Nickel Cobalt Sulfide Double-Shelled Hollow Nanospheres as

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Nickel Cobalt Sulfide Double-Shelled Hollow Nanospheres as Superior Bifunctional Electrocatalysts for Photovoltaics and Alkaline Hydrogen Evolution Yiqing Jiang, Xing Qian, Changli Zhu, Hongyu Liu, and Linxi Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18439 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Materials & Interfaces

Nickel Cobalt Sulfide Double-Shelled Hollow Nanospheres as Superior Bifunctional Electrocatalysts for Photovoltaics and Alkaline Hydrogen Evolution Yiqing Jiang, Xing Qian*, Changli Zhu, Hongyu Liu, and Linxi Hou* College of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou 350116, China *E-mails: [email protected]; [email protected]. Fax: +86-0591-2286 6244; Tel: +86-0591-2286 5220.

ABSTRACT: Transition metal chalcogenides with hollow nanostructures have been considered as promising substitutes as precious metal electrocatalysts for energy conversion and storage. We synthesized NiCo2S4 double-shelled ball-in-ball hollow spheres (BHSs) via a simple solvothermal route and applied them in both dye-sensitized solar cells (DSSCs) and hydrogen evolution reactions (HERs) at the same time, which were clean and sustainable ways to convert energy. Benefiting from their remarkable structure features and advantageous chemical compositions, NiCo2S4 BHSs composed of tiny crystals possessed large surface area, well-defined interior voids, and high catalytic activity. The DSSC with NiCo2S4 BHSs under 100 mW cm−2 irradiation got a power conversion efficiency (PCE) of 9.49% (Pt, 8.30%). Besides, NiCo2S4 BHSs as a HER catalyst also obtained a small onset overpotential (27.9 mV) and a low overpotential (89.7 mV at 10 mA cm−2) under alkaline conditions. Therefore, this work offers a sensible strategy to synthesize bifunctional electrocatalysts for DSSCs and HERs. KEYWORDS:

Nickel

cobalt

sulfide;

Double-shelled

hollow

nanospheres;

Bifunctional electrocatalyst; Photovoltaic performance; Hydrogen evolution reaction

1

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INTRODUCTION Researches on sustainable electrochemical energy conversions and storages have sparked huge interests of researchers for global environmental degradations and fast-growing energy consumptions.1,2 Photovoltaics and water electrolysis have stood out among the miscellaneous corresponding technologies, benefiting from their remarkable low-price, high-efficiency and environmentally friendly features.3,4 Typically, a counter electrode (CE), a dye-loaded TiO2 photoanode, and triiodide/iodide redox electrolyte constitute a dye-sensitized solar cell (DSSC).5–7 Meanwhile, a standard hydrogen evolution reaction (HER) device employs a three-electrode

system.

Their

performances

are

both

highly

decided

by

electrocatalysts.8 Apparently, an ideal electrocatalyst must have big surface area, excellent conductivity, high catalytic activity, and low price. Regrettably, conventional electrocatalysts of DSSCs and HERs are commonly constituted of rare Pt materials, which hinders their broad applications because of high prices and limited resources.9 Thus, the explorations of the base metal electrocatalysts for both DSSCs and HERs are desperately required. Nanomaterials with hollow nanostructures have been becoming increasingly crucial and attractive for the electrochemical energy conversion and storage, considering their advantageous structural features and fascinating physicochemical properties including big surface area, large pore volume, low density and short charge-transport lengths.10–12 Past decades have witnessed many successful syntheses of various hollow structures, such as nanocages,13–15 hollow spheres,16 and hollow tubular.17 Among them, compared with single-shelled hollow spheres, hollow nanospheres with two shells possessing lager functional surface area, have been considered as promising hollow structures. Besides, the chemical composition has been of equal importance. The transition metal compounds (TMCs) have been isolated for unique characters, such as low costs, Pt-like catalytic activities, and high conductivities.18,19 Almost all TMCs, including carbides,20 nitrides,21 phosphides,22 chalcogenides,23 borides24, and their hybrids,25 have been frequently searched. Metal sulfides have won intense attentions due to excellent performances, especially cobalt and nickel 2


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sulfides,12.18,24-31 such as CoS,27 CoS2,28 Co3S4,29 Co9S8,30 NiS2,31 and NiCo2S4.18 Only a small amount articles about a electrocatalyst which is suitable for both DSSC and alkaline HER at the same time, have been reported so far.32 In this paper, we reported a simple two-step solvothermal route to synthesize NiCo2S4 double-shelled ball-in-ball hollow spheres (BHSs), which served as the bifunctional

electrocatalyst

for

both

DSSCs

and

HERs

simultaneously.

NiCo-precursor solid nanospheres were synthesized and then converted to NiCo2S4 BHSs via a sulfidation process. The deep influence of structure features and chemical compositions was investigated when the performance of NiCo2S4 BHSs contrasts with that of CoS BHSs, Ni3S4 BHSs, NiCo2S4 solid spheres (denoted as NiCo2S4 SSs), and NiCo2S4 nanoparticles (denoted as NiCo2S4 NPs). As expected, NiCo2S4 BHSs in the two applications showed the highest catalyst activity compared with that of these comparisons. As the CE material in DSSCs, NiCo2S4 BHSs showed a favorable power conversion efficiency (PCE) of 9.49% (Pt, 8.30%). Besides, as the catalyst for alkaline HERs, NiCo2S4 BHSs also delivered exceptional performance with a good stability, a small overpotential (89.7 mV at 10 mA cm−2) as well as a low onset overpotential (27.4 mV).

EXPERIMENTAL SECTION Materials. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), thioacetamide (TAA, AR), and nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR) were gained from Aladdin Ltd. (Shanghai, China). The N719 dye and the commercial Pt/C (20 wt%) were acquired from Solarinox Ltd. (Switzerland) and Shanghai Hesen Electric Ltd. (China), respectively. Other reagents and solvents were obtained from Sinopharm Chemical Reagent Co. Ltd. (China). The fluorine-doped SnO2 (FTO) glasses with the sheet resistance of 15 Ω sq−1 were received from Nippon Sheet Glass (Japan). The FTO glasses were pretreated with detergent, deionized water (DI), acetone, and ethanol, and then cut into rectangle (1.5 cm × 1.0 cm). Synthesis. NiCo2S4 BHSs were fabricated by a simple two-step facile solvothermal route. 36.3 mg of Ni(NO3)2·6H2O and 72.7 mg of Co(NO3)2·6H2O (nNi : nCo = 1 : 2) 3



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were dissolved in a mixture including 8 mL of glycerate and 40 mL of isopropanol. The solution was transformed into a 50 mL teflon-lined stainless-steel autoclave and heated at 180 °C for 10 h. The pale brown precursor, namely NiCo-precursor, was washed with ethanol and then dried at 50 °C for 12 h under vacuum. In the next step, 60 mg of the as-obtained precursor and 100 mg of TAA were dispersed into 40 mL of ethanol under ultrasonication and stirred to form a uniform suspension. Meanwhile, the pale brown suspension turned yellow-green as time went by. Then the suspension was transformed into a 50 mL teflon-lined stainless-steel autoclave and heated at 160 °C for 6 h. The obtained production was centrifuged, cleaned, and dried at 50 °C for 12 h under vacuum. Finally, in order to increase its crystallinity, the production was annealed at 350 °C for 2 h at the heating rate of 1 °C min−1 under Ar atmosphere. In addition, the pure cobalt sulfide/nickel sulfide hollow spheres were prepared as contrasts. 109 mg of Co(NO3)2·6H2O or Ni(NO3)2·6H2O was used for the syntheses of CoS BHSs or Ni3S4 BHSs under the same conditions. Moreover, in order to explore the influence of the glycerate in the synthesis process, a reference was synthesized without glycerate to bring out NiCo2S4 solid spheres (denoted as NiCo2S4 SSs). In detail, 100 mg of TAA, 36.3 mg of Ni(NO3)2·6H2O, and 72.7 mg of Co(NO3)2·6H2O were dispersed into 48 mL of isopropanol. The mixture was transformed into a 50 mL teflon-lined stainless-steel autoclave and heated at 160 °C for 16 h. Besides, to study the advantages of the double-shelled hollow sphere structure, NiCo2S4 irregular shape nanoparticles (denoted as NiCo2S4 NPs) were prepared via the same process of NiCo2S4 SSs except using DI as the solvent. Characterizations and Measurements. The morphology and nanostructure of the products were observed by a field-mission scanning electron microscopy (SEM, S-4800, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDS) and a transmission electron microscope (TEM, TECNAI G2F20, FEI). The crystal structures of these samples were measured by X-ray diffraction (XRD, X’ Pert PRO, Cu Kα, λ = 0.15406 nm) in the range of 10–70° within 10 min. Special surface area and pore size distributions were studied by the Brunauer-Emmett-Teller (BET) sorptometer

(Micromeritics,

ASAP

2020M,

USA)

4


to

perform

nitrogen

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adsorption-desorption isotherms. The valence states were examined by utilizing X-ray photoelectron spectrometer (XPS, ESCALAB 250, Mg Kα, USA). All relevant measurements were carried through a CHI660E electrochemical work station (CH Instruments). (1) Dye-sensitized solar cells. A DSSC is composed of a counter electrode (CE), a photoanode, and a redox electrolyte. The CEs were prepared by a spin-casting technique. Specially, 100 mg of a sample was dispersed in 10 mL of ethanol. The mixture was ultrasonicated and stirred for 30 min to form a uniform ink. Next, 100 µL of this ink was spin-casted on a FTO glass substrate and dried at 120 °C for 15 min. The loading mass on every CE might be 0.45 mg cm–2. As a reference, 20 mM chloroplatinic acid in isopropanol solution was covered on these glasses, following with being heated at 450 °C in air to fabricate Pt CEs. The photoanodes were produced by a screen-printing route to cover a nanocrystalline TiO2 layer (thickness, ~12 µm) and a scattering TiO2 layer (thickness, ~4 µm) on the conductive side of the pretreated FTO glasses with 20 and 200 nm TiO2 sols, respectively. Subsequently, after being heated in air at 500 °C for 1 h, these FTO glasses were soaked in 0.04 M titanium tetrachloride solution at 70 °C for 60 minutes and then annealed again. Finally, these TiO2 photoanodes were sensitized by being soaked into 0.3 mM N719 dye in ethanol solution for one day in a dry and dark environment. The redox electrolyte consisted of 0.3 M DMPII, 0.05 M I2, 0.1 M LiI, and 0.5 M 4-tert-butylpyridine in acetonitrile solution. To assemble a sandwich-structured DSSC, this electrolyte was put into the vacuum space between a photoanode and a CE. The active area of a DSSC was 0.16 cm2. The photocurrent density-voltage (J-V) curves were collected via putting the solar cells under AM 1.5 G illumination (100 mW cm−2) irradiation. It gained from a standard solar simulator (Oriel 94023A, AAA Class, Newport Corp.), which was adjusted with a pattern solar cell (91150V, Newport Corp.). A classical system with three electrodes employed in cyclic voltammetry (CV). All electrodes were inserted into an electrolyte containing 0.1 M LiClO4, 10 mM LiI, and 1 mM I2 in acetonitrile solution. The scan potential range ranged from −0.5 to 0.9 V and the scan ratio was 50 mV s−1. For electrochemical impedance spectroscopy (EIS) and Tafel polarization 5



curves, traditional symmetrical dummy cells (CE/electrolyte/CE) were fabricated. The EIS was measured between 100 kHz and 100 mHz with an amplitude of 5 mV, while Tafel curves were operated with a scan rate of 10 mV s−1 between −1.0 V and 1.0 V. (2) Hydrogen evolution reaction. HER measurements utilized a three-electrode electrochemical system, which was constituted of a graphite plate, an Ag/AgCI electrode, a glass carbon electrode (GCE, 3 mm in diameter) loaded with an as-prepared sample, and 1.0 M KOH. Briefly, 2 mg of the sample, 40 µL of Nafion solution (5 wt%), 368 µL of water, and 92 µL of ethanol were mixed and then experienced sonication for half an hour to make an uniform ink. Then, this ink (5 µL) was covered on the GCE and dried naturally to get into a film covering on the GCE. This sample loading mass would be 0.283 mg cm–2. The commercial Pt/C (20 wt%) was used as a reference. Linear sweep voltammetry (LSV) was conducted in the potential from −1.8 V to −0.8V (vs. the Ag/AgCI electrode) with a scan rate of 5 mV s−1. The EIS data were measured from 100 mHz to 100 kHz with an AC amplitude of 5 mV at an overpotential of 150 mV. Electrochemical active surface area (ECSA) was linkable with electrochemical double-layer capacitance (Cdl), which was measured by CV with divided scan rates (10 ~ 130 mV s–2) and potential (0.1 ~ 0.2 V vs. RHE). Cdl was the slope of the values of current density [j/2 = (janodic – jcathoic)/2] at 0.15 V vs. RHE against the scan rates.33

RESULT AND DISCUSSION Catalyst Morphologies and Compositions. Figure 1 and Figure S1 (Supporting Information, SI) are SEM images of NiCo-precursor nanospheres, NiCo2S4 BHSs, CoS BHSs, Ni3S4 BHSs, NiCo2S4 SSs, and NiCo2S4 NPs at different magnifications. As Figure 2a−c show, the as-prepared uniform solid NiCo-precursor nanospheres have very smooth surfaces without distinct pores and the diameter is around 550 nm. After sulfidation process, uniform NiCo2S4 BHSs (Figure 1d−f) were successful synthesized with an obvious shell structure and much rougher surface. This rough surface and shells composed of tiny crystals both tremendously enlarge the surface area of this sample. As a contrast, NiCo2S4 SSs (Figure 1g−i) can hardly be observed 6


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the shell structure. Therefore, glycerate benefits forming ball-in-ball structure and tiny crystals. As shown in Figure S1a−c, CoS spheres do possess a ball-in-ball structure like NiCo2S4 BHSs. However, Ni3S4 spheres (Figure S1d and S1e) are not as homogeneous as NiCo2S4 BHSs and pieces of broken spheres can be observed which may be due to their poor stability.

Figure 1. SEM images of (a−c) NiCo-precursor nanospheres, (d−f) NiCo2S4 BHSs, (g−i) NiCo2S4 SSs.

Figure 2 and Figure S4 (SI) exhibit the XRD patterns. In Figure 2, all diffraction peaks of NiCo2S4 BHSs are assigned exactly to a cubic phase of NiCo2S4 (ICDD PDF No. 00-020-0782), which indicates the pure quality of the synthesized production.34 The main peaks at 16.3°, 26.8°, 31.6°, 38.3°, 47.4°, 50.5°, 55.3°, 65.1°, and 69.3° correspond to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (5 3 3), and (4 4 4) planes of the cubic NiCo2S4.35 As for the unitary metal sulfides, the diffraction peaks of Ni3S4 BHSs are assigned excellently to the planes of cubic Ni3S4 (ICDD PDF No. 00-047-1739) which is relatively close to the phase structure of NiCo2S4 BHSs.36 Several diffraction peaks of CoS BHSs at 30.6°, 35.3°, 46.9°, and 54.4° correspond to the (1 0 0), (1 0 1), (1 0 2), and (1 1 0) planes of hexagonal CoS (ICDD PDF No. 03-065-3418).37 Besides, XRD patterns of these precursors are shown in Figure S4a. A dull peak at low angle appears in all of these plots, which proves the existence of 7



metal alkoxides.38 In Figure S4b, little difference exists among all kinds of NiCo2S4 samples, which means that all samples have the same phase structure.

PDF # 00-020-0782 NiCo2S4

PDF # 03-065-3418 CoS

Intensity (a.u.) 10

20

30 40 50 2 Theta (degree)

533 444

422 102 511 110 440

220 100 311 101 400

PDF # 00-047-1739 Ni3S4

111

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

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60

70

Figure 2. XRD patterns of NiCo2S4 BHSs, CoS BHSs, and Ni3S4 BHSs.

An intuitive and visualized approach to investigate the interior structure of NiCo2S4 BHSs was provide by TEM images (Figure 3 and Figure S2). The SEM images clearly exhibit the shell structure of the resultant NiCo2S4 BHSs after ultrasound treatment in Figure 3a. In Figure 3b−d, the double-shelled hollow sphere structure exists in NiCo2S4 BHSs with the separation of hollow and solid parts.39 The diameter of the inner shell is 300 nm and that of the outer shell is approximately 550 nm. The thickness of outer thin shell is 10−30 nm in Fig 3d, which is quite less than the inner shell. To more closely observe the outer shell, the area in yellow box in Figure 3d was magnified to obtain Figure 3e. Remarkable, Figure 3e displays clearly the lattice fringe, which matches interplanar spacing of (4 0 0) planes of cubic NiCo2S4. Meanwhile, a lattice spacing (0.235 nm) in Figure 3f matches (3 1 1) planes. The elemental mapping images in Figure S5 (SI) and the EDS spectra in Figure S6 (SI) provide clear information of the element distribution and quantity of NiCo2S4 BHSs.

8


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Figure 3. (a) SEM image of NiCo2S4 BHSs through ultrasound process, (b−f) TEM images of NiCo2S4 BHSs with different magnifications.

The XPS survey was employed to explore the surface chemical compositions and the valence states of NiCo2S4 BHSs. The survey spectrum (Figure 4a) presents Co, Ni, S, C, and O element attendances. The curves of Ni 2p and Co 2p are both constituted with two spin-orbit doublets and a pair of shakeup satellites (marked as “Sat.”). The peaks of 2p1/2 at 871.0/874.3 eV and the peaks of 2p3/2 at 853.6/856.6 eV are characteristic of Ni2+/Ni3+ ions.36 Besides, the peaks of 2p1/2 at 797.8/794.0 eV and the peaks of 2p3/2 at 781.5/779.0 eV are characteristic of Co2+/Co3+ ions. The existence of S2− ions is proved by the character peaks appearing at 163.5 and 162.3 eV in Figure 4d. The broad satellite at 167.8 eV is for S species with high oxide state.35 Thus, there are Co2+, Co3+, Ni2+, Ni3+, S2−, and SOx2− ions on the surface composition of NiCo2S4 BHSs.

9



Figure 4. XPS spectra of NiCo2S4 BHSs: (a) survey spectrum, (b) Ni 2p, (c) Co 2p, and (d) S 2p.

Brunauer-Emmett-Teller Analysis. As Figure 5 shows, all the isotherms display the properties of the typical mesoporous structure.40 The Brunauer-Emmett-Teller (BET) surface areas of NiCo2S4 BHSs, CoS BHSs, Ni3S4 BHSs, NiCo2S4 SSs, and NiCo2S4 NPs are 65.4, 50.4, 37.9, 15.9, and 4.8 m2 g−1, respectively. Moreover, the pore size distributions calculated by Barrett-Joyner-Halenda (BJH) method are shown as the inset of Figure 6. Some pore diameters are smaller than 10 nm for the open space between the interconnected nanocrystals that form samples.38 Specially, the most pores of NiCo2S4 SSs are less than 10 nm, which can indicate their solid sphere structure.

10


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320

160 80

-1

NiCo2S4 BHSs

CoS BHSs

Ni3S4 BHSs

NiCo2S4 SSs

3

NiCo2S4 NPs

3 -3

240

Pore volumn (10 cm /g )

3

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


Quantity Adsorbed (cm /g STP)

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2 1 0 10 100 Pore diameter (nm)

0 0.0

0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po)

Figure 5. N2 adsorption-desorption isotherms and pore size distributions (inset) of all samples.

Formation Mechanism of NiCo2S4 BHSs. The synthetic process of NiCo2S4 BHSs is illustrated in Figure 6. Firstly, cobalt/nickel nitrate hexahydrate reacted with glycerate in isopropanol at 160 °C for 10 h and pale brown metal alkoxides, named as NiCo-precursor solid spheres, were got. Then, the NiCo-precursor solid spheres were converted into black NiCo2S4 BHSs via a solution sulfidation process. The nanoscale Kirkendall effect and the ion exchange reaction were employed to explain the generation of ball-in-ball structure.41,42 This process could be commonly divided into three stages in Figure 6. At stage 1, as temperature rising, the sulfide ions (S2−) were liberated from the decomposition of TAA and reacted with Ni2+ and Co2+ ions on the surface of the precursors to form NiCo2S4 shell. A continuous provision of S2− contributed to the steady growth of the NiCo2S4 shell. Meantime, since the movement of outward diffused metal ions (M2+) was faster than that of the inward diffused S2−. Based on the Kirkendall effect, the variation of diffusivity might lead to creation of “Kirkendall voids” during the chemical transformation.43 Therefore, a well-defined gap close to the surface was generated to obtain the NiCo-precursor@NiCo2S4 yolk-shell structure at stage 2. As the reaction processed, the outward diffusion of M2+ got slower and the empty gap was enlarged. While the core became into second shell, anion exchange reaction was completed and NiCo2S4 BHSs were finally produced at stage 3.44 11



Figure 6. Synthetic process of NiCo2S4 BHSs: (1) surface NiCo2S4 formed by ion exchange, (2) continuous growth of metal sulfides on the inner NiCo-precursor sphere, and (3) end of ion exchange reaction (M2+ is a sign of metal ions.).

Photovoltaic Performances of DSSCs. Figure 7a reveals J-V curves and Table 1 reveals several relevant key photovoltaic parameters of these DSSCs. NiCo2S4 SSs and NiCo2S4 NPs worked as comparisons to verify whether the double-shelled hollow spheres structure do enhance PCEs. Besides, the unitary metal sulfides were compared with the binary metal sulfides to research the influence of chemical composition. As a result, the PCEs of NiCo2S4 BHSs, CoS BHSs, Ni3S4 BHSs, NiCo2S4 SSs, NiCo2S4 NPs, and Pt based CEs were 9.49%, 9.09%, 8.84%, 7.58%, 7.46%, and 8.30%, respectively. The fill factors (FF) of NiCo2S4 BHSs, CoS BHSs, Ni3S4 BHSs, NiCo2S4 SSs, NiCo2S4 NPs, and Pt based CEs were 0.647, 0.628, 0.624, 0.650, 0.601, and 0.627, respectively. Obviously, the PCE of NiCo2S4 BHSs based DSSC is highest. The PCEs of CoS BHSs and Ni3S4 BHSs based DSSCs were both superior to that of Pt-based CEs. Hence, the binary metal sulfides owned higher catalytic activity than that of the unitary metal sulfides, partly for the binary metal sulfides possessing a more stable structure, rougher surface, and higher surface area, which means more electroactive sites and transfer channels.45 Besides, the low catalytic activities of NiCo2S4 SSs and NiCo2S4 NPs may own to this special hollow sphere structure does enhance PCEs. The NiCo2S4 BHSs based DSSCs yielded the highest Jsc (17.4 mA cm–2) and high FF (0.647), implying that NiCo2S4 BHSs had low diffusion impedance and high diffusion coefficient for redox species.46 The stability 12


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testing for NiCo2S4 BHSs in DSSCs was measured after 2000 times of CV scanning and its result was in Figure S7 and Figure S5a/b (SI). The comparisons of DSSC performances for as-obtained samples with other non-noble metal-based catalysts are shown in Table S1 (SI).

Table 1. Summary of photovoltaic parameters of DSSCs based on various CEs. CE

Jsc (mA cm–2)

Voc (mV)

FF

PCE (%)

NiCo2S4 BHSs

17.4

843

0.647

9.49

CoS BHSs

17.2

842

0.628

9.09

Ni3S4 BHSs

16.6

854

0.624

8.84

Pt

16.1

821

0.627

8.30

NiCo2S4 SSs

14.6

798

0.650

7.58

NiCo2S4 NPs

14.8

838

0.601

7.46

EIS Analysis of DSSCs. To investigate the electrochemical characters of these samples for I3− reduction, Nyquist plots of EIS (Figure 7b), consisting of two semicircles, which were modeled with an equivalent circuit diagram, were conducted on typical sandwich-like structure dummy cells .47 The plots of NiCo2S4 SSs, NiCo2S4 NPs, and the corresponding equivalent circuit diagram are displayed as insets in Figure 7b. Four relevant vital impedance properties are presented in the diagram as follows: charge transfer resistance (Rct) is the most significant parameter and reflects resistance between the interface of CE and electrolyte, which is the diameter of the right semicircle; Series resistance (Rs) includes the electrolytic resistance and the resistance of the cell substrate and electrocatalyst film, which corresponds to the intercept of the real axis of the high-frequency region; Nernst diffusion-limited impedance (ZN) of the I−/I3− redox couple attributes to the another semicircle; The corresponding constant phase element (CPE) represents a standard difference in perfect capacitance.48 Table 2 includes the related data. The Rs of all samples show 13



little difference and the effect of Rs can be omitted. Meanwhile, the corresponding Rct values are distinctly different and the low value of this crucial parameter manifests the high conductivity.49 NiCo2S4 BHSs based CE exhibited the lowest Rct value of 2.48 Ω, while CoS BHSs (3.27 Ω) and Ni3S4 BHSs (4.35 Ω) based CEs showed lower Rct values than that of Pt CE (6.86 Ω). Moreover, the Rct value of NiCo2S4 SSs (12.8 Ω) and NiCo2S4 NPs (50.9 Ω) based CEs were much larger than that of other samples. Therefore, the NiCo2S4 BHSs based CE had the lowest resistance when the electrons transfered from CE to electrolyte as well as the highest catalytic activity, which matched well with the results of J-V measurements. CV Investigations of DSSCs. To study the I−/I3− redox reactions kinetics, CV were measured with the dummy cells. The related results are displayed in Figure 7c and summed up in Table 2. Figure 7c shows two pairs of redox peaks on a CV curve, namely Red-1/Ox-1 and Red-2/Ox-2. The couple peaks at low potential correspond to Eq. (3) reaction while the other peaks correspond to Eq. (4) reaction.50 I3− + 2e− ↔ 3I−

Eq. (3)

3I2 + 2e− ↔ 2I3−

Eq. (4)

The cathodic peak current density (JRed-1), which is the current density at the Red-1 peak, is spotted as the key parameter for I3− reduction. The higher intensity of the absolute value of JRed-1 correspond to the higher reduction velocity of I3−. Besides, the peak-to-peak separation (Epp) is interrelated to the redox reaction reversibility and catalytic activity. The lower value of Epp means the higher catalytic activity.51 Evidently, the JRed-1 values increase in the order of NiCo2S4 NPs (1.52 mA cm−2) < NiCo2S4 SSs (3.31 mA cm−2) < Pt (3.66 mA cm−2) < Ni3S4 BHSs (4.09 mA cm−2) < CoS BHSs (4.20 mA cm−2) < NiCo2S4 BHSs (4.30 mA cm−2). The Epp values are NiCo2S4 NPs (358 mV) > NiCo2S4 SSs (303 mV) > Pt (206 mV) > Ni3S4 BHSs (193 mV) > CoS BHSs (191 mV) > NiCo2S4 BHSs (189 mV). Therefore, NiCo2S4 BHSs have the highest reduction velocity and catalyst activity.

14


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Figure 7. (a) J-V curves of DSSCs with all products; (b) EIS Nyquist plots of these samples and the corresponding equivalent circuit model (inset); (c) CV curves of these CEs for I−/I3− redox couples at a scan rate of 50 mV s–1; (d) CV curves of NiCo2S4 BHSs based CEs with different scan rates (50 ~ 150 mV s–1); (e) the relationship between redox current density and square root of scan rates of CVs for NiCo2S4 BHSs based CE; and (f) Tafel polarization curves for the dummy cells fabricated with these CEs.

Table 2. Summary of EIS, CV and Tafel Polarization parameters for various CEs.

15



Rs CE

Epp

log (J0/mA

Rct (Ohm) (Ohm)

Page 16 of 30

(mA)

cm−2)

log (Jlim/mA cm−2)

NiCo2S4 BHSs

32.5

2.48

189

0.676

1.98

CoS BHSs

32.2

3.27

191

0.643

1.92

Ni3S4 BHSs

32.3

4.35

194

0.603

1.82

Pt

32.2

6.86

206

0.513

1.65

NiCo2S4 SSs

32.6

12.8

303

0.258

1.54

NiCo2S4 NPs

32.9

50.9

358

0.134

1.32

Figure 7e exhibits the CV curves of NiCo2S4 BHSs based CEs at diverse scan rates. As the values of the square root of the scan rates increase, the absolute values of current density of the anodic and cathodic peaks climb. As Figure 7f shows, a fitting well linear relationship appears visibly between the redox current density and the square root of the scan rates.52 Based on the famous Langmuir isotherms principle, two critical decisions are drawn: Firstly, the way of iodide species diffusing in the electrolyte is the control step of I3− reduction reaction on the surfaces of CEs; Secondly, the adsorption of iodide species do not relate directly to the redox reaction on the surface of CEs.53 Tafel Polarization Measurements of DSSCs. The same symmetric cells in the EIS researches were utilized in Tafel polarization measurements. The corresponding plots are shown in Fig.7f. Table 2 includes those related parameters. The limiting diffusion current density (Jlim) is the intersection of anodic branch with y axis and the exchange current density (J0) is the slope for branch. The bigger J0 means the higher catalytic activity, while the higher Jlim means the higher ionic diffusion coefficient based on Eq. (5).54 As Table 2 shows, the values of J0 increase in the order of NiCo2S4 NPs (0.134 log (mA cm−2)) < NiCo2S4 SSs (0.258 log (mA cm−2)) < Pt (0.513 log (mA cm−2)) < Ni3S4 BHSs (0.603 log (mA cm−2)) < CoS BHSs (0.643 log (mA cm−2)) < NiCo2S4 BHSs (0.676 log (mA cm−2)). The values of Jlim increase in the order of NiCo2S4 NPs (1.32 log (mA cm−2)) < NiCo2S4 SSs (1.54 log (mA cm−2)) < Pt (1.65 log (mA cm−2)) 16


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< Ni3S4 BHSs (1.82 log (mA cm−2)) < CoS BHSs (1.92 log (mA cm−2)) < NiCo2S4 BHSs (1.98 log (mA cm−2)). The relationship between ionic diffusion coefficient and Jlim is shown in Eq. (5), while the relationship between EIS data and Tafel results is shown in Eq. (6):48 D = lJlim/2nFC

Eq. (5)

J0 = RT/nFRct

Eq. (6)

Where l is thespace thickness; D is the diffusion coefficient; n is the number of electrons involved in the reduction of triiodide at the electrode; C is the I3− concentration; R and F are the gas constant and Faraday’s constant; T is the temperature (298K).55 The catalytic activities reflected by Tafel polarization curves keep the same trend with that of EIS and CV results.

Table 3. The HER data of these electrocatalysts in alkaline conditions Catalysts

ƞonset (mV)

ƞ10

b (mV

Rct

Cdl (mF

J0 (mA

(mV)

decade−1)

(Ohm)

cm−2)

cm−2)

NiCo2S4 BHSs

27.9

89.7

60.4

25.4

21.5

0.275

CoS BHSs

46.1

107

60.9

26.3

10.5

0.155

Ni3S4 BHSs

90.5

184

74.8

36.3

9.20

0.0838

NiCo2S4 SSs

128

206

82.3

109

7.43

0.0757

NiCo2S4 NPs

159

258

101

133

3.78

0.0300

Electrochemical Performances of HERs. Recently, Co-based and Ni-based materials have been considered as non-noble-metal multifunctional electrocatalysts for various applications.25,26 The resultant samples have been proved possessing excellent electrochemical pseudocapacitive and photovoltaic properties.38 Its performance in alkaline-medium HER is worth to be explored owing to its outstanding conduction, great catalytic activity and exceptional stability.16.26 Therefore, the as-prepared samples NiCo2S4 BHSs, CoS BHSs, Ni3S4 BHSs, NiCo2S4 SSs, NiCo2S4 NPs, and Pt/C (reference) were researched as electrocatalysts for HER 17



in 1.0 M KOH solution. The corresponding significant figures are displayed in Figure 8 and Figure 9. Table 3 includes relative data. Polarization Curves for HERs. The typical polarization curves for all the samples are exhibited in Figure 8a. Two vital parameters can be observed: onset overpotential (ηonset) and another vital corresponding potential (η10, a potential at a current density of 10 mA cm−2).56,57 The lower ηonset and η10 present the higher electrocatalytic activity.58 Remarkably, NiCo2S4 BHSs exhibited the lowest ηonset of 27.9 mV (vs. RHE), while the higher ηonset values of 46.1 and 90.5 mV were occupied by CoS BHSs and Ni3S4 BHSs, respectively. However, the ηonset values of NiCo2S4 SSs and NiCo2S4 NPs are 128 and 159 mV, reflecting their poor HER activity. Moreover, the η10 values of NiCo2S4 BHSs, CoS BHSs, Ni3S4 BHSs, NiCo2S4 SSs, and NiCo2S4 NPs are 89.7, 107, 184, 206, and 258 mV, respectively. As a result, the lowest ηonset and η10 of NiCo2S4 BHSs indicate its excellent electrocatalytic property for HER in alkaline medium, which is compared with that of other base metal sulfide electrocatalysts in Table S2 (SI).

Figure 8. (a) Polarization curves for all samples with a scan rate of 5 mV s−1 in 1.0 M KOH; (b) Tafel plots for all samples; (c) Polarization curves of NiCo2S4 BHSs initial and after 2000 times CV cycles between−0.5 V and 0 V vs. RHE; and (d) Nyquist plots with an overpotential of 150 mV. 18


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Tafel plots (Figure 8b) probe above catalysts inherent properties. The Tafel slope is calculated by follow Eq. (7): ƞ = b log j + a

Eq. (7)

Where ƞ is the overportential, b is the Tafel slope and j is the current density. The small Tafel slope indicates advantageous and favourable electrochemical reaction kinetics.59 The b value of the standard reference is 35.5 mV decade−1, approaching towards the early records.60 In addition, the Tafel slope for NiCo2S4 BHSs (60.4 mV decade−1) is lesser than CoS BHSs (60.9 mV decade−1), Ni3S4 BHSs (74.8 mV decade−1), NiCo2S4 SSs (82.3 mV decade−1), and NiCo2S4 NPs (101 mV decade−1), suggesting that NiCo2S4 BHSs possess great reaction kinetics. Moreover, the exchange current density (J0) is calculated using extrapolation methods, which embodies the essential ratio when the electron transfers reversibly.61 Based on the Tafel equation, the J0 values of NiCo2S4 BHSs, CoS BHSs, Ni3S4 BHSs, NiCo2S4 SSs, and NiCo2S4 NPs are 0.275, 0.155, 0.0838, 0.0757, and 0.0300 mA cm−2, respectively. One is the initial and the other is in the back of CV scanning for 2000 cycles. It can be observed that the polarization curve has little difference after CV scanning for 2000 cycles in Figure 8c. Taking ηonset/η10 for examples, ηonset/η10 only has an increasement of 6.5/14.5 mV, which figures that NiCo2S4 BHSs arm with the satisfactory durability. The related SEM/TEM images of NiCo2S4 BHSs after stability testing are in Figure S5c/d. EIS Measurements for HERs. The EIS measurements (Figure 8d) for all samples were tested in 1.0 M KOH. Typically, a Nyquist plot is a semicircle and the charge transfer resistance (Rct) presences the semicircle diameter. Regularly, a small Rct reflects a fast charge transfer capacity.62 The Rct value of NiCo2S4 BHSs is 25.4 Ω, which is smaller than that of CoS BHSs (26.3 Ω), Ni3S4 BHSs (36.3 Ω), NiCo2S4 SSs (109 Ω), and NiCo2S4 NPs (133 Ω), signifying that NiCo2S4 BHSs have the highest catalytic activity for HER. The related electrical equivalent circuit models and the explanations of related electric elements in the ESI could be saw in Figure S8. Electrochemically Active Surface Area of HERs. The electrochemically active 19



surface area (ECSA) and the Cdl were assessed by CV between 0.1 and 0.2 V (vs. RHE) in a non-Faradaic region (Figure 9).63,64 Apparently, in Figure 9f, the Cdl values, the slopes of figure 9f, increase in the order of NiCo2S4 NPs (3.78 mF cm−2) < NiCo2S4 SSs (7.43 mF cm−2) < Ni3S4 BHSs (9.20 mF cm−2) < CoS BHSs (10.5 mF cm−2) < NiCo2S4 BHSs (21.5 mF cm−2). The ECSA and the roughness factor (RF) can be calculated from the Cdl according to Eq. (8) and Eq. (9): ECSA = CdlS/Cs RF = ECSA/S = Cdl/Cs

Eq. (8) Eq. (9)

Where S is the geometric area of the bare GCE, namely 0.0707 cm2 and Cs is the specific capacitance. According to typical reports, Cs is assumed for a smooth GCE to be 0.040 mF cm−2 in 1 M KOH.65,66 As a result of calculations, the ECSA values increase in the order of NiCo2S4 NPs (6.68 cm2) < NiCo2S4 SSs (13.1 cm2) < Ni3S4 BHSs (16.3 cm2) < CoS BHSs (18.6 cm2) < NiCo2S4 BHSs (38.0 cm2) and the RFs are as follow: NiCo2S4 NPs (94.5) < NiCo2S4 SSs (186) < Ni3S4 BHSs (230) < CoS BHSs (263) < NiCo2S4 BHSs (538). It can be noted that NiCo2S4 BHSs own the most active sites at the solid-liquid boundary surface.67 The above results proved the superiority of NiCo2S4 BHSs for DSSCs and HERs. The excellent performance can be chiefly assigned to advantageous structural features and chemical compositions. Particularly, the double shells formed from extremely small crystals enhance relatively the surface area, which provides a large quantity of electroactive sites for redox reflexes, leading to high chemical activity.68 Meanwhile, the hollow space could be viewed as a reservoir for electrolytes, which benefits inner reflex space for fast diffusions and related reactions. Moreover, the gaps between these nanoparticles could reduce charge transport lengths.69

20


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Figure 9. Cyclic voltammograms of (a) NiCo2S4 BHSs, (b) CoS BHSs, (c) Ni3S4 BHSs, (d) NiCo2S4 SSs, and (e) NiCo2S4 NPs at different scan rates between 10 and 130 mV s−1. (f) Current densities at 0.15 V vs. RHE of all products plotted as a function of scan rate and the linear fittings.

CONCLUSIONS In conclusion, a simple solvothermal route has been reported about producing a series double-shelled hollow nanospheres (BHSs), which contains synthesis of the precursors and the following sulfidation reaction. The role that glycerate played on the formation of this special architecture was investigated by NiCo2S4 solid spheres prepared without glycerate. Besides, to figure out the influences of the structure features

and

chemical

compositions,

NiCo2S4

nanoparticals

and

binary

cobalt/nickel-based sulfide BHSs worked as other references. It turns out the superior of the BHS structure and ternary chemical composition. Just as important, NiCo2S4 BHSs are ideal bifunctional catalysts using in DSSCs and HERs in 1.0 M KOH 21



solution with excellent performances. The DSSC with NiCo2S4 BHSs under the AM 1.5G irradiation achieved a high PCE of 9.49%, which outperformed that of Pt (8.30%). Meanwhile, NiCo2S4 BHSs as HER catalysts under alkaline condition also obtained an onset overpotential (27.9 mV) and a Tafel slope (60.4 mV decade−1). Therefore, those works provide considerable approaches to conveniently produce bifunctional electrocatalysts used in DSSCs and alkaline HERs with good performances.

ASSOCIATED CONTENT Supporting Information XRD patterns, SEM images, TEM images, elemental mapping images, EDS spectra, J-V curves after stability testing and relevant comparison lists about recent DSSCs and HERs performances. This information is available free of charge through the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected]; [email protected]. Fax: +86-0591-2286 6244; Tel: +86-0591-2286 5220.

NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS We highly appreciate the National Natural Science Foundation of China (Grant Nos. 21702031 and 21676057), the Project of Education Department of Fujian Province (Grant No. JK2017004), and the Open Test Project of the Valuable Instruments of Fuzhou University (Grant No. 2017T034) for their financial support.

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Nickel Cobalt Sulfide Double-Shelled Hollow Nanospheres as

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