Accepted Manuscript 3D porous Ni-Co-P nanosheets on carbon fiber cloth for efficient hydrogen evolution reaction Yumeng Tian, Jing Yu, Hongsen Zhang, Cheng Wang, Milin Zhang, Zaiwen Lin, Jun Wang PII:
S0013-4686(19)30132-X
DOI:
https://doi.org/10.1016/j.electacta.2019.01.101
Reference:
EA 33504
To appear in:
Electrochimica Acta
Received Date: 10 October 2018 Revised Date:
26 November 2018
Accepted Date: 20 January 2019
Please cite this article as: Y. Tian, J. Yu, H. Zhang, C. Wang, M. Zhang, Z. Lin, J. Wang, 3D porous NiCo-P nanosheets on carbon fiber cloth for efficient hydrogen evolution reaction, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.01.101. 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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3D Porous Ni-Co-P Nanosheets on Carbon Fiber Cloth for Efficient Hydrogen Evolution Reaction
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Yumeng Tiana,c, Jing Yu*a,c, Hongsen Zhanga,c, Cheng Wang*b, Milin Zhanga,e, Zaiwen Lina,c, Jun Wang*a,c,d
a. Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, China
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b. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Heilongjiang University, Harbin 150086, China
c. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001,
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China
d. Institute of Advanced Marine Materials, Harbin Engineering University, 150001, China. e. College of Science, Heihe University, Heihe 164300, China.
* Corresponding author
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Jun Wang, E-mail:
[email protected], Tel.: +86 451 8253 3026; Jing Yu, E-mail:
[email protected];
Abstract
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Cheng Wang, E-mail:
[email protected].
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Besides the urgent demand to improve the electrocatalytic performance of non-noble-metal catalysts, the decrease of catalysts load is also challenging for the radical design of electrocatalysts. In this work, porous Ni-Co-P nanosheet arrays were supported on carbon fiber cloth with an ultrathin film thickness of only ~62 nm. The cross-linked porous nanosheet structure greatly increases the interface area of electrolytes and catalysts. The superhydrophilic and aerophobic nature is advantageous to interface
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contact and bubble detachment. These characteristics encourage Ni-Co-P nanosheets to maintain high performance under the condition of low active substance loading. For
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example, the Ni-Co-P shows superior hydrogen evolution reaction (HER) performances in both alkaline and acidic media, especially in alkaline conditions, which delivers a current density of 10 mA cm-2 at a low overpotential of 57 mV along with the Tafel slope
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of 69 mV dec-1. This work provides a direction for the development of low-cost and
Keywords:
Nickel
cobalt
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efficient non-noble metal electrocatalysts with ultralow load contents. phosphide,
porous
nanosheet
arrays,
low
load,
superhydrophilic-aerophobic surface, hydrogen evolution reaction.
1. Introduction
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Progress in catalysis is driven by increasing energy demands and oil resource exhausting [1]. Electrochemical water-splitting is a universal and efficient method to obtain hydrogen fuel, which is a clean, effective, abundant energy resource and is
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promising to replace the gradually exhausted petroleum-based fuels. The electrocatalysis
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includes hydrogen evolution reaction (HER) that is a two-electron process and oxygen evolution reaction (OER) which is a four-electron process. However, the practical application of hydrogen production by electrochemical water-splitting is limited due to the strong thermodynamic uphill reaction of water electrolysis, which causes a larger overpotential than theoretical value (1.23V) [2-4]. Therefore, efficient electrocatalysts need to be developed to reduce the overpotentials and expedite reaction kinetics. Thus far, the most efficient catalyst for HER is platinum [5-7] and IrO2 and RuO2 are topgallant for 2
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OER [8, 9]. Although they exhibit high performances, the expensive cost and low content of nature restrict their application [10, 11]. Consequently, it is urgent to develop low-cost
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and efficient catalysts [12]. Recently, transitional metal phosphides have been studied as electrocatalysts for HER to replace Pt group materials. The hydrogenases-like catalytic mechanism endows
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them with conspicuous durability and high catalytic activity [13-17]. In spite of these
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researches, the HER performance of phosphides for electrochemical water-splitting is still unsatisfactory compared with noble metal-based catalysts. In order to further improve the HER performance of transition metal phosphides, we can start from the following two aspects. One is surface morphology construction, which could significantly increase
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active sites and promote mass/charge transfer. Nanosheet arrays have been proved to be ideal structure to expose abundant active sites [18-20]. Benefiting from the two-dimensional structure and porous architecture, the electrode realizes a fast electron
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and ion transfer, a large electroactive surface area and superior buffer space in the
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electrocatalysis process. Electrodeposition is an extensive and efficient method to synthetize functional films and construct surface morphology. Meanwhile, a suitable substrate is important for efficient electrodeposition. The in-situ deposition of active material on conductive substrate is favorable to the mass and charge diffusion. Yang et al. synthesized an amorphous CuO@Ni-P nanowire arrays on copper foam by electrodeposition. The as-fabricated 3D catalytic electrode showed excellent performance for both HER and OER in alkaline condition [21]. Liu et al. electrodeposited Ni on 3
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functionalized carbon fiber paper, followed by phosphorization. The as-synthesized CP@Ni-P exhibits high OER activity in 1M KOH and excellent HER activity in both
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alkaline and acidic media. Furthermore, CP@Ni-P showed outstanding durability [22]. The other is alloying that is a valid method to optimize the capability of heterogeneous catalysts and the surface features such as synergistic coupling effect, electronic structure
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and wettability [23-25]. In addition, NiCo-based compounds exhibit excellent
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electrocatalytic activity in most of past works [10, 26-28]. Chang et al. reported a ternary Ni2-xCoxP hybrid with graphene. They prove that the Co-doping in Ni2P is favorable to HER performance. Further density functional theory calculations illustrate that the Ni2-xCoxP catalysts deliver the proper trapping of Hads and facile desorption of the
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emerged H2, exhibiting a better HER activity than Ni2P under the reaction conditions [29]. Especially, besides the approach to improve the performance of nonprecious metal based materials, reducing the load of active substances is also an effective measure to reduce
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costs [30, 31].
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According to the above factors, we prepared ternary Ni-Co-P nanosheet network on carbon cloth through an electrodeposited method first, followed by a further phosphidation treatment. As a contrast, precursor solutions with different Ni/Co proportions were also used to prepare Ni-Co-OH precursors, and then explored the morphology and HER performance of corresponding phosphides. The Ni-Co-P-2 (synthesized by the precursor solutions with Ni:Co=1:2) shows the best HER performance in both alkaline media and acidic media with a current density of 10 mA 4
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cm-2 at the overpotentials of 57 mV and 74 mV, along with the Tafel slopes of 69 mV dec-1 and 88 mV dec-1, respectively. The as-prepared Ni-Co-P-2 also displays an excellent
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durability in in both alkaline and acidic environments.
2. Experimental section
Materials: Nickel nitrate hexahydrate [Ni(NO3)2·6H2O, ≥ 98%] and cobalt nitrate
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hexahydrate [Co(NO3)2·6H2O, ≥99%] were purchased from Damao Chemical Reagent
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Factory. Sodium hypophosphite [NaH2PO2·H2O, ≥99%] and lithium chloride anhydrous (LiCl, ≥95%) were purchased from Zhiyuan Chemical Reagent Company. Sulphuric acid (H2SO4) was purchased from Shanghai Chemical Reagent Company. Potassium hydroxide (KOH, ≥85%) was purchased from Guangfu Technology Company. All
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chemicals used in this work are analytical grade and without further purification. Synthesis of Ni-Co-P nanosheets: Ni-Co-OH nanosheets were prepared by an electrodeposition method onto carbon fiber cloth (1 × 1 cm2), which has been carefully
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cleaned by acetone, ethanol, and deionized water for several times to remove the surface
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impurities. The electrodeposition was conducted in a standard three-electrode system with bare CC, platinum foil, and saturated calomel electrode as working electrode, counter electrode, and reference electrode, respectively. Ni-Co-OHs were synthesized by different Ni/Co proportions precursor solutions (Table S1). The obtained products were named as Ni-OH, Co-OH, Ni-Co-OH-1, Ni-Co-OH-2, Ni-Co-OH-3, Ni-Co-OH-4, respectively. Then the obtained hydroxides were calcined at 300 °C in air for one hour. The corresponding oxides were named as Ni-O, Co-O, Ni-Co-O-1, Ni-Co-O-2, 5
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Ni-Co-O-3, Ni-Co-O-4. After being blown for 30 min by high-purity argon, the as-obtained sample was annealed at 400 °C for 2 h under Ar atmosphere with 5°C /min
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with 0.2 g sodium hypophosphite at upstream of the airflow. And then cooled down to room temperature. The corresponding phosphides are defined as Ni-P, Co-P Ni-Co-P-1, Ni-Co-P-2, Ni-Co-P-3, and Ni-Co-P-4, respectively. The mass loading of the active
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Ni-Co-P-2 on the carbon cloth was determined using microbalance and calculated to be
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about 0.27mg cm-2.
Characterizations: The morphology of products was characterized by scanning electron microscope (SEM, FEI Quanta 200F) and transmission electron microscope (TEM, JEOL JEM-2100). High-resolution TEM (HRTEM) and energy-dispersive X-ray spectroscopy
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(EDX) were conducted on JEOL JEM-2100. X-ray photoelectron spectroscopy (XPS) was recorded by PHI 5700 ESCA spectrometer. Static contact angles were tested by DataPhysics OCA20.
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Electrochemical Measurement: Electrochemical measurements were tested by CHI760E
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electrochemical workstation (Shanghai Chenhua Instrument, Inc.) at room temperature in a three-electrode system. The Ni-Co-P-2/CC was directly employed as working electrode. Graphite electrode and saturated calomel electrode were used as counter and reference electrodes, respectively. HER tests were separately evaluated in 1 M KOH and 0.5 M H2SO4 after being deaerated by high-purity nitrogen for 15 min. All potentials in this work were calibrated to RHE by the equation of E(RHE) = E(SCE) + 0.241 +0.059 pH. All linear scan voltammograms (LSV) were iR-corrected to eliminate the interference of 6
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solution resistance. The LSV curves were measured at a scan rate of 5 mV s-1. Electrochemical impedance spectroscopies (EIS) were performed with the frequency
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range of 0.01 Hz to 100 kHz at 5 mV AC voltage amplitude. The long-term durability tests were tested by the chronopotentiometric measurements. Because of the extremely low load of Ni-Co-P on CC, the reported current density was on account of geometrical
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area.
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3. Results and discussion
The fabrication process of porous Ni-Co-P nanosheets arrays is illustrated in Scheme 1, which is divided into the following three steps: (1) In order to obtain better surface morphology, Ni-Co-OH was first synthesized on carbon cloth by electrodeposition; (2)
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Then an oxidation treatment was carried out to inspire more active sites; (3) In the final phosphorization step, the obtained Ni-Co-O was reacted with the PH3 gas decomposed from NaH2PO2 to generate Ni-Co-P. Eventually, Ni-Co-P not only kept the original
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nanosheet configuration of Ni-Co-OH, but also derived porous structure, which would
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improve greatly the HER performance of Ni-Co-P. The morphologies of products are characterized by scanning electron microscopy
(SEM). As shown in Fig. 1a, the electrodeposited Ni-Co-OH-2 is composed of nanosheets which cover the conductive carbon fiber cloth substrate. These nanosheets are vertically and evenly distributed on carbon fiber cloth and cross-linked to form a grid-like structure. As expected, the morphology is well maintained after oxidation and phosphorization treatment, which are displayed in Fig. 1b and Fig. 1c. This unique 3D 7
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network configuration endows Ni-Co-P-2 with a large specific surface area and the structure of crosslinking is favorable to the mass and charge transfer, as well as the
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diffusion of electrolyte [32-34]. Furthermore, the active film thickness of Ni-Co-P-2 is ~62nm (Fig. 1d), which is much smaller than those of previously reported electrocatalysts prepared by electrodeposition technique, such as Co-P (1-3 µm) [35], Ni-P/NF (1.3 µm)
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[36], Ni3Se2/CF (~5 µm) [37]. The extreme low load amount of phosphides results in the
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difficulty to detect their XRD characteristic diffraction peaks (Fig. S1). The Ni-Co-OH precursors were prepared by various ratios of solutions under the same electrodeposition conditions for comparison. Then the corresponding oxide precursors and phosphides were prepared. The SEM images were provided in Fig. S2-S4. As seen in Fig. S2a-e, the
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Ni-Co-OH nanosheets on carbon fiber cloth are dense, and the top of the nanosheets turned curled and sparse after oxidation treatment (Fig. S3a-e). Ultimately, Ni-Co-P maintained the nanosheets structure of precursors after phosphorization treatment.
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The construction of these nanosheets was further characterized by transmission
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electron microscopy (TEM). As seen in Fig. 2a and b, Ni-Co-P-2 well retained the initial nanosheets morphology of Ni-Co-OH-2, echoing the results of the previous SEM analysis. Furthermore, the phosphorization process caused a porous structure while maintaining the basic nanosheet morphology. The pore structure is conducive to ion transfer during electrochemical reaction, and also increases the interfacial area of electrolyte and catalyst. Accordingly, the exposure of active sites and the electrochemically active surface area (ECSA) of Ni-Co-P-2 is bigger than precursors. The interplanar spacing of Ni-Co-P-2 is 8
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further confirmed by high-resolution TEM (HRTEM), which displays that the lattice fringe is 0.218 nm, corresponding to the (111) plane of NiCoP. The EDS elemental
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mappings of Ni-Co-P-2 from TEM indicate the homogenous distribution of Ni, Co and P in Ni-Co-P-2 nanosheet. In addition, the N2 adsorption/desorption curves were carried out to measure specific surface area and pore-size distribution (Fig. S5). The BET surface
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area of NiCoP-2 is calculated to be 16.32 m²/g and the average pore size is 5.7 nm.
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X-ray photoelectron spectroscopy (XPS) was then analyzed to investigate the elemental composition and chemical valence states of Ni-Co-P-2. The spectral peaks in survey spectrum (Fig. 3a) attest the existence of Ni, Co and P elements. The Ni 2p XPS spectrum shows two main peaks with binding energies at 856.4eV and 874.1 eV,
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corresponding to Ni 2p3/2 and Ni 2p1/2, respectively (Fig. 3b). As depicted in Fig. 3c, the peaks at 781.9 eV and 798.3 eV are attributed to Co 2p3/2 and Co 2p1/2, respectively. The P 2p spectrum exhibits peaks at 129.8 eV and 130.7 eV, reflecting the binding energies of
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P 2p3/2 and P 2p1/2 boned to Ni and Co in Ni-Co-P. The peaks located at 134.4 eV and 135.3 eV can be attributed to oxidized P species. The results indicate the binding energies
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of Ni (856.4 eV), Co (781.9 eV) are related to Ni2+ and Co2+ [38,39]. Both Ni 2p3/2 of 856.4 eV and Co 2p3/2 of 781.9 eV show positively shift relative to metallic Ni0 (852.3 eV) and metallic Co0 (777.9 eV) [40], respectively. Moreover, the P 2p3/2 of 129.8 eV has a negative shift compared to elemental P (130.2 eV). It is suggested that Ni and Co with partial positive charge (δ+) have an electron transfer to P with partial negative charge (δ-) [41]. For HER, Ni and Co centers with partial positive charge could serve as the 9
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hydride-acceptor centers while the negative P centers acts as the proton-acceptor centers. Significantly, the P center can lead to the generation of nickel and cobalt hydride, which
confirmed the successful synthesis of Ni-Co-P.
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could promote the hydrogen evolution by electrochemical desorption [42]. The XPS
The electrocatalytic HER properties of ternary Ni-Co-P-2, Ni-Co-O-2 precursor and
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homologous binary phosphides were investigated in a standard three-electrode system in 1M KOH. The linear sweep voltammetry (LSV) at the scan rate of 5 mV s-1 (Fig. 4a)
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shows that the overpotential of Ni-Co-P-2 to attain a current density of 10 mA cm-2 is 55 mV versus RHE, which is lower than that of the Ni-Co-O-2 precursor (221 mV) as well as corresponding binary phosphides (97 mV for Co-P and 133 mV for Ni-P).
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Significantly, this overpotential for HER in alkaline media is superior to much of reported non-noble metals catalysts, such as Ni2P nanoparticles (η10=250 mV) [43], CoP/CC (η10=209 mV) [44], Co2P@N, PPCN/CNTs (η10=154 mV) [45], Ni-Co-P Nanocubes
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(η10=150 mV) [46], multishelled Ni2P (η10=98 mV) [47], and Co2P hollow NPs (η10=95
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mV) [48] (the detailed performance comparison is listed in Table S2). To further research the kinetics of Ni-Co-P-2 for HER, Tafel slopes were calculated. As shown in Fig. 4b and S6, the resulting Tafel slopes are approximately 84, 93, 101, and 69 mV dec-1 for Ni-Co-O-2, Co-P, Ni-P, and Ni-Co-P-2, respectively. It can be seen that alloying has a certain impact on the enhancement of HER kinetics [49, 50]. The Tafel slope of 69mV dec-1 upon Ni-Co-P-2 illustrates that the HER mechanism of Ni-Co-P-2 corresponds to Volmer-Heyrovsky mechanism, and the desorption steps exist simultaneously with 10
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discharge steps [51-54]. The electrochemical impedance spectroscopy (EIS) at -0.15 V versus RHE was also
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carried out to further investigate the kinetics of the HER. Series resistance (Rs) originates from the resistance of the electrolyte solution, including connections of interfaces and electrode clips. Charge transfer resistance (Rct) issues from the charge transfer resistance
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between catalysts and current collector. The Rs and Rct values profoundly affect
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electrocatalytic kinetics. Generally, small values of Rs indicate a favorable bonding of catalyst and current collector, and small Rct values represent an expeditious charge transfer [55, 56]. Fig. 4c shows that the Rct value is 0.92 Ω for Ni-Co-P-2, much lower than Ni-Co-O-2 precursor (49.89 Ω) and homologous binary phosphides (1.83 Ω for
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Co-P; 4.07 Ω for Ni-P). These results certify the favorable HER kinetics and excellent electron transport of Ni-Co-P-2. In addition, the performance of catalysts is closely linked with electrochemical surface area (ECSA). To obtain the double-layer capacitance (Cdl)
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which is generally proportional to ECSA, the cyclic voltammetry technique (CV) with different scan rates was measured (Fig. S8). As displayed in Fig. 4d, Ni-Co-P-2 exhibits a
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high Cdl value of 6.82 mF cm-2 compared with Ni-CO-O-2 (0.78 mF cm-2), Co-P (1.01 mF cm-2) and Ni-P (0.61 mF cm-2), indicating Ni-Co-P-2 possesses a large ESCA. This is mainly because the porous nanosheets structure of Ni-Co-P-2 provides generous accessible active sites [57, 58], corresponding to the previous TEM. Furthermore, the long-term stability of Ni-Co-P-2 was estimated by the polarization curve of Ni-Co-P-2 after 2000 cycles at 100 mV s-2. As the Fig. 4e shows, the LSV curves before and after 11
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2000 cycles are almost coincide, suggesting the excellent long-term electrocatalysis stability. Chronopotentiometry durability test for 23 h at the current densities of 10 mA
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cm-2 and 100 mA cm-2 were further carried out (Fig. 4f). After 23 h, overpotentials were slightly increased but still maintain favorable performance. In addition, LSV curves and Tafel slopes of Ni-Co-P with different Ni/Co proportions were also carried out to explore
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the influence of bimetal ratio on the electrocatalytic performance (Fig. S9). In spite to
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own similar morphologies, NiCo phosphides exhibit different electrocatalytic activities toward HER, indicating the interaction between bimetal atoms, which would lead to positive effect on the catalytic performance of phosphides.
The electrocatalytic HER performances of ternary Ni-Co-P-2 and Ni-Co-O-2
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precursor in 0.5M H2SO4 were also carried out. The LSV curves at the scan rate of 5 mV s-1 were displayed in Fig. 5a. Ni-Co-P-2 requires overpotential of 74 mV to drive 10 mA cm-2, which is much smaller than that of Ni-Co-O-2 precursor (141 mV), indicating
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improved HER activity after phosphorization. Then Fig. 5b exhibits the Tafel plots. The
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Tafel slope of Ni-Co-P-2 is 88 mV dec-1, suggesting the HER mechanism of Ni-Co-P-2 is Volmer-Heyrovsky mechanism. While the Tafel slope of Ni-Co-O-2 precursor is 345 mV dec-1, demonstrating Ni-Co-P-2 possesses more favorable HER kinetics. To further investigate the kinetics of Ni-Co-P-2, the EIS test at -0.15 V versus RHE was also implemented (Fig. S10). The Rct of phosphide is obviously lower than oxide precursor, indicating superior charge transfer ability upon phosphide. The stabilities of Ni-Co-P-2 in the acid ambient were explored by continuous CV cycles at a scan rate of 100 mV s-2. 12
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There is almost no evident degeneration in LSV curves after 2000 cycles (Fig. 5c). Furthermore, the chronopotentiometry validates the favorable long-term stability of
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Ni-Co-P-2 (Fig. 5d). In addition, LSV curves for added four Ni-Co-P-2 samples synthesized by same technics were carried out and presented similar trend, which proved the favorable repeatability (Fig. S11).
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The superhydrophilic interface can enhance the electrocatalytic activity by
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maximizing the interfacial contact between the catalyst and the electrolyte. The static contact angle tests were carried out to measure the hydrophilicity of the products. As Fig. 6 shows, the contact angle of bare CC and Ni-Co-O-2 are 139° and 121.9°, respectively, indicating their hydrophobic nature. Meanwhile, the static contact angle of Ni-Co-P-2 is
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0°, suggesting a superhydrophilic surface, which results in a sufficient connection between the interface contact. In addition, gas bubbles adhered on electrode impedes decreases electrolysis efficiency seriously [59]. Hence, we measured contact angle
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between gas bubble and nanostructured film (Fig. 6b). The air bubble contact angle is
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145°, suggesting an aerophobic interface. The superhydrophilic and aerophobic nature of Ni-Co-P-2 can significantly improve the HER performance by promoting the electron transfer and mass-transfer kinetics, as well as the gas bubble detachment. The high performance of Ni-Co-P can be attributed to following factors: i) 3D
nanosheets and porous structure of Ni-Co-P-2 provides generous accessible active sites and propitious to electrolyte ion diffusion, as well as offers a buffer space in the electrocatalysis process that is beneficial to durability. ii) Synergism between nickel and 13
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cobalt bimetals optimize the capability of heterogeneous catalysts and surface features, accelerating HER reaction kinetics. iii) The binder-free construction method greatly
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improved the mechanical stability and electrical conductivity of catalyst. iv) The superhydrophilic interface of Ni-Co-P-2 is beneficial to the sufficient contact between
detachment, improving the HER performance.
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4. Conclusion
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electrolyte and electrocatalyst, and the aerophobic interface is beneficial to bubble
To conclude, Ni-Co-P nanosheets were successfully synthesized with a preeminent morphology structure. Benefiting from the 3D porous nanosheet arrays, bimetal effect and superhydrophilic nature, the as-prepared catalyst worked as an efficient catalyst for
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HER in both alkaline and acidic electrolyte with an extremely low load of active substances. In strong alkaline media, Ni-Co-P-2 electrode exhibited superior HER performance with a low overpotential of 57 mV to reach 10 mA cm-2 current density, as
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well as a small Tafel slope of 69 mV dec-1. Besides, it necessitates overpotential of 74
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mV to approach 10 mA cm-2 and the Tafel slope is 88 mV dec-1 in acidic medium. The Ni-Co-P-2 also affords a good stability in both alkaline and acidic media that remains for more than 23 h. The ternary Ni-Co-P-2 with 3D porous surface morphology construction and alloying-compositions provide a feasible device to improve the HER performance of non-noble metals catalyst in both alkaline and acidic medium. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC 14
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51603053), Natural Science Foundation of Heilongjiang Province (B2015021), and Fundamental Research Funds of the Central University and the Application Technology
Technology Development Program (JCKY2016604C006).
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Scheme 1. Schematic illustration for the preparation process of Ni-Co-P porous nanosheet on carbon
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fiber cloth.
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Fig. 1. SEM images of the as-prepared samples. (a) Ni-Co-OH nanosheet arrays on CC. (b) Ni-Co-O
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nanosheet arrays on CC. (c) Ni-Co-P porous nanosheet arrays on CC. (d) The cross-section SEM
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image of Ni-Co-P porous nanosheet arrays.
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Fig. 2. TEM images of (a) Ni-Co-OH nanosheet and (b) Ni-Co-P nanosheet. (c) HRTEM image and (d)
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EDS elemental mapping of Ni-Co-P for Ni, Co, P.
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Fig. 3. XPS spectra of Ni-Co-P-2. (a)XPS survey spectrum. High-resolution , (b) Ni 2p, (c) Co 2p, and
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(d) P 2p spectra.
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Fig. 4. HER performance in 1M KOH. (a) HER polarization curves (iR-compensated) with a scan rate
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of 5 mV s-1, (b) corresponding Tafel slopes and overpotentials at current density of 10 mA cm-2, (c) Nyquist plots and (d) double-layer capacitance of Ni-Co-O-2, Co-P, Ni-P and Ni-Co-P-2. (e) HER polarization curves of Ni-Co-P-2 before and after 2000 cycles. (f) Long-term stability of Ni-Co-P-2 at 10 mA cm-2 and 100 mA cm-2.
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Fig. 5. HER performance in 0.5M H2SO4. (a) HER polarization curves with a scan rate of 5 mV s-1, and (b) corresponding Tafel slopes of Ni-Co-O-2 and Ni-Co-P-2. (c) HER polarization curves of Ni-Co-P-2 before and after 2000 CV cycles. (d) Long-term stability of Ni-Co-P-2 at 10 mA cm-2 for
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Fig. 6. (a) Static contact angle of bare CC, Ni-Co-OH, Ni-Co-O and Ni-Co-P-2 and (b) Air bubble
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contact angle of Ni-Co-P-2.
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3. Exhibits high HER performance with ultralow load contents.
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4. Cross-linked porous nanosheet structure provides large electroactive surface area.