Supporting information for:

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1 Supporting information for: Electrochemical Dissolution of Iridium and Iridium Oxide Particles in Acidic Media: Transmission Electron Microscopy, Electrochemical Flow Cell Coupled to Inductively Coupled Plasma Mass Spectrometry and X-ray Absorption Spectroscopy Study Primož Jovanovič*, Nejc Hodnik *, Francisco Ruiz-Zepeda, Iztok Arčon,, Barbara Jozinović, Milena Zorko, Marjan Bele, Martin Šala, Vid Simon Šelih, Samo Hočevar and Miran Gaberšček *, Department of Materials Chemistry, National Institute of Chemistry Hajdrihova 19, 1000 Ljubljana, Slovenia Department of Catalysis and Chemical reaction Engineering, National Institute of Chemistry Hajdrihova 19, 1000 Ljubljana, Slovenia University of Nova Gorica, Vipavska 13, Nova Gorica, Slovenia Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia Department of Analytical Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI Ljubljana, Slovenia Legend: As prepared Ir nanoparticles: A-Ir Electrochemically oxidized Ir nanoparticles: E-Ir Thermally prepared IrO 2 nanoparticles: T-IrO 2 S1

2 Transmission electron microscopy analysis A-Ir sample: Figure S1: ADF images of A-Ir nanoparticles. Voids observed on Ir crystalites (white arrows), and some smaller nanoparticles (blue arrows). Figure S2: EDX chemical mapping and ADF image of A-Ir. Oxygen and Carbon signals seem to come from the same parts of the nanoparticle, only few spots show IrO x (signaled with arrows). S2

3 Figure S3: FFT (Ir zone axis [101]), atomic models and ADF image of the surface of one A-Ir nanoparticle, high index (-1-31) and (-1-51) planes are the most common facets. E-Ir sample: Figure S4: ADF and BF image of an E-Ir nanoparticle, showing a rough morphology. S3

4 Figure S5: (a) STEM-HAADF image of E-Ir particles. (b) and (c) Ir particles partially covered by a layer of Nafion with Ir single atoms and Ir clusters. (d) STEM-ADF and BF images of the surface of one of the Ir particles showing steps at the surface and a cover layer with Ir single atoms. Figure S6. FFT (Ir zone axis [101]), ADF image and atomic models of the suface of one E-Ir nanoparticle. (42-4), (53-5), (31-3) and (20-2) planes are the most common facets. Single atoms are also visible at the surface. S4

5 Figure S7: ADF image and EDX chemical mapping of an E-Ir nanoparticle. A small IrO x spot is observed at the edge of the surface of the nanoparticle (signaled by arrow). Figure S8: STEM-ADF image and EDX spectra of the cover layer with small atomic clusters observed in E-Ir samples. The presence of elements that compose nafion are identified in the spectra. S5

6 Figure S9: STEM-ADF images E-Ir nanoparticles (a) showing the cover layer. (b) Close up to reveal single atoms and clusters at the surface of the Ir crystalites. Figure S10: (a1), (b1) and (a2) STEM-ADF images of clusters and single atoms of Ir. (c1) and (c2) correlated Ir EELS signal from boxed areas, acordingly. S6

7 Scheme S11: Model of a stack of three wrinkled layers of nafion and the corresponding simulated STEM-ADF images. (a) Without Ir atoms and (b) with Ir atoms. Notice the high contrast in the ADF simulated image when Ir atoms are present. Scheme S12: Schematic representation of electron induced decomposition of surface oxides. S7

8 T-IrO2 sample: Figure S13: ADF image and EDX chemical mapping of a T-IrO2 nanoparticle. It can be seen that IrO2 forms a shell sorounding an Ir core. Figure S14: STEM-HAADF and TEM images displaying the morphologies of T-IrO2 nanoparticles. S8

9 Identical location transmission microscopy (IL-TEM) analysis Il-TEM analysis was performed before and after 200 actiation cycles (from 0.05 to 1.2V). The electrochemical pretreatment transforms an A-Ir analogue to E-Ir analogue. The electron microsocope was operated at 80 kv and the same region of interest (ROI) was tracked for observation before and after EC. This task was performed on several spots of the sample grid to check for consistency. Single atoms and clusters can be observed at the surface after the sample is submitted to EC (Figure S15 and Figure S16). Figure S15: Schematic diagram showing a stepwise demonstration of how IL-TEM analysis was performed. S9

10 Figure S16: IL-(S)TEM-HAADF images of an Ir particle in (a) before and in (b) after EC. A nanoparticle can be spoted on the surface in (a), while single atoms are observed at the surface in (b). Figure S17: IL-(S)TEM-ADF and EDX maps of Ir and O of an Ir particle in (a) before and in (b) after EC. S10

11 Identical location scanning electron microscopy (IL-SEM) analysis Thick catalyst films were deposited directly on graphite SEM holder (TedPellaInc.).The graphite holder was modified so it could be used as a working electrode for degradation experiments. First we cut down a cap on the original graphite holder and polished It on a plane writing cellulose paper. A hollow in the shape of letter X was made with scalpel.this helps us to locate the same location after the treatments. The graphite holder was rubbed into the catalyst so every part of the X-shaped hollow is filled with catalyst.to prevent carbon deposition in SEM experiments the graphite holde was dipped in isopropylalcohol (pro analysi, Merck).To prevent later sample detachment the graphite holder was additionally dipped in water(milli-q). After that it was heated to 100 C for 10 min in order to remove water and any isopropyl alcohol residues 1 4. After 10 min the samples were dried and then investigated using a scanning electron microscope (FE-SEMZeiss SUPRA 35VP) with an accelerating voltage of 7kV. Electrochemical flow cell measurements Figure S18: Typical cyclovoltammetric response in the presently used electrochemical flow cell configuration when cycling till a) 1.5 V and b) 1.6 V vs RHE. S11

12 Quasi in situ electrochemical XANES and EXAFS study The X-ray absorption spectra at Ir L3-edge of the metallic Ir (reffered in the main text as A- Ir), electrochemically pretreated Ir (E-Ir) and IrO 2 (T-IrO 2 ) catalysts and reference Ir compounds were measured at room temperature in fluorescence and/or transmission detection mode at the beamline P64 of PETRAIII at DESY, Hamburg. A Si (111) double crystal monochromator was used with energy resolution of about 1 ev at 11 kev. Higher-order harmonics were effectively eliminated by a flat Rh coated mirror installed in front of the monochromator. The size of the beam on the sample was about 1 mm in diameter. The intensity of the monochromatic X-ray beam was measured by two consecutive ionisation chambers, first filled with 20% of Kr and 80% N 2, and the second filled with Kr, at 1bar. Sample cell was placed between the two ionization cells. Fluorescence signal from the Ir in the sample was detected with PIPS fluorescence detector. Lower energy fluorescence lines from the elements in the cell walls (Al) were effectively filtered with 20 micron Al foil. The absorption spectra were measured in the energy region from -150 ev to ev relative to the Ir L3-edge (11215 ev) in a continuous scan mode. The scan duration was 300 s. The signal from detectors was integrated during scan in time domain to obtain energy steps of 0.5 ev. Electrochemical experiment was conducted as follows: A water suspension of Ir and IrO 2 catalysts were dropcasted on a working electrode of a 0.5 mm thick ceramic based commercially available electrochemical cell. After drying a thin layer of the catalyst film was obtained. Electrochemical cells consisted of screen-printed electrodes with carbon working (d = 4 mm) and counter electrode, and silver quasi-reference electrode (DropSens, Oviedo). Loading of each catalyst was set to 5 mgcm M HClO 4 was chosen as the working electrolyte. Electrochemical treatment was conducted under potentiostatic regime. In the case of Ir analogue 200 activation cycles were performed (analogously as described in the main text) before potentiostatic treatment. After each potential sequence the electrochemical cell was transferred to X-ray absorption chamber therefore electrochemical experiment and subsequent X-ray absorption spectroscopy analysis were performed in ex situ mode as schematically presented in Fig. S19. S12

13 Figure S19: Left-A screen printed three electrode system used for electrochemical treatment and for XAS analysis. Right-schematic diagram of electrochemical regime and XAS analysis performed in an ex situ mode. Two to six repetitions of the scans of Ir L3 XAS spectra were measured on the same spots on the sample to check stability of the sample and reproducibility of the spectra. There were no differences beyond noise level between the spectra of the repeated scans. The reference samples (metallic Ir, IrO 2 and IrBr 3 ) were prepared in the form of homogeneous pellets, pressed from micronized powder mixed with micronized carbon black, with the total absorption thickness of about 2 above the Ir L3-edge. The analysis of Ir L3-edge XANES and EXAFS spectra is performed with the Demeter (IFEFFIT) program package 5 in combination with FEFF6 program code 6 for ab initio calculation of photoelectron scattering paths. Ir L3-edge XANES results Changes in Ir valence state and local symmetry around Ir cation in the sample during electrochemical activations can be monitored with Ir L 3 XANES analysis. Normalised Ir L 3 - edge XANES spectra of the Ir and IrO 2 catalysts together with the reference samples (metallic Ir, IrBr 3, and IrO 2 ) with known Ir valence state (0, 3+, and 4+, respectively) are shown on Fig. S20. Significant differences between Ir L3-edge edge profiles and pre-edge resonance energy positions are observed in the XANES spectra of the Ir reference compounds. The XANES spectra of Ir catalyst in initial state of the material (before electrochemical treatment) is identical to the XANES spectrum of reference Ir metal. Ir L3-edge EXAFS analysis (not shown here) confirms that Ir is in metallic form with fcc crystal structure. After electrochemical treatment at different potentials small differences in the Ir XANES spectra are detected, indicating structural and valence changes of part of the Ir cations in the sample. S13

14 Principal component analysis (PCA) 5 is used in the first step to resolve the changes The variation of the edge profile in the set of Ir catalyst spectra is described by eigenvectors of the correlation matrix of the target XANES spectra, extracted and ordered by the amount of total variance they account for. The number of the eigenvectors above the noise level denotes the number of independent components with a physical meaning, possible Ir compounds in our case. The result of PCA, shown in Table S1, suggests that there are two or eventually three principal components in our six target spectra. In the next step of the analysis we used three reference XANES spectra in the linear combination fit (LCF) 5. Very good fits are obtained with spectrum of Ir metal, spectrum of IrBr 3 as reference for Ir(3+) and spectrum measured on IrO 2 catalyst in initial state as reference for Ir(4+). The quality of fits is illustrated on Fig. S21. The fit results, listed in Table 2, show that after the activation cycles about 20% of Ir cations, that were initially in metallic form, are oxidised to Ir3+ state, and a small part of about 5% is oxidised to Ir(4+). The XANES results are supported also by Ir L3-edge EXAFS analysis (not shown here), where we observed that average local structure around Ir cations in the sample after electrochemical treatment is partially changed: beside characteristic local structure of Ir cations in fcc crystal structure, with first Ir-Ir coordination shell at 2.70 A, oxygen atoms are detected in the nearest coordination shell at Ir-O distance of about 2.0 A. So, in this way EXAFS results also clearly indicate partial oxidation of Ir cation during electrochemical treatment. The XANES spectra of the IrO 2 catalysts in initial state of the material and after electrochemical activations at different potentials are all identical within noise level. The observation is confirmed also by principle component analysis, which shows that only one principle component can completely describe the whole set of six XANES spectra. The energy position of the pre-edge resonance coincides with that of IrO 2 reference compound, so we can conclude that Ir cations are in the form of Ir(4+) oxide, which does not change during electrochemical treatment. The result is confirmed also by Ir L3-edge EXAFS analysis (not shown here), where we observed that local structure around Ir cations in the sample is similar (not identical) to crystalline IrO 2, and its local structure remains unchanged during all steps of electrochemical treatment. S14

15 Table S1. Eigenvalues obtained by principal component analysis (PCA) of the Ir L 3 -edge XANES spectra of Ir catalysts samples: Eigenvalues 3. Variance Figure S20: Ir L3-edge XANES spectra of Ir and IrO 2 catalysts in initial state of the material and after electrochemical activations for 3 min at different potentials: 0.9 V, 1.2V, 1.6V and reduced at 0.05V vs RHE, and the reference samples (metallic Ir, IrBr 3, and IrO 2 ). The spectra are displaced vertically for clarity. S15

16 Figure S21: The Ir L3-edge XANES spectrum of the Ir catalyst after electrochemical activations for 3 min at potential of 0.9 V. Black dots - experiment; magenta dashed line best-fit linear combination of XANES profiles of Ir metal (75%), Ir(3+)Br 3 (20%) and IrO 2 (5%). Fit components are shown below. References (1) Zorko, M.; Jozinović, B.; Bele, M.; Hodnik, N.; Gaberšček, M. Ultramicroscopy 2014, 140, (2) Hodnik, N.; Zorko, M.; Bele, M.; Hočevar, S.; Gaberšček, M.; Hoc, S.; Gabers, M. J. Phys. Chem. C 2012, 116, (3) Hodnik, N.; Zorko, M.; Jozinović, B.; Bele, M.; Dražič, G.; Hočevar, S.; Gaberšček, M. Electrochem. commun. 2013, 30, (4) Hodnik, N.; Jovanovič, P.; Pavlišič, A.; Jozinović, B.; Zorko, M.; Bele, M.; Šelih, V. S.; Šala, M.; Hočevar, S.; Gaberšček, M. J. Phys. Chem. C 2015, 119 (18), (5) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12 (4), (6) Rehr, J. J.; Zabinsky, S. I.; Albers, R. C. Phys. Rev. Let. 1992, 69 (23), S16