Hierarchical Ni2P@NiFeAlOx Nanosheet Arrays as Bifunctional

Publication Date (Web): February 12, 2019. Copyright © 2019 American Chemical Society. *E-mail: [email protected] (F.L.). Cite this:Inorg. Chem. XX...
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Hierarchical Ni2P@NiFeAlOx Nanosheet Arrays as Bifunctional Catalysts for Superior Overall Water Splitting Zhi Gao, Feng-qing Liu, Li Wang, and Feng Luo* State Key Laboratory of Nuclear Resources and Environment, School of Chemistry, Biology and Materials Science, East China University of Technology, Nanchang 330013, P. R. China

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 02/12/19. For personal use only.

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ABSTRACT: Bifunctional electrocatalysts based on transition-metal phosphides are appealing for overall water splitting owing to their excellent electrical conductivity, low cost, and high stability. However, these specials are often restricted by some serious drawbacks such as its relatively poor activity for oxygen evolution reaction (OER) and its manufacture, which usually requires one to add additional large numbers of P sources and, consequently, inevitably leads to the release of flammable and detrimental PH3. Herein, we show an effective avenue to overcome these issues. For the first time, the in situ topological transformation of PO43−-intercalated NiFeAl-layered double hydroxide nanosheet arrays upon calcination under a H2 atmosphere is developed to fabricate supported nickel phosphide without any additional P source. The resulting phase affords unique Ni2P@NiFeAlOx core−shell nanosheet arrays, which exhibit an excellent performance for OER and hydrogen evolution reaction in 1.0 M KOH, with low overpotentials of 210 and 105 mV at 10 mA cm−2, respectively. Impressively, it can also serve as both a cathode and an anode to drive water splitting in alkaline media, giving 10 and 100 mA cm−2 at cell voltages of only 1.52 and 1.62 V, respectively. This value is better than the commercial criterion of the Pt/C//IrO2 counterpart and also ranks at the top level in all established bifunctional electrocatalysts. The outstanding performance of Ni2P@NiFeAlOx is mainly attributed to the synergistic effect from a highly dispersed Ni2P core and a thin NiFeAlOx shell, as well as the efficient mass transport of a hierarchical nanoarray framework.

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INTRODUCTION Electrochemical water splitting into clean hydrogen fuel has been regarded as a promising technology to address energy and environmental problems.1−3 Currently, noble-metal catalysts (e.g., Pt, IrO2, and RuO2) are generally required for an electrochemical water splitting device to reduce the overpotential and improve the reaction rate.4−6 However, the high cost and scarcity of such precious metals limit their large-scale application. Driven by these challenges, substantial efforts have been devoted to developing abundant and cost-effective nonnoble-metal catalysts for water splitting, and a great deal of progress has been achieved. It has been reported that transition-metal compounds, such as phosphides,7−12 sulfides,13−15 selenides,16 borides,17 and carbides,18,19 could efficiently catalyze hydrogen evolution reaction (HER), and transition-metal oxides/hydroxides exhibited remarkable performance for oxygen evolution reaction (OER).20−26 Nevertheless, it still remains a huge challenge to pair OER and HER catalysts under the same electrolyte for highly efficient water splitting because of the fact that these electrocatalysts show their best performances in different media (alkaline or acid media).27−30 Producing different single-function electrocatalysts for OER and HER will raise the cost because of the requirement of different processes and reaction devices. To this end, the development of bifunctional non-noble-metal © XXXX American Chemical Society

electrocatalysts to simultaneously meet high HER and OER performances in the same electrolyte to achieve alkaline water electrolysis is highly desirable and still remains a huge challenge. Owing to their versatility in chemical composition and architectural structure, layered double hydroxides (LDHs) have attracted extensive attention in electrochemical fields.31 In particular, the metal cations can be selectively introduced to the layer of LDHs with uniform and highly ordered states, which can endow LDHs with outstanding OER performance. For example, NiFe-based LDH nanomaterials and their derivates are shown to be some of the best OER catalysts.32 However, their poor ability for HER seriously restricts their application as bifunctional catalysts for overall water splitting.33 On the other hand, recent advances reveal that nickel phosphides possess excellent electrical conductivity, low cost, and high stability, suggesting their extremely promising potential as electrocatalysts. In this regard, translating LDHs to nickel phosphides received extensive attention, and a significant HER performance was observed.34 However, the preparation process is not a desirable approach because it usually requires a large amount of additional P sources (e.g., Received: November 29, 2018

A

DOI: 10.1021/acs.inorgchem.8b03327 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry NaH2PO2 and NH4H2PO2) and, consequently, results in flammable and poisonous PH3.35−37 Additionally, with this synthesis strategy, it is hard to create strong electronic interactions between nickel phosphide and the support because of the massive state of nickel phosphides, which seriously limits their efficiency for OER in alkaline media and thus retards their application as bifunctional electrocatalysts. To this end, the integration of OER (NiFe-based LDH or derived metal oxides) and HER (highly dispersed nickel phosphides) active sites in the same electrocatalyst based on a LDH precursor and simultaneous creation of enhanced synergistic effects and electron-transfer rate at the interfaces by an ecofriendly and new strategy should be an effective way of enhancing the performance for overall water splitting and is full of challenges. Herein, for the first time, using the lattice-confined Ni2+ cation and intercalated PO43− anion of NiFeAl-PO43−-LDH as Ni and P sources, respectively, the Ni2P@NiFeAlOx nanosheet arrays can be obtained through the in situ topological transformation of NiFeAl-PO43−-LDH upon calcination under a H2 atmosphere without any additional P source. The as-synthesized Ni2P@NiFeAlOx nanosheet arrays with a highly dispersed Ni2P core and a thin NiFeAlOx shell exhibit obviously synergistic effects toward overall water splitting. Additionally, the hierarchical nanoarray frameworks can facilitate mass diffusion in the water splitting. As a result, a two-electrode electrolyzer using Ni2P@NiFeAlOx as both the anode and cathode can afford 10 and 100 mA cm−2 watersplitting currents at cell voltages of only 1.52 and 1.62 V, respectively, outperforming most of the recently reported bifunctional electrocatalysts and the commercial Pt/C//IrO2 electrodes. In addition, the Ni2P@NiFeAlOx core−shell nanoarrays exhibit long-term stability and cycling durability for overall water splitting. This work offers a general, effective, and ecofriendly strategy to design hierarchical transition-metal phosphide nanosheet arrays via the in situ topological transformation of a LDH precursor, which can serve as highly efficient non-noble-metal bifunctional catalysts for practical hydrogen production by electrochemical water splitting.

Scheme 1. Synthetic Route for Ni2P@NiFeAlOx

NiFeAl-CO32−-LDH to NiFeAl-PO43−-LDH, Fourier transform infrared (FT-IR) characterization was performed. As shown in Figure S2, there is a broad absorption centered at about 3423 cm−1 due to the stretching vibration of hydroxyl groups from LDH supports and water. A small band at 1644 cm−1 is assigned to the bending vibration of water. In the case of NiFeAl-CO32−-LDH, the strong bands at 1360 and 802 cm−1 correspond to the stretching vibration of an interlayer CO32− anion. It is noticed that the bands at 1360 and 802 cm−1 thoroughly disappear in NiFeAl-PO43−-LDH, while a new strong stretching vibration at 1089 cm−1 related to the interlayer PO 4 3− can be obviously detected, strongly demonstrating the complete exchange of CO32− in the interlamination of NiFeAl-CO32−-LDH by PO43−. For Ni2P@ NiFeAlOx, the Ni2P phase is observable (JCPDS 03-0953) without the formation of Fe2P species (JCPDS 51-0943) because of the higher temperature requested for the reduction of Fe3+ species. Moreover, no other crystalline phase is detected in addition to Ni foam, indicating the amorphous state of the nanosheet matrix. It is found that the reduction temperature plays a key role in the formation of Ni2P nanoparticles and the preservation of an ordered array structure. At relatively low reduction temperature, such as 300 and 400 °C, the morphology of sample is unchanged compared to NiFeAl-PO43−-LDH; however, Ni2P nanoparticles are not detected (Figure S3a−d). When the temperature is raised to 500 °C, the hierarchical nanosheet arrays still can be retained and, excitingly, the nanoparticles are distinctly observed (Figures 1c and S3e,f). When the temperature is further increased to 600 and 700 °C, the nanosheet arrays are destroyed and the nanoparticles aggregate seriously (Figure S3g−j). Thus, the optimized reduction temperature is 500 °C. For the Ni2P@NiAlOx samples prepared using the same procedure as that for Ni2P@ NiFeAlOx without the addition of Fe, the nanosheets gather together and the Ni2P nanoparticles aggregate obviously (Figure S4), and the diffraction peaks related to the Ni2P phase in Ni2P@NiAlOx are higher than those in Ni2P@ NiFeAlOx (Figure S5), indicating that the introduction of Fe species can facilitate the dispersion of Ni2P nanoparticles and stabilize the hierarchical nanosheet arrays. Moreover, the actual

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RESULTS AND DISCUSSION The preparation process of the Ni2P@NiFeAlOx core−shell nanoarrays involves three steps (Scheme 1). First, the vertically oriented and ultrathin CO32−-intercalated NiFeAl-LDH nanosheet arrays with a thickness of about 10 nm and a smooth surface were uniformly grown on Ni foam by a solvothermal reaction (Figure 1a). Subsequently, NiFeAl-PO43−-LDH can be obtained by a simple ion exchange in a NaH2PO4 solution, which could perfectly maintain the morphologies of nanosheet arrays (Figure 1b). After that, NiFeAl-PO43−-LDH was reduced at 500 °C under a H2 atmosphere to produce uniform Ni2P nanoparticles, and, simultaneously, the ordered array structure was well maintained (Figure 1c, inset). The energydispersive X-ray spectroscopy (EDS) elemental mapping images of Ni2P@NiFeAlOx confirm the coexistence of Ni, Fe, Al, P, and O and their uniform distribution (Figure S1). Figure 1d illustrates X-ray diffraction (XRD) patterns of NiFeAl-CO32−-LDH, NiFeAl-PO43−-LDH, and Ni2P@NiFeAlOx. NiFeAl-CO32−-LDH and NiFeAl-PO43−-LDH can be indexed to the rhombohedral LDH phase. The interlayer distance (d003) calculated in NiFeAl-PO43−-LDH is 0.87 nm, which is larger than that in NiFeAl-CO32−-LDH (0.77 nm), implying the successful intercalation of a PO43− anion.38 Moreover, to further determine the total conversion from B

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Figure 1. SEM images for (a) NiFeAl-CO32−-LDH, (b) NiFeAl-PO43−-LDH, and (c) Ni2P@NiFeAlOx. (d) XRD patterns of NiFeAl-CO32−-LDH, NiFeAl-PO43−-LDH, and Ni2P@NiFeAlOx.

TEM images of Ni2P@NiFeAlOx show that some tiny cracks can be clearly observed on the NiFeAlOx shell surface, which indicates that a reactive intermediate can easily gain access to the Ni2P surface during the process of electrocatalytic water splitting, because of the presence of the unclosed shell of NiFeAlOx layers. Moreover, one single particle possesses clear lattice fringes with interplanar spacings of about 1.97 Å (Figure 1d), which is indexed to the (200) plane of the Ni2P phase (JCPDS 03-0953), further proving generation of the crystalline Ni2P phase in the Ni2P@NiFeAlOx sample (Figure 2d). The lattice fringes are not detected for the NiFeAlOx shell, proving its amorphous state, well-consistent with the XRD results. The mean size of Ni2P in Fe-free Ni2P@NiAlOx is also investigated (Figure S6), which is much larger than that in Ni2P@ NiFeAlOx (Figure 2), indicating that Al-containing components can act as both the support and physical spacer to inhibit the aggregation of Ni2P nanoparticles and improve the dispersion of Ni2P nanoparticles. To investigate the chemical composition of the designed samples and the synergistic effect of the core−shell structure, X-ray photoelectron microscopy (XPS) characterization was carried out. In the P 2p region of Ni2P@NiFeAlOx and Ni2P@ NiAlOx, the peak at the binding energy of 129.7 eV is attributable to P−Ni and another one at a higher binding energy of 134.0 eV suggests the formation of a P−O species that mainly comes from the oxidized surface of Ni2P nanoparticles due to exposure at the air atmosphere (Figure 3a).39 For the spectrum of Fe 2p (Figure 3b), the peaks of Fe 2p3/2 and Fe 2p1/2 in the Ni2P@NiFeAlOx sample appear at 711.9 and 725.5 eV, respectively, which are assigned to Fe3+ species.40 Moreover, no characteristic peaks related to FexP species were detected, coincident with the XRD results.41 The Ni 2p3/2 spectra of the Ni2P@NiFeAlOx and Ni2P@NiAlOx samples can be fitted into three different peaks. The binding energies at about 853.4 and 856.8 eV are associated with Ni−P and nickel oxide, respectively (Figure 3c).42 There is only one peak at a binding energy of 530.5 eV in the spectrum of O 1s (Figure 3d), which is characteristic of the lattice O, and the Al 2p spectrum determines the existence of Al3+ species (Figure

contents of metal elements in different samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the results are summarized in Table S1, which is in good agreement with the theoretical content. The transmission electron microscopy (TEM) images further determine the nanosheet morphology of the Ni2P@ NiFeAlOx sample, and the uniformly dispersed Ni2P nanoparticles are distinctly found (Figure 2a,b). As shown in Figure 2c, the clear boundary between the Ni2P core (diameter: ≈90.5 nm) and NiFeAlOx shell (thickness: ≈1.6 nm) can be observed. This distinctive core−shell structure is good for enhancing interfacial contacts and interactions, as well as improving the stability of Ni2P@NiFeAlOx. On the other hand,

Figure 2. TEM images of the Ni2P@NiFeAlOx sample. C

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Figure 3. XPS spectra of (a) P 2p, (b) Fe 2p, (c) Ni 2p3/2, and (d) O 1s in different samples.

Figure 4. (a) OER polarization curves and (b) overpotentials at 10 mA cm−2 for different catalysts. (c) Tafel plots for OER. (d) Nyquist plots for electrocatalysts. (e) Linear fitting of the capacitive currents as a function of the scan rate. (f) LSV curves for the Ni2P@NiFeAlOx before and after 1000 cycles of CV scans. Inset: Chronoamperometric curve of Ni2P@NiFeAlOx at the overpotentials of 210 and 280 mV.

S7).43,44 The above XPS results further demonstrate that the Ni2P@NiFeAlOx sample is composed of metal phosphide Ni2P and NiFeAlOx oxide. Noticeably, the binding energy of Ni 2p3/2 in Ni2P@NiFeAlOx obviously shifts to higher values compared to that in Ni2P@NiAlOx (Figure 3c), which is ascribed to smaller Ni2P nanoparticles in Ni2P@NiFeAlOx that can enhance the intimate contact of Ni2P and the support and

thus lead to strong electronic interaction between them. As demonstrated by the other composite systems, such a significant electronic interaction formed in electrocatalysts can improve the water splitting activity.45,46 On the basis of the above results, it can be concluded that the hierarchical core−shell nanoarrays are successfully fabricated in the Ni2P@NiFeAlOx electrode. The electroD

DOI: 10.1021/acs.inorgchem.8b03327 Inorg. Chem. XXXX, XXX, XXX−XXX

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still much larger than those of other catalysts at the same overpotential, indicating that Ni2P@NiFeAlOx is intrinsically more active than other samples. Moreover, the OER performances of a series of Ni2P@NiFeAlOx (3:n:1) samples with different Fe contents (n = 0, 0.5, 1, 1.5, 2) were also tested to investigate the compositional effect. As shown in Figure S13, the overpotential versus ratio shows an inverted volcano shape and Ni2P@NiFeAlOx with a Ni/Fe/Al molar ratio of 3:1:1 exhibits the best OER performance. The stability and durability are key indices in assessing the electrocatalysts. The long-term cycling test of Ni2P@NiFeAlOx was carried out, and the results reveal negligible decay of the current density before and after 1000 cycles of CV scans (Figure 4f). In addition, the chronoamperometric curves of Ni2P@NiFeAlOx measured at two different constant overpotentials of 210 and 280 mV can deliver stable current densities of 10 and 100 mA cm−2 during 24 h, respectively (Figure 4f, inset). The morphology, chemical composition, and crystalline phase of the spent Ni2P@NiFeAlOx sample after the long-term ability test of OER were determined by scanning electron microscopy (SEM), XPS, and XRD characterizations. The morphology remains nearly identical with that before, as demonstrated by the SEM image (Figure S14). For the Ni 2p3/2 spectrum (Figure S15a), the peak belonging to Ni2P disappears and the only peak attributed to the Ni2+ species still exists, indicating that the surface of the Ni2P nanoparticles may be oxidized to Ni(OH)2. As the TEM image of Ni2P@ NiFeAlOx after stability test shows (Figure S15b), Ni2P nanoparticles were covered by a thin layer with a thickness of ∼2.1 nm, larger than the thickness of the NiFeAlOx shell (∼1.6 nm), implying the generation of Ni(OH)2. The formed Ni(OH)2 layer can prevent further oxidation of Ni2P.48 Consistently, the binding energy at 129.7 eV for P−Ni disappears and the only peak at 134.0 eV assigned to P−O is still observable (Figure S16a). Moreover, no obvious change in the binding energy is found in the Fe 2p region (Figure S16b), suggesting the invariability of Fe3+ species. The only peak at higher binding energy (531 eV) in O 1s spectra demonstrates the formation of MOx/M−OH (M = Ni, Fe, Al) on the surface of the material (Figure S16c). However, the characteristic diffractions related to the MOx/M−OH (M = Ni, Fe, Al) phases are not found in the XRD pattern of Ni2P@NiFeAlOx (Figure S17), which should be due to the small amounts and their thin layers. These results prove that the thin MOx/M− OH (M = Ni, Fe, Al) layer is coated on the surface of the material during the OER process.49 Next to the OER performance, the HER activity of different electrocatalysts was also tested in a 1 M KOH aqueous solution. As shown in Figure 5a, the commercial 20 wt % Pt/C catalyst coated on Ni foam delivers the lowest overpotential to reach a current density of 10 mA cm−2. Ni2P@NiFeAlOx only needs an overpotential of 105 mV to afford 10 mA cm−2, which is lower than those of Ni2P@NiAlOx (125 mV), NiFeAlPO43−-LDH (145 mV), and Ni foam (240 mV). This is comparable and even superior to those of most recently reported HER catalysts (Table S3). Moreover, the HER performances of a single Ni2P and NiFeAlOx were also tested. As shown in Figure S18, the overpotentials of Ni2P and NiFeAlOx at a current density of 10 mA cm−2 are 138 and 183 mV, respectively, which are obviously higher than that of Ni2P@NiFeAlOx, highlighting the synergistic effect of Ni2P and NiFeAlOx in Ni2P@NiFeAlOx for HER. Ni2P@NiFeAlOx also displays the smallest Tafel slope of 106 mV dec−1

catalytic activity of Ni2P@NiFeAlOx toward OER was investigated in 1.0 M KOH using a typical three-electrode system with a scan rate of 5 mV s−1. For comparison, the OER activities of Ni2P@NiAlOx without the addition of Fe, NiFeAlPO43−-LDH nanosheet arrays, bare Ni foam, and commercial IrO2 supported on Ni foam with about 1 mg cm−2 loading were also tested under the same conditions. The IR correction is applied to all original data to reveal the intrinsic performance of the catalysts. As shown in Figure 4a,b, Ni2P@NiFeAlOx exhibits the lowest overpotential of 210 mV at a current density of 10 mA cm−2, which is 42, 76, 115, and 172 mV less than those of Ni2P@NiAlOx, NiFeAl-PO43−-LDH, commercial IrO2, and bare Ni foam, respectively. Furthermore, the OER activity of a single Ni2P and NiAlOx was respectively explored. As the polarizations depict in Figure S8, Ni2P@NiFeAlOx exhibits higher OER activity compared to Ni2P and NiAlOx, proving that the exceptionally excellent OER performance in Ni2P@NiFeAlOx originates from the synergistic effect between Ni2P and NiFeAlOx. The OER performance of NiFeAl-PO43−LDH reduced at different temperatures was compared (Figure S9). The result indicates that NiFeAl-PO43−-LDH reduced at 500 °C shows the most excellent OER performance, which indicates that the coexistence of ordered nanoarray structures and well-defined Ni2P nanoparticles is good for driving OER. Moreover, the OER activities of Ni2P@NiFeAlOx and Al-free Ni2P@NiFeOx were also explored. The Ni2P nanoparticles with smaller size in Ni2P@NiFeAlOx not only can expose more active sites but also can enhance the electronic interaction between Ni2P and the support. Thus, Ni2P@NiFeAlOx exhibits higher OER activity relative to Al-free Ni2P@NiFeOx (Figure S10). Noticeably, Ni2P@NiFeAlOx only requires overpotentials of 226, 260, and 280 mV to reach the current densities of 20, 50, and 100 mA cm−2, respectively. The OER activity of Ni2P@NiFeAlOx outperforms that of most state-of-the-art catalysts (Table S2). To gain further insight into different electrodes, the Tafel slope was investigated (Figure 4c). The Tafel slope in Ni2P@NiFeAlOx is only 54 mV dec−1, which is smaller than that in Ni2P@NiAlOx (62 mV dec−1), NiFeAlPO43−-LDH (88 mV dec−1), IrO2 (155 mV dec−1), and Ni foam (174 mV dec−1), demonstrating the faster OER kinetics in Ni2P@NiFeAlOx. Electrochemical impedance spectroscopy (EIS) measurements were performed at a constant overpotential of 210 mV to explore the charge-transfer kinetics. As shown in Figure 4d, the Nyquist plots reveal that the chargetransfer resistance in Ni2P@NiFeAlOx is much lower than those in Ni2P@NiAlOx, NiFeAl-PO43−-LDH, and Ni foam, indicative of the fast electron transfer between a highly dispersed Ni2P nanoparticle core and NiFeAlOx shell induced by the distinct core−shell structure in Ni2P@NiFeAlOx. The electrochemically active surface areas (ECSAs) of different catalysts were determined by the double-layer capacitance (Cdl) using the cyclic voltammetry (CV) method, which is proportional to the ECSAs of the electrocatalysts. As shown in Figures 4e and S11, Ni2P@NiFeAlOx exhibits the highest Cdl value (6.2 mF cm−2), which is about 1.5, 2, and 6 times those of Ni2P@NiAlOx (4.2 mF cm−2), NiFeAl-PO43−-LDH (2.9 mF cm−2), and Ni foam (1.0 mF cm−2), indicating that Ni2P@ NiFeAlOx can provide more active sites because of the ordered hierarchical nanoarrays and highly dispersed Ni2P nanoparticles. However, slight variation of the ECSA values should not lead to dramatic differences in the OER performance. Thus, the current density was normalized to ECSA.47 As shown in Figure S12, the current density of Ni2P@NiFeAlOx is E

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Figure 6. (a) Polarization curve for overall water splitting. (b) LSV curves for Ni2P@NiFeAlOx before and after 1000 cycles of CV scans. Inset: Chronoamperometric curves of Ni2P@NiFeAlOx at voltages of 1.52 and 1.62 V.

bifunctional electrocatalysts through in situ topological transformation.

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CONCLUSIONS In summary, we have developed an ingenious and simple strategy by in situ topological transformation of NiFeAl-PO43−LDH nanosheet arrays grown on Ni foam upon calcination under a H2 atmosphere to realize the ecofriendly synthesis of nickel phosphide material (Ni2P@NiFeAlOx core−shell nanoarrays). In the preparation process, any additional P source is not needed, which avoids a waste of P resources and the release of poisonous PH3. The obtained Ni2P@NiFeAlOx shows extremely high electrocatalytic activity and excellent durability toward OER and HER. More importantly, Ni2P@ NiFeAlOx can deliver 10 and 100 mA cm−2 at cell voltages of only 1.52 and 1.62 V, respectively, in a two-electrode alkaline water electrolyzer, which is lower than the integration of commercial Pt/C//IrO2 electrodes. The extraordinary electrocatalytic activity of Ni2P@NiFeAlOx for overall water splitting is related to the following factors: (1) the synergy between a highly dispersed Ni2P nanoparticle core and a thin NiFeAlOx shell, as created by strong electronic interactions between them in the core−shell structure; (2) the hierarchical nanoarray structure of Ni2P@NiFeAlOx can maximize accessible active sites, promote electrolyte penetration, and accelerate the release of bubbles formed on the surface of the electrode. The general strategy proposed in this work is promising for the development of heteroatom-doped, ecofriendly, and efficient catalysts for overall water splitting.

Figure 5. (a) HER polarization curves. (b) Tafel plots for HER. (c) Nyquist plots for electrocatalysts. (d) LSV curves for the Ni2P@ NiFeAlOx before and after 1000 cycles of CV scans. Inset: Chronoamperometric curves of Ni2P@NiFeAlOx at overpotentials of 105 and 215 mV.

compared to other samples (Figure 5b), implying a more favorable HER kinetic process. As shown in Figure 5c, Nyquist plots indicate the lower charge-transfer resistance in Ni2P@ NiFeAlOx relative to that in Ni2P@NiAlOx, NiFeAl-PO43−LDH, and Ni foam. The polarization curves of Ni2P@ NiFeAlOx before and after 1000 cycles of CV scans remain almost unchanged, and the chronoamperometric plots for 24 h at two different constant overpotentials of 105 and 215 mV can deliver stable current densities of 10 and 100 mA cm−2 for 24 h, respectively, strongly proving the outstanding stability and durability of Ni2P@NiFeAlOx (Figure 5d, inset). After the stability test of HER for 24 h, the crystalline phase and hierarchical morphology in the Ni2P@NiFeAlOx sample are well-maintained, as determined by XRD and SEM (Figures S19 and S20). On the basis of the results of the electrochemical performance presented above, it can be reasonably concluded that Ni2P@NiFeAlOx core−shell nanosheet arrays are highly active bifunctional catalysts for OER and HER. Thus, Ni2P@ NiFeAlOx was used as an anode and a cathode to drive the overall water splitting in a two-electrode system. For comparison, the commercial benchmark of IrO2//Pt/C on Ni foam was prepared. As depicted in Figure 6a, Ni2P@ NiFeAlOx can reach a current density of 10 mA cm−2 by applying a cell voltage of only 1.52 V, which is lower than that of the commercial Pt/C//IrO2 counterpart (1.55 V). It is worth noting that our catalyst is among the best compared to other bifunctional catalysts for overall water splitting in an alkaline solution (Table S4). Additionally, a negligible change is found for the polarization curve of overall water splitting before and after 1000 cycles of CV scans, and the chronoamperometric curves of Ni2P@NiFeAlOx measured at two different voltages of 1.52 and 1.62 V can deliver stable current densities of 10 and 100 mA cm−2 for 24 h, respectively (Figure 6b, inset). All of the above results prove that the Ni2P@NiFeAlOx electrode can present excellent performance for overall water splitting and good durability, which maps out a promising strategy for fabricating superior heteroatom-doped

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EXPERIMENTAL METHODS

Synthesis of NiFeAl-PO43−-LDH. In a typical process, Ni(NO3)2· 6H2O (1.5 mmol), Fe(NO3)3·6H2O (0.5 mmol), Al(NO3)3·9H2O (0.5 mmol), and urea (5 mmol) with a Ni/Fe/Al molar ratio of 3:1:1 were dissolved in 70 mL of deionized water and then transferred to a 100 mL Teflon-lined autoclave with 1 × 2 cm Ni foam leaching against the wall. The autoclave was aged at 150 °C for 10 h. After cooling to room temperature, the obtained NiFeAl-CO32−-LDH on Ni foam was washed with deionized water several times, then dried at 70 °C for 12 h, and subsequently suspended in 100 mL of deionized water with vigorous agitation. Then the NaH2PO4 solution (1 M) was dropwise added until pH = 4.5 under a N2 atmosphere to obtain NiFeAl-PO43−-LDH grown on Ni foam, followed by drying at 70 °C for 12 h in a vacuum oven. Synthesis of NiAl-PO43−-LDH and NiFe-PO43−-LDH. NiAlPO43−-LDH and NiFe-PO43−-LDH were prepared according the same procedure for NiFeAl-PO43−-LDH without the addition of Fe(NO3)3· 6H2O and Al(NO3)3·9H2O, respectively, in the synthesis process. Synthesis of Ni2P@NiFeAlOx, Ni2P@NiAlOx, and Ni2P@ NiFeOx. NiFeAl-PO43−-LDH, NiAl-PO43−-LDH, and NiFe-PO43−F

DOI: 10.1021/acs.inorgchem.8b03327 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry LDH were reduced at 500 °C for 2 h in a 10% H2/Ar atmosphere with a ramping rate of 2 °C/min to obtain Ni2P@NiFeAlOx, Ni2P@ NiAlOx, and Ni2P@NiFeOx, respectively. The amount of Ni2P@ NiFeAlOx, Ni2P@NiAlOx, and Ni2P@NiFeOx loading on Ni foam, which was determined by the weight difference of Ni foam before and after material growth, is approximately 1.0 mg cm−2. Preparation of Ni2P. Ni(NO3)2·6H2O (2.5 mmol) and urea (5 mmol) were dissolved in 70 mL of deionized water and then transferred to a 100 mL Teflon-lined autoclave with 1 × 2 cm Ni foam leaching against the wall. The autoclave was aged at 150 °C for 10 h. After cooling to room temperature, the obtained Ni(OH)2 on Ni foam was washed with deionized water several times and then dried at 70 °C for 12 h. Finally, Ni(OH)2 and 1 g of NaH2PO2 were placed in two separate porcelain boats with NaH2PO2 at the upstream side, followed by heating at 300 °C for 1 h at a heating rate of 2 °C min−1 under an Ar-flow atmosphere. The final obtained Ni2P was washed with deionized water and absolute ethanol several times. The amount of Ni2P loading on Ni foam determined by the weight difference of Ni foam before and after material growth is approximately 1.0 mg cm−2. Preparation of NiFeAlOx. NiFeAl-CO32−-LDH grown on Ni foam was heated at 500 °C for 2 h at a heating rate of 2 °C min−1 under an Ar atmosphere to obtain NiFeAlOx. The amount of NiFeAlOx loading on Ni foam determined by the weight difference of Ni foam before and after material growth is about 1.0 mg cm−2. Preparation of IrO2 and Pt/C Electrodes. The commercial IrO2 or Pt/C was dispersed in a mixed solution of 1 mL of ethanol and 50 μL of Nafion, and the formed solution was drop-casted onto Ni foam with a catalyst loading of 1.0 mg cm−2. Characterization. XRD measurements were carried out using a Bruker AXS D8 Discover diffractometer with a Cu Kα source (λ = 0.15406 nm) at a 2θ range of 3−70°. FT-IR spectra of samples were collected on a Bruker Vector-22 spectrometer. Elemental analysis was performed using a Shimadzu ICPS-7500 ICP-AES spectrometer, and the morphologies of the samples were determined by SEM (Hitachi S-4800) combined with EDS to determine the dispersions of different elements. TEM and high-resolution TEM (HRTEM) images were obtained on a JEOL2010F instrument. XPS characterization was tested using a Thermo VG ESCALAB250 spectrometer. XPS tests of all samples were performed under the same conditions, and the binding energy was carefully corrected, referring to C 1s at 284.6 eV. Moreover, in order to minimize the influence of radiation used on the electronic states of elements in the samples, the samples were initiated by a single-scan analysis (no more than 20 s) during the course of the XPS experiments. Electrochemical Measurements. All of the electrochemical tests were performed at room temperature on a CHI660E electrochemical workstation (CH Instruments Inc., Shanghai) in a three-electrode system using a 1 M KOH aqueous solution as the electrolyte, a carbon electrode as the counter electrode, a saturated calomel electrode as the reference electrode, and the as-prepared catalyst as the working electrode. Linear-sweep voltammetry (LSV) was conducted at a scan rate of 5 mV s−1 to obtain the polarization curves. CV was performed at a scan rate of 100 mV s −1 for 1000 cycles, and the chronoamperometric curves were recorded at overpotentials of 210 mV for OER and 105 mV for HER to determine the stability of Ni2P@NiFeAlOx. EIS measurements were carried out over a frequency range from 100 kHz to 0.1 Hz at overpotentials of 210 mV for OER and 105 mV for HER. The ECSAs of different catalysts were determined based on Cdl using a simple CV method in a nonFaradaic potential range of 1.23−1.33 V versus reversible hydrogen electrode (RHE). When the capacitive density was plotted at 1.28 V vs RHE against the scan rate (10, 20, 40, 60, and 80 mV s−1), a linear trend was observed. The slope of the fitted line is two times Cdl.

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SEM images, FT-IR spectra, XRD patterns, actual atomic ratios, XPS spectra, electrocatalytic results, and HRTEM image (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.L.). ORCID

Feng Luo: 0000-0001-6380-2754 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (Grants 21871047 and 21661001) and Natural Science Foundation of Jiangxi Province of China (Grant 20181ACB20003).

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REFERENCES

(1) Service, R. F. The Hydrogen Backlash. Science 2004, 305, 958− 961. (2) Gu, Y.; Chen, S.; Ren, J.; Jia, Y. A.; Chen, C. M.; Komarneni, S.; Yang, D. J.; Yao, X. D. Electronic Structure Tuning in Ni3FeN/r-GO Aerogel toward Bifunctional Electrocatalyst for Overall Water Splitting. ACS Nano 2018, 12, 245−253. (3) Tan, C. J.; Cao, X. H.; Wu, X.-J.; He, Q. Y.; Yang, J.; Zhang, X.; Chen, J. Z.; Zhao, W.; Han, S. K.; Nam, G.-H.; Sindoro, M.; Zhang, H. Recent Advances in Ultrathin Two-Dimensional Nanomaterials. Chem. Rev. 2017, 117, 6225−6331. (4) Li, S.; Zhang, L.; Lan, Y.; O’Halloran, K. P.; Ma, H.; Pang, H. Polyoxometalate-Encapsulated Twenty-Nuclear Silver-Tetrazole Nanocage Frameworks as Highly Active Electrocatalysts for the Hydrogen Evolution Reaction. Chem. Commun. 2018, 54, 1964− 1967. (5) Chen, D. J.; Chen, C.; Baiyee, Z. M.; Shao, Z. P.; Ciucci, F. Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices. Chem. Rev. 2015, 115, 9869−9921. (6) Wang, P. T.; Zhang, X.; Zhang, J.; Wan, S.; Guo, S. J.; Lu, G.; Yao, J. L.; Huang, X. Q. Precise Tuning in Platinum-Nickel/Nickel Sulfide Interface Nanowires for Synergistic Hydrogen Evolution Catalysis. Nat. Commun. 2017, 8, 14580−14588. (7) Chung, D. Y.; Jun, S. W.; Yoon, G.; Kim, H.; Yoo, J. M.; Lee, K.S.; Kim, T.; Shin, H.; Sinha, A. K.; Kwon, S. G.; Kang, K.; Hyeon, T.; Sung, Y.-E. Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst. J. Am. Chem. Soc. 2017, 139, 6669−6674. (8) Xiao, X. F.; He, C.-T.; Zhao, S. L.; Li, J.; Lin, W. S.; Yuan, Z. K.; Zhang, Q.; Wang, S. Y.; Dai, L. M.; Yu, D. S. A General Approach to Cobalt-Based Homobimetallic Phosphide Ultrathin Nanosheets for Highly Efficient Oxygen Evolution in Alkaline Media. Energy Environ. Sci. 2017, 10, 893−899. (9) You, B.; Jiang, N.; Sheng, M. L.; Bhushan, M. W.; Sun, Y. J. Hierarchically Porous Urchin-Like Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2016, 6, 714−721. (10) Zhang, X.; Zhou, F.; Pan, W. Y.; Liang, Y. Y.; Wang, R. H. General Construction of Molybdenum-Based Nanowire Arrays for pH-Universal Hydrogen Evolution Electrocatalysis. Adv. Funct. Mater. 2018, 28, 1804600−1804607. (11) Zhang, X. P.; Zhu, S. Y.; Xia, L.; Si, C. D.; Qu, F.; Qu, F. L. Ni(OH)2−Fe2P Hybrid Nanoarray for Alkaline Hydrogen Evolution Reaction with Superior Activity. Chem. Commun. 2018, 54, 1201− 1204.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03327. G

DOI: 10.1021/acs.inorgchem.8b03327 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(29) Zou, X. X.; Goswami, A.; Asefa, T. Efficient Noble Metal-Free (Electro)Catalysis of Water and Alcohol Oxidations by Zinc−Cobalt Layered Double Hydroxide. J. Am. Chem. Soc. 2013, 135, 17242− 17245. (30) Song, F.; Hu, X. L. Ultrathin Cobalt−Manganese Layered Double Hydroxide Is an Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2014, 136, 16481−16484. (31) Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124−4155. (32) Li, Y. M.; He, H. Y.; Fu, W.; Mu, C. Z.; Tang, X.-Z.; Liu, Z.; Chi, D. Z.; Hu, X. In-Grown Structure of NiFe Mixed Metal Oxides and CNT Hybrid Catalysts for Oxygen Evolution Reaction. Chem. Commun. 2016, 52, 1439−1442. (33) Luo, J. S.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Grätzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and EarthAbundant Catalysts. Science 2014, 345, 1593−1596. (34) Du, H. T.; Xia, L.; Zhu, S. Y.; Qu, F.; Qu, F. L. Al-Doped Ni2P Nanosheet Array: A Superior and Durable Electrocatalyst for Alkaline Hydrogen Evolution. Chem. Commun. 2018, 54, 2894−2897. (35) Zhang, F.-S.; Wang, J.-W.; Luo, J.; Liu, R.-R.; Zhang, Z.-M.; He, C.-T.; Lu, T.-B. Extraction of Nickel from NiFe-LDH into Ni2P@ NiFe Hydroxide as A Bifunctional Electrocatalyst for Efficient Overall Water Splitting. Chem. Sci. 2018, 9, 1375−1384. (36) Wang, K. W.; She, X. L.; Chen, S.; Liu, H. L.; Li, D. H.; Wang, Y.; Zhang, H. W.; Yang, D. J.; Yao, X. D. Boosting Hydrogen Evolution via Optimized Hydrogen Adsorption at the Interface of CoP3 and Ni2P. J. Mater. Chem. A 2018, 6, 5560−5565. (37) Yu, J. H.; Cheng, G. Z.; Luo, W. Hierarchical NiFeP Microflowers Directly Grown on Ni Foam for Efficient Electrocatalytic Oxygen Evolution. J. Mater. Chem. A 2017, 5, 11229−11235. (38) Luo, M.; Cai, Z.; Wang, C.; Bi, Y. M.; Qian, L.; Hao, Y. C.; Li, L.; Kuang, Y.; Li, Y. P.; Lei, X. D.; Huo, Z. Y.; Liu, W.; Wang, H. L.; Sun, X. M.; Duan, X. Phosphorus Oxoanion-Intercalated Layered Double Hydroxides for High-Performance Oxygen Evolution. Nano Res. 2017, 10, 1732−1739. (39) Li, P.; Zeng, H. C. Bimetallic Ni-Fe Phosphide Nanocomposites with A Controlled Architecture and Composition Enabling Highly Efficient Electrochemical Water Oxidation. J. Mater. Chem. A 2018, 6, 2231−2238. (40) Son, C. Y.; Kwak, I. H.; Lim, Y. R.; Park, J. FeP and FeP2 Nanowires for Efficient Electrocatalytic Hydrogen Evolution Reaction. Chem. Commun. 2016, 52, 2819−2822. (41) Liang, Y. H.; Liu, Q.; Asiri, A. M.; Sun, X. P.; Luo, Y. L. SelfSupported FeP Nanorod Arrays: A Cost-Effective 3D Hydrogen Evolution Cathode with High Catalytic Activity. ACS Catal. 2014, 4, 4065−4069. (42) Yu, X.-Y.; Feng, Y.; Guan, B. Y.; Lou, X. W.; Paik, U. Carbon Coated Porous Nickel Phosphides Nanoplates for Highly Efficient Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 1246−1250. (43) Gao, Z.; Yang, L.; Fan, G. L.; Li, F. Promotional Role of Surface Defects on Carbon-Supported Ruthenium-Based Catalysts in the Transfer Hydrogenation of Furfural. ChemCatChem 2016, 8, 3769− 3779. (44) Liu, Y. Y.; Sang, S. B.; Wu, Q. M.; Lu, Z. G.; Liu, K. Y.; Liu, H. T. The Electrochemical Behavior of Cl− Assisted Al3+ Insertion into Titanium Dioxide Nanotube Arrays in Aqueous Solution for Aluminum ion Batteries. Electrochim. Acta 2014, 143, 340−346. (45) Yu, J.; Zhong, Y. J.; Wu, X. H.; Sunarso, J.; Ni, M.; Zhou, W.; Shao, Z. P. Bifunctionality from Synergy: CoP Nanoparticles Embedded in Amorphous CoOx Nanoplates with Heterostructures for Highly Efficient Water Electrolysis. Adv. Sci. 2018, 5, 1800514− 1800522. (46) Zhao, Y. J.; Fan, G. L.; Yang, L.; Lin, Y. J.; Li, F. Assembling Ni−Co Phosphides/Carbon Hollow Nanocages and Nanosheets with Carbon Nanotubes into A Hierarchical Necklace-Like Nanohybrid for Electrocatalytic Oxygen Evolution Reaction. Nanoscale 2018, 10, 13555−13564.

(12) Du, H. T.; Kong, R.-M.; Guo, X. X.; Qu, F. L.; Li, J. H. Recent Progress in Transition Metal Phosphides with Enhanced Electrocatalysis for Hydrogen Evolution. Nanoscale 2018, 10, 21617−21624. (13) Gao, M.-R.; Liang, J.-X.; Zheng, Y.-R.; Xu, Y.-F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S.-H. An Efficient Molybdenum Disulfide/Cobalt Diselenide Hybrid Catalyst for Electrochemical Hydrogen Generation. Nat. Commun. 2015, 6, 5982−5988. (14) Long, X.; Li, G.; Wang, Z. L.; Zhu, H. Y.; Zhang, T.; Xiao, S.; Guo, W. Y.; Yang, S. H. Metallic Iron−Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media. J. Am. Chem. Soc. 2015, 137, 11900−11903. (15) Du, H. T.; Kong, R. M.; Qu, F. L.; Lu, L. M. Enhanced Electrocatalysis for Alkaline Hydrogen Evolution by Mn Doping in a Ni3S2 Nanosheet Array. Chem. Commun. 2018, 54, 10100−10103. (16) Wang, F. M.; Li, Y. C.; Shifa, T. A.; Liu, K. L.; Wang, F.; Wang, Z. X.; Xu, P.; Wang, Q. S.; He, J. Selenium-Enriched Nickel Selenide Nanosheets as a Robust Electrocatalyst for Hydrogen Generation. Angew. Chem., Int. Ed. 2016, 55, 6919−6924. (17) Masa, J.; Weide, P.; Peeters, D.; Sinev, I.; Xia, W.; Sun, Z. Y.; Somsen, C.; Muhler, M.; Schuhmann, W. Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution. Adv. Energy Mater. 2016, 6, 1502313−1502322. (18) Tang, Y. J.; Gao, M. R.; Liu, C. H.; Li, S. L.; Jiang, H. L.; Lan, Y. Q.; Han, M.; Yu, S. H. Porous Molybdenum-Based Hybrid Catalysts for Highly Efficient Hydrogen Evolution. Angew. Chem. 2015, 127, 13120−13124. (19) Li, J. S.; Wang, Y.; Liu, C. H.; Li, S. L.; Wang, Y. G.; Dong, L. Z.; Dai, Z. H.; Li, Y. F.; Lan, Y. Q. Coupled Molybdenum Carbide and Reduced Graphene Oxide Electrocatalysts for Efficient Hydrogen Evolution. Nat. Commun. 2016, 7, 11204−11211. (20) Gong, M.; Li, Y. G.; Wang, H. L.; Liang, Y. Y.; Wu, J. Z.; Zhou, J. G.; Wang, J.; Regier, T.; Wei, F.; Dai, H. J. An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452−8455. (21) Song, F.; Hu, X. L. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477−4485. (22) Cao, L.-M.; Wang, J.-W.; Zhong, D.-C.; Lu, T.-B. TemplateDirected Synthesis of Sulphur Doped NiCoFe Layered Double Hydroxide Porous Nanosheets with Enhanced Electrocatalytic Activity for the Oxygen Evolution Reaction. J. Mater. Chem. A 2018, 6, 3224−3230. (23) Yan, Z. H.; Sun, H. M.; Chen, X.; Liu, H. H.; Zhao, Y.; Li, H. X.; Xie, W.; Cheng, F. Y.; Chen, J. Anion Insertion Enhanced Electrodeposition of Robust Metal Hydroxide/Oxide Electrodes for Oxygen Evolution. Nat. Commun. 2018, 9, 2373−2381. (24) Jin, H. Y.; Wang, J.; Su, D. F.; Wei, Z. Z.; Pang, Z. F.; Wang, Y. In situ Cobalt-Cobalt Oxide/N-Doped Carbon Hybrids as Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 2688−2694. (25) Wang, H. T.; Lee, H.-W.; Deng, Y.; Lu, Z. Y.; Hsu, P.-C.; Liu, Y. Y.; Lin, D. C.; Cui, Y. Bifunctional Non-Noble Metal Oxide Nanoparticle Electrocatalysts Through Lithium-Induced Conversion for Overall Water Splitting. Nat. Commun. 2015, 6, 7261−7268. (26) Guo, X. X.; Kong, R.-M.; Zhang, X. P.; Du, H. T.; Qu, F. L. Ni(OH)2 Nanoparticles Embedded in Conductive Microrod Array: An Efficient and Durable Electrocatalyst for Alkaline Oxygen Evolution Reaction. ACS Catal. 2018, 8, 651−655. (27) Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Hong, G. S.; Dai, H. J. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296−7299. (28) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L. S.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274−10277. H

DOI: 10.1021/acs.inorgchem.8b03327 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (47) Liang, H. F.; Gandi, A. N.; Xia, C.; Hedhili, M. N.; Anjum, D. H.; Schwingenschlögl, U.; Alshareef, H. N. Amorphous NiFe-OH/ NiFeP Electrocatalyst Fabricated at Low Temperature for Water Oxidation Applications. ACS Energy Lett. 2017, 2, 1035−1042. (48) Li, Y. J.; Zhang, H. C.; Jiang, M.; Zhang, Q.; He, P. L.; Sun, X. M. 3D Self-Supported Fe-Doped Ni2P Nanosheet Arrays as Bifunctional Catalysts for Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1702513−1702520. (49) Jia, Y.; Zhang, L. Z.; Gao, G. P.; Chen, H.; Wang, B.; Zhou, J. Z.; Soo, M. T.; Hong, M.; Yan, X. C.; Qian, G. R.; Zou, J.; Du, A. J.; Yao, X. D. A Heterostructure Coupling of Exfoliated Ni−Fe Hydroxide Nanosheet and Defective Graphene as a Bifunctional Electrocatalyst for Overall Water Splitting. Adv. Mater. 2017, 29, 1700017−1700024.

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DOI: 10.1021/acs.inorgchem.8b03327 Inorg. Chem. XXXX, XXX, XXX−XXX