Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering

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Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering Wei Lan a, Dan Li a, Wanxin Wang a, Zhongqing Liu b, Hui Chen a, Yi Xu a,* a

Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, Sichuan, China b School of Chemical Engineering, Sichuan University, Chengdu, 610065, Sichuan, China

highlights
A hierarchical flower-like NieP-MWCNTs composite is prepared by one-step powder sintering.
NieP-MWCNTs catalyst shows highly activity and good stability for HER.
The HER performance of NieP-MWCNTs can be improved by adding MWCNTs.

article info

abstract

Article history:

Transition metal phosphides (TMPs) have been proven to be a high-performance non-noble

Received 24 July 2019

metal electrocatalyst with high activity and nearly ~100% Faraday efficiency. However,

Received in revised form

their applications remain faces great challenging due to its unsatisfactory exposure to

14 October 2019

catalytic active sites and electronic conductivity. Here, powder sintering was used to

Accepted 18 October 2019

prepare NieP-Multi-walled carbon nanotubes (NieP-MWCNTs) composite electrodes with a

Available online xxx

hierarchical flower-like structure with a large surface area, the composite electrodes were synthesized by phosphating mixed powder (porous nickel powder, red phosphorus and

Keywords:

MWCNTs). Due to the flower-like nanoplate hierarchical structure, the fast vectorial elec-

NieP-MWCNTs

tron transfer along the building block nanoplates was effectively induced and active sites

Powder sintering

were highly exposed. The resulting NixP-43 wt%MWCNTs shows high activity and dura-

Hierarchical structure

bility towards the Hydrogen evolution reaction (HER) under acidic conditions. It demands

Flower-like nanoplates

extremely low onset-potential of 29 mV, 96 mV to achieve a current density of 10 mA cm2.

Hydrogen evolution reaction

This work suggests an effective method to facilitate grain preferred orientation growth and exposure high activity active, experimental results demonstrate that the electrocatalytic performance of as-prepared NieP-MWCNTs were successfully optimized through the addition of MWCNTs, and might promote further study of the TMPs catalysts for HER. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail address: [email protected] (Y. Xu). https://doi.org/10.1016/j.ijhydene.2019.10.137 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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Introduction In recent years, with the increase of global energy demand, environmental issues and the accompanying climate changes are prompting scientists to search for environmentally friendly, renewable and sustainable alternative sources of energy to replace exhaustible fuel-based energy [1e7]. Hydrogen (H2) is an energy carrier with low density, abundant resources and high energy density (between 120 and 142 MJ kg1) [8]. As an efficient and clean secondary energy carrier, hydrogen is regarded as the future oil [9,10]. The hydrogen evolution reaction (HER) is one of the most promising strategies for the production of hydrogen, which uses the water as raw material [11,12]. Water splitting (H2O (l) /H2 (g) þ 1/2O2 (g)), an uphill reaction process with the energy required to water split is 237 kJ mol1 [13]. The process of HER not only has no emission of greenhouse gases and other polluted gases but also produces pure hydrogen with high efficiency [1]. One key challenge is to find economic, long-term stability, and efficient HER electrocatalysts for water electrolysis [8]. Currently, precious metals based catalysts are still the most efficient and sustainable catalysts for hydrogen evolution reaction (HER) [14,15] and oxygen evolution reaction (OER) [16]. However, the large-scale application of precious metals has greatly hindered due to their scarcity and high costs. Therefore, it is urgent to develop clean, cost-effective, earth-abundant and sustainable non-precious metals alternatives with comparable electrocatalytic activity and durability to noble metals based catalysts [17e19]. At present, TMPs are typical representatives of the nonprecious metal hydrogenation electrocatalysts [20e22]. TMPs are widely used as catalysts based on their non-precious metal free properties and high catalytic activities in HER, lithium ion batteries, hydrodenitrogenation, hydroprocessing, photocatalytic degradation and hydrodesulfurization [23e28]. In the literature reported so far, there are two main ways to prepare HER electrodes based on the transition metal phosphide catalysts: the first method involves the preparation of transition metal phosphides nanocatalysts by hydrothermal synthesis, followed by the use of Nafion or a polymeric binder to attach them to conductive carriers, such as glassy carbon [29,30] or Ti plate [31,32]; Another method aims to develop unbonded electrodes involve the synthesis of transition metal oxide/hydroxid (TMO/TMOH) nanostructures by hydrothermal processing [33,34] or electrodeposition on a current collector [35]. For example, Zhao et al. prepared the hierarchically porous Ni2P polyhedrons and nickel-cobalt bimetal phosphide nanotube via low-temperature phosphorization from a nickel centered metalorganic framework (MOF-74-Ni) and bimetallic metal-organic framework (MOF-74) precursors [27,36]. Furthermore, Huang et al. reported that Ni12P5 nanoparticles (NPs) could be used as efficient catalysts for hydrogen generation via electrolysis and photoelectrolysis [37]. The NixP on Ni foam or CNTs were assembled by electrodeposition using NiCl2$6H2O (or NiSO4$6H2O) and NaH2PO2$H2O as precursors [38,39]. The above methods are unsatisfactory in achieving preferred orientation growth of crystal grains and exposure of high activity sites, so that the electrocatalyst has better HER performance.

However, Liu and Rodriguez reported that the Ni2P (001) behaved similarly to the [NiFe] hydrogenase based on density functional theory (DFT) calculations, which predicts that Ni2P to be a highly active HER catalyst [1,40]. Similarly, Schaak et al. reported that nanostructured nickel phosphide (Ni2P) with a high active surface area and a high density of exposed (001) facets can be used as a potential HER catalyst to replace noble metals [41]. Moreover, the majority of the reported HER electrocatalysts are in the form of nanoparticles or nanostructures adhered to the conductive substrate. Among these electrodes, their microstructures are rather difficult to control, low conductivity and poor interfacial electron conduction ability during HER. Motivated by these predictions, as an excellent HER electrocatalyst, it needs to have a specific crystal structure to overcome the exposure of active sites, so that the high activity plane is exposed as much as possible will enhance its intrinsic catalytic activity and improve HER performance. Two-dimensional (2D) materials such as MoSx-Coated TiO2 Nanotube Arrays and a-MoO3-x nanosheets have been demonstrated to be a highly active electrocatalyst for HER [42,58]. Therefore, in this work, a three-dimensional (3D) NixPMWCNTs nanoflowers with higher catalytic activity by Powder Sintering is reported. Powder Sintering is an industrial technique for using metal powders (or mixtures of metal powders and non-metal powders) as raw materials, forming and sintering to obtain composite materials, metal materials, and various types of articles. The porous nickel spherical powder, red phosphorus and MWCNTs were uniformly mixed with anhydrous ethanol, after the mixed powders were vacuum dried and then subjected to a tableting operation, followed by phosphorization to obtain NixP-MWCNTs composite nanoflowers hybrid electrodes. The as-prepared composite materials have a three-dimensional hierarchical structure, which can be directly used as the electrode for HER. The addition of MWCNTs could effectively improve the conductivity of Nickel phosphide materials and promote the formation of flower-like nanoplates. Benefiting from the uniform composition and the hierarchical structure, the resulting NixP-MWCNTs composite exhibited highly dispersity, active sites with highly exposed, thus presenting excellent electrocatalytic performance and durability as a potential non-noble metal electrocatalyst for HER.

Experimental Reagents Porous nickel spherical powder (Particle size range is 10e80 mm) was prepared by corrosion of the gas atomized NieAl alloy powder with NaOH alkaline solution. Red phosphorus (P, AR), ethanol (C2H5OH, AR), sodium hydroxide (NaOH, AR) and sulfuric acid (H2SO4, 98%) were obtained from Chengdu Kelon chemical reagent factory. Multi-walled carbon nanotubes (MWCNTs, >98%, OD: 4e6 nm) were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. The platinum on carbon (Pt/C) was obtained from Sigma-Aldrich Co. LLC. All reagents are used without any pretreatment.

Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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Catalyst preparation Formation of three-dimensional self-supported skeleton The as-prepared porous nickel spherical powder and red phosphorus were placed in an agate mortar to be ground and uniformly mixed (the molar ratio of Ni-to-P is 1:1). Then, the mixed powder and MWCNTs were stirred in a beaker filled with anhydrous ethanol and vacuum dried subsequently (The calculation method of component design is shown in Table 1). When the mixed powders were completely dried, the tableting operation was performed. In the end, three-dimensional selfsupported Skeleton was obtained.

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0.5 M H2SO4. The Tafel analysis was performed in the potential range of open circuit voltage ±0.5 V with a scanning rate of 10 mV s1. The EIS measurement was conducted at a voltage of the open circuit voltage of each sample in the frequencies range of 102e105 Hz. The stability test was obtained by cyclic voltammetry (CV) scanning of 2000 cycles with a scan rate of 10 mV s1. Unless specified, all of the electrochemical measurements were performed using 0.5 M H2SO4 as the electrolyte solution.

Results and discussion

Synthesis of three-dimensional hierarchical structure NixPMWCNTs composite nanoflowers electrode

Physico-chemical characterization

The as-fabricated three-dimensional self-supported skeleton was placed in a corundum porcelain boat at the center of the tube furnace. The samples were heated at 700  C for 1 h with a heating rate of 10  C min1 under a constant flow of nitrogen at 10 mL min1. After naturally cooling to ambient temperature, the three-dimensional hierarchical structure NixPMWCNTs composite nanoflowers electrodes were prepared, the representative fabrication process is schematically shown in Fig. 1. Meanwhile, the optical photos of NixP-0wt% MWCNTs, NixP-41 wt%MWCNTs, NixP-43 wt%MWCNTs and NixP-45 wt%MWCNTs as shown in Fig. S1.

Fig. 2a shows, the porous nickel spherical powder mainly contains two phases of Ni and NiO. There existed typical diffraction peaks at 44.5 , 51.8 , 76.4 and 37.2 , 43.3 , 62.9 , respectively, and all the diffraction peaks matched well with the face-centered cubic structure of Ni (PDF #04-0850) and NiO (PDF #47-1049). When the content of MWCNTs is 0 wt%, the product of NixP-0wt%MWCNTs is Ni2P (Fig. 2a). The crystalline phase structure and purity of the as-prepared NixP-0wt% MWCNTs were characterized by XRD, there existed diffraction peaks located at 2q ¼ 30.5 , 31.8 , 35.4 , 40.7 , 44.6 , 47.4 , 54.2 , 55.1 , 66.4 , 72.7 and 74.8 indexed to the (110), (101), (200), (111), (201), (210), (300), (211), (310), (311) and (400) planes of NixP-0wt%MWCNTs, respectively, the diffraction peaks matched well with Ni2P (PDF #74-1385). No extraneous peaks exist, which indicates that the as-synthesized Ni2P is a pure phase. Proper porosity is very critical for porous nickel spherical powder to obtain a decent Specific surface area (SSA). As demonstrated in Fig. 2b, the N2 adsorption/desorption isotherms of the Porous Nickel Spherical exhibit the typical IV hysteresis loops, proving the existence of mesopores, the multipoint BET results show the SSA value of porous nickel spherical powder is 78.33 m2 g1. The pore size distribution (PSD) curves, obtained by using the Barret-JoynerHalenda (BJH) method, exhibit noticeable peaks centered at 6.5 nm, evidently demonstrating the mesopores (between 2 and 50 nm) of the porous nickel spherical powder (Fig. 2c). Based on its porous structure, it has a larger SSA value, so that it can be exposed more active sites than commercial Ni powders. The SEM image shows that the porous nickel spherical powder possessed a loose and porous surface, which greatly increased its SSA (Fig. 2c). The porous nickel skeleton could also be used as a precursor with high specific surface area and high catalytic activity for preparing HER electrode materials [43]. In our experiments, according to the molar ratio of Ni to P of 1:1 and different content of MWCNTs were added, the raw material components were designed to prepare electrode materials for HER. For example, the porous nickel spherical powder was mixed with red phosphorus and then subjected to a tableting operation, followed by a 700  C phosphorization process to obtain the NixP-0wt%MWCNTs electrode material. Fig. 2d shows SEM images of the NixP-0wt% MWCNTs at different magnification. The material has a threedimensional connected pore structure, which is beneficial in increasing specific surface area and exposing more active centers which could enhance its catalytic activity. To further

Catalysts characterizations X-ray diffraction (XRD) patterns were obtained on PANalytical Empyrean (Holland) X-Ray Diffractometer. The field emission scanning electron microscopy (FESEM) photographs, scanning electron microscopy (SEM) and elemental mapping images were recorded on a JEOL JSM-7800F Prime (Japan). Low and high-resolution transmission electron microscopy (TEM and HRTEM) photographs were all recorded on a JEOL JEM2100F (Japan) transmission electron microscope. X-ray photoelectron spectra (XPS) was conducted on a Thermo ESCALAB 250Xi (U.S.A.) spectrometer. Electrochemical measurements were carried out using a CHI660E electrochemical workstation in a standard threeelectrode system (Ag/AgCl electrode as the reference electrode, Pt as a counter electrode), including linear sweep voltammetry (LSV), Tafel curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV). All potentials were calibrated for the reversible hydrogen electrode (RHE) on the basis of the Nernst equation without iR correction: ERHE ¼ E Ag/AgCl þ 0.197 þ 0.059 pH. During LSV, the starting and ending voltages were 0.742 V and 0.242 V respectively, with a scanning rate of 10 mV s1 at ambient temperature in

Table 1 e Composition design scheme of composite materials. Samples NixP- 0 wt%MWCNTs NixP-41 wt%MWCNTs NixP-43 wt%MWCNTs NixP-45 wt%MWCNTs

Ni (g)

P (g)

MWCNTs (g)

0.39 0.39 0.39 0.39

0.21 0.21 0.21 0.21

0 0.42 0.45 0.49

Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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Fig. 1 e Schematic illustration for the synthesis of NixP-MWCNTs composite nanoflowers.

Fig. 2 e (a) XRD patterns of the NixP-0wt%MWCNTs, Porous Nickel Spherical. Standard crystal indices of Ni2P (PDF #74-1385), Ni (PDF #04-0850) and NiO (PDF #47-1049) are shown at the bottom. (b) N2 absorption/desorption isotherms. (c) The corresponding pore size distribution curves of Porous Nickel Spherical powder. Inset shows the SEM image of Porous Nickel Spherical powder. (d) SEM image and Enlarged view of NixP-0wt%MWCNTs. (e) TEM images of NixP-0wt%MWCNTs. Inset shows the HRTEM images region.

examine the three-dimensional connected pore structure composite of NixP-0wt%MWCNTs, TEM and HRTEM (Fig. 2e) characterizations were carried out. From the HRTEM image, clear crystal lattice fringes can be observed with a distance of approximately 0.211 nm, which is corresponding to the (111) plane of Ni2P. Furthermore, a series of experiments were conducted. Different mass fraction of MWCNTs were added to the mixed powder of the porous nickel and red phosphorus (the molar ratio of Ni-to-P is 1:1), the experimental results showed that the HER performance was optimal when MWCNTs with a mass fraction of 43% were added. The crystalline phase structure and purity of the three-dimensional hierarchical structure NixP-MWCNTs composite nanoflowers electrode synthesized by adding different mass fractions of MWCNTs were characterized by XRD (Fig. 3). But the product of NixP45 wt%MWCNTs is Ni2P, Ni12P5, Ni3P and MWCNTs coexisted (Fig. 3d). The diffraction peaks at 32.7 , 35.8 , 38.4 , 41.3 , 41.8 ,

44.4 , 47.0 , 49.0 , 54.1 , 56.2 , 59.7 , 68.6 , 72.4 , 74.0 and 74.8 were attributed to the (310), (301), (112), (202), (400), (330), (240), (312), (510), (501), (213), (620), (541), (352) and (004) planes respectively, the diffraction peaks matched well with Ni12P5 (PDF #74-1381). The diffraction peaks at 36.4 , 42.8 , 43.6 , 45.3 , 46.0 , 46.6 , 50.6 , 52.0 , 52.8 and 78.9 were indexed to the (301), (330), (112), (420), (202), (141), (222), (150), (312) and (143) planes respectively, the diffraction peaks matched well with Ni3P (PDF #74-1384). The diffraction peaks at 20.6 , 26.6 and 28.0 were indexed to the (311), (002) and (024) planes respectively, the diffraction peaks matched well with MWCNTs (PDF #82-0505). The diffraction peak of NixP-41 wt% MWCNTs is similar to that of NixP-45 wt%MWCNTs. However, when the content of carbon nanotubes is 43 wt%, the product of NixP-43 wt%MWCNTs is Ni2P, Ni12P5 and MWCNTs (Fig. 3b, c). It can be seen that as the content of MWCNTs increased, the material composition changed from single phase to multiphase.

Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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Fig. 3 e XRD patterns of (a) NixP-45 wt%MWCNTs, (b) NixP43 wt%MWCNTs, (c) NixP-41 wt%MWCNTs. Furthermore, the chemical states of Ni, P, C in the assynthesized NixP-43 wt%MWCNTs were investigated by Xray photoelectron spectroscopy (XPS), as shown in Fig. 4. The XPS survey spectrum (Fig. 4a) confirms the elemental composition of NixP-43 wt%MWCNTs with the peaks of Ni, P, C and O. Among which the C comes from MWCNTs, the O elements should be inherited from the NiO in the porous nickel powder precursor. The high-magnification XPS spectrum

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(Fig. 4b) shows that the C1s core level can be deconvoluted into two peaks. The main peak at 284.6 eV is assigned to an sp2 hybridized graphite-like carbon atom and the peak around 286.4 eV is attributed to the carbon atom bound to one oxygen atom by a single bond, which is consistent with the reported values for CNTs [44e46]. The High-magnification XPS spectrum in the Ni 2p region of the NixP-43 wt%MWCNTs is shown in Fig. 4c. For the Ni 2p3/2 energy level, three peaks can be observed at 852.6, 855.4 and 860.3 eV, which can be attributed to Nidþ in Ni12P5，Ni2þ in NiO and the satellite of the Ni 2p3/2 peak [27,37,47e49]. Three peaks are observed at 852.8, 855.6 and 860.5 eV, which can be attributed to Nidþ in Ni2P，Ni2þ in NiO and the satellite of the Ni 2p3/2 peak [27,37,47e49]. Similarly, three peaks are observed at 870.1, 873.8 and 878.8 eV (Fig. 4c) for the Ni 2p1/2 energy level, corresponding to Nidþ in Ni12P5, Ni2þ in surface NiO and the satellite of the Ni 2p1/2 peak，respectively [19,39e41]. Three peaks are observed at 870.1, 873.8 and 879.1 eV, which can be attributed to Nidþ in Ni2P, Ni2þ in NiO and the satellite of the Ni 2p1/2 peak [ [27,37,47e49]. The deconvoluted XPS spectrum of Ni 2p3/2 was adapted to analyze the surface composition of the as-synthesized NixP43 wt%MWCNTs (Fig. S2). The peak located at 852.6eV belongs to Nidþ in Ni12P5, and the peaks located at Lin 852.8eV resulted from Nidþ in Ni2P. The composition of the assynthesized is listed in Table S1 in the Supporting Information. This result suggests that the production yield of Ni2P is about 38.6% and Ni12P5 is 61.4% about on the surface of NixP43 wt%MWCNTs.

Fig. 4 e (a) XPS spectra of (a) survey spectrum, (b) C 1s, (c) Ni 2p and (d) P 2p regions for NixP-43 wt%MWCNTs. Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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For the P 2p energy level, there are two peaks around 129.5 and 133.8 eV can be observed (Fig. 4d), which can be assigned to Pd in Ni12P5, Pdþ in some oxidized P species (e.g., P2O5) [27,48e50]. Two peaks around 129.6 and 133.2 eV can be observed, which can be assigned to Pd in Ni2P, Pdþ in some oxidized P species [27,47,49,50]. Since the samples were exposed to the air, the oxidized P species forms on the surface of the NixP-43 wt%MWCNTs [50,51]. The Ni 2p3/2 peaks at 852.6 and 852.8 eV are very close to that of the Ni metal, which indicates that Ni species in NixP-43 wt%MWCNTs has a very small positive charge (Nidþ, 0
the presence of microscopic morphology, its electrocatalytic hydrogen evolution performance will be greatly improved. It also was found that the NixP-43 wt%MWCNTs has a three-dimensional hierarchical structure composite nanoflowers, and the MWCNTs wrapped around the nanoflowers (Fig. 6aec and Fig. S3). Meanwhile, the EDS mapping shows the distributed of Ni, P and C, which also indicates that Ni and P are uniformly distributed in MWCNTs (Fig. S4). As Fig. 6c shows, many MWCNTs and nickel phosphide nanocrystals could be observed. To further confirm the composition of NixP-43 wt%MWCNTs, the FESEM energy dispersive spectroscopy (EDS) analysis was performed. The result of elemental mapping images demonstrated that both Ni, P and C elements are uniformly distributed throughout the flowerlike nanoplates of NixP-43 wt%MWCNTs (Fig. 6d). The uniform distribution of MWCNTs in flower-like nanoplates is beneficial to increase its specific surface area and conductivity. Meanwhile, the growth of grains in NixP-43 wt%MWCNTs with preferential orientation and more highly active sites are exposed. To further examine the hierarchical structure of NixP-43 wt%MWCNTs, TEM and HRTEM (Fig. 7) characterizations were carried out. The MWCNTs and nickel phosphide

Fig. 5 e Low-magnification FESEM images, High-magnification FESEM images NixP-41 wt%MWCNTs (a, b), NixP-43 wt% MWCNTs (c, d), NixP-45 wt%MWCNTs (e, f). Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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Fig. 6 e (a) Low-magnification FESEM images, High-magnification FESEM images of MWCNTs layers (b) and NixP-MWCNTs composite nanoflowers (c), FESEM nanoflowers images, corresponding elemental mapping (d).

Fig. 7 e TEM images (a), HRTEM images and FFT patterns (b, c, d) (Insets) of NixP-43wt%MWCNTs.

Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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nanocrystals exhibited highly dispersity (Fig. 7a), which is consistent with the above EDS experimental results. From the HRTEM image, clear crystal lattice fringes can be observed with a distance of approximately 0.334 nm, 0.338 nm and 0.432 nm, which is corresponding to the (002) plane of MWCNTs (Fig. 7b), (001) plane of Ni2P (Fig. 7c) and (200) plane of Ni12P5 (Fig. 7d) respectively. As seen from the above, we further demonstrate that the NixP-MWCNTs composites nanoflowers have extremely high catalytic activity. Additionally, the three-dimensional hierarchically structure composite nanoflowers could accelerate the release of generated gas bubbles, provide fast vectorial electron transfer [52] along the flower-like nanoplates.

Electrocatalytic performance toward the HER The electrocatalysis performance of the as-prepared NixPMWCNTs composites toward HER was evaluated in a 0.5 M H2SO4 solution using a standard three-electrode system with a scan rate of 10 mV s1. Fig. 8a shows the polarization curve of NixP-0wt%MWCNTs, NixP-41 wt%MWCNTs, NixP43 wt%MWCNTs and NixP-45 wt%MWCNTs, which were

obtained by LSV measurements. CNTs (CNTs catalysts were prepared as described above) and commercial Pt/C catalysts were also evaluated for comparison. At the same time, all samples were performed at the same optimized loading of 0.18 mg cm2. As expected, both the CNTs and NixP-0wt% MWCNTs show no significant HER activity from 0.1 V to 0.5 V (RHE). The commercial Pt/C catalyst shows superior HER activity with a near-zero overpotential, which is consistent with previous reports [53]. Using porous spherical nickel powder and red phosphorous as precursors, the NixP0wt%MWCNTs has a three-dimensional porous structure, which is conducive to increasing the specific surface area and exposing more active metal centers to enhance the activity of the catalyst. After a certain amount of MWCNTs were added to the powder of nickel and phosphorus, it shows a markedly enhanced electrocatalytic performance close to the commercial Pt/C catalyst. As the addition of MWCNTs, the conductivity of NixP-MWCNTs composite materials were improved, which promoted charge transfer at the interface. Meanwhile, it also promoted the formation of the NixPMWCNTs composite hierarchical nanoflowers, which promoted the active sites were highly exposed. As shown in

Fig. 8 e (a) Polarization (LSV) curves of MWCNTs, NixP-0wt%MWCNTs, NixP-41 wt%MWCNTs, NixP-43 wt%MWCNTs, NixP45 wt%MWCNTs, and commercial Pt/C in 0.5 M H2SO4 solution at a scan rate of 10 mV s¡1. (b) Summary of onsetpotential, overpotential at j ¼ 10 mA cm¡2 and overpotential at j ¼ 20 mA cm¡2 for the HER catalyzed by NixP-0wt%MWCNTs, NixP41 wt%MWCNTs, NixP-43 wt%MWCNTs, NixP-45 wt%MWCNTs in 0.5 M H2SO4. (c) Tafel plots of MWCNTs, NixP-0wt% MWCNTs, NixP-41 wt%MWCNTs, NixP-43 wt%MWCNTs, NixP-45 wt%MWCNTs, and commercial Pt/C. (d) Electrochemical impedance spectra of NixP-0wt%MWCNTs, NixP-41 wt%MWCNTs, NixP-43 wt%MWCNTs and NixP-45 wt%MWCNTs. Inset shows the high-frequency region. Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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Fig. 8b, the NixP-43 wt%MWCNTs exhibits an HER onset overpotential as small as 29 mV, which is only slightly higher than that of commercial Pt/C electrodes, and can achieve the electrode current densities of 10 (j10) and 20 mA cm2 (j20) for HER at overpotential of 96 and 159 mV respectively. However, the onset overpotential of the other three electrodes and the overpotential required to achieve the same current density are all higher than the NixP-43 wt% MWCNTs, so the NixP-43 wt%MWCNTs has better electrocatalytic performance than the others. Furthermore, the electrocatalytic performance of the NixP-43 wt%MWCNTs is superior to most of the reported non-noble catalysts toward HER (Table S2). To further investigate the electrocatalytic activity of the fabricated electrode materials, the HER kinetics of the as-synthesized electrode were obtained by fitting the linear regions using the Tafel plots equation, h ¼ aþblog (j) (h is the overpotential, a the Tafel constant, b the Tafel slope, and j the current density). Fig. 8c shows the Tafel plots of MWCNTs, NixP-0wt%MWCNTs, NixP-41 wt% MWCNTs, NixP-43 wt%MWCNTs, NixP-45 wt%MWCNTs, and commercial Pt/C, the Tafel slopes are calculated to be 239, 157, 113, 127, 58, and 32 mV dec1, respectively. As expected, among the composite materials prepared above, the performance exhibited by NixP-43 wt%MWCNTs is optimal. The smaller the Tafel slope, the lower the overpotential of the catalytic process at the same kinetic current density. In the above analysis of composition and microstructure, we know that the composition and structure of the NixP-MWCNTs composite materials changed with the change of MWCNTs content. Among the four composite materials, NixP-43 wt% MWCNTs only contains two catalytical active phases Ni2P and Ni12P5, while several other composites contain other phases. Meanwhile, the hierarchically structure composite nanoflowers formed can expose the more active centers, which makes HER more efficient. As mentioned above, with the addition of MWCNTs, which promote the formation of hierarchical structure nanoflowers to exposure of high activity sites, and enhance the HER performance of the composite to make the hydrogen evolution reaction more complete. Obviously, the dramatic change of the Tafel slope of the composite materials by some change of content of MWCNTs. In acid solution, HER underwent a multistep electrochemical process that was two different mechanisms (VolmerHeyrovsky or Volmer-Tafel) with three possible reactions. The HER taking place on the commercial Pt/C surface with a low Tafel slope of 32 mV dec1 is close to previously reported values, suggesting that the well-known mechanism of VolmerTafel has a desorption step as the rate-determining step because of the high adsorbed hydrogen (Hads) coverage [54e57]. The apparent Tafel slope of the NixP-43 wt%MWCNTs is larger than that of the commercial Pt/C, but it is lower than several other electrode materials, suggesting that the more efficient HER process by the NixP-43 wt%MWCNTs, and both of them catalyzed the HER through the Volmer-Heyrovsky mechanism, and that the rate-determining step would be consistent with that of the desorption step [1,31,54,55,58]. The excellent HER electrocatalytic activity of the NixP-43 wt%MWCNTs can be attributed, on the one hand, to the self-supported 3D porous structure, which is beneficial in exposing active sites and

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facilitating the diffusion of electrolyte, thus enhancing the activity and conversion efficiency of the catalyst. On the other hand, the existence of the composite nanoflowers hierarchical structure with exposed active sites. Besides, all our tests are conducted under the same conditions and are comparable. We also try our best to eliminate the effect of electrolyte diffusion and we place a circulating water pump facing the working electrode into the electrolyte (Fig. S5). Meanwhile, the electrochemical tests are carried out after the open circuit potential (OCP) is stable. The electrochemical result with a pump is shown in Fig. S6 in this comment. As we can see in Fig. S6, the potential doesn't change higher. The Electrochemical impedance spectroscopy (EIS) measurement was performed on NixP-0wt%MWCNTs, NixP-41 wt %MWCNTs, NixP-43 wt%MWCNTs, NixP-45 wt%MW CNTs as shown in Fig. 8d. Experimental data were fitted with the Nyquist equivalent circuit shown in Scheme S1, where Rs is the electrolyte resistance, R1 and C1 are the charge transfer resistance and capacity inside the electrode, Rct is the charge transfer resistance at the interface between the electrode and the electrolyte, CPE is a constant phase element. The value of charge transfer resistance (Rct) is related to the electrocatalytic kinetics of the catalyst. A smaller Rct value means faster charge transfer, and its value can be calculated from the semicircle diameter in the Nyquist plot [59]. The conductivity of a metal conductor is an important factor for the charge transfer of electrocatalysts. Fig. 8d shows, among the four samples, the semicircle diameter of NixP-41 wt% MWCNTs, NixP-43 wt%MWCNTs and NixP-45 wt%MWCNTs are significantly reduced, indicating that the addition of MWCNTs can effectively reduce the electrochemical impedance of the interface between the electrode and the electrolyte, and accelerate the transmission of the interface electrons. At the same time, there are some differences in the electrochemical impedance difference between NixP-43 wt% MWCNTs and NixP-45 wt% -MWCNTs. The reason why NixP43 wt%MWCNTs is better may lie in its unique microstructure. The ohmic resistance in the high-frequency region and shows apparent change, again indicating outstanding electrochemical with the increase of MWCNTs. Electrochemical impedance parameters obtained by fitting the Nyquist plots to the equivalent circuit presented in Table S3 of NixP-0wt% MWCNTs, NixP-41 wt%MWCNTs, NixP-43 wt%MWCNTs, NixP-45 wt%MWCNTs. Generally, the Rct value of NixP-43 wt% MWCNTs is smallest and the total resistance is the smallest, indicating that the structure of flower-like and hierarchical nanoplates could enhance electron transfer along the building block during the HER process. At present, most coated electrodes require a polymer binder to combine electrocatalysts and substrates. However, it may have adverse effects, inhibit electrolyte diffusion and increase series resistance, and prevent the exposure of active sites. In our work, the fabricated electrode materials possessed outstanding conductivity and robust three-dimensional hierarchical structure NixP-MWCNTs composite nanoflowers, no polymer binder was used for electrode materials preparation, which could effectively decrease the series resistance and accelerate the release of generated gas bubbles and provide fast vectorial electron transfer, well agreement with the LSV and Tafel analysis.

Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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Fig. 9 e (a) Polarization curves of the NixP 43wt%MWCNTs electrode measured before and after 2000 continuous cycle of ¡1 potential sweeps at a scan rate of 10 mV·s . (b) FESEM image of the NixP-43wt%MWCNTs hierarchical structure flower-like nanoflowers after the cycling stability test. TEM (c) and HRTEM (d, e, f) images of the NixP 43wt%MWCNTs after the cycling stability test.

Due to the electrocatalyst should have enough stability in practical applications, the long-term durability of the NixP43 wt%MWCNTs toward HER measurement was performed using a standard three-electrode system. Fig. 9a shows the polarization curves of the NixP-43 wt%MWCNTs electrode before and after 2000 continuous cycles in 0.5 M H2SO4, the polarization curve shows only a negligible degradation of current density compared with the initial one. After the 2000 continuous cycles test, no clear change in morphology had happened, as demonstrated by FESEM images (Fig. 9b). To further investigate the stability of the NixP-43 wt% MWCNTs, the XRD, TEM images and HRTEM were performed after the 2000 continuous cycle measurement. Compared to the fresh NixP-43 wt%MWCNTs, it has the same XRD diffraction peaks for NixP-43 wt%MWCNTs after 2000 cycles (Fig. S7). TEM and HRTEM images after 2000 cycles (Fig. 9cef) also showed that the structure and chemical states of the NixP-

43 wt%MWCNTs were well preserved after 2000 CV cycles in acid medium. All these results from morphological and compositional characterization manifest that the high electrocatalytic activity and structural stability of the NixP-43 wt% MWCNTs make it a promising material for practical applications.

Conclusions In summary, an unprecedented powder metallurgy route to fabricate a three-dimensional hierarchical structure NixPMWCNTs composite nanoflowers hybrid electrodes with fully exposed active sites was developed. The material has good mechanical strength, which would be directly utilized as an electrode for HER. Meanwhile, experimental results demonstrate that the addition of MWCNTs has greatly enhanced the

Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137

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conductivity of Nickel phosphide materials, and promoted grains preferred orientation growth to form a hierarchical nanoflowers and that the material displayed perfect dispersity and highly exposed active sites. The synthesized NixP-43 wt% MWCNTs exhibited electrocatalytic activity toward HER with a low onset-potential of 29 mV and a small overpotential of 96 mV to reach a current density of 10 mA cm2 and Tafel slopes of 58 mV dec1, which showed excellent catalytic activity and outstanding long-term stability in acid medium. On the one hand, thanks to the addition of MWCNTs and the uniform distribution of precursor powders, the as-prepared NixP-43 wt%MWCNTs electrode exhibited highly dispersity and improved inherent catalytic activity. On the other hand, in view of the three-dimensional hierarchically structure NixP-MWCNTs composite nanoflowers, the long-term stability of NixP-43 wt%MWCNTs accordingly increased. The flowerlike nanoplates hierarchically structure with exposed high active sites of NixP-43 wt%MWCNTs facilitates the diffusion of electrolyte, accelerates the release of generated gas bubbles and vectorial electron transfer along the nanoflowers. This study promotes a pioneering way for preparing the threedimensional hierarchical structure nanoflowers composites as low-cost and high-efficient electrodes for water electrolysis, which found that this method facilitates more the exposure of highly active sites and improves catalytic performance.

Acknowledgements This work was financially supported by the National Key Project of Research and Development Program of China (Grant No.2016YFB1100202).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.137.

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Please cite this article as: Lan W et al., Multi-walled carbon nanotubes reinforced nickel phosphide composite: As an efficient electrocatalyst for hydrogen evolution reaction by one-step powder sintering, International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.10.137