Tunable Ag Micromorphologies Show High Activities for

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Tunable Ag Micro-morphologies Show High Activities for Electrochemical H Evolution and CO Electrochemical Reduction 2

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Jing-Fang Huang, and Yi-Ching Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00116 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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ACS Sustainable Chemistry & Engineering

Tunable Ag Micro-morphologies Show High Activities for Electrochemical H2 Evolution and CO2 Electrochemical Reduction Jing-Fang Huang* and Yi-Ching Wu Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, R.O.C. * E-mail: [email protected]

Contaminate-free nanostructured Ag is prepared from electrochemical redox-treated Ag by two different potential pulses (DP and r-DP) in an NaBr aqueous solution. Agr-DP and AgDP from r-DP and DP, respectively, are easily tuned for their activities in the hydrogen evolution reaction (HER) favoring or selective CO2 reduction reaction (CO2RR). Agr-DP shows Pt-like HER activity in H2SO4 aqueous solution, and AgDP is capable of electrochemically reducing CO 2 to CO with approximately 97.8% catalytic selectivity at a low overpotential of ~0.25 V in NaHCO 3 aqueous solution. The electrochemical signals for the first observed CO 2•− intermediate adsorption (CO2ads•−) and HCO3- adsorption (HCO3-ads) in the selected NaClO4 aqueous solution were used to understand the possible role of HCO3- in CO2RR.

KEYWORDS: Hydrogen Energy; Silver; Carbon Dioxide Reduction; Hydrogen Evolution Reaction; Nanoporous

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INTRODUCTION Extensive research into closing the anthropogenic carbon cycle using the electrochemical CO 2 reduction reaction (CO2RR) has attracted renewed attention for utilizing CO2, a major greenhouse gas that is captured at large emission sources.1-5 High-efficiency and low-cost catalysts are urgently required for developing a commercialized CO 2RR process. Several cathode materials, including Cu,6-11 Au12-14 and Ag,15-24 are promising electrocatalysts because of their high activities for CO2RR. Au and Ag are of particular interest, because both materials promote the CO 2RR to produce CO selectively with faradaic efficiencies (FEs) above 80%. Although Au exhibits a higher activity for CO2RR than Ag — the CO2RR on Au with a FE of more than 80% at a potential of 0.74 V vs. reversible hydrogen electrode (RHE) is more positive than that on Ag (-0.97 V vs. RHE)25 — its use is limited for large-scale applications, owing to its high price and low abundance.26-27 Recently, Ag-based catalysts have attracted considerable interest because of their lower cost compared with Au and the ability to selectively reduce CO 2 to CO. However, Ag-based catalysts require higher overpotential (η > 0.9 V) to attain efficiency and selectivity. At highly cathodic η, the hydrogen evolution reaction (HER),28-29 which is an accompanying reaction from H2O reduction, must also be suppressed. HER and CO 2RR are two well-known competing reactions on the cathode. Nanostructured Ag (Ag nano)-based catalysts like Ag nanoparticle and Ag nanoporous (Agnpo) catalysts, were developed to enhance the CO2RR owing to the significantly increased active sites on Ag nano.15-17,

30-33

Particularly for Agnpo, ultrahigh surface area and

increased active sites enable the CO2RR to be conducted efficiently and selectively at reduced η (0.37 V to 0.49 V) with more than 90% selectivity for CO production.16-17, 19 Lu et al. prepared Agnpo by dealloying an Ag–Al alloy precursor in an HCl aqueous solution.19 Agnpo was also produced from the potential anodized Ag in a NaOH aqueous solution or electrochemically treated


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Ag by an oxidation-reduction process in a KCl aqueous solution.16-17 Although it is possible that the high activities of Ag npo were caused by a nanostructured morphology effect, these Ag nano were prepared using a medium rich in Cl- or OH- ions. Cl- or OH- residues on Ag surfaces were believed to promote the CO2RR performance in related research.16-17 To understand the pure nanostructured morphology effect, an NaBr aqueous solution (NaBraq) was selected as a medium for Ag treatment to fabricate contaminant-free Agnpo in this study. AgBr is the most photosensitive among silver halides (AgX).34-35 When AgBr is exposed to sunlight, a decomposition reaction takes place, which results in the formation of Ag and Br2g, to reduce Br residues effectively. Unlike Pt-related noble metals, the poor HER activity of Ag in acidic environments makes it rarely considered as a potential HER electrocatalyst. In our previous studies, Ag treated by a repetitive electrochemical redox of AgBr/Ag through simple multiple-scan cyclic voltammetry (Ag CV) in NaBraq yielded a sponge-like Agnpo and exhibited Pt-like HER activity, but a CO2RR active like other Agnpo.36 In this work, two different potential treatments (Scheme 1) for easily tuning Ag as Pt-like HERactive or CO2RR-active are proposed. We also firstly demonstrates the micromorphology of Ag nano decides the main reaction trend between two competing reactions, HER and CO 2RR. HCO3containing aqueous electrolyte solutions (NaHCO 3aq or KHCO3aq) are common systems for the CO2RR. CO2RR rates in the HCO3-aq are higher than in other aqueous electrolyte solutions. The well-accepted roles of HCO3- are as a pH buffer or H+ donor. In previous studies, the high HCO3content in HCO3-aq limited understanding of the effect of HCO3-. Although the adsorption of HCO3(HCO3-ads) on Ag substrates has been observed in studies on the anion absorption phenomenon,37 studying the role of HCO3- in promoting the CO2RR is still rarely attempted because of the difficulty in directly probing this process. In particular, there is little electrochemical evidence suggesting the possible interaction between HCO 3-ads and CO2 in the CO2RR. To study the effect


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of HCO3-ads in the CO2RR, a NaClO4 aqueous solution (NaClO4aq) was selected as another studying system because of the weaker adsorption ability of ClO 4-.37 The hardly visible signal about the reaction intermediate, adsorbed reduced CO2 radical anion (CO2ads•−), first observed in the NaClO4aq assists in understanding the interaction between HCO 3-ads and CO2 on a contaminantfree Agnpo in the CO2RR. EXPEIMENTAL SECTION Chemicals. Deionized water (specific resistivity = 18.2 M cm) was used to prepare all solutions. All chemicals were of analytical grade unless otherwise stated. 95–98% H2SO4 (JT-Baker), 98% NaClO4 (Alfa), 99.0% NaBr (JT-Baker), and 99.8% NaHCO3 (Sigma-Aldrich) and 99.8% Pb(NO3)2 (JT-Baker) were used as received. Preparation of redox-treated silver electrodes. The electrochemical experiments were performed using a CHI 760C potentiostat/galvanostat and a three-electrode electrochemical cell. To prevent Cl- and Pt interferences, Hg/HgSO4 (0.5 M H2SO4) and a graphite rod (or a silver wire) were used as the reference electrode and counter electrode, respectively. The potentials were measured against the reference electrode and converted to the reversible hydrogen electrode (RHE) reference scale by ERHE = EHg/HgSO4 + 0.68 + 0.059pHelectrolyte. A polished Ag wire was sonicated in 70% ethanol (3 min) and deionized water (3 min), and dried before being used as the working electrode. Ag wire working electrodes were treated using DP and r-DP, respectively, in 0.5 M NaBraq (Scheme 1). For DP treatment, the anodic potential was 0.6 V vs. RHE for various periods (ta) and the cathodic potential was -0.3 V vs. RHE for 0.5 h (tc). For r-DP treatment, the repetitive DP sequences where each sequence was stepped to 0.6 V vs. RHE for t a = 3 s immediately followed the anodic process by stepping the potential to -0.3 V vs. RHE for tc = 5 s. The resulting AgDPt


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from DP and Agr-DPt from r-DP — t is the total ta (hour, h) in the DP and the r-DP — were then electrochemically cleaned by CV between 0.5 V and -0.4 V versus RHE, with a scan rate of 0.05 V/s in an Ar-purged 0.5-M H2SO4 until reproducible CVs were obtained. The microstructure image analyses of AgDPt and Agr-DPt were performed using a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) and a JEOL JIB-4601F — a multi-beam processing and observation system that incorporates a Schottky-type SEM and a high-power focused ion beam (FIB) column. The FIB was used for fine milling and performing 3D structure analysis with automatic cross-section milling, observation, and analysis at fixed intervals. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) were used for surfaceelement analysis of AgDPt and Agr-DPt. The XPS data were acquired using the ULVAC-PHI, PHI5000 VersaProbe/scanning ESCA microprobe. For X-ray diffraction (XRD) measurements (Cu Kα radiation), an Ag foil (10 × 10 mm) was used as a working electrode for the preparation of AgDPt and Agr-DPt. Lead underpotential deposition (Pb UPD). Electrochemical surface area (ECSA) determinations of AgDPt and Agr-DPt were performed by Pb UPD. An Hg/Hg2SO4 reference electrode and Pt counter electrode were used. Pb UPD was conducted in 1 mM Pb(NO3)2 + 1 mM HClO4 + 0.5 M NaClO4 solution, at a scan rate of 0.01 Vs -1. A typical three-electrode electrochemical cell was used for the electrochemical experiments with Ar degassing and with no stirring. The desorption peak of Pb UPD was integrated to calculate the ECSA assuming 420 μC cm−2 for Ag-based samples.38 Figure S4 shows that AgDPt and Agr-DPt have different ECSAs. Electrochemical characterization of hydrogen evolution reaction (HER). Pt and pretreated (Agi) and post-treated Ag wires (AgDPt and Agr-DPt) as working electrodes were electrochemically cleaned using a cycling potential between 0.5 V and -0.4 V versus RHE with a scan rate of 0.1 V


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s-1 for 20 times in Ar-purged 0.5 M H2SO4aq. The HER was examined using CV at a scan rate of 0.05 V s-1 and linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 in Ar-purged 0.5 M H2SO4aq at 25 °C. Electrochemical characterization of CO2 reduction reaction (CO2RR). AgDPt as working electrodes were electrochemically cleaned using a cycling potential between 0.5 V and -0.4 V versus RHE with a scan rate of 0.1 V s-1 for 20 times in Ar-purged 0.5 M H2SO4aq. The CO2RR was examined using CV at a scan rate of 0.05 V s -1 and linear sweep voltammetry (LSV) at a scan rate of 5 mV s -1 in 0.1 M NaHCO3aq saturated with CO2 (CO2-NaHCO3aq) and 0.1 M NaClO4aq saturated with CO2 (CO2-NaClO4aq), respectively, at 25 °C. To identify the CO2RR mechanism, LSV scans were replotted as potential versus log(current density (j)) to obtain Tafel plots. From the least-squares fits of the linear regions of the Tafel plots to the Tafel equation, Tafel slopes were determined. CO2 electrolysis was performed at room temperature in a gas-tight H-shaped electrochemical cell using a piece of nylon membrane filter (ULTIPOR 66N, NQ047100) as the separator. Each compartment contained 30.0 mL electrolyte and 30 mL headspace. Before electrolysis, the electrolyte was purged again with CO2 gas for 20 min and the headspace for at least 10 min. The gas-phase product was sampled every 30 min using a gas-tight syringe (Hamilton). A gas chromatograph (SHIMADZU, GC-Tracera) equipped with PLOT MolSieve 5A and with a barrier ionization discharge detector (BID) was used for quantifications. Helium (99.999%) was used as the carrier gas. The partial current density (jproduct, jCO and jH) for each evolved gaseous product was calculated using the following relationship: nF P

jproduct = [P] × flowrate × RT A

Equation 1


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where [p] (ppm) is the concentration of the product measured via GC from a calibration standard gas mixture, P is the pressure in the electrochemical cell headspace (1 atm), T is the temperature, R is the gas constant, n is the number of electrons transferred per equivalent of the given product, F is Faraday’s constant, and A is the surface area of the electrode. Faradaic efficiency values were determined by dividing jproduct by specific jtotal.

Scheme 1. Schematic diagrams for DP and r-DP treatments. RESULTS AND DISCUSSION Two different Ag nano surfaces were obtained by two different potential treatments – double potential pulse (DP) and repetitive DP (r-DP), in 0.5 M NaBraq (Scheme 1). For the DP treatment, a polished, clean Ag wire (Agi), was anodically treated at 0.6 V vs. RHE for various periods (ta) to form AgBr deposits (AgBr s) of different thicknesses; subsequently, AgBr s was cathodically reduced to Ag at -0.3 V vs. RHE for 0.5 h (tc). For r-DP treatment, Agi was treated by repetitive DP sequences, where each sequence was stepped to 0.6 V vs. RHE for ta = 3 s immediately following the anodic process, by stepping the potential to -0.3 V vs. RHE for tc = 5 s. The micromorphologies of the Ag electrodes treated by DP (Ag DP) and r-DP (Agr-DP) were examined


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Figure 1. SEM images of AgDP8h in (a) low magnification (scale bar: 5 μm) and (b) high magnification (scale bar: 1 μm), and Agr-DP8h in (c) low magnification (scale bar: 5 μm) and (d) high magnification (scale bar: 1 μm). Cross-sectional SEM images of (e) AgDP8h (depth = 62.4 μm) and (f) Agr-DP8h (depth = 3.7 μm). using scanning electron microscopy (SEM). AgDP8h and Agr-DP8h both showed highly rough irregular surfaces for AgDPt and Agr-DPt, respectively, for DP and r-DP treatments, where t is the total ta (hour, h) (Figures 1 and S1). The microstructure of AgDP8h showed a continuous porous


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network with a characteristic ligament size of ~30-50 nm and pore size of ~20-30 nm, similar to that of Ag treated by DP in NaClaq or NaOHaq.16-17 In particular, the Agr-DP8h surface (Figures 1c and 1d) showed coarsening of the interconnected Ag particles (150-200 nm) and growth of sintering necks. The micromorphology on Agr-DP was similar to that on AgCV reported in previous studies.36 It was caused by a seed-growth process in which the pre-electroxidative AgBr s serves as the seeds and Ag sources for the growth of Ag nanoparticles during the electroreduction of AgBr s. Only pure Ag without Br contamination was detectable on both Ag DP and Agr-DP by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) (Figure S2). Interestingly, Agr-DP and AgDP showed completely different reaction trends between two competing reactions (HER and CO2RR). The polarization curves and CVs for the HER on different electrode materials, including Pt, Agi, Agr-DP8h, and AgDP8h, were recorded in 0.5 M H2SO4aq (Figure 2a). Agr-DP8h showed Pt-like HER activity (the HER onset potential of the Agr-DP8h (-0.05 V) is very close to that of Pt (-0.02 V)), which was much better than that of Agi and AgDP8h. These results are consistent with previous findings on the exceptionally high HER activities on Ag CV.36 Surprisingly, an ultrahigh background current (0.5 V to 0.0 V vs. RHE) from double-layer charging was found on AgDP8h, which was much higher than that on Agi and Agr-DP8h (inset in Figure 2a). This was attributed to the ultrahigh surface area of Ag DP8h. The electrochemical surface area (ECSA) of AgDP8h (27 cm2) evaluated from Pb underpotential deposition (Pbupd) was 387- and 14fold higher than those of Agi (0.07 cm2) and Agr-DP8h (1.8 cm2) (Figure S3).38 The unrestricted increases in the thickness of AgBr s by the DP created a thick pore structure on the AgDP surface (Figure 1e), which in turn caused a significant increase in ECSA. However, the increase in the thickness of AgBrs was limited in the r-DP treatment (each DP sequence for the formation of AgBr s was limited to 3 s), resulting in a slight increase in the surface area of Ag r-DP


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2 Figure 2. (a) Polarization curves of various electrode materials for HER. The inset shows the corresponding CVs of the various electrode materials for the HER in 0.5 M H 2SO4 with a scan rate of 0.05 Vs-1 at 25 °C. (b) XRD patterns (Cu Kα) of Agi, Agr-DP, and AgDP. (Figure 1f). This also implies that the high surface area is not primarily responsible for the increase in HER activity. Typical characteristic Ag diffraction peaks at 2θ angles of 38.2°, 44.5°, 64.4°, and 77.4°, which correspond to the (111), (200), (220), and (311) crystal planes, respectively, can be detected from the X-ray diffraction (XRD) patterns. The XRD pattern for Ag r-DP is similar to that of Agi, in the metal form and as a polycrystalline fcc structure with no preferential crystallographic orientation (Figure 2b). The same ultrahigh HER activities on Agr-DP8h and AgCV strongly suggest that the key reason for the dramatically increased HER activity proposed in


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previous studies is the effectively increased surface active sites or “hot spots” (interconnected Ag nanoparticles) for H atom adsorption, but relating to the high surface area and crystallographic orientation.36 The electrocatalytic activities of Agi, Agr-DP8h, and AgDP8h for CO2RR were assessed in 0.1 M NaHCO3aq saturated with Ar (Ar-NaHCO3aq) and CO2 (CO2-NaHCO3aq), respectively (Figure 3a). In Ar-NaHCO3aq, polarization curves show that the cathodic current quickly rises from the HER initiated at approximately -0.4 V, especially on Agr-DP8h, indicating that the HER activity of AgrDP8h

is still superior to that of Agi and AgDP8h. Interestingly, the HER activity of AgDP is distinctly

suppressed with the increase of ta during the DP treatment. HER activity suppression is a key step to promote CO2RR; otherwise, a major portion of the cathodic current efficiency will be exhausted in generating H2 rather than reducing CO2. Based on the successful suppression of HER activity, the polarization curves on AgDPt (t are 1, 2, 4, and 8 h) exhibit cathodic currents for CO2 reduction initiated at approximately -0.38 to -0.45 V in the CO2-NaHCO3aq. The CO2RR performance is significantly improved with the increase of ta owing to the positive shift of the reduction potential for CO2RR with a current density of 5 mA cm-2 (from -0.9 V on Agi to -0.43 V on AgDP8h) and the significant enhancement of reduction current density for the CO 2RR. AgDP exhibits an XRD pattern different from those of Agr-DP and Agi. The intensity ratio of (111)/(200) on Ag DP is higher than those on Agr-DP and Agi. It is believed that higher index facets, Ag(111), relating to steps and active sites, enable the CO2RR to take place.15, 19 However, the increase of Ag(111) intensity still does not explain the suppression of HER activity on AgDP. AgDP8h shows the highest CO2RR performance in a series of AgDPt.


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Figure 3. (a) Polarization curves recorded on Agi and AgDPt (t: 1 h, 2 h, 4 h, and 8 h) with a scan rate of 0.05 Vs-1 at 25 °C in CO2-NaHCO3aq. The inset shows the corresponding polarization curves recorded on Agr-DP8h and AgDPt (t: 1 h, 2 h, 4 h, and 8 h) in Ar-NaHCO3aq. Faradaic efficiencies for (b) CO and (c) H2 on (blue circle) Agi and (red circle) AgDP8h. (d) Partial current density for CO production, jCO, vs. applied potentials on (blue circle) Agi and (red circle) AgDP8h. Inset: magnification of the plot of Agi. The CO2RR performances of AgDP8h and the Agi were further evaluated by controlled potential electrolysis in CO2-NaHCO3aq. The gas-phase and liquid-phase products were identified by a gas chromatography (GC) system equipped with a barrier ionization discharge detector (BID) and 1H NMR, respectively, during the CO2RR. As expected, the major products were CO and trace H 2,


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and no liquid-phase products could be detected after electrolysis at each applied potential. The FEs for the major products, CO (FECO) and H2 (FEH), during the CO2RR are plotted at various potentials for Agi and the AgDP8h in Figures. 3b and 3c. CO2RR initiated at -0.32 V vs. RHE on AgDP8h, which is a positive shift by about 200 mV compared with that of Ag i (-0.5 V vs. RHE). The FECO on AgDP8h sharply increased to 97.8% after the potential exceeded -0.39 V, but the FECO on Agi only gradually increased to 90% after the potential exceeded -0.85 V. The maximum FECO on AgDP8h was positively shifted by >450 mV compared to that of the Ag i. The increase in FECO was also accompanied by a decrease in FEH. The partial current density for CO production, jCO, vs. applied potentials indicated that the onset potential for CO 2RR on AgDP8h is -0.35 V vs. RHE (Figure 3d). The overpotential for CO production, ηCO, on AgDP8h (0.24 V), corresponding to the standard reduction potential E° = -0.11 V of the CO2RR vs. RHE, was reduced by 250 mV compared with that of Agi (0.49 V). The specific activity (SA), which represents the intrinsic activity of catalysts for CO production, was further evaluated from jCO normalized by the ECSA (Table S1). AgDP8h exhibits high SAs of 164.2 μA cm-2 and 350.7 μA cm-2 measured at -0.5 V and -0.6 V, respectively –91 and 57 times higher than those of Agi (1.8 μA cm-2 and 6.2 μA cm-2). There was no Br residue on the AgDP and Agr-DP obtained from NaBraq. The CO2RR performance on AgDP was even higher than on Ag nano from Cl- or OH- ions containing media reported in the literature (Table S2). This implies that the high CO 2RR performance on AgDP could be primarily attributed to the unique nanostructure. Interestingly, a broad reductive wave, c1, from 0.3 V to -0.3 V, was first observed, and grew with the increase of ta, besides the significant improvement of CO2RR activity (Figure 3a). c1 is ECSA-dependent and could be due to the reductive desorption of absorbents like HCO3-ads,37 and possibly related to the CO2RR performance. To study the effect


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E (V vs. RHE) Figure 4. CVs of AgDP8h recorded with a scan rate of 0.05 Vs -1 in various concentrations of NaHCO3 containing Ar-NaClO4aq. Inset: magnification of the red dash area. of unique adsorbents in the CO2RR, 0.1 M NaClO4aq was selected as the studying system because of the weaker adsorption ability of ClO4-. With Ar-degassed 0.1 M NaClO4aq (Ar-NaClO4aq), the CV of AgDP8h shows a broad reduction wave, c1’, from the reduction of ClO4- (from 0.3 V to -0.4 V) (Figure 4). After the addition of HCO3-, the reduction wave c1’ disappeared, and a new reduction wave c1 grew with the increase of NaHCO3 concentration. The c1 from the reductive desorption of HCO3- is similar to the unique reduction wave shown in Figure 3a. These results suggest that HCO3-ads occurs on AgDP and suppresses the reduction of ClO4-. Similar adsorption behavior was also observed on Agi (Figure S4). The CO2RRs on Agi and AgDP8h were also conducted in 0.1 M NaClO4aq (Figure 5). The CV of Agi shows a cathodic current for HER initiated at -0.7 V in ArNaClO4aq (Figure 5a). Unlike the results obtained in CO 2-NaHCO3aq, CO2RR cannot be solely observed on Agi with avoiding HER interference in the CO2-saturated 0.1 M NaClO4aq (CO2NaClO4aq). However, a new cathodic wave, c2, at -0.4 V for CO2RR was observed.


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-0.8

E (V vs. RHE)

25

6

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15

-2

20

j (mAcm )

2-

j (mAcm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[CO3 ]/mM 0 0.1 4 0.2 0.3 0.4 0.5 2

10 0.4

5

0.0

-0.4

E (V vs. RHE) c2

0 0.8

0.4

0.0

-0.4

-0.8

E (V vs. RHE) Figure 5. (a) CVs of Agi recorded with a scan rate of 0.05 Vs-1 at 25 °C in (black dash line) for Ar-NaClO4aq, (blue line) CO2-NaClO4aq, and (red line) 0.1 mM NaHCO3 containing CO2NaClO4aq. The inset shows the corresponding polarization curves recorded on Ag i in various concentrations of NaHCO3 containing CO2-NaClO4aq. (b) CVs of (red line) AgDP8h and (blue line) Agi recorded with a scan rate of 0.05 Vs -1 at 25 °C in CO2-NaClO4aq. The inset shows the corresponding polarization curves recorded on AgDP 8h in various concentrations of NaHCO3 containing CO2-NaClO4aq. This wave likely corresponds to the one-electron reduction of CO2 to the associated radical anion (CO2•−).39 In considering the well-accepted mechanism for CO2RR on Ag-based catalysts, a oneelectron transfer step forming adsorbed CO 2•− (CO2ads•−) is the initial step in the overall twoelectron reduction of CO2 to CO; subsequently, the intermediate CO2ads•− reacts with two protons and gains another electron to form CO.15-16, 40 In addition to nonaqueous systems,41 in view of the


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instability of the intermediate, signals of CO2ads•− are rarely observed during CO2RR, especially in NaHCO3aq. The formation of CO2ads•− is first observed because the high oxidation ability of ClO 4can reoxidize CO2ads•− back to CO2 and stop the protonation of CO2ads•− through the following EC’ process.38

This process also enhances the current signal for CO 2ads•− caused by reducing the CO2ads•− continually occupied on active sites. The first observed c2 related to CO2ads•− was tracked to understand the role of HCO3- in CO2RR. Interestingly, c2 positively shifts from -0.4 V to -0.3 V after the addition of 0.1 mM NaHCO3. The potential shift indicates that CO2RR is promoted only in the presence of trace HCO3-. The wave c1 for HCO3-ads disappeared in the presence of CO2. The disappearance of c1 and the trace HCO3--sensitive peak potential of c2 suggest that a possible chemical step related to the interaction between HCO 3-ads and CO2 occurs before the initial electrochemical reduction of CO2 to CO2•− in the CO2RR. This likely involves the binding of HCO3- to the Ag surface first, where the desorption process was varied by the interaction with the CO2 molecule through the formation of hydrogen bonding (H-bonding), allowing CO2 to access the electrode surface to precede the reduction of CO 2 or a possible surface complex “CO2-HCO3ads”

(Scheme 2). With increasing concentration of HCO3-, the peak current of c2 decreased, but the

peak potential remained constant. The increase of HCO 3- does not change the activity for the production of CO2ads•−, but a reduced CO2ads•− content causes the c2 peak current to decrease. These


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Scheme 2. Schematic of the role of HCO3- in the proposed CO2RR mechanism. results could indicate that HCO3-ads acts as an H+ donor on the Ag surface to promote the protonation of CO2ads•− and enables it to gain another electron from Ag to form CO at a higher concentration of HCO3-. In these results, the interaction between HCO 3-ads and CO2 is first demonstrated near the Ag surface (Scheme 2). The interaction assists the exchange equilibrium between HCO 3-ads and CO2.13 Surprisingly, the CO2RR on AgDP8h shows an unexpectedly superior performance than on Ag i in CO2-NaClO4aq (Figure 5b). Although some studies reported that CO2RR rates on Agi in HCO3-aq are higher than in other electrolyte aqueous solutions,42-43 CO2RR on AgDP8h shows the same high performance in NaClO4aq as in NaHCO3aq. The onset potential for CO2RR initiated at -0.38 V on AgDP8h in NaClO4aq is the same as in NaHCO3aq. Interestingly, the cathodic wave, c2, associated CO2ads•− is only present in NaClO4aq and positively shifts from -0.4 V on Agi to -0.2 V. Jiao’s group proposed that the improvement of CO2RR on Agnano was possibly from the increase in step site density for the CO2ads•− intermediate.15, 19 Tafel analyses were conducted in CO2-NaHCO3aq and


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CO2-NaClO4aq, respectively, to understand the CO2RR mechanism (Figure S5). In CO2-NaHCO3aq (Figure S5a), Tafel plots of Agi and AgDP8h, η vs. log(jco), are consistent with previous results that show one linear segment with a high slope (~140 mV/dec) for Ag i but two linear segments and a lower slope (~55 mV/dec) at low η and the rapid increase of the slope at high η for Ag DP8h. The lower slope shown for AgDP8h indicates that the rate-determining step (rds) is different from that on Agi, and it is believed that the initial CO2ads•− formation is more kinetic-favoring than a later protonation step because CO2ads•− could be stabilized on many surface active sites on AgDP8h.15, 19 In the results presented here, the electrolyte-independent CO2RR activity on AgDP8h suggested that many active sites on AgDP8h assist the initial step of the formation of CO2ads•− and primarily make this step kinetic-favoring. The c2 peak current decreased to disappear completely with increasing the HCO3- concentration to more than 0.5 mM. Based on the assumption about HCO 3-ads as the major H+ donor, the content of HCO3-ads is higher on AgDP8h than on Agi (Figure S4b) and more effectively conducts the protonation of the as-formed CO2ads•−. In CO2-NaClO4aq (Figure S5b), Tafel analyses of Agi and AgDP8h were conducted in various HCO3- concentrations. Tafel plots show that the slopes of Agi and AgDP8h are independent of the HCO3- concentration. This indicates that the microstructure of Ag primarily decides the rds in the CO 2RR. CONCLUSION In summary, Agnano with two different micromorphologies were prepared from electrochemically treated Ag by two different potential pulses (DP and r-DP). The contaminant-free Agnano was obtained from Ag treatment in NaBraq. The treated Ag selectively tuned their activities as HERfavoring or CO2RR-favoring. Agr-DP showed Pt-like HER activity owing to its microstructure comprising interconnected Ag grains for H atom adsorption. AgDP with a nanoporous structure exhibited an enhanced catalytic activity for CO 2RR, and a high selectivity for CO with FECO =


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97.8% at a ηCO as low as ~0.25 V. The signals correlated to the first observed CO 2ads•− and HCO3ads

in the selected NaClO4aq were used to understand the role of HCO 3- in CO2RR. It was

demonstrated that HCO3-ads assists the CO2 from the solution phase near the Ag surface through a possible interaction, e.g., H-bonding. It also acts as a proton donor in the subsequent protonation. ASSOCIATED CONTENT Supporting Information. SEM images of AgDP8h and Agr-DP8h, XPS spectra and the EDS plot for the treated Ag, Pb UPD on Agi, Agr-DP8h and AgDP8h, Comparison of HCO3- reduction desorption on Agi and AgDP8h, Tafel polarization curves of Agi and AgDP8h for CO2RR, CO2RR Performance on Agi and AgDP8h, Comparison of CO2RR catalytic performance of various Ag-based catalysts (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Acknowledgment This work was supported by the Ministry of Science and Technology of the Republic of China, Taiwan. References (1) Mariano, R. G.; McKelvey, K.; White, H. S.; Kanan, M. W., Selective increase in CO 2 electroreduction activity at grain-boundary surface terminations. Science 2017, 358, 1187-1191, DOI 10.1126/science.aao3691.


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For Table of Contents Use Only.

Contaminate-free nanostructured Ag with tunable HER or CO 2RR active were prepared to understand role of HCO3--promoters in the CO2RR.


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Tunable Ag Micromorphologies Show High Activities for

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