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Preferentially oriented Ag nanocrystals with extremely high activity and faradaic efficiency for CO2 electrochemical reduction to CO Xiong Peng, Stavros G. Karakalos, and William Earl Mustain ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16164 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Applied Materials & Interfaces

Preferentially oriented Ag nanocrystals with extremely high activity and faradaic efficiency for CO2 electrochemical reduction to CO

Xiong Peng1,2, Stavros G Karakalos1, William E. Mustain1,2* 1

College of Engineering and Computing, University of South Carolina, Columbia, SC 29208,

United States 2

Department of Chemical and Biomolecular Engineering, University of Connecticut, 191

Auditorium Drive, Storrs, Connecticut 06269, United States * Corresponding Author: [email protected]

Key words: CO2 reduction; electrocatalyst; silver; preferential orientation; faradaic efficiency

ACS Paragon Plus Environment

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Abstract: Selective electrochemical reduction of CO2 is one of the most important processes to study because of its promise to convert greenhouse gas to value-added chemicals at low cost. In this work, a simple anodization treatment was devised that first oxidizes Ag to Ag2CO3, then uses rapid electrochemical reduction to create preferentially-oriented nanoparticles of metallic Ag (PON-Ag) with high surface area as well as high activity and very high selectivity for the reduction of CO2 to CO. The PON-Ag catalyst was dominated by (110) and (100) orientation, which allowed PON-Ag to achieve a CO faradaic efficiency of 96.7% at an operating potential of -0.69 V vs. RHE.

This

performance is not only significantly higher than polycrystalline Ag (60% at -0.87 V vs. RHE), but represents one of the best combinations of activity and selectivity achieved to date – all with a very simple, scalable approach to electrode fabrication.


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1. Introduction Electrochemical reduction of carbon dioxide (CO2) is widely regarded as a promising pathway to produce value-added chemicals such as carbon monoxide (CO), formic acid (HCOOH), methane (CH4), and ethylene (C2H4), while simultaneously mitigating the greenhouse gas effect1–4. The electrochemical conversion of CO2 to CO is considered to be an important and prototypical example of what is possible since CO can be transformed into a variety liquid fuels that can be directly integrated into the global energy infrastructure, such as acetic acid and methanol5. Therefore, developing highly active and low cost electrocatalysts for CO2 conversion to CO is important for promoting technology transfer to realistic systems, commercialization and process scaleup. Various metallic electrocatalysts have been investigated for CO2 electrochemical reduction, and a diverse family of products are possible that can change drastically with electrode potential6–10 due to their different CO binding energy. Metals that bind CO too strongly produce mostly hydrogen from CO poisoning; metals that bind CO weakly produce mostly CO, as the CO is released from the surface before it is further reduced; metals that have intermediate CO binding energy produce various reduction products that participate in processes that require more than two-electrons9. Of the explored metals for electrochemical reduction of CO2, gold (Au) and silver (Ag) are reported to have the highest activity and selectivity for CO among the pure transition metals due to their distinctive CO binding energy. Compared to Au, the relatively low cost and higher abundance of Ag makes it a more promising candidate for commercial use. However, a large overpotential is required when using polycrystalline Ag and, though relatively high, the CO faradic efficiency (FE) is still insufficient2.


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In the 1990s, Hori and coworkers studied the orientation and potential dependence of CO2 electrochemical reduction on Ag single crystal surfaces in order to determine which crystalline facets are the most active11. Their results showed that Ag (110) and Ag (100) possess high activity for CO2 electrochemical reduction to CO. Since then, this has been explained by density functional theory (DFT) calculations have shown that the adsorbed carboxyl (*COOH) was most strongly bound to the Ag (110) facet, followed by Ag (100) and Ag (111), with the Ag (111) surface binding the *COOH the weakest12. However, translating their findings to producing highly active and selective nanoparticles has been elusive to date. This is not meant to suggest that controlled faceting of silver has not previously been somewhat successful. In fact, recently , Luo et al. synthesized triangular silver nanoplates to expose specific Ag (100) facets and edge sites13. Their catalyst was able to achieve a maximum CO faradaic efficiency of 96.8%, though at a very high overpotential of 0.746 V and a CO partial current density only ca. 1.25 mA/cm2. In another study, Jiao et al14. reported a Ag nanoporous catalyst that was formed through de-alloying of Ag-Al, resulting in a high density of step-edge sites on the nanoporous surface, leading to increased stabilization of certain reaction intermediates. Though this catalyst could enable the CO2 to CO transformation at lower overpotentials (0.49 V), it did so at the expense of the maximum faradiac efficiency (92%). Therefore, there is still a need to create Ag-based catalysts with both high faradaic efficiency and low overpotentials.

Another possible shortcoming of the above work is that both

approaches required complex synthesis routes that may inhibit their practical application. One promising pathway that is intrinsically low cost and relatively simple to produce catalysts with preferred orientation is rapid surface oxidation-reduction cycles. These so-called “oxidized-derived” metal nanoparticles can typically be created with


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preferential faceting on the electrode surface1. In fact, Au and Cu catalysts have already been demonstrated with low overpotential and high selectivity for CO production from CO26,15. However, conventional synthesis of preferentially-oriented nanoparticles (PON) involves a high temperature treatment and the use of oxygen surface species. Thermal oxidation of Ag is not preferred because Ag oxide (Ag2O) decomposes above 280 °C16, and higher temperatures are required in order to enable the desired rapid surface reconstruction. Therefore, it is important to develop a facile and reliable way to fabricate a PON-Ag catalyst that avoids thermal cycles as its driving force – such as electrochemistry. Zhou et al.17 created an oxidized-derived Ag catalyst through potential cycling and anodization of Ag foil in 0.1 M NaNO3 solution. Their treatment created particles with a preferred (220) surface orientation that were able to achieve comparable faradaic efficiency and overpotential to Jiao et al.14, 92.8 % and 0.5 V, respectively – though their CO partial current density was lower, ca. 3.4 mA/cm2. Lee et al.18 used a very similar method to form Ag nanosheets that were able to achieve approximately 95% CO FE at a very low overpotential of 0.29 V; however, the CO partial current density at this overpotential was very low (0.4 mA/cm2). The overarching goal of this work was to produce a PON-Ag electrocatalyst with low index preferential faceting in order to achieve the elusive combination of very high catalyst activity and very high faradaic efficiency.

This was achieved by electro-

anodization/reduction cycles with an Ag foil in 0.5 M KHCO3. Bicarbonate electrolyte was used with the intention of first oxidizing Ag to Ag2CO3 because it was believed that the rapid destruction of the monoclinic structure (in the subsequent electro-reduction) and recombination of Ag would result in more (100) and (110) surface orientation than the


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same process from cubic Ag2O. In addition to the preferential faceting, the method used in this study has another advantage over other existing approaches in that it does not require different electrolytes to conduct the anodization treatment and to perform CO2 reduction, which avoids introducing impurities and possible detachment of the oxidized Ag particles when changing the electrolyte for subsequent CO2 electrolysis. The resulting preferentially oriented Ag nanoparticles (PON-Ag) were both physically and electrochemically characterized using a comprehensive suite of tools including XRD, XPS, SEM, TEM, cyclic voltammetry, and chronoamperometry. The PON-Ag electrode showed a very high CO faradaic efficiency and low overpotential – in fact, the PON-Ag catalyst achieved one of the best combinations of activity and selectivity reported to date – all with simple, scalable electrode fabrication.

2. Experimental Section 2.1. Materials Silver foil (99.998%, 0.1mm thick, Alfa Aesar), platinum foil (99.9%, Aldrich), potassium bicarbonate (99.99%, metal basis Fisher), Nafion® 212 membrane (DuPont), 2-propanol (IPA) (Optima, Fisher), 18.2 MΩ deionized water (Millipore system), carbon dioxide (Airgas, 99.999%), sandpaper (CarbimetTM, Buehler), KimwipesTM (KIMTECH).

2.2. Electrode preparation Ag foil and Pt foil were mechanically polished using sandpaper until shiny and a visually smooth surface was obtained. They were then cleaned with DI water and IPAsoaked Kimwipes before each experiment. The PON-Ag electrode was prepared by first oxidizing the Ag foil to Ag2CO3 by applying a potential of 8.0 V vs. Ag/AgCl for 5 seconds in 0.5 M KHCO3 electrolyte. The cell voltage was selected in order to drive the


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Ag oxidation reaction at an adequate rate to create a homogeneous oxidized layer over a short time, which was necessary to ensure that Ag deposition did not occur on the counter electrode. Then, the electrode was reduced at the same potential as the CO2 electrochemical reduction experiments, which will be described later, also in in 0.5 M KHCO3 electrolyte. 2.3. Physical characterization X-ray diffraction (XRD) of the PON-Ag and Ag foil was performed using a Bruker D8 Advance X-ray diffractometer with a Cu Kα1 ceramic X-ray tube (λ=0.1540562 nm) and a LynxEye Super Speed detector to investigate the catalyst crystallinity and structure. Data was collected from 25° to 90° at a scan rate of 0.0285°/s. The PON-Ag surface morphology was imaged with a FEI Quanta FEG 250 scanning electron microscope (SEM). Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) images were acquired on a FEI Talos S/TEM. The TEM sample was prepared by abrasively removing the PON-Ag nanoparticles from the substrate. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos AXIS Ultra DLD XPS system with a monochromatic Al Kα source operated at 15 keV and 150 W, and a hemispherical energy analyzer. The x-rays were incident at an angle of 45o with respect to the surface normal. performed at a pressure below 10-9 mbar.

The analysis was

High resolution core level spectra were

measured with a pass energy of 40 eV and analysis of the data was carried out using XPSPEAK41 software. The XPS experiments were performed while using an electron gun directed on the sample, for charge neutralization.

2.4. Electrochemical reduction of CO2 and product analysis


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All of the electrochemical experiments were performed in an air-tight two compartment three-electrode electrochemical cell, similar to reference19 (electrochemical cell shown in Figure S1). A Pt foil and Ag/AgCl electrode were used as the counter and reference electrode, respectively. The cell was designed to have a working electrode active area of 4.5 cm2 (1.5 cm × 3 cm) and 8 mL of electrolyte (0.5M KHCO3) in each of the two compartments, along with a 2 mL headspace above the electrolyte. A Nafion 212 membrane was used as the separator between the working and counter electrodes. The CO2 added to the cell was regulated by a mass flow controller (MKS Instruments) at 20 sccm and flowed to both compartments throughout the electrolysis. The electrochemical measurements were carried out using an Autolab PGSTA302N potentiostat. All electrochemical data was collected vs. Ag/AgCl reference and converted to the reversible hydrogen electrode (RHE) by Equation 1. VRHE = VAg/AgCl + 0.199 + 0.059*pH

(1)

The pH for all experiments was measured to be 7.5. CO2 was purged for at least 20 min on both sides before each the potential was applied. Each electrolysis experiment was run chronoamperometrically for 120 min at potentials of -0.95, -1.0, -1.05, -1.1, -1.2, -1.3, 1.35, -1.4, -1.45, -1.5, -1.55, -1.6 and -1.75 V vs. Ag/AgCl. No stirring or other agitation was applied. The gas products were detected by sending the effluent from the working electrode compartment to a customized gas chromatograph (GC, Agilent Technologies 7890B series). CO2 and CO were detected and analyzed by a flame ionization detector (first fed to a methanizer internal to the GC to convert them to methane). Hydrogen, formaldehyde and methanol were detected and analyzed with a thermal conductivity detector. The GC was well calibrated using standard gases in the concentration range of interest. Samples were injected into the GC every 16 minutes (at 10, 26, 42, 58, 74, 90,


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and 106 min). The CO and H2 Faradaic Efficiency (FECO and FEH2, respectively) for each injection were calculated through the following equation:

vtotal × ω k × FEk (%) =

P × 2× F RT × 100

itotal

(2)

where vtotal is the volumetric flow rate, ωk is the volume concentration of CO or H2 in the exhaust gas from the electrochemical cell (GC data), itotal is the total reduction current at steady state,

P =14.85 PSI, R =8.314 J/mol/K, T =294 K and F =96485 C/mol e-.

2.5. ECSA estimation and surface roughness measurements The relative electrochemically active surface area (ECSA) and roughness factors of PON-Ag and pristine Ag foil electrodes were estimated by measuring the double layer capacitances by cyclic voltammetry (CV)6. CV measurements were made in the same electrolysis cell as the CO2 reduction experiments, 0.5 M KHCO3 electrolyte. The CVs were obtained within a potential range where no faradaic processes were occurring (-0.15 to -0.05 V vs. Ag/AgCl), and the geometric current density was plotted against the CV scan rate. The slope of the linear regression of scan rate versus current density was used to estimate the electrode double layer capacitance. 2.6. Determination of exposed crystalline facets of PON-Ag electrode One of the most common methods to investigate preferential faceting – especially for bulk materials – is XRD. However, Ag (110) and Ag (100) are forbidden facets, and thus will not show identifying peaks in the XRD pattern. On the other hand, electrochemical methods provide an option to reveal surface-specific information and one electrochemical method that has been shown in the literature to be effective in identifying


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the low index facets of Ag is CV in the presence of bicarbonate11. Since each of the exposed surface facets adsorb HCO3- with different free energy, CVs in the presence of bicarbonate show peaks corresponding to each facet at a distinct potential. Therefore, in this work, in order to identify and quantify the faceting of the silver nanoparticles, CVs were collected in N2-purged 0.5 M KHCO3 solution at scan rate of 50 mV/s11. 2.7. Electrochemical Impedance Spectroscopy (EIS) EIS was used to compare the charge transfer resistance of Ag and PON-Ag electrodes in the reacting environment for CO2 reduction to CO. The EIS frequency range was 10 kHz-0.1Hz. At each frequency, the cell response was measured in a potentiostatic mode by applying a 10 mV voltage perturbation to the cell and measuring its current response. The current and voltage response was used to create Nyquist plots to better visualize the impedance data.

3. Results and discussion 3.1. Preparation of PON-Ag and physical characterization A two-step method was designed to create PON-Ag electrodes for CO2 reduction from a polycrystalline Ag foil (Figures 1a and 1b). First, an Ag foil was submerged in the electrolyte where the potential was stepped to 8.0 V vs. Ag/AgCl for 5 seconds in 0.5 M KHCO3 electrolyte, creating a crystalline layer of Ag2CO3 on top of the Ag foil (Figure S2). Bicarbonate solution was chosen for the anodization electrolyte because a previous study showed that if the oxidized-derived Ag was produced from Ag2O20, a large portion of the resulting surface was Ag (111) terminated, which is unwanted because it is a lower activity surface orientation. This result makes sense because during the transition from the simple cubic Ag2O to face-centered cubic Ag, the corner site Ag


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atoms are preserved. However, if the Ag foil were instead anodized to Ag2CO3, the crystallography is completely disrupted (Figure S2) and the Ag atoms on the Ag (111) plane are removed (Figure 1a) due to crystallographic strain and anisotropic etching. In the second step, the electrode potential was driven negative in order to rapidly decompose the Ag2CO3 and reform metallic PON-Ag21, though now with preferential exposure of (110) and Ag (100) facets through aggregation along crystal defects intrinsic to the transformation of the Ag2CO3 monoclinic structure to face-centered cubic metallic Ag.

Figure 1. Physical characterization at various stages of synthesis. (a) XRD patterns of AD-Ag, PON-Ag and Ag foil; Representative SEM images of the (b) Ag foil surface and (c) (d) PON-Ag surface at multiple magnifications (e) Cross section of PON-Ag electrode.


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XRD patterns of the polished Ag foil, anodized Ag foil with surface Ag2CO3 (AD-Ag), and reduced PON-Ag are shown in Figure 1a. The AD-Ag foil showed peaks at 18.61°, 20.66°, 32.76°, 33.76°, 37.09°, 39.75°, 41.91°, 44.45° and 51.3°, which were assigned to the (020), (110), (-101), (-130), (200), (031), (220), (131) and (211) reflections of the monoclinic phase of Ag2CO3 (JCPDS card No.70-2184). The other peaks located at 44.06°, 64.2°, and 77.2° can ascribed to the diffractions of (200), (220) and (311) of the Ag remaining in the substrate. The AD-Ag XRD pattern confirmed that the Ag foil was electrochemically oxidized to Ag2CO3 in bicarbonate solution. As shown in the SEM images of the AD-Ag (Figure S1), the anodization treatment created a uniform layer of oxidized trigonal-shaped Ag2CO3 with a particle size around 1 micron. Both the PON-Ag and Ag foil XRD patterns showed five main peaks located at 37.88°, 44.06°, 64.2°, 77.2°, 81.36°, which can be ascribed to the corresponding (111), (200), (220), (311), (222) face cubic centered (fcc) Ag reflections (JCPDS Ag-04-0783). The XRD pattern for the PON-Ag shows that all of the AD-Ag was fully reduced to metallic Ag prior to the CO2 reduction experiments. As purchased, the Ag foil (Figure 1b) had a mostly smooth surface, though some features from mechanical polishing were still evident. After anodization/reduction treatment, the PON-Ag had a porous, nanostructured surface with uniformly distributed particles (Figure 1d) covering the electrode with an average thickness of 2.5 µm (Figure 1e), which enabled a much higher ECSA (ca. 50 times greater) than the starting Ag foil. Compared to the AD-Ag particles, after reduction the PON-Ag was columnar in shape and around 300 nm in length and 50 nm wide (Figure 1c). The PON-Ag catalyst that was created on the electrode also formed inter-particle pores that were a few hundred nm in diameter.


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Figure 2. Representative TEM images: (a) low resolution images displaying a wellconnected Ag nanoparticle architecture; (b), (c)，d) high resolution images showing the lattice parameters of the PON-Ag electrode; (e), (f) zoomed in areas of B and C, respectively, from (a) showing the grain boundaries formed after the electrode anodization/reduction treatment.

As shown in Figure 2a, the PON-Ag formed a layer of Ag particles with a dendrite-like morphology, which were consistent with the SEM results. High resolution TEM images (Figure 2b, 2c, 2d) of the PON-Ag showed that the lattice fringes clearly aligned parallel to each other, indicating that the PON-Ag was highly crystalline (singlecrystal-like), which was further confirmed by the XRD pattern14. The d-spacing of 0.21 nm, 0.25 nm and 0.15 nm measured by TEM corresponds to the (200), (111) and (220)


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facets of the Ag, respectively. Notably, both the Ag (110) and Ag (100) are not observable in the X-ray diffraction pattern since these reflections are forbidden. However, the (110) and (100) are reasonably assumed to exist in an amount proportional to that of (220) and (200), due to the fact that (110) and (100) are parallel to (220) and (200) in a face centered cubic, respectively. The PON-Ag electrode also exhibited interconnected nanocrystallites with grain boundaries between particles (Figure 2e, 2f) that could possibly contribute to the activity. However, the HRTEM images (Figure 2e and 2f) show only a small number of interconnected nanocrystals, thus the electrochemical area of the grain boundaries formed is pretty small and likely a minority contributor to the observed behavior.

Figure 3. (a) Voltammograms of PON-Ag in 0.5M KHCO3 saturated with N2 at a scanrate of 50 mV/s; High resolution of Ag 3d XPS spectra of the (b) Ag foil; (c) PONAg electrodes

In order to more precisely quantify the presence of (110) and (100) facets on the electrode surface, HCO3- adsorption/desorption experiments were performed on the PONAg catalysts, a technique that was reported by Hoshi et al.11 where the peaks observed in CVs can be quantitatively assigned to the individual low index Ag facets. As shown in Figure 3a, the PON-Ag electrode shows three redox peaks in the CV that can be assigned


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to the adsorption/desorption of HCO3- on: Ag (110) at -0.75 to -0.65 V, Ag (100) at -0.55 to -0.45 V, and Ag (111) at -0.35 to -0.25 V vs. SHE. It was observed that the peak intensity for (110) and (100) are larger and better defined compared to the (111) facet, suggesting preferential faceting of the Ag particles reduced from Ag2CO3. In addition to observing that the peaks were better defined, the HCO3- adsorption charge was determined from the anodic scan for Ag (110), Ag (100) and Ag (111). The calculated charges were 5.0*10-3 C, 7.7*10-3 C and 1.5*10-3 C, respectively, which shows that the Ag (110) and Ag (100) facets were each 3-5 times more prevalent than Ag (111), assuming complete coverage and identical packing density of HCO3- on each facet under the conditions employed. This result shows that our synthesis method was successful in creating preferentially faceted Ag (110) on the foil surface from the rapid electrochemical decomposition of AgCO3, something that was accomplished here for the first time – which can be useful not only for CO2 reduction but other reactions as well. XPS was used to probe the electronic structure and density of states of the Ag foil and the PON-Ag electrode. Figure 3b and Figure 3c shows that the Ag 3d XPS spectra could be deconvoluted into one main peak for both Ag 3d3/2 and Ag 3d5/2, indicating that only metallic Ag was present. The Ag 3d peaks of PON-Ag were shifted to lower binding energy than the Ag foil – probably caused by enhanced electron density on the electrode surface, which is believed to have a positive effect on CO2 reduction to CO22. Goodman and coworkers have shown that intermetallic electron transfer can cause a decrease in core-level binding energy, resulting in stronger chemisorption of CO due to the formation of high electron density of the host metal that allows facile π-backdonation into the adsorbed CO23. Thus, the downshift of binding energy of PON-Ag than Ag foil may help


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stabilize the key intermediate *COOH on the electrode surface, which helps the selective electrochemical conversion of CO2 to CO. 3.2. Electrochemical reduction of CO2 to CO The electrochemical reduction of CO2 was investigated on PON-Ag and two primary gas-phase products were found: CO and H2. The PON-Ag showed very high rates and CO selectivity. Figure 4a presents iR-corrected total reduction current densities (normalized to the geometric electrode area) as a function of potential, which were measured chronoamperometrically. The PON-Ag showed a much higher total reduction current density than the Ag foil, at least partly because of the enhanced surface area, which is discussed in detail in the Supporting Information and graphically shown in Figure S3. From this data, the CO partial current density was calculated using Equation 2, and the result is plotted in Figure 4b. The PON-Ag showed a higher CO partial current density than that of the polycrystalline Ag at all operating potentials, ranking the second highest among all of the materials reported in the literature to date (shown in Table S113,14,17,20,24,25). One of the most significant results of this work is that the onset potential for CO2 reduction on PON-Ag was high, meaning low overpotential, where a measureable CO partial current density (> -0.05 mA/cm2) was found at only 0.3 V vs. RHE (0.19 V overpotential). It should be noted that the polycrystalline Ag had a much lower cathodic onset potential (higher reaction overpotential), matching well with previous literature26. In order to gain kinetic insights into the enhanced catalytic activity of CO2 reduction to CO on PON-Ag from the electrochemical data, Tafel plots (potential vs. log of the partial current density) were created and are shown in Figure 4c. The PON-Ag electrode showed a Tafel slope of 115 mV/decade for CO production, which suggests an


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effective transfer coefficient of approximately 0.5 (Equation 3), indicating that the initial electron transfer to CO2 to form a surface adsorbed *COOH intermediate is the ratedetermining step, Equation 4 (γ=0, ν=ρ=1; β≡0.5). α eff =

2.303RT F ⋅ Tafel Slope

α eff =

γ + ρβ ν

(3)

(4)

Figure 4. Electrochemical characterization of polycrystalline Ag foil and PON-Ag electrodes showing: (a) total reduction current density; (b) CO partial current density as a function of potential (iR-corrected); (c) Tafel plot of overpotential as a function of CO partial current density; and (d) CO faradaic efficiency as a function of potential (current densities normalized by geometric surface area).

Where αeff is the effective transfer coefficient, γ is the number of steps preceding the rate determining step, ν is the stoichiometric coefficient, β is the reversible transfer coefficient


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and ρ is a coefficient equal to 1 if the RDS is an electron transfer step or 0 if it is a chemical step. The Tafel slope for the Ag foil was parallel to the PON-Ag, confirming that the underlying mechanism for CO2 reduction on both Ag electrodes was the same. The selectivity for CO2 reduction to CO was also investigated on the PON-Ag and polycrystalline Ag electrodes and is represented by plotting the CO faradaic efficiency (FECO) as a function of the operating potential in Figure 4d. Both PON-Ag and Ag foil showed a volcano-like potential dependence for FECO. The PON-Ag showed enhanced CO selectivity over the hydrogen evolution reaction at lower reaction overpotential than the polycrystalline Ag electrode. In addition, the maximum CO faradaic efficiency was 96.7% at an operating potential of -0.69 V (0.58 V overpotential), which was much higher than that of the polycrystalline Ag foil (60% at -0.87 V). It is also interesting to note that although the CO faradaic efficiency decreased from -0.72 to -0.9 V vs. RHE, the CO partial current density continued increasing in this potential range, indicating that the decrease in CO selectivity was not due to a decrease in the CO production rate, but a more rapidly increasing production rate for H2 (Figure S3) because of its high exchange current density. Moreover, the CO2 electrochemical reduction activity and selectivity of PON-Ag electrodes prepared in the work were compared with other top-performing silver-based electrocatalysts (shown in Table S113,14,17,20,24,25,18,27,28). Though it is difficult to directly compare the results in Table S1 due to the differing conditions (0.1M vs. 0.5M HCO3; with/without agitation), it is possible to say that the PON-Ag in this work showed one of the best performances to date – with a very attractive combination of low overpotential, high partial current density and high faradaic efficiency. In fact, only two reported Agbased catalysts14,18 have been able to achieve a combination of high faradaic efficiency


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and low overpotential comparable to this work. Further discussing the differences in the reacting conditions, previous studies indicated that the properties of electrode-electrolyte boundary layer could have significant impact on the FE and achievable maximum current density during CO2 electrochemical reduction29–31. Because of the large excess in local anions at these high concentrations, it is likely that the low CO2 concentration in the aqueous electrolyte is controlling the mass transport behavior.

Thus, stirring the

electrolyte increases the availability of CO2 to be dissolved and react at the electrode surface, with higher CO2 mass transport rates at higher RPM. Hence, our work can be favorably compared to all of the studies in Table S1 where the electrolyte was unagitated as well as all but one study where the electrolyte was agitated – the sole exception showing higher CO2 partial current density than this work was Jiao et al.14, where the electrolyte was stirred vigorously at 1200RPM. It is expected that the PON-Ag could perform even better than reported here if the reaction was performed under similar reaction conditions. One area where this work has a distinct advantage is that the PONAg catalyst was prepared using a simple and scalable method that be used to produced low cost nanoporous Ag for commercial applications14. Finally, the stability of the PON-Ag electrodes for CO2 electrochemical reduction was examined continuously over a two 2-hour period (7 injections) at each potential. The average COFE for these longer experiments is plotted versus potential in Figure 5a and the Tafel data is shown in Figure 5b. Even over two hours of operation, the PON-Ag electrode showed very good stability. For instance, the PON-Ag was able to achieve a COFE of 87% at an operating potential of -0.72 V vs. RHE, which was very similar to the initial performance data shown in Figure 4d. Also, the measured current and Tafel slope


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did not appreciably decline during the test – showing that the PON-Ag catalyst not only achieved very high performance, but excellent stability as well.

Figure 5 Electrochemical stability of PON-Ag electrodes showing the average performance over a two hour period: (a) CO faradaic efficiency; (b) Tafel current and slope (current densities normalized by geometric surface area).

4. Conclusion

In this work, an Ag electrode with preferential (100) and (110) faceting was prepared by a simple, scalable two-step method where the electrode was first anodized to Ag2CO3, then reduced back to nanocrystalline metallic Ag nanoparticles with high electrochemical surface area. The PON-Ag was able to achieve an extremely high faradaic efficiency of 96.7% at a low operating overpotential of 0.58 V (absolute potential of -0.69 V vs. RHE), as well as a very high areal CO partial current density of 4.4 mA/cm2). This work shows the promise for Ag-based catalysts to capture and reduce CO2 extremely efficiently – and to produce highly effective Ag catalysts without relying on exotic synthesis methods. Additionally, since the PON-Ag produced in this work is self-supported, and generated in-situ in the reacting environment, it may be possible in the future to eliminate the use of ionic polymers for electrode fabrication, which would reduce the cost and complexity for commercial use. It may also be possible to perform


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operando potential cycling to allow the PON-Ag to regenerate or self-heal the catalyst, and this will be probed in future work. Supporting Information CO2 electrolysis setup; SEM images of AD-Ag electrode; Electrochemical measurements, CO and H2 FE of the PON-Ag and Ag foil electrode; SEM images of PON-Ag electrode; Comparison Table.

Acknowledgements This work was supported by the U.S. Department Of Energy (DOE) award number DE-SC0010531. The author acknowledges the Center for Clean Energy Engineering at the University of Connecticut for use of the physical characterization equipment.

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Preferentially Oriented Ag Nanocrystals with ... - ACS Publications

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