Achieving Stable and Efficient Water Oxidation by Incorporating NiFe. Layered Double Hydroxide Nanoparticles into Aligned Carbon.

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1 Electronic Supplementary Material (ESI) for Nanoscale Horizons. This journal is The Royal Society of Chemistry 2015 Achieving Stable and Efficient Water Oxidation by Incorporating NiFe Layered Double Hydroxide Nanoparticles into Aligned Carbon Nanotubes Rong Chen, 1 Gengzhi Sun, 1,2 Cangjie Yang, 1 Liping Zhang, 1 Jianwei Miao, 1 Huabing Tao, 1 Hongbin Yang, 1 Jiazang Chen, 1 Peng Chen 1 * and Bin Liu 1 * 1 School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, , Singapore. 2 Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing , China. ChenPeng@ntu.edu.sg (P. Chen) and liubin@ntu.edu.sg (B. Liu) These authors contributed equally. Experimental Section Synthesis of NiFe-LDH nanoparticles All chemicals were purchased from Sigma-Aldrich and used without further purification. The NiFe-LDH nanoparticles were prepared through fast co-precipitation of Ni 2+ and Fe 3+ followed by sonication at room temperature. Briefly, a mixed solution containing 1.5 mmol of metal ions with different ratios of Ni(NO 3 ) 2 to Fe(NO 3 ) 3 (different metal cation ratios were studied to optimize the composition of NiFe-LDH for OER (Figure S3 & S4), marked as 10 mol% 40 mol% Fe in NiFe- LDH) were quickly added into 20 ml of NaOH aqueous solution (0.15 M) under vigorous stirring. The precipitate was then centrifuged and washed three times with deionized (DI) water. Subsequently, after sonicating over night at room temperature, a stable homogenous suspension was obtained. The homogenous feature was attributable to the positive zeta potential of ~ 32 mv. The suspension was stable in DI water, ethanol or dimethylformamide (DMF). For comparison,

2 hydrothermal treatments were also carried out at 100 o C to 120 o C for 24 h instead of sonication at room temperature. Materials characterization The morphology and structure of as-obtained catalysts were studied with field-emission scanning electron microscopy (FESEM, JEOL, JSM6700F) and transmission electron microscopy (TEM, JEOL 2100F). X-ray diffraction (XRD) patterns were recorded on Bruker AXS D8 Advance with Cu Kα radiation ( = Å). Detailed compositional information was probed by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific) at mbar using a monochromatic Al Kα X-ray beam ( ev). All binding energies were referenced to the C 1s peak ( ev) arising from adventitious hydrocarbons. Particle size distribution and zeta potential were determined by dynamic light scattering (DLS, Zeta PALS) using BI-200SM (Brookhaven, USA). Electrochemical measurement Electrochemical measurements were performed in a standard three-electrode cell using CHI 760D electrochemical workstation. Pt plate was used as the counter electrode and a saturated calomel electrode (in saturated KCl) as the reference electrode. All potentials were calculated with respect to reversible hydrogen electrode (RHE) based on: E(RHE) = E(Hg/HgO) ph V. The overpotential (η) was calculated by η(v) = E(RHE) V. The working electrode was prepared by loading 3 µl of the catalyst suspension onto glassy carbon electrode with area of cm 2 (catalyst loading: 0.05 mg/cm 2 ). The catalyst suspension was prepared by mixing 980 µl of 3.5 mg/ml NiFe- LDH suspension with 20 µl of 5 wt% Nafion solution under sonication for 1 h. To construct a

3 microfiber electrode, well-dispersed NiFe-LDH nanoparticles in DMF were drop-casted onto a multi-walled carbon nanotube (MWCNT) sheet on a polytetrafluoroethylene (PTFE) substrate, which was grown by a chemical vapor deposition method as described previously. 1 Subsequently after drying in vacuum, the NiFe-LDH@MWCNT sheet was peeled off and twisted into a fiber at a twisting speed of 200 rpm in 3 min. As MWCNT sheet is very thin and porous, NiFe-LDH nanoparticles can easily penetrate into the sheet. The NiFe-LDH nanoparticles will be located both inside and outside of the MWCNT bundles. During electrochemical testing, the working electrode was cycled at a rate of 20 mv/s until achieving stable cyclic voltammetry scans. The current density was calculated by dividing as-measured current with the geometric area of the microfiber electrode (A = Dh, where D and h are the diameter and length of the microfiber electrode, respectively). All electrochemical tests were performed in 1 M NaOH electrolyte, which was purged by O 2 for at least 30 min before data collection. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were recorded at a scan rate of 1 mv/s. AC impedance spectroscopy was acquired in a frequency range from 100 khz to 0.1 Hz at an amplitude of 10 mv. Faraday efficiency was measured using volumetric method as described before. 2

4 Figure S1. FESEM images of NiFe-LDH nanoparticles prepared under different conditions: (a) without sonication, (b) sonication at RT, (c) hydrothermal at 100, and (d) hydrothermal at 120. (e) XRD patterns, and (f) polarization curves of NiFe-LDH nanoparticles prepared at different temperatures (performances are tested on RDE electrode).

5 Figure S2. Photographs of NiFe-LDH nanoparticle suspension before and after sonication.

6 Figure S3. FESEM images of NiFe-LDH nanoparticles with different Fe contents. (a) 10 mol % Fe. (b) 20 mol % Fe. (c) 30 mol % Fe. (d) 40 mol % Fe. Insets: corresponding photographs of NiFe-LDH suspensions.

7 Figure S4. Polarization curves of NiFe-LDH with different Fe contents tested on RDE electrode. The percentage number represents mol % of Fe in NiFe-LDH.

8 Figure S5. Mass activity as a function of the loading amount of NiFe-LDH nanoparticles on glassy carbon electrode.

9 Figure S6. Polarization curves of microfiber electrode with varying NiFe- LDH composition from 50% to 95%. Inset shows the surface morphology of 95% NiFe- microfiber electrode. Deteriorating catalytic performance of 95% NiFe- microfiber electrode was due to poor conductivity of too thick NiFe LDH layer. The percentage number represents wt % of NiFe-LDH in the microfiber electrode.

10 Figure S7. Polarization curves of 90 wt % NiFe-LDH@MWCNT microfiber electrode before and after ir correction.

11 Figure S8. AC impedance spectrum of electrode made of several parallel microfibers at an overpotential of 195 mv. Inset shows a photograph of the electrode.

12 Figure S9. XPS spectra of NiFe-LDH before and after 24 h of electrolysis: (a) wide-scan spectra, (b) high resolution Ni 2P spectra, (c) high resolution Fe 2P spectra, and (d) high resolution O 1s spectra.

13 Table 1. Comparison of OER performance of NiFe-LDH in alkaline electrolyte. References Tafel slope (mv/dec) 10 ma/cm 2 (mv) η@ 30 ma/cm 2 (mv) Loading amount (mg/cm 2 ) Electrode Experimental conditions This work _ 0.05 NiFe-LDH nanoparticles on glassy carbon electrode 1 M NaOH This work _ NiFe-LDH@MWCNT microfiber electrode 1 M NaOH Ma, W.; et al. ACS Nano 2015, 9, _ 0.25 Alternately stacked NiFe-LDH nanosheets & Graphene on glassy carbon electrode 1 M KOH Lu, X.; et al. Nat. Commun. 2015, ~ 250 _ Amorphous NiFe hydroxide electrodeposited on Ni foam 1 M KOH Long, X.; et al. Angew. Chem. Int. Ed ~ FeNi-rGO LDH on Ni foam 1 M KOH 2014, 53, Song, F.; Hu, X. Nat. Commun. 2014, _ 0.07 Exfoliated NiFe-LDH nanosheets on glassy carbon electrode 1 M KOH Gong, M.; et al. J. Am. Chem. Soc. 2013, 135, ~ NiFe-LDH/CNT on glassy carbon electrode 1 M KOH Note: Ni foam is a three-dimensional conductive support, with thickness typically in the range of millimeters, which is much thicker than that of our microfiber electrode.

14 Movie S1. Evolution of oxygen bubbles at a current of 13.5 ma with single NiFe-LDH@MWCNT microfiber electrode. Movie S2. Evolution of oxygen bubbles at a current of 21 ma with NiFe-LDH@MWCNT microfiber (color: black) woven into fabrics (color: white). References: 1 L. X. Zheng, G. Z. Sun and Z. Y. Zhan, Small, 2010, 6, R. Chen, H.-Y. Wang, J. Miao, H. Yang and B. Liu, Nano Energy, 2015, 11,