CoP Nanoparticles Supported on Ti4O7 as the Electrocatalyst

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NiCoP/CoP nanoparticles supported on Ti4O7 as the electrocatalyst possessing an excellent catalytic performance toward hydrogen evolution reaction Dan Ma, Ruihao Li, Zhilin Zheng, Zhijun Jia, Kai Meng, Yi Wang, Guangming Zhu, Hui Zhang, and Tao Qi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02935 • Publication Date (Web): 29 Sep 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Sustainable Chemistry & Engineering

NiCoP/CoP nanoparticles supported on Ti4O7 as the electrocatalyst possessing an excellent catalytic performance toward hydrogen evolution reaction Dan Ma†,‡, Ruihao Li§, Zhilin Zheng†, Zhijun Jia†, Kai Meng†, Yi Wang†,*, Guangming Zhu‡,*, Hui Zhang†, Tao Qi† †

National Engineering Laboratory for Hydrometallurgical Cleaner Production

Technology, Institute of Process Engineering, Chinese Academy of Sciences, No.1 North 2nd Road, Haidian District, Beijing 100190, China ‡

School of Natural and Applied sciences, Northwestern Polytechnical University, No. 1

Dongxiang Road, Changan District, Xi’an 710072, China §

High School Affiliated to Fudan University, No. 383 Guoquan Road, Yangpu District,

Shanghai 200433, China *

Corresponding Authors: Y. Wang, Fax: (+86) 10 82544848-802, Tel: (+86) 10

82544967, E-mail: [email protected]; G. Zhu, E-mail: [email protected]

ABSTRACT NiCoP/CoP nanoparticles, NiCoP/CoP nanoparticles supported on carbon black (NiCoP/CoP-C) and titanium suboxide (NiCoP/CoP-Ti4O7), respectively, are prepared by a hydrothermal method followed with two steps of low temperature annealing processes. As hydrogen evolution reaction (HER) catalysts, the activities of the three catalysts are measured by linear sweep voltammetry (LSV), cyclic voltammetry (CV) ， electrochemical impedance spectroscopy (EIS) and so on. Impressively, NiCoP/CoP-Ti4O7 shows the best electrocatalytic performance for HER with an 1

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ultralow overpotential of 128 mV at 10 mA cm–2 along with negligible catalytic activity degradation after 1000 cycles of CV, which is partly ascribed to the high conductivity and corrosion resistance of Ti4O7. According to the characterization of X-ray photoelectron spectra (XPS)，it is found that NiCoP/CoP-Ti4O7 has a higher ratio of Ni2+ and surface defects, which can be also used to explain the reason why NiCoP/CoP-Ti4O7 owns the most preeminent HER activity among the three catalysts. Keywords:

NiCoP/CoP nanoparticles,

Ti4O7, Hydrogen

evolution

reaction,

Electrocatalysts, Electron interaction

INTRODUCTION The on-going exhaustion of tradition fossil fuels has created a strong demand for the alternative energy sources with high energy densities and eco-friendliness. Hydrogen, as a non-polluting and renewable clean energy, is regarded as one of the most promising alternatives for future fuel applications.45 At present, water electrolysis is the main technology to obtain hydrogen with high purity. However, HER on the cathode and oxygen evolution reaction (OER) on the anode need a large overpotential during electrochemical water splitting, and thereby limits the large-scale preparation of hydrogen.5-6 In this regard, the utilization of HER electrocatalysts is an efficient approach to reduce the large water-splitting overpotential, which can significantly improve the hydrogen generation efficiency and save electric energy. So far, there is no doubt that Pt presents the state-of-the-art electrocatalytic performance with the near-zero onset potential, the small η10 (43 mV) in 0.5 M H2SO4 and so on. In spite of this, the rarity and high cost of the noble metals and their derivatives severely 2


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impede their wide applications.7-9 Consequently, developing the low-cost and highly efficient electrocatalysts for the HER based on earth-abundant materials are highly desirable. During the past several years, inexpensive and plentiful transition metals (TMs, e.g. Ni, Co, Fe and Mn) have been intensively exploited as HER electrocatalysts, owing to their high current density at a low overpotential in HER caused by the unoccupied d orbitals and unpaired d electrons.10-13 Nevertheless, their HER catalytic performance, regarding lower overpotential and great durability, still needs amelioration to reach or surpass that of noble metal electrocatalysts. To fulfill this purpose, tremendous efforts have been devoted to improve the activities and stabilities of TMs for HER, and two major strategies have been proposed. Firstly, the intrinsic electrochemical activity of TM-based electrocatalysts can be significantly enhanced by the introduction of other catalytic components by forming metal alloys, metal oxides, phosphides or sulfides, and so forth.14-22 This is because the synergistic effect of combined catalytic component usually facilitates the adsorption of the H ions or the dissociation of water. Transition-metal phosphides (TMPs), such as Co1.33Ni0.67P,23 FexCo1-xP,24 Ni2P-CoP,25 Ni2P,26 Ni-Co-P27 and CoP28 displayed relatively superior activity in HER and considerable investigation and endeavor are on the way. For instance, the very recently reported nickel-cobalt phosphides (NiCoP) are the typical representatives.29 The phosphorus has the ability to trap positively charged protons and bond the atomic hydrogen strongly, as well as the moderate bonding to the reaction intermediates, thus further improving the catalytic performance.30 Secondly, 3



increasing the specific surface area of TM-based electrocatalysts is also beneficial to enhancing the activity of catalyst for HER. This can be achieved by reducing the size of catalyst to nanoparticles, using the template method to yield hollow or porous structures, or loading into other materials to promote the dispersion of main catalyst.31-34 In this work, to further improve the electrocatalytic performance of NiCoP/CoP, including the activity and the durability in strong acid, a viable measure is coupling NiCoP/CoP with conductive supports. It is well known that supporting materials can alter the electronic structures of main electrocatalysts and prevent them from aggregation and overlapping. Carbon black is commonly used as a supporting material because of its relatively large surface area, high conductivity (400-700 S m-1), as well as unique structure and so on.35 However, carbon black is liable to suffer from corrosion and this is unquestionable to cause a negative impact on HER performance. Because Ti4O7 has the characteristics of anti-corrosion, remarkable stability in acidic media and superior electrical conductivity (5000 S m-1),36-38 it has already been used as a good supporting material in various fields of electrochemistry, such as fuel cells,39-40 zinc-air batteries,41 and OER.36 However, up to now, there was no any report about the Ti4O7 used as a supporting material in HER. In this work, NiCoP/CoP catalyst supported on Ti4O7 was prepared by a hydrothermal method followed with two steps of low temperature annealing processes and its catalytic performance for HER was investigated.

EXPERIMENTAL SECTION 4


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Chemicals and Reagents. The chemicals used in the experiments were Ni(NO3)2·6H2O, Co(NO3)2·6H2O, CO(NH2)2, NaH2PO2·H2O, Nafion solution (0.5 wt% in isopropanol), deionized water, sulphuric acid (H2SO4), carbon black (Vulcan XC-72) and Ti4O7. The reagents were used without further purification. Synthesis of NiCo2O4, NiCo2O4 on C and Ti4O7, respectively. NiCo2O4 nanowires were synthesized by the hydrothermal method and calcination. First of all, 291 mg of Co(NO3)2·6H2O, 145 mg of Ni(NO3)2·6H2O and 300 mg of urea were dissolved in 35 mL deionized water and stirred for 30 min to form a homogeneous solution. After that, 53 mg of C or Ti4O7 was uniformly dispersed in the solution and sonicated for 30 min. Next, the as-prepared compounds were transferred into a 50 mL Teflon-lined stainless autoclave and heated at 120 °C for 6 h. Afterward, the autoclave was naturally cooled down to room temperature. The black precipitate was washed four to five times with deionized water and ethanol, respectively, and then dried in a vacuum oven at 60 °C for 8 h. Finally, it was annealed at 250 °C in N2 for 2 h with a heating rate of 2 °C min-1. For comparison, free NiCo2O4 was prepared using the same method without addition of supporting material. Synthesis of NiCoP/CoP, NiCoP/CoP on C and Ti4O7, respectively. 100 mg of NiCo2O4, NiCo2O4 on C and Ti4O7 were mixed with 500 mg sodium hypophosphite using a mortar to grind into powder, respectively. Subsequently, the samples were heated at 300 °C for 120 min under N2 atmosphere. The obtained 5



NiCoP/CoP on C and Ti4O7 were denoted as NiCoP/CoP-C and NiCoP/CoP-Ti4O7, respectively. Physicochemical Characterization. X-ray diffraction (XRD) was used to analyze the composition of these catalysts employing X’Pert-PRO MPD diffractometer with Cu-Kα and the 2θ ranged from 20° to 80°. Transmission electron microscopy (TEM, FEI Talos F200S), high-resolution transmission electron microscope (HRTEM) and selective area electron diffraction (SAED) were operated at an acceleration voltage of 200 kV. Field emission scanning electron microscopy (FE-SEM, quanta 400) was also used to characterize the microstructures of the catalysts. The elemental content analyses were performed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES, IRIS Intrepid II).

Chemical state analyses of Ni, Co and P in NiCoP/CoP-Ti4O7, NiCoP/CoP-C

and NiCoP/CoP were performed by X-ray photoelectron spectroscopy (XPS) using ULVAC-PHI X-ray photoelectron spectrometer (Japan). All XPS spectra were corrected using the C 1s line at 284.8 eV followed with background subtraction and curve fitting. Preparation of Working Electrode. The glassy carbon working electrode (GC, 4 mm diameter, CH Instruments Inc.) was polished carefully with 1.00, 0.30, and 0.05 µm alumina powder and then cleaned for several times with ethanol, HNO3, and deionized water, respectively. To prepare working electrode, catalyst ink was prepared by ultrasonically dispersing 5 mg of the as-prepared catalyst in 1 mL ethanol. 5 µL catalyst ink was dropped onto a GC 6


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working electrode (mass loading ≈ 0.199 mg cm-2) and then dried at room temperature. After that, 2 µL of Nafion (0.5 wt% in isopropanol) was dropped to fix the electrocatalysts. Electrochemical Measurements. Electrochemical measurements were carried out in a three-electrode cell using a electrochemical workstation (CHI760D, Chenhua, Shanghai) and the electrolyte was 0.5 M H2SO4 aqueous solution. Pt foil acted as the counter electrode and saturated calomel electrode (SCE) served as the reference electrode. To unify the standard of evaluation, all potentials appearing in this article were referred to the reversible hydrogen electrode (RHE), which can be calculated according to the equation: E (vs RHE) = E (vs SCE) + 0.0592 pH + 0.242 V (Equation 1).42 In 0.5 M H2SO4 aqueous solution, the pH of the electrolyte is zero. Namely, “E (vs RHE) = E (vs SCE) + 0.242 V” (Equation 2) acts as the criterion to unify potentials.

RESULTS AND DISCUSSION The XRD pattern of NiCoP/CoP was firstly measured to investigate the chemical composition and crystal structure. From Figure 1a, it can be seen that the peaks at 40.9°, 44.9° and 47.5° are related to the (111), (201) and (210) planes of NiCoP (PDF No. 71-2336),43 respectively, and the peaks at 31.7°, 36.2° and 48.1° match well with the (011), (111) and (211) planes of CoP (PDF No. 29-0497),44 respectively. The SEM image of NiCoP/CoP is displayed in Figure 1b, and NiCoP/CoP exhibits a aggregated nanoparticle morphology, with the particle size ranging from 50 to 300 nm. This is obviously different from the micron flower morphology of NiCo2O4-precursor (see 7



Figure S1). Figure 1c and the inset present the TEM and SAED image of NiCoP/CoP, respectively. It can be observed that many discrete spots have formed several polycrystalline rings and the polycrystalline rings can be assigned to the (111) and (201) planes of NiCoP and the (011) and (111) planes of CoP, respectively. HRTEM image of NiCoP/CoP (see Figure 1d) reveals lattice fringe with interplanar spacing of 0.22 nm, corresponding to (111) plane of NiCoP. Moreover, the energy dispersive X-ray (EDX) spectra of NiCoP/CoP were performed to confirm its component elements and their atomic ratio. As depicted in Figure 1e, the elements of Co, Ni and P coexist in the NiCoP/CoP, and the atomic ratio of Ni : Co : P is close to 1 : 2 : 2. Furthermore, the ratio of Ni : Co : P was further confirmed by ICP-AES and the mass ratio of Ni : Co : P is 19.65 : 39.12 : 22.98. Accordingly, the calculated atomic ratio of Ni : Co : P also approaches 1 : 2 : 2, implying that the molar ratio of NiCoP/CoP in the sample is close to 1:1. The EDX elemental mapping distributions (see Figure 1f) suggest that the component elements of Co, Ni and P are dispersed homogeneously and NiCoP/CoP is synthesized successfully.

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Figure 1. (a) XRD pattern, (b) SEM image, (c) TEM image, inset: SAED image, (d) HRTEM image, (e) EDX spectra, (f) EDX elemental mapping images of NiCoP/CoP. The composition and structure of the NiCoP/CoP-C and NiCoP/CoP-Ti4O7 samples were also investigated by XRD. As illustrated in Figure S2 (the XRD pattern of NiCoP/CoP is also shown for comparison), NiCoP/CoP-C exhibits diffraction peaks similar to NiCoP/CoP due to the low graphitization degree of C, while the NiCoP/CoP-Ti4O7 contains two kinds of diffraction peaks, which belong to NiCoP/CoP and Ti4O7 (PDF No. 50-0787), respectively. There are no peaks of any other phases or impurities, suggesting that the NiCoP/CoP, C and Ti4O7 did not react to form new phases. Based on the TEM images (see Figure S3a and Figure 2a), the NiCoP/CoP nanoparticles were successfully loaded onto the C and Ti4O7, respectively. The detailed crystal plane information for NiCoP/CoP, C and Ti4O7 is given in the corresponding HRTEM images of NiCoP/CoP-C and NiCoP/CoP-Ti4O7 (see Figure S3b and Figure 2b). 9



Figure 2. (a) TEM image, (b) HRTEM image, (c) EDX spectra and (d) EDX elemental mapping images of NiCoP/CoP-Ti4O7. It is notable that after loading onto the C, the NiCoP/CoP nanoparticles turn to be a smaller size of approximately 5∼10 nm. Meanwhile, the dispersion of NiCoP/CoP nanoparticles in NiCoP/CoP-C is more homogeneous than that in NiCoP/CoP-Ti4O7, which is attributed to the larger specific surface area of carbon black. The serious aggregation of NiCoP/CoP particles in NiCoP/CoP-Ti4O7 results from the small specific surface area of Ti4O7.40 EDX analyses (see Figure S3c-d and Figure. 2(c-d)) reveal that the NiCoP/CoP-C consists of Ni, Co, P and C, while the NiCoP/CoP-Ti4O7 is made up of Ni, Co, P, Ti and O. LSV curves were measured with a scanning rate of 5 mV s-1. From LSV curves shown in Figure 3a, we can get a preliminary estimation about the activity of the three electrocatalysts. The free NiCoP/CoP and NiCoP/CoP-C show the onset overpotential of 92 mV and 75 mV, respectively, while the NiCoP/CoP-Ti4O7 exhibits the exceptional onset overpotential of 48 mV, suggesting the outstanding hydrogen evolution activity of the last one. Additionally, to reach the same current density (10 10


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mA cm-2), the needed overpotentials for NiCoP/CoP-Ti4O7, NiCoP/CoP-C, and free NiCoP/CoP, are 128 mV, 156 mV and 171 mV, respectively. This further indicates that NiCoP/CoP-Ti4O7 possesses the highest catalyst activity among the three catalysts. Such excellent performance for NiCoP/CoP-Ti4O7 may be attributed to the higher conductivity of Ti4O7 as well as the synergistic effect of NiCoP/CoP and Ti4O7. In addition, NiCoP/CoP-C needs less overpotential to meet the same current density compared with NiCoP/CoP, which possibly results from the good dispersity of NiCoP/CoP on carbon black along with the high conductivity of carbon black. Besides, Tafel plots can be gained from LSV curves, which are deemed as a significant kinetic parameter to describe the detailed underlying mechanism. According to Figure 3b, NiCoP/CoP-Ti4O7 has a Tafel slope of 65.5 mV dec-1, which is the lowest as compared with NiCoP/CoP-C (79.3 mV dec-1) and NiCoP/CoP (86.3 mV dec-1), suggesting that NiCoP/CoP-Ti4O7 possesses much faster kinetics for HER. The lowest Tafel slope of NiCoP/CoP-Ti4O7 may be related to the electron interaction between NiCoP/CoP and Ti4O7 and superior electrical conductivity of Ti4O7 (5000 S m-1). Table S1 listsd the catalytic performance of some electrocatalysts reported in the literature towards HER in 0.5 M H2SO4. It can be found that although the loading mass of NiCoP/CoP-Ti4O7 is the least as compared to other reported catalysts, it owns quite good HER activities.

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Figure 3. (a) LSV curves, (b) Tafel plots, (c) EIS of NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, (d) LSV curve comparison before and after 1000 cycles of CV for NiCoP/CoP-Ti4O7. Not only Pt foil but carbon rod was used as the counter electrode for measuring the catalytic performances of the catalysts. As shown in Figure S4a, the free NiCoP/CoP and NiCoP/CoP-C show the onset overpotential of 98 and 78 mV, respectively, while the NiCoP/CoP-Ti4O7 exhibits the exceptional onset overpotential of 51 mV. To reach the same current density (10 mA cm-2), the needed overpotentials for NiCoP/CoP-Ti4O7, NiCoP/CoP-C, and free NiCoP/CoP, are 132, 165 and 180 mV, respectively. According to Figure S4b, NiCoP/CoP-Ti4O7 has a Tafel slope of 65.8 mV dec-1, which is the lowest as compared with NiCoP/CoP-C (79.8 mV dec-1) and NiCoP/CoP (86.9 mV dec-1). Thus, there are not obvious differences on the catalytic performances of the catalysts whether carbon rod or Pt foil acts as counter electrode in 12


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electrochemical measurements. To understand the kinetic process in depth, electrochemical impedance spectroscopy (EIS) measurements were carried out at -0.14 V vs RHE. The charge-transfer resistance (Rct) is related to the charged-transfer process of the electrode, which can be acquired from the diameter of the semicircles in the low frequency zone. From EIS results presented in Figure 3c, we can draw a distinct conclusion that NiCoP/CoP-Ti4O7 has the smallest Rct (115 Ω). As compared with NiCoP/CoP-C (131 Ω) and NiCoP/CoP (175 Ω), NiCoP/CoP-Ti4O7 shows a faster charge transfer and HER process, which may result from the superior electrical conductivity of Ti4O7. The durability is also a critical criterion to evaluate an electrocatalyst. To test the long-term stability, the electrocatalysts were cycled continuously for 1000 cycles of CV. From Figure 3d and Figure S5a-b , it is obvious that NiCoP/CoP-Ti4O7 shows the highest stability with a negligible shift of LSV curves after 1000 cycles CV in 0.5 M H2SO4, indicating that NiCoP/CoP-Ti4O7 has a good long-term operating stability during the electrochemical examination process. Conversely, NiCoP/CoP-C and NiCoP/CoP suffered from the drastic degradation of HER activities after 1000 cycles of CV. Additionally, the stability of the electrocatalysts was also evaluated through I-t measurements at -0.11 V vs RHE. As shown in Figure S6, after testing for 30 h, the current density of NiCoP/CoP-Ti4O7 was reduced from 3.23 mA cm-2 to 2.28 mA cm-2, exhibiting a slight current degradation of 29.4%. However, NiCoP/CoP-C and NiCoP/CoP suffered from a larger current attenuation of 48.7% and 57.2%, 13



respectively. The results of I-t measurements also demonstrate the excellent durability of NiCoP/CoP-Ti4O7, which may be ascribed to the good corrosion resistance of Ti4O7. In addition, the XRD patterns of the electrocatalysts after 30 h of I-t measurements are provided in Figure S7. The characteristic peaks of NiCoP/CoP were still preserved perfectly, but the peak intensity of NiCoP/CoP decreased a little. However, the characteristic peaks of Ti4O7 in NiCoP/CoP-Ti4O7 and C in NiCoP/CoP-C became clearer as compared with those in Figure S2. To investigate the effect of the loading of NiCoP/CoP on Ti4O7 on the catalytic activity and stability, NiCoP/CoP-Ti4O7-10% and NiCoP/CoP-Ti4O7-50% (the percentages represent the contents of Ti4O7) were also synthesized using the same method (see Experimental Section and Supporting Information). Their electrocatalytic performance was compared with that of NiCoP/CoP-Ti4O7 (the content of Ti4O7 is 30%). As illustrated in Figure S8a-b, NiCoP/CoP-Ti4O7 needs the least overpotential to meet the same current density and owns the lowest Tafel slope as compared with NiCoP/CoP-Ti4O7-10% and NiCoP/CoP-Ti4O7-50%. After testing for 20 h, NiCoP/CoP-Ti4O7-10% and NiCoP/CoP-Ti4O7-50% had more distinct attenuation of HER activities and suffered from a larger current degradation of 31.5% and 28.4%, respectively, while the corresponding value for NiCoP/CoP-Ti4O7 was only 24.1% (see Figure S8c). Moreover, as depicted in Figure S8d-f, NiCoP/CoP-Ti4O7 owns the highest durability among the three

catalysts. That is to say, 30% is the appropriate

amount of Ti4O7 for possessing the excellent HER activity in these three ratios of catalysts. 14


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X-ray photoelectron spectroscopy measurements are performed to investigate the surface elemental composition and chemical status so as to understand the electron interaction mechanism between NiCoP/CoP and Ti4O7. Figure 4a-c are the P 2p spectra of NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, respectively. The peaks of NiCoP/CoP-Ti4O7 at 129.7 eV, NiCoP/CoP-C at 130.2 eV, and NiCoP/CoP at 129.3 eV are attributed to Pδ- in the metal phosphides, while the peaks of NiCoP/CoP-Ti4O7 at 133.7 eV, NiCoP/CoP-C at 134.4 eV, and NiCoP/CoP at 133.7 eV correspond to phosphates.45 Figure 4d-f are the XPS spectra of Ni 2p regions of NiCoP/CoP-Ti4O7, NiCoP/C and NiCoP/CoP, respectively. The binding energies of Ni 2p3/2 and Ni 2p1/2 in NiCoP/CoP-Ti4O7 are 852.9 and 856.5 eV, 869.9 and 874.2 eV, respectively. As for NiCoP/CoP, the peaks for Ni 2p3/2 and Ni 2p1/2 locate at 852.9 and 856.6 eV, 870.1 and 874.1 eV, respectively. However, the peaks of Ni 2p1/2 and Ni 2p3/2 of NiCoP/CoP-C both shift to higher binding energies compared with the two formers.46 There is not straightforward distinction about the peak position between NiCoP/CoP-Ti4O7 and NiCoP/CoP, but the proportion of Ni2+/Ni3+. After a series of calculation, the ratios of Ni2+/Ni3+ were gained and increased in the order of NiCoP/CoP (1.78) ＜ NiCoP/CoP-C (1.88) ＜ NiCoP/CoP-Ti4O7 (2.68).

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Figure 4. (a-c) XPS spectra of P (2p) for NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, respectively, (d-f) XPS spectra of Ni (2p) for NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, respectively, and (g-i) XPS spectra of Co (2p) for NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, respectively. Additionally, as shown in Figure 4g-i, there is no obvious difference about the peak position of Co 2p spectra between NiCoP/CoP-Ti4O7 and NiCoP/CoP but the ratios of Co3+/Co2+. The ratios of Co3+/Co2+ were increased in the order of NiCoP/CoP (1.02) ＜ NiCoP/CoP-C (1.18) ＜ NiCoP/CoP-Ti4O7 (1.34). Figure S9a-b show the XPS spectra of Ti 2p regions of Ti4O7 and NiCoP/CoP-Ti4O7, respectively. The binding energies of Ti 2p3/2 and Ti 2p1/2 in Ti4O7 are 457.6 and 458.3 eV, 462.7 and 463.9 eV, respectively. As for NiCoP/CoP-Ti4O7, the peaks for Ti 2p3/2 and Ti 2p1/2 locate at 457.5 and 458.1 eV, 463.1 and 464.0 eV, respectively.47 The ratios of 16


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Ti4+/Ti3+ were increased in the order of Ti4O7 (0.43) ＜ NiCoP/CoP-Ti4O7 (0.53). All the differences of cation ratios may be related to the electron interaction between NiCoP/CoP and Ti4O7. Because the valence electron configuration of Ti is 3d24s2, Ti4O7 is betatopic configuration. The pristine NiCoP/CoP contains nickel and cobalt cations, which have unoccupied orbits. Hence, some electrons of Ti4O7 may have a trend to transfer to NiCoP/CoP, leading to the increase of Ti4+ in NiCoP/CoP-Ti4O7. This trend may have a greater impact on Ni3+ than Co3+ firstly owing to the stronger oxidizing ability of Ni3+, resulting in the increase of Ni2+ content. To keep the charge balance, the ratio of Co3+ in NiCoP/CoP-Ti4O7 increases accordingly. The mechanism is schematized in Figure 5.

Figure 5. The electronic effect schematic diagram between Ti4O7 and NiCoP/CoP. Huang et al.48 illustrated that the Ni2+ can help accelerate the kinetic rate of the HER (H2O + Ni2+ ↔ Ni2+−OHad + Had). Thus, the high content of Ni2+ in NiCoP/CoP-Ti4O7 also contributes to the excellent catalytic activity of this catalyst. In addition, from Soriano’s study, the more proportion the satellite peaks have, the more defects the catalysts have.49 The ratios of satellite peak for Ni were increased in the 17



order of NiCoP/CoP (0.47) ＜ NiCoP/CoP-C (0.50) ＜ NiCoP/CoP-Ti4O7 (0.51) while the ratios of satellite peak for Co were increased in the order of NiCoP/CoP (0.19) ＜ NiCoP/CoP-C (0.34) ＜ NiCoP/CoP-Ti4O7 (0.37). Obviously, among the three catalysts, NiCoP/CoP-Ti4O7 has the highest ratio of satellite peaks, suggesting that this catalyst contains the most defects. Usually, more defects are more favorable to improve the activity of electrocatalysts for the HER.50 Therefore, according to XPS analyses, the high content of Ni2+ and more defects can be used to explain the excellent HER performance of NiCoP/CoP-Ti4O7.

CONCLUSIONS In this work, NiCoP/CoP, NiCoP/CoP-C and NiCoP/CoP-Ti4O7 catalysts were prepared. Among these three catalysts, NiCoP/CoP-Ti4O7 exhibits an exceptional HER performance including the smallest onset potential, the best stability, and the minimum Rct and so on. After doping 30% Ti4O7 in NiCoP/CoP, the overpotential to reach 10 mA cm-2 has been reduced from 171 mV to 128 mV. The good catalytic performance may be partly attributed to the high conductivity and corrosion resistance of Ti4O7. In addition, according to the characterization of XPS, we find NiCoP/CoP-Ti4O7 has a higher ratio of Ni2+ and surface defects, which is possibly caused by the electron interaction and synergistic effect between NiCoP/CoP and Ti4O7. This also contributes to the excellent catalytic activity of NiCoP/CoP-Ti4O7. Thus, Ti4O7 is a good supporting material and NiCoP/CoP-Ti4O7 is a promising catalyst for HER. Supporting Information. 18


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Characterization data, including XRD, SEM, TEM, HRTEM, ICP and XPS; The catalytic activities and stabilities of the catalysts, including LSV curves, Tafel Plots and I-t curves; The catalytic activities of other reported electrocatalysts. This Supporting Information is available free of charge

via the Internet at

http://pubs.acs.org. Corresponding Authors: *

Y. Wang, Fax: (+86) 10 82544848-802. Tel: (+86) 10 82544967. E-mail:

[email protected] ; *G. Zhu, E-mail: [email protected] ORCID Y. Wang: 0000-0001-7975-091x Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors are grateful for the financial support by Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant No. QYZDJ-SSW-JSC021), Chinese National Programs for High Technology Research and Development (2014AA06A513), as well as by the 973 Program (Grant No. 2015CB251303).

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Figure 1

Figure 1. (a) XRD pattern, (b) SEM image, (c) TEM image, inset: SAED image, (d) HRTEM image, (e) EDX spectra, (f) EDX elemental mapping images of NiCoP/CoP.

Figure 2

Figure 2. (a) TEM image, (b) HRTEM image, (c) EDX spectra and (d) EDX elemental mapping images of NiCoP/CoP-Ti4O7.

24


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Figure 3

Figure 3. (a) LSV curves, (b) Tafel plots, (c) EIS of NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, (d) LSV curve comparison before and after 1000 cycles of CV for NiCoP/CoP-Ti4O7.

25



Figure 4.

Figure 4. (a-c) XPS spectra of P (2p) for NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, respectively, (d-f) XPS spectra of Ni (2p) for NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, respectively, and (g-i) XPS spectra of Co (2p) for NiCoP/CoP-Ti4O7, NiCoP/CoP-C and NiCoP/CoP, respectively.

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Figure 5.

Figure 5. The electronic effect schematic diagram between Ti4O7 and NiCoP/CoP.

TABLE OF CONTENTS (TOC) GRAPHIC

Synopsis: The electron interaction and synergistic effect between NiCoP/CoP and Ti4O7 lead to higher ratio of Ni2+ and surface defects, improving the performance of NiCoP/CoP-Ti4O7.

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CoP Nanoparticles Supported on Ti4O7 as the Electrocatalyst

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