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Morphology-Tuned Synthesis of Nickel Cobalt Selenides as Highly Efficient Pt-Free Counter Electrode Catalysts for Dye-Sensitized Solar Cells Xing Qian, Hongmei Li, Li Shao, Xiancai Jiang, and Linxi Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09966 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

Morphology-Tuned Synthesis of Nickel Cobalt Selenides as Highly Efficient Pt-Free Counter Electrode Catalysts for Dye-Sensitized Solar Cells Xing Qian,a Hongmei Li,a Li Shao, Xiancai Jiang, and Linxi Hou* School of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou 350116, China a

These authors contributed equally to this work.

*E-mail: [email protected]. Fax: +86-0591-2286 6244; Tel: +86-0591-2286 5220.

Abstract: In this work, morphology-tuned ternary nickel cobalt selenides based on different Ni/Co molar ratios have been synthesized via a simple precursor conversion method and used as counter electrode (CE) materials for dye-sensitized solar cells (DSSCs). The experimental facts and mechanism analysis clarified the possible growth process of product. It can be found that the electrochemical performances and structures of ternary nickel cobalt selenides can be optimized by tuning the Ni/Co molar ratio. Benefiting from the unique morphology and tunable composition, among the as-prepared metal selenides, the electrochemical measurements showed that the ternary nickel cobalt selenides exhibited a more superior electrocatalytic activity in comparison with binary Ni and Co selenides. In particular, the three dimensional dandelion-like Ni0.33Co0.67Se microspheres delivered much higher power conversion efficiency (9.01%) than that of Pt catalyst (8.30%) under AM 1.5G irradiation.

KEYWORDS: nickel cobalt selenides, dandelion-like structure, electrocatalytic activity, counter electrode, dye-sensitized solar cells

INTRODUCTION Under the background of environment problems and rapidly depletion of fossil fuels, it is an urgent need to seek alternative green energy sources. Dye-sensitized 1

ACS Paragon Plus Environment


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solar cells (DSSCs), as ideal substitutes for tradition silicon solar cells, have received global attention and extensive research due to their low cost, environmentally friendly character, easy preparation, and high power conversion efficiencies (PCEs).1–5 As an essential part of DSSC, the counter electrode (CE) has two significant role, including collecting the electrons from external circuit and catalyzing the reduction reaction of triiodide (I3–) to iodide (I–) in the electrolyte.6–9 Traditionally, platinum (Pt) has proven to be a preferable CE catalytic material for DSSC because of its excellent conductivity and electrocatalytic activity. But its drawbacks, such as low abundance ratio, expensive cost, and especially easy corrosion in the I3–/I– electrolyte, not only worsen the CE performance, but also limit their large-scale commercial application for DSSC.10–13 Therefore, much great effort is necessary to be taken to explore earth-abundant, low-cost, and highly efficient substitutes for Pt in DSSCs, including carbonaceous materials,14–17 conductive polymers,18–20 inorganic compounds,21–23 and their hybrid materials.24–29 As a class of functional materials, transition metal chalcogenides (TMC), possessing outstanding magnetic, electrical and optical properties, have been intensively researched in various applications including magnetic devices,30 electrochemical hydrogen evolution,31,32 supercapacitors,33,34 lithium-ion batteries,35 and DSSCs.21,22,36–39 Among them, stimulated by the charming intrinsic properties and remarkably high catalytic activity, some methods have been adopted to explore various binary selenides CE materials for DSSC. For example, Wang et al. synthesized the FeSe2 films with controllable morphology via a facial hydrothermal method for DSSCs, and the 3D flower-like FeSe2 shows a higher PCE of 8.00% than that of the DSSC using a Pt CE (7.87%).37 A series of Ni-Se alloy with different Ni content were synthesized by Duan et al. via a mild solution strategy in situ on FTO glasses and used directly as CEs in bifacial DSSCs and the Ni0.85Se alloy provided an impressive PCE of more than 10% from bifacial irradiation.40 Obviously, except the structural design, the chemical composition also plays a key role in the electrochemical performance of CE materials. To date, although transition metal 2


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chalcogenides have been extensively explored in the field of DSSC, such as NiS,7 FeS2,9 Co9S8,11 NiSe2,22 FeSe2,37 MoS2,41 and so on. However, some of them have showed unsatisfactory PCE values, amorphous nanostructures, and changeless compositions, and whether further tuning of the nanostructure of CE material with different element composition could strengthen its PCE value has rarely been systematically researched, as well as the growth mechanism of the majority of electroactive materials has not been discussed in detail. As a consequence, there is still a considerable development space to design and synthesis more efficient CE materials with various novel nanostructures and adjustable elementary composition. To strengthen the properties of the CE materials, the optimization of morphology and composition are indispensable. Among diverse morphologies, sperical micro/nanostructures with easy fabrications, perfect substrate matrices, and interesting surface morphologies have been widely studied in energy conversion/storage fields. Multi-shelled ZnO hollow microspheres, which has been prepared and used as the photoanodes by Tang and co-workers, showed a high PCE of 5.6%.42 Wang et al. have synthesized a porous Co3O4 hollow spheres, which showed outstanding catalytic activity for CH4 combustion.43 Nevertheless, it is still a challenge to synthesis dandelion-like and floccus-like microspheres in despite of some efforts have been made. Meaningfully, the dandelion-like microsphere assembled with nanotubes can provide more open space, charge transfer channels, and active sites to reinforce its catalytic performance.44 On the other hand, compared with binary compounds, ternary compounds (especially ternary Ni-Co compounds) demonstrated higher redox activity, superior electrochemical characteristics, and controllable structure owing to the appropriate synergistic effect of different electrocatalytic components or possible valence interchange.45–48 In recent years, various morphology of ternary Ni-Co compounds such as urchin-like,47,49 flower-like,50 nanoplatelet,46 nanotube,51,52 and hollow nanosprism53 have been investigated for photoelectric devices. For example, the 3D urchin-like NiCo2S4 nanostructure, which has been synthesized by Chen et al.47 showed higher electrochemical performance for high-rate pseudocapacitors than 3



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corresponding binary sulphides (Co9S8 and NiS). Banerjee et al.54 reported the self supported NiCo2S4 nanoneeds were directly formed on FTO substrates by anion exchange method and used as CE materials in DSSCs, giving a benign PCE of 6.9%, which is comparable to conventional Pt CE (7.7%). More recently, huo et al.50 used flower-like NiCo2S4/NiS microsphere as Pt-free CE with a high PCE of 8.8%, which is superior to Pt CE (8.1%). The above examples exhibited that ternary Ni-Co compounds may possess great prospect as CE materials in DSSC. To the extent of our knowledge, despite great efforts have been devoted to research the electrochemical performance of Ni-Co compounds, the synthesis of Ni-Co selenides based on tunable Ni/Co molar ratio is still limited. Besides, novel nanostructured nickel cobalt selenides which are supposed to further enhance the catalytic activity of CE have been not reported. Herein, we successfully fabricated ternary Ni-Co selenides with interesting morphologies based on different Ni/Co molar ratios (denoted as the name of NixCo1– xSe

(x = 0, 0.33, 0.5, 0.67, and 1.0) by a facile precursor transformation method,

including 3D dandelion-like Ni0.33Co0.67Se assembled with nanotubes, floccus-like Ni0.5Co0.5Se, Ni0.67Co0.33Se microsphere assembled with nanosheets, and Co3Se4 nanotubes. Then the as-prepared products were cast on FTO glass substrates by spin-casting technique. Based on the structural and compositional advantages, photovoltaic measurements manifested that the Ni0.33Co0.67Se delivered a PCE of 9.01%, which higher than that of Pt CE (8.30%) at the same conditions, exhibiting the superb electrocatalytic activity and high conductivity of ternary nickel cobalt selenides. These prominent features exhibit that ternary Ni-Co selenides are one of the most ideal electroactive materials for DSSC. Impressively, this work shows a simple and practical strategy to a large-scale synthesis of other efficient electrode materials with unique structure and desirable composition in ternary compound systems. EXPERIMENTAL SECTION Synthesis of NixCo1–xSe (x = 0, 0.33, 0.5, 0.67, and 1.0) composites. The NixCo1– xSe

compositions with different morphologies were synthesized by a facile precursor 4


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transformation method. The NixCo1–x(CO3)0.5OH precursors were first synthesized via a facial hydrothermal treatment process. Typically, the total mass of NiCl2·6H2O or/and CoCl2·6H2O was 10 mmol with the Ni/Co molar ratio of 1/0, 0.67/0.33, 0.5/0.5, 0.33/0.67, and 0/1 were firstly dissolved into 60 mL deionized water, respectively. To the same solution, 10 mmol of urea was added and intensively stirred for 15 min to form a homogeneous solution, which acted as the precipitant to yield OH– and CO32–, after that the above solution was poured into Teflon-lined stainless steel autoclave of 100 mL volume and heated at 130 °C in an air oven for 10 h. After cooling naturally to ambient temperature, the precipitation was centrifuged and washed respectively with distilled water and ethanol three times. Finally, after drying at 60 °C under vacuum for 12 h, the prescursor sample was obtained. In the next step, the The NixCo1–x(CO3)0.5OH precursors were transformed into NixCo1–xSe. In brief, 0.1 g of the precursor and 0.3 g Na2SeO3 were dispersed in 50 mL deionized water, then 10 mL of N2H4·H2O (80%) was added into the solution. After an intense stirring for 15 min, the above solution was poured into Teflon-lined stainless steel autoclave of 100 mL volume and heated at 180 °C in an air oven for 8 h. After cooling naturally to ambient temperature, the precipitation was centrifuged and washed respectively with distilled water and ethanol three times. Finally, after drying at 60 °C under vacuum for 12 h. The black samples were obtained and named as NiSe, Ni0.67Co0.33Se, Ni0.5Co0.5Se, Ni0.33Co0.33Se, and Co3Se4, respectively. Furthermore, in order to research the role of urea in the synthesis process, we chose Ni0.33Co0.67Se as a reference and prepared a counterpart (denoted as Ni0.33Co0.67Se-Ref) without urea through a one-step hydrothermal method. Namely, 0.1 g of NiCl2·6H2O and CoCl2·6H2O with the Ni/Co molar ratio of 0.33/0.67 and 0.3 g Na2SeO3 were dispersed in 50 mL deionized water, then 10 mL of N2H4·H2O (80%) was added into the solution. The following synthesis process was the same to that of NixCo1–xSe. Characterizations of Obtained Samples. The crystal structures and compositions of the products were characterized by X-ray diffraction (XRD, X'Pert PRO, Cu Kα, λ = 0.1542 Å). The surface morphologies of the products were measured with a 5



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field-mission scanning electro microscopy (SEM, S-4800, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX). The valence state of Ni0.33Co0.67Se was characterized by X-ray photoelectron spectrometer (XPS) analysis (ESCALAB 250, Mg Ka, USA). A N2 adsorption-desorption isotherms of products were performed by a Brunauer-Emmett-Telller (BET) sorptometer (Micromeritics, ASAP 2020M, USA). Fabrications of Counter Electrodes. The NiSe, Ni0.67Co0.33Se, Ni0.5Co0.5Se, Ni0.33Co0.67Se, and Co3Se4 based CEs were prepared on FTO glass substrates by a spin-casting technique. In detail, 0.1 g of the synthesized product was dispersed in 10 mL of ethyl alcohol and sonicated for 30 min to form a homogenous ink solution. Then as-prepared solution was cast on pre-cleaned FTO glass substrates by spin-casting at a rotating speed of 500 rpm for 12 s and heated at 120 °C for 10 min. Finally, the resultant CE films were prepared after the third layer was coated using the same processes. The loading mass for each sample was approximately 0.67 mg cm–2 on a FTO glass substrate. Moreover, as a reference for comparison studies, Pt CE was prepared by spin-casting 20 mM chloroplatinic acid (isopropanol solution) on the FTO glass and then thermal deposited at 450 °C in the air for 30 min. Fabrications of DSSCs. The TiO2 photoanode were prepared by screen-printing method, referring to our previous works.55,56 Briefly, a commercial 20 and 200 nm TiO2 sol (Heptachroma Corporation) were used to prepare the TiO2 film for the transparent nanocrystalline layer (~12 µm) and scattering layer (~4 µm) on the treated FTO conductive glass (treated with 0.05 M TiCl4 aqueous solution), respectively. Then, the TiO2 film was heated to 500 °C for 1 h before treated with 0.04 M TiCl4 aqueous solution at 70 °C for 1 h. Finally, the TiO2 electrode was sintered at 500 °C in a muffle furnace for 1 h again. The final TiO2 photoanodes were sensitized by soaking into a 0.3 mM N719 dye ethanol solution in room temperature overnight. The redox electrolyte contained 0.3 M DMPII, 0.1 M LiI, 0.05 M I2 and 0.5 M 4-tert-butylpyridine in acetonitrile solution and was injected into the interspace between the CE and the photoanode via capillary force. Thus, a sandwich structure of DSSC was obtained and its active area was 0.16 cm2. 6


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Characterization of the Counter Electrodes and DSSC. All of the electrochemical measurements were measured by the CHI660E electrochemical workstation (Chenhua, Shanghai). The photocurrent density–voltage (J–V) curves of the DSSCs were measured with a standard solar simulator (XM-500W, Trusttech) under 100 mW cm–2 irradiation calibrated with a standard silicon solar cell (91150V, Newport Corporation) and were not confirmed from independent certification laboratories. Cyclic voltammetry (CV) was performed in a three-electrode system to study the electrocatalytic ability of the CEs and the scanning potential range was from −0.5 V to 0.9 V at a scan rate of 50 mV s−1. The counter electrode, working electrode, and reference electrode are a Pt sheet, the as-prepared CE, and Ag/AgCl electrode, respectively.The electrolyte solution was made up of 10 mM LiI, 1 mM I2, and 100 mM LiClO4, where anhydrous acetonitrile was used as the solvent. The electrochemical impedance spectra (EIS) and Tafel polarization curves were conducted with a traditional CE symmetrical cell. For EIS measurements, the frequency range, bias voltage, and AC amplitude were 100 kHz−100 mHz, 0 V, and 10 mV, respectively. Meanwhile, the voltage range and scan rate of Tafel curve tests were −1.0 to 1.0 V and 10 mV s−1, respectively. A black mask was applied on the surface of DSSCs to avoid stray light and all solar cell tests were performed at room temperature. RESULT AND DISCUSSION Morphologies and Compositions. Figure 1 and Figure 1S (Supporting Information, SI) show the phase structures and composition of as-prepared products and their precursors, respectively. When CoCl2·6H2O was used to synthesize precursor, all the diffraction peaks in Figure S1 are assigned to Co(CO)0.35 Cl0.20 (OH)1.10·1.74H2O (PDF#38-0547). Turning the Ni/Co molar ratio to 0.33/0.67, the precursor Ni0.33Co0.67(CO3)0.5OH showed the same diffraction peaks as that of Co(CO3)0.5OH·0.11H2O (PDF#48-0083) except for lightly shifting to the low diffraction direction. This phenomenon showed the partial Ni ions was replaced by Co ions which did not change the crystal structures of these precursor samples and only 7



led a small change in the lattice parameter. This result can be attributed to the invariable formation of the carbonate hydroxides of Ni2+ or Co2+ due to the hydrolysis of urea in a hydrothermal condition.47 Additionally, as can be seen from Figure 1, the diffraction peaks at 39.1°, 53.1°, 60.5°, 71.7°, and 74.6° can be assigned to (111), (311), (31-3), (402), and (42-2) planes of cubic Co3Se4 (PDF#01-2805). Interestingly, all of the samples with different Ni/Co molar ratios possess similar XRD patterns. Meanwhile, we noticed that all of the diffraction peaks gradually shift to a low diffraction angle direction with the increasement of Ni element and finally reach 38.3°, 52.2°, 59.0°, 70.8°, and 72.8° corresponding to the (101), (102), (110), (103), and (201) planes of cubic NiSe (PDF#75-0610). Furthermore, compared to the XRD patterns of other four samples, the characteristic diffraction peaks of NiSe phase are more sharpen and stronger because Ni2+ is firmly stable than Co2+ even under high temperature environment. So a part of Co2+ in Co-based compounds can be easily oxidized to Co3+, while Ni ions still show a divalent state in Ni-based compounds.45 In view of Co2+ and Co3+ coexist in the selenides, as a result, Co3Se4 was successfully

71-1 42-2

31-3

Co3Se4

311

111

synthesized instead of CoSe in our experiment.

Intensity (a.u.)

Ni0.33Co0.67Se Ni0.5Co0.5Se

10

20

30 40 50 60 2 Theta (degree)

103 201

102

NiSe

110

Ni0.67Co0.33Se

101

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70

80

Figure 1. XRD patterns of the samples Co3Se4, Ni0.33Co0.67Se, Ni0.5Co0.5Se, Ni0.67Co0.33Se, and NiSe.

To investigate the morphologies of the as-prepared samples, the SEM images of the synthesized samples were shown in Figure 2. Figure S2 (SI) shows the morphologies 8


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of the needle-like precursors (Co(CO)0.35Cl0.20(OH)1.10·1.74H2O), which are composed of many nanorod bunches and the nanorods are bunched together in a radial fashion. In contrast, as can be seen from Figure 2a, the as-prepared Co3Se4 sample presents bunches of tubular structure are nanotubes instead of nanorods. Further, it also could be observed that the surface of the Co3Se4 nanotubes are very rough in the inset of Figure 2a, which manifested they are assembled with a mass of tiny nanoparticles. Additionally, the precursor of Ni0.33C0.67Se consists of a dandelion-like structure with many homogeneous nanorods radially grown from their common center (Figure 2b). After the hydrothermal treatment of precursor in selenide solution at 180 °C for 8 h, the Ni0.33Co0.67Se sample is similar to the precursor in morphology and structure with an average diameter of ~4 µm (Figure 2c). Figure 2e exhibits the nanorods of three Ni0.33Co0.67Se microspheres interacts each other. The surface of these Ni0.33Co0.67Se microspheres from the side and front views are observed in Figure 2d and its inset, respectively. It is clear to see that the dandelion-like Ni0.33Co0.67Se consists of uniform Ni0.33Co0.67Se nanotubes with diameter of ~100 nm. Besides, the Ni0.33Co0.67Se nanotube becomes shorter and coarser as well as the distance between nanotubes increases in comparison with its precursor, which can contribute to the contact of Ni0.33Co0.67Se CE and electrolyte thus enhancing the catalytic activity of Ni0.33Co0.67Se CE. Figure 2f and 2g are the SEM image of Ni0.5Co0.5Se sample, it can be found that the floccus-like microsphere with a loose surface structure and diameter of ~4 µm, and the Figure S3 (SI) shows the surface of Ni0.5Co0.5Se sample is composed of interconnected nanosheets. This unique structure was formed by its precursor (Ni0.5Co0.5(CO3)0.5OH) microsphere consists of numerous tentacles after the hydrothermal treatment (Figure S4, SI). In Figure 2h, the as-prepared sample also shows almost the same floccus-like microsphere structure as that of Ni0.5Co0.5Se but with compact surface texture, when the Ni/Co molar ratio is tuned to 0.67/0.33 (Ni0.67Co0.33Se). As shown in Figure 2i, the surface of NiSe sample has a lot of nanoparticles, which are irregularly aggregated.

9



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Figure 2. SEM images of (a) Co3Se4, (b) the Ni0.33Co0.67Se precursor, (c, d and e) Ni0.33Co0.67Se, (f and g) Ni0.5Co0.5Se, (h) Ni0.67Co0.33Se, and (i) NiSe.

Figure S5 (SI) shows the EDS spectrum of the Ni0.33Co0.67Se, Ni0.5Co0.5Se, and Ni0.67Co0.33Se, respectively. The obtained results clearly reveal that the three samples are composed of Ni, Co, Se, O, and C elements. What’s more, the atomic percent of every element are mentioned in the inset of Figure S5 (SI), displaying the Ni-Co-Se atom ratio of Ni0.67Co0.33Se, Ni0.5Co0.5Se, and Ni0.33Co0.67Se is approximately confirmed to be 1: 2: 6, 1: 1: 6, and 2: 1: 6, respectively. Besides, the EDS elemental mapping images of the sample Ni0.33Co0.67Se are shown in Figure S6 (SI), which indicates the Ni, Co, and Se elements are uniformly distributed in the samples. XPS spectra (Figure 3) was further conducted to confirm the composition and valence state of the Ni-Co selenides. The survey spectrum (Figure 3a) determines the existence of Ni, Co, Se, O, and C (as the reference) elements in Ni0.33Co0.67Se. The analysis of the high resolution XPS spectra of Ni, Co, and Se elements (Figure 3b, c, and d) were carried out using the Gaussian fitting method. Both of Ni 2p and Co 2p 10


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spectra can be fitted with two spin-orbit doublets as well as two shake-up satellites (expressed as “Sat.”). As demonstrated in Figure 3b of Ni 2p XPS, the peaks around 853.6 eV for Ni 2p3/2 and 871.0 eV for Ni 2p1/2 can be attributed to Ni2+ in the Ni-Co selenides while the peaks centered at around 856.3 eV in Ni 2p3/2 and 874.2 eV in Ni 2p1/2 are assigned to Ni3+.47,57 The spectrum of Co 2p was composed of Co 2p1/2 at 797.4 and 793.9 eV as well as Co 2p3/2 at 781.1 and 778.9 eV, which are characteristic of Co2+ and Co3+.51,58 Based on many previous researches, Ni element exhibits a stable divalent state in Ni-based compound and Co2+ can be easily oxidized into Co3+ in Co-based compound, while the Co3+ sites of Co3Se4 phase is replaced by some Ni atoms may result in the formation of Ni3+ ions because of the radius of Ni is almost equivalent with Co atom.45,57 Meanwhile, compared to the NiSe sample, the coexistence of Ni2+ and Ni3+ in Ni0.33Co0.67Se sample make the coordination mode of Ni ions with Se2– has changed, which is in accord with XRD results. Furthermore, the Se 3d5/2 and Se 3d3/2 peaks appear at 54.7 eV and 55.5 eV in the high resolution scan of Se 2p spectrum reflect the existence of Se2–, and the broad peak at 59.6 eV suggests the surface Se species with a high oxide state (SeOx).58 We noticed that there are Ni2+, Ni3+, Co2+, Co3+, and Se2– on the near-surface composition of the Ni0.33Co0.67Se hybrid. In the experiment, the integration of Co2+, Co3+, and Se2- result in the formation of Co3Se4 phase, while Co ions were partially replaced by Ni ions will not change the original Co3Se4 phase and consequently present NixCo3-xSe4 phase. Combining with EDS analysis and similar reports about Ni-Co compounds, the Ni0.33Co0.67Se sample can be indicated as NiCo2Se4 phase.59

11



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Figure 3. XPS spectra of the Ni0.33Co0.67Se (a) survey spectrum, (b) Ni 2p, (c) Co 2p, and (d) Se 3d.

Growth Mechanism of the Ni0.33Co0.67Se. Figure 4 shows the growth process of the Ni0.33Co0.67Se sample, which can be explained by the Kirkendall effect. 60,61 In the first hydrothermal process, the dandelion-shaped precursor (Ni0.33Co0.67 (CO3)0.5OH) microspheres were synthesized, where CoCl2·6H2O and NiCl2·6H2O are used to provide Co2+ and Ni2+, H2O serves as solvent, and urea is used as the precipitant to yield OH– and CO32–. The OH– and CO32– reacted with Co2+ and Ni2+ to form a lot of random nuclei at a low temperature, which was acted as the seed for the further growth of the crystal. As the temperature increased and time went on, the high concentration of seed resulted in the formation of 3D dandelion-shaped precursor microsphere which originated from the crystal growth of the primary nuclei and the further epitaxial process of these nanorods. The corresponding equation can be expressed as follows: CO(NH2)2 + H2O → CO2 +2NH3

(1)

CO2 +H2O → CO32– + 2H+

(2)

12


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NH3 + H2O → NH4+ + OH– 0.67Co2+

+

0.33Ni2+

(3) +

0.5CO32–

Ni0.33Co0.67(CO3)0.5OH·0.11H2O

OH–

+

+

0.11H2O

→

(4)

In the next step, the formation of the Ni0.33Co0.67Se was conducted through a hydrothermal treatment of the precursor in selenide solution, where hydrazine hydrate was used to reduce Na2SeO3. The selenide ion reacts with nanorods resulted in the formation of a thin layer of Ni0.33Co0.67Se nanocrystallites on their surface. The Ni0.33Co0.67Se nanocrystallites with many grain boundaries accelerated the diffusion of material through the Ni0.33Co0.67Se layer and the further reaction between selenide ion and precursor. The different diffusivities of the different components may result in the formation of tubular nanostructures, which was in good accordance with the Kirkendall effect. With the reaction continued, the dandelion-shaped Ni0.33Co0.67Se microspheres with many nanotubes can be obtained ultimately.

Figure 4. Schematic diagram for the fabrication of Ni0.33Co0.67Se.

13



Brunauer-Emmett-Teller Analysis. The nitrogen adsorption-desorption isotherms (Figure 5) were employed to investigate the specific surface areas of the five as-prepared samples. The obtained results exhibited that the Brunauer-Emmett-Teller (BET) specific surface areas of NiSe, Co3Se4, Ni0.5Co0.5Se, Ni0.67Co0.33Se, and Ni0.33Co0.67Se are 5.0, 10.1, 26.1, 28.9, and 35.6 m2 g–1, respectively. Clearly, among these metal selenides, the sample Ni0.33Co0.67Se possesses the highest specific surface area, which can increase the contact area of Ni0.33Co0.67Se CE and electrolyte solution and provide more active sites then finally improve the catalytic activity and

3 -1

150 120 90 60

Quantity adsorbed (cm3g-1STP)

electrochemical properties of DSSCs.

Quantity adsorbed (cm g STP)

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

Page 14 of 31

150 120

Ni0.67Co0.33Se NiSe

90 60

Co3Se4 Ni0.33Co0.67Se Ni0.5Co0.5Se

30 0 0.0

0.2 0.4 0.6 0.8 1.0 Relative pressure ( P/P0)

30 0 0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

Figure 5. N2 adsorption-desorption isotherms of Co3Se4, Ni0.33Co0.67Se, Ni0.5Co0.5Se, Ni0.67Co0.33Se (inset), and NiSe (inset) samples.

Electrochemical Impedance Analysis. To investigate the electrocatalytic activities of the synthesized metal selenides for I3– reduction, the Nyquist plots of electrochemical impedance spectroscopy (EIS) for CEs was conducted on the symmetrical

dummy

cells

fabricated

with

a

sandwich-like

structure

(CE/electrolyte/CE) (Figure 6). The relevant equivalent circuit diagram is shown as a inset in Figure 6, in which four impedance properties can be observed: Series resistance (Rs) is the intercept of the real axis of high-frequency, which is chiefly comprised of the bulk of CE material, the electrolytic resistance, and the substrate slide; Charge transfer resistance (Rct) is the diameter of high-frequency (left) 14


Page 15 of 31

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


semicircle and the corresponding constant phase element (CPE) indicating deviation from the ideal capacitance at the CE/electrolyte interface; While the second semicircle at the low-frequency (right) region represents Nernst diffusion limited impedance (ZN) of the I–/I3– couple of the electrolyte. The corresponding parameters were determined by fitting the impedance spectra and summarized in Table 1. Obviously, the Rs values of all CEs are almost uniform and have much smaller relative differences compared with their corresponding Rct values, which manifests the effect of Rs on photovoltaic performance can be ignored. While the Rct value of CE is the most important parameter in DSSC, the lower Rct value for CE material implies that it has eximious catalytic activity. At the same time, among these CEs, it was found that the ranking of the Rct values of different CEs is as follows: NiSe (13.9 Ω) > Co3Se4 (7.66 Ω) > Pt (2.88 Ω) > Ni0.67Co0.33Se (1.96 Ω) > Ni0.5Co0.5Se (1.50 Ω) > Ni0.33Co0.67Se (1.11 Ω), implying the catalytic activity increases in the reverse order of NiSe < Co 3Se4 < Pt < Ni0.67Co0.33Se < Ni0.5Co0.5Se < Ni0.33Co0.67Se. Clearly, The Rct values of the NixCo1– xSe

(x = 0.33, 0.5, and 0.67) are very close, which may be ascribed to the nearly

special surface and the chemical and physical similarities of Co and Ni element even at different compositions. Certainly, with the increase of Co content, the Rct of NixCo1–xSe (x = 0.33, 0.5, and 0.67) samples gradually decrease. It seems that too many Ni elements in Ni-Co selenides weaken their catalytic activity. Meaningfully, it is very important for us to find an appropriate Ni/Co molar ratio in Ni-Co based compounds. By now, many researchers have successfully prepared various Ni-Co based compounds with adjustable composition to achieve their best performance, such as Ni1.5Co1.5S4,45 NiCo2S4,62 CoNi2S4,63 and Ni0.48Co0.52S1.097.64 In addition, the Ni0.33Co0.67Se possesses the smallest Rct value due to the fast electron transfer channels and large surface areas, which implied it has relatively prominent catalytic activity and better charge-transfer abilities.65

15



15 12

-Z'' (Ohm)

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

Co3Se4

Ni0.33Co0.67Se

Ni0.5Co0.5Se

Ni0.67Co0.33Se

NiSe

Pt Rct

9

Page 16 of 31

Zn

Rs

6

CPE

3 0 28

32

36

40 44 48 Z' (Ohm)

52

56

Figure 6. Nyquist plots for dummy cells fabricated with Co3Se4, Ni0.33Co0.67Se, Ni0.5Co0.5Se, Ni0.67Co0.33Se, NiSe and Pt CEs and the set is the relevant equivalent circuit model.

Cyclic Voltammetry Analysis. Cyclic voltammetry (CV) measurements were carried out to elucidate the reaction kinetics of as-prepared CE materials in a three-electrode system (Figure 7). There are two pair redox peaks (Ox-1/Red-1, Ox-2/Red-2) on CV curves with the similar shapes for all of these electrodes, suggesting these CEs have similar electrochemical stability and catalytic activity during the redox process. The negative (left side) and positive pairs (right side) are expressed by equation 5 and 6 as below, respectively.5 I3– + 2e– ↔ 3I–

(5)

3I2 + 2e– ↔ 2I3–

(6)

In DSSCs, the cathodic peak current density (JRed-1) and the peak-to-peaks separation (Epp) are two crucial parameters for directly comparing electrocatalytic activities of different CEs, which are interrelated to the reduction velocity and the reversibility of the redox reaction, respectively. The higher values of JRed-1 and the smaller Epp values (Table 1) means the CE had a superior electrocatalytic activities for I3— reduction.66 The JRed-1 absolute values of NiSe (1.044 mA cm–2) < Co3Se4 (1.849 mA cm–2) < Pt (2.373 mA cm–2) < Ni0.67Co0.33Se (2.878 mA cm–2) < Ni0.5Co0.5Se 16


Page 17 of 31

(2.917 mA cm–2) < Ni0.33Co0.67Se (3.120 mA cm–2). Moreover, the order of Epp is NiSe (602 mV) > Co3Se4 (565 mV) > Pt (460 mV) > Ni0.67Co0.33Se (365 mV) > Ni0.5Co0.5Se (349 mV) > Ni0.33Co0.67Se (329 mV). Apparently, both of JRed-1 and Epp order are consistent with the results of EIS analysis. The ternary Ni-Co selenides show better intrinsic electrocatalytic activities than Pt and the binary selenides NiSe and Co3Se4, signifying that the coexistence Ni and Co ions in the selenides contribute to the reduction of I3—. The better intrinsic electrocatalytic activities may depend on the higher specific surface area and conductivity of the ternary Ni-Co selenides. In particular, the Ni0.33Co0.67Se sample delivered the highest JRed-1 and smallest Epp, exhibiting its excellent electrocatalytic performance and potential application in DSSC. 6

Co3Se4

-2

Current density (mA cm )

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


Ni0.33Co0.67Se

Ox-2

Ox-1

Ni0.5Co0.5Se

3

Ni0.67Co0.33Se NiSe Pt

0

Red-2

-3

Red-1

-6 -0.6

Epp

-0.3 0.0 0.3 0.6 0.9 Voltage (V vs Ag/AgCl)

1.2

Figure 7. CVs of the samples Co3Se4, Ni0.33Co0.67Se, Ni0.5Co0.5Se, Ni0.67Co0.33Se, NiSe, and Pt based CEs at a scan rate of 50 mV S–1.

Table 1. EIS, CV and Tafel Polarization parameters of DSSCs with different CEs Rct

JRed-1 (mA

Epp

Jlim/log

J0/log (mA

CES

Rs (Ohm)

(Ohm)

cm–2)

(mA)

(mA cm–2)

cm–2)

Co3Se4

29.94

7.66

1.85

565

1.66

0.38

Ni0.33Co0.67Se

30.40

1.11

3.12

329

1.95

0.61

Ni0.5Co0.5Se

30.38

1.50

2.92

349

1.84

0.57

Ni0.67Co0.33Se

30.04

1.96

2.88

365

1.77

0.50

17



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Page 18 of 31

NiSe

29.87

13.88

1.04

602

1.62

0.29

Pt

30.30

2.88

2.37

460

1.74

0.43

Figure 8a shows CV curves of Ni0.33Co0.67Se electrode in I–/I3– electrochemical system with different scan rates. It can be found that the cathodic peaks regularly shifted to the negative direction while the anodic peaks shifted to the position direction with the increasing scan rate. A good linear relationship between the peak current density and square root of the scan rate is observed in Figure 8b. Based on the diverse reports and the Langmuir isotherms principle concerning this linear relationship, which reveals two conclusions as follows: Firstly, the I3– reduction on the surface of Ni0.33Co0.67Se CE is controlled by means of diffusion of iodide species in the electrolyte; Secondly, the redox reaction on the surface of Ni0.33Co0.67Se CE has a little effect on the adsorption of iodide species and there is no specific interaction between the I–/I3– redox couple and Ni0.33Co0.67Se CE.67

Figure 8. (a) CVs of the Ni0.33Co0.67Se at different scan rate and (b) the relationship between redox current densities and the square root of the scan rates.

Tafel Polarization Curves Analysis. To further elucidate the catalytic activity of different CEs, the Tafel polarization curves measurements were performed on the symmetric cells used in EIS experiments (Figure 9). The limiting diffusion current density (Jlim, the intersection of anodic branch with y axis) and the exchange current 18


Page 19 of 31

density (J0, the slope for anodic or cathodic branch) are responsible for the catalytic activity of the CEs.68 As summarized in Table 1, the values of Jlim increased in the following order: NiSe (1.62 log (mA cm–2)) < Co3Se4 (1.66 log (mA cm–2)) < Pt (1.74 log (mA cm–2)) < Ni0.67Co0.33Se (1.77 log (mA cm–2)) < Ni0.5Co0.5Se (1.84 log (mA cm–2)) < Ni0.33Co0.67Se (1.95 log (mA cm–2)). Usually, the larger slope for anodic or cathodic branch, the higher J0 value and meanwhile, it is easy to understand that the J0 is inversely proportional to Rct and has a position correlation with the electrocatalytic activity of CE based on equation 7. Similarly, the sequence of increasing J0 values also are NiSe < Co3Se4 < Pt < Ni0.67Co0.33Se < Ni0.5Co0.5Se < Ni0.33Co0.67Se, so Ni0.33Co0.67Se is better electrocatalysts than other selenides based CEs in this paper. The conclusions derived from the Tafel and EIS as well as CV date are consistent. J0 = RT/nFRct

(7)

Where R is the gas constant, T is the temperature (298 K), F is Faraday’s constant, n (n = 2) is the number of electrons involved in the reduction of triiodide at the electrode, and Rct is the charge transfer resistance.

2

V=-0.12 V

3

log (J) (log (mA cm-2))

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


Jlim

1 0

Co3Se4

J0

Ni0.33Co0.67Se Ni0.5Co0.5Se Ni0.67Co0.33Se

-1

NiSe Pt

-2 -1.0

-0.5

0.0 0.5 Voltage (V)

1.0

Figure 9. Tafel polarization curves of the symmetric dummy cells from Co3Se4, Ni0.33Co0.67Se, Ni0.5Co0.5Se, Ni0.67Co0.33Se, NiSe, and Pt CEs.

Photovoltaic Performance Analysis. Figure 10 exhibits the photocurrent density-voltage (J-V) curves of the DSSCs assembled with NiSe, Co3Se4, Ni0.67Co0.33Se, Ni0.5Co0.5Se, Ni0.33Co0.67Se, and Pt based CEs, and the relevant 19



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

photovoltaic parameters such as open-circuit voltage (Voc), short-circuit photocurrent density (Jsc), fill factor (FF), and power conversion efficiency (PCE) are listed in Table 2. It can be seen that the structures and compositions of metal selenides have a vital effect on the performances of DSSCs. The PCEs of Co3Se4, Ni0.33Co0.67Se, Ni0.5Co0.5Se, Ni0.67Co0.33Se, NiSe, and Pt electrodes are 7.95, 9.01, 8.80, 8.59, 7.23, and 8.30%, respectively. Clearly, the DSSC based on Co3Se4 CE exhibited a higher PCE than that of NiSe, which was mostly due to the hollow nanostructure of Co3Se4 which possessed more charge transfer channels and electroactive sites.42,43 Additionally, the PCEs of the ternary Ni-Co selenide samples are obviously superior to these binary NiSe and Co3Se4 samples. This unique phenomenon is ascribed to the fact that ternary metal compounds have higher electrocatalytic activities than the corresponding binary systems which is usually revealed in many Ni-Co compounds.45,47 Furthermore, benefiting from the 3D loose surface structure and abundant valence states, the DSSC with Ni0.67Co0.33Se yielded a higher PCE of 8.59%, Jsc of 15.89 mA cm–2, and FF of 0.69, which surpassed that of the Pt CE. Interestingly, the PCE and Jsc increased gradually with a rise in Co content. The reason might be associated with the characteristic of morphology and the change of synergistic effect between Ni and Co ions in the ternary Ni-Co selenides.59 The highest PCE (9.01%) achieved by Ni0.33Co0.67Se may be attributed to the increased surface area and enhanced the electrocatalytic activity due to its unique structure and appropriate synergistic contribution between Ni and Co ions.50 Based on the above electrochemical analysis and SEM characterization of metal selenides, the outstanding catalytic performance of NixCo1-xSe (especially Ni0.33Co0.67Se) may depend on several main reasons: (1) the spherical microstructured NixCo1-xSe especially assembled with nanosheet and nanotube can provide more charge transport pathway and large contact area exposed to electrolyte, which contributes to the improvement of charge transfer ability and the increasement of electroactive sites;44,65 (2) some intrinsic performances of Ni-Co chalcogenides, for example high catalytic activity and good conductivity, make them be the ideal choices 20


Page 20 of 31

Page 21 of 31

for electrode materials in DSSC;51,62 (3) benefiting from the synergistic effect of Co and Ni ions, the Ni-Co selenides showed unique morphology and impressive electrochemical properties.64 As a demonstration, it has been proved that tuning the composition in the Ni-Co compounds is necessary, not only to design its structure but also to optimize its electrochemical performance. In order to research the role of urea in the synthesis process, the XRD pattern, SEM image, and J-V curves of Ni0.33Co0.67Se-Ref were exhibited and compared in Figure S7 and S8 (SI), respectively. For Figure S7, the diffraction peaks of Ni0.33Co0.67Se-Ref were almost the same to Ni0.33Co0.67Se, which indicated they have the same crystal structure. According to Figure S8, the Ni0.33Co0.67Se-Ref consists of abundant irregular particles, while Ni0.33Co0.67Se showed a regular 3D dandelion-like structure. The phenomenon indicated urea has an important effect on the morphology of sample, which is also good consistent with previous reports.49 Therefore, urea is an ideal precipitant in my work for the synthesis of precursor sample, which may determine the performance of product ultimately. Therefore, we also measured the J-V curves of DSSC with Ni0.33Co0.67Se-Ref (Figure S9, SI). Table S1 (SI) shows the corresponding parameters. Clearly, the Ni0.33Co0.67Se-Ref obtained PCE values of 8.05%, which is lower than that of Ni0.33Co0.67Se (9.01%) and Pt (8.30%). This result demonstrates that urea plays an important role in the morphology and electrocatalytic activity of sample. 20 -2

Current density (mA cm )

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


15 Co3Se4

10

Ni0.33Co0.67Se Ni0.5Co0.5Se

5

Ni0.67Co0.33Se

0 0.0

0.2

NiSe Pt

0.4

0.6

0.8

Voltage (V)

Figure 10. Photocurrent density-voltage curves of DSSCs with Co3Se4, Ni0.33Co0.67Se, Ni0.5Co0.5Se, Ni0.67Co0.33Se, NiSe, and Pt CEs under AM 1.5G illumination. 21



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

Page 22 of 31

Table 2. Photovoltaic parameters of DSSCs with different CEs CEs

Voc (mV)

Jsc (mA cm–2)

FF

PCE (%)

Co3Se4

793 ± 3

14.96 ± 0.18

0.67 ± 0.01

7.95 ± 0.14

Ni0.33Co0.67Se

789 ± 3

17.29 ± 0.11

0.67 ± 0.01

9.01 ± 0.07

Ni0.5Co0.5Se

783 ± 4

16.42 ± 0.13

0.69 ± 0.01

8.80 ± 0.10

Ni0.67Co0.33Se

784 ± 3

15.89 ± 0.13

0.69 ± 0.01

8.59 ± 0.09

NiSe

783 ± 5

14.54 ± 0.22

0.64 ± 0.01

7.23 ± 0.17

Pt

791 ± 2

15.33 ± 0.06

0.69 ± 0.01

8.30 ± 0.04

CONCLUSION In summary, we synthesized five novel transition metal selenides based on tunable Ni/Co molar ratios through a simple precursor conversion method. The electrochemical performances and structures of ternary nickel cobalt selenides can be optimized by tuning the composition of Ni/Co. Benefiting from the unique morphology and tunable composition, among the as-prepared metal selenides, the electrochemical measurements showed that the ternary nickel cobalt selenides had a more superior electrocatalytic activity in comparison with binary Ni and Co selenides. The catalytic activity of metal selenides for the reduction of I–/I3– redox couple in DSSC increased in the following order: NiSe < Co3Se4 < Pt < Ni0.67Co0.33Se < Ni0.5Co0.5Se < Ni0.33Co0.67Se. The 3D dandelion-like Ni0.33Co0.67Se microspheres showed the highest electrocatalytic activity, gaining the highest PCE (9.01%) which is much higher than that of Pt catalyst (8.30%). In addition, the study found that urea has a vital effect on the morphology and electrochemical performance of samples. Therefore, tuning the morphology and composition of transition metal selenides should be an important approach in the future for the design and preparation of electrocatalysts with both low cost and high activity in DSSCs. ASSOCIATED CONTENT Supporting Information 22


Page 23 of 31

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XRD pattern of the Co3Se4 precursor and Ni0.33Co0.67Se precursor; SEM image of Co3Se4 and Ni0.5Co0.5Se precursor; HRSEM image of Ni0.5Co0.5Se and Ni0.67Co0.33Se; EDS spectra of Ni0.33Co0.67Se, Ni0.5Co0.5Se, and Ni0.67Co0.33Se; the elemental mapping measurement of the Ni0.33Co0.67Se; the XRD pattern, SEM image, and photocurrent density-voltage curves of Ni0.33Co0.67Se-Ref; photovoltaic parameters of DSSCs with Ni0.33Co0.67Se and Ni0.33Co0.67Se-Ref CEs. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax: +86-0591-2286 6244; Tel: +86-0591-2286 5220.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We are grateful for the financial support from the National Natural Science Foundation of China (no.21376054).

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