Nitrogen-Doped Carbon

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Metal Doping Effect of the M-Co2P/NCNTs (M= Fe, Ni, Cu) Hydrogen Evolution Hybrid Catalysts Yuan Pan, Yunqi Liu, Yan Lin, and Chen Guang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02023 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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ACS Applied Materials & Interfaces

Metal Doping Effect of the M-Co2P/NCNTs (M= Fe, Ni, Cu) Hydrogen Evolution Hybrid Catalysts Yuan Pan, Yunqi Liu*, Yan Lin, Chenguang Liu* State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China University of Petroleum (East China), 66 West Changjiang Road, Qingdao, Shandong 266580, China

1

ACS Paragon Plus Environment


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ABSTRACT The enhancement of catalytic performance of cobalt phosphide-based catalysts for the hydrogen evolution reaction (HER) is still challenging. In this work, the doping effect of some transition metal (M= Fe, Ni, Cu) on the electrocatalytic performance of the M-Co2P/NCNTs hybrid catalysts for the HER was studied systematically. The M-Co2P/NCNTs hybrid catalysts were synthesized via a simple in situ thermal decomposition process. A series of techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectrometry (ICP-OES), transmission electron microscopy (TEM) and N2 sorption were used to characterized the as-synthesized M-Co2P/NCNTs hybrid catalysts. Electrochemical measurements showed that the catalytic performance according

the

following

order

of

Fe-Co2P/NCNTs

>

Ni-Co2P/NCNTs

>

Cu-Co2P/NCNTs, which can be ascribed to the difference of structure, morphology and electronic property after doping. The doping of Fe atoms promote the growth of the [111] crystal plane, resulting a large specific area and exposing more catalytic active sites. Meanwhile, the Feδ+ has highest positive charge among all the M-Co2P/NCNTs hybrid catalysts after doping. All these changes can be used to contribute the highest electrocatalytic activity of the Fe-Co2P/NCNTs hybrid catalyst for HER. Furthermore, an optimal HER electrocatalytic activity was obtained by adjusting the doping ratio of Fe atoms. Our current research indicates that the doping of metal is also an important strategy to improve the electrocatalytic activity for the HER. 2


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KEYWORDS:

Metal

doping

effect,

cobalt

phosphide,

hybrid

catalysts,

electrocatalytic activity, hydrogen evolution

1. INTRODUCTION Nowadays, with the shortage of energy reserves, the increase of environmental problems and the intensification of the conflict caused by energy security, the development of new renewable energy has become more and more urgent1. Hydrogen, which has been regarded as one of new energy sources due to its high energy density and cleanliness2. Water electrolysis is a mature and promising technology, however, the cathode has high overpotential, which leads to the excessive energy consumption3. Therefore, an efficient electrocatalyst is needed to decrease the overpotential and improve the efficiency of the hydrogen evolution. At present, Pt-based materials are the most efficient catalysts for the hydrogen evolution reaction (HER). However, it is difficult to realize the large-scale industrialization due to the scarcity and expensive of precious metals4. Thus, the exploration of non-noble metal catalysts with cheap and abundant reserves has become the current trend of hydrogen energy conversion. Transition metal phosphides (TMPs), especially cobalt phosphide-based materials, have received extensive attention as one of promising non-noble metal catalysts due to the low cost, high catalytic activity and excellent stability. Various strategies have been frequently applied to improve the HER performance of the cobalt phosphide catalysts. The first strategy is optimizing the microstructure of cobalt phosphide. For example, Jiang et al.5 studied the morphology effect of CoP on the electrocatalytic 3



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performance for the HER and concluded that CoP nanowires (NWs) showed the highest electrocatalytic performance than that of the corresponding nanosheets (NSs) and nanoparticles (NPs). Similarly, CoP with highly branched6 and nanotubes (CoP NTs)7 also exhibited the higher catalytic activity than that of the corresponding CoP NPs. Callejas et al.8 compared the catalytic activity of nanostructured Co2P and morphologically equivalent CoP for the HER and found that the catalytic performance can be increased with the increase of phosphorus content. The second strategy is constructing the nanocomposites between cobalt phosphide and carbon materials, which have large specific area and conductivity, such as carbon nanotubes (CNTs) and graphene. For example, Liu et al.9 prepared CoP/CNTs catalyst, which enhance the electrocatalytic activity effectively for the HER as compared with pure CoP NCs. Ma et al.10 also indicated that the HER performance can be enhanced after CoP NPs deposited on the reduced graphene oxide (RGO) sheets. Yang et al.11 demonstrated that CoP NSs grown on carbon cloth also have excellent electrocatalytic activity for hydrogen generation. Additionally, the doping of heteroatoms into carbon materials is another strategy to enhance the catalytic activity of TMPs catalysts. For instance, Liu et al.12 demonstrated that FeP/NCNT hybrid catalyst exhibits higher HER activity and stability than that of pure FeP NPs. Our previous researches also indicate that, after N-doped into reduced graphene oxide (RGO), the catalytic performance of Ni2P/NRGO hybrid catalyst can be enhanced sharply as compared with Ni2P/RGO hybrid catalyst13. Recently, some groups reported that the doping of non-noble metal cation and the 4


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anion can enhance the HER catalytic performance of electrocatalysts. Nelson et al.14 reported that the doping of sulfur into cobalt oxide nanoparticles without inducing secondary phase formation can change the HER performance. Dai et al.15 also demonstrated that the doping of Co atoms into MoS2 catalyst can enhance the catalytic performance. Wang et al.16 proved that Co-doped FeS2 NSs-CNTs hybrid catalyst also exhibits excellent HER activity and stability in strong acidic solutions from the experimental and density functional theory calculations. Similarly, Fe-doped NiS ultrathin NSs17, Co-doped MoP18, Cu-doped MoS2/rGO19, Ni/Co-doped WS220, Fe-doped Mo2C21, and Mo-edge terminated 3D MoS2/graphene22 catalysts were also reported as efficient hydrogen evolution catalyst. However, up to present, reports are rare on the different metal doping effect on the catalytic performance of M-Co2P/NCNTs hydrogen evolution hybrid catalysts. Based on the above research progress, herein, we study the doping effect of certain transition metal (M= Fe, Ni, Cu) on the electrocatalytic activity of the M-Co2P/NCNTs hybrid catalysts for HER. We adopt a facile in situ thermal decomposition approach to synthesize the M-Co2P/NCNTs hybrid catalysts using cobalt acetylacetonate as Co source, TPP as P source, M acetylacetonate as metal dopants in OAm solution of the NCNTs. Then the influence of different metal doping on electrocatalytic performance of such M-Co2P/NCNTs hybrid catalysts were systematically investigated. Due to the change of the structure, morphology and electronic property after metal doping, the electrocatalytic performance of such catalysts according the following order of Fe-Co2P/NCNTs > Ni-Co2P/NCNTs > 5



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Cu-Co2P/NCNTs. Based on the characterization results, the doping of Fe atoms promote the growth of the [111] crystal plane, resulting a large specific area and exposing more catalytic active sites. Furthermore, the Feδ+ has highest positive charge among all the M-Co2P/NCNTs hybrid catalysts after doping. All these changes can be used to contribute the highest catalytic performance of the Fe-Co2P/NCNTs hydrogen evolution hybrid catalyst. Furthermore, an optimal HER electrocatalytic activity was obtained by adjusting the doping ratio of Fe atoms.

2. EXPERIMENTAL 2.1 Synthesis of M-Co2P/NCNTs (M= Fe, Ni, Cu) hybrid catalysts Typically, a certain amount of M acetylacetonate, 0.257 g cobalt acetylacetonate (Co(acac)2, Aladdin, 97%), 2 g triphenylphosphine (TPP, Aladdin, CP), 10 mL oleylamine (OAm, Aladdin, 95%) and 40 mg nitrogen-doped carbon nanotubes (NCNTs, Aladdin, 95%) were mixture, stirred, heated to 320 oC and reacted 2 h under a flow of Ar in a four-neck flask. The addition quantity of M acetylacetonate was 0.2 mmol each time, namely the M: Co precursor ratio was kept at 0.2. A mixture solvent of hexane and ethanol (Vhexane: Vethanol = 1: 3) was used to wash the product by centrifugation. Finally, the M-Co2P/NCNTs hybrid catalysts can be synthesized by drying at 60 oC in vacuum for 24 h. Additionally, the Fe-Co2P/NCNTs hybrid catalyst with different Fe doping contents also can be obtained by adjusting the Fe: Co precursor molar ratio (Fe: Co = 0.05, 0.1, 0.2, 0.4), respectively, without changing the other synthetic conditions. Note: the atomic molar ratio of M: Co: P of the 6


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as-synthesized M-Co2P/NCNTs hybrid catalysts were confirmed from the ICP-OES elemental analysis results. 2.2 Catalysts characterization XRD was performed on a panalytical X'pert PROX-ray diffractometer (45 kV, 40 mA, Cu Kα radiation, λ = 0.15406 nm). XPS was executed on a VG ESCALABMK II spectrometer system (Al Kα, 1486.6 eV). ICP-OES was performed on a ThermoScientific iCAP6300 instrument. N2 sorption was tested on a ChemBET 3000 (Quantachrome, USA). TEM, high resolution (HRTEM) were carried out on an FEI Tecnai G2F20 electron microscope. 2.3 Electrochemical tests The electrochemical measurements of the M-Co2P/NCNTs (M= Fe, Ni, Cu) hybrid catalysts were tested in 0.5 M H2SO4 solution on a Gamry Reference 600 instruments at room temperature. The working electrode is the catalysts loaded on glassy carbon electrode (GCE), the reference electrode is the Ag/AgCl electrode and the counter electrode is the Pt electrode. The electrolyte was saturated with N2. Typically, the mixtures including catalyst (5 mg), ethanol (1 mL), and 0.5 wt % Nafion (80 µL) were ultrasonicated 30 min to form an ink solution. Then 5 µL of the above solution was dropped onto the GCE (loading: 0.2 mg·cm−2). The polarization curves of the as-synthesized M-Co2P/NCNTs hybrid catalysts were obtained by linear sweep Voltammetry (LSV). To ensure reproducibility of the results, the electrochemical measurements were repeated 5 times. AC impedance tests were performed at different potentials from 106 to 0.1 Hz. The stability measurements were performed by cyclic 7



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voltanmmetry (CV, sweep rate: 100 mV·s−1) and long-term chronoamperometry at the same potential of 0.2 V. The effective active surface area (ECSA) of M-Co2P/NCNTs hybrid catalysts was estimated according to the reported method by CVs with different scan rates in the vs. reversible hydrogen electrode (RHE) region of 0.1-0.2 V. All the potentials were calibrated in this work by the equation of E (RHE) = E (Ag/AgCl) + 0.222 V. The turnover frequency (TOF) values and the active site numbers (n) can be confirmed according to reported method23 by CV test in 1.0 M phosphate buffer solution (PBS, pH = 7) at a scan rate of 50 mV·s-1 from -0.2 to 0.6 V vs. the RHE.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of the M-Co2P/NCNTs catalysts The Fe-, Ni- and Cu-doped Co2P on NCNTs were prepared via a simple thermal decomposition process using Fe(acac)3/Ni(acac)2/Cu(acac)2 as metal dopants, Co(acac)2 as Co source and TPP as P source in OAm solution of the NCNTs at 320 oC for 2 h. The XRD patterns of the M-Co2P/NCNTs catalysts were presented in Figure 1. The (002) crystal plane of graphite can be found at 26.4° for all the catalysts. For the Fe-Co2P/NCNTs and Ni-Co2P/NCNTs catalysts, the peaks at 40.6°, 44.2°, 47.9°, 52.5° and 55.8° can be assigned to the (111), (021), (120), (002) and (030) crystal planes of hexagonal Co2P (PDF# 00-054-0413). No extraneous Fe or Ni peaks are observed, which indicates that Fe and Co atoms have been introduced into the crystalline structure of Co2P successfully. However, from the XRD pattern of the 8


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Cu-Co2P/NCNTs catalyst, beside the peaks of Co2P, we also found the diffraction peaks of elementary substance Cu. The peaks at 43.4° and 50.6° corresponding to the (111) and (200) crystal planes of cubic Cu (PDF# 01-085-1326), which can be attributed to the doping of Cu in Co2P/NCNTs catalyst reached saturated, and excess Cu aggregated to form nanoclusters. The element composition and electronic property of the M-Co2P/NCNTs hybrid catalysts were characterized by XPS. The survey spectra (Figure 2a) confirm the existence of the corresponding elements in M-Co2P/NCNTs hybrid catalysts. The element O in all spectra come from the residual oxygen containing functional groups on the NCNTs and the surface oxidation of M-Co2P/NCNTs hybrid catalysts due to air contact. Three peaks are observed in C 1s orbit at 284.5, 285.8 and 289.2 eV corresponding to the hybridized sp2, sp3 carbon atoms and C=O, respectively24, 25 (Figure 2b). Two peaks can be seen in N 1s orbit at 398.9 and 400.9 eV (Figure 2c) representing the pyridinic N and pyrrolic N species in M-Co2P/NCNTs hybrid catalysts. These two types of N species are also the active center for the HER, improving the electrocatalytic activity by interacting with H+.26 Two characteristic Co2P peaks at about 778.9 and 792.7 eV were observed of each hybrid catalyst (Figure 2d-f), which can be ascribed to the Co 2p3/2 and Co 2p1/2 energy levels. Two other peaks at about 781.4 and 797.6 eV correspond to oxidized Co species. The small satellites also can be observed at about 784.5 and 803.1 eV, respectively. The Co-P of Co2P also can be confirmed by the P 2p spectra in Figure 2g-i of the M-Co2P/NCNTs hybrid catalysts. Three peaks are observed at 129.1, 129.7 and 134.2 eV representing 9



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the P 2p3/2, P 2p1/2 in Co2P and oxidized P species27, respectively. For Fe 2p orbit (Figure 2j), the peaks at 711.1 and 722.7 eV are the Fe3/2 and Fe1/2 of Fe-P in Fe-Co2P/NCNTs hybrid catalyst. Similarly, for Ni 2p orbit (Figure 2k), the peaks at 852.6 and 870.1 eV represent the Ni3/2 and Ni1/2 of Ni-P in Ni-Co2P/NCNTs hybrid catalyst. Thus, it can be concluded that the Fe and Ni atoms have been introduced into the crystalline phase structure of Co2P based on these facts successfully. However, from the Cu 2p orbit (Figure 2l), the different electronic energy level structure can be observed. Besides the Cu3/2 and Cu1/2 of Cu-P at 934.3 and 954.5 eV, the oxidized Cu at 941.7, 962.6 eV in Cu-Co2P/NCNTs hybrid catalyst, another peak at 932.6 eV also can be fitted out, which can be attributed to metal Cu, this result is consist with the XRD analysis result. In addition, from the above fitting result, we also found that the binding energy of the Fe 2p3/2, Ni 2p3/2, Cu 2p3/2, and Co 2p3/2 are positively shifted compared with the metallic such as Fe (706.8 eV), Ni (852.2 eV), Cu (932.6 eV), and Co (778.1 eV) while the P 2p3/2 in all the M-Co2P/NCNTs hybrid catalysts have a lower electron level (129.1 eV) than red phosphorus (130 eV), which indicates that all the metals have a partial positive (δ+) in M-Co2P/NCNTs hybrid catalysts but all the P species have negative charge (δ-)28. Furthermore, the Feδ+ has the highest partial positive, which is beneficial for the electrocatalytic process for the HER29. However, the existence of metal Cu in Cu-Co2P/NCNTs catalyst is not conducive to the electrocatalytic reaction. The element composition and content of the M-Co2P/NCNTs hybrid catalysts were further estimated by ICP-OES analysis (Table S1). It can be estimated that the M: Co: 10


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P molar ratio are about 0.3:1.7:1, 0.3:1.7:1 and 0.4:1.9:1 for the Fe-Co2P/NCNTs, Ni-Co2P/NCNTs and Cu-Co2P/NCNTs hybrid catalysts, respectively. The Fe and Co atoms have the same content in M-Co2P/NCNTs hybrid catalysts. However, the higher content of Cu might be due to the contribution of excess Cu nanoclusters. Figure 3 shows the TEM, HRTEM and STEM-EDX mapping of the M-Co2P/NCNTs catalysts. It can be observed from the TEM of the Fe-Co2P/NCNTs catalyst (Figure 3a) that rod-like or layer-like structure Fe-Co2P with the length of about 45 nm decorated on NCNTs successfully. The HRTEM image (Figure 3b) suggests that the fringe spacing is 2.196 Å, representing the [111] crystalline plane of Co2P, which indicates that the doping of Fe atoms promote the growth of the [111] crystal plane. The corresponding STEM-EDX elemental mapping (Figure 3c) suggesting a uniform distribution of all the elements of the Fe-Co2P/NCNTs catalyst. For the Ni-Co2P/NCNTs catalyst, the different morphology structure can be observed. The as-synthesized Ni-Co2P exhibits monodisperse sphere-like structure with the particle size of about 15 nm (Figure 3d). But for Cu-Co2P/NCNTs catalyst, some large flower-like microspheres with the average size of about 200 nm were observed (Figure 3e), which can be attributed to the aggregation of excess Cu nanoclusters due to the strong intermolecular forces and high surface energy in the high boiling solution. Furthermore, the EDX spectra (Figure S1) also confirm the existence of all the elements of the M-Co2P/NCNTs catalysts. The textural information of the M-Co2P/NCNTs catalysts were confirmed by N2 adsorption-desorption experiments, the results were shown in Figure 4 and Table S2. 11



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A type-IV isotherm (Figure 4a) with obvious H3-type hysteresis loop at the relative pressure ranging from 0.5 to 1.0 can be observed for all of the M-Co2P/NCNTs catalysts, which suggests that the as-synthesized M-Co2P/NCNTs catalysts have a large number of macropores and mesopores30, 31. In addition, the adsorption quantity of Fe-Co2P/NCNTs and Ni-Co2P/NCNTs is higher than that of Cu-Co2P/NCNTs, which indicates that the former have larger surface area. Additionally, the corresponding pore-size distribution (Figure 4b) also shows that the average pore size are

14.1,

17.8 and 18.5 nm for Fe-Co2P/NCNTs, Ni-Co2P/NCNTs and

Cu-Co2P/NCNTs, respectively, further suggesting the mesoporous structure characteristic of the M-Co2P/NCNTs catalysts. In addition, the Fe-Co2P/NCNTs catalyst have the largest BET specific area of 48.4 m2·g-1, indicating the most exposed active sites and exhibiting the highest electrocatalytic performance for the HER32. 3.2 Electrocatalytic performance of the as-synthesized M-Co2P/NCNTs catalysts The electrocatalytic properties of the as-synthesized M-Co2P/NCNTs and 20 % Pt/C catalysts were further tested at room temperature in N2 saturated 0.5 M H2SO4 solution for the HER using three-electrode setup. The LSV curves (Figure 5a) indicate that the overpotential needed for 20 % Pt/C catalyst is nearly zero, suggesting the highest HER catalytic activity. By comparing M-Co2P/NCNTs catalysts, the onset overpotentials needed for Fe-Co2P/NCNTs, Ni-Co2P/NCNTs and Cu-Co2P/NCNTs catalysts are 25, 96 and 140 mV, respectively. When the current densities are 10 and 20 mA·cm−2, the Fe-Co2P/NCNTs catalyst only needed overpotentials of about 104 and 126 mV, however, the overpotentials needed for the Ni-Co2P/NCNTs are about 12


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230 and 278 mV, while the required overpotentials for the Cu-Co2P/NCNTs catalyst are about 270 and 301 mV, respectively. All these results demonstrated strongly that the Fe-Co2P/NCNTs catalyst shows the highest catalytic performance. Our previous research results indicate that the Co2P/NCNT hybrid catalyst also exhibits higher electrocatalytic activity with onset overpotential of 62 mV, however, our current research further indicates that the doping of Fe atoms lead to better performance with onset overpotential of 25 mV than the Co2P/NCNT hybrid catalyst, which indicate that the HER can be promoted effectively by the introduction of Fe atoms into Co2P/NCNT hybrid catalyst. Additionally, the catalytic performance of the Fe-Co2P/NCNTs hybrid catalyst is higher than that of the others metal-doped HER catalysts (Table S3), including CoMoS15, Fe-MoS3 film30,

31

, Co-MoS3 film34,

Ni-MoS3 film34, Ni-MoS235, NiWS20, Cu-MoS2/rGO19, CoMoP18, CoS2@MoS236, α-INS17, NiMoNx/C37, Co0.6Mo1.4N238, Au-MoS2 film39 and so on. The difference of the electrocatalytic activity for the HER among all the as-synthesized M-Co2P/NCNTs hybrid catalysts can be assigned to the doping of different metal atoms, which changed the electronic property of the catalysts. The electrocatalytic mechanism of the TMPs is analogous with the natural biology hydrogenase and metal complex catalysts due to their similar electronic structure. For hydrogenase, Nicolet et al.40 thought that the hydrogen evolution was catalyzed by using pendant bases near the metal catalytic active centers. For metal complex catalysts, Wilson et al.41 reported that the HER was occurred once it incorporates proton. For the as-synthesized M-Co2P/NCNTs hybrid catalysts in this work, a partial positive of all the metals and a 13



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negative charge of P species have been proved by the XPS. The metal centers are the hydride-acceptor and the P species are the proton-acceptor, respectively. In the process of catalytic protons reduction, the basic P centers collected protons, the metal centers acted as an electron collector, therefore, the protons can obtain electrons in this sites, promoting the formation of hydrogen42. Furthermore, due to the difference of doping metals, the Feδ+ has the highest partial positive, which is beneficial for the electrocatalytic process for the HER43. But for Cu-Co2P/NCNTs catalyst, the existence of metal Cu in is not conducive to the electrocatalytic reaction. Therefore, the catalytic performance of the as-synthesized M-Co2P/NCNTs hybrid catalysts according

the

following

order

of

Fe-Co2P/NCNTs

>

Ni-Co2P/NCNTs

>

Cu-Co2P/NCNTs. The Tafel plot the as-synthesized M-Co2P/NCNTs and 20 % Pt/C catalysts were shown in Figure 5b. The Tafel equation was used to fit the linear portion, and the obtained Tafel slope of the 20 % Pt/C, Fe-Co2P/NCNTs, Ni-Co2P/NCNTs and Cu-Co2P/NCNTs were 30, 68, 106, 112 mV·dec-1, respectively, revealing a Volmer-Heyrovsky mechanism. Additionally, Fe-Co2P/NCNTs catalyst show the smallest Tafel slope among all the M-Co2P/NCNTs catalysts, which indicates that it has the fastest hydrogen evolution efficiency. Figure 5c shows the exchange current density (j0) curves of the M-Co2P/NCNTs catalysts, the corresponding values were shown in Table 1. Obviously, the Fe-Co2P/NCNTs catalyst shows the largest j0 of 0.33 mA·cm-2 among the M-Co2P/NCNTs catalysts, indicating the highest HER catalytic performance. 14


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Additionally, we also normalized the obtained j0 by the BET specific area of the as-synthesized catalysts, the values were shown in Table 1. The improvement of BET specific area from Cu-Co2P/NCNTs to Fe-Co2P/NCNTs hybrid catalyst lead to the improvement of normalized j0 gradually, which suggests that the catalytic performance of the M-Co2P/NCNTs catalysts affected by the BET specific area. The Fe-Co2P/NCNTs hybrid catalyst exhibits the largest normalized j0 values, suggesting the highest catalytic activity among all the catalysts. It also can be seen that the BET specific area of the Fe-Co2P/NCNTs hybrid catalyst is 1.14 and 2.05 times than that of the Ni-Co2P/NCNTs and Cu-Co2P/NCNTs hybrid catalysts, but the normalized j0 value of the former is 5 and 5.97 times than that of the corresponding latter. All these results suggest that BET specific area of M-Co2P/NCNTs catalysts is not a dominant factor to affect the HER catalytic performance44. The double layer capacitance (Cdl), which is considered to evaluate the effective electrochemically active surface area (ECSA)45, was determined by testing the cyclic voltammetry (CV) scans (Figure S2) of the as-synthesized M-Co2P/NCNTs catalysts, as shown in Figure 5d. The results show that, the Fe-Co2P/NCNTs catalyst shows much larger Cdl (15.4 mF·cm-2), which is about 2.19 and 2.26 times than that of the Ni-Co2P/NCNTs

and

Cu-Co2P/NCNTs

catalysts,

which

reveals

that

the

Fe-Co2P/NCNTs catalyst presented the highest intrinsic HER catalytic performance. The TOF, which is a highly important parameter for evaluating the catalytic activity, was calculated according to the reported methods23. CV curves (Figure 5e) were tested in neutral PBS to calculate the number of active sites of the as-synthesized 15



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M-Co2P/NCNTs catalysts. The calculated active site numbers of the as-synthesized Fe-Co2P/NCNTs, Ni-Co2P/NCNTs, and Cu-Co2P/NCNTs hybrid catalysts are 3.48×10-8, 2.38×10-8, and 4.27×10-9 mol, respectively, which indicates that the Fe-Co2P/NCNTs hybrid catalyst has higher catalytic active sites. Figure 5f shows the relationship of TOF and potential. It can be seen that, when the TOF value reached 0.1 s-1, the Fe-Co2P/NCNTs, Ni-Co2P/NCNTs, and Cu-Co2P/NCNTs hybrid catalysts need 73, 171, and 192 mV overpotentials, further indicating the excellent electrocatalytic activity of the Fe-Co2P/NCNTs. Additionally, considering that the Co2P/NCNTs catalyst is also a good HER catalyst, we further tested the CV curve in neutral PBS (Figure S3a) of the Co2P/NCNTs hybrid catalyst to illustrate the positive role of the doping of Fe atoms. The calculated active site number of Co2P/NCNTs catalyst is 2.47×10-8 mol, lower than that of the Fe-Co2P/NCNTs catalyst, indicating that the latter has the higher catalytic performance. Simultaneously, at a certain overpotential of 0.15 mV vs. RHE, the TOF value (1.0 s-1) of Fe-Co2P/NCNTs catalyst is larger than that of the Co2P/NCNTs catalyst (0.32 s-1, Figure S3b), which further suggests that the Fe-Co2P/NCNTs catalyst exhibits the higher intrinsic catalytic activity for HER. The electrical conductivity of the as-synthesized M-Co2P/NCNTs catalysts was further compared by EIS test at the overpotential of 160 mV, the obtained Nyquist plots are shown in Figure 5g. It can be seen that, the semicircle diameter of the as-synthesized M-Co2P/NCNTs catalysts according to the following order of Cu-Co2P/NCNTs > Ni-Co2P/NCNTs > Fe-Co2P/NCNTs, indicating the strongest electron transfer ability of the Fe-Co2P/NCNTs. 16


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The kinetic of the Fe-Co2P/NCNTs hybrid catalyst for the HER was studied by the EIS experiment. The obtained Nyquist and Bode plots at different potentials were shown in Figure 6a-b, respectively. The Fe-Co2P/NCNTs hybrid catalyst has smaller semicircle diameter at high potential, which shows that the HER can be promoted at high potential. The Fe-Co2P/NCNTs hybrid catalyst shows two time constant behavior from the corresponding Bode plots, indicating the co-existence of the charge transfer resistance and the interface resistance, therefore, an equivalent electrical circuit (Figure 6c) consists of a series resistor and two parallel circuits was used to fit the kinetic process. Two parallel circuits are composed of resistors and constant phase elements (CPE). The Rs, R1, Rct represent the solution resistance arise from the electrolyte and all contact, the interface resistance produced from the surface between catalyst and GCE, the charge transfer resistance produced from the surface between catalyst and electrolyte, respectively. In general, the smaller of Rct, the faster of the charge transfer. From the fitting results (Table 2), it can be seen that the Rs values almost identical of about 10 Ω at different potentials. However, the Rct value are potential-dependent with the change of potential. The Rct has low value at the high potential, which indicates that the increase of the potential can accelerate the process of hydrogen evolution. Moreover, the Tafel slope of the Fe-Co2P/NCNTs hybrid catalyst can also be obtained by fitting the Rct value at various potentials46. The obtained log(1/Rct)-η plot of the Fe-Co2P/NCNTs hybrid catalyst is shown in Figure 6d. The generated Tafel slope was 65 mV·dec-1, this value is nearly the same as the Tafel slope produced from the polarization curves. 17



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Furthermore, the stability of the M-Co2P/NCNTs catalysts was tested by CV scanning 1000 cycles (Figure 7a) and long-term chronoamperometry (Figure 7b). The polarization curve of the Fe-Co2P/NCNTs hybrid catalyst was nearly not changed after 1000 cycles. The current density of the Ni-Co2P/NCNTs and Cu-Co2P/NCNTs catalysts only has a slight loss after 1000 cycles. All these results suggest that the stability of the M-Co2P/NCNTs catalysts is good in acid solution. Additionally, the Fe-Co2P/NCNTs catalyst has the largest current density. Meanwhile, all the M-Co2P/NCNTs catalysts can maintain their catalytic activity at least 60 000s at the established overpotential of 200 mV. However, a spot of catalyst dropped into the electrolyte after the produce of H2, which lead to the degradation of current density at the first 10000 s. Meanwhile, due to the accumulation and release of the generated H2 bubbles on the surface of the as-synthesized hybrid catalysts, the typical serrate shape can be observed from the I-t curves clearly47, 48. In order to study the electrocatalytic activity affected by doping content of Fe atoms, samples of the Fe-Co2P/NCNTs catalysts with various Fe contents were synthesized and the HER catalytic performance was evaluated. The elemental stoichiometric ratios of Fe: Co: P were confirmed by ICP-OES analysis (Table S1). Figure 8a shows that Fe0.3Co1.7P/CNTs catalyst has the best electrocatalytic performance for the HER. The catalytic activity first increased and then decreased gradually with increasing of Fe doping content. For example, the Fe0.1Co0.9P/CNTs, Fe0.2Co1.8P/CNTs, Fe0.3Co1.7P/CNTs, and Fe0.8Co1.9P/CNTs hybrid catalysts needed an overpotential of 153, 123, 104, and 114 mV to reach a current density of 10 mA·cm-1, 18


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respectively. The excessive doping of Fe atoms resulted in lower HER electrocatalytic activity due to the production of elemental Fe according to the XPS results (Figure S4). The Cdl of the Fe-Co2P/NCNTs catalysts with various Fe doping contents was also determined by testing CV scans (Figure S5a~b) to compare the HER catalytic activity, as shown in Figure S4c. As expected, the Fe0.3Co1.7P/CNTs catalyst shows much larger Cdl (15.4 mF·cm-2) than that of the Fe0.1Co0.9P/CNTs (9.94 mF·cm-2) and Fe0.8Co1.9P/CNTs catalysts (12.3 mF·cm-2), further indicating that the Fe doping content is an impotent factor to affect the electrocatalytic activity. Additionally, we further compared the number of active sites and TOF of the Fe-Co2P/NCNTs catalysts with various Fe doping contents, as shown in Figure 8b~c. The calculated active site numbers of the Fe0.1Co0.9P/CNTs, Fe0.2Co1.8P/CNTs, Fe0.3Co1.7P/CNTs, and Fe0.8Co1.9P/CNTs hybrid catalysts are 1.41×10-8, 2.14×10-8, 3.48×10-8, and 2.31×10-8 mol, respectively. When the TOF value reached 0.1 s-1, the needed overpotentials are 115, 88, 73, and 85 mV for the Fe0.1Co0.9P/CNTs, Fe0.2Co1.8P/CNTs,

Fe0.3Co1.7P/CNTs,

and

Fe0.8Co1.9P/CNTs

hybrid

catalysts,

respectively. All these results indicate that an optimal HER electrocatalytic activity was obtained for the Fe0.3Co1.7P/CNTs hybrid catalyst. Increasing or decreasing Fe doping ratio (compared to Fe0.3Co1.7P/CNTs) led to lower electrocatalytic prformance of the Fe-Co2P/CNTs hybrid catalysts.

4. CONCLUSIONS 19



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Three M-Co2P/NCNTs (M= Fe, Ni, Cu) hybrid catalysts were synthesized via a simple in situ thermal decomposition process successfully. Based on the experimental results, after metal doping, the change of structure, morphology and electronic property results in a different HER catalytic activity. Benefiting from the largest surface area and highest positive charge of Fe after doping, the obtained Fe-Co2P/NCNTs hybrid catalyst shows the highest HER catalytic performance among all the M-Co2P/NCNTs hybrid catalysts in acidic solution. Furthermore, an optimal HER electrocatalytic activity was obtained by adjusting the doping ratio of Fe atoms. Increasing or decreasing Fe doping ratio (compared to Fe0.3Co1.7P/CNTs) led to lower HER activity of the Fe-Co2P/CNTs hybrid catalysts. Due to the facile synthesis, low-cost and high electrocatalytic performance, the as-synthesized Fe-Co2P/NCNTs hybrid catalyst appears to be a promising HER electrocatalyst.

ASSOCIATED CONTENT Supporting Information EDX spectra, XPS analysis of the Fe-Co2P/NCNTs hybrid catalyst with different Fe doping content, cyclic voltammetry curves, ICP-OES analysis results, textural property data and the comparison of the catalytic performance data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]; 20


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E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We appreciate the financial support from the National Natural Science Foundation of China (Grants No. 21006128, 21176258) and the Fundamental Research Funds for the Central Universities (Grants No. 15CX06039A).

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FIGURE CAPTIONS Figure 1 XRD patterns of the as-synthesized M-Co2P/NCNTs hybrid catalysts. Figure 2 XPS spectra in the (a) survey, (b) C (1s), (c) N (1s), (d-f) Co (2p), (g-i) P (2p), (j) Fe (2p), (k) Ni (2p) and (l) Cu (2p) regions for the as-synthesized M-Co2P/NCNTs hybrid catalysts. Figure 3 (a) TEM, (b) HRTEM and (c) STEM-EDX mapping images of the as-synthesized Fe-Co2P/NCNTs hybrid catalyst. TEM images of the as-synthesized (d) 28


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Ni-Co2P/NCNTs and (e) Cu-Co2P/NCNTs hybrid catalyst. Figure 4 (a) N2 adsorption-desorption isotherms and (b) Barrett-Joyner-Halenda (BJH) pore-size distribution of the as-synthesized M-Co2P/NCNTs hybrid catalysts. Figure 5 (a) LSV curves of the as-synthesized M-Co2P/NCNTs hybrid catalysts and 20 % Pt/C catalyst in 0.5 M H2SO4 with a scan rate of 5 mV·s-1. (b) Tafel plots of the as-synthesized M-Co2P/NCNTs hybrid catalysts derived from the polarization curves. (c) Calculated exchange current density of the as-synthesized M-Co2P/NCNTs hybrid catalysts by using extrapolation methods. (d) The relationship of the as-synthesized M-Co2P/NCNTs hybrid catalysts between current density variation (∆J=Ja-Jc) and scan rate. (e) CVs of the as-synthesized M-Co2P/NCNTs hybrid catalysts and bare GCE recorded at pH = 7 with a scan rate of 20 mV·s-1. (f) Calculated TOFs for the as-synthesized M-Co2P/NCNTs hybrid catalysts in 0.5 M H2SO4. (g) Nyquist plots of the as-synthesized M-Co2P/NCNTs hybrid catalysts with an overpotential of 160 mV in 0.5 M H2SO4 . Figure 6 (a) Nyquist and (b) Bode plots (The fitting curves were shown as solid lines) (c) the equivalent electrical circuit of the as-synthesized Fe-Co2P/NCNTs hybrid catalyst at different potentials from 80 to 160 mV. (d) The log(1/Rct)-η fitting curve of the Fe-Co2P/NCNTs hybrid catalyst. Figure 7 (a) CV scanning 1000 cycles with scan rate of 100 mV·s-1 and (b) long-term chronoamperometry of the as-synthesized M-Co2P/NCNTs hybrid catalysts in 0.5 M H2SO4 solution. Figure 8 (a) LSV curves of the as-synthesized Fe-Co2P/NCNTs hybrid catalysts with 29



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different Fe doping contents in 0.5 M H2SO4 with a scan rate of 5 mV·s-1. (b) CVs of the as-synthesized Fe-Co2P/NCNTs hybrid catalysts with different Fe doping contents recorded at pH = 7 with a scan rate of 20 mV·s-1. (c) Calculated TOFs for the as-synthesized Fe-Co2P/NCNTs hybrid catalysts with different Fe doping contents in 0.5 M H2SO4.

Figure 1 XRD patterns of the as-synthesized M-Co2P/NCNTs hybrid catalysts.

30


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31



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Figure 2 XPS spectra in the (a) survey, (b) C (1s), (c) N (1s), (d-f) Co (2p), (g-i) P (2p), (j) Fe (2p), (k) Ni (2p) and (l) Cu (2p) regions for the as-synthesized 32


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M-Co2P/NCNTs hybrid catalysts.

33



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Figure 3 (a) TEM, (b) HRTEM and (c) STEM-EDX mapping images of the as-synthesized Fe-Co2P/NCNTs hybrid catalyst. TEM images of the as-synthesized (d) Ni-Co2P/NCNTs and (e) Cu-Co2P/NCNTs hybrid catalyst.

34


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Figure 4 (a) N2 adsorption-desorption isotherms and (b) Barrett-Joyner-Halenda (BJH) pore-size distribution of the as-synthesized M-Co2P/NCNTs hybrid catalysts.

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Figure 5 (a) LSV curves of the as-synthesized M-Co2P/NCNTs hybrid catalysts and 20 % Pt/C catalyst in 0.5 M H2SO4 with a scan rate of 5 mV·s-1. (b) Tafel plots of the as-synthesized M-Co2P/NCNTs hybrid catalysts derived from the polarization curves. (c) Calculated exchange current density of the as-synthesized M-Co2P/NCNTs hybrid catalysts by using extrapolation methods. (d) The relationship of the as-synthesized M-Co2P/NCNTs hybrid catalysts between current density variation (∆J=Ja-Jc) and scan rate. (e) CVs of the as-synthesized M-Co2P/NCNTs hybrid catalysts and bare GCE recorded at pH = 7 with a scan rate of 50 mV·s-1. (f) Calculated TOFs for the as-synthesized M-Co2P/NCNTs hybrid catalysts in 0.5 M H2SO4. (g) Nyquist plots of the as-synthesized M-Co2P/NCNTs hybrid catalysts with an overpotential of 160 mV in 0.5 M H2SO4 .

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Figure 6 (a) Nyquist and (b) Bode plots (The fitting curves were shown as solid lines) (c) the equivalent electrical circuit of the as-synthesized Fe-Co2P/NCNTs hybrid catalyst at different potentials from 80 to 160 mV. (d) The log(1/Rct)-η fitting curve of the Fe-Co2P/NCNTs hybrid catalyst.

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Figure 7 (a) CV scanning 1000 cycles with scan rate of 100 mV·s-1 and (b) long-term chronoamperometry of the as-synthesized M-Co2P/NCNTs hybrid catalysts in 0.5 M H2SO4 solution. 42


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Figure 8 (a) LSV curves of the as-synthesized Fe-Co2P/NCNTs hybrid catalysts with different Fe doping contents in 0.5 M H2SO4 with a scan rate of 5 mV·s-1. (b) CVs of the as-synthesized Fe-Co2P/NCNTs hybrid catalysts with different Fe doping contents recorded at pH = 7 with a scan rate of 20 mV·s-1. (c) Calculated TOFs for the as-synthesized Fe-Co2P/NCNTs hybrid catalysts with different Fe doping contents in 0.5 M H2SO4.

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Table 1 The catalytic activities of the M-Co2P/NCNTs hybrid catalysts for the HER. Catalyst

Fe-Co2P/NCNTs

Ni-Co2P/NCNTs

Cu-Co2P/NCNTs

log j

j0

Normalized j0

Cdl

(mA·cm-2)

(A·cm-2)

(A·cm-2BET)

( mF·cm-2)

-0.48

3.3×10-4

3.43×10-6

15.4

(SD=5.09E-3)

(SD=7.85E-6)

(SD=8.15E-8)

(SD=4.29E-4)

-1.24

5.8×10-5

6.86×10-7

7.02

(SD=7.07E-3)

(SD=1.02E-6)

(SD=1.21E-8)

(SD=2.07E-4)

-1.57

2.7×10-5

5.75×10-7

6.82

(SD=7.07E-3)

(SD=1.02E-4)

(SD=2.57E-6)

(SD=1.97E-4)

Note: SD - Standard deviation.

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Table 2 Values of elements in equivalent circuit resulted from fitting the EIS data. Potential

Rs

CPE1

(V) vs. RHE

(Ω)

(F·cm-2·Sn-1)

9.299

3.76E-05

(SD=0.026)

n1

R1

CPE dl

ndl

Rct

(Ω)

(F·cm-2·Sn-1)

0.8

2.091

2.09E-03

0.8

615.2

(SD=1.113)

(SD=0.138)

(SD=0.149)

(SD=0.016)

(SD=0.007)

(SD=0.033)

9.257

9.13E-05

0.8

2.514

2.81E-03

0.8

238.7

(SD=0.015)

(SD=0.48)

(SD=0.069)

(SD=0.074)

(SD=0.01)

(SD=0.004)

(SD=0.011)

9.596

2.36E-04

0.8

3.04

3.29E-03

0.8

100

(SD=0.012)

(SD=0.31)

(SD=0.054)

(SD=0.056)

(SD=0.009)

(SD=0.004)

(SD=0.006)

9.878

1.89E-05

0.8

3.062

3.71E-03

0.8

52.46

(SD=0.019)

(SD=0.51)

(SD=0.084)

(SD=0.089)

(SD=0.019)

(SD=0.009)

(SD=0.009)

10.38

1.03E-05

0.9

2.074

3.87E-03

0.62

30.61

(SD=0.015)

(SD=0.77)

(SD=0.085)

(SD=0.1)

(SD=0.039)

(SD=0.014)

(SD=0.014)

10.07

1.32E-04

0.8

4.065

4.14E-03

0.64

19.72

(SD=0.025)

(SD=0.69)

(SD=0.105)

(SD=0.115)

(SD=0.074)

(SD=0.032)

(SD=0.029)

(Ω)

-0.06

-0.08

-0.10

-0.12

-0.14

-0.16

Note: SD - Standard deviation.

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Table of Contents

47


Nitrogen-Doped Carbon

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