Composition-Dependent Electrocatalytic Activity of Cobalt Sulfides for

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Composition-Dependent Electrocatalytic Activity of Cobalt Sulfides for Triiodide Reduction in Dye-Sensitized Solar Cells Min Soo Kim, and Jin Ho Bang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08449 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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The Journal of Physical Chemistry

Composition-Dependent Electrocatalytic Activity of Cobalt Sulfides for Triiodide Reduction in DyeSensitized Solar Cells Min Soo Kim† and Jin Ho Bang†,‡,* Department of Bionano Technology and Department of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea AUTHOR INFORMATION Corresponding Author: Jin Ho Bang *Email: [email protected] †

Department of Bionano Technology, Hanyang University

‡

Department of Chemical and Molecular Engineering, Hanyang University

ACS Paragon Plus Environment

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ABSTRACT

A new nanoarchitecture of cobalt sulfide (CoSx) is designed by exploiting a Prussian blue analogue. Depending on the sulfidation temperatures, CoSx with different compositions and morphologies are obtained. This investigation of the composition-dependent electrocatalytic activity of CoSx for triiodide reduction reaction (IRR) reveals that sulfur-deficient CoSx is more active than sulfur-rich CoSx. When utilized in dye-sensitized solar cells (DSSCs), sulfurdeficient CoSx with a hollow nanocube morphology outperforms platinum (Pt), showing great promise as a Pt alternative. This composition dependency on IRR is attributed to different surface characteristics and electrical properties that vary with CoSx composition. This work highlights the importance of understanding the surface properties of sulfide-based electrocatalysts that are intimately dictated by their compositions as part of a new design principle for a highly active electrocatalyst.


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1. INTRODUCTION In electrocatalysis, a great deal of effort has recently been exerted to develop low-cost, highly active electrocatalysts that can potentially replace Pt.1-3 In DSSCs, for instance, various transition metal compounds based on earth-abundant elements have been explored to find a highly efficient catalyst that can work for the counter electrode (CE).4-14 This effort has been driven not only because of the scarcity of Pt, but also because of its limited chemical stability over time when in contact with iodine-based electrolytes.15 CoS was the first sulfide material explored as a Pt-free electrocatalyst in DSSCs.16 Since then, CoSx with different compositions (e.g., CoS, CoS1.0365, CoS1.097, Co3S4, and CoS2) and various nanostructures have been exploited in DSSCs and even in water electrolysis.17-36 Despite such efforts, not many aspects of the IRR activity of CoSx have been revealed to date. In particular, we noticed that no attention has been paid to the influence of the composition of CoSx on their IRR activity, while diverse stoichiometric ratios are available for CoSx.37 Because the physical properties of metal sulfides are closely linked to their stoichiometry,38-40 examining the composition dependency of the IRR activity is essential for new designs for a highly efficient CoSx electrocatalyst. In this report, we identify two advances for the development of CoSx electrocatalysts: i) the introduction of a novel nanostructure (hollow nanocubes) and ii) the description of a new insight into the effect of composition on the IRR activity of CoSx. To date, CoSx electrocatalysts for DSSCs have been prepared in the form of composites with carbon, films deposited electrochemically or wetchemically, or one-dimensional arrays.16-22,25,26,28,41 Unless supported by carbon, nanostructured CoSx is generally prepared in sub-micrometer-sized particles with limited porosity, hampering the full utilization of their active sites. To overcome such limitations, we herein propose hollow


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nanocubes because hollow nanostructures are an attractive choice for electrocatalysis owing to their excellent physical properties, including high surface area, improved mass transfer, and structural robustness against agglomeration.42,43 By virtue of these benefits, when utilized as an electrocatalyst in DSSCs, the CoSx hollow nanocubes yield a higher efficiency (8.48%) than Pt (8.01%), which is the highest power conversion efficiency (PCE) for the DSSCs harnessing CoSx-based CEs. In addition to this improvement, our synthesis approach yields mixed phase CoSx, which stimulated our interest in the composition effect on the IRR activity of CoSx. X-ray photoelectron spectroscopy (XPS) combined with electrochemical analysis sheds light on the relationship between the composition and the IRR activity. This approach revealed that sulfurdeficient CoSx is more active than its sulfur-rich counterparts, suggesting that sulfur is intimately associated with favorable surface characteristics and electrical properties. 2. EXPERIMENTAL SECTION 2.1 Synthesis of CoSx. Prussian blue analogue (Co3[Co(CN)6]2) was synthesized as described in the literature.44 Two aqueous solutions were prepared: 10 mL of 7.5 mM Co(CH3COO)2·nH2O solution (Solution A) and 10 mL of 4 mM K3[Co(CN)6]2 solution containing 0.3 g of polyvinylpyrrolidone (PVP) (Solution B). Solution A was slowly added to solution B using a syringe pump while being stirred for 10 min. The mixed solution was then left undisturbed for 4 h at room temperature to induce Ostwald ripening. The resulting precipitate was centrifuged, washed with deionized water and ethanol several times, and dried in air at 60 °C. To synthesize CoSx, Co3[Co(CN)6]2 was heated in an H2S atmosphere for 1 h at 300, 350, and 400 °C, respectively.


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2.2. Electrode Fabrication. Fluorine-doped SnO2 (FTO) glass was sonicated for 30 min in a mixed solution containing isopropanol, ethanol, and acetone (1:1:1 ratio). The pre-cleaned FTO glass was dipped in 40 mM TiCl4 solution at 70 °C for 30 min. Three layers of nanosized TiO2 paste (Solaronix T/SP) and two layers of scattering TiO2 paste (Dyesol DSL 18NR-AO) were applied to the FTO glass via a screen printing method. After screen-printing, the TiO2 layers were sintered at 550 °C for 1 h. The resulting TiO2 electrode was immersed in 40 mM TiCl4 solution at 70 °C for 30 min and subsequently annealed at 450 °C for 30 min. The prepared photoanode was treated with 0.1 M HNO3 solution for 15 min and immediately dipped in 0.5 mM N719 dye ethanol solution at 40 °C for 2 h.45 To prepare CoSx paste, 54 mg of CoSx and 3 mg of a carbon black (super P) were added to an aqueous solution containing 3 mg of carboxymethyl cellulose. The mixture was thoroughly mixed for 40 min to form a homogeneously blended paste. The CoSx paste was loaded on the cleansed FTO glass via the doctor-blade method using a micrometer adjustable film applicator to keep the film thickness identical for every trial. The thickness of the CoSx layer was set to 10 µm. The CoSx film was dried at 120 °C for 12 h in a vacuum oven before use. The standard Pt CE was prepared via the sputtering method. The photoanode and each CE were assembled into sandwich-type cells, and the space between the two electrodes was filled with an iodide-based electrolyte (Dyesol, ELHPE). Symmetric dummy cells were fabricated with two identical CEs with the iodide electrolyte filled in between. 2.3 Characterization and Electrochemical Analysis. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out using a field-emission scanning electron microscope (Hitachi S-4800) and a transmission electron microscope (JEOL 2010F). An X-ray diffractometer (Rigaku D/Max-2500/PC) was used to obtain diffraction patterns. Fourier


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transform infrared (FT-IR) spectra were recorded using a Varian 800 infrared spectrometer. Nitrogen adsorption/desorption isotherms were measured using a surface area analyzer (BELSORP-mini II) at 77.4 K. The Brunauer–Emmett–Teller (BET) method was employed to determine specific surface area, and the Barrett–Joyner–Halenda (BJH) method was used to analyze pore size distribution. XPS was performed using a PHI Versa Probe system with a 100 W Al Kα X-ray source. Cyclic voltammetry (CV) measurements were complemented at various scan rates in an electrolyte containing 50 mM LiI, 10 mM I2, and 500 mM LiClO4 in acetonitrile with the Ag/Ag+ quasi-reversible reference electrode and Pt mesh as the counter electrode using an electrochemical workstation (CHI 660D, CH Instruments). Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 100 kHz to 50 mHz with a perturbation amplitude of 5 mV, and Tafel polarization measurements were carried out in the voltage range of -0.6−0.6 V at a scan rate of 5 mVs-1. Current–voltage (J–V) response of DSSCs (the active area was 0.188 cm2) was measured using a Keithley 2400 source meter under illumination from a solar simulator (HAL-320, Asahi Spectra). A standard silicon diode (CS-20, Asahi Spectra) was used to calibrate the light intensity of the solar simulator to one sun condition (air mass 1.5G). 3. RESULTS AND DISCUSSION 3.1 Synthesis and Characterization of CoSx The CoSx hollow nanocubes were prepared by heating Co3[Co(CN)6]2, one of the old, traditional metal-organic frameworks (MOFs), under H2S gas flow at different temperatures. This self-templating approach using a MOF has become popular recently because of its versatility for diverse materials and ability to produce unique nanostructures that are not


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obtainable from conventional synthesis approaches.46,47 The cube-shaped Co3[Co(CN)6]2 serves as a structural frame for the synthesis of hollow nanocubes so long as the decomposition of bridging cyanide ligands is delicately controlled to avoid an abrupt structural collapse. A facecentered cubic phase of crystalline, microporous Co3[Co(CN)6]2 powder was prepared as noted in a previous report (see the characterization results in Figure S1).44 Figure 1 shows a schematic illustration of the formation of CoSx hollow nanocubes from Co3[Co(CN)6]2 and the TEM images of each stage of this conversion process as performed at 300 °C. Upon sulfidation, the solid nanocube-shaped Co3[Co(CN)6]2 begins to transform into CoSx while releasing gaseous products during the decomposition of the cyanide ligands. With time, a hollow void begins to form, and the CoSx grains composing the outer shell grow, eventually forming a rigid hollow frame. We speculate that the inner pressure built up by the decomposition of Co3[Co(CN)6]2 at the early stages of formation help to form the hollow void, while CoSx inherits the original morphology of Co3[Co(CN)6]2. Energy-dispersive X-ray spectroscopy (EDS) performed under a dark-field TEM mode confirms the formation of hollow nanocubes of CoSx (Figures S2-3).


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Figure 1. Time resolved TEM analysis during the conversion of Co3[Co(CN)6]2 into CoSx hollow nanocubes at 300 °C under hot H2S flow at (A) 0 min, (B) 10 min, (C) 20 min, and (D) 30 min. (E) Schematic illustration of the formation of CoSx hollow nanocubes.

Figure 2 shows the electron micrographs of CoSx obtained at different sulfidation temperatures. The SEM images in Figures 2A-D reveal that CoSx loses structural integrity as temperature increases. Unlike the CoSx heated at 300 °C (CoSx-300), where the nanocube-like morphology is fairly well-maintained, the structure of CoSx obtained at 350 °C (CoSx-350) collapses significantly, which may have resulted from the rupture of nanocubes induced by the higher internal pressure of evolved gases. When the temperature is elevated to 400 °C, the resulting CoSx (CoSx-400) appears more shrunken and almost loses its cube-like morphology. The average particle size and its distribution in Figure S4 supports the decrease in the particle size with temperature. The TEM images shown in Figures 2E-H are in good agreement with the SEM observation. Interestingly, we observe that while CoSx-350 mainly consists of deformed solid particles, ruptured hollow nanocubes are sporadically observed. This suggests that some nanocubes are capable of retaining their cubic morphology in spite of their abrupt structural collapse. There are no hollow nanocubes observed in CoSx-400; only severely collapsed solid particles are present. The fast Fourier transform (FFT) patterns (Figures 2I-L) of three CoSx specimens unveil other intriguing characteristics of CoSx. The hollow nanocubes (whether ruptured or not) are poly-crystalline and composed of two phases: CoS1.097 and Co3S4. The solid particles, however, are single-crystalline and contain only a single phase: CoS2. The analysis of lattice fringes in the high-resolution TEM images is consistent with this observation (Figure S5).


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Figure 2. SEM images, TEM images, and FFT patterns of CoSx obtained at different temperatures. (A) Co3[Co(CN)6]2, (B, E, I) CoSx-300, (C, F, G, J, K) CoSx-350, and (D, H, L) CoSx-400.

To further confirm this characterization, the X-ray diffraction (XRD) patterns of three CoSx samples are examined (Figures 3A-C). Broad diffraction peaks appear in CoSx-300, which are assigned to the CoS1.097 (JCPDS 19-0366) and Co3S4 (JCPDS 75-1561) phases. In CoSx-350, the diffraction peaks designated as the CoS2 (JCPDS 89-1492) phase are sharper than the peaks in CoSx-300, reflecting improved crystallinity. In addition, there are broad peaks present, assigned to the CoS1.097 and Co3S4 phases. In CoSx-400, much narrower and better-defined peaks


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are resolved, belonging to the CoS2 phase. Owing to the small crystallites developed in the CoSx samples, the diffraction peaks considerably overlap, particularly in CoSx-300 and CoSx-350. Therefore, a Rietveld refinement was performed to resolve the peaks and assess the peak assignment. The simulated lines match the measured lines quite well (Figure 3D), affirming the validity of our initial assignments. All results of the XRD analysis are also consistent with the FFT analysis. Nitrogen physisorption measurements were carried out to obtain the textual properties of three CoSx samples. Figures 3E and 3F show the adsorption/desorption isotherms and pore-size distribution, respectively. The type IV feature with noticeable hysteresis is observed only in CoSx-300, while CoSx-350 and CoSx-400 approximate the type III feature, revealing the nonporous nature of these two CoSx specimens. The specific surface areas of CoSx300, CoSx-350, and CoSx-400 are determined to be 45.8, 14.1, and 9.6 m2g-1, and their pore volumes are 10.5, 3.3, and 2.2 cm3g-1, respectively. The slightly higher surface area and larger pore volume of CoSx-350 in comparison to those of CoSx-400 are presumably caused by the presence of a small number of ruptured nanocubes. A bimodal curve whose peaks are centered at ca. 2 and 21 nm is observed in the pore-size distribution of CoSx-300, confirming the presence of both the hollow void at its center and mesopores in its shell.


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A

B

CoSx-350

Intensity (a.u.)

Intensity (a.u.)

CoSx-300

CoS1.097 (19-0366)

CoS1.097 (19-0366) Co3S4 (75-1561)

Co3S4 (75-1561)

20

30

40

50

60

70

CoS2 (89-1492)

80

20

30

2-Theta (Degree)

50

60

70

D

CoSx-300

Intensity (a.u.)

Intensity (a.u.) 20

30

40

50

60

70

80

2-Theta (Degree) 250

CoSx-350

CoSx-400

20

30

F0.020

40

50

CoSx-400 (Desorption)

100

70

80

CoSx-300 CoSx-350

0.015

dV/dlogD

150

60

2-Theta (Degree)

CoSx-300 (Adsorption) CoSx-300 (Desorption) CoSx-350 (Adsorption) CoSx-350 (Desorption) CoSx-400 (Adsorption)

200

80

y-obs y-calc Difference Bragg position

CoSx-400

CoS2 (89-1492)

E

40

2-Theta (Degree)

C

Volume Adsorbed (cm3g-1)

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


CoSx-400

0.010

0.005

50 0

0 0

0.2

0.4

0.6

0.8

1.0

1

Relative Pressure (P/P0)

10

100

Pore Diameter (nm)

Figure 3. XRD patterns of (A) CoSx-300, (B) CoSx-350, and (C) CoSx-400. (D) Rietveld analysis of the XRD patterns. (E) N2 physisorption isotherms and (F) pore-size distributions of CoSx-300, CoSx-350, and CoSx-400.


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3.2 Electrocatalytic Activity of CoSx The IRR activity of the three CoSx samples was initially assessed by CV measurements. There are two pairs of redox peaks in the cyclic voltammograms (Figure 4A) that correspond to the redox reaction of the I3-/I- couple in the low potential region and that of the I2/I3- couple in the high potential region.48 Hence, the redox pair appearing in the low potential region is related to the IRR activity of CoSx. The peak current density increases in the order of CoSx-300 > CoSx350 > CoSx-400, suggesting that CoSx-300 is the most IRR active. The inset in Figure 4A exhibits a linear dependency of the peak current density (Ip) on the square root of the scan rate (υ1/2), in which the anodic and cathodic peak currents obey the Randles-Sevcik equation: Ip = 2.69×105n2/3ACD1/2υ1/2

(1)

where n is the number of electrons involved in the charge transfer, A is the electrode area, C is the concentration of I3-, D is the diffusion coefficient, and υ is the scan rate. This implies that the IRR on CoSx is diffusion-limited, and hence the diffusion coefficients can be estimated from the slopes. The diffusion coefficients of CoSx-300, CoSx-350, and CoSx-400 are determined to be 3.38×10-6, 2.48×10-6, 2.30×10-6 cm2s-1, respectively, reflecting the large surface area and mesoporosity developed in CoSx-300. The IRR activity of CoSx was also examined by using a symmetric dummy cell. This experimental configuration is useful for the assessment of IRR activity in that any effects of photoelectrodes can be completely excluded, two identical electrodes can experience a uniform electrical field throughout the whole electrode surface, and only diffusion can govern mass transport in the electrolyte.49 Tafel polarization measurements are complementary as they are simple and intuitive, so that the relative comparison of electrocatalytic activity can be quickly


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made. Figure 4B shows the Tafel polarization curves of the three CoSx samples along with that of Pt. The exchange current (i0) determined by extrapolating the anodic and cathodic branches in the low overpotential region is related to the charge transfer resistance (Rct) for IRR. The limiting current density (Jlim) in the high overpotential region is associated with the diffusion coefficient (D) of I3- in the electrolyte via the following equations: = =

(2)

(3) 2

where R is the gas constant, T is the temperature, n is the number of electrons involved in the IRR, F is the Faraday constant, l is the distance between the electrodes in the symmetric dummy cell, and C is the concentration of I3-. A notably higher gradient of current is observed in CoSx300 compared to other counterparts, implying the greatest i0 (the smallest Rct) for CoSx-300. The i0 values are found in the order of CoSx-300> CoSx-350 > CoSx-400. The Jlim determined in the cells follows the same order, revealing that the most effective I3- diffusion takes place on CoSx300. This observation accords well with the CV measurements. These results were also crosschecked by EIS, which provides information about interfacial charge transfer and transport kinetics. Figure 4C displays the Nyquist plots of symmetric dummy cells, in which there are the intercept in the real axis (Z′) corresponding to the equivalent series resistance (Rs), the two following overlapped semicircles representing the electron transport resistance in the CoSx film (Rtms) and Rct, and the low frequency semicircle associated with the Nernst diffusion impedance (ZN), respectively.50,51 ZN reflects how fast I3- diffusion occurs in the electrolyte and is expressed in the following equation:49


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=

tanh !

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# (4) "

Herein, W is the Warburg parameter expressed as follows: = %/(' ( ' )√) , where k is the Boltzmann constant, T is the temperature, n is the number of electrons involved in the charge transfer, e is the elementary charge, C is the concentration of I3-, A is the electrode area, and D is the diffusion coefficient of I3-. KN is defined as D/δ2, where δ is the diffusion layer thickness. It is noteworthy that we used a different equivalent circuit for Pt because the interfacial structure in the thin, sputtered Pt film would obviously be different from those of thicker CoSx films (Figure S6 for the characterization of the Pt film).50 This difference is reflected in the Nyquist plot of Pt (Figure S5), where only two distinctive semicircles appear. The EIS parameters are extracted by fitting the spectra to the equivalent circuits (inset in Figures 4B and S7) and are summarized in Table 1. The Rct of CoSx-300 is as low as 0.78 Ω, and the ZN is 4.82 Ω, which are smaller than those of CoSx-350, CoSx-400, and even Pt. The overall trend of Rct and ZN in the EIS analysis is consistent with the trend of i0 and Jlim in the Tafel polarization measurements, affirming the superior electrocatalytic activity of CoSx-300 over Pt. Another noticeable disparity among the catalysts is the Rs values that can reflect the electrical conductivity of CoSx. The RS values are in the order of CoSx-300 > CoSx-350 > CoSx-400, suggesting a higher electrical conductivity for CoSx-300. On the other hand, the Rtms values are small and nearly identical; hence, their influence on the IRR activity seems to be negligible. The IRR activity of CoSx is indeed reflected in the J-V curves of full cells (Figure 4D). When a DSSC is equipped with a CE made of CoSx300, it outperforms all other counterparts. The photovoltaic parameters of each DSSC are summarized in Table 1. It is noteworthy that the PCE of the DSSC equipped with CoSx-300 is 8.49%, which is the highest PCE ever achieved with CoSx-based CEs (Table S1). The influence


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of CE on DSSC performance is primarily manifested in the short-circuit current (Jsc) and fill factor (FF). The highest Jsc and FF values in the DSSC with CoSx-300 are consistent with better IRR activity in the CoSx-300 as compared to those of CoSx-350 and CoSx-400.

Figure 4. (A) CVs of CoSx-300, CoSx-350, CoSx-400 recorded at a scan rate of 10 mVs-1 (inset: the peak current density vs the square root of scan rate). (B) Tafel polarization curves and (C) Nyquist plots (inset: equivalent circuit) of symmetrical dummy cells made of CoSx-300, CoSx350, CoSx-400, and Pt. (D) J-V curves of DSSCs with different CEs. Table 1. Solar Cell Performance Parameters of DSSCs with Different CEs. EIS Parameters for the Symmetric Dummy Cells Made of CoSx-300, CoSx-350, CoSx-400, and Pt.


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CEs

Jsc (mAcm-2)

Voc (V)

FF

CoSx-300

16.8±0.1

0.73±0.01

0.70±0.01

8.48±0.04

8.88

Rtrns Rct (Ω) ZN (Ω) (Ω) 0.35 0.78 4.82

CoSx-350

16.2±0.1

0.73±0.01

0.69±0.01

8.04±0.02

13.57

0.26

0.84

4.86

CoSx-400

14.8±0.1

0.73±0.01

0.68±0.01

7.42±0.04

15.81

0.4

1.40

4.93

Pt

15.8±0.1

0.74±0.01

0.69±0.01

8.01±0.04

9.97

-

2.68

5.91

PCE (%) Rs (Ω)

Olsen and coworkers reported that, unlike bulk Pt, the vapor-deposited Pt on FTO is prone to react with the iodide electrolyte and degrades over time;15 therefore, ensuring long-term stability in the iodide electrolyte is an important requirement of Pt-alternative electrocatalysts. To investigate the stability of CoSx against corrosion, continuous potential sweeps of each catalyst were performed (Figure S8). As anticipated, Pt shows a noticeable decrease in current density during 100 potential cyclings. All CoSx electrodes also show a slight decline in current density, but the degree of the degradation appears different. The decrease in CoSx-300 is much less pronounced than those in CoSx-350 and CoSx-400, implying the greater robustness of CoSx300 against corrosion. 3.3 Composition Dependent IRR Activity of CoSx While it is evident that the sulfidation temperature matters for the IRR activity of CoSx, it is difficult to elucidate the factors governing their IRR activity, because the effects from their different compositions and specific surface area are not independent in these electrochemical results. The composition of metal sulfides has a profound effect on their electrocatalytic activity,38,52-55 and the number of electrocatalytic active sites is generally proportional to the surface area of the electrocatalyst, albeit not necessarily linearly proportional. To explore the influence of the composition of CoSx, therefore, these two major factors should be decoupled. To this end, we prepared pure CoS1.097, Co3S4, and CoS2 powders whose compositions appeared in


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our CoSx samples (SEM images in Figure S9). These three specimens have similar specific surface areas (1.28, 6.98, and 1.66 m2g-1, respectively), and this allows us to examine the composition effect while excluding the influence of surface area. The performance of DSSCs are ranked CoS1.097 > Co3S4 > CoS2 (Figure 5A and Table 2), suggesting that sulfur-deficient CoSx possesses inherently higher electrocatalytic activity than its sulfur-rich counterparts. Note that despite the slightly greater surface area of Co3S4, it is inferior to CoS1.097, which clearly shows the composition-dependence of CoSx’s IRR activity. The electrochemical analysis based on the Tafel polarization, EIS, and CV measurements shows the same trend found in the J-V curves (Figures 5B-C, Table 2, Figure S10). The exchange current of CoS1.097 is remarkably higher than those of Co3S4 and CoS2, and the trend in Rct values determined by the EIS analysis are consistent with this observation. In addition, the Rs values are in the order of CoS1.097 > Co3S4 > CoS2, suggesting a higher electrical conductivity of CoS1.097 compared to Co3S4 and CoS2. While little is known about the electrical properties of CoSx, they are considered to be electrically conductive materials; the resistivity of CoS2 at 300 K is as low as ~160 mΩcm-1 and that of Co9S8 at the same temperature is 3.5 mΩcm-1.56 The resistivity of CoS1.097 is unavailable in the literature, but given the composition dependency on the resistivity above, we speculate that it would be close to the resistivity of Co9S8 rather than that of CoS2, which seems to appear in our EIS results as well. Based on this composition-dependent IRR activity of CoSx, we conclude that the greater electrocatalytic activity of CoSx-300 results from the combination of both the favorable sulfurdeficient composition for IRR (i.e., faster reaction kinetics and enhanced electrical conductivity) and its hollow interior that promotes the diffusion of I3-. These effects are manifested in the smallest Rct, Rs, and ZN in the EIS analysis (Table 1).


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Figure 5. (A) J-V curves of DSSCs with different CEs. (B) Tafel polarization curves and (C) Nyquist plots (inset: equivalent circuit) of symmetrical dummy cells made of CoS1.097, Co3S4, and CoS2. Table 2. Solar Cell Performance Parameters of DSSCs with Different CEs. EIS Parameters for the Symmetric Dummy Cells Made of CoS1.097, Co3S4, and CoS2 CEs.


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CEs

Jsc (mAcm-2)

Voc (V)

FF

CoS1.097

15.3±0.1

0.73±0.01

0.69±0.01

7.68±0.04

11.13

Rtrns Rct (Ω) ZN (Ω) (Ω) 1.25 0.95 5.64

Co3S4

14.8±0.2

0.70±0.01

0.71±0.01

7.29±0.11

18.64

0.29

1.65

7.43

CoS2

13.5±0.2

0.70±0.01

0.66±0.01

6.25±0.07

19.41

1.43

2.85

8.21

PCE (%) Rs (Ω)

While our investigation of composition-dependent IRR activity provides new insight into the electrocatalytic behavior of CoSx, the underlying cause of this effect remains undetermined. Since CoSx’s surface primarily serves as a collection of catalytic active sites, examining the surface characteristics of CoSx is important to gain some insight into this unresolved problem. Before moving into any experimental effort, however, it is vital to understand the theoretical aspects of the IRR mechanism. The electrochemical reduction of I3- (I3- (sol) + 2e- → 3I- (sol))57 has generally been known to occur via the following pathways:10,49 I3- (sol) ↔ I2 (sol) + I- (sol)

(5)

I2 (sol) + 2* → 2I*

(6)

I* + e- → I- (sol)

(7)

where * represents the electrochemically active sites and the sol stands for a solvent, which is typically acetonitrile. As the first step is very fast,57 the second and third steps, which correspond to the dissociation of I2 onto the catalyst’s surface and the subsequent one-electron transfer to each adsorbed iodine atom (I*), are known to dictate the overall IRR kinetics.10 Several computational studies combined with experimental results suggest that the adsorption energy of the I atoms on the catalyst’s surface may serve a descriptor that determines the IRR activity.10,5860

According to these early works, an optimal adsorption energy is required for a good catalyst

because the binding strength between I species and the surface for the adsorption of I atoms and


19


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the desorption of I* must be balanced to cause the IRR to proceed efficiently. In order words, the binding strength should be strong enough for the dissociative adsorption of I2 to occur in a thermodynamically favorable fashion. At the same time it should be sufficiently weak for I* to exothermically depart the surface in the form of I- (sol).10 In addition to this rigorous demand for binding strength, it is equally important to create more active sites (i.e., metal ions exposed on the surface) to fully harness the catalytic activity.10,58 We carried out XPS analysis to explore the surface characteristics of CoS1.097, Co3S4, and CoS2. The Co 2p spectra composed of Co 2p3/2 and Co 2p1/2 peaks (Figure 6A) can be deconvoluted into several peaks corresponding to Co−S and Co−O bonds.61,62 The first two peaks at 778.6 and 780.1 eV are linked to Co−S (red lines) and Co−S with surface-adsorbed hydroxide species (orange lines), respectively. The following two peaks at higher binding energy are attributed to Co−O bonds associated with CoO (olive lines) and Co(OH)x (blue lines). The S 2p spectra in Figure 6B can be deconvoluted into three doublets including S2- (red lines), S22(olive lines), and SOx2- (blue lines).24,38 The overall spectra feature of CoS1.097 and Co3S4 are similar, but that of CoS2 is strikingly different. While surface oxidation that results in the formation of Co−O bonds and the oxidized S species is rarely found in CoS1.097 and Co3S4, it is common in CoS2. In general, CoSx is subject to oxidation in air,17,23 and the surface oxidation is prone to occur in sulfur-rich CoSx,63 which appears to be true in our case as well. The presence of oxidized species on the surface on CoSx can have a significant influence on the IRR activity. A very recent report by Zhang et al. revealed that the presence of surface oxygen is fatal to the IRR activity of CoSx, which is intimately related to the adsorption energy of the I atom.64 The energy compensation from the bond formation between surface Co atoms and I atoms is not sufficiently large for the dissociation of I2 when the surface Co atoms are bound to oxygen atoms. This


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explains why the IRR activity of CoS2 is inferior to those of its two counterparts. Another useful piece of information the XPS analysis provides is the amount of surface Co because IRR takes place exclusively on the surface of CoSx and only Co can serve as a binding site for the I atoms. The amounts of surface Co in CoS1.097, Co3S4, and CoS2 were determined to be 11.95, 10.96, 8.82 atomic wt%, respectively, indicating that more active sites are available for sulfur-deficient CoSx. Along with the lower Rs, this higher exposure of Co atoms on the surface seems to be responsible for the higher IRR activity of CoS1.097 compared with Co3S4. It is noteworthy that in addition to the number of surface-exposed Co, considering its electronic structure (i.e., oxidation states) and local coordination configuration is necessary to fully elucidate the underlying factors dictating the IRR activity of CoSx. The relative portions of Co−S and Co−S with surfaceadsorbed hydroxide species in the XPS spectra are slightly different for CoS1.097 and Co3S4 (Table S2), hence this difference may affect the adsorption energy of the I atom. However, it is difficult to determine at present how these two states dictate the binding energy between Co and I, and how much influence this difference can exert on the IRR activity of CoS1.097 and Co3S4. This is partly because the mechanism of IRR on CoSx is unclear (even the mechanism on Pt is still under debate)48,49,57,65-67 and little has been known about the intermediates that may be formed during the IRR. For more insights, therefore, a rigorous surface characterization of CoSx with well-defined surface states along with computation study is required. Given our examination of the IRR activity (Figures 5B-C), we speculate that the lower electrical resistance of CoS1.097 than Co3S4 seems to play a more important role in determining the IRR activity than the difference in the oxidation states. The local coordination configuration governed by the morphology of CoSx (i.e., exposed facets on surface) may also have influence on the IRR activity. Major facets that could be found in CoS1.097, Co3S4, and CoS2 are listed in Figure S11, which


21


show different coordination numbers for Co. While this difference in the degree of coordinative unsaturation can affect the IRR activity as demonstrated in several previous works,58,60,68,69 the CoS1.097, Co3S4, and CoS2 synthesized in our work are composed of randomly oriented facets (Figure S9) unlike the well-defined morphologies in those reports. Therefore, the effect of local coordination configuration, if any, is expected to be trivial than other factors.

A

CoO Co(OH)x Background

Intensity (a.u.)

Raw Co-S Co-S

810

805

800

795

790

785

780

775

160

158

Binding Energy (eV)

B Intensity (a.u.)

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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Raw S2S22SOx2Background

172

170

168

166

164

162

Binding Energy (eV)

Figure 6. (A) XPS Co 2p and (B) S 2p spectra of CoS1.097, Co3S4, and CoS2.

4. CONCLUSION


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Hollow cobalt sulfide nanocubes are designed by a delicate sulfidation of Co3[Co(CN)6]2 and evaluated as an electrocatalyst for a triiodide reduction reaction. Depending on the sulfidation temperature, the resulting cobalt sulfides feature significantly different compositions and morphologies, which have a significant effect on the IRR activity. This study provides a new insight into cobalt sulfide-based electrocatalysts by investigating the composition dependency of their IRR activity. The IRR activity of sulfur-deficient cobalt sulfide is found to be superior to that of sulfur-rich cobalt sulfide, which is intimately associated with different surface characteristics and electrical conductivity of cobalt sulfides according to XPS and electrochemical analyses. This finding highlights the importance of controlling surface properties of sulfide-based electrocatalysts and calls attention to an approach for preventing surface oxidation that deteriorates catalytic activity. This implication should be taken into account when establishing designing principles for sulfide-based electrocatalysts that aim to replace Pt.

ASSOCIATED CONTENT Supporting Information. Characterization of Co3[Co(CN)6]2; additional TEM analysis results; solar cell performance comparison; Nyquist plots of Pt; Stability test results; SEM images and cyclic voltammograms of CoS1.097, Co3S4, and CoS2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2016R1A1A1A05005038 and 2008-0061891).

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Composition-Dependent Electrocatalytic Activity of Cobalt Sulfides for

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