Supporting Information

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1 Supporting Information SrNb 0.1 Co 0.7 Fe 0.2 O 3 d Perovskite as a Next-Generation Electrocatalyst for Oxygen Evolution in Alkaline Solution** Yinlong Zhu, Wei Zhou, Zhi-Gang Chen, Yubo Chen, Chao Su, Moses O. TadØ, and Zongping Shao* ange_ _sm_miscellaneous_information.pdf

2 Supporting information Experimental Section Catalysts synthesis. SrNb0.1Co0.7Fe0.2O3-δ (SNCF) powder was synthesised using a traditional solid-state reaction route. Briefly, stoichiometric amounts of SrCO3, Nb2O5, Co2O3 and Fe2O3 (all analytical grade, Sinopharm Chemical Reagent Co., Ltd.) were weighed, mixed and milled in a planetary mill (Fritsch, Pulverisette 6) using acetone as a solvent medium at 400 rpm for 1 h. After drying, the powder mixtures were calcined at 1200 C in air for 10 h to form the perovskite oxides. SNCF-BM powders were obtained by further ball-milling the calcined SNCF powders using ethanol as a solvent medium at 400 rpm for 1 h in a planetary mill (Fritsch, Pulverisette 6). The other perovskite catalysts used in this study for comparison, namely SrCo0.8Fe0.2O3-δ (SCF), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), PrBaCo2O5+δ (PBC), and LaNiO3-δ (LN), were synthesiszed by a standard combined EDTA citrate complexing sol-gel process. [1] The solid precursors provided by this method were calcined as follows: 1) in air at 1000 C for 5 h for SCF, 2) in air at 1000 C for 5 h for BSCF, 2) 1050 C for 5 h for PBC, and 3) 800 C for 5 h for LN. The commercial catalysts IrO2 and LiCoO2 were purchased from Aladdin Co., Ltd. Physicochemical characterization. The obtained samples were characterized by room temperature (RT) powder diffraction for phase identification and to assess phase purity. A diffractometer (Rigaku Smartlab, Cu Kα radiation, λ = Å) in Bragg-Brentano reflection geometry was used. The diffraction patterns were recorded by continuous scanning in the 2θ range of with an interval of Structure refinements of the XRD patterns were carried out using DIFFRACplus Topas 4.2 software. [2] During the refinements, general parameters such as the scale factor, background parameters, and the zero point of the counter were optimized. Le Bail refinement was used initially to determine the space group and to approximate the lattice parameters of the SNCF. Rietveld refinement was then performed to determine the general position of the atoms. The chemical compositions and surface element states were determined by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe spectrometer equipped with an Al Kα X-ray source). The specific surface areas of the catalysts were obtained with a Brunauer Emmet Teller (BET) analysis system with a N2 adsorptive medium. Fourier transform infrared (FT-IR) spectra were measured using a PerkinElmer Spectrum 100. Transmission electron microscopy (TEM) was conducted at 200 kv with a Philips Tecnai T30F field emission instrument equipped with a 2k-CCD camera. Mössbauer spectroscopy of the SNCF was recorded using a conventional constant acceleration spectrometer (OXFORD-MS500) with a gray source of 25 mci 57 Co in a palladium matrix moving at room temperature. The absorber was kept static in a temperature-controllable cryostat. All isomer shifts are quoted relative to a-fe at room temperature. The Mçssbauer spectrum was fitted using the MOSSWINN 3.0 program. The morphologies of the catalysts were observed using an environmental scanning electron microscope (ESEM, QUANTA-2000). The oxygen desorption properties were probed using oxygen temperature programmed desorption (O2-TPD). For a typical O2-TPD measurement, approximately 0.15 g of

3 each sample was loaded in a U-type quartz tube. The samples were pretreated by heating to 250 C with a heating rate of 10 C min -1 under pure Ar, and kept at this temperature for 2 h. After cooling the sample to room temperature, adsorption of O2 was carried out in flowing O2 for 1 hour. The gas was then switched to pure Ar until the baseline was stable. After these processes finished, the O2-TPD measurement was performed up to 700 C with a heating rate of 10 C min -1 in flowing pure Ar (15 ml min -1 ). The effluent gases were analysed by a mass spectrometer (MS, Hiden, QIC-20) to monitor the oxygen concentration. Electrode preparation and electrochemical characterization. Electrochemical measurements were performed at room temperature using a rotating disk working electrode made of glassy carbon (PINE, 5 mm diameter, cm 2 ) connected to a PARSTAT 2273 (Princeton Applied Research) advanced electrochemical system. The glassy carbon (GC) electrode was pre-polished with 50 nm α-al2o3 slurries on a polishing cloth and sonicated in ethanol for 5 min. The electrodes were finally rinsed with deionized water and dried before each test. A Pt foil and Ag/AgCl (3.5 M KCl) were used as the counter and reference electrodes, respectively. The potentials reported in our work are referenced to the reversible hydrogen electrode (RHE), calibrated as described below. The preparation method of the working electrodes containing the investigated catalysts was stated as follows. To remove any electrode conductivity limitations present in the thin film electrodes, all the catalysts were mixed with as-received conductive carbon (Super P Li) at a mass ratio of 1:1. Briefly, the electrocatalyst suspensions were prepared by sonication of a mixture of oxide (10 mg), conductive carbon (10 mg), Nafion solution (5 wt%, 100 μl) and ethanol (1 ml) for at least 1 h to generate a homogeneous ink. Next, a 5 μl aliquot of as-prepared catalyst ink was dropped on the surface of the RDE, yielding an approximate catalyst loading of mgtotal cm -2 (0.232 mgcat cm -2 ) and left to dry for the OER tests. The electrolyte was 0.1 M KOH aqueous solution (99.99% metal purity, ph ~12.6), which was saturated with O2 for ~30 min prior to each test and maintained under O2 atmosphere throughout. Linear sweeping voltammograms (LSVs) were performed RDE at 1600 rpm in O2-saturated 0.1 M KOH solution at a scan rate of 5 mv s -1 from 0.2 to 1.0 V versus Ag AgCl (3.5 M KCl). All potential values are ir-corrected to compensate for the effect of solution resistance, which were calculated by the following equation: EiR-corrected = E - ir, where i is the current, and R is the uncompensated ohmic electrolyte resistance (~45 Ω) measured via high frequency ac impedance in O2-saturated 0.1 M KOH. The accelerated stability tests were performed in O2-saturated 0.1 M KOH by potential cycling between 0.2 and 1 V versus Ag/AgCl (3.5 M KCl) at a scan rate of 100 mv/s for 1000 cycles. At the end of the cycles, the resulting electrodes were used for LSVs at a scan rate of 5 mv/s. The EIS data were fitted by an equivalent circuit consisting of an uncompensated solution resistance (Rs), a charge-transfer resistance (Rct), a pseudocapacitive element (Cp), and a constant-phase element (CPE), as shown below:

4 Calculation method. Details concerning the calculation of mass activity (MA) and specific activity (SA) are shown below. The values of mass activity (A g -1 ox) were calculated from the oxide catalyst loading m (0.232 mgcat cm -2 GEO,) and the measured current density J (ma cm -2 GEO) at η = 0.5 V by the following equation: mass activity = J/m. The values of specific activity (ma cm -2 ox) were calculated from the BET surface area SBET (m 2 g -1 ), oxide catalyst loading m (0.232 mgcat cm -2 GEO), and the measured current density J (ma cm -2 GEO) at η = 0.5 V by the following equation: specific activity = J/(10 m SBET). RHE Calibration. In all measurements we used Ag/AgCl (3.5 M KCl) as the reference electrode, which was calibrated with respect to RHE. The calibration was performed in a high purity hydrogen-saturated electrolyte with a platinum rotating disk electrode (PINE, 4 mm diameter, cm 2 ) as the working electrode. Cyclic voltammetry (CV) was run at a scan rate of 1 mv s -1, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reaction (Figure S2). In 0.1 M KOH solution, ERHE = EAg/AgCl V. Figure S1. XRD patterns of SNCF and SNCF-BM samples; the inset is a magnified section of XRD patterns, 2θ =

5 Figure S2. Potential calibration of the reference electrode in 0.1 M KOH solution. In this case, the potentials showed were calculated by the following equation: V RHE = V Ag/AgCl V. Figure S3. OER linear sweeping voltammograms (LSVs) on rotating disk electrode for other recent well-studied materials with high OER activity in O 2-saturated 0.1 M KOH solution at 1600 rpm., such as PrBaCo 2O 5-δ (PBC) with double perovskite, LiCoO 2 with spinel structure and LaNiO 3-δ (LN) with rhombohedral perovskite structure. These catalysts were tested under the same condition as SCNF, ruling out the influence of different electrode preparation details and electrode composition varied in literatures.

6 Figure S4. XRD patterns of the BSCF, SCF, PBC, LN and LiCoO 2 powders. Figure S5. Nitrogen adsorption-desorption isotherm curves of (a) SNCF, (b) BSCF, (c) IrO2 and (d) SNCF-BM catalysts; the inset shows the corresponding BJH pore size distribution plot.

7 Figure S6. SEM images of (a) SNCF and (b) SNCF-BM powders; the insert is a magnified image. catalysts Table S1. Comparison of OER Activity Data for Different Catalysts. η@ J = 10 ma cm -2 (V) Tafel slope (mv dec -1 ) Specific (ma cm -2 ox) Mass (A g -1 ox) SNCF BSCF IrO SNCF-BM Table S2. Oxygen non-stoichiometry (δ) and average valence states of Co&Fe (n) of SNCF. [3] Sample δ n SNCF

8 Figure S7. Mössbauer spectrum of SNCF. Profile Isomer shift δ (mm/s) Table S3. Mössbauer parameters of SNCF. Quadrupole splitting Qs (mm/s) Half-peak width FWHM (mm/s) Area Ratio Fe Fe References 1. Z. P. Shao, S. M. Haile, Nature 2004, 431, DIFFRACPlus TOPAS 4.2 software, Bruker-AXS GmbH, Karlsruhe, Germany, Y. L. Zhu, J. Sunarso, W. Zhou, S. S. Jiang, Z. P. Shao, J. Mater. Chem. A 2014, 2,