Supporting Information. For

Transcription

1 Supporting Information For Synergistic In-Layer Cobalt Doping and Interlayer Iron Intercalation Into Layered MnO2 Produces an Efficient Water Oxidation Electrocatalyst. Ian G. McKendry, Loveyy J. Mohamad, Akila C. Thenuwara, Tim Marshall, Eric Borguet, Daniel R. Strongin, Michael J. Zdilla Department of Chemistry, Temple University, 1901 N. 13th St, Phildadelphia, PA S1

2 Contents Experimental... 3 General... 3 Synthesis of cobalt-doped birnessite, Kx[ConMn(1-n)O2]... 3 Synthesis of iron-intercalated cobalt birnessite, KxFe[ConMn(1-n)O2]... 4 Synthesis of Nanosheets... 4 Assembly of NS to layered catalysts on FTO electrode... 4 X-ray Diffractometry:... 5 Raman Spectra:... 7 Transmission Electron Microscopy:... 8 Scanning Electron Microscopy: Energy-Dispersive X-ray Spectroscopy: X-ray Photoelectron Spectra: Nanosheet Assembly and Charcterization: Electrochemistry: Chronopotentiometry: Tafel Analysis BET Surface Area: S2

3 Experimental General All reagents were purchased from chemical vendors and used without further purification. X-ray diffraction (XRD) was performed on a Bruker D8 ADVANCE diffractometer using molybdenum Kα radiation from a sealed tube and processed using DIFFRACEVA software package. Transmission electron microscopy (TEM) images were collected using a JEOL JEM-1400 microscope operating at 120 kv. Elemental mapping was performed using STEM-EDS. Scanning electron microscopy (SEM) was performed on an Agilent 8500 FE-SEM operating at 1 kv. Samples were mounted on conductive carbon tape mounted on aluminum studs. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher K-Alpha+ with Al-Kα monochromatic X-ray source. Electrochemical measurements were performed with a CHI 660E electrochemical apparatus. Electrochemical studies were carried out in an aqueous solution of 1 M KOH in a three-electrode set-up with a glassy carbon working electrode, platinum counter electrode, and standard calomel electrode (SCE) reference with values reported versus the standard hydrogen electrode. Linear sweep voltammetry (LSV) measurements were carried out at 10 mv/s scan rates. The working electrode was prepared by drop casting 10 μl of catalyst ink. The ink was prepared by suspending 5 mg catalyst, 5 mg carbon powder, and 20 μl of nafion solution (5% in a mixture of lower aliphatic alcohols and water) in 1 ml isopropanol via sonication for 1 h. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed to quantify elemental composition using a Thermo Scientific icap 7000 Series ICP- OES. Raman spectra were collected using a LabRAM HR Evolution spectrometer with a laser excitation wavelength of 532 nm and 1800 g/mm diffraction grating providing a spectral resolution of ~ 1 cm -1. Laser light was focused on ~ 1 μm spot with an Olympus MPLN100x objective. Excitation intensity was maintained below damage threshold. Spectra were collected for at least 3 spots per sample to check for homogeneity. BET surface analysis was conducted with a Quadrasorb evo Gas Soption Surface Area and Pore Size Analyzer. Synthesis of cobalt-doped birnessite, Kx[ConMn(1-n)O2] Synthesis of cobalt-doped birnessite was carried out as previously reported based on modification of the McKenzie protocol. 40,41 Hydrochloric acid (1.50 M, 50.0 ml, 75.0 mmol) and an aqueous solution of cobalt (II) chloride hexahydrate (0.200 M, 50.0 ml, 10.0 mmol) were simultaneously added dropwise via syringe pumps at a rate of 1 ml/min each to a heated and stirring (80 C, 360 rpm) aqueous solution of potassium permanganate (0.200 M, ml, 50.0 mmol) in a 600 ml beaker. Heating continued for an additional 0.5 h after HCl and cobalt (II) chloride hexahydrate addition was completed. The resulting 350 ml solution was then covered with a watch glass to prevent excessive evaporation overnight and aged for 15 h at a cooled temperature of 45 C and a stirring speed of 360 rpm. The resulting suspension was filtered and the residue washed with deionized water via vacuum filtration and air-dried at room temperature to give 1:5 cobalt to manganese doped birnessite (Co-Birnessite). S3

4 Synthesis of iron-intercalated cobalt birnessite, KxFe[ConMn(1-n)O2] Iron(II) chloride ( g, mmol) was dissolved in 20 ml of deionized water in a 30 ml beaker stirring at 600 rpm. Following this, 9.5 µl of hydrazine (N2H4) solution (35 wt% in H2O) was added to the stirring iron(ii) chloride solution. A noticeable color change from yellow to pale blue was observed after the addition of hydrazine, indicating the formation of an iron-hydrazine complex, which has been shown to facilitate transfer of the cations into the interlayer of birnessite, replacing interstitial potassium ions. 31 The suspension was stirred for 10 min at a speed of 600 rpm. Next, 100 mg of cobalt-doped birnessite was added to the solution and stirred for an additional 1 h at a speed of 600 rpm at room temperature. Finally, the suspension was vacuum filtered, and the residue washed with excess deionized water and airdried at room temperature. Synthesis of Nanosheets Exfoliation of birnessite materials was performed by stirring (360 rpm) an aqueous solution of tetra-n-butylammonium hydroxide (TBAOH, 0.1 M) at room temperature for 2 days. The resulting suspension was centrifuged at 1400 rpm for 30 min to separate TBAOH from bulk and nanosheets (NS). The resulting pellet was resuspended in water and centrifuged at 1400rpm for 30 min to remove residual TBAOH surfactant. The washed pellet was resuspended in water and centrifuged at 6000 rpm for 10 min to give the suspended NS. S1 Assembly of NS to layered catalysts on FTO electrode The few-layer catalysts were prepared on a 1 cm x 1 cm fluorine doped tin oxide (FTO) substrate. The FTO was first coated with aqueous polyethylenimine (PEI) (2.6 mg/ml). After rinsing with deionized water and drying, the PEI-primed FTO was dipped in the NS solution for 1.5 min, removed, rinsed with deionized water, and dried. The substrate was then dipped in a 5 mm solution of the appropriate metal chloride (FeCl2 or KCl) for 1.5 min, removed, rinsed with deionized water, and dried. The process was then repeated with alternating immersion between NS and metal chloride solutions with water rinses in between until a total of 10 NS layers were assembled (Figure S14). 42 Samples were characterized using TEM and UV-Visible spectroscopy (Figure S14). References (S1) Kang, Q.; Vernisse, L.; Remsing, R. C.; Thenuwara, A. C.; Shumlas, S. L.; Mckendry, I. G.; Klein, M. L.; Borguet, E.; Zdilla, M. J.; Strongin, D. R. Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, S4

5 X-ray Diffractometry: Figure S1. XRD patterns of Birnessite, Fe-CoBirnessite, Co-Birnessite, and side product identified as magnetite and lepidocrocite iron oxy hydroxides. S5

6 (a.) (b.) Intensity (arb. units) Side Product Lepidocrocite Magnetite Intensity (arb. units) Magnetite Lepidocrocite (c.) ϴ (Degrees) (d.) 2ϴ (Degrees) Theoretical Lepidocrocite Theoretical Magnetite Intensity (arb. units) Intensity (arb. units) ϴ (Degrees) ϴ (Degrees) Figure S2. XRD pattern of (a.) side product identified as (c.) Lepidocrocite and (d.) Magnetite, overlay in panel (b). S6

7 Raman Spectra: (a.) (a.) (c.) (b.) (c.) Figure S3. Raman spectra of (a.) Co-Birnessite 29 and Fe-CoBirnessite (b.) magnetite, and (c.) lepidocrocite. Reference Spectra are taken from the RRUFF database 50, Database codes R (lepidocrocite) and R (magnetite). S7

8 Transmission Electron Microscopy: Figure S4. TEM images of Kx[Co0.16Mn0.84O2]. S8

9 Figure S5. TEM images of crude KxFe[Co0.16Mn0.84O2]. S9

10 Figure S6. TEM images of thoroughly washed KxFe[Co0.16Mn0.84O2]. S10

11 (a.) (b.) (c.) (d.) Figure S7. TEM images of (a.) Magnetite and (b.) (d.) Lepidocrocite. S11

12 Scanning Electron Microscopy: (a.) (c.) (b.) (d.) (c.) Figure S8. SEM images of (a.) Kx[Co0.16Mn0.84O2]. (b.) crude KxFe[Co0.16Mn0.84O2].and (c.) thoroughly washed KxFe[Co0.16Mn0.84O2].. S12

13 Energy-Dispersive X-ray Spectroscopy: Mn Fe Co Figure S9. EDS mapping of KxFe[Co0.16Mn0.84O2] showing a uniform distribution of Mn, Fe, and Co. Figure S10. EDS mapping of KxFe[Co0.16Mn0.84O2]. S13

14 (a.) (b.) Figure S11. TEM image of (a.) side lepidocrocite and magnetite phase with (b.) elemental analysis by EDS. S14

15 X-ray Photoelectron Spectra: (a.) (b.) Intensity (arb. units) Intensity (arb. units) Experimental Average Fit Lepidocrocite Experimental Average Fit Magnetite Binding Energy (ev) Binding Energy (ev) (c.) (d.) Experimental Average Fit Mn(IV) Mn(III) Mn(II) Intensity (arb. units) Intensity (arb. units) Binding Energy (ev) Binding Energy (ev) Figure S12. XPS of crude KxFe[Co0.16Mn0.84O2] depicting (a.) Fe 2p spectra fit to lepidocrocite (b.) Fe 2p spectra fit to magnetite (c.) Mn 2p spectra fit to birnessite and (d.) Co 2p spectra. S15

16 (e.) (f.) Experimental Average Fit Fe(III) Fe(II) Intensity (arb. units) Binding Energy (ev) Figure S12 (continued). XPS of crude KxFe[Co0.16Mn0.84O2] depicting (e.) O 1s spectra and (f.) Fe-CoBirnessite spectra fit with Fe(II) and Fe(III) peaks. S16

17 (a.) (b.) Intensity (arb. units) Intensity (arb. units) Experimental Average Fit Lepidocrocite Experimental Average Fit Magnetite Binding Energy (ev) Binding Energy (ev) (c.) (d.) Experimental Average Fit Mn(IV) Mn(III) Mn(II) Intensity (arb. units) Intensity (arb. units) Binding Energy (ev) Binding Energy (ev) Figure S13. XPS of washed KxFe[Co0.16Mn0.84O2] depicting (a.) Fe 2p spectra fit to lepidocrocite and (b.) Fe 2p spectra fit to magnetite. (c.) Mn 2p spectra fit to birnessite (d.) Co 2p spectra. S17

18 (e.) (f.) Experimental Average Fit Fe(III) Fe(II) Intensity (arb. units) Binding Energy (ev) Figure S13 (continued). XPS of washed KxFe[Co0.16Mn0.84O2] depicting (e.) O 1s spectra and (f.) Fe-CoBirnessite fit with Fe(II) and Fe(III) peaks. S18

19 Nanosheet Assembly and Charcterization: Figure S14. Layer-by-layer nanosheet assembly of birnessite. S19

20 Figure 14b. TEM images of assembled Kx[MnO2]. S20

21 Figure 14b (cont). TEM images of assembled KxFe[MnO2]. S21

22 Figure 14b (cont). TEM images of assembled Kx[Co0.3Mn0.7O2]. S22

23 Figure 14b (cont). TEM images of KxFe[Co0.3Mn0.7O2]. S23

24 Figure 14c. UV-vis absorption spectra in reflectance of assembled KxFe[MnO2]., Kx[Co0.3Mn0.7O2]., and KxFe[Co0.3Mn0.7O2].. S24

25 Electrochemistry: K xfe[co 0.16Mn 0.84O 2] K xfe[co 0.12Mn 0.88O 2] K x[co 0.12Mn 0.88O 2] K x[co 0.16Mn 0.84O 2] Figure S15. Electrochemical water oxidation activity of various preparations of Co- and Fe-Co-doped synthesized birnessite catalysts. S25

26 Figure S16. Electrochemical water oxidation activity of layer-by-layer assembled catalysts. 12 Current Density (ma/cm 2 ) Magnetite Lepidocrocite Potential (V) Figure S17. Electrochemical water oxidation activity of magnetite and lepidocrocite. S26

27 Chronopotentiometry: (a) (b) Figure S18. Chronopotentiometry performed at a current density of 5 ma/cm 2. (a) Enlarged section of first 2000 s (b) Full timecourse for 45000s. S27

28 Tafel Analysis Figure S19. Tafel Analysis of Kx[Co0.16Mn0.84O2]. S28

29 BET Surface Area: Table S1. Determined surface areas for Co0.15Mn0.85O2, Fe-CoBirnessite, Fe3O4 (magnetite), and γ-fe(o)(oh) (lepidocrocite). Sample Surface Area (m 2 /g) K x[co 0.16Mn 0.84O 2] 3.8 K xfe[co 0.16Mn 0.84O 2] 140 Fe 3O 4 23 γ-fe(o)(oh) 83 Inductively Coupled Plasma Optical Emission Spectrometry: Table S2. ICP-OES concentrations. Sample Co Concentration Mn Concentration Fe Concentration (mm) (mm) (mm) K x[co 0.16Mn 0.84O 2] Pre-washing K xfe[co 0.16Mn 0.84O 2] Post-washing K xfe[co 0.16Mn 0.84O 2] Table S3. Metal ion percentages based on ICP-OES. Sample Co Mn Fe (%) (%) (%) K[Co 0.16Mn 0.84O 2] Pre-washing K xfe[co 0.16Mn 0.84O 2] Post-washing K xfe[co 0.16Mn 0.84O 2] Table S4. Metal ion percentages based on ICP-OES of K[Co0.31Mn0.69O2] monolayer suspensions used for layer-by-layer assembly of K/ Co0.31Mn0.69O2 and Fe/ Co0.31Mn0.69O2. Sample Kx[Co0.31Mn0.69O2] nanosheets Co concentration Mn concentration Co:Mn (mm) (mm) S29