graphene supported cobalt phosphide nanoparticles for electrocatalytic hydrogen evolution reaction

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Multilevel N-doped carbon nanotube/graphene supported cobalt phosphide nanoparticles for electrocatalytic hydrogen evolution reaction Xinglong Guan, Jingwen Ma, Kai Li, Junmei Liang, Zhen Li, Wenchao Peng, Guoliang Zhang, Xiaobin Fan, Fengbao Zhang, Yang Li* Lab of Advanced Nano
structures & Transfer Processes, Department of Chemical Engineering, Tianjin University, Tianjin, 300354, China

highlights  A two-step strategy is used to synthesize composites.  The multilevel structure improve the electrical conductivity and reaction active area.  The hybrid exhibits high HER activities.  Interaction between CoP and carbon structure enhances the activity and stability.

article info

abstract

Article history:

Electrochemical water splitting plays an important role in alternative energy studies, since

Received 1 June 2019

it is highly efficient and environment-friendly. Accordingly, it is an ideal way of providing

Received in revised form

alternative to meet the urgent need of finding sustainable and clean energy. This study

16 August 2019

presents the fabrication of CoP attached on multilevel N-doped CNT/graphene (CoPeCNT/

Accepted 19 August 2019

NG) hybrids. The multilevel carbon structure can enhance electrical conductivity efficiently

Available online 26 October 2019

and increase the reaction active area immensely. The obtained electrocatalyst exhibits great electronic conductivity (17.8 s cm
1) and HER activity with low overpotential (155 mV

Keywords:

at 10 mA cm
2), low Tafel slope (69.1 mV dec
1) in 0.5 M H2SO4. In addition, the CoPeCNT/

Graphene

NG displays prominent electrochemical durability after 18 h.

Carbon nanotube

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Cobalt phosphide Multilevel structure Hydrogen evolution reaction

Introduction The severe depletion of fossil fuels and consequent pollution make the development of alternative clean energy a must. Hydrogen, an efficient zero-carbon-footprint fuel, has been

considered as one of the most advantageous alternatives [1]. Electrolyzing water is an effective and convenient way of producing hydrogen with high purity. Remarkably efficient electrocatalysts are urgently required in hydrogen evolution reaction (HER). Among various electrocatalysts, platinum-

* Corresponding author. E-mail address: [email protected] (Y. Li). https://doi.org/10.1016/j.ijhydene.2019.08.163 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

30054

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

based catalysts show an outstanding catalytic performance, but the high price and low abundance hinder their large-scale production and commercial application [2]. Therefore, it is necessary to develop non-noble metal catalysts with an efficient and inexpensive approach to promoting hydrogen production. Recently, transition-metal phosphides (TMPs), for instance, Ni2P [3], CoP [4], and FeP [5] have gained great attention because of their low price and excellent catalytic activity in HER. Among them, CoP has been most widely applied in HER. For example, Liu et al. reported porous carbon nanofiber with the appearance of lotus root functionalized as electrocatalyst to produce hydrogen in different pH environments [6]. Qian et al. developed CoP nanoparticles in combination with WSe2 nanoflakes for HER [7]. Despite evident advantages, TMPs are unable to conduct electricity effectively. In addition, they are characterized with low stability and small surface area, prohibiting their applications. To leap over the barriers, carbonaceous materials could be chosen as the ideal carriers. Graphene, a kind of typical 2D carbon materials, has been served as a promising support of TMPs nanoparticles for electrochemistry application on account of its vast catalytic surface area, excellent conductivity as well as great chemical stability [8]. Thus, graphene can prevent the TMPs nanoparticles from aggregation to maintain sufficient activity area, meanwhile, connect separate nanoparticles with each other to achieve better structural stability. Besides, carbon nanotube (CNT) with excellent mechanical flexibility and high conductivity, is another important carbon material for HER [9]. The transport of electrons through the carbon nanotubes shares great similarity with the movement of bullets in a trajectory, which means the process can decrease diffusion resistance while maintaining fast currents [10]. Additionally, many attempts have been made to synthesize multilevel structure hybrids [11]. In comparison with pure graphene or CNT, multilevel nanostructure can accelerate the ion transfer in the composite and enlarge active surface area. The combination of graphene and carbon nanotubes shows improved electrical conductivity, higher stability and catalyst activity due to synergism. The CNTs can promote electron transfer, and function as spacers to increase the basal spacing between graphene sheets to facilitate the mass transport of reactants. Besides, the multilevel carbon structure can efficiently avoid the aggregation of nanoparticles and expose more active sites, as well as increasing the stability by protecting the nanoparticles from corrosive effect of electrolyte. Moreover, Nitrogen heteroatoms doping is an efficient way to enhance electrical performance of carbon, owing to the augment of the number of active sites, which can cause hydrogen adsorption Gibbs freeenergy to abate [12,13]. In this work, the fabrication of cobalt phosphide nanoparticles attached to multilevel N-doped CNT/graphene nanostructures (CoPeCNT/NG) is illustrated. Cobalt metal as multifunctional active site catalyzes both the formation of CNT and the process of HER. The multilevel carbon structure as the catalyst substrate can enhance immensely electrical conductivity, increase the reaction activity area and prevent the aggregation of the CoP nanoparticles. As a result, the assynthesized CoPeCNT/NG material demonstrates excellent

HER performance in acidic electrolyte, displaying low overpotential and Tafel slope, as well as great stability.

Experimental Preparation of CoeCNT/NG Graphene Oxide (GO) was made by applying Hummers Method. Then, 174 mg cobalt acetate tetrahydrate (Co(Ac) 2$4H2O), 2 g urea and 200 mg GO were added into 40 mL deionized water. Subsequently, the admixture was stirred for 20 min and ultrasonicated for 20 min to produce a homogenous suspending liquid. After lyophilization, the well-mixed sample was annealed at 850  C for 120 min with a temperature increasing speed of 5  C min
1 in N2, and then cooled down to environment temperature to obtain CoeCNT/NG. For comparison, NG was fabricated by using the same preparation without Co(Ac)2$4H2O.

Preparation of CoPeCNT/NG To obtain CoPeCNT/NG, the CoeCNT/NG and NaH2PO2 at a mass ratio of 1:5 were put in a corundum boat. Then, the corundum boat was putted into tubular furnace and annealed at 450  C for 120 min at a heating rate of 2  C min
1 under N2 and naturally cooled to ambient temperature. For comparison purpose, CoPeRGO was prepared by repeating the procedures except for not adding urea, while CoP was formed without both urea and graphene.

Material characterization The crystal structures of obtained hybrids were measured by X-ray diffractometer (D8 Advance, Germany) with Cu Ka radiation. The surface morphology of materials was tested by field emission scanning electron microscopy (S-4800, Japan). The surface morphology and element composition of catalysts were tested by field emission transmission electron microscopy (JEM-2100F, Japan). The composition and valence distribution of elements were analyzed by X-ray photoelectron spectrometer (ESCALAB 250XI, America). All binding energies were modified by comparison with the standard of C 1s binding energy. N2 adsorption/desorption isotherms were performed on SSA-7000 instrument.

Electrochemical measurements The electrochemical measurements were tested by electrochemical workstation (CHI660E, Shanghai). With a typical three-electrode system, an Ag/AgCl electrode was used as reference electrode. The catalyst dropped on glassy carbon electrode (GCE) acted as working electrode and a graphite rod was used as counter electrode. In the study, every potential was modified based on reversible hydrogen electrode. Catalyst solutions were prepared by following: 5 mg of the catalyst, 0.75 mL distilled water, 0.25 mL isopropanol and 0.05 mL 5 wt% Nafion solution were mixed. The mixed solution was ultrasonicated for 40 min to form a uniform homogenous soliquoid. Then, 5 mL solution was dropped onto the surface of

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

GCE and dried in air naturally. EIS was examined in the sweeping frequency from 100 kHz to 0.1 Hz.

Results and discussions In this work, CoP attached on multilevel N-doped CNT/graphene (CoPeCNT/NG) hybrids is synthesized through a twostep strategy (Scheme 1). Firstly, the mixture of graphene oxide, urea and cobalt tetrahydrate acetate is ultrasonicated, freeze-dried and calcined to form CoeCNT/NG. Then, the resulting CoeCNT/NG is calcined with sodium hypophosphite at 450  C for 120 min to fabricate CoPeCNT/NG. The morphology and element composition of CoPeCNT/ NG is characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM). The SEM image of the N-doped graphene is rough and without any CNTs (Fig. S1a, Supporting Information). After adding cobalt, there are nanoparticles and curved CNTs decorated on the graphene sheets with carbon nanotubes (Fig. S1b, Supporting Information), which indicates that cobalt can facilitate the formation of carbon nanotubes. Compared to CoeCNT/NG, the structure of CoPeCNT/NG (Fig. 1a) does not have obvious change after phosphorization. Similarly, as presenting on TEM image of CoPeCNT/NG (Fig. 1b), the nanoparticles and hollow structural nanotubes are dispersed on the surface of graphene sheets. Notably, the CNTs show a bamboo-like structure. Another alternative sample was synthesized by using melamine instead of urea. The corresponding SEM (Fig. S2a, Supporting Information) image shows the existence of nanotubes on graphene sheets, indicating that the cobalt can catalyze the formation of CNTs from different carbon sources. Therefore, the formation mechanism of CNTs could be described as follow: Firstly, cobalt, as the nucleation site, catalyzes the growth of nanotube with urea, which acts as the sources of carbon and nitrogen. Then, the surface of cobalt is gradually covered by a carbon layer. When the cobalt nanoparticle is completely covered by carbon, the catalytic nanoparticle loses its activity immediately and the growth of CNTs stops simultaneously. Finally, the catalyst separates from the nanotubes [14]. The high-resolution transmission electron microscopy (HRTEM) images of the CoPeCNT/NG (Fig. 1c and d) provide proofs of the mentioned multilevel carbon structure formation mechanism. In Fig. 1c, the nanoparticles show well-resolved lattice fringes with a distance of 0.190 nm, in accordance with (211) crystal planes of CoP [15]. The shell on nanoparticles displays lattice distance is 0.347 nm, showing correspondence to (211) crystal planes of carbon [16]. Besides, the average size of CoP nanoparticles in Fig. 1b is about 28.12 nm. Compared to CoPeCNT/NG, the average

30055

nanoparticles size of CoP on CoP-RGO (Fig. S2b, Supporting Information) is much bigger. The comparison of nanoparticle size distribution is shown in Fig. S3. This result suggests the formation of carbon shell can prevent the nanoparticles’ aggregation so as to reduce the nanoparticles size. The larger number of active sites and the size-dependent variation in electronic structure on the smaller nanoparticles may contribute to the better catalytic properties. Furthermore, N doped carbon shell can provide more active sites, which is beneficial for HER [17]. In Fig. 1d, the nanotubes show wellresolved lattice fringes with a space of 0.346 nm, showing correspondence to (211) crystal planes of carbon. It should be noted that the crystal fringes along the curled edge of carbon nanotubes are discontinuous, proving that nitrogen atoms are doped in the CNTs [18]. Furthermore, corresponding quantitative energy dispersive X-ray spectroscopy (EDX) elemental mapping images (Fig. S4, Supporting Information) amply demonstrated that the C and N elements distribute on the nanotubes uniformly, indicating the successful synthesis of N-doped CNTs. Besides, according to coupled plasma optical emission spectrometer, the mass fraction of Co and P are 27.1 wt% and 13.9 wt%, respectivly. The atom ratio of Co and P is nearly 1:1, which indirectly proves the successful synthesis of CoP nanoparticles. In Fig. 1e, the wider distribution of elements shows the Co and P elements are evenly spread over the particles. N elements are distributed within the graphene, proving that graphene has been doped with N. The crystal structures of CoeCNT/NG and CoPeCNT/NG are characterized by Xeray diffraction (XRD) patterns. The XRD pattern of CoeCNT/NG (Fig. 2a, the blue line) demonstrates the existence of two peaks at 44.2 and 51.5 , related to the (111) and (200) planes of Co, respectively [19]. The reflection peaks of CoPeCNT/NG (red line) at 31.7 , 35.3 , 36.2 , 46.3 , 48.2 , 52.2 56.0 and 56.8 correspond to the (011), (200), (111), (112), (211), (103), (020) and (301) planes of CoP phase respectively [20]. Furthermore, no other peaks are found in XRD pattern of CoPeCNT/NG, indicating Co has been successful transformed into CoP nanoparticles. X-ray photoelectron spectroscopy (XPS) measurement is measured to analyze the composition and valence distribution of elements. The full XPS spectrum (Fig. 2b) shows that elemental of C, N, O, P and Co are embodied in the obtained materials. The high-resolution C 1s XPS spectrum (Fig. 2c) can be well fitted with four peaks at 284.5, 286.0, 286.9 and 289.2 eV, assigned to C]C, CeO/C]N, C]O/CeN and OeC]O, respectively [21]. The high-resolution N 1s XPS spectrum (Fig. 2d) with three diffraction peaks at 400.4, 398.5 and 401.3 eV to pyrrolic N, pyridinic N, and graphitic N, respectively [22]. The contents of three types of N are 7.7, 33.1, and 59.2 atom% respectively. Graphitic N and pyridinic N occupy

Scheme 1 e The preparation schematic of the CoP-CNT/NG catalyst.

30056

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

Fig. 1 e The morphology and element composition of CoPeCNT/NG. a) SEM image, b) TEM image and c), d) HRTEM images. e) HAADF-STEM image of CoP-CNT/NG and C, N, O, Co and P element mapping images of CoP-CNT/NG.

most type of the nitrogen in the obtained materials, which are regarded as the availably activate around carbon species for HER [23]. In Co 2p spectrum (Fig. 2e), a couple of peaks at 778.5 and 781.2 eV correspond to Co 2p3/2. The Co 2p1/2 region is corresponding to a diffraction peak located at 797.4 eV. The satellite peaks observed at 783.4, 786.5, and 802.8 eV are consistent with the shake-up excitation of high-spin Co2þ ions [24]. The spectrum of P 2p shows in Fig. 2f. Two strong peaks at 129.2 and 134 eV are corresponding to CoeP and PeO binding respectively, illustrating the successful phosphorization of Co [25]. The peak attributed to the P species of CoP (129.2 eV) is weak. It may own to that the CoP nanoparticles are covered by carbon shell, but XPS can only detect a few nanometers of the material surfaces [26]. Consistent with the previous analyses,

The XPS results suggest the successful synthesis of CoP-CNT/ NG. To further investigate the influences of multilevel structure for catalytic reaction, nitrogen adsorption/desorption isotherms are collected to evaluate the specific surface area and pore structure of CoP-RGO and CoP-CNT/NG (Fig. S5, Supporting Information). In Fig. S5a, CoP-CNT/NG indicates a typical Type IV adsorption/desorption isotherm, referring to mesoporous characteristics [4]. Fig. S5b exhibits the pore size of catalysts are mainly distributed from 0 to 5 nm. Besides, according to Brunauer-Emmett-Teller (BET) equation, CoPCNT/NG bears a surface area of 67.15 m2 g
1, much larger than CoP-RGO (36.39 m2 g
1). These data indicate that multilevel carbon structure features rich mesoporous channels and

30057

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

b)

30

35

40

45

50

55

60

65

70

2 Theta (degree)

c) Intensity (a.u.)

C=C

75

80

C 1s

0

280

e)

282

284

286

288

600

800

graphitic N pyridinic N

Co 2s O KLL 1000

N 1s

pyrrolic N

O-C=O

290

Binding energy (eV)

Co 2p3/2 Co 2p1/2

Intensity(a.u.)

400

Binding energy (eV)

d)

C-O/C=N/C-P C=O/C-N

200

Co LMM Co 2p3 Co 2p1

CoP-CNT/NG

O 1s

N 1s

Co-CNT/NG

C 1s

a)

292

294

Co 2p

395

400

405

Binding energy (eV)

f)

410

P 2p

P-O

Co-P 2+

high-spin Co ions

775

780

785

790

795

800

805

810

Binding energy (eV)

128

132

136

Binding energy (eV)

140

Fig. 2 e a) XRD patterns of Co-CNT/NG and CoP-CNT/NG. b) XPS survey. (cef) higheresolution scans of C1s, N 1s, Co 2p, P 2p of CoP-CNT/NG respectively.

large surface areas can improve charge transportation, and thus, promote HER. The electrocatalytic performances of catalysts for HER are measured in a 0.5 M H2SO4 solution and deposit on the glassy carbon electrode (GCE). Except for CoPeCNT/NG, commercial Pt/C (20 wt%), CoeCNT/NG, CoPeRGO, CoP and NG are employed as comparisons under identical conditions. Fig. 3a shows the polarization curves of samples at a rate of 5 mV s
1. It is obvious that commercial Pt/C performs the best catalytic activity with 32 mv at 10 mA cm
2, consistent with the previous research. CoeCNT/NG, CoPeRGO, CoP and NG show overpotential of 189, 192, 235 and 292 mV at a current density of 10 mA cm
2 (Fig. 3a). After phosphating, the CoPeCNT/NG

with a lower overpotential (h10 ¼ 155 mV), exhibits better HER catalytic than CoeCNT/NG, CoP and CoPeRGO. The introduction of P to CoP-CNT/NG may attributed to the further improved HER performance. Recent research has reported that different crystal facets of CoP nanostructures exhibit good catalytic activities for HER, such as (111), (110) and (011) facets [27]. Particularly, on (111) facets, the Co bridge sites and P top sites can absorb hydrogen with a near-zero free energy change, indicating that synergistic effect of Co and P atoms can improve HER activity. Besides, the interaction of multilevel N-doped CNT/graphene can accelerate the ion transfer in the composite and enhance the electrical conductivity effectively. The above factors are the reason for the excellent HER

30058

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

b)

J (mA cm-2)

-10 -20

Overpotential (V)

a)

0.4

Pt-C (20wt%) CoP-CNT/NG Co-CNT/NG CoP-GO CoP NG

0

0.2

-30 -40 -50

-0.5

-0.4

Pt-C (20 wt%) CoP-CNT/NG Co-CNT/NG CoP-GO CoP NG

-0.3

-0.2

E (V vs RHE)

-0.1

89.4 mV dec-1

82.2 mV dec-1 74.2 mV dec-1 69.1mV dec-1 32.5 mV dec-1

0.0 0.0

0.0

117.3 mV dec-1

0.2

0.4

0.6

0.8 -2

1.0

1.2

log j (mA cm )

Fig. 3 e a) polarization curves of NG, CoP, CoP-RGO, Co-CNT/NG, CoP-CNT/NG and PteC (20 wt%) respectively in 0.5 M H2SO4 and b) corresponding Tafel plots of catalysts.

120

▲ ▲ ▲ ▲ ▲

100

-Z'' (Ω)

80 60

CoP-CNT/NG Co-CNT/NG CoP-GO CoP NPG

b)20

40 20 0

0

20

40

60

80 100 120 140 160 180 200

Electrical Conductivity (s cm-1)

a)

16 12 8 4 0

NG

CoP

Co-CNT/NG CoP-RGO CoP-CNT/NG

Z' (Ω) Fig. 4 e a) EIS spectra of CoP-CNT/NG, Co-CNT/NG, CoP-RGO, CoP and NG with an overpotential of 200 mV. b) electrical conductivity of dense NG, CoP, CoP-RGO and CoP-CNT/NG thin films measured with a fourepoint probe.

performance. In addition, the CoP-CNT/NG shows the catalytic activity with 176 mV at a current density 10 mA cm
2 in 1.0 M KOH (Fig. S6, Supporting Information). Besides, the activity of the as-prepared CoPeCNT/NG is comsparable to the

h ¼ b logjjj þ a

20

CoP-CNT/NG Co-CNT/NG CoP-RGO CoP NG

-2

J (mA cm )

16 12

62.9 mF cm 47.8 mF cm

Hþ þ e
/Hads ðVolmer reactionÞ

(1)

-2

Hads þ Hþ ðaqÞ þ e
/H2 ðgÞ ðHeyrovsky reactionÞ

(2)

Hads þ Hads /H2 ðgÞ ðTafel reactionÞ

(3)

31.7 mF cm 19.9 mF cm

4

-2

-2

12.3 mF cm

0

50

100

150

200

Scan Rate (mV/s)

In the equation, b stands for the Tafel slope and j symbolizes the current density [34]. In general, the HER pathway in acid environment goes through two diverse mechanisms with three major reactions as follows:

-2

8

0

reported CoP based electrocatalysts (Table S1, Supporting Information) [28e33]. In addition, the Tafel slope can reflect the HER kinetics of electrocatalyst, which is calculated according to the equation as below：

-2

250

Fig. 5 e The Cdl of CoP-CNT/NG, CoP-CNT/NG, CoP-RGO, CoP and NG.

Firstly, the proton (Hþ) in the solution combines with an electron on the electrocatalyst to form a hydrogen atom (Hads). Secondly, for Hads, there are two possible reactions. In the case of low surface density, Hads and electron tends to combine together to form H2 (Heyrovsky reaction). Otherwise, two Hads will combine together to generate H2 (Tafel reaction). Usually,

30059

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

b)

0

40 30

J (mA cm-2)

-10

1st cycle 1500th cycle

-20 -30

J (mA cm-2)

a)

20 10 0

-10

-40 -50 -0.3

-20 -0.2

-0.1

Potential (V vs. RHE)

0.0

-30

0

5

10

Time (h)

15

20

Fig. 6 e a) The polarization curves contrast of the first cycle and the 1500th cycle. b) Timeedependent current density curve of CoP-CNT/NG over 18 h at ¡165 mV.

lower Tafel slope reflects faster rate of current density increase, which can also reflect a better electrocatalytic performance [35]. The Tafel slopes of the catalysts are shown in Fig. 3b. The Tafel slopes of Pt/C, CoPeCNT/NG, CoeCNT/NG, CoPeRGO, CoP and NG are 32.5, 69.1, 74.2, 82.2, 89.4 and 117.3 mV dec
1, respectively. The results reflect that the CoPeCNT/NG possesses great HER kinetics, which is controlled by the first step and the atom recombination step. The electrochemical impedance spectroscopy (EIS) tests are conducted at the same potential of 200 mV, to further investigate the great enhanced HER catalytic property of CoPeCNT/NG [36]. Fig. 4a shows the charge transfer resistance (Rct) of Hþ reduction at the electrode-electrolyte interface. The Rct of CoPeCNT/NG shows a lower value of Rct (31.2 U) than that of CoeCNT/NG (62.2 U), CoPeRGO (80.2 U), CoP (154.3 U) and NG (356.5 U). Such results indicate the synergistic effect between CNTs and graphene can improve the electrocatalytic conductivity and accelerate the ‘electrocatalytic kinetics of the reaction. For electrocatalyst reactions, the electrical conductivity is an important indicator, which can be accurately measured by FourePoint Probes [37]. Fig. 4b shows the electrical conductivity of CoPeCNT/NG, CoPeRGO, CoeCNT/NG, CoP and NG. The reported results are described by average values obtained by duplicating the measurement in each side of the sample films. The electrical conductivity of CoPeCNT/ NG is 17.8 s cm
1, which is significantly higher than CoeCNT/ NG (11.38 s cm
1), CoPeRGO (2.78 s cm
1), CoP (2.79 s cm
1) and NG (2.76 s cm
1). In conclusion, the existence of multilevel carbon structure significantly improves the conductivity of the composite, facilitating HER catalytic process. The electrochemical double-layer capacitance are also tested by CV to investigate the kinetics during the HER process [38]. The potential region of CV test should be within no occurrence of the apparent faradaic process. The electrochemical double-layer capacitance (Cdl) is measured to investigate the electrochemically active surface area (ECSA). The CV tests of the samples are recorded at different scan rates from 20 mV s
1 to 200 mV s
1 (Fig. S7, Supporting Information), and the results are shown in Fig. 5. In Fig. 5, the Cdl

of CoPeCNT/NG is 62.9 mF cm
2, which is much higher than that of CoeCNT/NG (47.8 mF cm
2), CoPeRGO (31.7 mF cm
2), CoP (12.3 mF cm
2) and NG (19.9 mF cm
2). The combination of graphene and CNTs, can increase the folds and the positions of margin, making more room for active sites of electrocatalysts. The electrocatalytic stability is considered as another significant criterion for catalyst application. In accordance with Fig. 6a, the CoPeCNT/NG exhibits great stability and durability as its catalytic activity barely decreases after 1500 cycles cyclic voltammetry scanning. The long-term chronoamperometry is another method to evaluate the stability and durability of catalysts. As shown in Fig. 6b, the CoPeCNT/NG maintains great catalytic activity for 18 h. Above results reflect that the CoPeCNT/NG possesses excellent constancy and stability in HER, which can be attributed to the carbon shell covering CoP nanoparticles. The SEM in Fig. S8 image indicates that after catalytic reaction, the structure of CoPeCNT/NG shows no obvious change. Besides, the repeated test guarantees the stable performance, credible results of the obtained hybrids. The statistical data of LSV testing and EIS testing have been supplied in Fig. S9, Supporting Information. In summary, the remarkable electrochemical performance of the CoPeCNT/NG is ascribed to the distinctive multilevel carbon structure. The combination of CNT and graphene not only features rich mesoporous channels, large surface areas and more active sites of electrocatalysts, but also immensely increases the electrical conductivity. Besides, the N doped graphene and CNT can also improve electrical performance. The carbon shells covering CoP nanoparticles can prevent them from leaching by acid in order to improve their stabilities.

Conclusion In this work, an efficient multilevel structure electrocatalyst has been synthesized and applied in catalyzing hydrogen evolution reaction. Besides, the CoP nanoparticles are

30060

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

positioned on the multilevel Nedoped CNT/graphene nanostructures. The distinctive multilevel structure is proved to display high electronic conductivity and abundant active sites for HER. Meanwhile, catalysts exhibit great HER activity with low overpotential, low Tafel slope and prominent electrochemical durability. Such an excellent HER catalytic can be attributed to the synergistic effects within multilevel carbon structures, which can improve surface areas and electron transfer. The synthesis method adopted here can be applied to other transition metals to fabricate CNT-graphene multilevel carbon material catalysts.

Acknowledgements This study was supported by the Specialized Research Funds for the National Natural Science Foundation of China (NO. 21506157), and the Program of Introducing Talents of Discipline to Universities (No. B06006).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.163.

references

[1] Song M, He Y, Zhang M, Zheng X, Wang Y, Zhang J, Han X, Zhong C, Hu W, Deng Y. Controllable synthesis of Co2P nanorods as high-efficiency bifunctional electrocatalyst for overall water splitting. J Power Sources 2018;402:345e52. [2] Zou X, Huang X, Goswami A, Silva R, Sathe BR, Mikmekova E, Asefa T. Cobalt-embedded nitrogen-rich carbon nanotubes efficiently catalyze hydrogen evolution reaction at all pH values. Angew Chem Int Ed 2014;53:4372e6. [3] Yang F, Chen Y, Cheng G, Chen S, Luo W. Ultrathin nitrogendoped carbon coated with CoP for efficient hydrogen evolution. ACS Catal 2017;7:3824e31. [4] Yan L, Dai P, Wang Y, Gu X, Li L, Cao L, Zhao X. In situ synthesis strategy for hierarchically porous Ni2P polyhedrons from MOFs templates with enhanced electrochemical properties for hydrogen evolution. ACS Appl Mater Interfaces 2017;9:11642e50. [5] Tian L, Yan X, Chen X. Electrochemical activity of iron phosphide nanoparticles in hydrogen evolution reaction. ACS Catal 2016;6:5441e8. [6] Lu H, Fan W, Huang Y, Liu T. Lotus root-like porous carbon nanofiber anchored with CoP nanoparticles as all-pH hydrogen evolution electrocatalysts. Nano Res 2018;11:1274e84. [7] Qian J, Li Z, Guo X, Li Y, Peng W, Zhang G, Zhang F, Fan X. CoP nanoparticles combined with WSe2 nanosheets: an efficient hybrid catalyst for electrocatalytic hydrogen evolution reaction. Ind Eng Chem Res 2018;57:483e9. [8] Liu C, Li F, Ma LP, Cheng HM. Advanced materials for energy storage. Adv Mater 2010;22:E28e62. [9] Oluigbo CJ, Xie M, Ullah N, Yang S, Zhao W, Zhang M, Lv X, Xu Y, Xie J. Novel one-step synthesis of nickel encapsulated carbon nanotubes as efficient electrocatalyst for hydrogen evolution reaction. Int J Hydrogen Energy 2019;44:2685e93.

[10] Kim NY, Recher P, Oliver WD, Yamamoto Y, Kong J, Dai H. Tomonaga-Luttinger liquid features in ballistic single-walled carbon nanotubes: conductance and shot noise. Rev Let 2007;99. 036802. [11] Liu J, Xu H, Li H, Song Y, Wu J, Gong Y, Xu L, Yuan S, Li H, Ajayan PM. In-situ formation of hierarchical 1D-3D hybridized carbon nanostructure supported nonnoble transition metals for efficient electrocatalysis of oxygen reaction. Appl Catal, B 2019;243:151e60. [12] Yang S, Chen L, Wei W, Lv X, Xie J. CoP nanoparticles encapsulated in three-dimensional N-doped porous carbon for efficient hydrogen evolution reaction in a broad pH range. Appl Surf Sci 2019;476:749e56. [13] Dahal B, Mukhiya T, Ojha GP, Muthurasu A, Chae S-H, Kim T, Kang D, Kim HY. In-built fabrication of MOF assimilated B/N co-doped 3D porous carbon nanofiber network as a binderfree electrode for supercapacitors. Electrochim Acta 2019;301:209e19. [14] Han DR, Luo CL, Dai YF, Zhu XF. A possible formation mechanism of double-walled and multi-walled carbon nanotube: a molecular dynamics study. Mater Res Express 2016;3. [15] Liu Y, Zhu Y, Shen J, Huang J, Yang X, Li C. CoP nanoparticles anchored on N,P-dual-doped graphene-like carbon as a catalyst for water splitting in non-acidic media. Nanoscale 2018;10:2603e12. [16] Meng T, Hao YN, Zheng L, Cao M. Organophosphoric acidderived CoP quantum dots@S,N-codoped graphite carbon as a trifunctional electrocatalyst for overall water splitting and Zn-air batteries. Nanoscale 2018;10:14613e26. [17] Ma J, Wang M, Lei G, Zhang G, Zhang F, Peng W, Fan X, Li Y. Polyaniline derived N-doped carbon-coated cobalt phosphide nanoparticles deposited on N-doped graphene as an efficient electrocatalyst for hydrogen evolution reaction. Small 2018;14. [18] Lee J-S, Jo K, Lee T, Yun T, Cho J, Kim B-S. Facile synthesis of hybrid graphene and carbon nanotubes as a metal-free electrocatalyst with active dual interfaces for efficient oxygen reduction reaction. J Mater Chem A 2013;1:9603. [19] Hou D, Zhou W, Zhou K, Zhou Y, Zhong J, Yang L, Lu J, Li G, Chen S. Flexible and porous catalyst electrodes constructed by Co nanoparticles@nitrogen-doped graphene films for highly efficient hydrogen evolution. J Mater Chem A 2015;3:15962e8. [20] Jiang J, Zhu K, Fang Y, Wang H, Ye K, Yan J, Wang G, Cheng K, Zhou L, Cao D. Coralloidal carbon-encapsulated CoP nanoparticles generated on biomass carbon as a high-rate and stable electrode material for lithium-ion batteries. J Colloid Interface Sci 2018;530:579e85. [21] Fan X, Wang X, Yuan W, Li CM. Diethylenetriaminemediated self-assembly of three-dimensional hierarchical nanoporous CoP nanoflowers/pristine graphene interconnected networks as efficient electrocatalysts toward hydrogen evolution. Sustainable Energy Fuels 2017;1:2172e80. [22] Le QJ, Huang M, Wang T, Liu XY, Sun L, Guo XL, Jiang B, Wang J, Dong F, Zhang YX. Biotemplate derived three dimensional nitrogen doped graphene@MnO2 as bifunctional material for supercapacitor and oxygen reduction reaction catalyst. J Colloid Interface Sci 2019;544:155e63. [23] Liang Z, Fan X, Lei H, Qi J, Li Y, Gao J, Huo M, Yuan H, Zhang W, Lin H, Zheng H, Cao R. Cobalt-nitrogen-doped helical carbonaceous nanotubes as a class of efficient electrocatalysts for the oxygen reduction reaction. Angew Chem 2018;57:13187e91. [24] Chang J, Xiao Y, Xiao M, Ge J, Liu C, Xing W. Surface oxidized cobalt-phosphide nanorods as an advanced oxygen

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 0 5 3 e3 0 0 6 1

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

evolution catalyst in alkaline solution. ACS Catal 2015;5:6874e8. Zhu P, Zhang Z, Zhao P, Zhang B, Cao X, Yu J, Cai J, Huang Y, Yang Z. Rational design of intertwined carbon nanotubes threaded porous CoP@carbon nanocubes as anode with superior lithium storage. Carbon 2019;142:269e77. Zhang M, Yu J, Ying T, Yu J, Sun Y, Liu X. P doped onion-like carbon layers coated FeP nanoparticles for anode materials in lithium ion batteries. J Alloy Comp 2019;777:860e5. Hu G, Tang Q, Jiang DE. CoP for hydrogen evolution: implications from hydrogen adsorption. Phys Chem Chem Phys 2016;18:23864e71. You B, Jiang N, Sheng M, Gul S, Yano J, Sun Y. Highperformance overall water splitting electrocatalysts derived from cobalt-based metaleorganic frameworks. Chem Mater 2015;27:7636e42. Tian L, Yan X, Chen X, Liu L, Chen X. One-pot, large-scale, simple synthesis of CoxP nanocatalysts for electrochemical hydrogen evolution. J Mater Chem A 2016;4:13011e6. Ma L, Shen X, Zhou H, Zhu G, Ji Z, Chen K. CoP nanoparticles deposited on reduced graphene oxide sheets as an active electrocatalyst for the hydrogen evolution reaction. J Mater Chem A 2015;3:5337e43. Zheng H, Huang X, Gao H, Lu G, Li A, Dong W, Wang G. Cobalt-tuned nickel phosphide nanoparticles for highly efficient electrocatalysis. Appl Surf Sci 2019;479:1254e61. Wang L, Cao J, Cheng X, Lei C, Dai Q, Yang B, Li Z, Younis MA, Lei L, Hou Y, Ostrikov K. ZIF-derived carbon nanoarchitecture as a bifunctional pH-universal

[33]

[34]

[35]

[36]

[37]

[38]

30061

electrocatalyst for energy-efficient hydrogen evolution. ACS Sustainable Chem Eng 2019;7:10044e51. Zhang Y, Sun H, Qiu Y, Zhang E, Ma T, Gao G-g, Cao C, Ma Z, Hu P. Bifunctional hydrogen evolution and oxygen evolution catalysis using CoP-embedded N-doped nanoporous carbon synthesized via TEOS-assisted method. Energy 2018;165:537e48. Jiao L, Zhou YX, Jiang HL. Metal-organic framework-based CoP/reduced graphene oxide: high-performance bifunctional electrocatalyst for overall water splitting. Chem Sci 2016;7:1690e5. Kim JK, Park S-K, Kang YC. Structure-optimized CoP-carbon nanotube composite microspheres synthesized by spray pyrolysis for hydrogen evolution reaction. J Alloy Comp 2018;763:652e61. Ren X, Wu D, Ge R, Sun X, Ma H, Yan T, Zhang Y, Du B, Wei Q, Chen L. Self-supported CoMoS4 nanosheet array as an efficient catalyst for hydrogen evolution reaction at neutral pH. Nano Res 2018;11:2024e33. ~ eda A, Gupta A, Engel JT, Blaikie BE, Kumar A, Castan Oliver DR. Direct contact four-point probe characterization of Si microwire absorbers for artificial photosynthesis. RSC Adv 2016;6:110344e8. Yu Y, Qiu X, Zhang X, Wu Z, Wang H, Wang W, Wang Z, Tan H, Peng Z, Guo X, Liu H. Metal-organic frameworks derived bundled N-doped carbon nanowires confined cobalt phosphide nanocrystals as a robust electrocatalyst for hydrogen production. Electrochim Acta 2019;299:423e9.