Supporting Information. Nature of Activated Manganese Oxide for Oxygen Evolution

Transcription

1 Supporting Information Nature of Activated Manganese Oxide for Oxygen Evolution Michael Huynh, a Chenyang Shi, b Simon J. L. Billinge, b,c Daniel G. Nocera*,a adepartment of Chemistry and Chemical Biology, Harvard University, Cambridge, MA b Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States. c Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States. dnocera@fas.harvard.edu S1

2 Index Page Figure S1. CV and Tafel plot (OER) of MnOx deposited by CV without buffer S3 Figure S2. Tafel plots (OER) of MP deposited MnOx, varying cathodic E S4 Figure S3. Faradaic efficiency of OER on activated MnOx by O 2 sensor S5 Figure S4. OER stability of activated MnOx at ph 7.0, 2.5, and 0.3 S6 Figure S5. Tafel plots of OER on activated MnOx before and after stability test S7 Figure S6. Electrochemical surface area measurements of FTO and C cloth S8 Figure S7. Electrochemical impedance spectroscopy of MnOx films S9 Figure S8. CV QCM i t plot of MnOx deposition in Mn 2+ solution with KNO 3 S10 Figure S9. QCM plot of multipotential deposition of activated MnOx S11 Figure S10. CV QCM plot in Mn 2+ -free KNO 3 solution Figure S11. CV QCM plot in Mn 2+ buffered by MeP i at ph 8.0 Figure S12. QCM plot of cathodic deposition at 0.4 V and with anodic pulse Figure S13. High-resolution XPS of manganese oxide control compounds Figure S14. PXRD patterns of MnOx thin films Figure S15. PXRD patterns of manganese oxide control compounds Figure S16. PDF of manganese oxide control compounds Figure S17. Tafel plot of as-deposited MnOx showing MnO 4 region at high E Table S1. Fitting parameters for electrochemical impedance spectroscopy S20 Table S2. Table S3. Pearson s coefficients of PDF pairs between MnOx and control samples Summary of PDF fitting results for MnOx samples and control samples S12 S13 S14 S15 S16 S17 S18 S19 S21 S22 S2

3 Figure S1. (a) Cyclic voltammogram of a 1 cm 2 FTO electrode in 0.5 mm Mn 2+ at 100 mv/s showing the first (blue line), intermediate (grey lines), and last scans (red line). (b) Tafel plots of oxygen evolution in 0.10 M P i and 1.73 M KNO 3 at ph 7.0 on MnOx films deposited by different deposition protocols: constant potential in MeP i buffer at ph 8 (123 mv/decade Tafel slope; blue ), CV (68 mv/decade; dark purple ), and CV without KNO 3 during deposition (74 mv/decade; light purple ). The OER activity of a blank FTO electrode (grey, ) is included for comparison. S3

4 Figure S2. Tafel plots of oxygen evolution in 0.10 M P i and 1.73 M KNO 3 at ph 7.0 on MnOx films prepared by multipotential deposition in 0.5 mm Mn 2+, 0.9 M KNO 3 solution with the first pulse at 1.1 V for 3 s and the second pulse for 2 s at a potential of: 0.4 V (92 mv/decade Tafel slope; blue ), 0.2 V (85 mv/decade; green ), 0 V (72 mv/decade; purple ), and 0.4 V (65 mv/decade; red ). A total of 50 pulse cycles were employed during multipotential deposition. S4

5 Figure S3. Faradaic efficiency of OER on activated MnOx films operated at 0.1 ma/cm 2 in 0.10 M P i and 1.75 M KNO 3 solution at ph 2.5 (red, ) and 7.0 (green, ). O 2 was detected by fluorescence sensor and theoretical O 2 traces (black, ) are calculated from total charge passed during chronoamperometry assuming 100% Faradaic efficiency. Plots are offset by 2 h for clarity. Background sensor drift and O 2 leak of electrolysis cell included for comparison (grey, ). S5

6 Figure S4. Electrochemical stability as measured by sustained chronoamperometry on FTO at 0.1 ma/cm 2 for activated MnOx (red, ), constant potential deposited MnOx (blue, ), IrOx (purple, - - -) and blank FTO (grey, ) in 0.10 M P i and 1.73 M KNO 3 at (a) ph 7.0 and (b) ph 2.5; (c) in 0.5 M H 2 SO 4 at ph 0.3. (d) Stability measurements were repeated using a higher surface area carbon cloth electrode (grey, ) at 1 ma/cm 2. S6

7 Figure S5. Tafel plots of oxygen evolution on activated MnOx before (red ) and after (light purple ) sustained chronoamperometry for 6 h at 0.1 ma/cm 2 in 0.10 M P i and 1.73 M KNO 3 at (a) ph 7.0 and (b) ph 2.5. S7

8 Figure S6. Double-layer capacitance measurements for determining the electrochemically active surface area (ECSA) of FTO (0.2 mf/cm 2 slope; grey, ) and carbon cloth (2.8 mf/cm 2 slope; red, ) in 1 M KNO 3. Current densities were recorded in a small non- Faradaic region near the open circuit potential of the electrode. S8

9 Figure S7. Electrochemical impedance spectroscopy evaluated at 1.65 V with superimposed 5 mv AC signal: (a, b) FTO, (c, d) MnOx deposited at constant potential, and (e, f) activated MnOx. (a, c, e) Bode plot featuring impedance (red, ) and phase data (blue, ). (b, d, f) Corresponding Nyquist plot where insets show expanded region at high frequencies. Data were fitted (black, ) to a (g) modified Randles circuit where R u is uncompensated resistance, R f is the Faradaic resistance, Y(α) is a constant phase element similar to a capacitor that includes solution and film capacitance, and W d is a Warburg element representing diffusion limitations during oxygen evolution. Fitted parameters are presented in Table S1. S9

10 Figure S8. Current time view (top) and corresponding change in mass as measured by a quartz crystal microbalance (bottom) on a Pt-sputtered quartz electrode in 0.5 mm Mn 2+ and 0.9 M KNO 3 solution at 50 mv/s scan rate. The scans progress from red to purple lines, and grey arrow indicates increase in mass over time. The CV view is shown in Figure 7. S10

11 Figure S9. Current time view (top) and corresponding change in mass as measured by a quartz crystal microbalance (bottom) on a Pt-sputtered quartz electrode performing multipotential deposition of MnOx by alternating between 1.1 V for 3 s and 0.4 V for 2 s in 0.5 mm Mn 2+ and 0.9 M KNO 3 solution. S11

12 Figure S10. Cyclic voltammogram (top) and corresponding change in mass as measured by a quartz crystal microbalance (bottom) on a Pt-sputtered quartz electrode in Mn-free 0.9 M KNO 3 solution at 50 mv/s scan rate. The scans progress from red to purple lines. S12

13 Figure S11. Cyclic voltammogram at 50 mv/s scan rate (top) and corresponding change in mass as measured by a quartz crystal microbalance (bottom) on a Pt-sputtered quartz electrode in a solution of 0.5 mm Mn 2+ and 50 mm MeP i at ph 8.0 (red, ) and without any Mn 2+ (grey, ). Arrows were added to show progression of the cycles. S13

14 Figure S12. Change in mass on a Pt-sputtered quartz electrode as measured by a quartz crystal microbalance for cathodic deposition at 0.4 V in: (a) Mn-free 0.9 M KNO 3 solution (grey) and with added 0.5 mm Mn 2+ (red) where the black arrow indicates when the electrode potential was switched to open circuit. (b) With added 0.5 mm Mn 2+ (red, ) where the black arrow denotes a 30 s anodic pulse at 1.0 V. The same anodic pulse on a fresh electrode (grey, ) without any prior cathodic deposition procedure is provided for comparison. S14

15 Figure S13. High-resolution XPS spectra in the (a) Mn 3s, (b) 2p, and (c) 3p regions for control manganese oxides: Mn IV O 2 (blue, ), Mn III 2O 3 (green, ), Mn II,III 3O 4 (red, ), and Mn II O (magenta, ). S15

16 Figure S14. Powder X-ray diffraction patterns of FTO (grey, ) and MnOx thin films electrodeposited by: constant potential (blue, ), multipotential cycling (red, ), and cathodization (green, ). S16

17 Figure S15. Powder X-ray diffraction patterns of control manganese oxide compounds: β- MnO 2 (pyrolusite, light blue, ; JCPDS ICDD card # ), δ-mno 2 (birnessite, blue, ; JCPDS # ), α-mn 2 O 3 (bixbyite, green, ; JCPDS # ), α- Mn 3 O 4 (hausmannite, red, ; JCPDS # ), and MnO (manganosite, magenta, ; JCPDS # ). Compounds were matched to JCPDS ICDD entries (grey ). S17

18 Figure S16. Pair distribution functions of total scattering collected from control manganese oxides: β-mno 2 (pyrolusite, light blue), δ-mno 2 (birnessite, blue, ), α-mn 2 O 3 (bixbyite, green, ), α-mn 3 O 4 (hausmannite, red, ), and MnO (manganosite, magenta, ). PDFs for MnOx samples are in Figure 13. S18

19 Figure S17. Tafel plot of constant potential deposited MnOx in 0.10 M Pi and 1.73 M KNO3 at ph 2.5 with slopes of 678 mv/decade (red, ) and 125 mv/decade (purple, ). The lower slope region at higher potentials is consistent with permanganate formation. S19

20 Table S1. Fitting parameters for electrochemical impedance spectroscopy evaluated at 1.65 V with superimposed 5 mv AC signal using a modified Randles circuit model (Figure S7g). Fit parameters a FTO constant potential MnOx activated MnOx R u (Ω) R p (Ω) Y (S s α ), α , , , 0.93 W d (S s 1/2 ) GoF Kramers-Kronig GoF a R u is uncompensated resistance, R f is the Faradaic resistance, Y(α) is a constant phase element similar to a capacitor that includes solution and film capacitance, α is a factor between 0 and 1 where 0 is an ideal resistor and 1 is an ideal capacitor, and W d is a Warburg element representing diffusion limitations during oxygen evolution. GoF stands for goodness of fit where smaller values correspond to better fitting. S20

21 Table S2. Pearson s product moment correlation coefficients calculated between PDFs of manganese oxide control samples and MnOx catalyst films. r range of PDF a 1-6 Å 1-10 Å 1-15 Å 1-20 Å 1 vs. β-mno vs. δ-mno b vs. α-mn 2 O vs. α-mn 3 O vs. MnO vs. β-mno vs. δ-mno vs. α-mn 2 O vs. α-mn 3 O vs. MnO vs. β-mno vs. δ-mno vs. α-mn 2 O vs. α-mn 3 O vs. MnO a The labels 1, 2, and 3 represent MnOx samples prepared by constant potential, cathodized, and multipotential deposition protocols, respectively. b Highest correlations are bolded. S21

22 Table S3. Summary of PDF fit results for MnOx catalyst samples. Constant Potential a Cathodized b Multipotential a R c w Phase 1: birnessite a (Å) b (Å) c (Å) β ( ) Size (Å) x (O OL ) z (O OL ) x (K IL ) z (K IL ) d x (O IL ) z (O IL ) Occ. (Mn) Occ. (K IL ) Occ. (O IL ) U 11 (Mn) (Å 2 ) U 22 (Mn) (Å 2 ) U 33 (Mn) (Å 2 ) U 11 (O) (Å 2 ) U 22 (O) (Å 2 ) U 33 (O) (Å 2 ) U iso (K IL ) (Å 2 ) U iso (O IL ) (Å 2 ) r (Mn O(1) 4) (Å) r (Mn O(2) 2) (Å) BVS e Phase 2: hausmannite a (Å) c (Å) y (O) z (O) S22

23 U iso (Mn) (Å 2 ) U iso (O) (Å 2 ) Size (Å) mole birnessite(%) 68.1 mass birnessite (%) 61.5 r (Mn tet O(1) 4) (Å) r (Mn oct O(2) 4) (Å) r (Mn oct O(3) 2) (Å) BVS (Mn tet ) 2.02 BVS (Mn oct ) 3.06 a Constant potential and multipotential deposited samples adopt the space group C2/m (No. 12) with Mn, O OL, K IL and O IL atoms sitting at (0, 0, 0), (x, 0, z), (x, 0, z) and (x, 0, z), respectively. Here OL and IL stand for octahedral layer and interlayer, respectively. The occupancy of intercalated K + was charge compensated by the loss of Mn ion, i.e. K occ = 2(1 Mn occ ). Anisotropic thermal factors were used for Mn and O in the MnO 6 octahedron while isotropic thermal factors were employed for intercalated K and O atoms. b For cathodized MnOx, a second phase of hausmannite was also considered where the space group is represented by I4 1 /amd (No. 141) with Mn1, Mn2, O atoms at (0, 0, 0), (1/2, 1/4, 1/8), (0, y, z), respectively. c R w is a measure of goodness-of-fit with lower values indicating better agreement. 1 d z (K IL ) was fixed to the value of z (O IL ) to ensure physical results. e Bond valence sum (BVS) is calculated using equation s = Σ exp((r 0 R)/B) where R 0 = Å, Å and Å for Mn 4+, Mn 3+, and Mn 2+, respectively, B = 0.37, and R is the Mn O bond length. 2 1 Petkov, V.; Gateshki, M.; Choi, J.; Gillan, E. G.; Ren, Y. J. Mater. Chem. 2005, 15, Brown, I. D.; Altermatt, D. Acta Crystallogr. B 1985, 41, S23