Effect of Chloride Anions on the Synthesis and. Enhanced Catalytic Activity of Silver Nanocoral

Transcription

1 Supporting Information Effect of Chloride Anions on the Synthesis and Enhanced Catalytic Activity of Silver Nanocoral Electrodes for CO 2 Electroreduction Polyansky* Yu-Chi Hsieh, Sanjaya D. Senanayake, Yu Zhang, Wenqian Xu, and Dmitry E. United States Chemistry Department, Brookhaven National Laboratory, Upton, New York , * dep@bnl.gov 1

2 Table of Contents Scanning electron microscopy (SEM) images of Ag nanoparticles (NPs)... 3 Electrochemical surface area determined by lead (Pb) underpotential deposition (UPD) measurements... 3 CO 2 reduction performance of as-prepared AgCl sample... 5 X-ray diffraction of Ag-based samples... 6 CO 2 reduction performance of Ag nano-corals after annealing in H CO 2 reduction performance of Ag NPs with different amount of surface chloride... 9 Absence of Pt for Ag nano-corals after CO 2 reduction Estimation of TOF and TON values for the Ag nano-coral catalyst References

3 Scanning electron microscopy (SEM) images of Ag nanoparticles (NPs) Figure S1. SEM images of the Ag NPs at (a) low-magnification, (b) high-magnification. The particle size of Ag NPs is 50 ± 24 nm. Electrochemical surface area determined by lead (Pb) underpotential deposition (UPD) measurements Figure S2. Lead (Pb) underpotential deposition (UPD) and bulk deposition in 1 mm Pb(acetate) mm HClO M NaClO 4 solution, at the sweep rate of 10 mv s 1, (a) in different potential ranges on the Ag NPs, (b) in the same potential range for Ag foil, Ag NPs, and Ag nano-coral samples. 3

4 Figure S2 presents the underpotential deposition (UPD) and the bulk deposition of Pb on the Ag-based catalysts surface. The cathodic peak at 0.4 to 0.5 V (vs. SCE) was associated with the stripping of bulk-deposited Pb, and the cathodic peak at 0.35 to 0.2 V (vs. SCE) corresponded to the stripping of underpotentially-deposited Pb. With the more negative potential range, that is, with higher overpotential for Pb bulk deposition, the stripping charge for deposited Pb increased. In contrast, the desorption/stripping peak for underpotentially deposited Pb showed the similar charge with different potential range, indicating that the Pb overlayer completely covers the Ag surface. The desorption peak of Pb UPD was integrated to calculate the electrochemical surface area assuming 420 μc cm 2 ec for Ag-based samples. Figure S2b shows that Ag foil, Ag NPs, and Ag nano-corals have different electrochemical surface area. Interestingly, the Ag nano-coral sample has a lower stripping charge for bulk-deposited Pb at 0.4 ~ 0.5 V, compared to Ag foil and Ag NPs, indicating the suppressed bulk deposition of Pb on Ag nano-corals, and therefore confirmed the existence of chloride anions on the surface of Ag nano-corals. 4

5 CO 2 reduction performance of as-prepared AgCl sample Figure S3. Total current density (left axis) and current efficiencies (right axis) for the formation of CO (red square) and H 2 (blue circle) as a function of time using as-prepared AgCl, measured in CO 2 -saturated 0.1 M KHCO 3 at 0.6 V (vs. RHE). The potential of 0.6 V reported for asprepared AgCl samples is not ir-corrected, due to the strong influence of the released Cl on the solution resistance. For the as-prepared AgCl sample the catalytic performance was measured in CO 2 - saturated 0.1 M pre-electrolyzed KHCO 3 (ph = 6.8) at 0.6 V for 2 h, and a 60 μl gas sample was taken every 15 min into the GC for product analyses. The bulk atomic ratio of Ag to Cl measured by X-ray energy-dispersive spectroscopy (EDS) was 52.4/47.6 for the as-prepared AgCl, and 99.4/0.6 for the AgCl after 30 min CO 2 reduction. The substantial decrease of the relative amount of Cl during the first 30 min of cathodic electrolysis further proves that the AgCl sample decomposes under cathodic potential forming mainly metallic Ag. However a small amount of chloride ions is still retained on the surface. 5

6 X-ray diffraction of Ag-based samples Figure S4. Synchrotron X-ray diffractograms of Ag foil (black), as-prepared AgCl (olive green), and Ag nano-corals (red), respectively. λ = Å. The peak positions for pure Ag (dark grey) and pure AgCl (green) phases are shown at the bottom. Table S1. Fitting results of X-ray diffractograms in Figure S4. The effective particle diameter was estimated from treatment of the XRD data with Rietveld whole-profile refinement using GSAS software 1,2. Sample Phase Lattice parameter (Å) Grain size (nm) Ag foil Ag >1000 As-prepared AgCl Ag >1000 AgCl ~200 Ag nano-corals Ag ~150 6

7 CO 2 reduction performance of Ag nano-corals after annealing in H 2 In order to further confirm the influence of surface Cl on the CO 2 reduction activity, the Ag nano-coral sample was annealed in a H 2 atmosphere at 250 for 1 hour, to remove the adsorbed Cl from the Ag surface while retaining the porous structure. 3 After the annealing in H 2, the surface area was reduced by 59%, consistent with the denser morphology (Figure S5a). The cyclic voltammetry curves of the Ag nano-coral sample before (red, Figure S5b) and after (turquoise, Figure S5b) H 2 annealing, measured in Ar-saturated 0.1 M KHCO 3 show that the annealed sample demonstrates higher activity for the hydrogen evolution reaction. In addition, the CO 2 reduction activity after H 2 annealing was measured in electrolyte solutions containing different ratios of KCl to KHCO 3, but maintaining the total concentration of both salts constant at 0.1 M. In the pure KHCO 3 solution, the sample shows a relatively low CO current efficiency of 41% and high H 2 current efficiency of 58% (Figure S5c). Interestingly, with increased Cl ion concentration in the electrolyte, the CO current efficiency increases and reaches a maximum of 63% in 0.05 M KCl M KHCO 3. The CO partial current density also increases with increased Cl ion concentration (turquoise, Figure S5c). This experiment supports the hypothesis that the chloride ions can suppress the hydrogen evolution reaction and therefore promote selectivity for the CO 2 reduction reaction. However, if the concentration of the Cl ion is increased above 0.05 M, the CO current efficiency and CO partial current density decreases. The volcano-shaped dependence of CO current efficiency and CO partial current density can be interpreted by considering the dependence of the amount of dissolved CO 2 on the amount of bicarbonate and the ph of the solution. Using the known value of the equilibrium constant of reaction (1), together with concentration values for bicarbonate and protons in the solution, the concentration of dissolved CO 2 can be calculated (Figure S5d). While the increased 7

8 amount of Cl further inhibits the hydrogen evolution reaction, it also leads to a drop in concentration of dissolved CO 2 leading to a decrease in catalytic current density and eventually CO partial current density. CO 2 + H 2 O HCO 3 + H + (1) K a(app) = [H+ ][HCO 3 ] [CO 2 ] = Figure S5. (a) Low-magnification and high-magnification (inset) SEM images of Ag nano-coral sample after annealing in H 2. (b) Cyclic voltammograms measured in Ar-saturated 0.1 M KHCO 3, for the Ag nano-coral sample before (red) and after (turquoise) annealing in H 2. (c) (Left axis) CO (orange) and H 2 (brown) current efficiencies, (Right axis) CO partial current density (turquoise) on the Ag nano-coral sample after annealing in H 2, as a function of Cl ion concentration in the electrolyte, measured in CO 2 -saturated electrolytes of x M KCl + (0.1 x) 8

9 M KHCO 3, at 0.6 V (with ir-correction) for 30 min. (d) total current density (left axis, turquoise) and the dissolved CO 2 concentration (right axis, violet), as a function of Cl ion concentration in the electrolyte, measured in CO 2 -saturated electrolytes of x M KCl + (0.1 x) M KHCO 3, at 0.6 V (with ir-correction) for 30 min. The electrochemical surface area of this sample determined by Pb UPD is 16.8 cm 2. CO 2 reduction performance of Ag NPs with different amount of surface chloride Additional experiments were carried out to investigate the correlation between surface chloride amount and CO 2 reduction activity. Instead of Ag foil, Ag NPs were used as the substrate material for better control of the chloride amount and to maintain the catalysts surface area. Ag NPs were immersed in aqueous 3.7 wt% HCl solution for different periods of time. The chemical reaction between the surface layer of silver oxide (Ag 2 O) and HCl is shown as follows. Ag 2 O + 2 HCl 2 AgCl + H 2 O (2) The atomic ratios of Cl:Ag on the NPs surface were determined by XPS experiments. After the immersion in HCl solution for 30 min, 1 h, 2 h, and 4 h, the Cl:Ag ratio was 0.065, 0.153, 0.182, and on Ag NPs surface, respectively. 9

10 Figure S6. Catalytic performance of Ag NPs as a function of surface Cl concentration. (a) Left vertical axis (black columns): The CO partial current density for Cl-modified Ag NPs (j CO ) normalized by the CO partial current density for unmodified Ag NPs (j CO (Ag NPs) ); Right vertical axis (red sphere): CO current efficiency (%) for Cl-modified Ag NPs in comparison to unmodified Ag NPs, as a function of the atomic ratios of Cl:Ag on NPs surface. (b) Lead (Pb) underpotential deposition (UPD) and bulk deposition, in 1 mm Pb(acetate) mm HClO M NaClO 4 solution, at the sweep rate of 10 mv s 1, in the same potential range, for unmodified Ag NPs (black) and Ag NPs immersed in HCl solution for 4 h (red). The electrochemical surface area determined by Pb UPD measurements is 18.5 cm 2 for unmodified Ag NPs, and 17.7 cm 2 for Ag NPs immersed in HCl solution for 4 h. 10

11 Absence of Pt for Ag nano-corals after CO 2 reduction Figure S7. Synchrotron sxps spectra for Ag nano-corals after 30 min (black, bottom) and after 72 h (red, top) of CO 2 reduction, exhibiting the absence of Pt 4f signals. Estimation of TOF and TON values for the Ag nano-coral catalyst The number of Ag atoms on the electrode surface can be obtained from the Pb UPD experiment (Figure S2). The Pb desorption peak at 0.35 to 0.2 V (vs. SCE) was integrated to calculate the electrochemical surface area assuming 420 μc cm ec 2 for Ag-based samples, resulting in mols cm 2 of Ag atoms. Based on the value of the specific activity of 162 ua cm 2, the amount of CO 2 reduced per second (assuming a two-electron reaction) is mols s 1 cm 2. The resulting value of turnover frequency (TOF) equals / , which is ~ 0.4 s 1. The turnover number (TON) values can be obtained as follows: 0.4 s s = 720 (over 30 min) 0.4 s s 0.85 (average drop in specific activity) = (over 72 h) 11

12 References (1) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, (2) Toby, B. H. J. Appl. Crystallogr. 2001, 34, (3) Bowker, M. J. Electron. Spectrosc. Relat. Phenom. 1986, 37,