Electronic Supplementary Information. Effect of Lattice Strain in Electrocatalytic Activity of IrO 2 for Water Splitting

Transcription

1 Electronic Supplementary Material (ESI) for ChemComm. This journal is The Royal Society of Chemistry 2018 Electronic Supplementary Information Effect of Lattice Strain in Electrocatalytic Activity of IrO 2 for Water Splitting Wei Sun a,, Zhiqiang Wang b,, Waqas Qamar Zaman a, Zhenhua Zhou a, Limei Cao a, Xue-Qing Gong b, Ji Yang a,* a State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Processes, School of Resources and Environmental Engineering. East China University of Science and Technology, 130 Meilong Road, Shanghai , P.R. China. b Key Laboratory for Advanced Materials, Center for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, 130 Meilong Road, Shanghai , P.R. China. * Corresponding author: yangji@ecust.edu.cn

2 Experimental methods Materials preparation All Iridium oxides are prepared via the hydrothermal synthesis. In detail, 3 ml of 56.7 mm iridium chloride (IrCl 3 3H 2 O, >98% purity) precursor was added to 10 ml of 0.1 M sodium hydroxide (NaOH, A.R. purity), and an additional 10 ml of deionized water was mixed into the solution. Then, the whole solution was transferred to a 40-mL Teflon-lined pressure vessel and loaded into an oven to heat the solution to 150 C for 720 min. The vessels were cooled naturally at room temperature. The precipitates were suction filtered and washed with deionized water twice to remove other ions. The remaining solid on the filter was dried to dehydration in an oven at 60 C at least 30 min. The dried powders were transferred to a crucible and annealed at different temperature for 6 h. Electrode preparation and Electrochemical Measurements The electrodes used for the electrochemical measurements are prepared as follows. 6 mg of fresh catalyst powders are dispersed in 1.5 ml of 2:1 v/v isopropanol/water and 15 ul Nafion solution added into the solution. Then ultrasonicating them for approximately 30 min to form a homogeneous ink. Next, 7.5 μl of ink deposited on tailored Ti plate. Here, a saturated calomel electrode (SCE) is employed as a reference and a polished and cleaned Pt foil with a 1.5 cm 1 cm reaction area were used for the counter electrode. The SCE was calibrated with respect to the RHE in all three types of ph solution using a high purity hydrogen saturated electrolyte with a Pt foil as the working electrode 1. The OER determinations under the 0.1 M perchloric acid (HClO 4, A.R. purity) solution. The working electrodes were cycled a least 5 times until the curves were observed to overlap in the curves; then, the data of the cyclic voltammetry (CV) and polarization curves were recorded at the specified scan rate. Materials Characterizations The crystal structure of the catalysts were investigated using powder X-ray diffraction (XRD) using a D/max2550 V apparatus with a Cu-Kα radiation source (λ= Å). A JEM-2100 transmission electron microscope was used to obtain the TEM images. Raman

3 spectra were determined using a iuvia refl instrument with a laser source wavelength of 514 nm. The surface properties of the catalysts were determined via X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250Xi instrument with a Al-Kα radiation source. The XPS spectra were calibrated with respect to C-1s at the binding energy of ev. The X-ray absorption data (XAS) at the Ir L III edge of the samples, which were mixed with LiF to reach 50 mg, were recorded at room temperature in transmission mode using ion chambers using the BL14W1 beam line of the Shanghai Synchrotron Radiation Facility (SSRF), China. Benchmarking calculations The different factors involved in Fig. 2c are benchmarked using the Un-A sample. In detail, for crystallinity calculations, the background of the peaks in XRD patterns is considered and deducted by K α function. Although the Un-A and A-200 are amorphous, a weak (101) peak is identified in the pattern, so that the crystallinity of all samples are calculated based on the (101) intensity. For the particle size, surface area, ECSAs and the 5d occupation states, the index is calculated as wi w Un A Where, w i is the property value of the samples. w Un-A is the non-annealing case. Specific activity: The specific activity 2 of catalyst can be defined by specific current density, j s, which is calculated by dividing the current density per geometric area (j geo ) at a given overpotential by the roughness factor (RF) of the surface. The RF is calculated by taking the estimated ECSA and dividing by the geometric area of the electrode, 0.25 cm 2. Thus, js is given by fellow equation. j s jgeo ECSA / 0.25 Here, j geo is the current density (ma cm -2 ) at the η=0.350 V.

4 Figures and Tables Fig. S1. XRD patterns of prepared IrO 2 oxides under different annealing temperatures. The vertical line corresponds to the standard IrO 2 diffraction peaks with card number of

5 Fig. S2. TEM images of the prepared IrO 2, clear crystal particle observed under 400 C. Table S1. Surface area and particle size of different IrO 2 samples. Samples BET surface area (m 2 g -1 ) TEM particle size a (nm) Scherrer particle size b (nm) Un-A NA A NA A A A a: Average particle size. K D B b: Scherrer equation: ; K=0.89, λ=0.154 nm, B is the peak cos( ) FWHM, θ is the diffraction position.

6 Fig. S3. (a)-(g) are the normalized CVs of Un-A, A-200, A-400, A-500, A- 600, A-700 and A-800, respectively. (h) The calculated ECSAs of different IrO 2 samples by using the green rejoin in normalized CV curves. The current determined by CV method includes two parts, capacitor current (i c ) and Faradic current (i F ); i c is linear based on the scan rate determined by i c =C d ν. The parameter C d is crucial factor associated with electrochemical active sites. Thus, it can be normalized by i/ν= C d (constant) +i F /ν both obtaining the C d value and marked Faradic current to determine the onset potential of OER activity. The region between vertical orange lines is chosen to estimate C d value. ECSAs = C d /C s, where C s = mf cm -2 based on the reported value 2. The loading mass of iridium dioxides is 0.05 mg in all samples.

7 Fig. S4. Normalized Ir-L III edge XANES spectra for the prepared materials. The upper left inset corresponds to white line region and (b) is their white line intensity ratio relative to Un-A.

8 Fig.S5. The contribution of these five factors to the OER activity. (a)- crystallinity, (b)-bet surface area (current density are normalized by BET), (c)-ecsas (current density are normalized by ECSAs), (d)-particle size, (e)- 5d electrons occupation states. Firstly, it clearly shows that higher crystallinity led to the lower OER activity. The smaller sizes or larger surface areas are usually to pursue to enhance catalysts OER activity. BET surface area, TEM particle size and ECSAs are used for such evaluations. However, we not find higher OER activity corresponding to large surface area or smaller particle size. Comparatively, the relationship between the ECSAs of catalyst and its OER activity is more closely related to BET. The critical point is the electronic structure of the activated sites 10. We use the 5d occupation states to describe this key point because Ir-5d electrons determine its valence states. It can clearly find that the filling of the 5d occupation states also not agreement with the OER activity trend, but more electrons on 5d states seems better for OER activity.

9 Fig.S6. The Rietveld refinement of XRD patterns of prepared IrO 2 samples. The R-factors and errors are shown in each figure.

10 Fig.S7. (a)-(c) are the fitting results of double Ir-Ir shell in the range of Å. The k range is 2-14 Å. (d) The cell parameters of IrO 2 prepared at different annealing temperature. The variation of c/a ratio is consistent with XRD refinements.

11 Fig. S8. Fitting results of Ir-O shell for different annealing temperature prepared IrO 2 by using IFEFFIT. IrO 2 fitting parameters are a=b= Å, c= Å, space=p42/mnm, Ir-O 2-c = Å; Ir-O 4-c = Å.

12 Table S2. Results of fitted Ir-O shell in EXAFS by IFEFFIT. Sample Scattered Coordination Distan Δσ 2 (Å 2 ) R-factor path a number b ce (Å) Un-A Ir-O A-200 Ir-O A-400 Ir-O 2-c Ir-O 4-c c Ir-O 4-c A-600 Ir-O A-800 Ir-O 2-c Ir-O 4-c Notes: a and b indicate the scattered path and coordination number, for Un-A and A-200 are amorphous, so we think that their Ir-O coordination type just has only one.

13 Fig. S9. XRD patterns of transition metals doped IrO 2. (a) 3 The Co-doped, a- pyrolyzed-iro 2, b-pyrolyzed-ir 0.7 Co 0.3 O x, c-pyrolyzed-irco 2 O x, d-leached- Ir 0.7 Co 0.3 O x, and (e)-pyrolyzed-co 3 O 4. (b) 4 Ni and (c) 5 Cu-doped cases. The characteristic diffraction peaks of IrO2 are indicated in the figures. We can clearly find that the doping of Co, Ni and Cu give rise to a shift to a high angle in the diffraction peak positions with a Miller index (h k l) of l 0. Thus, the c axis is reduced according to the Bragg equation.

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