reduced graphene oxide nanocomposite as counter electrode for high efficient dye-sensitized solar cells

Accepted Manuscript Nickel selenide/reduced graphene oxide nanocomposite as counter electrode for high efficient dye-sensitized solar cells Jia Dong, Jihuai Wu, Jinbiao Jia, Leqing Fan, Jianming Lin PII: DOI: Reference:

S0021-9797(17)30272-2 http://dx.doi.org/10.1016/j.jcis.2017.03.025 YJCIS 22122

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

19 January 2017 12 February 2017 4 March 2017

Please cite this article as: J. Dong, J. Wu, J. Jia, L. Fan, J. Lin, Nickel selenide/reduced graphene oxide nanocomposite as counter electrode for high efficient dye-sensitized solar cells, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.03.025

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Nickel selenide/reduced graphene oxide nanocomposite as counter electrode for high efficient dye-sensitized solar cells

Jia Dong, Jihuai Wu Jinbiao Jia, Leqing Fan, Jianming Lin Eng. Res. Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China

Abstract: Nickel selenide/reduced graphene oxide (Ni0.85Se/rGO) nanosheet composite is synthesized by a facile hydrothermal process and used as counter electrode (CE) for dye-sensitized solar cells (DSSC). The Ni0.85Se/rGO film spin-coated on FTO show prominent electrocatalytic activity toward I3–/I–. The electrocatalytic ability of Ni0.85Se/rGO film is verified by photocurrent-voltage curves, cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization curves. On account of its decent electrical conductivity and superior electrocatalytic activity, the DSSC using optimal Ni0.85Se/rGO CE achieves a power conversion efficiency (PCE) of 9.75%, while the DSSC based on sputtered Pt CE only obtains a PCE of 8.15%.

Keywords: dye-sensitized solar cell, counter electrode, nickel selenide, reduced graphene oxide

 Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22692229. E-mail address: [email protected] (J. Wu). 1

1. Introduction Dye-sensitized solar cell (DSSC) has received more and more attention for its application in green energy since its appearance in 1991 [1]. The best power conversion efficiency (PCE) of DSSC has come to 14.7% with a co-photosensitized method using alkoxysilyl-anchor dye and carboxy-anchor organic dye [2]. Generally, a DSSC is made up of three important components, dye-adsorbed TiO2 film, electrolyte which comprises redox couple of iodide/triiodide (I–/I3–), and a platinum (Pt) counter electrode (CE). Among them, CE adjusts the catalytic reduction of redox couples, exerting an influence on PCE [3]. On the other hand, Pt, the most-common used CE material, is high cost and low natural abundance, which impose restrictions on the commercialization of DSSC. Based on these thoughtfulness, many substitute materials have been proposed. These substitute materials cover carbon materials [4-7], alloy materials [8-10], conducting polymers [11-13] and transition metal compounds [14-16]. Also, transition metal compounds include carbides, nitrides and chalcogenide. Among these substitutes, transition metal compounds are less expensive and abundant on earth. What’s more, they have excellent electrocatalytic activity, indicating the high potential of replace Pt as CEs in DSSC. In addition, grapheme, as a two-dimensional material, is endowed with versatile physical properties and tunable photoelectrochemical properties. Even though graphene has a poor electro-catalytic activity, it has an admirable electro-conductivity and a large surface area due to its two-dimensional structure [17]. Most important of all, its large surface area can support catalytic material, causing more active sites whose role is reducing I3– to I– increase. Furthermore, a plenty of researches demonstrate that the introduction of graphene in CE materials can effectively enhance the electro-conductivity and catalytic activity of a CE, strengthening the power conversion efficiency (PCE) [18, 19]. Some research showed that nickel selenide together with graphene can be feasible replacements for 2

conventional Pt CE. In 2014, Zhang et al. synthesized microsphere NiSe2/rGO and octahedron NiSe2/rGO through hydrothermal route. When used as CE materials in DSSCs, microsphere NiSe2/rGO CE and octahedron NiSe 2/rGO CE achieved PCE values of 6.94% and 6.23% (The PCE of Pt CE under the same condition was 6.82%) [20]. In 2015, Zhang et al. prepared mesoporous Ni0.85Se nanospheres grown on graphene via hydrothermal approach and the obtained Ni0.85Se@rGO CE reached a PCE of 7.82% which was higher than that of Pt CE (7.54%) [21]. Later on, Xiao Zhang et al. prepared a series of Ni1-xSe@graphene composite and the optimized Ni1-xSe@graphene CE yielded a PCE of 7.37% which was a little higher than that of Pt CE (7.02%) [22]. In 2016, Zhang et al. synthesized NiSe-Ni3Se2 hybrid nanostructure CE on graphene with morphology of hollow hexagonal nanodisk and the best-performed NiSe-Ni3Se2/rGO CE reached a PCE of 7.83% which was higher than that of Pt CE (7.28%) [23]. In this work, nickel selenide/reduced graphene oxide (Ni0.85Se/rGO) nanosheet composite was prepared via facile hydrothermal method and used as electrocatalytic material for DSSC. The Ni0.85Se/rGO films on FTO were prepared by spin-coating method. Notably, the DSSC using optimal Ni0.85Se/rGO CE achieves a PCE of 9.75%, while the DSSC based on sputtered Pt CE only obtains a PCE of 8.15%.

2. Experimental 2.1 Synthesis of Ni0.85Se/rGO CEs Ni0.85Se/rGO nanosheet composite was prepared by a facile one-step hydrothermal process. In brief, 1.1885 g NiCl26H2O, 0.75 g PVP (polyvinylpyrrolidone, MW = 10000) and 50 ml GO aqueous suspension were added into 100 ml autoclave to form transparent solution A. 0.7896 g Se powder (99.999%), 0.5675 g NaBH4 (96%) and 25 mL GO aqueous suspension 3

were added into a beaker, reacted for 20 min to form a rufous solution B. Further, solution B was slowly added to solution A and stirred for another 10 min. Finally, the mixture was reacted at 180 ºC for 12 h. After cool down to room temperature, the black precipitate was collected by centrifugal separation at a rate of 10000 rpm for 10 min and washed several times by distilled water. Afterwards, the wet black products were dispersed directly in ethanol at a concentration of 0.06 gml–1, and ultra-sonic for about 120 min. Then the Ni0.85Se/rGO ink was obtained. Ni0.85Se/rGO CEs were fabricated by spin-coating Ni0.85Se/rGO ink on FTO conducting glass substrate (Fluorine doped tin oxide over-layer, sheet resistance 14 Ωcm–1) at a rate of 3000 rpm for 20 sec. These CEs were simply marked as Ni0.85Se/rGOx (x = concentration of GO aqueous suspension in hydrothermal reaction solution, mgml–1) CEs. The GO aqueous suspension was prepared by improved Hummer’s method [24] and the concentration of GO aqueous suspension was 0.05 mgml–1, 0.1 mgml–1 and 0.15 mgml–1. Ni0.85Se was prepared with the same method by changing GO aqueous suspension in hydrothermal reaction solution into distilled water. 2.2 Fabrication of DSSCs The dye-sensitized TiO2 photoanode and the as-prepared CEs were assembled to form DSSCs by sandwiching I–/I3– electrolyte. The dye-sensitized TiO2 photoanodes were prepared as our previous report [25] and the active areas were about 0.11 cm2. The I–/I3– electrolyte was a solution of 0.10 M tetramethyl ammonium iodide, 0.1 M tetraethyl ammonium iodide, 0.1 M tetrabutyl ammonium iodide, 0.1 M sodium iodide, 0.1 M potassium iodide, 0.1 M lithium iodide, 0.05 M iodine and 0.5 M 4-tert-butyl-pyridine in acetonitrile. The sputtered Pt CE was bought from Wuhan Lattice Solar Energy Technology Co. Ltd. 2.3 Characterizations and measurements X-ray diffraction (XRD, Cu Kα radiation, Smart Lab 3 kW, Rigaku, Japan) was used to 4

examine the composition of Ni0.85Se powder and Ni0.85Se/rGO powder. The morphologies of the as-prepared Ni0.85Se and Ni0.85Se/rGO were observed by transmission electron microscope (TEM) (JEM-2100, Japan). The optical transparency of Ni0.85Se CE and Ni0.85Se/rGO CEs was evaluated by Lamda 950 UV/Vis-NIR spectrophotometer. The photocurrent-voltage (J-V) curves were carried out by an AM 1.5G simulated solar light coming from an AAA solar simulator (Newport-94043A) with Keithley 2400 digital source meter. The cyclic voltammetry (CV) curves were carried out with a three-electrode system using CHI660E setup at a scan rate of 50 mVs–1. The three-electrode system used a Pt sheet as counter electrode, an Ag/AgCl electrode as reference electrode and various CEs as working electrode. The electrolyte in three-electrode system is 10 mM LiI, 1 mM I2 and 0.1 M LiClO4 in acetonitrile. Electrochemical impedance spectroscopy (EIS) measurements and Tafel polarization curves were tested with a Zennium electrochemical workstation (IM6) by assembling symmetric cell. The EIS measurements were measured at an amplitude of 5 mV in a frequency range from 100 mHz to 100 kHz and the results of EIS were analyzed with Zview software.

3. Results and discussion 3.1 Compositions The chemical compositions of the as-synthesized product were characterized by X-ray diffraction (XRD) and Raman spectroscopy. Fig. 1(a) shows the XRD pattern of Ni0.85Se and Ni0.85Se/rGO powder. It is clearly to see that the XRD results strongly exhibit the presence of Ni0.85Se in these four samples due to these XRD spectra can be well index to Ni0.85Se (JCPDS No. 18-0888). Fig. 1(b) shows the Raman spectroscopy of Ni0.85Se and Ni0.85Se/rGO films on FTO. There are two main peaks belonging to elemental Se at about 141 cm–1 and 235 cm–1 in 5

the Raman curves of Ni0.85Se CE [26, 27]. These two peaks also exist in Ni0.85Se/rGO CEs. Meanwhile, Ni0.85Se/rGO CEs have other two obvious peaks at 1345 cm–1 and 1590 cm–1, which are the D-band and G-band of rGO, respectively. The Raman spectra in Fig. 1(b) show that rGO is successfully introduced into Ni0.85Se CEs.

(a)

(b)

Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15 (1 1 0) (1 0 2)

D G

In te n s ity (a .u .)

In te n s ity (a .u .)

(1 0 1)

FTO Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15

(2 0 1) (2 0 2)

P DF #18-0888

10

20

30

40

50

60

70

0

80

2 T heta (degree)

500

1000

1500

2000

2500

3000

3500

-1

R aman s hift (c m )

Fig. 1. (a) XRD pattern of Ni0.85Se and Ni0.85Se/rGO powder, (b) Raman spectra for Ni0.85Se CE and Ni0.85Se/rGO CEs. 3.2 Morphology observation Fig. 2(a) and Fig. 2(b) show the morphologies of Ni0.85Se and Ni0.85Se/rGO measured by transmission electron microscope (TEM). The TEM images reveal that Ni0.85Se and Ni0.85Se/rGO are composed of a number of nanosheet. And the shape of Ni0.85Se/rGO nanosheet varies from Ni0.85Se nanosheet. While the shape of three kinds of Ni0.85Se/rGO nanosheet are more or less. Besides, the shape of Ni0.85Se nanosheet is much more uniform than Ni0.85Se/rGO nanosheet and Ni0.85Se/rGO nanosheet is fragmented. Furthermore, Fig. 2(c) and Fig. 2(d) show the presence of rGO and it was circled with red line partly. However, in the TEM image of Ni0.85Se/rGO0.05, rGO nanosheets are not obvious which may because the concentration of rGO is relatively low.

6

Fig. 2. TEM images of (a) Ni0.85Se, (b) Ni0.85Se/rGO0.05, (c) Ni0.85Se/rGO0.1, and (d) Ni0.85Se/rGO0.15. 3.3 Optical transmittance The optical transmittance spectrum is conducted to evaluate the transmittance of Ni0.85Se CE, Ni0.85Se/rGO CEs and sputtered Pt CE. The results are displayed in Fig. 3. Fig. 3 shows that the transmittance of Ni0.85Se CE is over 70% in the visible range (300–800 nm) and sputtered Pt CE exhibits a poor transparency because its surface likes a mirror and most of the sunlight will be reflected directly rather than penetrate. Moreover, Fig. 3 indicates that introducing rGO into CEs will lead to a dramatic loss of transparency, and the transparency of 7

CE is inversely proportional to the concentration of GO. The phenomenon of dramatic loss of transparency in this work indicates that fragmented nanosheet morphology is more conducive to the coverage of film on FTO substrate compared to uniform nanosheet morphology.

T ra n s m itta n c e (% )

90

60

Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15 FTO Sputtered Pt

30

0 300

400

500

600

700

800

Wavelength (nm)

Fig. 3. Transmittance spectra of FTO supported Ni0.85Se CE, Ni0.85Se/rGO CEs and sputtered Pt CE. 3.4 Photovoltaic performances The photovoltaic performance was characterized by photocurrent-voltage (J-V) curves. Fig. 4 presents the J-V curves of various CEs and the detailed parameters are listed in Table 1. According to Fig. 4(a), among Ni0.85Se CE and Ni0.85Se/rGO CEs, Ni0.85Se/rGO0.1 has the best photovoltaic performance (PCE = 9.75%, Voc = 0.751 V, Jsc = 19.94 mA·cm–2, FF = 0.652) when one simulated sunlight (AM1.5) irradiates from the front side. Meanwhile, the DSSC device with sputtered Pt CE gets a PCE of 8.15% under the same condition. The PCE of Ni0.85Se/rGO0.05 CE and Ni0.85Se/rGO0.15 CE are 9.34% and 8.89%, respectively. The photovoltaic performance of these five CEs follows the order of Ni0.85Se/rGO0.1 > Ni0.85Se/rGO0.05 > Ni0.85Se > Ni0.85Se/rGO0.15 > Sputtered Pt. Owing to the transparency of Ni0.85Se/rGO CE, the J-V curves with simulated sunlight (AM 1.5G) irradiates from the rear side are also tested and the results are shown in Fig. 4(b). It can be seen that the PCEs of 8

DSSCs with Ni0.85Se/rGO CEs are inferior to the device based on Ni0.85Se CE. The DSSC assembled with Ni0.85Se/rGO0.05 CE achieves the highest PCE of 4.05% as to three kinds of Ni0.85Se/rGO CE.

(b)

(a)

24

12

Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15

2

C u rre n t de n s ity (m A /c m )

-2

C u rre n t de n s ity (m A c m )

20

16

12

Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15 Sputtered Pt

8

4

Sputtered Pt

8

4

0

0 0.0

0.2

0.4

0.6

0.0

0.8

0.3

0.6

0.9

Voltage (V)

Voltage (V)

Fig. 4. Photocurrent-voltage (J-V) curves of the DSSCs based on different CEs under one sun illumination (AM 1.5G) (a) front irradiation, (b) rear irradiation, (c) box chart of the PCEs of the DSSC devices based on different CEs under front irradiation. Table 1 Photovoltaic parameters of the DSSCs based on different CEs Counter electrodes Irradiation VOC (V) JSC (mA·cm–2)

FF

PCE (%)

Front

0.743

17.86

0.675

8.96

Rear

0.715

9.51

0.772

5.25

Front

0.751

18.66

0.666

9.34

Ni0.85Se

Ni0.85Se/rGO0.05

9

Rear

0.720

8.06

0.696

4.05

Front

0.751

19.94

0.652

9.75

Rear

0.717

7.87

0.702

3.97

Front

0.765

18.13

0.641

8.89

Rear

0.715

6.90

0.713

3.52

Front

0.746

16.51

0.662

8.15

Rear

0.683

1.19

0.664

0.54

Ni0.85Se/rGO0.1

Ni0.85Se/rGO0.15

Sputtered Pt 3.5 Cyclic voltammetry Cyclic voltammetry is used to compare the electrocatalytic activity of Ni0.85Se CE, Ni0.85Se/rGO CEs and sputtered Pt CE. Fig. 5 shows cyclic voltammetry curves and the related data are listed in Table 2. In Fig. 5, two pairs of redox peaks can be seen and the redox peaks at more negative potentials (we noted as Red1 and Ox1) can be used to elevate the electrocatalytic activity of a CE since the main role of a CE is to reduce I3– to I–. Two prominent parameters that can evaluate electrocatalytic activity are peak current density of reduction peak Red1 (|Jred1|) and the peak-to-peak potential separation (Epp) between Red1 and Ox1. A higher peak current density and a lower Epp will lead to an improved electrocatalytic activity [28]. According to Fig. 5 and Table 2, the |Jred1| values of these five CEs follow the order of Ni0.85Se/rGO0.1 > Ni0.85Se/rGO0.05 > Ni0.85Se > Ni0.85Se/rGO0.15 > Sputtered Pt, and the Epp values follow the order of Ni0.85Se/rGO0.1 < Ni0.85Se/rGO0.05 < Ni0.85Se < Ni0.85Se/rGO0.15 < Sputtered Pt. Integrating the two cases, the electrocatalytic activity of CEs fabricated in this experiment follow the order of Ni0.85Se/rGO0.1 > Ni0.85Se/rGO0.05 > Ni0.85Se > Ni0.85Se/rGO0.15 > Sputtered Pt. This result is in good accordance with photovoltaic performances tested by J-V curves.

10

2

Ox2

-2

C u rre n t de n s ity (m A c m )

Ox1 1

0

Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15 Sputtered Pt

Red2 -1

Red1 -2 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

P otential ( V vs Ag/AgC l)

Fig. 5. Cyclic voltammetry curves for Ni0.85Se CE, Ni0.85Se/rGO CEs and sputtered Pt CE Table 2 Extracted data from CV and EIS curves. CEs

Jred1(mA·cm−2)

Epp (mV)

Rs (Ω·cm2)

Rct (Ω·cm2)

Ni0.85Se

–1.420

322.5

9.86

0.82

Ni0.85Se/rGO0.05

–1.455

312.5

9.49

0.75

Ni0.85Se/rGO0.1

–1.492

299.5

9.69

0.71

Ni0.85Se/rGO0.15

–1.370

329.5

11.04

1.08

Sputtered Pt

–0.864

545.0

14.59

2.07

3.6 Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is performed to further verify the electrocatalytic activity of a CE. The EIS curves are shown in Fig. 6 and the extracted data (Rs and Rct, obtained by fitting EIS spectra with Z-view software) are summarized in Table 2. The value of Rs is the intercept on the real axis and it represents series resistance. Rct is charge-transfer resistance at CE/electrolyte interface. According to Fig. 6 and Table 2, introducing a certain amount of rGO into Ni0.85Se CE helps to reduce the values of Rs and Rct. The Rs values of Ni0.85Se CE and Ni0.85Se/rGO CEs range from 9.49 Ω·cm2 to 11.04 Ω•cm2, indicating that Ni0.85Se CE and Ni0.85Se/rGO CEs have superior electron-conducting 11

ability. Besides, Ni0.85Se/rGO0.1 CE has the lowest Rct value of 0.71Ω·cm2 which means that among these five CEs, Ni0.85Se/rGO0.1 CE has the best charge-transfer ability. The Rct values of CEs related in this experiment keep to the order of Ni0.85Se/rGO0.1 < Ni0.85Se/rGO0.05 < Ni0.85Se < Ni0.85Se/rGO0.15 < Sputtered Pt.

3

Cdl

2

-Z '' ( Ωcm )

Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15 Sputtered Pt

Rs Rct

Zw

0 9

12

15

18

21

2

Z' (Ωcm )

Fig. 6. Nyquist plots of Ni0.85Se CE, Ni0.85Se/rGO CEs and sputtered Pt CE 3.7 Tafel polarization curves Fig. 7 depicts the Tafel polarization curves. In Tafel curves, we can get the information of exchange current density (J0). J0 is the slopes for anodic or cathodic branches and it concerns Rct by the formula: J0 = RT/nFRct [29]. In this formula, R represents gas constant, T represents absolute temperature, n represents the number of electrons referred to the reduction of I3–, and F is Faraday's constant. In Fig. 7, it can be seen that the values of J0 follow the order that Ni0.85Se/rGO0.1 > Ni0.85Se/rGO0.05 > Ni0.85Se > Ni0.85Se/rGO0.15 > Sputtered Pt, implying the values of Rct derived from Tafel curves follow the order of Ni0.85Se/rGO0.1 < Ni0.85Se/rGO0.05 < Ni0.85Se < Ni0.85Se/rGO0.15 < Sputtered Pt. The conclusion drawn from Tafel polarization curves is in good agreement with the conclusion obtained from EIS.

12

-2

L og(c u rre n t de n s ity (m A c m )

2

1

Ni0.85Se Ni0.85Se/rGO0.05 Ni0.85Se/rGO0.1 Ni0.85Se/rGO0.15 Sputtered Pt

0

-1

-2 -1

0

1

Voltage (V)

Fig. 7. Tafel polarization curves of the dummy cells with Ni0.85Se CE, Ni0.85Se/rGO CEs and sputtered Pt CE

4. Conclusions In conclusion, Ni0.85Se/rGO nanosheet composite were successfully prepared by a facile one-step hydrothermal process. Ni0.85Se/rGO films fabricated by spin-coating method were found to have excellent electron-conducting ability, wonderful electron transport ability and superior electrocatalytic activity. The DSSC assembled with Ni0.85Se/rGO0.1 CE achieves a power conversion efficiency of 9.75%, which is higher than that of sputtered Pt CE (8.15%) by 19.63%. The results and discussion in this experiment reveal that Ni0.85Se/rGO has a great potential to replace sputtered Pt as a CE in DSSC.

Acknowledgement The authors acknowledge the financial joint support by the National Natural Science Foundation of China (Nos. 91422301, 51472094, 61474047, 21301060, 61306077, U1205112), 13

the specialized research fund for the doctoral program of Higher University, Ministry of Education, China (NO. 20123501110001) and the funded projects about research and innovation ability cultivation for graduate students of Huaqiao University.

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Graphical Abstract:

-2

Current density (mA cm )

20

16

12

Counter electrode: Ni0.85Se Ni0.85Se/rGO Sputtered Pt

8

4

0 0.0

0.2

0.4

Photovoltage (V)

16

0.6

0.8