Co2P encapsulated by hollow N-doped carbon nanospheres for efficient hydrogen evolution

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Ultrafine and highly-dispersed bimetal Ni2P/Co2P encapsulated by hollow N-doped carbon nanospheres for efficient hydrogen evolution Xin-Yu Zhang a, Bao-Yu Guo a, Qin-Wei Chen a, Bin Dong a,*, Jia-Qi Zhang a, Jun-Feng Qin a, Jing-Yi Xie a, Min Yang a, Lei Wang b, Yong-Ming Chai a,**, Chen-Guang Liu a a

State Key Laboratory of Heavy Oil Processing, Institute of New Energy, China University of Petroleum (East China), Qingdao, 266580, PR China b Shandong Key Laboratory of Biochemical Analysis, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, PR China

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abstract

Article history:

Ultrafine Ni2P/Co2P nanoparticles encapsulated in hollow porous N-doped carbon nano-

Received 12 January 2019

spheres are synthesized through a facile two-step access. Firstly, metallic Ni and Co coated

Received in revised form

by hollow N-doped spheres as precursors are obtained through a high temperature calci-

6 April 2019

nation route of organic polymer and inorganic Ni and Co salts. Then bimetal Ni2P/Co2P

Accepted 10 April 2019

supported on N-doped carbon nanospheres are acquired by a facile phosphorization pro-

Available online 9 May 2019

cess. It is worth to note that aniline-pyrrole polymer can prevent fast growth and severe

Keywords:

nation of hollow polymer spheres lead to the formation of ultrathin NC shell on the surface

aggregation of Ni2P/Co2P, which implies more exposed active sites. Moreover, the calciHollow carbon spheres

of Ni2P/Co2P hybrids, which can tune electronic structures, improve the conductivity and

Ni2P

protect active sites from corrosion in harsh conditions. When used as HER catalyst, it

Co2P

displays remarkable catalytic activity in both acidic and alkaline solutions, which needs an

Carbon doping

onset potential of only 164 mV and 168 mV, respectively. Therefore, this work may propose

Hydrogen evolution reaction

a new strategy to design unique inorganic-organic heterostructures to combine ultrafine metal phosphides with porous carbon for efficient HER. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen (H2) as a sustainable and environmental friendly energy carrier has been considered as a promising alternative energy to replace fossil fuels in the future [1e6]. Electrolytic water splitting provides a friendly way for the production of

highly pure hydrogen [7e11]. To date, the noble metal platinum (Pt) has been widely considered as benchmark electrocatalysts with nearly zero onset potential, but the high cost and scarcity hinder its further application [12e18]. Therefore, it is necessary to develop effective and low-cost electrocatalyst for HER based on noble-metal-free materials.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Dong), [email protected] (Y.-M. Chai). https://doi.org/10.1016/j.ijhydene.2019.04.108 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Recently, much of attention has been focused on transition metal phosphides (TMPs) due to their hydrogenase-like reaction mechanism and earth-abundance [19e22]. And it has been reported that TMPs (M ¼ Co, Ni, Fe, Mn) can be competent for HER catalysts even though their performances are far from the noble Pt, which implies that there is still potential for improvement [23e29]. For example, Yang et al. reported that nickel phosphides exhibit high activity in a large rank of pH [30]. Gao et al. stated that cobalt phosphides are regarded as the most alternative electrocatalyst for HER because of P atom has more electronegativity, which could take electrons from metals and figure as proton carriers [31]. Liang et al. introduced that binary NiCoP could enhance the HER performance by realizing the low energy of hydrogen adsorption [32]. Besides, other studies showed that some substrates including graphene, carbon nanotubes and metal organic framework with high surface area, exhibit better performance for HER [33e39]. However, these works are all loading the catalyst on the surface of support and their stability is not well realized. Nanostructured catalysts for HER with controlled morphologies have attracted much attention because of their outstanding performance arising from their unique structure. Particularly, hollow spheres can provide a larger surface for catalyst to contact with the electrolyte and build a favorable catalytic interface, thereby significantly improving electrochemical performance [40e43]. For example, Zhang et al. developed petal-like molybdenum disulfide nanosheets inside hollow mesoporous carbon sphere (HMCS) that can effectively control and confine in situ growth of MoS2 nanosheets and obviously improve the conductivity and structural stability of the hybrid material [44]. Yang et al. reported that hollow porous N-doped carbon nanospheres can be used as a good precursor with porous structure allowing metal catalysts exist stably [45]. The stability of metal phosphides for HER may be enhanced by NC-modification to further enhance the performance. Herein, we design a new strategy synthesizing bimetallic Ni, Co and its corresponding phosphides encapsulated by hollow porous N-doped carbon (NC) nanospheres by a twostep access. The systematic synthesis process is illustrated in Scheme 1. Firstly, NiCo@NC nanospheres are obtained by carbonization of polymer nanospheres at a high temperature in an inert argon atmosphere. Secondly, binary Ni2P/Co2P@NC nanospheres are synthesized by a facile in-situ phosphorization process. The polymer can prevent fast growth and severe aggregation of Ni2P/Co2P hybrids, which implies more exposed active sites. Moreover, the formation of ultrathin NC

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shell on the surface of Ni2P/Co2P can tune electronic structures, improve the conductivity and protect active sites from corrosion in harsh conditions. Benefiting from the synergistic effect among binary Ni2P and CoP, and NC, rich active sites of Ni2P/Co2P@NC heterointerface, and N-doped carbon, the asprepared Ni2P/Co2P@NC nanospheres exhibit excellent HER performance and long-term durability in both acid and alkali, which requires an onset potential of only 164 mV and 168 mV in acidic and alkaline solution, respectively.

Experimental Synthesis of hollow polymer nanospheres Hollow polymer nanospheres were prepared using a low temperature polymerization method and the preparation steps were described as following. Firstly, 0.28 g of pyrrole, 0.388 g of aniline, and 0.12 g of TX-100 were dissolved in 60 ml deionized water. After vigorous stirring and sonication for 40 min, the homogeneous solution was placed in an ice-water bath and cooled for 30 min. At the same time, 1.91 g of ammonium persulphate dissolved in 20 ml of deionized water was also placed in an ice-water bath for 30 min. The above two precooled samples were mixed and stirred at room temperature for 0.5 min, and then reacted without stirring for 12 h in an ice-water environment. Finally, the obtained product was washed with deionized water for several times and dried in a vacuum oven at 60  C.

Synthesis of NiCo@NC nanospheres The above prepared hollow polymer nanospheres were dissolved in 20 ml of deionized water containing 0.5 g of Ni(NO3)2$6H2O and 0.5 g of CoCl2$6H2O. It was stirred and sonicated for 0.5 h each other and then dried at 60  C. Subsequently, 0.2 g of the obtained powder was placed in a clean porcelain and burned under an argon atmosphere at 500  C for 2 h, then heated up to 800  C for 4 h. After cooling to room temperature, the final product was washed for several times with deionized water and dried in a vacuum oven at 60  C.

Synthesis of Ni2P/Co2P@NC nanospheres For the preparation of Ni2P/Co2P@NC, 50 mg of NiCo@NC and 1 g of NaH2PO2$H2O were taken in two porcelain boats respectively and burned in argon atmosphere at 300  C for 2 h.

Scheme 1 e Schematic illustration of the synthesis process of Ni2P/Co2P@NC.

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After cooling to room temperature, the target sample was prepared. For comparison, Ni2P@NC and Co2P@NC were synthesized by using similar method for Ni2P/Co2P@NC without CoCl2$6H2O and Ni(NO3)2$6H2O respectively.

Synthesis of NC, Ni2P@NC, Co2P@NC as control samples NC nanopsheres were obtained similar to Ni2P/Co2P@NC without adding CoCl2$6H2O and Ni(NO3)2$6H2O. Ni2P@NC control sample was synthesized in the same way of Ni2P/ Co2P@NC nanospheres without adding CoCl2$6H2O. Co2P@NC control sample was synthesized in the same way of Ni2P/ Co2P@NC nanospheres without adding Ni(NO3)2$6H2O.

Characterizations X-ray powder diffraction (XRD) was used to explore the crystal information of the above samples on X'Pert PRO MPD with 2q range from 8 to 76 . X-ray photoelectron spectra (XPS) conducted on a VG ESCALABMK II photoelectron spectrometer (Al Ka of 1486.6 eV) was employed to analyze the surface element states of all samples. Scanning electron microscopy (SEM) and SEM mapping was conducted on Hitachi (S-4800) to explore the morphology and the distribution of various elements of the samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were utilized to observe the crystal structure of the samples on FEI Tecnai G2.

Electrochemical measurements The electrochemical measurements were conducted on a Gamry Reference 600 electrochemical workstation, in which a three-electrode system was adopted in a solution of 0.5 M H2SO4 and 1.0 M KOH, respectively. A carbon rod and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. For the preparation of the working electrode, 5 mg of powder with 0.5 ml of 5 wt % Nafion solution was dispersed in 1 ml of 1:1 (v/v) waterethanol by sonicating for 0.5 h to form a homogeneous ink. Then 5 mL of the ink was loaded onto the glassy carbon electrode with an area of 0.1256 cm2. Thus, the amount of powder (Ni2P@NC, Co2P@NC or Ni2P/Co2P@NC) loaded on GCE is 0.199 mg/cm2. The electrocatalytic activity was recorded by linear sweep voltammetry (LSV) with scan rate of 5 mV s1 [46,47]. Electrochemical impedance spectroscopy (EIS) data was performed at 0.3 V (vs RHE) in 0.5 M H2SO4 (1.35 V vs RHE in 1.0 M KOH) with frequency from 105 Hz to 0.1 Hz at an AC voltage of 5 mV. The electrochemical capacitance was determined with two different methods, which contains cyclic voltammo-grams (CVs) and electrochemical impedance spectroscopy (EIS) [48]. The catalyst stability test was obtained by cycling the potential from 0.2 to 0 V (vs RHE) in 0.5 M H2SO4 (from 1 to 1.1 V vs RHE in 1.0 M KOH) for 1000 cycles at a sweep rate of 100 mV s1. The iR correction to data with the series resistance (Rs) is performed by hcorr ¼ h e iRs [49]. In all experiments, it was necessary to pass H2 into the solution at the room temperature to get rid of the dissolved oxygen for 30 min. The potentials were corrected with respect to RHE based on the following Nernst equation: ERHE ¼ ESCE þ 0.243 þ 0.059 pH.

Results and discussion The samples were characterized by XRD to identify the crystallographic structure and the diffraction patterns are presented in Fig. 1. For NC hollow nanospheres, a broad peak at 26.5 can be indexed to the amorphous carbon matrix. After the calcination process of polymer spheres impregnated with Ni and Co ions, main peaks at 41.5 , 44.6 , 47.3 , 75.3 and 44.3 , 51.6 , 76.9 correspond to the (100), (002), (101), and (110) plane of metallic Co (PDF no. 01-001-1278) and (111), (200) and (220) plane of metallic Ni (PDF no. 03-065-0380), respectively. After phosphorization process for Ni@NC, the well-indexed diffraction peaks at 40.7 , 44.6 , 47.3 and 54.1 reveal the existence of Ni2P (PDF No.01-089-4864) and typical peaks at 40.83 , 44.85 , 48.3 and 55.4 are ascribed to Co2P (PDF No.00054-0413) species. For the Ni2P/Co2P@NC, it can be seen that the hybrids are composed of Ni2P and Co2P mixtures thus are denoted as Ni2P/Co2P@NC. In order to determine the valence and composition state of Ni2P/Co2P@NC, XPS data are investigated in Fig. 2. As shown in Fig. 2a, the Ni, Co, P, C, N, O elements can be clearly observed in the XPS survey of Ni2P/Co2P@NC. In Fig. 2b, Ni0 can be detected at 853.1 eV. In addition, the peaks at 871.1 eV and 854.4 eV are assigned to Ni2þ, the peaks at 874.0 eV and 856.4 eV are indexed to Ni3þ, respectively, which are in good accordance with the peaks of Ni2P [20]. As shown in Fig. 2c, the high resolution peak of Co 2p is typically assigned to 2p1/2 and 2p3/2, which is well consistent with cobalt element in Co2P. After deconvolution, the Co 2p1/2 and Co 2p3/2 can be fitted to Co2þ (798.2 eV and 782.2 eV) and Co3þ (798.8eV and 778.6 eV), respectively. The remaining two peaks at 803.8 eV and 786.5 eV are satellite peaks [21]. In Fig. 2d, the P 2p spectrum shows three main peaks at 134.1 eV, 130.6 eV and 129.7 eV. The peak at 129.7 eV is for P 2p3/2, 130.6 eV is for P 2p1/2 and 134.1 eV is for the oxidized metal phosphate species due to exposure at the air atmosphere [23]. For C 1s in Fig. 2e, it also shows four peaks at 286.4, 285.3, 284.7 and 288.9 eV, which are ascribed to eC¼O, eCeOe, sp2eC and CeN, respectively [13]. Fig. 2f shows the XPS spectra of N 1s, where the peaks at 400.9, 398.9, and

Fig. 1 e XRD patterns of NC, NiCo@NC, Ni2P@NC, Co2P@NC and Ni2P/Co2P@NC.

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Fig. 2 e (a) XPS survey spectra for Ni2P/Co2P@NC in (b) Ni 2p, (c) Co 2p, (d) P 2p, (e) C 1s and (f) N 1s.

398.3 eV can be attributed to quaternary-N, pyrrolic-N, and pyridinic-N, respectively [21]. For comparison, the XPS spectra of Ni, Co and P elements in pure metal phosphides (Ni2P or Co2P) have been also measured in Fig. S3-S5. Wherein the Ni peaks in Ni2P@NC have a little positive shift while the Co (in Co2P@NC) is in the opposite situation comparing to the Ni2P/

Co2P hybrids, which may due to the rearrangement of charge in the presence of phosphorus. And all of these XPS data indicates the successful synthesis of Ni2P/Co2P@NC. The morphology of NiCo@NC, Ni2P@NC, Co2P@NC and Ni2P/Co2P@NC are characterized by SEM, as shown in Fig. S1. Fig. S1a shows that NiCo@NC has a unique spherical structure

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Fig. 3 e SEM images of (a, b) Ni2P@NC, (c, d) Co2P@NC and (e, f) Ni2P/Co2P@NC. (g) SEM images and elemental mapping of Ni2P/Co2P@NC and (h) EDX of Ni2P/Co2P@NC.

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with the narrow diameter about 100 nm. Under higher magnification, Fig. S1b shows that the surface of NiCo@NC nanospheres is rough. After phosphorization for Ni@NC and Co@NC, Fig. 3aed shows that the obtained Ni2P@NC and Co2P@NC still remain spherical morphology with no apparent changes, which confirms the stable structure of polymer framework. Moreover, SEM images of binary Ni2P/Co2P@NC nanospheres are shown in Fig. 3e, f. It can be seen that the Ni2P/Co2P@NC nanospheres also exhibit the spherical morphology but with shrink diameter of 90 nm compared with NiCo@NC. In order to further confirm the existence and composition of elements of Ni2P/Co2P@NC, SEM elemental mappings and Energy Dispersive X-ray (EDX) are tested which

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demonstrate the homogeneous distribution of Co, Ni, P, C, and N elements (Fig. 3geh). At the same way, the core-shell-like nanostructure of NiCo@NC, Ni2P@NC, Co2P@NC and Ni2P/Co2P@NC are confirmed by TEM and HRTEM (Fig. 4). As shown in Fig. 4aed, uniform NC spheres with hollow structure are homogeneously distributed, which is identical with that of SEM results. And there are a large number of well-dispersed nanoparticles in each NC hollow sphere, which correspond to Ni/Co (Fig. 4a), Ni2P (Fig. 4b), Co2P (Fig. 4c) and the hybrids of Ni2P/Co2P (Fig. 4d, e). The HRTEM image of Fig. 4f clearly shows that the shell is composed of several layers of carbon, and the inside particles are composed of ultrafine Ni2P/Co2P with

Fig. 4 e TEM image of (a) NiCo@NC, (b) Ni2P@NC, (c) Co2P@NC and (d, e, f) Ni2P/Co2P@NC.

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apparent lattice fringes distance of 0.507 nm and 0.429 nm, indexed to (100) planes of Ni2P and (101) planes of Co2P, respectively. The NC shell coated and crosslinked with Ni2P and Co2P nanoparticles can effectively prevent the severe stack and excessive growth of active sites thus exposing large surface area, which are favorable for HER. Fig. 5 shows HER measurements results of NC, NiCo@NC, Ni2P@NC, Co2P@NC and Ni2P/Co2P@NC in the acidic solution. Fig. 5a shows LSV curves of all the samples. As expected, Ni2P/ Co2P@NC possesses smallest onset potential compared with NC, Ni2P @NC and NiCo@NC, which only needs an onset potential of 164 mV and an overpotential of 226 mV to reach 10 mA cm2, indicating the binary Ni2P/Co2P can synergetically improve the HER performance. The binary Ni2P/Co2P@NC exhibis superior activity than many of the recently reported non-noble metal HER catalysts listed in Table S1. In addition, Tafel plots of various samples in Fig. 5b are obtained by Tafel equation (h ¼ b log j þ a) to evaluate HER kinetics of asprepared catalysts. Consistent with LSV results, Tafel slope of Ni2P/Co2P@NC (64.9 mV dec1) is much smaller than that of Co2P@NC (69.6 mV dec1), Ni2P@NC (88.2 mV dec1), NiCo@NC

(140.1 mV dec1) and NC (171.2 mV dec1), indicating that Ni2P/ Co2P@NC has a fast kinetics for HER in acidic solution. To further explore the charge transport behavior, electrochemical impedance spectra (EIS) measurements of all samples are presented (Fig. 5c). The charge transport resistance (Rct) represents the electron transfer between catalyst and electrolyte and the Rct of five samples are increasing in the order of Ni2P/Co2P@NC, Co2P@NC, Ni2P@NC, NiCo@NC and NC. The smaller Rct of Ni2P/Co2P@NC indicates the fast charge transfer rate between Ni2P/Co2P@NC interfaces and electrolyte. Beyond that, the double-layer capacitances (Cdl) is also evaluated to calculate the electrochemically active surface area (ECSA) by two different methods (CVs and EIS) in 0.5 M H2SO4. As shown, the CVs results is 14.33 mF cm2 and the calculated ECSA is 114.09 cm2 while for EIS, the Cdl value is 14.15 mF cm2 and the ECSA is 112.71 cm2 (Table S2 and Table S3), which is in the acceptable limits within 15% and consistent with previous literature reports [48]. Compared with other samples in Fig. 5d, the Ni2P/Co2P@NC has a higher Cdl and ECSA, which may be ascribed to that bimetallic phosphides can be well-dispersed by organic polymer and porous

Fig. 5 e Electrochemical measurements of all samples in acidic solution. (a) Linear sweep voltammogram (LSV) curves, (b) Tafel plots, (c) Electrochemical impedance spectroscopy (EIS) results and (d) Cyclic voltammograms.

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Fig. 6 e Electrochemical measurements of all samples in alkaline solution. (a) Linear sweep voltammogram (LSV) curves, (b) Tafel plots, (c) Electrochemical impedance spectroscopy (EIS) results, and (d) Cyclic voltammograms.

carbon encapsulation, which is favorable for exposing rich active sites for enhanced HER. In addition, catalysts that have better performance both in acid and alkaline solutions are attracting more and more

attention. Therefore, we also examine the HER performance of Ni2P/Co2P@NC in 1 M KOH. At the same time, NC, NiCo@NC, Ni2P@NC and Co2P@NC are also tested for comparison. Fig. 6a shows the linear sweep voltammogram (LSV)

Fig. 7 e LSV curves of Ni2P/Co2P@NC before and after 1000 cycles in (a) acidic and (b) alkaline solutions.

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curves of all samples. It's clearly that NC almost has no catalytic activity for HER. However, the Ni2P/Co2P@NC needs a small overpotential of only 251 mV affording the current density of 10 mA cm2, which is lower than that of other catalysts. The intrinsic kinetics of samples for HER are also investigated by their Tafel plots. Fig. 6b shows lower Tafel slope of 81.64 mV dec1 for Ni2P/Co2P@NC compared with Co2P@NC of 128.48 mV dec1, Ni2P@NC of 159.68 mV dec1, NiCo@NC of 107.52 mV dec1 and NC of 199.21 mV dec1 in alkaline solution. In the same way, EIS is also tested to explore the charge transfer resistance of as-prepared samples. As shown in Fig. 6c, the Rct decrease in the order of NC, NiCo@NC, Ni2P @NC, Co2P@NC and Ni2P/Co2P@NC, which demonstrates the high charge transfer rate between Ni2P/ Co2P@NC heterointerfaces and electrolyte. In addition, we can find that the Ni2P/Co2P@NC has largest Cdl and highest electrochemically active surface area than that of other catalysts in Fig. 6d and Table S2-S3, which can provide more active sites to be exposed to the electrolyte. Besides the catalytic activity, a great stability is another important factor for HER. In Fig. 7, the polarization curves of Ni2P/Co2P@NC are measured by continuous CV at a scan rate of 100 mV s1 for 1000 cycles in acidic and alkaline solutions. And the LSV curve remains nearly unchanged with small loss of the current density. The main reason for the degradation may be ascribed to the production of large amount of H2 bubbles along with slightly exfoliation corrosion of catalyst from the GCE surface.

Conclusions In this work, binary Ni2P/Co2P nanoparticles encapsulated by hollow porous N-doped carbon (NC) nanospheres have been synthesized by a facile two-step access. The formation of ultrathin NC shell on the surface of Ni2P/Co2P hybrids can tune electronic structures, improve the conductivity and protect active sites from corrosion in harsh conditions. The obtained Ni2P/Co2P@NC shows excellent electrocatalytic HER performance with an onset potential of only 164 mV and 168 mV in acidic and basic media, respectively. The enhanced electrocatalytic performances could be ascribed to the synergistic effect among binary Ni2P and CoP, and NC, rich active sites of Ni2P/Co2P@NC heterointerface, and N-doped carbon. Therefore, using this strategy to design unique inorganic-organic heterostructures to combine ultrafine metal phosphides with porous carbon may be a new way to construct active heterointerface for efficient HER.

Acknowledgements This work is financially supported by Major Program of Shandong Province Natural Science Foundation (ZR2018ZC0639) and Shandong Provincial Natural Science Foundation (ZR2017MB059) and the National Natural Science Foundation of China (21776314) and the Fundamental Research Funds for the Central Universities (18 C  05016 A).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.108.

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