Novel Cobalt-Doped Ni0.85Se Chalcogenides (CoxNi0.85âxSe) as

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Energy, Environmental, and Catalysis Applications

Novel Cobalt-doped Ni0.85Se Chalcogenides (CoxNi0.85xSe) as High Active and Stable Electrocatalysts for Hydrogen Evolution Reaction in Electrolysis Water Splitting Wenjun Zhao, Shiquan Wang, Chuanqi Feng, Huimin Wu, Lei Zhang, and Jiujun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12797 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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ACS Applied Materials & Interfaces

Novel Cobalt-doped Ni0.85Se Chalcogenides (CoxNi0.85-xSe) as High Active and Stable Electrocatalysts for Hydrogen Evolution Reaction in Electrolysis Water Splitting Wenjun Zhaoa, Shiquan Wanga, Chuanqi Fenga, Huimin Wu*,a, Lei Zhang*,b,c, Jiujun Zhangc aHubei

Collaborative Innovation Center for Advanced Organic Chemical Materials & Key Laboratory

for Green Preparation and Application for Functional Materials, Ministry of Education & College of Chemistry & Chemical Engineering, Hubei University, Wuhan 430062, PR China bInstitute

for Sustainable Energy/College of Sciences, Shanghai University, Baoshan, Shanghai,

200444, China cEnergy,

Mining and Environment, National Research Council of Canada,Vancouver,BC,V6T1W5,

Canada * Corresponding author. Tel.: 86 18971479006; Fax: 86 27 88663043. E-mail address: [email protected]; [email protected]

1

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: In this paper, novel cobalt-doped Ni0.85Se chalcogenides (CoxNi0.85-xSe, x=0.05, 0.1, 0.2, 0.3, 0.4) are successfully synthesized and studied as high active and stable electrocatalysts for hydrogen evolution reaction (HER) in electrolysis water splitting. The morphologies, structures and composition of these as-prepared catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy. The electrochemical tests, such as linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry testing, are performed to evaluate these catalysts’ HER catalytic performance including activity and stability. The results indicate that a suitable doping can result in synergetic effect for increasing the catalytic performance. Among different catalysts, Co0.1Ni0.75Se shows the highest HER performance. After introducing the reduced graphene oxide into this catalyst as the support, the resulted Co0.1Ni0.75Se/rGO shows even better performance than unsupported Co0.1Ni0.75Se, which are confirmed by the reduction of HER overpotential of Co0.1Ni0.75Se/rGO to 103 mV compared to 153 mV of Co0.1Ni0.75Se at a current density of 10 mA/cm2, and the smaller Tafel slope (43 mV/dec) and kinetic resistance (21.34 Ω) than those of Co0.1Ni0.75Se (47 mV/dec, 30.23 Ω). Furthermore, the large electrochemical active surface area and high conductivity of such a Co0.1Ni0.75Se/rGO catalyst, induced by r-GO introduction, are confirmed to be responsible for the high HER performance. Keywords: Hydrothermal method; Reduced graphene oxide; Cobalt doping; Hydrogen evolution reaction; Electrocatalysis 2


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1. Introduction With the increased environment impact of fossil fuel energy usages, the research for alternative clean energy has become intensified in today’s world

1-3.

Hydrogen (H2)

generated from electrolysis water splitting by sustainable energy sources such as solar and wind has been considered to be the eternal fuel to replace fossil ones due to its high energy density and environmentally-friendly

3-5.

For electrolysis water splitting, Pt or

Pt-based materials have been known as the most active electrode (cathode) electrocatalysts for hydrogen evolution reaction (HER)

6-8.

However, because of

high-cost and scarcity of the Pt-based materials, their practical applications are severely limited. Therefore, finding non-precious metal catalysts to replace Pt is imminent 9-11. Regarding non-precious metal catalysts, transition metal dichalcogenides (TMDs), such as MoS2, WS2, MoSe2, WSe2 etc. in catalytic fields about a decade ago

12-16

have been known as the promising materials

17-18.

Among them, MoS2 and WS2 have been

demonstrated to have catalytic activities close to Pt-based materials

19-21.

Recently,

MoSe2 and WSe2 have also explored as HER electrocatalysts for water electrolysis splitting transition

22-24.

For example, Nørskov et al.

25

studied these two catalysts, and

showed that the edge sites of MoSe2 and WSe2 were HER active, and their activities were comparable or higher than those of both MoS2 and WS2. Furthermore, CoSe2 and NiSe2 were also considered to be promising candidates for HER, which exhibited high HER catalytic performance

26-31.

Unfortunately, there still exist gaps between these selenides

and Pt-based materials in catalytic effect. Therefore, further work that can significantly improve their catalytic performance needs to be carried out. With respect to this, heteroatom doping has been considered to be an effective way to improve the 3



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performance of catalysts through adjusting their electronic properties or surface structures 32-35.

For example, Zhang et al.

36

reported cobalt doped WSe2 catalyst, and showed that

the introduction of cobalt atom could change the electronic structure of WSe2, then reduce the overpotential of Co-doped WSe2 electrode, resulting in a higher HER activity. Some heteroatom-doped catalysts with excellent electrocatalytic performances have also been reported, which contributed to the development of HER catalysts 37-39. Furthermore, compositing supporting substrate with the catalyst can also promote the catalytic activity by enhancing active area and conductivity

40-41.

For instance, carbon

nanotubes and graphene oxide (GO) are appropriate conductive substrates 42. When these advanced carbon materials are combined with TMD catalysts, the supported catalysts 43.

formed can show enhanced HER activities

Particularly, graphene with single atomic

layer of sp2 carbon atoms has large theoretical surface area, excellent electrical conductivity and rapid charge mobility

44.

Some studies confirmed that the catalysts

composited with graphene can have significantly enhanced catalytic performance

45-46.

Therefore, graphene (graphene oxide) should be a rational supporting substrate for HER catalysts. Due to above analysis, the introduction of heteroatom and r-GO may increase the HER activities of TMD catalysts. Therefore, we speculated that doping cobalt atom into nickel selenide may improve its HER activities by regulating electronic properties or surface structures of nickel selenide. If r-GO was further introduced on this basis, the HER activities should be further improved because of the enhanced conductivity. In this paper, we prepare CoxNi0.85-xSe catalysts with different cobalt doping ratios for HER catalysts. Particularly, a reduced graphene oxide (r-GO) supported catalyst ( Co0.1Ni0.75Se/rGO catalyst) is also synthesized and deeply studied. Experiment results 4


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show that owing to the electronic effects introduced by cobalt atom, the catalytic activities of CoxNi0.85-xSe are higher than the original Ni0.85Se, and related to the doping ratios of cobalt. Among different Co-doped catalysts (CoxNi0.85-xSe), Co0.1Ni0.75Se shows the highest catalytic activity towards HER. To further improve Co0.1Ni0.75Se’s HER activity, we synthesize Co0.1Ni0.75Se with large specific surface area and high conductivity r-GO as support to obtained Co0.1Ni0.75Se/rGO catalyst. Experimental results show that the r-GO supporting can make Co0.1Ni0.75Se/rGO having less prone to aggregation and more exposed electrochemical active sites. Therefore, the HER performance of Co0.1Ni0.75Se/rGO is significantly improved, far more better than Co0.1Ni0.75Se.

2. Experimental 2.1. Chemical reagents Graphene oxide (GO) sheets were provided by XFNANO Materials Tech Co., Ltd (Nanjing city, China). Sodium selenite (Na2SeO3) was purchased from Xiya Reagent Research Center. The other reagents were supplied by Sinopharm Chemical Reagent Co., Ltd. All of them were analytical reagent grade and used without any further purification. Ultrapure water (18.25 mΩ/cm) manufactured by Aquapro water system was used throughout the experiments. 2.2. Synthetic method A one-step hydrothermal method was used to synthesize CoxNi0.85-xSe catalyst samples, where the x value is adjustable and represents different cobalt doping ratios (x (atom ratio) = 0.05, 0.1, 0.2, 0.3, 0.4). First, 0.238 g NiCl2·6H2O and different amounts of 5



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Co(NO3)2·6H2O (Table 1) were dissolved in 6 mL H2O with magnetic stirring for 30 minutes, then 0.845 g Na2SeO3, 24 mL ethanolamine (EA) and 8.5 mL N2H4•H2O were slowly injected into the mixture solutions, separately with a continuing stirring till homogeneous solutions were formed. Then the above solutions were separately poured into 100 mL Teflon-lined stainless-steel autoclaves and heated at 140 °C for 24 hours. The formed products were marked as Co0.05Ni0.8Se, Co0.1Ni0.75Se, Co0.2Ni0.65Se, Co0.3Ni0.55Se, Co0.4Ni0.45Se, respectively. For comparison, Ni0.85Se was also synthesized under the same conditions without adding Co(NO3)2·6H2O. The synthesis of Co0.1Ni0.75Se used a similar method described above. For Co0.1Ni0.75Se/rGO synthesis, first, 17 mg GO was dispersed in 10 mL H2O, and sonicated it into a homogeneous dispersion. Then this dispersion was slowly added into the already synthesized Co0.1Ni0.75Se homogeneous solution with stirring till a uniform dispersion was formed. The subsequent steps were the same as before for unsupported CoxNi0.85-xSe synthesis to form Co0.1Ni0.75Se/rGO catalyst sample. For comparison, Ni0.85Se/rGO without Co atoms was also synthesized under the same conditions. Material

doping ratio

Co(NO3)2·6H2O

NiCl2·6H2O

number

(At%)

(g)

(g)

Co0.05Ni0.8Se

5.9

0.018

0.238

0.845

Co0.1Ni0.75Se

11.8

0.039

0.238

0.845

Co0.2Ni0.65Se

23.5

0.09

0.238

0.845

Co0.3Ni0.55Se

35.3

0.159

0.238

0.845

Co0.4Ni0.45Se

88.9

0.259

0.238

0.845

Na2SeO3 (g)

Table 1 Different amounts of Co(NO3)2·6H2O added in the synthesis reaction.

6


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2.4. Characterization X-ray diffraction (XRD) patterns were collected on a GBC MMA X-ray diffractometer with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB

250Xi

device.

Transmission

electron

microscopy

(TEM)

and

energy-dispersive spectroscopy (EDS) were respectively performed on a Tecnai G2 F30 instrument and a JSM6510LV instrument. Raman spectra of the catalyst samples were recorded on a Jobin Yvon Horbia Confocal Micro Raman Spectrometer Model HR800 with a 532 nm diode laser excitation on a 300 lines/mm grating at room temperature. CHI 750E electrochemical workstation (CH Instrument Company, Shanghai, China) was employed for electrochemical measurements. All tests were carried out using a conventional three-electrode system with 0.5 M H2SO4 as the electrolyte solution. Saturated calomel electrode (SCE) and carbon rod were used as reference electrode and counter electrode. Working electrodes were made of the as-prepared catalysts. The catalyst inks were prepared by adding 5 mg corresponding catalyst to 0.5 mL absolute ethanol with 35 µL Nafion (5%). For individual electrode preparation, the ink was drop-casted onto a glass carbon electrode (GCE, diameter 3 mm). Current densities were calculated based on the geometric area (0.07 cm2). All of the measured potentials were converted to reversible hydrogen electrode (RHE) according to the Nernst equation (ERHE (V) = ESCE + 0.059 pH+ 0.241).

3. Results and discussion 3.1. Physical characterization Fig. 1 presents the preparation schematic of Co0.1Ni0.75Se/rGO though a hydrothermal 7



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method. The crystalline properties of as-prepared catalysts were investigated by XRD. Fig. 2(A) shows the XRD patterns of (a) Ni0.85Se, (b) Co0.05Ni0.8Se, (c) Co0.1Ni0.75Se, (d) Co0.2Ni0.65Se, (e) Co0.3Ni0.55Se, (f) Co0.4Ni0.45Se and (g) Co0.1Ni0.75Se/rGO. The prepared Ni0.85Se shows the peaks at 33.4°, 44.8°, 50.8°, 60.1°, 62.1°, 70.1°, corresponding to the (101), (102), (110), (103), (201), (202) lattice planes, respectively, well agreeing with the typical hexagonal structure of Co0.1Ni0.75Se (JCPDS No.18-0888)

28.

However, after

doping cobalt, the peaks of CoxNi0.85-xSe are shifted slightly. The degrees of deviation may be related to the amount of cobalt

47.

No additional peaks of other phases can

observed, confirming the successfully partial replacement of Ni by Co in Ni0.85Se lattice 48.

Moreover, no Co-Ni-Se ternary phase can be found, probably because the nanocrystal

was smaller or the phases are overlapped with Ni0.85Se phase 49. Fig. 2A also shows that after introducing rGO, there is no apparent peak of rGO in Co0.1Ni0.75Se/rGO pattern. This may be due to its low crystallinity, resulting in relatively low diffraction intensity 50. The slight shift of peak position may be related to the presence of r-GO

46.

In

electrochemical measurements, Co0.1Ni0.75Se/rGO catalyst exhibits better HER catalytic performance than other ones, as shown in Fig. S1.

Fig. 1. Schematic illustration for the preparation of Co0.1Ni0.75Se/rGO catalyst. 8


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(c) (d) (e) (f) (g)

Ni-Se Se-Se

Intensity (a.u.)

Intensity (a.u.)

(b)

Co-Se

(202)

(103) (201)

(102)

(a)

(110)

(B)

(101)

(A)

G

D

Co0.1Ni0.75Se/rGO Co0.1Ni0.75Se

Ni0.85Se JCPDS 18-0888

10

20

30

40

50

60

70

2 Theta (degree)

(C)

500

80 Ni 2p

Ni 2p1/2 satellite

880

870

860

(E)

810

Se 3d

60

58

56

54

Binding Energy (eV)

52

805

800

795

790

785

Binding Energy (eV)

780

(F)

775 C 1s

C-C=C

SeOX

62

2500 Co 2p

Intensity (a.u.)

Se 3d3/2 Se 3d5/2

2000

Co 2p1/2

850

Binding Energy (eV)

1500

Co 2p3/2

Intensity (a.u.)

Intensity (a.u.)

satellite

1000

Raman shift (cm-1)

(D)

Ni 2p3/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

294

C-O O-C=O

292

290

C=O

288

286

284

Binding Energy (eV)

282

280

Fig. 2. (A) XRD patterns of (a) Ni0.85Se, (b) Co0.05Ni0.8Se, (c) Co0.1Ni0.75Se, (d) Co0.2Ni0.65Se, (e) Co0.3Ni0.55Se, (f) Co0.4Ni0.45Se, and (g) Co0.1Ni0.75Se/rGO; (B) Raman spectras of Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO; XPS high-resolution spectras of (C) Ni 2p, (D) Co 2p, (E) Se 3d, and (F) C1s for Co0.1Ni0.75Se/rGO.

9



Fig. 2(B) presents the Raman spectra of Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO. The peaks at 271 cm-1 can be indexed to the Se-Se bending mode 51. The other two peaks at 186 cm-1 and 510 cm-1 belong to Ni-Se and Co-Se, respectively 52-53, which demonstrates that cobalt has been successfully doped into the compound. In addition, the Co0.1Ni0.75Se/rGO has other two special peaks at 1346 cm-1 and 1587 cm-1, which can be attributed to the D band and G band of rGO, respectively 53. While the Raman spectra of Co0.1Ni0.75Se do not show the two peaks. This result indicates that rGO is successfully introduced into Co0.1Ni0.75Se/rGO. To investigate the chemical compositions and valence states of catalysts, the XPS measurements were used in experiment. Fig. S2 shows the surface survey XPS spectra of Co0.1Ni0.75Se/rGO, indicating the existence of Co, Ni, Se, C elements in Co0.1Ni0.75Se/rGO. The peaks at 873.3 eV and 854.1 eV in Fig. 2(C) corresponded to Ni2+. The Ni 2p1/2 at 874.1 eV and Ni 2p3/2 at 856.4 eV may belong to Ni3+ from the surface oxide phase. The two satellite peaks at 879.8 eV and 861.6 eV were oxidation state of Ni2+

26.

Compared with pure binary selenide Ni0.85Se, the binding energy of Ni 2p after

cobalt doping is shifted slightly, as shown in Fig. S3(A). Fig. 2(D) presents Co 2p containing two major peaks. The Co 2p1/2 at 794.9 eV and Co 2p3/2 at 778.5 eV are most likely corresponding to Co3+. The Co 2p1/2 at 800.6 eV and Co 2p3/2 at 781.5 eV are most likely corresponding to Co2+

37, 46, 54-55.

The binding energy of Co 2p is also shifted

slightly compared to the pure Co0.85Se (Co 2p1/2 at 797.28 eV, Co 2p3/2 at 781.23 eV, respectively) 56. Se 3d5/2 and Se 3d3/2 located at 53.5 eV and 54.3 eV in Fig. 2(E) can be attributed to Se2- 57. Its binding energy is lower than that of Ni0.85Se, as indicated in Fig. S3(B). This may be due to the fact that selenium is more electronegative. Charge transfer 10


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from nickel and cobalt to selenium may result in chemical shifts. As a result, nickel and cobalt are slightly moved to higher binding energy, and the selenium is transferred to lower binding energy. This result indicates that Co0.1Ni0.75Se is a complete ternary selenide

58.

The other peak at 58.1 eV can be ascribed to surface oxidation SeOX. Fig.

S3(C) presents the survey spectra of Ni0.85Se, no Co 2p peak was found compared with that of Co0.1Ni0.75Se/rGO. Fig. 2(F) shows C 1s spectra, the peaks at 284.2 eV, 285.3 eV, 285.9 eV, and 288.3 eV are assigned to C-C=C, C-O, C=O, O-C=O, respectively. Comparing with the rGO data reported in previous study

52,

this result indicates the

successful formation of Co0.1Ni0.75Se/rGO. The XPS spectra of Co0.1Ni0.75Se were also collected, as shown in Fig. S4. The high-resolution spectra of Ni 2p, Co 2p and Se 3d are slightly deviated from that of Co0.1Ni0.75Se/rGO, indicating the presence of rGO. The morphology of Co0.1Ni0.75Se/rGO was also characterized by transmission electron microscopy (TEM). As shown in Fig. 3(A), the Co0.1Ni0.75Se nanoparticles are uniformly distributed on rGO without aggregation. This uniform distribution may mean the full exposure of catalytic active sites. However, Co0.1Ni0.75Se nanoparticles without rGO have a serious aggregation, as shown in Fig. S5(A), which can hinder the exposure of active sites. In addition, Fig. S7 shows the size distributions of Ni0.85Se, Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO. Co0.1Ni0.75Se nanoparticles have a smaller particle size than Ni0.85Se, which may be due to the doping of Co atom. The particle size of Co0.1Ni0.75Se/rGO is smaller than that of Co0.1Ni0.75Se and Ni0.85Se. This may be related to the support of rGO and indicates that rGO is key to confine the growth aggregation of particles and further obtain the small-sized Co0.1Ni0.75Se/rGO. Fig. 3(B) presents the high-resolution TEM (HRTEM) image of Co0.1Ni0.75Se/rGO. The distance between lattice fringes can be 11



respectively found to be 0.29 nm, 0.21nm, and 0.18 nm, which are related to the (101), (102), and (110) planes of Ni0.85Se (JCPDS No.18-0888), respectively, and consistent with the XRD results. The mapping of Co0.1Ni0.75Se/rGO testifies that all the elements have a homogeneous distribution in the catalyst, as shown in Figure 3(C) (D) (E) (F) (Fig. S5(B) shows the original SEM image). In addition, EDS analysis validates the elements atom ratio of Co0.1Ni0.75Se/rGO catalyst (shown in Table S1), which futher proves the existence of Co, Ni, Se, C, and O elements in the catalyst. O element may arise from surface oxidation of the catalyst 59. According to the analysis, the doping ratio of cobalt atom is 11% approximately in the Co0.1Ni0.75Se/rGO catalyst. This result is consistent with XPS and very close to the design results (11.8%, Table 1), indicating that the Co0.1Ni0.75Se/rGO catalyst was successfully prepared. The XRD results suggest that no additional peaks of other phases can observed, confirming the partial replacement of Ni by Co in Ni0.85Se lattice. The XPS results suggest that Co exists in Co0.1Ni0.75Se and the EDS mapping indicated that Co0.1Ni0.75Se is compound not mixed 46, 55, 60.

12


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Fig. 3. (A) TEM image, (B) HRTEM image, (C) Ni, (D) Se, (E) Co, and (F) C mapping images of Co0.1Ni0.75Se/rGO catalyst.

13



3.2. Electrochemical characterization The HER activities of as-prepared catalysts were measured by electrochemical experiments in 0.5 M H2SO4 solution. Through the test of linear sweep voltammetry (LSV), the HER overpotentials (η) of catalysts were directly compared. Normally, the lower overpotential corresponds to higher HER activity at the same current density ( jA) 21.

LSV curves of CoxNi0.85-xSe (x = 0.05, 0.1, 0.2, 0.3, 0.4, respectively) are presented in

Fig. S1(A). It can be clearly seen from the curves that these catalysts have various overpotentials but lower than the original Ni0.85Se. This may be related to different amounts of cobalt doping in the CoxNi0.85-xSe catalysts. The different amounts of Co in CoxNi0.85-xSe lead to the difference of their chemical composition, their surface structure and particle size may vary, which may be the reason for their different electrocatalytic properties. When current density jA is reached to -10 mA/cm2, Co0.1Ni0.75Se shows a lower η value than other CoxNi0.85-xSe catalysts, showing a higher HER activity. This means that they behave higher HER activity. Moreover, the η of Co0.1Ni0.75Se/rGO (103 mV) is obviously lower than that of Co0.1Ni0.75Se (153 mV) (Fig. 4(A)), suggesting that under the synergistic effect of rGO, Co0.1Ni0.75Se/rGO has a better HER activity than Co0.1Ni0.75Se. Although the overpotential of Pt/C is the lowest, that of the Co0.1Ni0.75Se/rGO is very close to it. To do a clear comparison, other noble metal-free electrocatalysts under the same conditions were also summarized in Table S2. It can be clearly seen that Co0.1Ni0.75Se/rGO shows a remarkable HER activity among those other catalysts. Tafel plots derived from LSV curves can evaluate the HER mechanism. The lower Tafel slope indicates that as overpotential increases, HER current increases more sharply, 14


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resulting in better HER performance

61.

The HER mechanism usually involves three

major reactions in acidic media (Equations 1~3) 62: H3O+ + e- ⇌ m-H +H2O

(Volmer reaction, 120 mV/dec)

H3O+ + e- + m-H ⇌ H2 + m + H2O 2 m-H ⇌ H2

(1)

(Heyrovsky reaction, 40 mV/dec)

(Tafel reaction, 30 mV/dec)

(2)

(3)

where e- represents electrons, m-H represents a hydrogen atom adsorbed onto catalyst m, and H2 represents the generated hydrogen molecule. According to Tafel equation (η = b log jA + a, and b is the Tafel slope), the Tafel slopes of Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO are 47 mV/dec and 43 mV/dec, respectively, lower than that of Ni0.85Se indicated in Fig. 4(B). This result suggests that these two catalysts follow the Volmer–Heyrovsky reaction during the HER process and the rate determining step is the slow hydronium ion combining with hydrogen atom reaction 63. Co0.1Ni0.75Se/rGO has a lower Tafel slope than all other catalysts (Fig. S1(B) and Table 2) and only a little higher than Pt/C (40 mV/dec), indicating its fast kinetics and excellent HER catalytic activity. The rapid HER process can lead to the reaction with a lower overpotential, being consistent with the result of LSV test. Electrochemical impedance spectroscopy (EIS) was performed at an overpotential of 210 mV to probe the HER catalytic kinetics

64.

Fig. 4(C) shows the Nyquist plots of

Ni0.85Se, Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO, respectively, with an equivalent circuit (shown in inset). The respective semicircle diameters are corresponded to the charge transfer resistances (Rct), which can evaluate the charge transfer limited process. A smaller diameter of semicircle represents a lower Rct for the HER. The Rct values of Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO are 21.34 Ω and 30.23 Ω, respectively, lower than 15



those of Ni0.85Se (48.24 Ω) and other as-prepared catalysts, as shown in Fig. S1(C) and Table 2. This implies that Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO have faster electron transfer rates than Ni0.85Se. This can be attributed to the electronic effect induced by the incorporation of cobalt, which can result in lower resistance. Furthermore, Co0.1Ni0.75Se/rGO exhibits even lower resistance than Co0.1Ni0.75Se. This should be attributed to the increased conductivity of rGO. The low resistance of Co0.1Ni0.75Se/rGO suggests its fast charge transfer and high HER kinetics.

-10

Ni0.85Se Co0.1Ni0.75Se

jA (mA/cm2)

-20

0.20

-30 -40 -50 -60 Co0.1Ni0.75Se/rGO

-70 -80

40

Pt/C

-0.3

-0.2

-0.1

43 mV/dec

0.15

Co0.1Ni0.75Se

0.05

Co0.1Ni0.75Se/rGO Pt/C

0.2

j (mA/cm2)

Ni0.85Se

10

0.6

j (mA/cm2)

0.8

1.0

1.2

2

1.71 mF/cm Ni0.85Se

2

1.3 mF/cm

Co0.1Ni0.75Se

12

0

0.4

(D)

15

Co0.1Ni0.75Se/rGO

20

40 mV/dec

Ni0.85Se

18

Co0.1Ni0.75Se

47 mV/dec

57 mV/dec

0.10

(C)

30

(B)

0.00 0.0

0.0

E (V vs. RHE)

Overpotential (V vs. RHE)

0 (A)

-Z'' / ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

Co0.1Ni0.75Se/rGO 2

0.53 mF/cm

9 6 3

0

10

20

30

40

Z' / ohm

50

60

70

2

4

16


6

8

Scan rate (mV/s)

10

Page 17 of 33

-8

0

(E) Co0.1Ni0.75Se/rGO

-10

-10

-12 -14

-12.58

-16

-12.60

5

10

Initial of Co0.1Ni0.75Se/rGO

-60

9.4

0

After 5000 cycles of Co0.1Ni0.75Se

-50

-12.66

-22

Initial of Co0.1Ni0.75Se

-40

-12.64

-20

(F)

-30

Bubble release Bubble accumulation

-12.62

-18

-24

jA (mA/cm2)

-20

jA (mA/cm2)

jA (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9.5

9.6

9.7

9.8

Time (h)

15

20

9.9

25

After 5000 cycles of Co0.1Ni0.75Se/rGO

-70

10.0 10.1

-80

30

-0.3

-0.2

-0.1

0.0

E (V vs. RHE)

Time (h)

Fig. 4. (A) LSV curves and (B) Tafel plots for Ni0.85Se, Co0.1Ni0.75Se, Co0.1Ni0.75Se/rGO and Pt/C; (C) Nyquist plots with an equivalent circuit (inset) of the Ni0.85Se, Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO; (D) Estimated Cdl and relative electrochemically active surface areas for Ni0.85Se, Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO; (E) i-t testing under static potential of 0.15 V vs. RHE. Inset is the enlargement of the area denoted by the rectangle; (F) LSV curves for Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO before and after 5000 cycles in the stability test.

To further understand the enhanced electrocatalytic performance of Co0.1Ni0.75Se/rGO, double layer capacitance (Cdl) was also measured to estimate electrochemical active surface areas (EASA) at the solid-liquid interface. The EASA is linearly proportional to Cdl (Cdl ∝ v × EASA, where v is scan rate)

65.

Larger EASA indicates more exposed

catalytic active sites, which can result in more efficient HER. As shown in Fig. 4(D), The Cdl is measured from double layer charging curves using cyclic voltammetry (CV) in a small potential range (0 to 0.2 V) (Fig. S6(A)). As the scan rate is increased from 2 to 10 mV/s, the corresponding currents are also increased. The relationships between the current and scan rate in this region are linear for all catalysts (other CVs are not presented 17



here). By calculating the slopes in Fig. 4(D), the Cdl of Co0.1Ni0.75Se/rGO is 1.71 mF/cm2, remarkably higher than Co0.1Ni0.75Se (1.3 mF/cm2) and Ni0.85Se (0.53 mF/cm2). This suggests that Co0.1Ni0.75Se/rGO can offer more active sites, which is consistent with the results of TEM, and greatly contribute to high HER activity. Moreover, the more active sites can lead to higher conductivity, being consistent with the results of EIS test. The results of other as-prepared catalysts are summarized in Fig. S1(D) and Table 2. Besides high catalytic activities, favorable stability towards HER is also critical for an efficient electrocatalyst. Chronoamperometry testing (i-t) at a constant potential (-0.21 V) was conducted to evaluate the stability of Co0.1Ni0.75Se/rGO catalyst, and the result is shown in Fig. 4(E). The current shows a slight drop at the beginning, which may be caused by the consumption of H+ or the remaining of H2 bubbles on the electrode surface 66.

The inset in Fig. 4(E) exhibits the typical processes of H2 bubble accumulation and

release. After 30 hours, the current tends to be stable and basically unchanged. At the same time, it can be seen that the Rct after 30 hours of i-t testing, only has a slightly increase (Fig. S6(B)). This suggests that Co0.1Ni0.75Se/rGO has a good stability. Moreover, the stability was further confirmed by the continuous CV measurement in range of -0.21-0.21 V at 50 mV/s for 5000 cycles. After these CV cycles, the LSV curves were re-recorded. Fig. 4(F) shows the LSV curves of Co0.1Ni0.75Se/rGO at the initial and after 5000 cycles. It can be seen that only 4 mV negatively shift at current density of 10 mA/cm2 can be observed. For comparison, the LSV curve of Co0.1Ni0.75Se was also tested for 5000 cycles. It can be seen that 12 mV of negative shift after 5000 cycles can be found at current density of 10 mA/cm2, indicating a poorer durability than Co0.1Ni0.75Se/rGO. This result reflects the excellent stability of Co0.1Ni0.75Se/rGO 18


Page 18 of 33

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catalyst. Tafel slope

Cdl ( mF/cm2 )

Rct (Ω)

57

0.53

48.24

172

52

1.14

36.90

Co0.05Ni0.8Se

183

63

0.80

46.16

Co0.1Ni0.75Se

153

47

1.30

30.23

Co0.2Ni0.65Se

168

50

1.19

34.58

Co0.3Ni0.55Se

175

61

1.13

39.79

Co0.4Ni0.45Se

178

55

0.85

42.37

Material number

η (mV)

Ni0.85Se

190

Ni0.85Se/rGO

（mV/dec）

Several bimetallic Co–Ni–Se materials have been extensively reported as effective electrocatalysts for water splitting

54-55, 67.

Compared with them, we prepared Co-doped

nickel selenide and composited it with r-GO. The prepared catalyst exhibits considerable electrocatalytic activity and stability. The above experimental results show that appropriate cobalt doping (11% doping ratio) in Ni0.85Se could improve the electrochemical performance by adjusting the electronic properties or surface structure of Ni0.85Se. According to previous reports, Co doping could further lower the kinetic barrier by promoting H-H bond formation on two adjacently adsorbed hydrogen37. This effect make the H2 generation process easier

68.

While the strong chemical and electronic

coupling between rGO and Co0.1Ni0.75Se can not only enhance electronic conductivity, but also increase its active areas 69, allowing more active sites to be exposed, leading to faster and more efficient HER process. As a result, Co0.1Ni0.75Se/rGO catalyst can give a better HER performance. Table 2 HER catalytic performance data of all as-prepared catalysts at 10 mA/cm2.

19



Co0.1Ni0.75Se/rGO

103

43

1.71

Page 20 of 33

21.34

4. Conclusions In this work, CoxNi0.85-xSe with different cobalt doping ratios are synthesized, and their catalytic performances are compared with respect to the hydrogen evolution reaction (HER) in electrolysis water splitting. Experiment results show that after doping cobalt, the CoxNi0.85-xSe catalysts exhibit different physical properties from the original Ni0.85Se, leading to different catalytic performances toward hydrogen evolution reaction (HER) in electrolysis water splitting. Among different catalysts, Co0.1Ni0.75Se give the highest HER catalytic activity. This indicates that a suitable doping can result in the synergetic effect for increasing the catalytic performance. This Co0.1Ni0.75Se catalyst is further studied by introducing the reduced graphene oxide (rGO) to form rGO supported catalyst (Co0.1Ni0.75Se/rGO).

HER tests show that this Co0.1Ni0.75Se/rGO can give a lower

overpotential and smaller Tafel slope than unsupported Co0.1Ni0.75Se catalyst. These may be because the introduction of rGO can effectively prevent Co0.1Ni0.75Se/rGO from aggregating, leading to more active sites exposure than Co0.1Ni0.75Se, and the increased EASA and conductivity. These together can give accelerated electron transfer rate in HER process, resulting in both high catalytic activity and stability.

ASSOCIATED CONTENT

Supporting Information Detailed structural and HER electrocatalytic performance of the prepared catalysts and their counterparts. 20


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Acknowledgements We acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21205030, 51402096), and by key project of Hubei provincial education department (D20171001), and Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices (201710), and (111 project, B12015).

21



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