NiFe Alloy Nanotube Arrays as Highly Efficient Bifunctional

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24096−24106

NiFe Alloy Nanotube Arrays as Highly Efficient Bifunctional Electrocatalysts for Overall Water Splitting at High Current Densities Chun-Lung Huang, Xui-Fang Chuah, Cheng-Ting Hsieh, and Shih-Yuan Lu* Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

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ABSTRACT: A bubble-releasing assisted pulse electrodeposition method was developed to create metallic alloy, NiFe, nanotube arrays in one step. The NiFe alloy nanotube array exhibited excellent bifunctional electrolytic activities, achieving low overpotentials of 100 mV for the hydrogen evolution reaction and 236 mV for the oxygen evolution reaction at 10 mA cm−2, both in 1 M KOH at room temperature. For overall water splitting, the NiFe alloy nanotube array delivered 10 mA cm−2 at an ultralow cell voltage of 1.58 V, among the top tier of the state-of-the-art bifunctional electrocatalysts. The NiFe alloy nanotube array also exhibited ultrastability at high current densities, experiencing only a minor chronoamperometric decay of 6.5% after a 24 h operation at 400 mA cm−2. The success of the present binder-free nanotube array-based electrode can be attributed to the much enlarged reaction surface area, one-dimensionally guided charge transport and mass transfer offered by the nanotube structure, and improved product crystallinity provided by the pulse current electrodeposition. The nanotube array structure proves to be a promising new architecture design for electrocatalysts. KEYWORDS: nanotube array, alkaline water electrolysis, hydrogen evolution reaction, oxygen evolution reaction, NiFe alloy

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INTRODUCTION With the worsening of fossil fuel depletion and overemission of carbon dioxide, development of clean alternative energy sources has become the top priority for the everlasting prosperity of mankind on Earth. Among the many promising candidates, hydrogen is the cleanest energy carrier of high gravimetric energy densities.1 Traditional hydrogen production processes, such as steam reforming of natural gases, are unsustainable, considering the use of fossil fuels as the raw material and emission of CO2 involved. Renewable energydriven electrolytic water splitting has been regarded as the most promising hydrogen production process for the green energy infrastructure. The electrolytic water splitting is composed of two working reactions, hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. To drive OER and HER, a minimum thermodynamic cell voltage of 1.23 V is required at room temperature. Extra resistances existing in the system however push the cell voltage to be significantly higher than 1.23 V, and the difference is called the overpotential. The overpotential, particularly that at the anode for the bottleneck OER, needs to be minimized to save electricity to make the process more economically feasible and competitive. Consequently, the development of high-efficiency electrocatalysts that can drive the water splitting at low overpotentials is most critical for the success of hydrogen production from electrolytic water splitting. In terms of electrolytic activities, noble-metal-based electrocatalysts, such as Pt for HER and IrO2 for OER, are the most prominent ones. For example, Audichon et al. synthesized an © 2019 American Chemical Society

RuO2/IrO2 core/shell structure with a surface modification/ precipitation method in ethanol to achieve an ultralow cell voltage of 1.5 V (vs RHE) to deliver a current density of 10.8 mA cm−2 in acidic media for overall water splitting.2 Wang et al. developed an electroless plating method to fabricate Pt nanospheres and nanotubes by using lipid vesicles and tubules as soft templates, achieving 10 mA cm−2 at ultralow overpotentials (η10) of 31 and 27 mV, respectively.3 Although noble-metal-based electrocatalysts exhibit excellent OER and HER performances, their high cost and Earth scarcity have detrimentally limited their commercial application potentials. The target is to develop cost-effective, highefficiency, stable electrocatalysts for HER and OER. In this regard, transition metals, particularly Fe-, Ni-, and Co-based electrocatalysts, including alloys,4 oxides,5 phosphides,6 sulfides,7 and so forth, have been widely studied and have exhibited excellent electrocatalytic performances toward water splitting. Some of the electrocatalysts show excellent electrocatalytic activities for both HER and OER and can be applied as bifunctional electrocatalysts for water splitting. Bifunctional electrocatalysts offer the advantage of material application convenience and many of them are multimetal-based materials such as Ni−Fe and Ni−Fe−M.8−10 Yu et al. coated NiFelayered double hydroxide nanosheets on Cu nanowires to exhibit an outstanding bifunctional feature in alkaline media with η10 of 199 and 116 mV for OER and HER, respectively.11 Received: April 4, 2019 Accepted: June 12, 2019 Published: June 12, 2019 24096

DOI: 10.1021/acsami.9b05919 ACS Appl. Mater. Interfaces 2019, 11, 24096−24106

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic of the Fabrication Process for the NiFe Nanotube Array

reactions, offers more reaction surface areas and stronger mass-transfer guiding ability than nanowires/nanorods and is expected to exhibit enhanced reaction activities. The NiFe alloy nanotube arrays were proved to be highly efficient and stable bifunctional electrocatalysts for overall water splitting. The product delivers a current density of 10 mA cm−2 at ultralow overpotentials of 236 and 100 mV for OER and HER, respectively, in 1 M KOH at room temperature. For overall water splitting, only 1.58 V is needed to reach 10 mA cm−2, comparable with the pairing of the two common benchmark electrodes: IrO2 for OER and Pt/C for HER.

Raja et al. grew water-stable NiFe-based bimetallic metal organic framework on the skeleton of nickel foam to achieve pronounced bifunctionality with η10 of 240 and 87 mV for OER and HER, respectively.12 Qin et al. used a hydrogen reduction method to coat a NiFeMo alloy onto a Ni foam, which showed ultralow η10 of 238 and 45 mV for OER and HER, respectively.13 One-dimensional (1D) nanostructured materials have attracted a wide range of research attention because of their spectacular physicochemical properties and suitability for the study of size-dependent transport phenomena.14 One-dimensional nanostructured materials, such as nanowires, nanotubes, and nanorods, hold great potentials for the fabrication of electronic, optoelectronic, and electromechanical devices of nanoscale dimensions15 but are rarely applied in electrolytic water splitting. A wide variety of processes have been developed to fabricate 1D nanostructured materials, including the hydrothermal process,16 liquid-phase reduction,17,18 chemical vapor deposition (CVD),19 electrodeposition,20−23 and so forth. Anodic aluminum oxide (AAO) membranetemplated electrodeposition is a simple method to fabricate uniform metal nanowire/nanorod arrays.23−27 Nevertheless, applications of metal nanowire/nanorod arrays in electrocatalysis have been rarely reported.28 Metallic 1D nanostructure arrays as electrocatalysts however possess several advantages as compared with the traditional powder-based ones. First, the high aspect ratio of the 1D nanostructure, particularly nanotubes, offers large extended surface areas to accommodate abundant active sites for electrocatalytic reactions. Second, the 1D-oriented structure provides fast guided charge transport and mass transfer. Third, the 1D nanostructure can be easily grown onto a substrate via CVD or electroplating, which expands its application potentials in industry. Fourth, binders, mostly nonconductive, are not required for metallic 1D nanostructure arrays, a big advantage for the charge transport and mechanical integrity of the electrocatalyst. In this study, a bubble-releasing assisted pulse electrodeposition method (Scheme S1) was developed to create metallic alloy, NiFe, nanotube arrays in one step. Unlike traditional nanotube array generation methods, normally in two steps with the assistance of sacrificial templates such as ZnO nanowires/nanorods,16,28 the present bubble-releasing assisted pulse electrodeposition method is simple, controllable, and capable of creating open-end tubular structures. The nanotube structure, with inner tube walls available for

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EXPERIMENTAL SECTION

Materials. Commercial AAO membranes were purchased from Whatman International Ltd. Acetone (C3H6O, 99%) and anhydrous ethanol (C2H5OH, 99.5%) were purchased from Echo Chemical Co., Ltd. Iron(II) sulfate heptahydrate (FeSO4·7H2O, 99%) and boric acid (H3BO3, 99.5%) were purchased from Showa Kako Corp. Nickel(II) sulfate hexahydrate (NiSO4·6H2O, 99%) was purchased from AENCORE CHEMICAL PTY., Ltd. Sulfuric acid (H2SO4, 98%) was obtained from J.T. Baker. Sodium dodecyl sulfate (SDS, 98%), sodium hydroxide (NaOH, 99%), and potassium hydroxide (KOH, 85%) were purchased from Honeywell International Inc. Twenty percent platinum on carbon black (Pt/C) and iridium(IV) oxide (IrO2, 99%) were purchased from Alfa Aesar. Deionized (DI) water (electrical resistance ∼18 MΩ) was produced with a Milli-Q Advantage A10 Water Purification System. All chemicals were used as received without further purification. Fabrication of NiFe Nanotube Array. NiFe nanotube arrays were fabricated by using commercial AAO porous membranes of 200−300 nm in pore size as the template through pulse current (PC) electrodeposition. First, a 300 nm thick Au layer was sputtered onto one side of the membrane to serve as a conducting base for later electrodeposition. A 20 μm thick Ni thin film was further electrodeposited on top of the Au layer to increase the mechanical strength of the Au base. A standard two-electrode cell was used for the electrodeposition of NiFe into the pore space of the AAO membrane by taking the Au/Ni film-supported AAO membrane as the working electrode and a Pt foil (1 cm × 1 cm) as the counter electrode. The Au/Ni film-supported AAO membrane was connected to the cell with a copper tape for electrodeposition. The electrolyte (200 mL) contained both Ni and Fe precursors by mixing appropriate amounts of NiSO4·5H2O, FeSO4·7H2O, 0.5 M H3BO3, and 0.2 g L−1 SDS in DI water, with the pH value of the solution adjusted to 2 with H2SO4. Three molar ratios of Ni versus Fe were investigated: 95/5, 90/10, and 80/20. The electrodeposition cell was kept in a temperature-controllable tank with a thermocouple immersed in the electrolyte. The electrodeposition of NiFe was carried out in the PC mode at 50 °C 24097


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by using an electrochemical analysis system (CHI). Three PC densities were used: −0.25, −1, and −2 A cm−2, with a duty cycle of 0.02 s (on)/1 s (off). For comparison purposes, Ni nanowire arrays were also fabricated with the same process, except that the electrolyte contained only 1 M NiSO4 and 0.5 M H3BO3, with the solution pH adjusted to 2 with H2SO4, and the duty cycle was set at 0.2 s (on)/0.8 s (off) at 0.05 A cm−2. The stirring speed of the electrolyte was maintained at 300 rpm. The above product electrodes were denoted as Ni-PC7500-0.05A, Ni95Fe5-PC24000-0.25A, Ni95Fe5-PC60001A, Ni95Fe5-PC3000-2A, Ni90Fe10-PC3000-2A, and Ni80Fe20PC3000-2A. The nomenclature is self-explanatory, with the number following Ni, Fe, and PC denoting the molar percentages of Ni and Fe precursors in the electrolyte and deposition time in seconds, respectively. The fabrication steps are illustrated in Scheme 1. Fabrication of Benchmark Electrodes of Pt/C and IrO2. For comparison purposes, graphite-supported benchmark electrodes were fabricated by loading graphite electrodes with 20 wt % Pt/C and IrO2 powders at 2.5 mg cm−2 for HER and OER, respectively. An amount of 10 mg catalyst was dispersed in 300 μL of 50% aqueous ethanol and 100 μL of polyvinylidene fluoride (0.3 wt % in Nmethylpyrrolidone as the binder) with ultrasonication for 30 min to form a homogeneous ink suspension. A 100 μL of the ink solution was drop-casted onto a graphite electrode, followed by drying at 70 °C in a vacuum oven to afford the benchmark electrodes. Characterizations. The crystallographic texture of the electrodeposited NiFe nanotube arrays was identified with an X-ray diffractometer (Rigaku Ultima IV, Rigaku Corporation) with Cu Kα radiation at a scanning rate of 2°/min. The morphology of the electrodes was observed with a field emission scanning electron microscope (Hitachi S-8010, Hitachi High-Technologies Corp.). Energy-dispersive X-ray spectrometry (EDXS) (EMAX-ENERGY, Horiba, Ltd.) was conducted to determine the elemental composition of the nanotubes with an accelerating voltage of 15 kV. For highresolution transmission electron microscopy (HRTEM) (JEOL 3000F, JEOL Ltd.) observation, an ethanol solution containing the nanowire/nanotube samples was drop-casted onto a Cu grid coated with a lacey carbon film and dried in air for 15 min. High-resolution X-ray photoelectron spectroscopy (ULVAC-PHI XPS, ULVAC-PHI Inc.) was conducted to study the chemical state of the elements in the electrode using monochromatized Al Kα X-ray as the excitation source. Electrochemical Measurements. The OER and HER performances were evaluated on a CHI6275D electrochemical workstation with a conventional three-electrode system. The as-prepared sample was used as the working electrode and a Pt foil (1 cm × 1 cm) or graphite electrode was used as the counter electrode for OER or HER, respectively, with an Hg/HgO electrode (RE-61AP) serving as the reference electrode. Prior to the OER and HER measurements, the working electrode was conditioned with 30 cycles of cyclic voltammetry at a scan rate of 100 mV s−1 to acquire stable electrode conditions and current densities. The linear sweep voltammetry (LSV) curves were recorded in potential windows of 0 to 1.0 V (vs Hg/HgO) and −0.8 to −1.5 V (vs Hg/HgO) for OER and HER, respectively, at a scan rate of 2 mV s−1. The potential values for OER and HER were reported, referring to the reverse hydrogen electrode (RHE) using the Nernst equation ERHE = EHg/HgO + 0.118 + 0.059pH, where EHg/HgO is the experimentally measured potential against the Hg/HgO reference electrode. The overpotential (η) was calculated using the equation η = ERHE − 1.23 for OER. The current density was iR-compensated. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 100 kHz to 100 mHz with a conventional three-electrode system, and the potential was set at a value high enough to ensure the occurrence of OER and HER. All EIS data were fitted with an equivalent circuit model to obtain the chargetransfer resistances. The overall water splitting measurement was performed in a two-electrode system. Prior to the overall water splitting test, the electrode was conditioned with cyclic voltammetry through the potential window of 1.0−2.6 V for 30 cycles with a scan rate of 100 mV s−1. The potential window of this measurement was set at 1−2.6 V with a scan rate of 2 mV s−1.

Research Article

RESULTS AND DISCUSSION The fabrication process of the NiFe nanotube array is illustrated in Scheme 1, and the details are described in the Experimental Section. Six products were fabricated, including Ni-PC7500-0.05A, Ni95Fe5-PC24000-0.25A, Ni95Fe5PC6000-1A, Ni95Fe5-PC3000-2A, Ni90Fe10-PC3000-2A, and Ni80Fe20-PC3000-2A. Ni-PC7500-0.05A serves as the control to investigate the effects of Fe incorporation. The next three products were designed to investigate the effects of the deposition current density. Note that, to keep the total deposition charge input constant, shorter deposition times were used for deposition at higher current densities accordingly. The final two products were fabricated, together with the fourth product, to study the effects of electrolyte composition at increasing Fe loading. Figure 1 shows the morphology and X-ray diffraction (XRD) patterns of the six products. The 1D nanostructure of the six

Figure 1. Top-view SEM images of Ni or NiFe 1D nanostructures: (a) Ni-PC7500-0.05A, (b) Ni95Fe-PC24000-0.25A, (c) Ni95Fe5PC6000-1A, (d) Ni95Fe5-PC3000-2A, (e) Ni90Fe10-PC3000-2A, and (f) Ni80Fe20-PC3000-2A, with the corresponding side-view SEM images as insets. (g) XRD patterns of the abovementioned Ni or NiFe 1D nanostructures.

products can be clearly seen from the top-view and side-view scanning electron microscopy (SEM) images, with uniform diameters of 200−300 nm and lengths of 10−12 μm. Further examination of the top-view images reveals the distinct morphological differences between the products. Ni-PC75000.05A, deposited at a low current density of 0.05 A cm−2, appears as a nanowire array. As for Ni95Fe5-PC24000-0.25A, Ni95Fe5-PC6000-1A, and Ni95Fe5-PC3000-2A, deposited at increasing deposition current densities, the morphology of the 1D nanostructure changes from being mostly nanowire to mostly nanotube. The morphology becomes completely 24098


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shift decreases and then increases again in accord with their Fe contents. The nanostructure of the three nanotube array products, Ni95Fe5-PC3000-2A, Ni90Fe10-PC3000-2A, and Ni80Fe20PC3000-2A, were further investigated with TEM to reveal the structural differences between them. Figure 2a shows the TEM

nanotube for Ni90Fe10-PC3000-2A and Ni80Fe20-PC30002A.29 Interestingly, from the side-view SEM images, the 1D nanostructure remains intact for nanowires as in Ni-PC75000.05A and Ni95Fe5-PC24000-0.25A but much less intact for more nanotube-dominated structures as in Ni90Fe10-PC30002A, Ni90Fe10-PC3000-2A, and Ni80Fe20-PC3000-2A. This phenomenon is a direct result of better mechanical strength associated with nanowire arrays than with nanotube arrays. Consequently, more structural damages occur to the nanotube array samples during the sample preparation process, which involves sample fracturing. The compositions of these 1D nanostructures were estimated with EDXS analyses, as shown in Figure S1, and the results are summarized in Figure S2. There are several points to note from Figure S2. First, for the Ni95Fe5 series products, the Fe content in the product is significantly higher than that in the electrolyte, with 5% in the electrolyte versus 8% in Ni95Fe5-PC24000-0.25A, 18% in Ni95Fe5-PC6000-1A, and 22% in Ni95Fe5-PC3000-2A. This is caused by the well-known anomalous deposition effect of Fe in NiFe co-deposition,30−33 favoring the deposition of Fe because of the local high pH environment inhibiting Ni deposition. Second, for the Ni95Fe5 series products, the Fe content increases with increasing deposition current densities, in good agreement with relevant data reported in the literature. This is caused by the stronger anomalous effect induced at higher deposition current densities.30−33 Third, for products Ni95Fe5-PC3000-2A, Ni90Fe10-PC3000-2A, and Ni80Fe20PC3000-2A, fabricated at increasing Fe loading in the electrolyte, the corresponding Fe content in the product first decreases and then bounces back up. This is a result of competition between several factors. First, in addition to the alloy deposition reaction, there is also a reduction of protons for hydrogen generation as a side reaction. This hydrogen generation reaction releases hydrogen bubbles, which can occupy the pore space of the AAO membrane, forcing the deposition and growth of NiFe to be along the pore walls only and thus creating the nanotube structure. This is why nanowires form at low deposition current densities and nanotubes form at high deposition current densities. Second, the released bubbles not only favor the formation of the nanotube structure but also hinder the mass transfer of the electrolyte. This would decrease the deposition of Fe as Fe deposition is more mass-transfer-limited than Ni.32 This explains why the Fe content of Ni90Fe10-PC3000-2A, 10%, is much less than that of Ni95Fe5-PC3000-2A, 22%. For Ni80Fe20-PC3000-2A, with much more Fe precursors in the electrolyte, twice of that for Ni90Fe10-PC3000-2A, the Fe content increases to 25%. Figure 1g shows the XRD patterns for all six products, with the standard XRD pattern of Ni, JCPDS 87-0712, included for comparison. The XRD pattern of Ni-PC7500-0.05A, a pure Ni nanowire array, agrees well with that of the standard one as expected. The minor diffraction peak, located at 2θ of 65° for the (220) crystalline planes of Au, is contributed by the Au layer. As shown above, with the loading of the Fe precursor, the Fe content of the product increases with increasing deposition current densities. As the atomic radius of Fe is larger than that of Ni, the substitutional alloying of Fe into Ni expands the crystalline lattices, leading to a slight left shift of the diffraction peaks. This left shift increases with increasing deposition current densities because of the increasing Fe content in the product, as evident in Figure 1g. As for Ni90Fe10-PC3000-2A and Ni80Fe20-PC3000-2A, the left

Figure 2. (a) TEM image of Ni95Fe5-PC3000-2A, with the corresponding SAED pattern as inset. (b) HRTEM image of Ni95Fe5-PC3000-2A, with the inset showing locally enlarged lattice fringes. (c) TEM image of Ni90Fe10-PC3000-2A, with the corresponding SAED pattern as inset. (d) TEM image of Ni80Fe20-PC3000-2A, with the corresponding SAED pattern as inset.

image of the nanotubes obtained from Ni95Fe5-PC3000-2A. It clearly shows that the nanotubes are not complete through pore structure. The bubbles released during the deposition process apparently are not strong enough to generate a complete nanotube structure. Figure 2b shows the HRTEM image of the partially hollow nanotube, from which an interlayer distance of 0.209 nm is determined, in good agreement with the d-spacing of the crystal planes of (111) of NiFe alloys. Figure 2c,d shows the TEM images of nanotubes obtained from Ni90Fe10-PC3000-2A and Ni80Fe20-PC3000-2A. The nanotube structure can be readily identified from the images, with a tube wall thickness of 105 nm for Ni90Fe10-PC3000-2A and 55 nm for Ni80Fe20PC3000-2A. Here, the tube wall of Ni80Fe20-PC3000-2A is thinner than that of Ni90Fe10-PC3000-2A, mainly because of the stronger bubble-releasing effect involved in higher Fe content electrolytes. Furthermore, high-angle annular darkfield scanning transmission electron microscopy (HAADF− STEM) images and the accompanying EDXS elemental mapping (Figures S3 and S4) confirm the nanotube structure and the uniform distributions of Ni and Fe in the tube shells of Ni95Fe5-PC3000-2A and Ni80Fe20-PC3000-2A, respectively. Ni80Fe20-PC3000-2A possesses a significantly thinner wall than Ni90Fe10-PC3000-2A and can offer more surface areas for water splitting reactions. The crystallinity of Ni80Fe20PC3000-2A, however, is inferior to that of Ni90Fe10-PC30002A, as can be judged from the selected-area electron diffraction (SAED) patterns of both products shown as insets in Figure 2c,d. It has been pointed out that crystallinity of the electrocatalyst plays an important role in water splitting, better 24099


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Figure 3. OER performances of Ni or NiFe 1D nanostructures. LSV curves of Ni or NiFe 1D nanostructures at (a) low current densities and (b) high current densities. (c) Tafel plots of Ni or NiFe 1D nanostructures. (d) Nyquist plots of Ni or NiFe 1D nanostructures recorded at 1.635 V (vs RHE) over a frequency range of 100 kHz to 100 mHz in 1 M KOH.

crystallinity being advantageous.34−36 The interplay among the surface area, crystallinity, and composition is complicated, from which the comparison performances of Ni90Fe10PC3000-2A and Ni80Fe20-PC3000-2A toward water splitting reactions are determined. The OER performances of all six products were measured in a conventional three-electrode electrochemical cell in 1 M KOH at room temperature. Figure 3a,b shows the LSV curves of the six products at small and large current density scales, respectively, from which the overpotentials achieved at low (10 mA cm−2) and high (400 mA cm−2) current densities can be readily determined. At 10 mA cm−2, Ni90Fe10-PC3000-2A and Ni95Fe5-PC3000-2A achieve the lowest overpotential of 236 mV, outperforming Ni-PC7500-0.05A (405 mV), Ni95Fe5-PC24000-0.25A (252 mV), Ni95Fe5-PC6000-1A (248 mV), and Ni80Fe20-PC3000-2A (256 mV). The LSV curve of Ni-PC7500-0.05A shows a pronounced oxidation peak located at around 270 mV, attributable to the oxidation of Ni2+ to Ni3+.37 This oxidation peak is followed by the emergence of OER, showing sharply increasing current densities. The two reactions are well separated for NiPC7500-0.05A, from which the overpotential for OER can be determined without the possible interference from the oxidation peak. When alloying the Ni nanowire array with Fe, the synergistic effect between Ni and Fe pushes the oxidation peak left toward the low-potential region and accelerates OER,12 making the two reactions partially overlapped with each other and the oxidation peak appearing as a shoulder of the OER segment, as evident from the LSV curves of the NiFe products. Under this circumstance, it is important to avoid the oxidation shoulder when determining the overpotential for OER. Generally speaking, alloying Ni with Fe greatly improves the OER activity of the product, as evident from Figure 3a,b.

The overpotentials were also determined in diluted electrolytes (0.1 M KOH), as shown in Figure S8. The trend is the same with that observed in 1 M KOH, with Ni90Fe10-PC3000-2A showing the lowest overpotential (η10 = 261 mV), outperforming Ni-PC7500-0.05A (η10 = 445 mV), Ni95Fe5PC24000-0.25A (η10 = 300 mV), Ni95Fe5-PC6000-1A (η10 = 281 mV), Ni95Fe5-PC3000-2A (η10 = 278 mV), and Ni80Fe20-PC3000-2A (η10 = 306 mV). For large-scale applications of the electrolytic water splitting for hydrogen production, the overpotentials at high current densities, such as 400 mA cm−2, denoted as η400, are most critical. Here, η400 is also determined in 1 M KOH and discussed. Again, Ni90Fe10-PC3000-2A gives the lowest overpotential, requiring only 371 mV to reach 400 mA cm−2, which outperforms Ni-PC7500-0.05A (η200 = 657 mV), Ni95Fe5-PC24000-0.25A (η 400 = 460 mV), Ni95Fe5PC6000-1A (η400 = 430 mV), Ni95Fe5-PC3000-2A (η400 = 410 mV), and Ni80Fe20-PC3000-2A (η400 = 407 mV). It is evident that the incorporation of Fe in Ni also greatly improves the OER activity of the product at high current densities. For the Ni95Fe5 series products, the products fabricated at higher deposition current densities, possessing higher Fe contents, exhibit progressively better OER activities. This trend is consistent with the finding observed in the literature, increasing the improvement with increasing Fe incorporation and reaching an improvement plateau at 20−35% Fe.38,39 Composition, however, is not the sole factor to determine the OER activity. Nanostructure and crystallinity also play important roles. It is interesting to compare η400 of Ni95Fe5PC24000-0.25A and Ni90Fe10-PC3000-2A, whose compositions are close−−8% Fe for Ni95Fe5-PC24000-0.25A and 10% for Ni90Fe10-PC3000-2A. The η400 value of Ni90Fe10PC3000-2A (371 mV) however is significantly lower than 24100


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Figure 4. HER performances of Ni or NiFe 1D nanostructures. LSV curves of Ni or NiFe 1D nanostructures at (a) low current densities and (b) high current densities. (c) Tafel plots of Ni or NiFe 1D nanostructures. (d) Nyquist plots of Ni or NiFe 1D nanostructures recorded at −0.165 V (vs RHE) over a frequency range of 100 kHz to 100 mHz in 1 M KOH.

versus scan rate, as shown in Figure S6. Cdl is directly proportional to ECSA and can be used as a comparison parameter. Here, Ni80Fe20-PC3000-2A exhibits the highest Cdl of 7.4 mF cm−2, outperforming Ni90Fe10-PC3000-2A (Cdl = 6.2 mF cm−2), Ni95Fe5-PC3000-2A (Cdl = 5.9 mF cm−2), Ni95Fe5-PC6000-1A (Cdl = 5.55 mF cm−2), Ni95Fe5PC24000-0.25A (Cdl = 5.3 mF cm−2), and Ni-PC7500-0.05A (Cdl = 3.8 mF cm−2). This is expected as Ni80Fe20-PC30002A is with a nanotube structure of thin tube walls and can accommodate abundant active sites. Ni90Fe10-PC3000-2A comes in second, with a Cdl slightly lower than that of Ni80Fe20-PC3000-2A, as it is with thicker tube walls. Nevertheless, the poor crystallinity of Ni80Fe20-PC3000-2A leads to OER performances inferior to those of Ni90Fe10PC3000-2A. These Cdl values were further used to normalize the current densities achieved at an overpotential of 350 mV to serve as a measure of intrinsic specific activities of the electrodes. The results are summarized in Table S3 for comparison. Evidently, Ni90Fe10-PC3000-2A stands out as the most active electrode, as expected, and the trend is consistent with that of TOF and MA. To further examine the OER activity, Tafel slopes, signifying the increase in the overpotential needed to achieve a ten-fold increase in current densities, were determined from recasting the LSV data into an overpotential versus log(current density) plot, as shown in Figure 3c, whose slope is the Tafel slope. Small Tafel slopes signify better electrochemical activities. From Figure 3c, the Tafel slopes of Ni-PC7500-0.05A, Ni95Fe5-PC24000-0.25A, Ni95Fe5-PC6000-1A, Ni95Fe5PC3000-2A, Ni90Fe10-PC3000-2A, and Ni80Fe20-PC30002A are 117, 64, 54, 51, 45, and 57 mV dec−1, respectively, with again Ni90Fe10-PC3000-2A exhibiting the best OER activity and all NiFe alloy products largely outperforming the pure Ni product.

that of Ni95Fe5-PC24000-0.25A (460 mV). This can be attributed to the nanotube structure of Ni90Fe10-PC3000-2A, offering enlarged surface areas to accommodate abundant active sites to boost the OER efficiency. Furthermore, both Ni90Fe10-PC3000-2A and Ni80Fe20-PC3000-2A possess a nanotube structure, and Ni80Fe20-PC3000-2A is with a much higher Fe content (25%) than that of Ni90Fe10-PC3000-2A. Nevertheless, η400 of Ni90Fe10-PC3000-2A (371 mV) is lower than that of Ni80Fe20-PC3000-2A (407 mV). This may be attributed to the better crystallinity of Ni90Fe10-PC3000-2A, as discussed in an earlier section, outweighing the effect of high Fe content of Ni80Fe20-PC3000-2A on OER activities. In summary, many factors affect the OER activities of alloy catalysts, including composition, nanostructure, crystallinity, and so forth. We further compare the electrocatalytic performances of the samples in terms of turnover frequencies (TOF) and mass activities (MA), with the overpotential fixed at 350 mV and the mass loading estimated from the population densities, geometric dimensions, and compositions of the nanowire/nanotube arrays. The results are summarized in Tables S1 and S2. Evidently, the trends of TOF and MA are in good agreement with that of the overpotentials. Ni90Fe10PC3000-2A exhibits the highest intrinsic catalytic activity among all tested samples, with a TOF of 0.39 10−1 s−1 and an MA of 129.7 A g −1 . In addition to TOF and MA, electrochemically active surface areas (ECSA) were also estimated to correlate with the electrochemical performances of the product electrodes. To determine ECSA, cycling voltammograms were first recorded at increasing scan rates for the sample electrodes in the potential window of 0−0.1 V (vs Hg/HgO) in 1 M KOH, as shown in Figure S5. Doublelayer capacitances (Cdl) of the sample electrodes were then obtained from the slopes of the fitted lines of capacitive current density differences (ΔJ), collected at 0.05 V versus Hg/HgO, 24101


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ACS Applied Materials & Interfaces To further investigate the kinetics of OER, EIS was conducted at 1.635 V (vs RHE) over a frequency range of 100 kHz to 100 mHz in 1 M KOH. The applied potential of 1.635 V was chosen to ensure the occurrence of OER for all products. The resulting Nyquist plot was curve-fitted with an equivalent circuit model shown in Figure 3d. The present equivalent circuit model has been commonly adopted for modeling the electrochemical reactions involved in electrolytic water splitting,12 and it includes electronic units of system resistances, electrode porosity resistance, charge-transfer resistance, constant phase element, and double-layer capacitance, denoted as Rs, Rp, Rct, CPE, and Cdl, respectively. The critical parameter Rct, quantifying the resistances present to the key charge-transfer reaction (OER or HER), was determined and summarized in Table S3 for comparison. Evidently, Ni90Fe10-PC3000-2A possesses the lowest Rct value of 0.15 Ω, as expected, outperforming Ni-PC7500-0.05A (Rct = 2.11 Ω), Ni95Fe5-PC24000-0.25A (Rct = 0.59 Ω), Ni95Fe5PC6000-1A (Rct = 0.22 Ω), Ni95Fe5-PC3000-2A (Rct = 0.17 Ω), and Ni80Fe20-PC3000-2A (Rct = 0.27 Ω). The present NiFe 1D nanostructure arrays not only are excellent electrocatalysts for OER but also perform well in HER. Figure 4a,b shows the LSV curves from which the overpotentials at low (10 mA cm−2) and high (400 mA cm−2) current densities are determined. Evidently, pure Ni nanowire arrays (Ni-PC7500-0.05A) exhibit the lowest overpotential at 10 mA cm−2 (only 75 mV), outperforming all NiFe 1D nanostructure arrays, including Ni95Fe5-PC24000-0.25A (132 mV), Ni95Fe5-PC6000-1A (153 mV), Ni95Fe5-PC3000-2A (201 mV), Ni90Fe10-PC3000-2A (100 mV), and Ni80Fe20PC3000-2A (231 mV). Evidently, the incorporation of Fe into Ni deteriorates the HER performance of the product, in accord with the finding reported in the literature.40 The value of η10 in fact increases with increasing Fe content, from 132 mV for Ni95Fe5-PC24000-0.25A (8% Fe) to 201 mV for Ni95Fe5PC3000-2A (22%). Nevertheless, among the five NiFe 1D nanostructure arrays, Ni90Fe10-PC3000-2A performs the best, attributable to its low Fe content of 10% and nanotube structure for enlarged reaction surface areas over the nanowire structure. There are also studies claiming that alloying Ni thin films with Fe improves the HER performance,41,42 which is contradictory to our finding with the 1D nanostructure arrays of Ni or NiFe. This discrepancy may arise from the detailed crystalline structure of the sample. For the present study, the 1D nanostructure is created with PC electrodeposition, from which nanoscale twin structures possessing twin boundaries (TBs) may form within the product.25 TBs are generated when two separate crystals share some of the same crystal lattice points in a symmetrical manner, leading to the intergrowth of two separate crystals in a variety of specific configurations. The presence of TBs can induce drastic effects on material properties, such as enhancements in mechanical strength, thermal stability, and electromigration resistances.43−45 As shown in Figure S7a,b, the Ni nanowires detached from NiPC7500-0.05A possess nanoscale twin structures, and the size of the T B spacing is around 25−66 nm. When the TB spacing is larger than 10 nm, the surface of the product, nanowires here, has a tendency to be modified to high-energy facets,46,47 which can readily trigger heterogeneous reactions such as HER. Once alloyed with Fe, the grain size of the product would be decreased because of the formation of solid solutions. This would lead to reduction in twinning propensity because of the high activation stress needed for twinning in small grains.48

Consequently, the overpotential for HER increases with increasing Fe content in the 1D NiFe alloy nanostructure. The overpotentials were also determined in diluted electrolytes (0.1 M KOH), as shown in Figure S9. The trend is the same with that observed in 1 M KOH, with Ni-PC7500-0.05A showing the lowest overpotential (η10 = 142 mV) and Ni90Fe10-PC3000-2A coming in second (η10 = 164 mV), outperforming Ni95Fe5-PC24000-0.25A (η10 = 190 mV), Ni95Fe5-PC6000-1A (η10 = 225 mV), Ni95Fe5-PC3000-2A (η10 = 255 mV), and Ni80Fe20-PC3000-2A (η10 = 286 mV). The overpotentials at 400 mA cm−2 are also determined in 1 M KOH for practical application consideration. The pure Ni nanowire array, Ni-PC7500-0.05A, still exhibits the lowest η400 of 352 mV, outperforming all NiFe 1D nanostructure arrays. Nevertheless, Ni90Fe10-PC3000-2A gives the same level of performance of 360 mV, the lowest among the five NiFe products, with 370 mV for Ni95Fe5-PC24000-0.25A, 395 mV for Ni95Fe5-PC6000-1A, 458 mV for Ni95Fe5-PC3000-2A, and 483 mV for Ni80Fe20-PC3000-2A. The HER performances of Ni90Fe10-PC3000-2A, although not as good as that of the pure Ni nanowire array, compete favorably with many state-of-the-art non-noble metal-based HER electrocatalysts in terms of η10, such as Ni/Ni(OH)2/graphite (∼140 mV),49 NiCo2O4 (110 mV in 1 M NaOH),50 TiO2@Co9S8 corebranch arrays (139 mV),51 Ni-BDT-A (80 mV),52 NiCu@C-1 (74 mV),53 porous NiFe-oxide nanocubes (197 mV),54 Pt@ PCM (139 mV),55 and CoMoS-24 h (98 mV).56 Tafel slopes, as shown in Figure 4c, are next discussed. As expected, the pure Ni nanowire array possesses the smallest Tafel slope (71 mV dec−1), outperforming all NiFe 1D nanostructure arrays. Ni90Fe10-PC3000-2A again exhibits the smallest Tafel slope of 78 mV dec−1 among the five NiFe 1D nanostructure arrays, with 88 mV dec−1 for Ni95Fe5-PC24000-0.25A, 94 mV dec−1 for Ni95Fe5-PC6000-1A, 106 mV dec−1 for Ni95Fe5-PC30002A, and 127 mV dec−1 for Ni80Fe20-PC3000-2A. Furthermore, EIS was conducted at −0.165 V (vs RHE) for all six products over a frequency range of 100 kHz to 100 mHz in 1 M KOH to extract the critical parameter Rct. The resulting Nyquist plots are presented in Figure 4d, and the determined Rct values are summarized in Table S4. As expected, the pure Ni nanowire array exhibits the smallest Rct value (3.33 Ω), outperforming all NiFe 1D nanostructure arrays. Ni90Fe10PC3000-2A again exhibits the smallest Rct value of 4.01 Ω among the five NiFe 1D nanostructure arrays, in accord with the results of overpotentials and Tafel slopes. In addition, the estimated TOF and MA, as listed in Tables S1 and S2, also show the same trend as those of the overpotential and Rct. Ni90Fe10-PC3000-2A exhibits the highest TOF of 0.46 10−1 s−1 and MA of 377.7 g A−1 among the five NiFe products, which can be attributed to its low Fe content and nanotube structure. In addition, Cdl-normalized current densities achieved at an overpotential of 350 mV, as summarized in Table S3, also show the same trend as those of TOF and MA. From the above results, Ni90Fe10-PC3000-2A shows excellent electrochemical performances for both OER and HER and is applied as a bifunctional electrocatalyst for overall water splitting in 1 M KOH. The electrochemical performances of the Ni90Fe10-PC3000-2A//Ni90Fe10-PC3000-2A couple are measured and compared with those of the other five 1D nanostructure-based couples. The pairing of the benchmark electrodes, Pt/C for HER and IrO2 for OER, for overall water splitting was also characterized and included for comparison. Figure 5a,b shows the LSV curves recorded for 24102


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Figure 5. Overall water splitting performances of Ni or NiFe 1D nanostructures and Pt/C//IrO2. LSV curves of Pt/C//IrO2, Ni-PC7500-0.05A// Ni-PC7500-0.05A, Ni95Fe5-PC24000-0.25A//Ni95Fe5-PC24000-0.25A, Ni95Fe5-PC6000-1A//Ni95Fe5-PC6000-1A, Ni95Fe5-PC3000-2A// Ni95Fe5-PC3000-2A, Ni90Fe10-PC3000-2A//Ni90Fe10-PC3000-2A, and Ni80Fe20-PC3000-2A//Ni80Fe20-PC3000-2A couples in 1 M KOH at (a) low current densities and (b) high current densities. (c) Corresponding Tafel plots. (d) Chronoamperometric stability test of the Ni90Fe10PC3000-2A//Ni90Fe10-PC3000-2A couple for 24 h operation at 50, 100, and 400 mA cm−2.

the seven couples at low and high current densities from which the cell voltages needed to achieve the current densities of 10 and 400 mA cm−2 can be determined. The Ni90Fe10-PC30002A//Ni90Fe10-PC3000-2A couple needs only 1.58 V to deliver a current density of 10 mA cm−2, close to 1.55 V achieved by the Pt/C//IrO2 couple and much smaller than those of the other five couples, 1.69 V for Ni-PC7500-0.05A// Ni-PC7500-0.05A, 1.62 V for Ni95Fe5-PC24000-0.25A// Ni95Fe5-PC24000-0.25A, 1.64 V for Ni95Fe5-PC6000-1A// Ni95Fe5-PC6000-1A, 1.67 V for -Ni95Fe5-PC3000-2A// Ni95Fe5-PC3000-2A, and 1.69 V for Ni80Fe20-PC30002A//Ni80Fe20-PC3000-2A. The performance of the Ni90Fe10-PC3000-2A//Ni90Fe10-PC3000-2A couple in fact competes favorably with many recently reported state-of-theart bifunctional electrocatalysts, including Ni−Co−P HNBs (1.62 V),57 Ni11(HPO3)8(OH)6/NF (1.6 V),58 δ-FeOOH NSs/NF (1.62 V),59 CoP@3D Ti3C2-MXene (1.58 V),60 FeNi@NC-CNTs/NF (1.98 V@145 mA cm−2),61 NDCHN-35 (1.701 V),62 NiCoS (1.58 V),63 and EBP@NG (1:8) (1.59 V).64 For more details, please refer to Table S8. At high current densities, such as 400 mA cm−2, the Ni90Fe10PC3000-2A//Ni90Fe10-PC3000-2A couple needs only 2.16 V to reach 400 mA cm−2, again close to 2.06 V achieved by the Pt/C//IrO2 couple, and outperforms the five comparison couples, 2.86 V@200 mA cm−2 for Ni-PC7500-0.05A//NiPC7500-0.05A, 2.19 V for Ni95Fe5-PC24000-0.25A// Ni95Fe5-PC24000-0.25A, 2.21 V for Ni95Fe5-PC6000-1A// Ni95Fe5-PC6000-1A, 2.2 V for Ni95Fe5-PC3000-2A// Ni95Fe5-PC3000-2A, and 2.26 V for Ni80Fe20-PC30002A//Ni80Fe20-PC3000-2A. Tafel slopes were also determined for the seven couples for overall water splitting, and the results are presented in Figure

5c. The Ni90Fe10-PC3000-2A//Ni90Fe10-PC3000-2A couple exhibits a small Tafel slope of 147 mV dec−1, only slightly larger than 125 mV dec−1 of the Pt/C//IrO2 couple but smaller than the five comparison couples, 464 mV dec−1 for Ni-PC7500-0.05A//Ni-PC7500-0.05A, 150 mV dec−1 for Ni95Fe5-PC24000-0.25A//Ni95Fe5-PC24000-0.25A, 157 mV dec−1 for Ni95Fe5-PC6000-1A//Ni95Fe5-PC6000-1A, 160 mV dec−1 for Ni95Fe5-PC3000-2A//Ni95Fe5-PC30002A, and 166 mV dec−1 for Ni80Fe20-PC3000-2A//Ni80Fe20PC3000-2A. The Ni90Fe10-PC3000-2A//Ni90Fe10-PC30002A couple also exhibits excellent stability, experiencing a minor chronoamperometric decay after 24 h of continuous operation at a high current density of 400 mA cm−2 and a negligible decay at 50 and 100 mA cm−2, as shown in Figure 5d. The morphological, compositional, and crystalline phase stabilities of Ni90Fe10-PC3000-2A were further examined with SEM (Figure S10), EDXS (Figures S11 and S12), and XRD (Figure S13), respectively. It is evident from Figure S10 that the nanotube array structure of Ni90Fe10-PC3000-2A was well maintained after the stability test. The composition of the electrocatalyst also remains the same, as can be judged from Figures S11 and S12. Figure S13 shows similar XRD patterns for the samples before and after the stability test, indicating the stable crystalline phase. Also examined are the high-resolution XPS spectra of Ni 2p3/2 and Fe 2p3/2 before and after the stability test, as compared in Figure S14. As shown in Figure S14a, for the sample before the test, the Ni 2p3/2 peak can be deconvoluted into three constituent peaks for Ni metal (852.1 eV), NiO (853.9 eV), and Ni(OH)2 (855 eV).65,66 As for the Fe 2p3/2 spectrum shown in Figure S14b, the constituent peaks for Fe metal (706.9 eV), FeO (709.5 eV), Fe2O3 (711 eV), and FeOOH (712.1 eV) can be identified.67,68 It is to be noted that 24103


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transfer, proves to be a promising new catalyst architecture design for electrocatalytic processes.

before the stability test, a thin oxidation layer is formed on the surface of the NiFe alloy as it is exposed to air and only minor amounts of Ni and Fe were detected with XPS. After the stability test (Figure S14b,d), large amounts of NiOOH (856 eV) and FeOOH (712.1 eV) were detected, indicating the formation of active species during water splitting. The atomic percentages of the contributing species of Ni 2p3/2 and Fe 2p3/2 before and after the stability tests are determined from the XPS spectra (Figure S14). The results are summarized in Tables S6 and S7 for comparison. Evidently, the valence states of both Ni and Fe become more positive after the stability tests, with the increasing dominance of the 3+ state contributed by the active species, NiOOH and FeOOH. Furthermore, it has been confirmed with DFT simulations that the presence of Fe sites in NiOOH optimizes the critical adsorption energies of OER intermediates (OH, O, OOH), thereby reducing the required overpotentials for OER.69 This conclusion is in good accord with the finding of the present work. We further determined Faradaic efficiency of the overall water splitting reaction of the Ni90Fe10-PC3000-2A// Ni90Fe10-PC3000-2A couple. The production rates of H2 and O2 when operated at 50 mA cm−2 for 60 min were measured with a gas chromatography analyzer. The results are compared with the theoretical values calculated from the measured current densities. As shown in Figure S15, the agreement between the measured and calculated values is excellent, and the ratio of the H2 and O2 production rates matches well with the theoretical value of 2, suggesting Faradaic efficiencies of close to 100%. The results indicate that no side reactions and gas crossover occur during the water splitting operation. Last but not least, a video (Video S1) is provided in the Supporting Information for a simple demonstration of the power-to-gas (P2G) and gas-to-power (G2P) approach for future green and sustainable energy infrastructure.70 Here, for the P2G process, solar cells drive water electrolysis to generate hydrogen, corresponding to storing electrical energy in the form of chemical energy. As for the G2P process, hydrogen is fed into fuel cells to convert chemical energy back into electrical energy. In the video, a full cell, Ni90Fe10-PC3000-2A//Ni90Fe10-PC3000-2A, was driven with a solar panel illuminated with simulated sun light to produce hydrogen that was in turn fed into a fuel cell to generate electricity to drive an electric fan.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05919. The Supporting Information is available free of charge on ACS publications website: Schematic for the formation of NiFe nanotube arrays; SEM images and EDXS spectra of NiFe 1D nanostructures; atomic ratios of Ni−Fe of Ni or NiFe 1D nanostructures; HAADF− STEM images and EDXS elemental mappings of Ni90Fe10-PC3000-2A and Ni80Fe20-PC3000-2A; estimated mass loadings, TOF, MA, and Cdl normalized current densities for OER and HER measured at an overpotential of 350 mV; cyclic voltammograms recorded at increasing scan rates in 1 M KOH; capacitive current density difference achieved at 0.05 V (vs Hg/HgO) versus scan rates in 1 M KOH; chargetransfer resistances for OER and HER from fitting the Nyquist plots with an equivalent circuit model; TEM images of Ni-PC7500-0.05A; LSV curves of Ni or NiFe 1D nanostructures in 0.1 M KOH for OER and HER; SEM images, EDXS spectra, atomic ratios, XRD patterns, high-resolution XPS spectra, atomic percentages of the contributing species of Ni 2p3/2 and Fe 2p3/2 of Ni90Fe10-PC3000-2A before and after stability tests; experimental and theoretical amounts of H2 and O2 production by the Ni90Fe10-PC3000-2A//Ni90Fe10PC3000-2A couple for overall water splitting at 50 mA cm−2; and comparison table of overall water splitting performances for bifunctional electrocatalysts (PDF) Video demonstration of the P2G and G2P approach using the Ni90Fe10-PC3000-2A//Ni90Fe10-PC30002A couple (MP4)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shih-Yuan Lu: 0000-0003-3217-8199

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Notes

The authors declare no competing financial interest.

CONCLUSIONS A unique electrocatalyst structure of nanotube array was successfully fabricated with an AAO membrane-templated high current density pulse electrodeposition method. Suitably balancing the hydrogen bubble generation and NiFe alloy deposition leads to the formation of NiFe nanotube arrays, and the NiFe nanotube array was proven to be an outstanding bifunctional electrocatalyst for overall water splitting. At optimal fabrication conditions, the resulting NiFe nanotube array delivers 10 mA cm−2 at an overpotential of 236 mV for OER and 100 mV for HER in 1 M KOH, together requiring only an ultralow cell voltage of 1.58 V to reach 10 mA cm−2 for the overall water splitting. The stability of the NiFe nanotube array is excellent even when operated at high current densities, exhibiting only a slight chronoamperometric decay of 6.5% after a 24 h operation at 400 mA cm−2. This binder-free NiFe nanotube array structure, offering much enlarged reaction surface areas and 1D guided charge transport and mass

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ACKNOWLEDGMENTS The authors acknowledge the financial support offered by the Ministry of Science and Technology of Taiwan, ROC, Chang Chun Petrochemical Corporation, and Swancor Ind. Co., Ltd. under grant MOST 106-2622-8-007-017.

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REFERENCES

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NiFe Alloy Nanotube Arrays as Highly Efficient Bifunctional

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