Single-Atom Au/NiFe Layered Double Hydroxide Electrocatalyst: Probing the Origin of Activity for Oxygen Evolution Reaction

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1 Single-Atom Au/NiFe Layered Double Hydroxide Electrocatalyst: Probing the Origin of Activity for Oxygen Evolution Reaction Jingfang Zhang,, Jieyu Liu,, Lifei Xi,, Yifu Yu, Ning Chen, Weichao Wang,,, * Kathrin M. Lange,,+ and Bin Zhang,, * Shuhui Sun, п Department of Chemistry, School of Science, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin , China Department of Electronics, Nankai University, Tianjin , China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin , China Canadian Light Source, Saskatoon, S7N 2V3, Canada п Institut National de la Recherche Scientifique-Énergie Matériaux et Télécommunications, Varennes, Quebec J3X 1S2, Canada Young Investigator Group Operando Characterization of Solar Fuel Materials (EE-NOC), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin 12489, Germany + Universität Bielefeld, Physikalische Chemie, Universitätsstr. 25, D Bielefeld, Germany S1

2 Supporting Information Materials Synthesis Preparation of NiFe LDH on Ti mesh. The NiFe LDH was prepared using a modified method which was reported by previous literature. 1 In a typical experiment, 0.77 mmol Ni(NO 3 ) 2 6H 2 O, 0.11 mmol Fe(NO 3 ) 3 9H 2 O, 4 mmol urea and 1.6 mmol NH 4 F were dissolved in 16 ml distilled water under vigorous stirring for 30 min, and then a cleaned Ti mesh substrate was immersed into above solution. Then the mixture was transferred into a 20 ml Teflon-lined autoclave and maintained at 120 ºC for 6 h. The system was allowed to cool down to room temperature, and the final product was washed with water and ethanol for several times and then dried in vacuum overnight for further characterizations. The mass loading of NiFe LDH on Ti mesh is determined by the mass differences before and after the growth of NiFe LDH. The typical mass loading is about 2 mg/cm 2. Preparation of single-atom Au supported on NiFe LDH/Ti mesh. Single-atom Au was supported on NiFe LDH by a simple electrodeposition method. In a typical procedure, a piece of NiFe LDH on Ti mesh was used as the working electrode. The electrolyte solution consists of 0.05 M NaCl and 0.3 mm HAuCl 4. Single-atom Au was deposited on NiFe LDH by stepping the potential to -0.6 V vs. SCE for 5 s, followed by stepping back to -0.2 V vs. SCE for 5 s for five cycles. Structural Characterization The scanning electron microscopy (SEM) images were taken with a Hitachi S-4800 scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with FEI Tecnai G2 F20 system equipped with GIF 863 Tridiem (Gatan). High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) was performed on JEOL ARM200F equipped with double aberration correctors and a cold S2

3 field emission gun. The X-ray diffraction (XRD) was recorded with a Bruker D8 Focus Diffraction System using a Cu Kα source (λ = nm). Raman spectra were recorded on a RENISHAW invia reflex Raman Microscope at excitation laser wavelength of 532 nm. Inductively coupled plasma mass spectrometry (ICP-MS) was carried out on an Agilent 7700x gas chromatograph equipped with an Auto sampler Injector. XAS test details X-ray absorption spectroscopy (XAS) of the metal edge was performed using the KMC2 beamline of the Helmholtz Zentrum Berlin (HZB) and the HXMA beamline of the Canadian Light Source (CLS). The Au L 3 -edge spectra were obtained in fluorescence mode using a Si drift detector (XFlash, Bruker) or a 32 Ge detector (Oxford). The Ni and Fe K-edge spectra were obtained in transmission mode. Reference metal foils (Au, Ni and Fe) were measured for energy calibration. The XAS data were processed with the ATHENA program. The Au XANES spectra were modeled using finite difference method for near-edge structure (FDMNES). Electrochemical Measurements Electrochemical measurements were carried out in a typical three-electrode cell consisting of a working electrode, a glassy carbon counter electrode, and a Hg/HgO (1 M KOH) reference electrode using an electrochemical workstation (CHI 660D, CH Instruments, Austin, TX). The single-atom Au/NiFe LDH on Ti mesh were directly used as the working electrode. All the loading mass of the catalysts on the Ti mesh was about 2 mg cm -2. All the potentials in the text, if not specified, were recorded relative to the reversible hydrogen electrode (vs. RHE) and the current density was normalized to the geometrical surface area. OER measurements were carried out in the presence of Ar-saturated 1 M KOH as electrolyte (The KOH electrolyte was purified prior to use according to a reported method, 2 and the electrochemical cell was S3

4 cleaned with H 2 SO 4 prior to each measurement.). Cyclic voltammetry (CV) curves were recorded at a scan rate of 5 mv s -1 with 95% ir-compensation. The potential E was calculated using the equation E = E measured ir, where E measured is the potential recorded vs. RHE, i is the current, and R is the ohmic drop tested by impedance spectroscopy. Chronopotentiometric and chronoampermetric measurements were obtained under the same experimental setup without compensating ir drop. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at 1.48 V (vs. RHE) from 1000 KHz to 0.1 Hz. The Faradaic efficiency was calculated by comparing the amount of gas theoretically calculated and experimentally measured. The gas experimentally generated from the water splitting was collected by water-gas displacing method. The theoretical amount of O 2 was calculated by applying the Faraday law. Calculation of ECSA The electrochemically active surface area (ECSA) was measured by CV at no apparent Faradaic potential range of V with different scan rates of 40, 60, 80, 100 and 120 mv s -1. By plotting the current density j ((j a -j c )/2) at 1.11 V against the scan rate, the linear slope is the double layer capacitance (C dl ). The ECSA was calculated by dividing C dl by the specific capacitance value for a flat standard with 1 cm 2 of real surface area. The specific capacitance for a flat surface is normally taken to be in the range of µf cm In this study, we assume 40 µf cm -2 for the calculation of ECSA. Turnover Frequency (TOF) Calculation Turnover frequency (TOF) was calculated according to the following equation (1): T F = 4 F (1) where j is the measured geometrical current density at a given overpotential of 280 mv, A is the surface area of the electrode, the number 4 represents four electron S4

5 transfer for per mole of O 2, F is the Faraday constant, and n is the number of moles of the Fe atom on the electrode. The Fe content was obtained from ICP-MS results. Computational Methods Density functional theory calculations plus Hubbard-U and dispersion interactions (i.e., van der Waals effects) (DFT + U + vdw) were carried out with the Perdew-Burke-Ernzenhof 4 (PBE) exchange correlation functional and projector-augmented wave (PAW) pseudopotentials 5, as implemented in the Vienna Ab initio Simulation Package (VASP) code. 6 An energy cutoff of 400 ev was applied for the plane-wave basis set. The effective U-J terms were fixed at 6.6 ev and 3.5 ev for Ni and Fe, respectively, as obtained within linear response theory on respective pure systems. 7 All calculations were spin polarized. To simulate the catalytic process on the surface, slab models with vacuum thickness larger than 16 Å were constructed. The Brillouin zone was sampled by a Γ-centered k-mesh of The convergence criterion for energy was 10 4 ev between two electronic steps. For structural optimization, the maximum force on each atom was less than 0.05 ev/å. The OER could occur in the following elementary steps in alkaline conditions: * + H - H * + e - (2) H * + H - * + H 2 + e - (3) * + H - H * + e - (4) H * + H - * H 2 + e - (5) where * stands for an active site on the surface, OH *, O *, OOH * are the adsorbed intermediates. S5

6 Figure S1. (a) Structural scheme of NiFe LDH (denoted as NiFeOOH-NiFe LDH). (b) The free energy diagram of OER over the surface of NiFeOOH-NiFe LDH model. A slab with NiFe LDH bottom layer and NiFeOOH stepped surface was constructed to simulate the reaction condition of NiFe LDH during OER process where only the top-most surface is oxidized to NiFeOOH. The corresponding simulation model is denoted as NiFeOOH-NiFe LDH. The rate-determining step is the second release of an electron with an overpotential of 0.26 V. Figure S2. (a) Proposed OER pathways with OH *, O * and OOH * intermediates on s Au/NiFeOOH model. (b) The free energy diagram for OER over s Au/NiFeOOH surface. A single Au atom is adsorbed on the top of the surface tetrahedral Ni atom neighboring to the Fe site with the Au-Ni distance of 2.57 Å. Au atoms become spontaneously negatively charged (-0.26 e) via electron transfer from Ni. The other two Ni cations are occupied by one water molecule respectively. Hydrogen bonds S6

7 develop among some of these solvent water molecules, the adsorbed oxygen-containing species, and the surface lattice oxygen atoms, making the slabs more stable. The first release of an electron originates from the adsorbed OH. An O-Au bond then forms between the O adatom and Au after the second release of an electron. Another OH - loses an electron to transform into an adsorbed OOH combining with the O adatom on the top of the exposed Fe site. The Au atom moves to the oxygen atom adjacent Ni atom spontaneously. After the forth electron release, O 2 forms and desorbs from the surface, thus allowing another solvent OH - to be adsorbed on the active site. The adsorption configurations of the OER intermediates are shown in Figure S2a. The single Au atom moves around the adsorption site during OER process. The OER activity of NiFeOOH attributes to the synergetic effect of Au, Fe, and Ni ions. The rate-limiting step is the O 2 formation, the corresponding overpotential is 0.23 V. Figure S3. (a) Photograph of NiFe LDH on Ti mesh. SEM image (b), TEM image (c), Raman spectrum (d) and XRD pattern (e) of NiFe LDH. As shown in Figure S3a, the as-prepared sample is homogeneously grown on Ti mesh. SEM and TEM images in Figure S3b, c reveal the formation of nanosheet arrays on a large scale. Raman spectrum (Figure S3d) and XRD pattern (Figure S3e) S7

8 further confirm the structure of NiFe LDH. Figure S4. SEM image (a), XRD pattern (b), and HAADF-STEM image (c) of s Au/NiFe LDH. The brighter white dots marked by the red circles in (c) represent the single Au atoms. Figure S5. Comparison of Au L 3 -edge XANES spectra simulated with the theoretical structure by adjusting Au-O bond distance between 1.88 and 2.06 Å along the crystallography c-axis direction. The progressive changing in the Au-O bond distance induces systematic XANES feature changing in terms of feature position, feature symmetry and feature relative intensity. The simulated spectrum with an Au-O bond distance of 1.88 Å could best reproduce the main features of the experimental spectrum. S8

9 Figure S6. Chronopotentiometry curves (without ir-compensation) of s Au/NiFe LDH and pure NiFe LDH at a constant current density of 50 ma cm -2. When biased galvanostatically at a constant current density of 50 ma cm -2, the s Au/NiFe LDH and NiFe LDH needed potentials of 1.55 V and ~1.61 V, respectively. The required lower potential for s Au/NiFe LDH catalyst indicates that s Au/NiFe LDH has higher activity than pure NiFe LDH. Figure S7. The current density (j) (left) and turnover frequency (TOF) (right) at an overpotential of 280 mv for s Au/NiFe LDH and pure NiFe LDH. The larger TOF (0.110 s -1 ) of s Au/NiFe LDH than that of NiFe LDH (0.018 s -1 ) is determined at the overpotential of 280 mv, suggesting a faster OER kinetic rate of s Au/NiFe LDH. S9

10 Figure S8. Typical CV curves of s Au/NiFe LDH (a) and NiFe LDH (b) in 1 M KOH with different scan rates (40, 60, 80, 100 and 120 mv s 1 ). (c) The current density at 1.11 V with respect to scan rate for s Au/NiFe LDH and NiFe LDH. The electrochemically active surface area (ECSA) is calculated on the basis of measured double-layer capacitance using CVs (Figure S8). By calculating the slope from the linear relationship of the current density against the scan rate (Figure S8c), the electrical double layer capacitor (C dl ) of s Au/NiFe LDH and NiFe LDH were confirmed to be 0.49 and 0.45 mf cm -2, respectively. As a result, the ECSAs of the s Au/NiFe LDH and NiFe LDH were calculated to be and cm 2, respectively. We also investigated mass and specific activities for s Au/NiFe LDH and NiFe LDH. The results are shown in Table S1. The mass activity of s Au/NiFe LDH is 64.9 A g -1 at an overpotential of 280 mv, which is ca. 6.0 times higher than NiFe LDH (10.8 A g -1 ). Meanwhile, the current densities normalized to ECSA were calculated to reflect their intrinsic activity. Consistently, the specific activity of s Au/NiFe LDH (10.6 ma cm -2 ) is about 5.6 times as large as that of NiFe LDH (1.9 ma cm -2 ). Above all, s Au/NiFe LDH exhibits a significantly higher catalytic activity compared with NiFe LDH. S10

11 Figure S9. (a) HAADF-STEM image of Au-15/NiFe LDH. (b) TEM image of Au-25/NiFe LDH. Figure S10. CV curves of three samples synthesized by different electrodeposition time (i.e. 5 cycles, 15 cycles, and 25 cycles) in 1 M KOH at a scan rate of 5 mv s -1. The s Au/NiFe LDH was electrodeposited by 5 cycles. In order to study the influence of Au loading amounts on the OER activity, we increased the electrodeposition time of Au on NiFe LDH, i.e. from primary 5 cycles (5 cycles was adopted to synthesize s Au/NiFe LDH) to 15 cycles (denoted as Au-15/NiFe LDH) and 25 cycles (denoted as Au-25/NiFe LDH). HAADF-STEM and TEM characterizations were performed on Au-15/NiFe LDH and Au-25/NiFe LDH to investigate their size and morphology, as shown in Figure S9. Figure S9a clearly displays the bright dots of nanoclusters with an average size of 1.4 nm (marked with red circles) in Au-15/NiFe LDH. Nanoparticles with an average size of 12.2 nm were S11

12 observed for Au-25/NiFe LDH (Figure S9b). It can be seen that the Au size become larger with increased Au loading amount. Then the OER activities of three samples were measured, as shown in Figure S10. There is no obvious difference in OER activities between s Au/NiFe LDH and Au-15/NiFe LDH, suggesting that Au single-atom and nanocluster take the similar responsibility for improving OER activity of NiFe LDH. When the electrodeposition time is increased from 15 to 25 cycles, the OER activity is decayed. These results suggest that s Au/NiFe LDH show the advantage not only in improving OER activity but also lowering Au usage. Figure S11. Tafel plots of s Au/NiFe LDH and NiFe LDH. The Tafel slope for s Au/NiFe LDH is 36 mv dec -1, which is much smaller than that of NiFe LDH (60 mv dec -1 ). Without Au, the Tafel slope is 60 mv dec -1, which is the theoretical value involving a chemical step subsequent to the first electron transfer step (the formation of O* from OH*). That is, the surface OH species is rearranged via a surface reaction. The Tafel slope decreases to 36 mv dec -1 after the decoration by single-atom Au, implying that the rate-limiting step changes to the second electron transfer step (the formation of OOH* from O*). S12

13 Figure S12. Nyquist plots at 1.48 V (vs. RHE.) for s Au/NiFe LDH and NiFe LDH. s Au/NiFe LDH has a smaller charge-transfer resistance at the interface between electrolyte and catalyst, indicating a faster charge-transfer rate of s Au/NiFe LDH. Figure S13. The amount of gas theoretically calculated and experimentally measured versus time for OER of s Au/NiFe LDH. Figure S14. HAADF-STEM image of s Au/NiFe LDH after a series of electrochemical measurements. S13

14 Figure S15. The Au L 3 -edge XANES spectra for as-prepared and post-oer s Au/NiFe LDH. The apparent similarity of Au L 3 -edge XANES spectra of as-prepared and post-oer s Au/NiFe LDH reveals the strong structural stability of single-atom Au in s Au/NiFe LDH during OER. Figure S16. Ex situ Ni (a) and Fe (b) K-edge XANES spectra of the as-prepared and post-oer s Au/NiFe LDH. The slightly shift of the absorption edge in the Ni K-edge spectrum to higher energy position indicates that Ni are partially oxidized after OER catalysis, which may be attributed to the transformation of NiFe LDH to NiFeOOH during OER. For Fe K-edge XANES, there is no noticeable energy shifts. Note that the shift amplitude of the absorption edge appeared in our results is less pronounced than that measured by in situ characterization in a previous report. 8 The discrepancies might come from S14

15 the fact that we did not measure the samples under applied potential, and the higher oxidation state of Ni and Fe were unstable and reduced to their corresponding balanced state, once the surfaces of s Au/NiFe LDH catalysts were exposed to moisture in ambient conditions. 9 The above results indicate that oxidation of NiFe LDH to NiFeOOH occurs during OER. Figure S17. Band structures of s Au/NiFeOOH-NiFe LDH with O *. The green lines show the total contribution of the system. Red points represent the contribution of (a)-(c) 2p orbitals of the oxygen atom which is connected with the adsorbed Au atom, (d) 6s orbitals and (e)-(i) 5d orbitals of the adsorbed Au atom. The point size denotes the contribution ratio. Blue dashed lines denote the Fermi level. S15

16 Figure S18. Schematic of p-d hybridization mechanism of Au and O. Except for the dz 2 orbital, all other d orbitals of Au are fully occupied by electrons (as in the atomic Au). dz 2 orbital of Au and p z orbital of O are hybridized and form energetically separated σ bonding state at -5.6 ev and σ* anti-bonding state at 0.2 ev (Figures S17-18). The other orbitals of Au and O have very weak hybridization. As shown in Figure S17, after the formation of σ bonding and σ* antibonding, electrons belong to dz 2 orbital of Au (2 e) and p z orbital of O (4/3 e) will be redistributed. 2 of the 10/3 electrons possess the low energy σ state, while, because of the high energy of σ* level, the other 4/3 electrons will transfer from σ* anti-bonding state to the not fully occupied low energy p x and p y states of O, which cause the electron density increase in the horizontal direction of O. The electron decrease of the upper and lower part of Au and O originates from the electron transformation from dz 2 orbital of Au and p z orbital of O to other orbitals. S16

17 Table S1 Comparison of OER activities of s Au/NiFe LDH with NiFe LDH at an overpotential of 280 mv in 1 M KOH. Specific Activity / ma cm -2 Materials Mass Activity / A g -1 a) b) j Geo j ECSA s Au/NiFe LDH NiFe LDH a) Data were calculated from geometric area; b) Data were determined by electrochemically active surface area (ECSA). Reference (1) Liu, X.; Wang, X.; Yuan, X.; Dong, W.; Huang, F. J. Mater. Chem. A 2016, 4, 167. (2) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. J. Am. Chem. Soc. 2014, 136, (3) Benck, J. D.; Chen, Z. B.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. ACS Catal., 2012, 2, (4) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, (5) P. E. Blöchl Phys. Rev. B 1994, 50, (6) (a) G. Kresse; Furthmiiller, J. Comp. Mater. Sci. 1996, 6, 15; (b) G. Kresse; J. Furthmüller Phys. Rev. B 1996, 54, (7) Cococcioni, M.; de Gironcoli, S. Phys. Rev. B 2005, 71, (8) Wang, D.; Zhou, J.; Hu, Y.; Yang, J.; Han, N.; Li, Y.; Sham, T.-K. J. Phys. Chem. C 2015, 119, (9) Dionigi, F.; Strasser, P. Adv. Energy Mater. 2016, 6, S17