SUPPORTING INFORMATION

Transcription

1 SUPPORTING INFORMATION Low Overpotential High Activity Mixed Manganese and Ruthenium Oxide Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Michelle P. Browne 1,2,3*, Hugo Nolan 2,3, Georg S. Duesberg 2,3, Paula E. Colavita 2,3 and Michael E. G. Lyons 1,2,3* 1 Trinity Electrochemical Energy Conversion and Electrocatalysis (TEECE) Group, School of Chemistry and AMBER National Centre, Trinity College Dublin, College Green, Dublin 2. 2 School of Chemistry, Trinity College Dublin, College Green, Dublin 2. 3 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and BioEngineering Research (AMBER) Centre, Trinity College Dublin, Dublin 2, Ireland *brownem6@tcd.ie and melyons@tcd.ie The Electrochemical Surface Area (ECSA) was determined using capacitance measurements, in a manner similar to Bell et al. and Jaramillo et al., 1-3. Briefly, ECSA was determined from the charging currents of CVs performed in a potential region where no Faradaic processes occur. LSV curves presented in Figure S10-S11 are normalised by ECSA, yielding interesting results. It should be noted that this has slightly changed the ranking of some of the studied electrocatalysts than by geometric area, in main manuscript. However, the overall conclusion is unchanged; some of the mixed Mn/Ru oxides perform better than the pure RuO 2. An example for calculating the double layer capacitance and, hence, the ECSA can be observed below: Figure S1. Double layer capacitance ( ) measurement was calculated by measuring the charging currents for the Cyclic Voltammograms shown in (a) for Mn:Ru 75:25 at an annealing temperature of 350 ᵒC taken at a range of scan rates (1 mv(-), 2 mv(-), 5 mv(-), 10 mv(-), 20 mv(-)and 50 mv(-)), and (b) Capacitance curves for this data were then plotted and the slope determined. The calculated capacitance values for a range of mixed oxides prepared at 350 and 450 C are shown in the table below. 1

2 Table S1: Double layer capacitance values for the pure and mixed oxides prepared at 350 and 450 C Material Mn % Precursor 350 ᵒC Double layer capacitance (μf) 450 ᵒC Double layer capacitance (μf) ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.07 The ECSA is then calculated from the equation as follows 1 ; = 2

3 Figure S2: FTIR spectra of pure (a) Mn (b) mixed Mn/Ru 50:50 and(c) pure Ru materials at the annealing temperatures of 350 ᵒC (Black) and 450 ᵒC (red). Table S2: FTIR peaks for Mn and Ru oxides Wavenumber (cm -1 ) Mn 3 O 4 (red) Mn 2 O 3 (green) RuO 2 (blue) FTIR analysis reveals that, at 350 ᵒC, the Mn x O y formed is predominately Mn 3 O 4 with minimal amounts of Mn 2 O 3, Figure S1. The peak observed at 514 cm -1 is attributed to the Mn-O vibrational mode in the octahedral lattice environment of the Mn 3 O 4 structure. The vibrational mode at 616 cm -1 can be assigned to the Mn-O bond in the tetrahedral environment of the Mn 3 O 4. These peaks are consistent with FTIR peaks assigned to Mn 3 O 4 in the literature. 4-6 At the lower end of the fingerprint region, a small vibrational peak at 447 cm -1 is attributed to a vibrational mode belonging to Mn 2 O 3, however, this peak is very weak. 1 For oxide films formed at 450 ᵒC, the vibrational peak at ca. 451 cm -1 observed in the FTIR spectrum is marginally enhanced, thereby indicating that the amount of Mn 2 O 3 present in the surface film has increased, but the layer is largely Mn 3 O 4. Peaks observed for Mn 2 O 3 are usually at 532 cm -1 can be attributed to Mn-O bending vibrations of Mn 2 O 3, while those at 573 cm -1 and 673 cm - 1 can be assigned to the Mn-O stretching vibrations of Mn 2 O 3. 7 In contrast, RuO 2 is formed at all the annealing temperatures examined, which correlates with previous work reported from our laboratory on thermally prepared RuO 2. 8,9 The RuO 2 IR vibrations are usually found between cm ,11 All bands are weak.. RuO 2 vibrations in the fingerprint region seem to be scarcely documented; however vibrations from Ru-oxide materials can be observed in FTIR spectra from other authors. 3

4 Figure S3. Survey Spectrums for (a) 350 and (b) 450 showing core level peaks in the Mn 100, Mn 50 and Mn 0 samples Figure S4. Representative XPS spectra of Mn core level for (a) Mn 100 (b) Mn50 and (c) Mn0 at an annealing temperature of 350 C. Representative XPS spectra of Ru core level for (d) Mn 100 (Blue box indicates Ti2p core level) (e) Mn50 and (f) at an annealing temperature of 350 C 4

5 Figure S5: XPS Mn and Ru atomic percentages at the annealing temperatures of 350 ᵒC (black) and 450 C (red) 5

6 Figure S6: XRD spectra of (a) pure Mn (b) mixed Mn/Ru 50:50 and (c) pure Ru materials at the annealing temperatures of 350, 450 and 600 ᵒC. The 2Ɵ values of 18.0ᵒ, 28.9ᵒ, 32.4ᵒ, 36.1ᵒ, 44.4ᵒ, 58.5ᵒ, 59.9ᵒ and 64.6ᵒ for the pure Mn x O y materials annealed at 350 and 450 ᵒC can be assigned to Mn 3 O 4 (JCPDS no ) corresponding to the (101), (112), (103), (211), (220), (321), (224) and (400) planes, respectively. The (211) plane was used to calculate the crystallite sizes of the Mn x O y materials prepared at 350 ᵒC and 450 ᵒC, this resulted in crystallite sizes of 9.9 nm and 7.4 nm respectively. Further XRD studies were conducted on the RuO 2 powders prepared at the same annealing temperatures. At the annealing temperatures of 350 ᵒC and 450 ᵒC, the 2Ɵ peaks, in Figure S2 at 28.12ᵒ, 35.35ᵒ, 40.24ᵒ, 54.51ᵒ, 58.18ᵒ, 65.93ᵒ, 67.31ᵒ, 69.65ᵒ and 74.12ᵒ correspond to the (110), (101), (200),(211),(220),(310),(112),(301) and (202) planes indexed to RuO 2 (JCPDS card no ). In the RuO 2 material at 450 ᵒC the peaks at 39.1ᵒ and 60.0ᵒ correspond to planes indexed to metallic Ru. 12 The crystallite sizes of the RuO2 of nm and nm were calculated at the annealing temperatures of 350 and 450 ᵒC respectively. 6

7 Figure S7: Raman Spectrum of pure Mn, pure Ru and the mixed Mn/Ru OER electrocatalysts at the annealing temperatures of 350 and 450 ᵒC. Red areas indicate peaks characteristic of Mn 3 O 4, while the blue areas highlight RuO 2 peaks. 7

8 Table S3: Raman peak positions Compound Characteristic peak positions (cm -1 ) Mn 3 O 4 (red) RuO 2 (blue) Mn 3 O 4 exhibits two weak Raman peaks between cm -1 and cm -1 with a strong peak at 654 cm -1. The Raman spectrum of Mn 2 O 3 is known to have weak signals at 311 cm -1, 653 cm -1 and 697 cm -1. Pure RuO 2 consists of three Raman bands at approximately 514 cm -1, 634 cm -1 and 703 cm - 1. The peaks associated with Mn 3 O 4, Mn 2 O 3 and RuO 2 are marked by red, turquoise and cyan boxes, respectively, in the spectra shown in figure S3. From analysing these spectra it was found that for electrocatalysts prepared at annealing temperatures of 350 and 450 C, characteristic peaks for Mn 3 O 4 were predominantly observed. 13 This is consistent with the findings from other physical characterisation such and XRD and FTIR. 8

9 Figure S8. SEM image for Ru electrocatalyst at 350 ᵒC Figure S9. (a) SEM image for Mn/Ru 50:50 electrocatalyst at 350 ᵒC (b) EDX spectra for Mn/Ru 50:50 (c) EDX mapping for ruthenium (d) EDX mapping for manganese (e) EDX mapping for titanium. 9

10 Figure S10. Oxygen overpotential measured at a current density of 10 ma cm -2 for catalysts normalized by ECSA. Figure S11. Current density at an overpotential of 300 mv. All catalysts normalized by ECSA. 10

11 96 mv dec mv dec mv dec mv dec mv dec mv dec -1 Figure S12. Tafel Slope Plots for all electrocatalysts at the annealing temperatures of (a) 350 ᵒC and (b) 450 ᵒC 11

12 Figure S13. CV of RuO 2 at the annealing temperature of 350 ᵒC to show the potential limits from where the charge is taken. The potential limits/boundaries for the calculation of the charge are taken from the oxidation sweep of the CV. The limits on all CV started before HER and ended before OER. All of the redox peaks were taken into account when calculating the charge, observed above. All active sites for the redox processes are assumed to also be active for the OER. Figure S14 Charge vs Mn % of the materials in this study at the annealing temperature of (a) 350 ᵒC and (b) 450 ᵒC OER may happen at one of the metal oxides before the other however, more in-situ experiments or DFT studies would be needed to determine this. The faradaic efficiency of the OER was close to 100% (also no corrosion observed), allowing the use of the above TOF formula. For the TOF values only, the charge scales to activity. However using voltammetric charge assumes the same actives sites used in the redox chemistry are the same sites used in OER, which may not be the cases. In general we believe the overpotential at 10mA cm -2 to be a better indication of OER activity. 12

13 Figure S15. Galvanostatic lifetime test at a fixed current density of 10 ma cm -2 for the 90:10 Mn/Ru and the pure Ru oxide prepared at an annealing temperature of C for 14.5 hours. 13

14 Figure S16. Cyclic Voltammetry of (a) Mn 100 (b) Mn 50 and (c) Mn 0 at the annealing temperature of 350ᵒC 14

15 Figure S17. LSV of the Ti support From the XPS, Figure S2, the Ti2p core level peak can be observed in the Mn 100 sample therefore it may be possible that Ti substrate could be in contact with the NaOH electrolyte. The LSV of the Ti substrate was recorded to ensure the Ti substrate did not influence the OER. The LSV of the Ti substrate shows only a small increase in current density (~ 1 ma cm -2 ) at a high overpotential of 0.9 V. This would indicate that the Ti substrate does not influence any of the pure and mixed Mn/Ru oxide towards the OER. 15

16 Proposed Mechanism Mn oxide The OER mechanism for Mn x O y could involve the generation of an anionic Mn(IV) surfaquo precursor followed by two electrochemical steps (the first of which being slow and rate determining) involving generation of Mn(V) and Mn(VI) surfaquo species. The unstable Mn(VI) species subsequently decomposes to di-oxygen with the concomitant regeneration of the anionic Mn(IV) surfaquo species: (Surf-O-) 2 Mn(OH) 2 (OH 2 ) 2 + OH - (Surf-O-) 2 MnO - OH(OH 2 ) 2 + H 2 O (Surf-O-)2MnO-OH(OH2)2 + OH - (Surf-O-) 2 MnO - (OH) 2 OH 2 + H 2 O + e - (Surf-O-) 2 MnO - (OH) 2 OH 2 (Surf-O-) 2 MnO(OH) 2 OH 2 + e - (Surf-O-) 2 MnO(OH) 2 OH OH - (Surf-O-) 2 MnO - (OH)(OH 2 ) 2 + H 2 O + O 2 + 2e - The above mechanism all proceeds while attached to the electrode surface (surf), similar to the RuO 2 mechanism. Therefore the Mn oxide species are linked via two bridging oxygen species denoted as ( O-) to the bulk oxide lattice. For hydrous Mn oxide, green and pink colors are generated from the electrode into the NaOH solution during OER potentials, which could indicate higher manganese oxidation states, Figure S18. The hydrous Mn oxide was produced, in our lab, by cycling a Mn metal wire in NaOH to produce a hydrated layer. Symons et al. have previously reported that Mn(VII) generates oxygen in solution phase using spectroscopic techniques. 14 These thermally prepared Mn oxides produced in this paper could generate oxygen similar to Symons at el. but with the higher oxidation state bound to the electrode surface, see mechanism scheme. More detailed in-situ analysis will be taken to investigate the thermally prepared Mn oxide mechanism to determine if it undergoes an OER mechanism similar to hydrous Mn or electrodeposited Mn oxide. 15,16 Figure S18. Pink colour generated in solution during the OER regime from a hydrous Mn oxide. 16

17 Table S4: Nomenclature of OER materials Nomenclature for materials Nomenclature (Figure 2,3 and 4) % of Mn compound in initial paste % of Ru compound in initial paste 100 Mn Mn Mn Ru Mn Mn Ru Mn Mn Ru Mn Mn Ru Mn Mn Ru Mn Ru Mn

18 Table S5: Tafel Slope and overpotential values of all Electrocatalysts in the study Material(Mn/Ru) Annealing temp (degrees) Tafel Slope (mv dec -1 ) Initial OER η (V) 100/0 96± ± /10 84± ± /25 70± ± / ± ± /75 68± ± /90 67± ± / ± ± /0 110± ± /10 89± ± /25 74± ± / ± ± /75 65± ± /90 75± ± /100 48± ±

19 Table S6: Comparison table of the best performance OER catalysts Material Reference Overpotential (V) 10 ma cm -2 Mn 25 Ru 450 This work ± M NaOH Mn 3 O M KOH Mn 2 O ( 20 ma cm -2 ) Mn 3 O 4 CeS M KOH 1 M NaOH Mn 350 This work ± M NaOH α MnO AMO (Amorphous) M KOH β MnO δ -MnO MnO x ± 22 ( 1 ma cm -2 ) Ni 0.9 Fe 0.1 O x M KOH IrO ± Nickel Iron - bulk M KOH Nickel Iron - nanosheets Ba 0.5 Sr 0.5 Co 0.8 Fe 0. 2O 3δ 22 ~ M KOH IrO ± 0.04 (not stable) NiFe x 0.35 ±0.01 NiCex ± 0.03 NiOx 0.42 ± M NaOH NiLaOx 0.41 ± 0.03 CoO x 0.39 ±0.04 NiCuO x 0.41 ±0.01 NiCoO x 0.38 ±0.03 Mn/Co2O4/CNT 23 ~ (vs. Ag/AgCl) (carbon nanotube) Mn/Co2O4/N-rGO (nitrogen-doped reduced graphene oxide) ~ (vs. Ag/AgCl) 0.1 M KOH 24 CoV2O Cobalt O x 100% 0.27 ± 0.02 Iron O x 100% 0.41 ± 0.02 Nickel O x 100% 0.28 ± 0.01 FeO 100% 3 ~ NiO 100% ~ Ni 0.6 Fe M KOH 1 M KOH 19

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