N, S-codoped graphene loaded Ni-Co bimetal sulfides for enhanced oxygen evolution activity

Journal Pre-proofs Full Length Article N, S-codoped Graphene Loaded Ni-Co Bimetal Sulfides for Enhanced Oxygen Evolution Activity Rongxian Zhang, Shiqing Cheng, Na Li, Wentao Ke PII: DOI: Reference:

S0169-4332(19)32962-9 https://doi.org/10.1016/j.apsusc.2019.144146 APSUSC 144146

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

18 June 2019 5 September 2019 21 September 2019

Please cite this article as: R. Zhang, S. Cheng, N. Li, W. Ke, N, S-codoped Graphene Loaded Ni-Co Bimetal Sulfides for Enhanced Oxygen Evolution Activity, Applied Surface Science (2019), doi: https://doi.org/10.1016/ j.apsusc.2019.144146

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© 2019 Published by Elsevier B.V.

Rongxian Zhang,* Shiqing Cheng, Na Li, Wentao Ke

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China, E-mail: [email protected]

1

−

The improved electrocatalytic activity of can be attributed to the improved electrochemical active specific area due to the synergistic effect between nickel sites and cobalt sites, the substrate effect of N, S co-doped graphene.

Keywords:

; graphene; catalysis; sulfide; cobalt

2

3

4

thioacetamide as a sulfur source for the synthesis

2.1. 5

obalt chloride hexahydrate (CoCl2·6H2O), Nickel chloride hexahydrate (NiCl2·6H2O) and thioacetamide (CH3CSNH2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium hydroxide (KOH) was purchased from Alfa Aesar. Synthesis of N, S-codoped graphene (Noted as NSG)



6

The obtained product was denoted as

α λ

θ

α

7

μ

μ μ

0.22 −2

η

8

thioacetamide was used as the doping agent for the introduction of nitrogen and sulfur in graphene nanosheets. Ni-Co sulfide was then loaded on the doped graphene nanosheets also with thioacetamide as sulfur source. The strong binding ability of sulfur, nitrogen on doped graphene nanosheets will cause the richness of nickel or cobalt ions on the doped sites, inducing the in situ formation of Ni-Co sulfides on the surface. Noted that during the reaction, toxic H2S gas will deliver.

CoS2 JCPDS 41-1471

Intensity (a.u.)

CoS2/NSG CoS2-NiCo2S4/NSG NiS2/NSG NSG NiCo2S4 JCPDS 20-0782 NiS2 JCPDS 89-3058

20

30

40

50

60

70

80

2 Theta (degree) Fig. 1. XRD patterns of the synthesized products, NiS2/NSG, CoS2/NSG, NSG and CoS2-NiCo2S4/NSG. In addition, the standard patterns of CoS2 (JCPDS: 41-1471), NiCo2S4 (JCPDS: 20-0782), and NiS2 (JCPDS: 89-3058) are also shown.

The crystal phases of the prepared composites were firstly investigated by X-ray powder diffraction (XRD). The patterns of CoS2/NSG and NiS2/NSG products can be easily indexed into cubic phase CoS2 (JCPDS: 41-1471) and NiS2 (JCPDS: 89-3058), 9

respectively. When both of cobalt and nickel salts are involved in the synthesis (sample of CoS2-NiCo2S4/NSG), the diffraction peaks present the mixture of cubic phase CoS2 and cubic phase NiCo2S4 (JCPDS: 20-0782). The standard XRD patterns of CoS2, NiS2, and NiCo2S4 only show slight difference among them. The existence of NiCo2S4 in the composite can be confirmed by the peak at 2θ = 50.5o that doesn’t overlay with any other peaks from CoS2 or NiS2. The absence of the peak at 53.4o that is featured for NiS2 indicates that the CoS2-NiCo2S4/NSG sample doesn’t contain NiS2 phase. All of these suggest the formation of NiCo2S4 and CoS2 in the bimetallic sulfide sample. In particular, the diffraction peaks distributed at 31.6º, 38.3º, 50.5º, and 55.3º are corresponded to the (311), (400), (511), and (440) crystal planes of cubic NiCo2S4 (JCPDS: 20-0782); while the peaks at 32.3º, 36.2º, and 54.9º can be indexed into the (200), (210), and (311) crystal planes of cubic CoS2, respectively. In addition, the N, S-codoped graphene only shows a wide shoulder peak at about 24o. Similarly, in the sulfide loaded composites, no sharp peaks corresponding to carbon was observed, suggesting the non-stacking feature of the doped graphene in the sample. It should be noted that the presence of NSG substrate can influence the formation of sulfides. If without them in the reaction system, the formed bimetal sulfides only contain NiCo2S4 phase (Fig. S1, see Supporting Information).

10

Fig. 2. SEM images of a) CoS2/NSG and b, c) CoS2-NiCo2S4/NSG products. d, e) TEM and f) HRTEM images of CoS2-NiCo2S4/NSG products. g) Element mapping analysis of the CoS2-NiCo2S4/NSG product with K edges for the elements of carbon, nitrogen, sulfur, cobalt, and nickel.

Fig. 2a shows the SEM images of CoS2/NSG product, demonstrating the composites of spherical-like CoS2 particles with size of 150-250 nm and sheet-like graphene. When nickel ions are involved in the reaction system, the formed CoS2NiCo2S4/NSG product show similar microscale morphology as that of CoS2/NSG sample. In the CoS2-NiCo2S4/NSG product, the involved spherical-like metal sulfide particles show relatively bigger size of ~200-300 nm (Fig. 2b and c). These sulfide submicron-particles are wrapped with N, S-codoped graphene, indicating the formation of hybrids. As shown by the TEM image in Fig. 2d, the spherical particles 11

are actually composed by sheet-like units with size of 30-70 nm, forming porous structure. In addition, on the graphene nanosheets, there are also many wrapped tiny sheet-like blocks (Fig. 2e). High-resolution TEM of the CoS2-NiCo2S4/NSG product presents two sets of well-defined clear lattice fringes, showing the high crystallinity. The lattice fringes with spacing of 0.225 nm are corresponded to (211) crystal plane of cubic CoS2, while that of 0.332 nm can be indexed into (220) crystal plane of NiCo2S4. These results further confirm the successful synthesis of CoS2NiCo2S4/NSG product. If without graphene substrate, single phase NiCo2S4 is formed, which shows unique submicron-structure (with size of ~5 μm) composed by interpenetrating nanosheets (Fig. S2, see SI). As for the product of NiS2/NSG, SEM image shown in Fig. S3 (see SI) also suggests the formation of composites composed by sheet-like graphene and quasi-cubic sulfide particles. EDS analysis was then used to further define the elemental composition of the CoS2-NiCo2S4/NSG product. The EDS spectrum shows the existence of C, N, S, Co and Ni elements. The element mapping of the composite is shown in Fig. 2g. As shown by the elemental mapping, a uniform and continuous distribution of C and N is observed, while S, Co, and Ni are relatively concentrated. This is well consistent to the composite product, that is, CoS2-NiCo2S4 loaded on N, S codoped graphene. In addition, ICP analysis was also conducted, which shows Co:Ni molar ratio of 3.7: 1 for the CoS2-NiCo2S4/NSG product. This molar ratio is different from the dosage of Co2+ and Ni2+, possibly due to the different formation ability of cobalt sulfide and nickel sulfide in the reaction system. 12

Co(II)

sat.

sat.

Intensity (a.u.)

b)

Co(III)

Co 2p

Intensity (a.u.)

a)

CoS2/NSG

CoS2-NiCo2S4/NSG 810

800

Ni 2p

Ni(0)

CoS2-NiCo2S4/NSG 790

S-O species

882

780

2-

S species

S 2p

864

858

852

Pyrrolic-N Graphite-N

Intensity (a.u.)

Intensity (a.u.)

870

d)

CoS2-NiCo2S4/NSG

Pyridinic-N

CoS2/NSG

CoS2-NiCo2S4/NSG

NiS2/NSG 171

876

Binding Energy (eV)

CoS2/NSG

174

Ni(II)

NiS2/NSG

Binding Energy (eV)

c)

Ni(III)

NiS2/NSG 168

165

408

162

404

400

396

392

Binding Energy (eV)

Binding Energy (eV)

Fig. 3. Detailed XPS spectra of the typical three products of CoS2/NSG, NiS2/NSG, and CoS2-NiCo2S4/NSG. a) Co 2p, (b) Ni 2p, (c) S 2p and (d) N 1s

XPS was then performed to determine the surface composition and chemical state of the products. The Co 2p spectra of the two products (CoS2/NSG and CoS2/NiCo2S4/NSG) can be deconvoluted into several bands, including pairs of fitting peaks for Co(II) and Co(III) and shakeup satellites (Fig. 3a) [36, 37]. It should be noted that after the introduction of Ni species, the XPS bands for both of Co(II) and Co(III) species red-shifts to lower binding energy (for example, the Co(III) peak of 13

779.5 eV shifts to 778.4 eV; the Co(II) peak of 781.8 eV shifts to 780.9 eV), suggesting the strong influence of involved Ni on Co sites [38, 39]. Fig. 3b presents the comparison of Ni XPS bands in the products of NiS2/NSG and CoS2NiCo2S4/NSG. The Ni XPS band in the CoS2-NiCo2S4/NSG composite is relatively weak, suggesting less amount of Ni on the surface. XPS spectrum of Ni 2p displays two spin-orbit doublets characteristic of Ni(II) and Ni(III) (Fig. 3b), including Ni(II) at 854.4 eV for Ni 2p3/2 and 873.0 eV for Ni 2p1/2, and Ni(III) at 856.3 eV for Ni 2p3/2 and 877.2 eV for Ni 2p1/2 [40]. Compared to the main peak at 854.4 eV for NiS2/NSG product, the corresponding XPS peak of CoS2-NiCo2S4/NSG products slightly blueshifts. The blue-shift of Ni XPS band and the red-shift of Co band indicate the electron transfer behavior from nickel to cobalt sites in the Ni-Co sulfide. In addition, it seems that there is a metallic Ni peak at 851.5 and 869.1 eV in the XPS band of CoS2-NiCo2S4/NSG composite [41]. The S 2p XPS bands for the three products can also be fitted into four peaks (Fig. 3c). The peak at 166-172 eV indicates the presence of oxygen-sulfur (O-S) species, which may be due to the surface oxidation [42]. The peak at 161-165 eV can be indexed to metal-sulfur species in metal sulfides. In addition, for the product of CoS2NiCo2S4/NSG composite, the obvious red-shift of XPS band for metal sulfides may suggest the presence of sulfur ion in a low coordination state at the surface. The deconvolution of N 1s spectra (Fig. 3d) reveals the presence of three types of nitrogen, namely, pyridinic N (398.6 eV), pyrrolic N (400.0 eV) and graphitic N (401.8 eV), respectively, suggesting the incorporation of nitrogen in carbon matrix 14

[43-45]. Among the three samples, the relatively different ratios of various nitrogen species in the XPS spectrum suggest the different loading location. Doped nitrogen in carbon framework can potentially increase the active centers and thereby benefit the electrochemical catalytic activity. Fig. S4 shows Raman spectra of CoS2/NSG and CoS2-NiCo2S4/NSG products. The Raman spectrum show the typical peaks of D and G bands at 1345 and 1600 cm-1, suggesting the presence of graphene in the products.

b 0.40 CoS2/NSG

80

Over potential (V)

Current density (mA cm-2)

a) 100 NiS2/NSG CoS2-NiCo2S4/NSG CoS2-NiCo2S4/rGO NiCo2S4

60 40 20

S 103 N iC o2 4

0.35

-1 dec

-1 V d ec G 69 .6m -1 N iS2/N S V dec 1 0 8 .9m G S -1 C o S2/N c V de 1 . 7m 6 1 /rGO o S4 -NiC 2 S o 2 C

0.30 0.25 0.20

-1

m V de c S /N SG 62 .8 C oS2-N iC o2 4

0

0.15 1.2

1.3

1.4

1.5

1.6

1.7

1.8

0.0

0.1



0.3

0.4

0.5



d)

-2

160

over-potentials for 10 mA cm

Tafel slopes

Tafel slopesmA dec-1 Mass activity(A g-1)

c)440

0.2

Log (j/mA cm-2)

Potential (V vs RHE)

mass activity

150

140

400

120

120 360



Overpotential (V)

.4 m V

90

100 320

80 60

280

40

TOF s-1

TOFNi

0.20 0.15 0.10 TOFCo

0.05

TOFCo

TOFCo+Ni

TOFNi

NiCo2S4 CoS2-NiCo2S4/NSG CoS2/NSG CoS2-NiCo2S4/rGO NiS2/NSG 

f) 3.5 Current density (mA cm-2)



30 0

CoS2-NiCo2S4/NSG CoS2/NSG NiCo2S4 NiS2/NSG CoS2-NiCo2S4/rGO

e) 0.25

60

0.00

2.8 2.1

CoS2-NiCo2S4/NSG CoS2-NiCo2S4/rGO CoS2/NSG NiS2/NSG NiCo2S4

1.4 0.7 0.0 15

CoS2/NSG

NiS2/NSG

30

45

60

Scanning Rate (mV s-1)

CoS2-NiCo2S4/NSG

15

75

Fig. 4. Electrochemical catalytic activity of the obtained products for OER. a) LSV curves. b) Tafel plots. Comparison of c) overpotential for 10 mA cm-2 and Tafel slopes, d) mass activity at overpotential of 350 mV, e) TOF at overpotential of 350 mV for all of the samples. d) Double layer capacitance (Cdl) value obtained by plotting of the current density vs scanning rate.

Linear sweep voltammetry (LSV) measurements were then carried out to detect the electrocatalytic activity of the obtained sulfide products for OER in 1 M KOH (as shown in Fig. 4). Apparently, the bimetal sulfide product (CoS2-NiCo2S4/NSG) shows much higher electrocatalytic activity than those of monometallic counterparts (CoS2/NSG, NiS2/NSG). The performance of the catalysts the overpotential at current density of 10 mA cm−2 [46]. As shown by Fig. 4a and c, the CoS2-NiCo2S4/NSG catalyst needs a smallest overpotential of 272 mV to drive current density of 10 mA cm−2, while the values are 357 and 386 mV on monometallic counterparts of CoS2/NSG and NiS2/NSG catalysts, respectively (Fig. 4c), demonstrating that the bi-metal composition is needed for the excellent catalytic activity (Fig. 4a-e). The N, S-codoped graphene substrate highly influences the electrocatalytic activity. For comparison, we then investigated the catalytic activity of CoS2-NiCo2S4 product loaded on reduced graphene oxide substrate prepared with similar route. At current density of 10 mA cm-2, the synthesized CoS2-NiCo2S4/rGO catalyst needs an over-potential of 335 mV, which is much higher than that of CoS2-NiCo2S4/NSG 16

catalyst (272 mV for 10 mA cm-2), indicating that N, S-codoped graphene is much more favorable for the catalytic process than reduced graphene oxide. In the absence of carbon substrate, the thus obtained bimetallic sulfide (NiCo2S4) only shows relatively weaker catalytic activity, suggesting the critical role of the graphene substrate for catalytic activity (Fig. 4c). Tafel plots are obtained to study the reaction kinetics according to the corresponding LSVs shown in Fig. 5a. The linear portions of these Tafel plots (Fig. 4b) were fitted by Tafel equation, η = blogj + a, where η is the over-potential, j is the current density, and b is the Tafel slope. Tafel slope of the optimized CoS2NiCo2S4/NSG catalyst (62.8 mV dec-1) is much lower than those of CoS2/NSG (108.9 mV dec-1), NiS2/NSG (69.6 mV dec-1), CoS2/NiCo2S4/rGO (161.7 mV dec-1), and NiCo2S4 (103.4 mV dec-1), indicating its favorable reaction kinetics and better OER properties (Fig. 4a-c). Detailed comparison of these catalysts is also based on mass activity and turn over frequency (TOF) values. Fig. 4d shows the mass activity of various catalysts, which were calculated with the current densities at an overpotential of 350 mV. The mass activity of CoS2-NiCo2S4/NSG catalyst was found to be 148.2 A g-1 (Fig. 4d), which is the maximum value among all of investigated sulfide samples. This indicates that for the same amount of mass loading, the CoS2-NiCo2S4/NSG catalyst is the best choice as compared to other contrast samples. Based on the involved metal ion content in the catalysts loaded on the electrodes and the current density at over-potential of 350 mV, TOF values for the CoS2/NSG, NiS2/NSG, and CoS2-NiCo2S4/NSG catalysts are 17

estimated and compared in Fig. 4e. The optimized catalyst, CoS2-NiCo2S4/NSG, affords TOF values of 0.0545 s-1 on both of metal sites (cobalt and nickel) (TOFCo+Ni) and 0.0697 s-1 on the involved cobalt sites (TOFCo), much higher than those of CoS2/NSG (TOFCo of 0.026 s-1) and NiS2/NSG (TOFNi of 0.013 s-1) catalysts. All of these comparison results indicate the excellent catalytic reactivity of CoS2NiCo2S4/NSG product towards water oxidation reaction. To check the origin of the excellent electrocatalytic activity of CoS2NiCo2S4/NSG product, electrochemically active surface area (ECSA) of the samples were then evaluated by the electrochemical doubleǦ layer capacitance method (Cdl), considering the fact that the (Cdl) value is proportional to ECSA value [47-49]. Fig. S5 shows the cyclic voltammetry curves of the catalysts measured in the range without redox processes at increasing scan rates (see SI). As shown in Fig. 4f, the CoS2-NiCo2S4/NSG product shows much higher Cdl value of 35 mF cm-2 suggesting that the CoS2-NiCo2S4/NSG product presents more accessible electrochemically active sites on the surface. In contrast, other contrast samples show Cdl values in the range of 0.9-8.7 mF cm-2,

CoS2-NiCo2S4/NSG

electrochemically active surface area is highly electrochemically active surface area suggests the high conductivity and more electrochemical active sites on the surface. The high conductivity is due to the presence of N, S, co-doped graphene; while the synergistic effect between nickel sites and cobalt sites would induce more electrochemical active sites on the surface. electrochemically active surface area of the optimized catalyst should be 18

responsible for the excellent electrocatalytic activity.

b) Currsnt density (mA cm-2)

Current density (mA cm-2)

a) 80 CoS2-NiCo2S4/NSG 60

97.6%

40 20

99.0%

0

0

2

4 6 Time (h)

8

10

150 120

the original LSV curve after 2000 CV cycles

90 60 30 0 1.20

1.35

1.50

1.65

1.80

Potential (V vs RHE)

Fig. 5. Electrocatalytic stability of the optimized catalyst CoS2-NiCo2S4/NSG. a) i-t curves and b) LSV curves before and after 2000 CV cycles.

-term stabilities of the CoS2-NiCo2S4/NSG catalyst were then tested by a potentiostatic method in 1 M KOH electrolyte. The result is shown in Fig. 5a. It can be seen that after 10 hours of continuous testing at a constant potential (1.50 and 1.62 V, corresponding to current densities of 10 and 50 mA cm-2), the current density showed only few losses of 1.0 % and 2.4 %, respectively, suggesting that the current density or in other words the OER activity of the catalyst is stable. In our previous study, we have investigated the catalytic activity and stability of commercial RuO2 catalyst for OER, which show an overpotential of ~380 mV to drive a current density of 10 mA cm -2 and ~6 % of current loss after 6 h of OER [50]. This result suggests that the designed and synthesized CoS2NiCo2S4/NSG catalyst has higher catalytic activity and stability than that of benchmark RuO2 catalyst in KOH electrolyte. Accelerated stability was further 19

assessed in 1 M KOH at room temperature. As shown in Fig. 5b, after 2000 continuous potential cycles, the CoS2-NiCo2S4/NSG catalyst exhibits few potential increases to achieve the current density of 10 mA cm-2, further showing their superior OER electrocatalytic stability.

Fig. 6. a) SEM, b) TEM, and c) HRTEM of CoS2-NiCo2S4/NSG product after 10 h of OER operation.

20

b)

a) Co(III)

Co 2p1/2

810

804

Ni 2p

Co 2p3/2

Intensity (a.u.)

Intensity (a.u.)

Co 2p

798

792

786

Sat.

888

780

Ni (III)

880

Sat.

872

864

856

Binding Energy (eV)

Binding Energy (eV)

c) S 2p

Intensity (a.u.)

SOx

Sat.

172

170

168

166

Binding Energy (eV)

Fig. 7. a) Co 2p, b) Ni 2p and c) S 2p regions of the XPS spectra for the products of CoS2-NiCo2S4/NSG after OER.

21

4.

Conclusions In conclusion, high-performance oxygen evolution catalyst composed by loading

of Ni-Co sulfide materials on N, S co-doped graphene nanosheets were designed and prepared. The thus obtained composites contain CoS2 and NiCo2S4, which show microstructures of sheet-like units. The optimized catalyst, CoS2-NiCo2S4/NSG, exhibits a lower overpotential of 272 mV for current density of 10 mA cm-2 and smaller Tafel slope of 62.8 mV dec-1. This catalytic activity is much better than the counterparts of CoS2/NSG and NiS2/NSG. The excellent electro- catalytic activity can be attributed to the improved electrochemical active 22

specific area due to the synergistic effect between nickel sites and cobalt sites, the substrate effect of N, S co-doped graphene. This study would provide an efficient route for the development of excellent OER electrocatalysts.

Conflicts of interest The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful for National Natural Science Foundation of China (No. 21776115) and Jiangsu Natural Science Foundation (No. BK20161343). Six talent peaks project in Jiangsu Province (XCL-2018-017).

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Figure Captions: Fig. 1. XRD patterns of the synthesized products, NiS2/NSG, CoS2/NSG, and CoS2NiCo2S4/NSG. In addition, the standard patterns of CoS2 (JCPDS: 41-1471), NiCo2S4 (JCPDS: 20-0782), and NiS2 (JCPDS: 89-3058) are also shown. Fig. 2. SEM images of a) CoS2/NSG and b, c) CoS2-NiCo2S4/NSG products. d, e) TEM and f) HRTEM images of CoS2-NiCo2S4/NSG products. g) Element mapping analysis of the CoS2-NiCo2S4/NSG product with K edges for the elements of carbon, nitrogen, sulfur, cobalt, and nickel. Fig. 3. Detailed XPS spectra of the typical three products of CoS2/NSG, NiS2/NSG, and CoS2-NiCo2S4/NSG. a) Co 2p, (b) Ni 2p, (c) S 2p and (d) N 1s. Fig. 4. Electrochemical catalytic activity of the obtained products for OER. a) LSV curves. b) Tafel plots. Comparison of c) overpotential for 10 mA cm-2 and Tafel slopes, d) mass activity at overpotential of 350 mV, e) TOF at overpotential of 350 mV for all of the samples. d) Double layer capacitance (Cdl) value obtained by plotting of the current density vs scanning rate. Fig. 5. Electrocatalytic stability of the optimized catalyst CoS2-NiCo2S4/NSG. a) i-t curves and b) LSV curves before and after 2000 CV cycles. Fig. 6. a) SEM, b) TEM, and c) HRTEM of CoS2-NiCo2S4/NSG product after 10 h of OER operation. Fig. 7. a) Co 2p, b) Ni 2p and c) S 2p regions of the XPS spectra for the product of CoS2-NiCo2S4/NSG after OER.

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Highlights

● CoS2-NiCo2S4 with sheet-like units loaded on N, S co-doped graphene were synthesized. ● To drive current density of 10 mA cm-2, the optimized bimetal sulfide catalyst only needs overpotential of 272 mV. ● The optimized bimetal sulfide shows much higher electrochemical active specific area. ● The more effective role of N, S co-doped graphene than reduced graphene oxide for oxygen evolution was demonstrated.

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