductive substrate for mechanical durability. Sn dendrite grown on Sn foil showed the maximum formate FE of 71.6% with high production rate [228.6 μmol/(h cm2)]. However, the electrodes must be heat-treated at high temperature (>180 °C) because the oxide layer is essential to stabilize the CO2•− intermediate.22 Zn dendrite23 and porous Cu24 were directly grown on metallic substrate via electrodeposition. However, those catalysts showed poor formate selectivity (FE < 30%). Recently, OD-Cu and OD-Pb were prepared by reducing copper oxide and lead oxide at room temperature.25,26 Because the oxide-derived metal catalysts have a very large surface area and expose a large number of active sites on the surface, both Received: July 16, 2017 Accepted: November 27, 2017

A

DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustrations of fabrication processes of nanoporous In-Sn alloy. ITO BRs were grown on Cu foil, followed by electroreduction. (b) Cyclic voltammogram of ITO BRs in a N2-purged 0.1 M KHCO3 solution in a potential range from +0.2 to −1.2 VRHE with scan rate of 50 mV/s. (c) Current density measured during electroreduction of ITO BRs. Reduction potential of −0.3 VRHE (60 min) was applied for 1-step reduction. Two potentials of −0.2 (30 min) and −0.3 (30 min) VRHE were applied for 2-step reduction. Three different potentials of −0.2 (20 min), −0.25 (20 min), and −0.3 (20 min) VRHE were sequentially applied for 3-step reduction. SEM images of (d) ITO BRs, and nanoporous In−Sn alloys fabricated by (e) 1-step reduction, (f) 2-step reduction, and (g) 3-step reduction. Inset: distribution of diameter of In−Sn NPs.

the FE and the production rate were much higher than those of bulk foils.27−31 However, Pb is a toxic metal that is incompatible with practical applications. Moreover, no work has been performed to control the morphology of oxidederived metal catalysts. Therefore, the relationship between morphology and catalyst performance is not clearly demonstrated. It is limited to improve the catalytic activity by adjusting the morphology of the oxide-derived metal catalysts. Herein, we report a novel method to fabricate oxide-derived nanoporous In−Sn alloy induced by electroreduction of indium tin oxide (ITO) in aqueous electrolyte. During the electroreduction process, a local electric field and joule-heating were formed at the tip of nanostructure, resulting in a metallic nanoporous structure. Manufacturability of nanoporous In−Sn

alloys was evaluated by varying the reduction potentials of indium tin oxide nanobranches (ITO BRs), and a design rule was provided. Because the morphology of nanoporous In−Sn alloy could be precisely controlled by changing the reduction potential and the time, it was possible to identify the unknown relationship between size of nanopores and catalytic activity of CO2 reduction in monolithic electrocatalysts.

2. EXPERIMENTAL SECTION 2.1. Preparation of Nanoporous In−Sn Alloy. Indium tin oxide nanobranches (ITO BRs) were fabricated by an electron beam evaporation of tin-doped (10 wt %) indium oxide on copper foil substrates.33,34 ITO BRs were grown at a rate of 1 nm/s at 400 °C for the well-developed branches. One side of the substrate was exposed B

DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) X-ray diffraction patterns and (b) XPS spectra of ITO BRs, In−Sn NPs (1-step and 3-step), thermally evaporated In−Sn, and In and Sn films. (c) O 1s spectra and (d) peak intensity ratio of O−In/OH−. most widely used in electrochemical CO2 reduction experiments.1,25 The active area of prepared electrode was 0.50 cm2 (a circular shape with 8 mm diameter). Each potential was applied for 30 min to measure Faradaic efficiency and production rates. The amount of gas product was measured by gas chromatography equipment, and the production rate was calculated by dividing the number of moles by the reaction time of 30 min. The measured potential (VAg/AgCl, V) could be converted to the reversible hydrogen electrode (VRHE, V) using the Nernst function: VRHE = VAg/AgCl + 0.222 + 0.0592 × pH. The EIS spectra were measured in a frequency range from 1 MHz to 1 Hz at a potential of −1.2 VRHE with an amplitude of 30 mV. All the measurement were conducted under ambient pressure at room temperature. 2.4. Product Analysis. Gas products were analyzed with gas chromatography (Inficon, 3000 Micro GC) equipped with two thermal conductivity detectors (TCDs) connected to a Molsieve column and Plot U column. Before the beginning of the electrochemical reduction, CO2 flowed for 10 min for removal of other gases. During the reaction, gas was kept circulated within the reactor with a flow rate of 50 mL/min. After the reaction, gas products were pumped into the gas chromatography instrument for analysis. Five samples were analyzed for each potential. Liquid products were analyzed using a 600 MHz 1H 1D liquid NMR spectrometer (Bruker) at 25 °C. The standard solution consisted of 5 mM dimethyl sulfoxide (SigmaAldrich) and 5 mM phenol (Sigma-Aldrich) in D2O as solvent (SigmaAldrich). The amount of products was calculated by integrating areas of the products with that of phenol or dimethyl sulfoxide.

with a 0.56 cm2 area by masking the other side with an epoxy. For conversion of ITO BRs to In−Sn alloy nanoparticles, electrochemical reduction was conducted in the N2-purged 0.1 M KHCO3 electrolyte. For the electroreduction process, three different methods of 1-step, 2step, and 3-step reduction were carried out using a potentiostat (Ivium Stat). For the 1-step electroreduction method, a constant potential of −0.3 VRHE was applied to ITO BRs for 60 min. Two different potentials of −0.2 and −0.3 VRHE were applied for the 2-step reduction, and three different potentials of −0.2, −0.25, and −0.3 VRHE were applied for the 3 step reduction. The prepared samples were rinsed with deionized water and dried at room temperature. Reproducibility of all the samples was checked more than 3 times by SEM before electrochemical measurements. 2.2. Characterizations. Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDS) were observed using a PHILIPS XL30S with an accelerating voltage of 5 kV and a working distance of 6 mm. X-ray photoelectron spectroscopy (XPS) was conducted on the 4D beamline at the Pohang Accelerator Laboratory with a base pressure of 5 × 10−10 Torr. X-ray diffraction (XRD) patterns were obtained from a diffraction meter (Rigaku, Dmax-2500pc). The size of nanoparticles was defined using a computer-based image analyzer (Leopard 2009). 2.3. Electrochemical Measurements. Measurements for electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), Faradaic efficiency (FE), and production rates were conducted in the H-type cell separated by a Nafion membrane with a three-electrode system. A Ag/AgCl filled with 1 M KCl was used for the reference electrode, and Pt mesh was used for the counter electrode. The electrolyte used for electrochemical measurements was an aqueous solution of 0.1 M KHCO3 (Sigma-Aldrich, 99.95%) prepared from dissolving the solid salt in DI water. The electrolyte was saturated with N2 or CO2 gas prior to measurements. The reason for using the Ag/ AgCl reference electrode and KHCO3 aqueous solution is that it is

3. RESULTS AND DISCISSION 3.1. Control the Morphology of Nanoporous In−Sn Alloy. For fabrication of the monolithic nanoporous In−Sn alloys, ITO BRs (Figure S1) were grown on the conductive Cu C

DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

For quantification of the atomic composition of prepared samples, synchrotron X-ray photoelectron spectroscopy (XPS) in the 4D beamline in Pohang Accelerator Laboratory was conducted (Figure 2b−d). ITO BRs showed a clear O 1s peak at 531 eV, Sn 3d peaks at 485 and 493 eV, and In 3d peaks at 444 and 452 eV (Figure 2b). In contrast, In−Sn NPs exhibited a large decrease in intensity of In 3d and Sn 3d peaks due to absorption of oxygen species in aqueous solution. The thickness of the oxide or hydroxide layer is expected to be thicker than 2 nm for shielding the photoelectron from In and Sn. Also, vacuum-deposited In−Sn, and In and Sn films were oxidized as confirmed by the presence of the O 1s peak. For elucidation of the bonding state of oxygen species on the surface, deconvolution of the O 1s peak was carried out with three binding states of O−In, O vacancy, and OH−In (Figure 2c).32 The ITO BRs showed the highest intensity of O−In, meaning that oxygen atoms mainly bond to indium rather than forming a hydroxide. On the other hand, the O−In bond almost vanished, and intensity of In−OH bonds increased after electroreduction. The ratio of O−In and OH−In bonds was 1.63 for ITO BRs, 0.03 for In−Sn NPs (1-step), 0.10 for In−Sn NPs (3-step), and 0.008 for In−Sn film (Figure 2d). These XPS results confirm that the ITO BRs were reduced to metallic In− Sn alloy. Also, it is found that the In−Sn alloys spontaneously formed a hydroxide layer on the surface. Even though the surfaces of nanoporous In−Sn alloy were changed to metal, a large amount of oxygen species remained in the bulk region of nanoporous In−Sn alloy as made evident by energy-dispersive X-ray spectroscopy (Figure S8). 3.3. Electrochemical CO2 Reduction. Electrochemical CO2 reduction activities were evaluated by linear sweep voltammetry in N2- and CO2-purged 0.1 M KHCO3 solution (Figure 3a). All the samples exhibited larger negative currents in the CO2-purged electrolyte than in the N2-purged one. In the CO2-purged electrolyte, In−Sn NPs (1-step) showed a current density of −3.2 mA/cm2 at −1.2 VRHE whereas In−Sn NPs (3-step) recorded a much higher current density of −9.6 mA/cm2 because of increased porosity and enlarged surface area. Current densities of In−Sn film, In film, and Sn film were −6.6, −5.1, and −5.4 mA/cm2, respectively. For verification of the enhanced surface area of nanoporous In−Sn NPs, charge/discharge curves were measured on a potential range from 0.44 to 0.64 VRHE for various scan rates (Figure S9). Then, current density measured at 0.54 VRHE was plotted as a function of scan rate (Figure 3b). Because the slope of the current density−scan rates curve indicates a double-layer capacitance which linearly correlates to electrochemical surface area, porosity of electrocatalysts could be determined. The double-layer capacitance of nanoporous In−Sn NPs (1067 μF/ cm2) was ∼10 times larger than that of In−Sn film (110 μF/ cm2), meaning that surface area of the nanoporous In−Sn NPs is at least 10 times larger than that of the In−Sn film. Electrochemical impedance spectroscopy measurements were carried out to investigate the kinetics of electron-transfer processes. The Nyquist impedance plots were obtained for the samples at a potential of −1.2 VRHE in CO2-purged 0.1 M KHCO3 solution in a frequency range from 200 kHz to 1 Hz with an amplitude of 30 mV (Figure 3c). Because the EIS spectra were measured at a certain negative potential (−1.2 VRHE), the three-electrode system was selected to prevent the potential from being different from sample to sample. When the Nyquist plot is observed, at the high-frequency region all examined samples establish a 45° with the Zre axis. A first time

foil substrates and reduced in electrolyte (Figure 1a). Because a curved surface with high curvature locally concentrates an electric field (Figure S2), a large amount of electrons would be clouded at the tip of ITO BRs. This large current flow at the tip results in local joule-heating. Cyclic voltammetry of ITO BRs was obtained in aqueous 0.1 M KHCO3 solution (Figure 1b) with a scan rate of 50 mV/s. An obvious reduction current was found in the first negative potential scanning, and the reduction current was almost zero after the second cyclic scan. This result means that one cyclic scanning from +0.2 to −1.2 VRHE is enough for reducing the ITO BRs to metallic In−Sn. However, the twig shape of ITO BRs was collapsed after one cyclic scan (Figure S3) because of a large cathodic current. For prevention of the collapsing, the reduction potential and the time were tuned for controlling the reaction kinetics. Because a low reduction potential generates a low cathodic current and a slow reaction rate, we chose the negative potential range from −0.2 to −0.3 VRHE which is nearly the onset potential of ITO reduction (Figure S4). For the electroreduction process, three different methods of 1-step, 2step, and 3-step reduction were conducted (Figure 1c). For the 1-step method, a constant potential of −0.3 VRHE was applied to ITO BRs for 1 h. At the beginning of the reduction, cathodic current density was as large as −1.1 mA/cm2; thus, the nanostructures could be damaged. Two different potentials of −0.2 and −0.3 VRHE were applied for 2-step reduction, and this method showed a peak current density of −0.6 mA/cm2. For the 3-step reduction method, reduction potentials of −0.2, −0.25, and −0.3 VRHE were sequentially applied to ITO BRs for 20 min. The maximum current density (−0.48 mA/cm2) was smaller than those for the other two methods. The shape of the In−Sn alloy formed after the reduction reaction is highly dependent on the reduction conditions of ITO BRs. As shown in Figure 1d−g, after 1-step reduction, ITO BRs (Figure 1d) were changed to large In−Sn NPs (Figure 1e), and the average diameter (Davg) of In−Sn NPs was 232 nm. When 2-step (Figure 1f) and 3-step (Figure 1g) reductions were conducted, Davg of the In−Sn NPs was decreased to 145 and 65 nm, respectively. As the average size of the nanoparticles becomes bigger, the standard deviation and error range increase. Since the parameter that can easily compare the morphology of nanoporous samples is the average particle diameter, we discussed the relationship between average diameter and catalytic characteristics. From these result, it is clearly made evident that the sequential increment of negative potential is a key technology to control the morphology of nanoporous In−Sn alloy. 3.2. Crystal Structure and Atomic Composition of Nanoporous In-Sn Alloy. X-ray diffraction (XRD) patterns of ITO BRs, In−Sn NPs, thermally evaporated In−Sn, and In and Sn films were measured (Figure 2a). The ITO BRs showed the peaks of ITO (222) and ITO (400) with a Cu (220) peak from Cu foil substrate. The In−Sn NPs showed In (100) and In3Sn (111) diffraction patterns, indicating that In−Sn NPs have mixed phases of indium and In3Sn alloy. For the thermally deposited In−Sn film (Figure S6), In3Sn (200) peak was detected at 36.8° (Figure S7). The XRD results confirm that the thermally evaporated In−Sn films are composed of a mixed phase of indium and In3Sn. In film showed characteristic peaks of In (100) and In (002), and Sn film also had a polycrystalline phase of Sn (200) and Sn (101). D

DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

those of In−Sn, In, and Sn films, meaning that the nanoporous In−Sn electrodes have lower charge-transfer resistance. Faradaic efficiency (FE) of electrodes was tested at different applied potentials from −0.4 to −1.2 VRHE (Figure 4). Notably, the In−Sn NPs (1-step; Figure 4a), In−Sn NPs (3-step; Figure 4b), and In−Sn film (Figure 4c) reduced a large amount CO2 to CO in the range from −0.4 to −0.8 VRHE reduction potential. The maximum FECO was 28.1% for In−Sn NPs (1-step), 24.4% for In−Sn NPs (3-step), and 28.6% for In−Sn film at −0.5 VRHE (summarized in Table S1). In contrast, maximum FECO was 9.3% for In film (Figure 4d) and 14.1% for Sn film (Figure 4e). It is clear that presence of Sn along with In enhances catalytic activity toward CO production. At the potential of −0.6 VRHE, formate production was not observed on In−Sn NPs (1-step), and In−Sn and In film. Sn film also showed small FEHCOO− of 10.4% at −0.6 VRHE, but In−Sn NPs (3-step) exhibited FEHCOO− of 29.1%. At the high potential of −1.1 VRHE, the maximum FEHCOO− was 56.1% for In−Sn NPs (1step), 78.6% for In−Sn NPs (3-step), 55.9% for In−Sn film, 92.3% for In film, and 70.9% for Sn film. The reason why the nanoporous In−Sn alloy (1-step) has lower FE than the In−Sn film can be various parameters such as composition, crystal orientation, and morphology. It was found by XPS measurement that the nanoporous In−Sn alloy (1-step) had a larger amount of In than the In−Sn film. In addition, In3Sn (111) was mainly detected in the nanoporous In−Sn alloy, whereas In3Sn (200) was detected in the In−Sn film. It is considered that these differences in composition and the crystallinity have more influence on the catalyst characteristics than the increase of the surface area. However, when the reduction method was changed from 1-step to 3-step, the FE dramatically increased. This means that the effect of morphology below the critical particle size becomes an overwhelmingly important factor as compared to the parameters such as composition and crystallinity. To optimize the porosity, we controlled the morphology of In−Sn alloy and found that porosity of In−Sn alloys plays a key role in electrocatalytic CO2 reduction. As the size of In−Sn NPs decreased from 1176 to 65 nm, FEHCOO− values were gradually increased from 42.4% to 78.6% (Figure S10). From these results, one can conclude that the small In− Sn NPs with large porosity increase the formate selectivity because the number of active sites such as edge and corner increased with decreasing the particle size.17,19 Although the Faradaic efficiency could be further improved by tuning the morphology, fine-tuning of morphology is beyond the scope of this work. The production rate was plotted as a function of potential range from −0.4 to −1.2 VRHE (Figure 5). Although the maximum FEHCOO− of In−Sn NPs (3-step; 78.6%) was lower than that of In film (92.3%) at −1.1 VRHE, the out-standing current density of In−Sn NPs (3-step) demonstrated a superior production rate of formate [79.5 μmol/(h cm2)] which is about 1.6 times higher than the maximum production rate of In film [50.0 μmol/(h cm2)] and Sn film [49.4 μmol/(h cm2)]. As the particle size of the In−Sn NPs becomes smaller, and the nanoporous shape develops, both the FE and the production rate increased. The FE was improved because new active sites such as edge or corner sites are formed on the surface, and the production rate is increased because the surface area involved in the CO2 reduction reaction increased. 3.4. Stability of Nanoporous In−Sn Alloy. Stability of electrodes in aqueous solution is an important factor for practical applications. For evaluation of the stability of In−Sn

Figure 3. Electrochemical CO2 reduction activities of ITO BRs, In−Sn NPs, and thermally evaporated In−Sn, In, and Sn films. (a) Linear sweep voltammetry curves in N2-purged (dashed line) and CO2purged (solid line) 0.1 M KHCO3 electrolyte at a scan rate of 50 mV/ s. (b) Current density plots at various CV scan rates. The current densities were obtained from the double-layer charge/discharge curves at 0.54 VRHE in N2-purged 0.1 M KHCO3 electrolyte. (c) Nyquist impedance plots of electrodes in CO2-purged solution in a frequency range from 1 MHz to 1 Hz with 30 mV amplitude at −1.2 VRHE.

constant at the high frequency range is characterized to a typical electrochemical behavior of a porous electrode.35−37 This first depressed semicircle indicates a capacitive/resistive behavior between the solution, reference electrode, and porous electrode constituted by the samples. At the low (or middle-tolow) frequency range a second time constant is clearly characterized. This probably will suggest a capacitance and a resistor, or is associated with a Warburg element when a typical equivalent circuit is commonly used.38−40 Although an equivalent circuit has not extensively been used in this present investigation, a qualitative analysis of these Nyquist plots clearly indicates that both the examined In−Sn alloys (1- and 3-step) exhibit impedance arcs in the second semicircle as compared to E

DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Faradaic efficiencies of (a) In−Sn NPs (1-step), (b) In−Sn NPs (3-step), (c) In−Sn film, (d) In film, and (e) Sn film at various potentials ranging from −0.4 to −1.2 VRHE.

Figure 5. Production rates of formate. The CO2 reduction was performed for 30 min at various constant potentials from −0.4 to −1.2 VRHE in CO2-purged 0.1 M KHCO3 electrolyte.

NPs (3-step), electrochemical reduction was performed for 12 h at a constant potential of −1.2 VRHE in CO2-purged 0.1 M KHCO3 electrolyte. For the current density measurement, CO2 gas was kept bubbled in the electrolyte to prevent the changing of the pH value during the reaction. Current density of nanoporous In−Sn alloy was almost constant for 12 h, indicating that the electrochemical reaction was stable under long-term operation (Figure 6a). For the FE measurement, the electrolyte was replaced with clean CO2-purged KHCO3 solution after finishing the reaction for 1 h because of the increasing concentration of the products in the reactor. At the initial stage of reaction (1 h), In−Sn NPs (3-step) showed FEHCOO− of 74.0%. However, it was slightly decreased to 63.0% after 3 h of reaction. Then, FE of formate was maintained and showed 59.1% after 12 h of operation. SEM images of In−Sn NPs (3-step) were characterized after electrochemical reaction for 4, 8, and 12 h (Figure 6b). After 4 h, the spherical NPs on the surface were a little damaged. The change in surface morphology could be a factor for the decreased FE. However, after reaction for 4, 8, and 12 h, the NPs maintained the

Figure 6. Stability of In−Sn NPs (3-step) was evaluated at −1.2 VRHE in a CO2-purged 0.1 M KCHO3 electrolyte. (a) Current density (solid line) and Faradaic efficiency of formate (points) were plotted as a function of operation time. (b) SEM images of In−Sn NPs before reaction and after 4, 8, 12 h of reaction.

identical morphology. Also, no detachment of catalysts from the substrate was observed even after a long-term electrochemical measurement. F

DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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4. CONCLUSIONS In summary, we fabricated a monolithic nanoporous In−Sn alloy (a network of nanoparticles) by reducing the ITO BRs grown on conductive Cu foil substrate. During the electroreduction process, a local electric field and joule-heating resulted in a nanoporous structure. It is clearly made evident that the sequential increment of reduction potential is a key technology to control the morphology of nanoporous In−Sn alloy. Because the morphology of nanoporous In−Sn alloy could be easily controlled by tuning the reduction conditions, the relationship between size of nanopores and catalytic activity could be identified. As the size of In−Sn NPs decreased from 1176 to 65 nm, formate Faradaic efficiency could be improved from 42.4% to 78.6% at −1.1 VRHE. The novel method to fabricate oxide-derived nanoporous metal alloys opens up the possibility of highly selective, nontoxic, and stable electrocatalysts for CO2 reduction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10308. TEM, SEM, XRD, EDS, and electrochemical measurements of nanoporous In−Sn alloy (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82(0)54-279-2152. Fax: +82(0)54-279-5242. ORCID

Jong-Lam Lee: 0000-0003-4502-9758 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported in part by the Samsung Research Funding Center of Samsung Electronics under Project SRFC-MA1402-14.

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REFERENCES

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DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b10308 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX