Nickel–Iron (Oxy)hydroxide

Feb 15, 2018 - Abstract Image. Designing well-controlled hierarchical structures on micrometer and nanometer scales represents one of the most importa...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 8585−8593

Radially Aligned Hierarchical Nickel/Nickel−Iron (Oxy)hydroxide Nanotubes for Efficient Electrocatalytic Water Splitting Zhihan Wu, Zhiqiang Wang, and Fengxia Geng* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: Designing well-controlled hierarchical structures on micrometer and nanometer scales represents one of the most important approaches for upgrading the catalytic abilities of electrocatalysts. Although NiFe (oxy)hydroxide has been widely studied as a water oxidation catalyst due to its high catalytic capability and abundance, its structural manipulation has been greatly restricted due to its inherent crystallographic stacking feature. In this work, we report for the first time the construction of a nanotube structure of NiFe (oxy)hydroxide with an inner Ni-rich layer, which was radially aligned on a macroporous nickel foam. Such a hierarchically structured material realized several crucial factors that are essential for excellent catalytic behaviors, including abundant catalytic sites, a high surface area, efficient ionic and electronic transport, etc., and the designed catalyst exhibited competitive electrocatalytic activity for reaction of not only oxygen evolution but also hydrogen evolution, which is very rare. As a result, this novel material was well-suited for the use as a bifunctional catalyst in an integrated water-splitting electrolyzer, which could be even driven by a single AA battery or a 1.5 V solar cell, outperforming a benchmark catalyst of noble-metal ruthenium−platinum combinations and most state-of-the-art electrocatalysts. The work provided important suggestions for the rational modulation of catalysts with new structures targeted for high-performance electrodes used in electrochemical applications. KEYWORDS: (oxy)hydroxide, NiFe, two-dimensional sheets, electrocatalyst, nanostructuring, water splitting

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INTRODUCTION Electrochemical water splitting to store electric energy in the form of chemical bonds holds the ultimate potential to provide future sustainable and regenerative energy supplies.1,2 The core reactions of the electrocatalytic oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), which involve breakage of O−H bonds and formation of O−O double bonds accompanied by the release of protons and electrons, are unfortunately kinetically sluggish, for which the practical application of water splitting demands advanced, stable, and efficient electrocatalysts to expedite the electrolysis rates and minimize dynamic overpotentials.3 Although Pt- and Ru/Irbased materials are state-of-the-art catalysts and respectively exhibit satisfactory catalytic activity toward the HER and OER reactions, the high costs, as well as the natural scarcity, of these noble metals pose substantial obstacles to their large-scale applications for electrolysis. Recently, earth-abundant transition metal-based compounds have been extensively pursued as promising inexpensive alternatives, including transition metal (oxy)hydroxides/oxides for OER and chalcogenides, phosphides, and carbides for HER. 4−7 However, the real implementation of these materials as bifunctional catalysts in a full cell for overall water splitting is seriously impeded by the mismatch between the optimal electrolytes for the reactions in the two directions, OER catalysts perform the best under alkaline conditions, whereas HER catalysts prefer acidic media.8,9 Meanwhile, the use of electrodes that are made © 2018 American Chemical Society

from two different materials for individual HER and OER reactions would inevitably introduce cost concerns and manufacturing complexities. Hence, it is seriously urgent to explore bifunctional electrocatalysts with high activity toward both the OER and HER reactions under the same conditions. Among the numerous suggested transition metal-based compounds, nickel−iron (NiFe) (oxy)hydroxides have been intensely investigated due to their superior catalytic characteristics and rich abundance in nature.10,11 It is noteworthy that although this class of materials is well known to exhibit a competitive OER performance, the activity toward the HER reaction in the same alkaline media is generally believed to be quite poor. The origin for its outstanding oxygen electrocatalysis activity has been recently reported to arise from the reactive Fe species at the edge/defect sites.12 Numerous efforts have been made to tune its catalytic activities, including morphological manipulation,13 defect engineering,14 heteroatom doping,15 etc., among which varying morphologies or regulating multiscale structures have proven to be very simple and effective by enabling sufficient mass transport, abundant catalytically active sites, and good accessibility of catalyst surfaces. For example, NiCo2O4, a typical OER catalyst, simultaneously exhibited an excellent HER behavior when the Received: November 7, 2017 Accepted: February 12, 2018 Published: February 15, 2018 8585

DOI: 10.1021/acsami.7b16953 ACS Appl. Mater. Interfaces 2018, 10, 8585−8593

Research Article

ACS Applied Materials & Interfaces

Figure 1. Morphology and structural characterization of the hierarchical hollow Ni/NiFe (oxy)hydroxide. (a) Schematic illustration of the heterostructured nanotubes, which are radially aligned on the Ni foam. (b, c) Scanning electron microscopy (SEM) and (d, e) transmission electron microscopy (TEM) images of the nanotubes. Inset in (d): corresponding selected-area electron diffraction (SAED) pattern. The rings indicated with 1, 3, and 4 were from (10), (11), and (20) of lactic dehydrogenase (LDH) in-plane structure, and the one marked with 2 should be ascribed to (111) of Ni particles. (f) High-angle annular dark field scanning TEM image and elemental line scanning at the wall edge region, and (g−i) zoom-in TEM views for NiFe (oxy)hydroxide, crystalline Ni, and the interface area.

material with a one-dimensional nanowire morphology was built into a hollow and porous cuboid assembly.16 MoS2 sheets manifested significantly enhanced HER electrocatalysis by modulating them into a uniform mesoporous foam.17 However, for NiFe (oxy)hydroxide, because of the crystallographic layerstacking structure, it is preferably crystallized in the form of sheets or plates with strong shape anisotropy, which naturally aggregates into flat films that lay flat on an electrode substrate.11,18,19 Thus, the delicate structural design of NiFe (oxy)hydroxide for enhancing the catalytic activity toward water splitting still remains a great challenge. In this work, utilizing a simple template strategy, we present the synthesis of a novel hierarchical Ni/NiFe (oxy)hydroxide nanotube heterostructure, which is radially aligned on a Ni foam for highly efficient OER and HER processes, as schematically illustrated in Figure 1a. Ni was in the form of nanoparticles and served as seeds for the perpendicular growth of NiFe (oxy)hydroxide, which was a porous agglomeration of ultrathin sheets with a thickness of only a few molecular layers. The constructed novel structure possessed three important advantages: (i) multiscale porosity, provided by the macro- and mesoporous support of the Ni foam in combination with the nanoscale assembly and the constructed nanotube structure, which facilitated the efficient penetration and fast transport of liquid electrolyte ions and the release of generated H2/O2 bubbles; (ii) the ultrathin thickness of the NiFe (oxy)hydroxide sheets and the unusual vertical growth, which vastly increased the number of active sites, that is, on the whole sheet surfaces besides those at the edges/defects for normal “bulk” materials; and (iii) the inner crystalline Ni-rich layer, which ensured

highways for electron conductivity, thus significantly enhancing the intrinsic catalytic activity. This hierarchical Ni/NiFe nanotube electrocatalyst delivered highly competing activities toward OER, HER, and the overall water-splitting reactions in alkaline electrolytes (details are in the respective tables in the Supporting Information). Due to the excellent catalytic activity, an alkaline electrolyzer with a deliberately designed hierarchical structure as a bifunctional catalyst can even be well operated by employing a single AA battery or a solar panel with a nominal voltage of ∼1.5 V at room temperature, which is considerably lower than most state-of-the-art water-splitting electrocatalysts. The successful material design and the demonstration of great potential as efficient electrocatalysts may open new opportunities for the design of novel architectures of two-dimensional sheet materials as high-performance electrodes used in electrochemical applications.

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RESULTS AND DISCUSSION The radially aligned Ni/NiFe (oxy)hydroxide nanotubes were supported on a nickel foam and the fabrication process is schematically illustrated in the Supporting Information, successively electrodepositing ZnO, Ni, and NiFe (oxy)hydroxide structures followed by a final removal of the ZnO core in NaOH. The characterization details for the products at the intermediate reaction steps are provided in the Supporting Information (Figures S1 and S2). The evolution of the sample surface characteristics at the progressing steps was examined using the scanning electron microscopy (SEM) technique. ZnO was grown as nanorods with an almost standard hexagonal prismatic shape, with dimensions for the base and side length of 8586

DOI: 10.1021/acsami.7b16953 ACS Appl. Mater. Interfaces 2018, 10, 8585−8593

Research Article

ACS Applied Materials & Interfaces

Figure 2. XPS spectra: high-resolution (a) Ni 2p and (b) Fe 2p for the Ni/NiFe (oxy)hydroxide heterostructured nanotube (upper profiles) compared with those for neat NiFe (oxy)hydroxide without the Ni seed layer (bottom profiles). The dashed lines highlight that the peak positions shifted to higher binding energies compared with neat NiFe (oxy)hydroxide without Ni hybridization.

shell of thin-sheet stacking assemblies, corresponding to the Nirich and NiFe (oxy)hydroxide platelet layers, respectively, which was also supported by the elemental line scanning analysis across the tube (Figure 1f). Thickness of the inner and outer layers was averagely 30−50 and 50−80 nm, respectively, in rough consistency with the SEM results (vide ante). The inset in Figure 1d is the corresponding selected-area electron diffraction (SAED) pattern, containing continuous diffraction rings with composite features attributable to the in-plane structure of NiFe (oxy)hydroxide and Ni, which additionally evidenced the heterostructure formation of Ni/NiFe (oxy)hydroxide. The EDS elemental analysis of the NiFe (oxy)hydroxide region and the entire wall gave the Ni/Fe atomic ratios of 1.5:1 and 6.5:1, respectively (Supporting Information, Figure S6), for which the (oxy)hydroxide can be averagely formulated as Ni1.5Fe with a Ni/Ni1.5Fe (oxy)hydroxide molar ratio of 5:1. The magnified view of the NiFe (oxy)hydroxide region showed that the platelets were composed of stacked sheetlike subunits, and each NiFe (oxy)hydroxide sheet manifested a high transparency under the electron beam (Figure 1g), indicative of their ultrathin feature, which would help increase the catalytically active surface sites. For each Ni particle, clear lattice fringes in hexagonal close-packed symmetry were detected, as manifested in Figure 1h, which may be ascribed to the (111) plane of the face-centered cubic Ni. The highly crystalline nature ensures fast electron transport along the nanotube. In addition, Ni acted as a seed layer for the growth of NiFe (oxy)hydroxide, which was critical for obtaining the perpendicularly grown platelet-like structure. Without the Ni nanoparticles as seeds, the produced NiFe (oxy)hydroxide under the same growth conditions was in a sheet morphology and directly wrapped the nickel foam or ZnO (Supporting Information, Figure S7). The above-mentioned results indicate that the NiFe (oxy)hydroxide thin sheets can achieve an in situ perpendicular growth only on the Ni nanoparticle seeds, which may be explained by the classic solid−liquid interfacial nucleation growth theory.23 In the deposition of NiFe (oxy)hydroxides, the Ni seeds served as interfacial nucleation sites, and the Fe ions preferably hydrolyzed on their surfaces, accelerating the generation of the Ni/NiFe (oxy)hydroxide heterostructure. At the interface, some Ni species may diffuse into the NiFe (oxy)hydroxide region, thus forming a 2−5 nm transition layer (Figure 1i). The presence of the transition layer at the interface buffers the occurrence of severe lattice strains and enables close contact and strong coupling at the interface, providing enormous possibilities for the enhancement of catalytic performance.

300 and 900 nm, respectively. From the X-ray diffraction (XRD) characterization results, it was determined that ZnO was in a hexagonal wurtzite structure. The preferential growth in a hexagonal prism morphology was potentially driven by its crystallographic structure; the exposed six side planes were {11̅00}, and the basal plane was (0001).20 The surfaces were relatively smooth. The growth of Ni in NiSO4·6H2O and NH4Cl gave a conformal coating of nanoparticles on the side planes, resulting in thicker nanorods with relatively rough surfaces. The nanoparticle thickness was on average 30−50 nm. Ni was well crystallized into a face-centered cubic phase, displaying obvious diffractions corresponding to (111) and (200) planes (JCPDS: 04-0850). An obvious peak broadening was observed, which should be related to small crystallite sizes. The average size for the primary Ni nanoparticles was estimated via the Scherrer equation using the (111) diffraction, giving a value of approximately 21 nm. The following electrodeposition of NiFe (oxy)hydroxide on Ni nanoparticles yielded perpendicularly grown platelets with lateral dimensions of 50−70 nm. No additional diffractions were detected after depositing NiFe (oxy)hydroxide, which could be explained by the possible incorporation of disorder/defects in the sample along with preferentially perpendicular growth orientation of the plates on substrate. After removing the ZnO template, a hollow Ni/NiFe (oxy)hydroxide tube structure radially aligned on the Ni foam was readily obtained (Figure 1b,c). The tube length did not substantially change during the process and was ∼1 μm (Supporting Information, Figure S3). In the Raman spectrum collected for the final product in Figure S4, whereas the band centered at 438 cm−1 could be attributed to the Ni−O vibrations, the appearance of signals at 537 and 705 cm−1, which have been mostly observed at high Fe content, was additional evidence of the Ni−Fe mixed (oxy)hydroxide formation.21,22 The broad feature of the two peaks suggested a high degrees of defection or disorder, which is beneficial for improving the catalytic performance of the materials. Energydispersive X-ray spectroscopy (EDS) showed that Ni, Fe, and O were the major elements, all of which were homogeneously distributed and covered almost the entire foam substrate (Supporting Information, Figure S5). This implied that the sample was evenly deposited and distributed over the Ni foam. The hierarchical organization and structural information for the obtained NiFe (oxy)hydroxide nanotubes was further revealed using transmission electron microscopy (TEM). A low-magnification image, displayed in Figure 1d, showed a clear contrast difference, confirming the hollow feature of the interior. From the magnified view (Figure 1e), it was evident that the wall was composed of an inner thick shell and an outer 8587

DOI: 10.1021/acsami.7b16953 ACS Appl. Mater. Interfaces 2018, 10, 8585−8593

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Figure 3. Electrocatalytic activities and stability of the hierarchical Ni/NiFe (oxy)hydroxide nanotubes. (a) Linear sweeping voltammetry curves and (b) Tafel slopes for the designed nanotube heterostructure compared with individual components, neat Ni and NiFe (oxy)hydroxide, and RuO2, toward the cathodic OER reaction. (c) HER linear sweeping voltammetry curves and (d) Tafel slopes for the nanotube heterostructure compared with individual components, neat Ni and NiFe (oxy)hydroxide, and Pt. (e) Histogram summary of the catalyst performance, overpotential required at a current density of 10 mA cm−2 and Tafel slope values. (f) Chronopotentiometric curves for durability testing in the OER and HER reactions.

binding energies of 0.5 eV, suggesting strong electronic interactions between NiFe (oxy)hydroxide and the Ni seed layer. The strong coupling at the interface and, thus, close electron transfer would greatly enhance the catalytic activity of the novel heterostructured nanotube electrode. The electrocatalytic activity toward OER and HER reactions was examined in alkaline aqueous KOH employing a typical three-electrode electrochemical configuration. The setup details are thoroughly addressed in the Experimental Section. The strongly alkaline media may to some extent bring about deprotonation of (oxy)hydroxide layers, which has been recently reported to be advantageous to promote eletrocatalytic reactivities.25,26 The performance of individual components, including neat (oxy)hydroxide, neat Ni, and benchmark noblemetal catalysts, Pt for HER and RuO2 for OER, were also studied for comparison purposes. Figure 3a depicts the collected typical OER linear sweep voltammetry (LSV) curves at a rate of 5 mV s−1. Before data collection, several continuous electrochemical conditioning cycles were conducted to ensure stability and reversibility. A sweeping rate of 5 mV s−1 was also believed to be low enough to reach the steady state for accurate analysis. Interestingly, a distinct redox peak related to Ni oxidation from Ni2+ to Ni3+ was clearly observed between 1.35 and 1.45 V before the oxygen evolution, which was barely detectable in the curves for individual components, neat (oxy)hydroxide and neat Ni, which is indicative of the

To prove the occurrence of electronic interactions at the interface between the Ni-rich layer and NiFe (oxy)hydroxide, the X-ray photoelectron spectroscopy (XPS) spectrum of the nanotube heterostructure was collected and compared with that of the NiFe (oxy)hydroxide directly grown on the template with no Ni seed at the sheet root. The survey spectrum confirmed the presence of Ni, Fe, and O (Supporting Information, Figure S8). In addition, the high-resolution O 1s indicated the presence of three species, namely, oxygen coordinated to Ni/Fe (529.8 eV), oxygen in −OH (531.4 eV), and oxygen for physically adsorbed and/or crystalline water (533.2 eV, Supporting Information, Figure S8), which corroborates the formation of mixed Ni−Fe (oxy)hydroxide. The atomic Ni/Fe concentration ratio was found to be 1.86:1, which is approximately consistent with the EDS results of Ni1.5Fe (oxy)hydroxide. The high-resolution Ni 2p and Fe 2p spectra, as depicted in Figure 2a,b, displayed 2p3/2 and 2p1/2 spin−orbit doublets along with two shakeup satellites, which is characteristic of Ni and Fe mostly in the +2 and +3 oxidation states.24 Ni0+ was not detected because XPS is a surfacesensitive analytical technique, and the Ni-rich layer was practically buried under NiFe (oxy)hydroxide. Notably, in comparison with the neat NiFe (oxy)hydroxide with no hybridization with Ni, it was discerned that the Ni 2p3/2 and 2p1/2 peaks both shifted to the higher binding energies of ∼0.3 eV and the Fe 2p3/2 and 2p1/2 peaks shifted to the higher 8588

DOI: 10.1021/acsami.7b16953 ACS Appl. Mater. Interfaces 2018, 10, 8585−8593

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ACS Applied Materials & Interfaces

Figure 4. Overall water-splitting catalytic performance in a 1.0 M aqueous KOH electrolyte. (a) Polarization curves for overall water splitting of the Ni/NiFe nanotube heterostructure compared with the Pt/C//RuO2 couple (dashed line). (b) Comparison of the electrochemical overall watersplitting performance for the Ni/NiFe nanotube with some reported examples of bifunctional electrocatalysts. (c) Chronopotentiometric curves recorded at a current density of 10 mA cm−2. Digital images of the water-splitting device powered by (d) an AA battery and (e) a solar panel with a nominal voltage of 1.5 V.

from which it was evident that the overpotential required to reach 10 mA cm−2 was lowered to only 154 mV, approaching that of the Pt catalyst, 65 mV, and significantly lower than those for neat Ni, partially oxidized Ni under the same condition, and NiFe (oxy)hydroxide, 270, 217, and 305 mV, respectively. This gentle oxidization process does not significantly change NiFe (oxy)hydroxide and its catalytic behavior. The OER performance was not affected by the gentle oxidation (Supporting Information, Figure S10), which may be because the oxidation did not significantly damage the Ni/NiFe (oxy)hydroxide interface. With this structure modulation, the reaction kinetics were also greatly enhanced, as suggested by the Tafel slopes drawn in Figure 3d. The value for the heterostructured nanotube was measured to be 71 mV dec−1, which is significantly lower compared with the values of 130, 113, and 143 mV dec−1 for neat Ni, partially oxidized Ni, and neat NiFe (oxy)hydroxide, respectively, suggesting a combined Volmer− Heyrovsky mechanism for hydrogen production. The Ni/NiO sites were believed to facilitate hydrogen adsorption and accelerate the Volmer process, thus yielding a higher level of the HER performance.27 As a growing body of evidences has proved that Fe within the (oxy)hydroxide plays a critical role in the electrocatalytic reactions,12,28 the effect of Fe content was studied. A higher Fe substitution produced improved electrocatalytic activity until reaching the saturation limit of Ni1.5Fe; further increase in the Fe content unfortunately would result in decreased performance, which may be due to the formation of Fe-rich phases (Supporting Information, Figure S11), which is in good consistency with previous studies.29 In addition to the overall high electrocatalytic activity, as summarized in Figure 3e and the comparison tables in the Supporting Information, the hybrid electrode also featured excellent stability, which is critical for practical applications. To assess the catalytic stability for the electrolysis of OER and HER, chronopotentiometric curves of the Ni/NiFe (oxy)hydroxide nanotube catalyst were recorded at constant current

impressively increased surface area and active sites due to the formation of this hierarchical nanotube structure. To avoid a possible overlap of the oxidation current with OER currents, the voltage was reversely scanned from the positive to the negative direction, as drawn in the inset, from which the potential required to achieve a current density of 10 mA cm−2 was determined to be ca. 1.435 V. Thus, the overpotential was extremely low at only 205 mV, nearly 88 mV lower than that for commercial RuO2 (293 mV) and outperforming the individual components, including neat Ni (371 mV) and NiFe (oxy)hydroxide (289 mV). Then, the OER current quickly increased, reaching a high current value of 300 mA cm−2 by only applying an additional 95 mV, i.e., an overpotential of 300 mV. At this voltage, neat Ni, neat NiFe (oxy)hydroxide, and RuO2 catalysts delivered much smaller current densities of 12, 60, and 48 mA cm−2, respectively, which additionally confirmed the impressively improved catalytic activity achieved by designing this heterostructured nanotube structure. The electrocatalytic kinetics was inspected by extracting the slopes from the Tafel plots depicted in Figure 3b. The value for the nanotube heterostructure was the lowest among the catalysts, 53 mV dec−1, contrasting with 151, 75, and 71 mV dec−1 for neat Ni, NiFe (oxy)hydroxide, and RuO2, respectively, which is a clear indication of the accelerated wateroxidation reaction kinetics for our designed electrode. The electrocatalytic HER activities of our designed heterostructured nanotube were also assessed in an alkaline environment. Although NiFe (oxy)hydroxide has been generally believed to be a poor HER electrocatalyst, this hierarchical heterostructured nanotube manifested a catalytic activity comparable to most recently reported HER catalysts, achieving a current density of 10 mA cm−2 at an overpotential of only 210 mV (Supporting Information, Figure S9). In addition, the HER catalytic behavior can be further ameliorated by partially oxidizing the Ni seeds via thermal treatment at 60 °C. Figure 3c draws the polarization curves after oxidation, 8589

DOI: 10.1021/acsami.7b16953 ACS Appl. Mater. Interfaces 2018, 10, 8585−8593

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ACS Applied Materials & Interfaces

constructed nanotube structure provided an abundant accessible surface for the effective penetration of liquid electrolytes and increased the number of catalytic sites, which was crucial for the electrochemical water splitting. The electrochemical double-layer capacitance was measured, and the heterostructure showed a high value of 3.42 mF cm−2, which is considerably larger compared with the values of 0.61 and 1.13 mF cm−2 for neat Ni and the (oxy)hydroxide, respectively (Supporting Information, Figure S14). Particularly, the ultrathin thickness of NiFe (oxy)hydroxide sheets ensured the exposure of a multitude of potentially reactive sites. Whereas the reactive sites for bulk materials are generally limited to edges/defects, the reaction centers in such two-dimensional sheet materials are assumed to be much enlarged with additional sites on the whole sheet surfaces.25 Second, the Ni core endowed electron highways, as proven by the electrochemical impedance measurement (Supporting Information, Figure S15), which activated more active sites on NiFe (oxy)hydroxide and enhanced the electrocatalytic activities. The generation of new water-splitting active sites can be to some extent deduced from the cyclic voltammograms (Supporting Information, Figure S16). Whereas the reduction and oxidation pairs for Ni2+/Ni3+ were located at 0.385 and 0.491 V for neat Ni and at 0.348 and 0.399 V for neat NiFe (oxy)hydroxide, the redox peaks of the Ni/NiFe nanotube catalyst were found to be centered at approximately 0.321 and 0.515 V. Third, based on the characterization results, strong electronic coupling and electronic transfer were present at the interface between Ni and NiFe (oxy)hydroxide, which can synergistically work to tune the absorption or binding energies of reaction intermediates on the catalyst surface and to optimize the corresponding activity. This phenomenon has been recorded in many heterostructured systems, for example, in MoS2/Ni3S2 heterostructures on Ni foam,38 Co9S8/Ni3Se2 hybrid electrocatalyst on exfoliated graphene foil,39 and in core−shell Cu/NiFe LDH nanostructure supported on Cu foam,40 among others.41−43 With the interplaying advantages integrated, the novel architecture maximized the number of catalytic sites, enhanced the catalytic activities, and optimized the electron transport, resulting in the unprecedented OER, HER, and oversplitting performances of NiFe (oxy)hydroxide.

densities. As drawn in Figure 3f, the potential and, thus, the catalytic activity were retained with no noticeable degradation in the reaction process for at least 12 h. Importantly, no appreciable degradation was found even at high current densities of 50 and 100 mA cm−2 in water oxidation. Furthermore, no obvious difference was discerned in the polarization curves before and after continuous cyclic voltammetric scanning of 1000 cycles, and the sample morphology and microstructure remained almost unchanged (Supporting Information, Figures S12 and S13), which further confirms the excellent durability of our designed electrode. From the above-mentioned electrochemical study results, it was evident that the devised material was a very competitive bifunctional electrocatalyst that is efficient for both OER and HER reactions. Therefore, an integrated electrolyzer was further assembled in a two-electrode configuration with the Ni/NiFe nanotube catalyst serving as both the anode and cathode. A cell with commercial Pt/C and RuO2 catalyst electrodes as the cathode and anode, respectively, was constructed as a benchmark reference. Figure 4a draws the linear sweeping voltammetry curves of H2O electrolysis in a 1.0 M KOH aqueous electrolyte solution using the Ni/NiFe nanotube electrocatalyst compared with the Pt/C//RuO2 couple. Clearly, the Ni/NiFe nanotube had excellent activity, delivering a current density of 10 mA cm−2 by applying a potential of just 1.56 V. Using a comprehensive comparison, it was revealed that the overall water-splitting activity of the Ni/ NiFe nanotube was greatly superior to that of the Pt/C//RuO2 couple (ca. 1.69 V) and also comparable to the recently reported bifunctional water-splitting electrocatalysts (Figure 4a,b and Table S3, Supporting Information) such as ultrasmall NiFeOx nanoparticles (1.51 V),30 NiFe LDH grown on NiCo2O4 nanowire arrays supported by Ni foam (1.60 V),31 NiFe LDH nanosheet arrays on Ni foam (1.70 V),32 porous MoO2 nanosheets (1.53 V),33 Ni2P nanoparticles embedded in N-doped carbon matrices (1.63 V),34 etc.35−37 Over a 10 h galvanostatic electrolysis at an applied current of 10 mA cm−2, the Ni/NiFe nanotube presented excellent durability in the overall water-splitting performance with negligible degradation (Figure 4c). Because the onset potential for the electrolyzer was impressively low, even below 1.36 V, such water electrolysis can be driven even using a single-cell AA battery with a nominal voltage of 1.5 V at room temperature, and large quantities of H2 and O2 bubbles can be clearly observed by the naked eye on the surfaces of the respective Ni/NiFe nanotube cathode and anode (Figure 4d and Movie S1 in the Supporting Information). It should be mentioned that an integrated twoelectrode eletrolyzer powered with an AA battery was also realized on some state-of-the-art excellent bifunctional electrocatalysts, including ultrasmall NiFeOx nanoparticles,30 porous MoO2 nanosheets,33 and NiFe-LDH sheets/graphene superlattice.8 More importantly, without the electricity input, the two-electrode electrolysis cell can be operated well by connecting it with a solar cell. When light was on, the bubbles were immediately generated at both electrodes (Figure 4e). The results unambiguously demonstrated the excellent catalytic activity of the designed heterostructured nanotube electrode and its great potential as an efficient catalyst for overall water splitting in an alkaline environment. The designed heterostructured electrode exhibited very competitive OER, HER, and overall splitting catalytic activities, and the improved performance can be ascribed to the following three factors. First, the multiscale hierarchical assembly and

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CONCLUSIONS In summary, we have developed a unique hollow Ni/NiFe (oxy)hydroxide heterostructure that is radially aligned on a Ni foam as a highly efficient bifunctional catalyst electrode for overall water splitting. The inner Ni-rich layer was made of highly crystalline nanoparticles, and NiFe (oxy)hydroxide was composed of ultrathin sheet subunits. Such a novel structure possessed a rich porosity, a large electrochemically active surface, and an excellent electron conductivity, from which the heterostructured electrode exhibited a combined high OER catalytic activity and a great HER enhancement in the same alkaline electrolyte. Therefore, the electrode was employed as a highly efficient bifunctional catalyst for water splitting in an integrated cell. Furthermore, it is very significant that the device can be driven by a single AA battery or a 1.5 V solar panel. This work highlighted a new strategy for designing a material with a controlled structure, which is of paramount importance for obtaining a highly efficient catalyst that is promising for largescale and real-world water-splitting electrolyzers. 8590

DOI: 10.1021/acsami.7b16953 ACS Appl. Mater. Interfaces 2018, 10, 8585−8593

Research Article

ACS Applied Materials & Interfaces

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gun operating at an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) analysis of the samples was conducted on an Escalab 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc.) using 300 W Al Kα radiation in an ultrahigh vacuum. The standard deviation for the binding energies was 0.1 eV. The Raman spectra were collected using a confocal LabRAM HR800 spectrometer using an excitation wavelength of 633 nm provided by a He−Ne laser.

EXPERIMENTAL SECTION

Materials. The chemicals for synthesizing the Ni/NiFe (oxy)hydroxide nanotube, including zinc nitrate hexahydrate [Zn(NO3)2· 6H2O, ≥98%], ammonium nitrate (NH4NO3, ≥98%), nickel sulfate hexahydrate (NiSO4·6H2O, ≥98%), ammonium chloride (NH4Cl, ≥98%), iron nitrate hexahydrate [Fe(NO3)3·9H2O, ≥98%], nickel nitrate hexahydrate [Ni(NO3)2·6H2O, ≥98%], and potassium hydroxide (KOH, ≥98%), were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. The noble metal benchmark catalysts, ruthenium oxide (RuO2) nanoparticles, and platinum (Pt), nominally 20 wt % on carbon black, were obtained from Aladdin Reagent Co., Ltd. and Alfa Aesar, respectively. All of the chemicals were used as received without purification. Milli-Q water was used in all of the experiments. Ni foam (thickness: 1.0 mm, bulk density: 0.1−0.8 g cm−3, number of pores per inch: 5−120) was obtained from Yierda Electronic (Kunshan) Co. Ltd. Synthesis of the Ni/NiFe Nanotube Heterostructure. In a typical synthesis, a piece of Ni foam (2 cm × 1 cm) was pretreated using sonication in an aqueous 5% HCl solution for 15 min, followed by rinsing with acetone in an ultrasonic bath for 10 min before washing with ultrapure water several times. ZnO nanorod arrays were prepared on Ni foam using the chronopotentiometry method, in a mixture solution of NH4NO3 (0.05 M) and Zn(NO3)2·6H2O (0.01 M) with a current density of 1.0 mA cm−2 at 78 °C for 60 min. For the Ni wrapping on ZnO, an electrodeposition was conducted in a solution of NiSO4·6H2O (0.02 M) and NH4Cl (0.03 M) with a current density of 0.9 mA cm−2 for 15 min, with the prepared ZnO on nickel foam used as the working electrode. NiFe (oxy)hydroxide was subsequently grown via the co-electrodeposition method. The ZnO/ Ni sample was used as the working electrode, and the electrodeposition was performed in a solution of Ni(NO3)2·6H2O (3 mM) and Fe(NO3)3·9H2O (3 mM) at −1.0 V (vs Ag/AgCl) for 350 s. In all of the electrodepositions, Ag/AgCl was used as the reference electrode. To make a hollow structure, the ZnO core was removed by immersing the sample in a 1.0 M NaOH solution for 3.5 h, yielding the desired heterostructured nanotube structure. For comparison studies, electrodes of neat NiFe (oxy)hydroxide and neat Ni were also fabricated using a similar process described above, respectively, without the Ni and NiFe (oxy)hydroxide deposition step. Electrochemical Test. All of the electrochemical measurements were conducted using a three-electrode system on an electrochemical analyzer (CHI 660E, CH Instruments, Shanghai). The Ni/NiFe nanotube supported on Ni foam was directly used as the working electrode, and the working surface area of the electrode was maintained at 1 (=2 × 0.5) cm2. A Hg/HgO electrode and a graphite rod were employed as the reference and counter electrodes, respectively. All of the potentials were manually iR-corrected and converted to the reversible hydrogen electrode (RHE) via the Nernst equation (ERHE = EHg/HgO + 0.059 × pH + 0.098). The overpotential (η) values were calculated from η = ERHE − 1.23 V. For fabricating RuO2 and Pt/C electrodes, commercially available powders were dropcasted on Ni foam, and the electrode surface area was controlled to be the same as that for the Ni/NiFe nanotube. The suspension for dropcasting was made by dispersing the powder of noble-metal catalysts in a mixture aqueous solution of Nafion and ethanol, followed by sonication of 30 min to achieve homogeneity. All of the electrochemical measurements were carried out at 25 °C, and 1.0 M aqueous KOH was used as an electrolyte throughout. The electrolytes were purged with N2 and O2 for the HER or OER measurements, respectively. Characterization. The X-ray diffraction (XRD) patterns were collected using a powder X-ray diffraction system (XRD, X’Pert-Pro MPD) employing a Cu Kα radiation (λ = 0.154 nm). The scanning electron microscopy (SEM) images for obtaining information on the sample morphologies were taken with a Hitachi SU8010 electron microscope. After drop-drying the ethanol suspension of the catalyst on a copper grid, transmission electron microscopy (TEM) imaging and a high-resolution TEM (HR-TEM) study were carried out using an FEI Tecnai G2 F20 S-TWIN TMP equipped with a field emission

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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.7b16953. Schematic diagram for the synthesis process; digital images and more SEM and XRD data for the samples at the progressive growth steps, Raman, elemental mapping, and XPS analysis for the final product of Ni/NiFe (oxy)hydroxide nanotubes, more electrochemical data, including HER/OER LSV curves of the Ni/NiFe (oxy)hydroxide before and after oxidation treatment, optimization of Fe content, comparing LSV curves and morphology before and after cyclic electrocatalytic reactions, estimation of electrochemical double-layer capacitances, Nyquist plots, and CV curves, tables comparing electrocatalytic water-splitting metrics of our sample with some examples in recent references (PDF) Integrated water-splitting electrolyzer driven by an AA battery (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fengxia Geng: 0000-0001-5557-4165 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (51402204 and 51772201), Thousand Young Talents Program, Jiangsu Specially-Appointed Professor Program, and a project funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

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REFERENCES

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