Synergistically Enhanced Electrochemical Performance of Ni3S4âPtX

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Synergistically enhanced electrochemical performance of Ni3S4-PtX (X=Fe, Ni) heteronanorods as heterogeneous catalysts in dye-sensitized solar cells Shoushuang Huang, Dui Ma, Zhangjun Hu, Qingquan He, Jiantao Zai, Dayong Chen, Huai Sun, Zhiwen Chen, Qiquan Qiao, Minghong Wu, and Xuefeng Qian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05418 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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ACS Applied Materials & Interfaces

Synergistically Enhanced Electrochemical Performance of Ni3S4-PtX (X=Fe, Ni) Heteronanorods as Heterogeneous Catalysts in DyeSensitized Solar Cells Shoushuang Huang,† Dui Ma,‡ ZhangJun Hu,† Qingquan He,† Jiantao Zai,‡ Dayong Chen,† Huai Sun, ‡ Zhiwen Chen,*, † Qiquan Qiao,§ Minghong Wu,* ,† Xuefeng Qian*, ‡ †

School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444,

China. E-mail: [email protected]; [email protected]; ‡

Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and Chemical

Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200240, China. E-mail: [email protected]; §

Center for Advanced Photovoltaics, South Dakota State University, Brookings, SD, 57007,

USA. KEYWORDS: hybrids, counter electrode, electrocatalytic, solar cells, energy conversion

ABSTRACT: Platinum (Pt) -based alloys are considerably promising electrocatalysts for the reduction of I‒/I3‒ and Co2+/Co3+ redox couples in dye-sensitized solar cells (DSSCs). However,

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it is still challenging to minimize the dosage of Pt to achieve comparable or even higher catalytic efficiency. Here, by taking full advantages of the Mott-Schottky (M-S) effect at metalsemiconductor interface, we successfully strategize a low-Pt-based M-S catalyst with enhanced electrocatalytic performance and stability for the large-scale application of DSSCs. The optimized M-S electrocatalyst of Ni3S4-Pt2X1 (X=Fe, Ni) heteronanorods are constructed by rationally controlling the ratio of Pt to transition metal in the hybrids. It was found that the electrons transferred from Ni3S4 to Pt2X1 at their interface under the Mott-Schottky effect results in the concentration of electrons onto Pt2X1 domains, which subsequently accelerates the regeneration of both I‒/I3‒ and Co2+/Co3+ redox shuttles in DSSCs. As a result, the DSSC with Ni3S4-Pt2Fe1 manifests an impressive power conversion efficiency (PCE) of 8.78% and 5.58% for iodine and cobalt-based electrolyte under AM1.5G illumination, respectively. These PCEs are obviously superior over the ones with Ni3S4-Pt, PtFe, Ni3S4 and pristine Pt electrodes. The strategy reported here is able to be further expanded to fabricate other low-Pt-alloyed M-S catalysts for wider applications in the fields of photocatalysis, water splitting, and heterojunction solar cells.

1.

INTRODUCTION

Dye-sensitized solar cells (DSSCs) have attracted considerable research interests as one kind of promising device for the conversion of solar energy directly to electrical energy.1-3 A traditional DSSC device consists of a N719-sensitized TiO2 porous film, an electrolyte containing I‒/I3‒ or Co2+/Co3+ redox couples, and counter electrode (CE). CEs collect electrons from the external circuit and reduce the I3‒ or Co3+ ions into I– or Co2+ in the electrolyte solution, playing key roles in improving and maintaining the photovoltaic performance of DSSC.4 In past years, carbon materials,5-7 organic polymers3,

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and inorganic compounds (such as sulfide,9 selenides,10-11


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carbides12 and oxides13) have been extensively explored as CE catalysts in DSSCs, but they still suffer from unsatisfactory PCE and long-term stabilities. Pt is by far the most preferred CE electrocatalyst for DSSCs, because of its electrocatalytic activity, high electron-conducting ability and fast charge-transfer capability.14 However, the high cost and rareness of Pt inevitably impedes the commercial application of DSSCs. To reduce the amount of Pt usage while maintaining its high electrocatalytic activity, two general strategies can be employed, including (1) enhancing the intrinsic activity of the active sites per unit geometric area15 and (2) alloying Pt with lower cost transition metals.16-18 For the latter, rationally synthesis of bimetallic or multimetallic Pt-based alloy catalysts not only makes possibility to reduce the dosage of Pt but also offers the chance to promote their electronic and chemical properties, enabling the engineering of Pt-based catalysts with improved activities and stabilities. For example, DSSC with PtNi0.33 alloy CE produces an outstanding PCE of 10.38% under AM1.5G radiation (100 mW cm-2), much higher than pure Pt CEs (6.75%).19 The device with a Pt9Fe1 CE also manifests a superior efficiency of 8.01%, yielding 10.6% and 18.8% enhancements compared with a Pt7Fe3 and Pt CE, respectively.20 Further study indicates that these enhanced catalytic activities are mainly derived from the good accordance of the work function of Pt-based alloy CEs with the redox potential of I–/I3–, which causes a weaker energy drop and thus facilitates the charge transfer from CE to electrolyte.21 Therefore, it provides a promising strategy to develop cost-effective and robust CE materials by alloying of Pt with transition metal species in DSSCs. Although great successes have been obtained for Pt-based alloy CEs, there are still some issues limiting the efficiency of DSSCs. For instance, the low surface-to-volume ratio of an alloy CE always results in unsatisfactory charger-transfer ability and therefore electrocatalytic activity. Besides, the high PCE can only be obtained when the thickness of a Pt-based alloy CE is up to


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few tens of nanometers, which means that the dosages of Pt species in alloyed electrocatalysts are still restricting the commercialization of DSSCs. Recently, various composites were developed to compensate the deficiency of the individual Pt-based alloy CE.18, 22-23 However, most composites are just prepared by physically mixing Pt-based alloy with two or even more materials, which results in poor catalytical performances due to their weak interactions between components.24 In contrast, metal-semiconductor based Mott-Schottky (M-S) catalysts provide an idea platform to tailor the catalytic activity of metal alloys, because the formed M-S effect can render an enhanced inner electric field at their interfaces and promote the transfer of electrons between metals and semiconductors.25-26 As a result, outstanding catalytic properties can often be obtained. For example, our recent work has revealed that the Ni3S4-PtCo heteronanorods exhibited preferable electrocatalytic activity toward the reduction of I3‒ ions due to the formed M-S heterojunctions.27 It’s well known that the good matching of work function between metal and semiconductor is critical for the formation of M-S heterojunctions. Up to now, most work is focused on the regulation of the shape, size, and components of semiconductive supports to obtain desired work functions.28 However, this is tedious and sometimes it cannot get satisfied results because of their unique physicochemical properties. In contrast, we find that it is a facile method to tune the work function of metal alloy for the constructing M-S heterojunctions, because the work function of Pt-based alloy can be simply tuned by the amount of metal precursor used in the synthesis. However, how to construct the M-S catalyst with the minimal amount of Pt is still unclear, and is definitely worth exploring. Recently, one-dimensional (1D)-based nanostructures have been extensively applied in the fields of water splitting,29-30 hydrogen evolution reaction,31 as well as CE catalysts in DSSCs,32-33 and attained satisfactory results due to a lot of advantages, such as the direct electrical pathways


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and abundant active sites on the exposed surfaces. Therefore, it is reasonably expected that 1Dnanostructures based CE catalysts are efficient and robust catalysts in DSSCs. On the basis of the above investigations, the design and synthesis of 1D M-S CE catalysts is a promising way to significantly improve the catalytic activity, stability and related photovoltaic performance of DSSCs. Herein, an optimized M-S electrocatalyst of Ni3S4-Pt2X1 (X=Fe, Ni) heteronanorods was constructed and employed as CE catalysts in DSSCs. The DSSCs with Ni3S4-Pt2Fe1 showed an impressive PCE of 8.78% for the I‒/I3‒ redox couple and 5.58% for the Co3+/Co2+ redox couple under illumination of AM1.5G (100 mW cm-2), much higher than the one with pure Pt CE in the corresponding electrolyte system. The excellent catalytic performance and stability of the Ni3S4PtX (X=Fe, Ni) heteronanorods in both of the systems make them promising alternatives to Pt in DSSCs. 2.

EXPERIMENT SECTION

Preparation of Ni3S4 nanorods, Ni3S4-Pt and Ni3S4-PtX (X=Fe, Ni) heteronanorods: The Ni3S4 nanorods (NRs) were synthesized following Liu’s procedure.34 The Ni3S4-PtX (X=Fe, Ni) heteronanorods and Ni3S4-Pt heteronanorods were prepared via the nucleation of PtX (X=Fe, Ni) and Pt on the surface of the Ni3S4 NRs.35-36 In a typical process, 0.2 mL oleic acid and 0.2 mmol 1, 2-hexadecanediol was mixed with 10.0 mL phenyl ether and then heated up to 200oC under nitrogen atmosphere. In parallel, 100 mg Ni3S4 nanorods, 2.0 mL oleylamine, 3.0 mL 1, 3dichlorobenzene, 25.0 mg Pt(acac)2 (48% min, Alfa) and different amounts of Fe(acac)3 or Ni(ac)2 were mixed together and sonicated for 30 minutes to help the dissolution of metal salts. The Ni3S4 suspension containing the metal precursors was then injected into the above solution at 200oC. After 10 min, the reaction was stopped by removing the flask from the heating source. Finally, products were collected by adding 30 mL of ethanol and then centrifuging at 5000 rpm


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for 5 min. The synthesis strategy of Ni3S4-Pt heteronanorods was the same as that for the Ni3S4PtX (X=Fe, Ni) nanorods except the addition of transition metal precursor. Synthesis of PtX (X=Fe, Ni) and Pt nanocrystals: In a typical synthesis of PX (X=Fe, Ni) nanocrystals, 0.2 mL of oleylamine, 0.2 mL of oleic acid, and 0.2 mmol of 1, 2-hexadecanediol were dissolved in 10 mL diphenyl ether. The solution was degassed under vacuum at 60°C for 30 min and then heated up to 200°C under N2 atmosphere. In parallel, 2.0 mL oleylamine, 3.0 mL 1, 3-dichlorobenzene, 25.0 mg Pt(acac)2 and different amounts of Fe(acac)3 or Ni(ac)2 were mixed together and slowly heated to 80oC for certain minutes to promote the dissolution of metal salts. Then the metal precursor was swiftly injected into the above diphenyl ether solution, and the color changed from yellow to black within 30 s. The reaction was allowed to proceed at 200oC for 10 min. Then, the products was precipitated with methanol followed by centrifugation and removal of the supernatant. Keeping other reaction parameters similar, Pt nanocrystals can be obtained except the addition of transition metal salts and oleic acid. Preparation of Ni3S4, Pt, Ni3S4-Pt and Ni3S4-PtX (X=Fe, Ni) CEs: All CEs were fabricated by spin-coating the as-synthesized nanocrystals “ink” (~ 100 mg mL-1) onto a cleaned FTO substrate (3 cm × 3.0 cm) at 500 rpm for 30 s at room temperature (SpinMaster 100, Chemat Technology, Inc.). In order to improve the conductivity of the CEs, the obtained films were dipped in 1, 2-ethanedithiol/acetonitrile solution (10 mM) for 30 s and (NH4)2S/formamide solution for 60 s,37-38 and subsequently calcined at 420oC for 30 min under nitrogen atmosphere. Fabrication of DSSCs: The iodine electrolyte-based DSSCs were fabricated as reported in our previous work.27, 39 Briefly, the photoanodes were prepared by immersing the commercial TiO2 films into a 0.5 mM ethanol solution of N719 dye for 24 h at room temperature. After that, the DSSCs device was assembled by attaching the N719-sensitized TiO2 porous film with the CEs


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with a 60 µm thick hot-melt film as spacer. The internal space of the cells were filled with redox electrolyte which is composed of 0.03 M I2, 0.6 M 1-butyl-3-methylimidazolium iodide, 0.5 M of 4-tert-butyl pyridine and 0.1 M guanidinium thiocyanate in acetonitrile. The cobalt-electrolyte based DSSC was also fabricated with a YD2-o-C8 dye-sensitized TiO2 photoanode, a CE, and an electrolyte containing 0.22 M cobalt (II), 0.055 M cobalt (III), 0.2 M 4-tert-butylpyridine and 0.1 M LiOCl4 with anhydrous acetonitrile as the solvent. The symmetrical dummy cell with active area of 0.25 cm2 was prepared by two identical CEs, and the redox electrolyte is similar to the one used in assembling complete DSSCs. Characterizations: The crystallographic structure of the as-synthesized Ni3S4 and Ni3S4-PtX (X=Fe, Ni) were characterized by power X-ray diffraction on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. Transmission electron microscopy (TEM) and Highresolution transmission electron microscope (HRTEM) images were taken with a JEOL-2100F microscope with an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) was carried out on a VG Scientific ESCLAB 220iXL X-ray photoelectron spectrometer. The photocurrent-voltage (J-V) curves of the assembled DSSCs with an active area of 0.16 cm2 were recorded under AM 1.5G illumination on a 94023A Oriel sol3A solar simulator. Incident phototo-current conversion efficiency (IPCE) curves were obtained at the short-circuit condition on an IPCE measurement systems. Cyclic voltammetry (CV) was performed in anhydrous acetonitrile solution containing 1 mM I2, 10 mM LiI and 0.1 mM LiClO4 with a scan rate of 50 mV s-1, where platinum nets electrode and the Ag/AgCl couple was used as counter and reference electrode, respectively. The electrochemical impedance spectroscopy (EIS) measurements were carried out with dummy cells by using a Zahner electrochemical workstation (Zahner Co., Germany) with the frequency ranging from 106 Hz to 0.1 Hz with an AC


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modulation signal of 10 mV, in dark condition. Tafel-polarization measurement were recorded at a scan rate of 50 mV s-1 in the symmetrical dummy cell. 3.

RESULT AND DISSCUSSIONS

Synthesis and characterization of Ni3S4-PtX (X=Fe, Ni) heteronanorods

Figure 1. Representative TEM images of the as-prepared Ni3S4 nanorods (a) and Ni3S4-Pt2Fe1 heteronanorods (b); HRTEM image of Ni3S4-Pt2Fe1 heteronanorods (c); Selected area FFT pattern from the Pt2Fe1 (d) and Ni3S4 (e) part of the heterostructure; XRD pattern (f) of the Ni3S4 nanorods and Ni3S4-Pt2Fe1 heteronanorods. TEM image reveals that the as-synthesized Ni3S4 nanorods (NRs) are highly monodispersed with an average length of ~34.0 nm and an average diameter of 9.0 nm (Figure 1a). The Ni3S4 NRs are then employed as the supports to produce 1D Ni3S4-PtFe heteronanorods. As shown in Figure 1b, no obvious size and shape of the Ni3S4 NRs are observed after metal nanocrystals


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deposition. However, many higher-contrast spots with sizes between 2 and 4 nm homogeneously distribute over the Ni3S4 nanorods surface (Figure 1b-c), which can be attributed to PtFe due to their higher electron density. The typical high-resolution (HRTEM) micrograph of a single Ni3S4-PtFe heteronanorod is presented in Figure 1c. In the middle, FFTs from the Ni3S4 and PtFe are shown. The FFT spectrum of PtFe nanocrystal in Figure 1d implies that it comprises a face centered cubic phase (space group = Pm-3m) and visualized along its [011] zone axis. While, a FFT spectrum obtained from the Ni3S4 NRs reveals its cubic phase (space group = Fd-3m) and visualized the [111] zone axis (Figure 1e). These results are in accordance with X-ray diffraction (XRD) pattern (Figure 1f) and X-ray photoelectron spectroscopy (XPS) (Figure S1), indicating that the sample is indeed composed of crystalline phases of Ni3S4 (JCPDS No. 76-1813, a = b = c = 9.457 Å) and PtFe (JCPDS No. 29- 0716, a = b = c = 3.866 Å). Additionally, the composition

Figure 2. TEM images of the as-prepared Ni3S4-Pt (a) andNi3S4-PtFe heteronanorods (b-c) with different nominal Pt/Fe ratios: (b) Pt/Fe = 2; (c) Pt/Fe = 1; (d) Pt/Fe = 0.5.


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of the PtFe on the Ni3S4 nanorods could be tuned by the amounts of Fe(acac)3 added in the synthesis. By keeping the amount of Pt(acac)2 constant at 25.0 mg and changing the amount of Fe(acac)3 from 13.5 mg to 27.0 mg, Fe contents in the products were controlled from 31.5% (Ni3S4-Pt2Fe1) to 63.2% (Ni3S4-Pt1Fe2) (Table S1). By using the similar alloying mechanism, high-quality Ni3S4-Pt2Ni1 heteronanorods can also be synthesized (Figure S2 and S3). These PtX (X=Fe, Ni) alloy nanocrystals have very similar size (5.5 nm) and size distribution despite the changes in composition. Application of Ni3S4-PtX (X=Fe, Ni) heteronanorods in iodine electrolyte-based DSSCs DSSCs with sandwich-structure are assembled and tested under 1 sun illumination. The J-V curves of DSSCs with Ni3S4-PtX (X=Fe, Ni), Ni3S4-Pt, Ni3S4, and Pt CEs are shown in Figure 3a, and the related photovoltaic parameters are summarized in Table 1. The DSSC using Ni3S4-Pt heteronanorods produces an open circuit voltage (Voc) of 754 mV, a short circuit current density (Jsc) of 16.52 mA cm-2, a fill factor (FF) of 0.69, thus resulting an overall PCE of 8.60%. This is an improved photovoltaic performance compared to that of the devices employing Ni3S4 nanorods (Voc = 748 mV, Jsc= 15.54 mA cm-2, FF = 0.63, η = 7.32%) and Pt (Voc = 761 mV, Jsc = 15.86 mA cm-2, FF = 0.65, η = 7.84%) as CE catalysts. These devices have comparable Voc (750 mV), apparently, the improvement of PCE is mainly contributed by the improvement of Jsc and FF.40 The increase of Jsc can be attributed to the accelerated regeneration of N719 dye molecules by I–species. While, the improvement of FF is caused by the reduced series resistance, chargetransfer resistance at the CE/electrolyte interface as well as diffusion resistance of I3− in the electrolyte.41-42 These results indicate that the Ni3S4-Pt hybrids can effectively improve the electrocatalytic activity toward the reduction of I3‒ ions in DSSCs. Additionally, it’s found that


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Figure 3. (a) J-V curves of the DSSCs using Ni3S4-PtFe, Ni3S4-Pt, Pt2Fe1, Ni3S4 and Pt CEs, measured at AM1.5G illumination; (b) IPCE spectrum of the corresponding DSSCs. Table 1 Photovoltaic performance obtained from DSSCs with various CEs with iodine-based electrolytes under AM1.5G illumination, and the electrochemical parameters of symmetric dummy cells with different CEs.

CEs

Voc

Jsc

(mV)

(mA cm-2)

Ni3S4-Pt2Fe1

739 ± 1.73

16.78 ± 0.12

Ni3S4-Pt

751 ± 2.38

Ni3S4-Pt1Fe1

FF

PCE

Rs

Rct

ZN

Epp

(%)

(Ω cm2)

(Ω cm2)

(Ω cm2)

(mV)

0.71 ± 0.00

8.79 ± 0.04

6.20

0.66

0.63

299

16.51 ± 0.06

0.69 ± 0.00

8.60 ± 0.02

6.25

0.72

0.68

308

752 ± 2.50

16.29 ± 0.07

0.68 ± 0.01

8.34 ± 0.03

6.29

0.78

0.71

312

Ni3S4-Pt1Fe2

756 ± 2.98

16.04 ± 0.08

0.66 ± 0.01

8.01 ± 0.03

6.22

0.84

0.82

316

Pt2Fe1

754 ± 2.21

15.94 ± 0.08

0.66 ± 0.01

7.91 ± 0.02

6.16

0.95

0.96

339

Pt

761 ± 2.50

15.82 ± 0.06

0.65 ± 0.01

7.83 ± 0.05

6.14

1.06

1.08

344

Ni3S4

747 ± 4.51

15.53 ± 0.05

0.63 ± 0.01

7.31 ± 0.03

6.46

1.62

1.16

403

the ratio of Pt to transition metal in the heteronanorods has a big influence on the PCE of DSSCs. The DSSCs using Ni3S4-Pt2Fe1 and Ni3S4-Pt2Ni1 CEs produce a higher PCE of 8.78% (Voc = 739 mV, Jsc = 16.73 mAcm-2, FF = 0.71) and 8.67% (Voc = 742 mV, Jsc = 16.69 mA cm-2, FF = 0.70) than that of Ni3S4-Pt (Figure S4 and Table S2), respectively. However, further increasing the ratio of transition metal in the hybrids, i.e., Ni3S4-Pt1Fe1 and Ni3S4-Pt1Fe2 CEs, causes a drop in


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the final PCE. Additionally, the IPCE curves of the DSSCs using Ni3S4, Pt, Ni3S4-Pt and Ni3S4PtFe CEs are also measured in the wavelength range of 400 ~ 800 nm under the short-circuit condition. As shown in Figure 3b, all devices exhibit good IPCE values from 400 to 600 nm and the quantum efficiency are maximized at about 520 nm. These results imply that the as-prepared CEs can provide sufficient and fast regeneration of I3– ions in the electrolyte.43-44 Especially, the IPCE value for the DSSCs with Ni3S4-Pt2Fe1 is significantly higher than the others, which means that the the Ni3S4-Pt2Fe1 CE can significantly accelerate the generation of electrons into photocurrent. Obviously, the IPCE is in good agreement with the difference of PCE and Jsc values, indicating their catalytic activity for the reduction of I3– ions follows the same trend.

Figure 4. CV curves of the Ni3S4-PtFe, Ni3S4-Pt, Pt2Fe1, Ni3S4 and Pt CEs in an iodine-based electrolyte with a scanning rate of 50 mV s-1. Cyclic voltammetry (CV) is conducted to characterize the electrocatalytic activity of the Ni3S4-PtFe, Ni3S4-Pt, Pt2Fe1, Ni3S4 and Pt CEs toward the reduction of I3‒ ions at room temperature. As shown in Figure 4, all electrodes exhibit two typical pairs of oxidation-reduction peaks (Red-A: I3‒ + 2e = 3I‒; Ox-A: 3I‒ - 2e = I3‒; Red-B: 3I2 + 2e = 2I3‒, Ox-B: 2I3‒ - 2e = 3I2),


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indicating the same electrocatalytic mechanism towards I3‒ ions and good reversibility they have. The peak current density of Red-A and peak-to-peak separation (Epp) between Red-A and Ox-A are the two important parameters for comparing the catalytic activities of different CEs.45-46 Generally, a larger peak current density of Red-A means the better catalytic activity the CE has, and a smaller Epp value indicates a smaller overpotential for the reduction reaction.12, 47 The Ni3S4-Pt CE produces a higher peak current density (2.27 mA cm-2) than that of Ni3S4 (1.63 mA cm-2) and Pt CE (1.75 mA cm-2). Additionally, the Epp of Ni3S4-Pt CE is approximately 308 mV, also smaller than Ni3S4 (403 mV) and Pt CE (344 mV). The higher peak current density and lower Epp value indicate the Ni3S4-Pt CE presents enhanced catalytic activity and a lower overpotential loss toward the reduction of I3− ions, which is a paramount prerequisite for an excellent CE in DSSCs. After carefully regulating the ratio of Pt/Fe in the heteronanorods, the resulting Ni3S4-Pt2Fe1 CE provides a higher peak current density and lower Epp value than the Ni3S4-Pt, Ni3S4-Pt1Fe1 and Ni3S4-Pt1Fe2 CEs, which confirms that it has the best electrocatalytic activity toward the reduction of I3– ions. Additionally, the stacking CV plots of Ni3S4-Pt2Fe1 CE were recorded at a scan rate from 10 to 100 mV s-1 (Figure S5). The good linear relationships between the peak current density and the square root of corresponding scan rate suggests the reduction process of I–/I3– redox couples at Ni3S4-Pt2Fe1 CEs obeys a diffusion-controlled mechanism, and there is no specific interaction between Ni3S4-Pt2Fe1 CE and I–/I3– redox couples.11, 45 EIS measurements are further employed to investigate internal resistance and charge transfer kinetics at the CE/electrolyte interface. The Nyquist plots for symmetric cells (CE/electrolyte/CE) conducted in dark at a bias voltage of 0 V are illustrated in Figure 5a.

48-49

The values of Rs

(series resistance), Rct (charge-transfer resistance) and ZN (nernst diffusion impedance) are


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obtained by fitting the Nyquist plots with the software of Zview 3.2 version, and summarized in Table 1. The Rct value of Ni3S4-Pt2Fe1 CE is 0.66 Ω cm2, smaller than those of Ni3S4-Pt (0.72 Ω cm2), Ni3S4-Pt1Fe1 (0.78 Ω cm2), Ni3S4-Pt1Fe2 (0.84 Ω cm2), Pt2Fe1 CE (0.95 Ω cm2) and Pt CE (1.06 Ω cm2), which implies that the Ni3S4-Pt2Fe1 CE has higher electrocatalytic activity than the others. This rapid conversion of I3‒ ions into I‒ ions is beneficial for dye recovery, which facilitate the photogenerated electrons flow from the N719 molecules to the conduction band of TiO2.50 In addition, the ZN value for the seven symmetric cells increase by a sequence of Ni3S4Pt2Fe1 (0.63 Ω cm2) < Ni3S4-Pt (0.68 Ω cm2) < Ni3S4-Pt1Fe1 (0.71 Ω cm2) < Ni3S4-Pt1Fe2 (0.82 Ω cm2) < Pt2Fe1 (0.96 Ω cm2) < Pt (1.08 Ω cm2) < Ni3S4 (1.16 Ω cm2), revealing a faster diffusion velocity of the redox species in the electrolyte for Ni3S4-Pt2Fe1 CE.51 These results confirm that the as-synthesized Ni3S4-Pt2Fe1 heteronanorods can reduce the charge-transfer resistance at the CE/electrolyte interface, and is conducive for the improvement of photovoltaic performance of DSSCs. EIS analysis of complete DSSCs were further studied under AM1.5G illumination at a bias voltage of the open circuit voltage. As shown in Figure 5b and Table S3, the charge transport impedances at the CE/electrolyte interface for complete DSSCs (Rct1) are much larger than the ones in symmetric cells. However, the Rct1 value for Ni3S4-Pt2Fe1 CE is still smaller than the other CEs, which confirms the Ni3S4-Pt2Fe1 CE has the best electrocatalytic activity for the reduction of I3– ions in DSSCs.52 Additionally, the constant phase element magnitude of CE (CPE1) decrease in the order of Ni3S4-Pt2Fe1 (48.88 mF cm-2) > Ni3S4-Pt (44.53 mF cm-2) > Ni3S4-Pt1Fe1 (39.16 mF cm-2) > Ni3S4-Pt1Fe2 (35.89 mF cm-2) > Pt2Fe1 (32.44 mF cm-2) > Pt (28.57 mF cm-2) > Ni3S4 (24.45 mF cm-2), suggesting a same order of the active surface area. Furthermore, the lower Nernst diffusion impedance (W) of the complete DSSCs using


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Figure 5. (a) Nyquist plots for the symmetric dummy cells conducted in dark with a bias voltage of 0 V and (b) Nyquist plots of complete DSSCs under illumination of 100 mW cm-2 with a bias voltage of the open circuit voltage. The inset shows the relevant equivalent circuit mode by the Z-view software. Ni3S4-Pt2Fe1 CE (1.39 Ω cm2) compared to that of Ni3S4-Pt (1.56 Ω cm2), Ni3S4-Pt1Fe1 (1.53 Ω cm2), Ni3S4-Pt1Fe2 (1.54 Ω cm2), Pt2Fe1 (1.71 Ω cm2), Pt (1.88 Ω cm2) and Ni3S4 (3.33 Ω cm2) CEs, indicating the diffusion of I3– ions on Ni3S4-Pt1Fe1 CE is more fluent. The conclusions of the catalytic activity obtained from the EIS measurements with symmetric dummy cells and complete DSSCs are consistent. Tafel polarization measurements are also performed to confirm the excellent activities of the Ni3S4-PtX heteronanorods for the reduction of I3– ions. As shown in Figure 6, the slope of a tangent for Ni3S4-Pt2Fe1 CE is slightly higher that of Ni3S4-Pt, Ni3S4-Pt1Fe1, Ni3S4-Pt1Fe2, Pt2Fe1, Pt and Ni3S4 CEs, which implies that the Ni3S4-Pt2Fe1 CE has the largest exchange current density (J0) and thus the best electrocatalytic activity.53 Additionally, the intersection of the cathodic branch with the y-axis in the diffusion zone can be considered as the limiting diffusion current density (Jlim), a parameter depending on the diffusion coeffcient (Dn) of the redox couple


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in electrolyte and CE catalysts. Typically, Jlim is proportional to Dn: Jlim = 2nFCDn/l, where n is the number of electrons involved in the reduction of I3−, C is I3− concentration, and l is the distance between the electrodes in a dummy cell. Dn can also be obtained by the Randles-Sevcik theory: Jred = Kn1.5ACDn0.5ν0.5,54 where Jred is the peak current density of Red-A, K is 2.69×105, A is the active area of dummy cell (0.25 cm2), and ν is the scan rate for CV plots. The calculated Dn values are in a decreasing order: 8.88×10-7 cm-2 s-1 (Ni3S4-Pt2Fe1) > 7.12×10-7 cm-2 s-1 (Ni3S4Pt) > 6.25×10-7 cm-2 s-1 (Ni3S4-Pt1Fe1) > 5.59×10-7 cm-2 s-1 (Ni3S4-Pt1Fe2) > 4.75×10-7 cm-2 s-1 (Pt2Fe1) > 3.84×10-7 cm-2 s-1 (Pt) > 3.84×10-7 cm-2 s-1 (Ni3S4). The highest Dn of Ni3S4-Pt2Fe1 CE among the CEs implies its highest diffusion velocity, which is well in agreements with the results from CV and EIS characterization.

Figure 6. Tafel polarization curves of the symmetric dummy cells fabricated by two identical CEs. The electrochemical stability of counter electrode catalysts is another important parameter for evaluation of the potential applications in DSSCs. To check the electrochemical stability of the as-prepared samples in the electrolyte, CEs are subjected to sequential scanning CVs for 50 cycles (Figure S6). One can see that the current densities and the Epp value of Ni3S4-Pt2Fe1 and


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Ni3S4-Pt CEs have nearly no change after 50 cycles. However, the Epp value increases noticeably for Pt2Fe1, Pt and Ni3S4 CE. These results demonstrate that the Ni3S4-Pt2Fe1 and Ni3S4-Pt CEs have better corrosion resistance than that of Ni3S4 and Pt CEs in the iodine-based electrolyte. Moreover, symmetrical dummy cells (CE/electrolyte/CE) were also subjected to sequential scanning of EIS for 10 cycles (Figure S7). There are no obvious changes in both of the Rs and ZN for all CEs, suggesting that the potential cycling exerts negligible influence on the series resistance as well as the mass transport between the CEs and redox pairs. However, the Rct increase markedly after 10 times repeated scanning. The Rct for Ni3S4-Pt2Fe1, Ni3S4-Pt, Pt2Fe1, Pt and Ni3S4 CEs increased by 0.05, 0.08, 0.42, 0.49 and 0.68 Ω cm2, respectively. On the basis of the above investigations, we can make a conclusion that Ni3S4-Pt2Fe1 CEs have better electrochemical stability than that of Ni3S4, Pt2Fe1 and pristine Pt counter electrodes in the corrosive I3–/I– electrolyte system, which are in good accordance with the results from multicycle CV. Application of Ni3S4-PtX heteronanorods in cobalt electrolyte-based DSSCs Iodide-based redox shuttle (I‒/I3‒) is prevalent in DSSCs, but I‒/I3‒ electrolyte solution suffers from several disadvantages obstructing the commercializing development of DSSCs, for example, a significant loss in open circuit voltage and short circuit current density, the extensive corrosion to metal CEs and absorption of visible light.55-57 To overcome these issues, Co2+/Co3+,5 disulfide/thiolate,58 SeCN−/(SeCN)59 and Br‒/Br3‒52 have been extensively explored as alternative redox couples to further improve the performance of DSSCs. Among them, the Co2+/Co3+ redox couples have attracted the most attention because of its unique merits,60 such as the reduced corrosivity towards metallic conductors and higher redox potential, thus a high PCE of 12.3% has been achieved for the porphyrin-sensitized DSSCs.61 Herein, the Ni3S4-PtX (X=Fe, Ni) CEs


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Figure 7. (a) J-V characteristics of DSSCs with the Co2+/Co3+ electrolyte with various CEs. (b) Nyquist plots, (c) bode spectrum and (d) Tafel polarization curves of the cobalt electrolyte-based symmetrical cells. Table. 2 Photovoltaic performance of cobalt-electrolyte based DSSCs, and the electrochemical parameters of the symmetrical cells with various CEs. CEs

Voc

Jsc

FF

PCE

Rs

Rct

ZN

τ1

(mV)

(mA cm-2)

(%)

(Ω cm2)

(Ω cm2)

(Ω cm2)

(ms)

Ni3S4-Pt2Fe1

782 ± 4.65

10.31 ± 0.11

0.69 ± 0.01

5.56 ± 0.04

6.81

2.21

19.50

0.43

Ni3S4-Pt

788 ± 3.10

9.91 ± 0.07

0.68 ± 0.00

5.32 ± 0.04

6.84

3.45

21.11

0.96

Pt2Fe1

789 ± 5.97

9.65 ± 0.01

0.67 ± 0.01

5.11 ± 0.03

6.57

4.97

26.84

1.35

Ni3S4

780 ± 3.10

8.67 ± 0.07

0.65 ± 0.00

4.44 ± 0.05

7.35

10.61

33.82

2.89

Pt

783 ± 3.77

9.27 ± 0.12

0.66 ± 0.01

4.85 ± 0.06

6.52

6.54

30.77

1.63


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were introduced into a Co2+/Co3+ redox couple to test their photovoltaic performance. As shown in Figure 7a and Table 2, the DSSC with Pt CE produce a Voc of 792 mV, a Jsc of 9.27 mA cm-2, a FF of 0.66, resulting an overall PCE of 4.85%. In contrast, the devices using Ni3S4-Pt2Fe1, Ni3S4-Pt2Ni1 and Ni3S4-Pt CEs result in a PCE of 5.58%, 5.46% and 5.32%, respectively (Figure S8 and Table S4). These results indicate the as-designed Ni3S4-PtX heteronanorods also have excellent electrocatalytic activity of for the reduction of the Co3+ ions in the cobalt-based electrolyte. To confirm their high catalytic activity for Co2+/Co3+ reduction, EIS and Tafel tests are employed with symmetrical cells. The Ni3S4-Pt2Fe1 CE exhibits the lowest Rct and ZN among the Ni3S4-Pt, Ni3S4 and Pt CEs, indicating it has the highest electrocatalytic activity toward the reduction of Co3+ ions (Figure 7b and Table 2). However, when compared with its catalysis toward I‒/I3‒ redox couples, the Ni3S4-Pt2Fe1 CEs have a larger ZN value. This is probably one of the main reasons for the lower PCE (3.69%) of the cobalt-mediated DSSCs than the I3‒/I‒ based DSSCs (8.78%), i.e., the larger size of the Co2+/Co3+ redox shuttles than I3‒/I‒ redox shuttles used in the electrolyte inevitably lead to lower mass transport and ion mobility. Therefore, the ineffcient mass transport and charge transfer rate lead to the unsatisfied regeneration of dye molecules and limited PCE.62 The above conclusion is supported by the electrons lifetime (τ1) participated in the Co3+ reduction reaction between CE and electrolyte. A smaller τ1 often means the less time is required for the reduction of Co3+ by CEs.63 The lower τ1 value for Ni3S4-Pt2Fe1 (0.43 ms) and Ni3S4-Pt (0.96 ms) implies an enhanced reduction reaction kinetics and higher electrocatalytic activity toward the reduction of Co3+. Additionally, the relatively larger slope of the cathodic branch of the Ni3S4-Pt2Fe1 CE than Ni3S4-Pt, Pt2Fe1, Pt and Ni3S4 CEs in the Tafel


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curves (Figure 7d) also demonstrate that the Ni3S4-Pt2Fe1 CE has superior electrocatalytic performance toward the Co3+ reduction. Electrocatalytic behaviour analysis of the M-S heterojunction Taking the results of the electrochemical characterization into consideration, including PCE, CV, EIS and Tafel polarization, the as-synthesized Ni3S4-PtX (X=Fe, Ni) heteronanorods indeed exhibit enhanced catalytic activity and stability toward the regeneration of I3– and Co3+ redox shuttles in DSSCs. To further illustrate the advantages of the Ni3S4-PtX (X=Fe, Ni) heteronanorods in catalyzing the reduction of I3‒ and Co3+ ions, the work functions of Ni3S4 and PtX alloys were subsequently conducted to analyze the structural features of the as-synthesized heteronanorods according to the reported literatures.16, 64 The detailed theoretical computation method was provided in the Supporting Information (Figure S9). As a p-type semiconductor, Ni3S4 has a band gap of 1.74 eV with a conduction band edge position at -3.59 eV versus vacuum level (Figure S10). In addition, the work functions for Ni3S4 (111), Pt (111), Pt2Fe1 (111), Pt1Fe1 (111), Pt1Fe2 (111) and Pt2Ni1 (111) surfaces are found to be 5.12 eV, 5.51 eV, 5.26 eV, 4.83 eV, 4.43 eV and 5.22 eV respectively. For the Ni3S4-Pt, Ni3S4-Pt2Fe1 and Ni3S4-Pt2Ni1 heterojunctions system, the work function of Pt, Pt2Fe1 and Pt2Ni1 is higher than that of Ni3S4. Therefore, a weak electric field comes into being at their interface and renders the electrons flow from the Ni3S4 to the Pt or Pt2Fe1 or Pt2Ni1 until their Fermi levels are aligned (Figure 8a).26, 65 As a result, the Ni3S4 is positively charged and the Pt, Pt2Fe1 and Pt2Ni1 is negatively charged near the heterojunction interface due to electrostatic induction.64 In this regard, the accumulated electrons in Pt, Pt2Fe1 and Pt2Ni1 would arouse the absorption of I3– or Co3+ ions from the electrolyte. Obviously, the number of I3–or Co3+ ions adsorbed by Pt, Pt2Fe1 and Pt2Fe1 should be proportional to the number of accumulated electrons in their domains. In present work, although


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Figure 8. (a) The band configuration of Ni3S4 and work functions of Pt-based alloy. (b) Schematic illustration of Mott-Schottky-type contact of Ni3S4-Pt2X1 (X = Fe, Ni). E0 is the vacuum level, Ev is the top of the valence band of Ni3S4, Eg is the band gap, Ec is the bottom of the conduction band of Ni3S4, EF are the Fermi levels of Ni3S4, and Φ is the work function. a stronger Mott-Schottky effect may be formed between Ni3S4 and Pt due to the lower work functions of Pt2Fe1 and Pt2Ni1 compared with Pt, the Ni3S4-Pt2Fe1 and Ni3S4-Pt2Ni1 CEs still exhibit better catalytic activity and photovoltaic performance than Ni3S4-Pt CEs. This can be explained by the fact that the Pt in PtX (X = Fe, Ni) alloy has a rich electronic structure due to the partial electrons transfer from Fe to the near-surface of Pt.66-67 It is experimentally supported by the XPS spectrum, in which the Pt0 4f7/2 peak in the Ni3S4-Pt2Fe1 samples is slightly shifted to higher binding energies than that of Ni3S4-Pt (Figure S11). Moreover, the better matching of work functions of the Pt2Fe1 and Pt2Ni1 with the redox potential of I–/I3– means a weaker energy drop, which facilitates the charge transfer from CE surface to I3–. Therefore, the Ni3S4-Pt2Fe1 and Ni3S4-Pt2Ni1 CEs exhibit lower charge transfer resistance, faster reaction kinetics, higher diffusion coefficient and better photovoltaic performance than that of Ni3S4-Pt CE. In contrast, the work functions of Pt1Fe1 and Pt1Fe2 are lower than that of Ni3S4, indicating that the holes will


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flow from the Ni3S4 to the Pt1Fe1 or Pt1Fe2 domains, which results in a ohmic contact and subsequent decrease its catalytic activity. These results further demonstrate the key role of the M-S effect in promoting the catalytic activity as well as photovoltaic performance of DSSCs.

Figure 9. (a) PL emission spectra of pure Ni3S4 and Ni3S4-Pt2Fe1 samples under 550 nm excitation at room temperature; (b) Surface photo-voltage spectrum of the as-synthesized Ni3S4 nanorods and Ni3S4-Pt2Fe1 heteronanods. The above calculation of the formation of M-S heterojunctions between Ni3S4 and Pt2Fe1 is strongly supported by the decreased photoluminescence (PL) intensity of Ni3S4. The PL spectrum of pure Ni3S4 nanorods and Ni3S4-Pt2Fe1 heteronanorods were recorded with an excitation wavelength of 500 nm. As shown in Figure 9a, the pure Ni3S4 nanorods have an emission centered at around 750 nm due to the band gap recombination of electron-hole pairs. In contrast, no detectable PL was observed for the Ni3S4-Pt2Fe1 heteronanorods. This result indicates that the Pt2Fe1 nanocrystals loaded onto the surface of Ni3S4 nanorods could significantly decrease the charge recombination rates, which demonstrates the efficient charge transfers between the Ni3S4 and Pt2Fe1.68 Additionally, the surface photovoltage spectrum of the Ni3S4-Pt2Fe1 heteronanorods exhibits significant surface photovoltage effects between 300 and


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450 nm, which directly hints that it has enhanced charge separation at the surface of the sample due to the formation of Ni3S4-Pt2Fe1 heterojunctions (Figure 9b).65 All these results demonstrate that Mott-Schottky heterojunction is formed at the interface of Ni3S4 and the Pt2Fe1 NPs. Based on the above theoretical calculation and electrochemical analysis, Ni3S4-Pt2X1 (X=Fe, Ni) CEs indeed display enhanced electrocatalytic performance in DSSCs due to its unique hybrid structures and composite character. Firstly, the 1D Ni3S4 nanorods have more active sites on the exposed surface, providing larger electrochemical active sites and shorter electron diffusion path to catalyze the reduction of I3−. Secondly, the existence of M-S heterojunctions between Ni3S4 and Pt2X1 alloy render an enhanced inner electric field at their interfaces, which could improve the electron transfer over the whole electrochemical reactions and then accelerate the catalytic reactions. Thirdly, the work function of Pt2X1 (X= Fe, Ni) matches better with the potential of I3−/I− compared with pristine Pt, which is favorable for the electron transport from CE to electrolyte. 4.

CONCLUSION In summary, monodisperse Ni3S4 nanorods are synthesized via a hot-injection method and

further employed as matrixes to produce Ni3S4-PtX (X= Fe, Ni) heteronanorods. The synergistic integration fully exploits the benefits of both structural and compositional characteristics of the heteronanorods, namely, the direct electrical pathways of 1D nanostructure, optimized work function of metal alloy as well as the fast charge-transfer ability between Ni3S4 nanorods and Ptbased alloy, endowing the M-S catalysts with excellent catalytic activity and stability for the reduction of both I3‒ and Co3+ ions in DSSCs. As a result, the Ni3S4-Pt2Fe1 heteronanorods showed a impressive PCE of 8.78% for the I3–/I– redox couple and 5.58% for the Co3+/Co2+ redox couple under AM1.5G illumination, which is superior to the pristine Pt CE in the


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corresponding electrolyte system. This study presented here is far from being fully optimized, but provide a promising approach to reduce the cost and to improve the photovoltaic performance of DSSCs by using earth abundant, more effciently catalytic semiconductor-metal alloy M-S heterojunctions.

ASSOCIATED CONTENT Supporting Information. XPS spectrum of the Ni3S4-Pt2Fe1 heteronanorods; TEM, HRTEM and XRD of the Ni3S4-Pt2Ni1 heteronanorods; CV, EIS, Tafel polarization and J-V curve of Ni3S4-Pt2Ni1 heteronanorods in iodine and cobalt-based electrolyte; multiple CV in acetonitrile solution and EIS spectrum of the Ni3S4-Pt2Fe1 CE; the work function of Ni3S4 (111), Pt (111) and Pt-based alloy; AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by National Natural Science Foundation of China (21601120, 11375111, 11428410 and 41430644), the Project Funded by China Postdoctoral Science


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Foundation (2017M610244), the Science Technology Commission of Shanghai Municipality (14DZ2250800, 17ZR1410500 and 14JC1402000) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13078).

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Synergistically Enhanced Electrochemical Performance of Ni3S4âPtX

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