Hollow hierarchical structure Co0.85Se as efficient electrocatalyst for the triiodide reduction in dye-sensitized solar cells

Solar Energy 178 (2019) 241–248

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Hollow hierarchical structure Co0.85Se as efficient electrocatalyst for the triiodide reduction in dye-sensitized solar cells

T

Rongfang Zhaoa,b, Dongmei Tanga, Long Huana, Qianhui Wua, Wenlong Lia, Xiue Zhanga, ⁎ ⁎ Ming Chena, , Guowang Diaoa, a b

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China

ARTICLE INFO

ABSTRACT

Keywords: Cobalt selenides Hollow hierarchical structure Triiodide reduction Electrocatalytic activity Dye-sensitized solar cells

The exploration of nonprecious metal-based electrocatalysts with high efficiency for the triiodide reduction is critical for the practical applications of the dye-sensitized solar cells. Herein, we develop a facile one-step hydrothermal method to synthesize hollow hierarchical structure Co0.85Se. Under the methanol-water reaction system, the product named as hollow hierarchical structure Co0.85Se-M has the largest specific surface area (215.36 m2 g−1) and the best crystallinity than other products obtained from other alcohol-water reaction systems. When this electrocatalyst is applied as a counter electrode for the dye-sensitized solar cells, it exhibits a small peak-to-peak separation (Epp, 97 mV) for the reduction of I3−/I− redox couple. It is found that the catalytic activity of Co0.85Se is closely dependent on the crystallinity. Moreover, the reactivity pathway is identified by density functional theory, which confirms that triiodide is reduced to iodide ion on Co0.85Se with a smaller energy barrier (∼0.65 eV) than on Pt (∼1.18 eV). Both experimental and theoretical results demonstrate Co0.85Se-M as an ideal counter electrode material for the dye-sensitized solar cells with a higher power conversion efficiency (8.76%) than Pt counter electrode (7.20%).

1. Introduction Solar energy, as a cleaner and the most abundant renewal energy (Yun et al., 2015), has been considered as an ideal energy carrier alternative to meet the global rapidly energy needs humankind and environmental concerns. Dye-sensitized solar cells (DSSCs), the thirdgeneration photovoltaic device with a power conversion efficiency of 13% (Mathew et al., 2014), have gained immense attention since it reported by O’Regan and Grätzel with landmark efficiency of ∼7% (O’Regan and Grätzel, 1991). The counter electrode (CE), as one key section in DSSCs, plays a crucial role in catalyzing the reduction of the oxidized sate to the reduced sate for a redox couple (eg., I3−/I−) (Hauch and Georg, 2001; Li et al., 2009). Noble metal (eg., Pt) owing high catalytic activity is conventionally used as electrocatalysts for the triiodide reduction. However, high cost, scarcity in nature and poor long-term stability against the iodine-based electrolyte of the precious noble metals catalytic limit the large-scale applications of DSSCs. Therefore, it is a challenging task to develop nonprecious metal-based catalysts with low cost and high performance, aiming the industrialization of DSSCs.

⁎

Recently, varieties of earth-abundant materials, such as carbon (Batmunkh et al., 2015; Dong et al., 2011; Roy-Mayhew et al., 2010), sulfides (Cui et al., 2016; Sun et al., 2011; Wang et al., 2009), nitride (Li et al., 2011; Wu et al., 2012), and selenides (Duan et al., 2014; Liu et al., 2015; Ramasamy et al., 2015; Kukunuri et al., 2015; Xin et al., 2011; Jin et al., 2017) have been introduce into DSSCs. As an important class of chalcogenides, metal selenides, such as NiSex (Gong et al., 2013; Zhang et al., 2016), CoxSe (Gong et al., 2012; Jiang and Hu, 2015; Sun et al., 2014), FeSex (Huang et al., 2015; Liu et al., 2015), SnSex (Liu et al., 2015), and so on (Chen et al., 2014; Wang et al., 2013), have been widely applied as electrocatalysts for DSSCs. Among these materials, cobalt selenide has demonstrated superb electrocatalytic activity for the triiodide reduction. Wang’s group first expanded their applications as CEs in DSSCs by using in situ-grown Co0.85Se nanosheets and Ni0.85Se on the FTO substrate (Gong et al., 2012). Zou et al. directly grew single-crystalline metal selenium nanosheets on metal fiber to fabricate low-cost, high-performance fibershaped DSSCs (Chen et al., 2016). Compared with nanoparticles, nanosheets display a larger contact area with the conductive substrate, facilitating electron transfer from the substrate to the catalyst, thus

Corresponding authors. E-mail addresses: [email protected] (M. Chen), [email protected] (G. Diao).

https://doi.org/10.1016/j.solener.2018.12.031 Received 18 July 2018; Received in revised form 27 November 2018; Accepted 13 December 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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enhancing the catalytic activity. In addition, the chemical state on the surface of catalyst could affect the adsorption ability and catalysis active site for triiodide ion. From recent advances, the morphology and crystallinity all play key roles in the electrochemical performance for CE materials (Qian et al., 2016; Xia et al., 2017). However, few studies have focused on the effect of morphology and crystallinity of Co0.85Se on catalytic activity at the same time. In this paper, we synthesized the hollow hierarchical structure (HHS) Co0.85Se-M via a one-step, facile hydrothermal method under a methanol-water reaction system. By combining the analysis of characterization and performance, the HHS Co0.85Se-M displays high specific surface area for the extraordinary morphology, allowing the effective contact area to electrolyte. Moreover, well-crystallinity Co0.85Se-M is conducive to the adsorption and the reduction of triiodide ions, which exhibits much higher activity than the partially crystallinity catalysts. Lastly, we explored the reaction pathway on the surface of catalytic via DFT calculations, which confirmed that the triiodide reduction on Co0.85Se catalyzer has a smaller energy barrier (∼0.65 eV) than that on Pt (∼1.18 eV).

method. The TiO2/FTO glass was heated to 450 °C for 30 min, which sensitized by soaking into 0.5 mM N719 dye acetonitrile solution for 24 h (Zhao et al., 2016). The redox electrolyte consists of 0.01 M LiI, 0.05 M I2, 0.6 M 1-butyl-3-methylimidazolium iodide (BMPⅡ), and 0.05 M 4-tert-butyl pyridine (TBP). The interspace between the CEs and the photoanodes was full with electrolyte to fabricate a sandwich structure DSSCs. 2.5. Device characterization and electrochemical tests Current-Voltage measurement was recorded by a Keithley model 2400 digital source mater. An oriel Sol 3A solar simulator was used to irradiate the cells with 0.25 cm2 effective cell area. The photovoltaic performance of the fabricated cells was recorded by Keithley 2400 under illumination of 100 mW cm−2 at the range of 300–1100 nm using a solar simulator (Newport). Cyclic voltammetry (CV) was carried out in a three-electrode system containing an electrolyte solution making up of 10 mM LiI, 1 mM I2 and 100 mM LiClO4. The scanning potential range was from −0.3 to 1 V at a scan rate of 50 mV s−1, Pt wire as the CE, Ag/AgCl as reference electrode and as-prepared CE as working electrode. The electrochemical impedance spectra (EIS) and Tafel polarization curves were conducted with a symmetrical dummy cell assembling with two identical CEs. In EIS measurements, frequency range, bias voltage, and ac amplitude were scanned from 100 mHz to 100 kHz, 0 V and 10 mV, respectively. The voltage range and scan rate of Tafel cure tests were from −1.0 to 1.0 V at 10 mV s−1. All of the electrochemical measurements were measured by an electrochemical workstation (ZAHNER ZEN NIUM CIMP-1, Germany).

2. Experimental section 2.1. Synthesis of HHS Co0.85Se The HHS Co0.85Se with different morphologies were synthesized via a facile one-step hydrothermal process. 0.15 g Co(NO3)2·6H2O firstly dissolved into 25 mL of methanol under intensively stirring for 10 min, forming a peach pink homogenous solution. Next, the aqueous solution (0.09 g Na2SeO3 dispersed in 2 mL deionized water) quickly added the above homogenous solution, appearing a nicely purple turbid liquid with stirring 0.5 h. Then, NH2NH2·H2O as reducing agent was added and the color became to khaki. Finally, the above solution was poured into a Teflon-lined stainless still autoclave and heated at 120 °C for 2 h. The precipitate was collected by centrifuged and washed with ethanol after cooling to ambient temperature. The HHS Co0.85Se with different morphologies were synthesized by replacing solvent as ethanol, npropanol and n-butanol.

2.6. DFT calculation All calculation results were obtained by using the DMol3 code of Materials Studio (Delley, 1996; Delley, 2000), which is based on the density functional theory (DFT) methods. The generalized gradient approximation (GGA) with the functional parametrization of Perdew–Burke–Ernzerhof (PBE) were utilized in this paper to calculate the exchange-correlation energy (Perdew et al., 1996). In addition, the double numerical basis set with polarization functions (DNP) and DFT semicore pseudopotential (DSPP) were employed. For the geometry optimizations, the Brillouin-zone was performed by a Monkhorst-Pack k-point grid of 2 × 2 × 1.

2.2. Characterization The morphology and microstructure of HHS Co0.85Se were carried out with field-mission scanning electron microscopy (SEM, Zeiss, Supra55, Germany) and transmission electron microscopy (TEM, JEM2100, Japan). High-resolution TEM (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed on FEI Tecnai G2 F30 STWIN (USA) operating at 300 kV. X-ray diffraction (XRD) data were obtained with an X-ray diffractometer (D8, advance, Bruker) with Cu Kα radiation (λ = 0.1514 nm). X-ray photoelectron spectroscopy (XPS) was recorded by Themo Escalab 250 system using Al Kα radiation (1486.6 eV). N2 adsorption-desorption isotherm of products was performed by Brunauer-Emmett-Teller (BET) technique (ASAP 2020, HD88).

3. Results and discussion 3.1. Characterization of HHS Co0.85Se Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to investigate the morphologies of the synthesized samples. Pompon-like HHS Co0.85Se catalysts were synthesized by a facile hydrothermal reaction under methanol-water reaction system (named as Co0.85Se-M). As shown in Fig. 1a–b, pompon-like HHS Co0.85Se-M with a uniform size distribution and good dispersibility is made up of many hollow bunches with a uniform average size together in a radial fashion. Furthermore, it can be found that each bunch consists of graphene-like 2D nanosheets, which self-crimp to form a unique hollow hierarchical structure (Fig. 1c–e). Contributing to the extraordinary structure, the pompon-like HHS Co0.85Se-M possess a large specific surface area (215.36 m2 g−1, Fig. S2a). The atomic ratio of Co and Se for HHS Co0.85Se-M is close to 0.85:1 (Fig. S3). The diffuse rings in the select-area electron diffraction (SAED) pattern (Fig. 1g) confirm good crystallization of the pomponlike HHS Co0.85Se-M. The Debye-Scherrer ring patterns of (1 0 1), (1 0 2), (1 1 0), (1 0 3), and (2 0 2) are well indexed with the standard hexagonal Co0.85Se. The continuous lattice distance of 0.262, 0.200, and 0.180 nm in the high-resolution TEM (HRTEM) image (Fig. 1f) is well corresponding to the d-spacing of (1 0 1), (1 0 2), and (1 1 0)

2.3. Preparation of Co0.85Se counter electrode HHS Co0.85Se was coated on FTO glass substrates by spin-coating technique. In detail, 0.01 g synthesized products dispersed in 1 mL deionized water under sonication for 1 h to form a homogeneous ink solution. Then the solution was coated on FTO glass substrates by spincoating. Moreover, Pt CEs as a reference for comparison studies prepared by spin-coating 50 mM H2PtCl6 isopropanol solution on the FTO glass and then thermally deposited at 400 °C for 15 min under the air. 2.4. Fabrication of DSSCs The TiO2 photoanodes were prepared by the screen-printing 242

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Fig. 1. (a–c) SEM images, (d–e) TEM images, (f) HRTEM image, (g) SAED pattern of HHS Co0.85Se-M, (h–j) Annular dark-field STEM image and the corresponding EDX element mappings of Co, Se.

crystallographic planes of Co0.85Se. The structure of one bunch in pompon-like HHS Co0.85Se-M is further confirmed by the element mapping from energy-dispersive X-ray (EDS) with the annular darkfiled STEM, which show the presence and distribution of Co and Se elements (Fig. 1i–j). It is obvious that the bunches consist of graphenelike 2D nanosheets to form irregular hollow center. The different morphologies and crystallinity of Co0.85Se were synthesized by changing the types of alcohol. Subsequently, the products named as Co0.85Se-E, Co0.85Se-P and Co0.85Se-B were prepared in ethanol, n-propanol, and n-butanol, respectively. As shown in Fig. 2a1–a3, the morphology of HHS Co0.85Se-E is close to spindleshaped. It is clear that the thin slice as graphene-like 2D nanosheets crimped and twined with each other to form the hollow bunch (Fig. 2a4, Fig. S1a). The HRTEM image and SAED pattern (Fig. S1d and g) show the lower crystallinity than Co0.85Se-M. With the increase of the alkyl chain, the products tend to form irregular aggregation, as shown in Fig. 2b1–b4 and Fig. 2c1–c4. Compared with Co0.85Se-M, the crystallinities of Co0.85Se-P and Co0.85Se-B decrease through the analysis of HRTEM images and SAED patterns (Fig. S1). Moreover, the specific surface area of Co0.85Se-E, Co0.85Se-P, and Co0.85Se-B is 173.03, 153.60, and 110.74 m2 g−1, respectively (Fig. S2). The decrease of specific surface area is attributed to the increase of the degree of aggregation. The crystallinity and chemical state were confirmed by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD spectrum of Co0.85Se-M in Fig. 3a exhibits distinct peaks, indicating its well-crystallinity nature in terms of these acuminate peaks. The peaks located at 33.3°, 44.8°, and 50.9° are assigned to (1 0 1), (1 0 2), and

(1 1 0) matching the hexagonal crystal structure of Co0.85Se (JCPDS card no. 52-1008) (Yuan et al., 2017; Chiu et al., 2016). However, as shown in Fig. 3b–d, only one peak of (1 0 1) crystallographic planes appears, meanwhile the characteristic diffraction peak of (1 0 2) and (1 1 0) are hardly distinguished, which indicate the poor crystallization of Co0.85Se-E, Co0.85Se-P and Co0.85Se-B (Huang et al., 2018; Lee et al., 2014). So, the different alcohol-water reaction systems not only affect the morphologies, but also change the crystallinity of Co0.85Se. With the increase of the alkyl chain of alcohol, the non-crystallization of the products increases. It is widely known that the electrocatalytic performance of an electrocatalyst determined by the surface composition and chemical state. XPS spectra were further identified the surface chemical states of Co0.85Se. The survey spectra (Fig. 3e, black line) of Co0.85Se-M determine the existence of Co and Se. In the Co 2p spectra (Fig. 3f, black line), Co 2p1/2, Co 2p3/2 and two satellite peaks (marked as ‘sat’) are detected, respectively. The binding energies of ∼778.9, ∼781.1, ∼793.7, and ∼797.3 eV should be assigned to Co3+ 2p3/2, Co2+ 2p3/2, Co3+ 2p1/2, and Co2+ 2p1/2, which are best fitted with two spin-obit doublets characteristic of Co2+ and Co3+ (Zhang et al., 2018; Meng et al., 2017). In addition, the peak in the range of 58.0–62.0 eV is the characteristic peak of Co 3p, as shown in Fig. 3g (black line) (Jiang and Hu, 2015; Yao et al., 2016). The binding energy of 54.0–56.0 eV in the high-resolution scan reflects the existence of Se2− being consistent with Se 3d5/2 and Se 3d3/2 (Meng et al., 2017; Yu et al., 2017). The peak of Se 3d for Co0.85Se-M, Co0.85Se-E, Co0.85Se-P, and Co0.85Se-B occur a certain extent shift to the direction of low binding energy. The decrease of binding energy should be attributed to the crystallographic decline 243

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Fig. 2. (a1–a3) SEM images and (a4) TEM image of Co0.85Se-E, (b1–b3) SEM images and (b4) TEM image of Co0.85Se-P, (c1–c3) SEM images and (c4) TEM image of Co0.85Se-B.

from Co0.85Se-M to Co0.85Se-B. The large binding energy of Se 3d for Co0.85Se-M means the large electronegativity, which is conducive to the adsorption and the reduction of triiodide ion (Liu et al., 2015). The hypothetical adsorption model of triiodide ion on Co0.85Se (1 0 1) is shown in Fig. S4, in which the interaction between Se atom and triiodide ion is ascribed to the electron-coupling. The remarkable affinity between the surface of Co0.85Se and triiodide ion is conducive to subsequent catalytic reactions.

(Fig. S5b).

Ip = n2F 2

(1)

0/ RT

where Ip denotes the current of redox peak; n, electron number; F, Faraday’s constant; ν, scan rate; Γ0, the adsorbance of reactant; R, the gas constant; and T, the temperature. On the basic of the above equation (Tang et al., 2010), the reduction on the surface of HHS Co0.85Se-M CEs is controlled by adsorption of I3− on counter electrode and no effected by adsorption of iodide species. Electrochemical impedance spectroscopy (EIS) measurements were carried out to investigate the reaction kinetics of counter electrode via the symmetric dummy cells with two identical electrodes (CE/electrolyte/CE). Nyquist plots for the cells with different CEs appear two semicircles, which extract the main parameters by fitting these spectra into an equivalent, as shown in Fig. 5(a) and Fig. S4. The intercept of Nyquist plots (left side) with the horizontal axis is the series resistance (Rs). The high-frequency semicircle represents the charge resistance (Rct) between CE and electrolyte. The low-frequency semicircle (right side) corresponds to the diffusion resistance (ZN) of I3−/I− couple in electrolyte. The values of two semicircles as shown in Fig. 5(a) are twice as much as Rct and ZN due to two identical electrodes. The parameters were summarized in Table 1. It is worth pointing out that the smaller Rct implies that the CE has higher electrocatalytic activity. According to the Rct values, the catalytic activity is in order of Co0.85Se-M > Co0.85Se-E > Co0.85Se-P > Co0.85Se-B > Pt. The order of ZN values, Co0.85Se-M < Co0.85Se-E < Co0.85Se-P < Co0.85Se-B < Pt, means that the diffusion coefficients (D) of triiodide vary in opposite order by judging from Eq. (2) (Hauch and Georg, 2001);

3.2. Electrocatalytic activity and photovoltaic performance of Co0.85Se CEs The primary electrochemical parameters, such as the peak-to-peak separation (Epp) and the cathodic peak current density (JRed-1), were used to investigate the electrocatalytic activity of several kinds of Co0.85Se via cyclic voltammetry (CV). In Fig. 4, the first pair of redox peaks (Ox-1/ Red-1) is assigned to I3− + 2e− ↔ 3I−. The second redox peaks are attributed to the reaction of 3I2 + 2e− ↔ 2I3− (Huo et al., 2015; Menshykau and Compton, 2008), The values of peak-to-peak separation (Epp) of the first redox peaks were shown in Table 1. Pt CE shows the largest Epp (227 mV), however, the values of Epp for all Co0.85Se products are smaller than that of Pt CE, especially, Co0.85Se-M with the smallest Epp (97 mV). In addition, the CV curves show the values of |Jred-1| in the order as Co0.85Se-M (1.14 mA cm−2) > Co0.85Se-E (0.96 mA cm−2) > (0.92 mA cm−2) > Co0.85Se-B (0.86 mA cm−2) > Pt Co0.85Se-P (0.43 mA cm−2), contrary to the order of Epp. The larger |JRed-1| and the smaller Epp all indicate that HHS Co0.85Se-M exhibits higher electrocatalytic activity than Pt CE, which could profit from the large specific surface area of Co0.85Se-M for triggering I3− reduction and diffusion. CV curves of HHS Co0.85Se-M CE with different scan rates reveal that the reduction peaks (Red-1) shift to the negative direction (Fig. S5a). The peak current and the scan rate demonstrate a good linear relationship

ZN= 244

kT tanh n2e02 cA i D

i D

(2)

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Fig. 3. XRD spectra of (a) Co0.85Se-M, (b) Co0.85Se-E, (c) Co0.85Se-P, and (d) Co0.85Se-B. XPS spectra: (e) survey spectra, (f) Co 2p and (g) Co 3p and Se 3d of Co0.85SeM, Co0.85Se-E, Co0.85Se-P, and Co0.85Se-B.

different CEs. In addition, the limiting current density (Jlim) is expressed by Eq. (3). Jlim is directly proportional to D and reciprocal to ZN. As shown in Fig. 5(b), the order of Jlim value is Co0.85Se-M > Co0.85SeE > Co0.85Se-P > Co0.85Se-B > Pt, which means Co0.85Se-M with the smallest ZN has the highest electrocatalytic activity. Therefore, Co0.85Se-M with the smallest Rct and ZN has the prominent catalytic activity and charge transfer abilities. Furthermore, the stability of HHS Co0.85Se-M CEs has been researched and the results are shown in Figs. S6 and S7. The coincident curves of CV and EIS demonstrate the good stability of Co0.85Se-M. The exchange current density (J0), closely relating to electrocatalytic properties of the CE materials, could be measured by Tafel polarization curves. J0 value is the intercept of the line at zero potential with the horizontal line potentials (as shown in Fig. S8). As shown in Fig. 5b, it is obvious that the order of J0 is Co0.85Se-M > Co0.85SeE > Co0.85Se-P > Co0.85Se-B > Pt. The larger slope for cathodic branch and the higher J0 indicate that CE has the higher electrocatalytic performance. Moreover, J0 can be calculated by Eq. (4) (Wang et al., 2009), which have the same order obtained by Tafel polarization curves. Combining with the characterization of catalysts and the analysis of electrocatalysis activity, the HHS Co0.85Se-M shows the highest catalysis ability which can be attributed to the following aspects: (1)

Fig. 4. CV curves of Pt, Co0.85Se-M, Co0.85Se-E, Co0.85Se-P, Co0.85Se-B.

where e0 is the elementary charge; k, the Boltzmann; c, the concentration of I3−; A, the electrode area; ω, the angular frequency; δ, the thickness of the diffusion layer. The analysis result indicates that the values of D increase with the increase of the electrocatalytic activity of

Table 1 Photovoltaic parameters of the DSSCs with Pt, Co0.85Se-M, Co0.85Se-E, Co0.85Se-P and Co0.85Se-B as CEs. CEs

Voc (V)

Pt C0.85Se-M C0.85Se-E C0.85Se-P C0.85Se-B

0.75 0.76 0.76 0.75 0.76

± ± ± ± ±

0.01 0.00 0.00 0.01 0.01

Jsc (mA cm−2)

FF

15.23 17.04 16.82 16.62 15.39

0.63 0.68 0.63 0.62 0.64

± ± ± ± ±

0.08 0.10 0.10 0.15 0.12

η (%) ± ± ± ± ±

0.01 0.00 0.00 0.00 0.01

7.20 8.76 8.07 7.78 7.58

± ± ± ± ±

0.05 0.08 0.10 0.10 0.10

245

Epp (mV)

Rct (Ω)

227 ± 0.65 97 ± 0.58 120 ± 0.80 141 ± 0.86 167 ± 0.95

6.67 2.93 3.60 3.73 4.87

± ± ± ± ±

0.02 0.00 0.01 0.02 0.02

ZN (Ω)

logJ0 (log(mA cm−2))

30.15 ± 0.01 8.99 ± 0.01 11.03 ± 0.02 12.91 ± 0.03 21.44 ± 0.02

0.74 1.10 1.01 0.97 0.87

± ± ± ± ±

0.01 0.00 0.01 0.01 0.02

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The photovoltaic characteristics, including the short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE, η) listed in Table 1. The η values of Co0.85SeM, Co0.85Se-E, Co0.85Se-P, Co0.85Se-B, and Pt CE are 8.76%, 8.07%, 7.78%, 7.58%, and 7.20%, respectively. The DSSCs with HHS Co0.85SeM as CE exhibited the highest η than other CEs, which should be attributed to the best crystallinity and the most electroactive sites. Furthermore, the DSSCs with Co0.85Se-M as CE has the highest Jsc of 17.04 mA cm−2. Incident photo-to-current efficiency (IPCE) also is an important parameter for photovoltaic property. The maximum IPCE value of the DSSCs with HHS Co0.85Se-M as CE is 79.63% at 540 nm. It is clear that the DSSCs with HHS Co0.85Se-M have the largest Jsc, η and IPCE values at the same time, which indicates the correlation between different photovoltaic parameters of CE materials. 3.3. Reaction mechanism analysis by DFT calculations The effective energy barrier is the energy between the most stable and the highest transition state, or refers to as the activation energy (Ea) of the transition state, which reveals that the catalytic reaction is underway according to the level of difficulty (Kattel et al., 2017; Lin et al., 2017; Liu et al., 2017). Electrocatalytic activity of catalyst strongly depends on the value of activation energy. In general, the smaller Ea enables a better catalytic activity. We further carried out density functional theory (DFT) to investigate the reaction mechanism of the reduction for I3−. On the basic of our experimental characterization results, we constructed the catalyst model on Co0.85Se (1 0 1) (Fig. S10) and Pt (1 1 1). Following previous studies and adjusting of calculation model, one main reaction pathway for I3− reduction to I− is selected in our calculations: I2 and I* as intermediate, as following (where * represent the surface adsorption site; TS, transition state):

Fig. 5. (a) Nyquist plots and (b) Tafel polarization plots for dummy cells fabricated with Pt, Co0.85Se-M, Co0.85Se-E, Co0.85Se-P, Co0.85Se-B.

High specific surface areas due to the unique hollow hierarchical structure allow the effective contact area to electrolyte. (2) Well-crystallinity brings about the high binding energy of Se 3d, which is conducive to the adsorption and the reduction of triiodide ion.

I3

I2 + I (TS 1)

2ne0 DNA l

(3)

I2 + e

I + I (TS 2)

J0 = RT / nFR ct

(4)

I +e

I (TS 3)

Jlim=

As shown in Figs. 7 and S10, the first step is the decomposition of I3− into I− and I2 (Signed as TS-1, Fig. S11) on Co0.85Se (1 0 1) with energy barriers of 0.65 eV, which is smaller than that on Pt (1 1 1) (1.18 eV). Then, I2 obtains one electron to generate I* and I− (Signed as TS-2, Fig. S12). Lastly, I* gains another electron to form I−, which departs from the surface adsorption site of Co0.85Se or Pt (Signed as TS3, Fig. S13) and diffuses into the electrolyte. The Ea values (as shown in Table S2) of Co0.85Se for TS-2 and TS-3 is approximately equal to zero which means these steps can immediately react (and without barrier). The effective energy barriers for all reaction steps imply that the ratelimiting step of the reduction is the dissociation of I3− (TS-1). The smaller Ea value of the transition state (TS-1) has, the stronger

The above results of the electrochemical performance for several CEs materials demonstrate that the well crystalline HHS Co0.85Se-M CEs has the most outstanding electrocatalytic activity for the triiodide reduction than the other partial crystallinity HHS Co0.85Se and Pt CE. Besides the electrochemical ability, the excellent durability photovoltaic performance is also required for an applicable CE material in DSSCs. Various materials were explored as a counter electrode for DSSCs. Photovoltaic performances of solar cells were evaluated under illuminated 1.5 G sunlight (100 mW cm−2 intensity) and the spectra wavelengths range from 300 to 1100 nm, as shown in Fig. 6 and Fig. S9.

Fig. 7. Reaction pathways for I3− ion reduction into I− on Pt (1 1 1) and Co0.85Se (1 0 1).

Fig. 6. J-V curves of DSSCs with Pt, Co0.85Se-M, Co0.85Se-E, Co0.85Se-P, and Co0.85Se-B CEs under AM 1.5 G illumination. 246

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electrocatalytic activity has. In this work, the DFT calculation reveals that Co0.85Se possesses a much stronger electrocatalytic activity than Pt, which agrees pretty well with the electrochemical results. In addition, the Co0.85Se-M with the large specific surface areas has more surface adsorption site and the better crystallinity for catalytic reaction than other catalysts. Base on the structural analysis and the theoretical calculation, Co0.85Se-M has the strongest catalytic activity than other Co0.85Se materials and Pt.

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4. Conclusion In summary, we designed and synthesized pompon-like HHS Co0.85Se via a facile one-step hydrothermal method with methanol as solvent. The different morphologies were obtained by changing the solvents, such as ethanol, n-propanol and n-butanol. The electrochemical measurements showed that pompon-like HHS Co0.85Se-M as CE material has the smallest Epp, Rct and the highest J0. The solar cell, HHS Co0.85Se-M as CE, shows highest conversion efficiency (8.76%). By combining the BET, XRD, XPS, and DFT calculation, the superior electrocatalytic activity of the pompon-like HHS Co0.85Se-M for the reduction of I3− into I− can be attributed to the following aspects: (1) High specific surface areas allows the effective contact area to electrolyte. (2) Well-crystallinity Co0.85Se-M brings about the high binding energy of Se 3d, which is conducive to the adsorption and the reduction of triiodide ion. (3) The rate-limiting step of the dissociation of I3− (TS1) on Co0.85Se with the smaller energy barrier benefits the reduction process. Given the low cost and easy fabrication strategy, our work should be a promising approach in the future to obtain highly efficient electrocatalytic activity CE materials for DSSCs. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21773203), the Natural Science Foundation of Jiangsu Province (BK20161329), the Innovation Projects in Jiangsu Province (KYZZ16_0491), the Jiangsu Province Fifth issue of 333 High level talents training project (Grant No. BRA2016202), the Application Research Porgram of Nantong (Grant No. MS12015028), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2018.12.031. References Batmunkh, M., Biggs, M.J., Shapter, J.G., 2015. Carbon nanotubes for dye-sensitized solar cells. Small 11, 2963–2989. Chen, H., Xie, Y., Cui, H., Zhao, W., Zhu, X., Wang, Y., Lu, X., Huang, F., 2014. In situ growth of a MoSe2/Mo counter electrode for high efficiency dye-sensitized solar cells. Chem. Commun. 50, 4475–4477. Chen, L., Yin, H., Zhou, Y., Dai, H., Yu, T., Liu, J., Zou, Z., 2016. In situ direct growth of single crystalline metal (Co, Ni) selenium nanosheets on metal fibers as counter electrodes toward low-cost, high-performance fiber-shaped dye-sensitized solar cells. Nanoscale 8, 2304–2308. Chiu, I.T., Li, C.T., Lee, C.P., Chen, P.Y., Tseng, Y.H., Vittal, R., Ho, K.C., 2016. Nanoclimbing-wall-like CoSe2/carbon composite film for the counter electrode of a highly efficient dye-sensitized solar cell: a study on the morphology control. Nano Energy 22, 594–606. Cui, X., Xie, Z., Wang, Y., 2016. Novel CoS2 embedded carbon nanocages by direct sulfurizing metal-organic frameworks for dye-sensitized solar cells. Nanoscale 8, 11984–11992. Delley, B., 1996. Fast Calculation of electrostatics in crystals and large molecules. J. Phys. Chem. 100, 6107–6115. Delley, B., 2000. From molecules to solids with the DMol3 approach. J. Chem. Phys. 113, 7756–7764. Dong, P., Pint, C.L., Hainey, M., Mirri, F., Zhan, Y., Zhang, J., Pasquali, M., Hauge, R.H., Verduzco, R., Jiang, M., Lin, H., Lou, J., 2011. Vertically aligned single-walled carbon nanotubes as low-cost and high electrocatalytic counter electrode for dye-sensitized

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