Supporting Information

Transcription

1 Supporting Information Defect-Rich 2D Material Networks for Advanced Oxygen Evolution Catalysts Bowei Zhang, Zhiyuan Qi, # Zishan Wu, Yu Hui Lui, Tae-Hoon Kim, # Xiaohui Tang, Lin Zhou, # Wenyu Huang, # and Shan Hu* Department of Mechanical Engineering and Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States # Ames National Laboratory, U.S. Department of Energy, Ames, Iowa 50011, United States Department of Chemistry and Energy Sciences Institute, Yale University, New Haven, Connecticut 06511, United States Corresponding to: shanhu@iastate.edu 1

2 CONTENTS EXPERIMENTAL METHODS...3 Figure S1 Photographs of (a) cleaned nickel foam, (b) Co-MOF, and (c) D-U-Co(OH) Figure S2 SEM images of Co-MOF nanosheet arrays grown on nickel foam....5 Figure S3 XRD pattern of ZIF-L-Co MOF formed on (a) nickel foam support and (b) in the solution....6 Figure S4 HAADF-STEM EDX elemental mapping of Co-MOF....6 Figure S5 Photographs of electrodes....7 Figure S6 SEM image of the Co-MOF-derived nanosheet arrays....7 Figure S7 HAADF-STEM image of the Co-MOF-derived ultrathin nanosheet-networks Table S1 Crystal information used for indexing of Co(OH) Figure S10 High-resolution HAADF-STEM image of D-U-Co(OH) 2 and the corresponding FFT pattern Figure S11 SEM images of T-Co(OH) Figure S12 XRD patterns of D-U-Co(OH) 2 and the T-Co(OH) 2 powders Figure S13 (a-b) TEM images of the T-Co(OH) Supplementary discussion Figure S14 Chemical composition analysis of D-U-Co(OH) Figure S15 Characterizations of D-U-Co(OH) 2 after the OER stability test Figure S16 Tafel slopes of D-U-Co(OH) 2 and T-Co(OH) Figure S17 Nyquist plots of D-U-Co(OH) 2 and T-Co(OH) 2 measured at the potential of 1.55 V vs RHE Figure S18 (a) Polarization plots of D-U-Co(OH) 2 recorded in the purified (Fe-free) 1M KOH, unpurified 1M KOH, and the 2 mm Fe + unpurified 1M KOH electrolytes; respectively. (b) The LSV plots of D-U-Co(OH) 2 and the electrodeposited CoO xh y in purified 1M KOH media. All the plots were recorded without applying ir-correction. Inset SEM image in (b) shows the morphology of electrodeposited CoO xh y on Ni foam Figure S19 TEM image and the FFT patterns of the ultrathin nano-polycrystalline CoNi hydroxide Figure S20 STEM-EDX spectrum of the ultrathin nano-polycrystalline CoNi hydroxide Figure S21 The polarization curve of the defect-rich ultrathin CoNi hydroxide (D-U-CoNi-OH) in 1 M KOH before and after a 90% ir-correction Figure S22 Tafel slope of D-U-CoNi-OH during OER, which is about 57 mv dec Figure S23 Stability of D-U-CoNi-OH toward OER Figure S24 Polarization plots of D-U-CoNi-OH recorded in the purified 1M KOH, unpurified 1M KOH, and 2 mm Fe + unpurified 1M KOH electrolytes, respectively, without applying ir correction Figure S25 SEM images of Co-MOF nanoarrays grown on carbon cloth Figure S26 SEM images of Co-MOF nanoarrays grown on titanium mesh Figure S27 Photograph of the Co-MOF grown on glass beaker wall Table S2 Comparison of the state-of-the-art Co oxides/hydroxides-based electrocatalysts for OER Table S3 Comparison of mass activity@η = 300 mv

3 EXPERIMENTAL METHODS Materials and Chemicals. Nickel foam (thickness: 1.6 mm, bulk density: 0.45 g cm -3 ), commercial RuO2 powder, 2-methylimidazole (99%), urea (98%), and cobalt chloride hexahydrate (> 97 %) were provided by Sigma-Aldrich. Nickel nitrate hexahydrate (99.99%), iron nitrate nonahydrate (99.99%), titanium mesh, carbon cloth, and potassium hydroxide (Fe < 0.001%) were purchased from Fisher Scientific. All chemicals used in this work are of analytical grade. The water used through this work was purified by Milli-Q system (resistivity > 18 MΩ cm). Synthesis of Co-MOF Nanoarrays. An aqueous solution containing 2-methylimidazole (C4H6N2, 40 ml, 0.4 M) was added into the aqueous solution of CoCl2 6H2O (40 ml, 0.05 M), and then a piece of clean Ni foam was immersed into the mixture solution. After reaction for 12 h without stirring, the sample was then taken out, washed by deionized water, and vacuum dried overnight. Synthesis D-U-Co(OH)2 and D-U-CoNi-OH Nano-networks. A piece of Co-MOF@Ni foam was etched by immersing into an ethanol solution (100 ml) containing cobalt chloride hexahydrate (0.6 g) with stirring for 30 min. The obtained D-U-Co(OH)2 sample was then taken out, washed with ethanol, and dried in air. Decreasing the reaction time to 10 min will result in insufficient etching. The D-U-CoNi-OH sample was synthesized according to a similar method with above via substituting the Co 2+ in ethanol by Ni 2+ (nickel nitrate hexahydrate, Fisher Scientific) during the etching process. Purification of 1M KOH electrolyte. To prepare cobalt hydroxide powder, 10 mmol urea and 4 mmol CoCl2 6H2O were dissolved in 40 ml water. The solution was then transferred into a 50 ml Teflon-lined stainless-steel autoclave and kept at 120 for 6 h. After that the cobalt hydroxide powder was collected by centrifugal method and then dried in vacuum oven at 60 for 6 h. The obtained cobalt powder (~ 0.5 g) was dispersed in 50 ml 1M KOH electrolyte and kept for 2 days. After that the cobalt hydroxide was removed from the electrolyte by centrifugal method. Note that all the vessels were washed by hydrochloride acid before use. Electrodeposition of cobalt hydroxide on Ni foam. The cobalt hydroxide was prepared by galvanostatic electrolysis in solution of 0.02 M Co M NH4NO3 at 2 ma cm -2 at room-temperature on Ni foam and then dried at 60 C for 6 h in vacuum oven to remove the excessive water. 3

4 Materials Characterization. XRD tests were conducted using a Siemens D500 X-ray diffractometer under Cu Kα radiation. Morphologies were examined by a FEI Quanta 250 field-emission scanning electron microscopy (FE-SEM) and FEI Titan Themis 300 Cubed probe aberration corrected high-angleannular-dark-field scanning transmission electron microscopy (HAADF-STEM). Energy-dispersive X- ray (EDX) mappings were taken under a scanning TEM model. Samples for STEM were prepared by drop-drying their diluted ethanol suspensions onto carbon-coated copper grids. Chemical compositions were analyzed by an X-ray photoelectron spectroscopy (XPS, AMICUS ESCA 3400) with Mg as excitation source. Atomic Force Microscopy (Bruker Dimension Icon AFM) characterization was conducted under a scanning-assisted mode. Electrochemical Measurements. Electrochemical tests of linear scan voltammetry (LSV), cyclic voltammetry (CV), and chronopotentiometry were conducted on a Gamry Reference 1000 potentiostat using a three-electrode setup in an argon-saturated 1M KOH solution. The electrolyte used through this work is unpurified 1M KOH solution unless otherwise noted. A calibrated Hg/HgO (1M NaOH) and Pt wire were used as the reference and counter electrodes, respectively. LSV were performed at a scan rate of 5 mv s -1 in the range of 1.61 V to 0.8 V vs. RHE. RuO2 ink was prepared by dispersing 10 mg commercial RuO2 powder into the mixture solution of 950 µl ethanol + 50 µl Nafion, and then coated onto Ni foam with a mass loading of about 0.7 mg cm -2 (the same with D-U-Co(OH)2) and dried in vacuum oven at 60. Nyquist plots were conducted from Hz to 0.1 Hz at a potential of 1.55 V vs. RHE. Calculations of Intrinsic Activity, Turnover Frequency (TOF), and Mass Activity. The Intrinsic activity is achieved via normalizing the polarization curve using the corresponding electrochemical surface area. 46 TOF value is calculated according to the equation: TOF = j A, where j is the current density; 40,47 A is the geometric area of the electrode; F is the Faraday constant (96485 C mol -1 ); n = m/m, n is the number of moles of the active materials, m is the mass of active material, and M is molecular weight. The total mass of active catalysts is used for the mass activity calculations in this work. 4Fn 4

5 Figure S1 Photographs of (a) cleaned nickel foam, (b) Co-MOF, and (c) D-U-Co(OH)2. Figure S2 SEM images of Co-MOF nanosheet arrays grown on nickel foam. 5

6 Figure S3 XRD pattern of ZIF-L-Co MOF formed on (a) nickel foam support and (b) in the solution. The three strong peaks between 40 and 80 degrees are indexed to metallic Ni substrate. Figure S4 HAADF-STEM EDX elemental mapping of Co-MOF. Note: the oxygen is from the carbon-coated copper grid rather the CO-MOF because the oxygen dots are homogeneously distributed in the whole image. 6

7 Figure S5 Photographs of electrodes. (a) Directly immerse Ni foam into the mix solution of Co 2+ and 2-methylimidazole for 6 hours. (b) Nickel foam was initially immersed into the water solution of 2-methylimidazole for overnight and then immersed into the mix solution of Co 2+ and 2- methylimidazole for another 6 hours. The concentrations of Co 2+ / 2-methylimidazole in the two solutions are the same, respectively. Figure S6 SEM image of the Co-MOF-derived nanosheet arrays with rough surface by in situ etching at room-temperature. 7

8 Figure S7 HAADF-STEM image of the Co-MOF-derived ultrathin nanosheet-networks. The red circles show the curly edge of the nanosheets. The yellow lines show the more reliable thickness of the nanosheets. Figure S8 Atomic Force Microscopy (AFM) images and the height profile of the ultrathin nanosheets. The nanosheets were peeled off from the nickel foam substrate by sonication. 8

9 Figure S9 (a) SEM image and (b) the corresponding EDX spectra for the Co-MOF after etching for only 10 min. Note: The Iridium (Ir) is from the conductive coating sputtered on the surface and the Ni element peak is from the Ni foam substrate. Table S1 Crystal information used for indexing of Co(OH)2. Co-H-O Phase: Co(OH) 2 published crystallographic data ( J. Magn. Magn. Mater. (1996) 154, L7-L11.) Space group p-3m1(164) Cell parameters a= 0.317, b=0.317, c= nm, α = 90, β=90, γ=120 V = nm 3, a/b = 1., b/c = 0.682, c/a = Standardized crystallographic data Space group p-3m1(164) Cell parameters a= 0.317, b=0.317, c= nm, α = 90, β=90, γ=120 V = nm 3, a/b = 1., b/c = 0.682, c/a = Site Elements Wyck. Sym. x y z Atom coordination from prototype entry O1 O 2d 3m. 1/3 2/ Co1 Co 1a -3m Niggli-reduced cell Cell parameters a= 0.317, b=0.317, c= nm, α = 90, β=90, γ=120 V = nm 3, a/b = 1, b/c = 0.682, c/a =

10 Growth of Co-MOF nanosheets on Ni foam. Immersing the cleaned Ni foam into a mixed aqueous solution of CoCl 2 6H 2O (0.05 M) and 2-methylimidazole (0.4 M) for 12 h at room temperature without stirring. (Co 2+ : 2-methylimidazole = 1:8) A solubility equilibrium exists in the aqueous solution: Co Cl + 4(N NH) Co(N N) 2 + 2(NH NH) + Cl (1) To guarantee the formation of Co(N-N) 2 (Co-MOF), the molar ratio of Co 2+ : 2-methylimidazole should be less than 1: 4. The ratio of Co 2+ : 2-methylimidazole is 1: 8 in this work, the H + produced during the formation process of Co- MOF is therefore consumed by the excessive 2-methilymidazole in the solution. Co (N NH) Co(N N) 2 + 2H + (2) (N-NH) + Cl + H + (NH NH) + Cl (3) Note: (N N) represents the deprotonated 2-methlymidazole: (N-NH) represents the 2-methlymidazole: (NH NH) + represents the pronated 2-methlymidazole: 10

11 Figure S10 High-resolution HAADF-STEM image of D-U-Co(OH)2 and the corresponding FFT pattern. The randomly oriented grain is less than 5 nm in size. Figure S11 SEM images of T-Co(OH)2 control sample, which was synthesized by immersing Co-MOF into 1M KOH electrolyte with stirring. The thickness of the T-Co(OH)2 nanosheets is between 60 ~ 200 nm. 11

12 Figure S12 XRD patterns of D-U-Co(OH)2 and the T-Co(OH)2 powders. Figure S13 (a-b) TEM images of the T-Co(OH)2.The inset image in (b) is the corresponding FFT pattern. The nanosheets were peeled off from the Ni foam substrate by sonication. Supplementary discussion: The red circles in the inserted FFT pattern index well to Co(OH)2. The yellows dots can be assigned to CoOxHy, which might be aroused by the high-voltage beam during HR-TEM characterization. 12

13 Figure S14 Chemical composition analysis of D-U-Co(OH)2. (a) XPS survey spectra of D-U- Co(OH)2. High resolution XPS spectra for (b) Co 2p and (c) O 1s of D-U-Co(OH)2. a b c Figure S15 Characterizations of D-U-Co(OH)2 after the OER stability test. (a-b) SEM images and (c) STEM images. Inset in (c) is the corresponding FFT pattern. 13

14 Figure S16 Tafel slopes of D-U-Co(OH)2 and T-Co(OH)2. The two electrodes have similar Tafel slopes in the range of 131 ~ 137 mv dec -1. Figure S17 Nyquist plots of D-U-Co(OH)2 and T-Co(OH)2 measured at the potential of 1.55 V vs RHE. Inset is the equivalent circuit. The Nyquist plots suggest that both D-U-Co(OH)2 and T-Co(OH)2 have very small resistance and thus excellent conductivity, which can be attributed to the close integration with conductive 3D Ni foam substrate. 14

15 Figure S18 (a) Polarization plots of D-U-Co(OH)2 recorded in the purified (Fe-free) 1M KOH, unpurified 1M KOH, and the 2 mm Fe + unpurified 1M KOH electrolytes; respectively. (b) The LSV plots of D-U-Co(OH)2 and the electrodeposited CoOxHy in purified 1M KOH media. All the plots were recorded without applying ir-correction. Inset SEM image in (b) shows the morphology of electrodeposited CoOxHy on Ni foam. Figure S19 TEM image and the corresponding FFT patterns of the ultrathin nano-polycrystalline CoNi hydroxide. The different FFT pattern from different area demonstrate its nanopolycrystalline nature. 15

16 Figure S20 STEM-EDX spectrum of the ultrathin nano-polycrystalline CoNi hydroxide. The peaks of Cu are originated form the copper grid. Figure S21 The polarization curve of the defect-rich ultrathin CoNi hydroxide (D-U-CoNi-OH) in 1 M KOH before and after a 90% ir-correction. The mass loading of D-U-CoNi-OH is only 0.27 mg cm

17 Figure S22 Tafel slope of D-U-CoNi-OH during OER, which is about 57 mv dec -1. Figure S23 Stability of D-U-CoNi-OH toward OER. The stability was demonstrated by comparing the initial polarization curve and a curve recorded after a long-time test at a constant j of 10 ma cm -2 for over 20 hours (left). The constant potential tests were conducted at 1.5 V and 1.6 V, respectively, in a purified 1M KOH (right). 17

18 Figure S24 Polarization plots of D-U-CoNi-OH recorded in the purified 1M KOH, unpurified 1M KOH, and 2 mm Fe + unpurified 1M KOH electrolytes, respectively, without applying ir correction. Figure S25 SEM images of Co-MOF nanoarrays grown on carbon cloth. 18

19 Figure S26 SEM images of Co-MOF nanoarrays grown on titanium mesh. Figure S27 Photographs of the Co-MOF grown on glass beaker wall. 19

20 Table S2 Comparison of the state-of-the-art Co oxides/hydroxides-based electrocatalysts for OER in alkaline medium (1M K/NaOH). (j: current density; η: overpotential). Catalyst Mass loading (mg cm -2 ) η (mv)@10 ma cm -2 η = 300 mv TOF (s -1 ) Reference 0.014@ η = D-U-Co(OH) ± mv This work 0.092@ η = D-U-CoNi-OH 0.27 ± mv ~ 0.004@ η = Exfoliated Co-Co LDH mv Nat. Commun. 5 (2014) @ η = Exfoliated Ni-Co LDH mv Nano Lett. 2015, 15, Exfoliated NiCo LDH @ η = 300 Adv. Mater. 2016, 28, 3DGN/single layer CoAl-NS mv Exfoliated Co(OH) s-co(oh)2/swnts LDH FeCo/GCE @ η = 300 mv Gelled FeCo/GCE @ η = 300 mv Gelled FeCoW/GCE 0.46@ η = mv 0.21 LDH FeCo/Gold foam Gelled FeCo/Gold foam Adv. Energy Mater. 2018, Science 2016, 352, 333. Gelled FeCoW/Gold foam CoMn LDH after AC @η = 300 mv Co(OH) ~ 0 -- α-co(oh) ~ 0 -- α-co(oh)2-cl @ η = 320 mv Co(OH)x-NCNT Co(OH)2@NCNTs@NF NiFe-LDH/CNT/CFP J. Am. Chem. Soc. 2014, 136,16481 ACS Appl. Mater. Interfaces 2017, 9, J. Mater. Chem. A 2017, 5, J. Mater. Chem. A 2016, 4, ACS Appl. Mater. Interfaces 2016, 8, Nano Energy 47 (2018) 96. J. Am. Chem. Soc. 2013, 135,

21 CoFe LDHs CoFe LDHs-Ar Amorphous NiCo2.7(OH)x η = 300 mv 4.78@ η = 300 mv 0.18@ η = 350 mv Co-Fe LDH/rGO Co3O4/N-rmGO N-CG-CoO ~ CoOx@CN/NF CC@NiCo2O Co-Mo-O NSs@NF Angew. Chem. Int. Ed. 2017, 56, Adv. Energy Mater. 2015, 5, Adv. Mater. Interfaces 2016, 3, Nat. Mater. 2011, 10, 780. Energy Environ. Sci. 2014, 7, 609. J. Am. Chem. Soc. 2015, 137, Adv. Energy Mater. 2017, 7, Nano Energy 45 (2018)

22 Table S3 Comparison of mass = 300 mv with the state-of-the-art Co oxides/hydroxidesbased electrocatalysts for OER in alkaline medium (1M K/NaOH). Catalyst Mass loading (mg cm -2 ) j = 300 mv Mass activity (A g -1 ) D-U-Co(OH) ± D-U-CoNi-OH 0.27 ± Reference This work ACS Appl. Mater. Co(OH)x-NCNT Interfaces 2016, 8, Adv. Energy Mater. 2018, Exfoliated Co(OH) Exfoliated NiCo LDH Nano Lett. 2015, 15, NiCo2.7(OH)x amorphous nanocages Exfoliated Co-Co LDH s-co(oh)2/swnts Ni0.75Fe0.25OOH 117 Gelled FeCo Annealed FeCoW 428 LDH FeCo/GCE CoMn LDH α-co(oh)2-cl Exfoliated Ni-Co LDH CoFe LDHs Co-Fe LDH/rGO Ag NW@Co NS Adv. Energy Mater. 2015, 5, Nat. Commun. 5 (2014): Adv. Energy Mater. 2018, Science 2016, 352, 333. J. Am. Chem. Soc., 2014, 136, J. Mater. Chem. A, 2016,4, Nat. Commun. 5 (2014): Angew. Chem. Int. Ed. 2017, 56, Adv. Mater. Interfaces 2016, 3, Dalton Transactions 45 (2016) Co(OH)2@NCNTs@NF Nano Energy 47 (2018) 96. 3DGN/single layer CoAl- NS Adv. Mater. 2016, 28, Co3O4/N-rmGO Nat. Mater. 2011, 10, 780. N-CG-CoO ~ CoOx@CN/NF CC@NiCo2O Energy Environ. Sci. 2014, 7, 609. J. Am. Chem. Soc. 2015, 137, Adv. Energy Mater. 2017, 7,