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Iron Hydroxide-Modified Nickel Hydroxylphosphate Single-Wall Nanotubes as Efficient Electrocatalysts for Oxygen Evolution Reactions Wei Bian, Yichao Huang, Xiaobin Xu, Muhammad Aizaz Ud Din, Gang Xie, and Xun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18875 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Materials & Interfaces

Iron Hydroxide-Modified Nickel Hydroxylphosphate Single-Wall Nanotubes as Efficient Electrocatalysts for Oxygen Evolution Reactions Wei Bian,†, ‡ Yichao Huang,† Xiaobin Xu,† Muhammad Aizaz Ud Din,† Gang Xie,‡ and Xun Wang*,† † Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China. ‡ College of Chemistry and Materials Science, Northwest University, Xi’an, 710069, China KEYWORDS nickel hydroxylphosphate/iron hydroxide • single-wall nanotubes • synergistic effect • controllable synthesis • oxygen evolution reaction

ABSTRACT: Developing efficient electrocatalysts for the oxygen evolution reaction (OER) is of great significance for the future renewable energy applications. Herein, efficient OER electrocatalysts based on iron hydroxide-modified nickel hydroxylphosphate (NiPO/Fe(OH)x) single-wall nanotubes (SWNTs) have been prepared by a facile stepwise surfactant-free solvothermal strategy, which possess diameters of about 6 nanometers and lengths of about several micrometers. Benefitting from the synergistic effect between iron hydroxides and NiPO SWNTs, the as-prepared NiPO/Fe(OH)x SWNTs exhibit higher OER activity than primary NiPO SWNTs. Furthermore, the OER activity with different Fe contents display a volcano-type shape,

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and the optimized NiPO/Fe(OH)x SWNTs persent excellent activity with a low overpotential of 248 mV to deliver current density of 10 mA cm-2, and 323 mV to achieve a large current density of 100 mA cm-2, as well as a remarkably low Tafel slope of 45.4 mV dec-1 in 1 M KOH electrolyte. The present work provides valuable insights to improve OER performance by rationally surface modification.

INTRODUCTION Electrocatalytic water splitting has been considered to be a promising technology for the future renewable energy applications.1,2 However, oxygen evolution reaction (OER) was an important bottleneck for water splitting, since the overpotential loss of OER is usually higher than that of the hydrogen evolution reaction (HER).3 Noble metals, such as iridium and ruthenium oxides, have been proved to be highly efficient OER electrocatalysts. Nevertheless, the high cost and low abundance greatly hinder their large-scale applications.4,5 Alternatively, a great number of first-row transition metal (TM) compounds were used as electrocatalysts for OER, including their hydroxides,6-9 oxides

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phosphates,16-19 phosphides20-22 and borates,23-26 owing to their

low cost, earth abundance, and stability in alkaline solution.27-33 Among them, non-precious nickel and iron-based compounds presented the most active electrocatalytic activity and thus aroused great research interests.34-38 In addition, the structure of electrocatalysts has a great influence on performance, single-wall nanotubes have been demonstrated to exhibit extraordinary performance in a wide range of applications, which was attributed to their unique one-dimensional structures, large specific surface area and high exposure of active defects.39 However, complex synthesis process and undesirable oxygen evolution properties restrict the application of the nickel hydroxylphosphate single-wall nanotubes.19 Therefore, seeking a facile


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synthesis method of nickel hydroxylphosphate single-wall nanotubes and greatly improving the OER performance are very meaningful. Furthermore, Fe played an important role in enhancing the activity for the Ni-based OER electrocatalysts. Recent studies indicated that nickel-iron layer double hydroxide (LDH) exhibited improvement for OER activity than corresponding single-metal hydroxide.37,40 Similarly, the iron inclusion in Ni-Fe oxide41 and oxyhydroxide42 electrodes have further demonstrated that iron was the key to water oxidation, which can not only improve the electrical conductivity but also influence their Ni(OH)2/NiOOH redox characteristic and give a dramatic increase in activity of NiOOH, leading to decrease OER overpotential. Besides, an electrodeposition method was applied to prepare iron-doped nickel phosphate on nickel foam (NF), which presented enhancement of OER performance than primary nickel phosphate due to the synergistic effect between nickel phosphate formation and iron incorporation.43 However, the inevitable aggregation and operational complexity hinder their applications. Therefore, developing a facile and moderate strategy to fabricate well-defined iron species modified nickel hydroxylphosphate single-wall nanotubes as efficient electrocatalyst remains a big challenge. Herein, we reported a facile stepwise surfactant-free solvothermal strategy to synthesize iron hydroxides-modified nickel hydroxylphosphate (NiPO/Fe(OH)x) single-wall nanotubes. The morphology, composition and electrocatalytic property prior to and after iron hydroxides deposited onto the NiPO SWNTs were investigated in detail. As the ultrathin 1D structure exposed more active sites and amorphous iron hydroxides coating, resulting in the activation of Ni(OH)2/NiOOH redox. The as-obtained NiPO/Fe(OH)x SWNTs exhibited greatly enhancement of OER performance. Furthermore, our results demonstrated that the OER activity of NiPO/Fe(OH)x SWNTs with different Fe contents presented a volcano-type shape. The


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optimized NiPO/Fe(OH)x-30 SWNTs catalyst (with a molar ratio of Ni/Fe=1.62) exhibited the highest OER activity with an overpotential of 248 mV at a current density of 10 mA cm-2 and 323 mV to achieve 100 mA cm-2, as well as a low Tafel slope of 45.4 mV decade-1, which was better than that of benchmark IrO2 catalyst (ca.50 mVdec-1).44 EXPERIMENTAL SECTION Materials. Nickel chloride hexahydrate (NiCl2·6H2O), H3PO4 (wt, 85%), Iron chloride hexahydrate (FeCl3·6H2O), dimethylformamide (DMF) and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Iridium dioxide (IrO2) was purchased from Shanghai Macklin Biochemical Co., Ltd. All the chemicals were used without further purification. The water (18 MΩ·cm) used in all experiments was prepared by passing through an ultra-pure purification system (Aqua Solutions). Synthesis of NiPO SWNTs. In a typical procedure, NiPO SWNTs were synthesized as follows, nickel chloride hexahydrate (NiCl2·6H2O, 0.1 mmol) was added to a 10 ml capacity Teflon-lined autoclave. Then 5 ml DMF and 2 ml deionized water were added in sequence. After stirring for 15 min, 0.1 mmol H3PO4 (wt, 85%) was further added to the above-mentioned solution, and stirring for another 15 min. The autoclave was sealed and heated at 140 °C for 32 h. After cooling to room temperature, the products were collected via centrifugation at 6000 rpm for 4 min and further washed with ethyl alcohol 3 times. The palegreen precipitate was dried in vacuum drying oven at 30 ℃ overnight.

Synthesis of NiPO/Fe(OH)x-30 and NiPO/Fe(OH)x-X SWNTs. The as-prepared NiPO SWNTs 25 mg were dispersed in the 3 ml deionized water, 30 umol FeCl3·6H2O were dissolved in the 4 ml deionized water keeping ultrasound for 5 min, adding to the above-mentioned


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solution. Then using sodium hydroxide adjusted pH to 4-5 when stirring. The resulting suspension was transferred to 10 mL Teflon-lined stainless-steel autoclaves and then heated at 60 °C for 4 h. Finally, the mixture was washed via centrifugation at 6000 rpm for 4 min with ethyl alcohol for three times and then faint yellow precipitate was dried in the vacuum drying oven at 30 °C overnight. For the synthesis of NiPO/Fe(OH)x-X (X=10, 20, 40, 50) nanotubes, the process was similar to NiPO/Fe(OH)x except using the amount of FeCl3·6H2O 10 umol, 20 umol, 40 umol, 50 umol, respectively. Characterization. TEM images were taken with a HITACHI H-7700. HRTEM (highresolution transmission electron microscopy) images, and energy-disperse X-ray (EDS) spectra were taken with a Tecnai G2 F20 S-Twinhigh-resolution transmission electron microscope at 200 KV equipped with HAADF-STEM. Powder X-ray diffraction (PXRD) pattern was carried on a BrukerD8 Advance using Cu Kα radiation (λ= 1.5418 Å). XPS were recorded on a PHI Quantera SXM spectrometer using monochromatic Al Kα X-ray sources (1486.6 eV). Fouriertransformed infrared resonance (FT-IR) spectra were performed in transmission mode on a Perkin-Elmer Spectrum 100 spectrometer (Waltham, MA, USA). Electrochemical studies were carried out on a CHI660E B15057 electrochemistry workstation (CH Instruments, Inc., Shanghai). ICP-OES was measured on ICAP 6300 (Thermo Fisher corporation). Electrochemical measurements. In a typical sample preparation, the products were dried by the vacuum drier at 30 °C for several hours. Afterward, 5 mg products and 1 mg carbon (Vulcan XC-72) were dispersed in 600 ul isopropanol and 400 ul deionized water and sonicated for 30 min. Then 8 µL dispersion was dropped on the glassy carbon electrode (5 mm in diameter) with a surface area of 0.196 cm2, which was used as the working electrode with a loading of 0.2 mg cm-2 catalysts. After the electrode surface was dried, 2 µL nafion@ethanol (nafion: 0.5%wt) was


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dropped to cover the electrode surface. For the fabrication of IrO2/C working electrode, only changing products with 5 mg commercial powdered IrO2, other processes remain unchanged. The electrochemical tests were performed in a three-electrode electrochemical cell (Pine Instruments) using a Pt wire and a saturated calomel electrode (SCE) as a counter electrode and the reference electrode, respectively. All the electrochemical tests were operated in O2 presaturated 1 M KOH electrolyte on a CHI660E B15057 electrochemistry workstation. All potentials measured were calibrated to the reversible hydrogen electrode (RHE) using the following equation: E(RHE) = E(Hg/HgCl2)+ 0.059pH +0.2415. For oxygen evolution reaction (OER) tests, first of all, the electrochemical accessibility of the working electrode was activated by potential cycling between 1.1 and 1.6 V (vs. RHE) at 100 mV s-1 in 1 M KOH until stable voltammogram curves were obtained, and the current density was calculated on the basis of the projected area, which is the area of the glassy carbon electrode (0.196 cm2). Then, the polarization curves and Tafel plots were recorded at scan rates of 5 mV s-1 and 1 mV s-1, respectively. All the potentials and voltages are 95% IR corrected unless noted. The stability tests were performed by chronoamperometry for 4 hours. Electrochemical impedance spectra (EIS) experiments were performed with the three-electrode cell system in 1M KOH at an open circuit voltage with frequency from 0.1 to 100,000 Hz. The electrochemical double-layer capacitance was determined from the CV curves measured in a potential range according to the following equation: Cdl = Ic/ν, where Cdl, Ic, and ν are the double-layer capacitance (mF cm-2) of the electroactive materials, charging current (mA cm-2), and scan rate (mV s-1), respectively. RESULTS AND DISCUSSION Scheme 1 The synthesis strategy of iron hydroxide-modified nickel hydroxylphosphate single-wall nanotubes.


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The facile two-step synthesis of NiPO/Fe(OH)x SWNTs is schematically shown in Scheme 1. NiPO SWNTs were first synthesized in DMF/H2O mixed solvents (Scheme 1a). The morphology of the NiPO SWNTs was characterized by transmission electron microscopy (TEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The structure of pure NiPO SWNTs was verified by TEM images (Figure 1a) and HAADF-STEM image (Figure 1b). TEM and STEM images (Figure S1 and Figure S5d) indicated that the obtained nanotubes are highly uniform with a length of about 5 micrometers. A higher magnification TEM image (inset in Figure 1a) reveals that the diameter of nanotube was about 5 nanometers. X-ray diffraction (XRD) pattern (Figure S3) indicated that


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Figure 1. (a) TEM image and (b) STEM image of NiPO SWNTs, the inset of (a) is the high magnification image. (c) EDX spectrum of NiPO SWNTs (the signal of Cu and carbon derive from the copper grid substrate). (d) EDX linear scanning for Ni, P and O along the line as indicated in (b). (e). STEM image and EDX elemental maps of a bundle of NiPO SWNTs.

the NiPO SWNTs existed in a low crystal-linity state due to the ultrathin nanostructure. The components were further analyzed by the energy-dispersive X-ray spectroscopy (EDX) spectrum (Figure 1c) and inductively coupled plasma (ICP) optical emission spectrometry (Table S1), which showed that the ratio of Ni and P was about 1.5. Furthermore, the Energy dispersive spectroscopy linear scanning data (Figure 1d) and Energy-dispersive X-ray spectroscopy (EDX) mapping of a bunch of SWNTs demonstrated that Ni (green), P (light blue), and O (saffron yellow) were distributed uniformly (Figure 1e). Otherwise, by analyzing the result of TGA curves, we found that the mass of the samples decreased by 30%. The rapid decease nearly 20 wt% at approximately 230°C may come from the removal of absorbed water molecules. As for the second step, with probably 10 wt% weight loss can be ascribed to the decomposition of the


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hydroxyl from 230°C -600°C. Thus the molar ratio between phosphate and hydroxyl was 1:1 (Figure S4), the formula of this hydroxylphosphate was given of Ni2(OH)(PO4).45 To better understand the formation process of NiPO SWNTs, time-dependent experiments were carried out (Figure S5). Interestingly, only bulk sphere could be obtained in 30 min (Figure S5a). A lot of short nanotubes could be found on the surface of the sphere when the reaction time reached 5 hours (Figure S5b). Eventually, most of the big sphere disappeared and purity of the sample was almost entirely nanotubes after 32h (Figure S5c and S5d). Based on the structural evolution, it can be inferred that the single-wall nanotubes derived from the bulk sphere, which was formed firstly and then slowly dissolved in the DMF/H2O mixed solvents to form the targeted NiPO SWNTs. Moreover, the volume ratios between DMF and H2O and reaction temperatures have a great influence on the final products purity and the formation of architecture (Figure. S6 and S7) Considering that the NiPO SWNTs could be well dispersed in aqueous solution, we designed a surface-coating method to make FeCl3·6H2O hydrolysis gradually in moderate and weak acid environment, and then deposited on the surface of NiPO SWNTs. After treating with different amount of FeCl3·6H2O, series of uniform NiPO/Fe(OH)x SWNTs could be obtained (Scheme 1b). In order to facilitate the description, the optimized sample NiPO/Fe(OH)x-30 was given.In contrast to the structural similarity of NiPO SWNTs, TEM image (Figure 2a) showed that the entire surface of NiPO/Fe(OH)x-30 nanotube became very rough and remained the length of micrometers, the NiPO SWNTs were homogeneously covered by some Fe compounds, which has been further confirmed by higher magnification TEM and a slightly larger diameter of 6 nanometers for SWNTs can be obtained (inset in Figure 2a). Furthermore, SEM images implied the sample as one-dimensional nanostructures (Figure S2). The XRD pattern of NiPO/Fe(OH)x-


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30 SWNTs were almost the same as that of NiPO SWNTs, which also indicated the low crystalline of NiPO/Fe(OH)x-30 SWNTs, and the amorphous state of the coated Fe species (Figure S3). The EDX spectrum (Figure 2b) and linear scanning data (Figure S8b) verified the existence of Fe


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Figure 2 (a) TEM image of NiPO/Fe(OH)x-30 SWNTs, the inset of (a) is the high magnification image of the single nanotube coated by iron hydroxides. (b) EDX spectrum of NiPO/Fe(OH)x-30 SWNTs (the signal of Cu and carbon derive from the copper grid substrate). (c) Nitrogen adsorption–desorption isotherm of the NiPO/Fe(OH)x-30 SWNTs, the inset of (c) is the BJH pore size distribution. High-resolution XPS spectra of (d) Ni 2p (e) O 1s, (f) P 2p of NiPO and NiPO/Fe(OH)x-30 SWNTs; (g) Fe 2p for NiPO/Fe(OH)x-30 SWNTs.

species. Meanwhile, energy-dispersive X-ray spectroscopy (EDX) mappings demonstrated that Fe (yellow) specieswere distributed on the surface of the NiPO SWNTs (Figure S8d). The architectures and components of NiPO/Fe(OH)x SWNTs with different Fe amounts were shown in Figure S9 and Table S1. It can be seen that the surface of NiPO SWNTs became more and more rough when the Fe contents increased. However, excess iron will lead to the aggregation of final products. It has been found that NiPO/Fe(OH)x-50 was the limit of Fe contents to maintain the well-defined nanotube structures. In addition, the nanotubes were formed in the aqueous solution of DMF and water, as the reaction progressed, it was due to the production of volatile gas HCl, so that the nanotubes formed a porous nanostructure.46 The NiPO/Fe(OH)x-30 SWNTs showed a large surface area of about 285 m2 g-1 and the corresponding Barrett–Joyner–Halenda (BJH) pore size of about 2.7 nm (Figure 2c), larger surface area and similar pore size for NiPO SWNT were also achieved (Figure S10). The high surface area and porous structures of the ultrathin nanotube structure may provide efficient transport of electrons and ions, leading to the high electrochemical activity. X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface chemical states of the NiPO SWNTs before and after iron hydroxides coating. The existence of iron was revealed for NiPO/Fe(OH)x-30 nanotubes in the XPS survey spectrum (Figure S11). The peaks located at ∼856.5 eV (2p3/2), ∼874.2 eV (2p1/2) and corresponding satellite peaks at ∼862.5.3 eV


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and ∼880.6 eV confirmed Ni in oxidation state (Ni2+) located on the surface. After iron hydroxide modification, a small up-shifts of binding energy was discovered, which proved the interactions between Fe and NiPO SWNTs (Figure 2d).47,48 For NiPO SWNTs, the O 1s XPS peak at ∼531.2 eV was assigned to phosphate and hydroxide compounds (Figure 2e) and the P 2p XPS peak at ∼133.6 eV can be ascribed to the pentavalent phosphorous in the materials (Figure 2f), indicating the formation of NiPO SWNTs.18 Meanwhile, FT-IR spectra revealed that NiPO and NiPO/Fe(OH)x-30 nanotubes had very similar absorption bands around 1000 cm-1 that could be assigned to the tetrahedral PO4 stretching vibrations (Figure S12).49


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Figure 3. (a) Linear sweep voltammetry curves toward OER in the presence of NiPO/Fe(OH)x-30/C (black), NiPO (red), and IrO2/C (blue) and (b) Tafel plots of all catalysts in 1M KOH electrolytes. (c) Comparison of potentials required to reach j = 10 mA cm-2 and Tafel slopes for all as-obtained catalysts. (d) EIS Nyquist plots for different electrocatalysts recorded at open circuit voltage (e) Time dependence of the current density under a static overpotential of 248 mV in 1M KOH solution for 10 hours. (f) Polarization curves of NiPO/Fe(OH)x30/C before and after CV testing of 1000 cycles in 1M KOH solution.


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Consistent with the Ni peaks, both O 1s and P 2p peaks in NiPO/Fe(OH)x-30 SWNTs were found profile shifted to higher binding energies, which meant the surface chemical states of the NiPO SWNTs have been changed with the deposition of iron hydroxide. Due to the interaction between NiPO SWNTs and iron hydroxides, and thus OER performance of NiPO/Fe(OH)x-30 SWNTs achieved further improvement. Two characteristic peaks of Fe 2p XPS spectra were located at ∼711.9 eV (Fe 2p3/2) and ∼725.6 eV (Fe 2p1/2), indicating the oxidation state of iron is primarily Fe3+ species in NiPO/Fe(OH)x-30 SWNTs, in accord with recent reports (Figure 2g).50 The OER performances of the NiPO, NiPO/Fe(OH)x-30 SWNTs as well as commercial IrO2 were conducted in a standard three electrode system in O2-saturated 1M KOH solution at room temperature. All the tested samples were all mixed with carbon powder, which has been demonstrated to be an effective and simple method to enhance the conductivity of the mixed system proved by our previous research (Figure S13-S15).51 Figure 3a shows the typical linear sweep voltammetry (LSV) curves in 1 M KOH with NiPO/Fe(OH)x-30/C, NiPO/C and IrO2/C as electrocatalysts, with the inset displaying the initial details of the LSV curves, it showed the polarization curves of NiPO/Fe(OH)x-30/C presented a sharp onsetpotential of OER current at ∼1.47 V (vs. RHE) and very lower than NiPO/C (1.51V) and IrO2/C (1.49V). NiPO/Fe(OH)x30/C exhibited significantly enhanced electrocatalytic activity in the OER displaying the smallest overpotential requirement of 248 mV to obtain a current density of 10 mA cm-2, while the NiPO/C and IrO2/C needed a relatively larger overpotential requirement of 360 mV and 330 mV, respectively. The OER performance of the different catalysts was further investigated in terms of their Tafel slopes (η =b log j + a). The Tafel slope of the NiPO/Fe(OH)x-30/C was measured to be 45.4 mV dec-1, smaller than that of NiPO/C 75.5 mV dec-1 and IrO2/C 55.4 mV/dec-1 (Figure 3b and Figure 3c), indicating despite a similar nanotube catalytic structures, as the introduction


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of proper iron hydroxides, more rapid OER rates can be achieved using NiPO/Fe(OH)x-30/C as electrocatalyst.52 Electrochemical impedance spectroscopy (EIS) unravel that the charge-transfer resistance of the NiPO/Fe(OH)x-30/C exhibited a significantly decrease in comparison with NiPO/C (Figure 3d). The finding suggested that NiPO/Fe(OH)x-30/C had the fastest charge transfer process

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Figure 4 (a) Linear sweep voltammetry curves toward OER in the presence of NiPO/Fe(OH)x-10/C, NiPO/Fe(OH)x-20/C, NiPO/Fe(OH)x-30/C, NiPO/Fe(OH)x-40/C, NiPO/Fe(OH)x/C-50 and (b) Tafel plots of all catalysts in 1M KOH electrolytes. (c) Current densities of different Fe content catalysts at an overpotential of 320 mV. (d) EIS Nyquist plots for different electrocatalysts recorded at open circuit voltage.


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three catalysts, which may be attributed to the rough surface and the strong coupling between iron hydroxides and NiPO SWNTs at unique one-dimensional electronic structure and was in good agreement with the outstanding OER property. This was supported by the electrochemically active surface areas (ECSA), which was estimated by using the electrochemical double-layer capacitance (Cdl, Figure S16). Furthermore, the OER performance of different morphologies during the formation process of NiPO SWNTs were shown in Figure S17, indicating that as the reaction time increases, more nanotubes were available, and the performance increased until the reaction ended all the samples were nanotubes, and this was successfully explained by electrochemically active surface areas (ECSA) (Figure S18). In addition to the catalytic activity, stability was another important parameter to estimate an electrocatalyst, long term electrochemical stability of the NiPO/Fe(OH)x-30/C was investigated for OER at a static potential of 1.478 V versus RHE. After 10 h, the current density can still remained around 75% (Figure 3e) and the nanotubes structure was still existent, FT-TR further demonstrated that the phosphate is still retained, indicating that although the sample was ultrathin structure, it still had structural maintenance after long time stability (Figure S19), whereas the current losses of NiPO/C (39.8%) and IrO2/C (52.2 %) were much higher for 4 hours (Figure S20). Moreover, the OER polarization curve of NiPO/Fe(OH)x-30/C after 1000 potential cycles almost overlaped with the intial one (Figure 3f). As shown in Table S2, the OER activity of NiPO/Fe(OH)x/C was comparable to that of most active non-precious metal electrocatalysts . To give more lights, the influence of iron hydroxides for OER performance was better illustrated by the electrochemical analysis of NiPO/Fe(OH)x-X/C with different iron contents (Figure 4). Obviously, the oxidation peaks located in 1.33 V vs RHE are ascribed to Ni(OH)2/NiOOH redox characteristics for NiPO/C, which proceeds as Ni(OH)2 + OH- ↔


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NiOOH + H2O + e- in alkaline electrolytes.53,54 As the formula of this hydroxylphosphate was given of Ni2(OH)(PO4), considering that the FT-IR spectrum and the structure of the electrocatalysts were basically matained before and after the stability, the essence of the sample has not changed. We inferred that nickel hydroxylphosphate exhibited as a catalyst precursor, nickel hydroxide in the sample acts as the real catalytic site to produce oxygen with the continuous activation of the catalyst, and the phosphate roots are preserved at the end of the reaction. After a significant amount of iron hydroxides coated into the NiPO SWNTs, the Ni(OH)2/NiOOH redox couple shifts to higher potentials, indicating synergistic effect between Ni and Fe, consistent with previous reports.41 Eventually, all the well-defined NiPO/Fe(OH)xX/C with different Fe contents exhibited higher OER activity than NiPO/C (Figure. 4a and Figure. 4b). Meanwhile, we found the OER activity of NiPO/Fe(OH)x-X/C depended on the Fe contents in a volcano-type shape changing following the Ni/Fe molar ratio as a function of the amount of Fe is in good agreement with the trends in the OER activity (Figure 4c), which was consistent with the charge-transfer resistance (Figure. 4d and Figure S21). NiPO/Fe(OH)x/C-30 showed the best OER performance when Ni/Fe=1.62, suggesting that the addition of the right amount of Fe, optimizing the binding energy of surface oxygen intermediates and leads to improved kinetics, which is key factor for improving the OER activity.55 With more Fe content (Ni/Fe=1.26), Ni(OH)2/NiOOH redox couple peak area decreased, and Ni/Fe=1.09, the oxidation wave was almost no longer visible due to its coincidence with the rapid rise in the OER current, resulting in a decreased activity than the best one since the decreasing quantity of Ni sites as excess Fe was coated.41 Based on the discussion mentioned above ,excellent OER activity of NiPO/Fe(OH)x/C might be attributed to the following sides. First, suitable amount of iron hydroxides coated on the NiPO SWNTs contributed to the enhanced electrocatalytic activity to a


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great extent since Fe exerted a partial-charge-transfer activation effect on NiPO SWNTs.42 Second, ultrathin 1D architecture with high specific surface area can provide more active sites and the mixture of carbon is expected to improve the charge transfer efficiency and enhance structural stability. CONCLUSIONS In summary, well-defined iron hydroxides-modified nickel hydroxylphosphate single-wall nanotubes as efficient electrocatalysts for OER were successfully fabricated by a facile stepwise surfactant-free solvothermal strategy. Our results demonstrated that the surface of NiPO SWNTs coated by an amorphous iron hydroxides layer was the key to improve OER activity. Moreover, the NiPO/Fe(OH)x-X SWNTs exhibited highly effective electrocatalytic OER activity compared to bare NiPO SWNTs due to the unique hollow 1D structure and the strong synergistic effect between iron hydroxides and NiPO SWNTs. The optimized NiPO/Fe(OH)x-30 SWNTs showed a very small overpotential of 248 mV to attain a current density of 10 mA cm-2 and Tafel slope of 45.4 mV dec-1, which was much than that of commercial IrO2. Such a facile surface modification route provides valuable insights for the rational design of highly active electrocatalysts toward renewable energy applications. ASSOCIATED CONTENT Supporting Information. TEM, SEM, XRD, TGA analysis on the nanotubes and or their intermediates; Analysis on nanotubes before and after electrocatalytic test. This material is available free of charge on the ACS publication website. AUTHOR INFORMATION


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Corresponding Author * Xun Wang E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by NSFC (21431003, 21521091), China Ministry of Science and Technology under Contract of 2016YFA0202801. REFERENCES 1. Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. 2. Mallouk, T. E. Divide and conquer. Nat. Chem. 2013, 5, 362-363. 3. Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L. A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II. Nat. Chem. 2012, 4, 418-423. 4. Song, F.; Hu, X. Ultrathin Cobalt-Manganese Layered Double Hydroxide Is an Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2014, 136, 16481-16484. 5. Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of elec-trocatalysts for oxygen- and hydrogen-involving energy con-version reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. 6. Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured α Nickel-Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077-7084. 7. Zou, S.; Burke, M. S.; Kast, M. G.; Fan, J.; Danilovic, N.; Boettcher, S. W. Fe (Oxy)hydroxide Oxygen Evolution Re-action Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution. Chem Mater. 2015, 27, 8011-8020. 8. Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt-Iron (Oxy)hydroxide Oxygen Evolu-tion Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648. 9. Song, F.; Hu, X. Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis. Nature Commun. 2014, 5, 4477.


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