Polyoxometalate Promoted Electrocatalytic CO2 Reduction at

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Polyoxometalate Promoted Electrocatalytic CO2 Reduction at Nanostructured Silver in Dimethylformamide Si-Xuan Guo, Fengwang Li, Lu Chen, Douglas R. MacFarlane, and Jie Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01042 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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ACS Applied Materials & Interfaces

Polyoxometalate Promoted Electrocatalytic CO2 Reduction at Nanostructured Silver in Dimethylformamide

Si-Xuan Guo, Fengwang Li, Lu Chen, Douglas R. MacFarlane and Jie Zhang* School of Chemistry and ARC Centre of Excellence for Electromaterials Science Monash University, Clayton, Victoria 3800, Australia

E-mail: [email protected]

KEYWORDS: CO2 reduction, electrocatalysis, silver, polyoxometalate, dimethylformamide

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ABSTRACT Electrochemical reduction of CO2 is a promising method to convert CO2 into fuels or useful chemicals, such as carbon monoxide (CO), hydrocarbons and alcohols. In this study, nanostructured Ag was obtained by electrodeposition of Ag in the presence of a Keggin type polyoxometalate, [PMo12O40]3- (PMo). Metallic Ag is formed upon reduction of Ag+. Adsorption of PMo on the surface of the newly formed Ag lowers its surface energy thus stabilizes the nanostructure. The electrocatalytic performance of this Ag-PMo nanocomposite for CO2 reduction was evaluated in a CO2 saturated dimethylformamide medium containing 0.1 M [n-Bu4N]PF6 and 0.5% (v/v) added H2O. The results show that this Ag-PMo nanocomposite can catalyze the reduction of CO2 to CO with an onset potential of -1.70 V vs. Fc0/+, which is only 0.29 V more negative than the estimated reversible potential (⁃1.41 V) for this process, and 0.70 V more positive than that on bulk Ag metal. High faradaic efficiencies of about 90% were obtained over a wide range of applied potentials. A Tafel slope of 60 mV dec-1 suggests that rapid formation of *CO2˙- is followed by the rate determining protonation step. This is consistent with the voltammetric data which suggest that the reduced PMo interacts strongly with CO2 (and presumably CO2˙-) and hence promotes the formation of CO2˙-.


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1. INTRODUCTION Carbon dioxide (CO2) is a notorious gas which contributes to the global warming. With the implementation of carbon capture technology, CO2 is now widely considered as a cheap and abundant C1 feedstock for the synthesis of fuels and other valuable chemicals. Due to its high stability, catalysts have been developed and studied in the reduction of CO2 to fuels or useful chemicals

by

photochemical,1-4

thermochemical4

and

electrochemical

methods.5-8

Photochemical conversion of CO2 suffers low efficiency, while the thermochemical route requires high temperature and pressure, which is not energy efficient for large scale production.4 In contrast, electrochemical reduction of CO2 is a promising method to convert CO2 into fuels or other useful chemicals, since the reaction can be conducted under ambient conditions with reasonably high energy efficiency, the reaction rate can be controlled by varying the applied potential, and the CO2 reduction products generated at the cathode can be easily separated from the oxidation products of the counter reaction at the anode by using a two-compartment electrolysis cell. Moreover, electrochemical conversion of CO2 can provide an appealing mechanism to store intermittent energy from renewable sources, such as solar and wind, potentially as part of a closed cycle. Bulk metal catalysts have been investigated for CO2 reduction extensively in the past decades,5,

7, 9-10

and have shown great potential in electrochemical conversion of CO2 into

fuels and useful chemicals. A variety of products such as carbon monoxide (CO), hydrocarbons and alcohols have been obtained depending on the catalyst materials and the reaction media.7, 9 CO is an important product, because it is a key starting material in bulk chemicals manufacturing, for example, aldehydes and acetic acid,11 and is often used as the feedstock in the Fischer–Tropsch process, a well-known and well-characterized process that has been used in industry to produce chemicals and synthetic fuels from synthesis gas, a mixture of CO and H2 with an appropriated ratio, for many decades. Among all bulk metallic 3 ACS Paragon Plus Environment


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catalysts, Au is a superior CO formation electrocatalyst.9 However, silver is of particular interest and has received much attention, since it can reduce CO2 to CO with good selectivity under moderate overpotentials in comparison to other metallic catalysts,7, 9-10, 12 and also costs much less than other precious metal catalysts, such as Au. Furthermore, first principles calculation by Back et al.13 suggests that, unlike Au, reducing the size of Ag enhances its activity towards CO formation without increasing the kinetics of unwanted hydrogen evolution reaction (HER). Consequently, Ag nanoparticle is a better electrocatalyst than Au nanoparticle in this regard. To create stable nanostructured Ag, several strategies have been developed. Notably, Jiao and co-workers have synthesized a nanoporous Ag electrocatalyst with highly curved internal surface, which produced CO with a faradaic efficiency of 92% at a rate over 3,000 times higher than the polycrystalline Ag electrode at overpotentials smaller than 0.5 V in aqueous media.14 The much enhanced performance at lower overpotential was ascribed to the greater stabilization of the intermediate CO2•- at the highly curved surface. Smith and co-authors achieved a CO faradaic efficiency of 80% in aqueous media at a moderate overpotential of 0.49 V with an oxide-derived Ag electrode, and attributed the improved catalytic activity to the enhanced stabilization of COOH˙ intermediate, and the high local pH near the highly nanostructured catalyst surface which facilitates the catalytic activity for the reduction of CO2 while suppressing the HER.15 Kim et al. reduced CO2 to CO in aqueous HCO3- media with high selectivity and efficiency using 5 nm diameter Ag nanoparticles supported on carbon as the electrocatalyst.16 To further enhance the catalytic activity of Ag nanocatalysts, the Kenis group has synthesized Ag nanoparticles supported on TiO2 (Ag/TiO2) and studied its catalytic activity toward CO2 reduction. Ag/TiO2 showed similar activity at lower overpotential at a loading of only ¼ of the unsupported Ag nanoparticles.17 It was proposed that Ag promotes the formation of CO while TiO2 stabilizes the intermediate and serves as redox electron carrier to assist CO2 reduction. Hsieh et al.


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reported that the adsorbed Cl- ions significantly enhances the activity and selectivity of CO2 reduction catalyzed by AgCl derived Ag nanocoral electrodes.18 Polyoxometalates (POMs) are a class of molecular oxides with high negative charges, which consist of group 5 or 6 transition metal oxyanions linked by shared oxygen atoms. They have a wide structural versatility and rich electrochemistry, and can undergo multiple electron reduction without decomposition. In recent years, they have been used as the reducing agent and stabilizer for the formation of nanoparticles/nanocatalysts.19-20 For example, Zhang et al. have used a Keggin type polyoxometalate [α-SiW12O40]4– (SiW) for the synthesis of palladium nanoparticles.21 Nadjo et al. have used a MoV–MoVI mixed-valence POMs to synthesize nanostructured Ag,22 and a Keggin type POM [PW12O40]3– (PW) for the synthesis of silver with hierarchical dendritic structures.23 In the previous study,24 we have used [α-SiW12O40]4– as an electron transfer mediator to facilitate the electrocatalytic reduction of CO2 by bovine serum albumin stabilized silver nanoclusters, and found that SiW has strong interaction with CO2, and the more electron reduced form has stronger interaction with CO2. However, SiW does not catalyze CO2 reduction. Evidently, there has been growing interest in recent years in the development of oxide derived/supported nanocatalysts to explore the synergistic effect of oxides and metals for CO2 reduction.25-27 In this paper, we report the electrodeposition of Ag-[PMo12O40]n- (Ag-PMo) nanocomposite using PMo as the stabilizer and its application as an electrocatalyst for the reduction of CO2 in dimethylformamide (DMF) solution containing 0.1 M [n-Bu4N]PF6 as the supporting electrolyte in the presence of 0.5% (v/v) added water as the source of protons. DMF was chosen as the solvent medium for CO2 reduction in this study, because (1) the solubility of CO2 in DMF is high (~0.23 M at 1 atm),28 (2) the proton source can be readily controlled in DMF, so that the competing hydrogen evolution reaction can be minimized, and (3) PMo is not stable in aqueous solutions at neutral pH.29-30 The electrodeposited Ag-PMo 5 ACS Paragon Plus Environment


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nanocomposites show an onset potential about 0.70 V more positive than that observed on a polycrystalline Ag electrode, and are able to reduce CO2 to CO with a faradaic efficiency of around 90% over a wide potential range. Mechanistic investigation suggests that PMo facilitates the adsorption and stabilization of the intermediate CO2•-, which in turn lowers the overpotential required for the reduction of CO2.

2. EXPERIMENTAL SECTION 2.1. Materials. AgNO3 (99%), phosphomolybdic acid (H3[PMo12O40], PMo, 99.9%,), silicotungstic

acid

(H4[α-SiW12O40],

SiW,

99.9%),

phosphotungstic

acid

(H3[PW12O40]·xH2O, PW, 99.995%), ferrocene (Fc, ≥ 98%) and sulfuric acid (H2SO4, 95% 98%) from Sigma-Aldrich, and CO2 (Food grade, Aligal, Air Liquide) were used as supplied. Dimethylformamide (DMF, 99.8%, Merck) was dried over 4 Å molecular sieves for at least 2 days before use. Tetrabutylammonium hexafluorophosphate ([n-Bu4N]PF6, GFS) was recrystallized twice from hot ethanol. Deionized water from a MilliQ-MilliRho purification system (resistivity 18 MΩ·cm) was used to prepare all aqueous solutions. Unless otherwise stated, DMF solutions used for the experiments contain 0.5% (v/v) added H2O. To synthesize [n-Bu4N]3[PMo12O40] that is soluble in DMF, aqueous solutions of [nBu4N]Cl and H3[PMo12O40] were mixed with a molar ratio of 3.2:1. This ratio is slightly higher than the stoichiometric ratio to ensure high yield of [n-Bu4N]3[PMo12O40]. Upon mixing, a light yellow precipitate was formed, which was then separated using a centrifuge at a rotation rate of 3,000 rpm. The solid [n-Bu4N]3[PMo12O40] product obtained was washed a few times with ethanol and deionized water to remove unreacted precursors, and then dried in oven at 110 °C for 3 hrs. 2.2. Electrochemistry. All electrochemical experiments were undertaken at 22 ± 2 °C using a CHI 760E electrochemical workstation (CH Instruments, Austin, TX). A standard 6 ACS Paragon Plus Environment

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three-electrode electrochemical cell arrangement was employed using a glassy carbon (GC) working electrode, a Pt wire counter electrode. A Pt wire placed in DMF (0.1 M [nBu4N]PF6), but separated from the analyte by a glass frit was used as a quasi-reference electrode for studies carried out in DMF solutions. The reference potential was calibrated against that of the Fc/Fc+ (Fc = ferrocene) redox couple as an internal reference31 from measurements made on the oxidation of 1.0 mM Fc present in the same solution. For electrodeposition of Ag-PMo and other silver nanocomposites in aqueous solution, a Pt wire separated from the analyte by a glass frit was used as a quasi-reference electrode. This reference electrode setup provides a highly stable reference potential with a variation within ± 2 mV in the measurement timescale. This quasi-reference electrode was used instead of more commonly used Ag/AgCl reference electrode to avoid the formation of AgCl precipitate due to the transfer of Cl- from the reference electrode to the electrolyte solution. The potential of this quasi-reference electrode is calibrated against that of an Ag/AgCl (3 M KCl) reference electrode. Prior to voltammetric experiments or electrodeposition, the working electrode was polished with 0.3 µm alumina on a clean polishing cloth (Buehler, USA), rinsed with water, sonicated in water thoroughly to remove alumina, rinsed successively with water and acetone and finally dried under nitrogen gas. For electrochemical studies (except bulk electrolysis), the solution was purged with nitrogen or carbon dioxide for at least 15 min before measurement, and then the electrochemical cell was kept under a slightly positive pressure of nitrogen or carbon dioxide at all times. Controlled potential bulk electrolysis was carried out in an airtight H-cell with two compartments separated by a frit. The solution was purged with CO2 for 30 min and then sealed properly. Two pieces of 2 cm long graphite rods (2 mm diameter) modified with Ag-



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PMo nanocomposites were connected together and used as the working electrode and a large Pt mesh as the counter electrode, along with the same reference electrode as used in voltammetric studies. The gas products, i.e. carbon monoxide and hydrogen, obtained by controlled potential bulk electrolysis were analyzed by gas chromatographic technique on an Agilent 7820 gas chromatograph with a HP-PLOT Molesieve/5Å column equipped with a thermal conductivity detector (TCD). Helium gas of 99.99% purity was used as the carrier gas for CO detection, while nitrogen gas (99.99%) was used as the carrier gas for H2 detection. The liquid products were analyzed by 1H NMR on a Bruker DRX400 at a frequency of 400.2 MHz with added DMSO as the internal standard. 2.3. Electrodeposition of Ag-PMo nanocomposites. The electrodeposition of Ag-PMo nanocomposite was carried out in an aqueous solution typically containing 2.0 mM AgNO3 and 5.0 mM PMo (pH ~ 1.75) where PMo is stable,29 at a constant potential to reduce Ag+ to Ag and PMo to PMon- simultaneously for typically 100 s to obtain films with optimal catalytic activity and stability. For the deposition on ITO glasses or graphite rods for the purposes of SEM, TEM, XRD and bulk electrolysis, a deposition time of 500 s was used. The modified electrodes were then rinsed thoroughly with water and dried in air before use. Other silver composites formed with SiW, PW or PMo with a different concentration were synthesized in a similar manner. The electroactive surface area of Ag-PMo nanocomposite modified electrode was determined using a well-established lead (Pb) underpotential deposition (UPD) method reported in the literature.18 In brief, a solution containing 1.0 mM Pb(acetate)2, 1.0 mM HClO4 and 0.50 M NaClO4 was used, together with a Hg/Hg2SO4 (saturated K2SO4) electrode (CH Instruments) as the reference electrode. The Pb UPD was carried out by scanning the potential between -0.6 V and -0.85 V or -1.0 V vs. Hg/Hg2SO4 (saturated K2SO4) electrode at a scan rate of 10 mV s-1. The Pb UPD desorption peak in the potential


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range of -0.85 to -0.65 V was integrated and the electroactive surface area was calculated using the value of 420 µC cm-2 for Ag based samples.18 2.4. Characterization of Ag-PMo nanocomposites. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectra (EDX) of the electrodeposited Ag-PMo and other silver nanocomposites modified ITO glasses were obtained using a FEI Nova NanoSEM 450 FEG SEM equipped with Bruker Quantax 400 X–ray analysis system. EDX analysis was conducted at 10 keV. High-resolution transmission electron microscopy (HRTEM) images were obtained on a FEI Tecnai G2 T20 TWIN TEM. X-Ray Diffraction (XRD) measurements were undertaken with a Bruker D2 X-ray powder diffractometer (Cu Kα1 radiation) using a scan rate of 0.5 degree per minute.

3. RESULTS AND DISCUSSION 3.1. Electrodeposition and characterization of Ag-PMo nanocomposites. To obtain the information about the reduction potentials of Ag+ and PMo needed for electrodeposition of Ag-PMo nanocomposites, cyclic voltammetric experiments were undertaken in an aqueous solution containing 2.0 mM AgNO3 and 5.0 mM PMo using a glassy carbon (GC) electrode. The speciation distribution of PMo is highly pH dependent. In this aqueous solution (pH ~ 1.75), [PMo12O40]3- and its protonated forms are the dominant species.29-30 Typical voltammograms obtained are shown in Figure 1. The magnitude of the peak current associated with the two chemically reversible redox processes (I/I' and II/II') observed at the potential between 0.56 and 0.26 V (vs. Ag/AgCl, 3 M KCl) are too small for dissolved PMo in comparison with the values estimated based on the Randles-Sevcik equation.32 A linear increase of their peak currents with the scan rates in the range of 20 - 200 mV s-1 suggests that they are associated with the PMo adsorbed on the electrode surface. A stronger interaction of the reduced forms of PMo with the electrode surface causes a positive shift in 9 ACS Paragon Plus Environment


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the reversible potentials of these surface confined processes in comparison with the processes associated with their dissolved counterparts. This observation is consistent with the previous results obtained by other researchers which suggest that PMo can strongly adsorb to electrode surfaces due to its high charge density.33 These surface confined processes due to the strong adsorption of PMo were also observed on other electrode materials such as Au and Ag (results not shown), and is expected to occur on the nanostructured Ag surface. The reduction process with a peak potential of about 0.17 V (process III), which is absent in the solution containing only 5.0 mM PMo, is due to the reduction of Ag+ to Ag(0), while other reduction processes are assigned to the reduction of PMo in the solution phase coupled with proton transfer.34

Figure 1. Cyclic voltammograms obtained at a GC electrode (3 mm diameter) in an aqueous solution containing 2.0 mM AgNO3 and 5.0 mM PMo (―), and 5.0 mM PMo only (―) in the absence of other added supporting electrolyte. Scan rate: 100 mV s-1.

Ag-PMo nanocomposites were electrodeposited potentiostatically in an aqueous solution containing 2.0 mM AgNO3 and 5.0 mM PMo. Different deposition potentials in the range between 0.26 V and -1.44 V were employed to generate a series of reduced forms of PMo with different capping and reducing power to assist the formation of nanostructured Ag. When the deposition potential was more positive than 0.26 V, no Ag was deposited. When 10 ACS Paragon Plus Environment

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the applied potential is between 0.24 and 0.15 V, a shiny silver film was obtained, indicating the deposition of bulk metal rather than nanostructured Ag. Nanostructured materials were obtained judging based on the color of the deposits when the deposition potential was more negative than 0.06 V (after the 1st PMo reduction process IV). In this potential region, both heterogeneous reduction of Ag+ at the electrode surface and homogeneous reduction of Ag+ by the electroreduced PMo (PMon-) occur simultaneously. PMon- are expected to be better stabilizing agents for Ag nanostructures as compared to the initial oxidized form due to their high charge density which favors a strong electrostatic interaction with Ag. However, when the applied potential was more negative than -0.64 V (Figure 1), rapid formation of bubbles on the electrode surface was observed due to the hydrogen evolution process. This may cause some detachment of the deposited nanocomposite film. To obtain morphological information of the electrodeposited materials, SEM characterization was undertaken. The SEM image (Figure 2a) of the Ag-PMo nanocomposite obtained at a deposition potential of -0.24 V in a solution containing 2.0 mM AgNO3 and 5.0 mM PMo shows the formation of dendrite-like Ag nanostructure with uniform nanocrystals of size about 20 nm. This type of dendrite-like nanostructures is usually obtained in the presence of surfactants or hard templates.35-36 This result suggests that PMo plays an important role in controlling the morphology of the nanostructures. It is well-known that bare nanoparticles are not stable due to their high surface energy, hence, have a tendency to lower their very high surface energy by coagulation and agglomeration or by sorption of molecules, such as PMo, from their surroundings. Therefore, the strong adsorption of PMo on the newly formed Ag nanoparticles can facilitate the nanoparticle formation and stabilize them. To study the effect of PMo concentration on the formation of the well-defined nanostructure, electrodeposition was also carried out in a solution containing 2.0 mM AgNO3 and different concentrations of PMo (0.02 to 5.0 mM) at the same applied potential (-0.24 V). In the 11 ACS Paragon Plus Environment


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presence of PMo at a concentration as low as 0.02 mM, dendrite-like nanostructure was again obtained (Figure 2b), which is similar to that obtained in the presence of 5.0 mM PMo, indicating that the nanostructure is not strongly influenced by the PMo concentration in this range. The SEM image of the deposited Ag films obtained when PMo was absent or was replaced with an equal concentration of H2SO4 (Figure S1 (see the Supporting Information)) shows the absence of well-defined nanostructure. This result again confirms that PMo plays an important role in controlling the morphology of the nanostructures. The EDX spectra (Figures S2 a and b) were also recorded to obtain the elemental composition of the nanocomposites, which confirm the presence of both silver and molybdenum on the deposited films obtained in the presence of both concentrations of PMo. To further reveal the morphological information of the films, high resolution TEM images (Figure 3) were taken from the nanocomposites obtained by electrodeposition in solutions containing 2.0 mM AgNO3 and either 0.02 mM or 5.0 mM PMo. Oval shaped nanoparticles with size of 20 nm × 30 nm were observed, which was not strongly dependent on the concentration of PMo and is in agreement with the SEM results (Figure 2). XRD spectra were also recorded to obtain the crystal facet information of the films. XRD patterns (Figure S3a) show predominantly the Ag(111) facet with a small amount of Ag(200), (220) and (311) facets. Average crystallite size was calculated to be around 28 nm using the Scherrer equation,37 which is consistent with the results obtained from SEM and TEM. XRD spectrum obtained with Ag electrodeposited in an aqueous solution containing 2 mM AgNO3 and 0.02 mM H2SO4 (instead of 0.02 mM H3PMo12O40) shows similar patterns (Figure S3b) and thus suggests that the presence of PMo does not greatly influence the preferred crystal orientation of Ag deposits.


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Figure 2. SEM images of Ag-PMo nanocomposites modified ITO glasses, obtained by electrodeposition at a potential of -0.24 V (vs. Ag/AgCl, 3 M KCl) for 500 s in an aqueous solution containing 2.0 mM AgNO3 and either (a) 5.0 mM or (b) 0.02 mM PMo.

Figure 3. TEM images of Ag-PMo nanocomposites obtained by electrodeposition in aqueous solutions containing 2.0 mM AgNO3 and (a) 5.0 mM or (b) 0.02 mM PMo at a potential of ⁃0.24 V for 500 s.

3.2. Electrocatalytic reduction of CO2 in DMF (0.1 M [n-Bu4N]PF6) in the presence of 0.5% added H2O. Electrocatalytic activity of Ag-PMo nanocomposites for CO2 reduction were then investigated in DMF in the presence of 0.5% (v/v) added H2O. The amount of added H2O of 0.5% (v/v) was chosen to provide adequate proton source since its molar concentration is 0.275 M, which is comparable to that of CO2 (~0.23 M) in CO2 saturated DMF,28 and is the optimum amount for the generation of CO2 reduction products.38 Under a N2 atmosphere, two surface-confined redox processes are observed at potentials of about 13 ACS Paragon Plus Environment


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0.96 V and -1.65 V vs. Fc0/+ scale (Figure 4). These are the characteristic redox processes of PMo, which again confirm the presence of PMo in the molecular form in the modified film and its stability in organic solvents or organic solvent-water mixtures.39-41 Under a CO2 atmosphere, much larger capacitive current was observed (Figure 4), indicating the change of local dielectric environment due to the strong interaction between PMo and CO2, presumably through the formation of a POM-bound monodentate carbonate species, similar to that reported previously for the CO2 reduction on a Sn/SnOx catalyst.42-43 When the potential is scanned to more negative than the onset potential of about ⁃1.7 V vs. Fc0/+, a sharp increase in current was observed, which was not seen under a N2 atmosphere or under a CO2 atmosphere in the absence of Ag-PMo, and is assigned to the reduction of CO2 catalyzed by the Ag-PMo nanocomposites. The results also showed that better catalytic activities were obtained when the deposition potential was between -0.14 and -0.64 V, which implies that nanostructured Ag is more catalytically active than bulk Ag.

Figure 4. Comparison of cyclic voltammograms obtained at an Ag-PMo nanocomposite modified GC electrode in DMF (0.1 M [n-Bu4N]PF6) with 0.5% (v/v) added H2O under a CO2 (―) or a N2 (―) atmosphere. Scan rate: 100 mV s-1.

To confirm the important role of PMo in the electrocatalytic reduction of CO2, a range of control experiments were undertaken: (a) The deposition of Ag was firstly carried out in a 14 ACS Paragon Plus Environment

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2.0 mM AgNO3 solution in the absence of PMo at a few chosen potentials in the range between 0.26 V and -1.44 V. Shiny silver films were obtained when the applied potential was more negative than the Ag+ reduction potential of about 0.26 V. The catalytic activities of the Ag films formed in the absence of PMo were also studied in DMF. Much more negative onset potentials for CO2 reduction were observed (Figure S4), regardless of the potentials chosen for electrodeposition of Ag films, as compared to the Ag-PMo nanocomposite modified electrode, indicating much lower catalytic activities in the absence of PMo; (b) The catalytic activity of the Ag films deposited in the presence of 5.0 mM H2SO4 at -0.24 V was also studied in DMF. Again, much more negative onset potential for CO2 reduction in DMF was observed (result not shown). The enhanced catalytic activity of Ag-PMo nanocomposite towards CO2 reduction is not attributed to the Ag facet since the presence of PMo does not greatly influence the preferred crystal orientation of Ag deposits, as discussed earlier. Since PMo plays an important role in electrocatalytic reduction of CO2, voltammetric experiments were also undertaken to investigate the effect of PMo concentration in the deposition solution on the catalytic activity of Ag-PMo nanocomposites towards CO2 reduction and to optimize the performance of the Ag-PMo nanocomposite. An Ag-PMo nanocomposite modified electrode obtained in the presence of PMo with a concentration as low as 0.02 mM in the deposition solution showed a much more positive onset potential of about -1.75 V and significantly increased catalytic current (16.7 mA cm-2 at -2.5 V) compared to those obtained at Ag films deposited in the absence of PMo (onset potential of 2.06 V, 2.8 mA cm-2 at -2.5 V) (Figure S4). This enhanced CO2 reduction performance may be attributed to the formation of different morphological structure of Ag film and the presence of PMo in the film, evidenced by the increase in capacitive current and presence of the characteristic redox processes of PMo in the potential range of -0.7 V to -1.1 V (inset of Figure S4). A slight decrease in the magnitude of reduction current was observed upon 15 ACS Paragon Plus Environment


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cycling of potential presumably due to the slow dissolution of PMo under cyclic voltammetric conditions. When the concentration of PMo is increased from 0.02 mM to 5.0 mM, the capacitive current further increases, along with the increase in catalytic current for CO2 reduction. The onset potential also shifts positively by ~ 200 mV, even though the morphology of the nanocomposite is not strongly influenced by the concentration of PMo in this range. Further increase of PMo to 10 mM does not lead to a further improvement of catalytic performance of the modified electrode in terms of either the magnitude or stability of catalytic current. Therefore, 5.0 mM PMo was used for Ag-PMo nanocomposite deposition in the following studies. To investigate whether other POMs also enhance the performance of Ag for CO2 reduction, other types of POMs, such as SiW and PW were also used for the formation of AgPOM composites under similar conditions. In DMF (0.1 M [n-Bu4N]PF6) solution with 0.5% added H2O under CO2 atmosphere, the Ag-SiW nanocomposite modified electrode also shows two pairs of surface-confined, characteristic redox processes of SiW at potentials of about -1.04 and -1.56 V, indicating the presence of SiW in the modified film (Figure S5). These formal potential values (E0', taken as the average of oxidation and reduction peak potentials) are about 250 mV more positive than those obtained on an AgNC@BSA-SiW adduct modified electrode under similar conditions (-1.28 and -1.70 V).24 This is due to the fact that SiW in the Ag-SiW composite is in a different environment than that in the AgNC@BSA-SiW adduct. Under the same conditions, Ag-PW nanocomposite modified electrode also shows a few pairs of surface confined characteristic redox processes of PW, confirming the presence of PW in the modified electrode (Figure S5). However, their catalytic activities towards electrochemical reduction of CO2 are much lower than that of the Ag-PMo nanocomposites (Figure 4), with much more negative onset potentials of about -2.20 V and -2.15 V for Ag-SiW and Ag-PW composites, respectively. The significantly improved 16 ACS Paragon Plus Environment

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catalytic activity of Ag-PMo nanocomposite for the reduction of CO2 compared to other AgPOM composites could be attributed to the much higher charge density and stronger basicity associated with PMo, which promotes the formation and stabilizes the nanocomposite and also enhances the interaction between the catalyst and CO2 as discussed later. Compared with a bare polycrystalline silver disk electrode (Figure S5), Ag-PMo nanocomposite modified electrode has shown a 700 mV positive shift in the onset potential for CO2 reduction (Figure 4). The stability of the Ag-PMo nanocomposites for the catalytic reduction of CO2 was examined by potentiostatic i-t curve at an applied potential of -1.9 V (Figure S6). A current density (i) of about 3.5 mA cm-2 was maintained throughout the experiment period of more than 3 hours without obvious decay. To confirm the stability of PMo during this measurement, cyclic voltammograms were obtained in strong acid (0.1 M H2SO4 aqueous solution) before and after the potentiostatic experiment. The voltammograms (Figure S7) confirm the presence of [PMo12O40]3- as the major PMo species in the Ag-PMo nanocomposites.44,45 The fact that similar characteristic surface-confined redox processes of [PMo12O40]3- were obtained before and after the long-term potentiostatic experiment also confirms the stability of PMo. Under a N2 atmosphere under similar conditions, an initial current spike is observed due to the double layer charging process. After that (~10 seconds), the current decays to almost zero since there is no faradaic process present. These results also confirm that the current obtained under a CO2 atmosphere is indeed due to the catalytic reduction of CO2. 3.3. Controlled potential bulk electrolysis and product identification. CO2 reduction can undergo multiple reaction pathways to form a wide variety of products. To identify the pathway that the reaction undergoes under current conditions and to obtain the faradaic efficiency for the CO2 reduction reaction, controlled potential bulk electrolysis was therefore 17 ACS Paragon Plus Environment


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undertaken in DMF (0.1 M [n-Bu4N]PF6) with 0.5 % added H2O at different applied potentials. After electrolysis, the head-space gaseous products obtained were analyzed by GC. The results suggest that CO is the main product with a faradaic efficiency of 90 ± 5% over the potential range from -1.9 to -2.5 V vs. Fc0/+. In addition to CO, H2 was also detected with a faradaic efficiency of 8 ± 3%. The liquid phase samples were also analyzed using both proton and carbon NMR, however, no CO2 reduction product was detected. The SEM images (not shown) obtained after controlled potential electrolysis show the same dendrite morphology with the presence of Mo in the EDX spectrum, suggesting that the catalyst stays intact during the long term electrolysis. A bulk electrolysis in N2 saturated DMF in the presence of 0.5 % added H2O was also carried out using the same catalyst at a more negative potential of -2.5 V vs. Fc0/+ to obtain sufficient magnitude of current. Hydrogen was the only product obtained, and no CO was detected under our experimental conditions, which confirms that the CO product obtained under a CO2 atmosphere is indeed produced from the reduction of CO2 catalyzed by Ag-PMo nanocomposite catalyst. In CO2 saturated DMF (0.5% v/v added H2O), H2CO3 is the strongest acid with a pKa of 7.37. On this basis, the standard potential for CO2/CO was theoretically estimated by Savéant and co-workers to be -0.690 V vs. NHE28 or -1.41 V vs. Fc0/+, knowing that the potential of Fc0/+ is 0.72 V vs. NHE in this medium.46 It should be noted that Matsubara et al.47 recently found that the experimentally determined standard potential in wet CH3CN is 0.28 V more negative than that estimated by Savéant and co-workers. Similar difference with experimental and estimated values may also be expected in wet DMF. The onset potential for electrochemical reduction of CO2 to CO catalyzed by Ag-PMo nanocomposite (Figure 4) was therefore only 0.29 V more negative than the reversible potential for this process. This onset potential value is much more positive (about 0.24 V) than that obtained at the AgNC@BSASiW adduct modified electrode under similar conditions.24 In the above discussion, the 18 ACS Paragon Plus Environment

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change of local proton availability due to the consumption of proton in the formation of CO (or H2 to a less extent) is omitted due to the low electrochemical reaction rate (i.e. low current density) in the potential region close to the onset potential. Controlled potential bulk electrolysis under a CO2 atmosphere was also conducted at the Ag nanoparticle modified graphite rods deposited from a solution containing 2.0 mM AgNO3 and 5.0 mM H2SO4. A much more negative potential was required in order to get a reasonable current magnitude for electrolysis. At an applied potential of -2.5 V, the main product obtained was CO, with a faradaic efficiency of 64 ± 5 %, which is considerably lower than that obtained at an Ag-PMo nanocomposite modified electrode at an applied potential of -1.9 V where twice the current magnitude was obtained. H2 was also detected with a faradaic efficiency of 29 ± 3 %. The liquid phase sample was again analyzed using NMR, and no CO2 reduction product was detected. 3.4. Mechanistic studies. The above results show that Ag-PMo nanocomposite is far more active than bulk Ag for CO2 reduction. For example, the current density measured at 2.5 V vs. Fc0/+ are -20.3 and -0.4 mA cm-2 for Ag-PMo modified GC and bare Ag electrodes, respectively. To investigate whether this observed enhancement is due to the increase in electroactive surface area associated with the nanostructured Ag-PMo, the electroactive surface area was determined by the Pb underpotential deposition (UPD) method18 and calculated to be 0.64 cm2 using the charge of 420 µC cm-2 for the desorption of a monolayer of Pb at Ag surface. This value is 9 times the geometric area of the electrode. Therefore, the enhancement in CO2 catalytic reduction property associated with Ag-PMo nanocomposite cannot be solely attributed to the increase in electroactive surface area. It should be noted that the bulk deposition of Pb at Ag-PMo nanocomposite was hindered as evidenced by the smaller amount of charge associated with the stripping peak of the bulk Pb at the potential of ⁃0.82 V vs. Hg/Hg2SO4 (sat. K2SO4) (Figure S8), suggesting the presence of foreign species, 19 ACS Paragon Plus Environment


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i.e. PMo, at the surface of Ag nanoparticles. Similar behavior was also found previously in the case of AgCl derived Ag nanocatalyst.18 Previously, we found that one of the key roles of SiW in AgNC@BSA-SiW nanocomposite in enhancing the catalytic activity of Ag nanoclusters was to provide CO2 binding sites, but SiW does not catalyze this reaction.24 In order to investigate whether this is also the case here, cyclic and near steady state RDE voltammetry of [n-Bu4N]3PMo were carried out under the same conditions as those for the Ag-PMo nanocomposites, i.e. in DMF (0.1 M [n-Bu4N]PF6) with 0.5 % (v/v) added H2O under N2 and CO2 atmospheres. Cyclic voltammograms obtained under a N2 atmosphere (Figure S9a) show three well-defined chemically reversible redox processes I/I', II/II' and III/III' in the potential range of 0 to -2.6 V, with the reversible potentials of -0.25 V, -0.75 V and -1.50 V, respectively. Some small processes observed may be attributed to the strong adsorption of PMo at the electrode surface and the acid-base chemistry of PMo in the presence of 0.5% added water.48 PMo is stable in the potential range studied, since the two oxidation processes at potentials of -0.22 V and -0.71 V stayed the same even when the potential was scanned to -2.6 V. Under a CO2 atmosphere, the first two redox processes remained almost unchanged, while the third one has now shifted about 560 mV more positive with two new additional well-defined redox processes 4/4' and 5/5' emerged in the potential range studied (Figure S9b). The positive shifts in redox potentials in the presence of CO2 are attributed to the increase of Lewis acidbase interaction between CO2 (and presumably CO2•-) and PMo and its reduced forms.24 Although PMo and its reduced forms are negatively charged, the charge density on PMo is low as it is a very large molecular oxide with a formula weight of about 1822. Moreover, they normally form ion pairs with the electrolyte cations. Thus, the electrostatic repulsion between CO2•- and PMo is not expected to significantly weaken the interaction between them. However, no catalytic current was obtained under CO2 atmosphere, suggesting that PMo has


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Lewis acid-base interaction with CO2 but does not catalyze its reduction under the conditions studied. To provide further mechanistic insight, Tafel analysis was performed. To simplify the data analysis, the solution was stirred during voltammetric measurements to minimize concentration polarization and data only obtained in the relatively low overpotential region were used. The overall current density obtained from cyclic voltammetric experiments was used instead of the partial current density for CO for Tafel analysis to obtain the Tafel slope. It is valid to do so since the bulk electrolysis results suggested that the faradaic efficiency for CO formation is essentially potential independent and is close to 100%. The Tafel plot of overpotential (η) versus current density by geometric area of the electrode for the Ag-PMo nanocomposite modified electrode shows a slope of 60 mV dec-1 over the overpotential range of 0.46 V to 0.9 V (Figure S10). Based on literature reports,12-13, 15, 49 the mechanism for CO2 reduction to form CO can be described in Equations 1 – 5 below, CO2 + * ⇌ *CO2

(1)

*

(2)

*

(3)

*

(4)

*

(5)

CO2 + e- → *CO2•CO2•- + H2CO3 → *HCO2• + HCO3HCO2• + H2CO3 + e- → *CO + HCO3- + H2O CO ⇌ CO + *

where * and superscript

*

denote an adsorption vacancy site and an adsorbed species,

respectively. It is commonly assumed that adsorption of CO2 (Equation 1) is followed by a one electron step to form adsorbed CO2•- (Equation 2). The standard potential of the CO2/CO2•- couple was estimated to -2.21 V vs. SCE (-2.41 V vs. Fc0/+) in DMF containing 0.1 M [n-Bu4N]ClO4 as the electrolyte.50 This reduction process could occur at a less negative potential if the interaction between the adsorbed CO2•- radical anion and the active atoms of 21 ACS Paragon Plus Environment


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the catalyst increases.18, 51 This radical anion can obtain a proton from H2CO3 to form HCO2• (Equation 3), which can then undergo a further one electron reduction to form CO.52 It has been found that either the formation of CO2•- (Equation 2) or the protonation of CO2•(Equation 3) are the rate limiting steps, with an indicative Tafel slope of 117 or 59 mV dec-1 at 22 °C,53 respectively. The Tafel slope of 60 mV dec-1 is very close to the theoretical value of 59 mV dec-1, suggesting that the formation of CO2•- is fast at the Ag-PMo nanocomposite modified electrode. This is attributed to a stronger interaction between PMo and CO2•- which reduces the overpotential of this reaction as suggested by the results shown in Figure S9. Consequently, protonation of CO2•- becomes rate limiting. Based on this mechanism, in principle, a reduction peak could be observed at a sufficiently negative potential where depletion of CO2 occurs. This reduction peak was not observed in our study since voltammetric experiments were undertaken in the potential region (i.e. > -2.5 V vs. Fc0/+) where PMo is stable.

4. CONCLUSIONS An Ag-PMo nanocomposite has been synthesized by electrochemical deposition where PMo adsorbs to the surface of Ag, serves as a stabilizing agent and is responsible for the dendrite morphology formed. The presence of PMo in the Ag-PMo nanocomposite has significantly reduced the overpotential for CO2 reduction and improved the current density. This catalyst has demonstrated highly attractive properties for CO2 reduction to CO in water-containing DMF. High faradaic efficiencies for CO formation of 90 ± 5% were obtained over a wide potential range in CO2 saturated DMF (0.1 M [n-Bu4N]PF6 and 0.5% (v/v) H2O). The onset potential of -1.70 V vs. Fc0/+ obtained is 0.70 V more positive than that obtained at a bulk Ag electrode, and only 0.29 V more negative than the estimated reversible potential (-1.41 V vs. Fc0/+) associated with the CO2/CO process in dimethylformamide (containing 0.1 M [n22 ACS Paragon Plus Environment

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Bu4N]PF6 and 0.5% (v/v) H2O).28 Voltammetric studies reveal that the enhanced catalytic activity associated with Ag-PMo is attributed to the strong interaction between the reduced PMo and CO2 (and presumably CO2˙- as well), which lowers the activation energy for CO2˙formation. This is consistent with the results obtained from Tafel analysis which suggests that rapid formation of *CO2˙- is followed by the rate determining protonation step.

ASSOSIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at DOI: . SEM images, SEM-EDX spectra, and XRD analysis to characterize the morphology and composition of various composites used in this work; voltammetric characterizations of various composites, solution phase voltammograms of PMo in DMF, under-potential deposition of Pb on Ag-PMo nanocomposite for the estimation of electroactive surface area, and Tafel plot.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS



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The authors thank the Australian Research Council for financial support through the ARC Centre of Excellence for Electromaterials Science (grant No. CE140100012) and for D.R.M’s Australian Laureate Fellowship (award No. FL120100019), and the use of facilities within the Monash X-Ray Platform and the Monash Centre for Electron Microscopy.


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Polyoxometalate Promoted Electrocatalytic CO2 Reduction at

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