Facile one-step synthesis of phosphorus-doped CoS2 as efficient electrocatalyst for hydrogen evolution reaction

Accepted Manuscript Facile one-step synthesis of phosphorus-doped CoS2 as efficient electrocatalyst for hydrogen evolution reaction Jingyan Zhang, Yuchan Liu, Baorui Xia, Changqi Sun, Yonggang Liu, Peitao Liu, Daqiang Gao PII:

S0013-4686(17)32394-0

DOI:

10.1016/j.electacta.2017.11.043

Reference:

EA 30633

To appear in:

Electrochimica Acta

Received Date: 2 September 2017 Revised Date:

3 November 2017

Accepted Date: 7 November 2017

Please cite this article as: J. Zhang, Y. Liu, B. Xia, C. Sun, Y. Liu, P. Liu, D. Gao, Facile one-step synthesis of phosphorus-doped CoS2 as efficient electrocatalyst for hydrogen evolution reaction, Electrochimica Acta (2017), doi: 10.1016/j.electacta.2017.11.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Facile One-step Synthesis of Phosphorus-doped CoS2 as Efficient Electrocatalyst for Hydrogen Evolution Reaction

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Jingyan Zhang, Yuchan Liu, Baorui Xia, Changqi Sun, Yonggang Liu, Peitao Liu, Daqiang Gao*

Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University,

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* Corresponding author: [email protected]

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Lanzhou 730000, P. R. China

ABSTRACT

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Synthesizing and designing of the hydrogen evolution reaction (HER) electrode composed of earth-abundance elements is the current research topics. Among the many discovered catalysts, cobalt disulfide has outstanding performance for HER due to its catalytic activity cobalt sites. Here, phosphorus-doped cobalt disulfide (P doped CoS2) samples are successfully synthesized via simple one-step hydrothermal method. By using them as the electrocatalysts, the most efficient electrode we obtained shows the excellent electrocatalytic activity, with low overpotential of 53 mV to achieve a 10 mA/cm2 current density, a small Tafel slope of 57 mV/dec and a high stability after 10000 cycles. First-principle calculations results indicate that phosphorus dopants could improve the electrocatalytic activity of hydrogen evolution by lowing the free energy for atomic hydrogen adsorption (∆G∗ ) at the Co sites. Moreover, the metallic P doped CoS2 could promote electron transfer and offers faster HER kinetics, thereby leading to better HER activity. This finding demonstrates an effective way to synthesis nonmetal doped CoS2 catalysts via one-step hydrothermal method and further improve the HER performance of CoS2 under the guidance of theoretical calculations. Keywords: Hydrogen evolution reaction; P-doped CoS2; Efficient electrocatalyst; First-principle calculations. 1. Introduction Energy problems have gradually risen to the hot issue of society. On one hand, traditional energy sources such as oil, coal, natural gas are not renewable. On the other hand, some harmful gases produced during the use of traditional energy are

ACCEPTED MANUSCRIPT seriously undermining the atmospheric environment, and even caused a series of diseases [1,2]. Seeking new renewable clean energy to replace traditional energy is necessary. As an energy carrier, hydrogen is favored by researchers because of its high calorific value and friendly products [3]. It is a viable way to obtain hydrogen from splitting water [4,5]. However, electrolysis water demands to add a gate voltage,

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which still needs to spend lots of power. So many researchers are committed to finding a catalyst to reduce the gate voltage and energy consumption in order to make produce hydrogen industrially possible. At present, it is reported that the most efficient catalysts are Pt-based alloys [6,7]. Whereas, as a noble metal, Pt is extremely

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expensive and scarce on the earth, which severely limits its application [8,9]. Therefore, finding new catalyst composed of high-abundance elements is indispensable. In recent decades, sulfides [10-12], selenides [13,14] and phosphides

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[15,16] have been reported in succession, including MoS2 [17], VS2 [18], CoS2 [19], MoSe2 [20] and other catalysts [21-24]. Surprisingly, some transition metal compounds have even been achieved excellent properties near and on par with noble metals [25-27].

Studies have shown that the incorporation of a third hetero-atom in a binary compound can alter the bandgap of the compound, which is helpful for improving the

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electrocatalytic activity of the catalyst. The types of doping are divided into anionic doping and cationic doping. Cationic dopants such as Cu [28,29], Mn [30], Ni [31,32], Co [33,34], anion doped with B [35], N [36], S [37], P [38]. Moderate doping can effectively improve the catalytic efficiency of the compound. According to the doping

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theory [39], the atomic radius and electronegativity of the P atom are close to the S atom, it is feasible to dip the P atom into the sulfide, where the P atom can substitute

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the S atom vacancy to form a new bond with another element, triggering lattice distortion. It is possible to provide some new active sites for the hydrogen evolution reaction and decrease the Gibbs free energy (∆G∗ ), which will be conducive to the improvement of catalytic performance. Unlike other semiconductor materials, CoS2 is considered as a promising

electrode material due to the amazing metal-like conductivity according recent reports [40,41]. Therefore, it is meaningful to further improve the electrocatalytic efficiency of CoS2 by doping. Recently, Ouyang et al. reported that P-doped CoS2 show higher activity for the HER. However, the systematic study of P concentration dependence on their HER properties, as well as, the deep theoretical explanations are undone. To

ACCEPTED MANUSCRIPT further study how to the P dopants affect the hydrogen evolution performance of CoS2, series P-doped CoS2 catalysts are prepared by a simple hydrothermal method and their electric catalytic properties for hydrogen evolution reaction (HER) are studied. By using them as the electrocatalysts, P doped CoS2 catalysts show the better HER performance than that of pure CoS2. First principle calculation results indicate that P

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dopants results in a much reduced |∆G∗ | values in CoS2. Besides, the P doped CoS2 samples also show metallic nature, which could promote the speed of electron transfer and offer faster HER kinetics. Thus, both our experimental and theoretical results demonstrate that the electrocatalytic activity of the CoS2 could be further enhanced by

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the P doping and the P doped CoS2 catalysts show the potential applications in further water splitting.

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2. Experimental

2.1. Synthesis of CoS2 and P doped CoS2

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Both pure CoS2 and P doped CoS2 are synthesized by one step hydrothermal method. Firstly, 0.5945 g of cobalt chloride hexahydrate (CoCl2·6H2O, 0.0025 mol) and 0.62 g of sodium pentahydrate (Na2S2O·5H2O, 0.0025 mol) were dissolved into 60 mL deionized water and stirred half an hour with a magnetic stirrer at room temperature to form a uniform pinkish solution. 0 g (~0 mol) 0.265 g (~0.0025 mol) 0.530 g (~0.005 mol) 1.060 g (~0.01 mol) 2.120 g (~0.02 mol) sodium phosphate (NaH2PO2·H2O) are weighted, respectively, added to the precursor solution and stirred until dissolved. Then the solution was transferred to 100 mL Teflon-lined stainless autoclaves one by one, reacted at 180°C for 12h. They were taken out after cooling to room temperature, washed two or three times with anhydrous ethanol and deionized water respectively, placed in the constant temperature oven at 60°C for 12h to dry. Finally we got five black powder samples that were labeled severally P0, P1, P2, P3 and P4 according to the order from less to more of phosphorus content, where the P concentrations are 0 at.%, 1.23 at.%, 2.28 at.% and 4.46 at.%, respectively. 3. Results and discussion Fig. 1a shows the X-ray diffraction (XRD) patterns of pure CoS2 (JCPDS 41-1471) and P doped CoS2 catalysts. It can be seen that all the samples are cubic structure with the lattice parameter a = b = c = 5.538 Å, belonging to the Pa-3 space group. There are two main peaks at 2 theta angle of 32.30º and 46.32º, which correspond to (200) and (220) faces. Besides, (111), (210), (211), (221), (311) crystal faces can be found at 2 theta angle of 27.93º, 36.24º, 39.83º, 49.33º, 54.94º. The location of the peaks shows

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no obviously shift after doping P and there are no P-related new phases, indicating that P elements are successfully doped into the structure of CoS2. As further increasing the P dopants, some new phases appeared shown in Fig. S1 for sample P4. Therefore here we just study the samples of P0~P3. Figs. 1b is the scanning electron microscope (SEM) picture of sample P3. It can be seen that the microstructure is the block stacked by the nanoparticles. There are some gaps between blocks and blocks. Fig. 1c shows the transmission electron microscopy (TEM) image of sample P3. As can be seen that it is piled up by many thin slices made up a lot of nanoparticles, but only a few are dispersed by ultrasound, just as pointed by the arrow in the picture. Which is consistent with the results of SEM. To assess the distribution of elements in the material, EDS mapping was performed for sample P3. As shown in Fig. 1d, both Co and S are uniformly distributed in the entire detected region. Meanwhile, the distribution of incorporated P atoms on the structure is quite homogeneous.

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Fig. 1. (a) XRD of pure CoS2 and P-doped CoS2. (b) SEM image of P3. (c) TEM image of P3. (d) EDS elemental mapping images of P3.

The Raman spectra of CoS2 and P doped CoS2 are also recorded and the results are shown in Fig. 2a. There are two obvious peaks at about 384 cm-1 and 284 cm-1 , corresponding to the out-of-plane Ag and in plane Eg vibrational of CoS2 [42]. It is obvious that the peaks shift to higher wave number after doping with P. But with the increase of P doping content, the peaks shifted smaller and smaller. The P1 with the lowest P content shows the most peak shift, while the P3 with the most P content has the least deviation. Even the peaks for P3 are nearly equivalent in position to P0 although much decreased in magnitude. It probably indicates a much more complex change in the sample than a simple linear of even proportional shift in vibrational modes upon P incorporation. To further characterize the chemical composition and

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elemental state of P doped CoS2, X-ray photoelectron spectroscopy (XPS) measurements are applied. As shown in Fig. 2b, the wide spectrum is recorded from 0 to 900 eV, indicates the presence of Co, S, P, C and O elements, where oxygen is derived from adsorbed oxygen or the sample is slightly oxidized, C is the carbon used for calibration. It reveals that the sample is consisted of Co, S and P elements. Fig. 2c-f are the high resolution spectra of the calibration C, Co, S, and P, respectively. There are two important peaks at 778.7eV and 793.9 eV assigned to the Co 2p3/2 and Co 2p1/2 for pure CoS2 in Fig. 2d, which is consistent with previous report [43]. Compared with pure CoS2, P-doped CoS2 shows the peak shift about -0.2 eV for Co 2p. That means there are new Co-P bonds formed after introducing P dopants, which subsequently affects the valence state of Co (the peak position of Co shift to lower energy state). In Fig. 2e, both samples exhibit S 2p1/2 and S 2p3/2 located at 163.9 eV and 162.8 eV. Besides, peaks attributed to sulfur oxide chemical bond in CoS2 are found. The P resolution spectrum shows two major peaks corresponding to P 2p1/2 and P 2p1/2 orbital, which are close to the previously reported values of P doped CoS2 [44].

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Fig. 2. (a) Raman spectrum of pure CoS2 and P-doped CoS2. (b) XPS wide spectrum for sample P3. (c) High resolution XPS spectrum of the C 1s. High resolution XPS spectra of the (d) Co 2p (e) S 2p (f) P 2p of the pure CoS2 (P0) and P-doped CoS2 (P3).

In order to investigate the role of P dopants on the hydrogen evolution reaction (HER) activity of CoS2, a series of electrochemical measurements are designed and carried out. Here, the HER activity of CoS2 and P doped CoS2 are using a typical three-electrode system in 0.5 M H2SO4 (pH = 0) with a scan rate of 2 mV/s−1. In detail, the three-electrode system is composed of counter electrode (CE: graphite electrode), reference electrode (RE: Ag/AgCl electrode) and working electrode (WE: glassy carbon electrode). First of all, Linear-sweep voltammetry (LSV) measurements are performed to investigate the HER performance of CoS2 and P doped CoS2 catalysts. Fig. 3a shows the LSV polarization curves of the catalysts. Here, the LSV polarization curves have corrected for solution resistance (2~3 ohm). Obviously, the sample P3 has

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the most optimistic catalytic activity in all samples. Its overpotential is only 53 mV at the current density of 10 mA/cm2, which is far better than pure CoS2 that it needs an overpotential of 239 mV to reach a current density of 10 mA/cm2. Similarly, if we want to reach a current density of 100 mA/cm2, the overpotential of 365 mV and 118 mV are required for pure CoS2 and the sample P3 with the largest amount of phosphorus. It suggests that sample P3 act as a high-performance cathode for HER. In addition, the Tafel slope is also an important parameter for evaluating the hydrogen electrocatalytic activity. Using the Tafel formula and the polarization curve, the Tafel slope can be obtained. As shown in Fig. 3b, the slope of sample P3 is 57 mV/dec, about one-second of pure CoS2 (~105 mV/dec). Besides, the Tafel slope is 80 mV/dec for P1, 89 mV/dec for P2, which still much better than pure CoS2. The electrochemical impedance spectroscopy is tested to characterize the impedance properties of catalysts. As shown in Fig. 3c, it is obviously that the properties of samples after P-doping are improved especially for sample P3. The circuit in Fig. 3c can be used to simulate its resistance properties. The resistance is related to the interface matching of the sample and the electrolyte. Rct reflects the electrocatalytic kinetics, and a lower value of Rct indicates a faster reaction rate [45]. Fig. 3f reflects the Rct value for each sample, and the Rct value of sample P3 is 12.71 ohm, which is smaller than the Rct of sample P1 ~36.53 ohm P2 (~27.71 ohm) and pure CoS2 (~138.2 ohm). The sample P3 has the fastest reaction rate owning to its the smallest Rct value among all samples, which reveals the superior performance of P3 samples once again. Other electrocatalytic parameters for P doped CoS2 catalysts are shown in Table 1.

Fig. 3. (a) Polarization curves of pure CoS2 and P-doped CoS2 in 0.5 M H2SO4 at a scan rate of 2 mV/s-1. (b) Corresponding Tafel plots obtained using slow-scan rate polarization curves. (c) Electrochemical impedance spectra of different electrodes at -0.28 V versus RHE. (d) Plot of charge transport resistance of different samples.

ACCEPTED MANUSCRIPT Table 1. Electro-catalytic parameters for samples of pure CoS2 and P doped CoS2 catalysts. η at J = 100 mA/cm2 [mV]

Tafel slope (mV/dec)

Exchange current density2 (mA/cm )

Potential at the TOF of 0.725 s-1 (mV)

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239

365

105

0.036

307

P1

169

301

89

0.156

236

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144

242

80

0.489

189

P3

53

118

57

0.546

81

Pt/C(20%)

10

97

30.6

0.710

0

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Catalyst

η at J = 10 mA/cm2 [mV]

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Besides, to evaluate the effective electrochemical surfaces of the catalysts, the electrochemical double layer capacitance (Cdl) [46] is measured and the results are shown in Fig. 4a. The Cdl is calculated to be 49 mF/cm2 for P3, close to ten times of CoS2 (~5 mF/cm2). Moreover, the turn over frequency (TOF) for each active site is estimated using the methods reported previously to further investigate the intrinsic catalytic of P3. As the most active catalyst, Pt/C shows a TOF of 0.725 s-1 at η = 0 mV [47]. To achieve a TOF of 0.752 s-1, P3 needs an overpotential of about 87 mV, much smaller than that CoS2 (~ 276 mV). To reach a TOF of 5 s-1, the P3 electrode needs an overpotential of 234 mV, smaller than P2 (335 mV), P1 (407 mV) and CoS2 (486 mV). Finally, catalytic stability is measured by two methods. We can see the LSV curves almost coincide with the previous before 10000 CV scanning at a scan rate of 100 mV/s within the potential from 0 to 0.2 V versus RHE. In Fig. 4d, under constant potential operation of 200 mV vs. RHE, the current density reduction of P3 is less than 1% after 80000 s. In contrast, the current density of pure CoS2, P1 and P2 decreased by 78 %, 13 %, 5 % at the same test condition, revealing the perfect stability of catalyst P3.

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Fig. 4. (a) The capacitive current density as a function of scan rate for pure and P doped CoS2 catalysts. (b) Turn over frequency curves of all samples normalized by the active sites. (c) Polarization curves of all the catalysts before and after 10000 CV cycles. (d) Time-dependent current density of pure and P doped CoS2 catalysts .

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Xie et al. synthesized P doped molybdenum dioxide on Mo foil by using hydrothermal reaction and post-sintering. The MoO2Px/Mo electrode showed overpotential of 135 mV at current density of 10 mA/cm2, Tafel slope of 62 mV/dec [48]. Phosphorus-Mo2C@carbon nanowires were also synthesized, where the optimized catalyst of [email protected] needs an overpotential of 89 mV to reach a current density of 10 mA/cm2, it has a small Tafel slope of 42 mV/dec and a electrochemical capacitance of 15.6 mF/cm2 [49]. Ouyang et al. fabricated P doped CoS2 through a low temperature method including a series procedures. Its overpotential is 67 mV at the current density of 10 mA/cm2 and the Tafel slope is about 58 mV/dec [22]. For our optimized P doped CoS2, the overpotential is about 53 mV at current density of 10 mA/cm2, Tafel slope of 57 mV/dec, a electrochemical capacitance of 49 mF/cm2, which is superior or comparable to that of the P doped catalysts mentioned above. The CoPS (P:S=1:1) catalyst (overpotential is about 61 mV at current density of 10 mA/cm2, Tafel slope is 48 mV/dec.) and Se doped Ni2P (overpotential is about 102 mV at current density of 10 mA/cm2, Tafel slope is 42 mV/dec) [50,51], whose hydrogen evolution efficiency is similar to our optimized sample. In contrast, P doped CoS2 catalysts in this work can be obtained only by a sample one-step hydrothermal method, and its catalytic efficiency is excellent, revealing that P doped CoS2 catalysts can be the good potential catalysts for HER. Further To understand the effects of P dopants on the outstanding HER catalytic efficacies of the CoS2, density functional theory (DFT) simulations are carried out. It is proposed that hydrogen evolution activity of the catalysts is strongly correlated with the chemisorption energy of atomic hydrogen to the electrocatalyst surface [52],

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therefore, we here calculate the free energy for atomic hydrogen adsorption (∆G∗ ) on the [001] surface of P doped CoS2 and the main results are shown in Fig. 5. It can be seen that the ∆G∗ for hydrogen adsorption at the Co sites of P doped CoS2 (-0.3 eV) shows a slight decrease compare to that of pure CoS2 (0.41 eV). At the same time, the P dopants show the calculated ∆G∗ of 0.2 eV. Moreover, the ∆G∗ at the adjacent Co sites shows the variation of further decreasing to 0.15 eV with hydrogen adsorption at the doped P sites. As we all know that the smaller |∆G∗ | enables a better activity toward HER, and an optimal HER activity can be achieved at |∆G∗ |  0 eV due to the balanced proton reduction rate and the removal of adsorbed hydrogen from the catalyst surface. Our calculation results indicate that the H binding goes from being too weakly bound (CoS2) to being very strongly bound (P doped CoS2) at Co sites, which may be caused by the valence state changing to form the P-Co bond. Compared to the Co sites in P doped CoS2, P dopants can be the new active site with the calculated ∆G∗ of 0.2 eV, which may enhance the HER performance of the P doped CoS2. At the same time, owing to the formed P-H band, P atom provide less electrons to adjacent Co atoms. Therefore, under the condition of P site occupied H, the valence state of adjacent Co atoms are not the same as P doped CoS2, but similar to the pure CoS2, which corresponding shows the ∆G∗ of 0.15 eV with hydrogen adsorption at the doped P sites. These results indicate that P dopants not are the new active sites, but also can further activate the adjacent Co atoms, which is well agree with the experimental results of boserving the excellent HER performance in P doped CoS2 catalysts..

Fig. 5. Free-energy diagram for H∗ adsorption at the Co site on the [001] surface of CoS2, and at the Co site, P site, and Co site after H∗ at P site on the [001] surface of P doped CoS2 and their optimal atomic structures.

Further, systematic density functional theory is firstly carried out to determine the influences of P dopants on the electronic structure of CoS2, because the conductive of the catalyst is also an important issuse to determine their catalytic activity. Here, a 2×2×2 supercell of CoS2 and P doped CoS2 (one S atom replaced by

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one P atom) to study their electronic structures. The calculated electron bands of CoS2 and P doped CoS2 shown in Figs. 6c and 6d indicate that there is no band gap exist in P doped CoS2, similar to pure CoS2, revealing its metallic nature, which would facilitate the electron transfer in catalysis during the catalytic reaction. Moreover, the partial charge density of the P-doped CoS2 for the bands near Fermi level is shown in Fig. 6e to visualize the distribution of conducting charges over the material. The conducting charges uniformly spread over the bulk P doped CoS2, which can effectively facilitate the charge transfer over the catalysts and the electrode and thereby enhance the catalytic activity in HER.

Fig. 6. The optimized atomic structures of (a) CoS2 and (b) P doped CoS2 with the 2×2×2 supercell. The calculated electronic bands of (c) CoS2 and (d) P doped CoS2, respectively. (e) The calculated density of states (DOS) plots of the bulk CoS2 and P doped CoS2 and the partial charge density for P doped CoS2 with the isosurface value of 0.01 e/bohr3.

4. Conclusions

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In summary, P-doped CoS2 catalysts are successfully prepared by simple one-step hydrothermal method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy are used to characterize the composition, structure and morphology of samples. The electrochemical measurement results show that P dopants could greatly improve the hydrogen evolution reaction activity efficiency of cobalt disulfide. The best performance P-doped CoS2 catalyst in this experiment demands an overpotential of only 46 mV at the current density of 10 mA/cm2 in 0.5 M H2SO4 solution. Meanwhile it has a small Tafel slope of 53 mV/dec and the outstanding stability at an overpotential of 200 mV for 80000 s. These excellent properties are derived from the P dopants. According to theoretical analysis, the ∆G∗ for hydrogen adsorption at the adjacent Co sites decreases to 0.15 eV from 0.41 eV (Pure CoS2) after P doping. At the same time, the doped P atoms could be the new activate sites. The P doped CoS2 maintains its metallic nature because the band gap does not appear in P doped CoS2 compared with pure CoS2, which would be beneficial for the electron transfer in catalysis during the catalytic reaction. In this work we obtained the theoretical support that P doping can improve the performance of CoS2, and selecting the optimizing P-contents in series of P doped CoS2 catalysts. We believe the more excellent catalysts will be found under the guidance of the theoretical calculation. Acknowledgments

References

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This work is supported by the National Natural Science Foundation of China (Grant No. 11474137 and 21571089), the Fundamental Research Funds for the Central Universities (GrantNo.lzujbky-2014-27, No.lzujbky-2016-130 and lzujbky-2016-k02) and the China Scholarship Council (CSC).

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ACCEPTED MANUSCRIPT Facile One-step Synthesis of Phosphorus-doped CoS2 as Efficient Electrocatalyst for Hydrogen Evolution Reaction

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Jingyan Zhang, Yuchan Liu, Baorui Xia, Changqi Sun, Yonggang Liu, Peitao Liu, Daqiang Gao*,

Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University,

* Corresponding author : [email protected]

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Lanzhou 730000, P. R. China

P doped CoS2 catalysts are prepared by a simple one-step hydrothermal method.

2.

P dopants promote electron transfer and activate more HER catalytic sites in

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1.

CoS2.

The ∆Gୌ∗ at the Co site after P doped (0.15 eV) is lower than that of pure CoS2

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(0.41 eV).

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3.

1