Hierarchically Structured Ni Nanotube Array-Based Integrated

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Hierarchically structured Ni nanotube arraysbased integrated electrodes for water splitting Xue Teng, Jianying Wang, Lvlv Ji, Weiqiang Tang, and Zuofeng Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03499 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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ACS Sustainable Chemistry & Engineering

Hierarchically structured Ni nanotube arrays-based integrated electrodes for water splitting Xue Teng, Jianying Wang, Lvlv Ji, Weiqiang Tang and Zuofeng Chen* Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China *[email protected] (Z.-F. C.) ABSTRACT The development of high-performance nonprecious electrocatalysts for overall water splitting has attracted increasing attention but remains a vital challenge. Herein, we report a ZnO-based template method to fabricate Ni nanotube arrays (NTAs) anchored on nickel foil for applications in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Based on this precursor electrode, the three-dimensional NiSe2 NTAs of unique sandwich-like coaxial structure have been fabricated by electrodepositon of NiSe2 on Ni NTAs, which exhibits high performance toward the HER in both acidic and alkaline media. The method based on Ni NTAs can be readily extended to fabricate Ni2P NTAs by gas-solid phosphorization for the HER and NiFeOx NTAs by anodic co-depositon of Ni and Fe for the OER. Consequently, an alkaline electrolyzer has been constructed using NiFeOx NTAs and NiSe2 NTAs as anode and cathode, resepctively, which can realize overall water splitting with a current density of 100 mA cm−2 at an overpotential of 510 mV. 1

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KEYWORDS water splitting; Ni nanotube arrays; selenide; phosphide; NiFeOx film

INTRODUCTION With the increasing global energy demand and environmental crisis, water splitting to produce hydrogen has attracted great research attention in recent years. The water splitting process can be divided into the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), which needs an applied voltage of at least 1.23 V to provide the thermodynamic driving force theoretically.1 Although Pt-based materials exhibit the highest activity toward the HER,2 and iridium oxide (IrO2) and ruthenium oxide (RuO2) are regarded as the most efficient OER electrocatalysts,3 the scarcity and high cost of these materials have severely impeded their wide application. These limitations necessitate efficient HER and OER catalysts made from earth-abundant elements and, indeed, great progress has been made during the past years in developing nonprecious metal catalysts with high activity for both HER (carbide,4,5 nitride,6 phosphide,2,7-9 and chalcogenide10-12) and OER (transition metal hydroxide,13-15 oxide16,17 and the layer structure type-family catalysts4,18,19). For example, as a member of inexpensive TMDs (transition metal dichalcogenides), NiSe2 is a Pauli paramagnetic metal with a resistivity below 10‒3 Ω cm.20 For its metallicity, NiSe2 has been utilized as a Pt-free counter electrode of dye-sensitized solar cells21 and as an energy storage material.22 Admittedly, its potential utilization as a HER catalyst has also been explored, and NiSe2 catalysts were found to perform remarkably well toward the HER in both acidic and basic solutions and demonstrated 2


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impressive stability.23-25 On the other hand, in basic and weakly basic electrolytes, oxides and hydroxides of the first row transition metals (Fe, Co, Ni, Mn) show promising performances and thus have received great attention.26-28 It has now been well-established that incorporation of Fe into Ni can significantly increase the OER activity by utilizing the synergistic effect of metal-metal interactions.29-32 The morphological architecture of a catalyst affects its specific surface area and thus impacts its catalytically active sites to some extent. In order to increase surface active sites, materials of various structures, such as nanoparticles,33 nanowires,34,35 nanosheets,15,36 polyhedra,37 nanodots,34 and nanotubes38-42 have been fabricated. Thus, controlling the structural morphology is most desirable for nonprecious catalysts in water splitting systems. Electrodeposition or electrochemical synthesis with its intrinsic advantages is an ideal method to produce catalysts with individual or multiple components. In this study, the self-standing three-dimensional Ni nanotube arrays (NTAs) are constructed electrochemically by a ZnO template method, which is then followed by NiSe2 coating by electrodeposition. In this strategy, Ni NTAs were employed as multifunctional inner layer to provide a large specific surface area and fast electron transport and support the outermost NiSe2 layer. Due to the effective utilization of the “core” and “shell” materials and their strong synergistic effects, the as-prepared self-standing three-dimensional NTAs electrode exhibits high performance toward the HER. Similarly, we obtained Ni2P NTAs, another electrode with excellent HER activity by a facile one-step gas-solid phosphorization of the Ni NTAs precursor 3



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electrode. On the basis of the hierarchically structured 3D Ni NTAs electrode, we further developed a convenient and straightforward method to fabricate a NiFeOx NTAs electrode for the OER, which was realized by a one-step anodic electrodeposition of the NiFeOx film on the Ni NTAs electrode. The electrode material maintains the morphology of nanotube arrays and exhibits an excellent catalytic performance toward the OER in alkaline solution. With both NiFeOx NTAs and NiSe2 NTAs electrocatalysts available, we assembled an alkaline electrolyzer for overall water splitting which could deliver a current density of 100 mA cm−2 at an applied cell voltage of only 1.74 V. Considering the versatility and the unique structure of Ni NTAs as the electrode substrate framework and the excellent catalysis performances, this study will open up new avenues for design of various novel electrocatalysts for efficient water splitting.

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Scheme 1. Schematic illustration of the fabrication of the electrocatalysts for both OER and HER.

RESULTS AND DISCUSSION Preparation and Characterization. The Ni NTAs-based catalyst electrodes were facilely synthesized by procedures as illustrated in Scheme 1 and detailed in Supporting Information: (1) In the first step, ZnO nanorod arrays (NRAs) were electrodeposited onto the Ni foil substrate as template. Figure 1A shows SEM image of ZnO NRAs, which are grown uniformly on the Ni foil substrate with an average diameter of ~ 300 nm and an average length of ~ 2 µm. The XRD pattern in Figure 5



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S1A indicates the successful fabrication of ZnO NRAs at Ni foil. (2) In the second step, a layer of metallic Ni was electrodeposited onto ZnO NRAs. Figure 1B shows that Ni nanoparticles are preferentially coated on the surface of ZnO NRAs. To verify the successful deposition of metallic Ni, we intentionally electrodeposited Ni NTAs on the FTO (F-doped SnO2 glass) substrate by the same procedure, which could eliminate the influence of Ni substrate. As shown in Figure S1B, the three sharp characteristic peaks at around 44.6°, 51.97° and 76.59° prove the coating of metallic Ni. (3) Finally, the ZnO NRAs template was dissolved by dipping in 2 mM H2SO4 aqueous solution. Figure 1C shows that, after dissolving ZnO core, Ni NTAs with open end are obtained and the nanoarray density is generally unchanged. The inner diameters and wall thicknesses of the resultant Ni nanotubes are approximately 300 and 100 nm, respectively. Figure S2 shows high-resolution TEM (HRTEM) image of the Ni nanotube. The interplanar spacings are determined to be 0.20 and 0.175 nm, which are identical to Ni (111) and Ni (200) lattice fringes, respectively. The inset in Figure S2 shows the selected area electron diffraction (SAED) pattern of Ni nanotube, which indicates that the Ni nanotube is of polycrystalline structure.

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Figure 1. SEM images of ZnO NRAs (A), Ni@ZnO NRAs (B), Ni NTAs (C); insets show magnified morphology. SEM images of NiSe2 NTAs (D,E) and Ni2P NTAs (G,H). HRTEM images of NiSe2 NTAs (F) and Ni2P NTAs (I); insets show SAED patterns.

The NiSe2 NTAs electrocatalyst was obtained by electrodeposition of NiSe2 on the Ni NTAs precursor electrode. As shown in Figures 1D and 1E, the SEM images of NiSe2 NTAs at different magnifications reveal uniform NiSe2 nanoparticles on the surface of Ni NTAs. A HRTEM image of the material is shown in Figure 1F. The interplanar spacing of NiSe2 is determined to be 0.27 nm, which is identical to the lattice plane of NiSe2 (210). The NiSe2 NTAs material is of polycrystalline in nature 7



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as evidenced by the defined diffraction rings in the SAED pattern in the inset. Figure 2A shows the XRD pattern of NiSe2 NTAs. The diffraction peaks are related to (210), (311), (400) and (420) of NiSe2 (JCPDS No. 89-7161) along with the background of Ni foil. Figure 2B shows the energy dispersive X-ray (EDX) spectrum, which indicates the co-existence of Ni and Se with small O signal from surface oxidation. The XPS analysis was also carried out to probe the surface chemistry of NiSe2 NTAs. In general, all of the X-ray photoelectron spectroscopy (XPS) peaks are assignable on the basis of the elements detected by EDX. In Figure 2C, the Ni 2p3/2 and Ni 2p1/2 peaks appear at 855.7 and 873.6 eV, and the satellite peaks at 862.4 and 879.2 eV are characteristics of Ni2+.43 In Figure 2D, the peak near 54.3 eV is consistent with the binding energy of Se2– and the peak at 58.9 eV implies the surface oxidation of Se species.24,44

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Figure 2. (A) XRD pattern and (B) EDX elemental analysis of NiSe2 NTAs. (C) Ni 2p and (D) Se 3d XPS of NiSe2 NTAs. HER Electrocatalysis. The HER electrocatalytic activity of the as-prepared NiSe2 NTAs electrode was first evaluated in acidic media. As a benchmark electrocatalyst, the commercial 20 wt% Pt/C was examined under the same experimental conditions. For comparison, the performances of pure Ni foil, Ni NTAs and NiSe2/Ni foil (NiSe2 electrodeposited on the bare Ni foil) were also examined. The polarization curves of these electrodes are shown in Figure 3A. As expected, 20 wt% Pt/C electrode exhibits high HER catalytic activity with negligible overpotential. Although the bare Ni foil shows poor HER activity, the Ni foil loaded with Ni NTAs shows improved electrocatalytic activity with an overpotential of 212 mV to reach a 9



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current density of 10 mA cm-2. After electrodeposition of NiSe2, the resultant electrode performs an onset overpotential of around 90 mV to achieve a current density of 10 mA cm‒2, which is compared favorably to many of nonprecious HER catalysts in acidic media.45 The performance of NiSe2 NTAs is also greatly superior to NiSe2/Ni foil, indicating the important role of the 3D NTAs support in improving the catalytic performance. Figure 3B shows the Tafel plots of various electrodes. The measured Tafel slope of the 20 wt% Pt/C was about 34 mV dec−1, which is consistent with the reported values.46 The pure Ni foil and Ni NTAs electrode show large Tafel slopes of 136 mV dec−1 and 124 mV dec−1, respectively. By contrast, NiSe2/Ni foil and NiSe2 NTAs exhibit smaller Tafel slopes of 82 mV dec−1 and 79 mV dec−1, respectively. Durability and stability are very essential aspects to evaluate the performance of a catalyst in practical applications. The stability of NiSe2 NTAs was assessed in a long-duration controlled potential electrolysis experiment. As shown in the inset in Figure 3C, the time-dependent current density is sustained at 10 mA cm−2 for 12 h under a static applied overpotential of 110 mV. In addition, after controlled potential electrolysis, the polarization curve of NiSe2 NTAs nearly overlaps with the initial one, indicating an excellent durability. The SEM image in Figure S3 and the XRD pattern in Figure S4 confirm further the high stability of the NiSe2 NTAs after the electrolysis experiment. To further evaluate the efficiency of the catalytic reaction at the NiSe2 NTAs electrode, an electrochemical impedance spectroscopy (EIS) was applied at an 10


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overpotential of 150 mV in 0.5 M H2SO4 (Figure S5A). As compared to Ni foil, the NiSe2 NTAs show a much lower charge transfer resistance (100 Ω for NiSe2 NTAs and 1200 Ω for Ni foil) in the Nyquist plot, indicating the favorable charge transport kinetics. To further gain insight into the catalytic activity of NiSe2 NTAs, EIS measurements were conducted at various overpotentials in 0.5 M H2SO4, and the Nyquist plots of the EIS response are shown in Figure S5B. The charge transfer resistance decreases as the overpotential increases. Water splitting to produce hydrogen includes both water oxidation and water/proton reduction half reactions, and most water oxidation electrocatalysts, especially the commonly used transition metal oxides, function efficiently only in alkaline media. To couple with water oxidation and realize water splitting, HER electrocatalysts that can operate in an alkaline solution are highly desirable. Figures 3D and 3E show the results of electrochemical measurements with NiSe2 NTAs in 1 M KOH, which feature an impressive overpotential of 88 mV at 10 mA cm‒2 and a Tafel slope of 78 mV dec-1 in the alkaline medium. Moreover, the NiSe2 NTAs electrode also exhibits excellent stability under such alkaline conditions, as shown in Figure 3F. As listed in Table S1, the catalytic performance of our NiSe2 NTAs electrode is superior to the most NiSe2-based electrocatalysts in both acidic and alkaline media. The remarkable features of high activity and strong durability suggest that the as-prepared NiSe2 NTAs electrode is a promising candidate to catalyze the HER. We tentatively attribute the high catalytic property of the catalysts to the following three 11



factors: (1) The 3D NTAs structure with the large specific surface area and rough surface will favor the electroactive species to touch the catalyst and participate in the HER sufficiently, facilitating the motion of active species.47,48 (2) The nanotube array structure improves electrical conductivity owing to the metallic Ni nanotube arrays directly anchored to the conductive Ni foil. Furthermore, this also prevents using polymer binder and conductive additives.48-51 (3) The coating of dense and uniform NiSe2 NTAs could effectively resist the acidic corrosion to the Ni foil. D

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20% Pt/C, Ni foil, Ni NTAs, NiSe2/Ni foil and NiSe2 NTAs. (C,F) iR-corrected HER polarization curves for NiSe2 NTAs before and after 12 h electrolysis; Insets show time-dependent current density curves of NiSe2 NTAs under an overpotential of 110 mV. Measurements in (A,B,C) were performed in 0.5 M H2SO4 and (D,E,F) in 1 M KOH.

To demonstrate the versatility of the as-prepared Ni NTAs precursor electrode to make high-performance electrocatalysts, we also fabricated Ni2P NTAs and use them for the HER. The Ni2P NTAs electrode was facilely fabricated via a direct phosphorization process of Ni NTAs (see Experimental Section in the Supporting Information). As revealed by SEM images in Figures 1G and 1H, the resultant Ni2P NTAs exhibit nanotube structure with uniform sizes, which is similar to NiSe2 NTAs discussed above. In Figure S6, the EDX spectrum of Ni2P NTAs confirms the presence of P along with O element due to the surface oxidation. In Figure S7, the XRD pattern of Ni2P NTAs confirms the formation of Ni2P, and the diffraction peaks related to (111), (201), (210) and (300) of Ni2P (JCPDS No. 03-0953) were obvious. The HRTEM image taken from the Ni2P NTAs (Figure 1I) further provides insight into the crystalline morphology and microstructural details, presenting lattice distances of 0.22 nm, 0.20 nm and 0.19 nm corresponding to the (111), (201) and (210) planes of Ni2P, respectively. The SAED pattern in the inset of Figure 1I shows that Ni2P is of polycrystalline structure. The catalytic performance of Ni2P NTAs toward the HER was also evaluated in both acidic and alkaline media. As shown in Figures S8A and S8B, the electrode is 13



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highly active toward the HER in 0.5 M H2SO4, which requires a small overpotential of 89 mV for driving a cathodic current density of 10 mA cm‒2 and shows a Tafel slope of 59 mV dec–1 obtained from the polarization curve using a linear fit applied to points in the Tafel region. It is worth to mote that the Tafel slope could also be obtained by EIS analysis, which could eliminate the effects of some arbitrary factors, such as different choices of overpotential region and different means for

iR-compensation. Figure S9 shows the Nyquist plots of the Ni2P NTAs electrode at different overpotentials. By fitting the experimental data, a model of two time-constants can be used to describe the HER behavior. According to the Nyquist plots and the corresponding equivalent circuit model, the value of the charge transfer resistances (Rct) at different overpotentials (η) can be conveniently extracted.52 The plot of η versus logRct−1 shows a Tafel slope of 63 mV dec−1 for Ni2P NTAs in 0.5 M H2SO4. This value was slightly higher than that obtained based on the polarization curve but is considered to be more comprehensive. The electrocatalytic performance of Ni2P NTAs was also assessed in 1 M KOH solution, as shown in Figure S8D. The electrode requires a very low overpotential of 80 mV to deliver the current density of 10 mA cm–2, thus, it appeared to be a highly active electrocatalyst for the HER in a strong alkaline electrolyte. Figure S8E provides the Tafel plot of Ni2P NTAs, which reveals a low Tafel slope of 70 mV dec–1. In Table S2, the comparison of Ni2P NTAs with other reported Ni2P materials demonstrates clearly the excellent catalytic performance in both acidic and alkaline media. 14


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OER Electrocatalysis. With Ni NTAs available, we further prepared the OER electrode (NiFeOx NTAs) by an anodic deposition procedure in 2 M Na2CO3 (pH 10.8) at 1.3 V, which formed the superficial NiFeOx catalyst on Ni NTAs (see Experimental Section in the Supporting Information). The anodic deposition procedure has been widely employed to in situ deposit metal oxides onto electrodes for the OER. In this study, high concentrations of carbonate anions are essential in complex ion formation to avoid precipitation of Fe(III).53 The SEM images in Figures 4A and 4B show that the nanotube array structure is maintained with NiFeOx coated Ni NTAs. In Figure 4C, the XRD pattern of the electrodeposited film shows no peaks indicative of crystalline phases other than the peaks associated with the Ni substrate. In addition, the SAED pattern of NiFeOx film scraped from a planar FTO also confirms that the NiFeOx catalyst is amorphous, as shown in Figure S10. The EDX result in Figure S11 shows that the amorphous solid was a mixture containing Ni, Fe, and O. Furthermore, the XPS analysis was carried out to probe the surface chemistry of NiFeOx NTAs. In Figure 4D, the Ni 2p XPS displays two spin-orbit doublets at 855.8 and 873.1 eV and two shake-up satellites at 861.6 and 879.4 eV,48,54 which are consistent with Ni oxide and Ni hydroxide or oxyhydroxide.55,56 In Figure 4E, the binding energies of Fe 2p of the electrodeposited catalyst film suggest that the valence state of Fe is Fe(III) and the fitting peaks centered at approximately 710.5, 711.8, 714.1, and 725.2 eV are consistent with the presence of both Fe oxide and hydroxide/oxyhydroxide.53,57,58 Complementary to the Ni 2p and Fe 2p XPS data, O 1s XPS in Figure 4F displays three oxygen contributions. 15



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Specifically, O(1) at 529.9 eV and O(3) at 532.2 eV are consistent with the oxygen in hydroxide or oxyhydroxide, which are due to oxygen atoms at the surface substituted by hydroxyl or oxyhydroxyl groups; the O(2) at 531.3 eV is ascribed to some defect sites with low oxygen coordination and adsorbed oxygen.48,59,60

Figure 4. (A,B) SEM images of NiFeOx NTAs of different magnifications. (C) XRD pattern of NiFeOx NTAs. (D) Ni 2p, (E) Fe 2p, (F) O 1s XPS of NiFeOx NTAs. 16


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The OER performances of various electrodes were examined in 1 M KOH solution. As shown in Figure 5A, the broad pre-waves observed at Ep,a = 1.35 - 1.5 V correspond to the formation of Ni(III) species, which are the active sites to catalyze the OER. The NiFeOx NTAs electrode exhibits a catalytic onset overpotential of 250 mV toward the OER in 1 M KOH. At higher potentials, a rapid increase in catalytic current is observed and a current density of 100 mA cm‒2 is obtained at an overpotential of only 300 mV. Comparing to the performance of the pure Ni-based electrodes, the introduction of Fe and the synergistic effect between Ni and Fe promote the formation and stabilization of the catalytically active species NiOOH in the electrochemical process, contributing to an enhancement in the OER catalytic activity.32,61,62 Because of the high specific surface area of 3D NTAs as shown in Figure S12, the performance of NiFeOx NTAs is also greatly superior to that of NiFeOx/Ni foil. The OER performance of NiFeOx NTAs was further investigated by Tafel measurements. Figure 5B shows that the Tafel slope of the NiFeOx NTAs electrode is approximately 49 mV decade‒1 in 1 M KOH solution. The small overpotential and Tafel slope make the NiFeOx NTAs electrode among the best reported NiFeOx catalysts at 2D planar substrates (Table S3). Figure 5C shows a multistep chronopotentiometric curve obtained at the NiFeOx NTAs electrode in 1 M KOH solution. The current was increased from 200 to 2000 mA cm‒2 with an increment of 200 mA cm‒2 per 600 s. At the start of 200 mA cm‒2, the potential immediately levelled off at 1.56 V and remained constant for the rest of 17



the 600 s. The response potentials remained constant at each step, indicating the high stability of the electrode within a wide range of current densities. The step height was decreased with increasing the current density, consistent with the Tafel behavior. In Figure S13, the charge transfer resistance decreased as the overpotential increased, indicating the efficient charge transfer and mass diffusion across the 3D electrode. The stability of the NiFeOx NTAs electrode was further assessed by a long-duration controlled potential electrolysis experiment. As shown in the inset in Figure 5D, the time-dependent current density curve keeps steady at ~35 mA cm−2 under a static overpotential of 260 mV for over 12 h. The remarkable features of the as-prepared NiFeOx NTAs electrode (high OER activity, favorable kinetics and strong durability) makes it very promising for OER in water splitting. B 0.4 -1

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Ni NTAs, NiFeOx/Ni foil and NiFeOx NTAs in 1 M KOH. (C) Multicurrent process obtained with NiFeOx NTAs in 1 M KOH. (D) iR-corrected OER polarization curves for NiFeOx NTAs before and after electrolysis under an overpotential of 260 mV for 12 h; inset shows the time-dependent current density curve.

Overall Water Splitting. Inspired by the excellent catalytic performances of NiFeOx NTAs for OER and NiSe2 NTAs for the HER in alkaline media, an alkaline electrolyzer employing NiFeOx NTAs as anode and NiSe2 NTAs as cathode was constructed. In Figure 6A, the LSV shows that a current density of 100 mA cm‒2 was achieved at a cell voltage of approximately 1.74 V, corresponding to a combined overpotential of 510 mV. This performance corresponds to an anodic overpotential of 310 mV to achieve an OER current density of 100 mA cm−2 at NiFeOx NTAs and a cathodic overpotential of 200 mV to achieve the same HER current density at NiSe2 NTAs. Vigorous gas bubbles were observed at both cathode and anode during the LSV measurement. Comparison of catalytic performance of various electrolyzers for overall water splitting is shown in Table S4. As shown in Figure 6B, the electrolyzer also exhibits excellent catalytic stability. It can deliver a sustained current density of ~ 10 mA cm‒2 at an applied cell voltage of 1.65 V for 15 h in 1 M KOH. In addition, the nearly overlapped polarization curves collected before and after 15 h electrolysis test further confirms the robustness of our electrolyzer for overall water splitting.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.3

1.4 1.5 1.6 1.7 Cell Voltage (V)

1.8

1.2

1.4 E (V)

1.6

1.8

Figure 6. (A) LSV of the NiFeOx NTAs||NiSe2 NTAs electrolyzer. (B) LSVs of the NiFeOx NTAs||NiSe2 NTAs electrolyzer before and after 15 h electrolysis at 1.65 V; inset shows the time-dependent current density curve.

CONCLUSIONS In summary, the precursor electrode of Ni nanotube arrays (NTAs) was fabricated by a template method, which was realized by electrochemical synthesis of ZnO nanorod arrays (NRAs) on a Ni foil substrate, followed by electrodepositing a layer of metallic Ni and dissolving ZnO NRAs. A NiSe2 NTAs electrode of sandwich-like coaxial structure was then fabricated by electrodeposition of NiSe2 onto Ni NTAs and exploited as a highly effective HER electrode. By comparing with other HER electrocatalysts, the well-defined NiSe2 NTAs electrode exhibits superior HER performance in both acidic and alkaline solutions. To prove versatility of the method developed, we also fabricate Ni2P NTAs by gas-solid phosphorization for HER and NiFeOx NTAs by anodic co-deposition of Ni and Fe for OER, both based on the Ni NTAs precursor electrode. Finally, an alkaline electrolyzer combining both NiFeOx 20


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NTAs OER electrode and NiSe2 NTAs HER electrode was constructed to realize efficient overall water splitting, reaching a current density of 100 mA cm−2 at an overpotential of 510 mV. The simplicity and versatility of the preparation approach and the high performance of the electrodes with unique microstructures are appealing and may be of value for practical applications in energy conversion and storage.

EXPERIMENTAL All experimental details are included in the Supporting Information.

SUPPORTING INFORMATION Experimental section and additional information as noted in the text.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573160, 21405114), The Recruitment Program of Global Youth Experts by China, and Science & Technology Commission of Shanghai Municipality (14DZ2261100).

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For Table of Contents Use Only

Synopsis An alkaline electrolyzer was constructed for efficient water splitting based on nickel nanotube arrays (Ni NTAs) as the electrode substrate framework.

31


Hierarchically Structured Ni Nanotube Array-Based Integrated

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