In situ electrodeposition of nickel cobalt selenides on FTO as an efficient counter electrode for dye-sensitized solar cells

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In situ electrodeposition of nickel cobalt selenides on FTO as an efficient counter electrode for dye-sensitized solar cells Qing-Song Jiang a,b,*, Wenjie Cheng a, Jing Wu a, Wenbo Li a, Jie Zhu a, Yufu Zhu c, Zhengqing Yan a, Guohua Lin a,b, Yulin Zhang a,b a

Faculty of Electronic Information Engineering, Huaiyin Institute of Technology, Huai’an 223003, China Jiangsu Engineering Laboratory for Lake Environment Remote Sensing Technologies, Huaiyin Institute of Technology, Huai’an 223003, China c Faculty of Mechanical & Material Engineering, Huaiyin Institute of Technology, Huai’an 223003, China b

highlights

graphical abstract


NixCoySe films are synthesized by a potential reversal electrodeposition technique.
NixCoySe array is designed by depositing

optimized

NixCoySe

film on polystyrene array.
Light absorption of DSC with NixCoySe array is enhanced by light scattering effect.
PCE of DSC with optimized NixCoySe CE is 7.40%, and larger than Pt-based device (6.32%).
PCE of DSC with NixCoySe array is 7.64%, and 20.9% larger than Ptbased device.

article info

abstract

Article history:

Construction of transition metal selenides with high electrocatalytic performance is of

Received 24 May 2019

significant importance, but it is still a challenge to develop the corresponding counter

Received in revised form

electrodes (CEs) by an electrodeposition technique. In the present work, nickel cobalt

7 July 2019

selenide (NixCoySe) films are prepared in situ on fluorine-doped tin oxide (FTO) glasses

Accepted 13 July 2019

through a potential reversal electrodeposition technique. The morphology and electronic

Available online 9 August 2019

structure of NixCoySe films can be tuned by controlling the Ni/Co molar ratio in electro-

Keywords:

interaction between the Ni and Co elements displays numerous particles composed of

Counter electrodes

sheets attached with nanocrystals, resulting in the more electrocatalytic active sites.

Nickel cobalt selenides

Benefiting from the unique morphology and optimized synergistic effect, NixCoySe-6 CE

plating solution. Specially, NixCoySe-6 film (the Ni/Co molar ratio of 1:1) with the optimized

* Corresponding author. Faculty of Electronic Information Engineering, Huaiyin Institute of Technology, Huai’an 223003, China. E-mail address: [email protected] (Q.-S. Jiang). https://doi.org/10.1016/j.ijhydene.2019.07.109 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Potential reversal electrodeposition

exhibits superior electrocatalytic activity for the triiodide reduction. Then, the dye-

technique

sensitized solar cell (DSC) fabricated by NixCoySe-6 CE has demonstrated a power con-

Array

version efficiency (PCE) over 7.40%, which is higher than that of platinum (Pt)-based device

Synergistic effect

(6.32%). Furthermore, NixCoySe-6 array CE is also prepared by using polystyrene array as template. The PCE of the DSC with NixCoySe-6 array CE reaches its maximum value of 7.64% and 20.9% larger than that of Pt-based device. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Dye-sensitized solar cells (DSCs) as one of the promising third generation solar cells have drawn extensive attention because of their low-cost, simple manufacturing process, environmental friendliness, and theoretical high PCE [1e3]. A typically DSC is consisted of N719 dye-sensitized titanium oxide, liquid electrolytes based on iodide/triiodide, and Pt CE on FTO glass [4]. As a crucial part of DSCs, CE generally is an efficient electrocatalyst for collecting electrons and facilitating the reduction of triiodide ions, and exerts a great influence on the photovoltaic performance of DSCs [5]. Taking the catalytic activity and electrical conductivity, Pt electrode has served as a champion CE in DSCs. However, the resource scarcity and expensive fabrication cost of Pt CE have seriously impeded the large-scale application of DSCs [6]. Therefore, many efforts have been devoted to developing high-performance alternative candidates, such as carbonaceous materials [7], organic polymers [8], inorganic compounds (carbides, nitrides, oxides, sulfides, selenides, tellurides, and phosphides) [9e15], and alloy materials [16]. Among the above alternative candidates, transition metal selenides as one of the most efficient Pt-free CE materials have exhibited the remarkable electrocatalytic activity for the triiodide reduction, due to the 3d electron configuration of transition metal and high electronic conductivity of selenium element [17e19]. Recently, binary selenides, such as Cu3Se2, FeSe2, MoSe2, NiSe, Ni0.85Se, Co0.85Se, CoSe2, have been explored in detail as CE materials of DSCs [20e26]. In particular, nickel selenides and cobalt selenides have been extensively investigated because of their abundant resource, high electrocatalytic performance, and low toxicity. Therefore, the morphology and phase of nickel selenides and cobalt selenides have been explored to improve their electrocatalytic activity [27e29]. Furthermore, the corresponding DSCs have demonstrated the comparable PCE with Pt-based DSC. Unfortunately, although nickel selenides and cobalt selenides provide the high electrocatalytic activity, the photovoltaic performance of the corresponding DSCs is still far from being satisfactory. In recent years, it is interestingly found that ternary nickel cobalt selenides have shown the higher electrocatalytic performance than that of binary nickel selenides and cobalt selenides [30]. Construction of ternary nickel cobalt selenides is served as an effective strategy to enhance their catalytic performance owing to the synergistic effect between Co and Ni elements [31e33]. The synergistic effect can be ascribed to the strong interaction of electron spin states, which is beneficial to provide the more catalytic active sites [34e36]. Ternary nickel cobalt selenide microspheres and Ni-Co-Se alloy hollow

microspheres were fabricated through the precursor conversion method and hydrothermal method, and the corresponding DSCs shown the higher photovoltaic performance than that of the Pt-based DSC [37,38]. The PCE of quasi-solidstate DSCs fabricated by CoxNiySe and NiCo2Se4 CEs had been also improved by comparing with those of DSCs based on binary nickel selenide CEs and cobalt selenide CEs, which were synthesized by solvothermal and hydrothermal methods [39,40]. From Table S1, it is obvious found that ternary nickel cobalt selenides exhibit the higher electrocatalytic activity than that of Pt CE, enhancing the photovoltaic performance of the corresponding DSCs. However, the fabrication technique of ternary nickel cobalt selenides is still needed to further explore by applying the simple synthesis process. In view of the fabrication process, electrodeposition technique is regarded as an effective synthesis approach to design nickel cobalt selenides, due to its low fabrication cost, simple experimental equipment, mild reaction condition, and short reaction time [41]. Furthermore, a potential reversal electrodeposition technique has been proposed to fabricate high-quality transition metal sulfides through removing undesirable elemental metal [42]. In this regard, it is urgent necessary to design nickel cobalt selenides by the potential reversal electrodeposition technique, and investigate their electrocatalytic activity. In this study, we report for the first time on the preparation of nickel cobalt selenide (NixCoySe) films by the potential reversal electrodeposition technique. The effect of the Ni/Co molar ratio in the electroplating solution on the morphology and electronic structure of NixCoySe films has been explored. The results show that NixCoySe-6 CE exhibits the higher electrocatalytic performance for the triiodide reduction than that of others NixCoySe CEs and Pt CE, due to the unique morphology and optimized synergistic effect. In addition, NixCoySe-6 arrays are also fabricated by using polystyrene arrays as templates, and possess the more porous structure. Comparing with NixCoySe-6 CE, the electrocatalytic activity of NixCoySe-6 array CE has been further improved. As a result, the PCE of the DSC fabricated by NixCoySe-6 array CE as high as 7.64% has been achieved, and larger than those of DSCs with NixCoySe-6 CE (7.40%) and Pt CE (6.32%).

Experimental Preparation of NixCoySe films All the chemicals were of analytical grade, and purchased from Sinopharm Chemical Reagent Co., Ltd of China. The

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electrodeposition process was shown as follows. The electroplating solution was consisted of 2.5 mM selenium oxide, 12.5 mM potassium chloride and 5 mM metal salts (nickel chloride hexahydrate and cobalt chloride hexahydrate) in 50 mL of deionized water and ethanol (4:1, v/v). The cleaned FTO glass (1.5 cm  2.5 cm), Ag/AgCl electrode, and Pt wire were served as the working electrode, reference electrode, and counter electrode, respectively. The electrodeposition potential was provided from multi-potential steps with threeelectrode system from CHI660E electrochemical workstation. The two electrodeposition potentials were set as 1.2 V and 0.2 V, respectively. The corresponding electrodeposition times were set as 6 s and 24 s, respectively. The electrodeposition cycle was set as 12-cycle. Then, NixCoySe films were obtained by washing the electrodeposited FTO glasses with deionized water. NixCoySe films with the different Ni/Co ratios were fabricated by adjusting the Ni/Co molar ratio in the electroplating solution, and defined as NixCoySe-z (z ¼ 0, 3, 4, 6, 9, 12, 18) films. The Ni/Co molar ratios of NixCoySe-0, NixCoySe-3, NixCoySe-4, NixCoySe-6, NixCoySe-9, NixCoySe-12, and NixCoySe-18 were 0:1, 1:2, 2:3, 1:1, 3:2, 2:1, and 1:0, respectively.

Preparation of NixCoySe-6 array An emulsifier-free emulsion polymerization method was applied to synthesize polystyrene (PS) spheres with the diameter of 450 nm [43]. The air/liquid/solid interface selfassembly method had been demonstrated an effective approach to fabricate two-dimensional PS array on FTO glass [44]. PS array was heat treated at 60  C for 12 h, and used as working electrode. 5 mM metal salts were composed of 2.5 mM nickel chloride hexahydrate and 2.5 mM cobalt chloride hexahydrate in the electroplating solution. Then, all the electrodeposition process was similar with that of NixCoySe films. As last, the electrodeposited PS array was immersed into tetrahydrofuran for ten mins to obtain NixCoySe-6 array. In addition, Pt CE was fabricated by a radio frequency magnetron sputtering system [45].

Characterization The crystalline structure of NixCoySe films and NixCoySe-6 array was characterized by X-ray powder diffraction (XRD, D8-Discover, Bruker) equipped with Cu Ka radiation (l ¼ 0.154 nm). The morphology of NixCoySe films and NixCoySe-6 array was characterized by a scanning electron microscope (SEM, JEOL 7800F). The microstructure of NixCoySe-6 film and NixCoySe-6 array was characterized by a transmission electron microscope (TEM, JEOL 2100F). The element mapping of NixCoySe-6 film and NixCoySe-6 array was obtained through an energy dispersive X-ray spectrometer attached to SEM. The chemical composition and chemical states of NixCoySe-6 array were determined via X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Escalab 250Xi). The transmittance spectra of PS array, NixCoySe-6 film and NixCoySe-6 array were analyzed by UV-vis-NIR spectrophotometer (UV-3600, Shimadzu). The photovoltaic performance of as-assembled DSCs and the electrocatalytic activity of asfabricated CEs were investigated by CHI660E electrochemical workstation. The power of incident light from Xenon light source was calibrated to 100 mW cm2 by a photosensitive diode. The photocurrent-voltage (J-V) curves of as-assembled DSCs were obtained at the scan rate of 40 mV s1. For cyclic voltammetry (CV) measurement, the electrolyte solution was consisted of 10.0 mM LiI, 1.0 mM I2, and 0.1 M LiClO4 in anhydrous acetonitrile. Three-electrode system was composed of Ag/Agþ electrode (10 mM AgNO3 in the electrolyte solution), CE, and Pt electrode, which correspond to reference electrode, working electrode, and counter electrode, respectively. The scan rate was set as 50 mV s1. The scan potential was varied from 0.6 V to 1.2 V. The symmetrical dummy cell was assembled by the identical as-fabricated CEs, and applied to obtained electrochemical impedance spectroscopy (EIS) and Tafel polarization curves. EIS spectra were recorded at the amplitude of 10 mV and the scan frequency from 0.1 Hz to 100 kHz. Tafel curves were recorded at the scan rate of 10 mV s1 and the scan potential from 1 V to 1 V.

Preparation of DSCs

Results and discussion TiO2 films with the thickness of 10 mm were fabricated on FTO glasses through a doctor-blade method, and sintered at 500  C for 15 min. 0.5 mM N719 dye solution was prepared by adding 0.0295 g of N719 dye (cis-bis(isothiocyanato)bis(2,20 - bipyridyl4,40 -dicarboxylato)-ruthenium(II)bis-tetrabutylammonium) into 50 mL of ethanol by stirring 12 h at room temperature. Then, TiO2 films were immersed in N719 dye solution for 24 h at 60  C. After the sensitized process, TiO2 photoanodes were washed with ethanol and heat treated at 60  C for 10 min. A typical DSC with an open structure was assembled as follows: TiO2 photoanode was laid on an opaque plate with a circular hole of 0.5 cm in diameter. CE was placed on the TiO2 photoanode by fixing with a clamp. Liquid electrolytes composed of the I/I 3 redox couple were purchased from Kunshan Sunlaite New Energy Technology Co., LTD (China), and injected into the internal space between TiO2 photoanode and CE. The effective electrode area of DSCs was approximated 0.2 cm2.

NixCoySe films are designed and fabricated by the potential reversal electrodeposition technique, and served as efficient Pt-free CEs of DSCs. The overall fabrication process of NixCoySe films and NixCoySe-6 array is shown in Scheme 1. On one hand, the cleaned FTO glasses are applied as substrates to grow NixCoySe films by the electrodeposition technique. The chemical composition of NixCoySe films is determined by adjusting the Ni/Co molar ratio in the electroplating solution. The digital photograph of NixCoySe films is shown in Fig. S1(a). The clearly deepened color indicates the formation of NixCoySe films. On the other hand, PS array composed of monodisperse PS spheres is applied as substrate to grow NixCoySe-6 nanomaterials, which is assembled on FTO glass through the air/liquid/solid interface self-assembly method [44]. Then, NixCoySe-6 array is obtained by removing PS spheres. Figs. S1(b) and (c) show the digital photograph of NixCoySe-6 film and NixCoySe-6 array, suggesting their high transparency.

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Scheme 1 e Schematic diagram of the fabrication process of NixCoySe films and NixCoySe-6 array.

Meanwhile, NixCoySe-6 film and NixCoySe-6 array are chosen as the representative CEs to investigate their electrocatalytic performance for the triiodide reduction. The surface morphology of NixCoySe films and NixCoySe-6 array are shown in Fig. 1 and Fig. S2. It is obvious that NixCoySe-0 (cobalt selenide) film grows on the surface of FTO glass, and exhibits numerous particles with different sizes. Then, NixCoySe-3 film has agglomerated together from particles, as shown in Fig. 1(b). With increasing the Ni content, the particles are interconnected by sheets for NixCoySe-4 film. Interestingly, the SEM image of NixCoySe-6 film (Fig. 1(d))

exhibits numerous interconnected particles, which are consisted of sheets attached with nanoparticles. The more sheets and nanoparticles are beneficial to provide the more electrocatalytic active sites. Subsequently, the interconnected particles are gradually densified by increasing the Ni/Co molar ratio in electroplating solution. Fig. 1(g) indicates that NixCoySe-18 (nickel selenide) film is also consisted of lots of particles with different sizes. PS arrays are assembled on the surface of FTO glass, and shown in Fig. S2(a). Clearly, PS arrays possess the highly ordered structure. Low-magnification SEM image (Fig. S2(b)) suggests the uniform growth of NixCoySe-6

Fig. 1 e SEM images of (a) NixCoySe-0 film, (b) NixCoySe-3 film, (c) NixCoySe-4 film, (d) NixCoySe-6 film, (e) NixCoySe-9 film, (f) NixCoySe-12 film, (g) NixCoySe-18 film, and (h) NixCoySe-6 array. (i) Transmittance spectra of PS array, NixCoySe-6 film, and NixCoySe-6 array.

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array. High-magnification SEM image (Fig. 1(h)) confirms that NixCoySe-6 array is composed of numerous interconnected particles, which are similar with the morphology of NixCoySe6 film. As shown in Fig. S3, the chemical composition of NixCoySe-6 film and NixCoySe-6 array are Ni, Co, and Se elements, which are uniformly distributed over the corresponding samples. Fig. 1(i) shows transmittance spectra of PS array, NixCoySe-6 film, and NixCoySe-6 array. It is obvious that NixCoySe-6 film and NixCoySe-6 array display the large transmittance, which is consistent with their digital photograph (Fig. S1). Furthermore, below the wavelength of 660 nm, the transmittance of NixCoySe-6 array has a significant decrease comparing with NixCoySe-6 film. The result indicates that the ordered structure of NixCoySe-6 array can be beneficial to increase the light harvesting efficiency of photoanode for DSCs by the scattering effect [46]. The microtopography and microstructure of NixCoySe-6 film are characterized by TEM, as can be seen in Fig. 2. It is found that the interconnected particles are formed from the accumulation of numerous sheets and particles, which are decorated with nanocrystals. The possible contribution of the structure characterization for the nanocrystals is the more electrocatalytic active sites. Then high-resolution TEM images of NixCoySe-6 film display the obviously lattice fringes, implying that NixCoySe-6 film has the good crystallinity. The interplanar spacings (Fig. (2c)) are approximately 0.274 nm, 0.273 nm, and 0.206 nm, respectively. The interplanar spacings of 0.274 nm and 0.273 nm can be indexed to the (101) planes of Co0.85Se (JCPDS No. 52-1008) or Ni0.85Se (JCPDS No.

18-0888) [27]. And the interplanar spacing of 0.206 nm is corresponded to the (102) planes of Co0.85Se (JCPDS No. 52-1008) or Ni0.85Se (JCPDS No. 18-0888) [27]. In addition, TEM images of NixCoySe-6 array shows the two-dimensional hexagonal close-packed structure, shown in Fig. S4(a). The highresolution TEM images of NixCoySe-6 array also display lattice fingers with the interplanar distances of 0.204 nm, 0.268 nm, and 0.273 nm, corresponding to the (102), (101), and (101) planes of Co0.85Se (JCPDS No. 52-1008) or Ni0.85Se (JCPDS No. 18-0888), respectively [27]. The change of the interplanar spacing implies the synergistic effect between Co and Ni ions in as-prepared NixCoySe films and NixCoySe-6 array [31]. However, the XRD patterns of NixCoySe films and NixCoySe-6 array just show the characteristic diffraction peaks of FTO glass, as shown in Fig. S5. The reason is likely that the asprepared NixCoySe films and NixCoySe-6 array possess the thin thickness, which are agreement with the results from Fig. 1(i) and Fig. S1. XPS spectra of NixCoySe-6 film are shown in Fig. 3, and applied to confirm the surface chemical composition and element valent states. In high-resolution Co 2p spectrum (Fig. 3(a)), it can be seen that two shake-up satellite peaks have been observed at 802.8 eV and 786.2 eV, and marked as “Sat.”. Then, the characteristic peaks of Co 2p1/2 and Co 2p3/2 are all split into two peaks, respectively. The binding energies of Co 2p1/2 are at approximately 797.6 eV and 793.9 eV, which are corresponded to Co2þ and Co3þ species, respectively [30,47]. The binding energies of Co 2p3/2 at 781.5 eV and 778.8 eV are also assigned to Co2þ and Co3þ species, respectively [38,48].

Fig. 2 e TEM images (a, b) and high-resolution TEM image (c) of NixCoySe-6.

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Fig. 3 e XPS spectra of NixCoySe-6 film: (a) Co 2p, (b) Ni 2p, and (c) Se 3d and Co 3p.

Fig. 3(b) shows the high-resolution Ni 2p spectrum. Apart from the shake-up satellite peaks at 880.9 eV and 861.9 eV, the two peaks at 874.2 eV and 856.4 eV are ascribed to Ni 2p1/2 and Ni 2p3/2 for Ni2þ species, respectively [37,49]. Furthermore, the other two peaks at 871.2 eV and 853.7 eV are ascribed to Ni 2p1/ 3þ species, respectively [39]. These results 2 and Ni 2p3/2 for Ni indicate that valence states of Ni and Co elements are þ2 and þ3 in NixCoySe-6 film. In addition, Fig. 3(c) shows the highresolution Se 3d and Co 3p spectrum. The broad peak observed at 59.8 eV is ascribed to Co 3p [39]. The peak located at 59.0 eV is corresponded to the surface oxidation state of Se species [32]. Moreover, the two peaks are observed at 55.1 eV and 54.3 eV, which are assigned to Se 3d3/2 and Se 3d5/2 [32,50]. The two split peaks from Se 3d indicate that the valence state of Se element is 2. The above analysis suggests that NixCoySe-6 film is successfully fabricated on FTO glass by the potential reversal electrodeposition technique. NixCoySe films with different Ni/Co ratios are directly served as CEs of DSCs. Then NixCoySe CE, photoanode, and liquid electrolytes are assembled into a sandwich-type cell with an open structure. The photovoltaic performance of DSCs has been analyzed by comparing the short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE, which can be obtained from J-V curves of as-assembled DSCs [51]. Fig. 4(a) shows the PCE distribution of DSCs based on NixCoySe-z (z ¼ 0, 3, 4, 6, 9, 12, and 18) CEs. Obviously, the PCE of the DSC based on NixCoySe-0 CE reaches 6.84%, which is larger than that of the Pt-based device (6.32%). Meanwhile, the PCE of DSCs based on NixCoySe CE increases with increasing the Ni content. When the Ni/Co molar ratio reaches 1, the DSC based on NixCoySe-6 CE exhibits the highest photovoltaic performance with the PCE as high as 7.40%. However, the PCE

of DSCs based on NixCoySe CE decreases with the further increase of the Ni content in electroplating solution. Furthermore, the DSC based on NixCoySe-18 CE has also achieved 6.51% of PCE. In regards of the photovoltaic performance of DSCs, we suppose that NixCoySe-6 CE shows the higher electrocatalytic activity than that of Pt and the others NixCoySe CEs, due to the more electrocatalytic active sites and optimized synergistic effect [37e40]. Then, the fabrication parameters of the NixCoySe-6 film are applied to design NixCoySe-6 array CE by using PS array as substrate. Fig. 4(b) shows J-V curves of DSCs based on Pt, NixCoySe-6, and NixCoySe-6 array CEs. The photovoltaic performance is summarized in Table 1. It is found that the DSC assembled from NixCoySe-6 array CE yields a remarkable PCE of 7.64% (Jsc ¼ 17.27 mA cm2, Voc ¼ 0.66 V, and FF ¼ 0.67), which is higher than that of the DSC assembled from NixCoySe6 CE (PCE ¼ 7.40%, Jsc ¼ 16.99 mA cm2, Voc ¼ 0.66 V, and FF ¼ 0.66). This is attributed to the ordered porous structure of NixCoySe-6 array, resulting in the high light harvesting efficiency of photoanode for DSCs [46]. Then, the repeatability of DSCs based on NixCoySe-6 and NixCoySe-6 array CEs are shown in Figs. S6 and S7. Obviously, the average Jsc, Voc, FF, and PCE values of the DSC based on NixCoySe-6 CE are 16.89 mA cm2,

Table 1 e Photovoltaic parameters of DSCs with Pt, NixCoySe-6, and NixCoySe-6 array CEs. CEs Pt NixCoySe-6 NixCoySe-6 array

Jsc(mA cm2)

Voc(V)

FF

PCE(%)

14.09 16.99 17.27

0.66 0.66 0.66

0.68 0.66 0.67

6.32 7.40 7.64

Fig. 4 e (a) The relationship between the Ni/Co ratio and PCE of DSCs with NixCoySe CEs. (b) J-V curves of DSCs based on Pt, NixCoySe-6, and NixCoySe-6 array CEs. (c) PCE distribution of ten DSCs with NixCoySe-6 and NixCoySe-6 array CEs.

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0.66 V, 0.66, and 7.41%, respectively. Meanwhile, the DSC based on NixCoySe-6 array CE has the high average photovoltaic performance with Jsc ¼ 17.02 mA cm2, Voc ¼ 0.67 V, FF ¼ 0.67, and PCE ¼ 7.63%. Therefore, the PCE of the DSCs assembled from NixCoySe CEs is increased from 7.41 ± 0.15% to 7.63 ± 0.25%, as shown in Fig. 4(c). The electrocatalytic performance of Pt, NixCoySe-6, and NixCoySe-6 array CEs is evaluated by conducting CV measurement at scan rate of 50 mV s1, as can be seen in Fig. 5(a). It is found that three CV curves possess the similar shape, which can be separated into two pairs of the redox peaks. The redox peaks for the left (right) pair are resulted from the redox     reaction of I 3 þ2e #3I (3I2þ2e #2 I3 ) [52,53]. Therefore, the redox peaks for the left pair are employed to investigate the electrocatalytic activity of as-prepared CEs by comparing the reduction peak current density (absolute value, |JRed|) and peak-to-peak potential separation (Epp). Normally, the larger

value of |JRed| represents the larger reaction rate, and the smaller value of Epp represents the larger electrochemical rate constant [52]. As shown in Fig. 5(b), the |JRed| value obeys an order of Pt < NixCoySe-6 NixCoySe-6>NixCoySe-6 array. As previously mentioned, the sheets and nanocrystals of NixCoySe-6 CE with the optimized synergistic effect produce the more electrocatalytic active sites, resulting in the high electrocatalytic activity. Furthermore, the porous structure of NixCoySe-6 array CE is beneficial to provide the more diffusion channels for I/I 3 redox couple. As a result, NixCoySe-6 array CE shows the larger reaction rate and electrochemical rate constant, implying the higher electrocatalytic activity for the triiodide reaction compared with Pt and NixCoySe-6 CEs. Fig. 5(c) and (e) show the CV curves of NixCoySe-6 and NixCoySe-6 array CEs at the scan rates of 35 mV s1, 50 mV s1,

Fig. 5 e (a) CV curves of Pt, NixCoySe-6, and NixCoySe-6 array CEs at a scan rate of 50 mV s¡1. (b) A comparison diagram of | JRed| and Epp of Pt, NixCoySe-6, and NixCoySe-6 array CEs. CV curves of (c) NixCoySe-6 and (e) NixCoySe-6 array CEs at different scan rates. The relationship between the redox peak current densities and the square root of the scan rate for (d) NixCoySe-6 and (f) NixCoySe-6 array CEs.

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65 mV s1, 80 mV s1. It can be seen that all the CV curves show the similar shape composed of the two oxidation peaks and two reduction peaks. The |JRed| and the oxide peak current density values gradually increase with increasing the scan rate. And the Epp value has the same increase trend. Then, the current densities of the redox peaks at the left pair for NixCoySe-6 and NixCoySe-6 array CEs are related to the square root of the scan rate, as can be seen in Fig. 5(d) and (f). The linear relationship suggests that the redox reaction of the I/I 3 redox couple at the CE/electrolytes interface is described as the diffusion limitation in the electrolytes [37]. And there is no other interaction between as-prepared CEs and the I/I 3 redox couple, implying their good stability. Furthermore, the reduction peak current at the left pair (Ip) can be expressed by diffusion coefficient (D) of I/I 3 redox couple, according to the Randles-Sevcik equation: Ip ¼ 2.69  105n1.5ACD0.5v0.5 [54]. Where n represents the number of electrons in the redox reaction, A is the area of CE, C is the triiodide concentration, v is the scan rate. Therefore, the D value from theoretical calculation follows an order of Pt < NixCoySe-6
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basis of the equivalent circuit model (inset of Fig. 6(a)), all the Nyquist plots have been fitted through Z-view software. There are three parameters of the series resistance (Rs), charge transfer resistance (Rct), and Nernst diffusion impedance (ZN) from the equivalent circuit model [55]. It is found that the Rs values of as-prepared CEs is larger than that of Pt CE, due to the weaker electronic conductivity of NixCoySe nanomaterials. The electronic conductivity of Pt, NixCoySe-6, and NixCoySe-6 array CEs is also analyzed from I-V curves by a two-probe method, as shown in Fig. S8 [56e59]. Obviously, the electronic conductivity has an order of Pt > NixCoySe-6 array > NixCoySe-6, which is consistent with the change trend of the Rs values. More importantly, the Rct and ZN values are generally applied to evaluate the electrocatalytic ability of asprepared CEs, as listed in Table S2 [60]. The electron transfer ability from CE materials to liquid electrolytes is represented as the Rct value. The slower electron transfer process implies the larger Rct value [60]. From Fig. 6(c) and Table S2, the Rct value follows a sequence of Pt > NixCoySe-6>NixCoySe-6 array. This reveals that NixCoySe-6 array CE has the stronger electron transfer ability for the reduction of triiodide than that of Pt and NixCoySe-6 CEs, due to the more electrocatalytic active sites resulting from sheets, nanocrystals, and synergistic effect. The diffusion ability of I/I 3 redox couple in CE materials is related to the ZN value [40]. The smaller ZN value is beneficial to improve the diffusion ability of I/I 3 redox couple. Therefore, the D value is inversely proportional to the ZN value [42]. As shown in Fig. 6(d) and Table S2, the ZN value decreases

Fig. 6 e (a) Nyquist plots of EIS and (b) Tafel polarization curves for symmetrical dummy cells based on Pt, NixCoySe-6, and NixCoySe-6 array CEs. (c) A comparison diagram of I0 and Rct for three CEs. (d) A comparison diagram of Jlim and ZN for three CEs. The inset of (a) is the equivalent circuit model. In (a), the symbols are the experimental data, while the solid lines are the simulated data.

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in the order of Pt > NixCoySe-6>NixCoySe-6 array, indicating that NixCoySe-6 array shows the larger diffusion ability. In additional, the change trend of the D value is in agreement with that from CV results. Thus, NixCoySe-6 array CE exhibits the high electrocatalytic activity for the reduction of triiodide. Tafel polarization curves of Pt, NixCoySe-6 and NixCoySe-6 array CEs are also applied to analyze their charge transfer kinetics for the I/I 3 redox couples, as can be seen in Fig. 6(b). In the polarization zone, the exchange current density (J0) is obtained via the linear extrapolation [39]. The larger J0 value indicates the faster electron transfer process. The increase order of the J0 value is shown as follows: Pt < NixCoySe6
Conclusions In summary, we have successfully employed the potential reversal electrodeposition technique to fabricate NixCoySe films and NixCoySe-6 array in situ on FTO glass. On one hand, NixCoySe films with different Ni/Co ratio have been designed to control their morphology and electronic structure. Specially, NixCoySe-6 film with the optimized synergistic effect shows numerous particles composed of sheets and nanocrystals, which is beneficial to provide the more catalytic active sites. As a result, NixCoySe-6 CE exhibits the higher electrocatalytic activity for the triiodide reduction than that of Pt and others NixCoySe CEs, resulting in the higher PCE of 7.40% for the corresponding DSC. On the other hand, NixCoySe-6 array CE has also been fabricated to improve the Jsc of DSCs by enhancing the light harvesting efficiency of photoanode. Thus, the DSC based on NixCoySe-6 array CE has yielded a remarkable photovoltaic performance with a PCE of 7.64%, which is 20.9% larger than that of Pt-

based device (6.32%). These results are helpful to design and synthesize the ternary selenides with high electrocatalytic activity, which are served as the promising candidates for Ptfree CE materials in DSCs.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 61804062, 61801188, 61704063); Jiangsu provincial university natural science research major program, China (No. 17KJA510001); the Science Research Foundation of Huaiyin Institute of Technology (No.17HGZ003); Jiangsu Overseas Visiting Scholar Program for University Prominent Young & Middle-aged Teachers and Presidents.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.109.

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