In Situ L-edge XAS Study of a Manganese Oxide Water Oxidation

Transcription

1 In Situ L-edge XAS Study of a Manganese Oxide Water Oxidation Catalyst Lifei Xi, + Christoph Schwanke, + Jie Xiao, Fatwa F. Abdi, Ivelina Zaharieva and Kathrin M. Lange +* + Operando Characterization of Solar Fuel Materials, Institute for Solar Fuels, Institute Methods for Material Development, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany, Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, Berlin, Germany. Supporting Figures and Table Figure S1. Schematic illustration of the transmission cell used in this study. The beam intensity with electrolyte is 70 times lower than that without electrolyte. Photo of in situ/operando PEC S1

2 transmission/fluorescence cell used for soft XAS. 1 Inset: the cell is opened for changing membranes. Note: The ultrathin Au/Si3N4 window has high transparency to X-ray, thus enable XAS spectra to be collected via transmission mode. The gap between two membranes is controlled by a helium pressure system. MnPi after 40 min FTO only Intensity (a.u.) (deg) Figure S2. X-ray diffraction (XRD) patterns of MnPi prepared by electrodeposition at 1.05 V for 40 min and pure FTO/glass substrate. (c) Figure S3. SEM image of MnPi on FTO/glass substrates: before and (b-c) after electrolysis of 5 h. Note: the red dotted box region in is further tested in high magnification and presents in S2

3 (c). It can be seen that the extended electrolysis of MnPi film results in a rough surface and irregular particle formation. The particle size is around nm. Figure S4. Linear combination fitting of Mn L-edge spectra of the MnPi films at OCP after freshly prepared and under 1.2 V for 51 min from the spectra of the manganese oxide powders. It can be seen in Figure 4a that the fitting spectrum contains a bit of uncertainty. The small mismatch in intensity at ev peak may originate from the different experimental resolution of experi-mental and reference spectra. 0,55 0,50 Fresh Activated 0,45 (V) 0,40 0,35 0,30 0,25-5,0-4,8-4,6-4,4-4,2-4,0-3,8 log(j) (A cm -2 ) Figure S5. Tafel plots of oxygen evolution of MnPi films before and after activation tested in 0.10 M KPi and 2.0 M KNO3 at ph7.0 without stirring. The overpotential η is obtained by subtracting the thermodynamic potential of water oxidation from the applied potential. The Tafel slope is proposed as a primary activity descriptor because it is invariant to the number of active sites (vide infra). 2 Lower slope indicates better catalysis. The Tafel slopes of MnPi films after activation at 1.2 V for 1 h and as freshly deposited film are 168 and 273 mv/decade. It can be seen that after activation, the Tafel slope is reduced and thus the film becomes more active. The reason of the relatively high Tafel slope values obtained in this study can be the deposition conditions, eg. potential and thickness. S3

4 Figure S6. Linear combination fitting of Mn L-edge spectra of the MnPi films after kept at 1.5 V for 20 min from the spectra of the manganese oxide powders. Current density (ma/cm 2 ) Time (min) Figure S7. Current vs time (I-t) profile of MnPi film tested under +1.5 V vs NHE in 0.10 M KPi and 2.0 M KNO3 at ph7.0. Note: the abrupt changes in the I-t curve (e.g. at minute ~9 and ~35) are due to gas generation. No such changes was observed when lower potentials were applied. A photo of oxygen bubble generated from MnPi film operated. We attempted to test Faradaic efficiency of oxygen evolution reaction but failed due to small area of catalyst in our transmission cell. We used gold patterned Si3N4 membranes which have a catalyst coated area of 0.09 cm 2. Table S1. Basic electronic and structure information of manganese oxide reference powders used in this study. Compound Syngony Space group Mn site symmetry Mn formal oxidation state d-band occupancy Spin state Ref. MnO Cubic fm-3m Octahedron 2+ 3d 5 High spin 3 S4

5 Distorted Octahedron Mn 3O 4 tetragonal I4 1/amd Mn 3+ : weakly distorted octahedron sites [MnO6]; Mn 2+ : tetrahedral sites [MnO4] 2+, 3+ High spin 3, 4 Mn 2O 3 Cubic 1a-3 Octahedron, but prone to Jahn-Tellar distortion or distorted Octahedron 3+ 3d 5 High spin ground state mixed with low-spin configuration 3 Birnessite Monoclinic prismatic (pseudohexagonal) 2/m layers of edge-sharing MnO6 octahedron with an interlayer of hydrated cations 3+, 4+ High spin 5, 6, 7 MnO 2 tetragonal P4 2/mnm Distorted Octahedron 4+ 3d 3 High spin 3 Note that for birnessite, which has a two-dimensional layered structure consisting of edge-sharing MnO6 octahedrons with an interlayer of cations and water, the average oxidation state normally falls between 3.6 and 3.8. This represents a predominance of Mn 4+ with minor amounts of Mn 3+. 6,7 References (1) Schwanke,C.; Xi, L.; Lange, K. M. A soft XAS Transmission Cell for Operando Studies. J. Synchrotron Rad. 2016, 23, (2) Huynh, M.; Shi, C.; Billinge, S.J.; Nocera, D.G. Nature of Activated Manganese Oxide for Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, (3) Qiao, R.; Chin, T.; Harris, S. J.; Yan, S.; Yang, W.L. Spectroscopic Fin-gerprints of Valence and Spin States in Manganese Oxides and Fluorides. Current Applied Physics 2013, 13, (4) Ramírez, A.; Hillebrand, P.; Stellmach, D.; May, M. M.; Bogdanoff, P.; Fiechter, S. Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water. J. Phys. Chem. C 2014, 118, (5) Lucht, K. P. ; Mendoza-Cortes, J. L. Birnessite: A Layered Manganese Oxide To Capture Sunlight for Water-Splitting Catalysis. J. Phys. Chem. C 2015, 119, (6) Ching, S.; Petrovay, D. J.; Jorgensen, M. L.; Suib, S. L. Sol Gel Synthesis of Layered Birnessite-Type Manganese Oxides. Inorg. Chem. 1997, 36, (7) Golden, D. C.; Dixon, J. B.; Chen, C. C. Ion Exchange, Thermal Transformations, and Oxidizing Properties of Birnessite. Clays Clay Miner. 1986, 34, S5