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Nickel-iron diselenide hollow nanoparticles with strongly hydrophilic surface for enhanced oxygen evolution reaction activity
Accepted Manuscript Nickel-iron diselenide hollow nanoparticles with strongly hydrophilic surface for enhanced oxygen evolution reaction activity Lin Lv, Zhishan Li, Yunjun Ruan, Yaoxing Chang, Xiang Ao, Jian-Gang Li, Zhaoxi Yang, Chundong Wang PII:
S0013-4686(18)31806-1
DOI:
10.1016/j.electacta.2018.08.039
Reference:
EA 32473
To appear in:
Electrochimica Acta
Received Date: 21 May 2018 Revised Date:
6 July 2018
Accepted Date: 8 August 2018
Please cite this article as: L. Lv, Z. Li, Y. Ruan, Y. Chang, X. Ao, J.-G. Li, Z. Yang, C. Wang, Nickel-iron diselenide hollow nanoparticles with strongly hydrophilic surface for enhanced oxygen evolution reaction activity, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.08.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Nickel-iron diselenide hollow nanoparticles with strongly hydrophilic surface for enhanced oxygen evolution reaction activity
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Lin Lv, Zhishan Li, Yunjun Ruan, Yaoxing Chang, Xiang Ao, Jian-Gang Li, Zhaoxi Yang and Chundong Wang*
School of Optical and Electronic Information, Huazhong University of Science and Technology,
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Wuhan 430074, P.R. China
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ABSTRACT
It is highly desired while remains challenging to explore stable, earth-abundant, low-cost, and high-efficient electrocatalysts towards eco-friendly utilization of green energy. In this study, we report a one-pot hydrothermal synthesis of polyvinyl-pyrrolidone (PVP)-decorated nickel-iron
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diselenide hollow nanoparticles with strongly hydrophilic surface, the hollow architecture of which could be assigned to the Kirkendall effect. As the lactam groups in PVP are strongly polar and incline to interact with water molecules, the surface wettability of the electrocatalyst was
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effectively improved after PVP was introduced. Compared with the pristine one, such PVP
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decorated nickel-iron diselenide hollow nanoparticles demands only a low overpotential of 255 mV to drive a geometrical current density of 10 mA cm-2 in 1 M KOH aqueous solution. Moreover, this PVP involved electrocatalyst yields a low Tafel slope of 56 mV dec-1 and possesses remarkably long-term durability. This surface engineering insight provides an indication for fabrication of high-efficient OER electrocatalysts. Keywords: Electrocatalyst, Selenide, Hollow structures, Hydrophilic surface, Kirkendall effect *Corresponding author. E-mail address: [email protected] (C.D. Wang). 1
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Introduction Due to the limited storage of natural energy resources, such as coal, petroleum and natural
gas in the earth crust, together with the environmental contamination of their combustion, the
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exploration of clean and renewable fuel has attained wild popularity [1-3]. Electrochemical water splitting towards generation of hydrogen is broadly acknowledged to be one of the most promising strategies [4, 5]. As a half reaction of water oxidation, oxygen evolution reaction (OER) at anodic
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part severely hinders the whole water splitting process because of its sluggish kinetics, which
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involves four electron (4e) process, compared with the counterpart two electron (2e) process at the cathode, leading to its high overpotential and decreased power conversion efficiency [6, 7]. Hence, exploring high-efficient electrocatalysts for OER becomes greatly imperative. To date, noble metal-based compounds, such as RuO2 and IrO2, exhibit great catalytic activities, and have been
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considered as the state-of-the-art electrocatalysts for OER [8]. However, the high cost and scarcity of noble metals, as well as the intrinsic instability under high anodic potential, in deed, dramatically restrict its practical large-scale application [9, 10]. Very recently, electrocatalysts
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based on earth-abundant 3d transition metals, such as metal oxides/hydroxides [11-14], nitrides
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[15], borides/borates [16, 17], sulfides [18, 19], selenides [20, 21], metal organic frameworks (MOFs) [22] and phosphides [23, 24] have been widely investigated, among which, particularly, 3d metal selenides have attracted intensive attention because of their remarkable catalytic performance [25, 26]. The excellent OER catalytic activities of 3d metal selenides have been ascribed to their various valence states of the cations and intrinsic metallic nature, which can provide multiple active sites and facilitate electron transport, respectively [27, 28]. Previously, tremendous efforts have been devoted to rationally design and construct hollow
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ACCEPTED MANUSCRIPT structures, aiming to expose more active sites to electrolytes, and reduce the ions/electrons transfer distance, which could essentially enhance the intrinsic catalytic activity of these electrocatalysts. [20, 29]. By controllable chemical etching and a sequent selenation process, Nai et al. successful
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fabricated Ni-Fe mixed diselenide with hollow nanocages, delivering excellent catalytic activity [30]. Liu and co-workers prepared CoSe2 hollow microspheres from metal-organic framework (MOF)-templates [31]. Shinde et al. synthesized porous Ni-Co perselenide hollow microparticles
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with metal acetate hydroxides as templates [29]. Nevertheless, it is noteworthy that conventional
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preparations of hollow structured metal selenides are complicated and time-consuming. Besides, hydrophilic property is particularly important for catalysts as electrolyte ions will be allowed to directly contact with the surface of the catalysts so that more active sites could be activated [32]. Zhao and his co-workers decorated Ti3C2 (containing hydrophilic groups) on the two-dimensional
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MOF nanosheets, exhibiting decent electrocatalytic activities [33]. Ding et al. proposed a scalable way for bridging single cobalt ions on carbon-nanotube (CNT) surface by using polymerized ionic liquid, evidencing the role of the hydrophilic groups for the enhanced OER performance [34].
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Hence, it is imperative to synthesize hydrophilic catalysts with facile approaches and understand
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the real function of the hydrophilicity for the catalytic OER activity. Herein, we report well-defined Ni-Fe diselenide hollow nanoparticles (termed as P-NFSHPs)
modified by polyvinyl pyrrolidone (PVP) with a strongly hydrophilic feature as the electrocatalyst for OER. Kirkendall effect is the main reason for the one-step formation of the hollow architecture, which involves a fast ion-exchange process between metal ions and Se ions. To the best of our knowledge, such type of one-step and in situ synthesis of Ni-Fe diselenide hollow nanoparticles with strongly hydrophilic property was firstly reported. Compared with the pristine catalyst, that is,
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ACCEPTED MANUSCRIPT Ni-Fe diselenide hollow nanoparticles without decoration of PVP (denoted as NFSHPs), the as-prepared P-NFSHSs exhibited enhanced OER performance with a low overpotential of 255 mV (vs. RHE) at 10 mA cm-2, a low Tafel slope of 56 mV dec-1 and decent long-term stability (only 13
2.
Experimental Section
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mV degradation after 12-h electrolysis).
2.1. Synthesis of Ni-Fe diselenide hollow nanoparticles
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All chemical reagents were directly used without any further purification.
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NiFe diselenide hollow nanoparticles were rapidly synthesized via one-pot hydrothermal method. In a typical procedure, Ni(NO3)2•6H2O and Fe(NO3)3•9H2O with different molar ratios of Ni/Fe (1:0, 3:1, 1:1, 1:3, 0:1 at a constant total metal ion molar mass of 1 mmol), different amount of polyvinyl pyrrolidone (PVP; 0, 11, 22 and 44 mg) and 2 mmol SeO2 powder were dissolved in
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45 mL deionized (DI) water. Then, the mixed solution was stirred for 15 minutes to form a homogeneous solution. Next, 15 mL N2H4•H2O (85 wt%) was slowly injected to the above solution. After stirring for 10 minutes, the solution was sealed, transferred to a 100 mL
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Teflon-lined stainless steel autoclave and maintained at 150 oC for 1 h. After cooling down to
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room temperature naturally, the precipitate was collected and washed with DI water and ethanol absolute three times, respectively, and then dried at 60 oC in an oven overnight. 2.2. Material Characterizations The crystalline structures of the as-prepared samples were characterized with an X-ray
diffractometer (XRD; Philips, X’pert Pro, Cu Kα, λ=1.5406 Å). The morphologies of the catalysts were recorded by field emission scanning electron microscopy (FESEM; Hitachi S-4800) and transmission electron microscopy (TEM; Tecnai G2 F30). The surface chemical states of the
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ACCEPTED MANUSCRIPT catalysts were examined by X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD-600W, Al Kα (1486.6 eV) X-ray source). Fourier transform infrared spectrometry spectra (FT-IR) measurement was conducted on Shimadzu FTIR-8400s (Japan). Contact angles were
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obtained on the optical contact angle measuring device (DSA 30, Germany, KRUSS GmbH). 2.3. Electrochemical Measurements
The electrocatalytic performance of the catalysts were evaluated in a standard three-electrode
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configuration with a CHI 760E electrochemical workstation (Chenhua, Shanghai), in which a Pt
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plate and a Ag/AgCl electrode served as the counter electrode and the reference electrode, respectively; the glassy carbon (GC, 5 mm diameter, 0.196 cm-2) disk electrode (loaded with catalyst) linked to a rotating-disk device was used as the working electrode, and 1 M KOH aqueous solution as the electrolytes. For preparation of the working electrode, the catalyst (5 mg),
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conductive acetylene black (1 mg) and Nafion solution (5 wt%, 30 µL) were dispersed in ethanol absolute (970 µL) to form uniform suspension. Homogenous catalyst ink was obtained by ultrasonicating the suspension for one hour. 10 µL ink was dropped on the GC electrode with a
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pipette and dried at ambient atmosphere, resulting in a refined working electrode with a loading
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mass of 0.255 mgcat cm-2, left for OER test. The preparation of RuO2-modified GC was conducted with the same procedure. Prior to the test, the GC disk electrode was pre-polished and washed with ethanol absolute and DI water. The cyclic voltammetry (CV) measurement was repeated for 20 cycles within the potentials ranging from 1.0 to 1.8 V vs. RHE at 50 mV s-1 to activate the catalysts and obtain repeatable polarization curves. Linear sweep voltammetry (LSV) was recorded at 5 mV s-1 using a rotating disk electrode with a rotation rate of 1600 rpm to release the oxygen bubbles absorbed on the electrode surface. All LSV curves were corrected with 95%
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ACCEPTED MANUSCRIPT IR-compensation to neutralize the losses of ohmic potential drop which contributed from the electrolyte resistance. All potential values in this work were calibrated according to the equation: ERHE = EAg/AgCl+0.197V+0.059pH. In addition, the experimental details for effective
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electrochemical active surface area (ESCA), AC impedance, turnover frequency and faradaic efficiency were depicted in Supporting Information.
3.
Results and discussion
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As shown in Fig. 1, P-NFSHPs were prepared via a facile one-step hydrothermal method.
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The detail formation process are suggested as follows: First, SeO2 was reduced to the elementary selenium, i.e. α-Se nanoparticles by N2H4•H2O at room temperature, forming a homogenous suspension with the color of brick red [35]; Second, metal cations (Ni2+ and Fe3+) were adsorbed on the surface of α-Se nanoparticles and surrounded with PVP macromolecules; Third, the α-Se
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was further gradually reduced to Se2- anions by the excessive N2H4•H2O with strong reducibility and alkalinity[36], and Se2- anions diffuse outward faster than the metal cations (Ni2+ and Fe3+) do inward; Finally, the α-Se core was completely dissolved, and the hollow nanoparticles were
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formed. The proposed formation mechanism of the selenides could be associated with the
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chemical equations as follows [37]:
SeO2 + N2H4 → Se + N2 + 2H2O
(1)
3Se + 6OH− → 2Se2−+SeO32−+3H2O
(2)
SeO32− + N2H4 → Se + N2↑ + H2O + 2OH−
(3)
Se2− + Se + Ni2+ → NiSe2
(4)
To clarify the crystalline structure of the as-prepared samples, XRD characterizations were carried out. In Fig. 2a, peaks at 30.0°, 33.6°, 36.9°, 50.7°, 55.5° and 57.8° (marked with ◊) were clearly
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ACCEPTED MANUSCRIPT observed, indexing to (200), (210), (211), (311), (023) and (321) planes of cubic NiSe2 (JCPDS Card No. 65-1843), and peaks at 29.3°, 31.1°, 34.6°, 36.1° and 48.0° (marked with #) were indexed to cubic FeSe2 (JCPDS Card No. 65-1455). On the other hand, we noticed that all the
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PVP-modified Ni-Fe diselenides possess cubic phase, the patterns of which are composed of both NiSe2 and/or FeSe2. When iron was introduced, the crystalline structure was changed from monometallic NiSe2 to FeSe2, being evidenced from the appearance of peaks at 31.1°, 34.6°, 36.3°
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and 48° (assigned to FeSe2) and the decline in peaks at 30.0°, 33.6° and 55.5° (associated to
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NiSe2), which eventually disappeared. Field-emission scanning electron microscopy (FESEM) was conducted to record the morphology of the as-prepared PVP-modified Ni-Fe diselenide hollow nanoparticles. As shown in Fig. 2b, PVP-modified Ni-Fe diselenides are uniform nanoparticles with sizes ranging from 40 to 90 nm. On the surface of these nanoparticles, thin
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lucent layers were observed, which should be the concentrated PVP. The PVP micelles interconnect the Ni0.75Fe0.25Se2 nanoparticles to establish ball clusters and form a conductive framework, which is beneficial to the electron transfer. For comparison, FESEM images of
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PVP-modified Ni-Fe diselenide nanoparticles (Fig. S1) and Ni0.75Fe0.25Se2 nanoparticles without
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PVP (Fig. S2) were also collected. To more closely identify the hollow feature of the as-prepared P-NFSHPs, transmission electron microscope (TEM) measurement was also implemented. The low-magnification image further confirms that the as-prepared P-NFSHPs are hollow nanoparticles with diameters ranging from 40 to 90 nm (Fig. 2c), which is well consistent with the FESEM observation. In a further magnified TEM image (Fig. 2d), a typical nanoparticle was captured, where holes and quasi-rhombus structure feature can be identified, and the formation of the quasi-rhombus structure could be evolved which is induced by the intrinsic cubic phase [38].
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ACCEPTED MANUSCRIPT To distinctly observe the hollow structure of P-NFSHPs, a typical TEM image was captured to present the void space (Fig. S3). Noteworthy, such kind of hollow nature not only inhibits the collapse of the structures and the aggregation of the nanocrystals, but also provide more accessible
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channels for mass transfer and ions diffusion [39, 40]. HRTEM image was captured from one edge of the quasi-rhombus nanostructure (marked with red circle in Fig. 2d). The measured interplanar spacing of 0.26 nm is indexed to the (210) plane of cubic NiSe2, indicating a crystalline state of
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the compound (shown in Fig. 2e). One thin amorphous layer was observed (highlighted with dash
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line) on the surface of Ni-Fe diselenides, which is caused by the oxidation of the sample because of its exposure to the air. Besides, we also noticed some species on the lattice are amorphous, which should be assigned to PVP, manifesting the hollow nanoparticles were firmly encapsulated by PVP. Fig. 2f shows the selected area electron diffraction (SAED) patterns, where the multiple
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bright rings were observed, suggesting a polycrystalline nature of Ni-Fe diselenides. From these rings, the interplanar spacing could be derived to be 2.62, 2.37, 1.78, 1.54 and 1.27 Å, well indexing to (210), (211), (311), (321) and (332) planes, which is consistent with the XRD results.
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To confirm the elemental composition, EDS analysis were carried out in TEM instrument.
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Obviously, Ni, Fe, Se, O, C, N and Cu elements were identified (Fig. S4), in which O element is ascribed to be the surficial oxidation of Ni-Fe diselenide hollow nanoparticles, C and N elements evidence the existence of PVP, and C and Cu come from TEM grid. The atomic ratio of Ni/Fe by EDS-TEM was detected to be 5.2:1, which varied a lot from the starting materials because of the rapid nucleation and crystallization process. Moreover, the hollow nature of our as-synthesized nanoparticle is also validated in high-angle annular dark-field (HAADF) TEM image (Fig. 2g), in which two clearly observed void spaces are captured (highlighted with arrows in Fig. 2g). In the
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ACCEPTED MANUSCRIPT elemental mappings (Fig. 2g and Fig. S5), it shows that Ni, Fe, Se, O, C and N are evenly distributed over the whole nanoparticles, indicating the uniformity of the as-prepared P-NFSHPs. Interestingly, we found that the encapsulation of PVP effectively improve the hydrophilia nature
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of NFSHPs (Fig. 2h). A much smaller contact angle was observed for the sample of P-NFSHPs (15.9°) compared with the pristine one (76.8°), indicating that P-NFSHPs possess a much better surface wettability.
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X-ray photoelectron spectroscopy (XPS) was further employed to understand the chemical
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states of the as-prepared P-NFSHPs (Fig. 3 and Fig. S6). In the core level of Ni 2p spectrum (Fig. 3a), four peaks at 853.14 eV, 855.73 eV, 870.43 eV and 873.59 eV were resolved, corresponding to Ni2+ 2p3/2, Ni3+ 2p3/2, Ni2+ 2p1/2 and Ni3+ 2p1/2, respectively; and one additional pair of satellite peaks at 862.50 eV and 880.39 eV were deconvoluted as well [41-43]. Besides, another
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pair of peaks at 859.86 eV and 877.76 eV were identified, ascribing to γ-NiOOH, which are ascertained to be conducive to the acceleration of OER process [41]. For the Fe 2p spectrum (Fig. 3b), two spin-orbit doublets corresponding to Fe2+ 2p and Fe3+ 2p are deconvoluted. Furthermore,
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the peaks at 706.63 eV and 720.25 eV are associated with Fe2+ 2p3/2 and Fe2+ 2p1/2, while the
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peaks at 712.09 eV and 724.48 eV are related to Fe3+ 2p3/2 and Fe3+ 2p1/2 [41, 44]. In the Se 3d spectrum (Fig. 3c), four peaks at 54.16, 55.11, 58.33 and 59.19 eV were observed, among which the peaks at 54.16 and 55.11 eV are ascribed to metal-selenium bonds [32, 45], while the peaks at 58.33 and 59.19 eV correspond to the oxidative Se species [20, 46]. Fourier transform infrared (FTIR) spectra was further examined to trace the composition of the as-prepared samples. As shown in Fig. 3d, three obvious newborn peaks were identified at ca. 1674 cm-1, 1424 cm-1, 1289 cm-1 in P-NFSHPs, which respectively corresponds to C=O stretching vibration, H-C-H bending
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ACCEPTED MANUSCRIPT and C−N vibration [47, 48]. For comparison, the FTIR spectra were also collected in NFSHPs and PVP. No peaks were identified in NFSHPs, while three strong peaks located at the same place were discerned, further evidencing the presence of PVP on the surface of P-NFSHPs. Moreover,
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the Brunauer-Emmett-Teller (BET) surface areas of NFSHPs and P-NFSHPs were also measured and determined to be 15.79 and 18.80 m2 g-1 (Fig. S7). Apparently, the total pore volume of P-NFSHPs is only slightly enlarged after the decoration of small amount of PVP macromolecules
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compared to that of NFSHPs. Thus, it informs that the enhanced OER performance should be
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contributed by the improved surface wettability rather than the enlarged specific surface area. The OER activities of the as-prepared electrodes were investigated in 1.0 M KOH aqueous solution in a standard three-electrode configuration. Prior to test, the electrodes were first electrochemically activated in O2-saturated 1.0 M KOH electrolyte at a scan rate of 50 mV s-1 until
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stable cyclic voltammetry curves were obtained. As shown in Fig. 4a, of all the as-prepared catalysts, PVP-modified Ni0.75Fe0.25Se2 exhibited the lowest overpotential of 272 mV at the current density of 20 mA cm-2, relative to the other Ni-Fe diselenides with different metal atomic
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ratios, i.e. NiSe2 (379 mV), Ni0.5Fe0.5Se2 (338 mV), Ni0.25Fe0.75Se2 (344 mV) and FeSe2 (372 mV).
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Furthermore, to clarify the role of PVP on the OER activity, the linear sweep voltammeter (LSV) was performed on PVP (with different amount)-modified NFSHPs (termed as P0.5-NFSHPs, P-NFSHPs and P2-NFSHPs). For comparison, the polarization curve of the state-of-the-art commercial RuO2 was also collected. In Fig. 4b, it could be easily found that P-NFSHPs delivered the best performance with a low overpotential of 272 mV at the current density of 20 mA cm-2 compared with other samples, i.e. pristine NFSHPs (301 mV), P0.5-NFSHPs (276 mV), P2-NFSHPs (331 mV) and RuO2 (333 mV). LSV curve of P-NFSHPs without IR-compensation
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ACCEPTED MANUSCRIPT was also presented for comparison (Fig. S8). The variations of the electrocatalytic behavior of NFSHPs upon PVP decoration could also be reflected from cyclic voltammetry (CV) curves (Fig. S9), in which it shows that the redox reaction is more abundant after appropriate amount of PVP
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was involved. Nonetheless, on the other hand, if excessive PVP was introduced, the electrocatalytic activity will be dramatically decreased. In view of these observations, we can conclude that the OER activity of NFSHPs is sensitive to the PVP, the reason of which could refer
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to the fact that appropriate amount of PVP could effectively enhance surface wettability, allowing
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more electrolyte ions contact and consequently promoting the ion/mass transfer rate. However, PVP itself has no essential contribution to OER activity for the fact that too much PVP introduced will deteriorate the OER performance. To assess the OER kinetics of the samples, the Tafel plots were evaluated and exhibited in Fig. 4c. Among all the catalysts, the as-prepared P-NFSHPs
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possess the lowest Tafel slope (56 mV dec-1), with others being 80 mV dec-1 (NFSHPs), 73 mV dec-1 (P0.5-NFSHPs), 64 mV dec-1 (P2-NFSHPs) and 62 mV dec-1 (RuO2), informing the accelerated catalytic kinetics toward OER for the PVP modified NFSHPs. To further investigate
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the intrinsic electrocatalytic activity of the catalysts NFSHPs and P-NFSHPs, the corresponding
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turnover frequencies (TOFs) were inspected (Fig. S10). The TOF of P-NFSHPs was calculated to be 0.099 s-1 at an overpotential of 300 mV, which is much larger than that of the pristine NFSHPs (0.042 s-1). Furthermore, the Faradaic efficiency of P-NFSHPs was determined by a rotating ring-disk electrode and measured to be ca. 99.3% (Fig. S11), demonstrating a desirable four-electron process with negligible peroxide intermediates. Of note, the OER activity of our as-prepared P-NFSHPs is comparable to or even superior than most of the reported Ni-, Co- or Fe-based compounds. More detail comparison of the OER activities can be consulted in Table S1.
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ACCEPTED MANUSCRIPT To further understand the influence of PVP on the intrinsic catalytic activity, the electrochemical active surface areas (ECSAs) were evaluated from the double-layer capacitance (Cdl) (Fig. S12). As shown in Fig. 4d, the Cdl of P-NFSHPs increases to 1.70 mF cm-2 from 1.04 mF cm-2 for the
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pristine NFSHPs, indicating the effective ECSA of P-NFSHPs was effectively enlarged, and more catalytic sites can be activated after the decoration of hydrophilic PVP. Additionally, electrochemical impedance spectroscopy (EIS) test was performed to disclose the difference of the
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transport kinetics of P-NFSHPs and NFSHPs. As the diameter of the semicircle in the frequency
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range is much smaller for P-NFSHPs compared with the case of the pristine NFSHPs (Fig. 4e), small charge transfer resistance of P-NFSHPs (12.8 Ω vs. 25.4 Ω for the pristine one) is validated, informing a faster charge transfer rate of P-NFSHPs, and the reason of which could refer to the hydrophilic feature of the catalyst after the involvement of PVP. Inset in Fig. 4e is the fitting
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equivalent circuit model [49]. As durability of electrocatalysts is another crucial factor for large-scale industrial production, we further implemented chronopotentiometric measurement (j-t) at a constant current density of 10 mA cm−2 in 1 M KOH to evaluate the stability of P-NFSHPs.
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Fig. 4f displays the measurement results, where it shows that after a 12-hour chronopotentiometric
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test, a dinky stability decay of 13 mV (4.8%) was discerned. Moreover, the stability assessment of the P-NFSHPs was also carried out by cyclic voltammetry. As shown in the inset of Fig. 4f, the overpotential is only slightly enlarged from 255 mV to 270 mV after cycling for 12 h, which is well consistent with the chronopotentiometric test result. It is worth noting that the NFSHPs possess a larger degradation of 12.2% (from 281 to 315 mV) than that of P-NFSHPs (4.8%) (Fig. S13), which could be attributed to the enhanced surface wettability of NFSHPs, making the gas bubble prevent the contact between electrode surface and electrolytes. In addition, FESEM images
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ACCEPTED MANUSCRIPT of the P-NFSHPs and NFSHPs after stability test were collected (Fig. S14) to evidence the effect of PVP on stability. It shows that the P-NFSHPs possess more intact microstructures of nanoparticles than NFSHPs, which may well be attributed to the protection of the outer PVP
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macromolecules. To address the advantage of the hydrophilic surface on the catalytic activity of P-NFSHPs, we propose a possible mass transfer route for PVP-modified Ni-Fe diselenide hollow
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nanoparticles (Fig. 5). For the improved hydrophilic property of P-NFSHPs, it could be well
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understood as lactam groups in PVP are strongly polar and favor to interact with polar water molecules. As such, when the surface of the selenide hollow nanoparticles is encapsulated with PVP, the hydrophily of NFSHPs was effectively improved compared with the pristine one (evidenced by the contact angle measurement result in Fig. 2h). It imparts us that the electrolyte
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ions will be allowed to directly contact with the electrode surface and enhance the adsorption/transfer efficiency of the ion species, which would eventually accelerate the kinetics of OER and improve the catalytic activity. Noteworthy, because the PVP itself does not have
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catalytic activity, thus, when excessive PVP molecules was involved, the thick PVP layer on the
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catalyst surface will strongly couple with electrolyte ions species and inhibit the later entrance of electrolyte ions into the deep layer of catalyst, lowering the reaction efficiency.
4.
Conclusions
In summary, PVP functionalized Ni-Fe diselenide hollow nanoparticles with strongly
hydrophilic surface were directly synthesized via one-pot hydrothermal approach as a highly efficient electrocatalyst towards OER. The as-prepared P-NFSHPs delivers low overpotential (η @10 mA cm-2=255 mV), small Tafel slope (56 mV dec-1), and decent long-term stability (13 mV
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ACCEPTED MANUSCRIPT degradation after 12-h electrolysis). The advanced catalytic behavior could be assigned to the introduced PVP on the electrocatalyst surface, which improves the adsorption/transfer efficiency of the ion species because of the enhanced hydrophilic nature which would eventually accelerate
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the kinetics of OER. We suppose that our work would push boundaries for configuration of high-efficient OER catalyst based on engineering its surface with polymers or other surface/interface modification.
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Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (NSFC Grants No.51502099), ‘the Fundamental Research Funds for the Central Universities’, HUST: 2018KFYYXJJ051. C. D. Wang particularly appreciates the funding support from the Hubei “Chu-Tian Young Scholar” program. The authors appreciate the technical support from the
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Analytical and Testing Center of Huazhong University of Science and Technology.
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Figures and Captions
Fig. 1. Schematic illustration for the synthesis of the PVP-modified Ni-Fe diselenide hollow
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nanoparticles.
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Fig. 2. (a) XRD patterns of PVP-modified Ni-Fe diselenides with different metal atomic ratios
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(1:0, 3:1, 1:1, 1:3 and 0:1). (b) FESEM image of PVP-modified Ni0.75Fe0.25Se2. (c, d) TEM images of PVP modified Ni0.75Fe0.25Se2 hollow nanoparticles with different magnifications. (e) HRTEM
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image of PVP modified Ni0.75Fe0.25Se2 hollow nanoparticles. (f) The corresponding SAED pattern. (g) HAADF-TEM image of P-NFSHPs and the corresponding EDS elemental mapping images. (h)
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Contact angle measurements of PVP modified Ni0.75Fe0.25Se2 (up) and Ni0.75Fe0.25Se2 without PVP modification (bottom).
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Fig. 3. XPS spectra of P-NFSHPs for (a) Ni 2p, (b) Fe 2p and (c) Se 3d. (d) FT-IR spectra of PVP,
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NFSHPs and P-NFSHPs.
Fig. 4. (a) Polarization curves of PVP-modified Ni-Fe diselenides with different metal atomic ratios. (b) Polarization curves of Ni0.75Fe0.25Se2 and different amount of PVP modified Ni0.75Fe0.25Se2. (c) Tafel plots of PVP-modified Ni-Fe diselenides, Ni0.75Fe0.25Se2 without
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ACCEPTED MANUSCRIPT modification of PVP and commercial RuO2. (d) Plots of the current density (at 1.15 V) vs. the scan rate. (e) Nyquist plots of NFSHPs and (f) Chronopotentiometric measurements of P-NFSHPs at 10
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mA cm-2 (Inset is the LSV curves of P-NFSHPs after 12-h measurement).
Fig. 5. Schematic mechanism diagram of the functional PVP on Ni-Fe diselenide hollow
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nanoparticles toward OER.
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Table of Contents Graphic
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S0013-4686(18)31806-1
DOI:
10.1016/j.electacta.2018.08.039
Reference:
EA 32473
To appear in:
Electrochimica Acta
Received Date: 21 May 2018 Revised Date:
6 July 2018
Accepted Date: 8 August 2018
Please cite this article as: L. Lv, Z. Li, Y. Ruan, Y. Chang, X. Ao, J.-G. Li, Z. Yang, C. Wang, Nickel-iron diselenide hollow nanoparticles with strongly hydrophilic surface for enhanced oxygen evolution reaction activity, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.08.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Nickel-iron diselenide hollow nanoparticles with strongly hydrophilic surface for enhanced oxygen evolution reaction activity
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Lin Lv, Zhishan Li, Yunjun Ruan, Yaoxing Chang, Xiang Ao, Jian-Gang Li, Zhaoxi Yang and Chundong Wang*
School of Optical and Electronic Information, Huazhong University of Science and Technology,
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Wuhan 430074, P.R. China
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ABSTRACT
It is highly desired while remains challenging to explore stable, earth-abundant, low-cost, and high-efficient electrocatalysts towards eco-friendly utilization of green energy. In this study, we report a one-pot hydrothermal synthesis of polyvinyl-pyrrolidone (PVP)-decorated nickel-iron
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diselenide hollow nanoparticles with strongly hydrophilic surface, the hollow architecture of which could be assigned to the Kirkendall effect. As the lactam groups in PVP are strongly polar and incline to interact with water molecules, the surface wettability of the electrocatalyst was
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effectively improved after PVP was introduced. Compared with the pristine one, such PVP
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decorated nickel-iron diselenide hollow nanoparticles demands only a low overpotential of 255 mV to drive a geometrical current density of 10 mA cm-2 in 1 M KOH aqueous solution. Moreover, this PVP involved electrocatalyst yields a low Tafel slope of 56 mV dec-1 and possesses remarkably long-term durability. This surface engineering insight provides an indication for fabrication of high-efficient OER electrocatalysts. Keywords: Electrocatalyst, Selenide, Hollow structures, Hydrophilic surface, Kirkendall effect *Corresponding author. E-mail address: [email protected] (C.D. Wang). 1
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Introduction Due to the limited storage of natural energy resources, such as coal, petroleum and natural
gas in the earth crust, together with the environmental contamination of their combustion, the
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exploration of clean and renewable fuel has attained wild popularity [1-3]. Electrochemical water splitting towards generation of hydrogen is broadly acknowledged to be one of the most promising strategies [4, 5]. As a half reaction of water oxidation, oxygen evolution reaction (OER) at anodic
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part severely hinders the whole water splitting process because of its sluggish kinetics, which
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involves four electron (4e) process, compared with the counterpart two electron (2e) process at the cathode, leading to its high overpotential and decreased power conversion efficiency [6, 7]. Hence, exploring high-efficient electrocatalysts for OER becomes greatly imperative. To date, noble metal-based compounds, such as RuO2 and IrO2, exhibit great catalytic activities, and have been
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considered as the state-of-the-art electrocatalysts for OER [8]. However, the high cost and scarcity of noble metals, as well as the intrinsic instability under high anodic potential, in deed, dramatically restrict its practical large-scale application [9, 10]. Very recently, electrocatalysts
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based on earth-abundant 3d transition metals, such as metal oxides/hydroxides [11-14], nitrides
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[15], borides/borates [16, 17], sulfides [18, 19], selenides [20, 21], metal organic frameworks (MOFs) [22] and phosphides [23, 24] have been widely investigated, among which, particularly, 3d metal selenides have attracted intensive attention because of their remarkable catalytic performance [25, 26]. The excellent OER catalytic activities of 3d metal selenides have been ascribed to their various valence states of the cations and intrinsic metallic nature, which can provide multiple active sites and facilitate electron transport, respectively [27, 28]. Previously, tremendous efforts have been devoted to rationally design and construct hollow
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fabricated Ni-Fe mixed diselenide with hollow nanocages, delivering excellent catalytic activity [30]. Liu and co-workers prepared CoSe2 hollow microspheres from metal-organic framework (MOF)-templates [31]. Shinde et al. synthesized porous Ni-Co perselenide hollow microparticles
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with metal acetate hydroxides as templates [29]. Nevertheless, it is noteworthy that conventional
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preparations of hollow structured metal selenides are complicated and time-consuming. Besides, hydrophilic property is particularly important for catalysts as electrolyte ions will be allowed to directly contact with the surface of the catalysts so that more active sites could be activated [32]. Zhao and his co-workers decorated Ti3C2 (containing hydrophilic groups) on the two-dimensional
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MOF nanosheets, exhibiting decent electrocatalytic activities [33]. Ding et al. proposed a scalable way for bridging single cobalt ions on carbon-nanotube (CNT) surface by using polymerized ionic liquid, evidencing the role of the hydrophilic groups for the enhanced OER performance [34].
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Hence, it is imperative to synthesize hydrophilic catalysts with facile approaches and understand
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the real function of the hydrophilicity for the catalytic OER activity. Herein, we report well-defined Ni-Fe diselenide hollow nanoparticles (termed as P-NFSHPs)
modified by polyvinyl pyrrolidone (PVP) with a strongly hydrophilic feature as the electrocatalyst for OER. Kirkendall effect is the main reason for the one-step formation of the hollow architecture, which involves a fast ion-exchange process between metal ions and Se ions. To the best of our knowledge, such type of one-step and in situ synthesis of Ni-Fe diselenide hollow nanoparticles with strongly hydrophilic property was firstly reported. Compared with the pristine catalyst, that is,
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2.
Experimental Section
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mV degradation after 12-h electrolysis).
2.1. Synthesis of Ni-Fe diselenide hollow nanoparticles
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All chemical reagents were directly used without any further purification.
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NiFe diselenide hollow nanoparticles were rapidly synthesized via one-pot hydrothermal method. In a typical procedure, Ni(NO3)2•6H2O and Fe(NO3)3•9H2O with different molar ratios of Ni/Fe (1:0, 3:1, 1:1, 1:3, 0:1 at a constant total metal ion molar mass of 1 mmol), different amount of polyvinyl pyrrolidone (PVP; 0, 11, 22 and 44 mg) and 2 mmol SeO2 powder were dissolved in
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45 mL deionized (DI) water. Then, the mixed solution was stirred for 15 minutes to form a homogeneous solution. Next, 15 mL N2H4•H2O (85 wt%) was slowly injected to the above solution. After stirring for 10 minutes, the solution was sealed, transferred to a 100 mL
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Teflon-lined stainless steel autoclave and maintained at 150 oC for 1 h. After cooling down to
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room temperature naturally, the precipitate was collected and washed with DI water and ethanol absolute three times, respectively, and then dried at 60 oC in an oven overnight. 2.2. Material Characterizations The crystalline structures of the as-prepared samples were characterized with an X-ray
diffractometer (XRD; Philips, X’pert Pro, Cu Kα, λ=1.5406 Å). The morphologies of the catalysts were recorded by field emission scanning electron microscopy (FESEM; Hitachi S-4800) and transmission electron microscopy (TEM; Tecnai G2 F30). The surface chemical states of the
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obtained on the optical contact angle measuring device (DSA 30, Germany, KRUSS GmbH). 2.3. Electrochemical Measurements
The electrocatalytic performance of the catalysts were evaluated in a standard three-electrode
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configuration with a CHI 760E electrochemical workstation (Chenhua, Shanghai), in which a Pt
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plate and a Ag/AgCl electrode served as the counter electrode and the reference electrode, respectively; the glassy carbon (GC, 5 mm diameter, 0.196 cm-2) disk electrode (loaded with catalyst) linked to a rotating-disk device was used as the working electrode, and 1 M KOH aqueous solution as the electrolytes. For preparation of the working electrode, the catalyst (5 mg),
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conductive acetylene black (1 mg) and Nafion solution (5 wt%, 30 µL) were dispersed in ethanol absolute (970 µL) to form uniform suspension. Homogenous catalyst ink was obtained by ultrasonicating the suspension for one hour. 10 µL ink was dropped on the GC electrode with a
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pipette and dried at ambient atmosphere, resulting in a refined working electrode with a loading
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mass of 0.255 mgcat cm-2, left for OER test. The preparation of RuO2-modified GC was conducted with the same procedure. Prior to the test, the GC disk electrode was pre-polished and washed with ethanol absolute and DI water. The cyclic voltammetry (CV) measurement was repeated for 20 cycles within the potentials ranging from 1.0 to 1.8 V vs. RHE at 50 mV s-1 to activate the catalysts and obtain repeatable polarization curves. Linear sweep voltammetry (LSV) was recorded at 5 mV s-1 using a rotating disk electrode with a rotation rate of 1600 rpm to release the oxygen bubbles absorbed on the electrode surface. All LSV curves were corrected with 95%
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electrochemical active surface area (ESCA), AC impedance, turnover frequency and faradaic efficiency were depicted in Supporting Information.
3.
Results and discussion
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As shown in Fig. 1, P-NFSHPs were prepared via a facile one-step hydrothermal method.
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The detail formation process are suggested as follows: First, SeO2 was reduced to the elementary selenium, i.e. α-Se nanoparticles by N2H4•H2O at room temperature, forming a homogenous suspension with the color of brick red [35]; Second, metal cations (Ni2+ and Fe3+) were adsorbed on the surface of α-Se nanoparticles and surrounded with PVP macromolecules; Third, the α-Se
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was further gradually reduced to Se2- anions by the excessive N2H4•H2O with strong reducibility and alkalinity[36], and Se2- anions diffuse outward faster than the metal cations (Ni2+ and Fe3+) do inward; Finally, the α-Se core was completely dissolved, and the hollow nanoparticles were
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formed. The proposed formation mechanism of the selenides could be associated with the
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chemical equations as follows [37]:
SeO2 + N2H4 → Se + N2 + 2H2O
(1)
3Se + 6OH− → 2Se2−+SeO32−+3H2O
(2)
SeO32− + N2H4 → Se + N2↑ + H2O + 2OH−
(3)
Se2− + Se + Ni2+ → NiSe2
(4)
To clarify the crystalline structure of the as-prepared samples, XRD characterizations were carried out. In Fig. 2a, peaks at 30.0°, 33.6°, 36.9°, 50.7°, 55.5° and 57.8° (marked with ◊) were clearly
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PVP-modified Ni-Fe diselenides possess cubic phase, the patterns of which are composed of both NiSe2 and/or FeSe2. When iron was introduced, the crystalline structure was changed from monometallic NiSe2 to FeSe2, being evidenced from the appearance of peaks at 31.1°, 34.6°, 36.3°
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and 48° (assigned to FeSe2) and the decline in peaks at 30.0°, 33.6° and 55.5° (associated to
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NiSe2), which eventually disappeared. Field-emission scanning electron microscopy (FESEM) was conducted to record the morphology of the as-prepared PVP-modified Ni-Fe diselenide hollow nanoparticles. As shown in Fig. 2b, PVP-modified Ni-Fe diselenides are uniform nanoparticles with sizes ranging from 40 to 90 nm. On the surface of these nanoparticles, thin
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lucent layers were observed, which should be the concentrated PVP. The PVP micelles interconnect the Ni0.75Fe0.25Se2 nanoparticles to establish ball clusters and form a conductive framework, which is beneficial to the electron transfer. For comparison, FESEM images of
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PVP-modified Ni-Fe diselenide nanoparticles (Fig. S1) and Ni0.75Fe0.25Se2 nanoparticles without
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PVP (Fig. S2) were also collected. To more closely identify the hollow feature of the as-prepared P-NFSHPs, transmission electron microscope (TEM) measurement was also implemented. The low-magnification image further confirms that the as-prepared P-NFSHPs are hollow nanoparticles with diameters ranging from 40 to 90 nm (Fig. 2c), which is well consistent with the FESEM observation. In a further magnified TEM image (Fig. 2d), a typical nanoparticle was captured, where holes and quasi-rhombus structure feature can be identified, and the formation of the quasi-rhombus structure could be evolved which is induced by the intrinsic cubic phase [38].
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channels for mass transfer and ions diffusion [39, 40]. HRTEM image was captured from one edge of the quasi-rhombus nanostructure (marked with red circle in Fig. 2d). The measured interplanar spacing of 0.26 nm is indexed to the (210) plane of cubic NiSe2, indicating a crystalline state of
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the compound (shown in Fig. 2e). One thin amorphous layer was observed (highlighted with dash
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line) on the surface of Ni-Fe diselenides, which is caused by the oxidation of the sample because of its exposure to the air. Besides, we also noticed some species on the lattice are amorphous, which should be assigned to PVP, manifesting the hollow nanoparticles were firmly encapsulated by PVP. Fig. 2f shows the selected area electron diffraction (SAED) patterns, where the multiple
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bright rings were observed, suggesting a polycrystalline nature of Ni-Fe diselenides. From these rings, the interplanar spacing could be derived to be 2.62, 2.37, 1.78, 1.54 and 1.27 Å, well indexing to (210), (211), (311), (321) and (332) planes, which is consistent with the XRD results.
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To confirm the elemental composition, EDS analysis were carried out in TEM instrument.
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Obviously, Ni, Fe, Se, O, C, N and Cu elements were identified (Fig. S4), in which O element is ascribed to be the surficial oxidation of Ni-Fe diselenide hollow nanoparticles, C and N elements evidence the existence of PVP, and C and Cu come from TEM grid. The atomic ratio of Ni/Fe by EDS-TEM was detected to be 5.2:1, which varied a lot from the starting materials because of the rapid nucleation and crystallization process. Moreover, the hollow nature of our as-synthesized nanoparticle is also validated in high-angle annular dark-field (HAADF) TEM image (Fig. 2g), in which two clearly observed void spaces are captured (highlighted with arrows in Fig. 2g). In the
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of NFSHPs (Fig. 2h). A much smaller contact angle was observed for the sample of P-NFSHPs (15.9°) compared with the pristine one (76.8°), indicating that P-NFSHPs possess a much better surface wettability.
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X-ray photoelectron spectroscopy (XPS) was further employed to understand the chemical
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states of the as-prepared P-NFSHPs (Fig. 3 and Fig. S6). In the core level of Ni 2p spectrum (Fig. 3a), four peaks at 853.14 eV, 855.73 eV, 870.43 eV and 873.59 eV were resolved, corresponding to Ni2+ 2p3/2, Ni3+ 2p3/2, Ni2+ 2p1/2 and Ni3+ 2p1/2, respectively; and one additional pair of satellite peaks at 862.50 eV and 880.39 eV were deconvoluted as well [41-43]. Besides, another
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pair of peaks at 859.86 eV and 877.76 eV were identified, ascribing to γ-NiOOH, which are ascertained to be conducive to the acceleration of OER process [41]. For the Fe 2p spectrum (Fig. 3b), two spin-orbit doublets corresponding to Fe2+ 2p and Fe3+ 2p are deconvoluted. Furthermore,
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the peaks at 706.63 eV and 720.25 eV are associated with Fe2+ 2p3/2 and Fe2+ 2p1/2, while the
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peaks at 712.09 eV and 724.48 eV are related to Fe3+ 2p3/2 and Fe3+ 2p1/2 [41, 44]. In the Se 3d spectrum (Fig. 3c), four peaks at 54.16, 55.11, 58.33 and 59.19 eV were observed, among which the peaks at 54.16 and 55.11 eV are ascribed to metal-selenium bonds [32, 45], while the peaks at 58.33 and 59.19 eV correspond to the oxidative Se species [20, 46]. Fourier transform infrared (FTIR) spectra was further examined to trace the composition of the as-prepared samples. As shown in Fig. 3d, three obvious newborn peaks were identified at ca. 1674 cm-1, 1424 cm-1, 1289 cm-1 in P-NFSHPs, which respectively corresponds to C=O stretching vibration, H-C-H bending
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ACCEPTED MANUSCRIPT and C−N vibration [47, 48]. For comparison, the FTIR spectra were also collected in NFSHPs and PVP. No peaks were identified in NFSHPs, while three strong peaks located at the same place were discerned, further evidencing the presence of PVP on the surface of P-NFSHPs. Moreover,
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the Brunauer-Emmett-Teller (BET) surface areas of NFSHPs and P-NFSHPs were also measured and determined to be 15.79 and 18.80 m2 g-1 (Fig. S7). Apparently, the total pore volume of P-NFSHPs is only slightly enlarged after the decoration of small amount of PVP macromolecules
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compared to that of NFSHPs. Thus, it informs that the enhanced OER performance should be
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contributed by the improved surface wettability rather than the enlarged specific surface area. The OER activities of the as-prepared electrodes were investigated in 1.0 M KOH aqueous solution in a standard three-electrode configuration. Prior to test, the electrodes were first electrochemically activated in O2-saturated 1.0 M KOH electrolyte at a scan rate of 50 mV s-1 until
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stable cyclic voltammetry curves were obtained. As shown in Fig. 4a, of all the as-prepared catalysts, PVP-modified Ni0.75Fe0.25Se2 exhibited the lowest overpotential of 272 mV at the current density of 20 mA cm-2, relative to the other Ni-Fe diselenides with different metal atomic
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ratios, i.e. NiSe2 (379 mV), Ni0.5Fe0.5Se2 (338 mV), Ni0.25Fe0.75Se2 (344 mV) and FeSe2 (372 mV).
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Furthermore, to clarify the role of PVP on the OER activity, the linear sweep voltammeter (LSV) was performed on PVP (with different amount)-modified NFSHPs (termed as P0.5-NFSHPs, P-NFSHPs and P2-NFSHPs). For comparison, the polarization curve of the state-of-the-art commercial RuO2 was also collected. In Fig. 4b, it could be easily found that P-NFSHPs delivered the best performance with a low overpotential of 272 mV at the current density of 20 mA cm-2 compared with other samples, i.e. pristine NFSHPs (301 mV), P0.5-NFSHPs (276 mV), P2-NFSHPs (331 mV) and RuO2 (333 mV). LSV curve of P-NFSHPs without IR-compensation
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ACCEPTED MANUSCRIPT was also presented for comparison (Fig. S8). The variations of the electrocatalytic behavior of NFSHPs upon PVP decoration could also be reflected from cyclic voltammetry (CV) curves (Fig. S9), in which it shows that the redox reaction is more abundant after appropriate amount of PVP
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was involved. Nonetheless, on the other hand, if excessive PVP was introduced, the electrocatalytic activity will be dramatically decreased. In view of these observations, we can conclude that the OER activity of NFSHPs is sensitive to the PVP, the reason of which could refer
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to the fact that appropriate amount of PVP could effectively enhance surface wettability, allowing
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more electrolyte ions contact and consequently promoting the ion/mass transfer rate. However, PVP itself has no essential contribution to OER activity for the fact that too much PVP introduced will deteriorate the OER performance. To assess the OER kinetics of the samples, the Tafel plots were evaluated and exhibited in Fig. 4c. Among all the catalysts, the as-prepared P-NFSHPs
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possess the lowest Tafel slope (56 mV dec-1), with others being 80 mV dec-1 (NFSHPs), 73 mV dec-1 (P0.5-NFSHPs), 64 mV dec-1 (P2-NFSHPs) and 62 mV dec-1 (RuO2), informing the accelerated catalytic kinetics toward OER for the PVP modified NFSHPs. To further investigate
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the intrinsic electrocatalytic activity of the catalysts NFSHPs and P-NFSHPs, the corresponding
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turnover frequencies (TOFs) were inspected (Fig. S10). The TOF of P-NFSHPs was calculated to be 0.099 s-1 at an overpotential of 300 mV, which is much larger than that of the pristine NFSHPs (0.042 s-1). Furthermore, the Faradaic efficiency of P-NFSHPs was determined by a rotating ring-disk electrode and measured to be ca. 99.3% (Fig. S11), demonstrating a desirable four-electron process with negligible peroxide intermediates. Of note, the OER activity of our as-prepared P-NFSHPs is comparable to or even superior than most of the reported Ni-, Co- or Fe-based compounds. More detail comparison of the OER activities can be consulted in Table S1.
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ACCEPTED MANUSCRIPT To further understand the influence of PVP on the intrinsic catalytic activity, the electrochemical active surface areas (ECSAs) were evaluated from the double-layer capacitance (Cdl) (Fig. S12). As shown in Fig. 4d, the Cdl of P-NFSHPs increases to 1.70 mF cm-2 from 1.04 mF cm-2 for the
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pristine NFSHPs, indicating the effective ECSA of P-NFSHPs was effectively enlarged, and more catalytic sites can be activated after the decoration of hydrophilic PVP. Additionally, electrochemical impedance spectroscopy (EIS) test was performed to disclose the difference of the
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transport kinetics of P-NFSHPs and NFSHPs. As the diameter of the semicircle in the frequency
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range is much smaller for P-NFSHPs compared with the case of the pristine NFSHPs (Fig. 4e), small charge transfer resistance of P-NFSHPs (12.8 Ω vs. 25.4 Ω for the pristine one) is validated, informing a faster charge transfer rate of P-NFSHPs, and the reason of which could refer to the hydrophilic feature of the catalyst after the involvement of PVP. Inset in Fig. 4e is the fitting
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equivalent circuit model [49]. As durability of electrocatalysts is another crucial factor for large-scale industrial production, we further implemented chronopotentiometric measurement (j-t) at a constant current density of 10 mA cm−2 in 1 M KOH to evaluate the stability of P-NFSHPs.
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Fig. 4f displays the measurement results, where it shows that after a 12-hour chronopotentiometric
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test, a dinky stability decay of 13 mV (4.8%) was discerned. Moreover, the stability assessment of the P-NFSHPs was also carried out by cyclic voltammetry. As shown in the inset of Fig. 4f, the overpotential is only slightly enlarged from 255 mV to 270 mV after cycling for 12 h, which is well consistent with the chronopotentiometric test result. It is worth noting that the NFSHPs possess a larger degradation of 12.2% (from 281 to 315 mV) than that of P-NFSHPs (4.8%) (Fig. S13), which could be attributed to the enhanced surface wettability of NFSHPs, making the gas bubble prevent the contact between electrode surface and electrolytes. In addition, FESEM images
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ACCEPTED MANUSCRIPT of the P-NFSHPs and NFSHPs after stability test were collected (Fig. S14) to evidence the effect of PVP on stability. It shows that the P-NFSHPs possess more intact microstructures of nanoparticles than NFSHPs, which may well be attributed to the protection of the outer PVP
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macromolecules. To address the advantage of the hydrophilic surface on the catalytic activity of P-NFSHPs, we propose a possible mass transfer route for PVP-modified Ni-Fe diselenide hollow
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nanoparticles (Fig. 5). For the improved hydrophilic property of P-NFSHPs, it could be well
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understood as lactam groups in PVP are strongly polar and favor to interact with polar water molecules. As such, when the surface of the selenide hollow nanoparticles is encapsulated with PVP, the hydrophily of NFSHPs was effectively improved compared with the pristine one (evidenced by the contact angle measurement result in Fig. 2h). It imparts us that the electrolyte
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ions will be allowed to directly contact with the electrode surface and enhance the adsorption/transfer efficiency of the ion species, which would eventually accelerate the kinetics of OER and improve the catalytic activity. Noteworthy, because the PVP itself does not have
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catalytic activity, thus, when excessive PVP molecules was involved, the thick PVP layer on the
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catalyst surface will strongly couple with electrolyte ions species and inhibit the later entrance of electrolyte ions into the deep layer of catalyst, lowering the reaction efficiency.
4.
Conclusions
In summary, PVP functionalized Ni-Fe diselenide hollow nanoparticles with strongly
hydrophilic surface were directly synthesized via one-pot hydrothermal approach as a highly efficient electrocatalyst towards OER. The as-prepared P-NFSHPs delivers low overpotential (η @10 mA cm-2=255 mV), small Tafel slope (56 mV dec-1), and decent long-term stability (13 mV
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ACCEPTED MANUSCRIPT degradation after 12-h electrolysis). The advanced catalytic behavior could be assigned to the introduced PVP on the electrocatalyst surface, which improves the adsorption/transfer efficiency of the ion species because of the enhanced hydrophilic nature which would eventually accelerate
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the kinetics of OER. We suppose that our work would push boundaries for configuration of high-efficient OER catalyst based on engineering its surface with polymers or other surface/interface modification.
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Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (NSFC Grants No.51502099), ‘the Fundamental Research Funds for the Central Universities’, HUST: 2018KFYYXJJ051. C. D. Wang particularly appreciates the funding support from the Hubei “Chu-Tian Young Scholar” program. The authors appreciate the technical support from the
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Figures and Captions
Fig. 1. Schematic illustration for the synthesis of the PVP-modified Ni-Fe diselenide hollow
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nanoparticles.
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Fig. 2. (a) XRD patterns of PVP-modified Ni-Fe diselenides with different metal atomic ratios
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(1:0, 3:1, 1:1, 1:3 and 0:1). (b) FESEM image of PVP-modified Ni0.75Fe0.25Se2. (c, d) TEM images of PVP modified Ni0.75Fe0.25Se2 hollow nanoparticles with different magnifications. (e) HRTEM
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image of PVP modified Ni0.75Fe0.25Se2 hollow nanoparticles. (f) The corresponding SAED pattern. (g) HAADF-TEM image of P-NFSHPs and the corresponding EDS elemental mapping images. (h)
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Contact angle measurements of PVP modified Ni0.75Fe0.25Se2 (up) and Ni0.75Fe0.25Se2 without PVP modification (bottom).
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Fig. 3. XPS spectra of P-NFSHPs for (a) Ni 2p, (b) Fe 2p and (c) Se 3d. (d) FT-IR spectra of PVP,
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NFSHPs and P-NFSHPs.
Fig. 4. (a) Polarization curves of PVP-modified Ni-Fe diselenides with different metal atomic ratios. (b) Polarization curves of Ni0.75Fe0.25Se2 and different amount of PVP modified Ni0.75Fe0.25Se2. (c) Tafel plots of PVP-modified Ni-Fe diselenides, Ni0.75Fe0.25Se2 without
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ACCEPTED MANUSCRIPT modification of PVP and commercial RuO2. (d) Plots of the current density (at 1.15 V) vs. the scan rate. (e) Nyquist plots of NFSHPs and (f) Chronopotentiometric measurements of P-NFSHPs at 10
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mA cm-2 (Inset is the LSV curves of P-NFSHPs after 12-h measurement).
Fig. 5. Schematic mechanism diagram of the functional PVP on Ni-Fe diselenide hollow
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nanoparticles toward OER.
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Table of Contents Graphic
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