Supporting Information

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1 Supporting Information Mo- and Fe-modified Ni(OH) 2 /NiOOH nanosheets as highly active and stable electrocatalysts for oxygen evolution reaction Yanshuo Jin a, Shangli Huang b, Xin Yue a, Hongyu Du c, Pei Kang Shen*,a,b a School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 51275, P. R. China. b Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University, Nanning, 534, P. R. China. c School of Physics, Sun Yat-sen University, Guangzhou 51275, P. R. China. These authors contributed equally. *pkshen@gxu.edu.cn ; stsspk@mail.sysu.edu.cn S1

2 Experimental Section Materials: Ammonium molybdate ((NH 4 ) 6 Mo 7 O 24 4H 2 O, AR), iron chloride (FeCl 3 6H 2 O, AR), nickel chloride (NiCl 2 6H 2 O, AR) and urea (CO(NH 2 ) 2, AR) were purchased from Tianjin Damao Chemical Reagent Factory. Nickel foams were purchased from Kunshan Jiayisheng Electronics Co., Ltd. Hydrochloric acid (HCl solution, 36-38%, AR) and potassium hydroxide (KOH, AR) were purchased from Guangzhou Chemical Reagent Factory. The deionized water was purified by the RO-DI system. Synthesis of Ni(OH) 2 /NiOOH nanosheets: 5 mg NiCl 2 6H 2 O, 5 mg CO(NH 2 ) 2 and 15 ml deionized water were transferred into a 5 ml hydrothermal synthesis reactor. Eight pieces of nickel foams (1. mm 1. mm 1. mm) after being immersed in 1% HCl solution and cleaned by deionized water were also transferred into the reactor. The reactor was maintained at 15 C for 12 hours. When cooled down to room temperature, these nickel foams with Ni(OH) 2 were took out from the reactor and cleaned with deionized water for several times. And then the electrochemical oxidation treatment was applied in a typical three-electrode setup in O 2 -saturated 1 M KOH solution at 25 C. An electrochemical oxidation treatment of chronoamperometry of 1.6 V vs. RHE was applied on a Bio-logic VMP3 electrochemical analyzer for one hour. The counter electrode is a graphite rod and the reference electrode is reversible hydrogen electrode (RHE). Synthesis of Fe:Ni(OH) 2 /NiOOH nanosheets: 3 mg FeCl 3 6H 2 O and 15 ml deionized water were transferred into a 5 ml hydrothermal synthesis reactor. Eight pieces of Ni foams with Ni(OH) 2 were also transferred into the reactor. The reactor was maintained at 15 C for 12 hours. When cooled down to room temperature, these Ni foams with Fe:Ni(OH) 2 were took out from the reactor and cleaned with deionized water for several times. And then the electrochemical oxidation treatment was applied in a typical three-electrode setup in O 2 -saturated 1 M KOH solution at 25 C. An electrochemical oxidation treatment of chronoamperometry of 1.6 V vs. RHE was applied on a Bio-logic VMP3 electrochemical analyzer for one hour. The counter electrode is a graphite rod and the reference electrode is reversible hydrogen electrode (RHE). Synthesis of Mo:Ni(OH) 2 /NiOOH nanosheets: 25 mg (NH 4 ) 6 Mo 7 O 24 4H 2 O and 15 ml deionized water were transferred into a 5 ml hydrothermal synthesis reactor. Eight pieces of Ni foams (1. mm 1. mm 1. mm) after being immersed in 1% HCl solution and cleaned by deionized water were also transferred into the reactor. The reactor was maintained at 15 C for 12 hours. When cooled down to room temperature, these Ni foams with Mo:Ni(OH) 2 were took out from the reactor and cleaned with deionized water for several times. And then the electrochemical oxidation treatment was applied in a typical three-electrode setup in O 2 -saturated 1 M KOH solution at 25 C. An electrochemical oxidation treatment of chronoamperometry of 1.6 V vs. RHE was applied on a Bio-logic VMP3 electrochemical analyzer for one hour. The counter electrode is a graphite rod and the reference electrode is reversible hydrogen electrode (RHE). S2

3 Synthesis of MoFe:Ni(OH) 2 /NiOOH nanosheets: 3 mg FeCl 3 6H 2 O and 15 ml deionized water were transferred into a 5 ml hydrothermal ynthesis reactor. Eight pieces of Ni foams with Mo:Ni(OH) 2 were also transferred into the reactor. The reactor was maintained at 15 C for 12 hours. When cooled down to room temperature, these Ni foams with MoFe:Ni(OH) 2 were took out from the reactor and cleaned with deionized water for several times. And then the electrochemical oxidation treatment was applied in a typical three-electrode setup in O 2 -saturated 1 M KOH solution at 25 C. An electrochemical oxidation treatment of chronoamperometry of 1.6 V vs. RHE was applied on a Bio-logic VMP3 electrochemical analyzer for one hour. The counter electrode is a graphite rod and the reference electrode is reversible hydrogen electrode (RHE). Characterizations: SEM measurements were performed on a Quanta 4/INCA/HKL scanning electron microscope. XPS measurements were performed on an ESCALAB 25 spectroscopy. Before XRD measurements and TEM measurements, material samples were achieved by long-time and high-powered ultrasonic treatment. XRD data were acquired on a D-MAX 22 VPC diffractometer with Cu Ka radiation (λ=1.5456å). TEM measurements were carried out on a Tecnai G2 F3 electron microscopy. Raman data were acquired on a Renishaw invia spectrometer. Electrochemical measurements: Electrochemical measurements were conducted on a Bio-logic VMP3 electrochemical analyzer. The OER electrochemical measurements were tested in a typical three-electrode setup. The counter electrode is a graphite rod and the reference electrode is reversible hydrogen electrode (RHE). Both polarization curves and CV curves at a scan rate of 1 mv s -1 were measured in O 2 -saturated 1 M KOH solution at 25 C. The stability tests were conducted using chronopotentiometric measurements. All of the potentials are ir-corrected. Computation methods: In this work, the first-principle calculations of density functional theory plus Hubbard U (DFT+U) were performed on Vienna ab initio simulation package(vasp) code, with a 3x3x1 k-point Monkhorst-Pack mesh for the first Brilliouin Zone and plane wave basic set with the kinetics cutoff energy of 4 ev. Methfessel-Paxton smearing of order 2 with a value of sigma.2 ev was applied to accelerate convergence. The core electronics were described by the projector augmented wave methods. For better discription of Fe, Ni, Mo of transition-metal electrons, the Hubbard U term, fixed Ueff(Fe)=4.eV, Ueff(Ni)=6.eV, Ueff(Mo)=3.5eV, were added to generalized gradient approximation(gga) with Perdew-Burke-Ernzerhof(PBE) functional by the rotational invariant approach methods by Dudarev et al. Spin-polarized were considered in all calculations. The atom positions were fully relaxation until the Hellman-Feynman force acting on each atom is less than.5 ev/å. The initial model was adopted by Alexis T. Bell previous reported. The doped model of Mo:NiOOH was constructed by substitute one Ni atom by one Mo atom. The doped model of Fe:NiOOH was constructed by substitute one Ni atom by one Fe atom. The doped model of MoFe:NiOOH was constructed by substitute two Ni atoms by one Fe atom and one Mo atom. The formation energy was calculated as follow, E f = E(M:NiOOH) - E(NiOOH) + Δn Ni μ Ni Δn M μ M. S3

4 where, E(M:NiOOH) and E(NiOOH) are the total energies of the supercell with and without metal dopants, respectively, Δn is the number of ions (Mo, Fe or Ni) replaced in the system, and μ is the chemical potential of Mo or Ni. Previous computational studies show the volcano-type curve towards OER activity. The data of NiOOH and Fe:NiOOH are acquired from Alexis T. Bell s work. Based on their method, our calculations revealed that MoFe:NiOOH has the best adsorption energies for OER intermediates and a concomitant increase in OER activity. Gibbs free energy was calculated ( G) by this equation: G = E + E ZPE - T S. Where E is directly determined by analyzing total energy as above described. T is system temperature (298.15K, in this work). E ZPE is difference between the adsorbed state and gas phase, and it calculated by summing vibrational frequency for all norm model. The OER is processed in following four electron pathway: OH - + * = OH* + e - (1) OH* + OH - = O* + H 2 O + e - (2) OH - + O* = OOH* + e - (3) OH - + OOH* = O 2 + H 2 O + e - (4) The theoretical overpotential η obtained from Gibbs free energy differences G i (i=1,2,3,4), as follow, η = max [ G 1, G 2, G 3, G 4 ]/e-1.23[v] For MoFe:NiOOH, G 1 = 1.64 ev; G 2 = 1.63 ev; G 3 = 1.43 ev; G 4 =.22 ev; η =.41V. S4

5 Table S1. Comparisons of OER performance for various 3d-metal-based electrocatalysts in alkaline condition (η 1 : overpotential at the current density of 1 ma cm -2 ; η 1 : overpotential at the current density of 1 ma cm -2 ). Catalysts η 1 (mv) η 1 (mv) Electrolyte Ref. MoFe:Ni(OH) 2 /NiOOH M KOH This work NiCo-LDH 33 1 M KOH 1 3DGN/CoAl-LDH 31 1 M KOH 2 CoMn-LDH M KOH 3 W.5 Co.4 Fe.1 /NF M KOH 4 NiFe-LDH/CFP 26 1 M KOH 5 Co 3 O SC 43 1 M KOH 6 Co 3 S 4 nanosheet M KOH 7 Ni-Co oxides layers M NaOH 8 FeOOH/CeO M NaOH 9 Ni-Fe-OH@Ni 3 S 2 /NF 3 1 M KOH 1 NiFe/NF 37 1 M KOH 11 CoMnCH 34 1 M KOH 12 S5

6 Table S2. The calculated data of the dopant formation energies. M (metal) E(NiOOH)/ ev E(M:NiOOH)/ ev μ Mo /ev μ Fe /ev μ Ni /ev E f /ev Mo Fe Mo-Fe S6

7 a b Intensity / a.u. MoFe:Ni(OH) 2 Intensity / a.u. MoFe:NiOOH/Ni(OH) Raman shift / cm Raman shift / cm -1 Figure S1. Raman spectra of a) MoFe:Ni(OH) 2 and b) MoFe:Ni(OH) 2 /NiOOH nanosheets. S7

8 Figure S2. SEM images of MoFe:Ni(OH) 2 /NiOOH nanosheets. S8

9 Figure S3. TEM images of MoFe:Ni(OH) 2 nanosheets. S9

10 Figure S4. TEM images of MoFe:Ni(OH) 2 /NiOOH nanosheets. S1

11 MoFe:Ni(OH) 2 Intensity / a.u Lattice Spacing / nm Figure S5. HRTEM images of MoFe:Ni(OH) 2 nanosheets. S11

12 Figure S6. TEM images of Ni(OH) 2 /NiOOH nanosheets. S12

13 Figure S7. TEM images of Mo:Ni(OH) 2 /NiOOH nanosheets. S13

14 S14

15 MoFe:NiOOH/Ni(OH) 2 Ni Intensity / a.u. Cu Ni Cu O Mo Ni Fe Mo K Cu Energy / kev Figure S9. EDX spectrum of MoFe:Ni(OH) 2 /NiOOH nanosheets. S15

16 Current density / ma cm Mo:NiOOH/Ni(OH) -1 2 NiOOH/Ni(OH) 2 MoO Potential / V vs. RHE Figure S1. CV curves of Mo:Ni(OH) 2 /NiOOH, Ni(OH) 2 /NiOOH and commercial MoO 3. S16

17 MoO 3 Intensity / a.u. (2) (111) (11) (6) (4) (21) (2) (12) (81) (62) θ θ / Figure S11. XRD pattern of commercial MoO 3. S17

18 Current density / ma cm Mo:NiOOH/Ni(OH) 2 MoFe:NiOOH/Ni(OH) Potential / V vs. RHE Figure S12. CV curves of Mo:Ni(OH) 2 /NiOOH and MoFe:Ni(OH) 2 /NiOOH. S18

19 Mo 3d Intensity / a.u. Mo Binding Energy / ev Figure S13. XPS spectra of Mo 3d for MoFe:Ni(OH) 2 /NiOOH. S19

20 5 NiOOH/Ni(OH) 2 4 Mo:NiOOH/Ni(OH) 2 Fe:NiOOH/Ni(OH) 2 -Z'' / ohm 3 2 MoFe:NiOOH/Ni(OH) Z' / ohm Figure S14.The Nyquist plot of Ni(OH) 2 /NiOOH, Mo:Ni(OH) 2 /NiOOH, Fe: Ni(OH) 2 /NiOOH and MoFe: Ni(OH) 2 /NiOOH. at η=3 mv. S2

21 Current density / ma cm Potential / V vs. RHE 1 mv/s 2 mv/s 3 mv/s 4 mv/s 5 mv/s J(1.41V vs. RHE) / ma cm Scan rate / mv s -1 NiOOH/Ni(OH) F cm -2 Figure S15. CV curves measured from 1 to 5 mv s -1 and corresponding j geo vs. scan rates plots of Ni(OH) 2 /NiOOH. S21

22 Current density / ma cm Potential / V vs. RHE 1 mv/s 2 mv/s 3 mv/s 4 mv/s 5 mv/s J(1.41V vs. RHE) / ma cm Mo:NiOOH/Ni(OH) 2.95 F cm Scan rate / mv s -1 Figure S16. CV curves measured from 1 to 5 mv s -1 and corresponding j geo vs. scan rates plots of Mo:Ni(OH) 2 /NiOOH. S22

23 W V B B S 2 Current density / ma cm Potential / V vs. RHE 1 mv/s 2 mv/s 3 mv/s 4 mv/s 5 mv/s J(1.41V vs. RHE) / ma cm Fe:NiOOH/Ni(OH) F cm Scan rate / mv s -1 Figure S17. CV curves measured from 1 to 5 mv s -1 and corresponding j geo vs. scan rates plots of Fe:Ni(OH) 2 /NiOOH. S23

24 . Current density / ma cm Potential / V vs. RHE 1 mv/s 2 mv/s 3 mv/s 4 mv/s 5 mv/s J(1.41V vs. RHE) / ma cm MoFe:NiOOH/Ni(OH) 2.97 F cm Scan rate / mv s -1 Figure S18. CV curves measured from 1 to 5 mv s -1 and corresponding j geo vs. scan rates plots of MoFe:Ni(OH) 2 /NiOOH. S24

25 Current Density / ma cm Potential / V vs. RHE Figure S19. Polarization curve of Ni foam in 1M KOH for OER. Current density / ma cm Potential / V vs. RHE 1 mv/s 2 mv/s 3 mv/s 4 mv/s 5 mv/s J(1.41V vs. RHE) / ma cm Ni foam.1 F cm Scan rate / mv s -1 Figure S2. CV curves measured from 1 to 5 mv s -1 and corresponding j geo vs. scan rates plots of Ni foam. 1. Amount of gas (mmol) Measured O 2 Caculated O Time (min) Figure S21. The amount of O2 theoretically calculated and experimentally measured versus time for OER of the MoFe:Ni(OH) 2 /NiOOH. The currenrt density is 1 ma cm-2 for 6 minutes. S25

26 Intensity / a.u θ θ / Figure S22. XRD pattern of MoFe:Ni(OH) 2 /NiOOH nanosheets after the chronopotentiometric reaction. Intensity / a.u Raman shift / cm -1 Figure S23. Raman spectrum of MoFe:Ni(OH) 2 /NiOOH nanosheets after the chronopotentiometric reaction. Figure S24. SEM image of MoFe:Ni(OH) 2 /NiOOH nanosheets after the chronopotentiometric reaction. S26

27 2 nm 1 nm 5 nm Figure S25. TEM images of MoFe:Ni(OH)2/NiOOH nanosheets after the chronopotentiometric reaction. Figure S26. HAADF STEM images and the corresponding EDX mappings of MoFe:Ni(OH)2/NiOOH. Intensity / a.u. Ni Cu Ni O Cu Si Mo Ni Fe 2 4 Mo Cu K Energy / kev Figure S27. EDX spectrum of MoFe:Ni(OH)2/NiOOH nanosheets after the chronopotentiometric reaction. S27

28 a Current Density / ma cm Potential / V vs. RHE b -Z'' / ohm Z' / ohm Figure S28. Polarization curve and the Nyquist plot of MoFe:Ni(OH) 2 /NiOOH after the chronopotentiometric reaction. Current Density / ma cm before after Potential / V vs. RHE Figure S29. Polarization curves of MoFe:Ni(OH) 2 /NiOOH before and after the chronopotentiometric reaction. S28

29 Figure S3. Model structure of MoFe:NiOOH for OER activity trend. The model of MoFe:NiOOH was constructed by substitute two Ni atoms by one Fe atom and one Mo atom. S29

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