Novel Cobalt-doped Ni0.85Se Chalcogenides (CoxNi0.85-xSe) as

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 40491−40499

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Novel Cobalt-Doped Ni0.85Se Chalcogenides (CoxNi0.85−xSe) 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 2018.10:40491-40499. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.

†

Hubei 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 ‡ Institute for Sustainable Energy/College of Sciences, Shanghai University, Baoshan, Shanghai 200444, China § Energy, Mining and Environment, National Research Council of Canada, Vancouver, British Columbia V6T1W5, Canada S Supporting Information *

ABSTRACT: In this paper, novel cobalt-doped Ni0.85Se chalcogenides (CoxNi0.85−xSe, x = 0.05, 0.1, 0.2, 0.3, and 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 Xray 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 (rGO) 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 rGO introduction, are confirmed to be responsible for the high HER performance. KEYWORDS: hydrothermal method, reduced graphene oxide, cobalt doping, hydrogen evolution reaction, electrocatalysis

1. INTRODUCTION With the increased environmental 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 because of its high-energy density and environmental friendliness.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 © 2018 American Chemical Society

of the Pt-based materials, their practical applications are severely limited. Therefore, finding nonprecious metal catalysts to replace Pt is imminent.9−11 Regarding nonprecious metal catalysts, transition-metal dichalcogenides (TMDs), such as MoS2, WS2, MoSe2, WSe2, and so forth,12−16 have been known as the promising materials in catalytic fields about a decade ago.17,18 Among them, MoS2 Received: July 28, 2018 Accepted: November 1, 2018 Published: November 1, 2018 40491

DOI: 10.1021/acsami.8b12797 ACS Appl. Mater. Interfaces 2018, 10, 40491−40499

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ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION

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 the 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 performance of catalysts through adjusting their electronic properties or surface structures.32−35 For example, Zhang et al.36 reported cobaltdoped WSe2 catalyst and showed that the introduction of cobalt atom could change the electronic structure of WSe2 and then reduce the overpotential of the 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 formed can show enhanced HER activities.43 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 (GO) should be a rational supporting substrate for HER catalysts. Because of the above analysis, the introduction of heteroatom and rGO 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 rGO 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 GO (rGO)-supported catalyst (Co0.1Ni0.75Se/rGO catalyst) is also synthesized and deeply studied. Experimental results 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 toward HER. To further improve Co0.1Ni0.75Se’s HER activity, we synthesize Co0.1Ni0.75Se with large specific surface area and high conductivity rGO as a support to obtain Co0.1Ni0.75Se/rGO catalyst. Experimental results show that the rGO 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.

2.1. Chemical Reagents. 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 of 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, and 0.4]. First, 0.238 g of NiCl2·6H2O and different amounts of Co(NO3)2·6H2O (Table 1) were dissolved in 6

Table 1. Different Amounts of Co(NO3)2·6H2O Added in the Synthesis Reaction material number

doping ratio (at. %)

Co(NO3)2·6H2O (g)

NiCl2·6H2O (g)

Na2SeO3 (g)

Co0.05Ni0.8Se Co0.1Ni0.75Se Co0.2Ni0.65Se Co0.3Ni0.55Se Co0.4Ni0.45Se

5.9 11.8 23.5 35.3 88.9

0.018 0.039 0.09 0.159 0.259

0.238 0.238 0.238 0.238 0.238

0.845 0.845 0.845 0.845 0.845

mL of H2O under magnetic stirring for 30 minutes; then, 0.845 g of Na2SeO3, 24 mL of ethanolamine, and 8.5 mL of 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 h. The formed products were marked as Co0.05Ni0.8Se, Co0.1Ni0.75Se, Co0.2Ni0.65Se, Co0.3Ni0.55Se, and Co0.4Ni0.45Se. 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 of GO was dispersed in 10 mL of 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 the Co0.1Ni0.75Se/rGO catalyst sample. For comparison, Ni0.85Se/rGO without Co atoms was also synthesized under the same conditions. 2.3. Characterization. X-ray diffraction (XRD) patterns were collected on a GBC MMA X-ray diffractometer with Cu Kα 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 HORIBA Confocal Micro Raman Spectrometer model HR800 with a 532 nm diode laser excitation on a 300 lines/mm grating at room temperature. A 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, respectively. Working electrodes were made of the as-prepared catalysts. The catalyst inks were prepared by adding 5 mg of corresponding catalyst to 0.5 mL of absolute ethanol with 35 μL Nafion (5%). For individual electrode preparation, the ink was drop-casted onto a glass carbon electrode (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). 40492


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

Figure 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 spectra of Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO; and XPS high-resolution spectra of (C) Ni 2p, (D) Co 2p, (E) Se 3d, and (F) C 1s for Co0.1Ni0.75Se/rGO.

3. RESULTS AND DISCUSSION 3.1. Physical Characterization. Figure 1 presents the preparation schematic of Co0.1Ni0.75Se/rGO though a hydrothermal method. The crystalline properties of as-prepared catalysts were investigated by XRD. Figure 2A 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°, and 70.1°, corresponding to

the (101), (102), (110), (103), (201), and (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 be observed, confirming the successfully partial replacement of Ni by Co in the Ni0.85Se lattice.48 Moreover, no Co−Ni−Se ternary phase can be found, probably because the nanocrystal 40493


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ACS Applied Materials & Interfaces was smaller or the phases are overlapped with the Ni0.85Se phase.49 Figure 2A also shows that after introducing rGO, there is no apparent peak of rGO in the 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 rGO.46 In electrochemical measurements, the Co0.1Ni0.75Se/rGO catalyst exhibits better HER catalytic performance than other ones, as shown in Figure S1. Figure 2B 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 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 and 1587 cm−1, which can be attributed to the D band and G band of rGO, respectively,53 whereas 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 the experiment. Figure S2 shows the surface survey XPS spectra of Co0.1Ni0.75Se/rGO, indicating the existence of Co, Ni, Se, and C elements in Co0.1Ni0.75Se/rGO. The peaks at 873.3 and 854.1 eV in Figure 2C are 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 and 861.6 eV were the 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 Figure S3A. Figure 2D 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).56 Se 3d5/2 and Se 3d3/2 located at 53.5 and 54.3 eV, respectively, in Figure 2E can be attributed to Se2−.57 Its binding energy is lower than that of Ni0.85Se, as indicated in Figure S3B. This may be due to the fact that selenium is more electronegative. Charge transfer 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 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. Figure S3C presents the survey spectra of Ni0.85Se; no Co 2p peak was found compared with that of Co0.1Ni0.75Se/ rGO. Figure 2F shows C 1s spectra; the peaks at 284.2, 285.3, 285.9, 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 the 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 Figure 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 TEM. As shown in Figure 3A, 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

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

nanoparticles without rGO have a serious aggregation, as shown in Figure S5A, which can hinder the exposure of active sites. In addition, Figure 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. Figure 3B presents the high-resolution TEM (HRTEM) image of Co0.1Ni0.75Se/rGO. The distance between lattice fringes can be found to be 0.29, 0.21, and 0.18 nm, which are related to the (101), (102), and (110) planes of Ni0.85Se (JCPDS no. 180888), respectively, and consistent with the XRD results. The mapping of Co0.1Ni0.75Se/rGO testifies that all of the elements have a homogeneous distribution in the catalyst, as shown in Figures 3C−F (Figure S5B shows the original scanning electron microscopy image). In addition, EDS analysis validates the elements atom ratio of the Co0.1Ni0.75Se/rGO catalyst (shown in Table S1), which further 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 be observed, confirming the partial replacement of Ni by Co in the 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 3.2. Electrochemical Characterization. The HER activities of as-prepared catalysts were measured by electro40494


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Figure 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; and (E) i−t testing under static potential of 0.15 V vs RHE. The 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.

catalysts under the same conditions are 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, resulting in better HER performance.61 The HER mechanism usually involves three major reactions in acidic media (eqs 1−3)62

chemical 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, and 0.4) are presented in Figure S1A. 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 its 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 have higher HER activity. Moreover, the η of Co0.1Ni0.75Se/rGO (103 mV) is obviously lower than that of Co0.1Ni0.75Se (153 mV) (Figure 4A), 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 electro-

H3O+ + e− F m − H + H 2O (Volmer reaction, 120 mV/dec)

(1)

H3O+ + e− + m − H F H 2 + m + H 2O (Heyrovsky reaction, 40 mV/dec) (2)

2m − H F H 2 (Tafel Reaction, 30 mV/dec)

(3)

−

where e represents electrons, m−H represents a hydrogen atom adsorbed onto catalyst m, and H2 represents the generated hydrogen molecule. According to the Tafel equation (η = b log jA + a, and b is the Tafel slope), the Tafel slopes of 40495


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which are 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 Figure S1D and Table 2. Besides high catalytic activities, favorable stability toward 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 the Co0.1Ni0.75Se/ rGO catalyst, and the result is shown in Figure 4E. 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 Figure 4E exhibits the typical processes of H2 bubble accumulation and release. After 30 h, the current tends to be stable and basically unchanged. At the same time, it can be seen that the Rct after 30 h of i−t testing only has a slight increase (Figure S6B). 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 to 0.21 V at 50 mV/s for 5000 cycles. After these CV cycles, the LSV curves were re-recorded. Figure 4F shows the LSV curves of Co0.1Ni0.75Se/rGO at the initial and after 5000 cycles. It can be seen that only 4 mV negative shift at a 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 a current density of 10 mA/cm2, indicating a poorer durability than Co0.1Ni0.75Se/rGO. This result reflects the excellent stability of the Co0.1Ni0.75Se/rGO catalyst. 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 rGO. 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 hydrogens.37 This effect makes the H2 generation process easier.68 Whereas 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, the Co0.1Ni0.75Se/rGO catalyst can give a better HER performance.

Co0.1Ni0.75Se and Co0.1Ni0.75Se/rGO are 47 and 43 mV/dec, respectively, lower than that of Ni0.85Se indicated in Figure 4B. 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 (Figure S1B and Table 2) Table 2. HER Catalytic Performance Data of All AsPrepared Catalysts at 10 mA/cm2 material number

η (mV)

Tafel slope (mV/dec)

Cdl (mF/cm2)

Rct (Ω)

Ni0.85Se Ni0.85Se/rGO Co0.05Ni0.8Se Co0.1Ni0.75Se Co0.2Ni0.65Se Co0.3Ni0.55Se Co0.4Ni0.45Se Co0.1Ni0.75Se/rGO

190 172 183 153 168 175 178 103

57 52 63 47 50 61 55 43

0.53 1.14 0.80 1.30 1.19 1.13 0.85 1.71

48.24 36.90 46.16 30.23 34.58 39.79 42.37 21.34

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 Figure 4C shows the Nyquist plots of Ni0.85Se, Co0.1Ni0.75Se, and Co0.1Ni0.75Se/rGO with an equivalent circuit (shown in the 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 and 30.23 Ω, respectively, lower than those of Ni0.85Se (48.24 Ω) and other as-prepared catalysts, as shown in Figure S1C 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. 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 (EASAs) at the solid−liquid interface. The EASA is linearly proportional to Cdl (Cdl ∝ v × EASA, where v is the scan rate).65 Larger EASA indicates more exposed catalytic active sites, which can result in more efficient HER. As shown in Figure 4D, the Cdl is measured from double-layer charging curves using cyclic voltammetry (CV) in a small potential range (0−0.2 V) (Figure S6A). 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 here). By calculating the slopes in Figure 4D, 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,

4. CONCLUSIONS In this work, CoxNi0.85−xSe with different cobalt doping ratios is synthesized and their catalytic performances are compared with respect to the HER in electrolysis water splitting. Experimental 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 HER in electrolysis water splitting. Among different catalysts, Co0.1Ni0.75Se gives 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 rGO to form rGO-supported catalyst (Co0.1Ni0.75Se/rGO). HER tests show that this Co0.1Ni0.75Se/ 40496


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rGO can give a lower overpotential and smaller Tafel slope than the 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 electrontransfer rate in HER process, resulting in both high catalytic activity and stability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b12797. Detailed structural and HER electrocatalytic performance of the prepared catalysts and their counterparts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86 18971479006. Fax: 86 27 88663043 (H.W.). *E-mail: [email protected] (L.Z.). ORCID

Huimin Wu: 0000-0002-8021-5241 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (grant no. 21205030 and 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).

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Novel Cobalt-doped Ni0.85Se Chalcogenides (CoxNi0.85-xSe) as

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