One-Pot Solvothermal in Situ Growth of 1D Single-Crystalline NiSe on

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One-Pot Solvothermal in Situ Growth of 1D Single-Crystalline NiSe on Ni Foil as Efficient and Stable Transparent Conductive Oxide Free Counter Electrodes for Dye-Sensitized Solar Cells Chao Bao,† Faxin Li,† Jiali Wang,† Panpan Sun,† Niu Huang,† Yihua Sun,† Liang Fang,‡ Lei Wang,‡ and Xiaohua Sun*,† †

College of Materials and Chemical Engineering, College of Mechanical and Power Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, China Three Gorges University, Yichang 443002, China ‡ GuangXi Key Laboratory of New Energy and Building Energy Saving, Guilin University of Technology, Guilin 541004, China S Supporting Information *

ABSTRACT: One-dimensional single-crystal nanostructural nickel selenides were successfully in situ grown on metal nickel foils by two simple one-step solvothermal methods, which formed NiSe/Ni counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The nickel foil acted as the nickel source in the reaction process, a supporting substrate, and an electron transport “speedway”. Electrochemical testing indicated that the top 1D single-crystal NiSe exhibited prominent electrocatalytic activity for I3− reduction. Due to the metallic conductivity of Ni substrate and the outstanding electrocatalytic activity of single-crystal NiSe, the DSSC based on a NiSe/Ni CE exhibited higher fill factor (FF) and larger short-circuit current density (Jsc) than the DSSC based on Pt/FTO CE. The corresponding power conversion efficiency (6.75%) outperformed that of the latter (6.18%). Moreover, the NiSe/Ni CEs also showed excellent electrochemical stability in the I−/I3− redox electrolyte. These findings indicated that single-crystal NiSe in situ grown on Ni substrate was a potential candidate to replace Pt/TCO as a cheap and highly efficient counter electrode of DSSC. KEYWORDS: dye-sensitized solar cells, counter electrode, nickel selenide, electrocatalytic activity, in situ growth electrocatalytic layer.10 Yun et al.5 pointed out that in situ growth was a very simple and effective method for fabrication of CEs with lower Rs. In addition, the conductive substrate is also an important factor affecting the FF and the cost of DSSCs. Several metal materials, such as stainless steel,11,12 Ti foils,13 Ni foam,14 and Mo sheets,15 have been used to act as the substrates of counter electrodes due to their low sheet resistance and superior corrosion resistances in I−/I3− electrolyte.16,17 Furthermore, comparable performances with TCOsupported Pt CEs have been achieved. So, in situ growth of electrocatalytic materials on metal substrates may be a promising approach to fabricate cost-effective and highperformance CEs. So far, plenty of substitutes for Pt have been researched as counter electrode materials of DSSCs, such as carbides,4,18 nitrides,19,20 sulfides,21,22 selenides,23,24 carbon,25,26 and conductive polymers.27,28 Among them, selenides have attracted a lot of attentions because of their low price and distinctive electronic properties.24 Zhang et al. fabricated high-performance mesoporous Ni0.85Se@graphene CEs in DSSCs.29 Sun et

1. INTRODUCTION In recent years, cheap and superior counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) have attracted much research attention.1−3 As a significant component of DSSCs, the CE plays the important roles of transporting and collecting external electrons and electrocatalytically reducing I3− to I− between the CE and the electrolyte interface.4 Thus, the ideal CE should have high conductivity, high electrocatalytic activity for reducing I3− to I−, and excellent electrochemical stability in the redox electrolyte solution.5 At present, in the research field of the CEs, transparent conductive oxide (TCO)-supported noble-metal platinum (Pt) is still the standard. However, the high price of TCO and the scarcity of Pt hinder their widespread use in the CEs of DSSCs, so it is crucial to search and develop cost-effective and high-performance materials to replace TCO and Pt for the future practical application of DSSCs. In previous work, much cheaper active materials substituting for Pt were prepared and then were coated on the conducting substrates to prepare CEs via spray-coating,6,7 spin-coating,8 or the docter-blade method,9 etc. However, the prepared CEs usually showed relative high series resistance (Rs), which would result in a low fill factor (FF) for DSSCs. It may be partly due to the weak connection between the substrate and the © XXXX American Chemical Society

Received: August 14, 2016 Accepted: November 16, 2016 Published: November 16, 2016 A

DOI: 10.1021/acsami.6b10198 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

vacuum oven to create the counter electrodes. In the second preparation method, the precursor solution was made by agitating 0.75 mmol of selenium powder into a mixing aqueous solution consisting of 28 mL of deionized water and 2 mL of hydrazine hydrate. The subsequent steps of the preparation process were the same as those of the first method. The as-synthesized nickel selenide counter electrodes with the first and second methods were labeled as NiSe-sb and NiSe-hh, respectively. As references, Pt CE was prepared by a sol− pyrolysis method. The 2-propanol solution of H2PtCl6 was spun onto FTO substrates and then pyrolyzed at 385 °C for 30 min. 2.3. Assembly of DSSC. Dye-sensitized nanocrystalline TiO2 photoanodes and the iodine-based electrolyte solution were prepared according to our previous preparation procedure.22 During the testing of the photovoltaic properties, the dye-loaded TiO2 electrode, a drop of iodine-based electrolyte solution, and a counter electrode were assembled into a sandwich-structured DSSC employing a metal plate as a mask with 0.25 cm2 irradiated area. 2.4. Characterization. XRD patterns were recorded on a Rigaku D/max-2500 X-ray diffractometer with Cu Kα radiation in the range of 20°−80°. X-ray energy dispersion spectroscopy (EDS) and scanning electron microscope (SEM) were performed by a Sirion FEG FESEM to examine the element distribution and morphologies of samples. Xray photoelectron spectroscopy (XPS) was obtained to analyze the chemical states and compositions of the CEs with an ESCALAB 250Xi analyzer (Thermo Scientific). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM 2100F microscope. The photocurrent density−voltage (J−V) curves of DSSCs and the electrochemical properties of the as-prepared CEs were characterized by the CHI660D electrochemical workstation (ShangHai). Detailed test methods and processing parameters can be seen in our previous work.22

al. synthesized CoSe2 nanorods used in the CE of DSSCs and demonstrated a high photoelectric conversion efficiency.24 Tang et al. prepared CEs of binary-alloy metal selenides for bifacial DSSCs and obtained high optical transparency.30−33 These researches demonstrated that the metal selenides presented good electrocatalytic performance for the triiodide reduction and can be potential materials for CEs of DSSCs. On the other hand, the excellent morphology of CE materials can also improve the electrocatalytic performance. One-dimensional (1D) single-crystal nanowire or nanorod can provide short electron transport pathway and large catalytic active area for reduction of triiodide;34 thus, 1D single-crystal nanostructural metal selenides would be an ideal and promising candidate for CEs of DSSCs. But, up to now, there are still no reports on the in situ prepariation of 1D single-crystal nanostructural NiSe on Ni foil as a CE for DSSCs. In this work, 1D single-crystal nanostructural nickel selenides were successfully in situ grown on metal nickel foils by two simple one-step solvothermal methods without further posttreatment, which formed a NiSe/Ni counter electrode. The nickel foil acted as a nickel source in the reaction process, a supporting substrate, and an electron transport “speedway”. The top 1D single-crystal nanostructural NiSe functioned as an electrocatalytic center for I3− reduction. Very small series resistance (Rs) was obtained for the superior electronic conductivity of Ni foil, and the strong adhesion of NiSe/Ni is due to the in situ growth. The excellent electrocatalytic activity of single-crystal NiSe leads to a low charge-transfer resistance (Rct). Therefore, the DSSC based on a NiSe/Ni CE exhibited higher fill factor and short-circuit current density than the DSSC based on Pt/FTO CE. The corresponding photoelectric conversion efficiency (6.75%) outperformed the latter (6.18%). Moreover, the NiSe/Ni CE also showed superior electrochemical stability in the I−/I3− redox electrolyte.

3. RESULTS AND DISCUSSION 3.1. Characterization of Materials. Figure 1 shows the Xray diffraction (XRD) patterns of the metallic nickel substrate

2. EXPERIMENTAL PROCEDURE 2.1. Materials. All the chemicals and solvents used were of analytical grade, without any further purification. Selenium powder (99%) was purchased from Tianjin TIANLI Chemical Reagents Ltd. Nickel foil (99%) was obtained from Shanghai Maikun Chemical Co., Ltd. Absolute ethanol, acetonitrile, acetic acid, polyethylene glycol (PEG, MW = 20 000), nitric acid, hydrochloric acid, Triton-X100, hydrazine hydrate (85 wt %), polypropylene carbonate, tetrabutyl titanate, guanidinium thiocyanate, lithium perchlorate, sodium borohydride, and all solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The Ru dye N719 was obtained from Solaronix. 4-tert-Butylpyridine and lithium iodide (LiI, 99%) were purchased from Beijing InnoChem Science & Technology Co., Ltd. Iodine (I2, 99.8%) was obtained from Beijing Yili Fine Chemicals Co., Ltd. Fluorine-doped stannic anhydride conductive glass (FTO), purchased from Nippon Sheet Glass (NSG), was used as the substrate for the photoanode (mesoporous nanocrystalline TiO2 film). 2.2. Preparation of Counter Electrodes. Nanostructural nickel selenide was in-situ grown on nickel substrates with two kinds of solvothermal methods. Before preparation, some piece of nickel foil (1.5 × 3 cm2) were washed with sodium hydroxide, hydrochloric acid, and deionized water. In a typical preparation, 1.6 mmol sodium borohydride and 0.75 mmol of selenium powder were put into 30 mL of deionized water with stirring for 0.5 h. Then, the precursor solution was transferred into a 50 mL hydrothermal autoclave, and a piece of treated Ni foil was soaked in precursor solution. The sealed autoclave was heated from room temperature to 130 °C and then kept at 130 °C for 12 h. After air-cooling down to room temperature, the reacted Ni foil was taken out from the autoclave, rinsed several times with deionized water and ethanol, and finally dried at 60 °C for 8 h in a

Figure 1. XRD patterns of Ni foil and two nickel selenide samples growing on nickel substrates.

and two kinds of nickel selenide samples growing on nickel substrates. These samples all shows three very sharp diffraction peaks at 44.58°, 51.88°, and 77.21°, respectively, which can be indexed well to the (111), (200), and (220) planes of the metallic nickel (JCPDS No. 89-7128). The rest of the diffraction peaks are assigned to the NiSe (JCPDS No. 750610), and no other diffraction peaks of impurities are observed. All these observations indicate that both methods can in situ prepare the NiSe on Ni substrate. In addition, the B


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Figure 2. SEM images of NiSe-sb (a, c) and NiSe-hh (b, d) samples at different magnifications and energy dispersive spectroscopy (EDS) analysis of NiSe-sb (e) and NiSe-hh (f).

spectra of two NiSe samples all demonstrate the existence of Se and Ni elements, and the intensity ratio of Ni and Se elements for the NiSe-hh sample is obviously higher than that of the NiSe-sb sample, which may be mainly associated with the surface microstructure of the two NiSe samples (as shown in Figure 2a−d). The EDS spectrum of the NiSe-hh sample may be more influenced by the nickel substrate due to its less coverage with NiSe nanorods. Obviously, one cannot obtain accurate atomic ratios due to the influence of the nickel substrate. The as-prepared NiSe nanostructures were further characterized by TEM. Figure 3a,b shows typical TEM images of the NiSe-sb nanowires and NiSe-hh nanorods, respectively. It can be seen that the NiSe-sb nanowires show lengths of several hundred nanometers, which is much longer than that of NiSehh nanorods, and have similar or largish diameters compared with those of the NiSe-hh nanorods, in good accord with SEM observation. The HRTEM images of a NiSe-sb nanowire and a NiSe-hh nanorod (Figure 3c,d) both demonstrate the singlecrystal nature and lattice fringes with an interplanar spacing of 2.649 and 2.656 Å, respectively, which is close to the standard

NiSe-sb sample shows higher diffraction intensity than the NiSe-hh sample, which indicates that the crystallinity of NiSesb sample is better than that of the NiSe-hh sample. Furthermore, the diffraction intensity for the (110) plane of the NiSe-sb sample is a bit stronger than that for the (101) plane, which is different from that of the NiSe-hh sample and the JCPDS (No. 75-0610) standard file, indicating that NiSe-sb crystal preferred growing along the [110] crystal orientation.35 The surface morphologies of two NiSe samples are shown in Figure 2 with different magnification. It is observed that largearea NiSe nanowires grow vertically on the nickel substrate for NiSe-sb sample, which is beneficial to increase the surface area of the counter electrode and can provide facile channels for electron transfer.36,37 The NiSe-hh sample shows different morphology at different sites, i.e., there are some small and compact nanorods in one place; however, there are some big grains like stone elsewhere. Obviously, the length of the NiSehh nanorods is much less than that of NiSe-sb nanowires. EDS measurements were performed to characterize the chemical compositions of the two NiSe samples. As shown in Figure 2e,f, besides Pt, C, and O elements (introduced by testing), the EDS C


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Figure 3. TEM images (a, b) and HRTEM images (c, d) of NiSe-sb nanowires and NiSe-hh nanorods, respectively. The insets shows the corresponding FFT pattern.

Figure 4. Full and high-resolution XPS spectra of NiSe-sb (a−c) and NiSe-hh (d−f).

hydrothermal procedure are described below for the first preparation method using the NaBH4 reducing agent

value (2.6781 Å) for the (002) plane of hexagonal NiSe. Thus, both lattice fringes can be assigned to the (002) plane of NiSe. The inset images of Figure 3c,d are the corresponding fast Fourier transform (FFT) patterns. They can be indexed to single-crystal NiSe with the growth orientation along [110] and [100], respectively. The longer NiSe-sb nanowire with [110] growth orientation should show a stronger diffraction intensity for the (110) plane of the NiSe. This is consistent with the XRD analysis data. It can be seen that the two kinds of as-prepared NiSe exhibited different diffraction intensities and morphologies, which should be related with the different reducing agents, i.e., NaBH4 and N2H4·H2O. They played important roles in preparing NiSe/Ni counter electrodes. According to previous research,38−40 the reactions for generating NiSe in the

NaBH4 + Se + 3H 2O → NaHSe + 3H 2 + H3BO3

(1)

3NaHSe + 3Ni + H3BO3 → 3NiSe + Na3BO3 + 3H 2 (2)

and for the second preparation method using the N2H4·H2O reducing agent 5N2H4 + 4H 2O + N2 + 2Ni ⇌ 2[Ni(N2H4)3 ]2 + + 4OH−

3Se + 6OH− → 2Se2 − + SeO32 − + 3H 2O D

(3) (4)


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ACS Applied Materials & Interfaces [Ni(N2H4)3 ]2 + + Se2 − → NiSe + 3N2H4

(5)

Both methods prepared 1D nanostructural NiSe, which may be associated with the geometry of Ni foil and nonequilibrium growth. Zhang et al. found that the geometry of a foil favored a more or less unidirectional diffusion of metal ions, leading to the orientated growth.41 The nonequilibrium growth would generate 1D nickel selenide by virtue of ion-oriented aggregation.41 However, the reducibility of NaBH4 is stronger than that of the hydrazine hydrate. In the first preparation method, Se powder can be reduced to Se2− (i.e., NaHSe) with the reduction of NaBH4. The NaHSe can directly react with metal Ni foil and generate NiSe on Ni foil. In the second preparation method, hydrazine is believed to promote the dissolution and reduction of selenide and coordinate the nickel ions by forming the complex [Ni(N2H4)3]2+.41 NiSe can be produced with longer reaction steps involving a complexing reaction. So, in the same situation, the first process is more likely to generate NiSe than the second, and the NiSe nanowires can grow longer and show higher diffraction intensity (i.e., higher crystallinity) in the first preparation process. To further evaluate the chemical states of Ni and Se elements, the XPS spectra of the as-prepared NiSe samples were collected. Figure 4a,d shows the full XPS spectra of two NiSe-based samples, which presents the characteristic peaks of Se and Ni and also the C 1s and O 1s signals attributed to contamination/surface oxidation of the samples exposed to the air after preparation.42 In the Ni 2p and Se 3d high-resolution spectra, the Ni 2p3/2 and Ni 2p1/2 centered at 855.4 and 873.2 eV (as shown in Figure 4b,e) correspond to Ni2+, whereas the peak at about 54.3 eV (as shown in Figure 4c,f) belongs to Se 3d, indicating the −2 valence of Se.40 These peaks are very consistent with the XPS spectral characteristic of NiSe. For the NiSe-hh sample, there are two other peaks located at 852.9 and 870.3 eV in Figure 4e, which can be assigned to metallic Ni 2p coming from Ni substrate.35 Correspondingly, only one peak at 852.9 eV (as shown in Figure 4b) appears in the NiSe-sb sample, which may be due to its large-area nanowires covering the nickel substrate. In addition, the peak near 58.9 eV in the Se 3d high-resolution spectrum (as shown in Figure 4c) for the NiSe-sb sample indicates the presence of a surface oxidation state for Se species,43 but this peak is not obvious for the NiSe-hh sample. All these findings demonstrate that we have successfully prepared both NiSe samples, which is consist with the XRD analysis, and that the NiSe-hh sample contains less surface oxidation state of the Se species. 3.2. Electrochemical Properties of CEs. To investigate the electrocatalytic activity of the as-prepared samples, the EIS measurements were first performed with a CE/electrolyte/CE structure. The typical Nyquist plots and the equivalent circuit are shown in Figures 5 and s1 (Supporting Information). There are two semicircles in the Nyquist plots for Pt- and NiSe-based CEs. The first semicircle, located in the high-frequency region, is closely related with the charge-transfer resistance (Rct) at the CE/electrolyte interface. The value of Rct reflects the electrocatalytic activity of the counter electrode.44 The left intercept on the real axis in the plots reveals the ohmic series resistance (Rs), which is related to the conductivity of CEs. The another semicircle in the low-frequency region is assigned to the Nernst diffusion resistance (Zw) of the redox couple in the electrolyte.45 According to the equivalent circuit in the inset of

Figure 5. Nyquist plots of EIS for the symmetrical dummy cells constructed with two identical Pt, Ni, and NiSe CEs. The inset gives the equivalent circuit.

Figure 5, the results of EIS analysis fitted by Z-view software are listed in Table 1. From the simulated data, The Rs of the Niand NiSe-based CEs is only around 1 Ω·cm2, which is much smaller than that of the Pt CE (20.7 Ω·cm2). Apart from the external wires and clips, the conductivity of CE also has a significant impact on Rs.46,47 Nanostructural single-crystal NiSe in situ grows on Ni substrate with a low resistor, which gives the NiSe/Ni counter electrode a better electrical conductivity, which can significantly affect the fill factor (FF) of DSSCs. It is well-known that the smaller Rct value of CE indicates its better catalytic performance.48,49 The Rct value of the Pt, NiSe-hh, NiSe-sb, and Ni CEs is 3.12, 1.26, 0.2, and 6.56 × 104 Ω·cm2, respectively. This indicates that the NiSe-based CEs have higher electrocatalytic activity than Pt and Ni CEs, and the electrocatalytic activity of the NiSe-sb CE is the highest among these CEs. However, the value of the Nernst diffusion resistance (Zw) of NiSe-based CEs is higher than that of Pt CE, which may be related to the stronger adsorption of I3− on the metal selenide CE.24 The sum (Rsum) of Rs, Rct, and Zw is a part of the whole series resistance of DSSC (including anode, counter electrode, and electrolyte).49,50 Obviously, smaller Rsum can promote the larger FF of the solar cell. It can be seen from Table 1 that the Rsum of NiSe-based CEs is smaller than that of Pt and Ni CEs, so the NiSe-based devices have larger FF than Pt- and Ni-based devices. Tafel polarization measurements were also carried out to further estimate the electrocatalytic performance of the Pt-, Ni-, and NiSe-based CEs (as shown in Figure 6). The exchange current density (J0), an important parameter reflecting the electron transfer kinetics at the CE/electrolyte interface, can be obtained from the intersection point of the linearly extrapolated Tafel polarization curve at the Tafel zone with the perpendicular line at zero potential. A higher J0 indicates a better electrocatalytic activity of a CE.51 It can be seen that the NiSe-sb CE shows the highest J0, followed by the NiSe-hh, Pt, and Ni CEs. This indicates that the NiSe-sb CE possesses the best electrocatalytic activity among these CEs. The J0 is inversely proportional to the charge-transfer resistance (Rct).22 In general, the order of J0 for different electrodes is wellconsistent with the Rct in EIS measurements. Another important parameter, the limiting diffusion current density (Jlim), can be extracted from the Tafel graph when its slope seems close to zero. It reflects the diffusion velocity of the redox couple in the electrolyte. A large Jlim indicates a large diffusion E


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Table 1. Impedance Parameters of the Symmetrical Dummy Cells and the Photovoltaic Performance Data of DSSCs Based on Different CEs sample

Rs (Ω·cm2)

Rct (Ω·cm2)

Zw (Ω·cm2)

Rsum (Ω·cm2)

Voc (V)

Jsc (mA·cm2)

FF

η (%)

Pt NiSe-sb NiSe-hh Ni

20.7 1.02 1.04 0.96

3.12 0.20 1.26 6.56 × 104

2.97 4.14 9.17

26.79 5.36 11.47

0.62 0.62 0.62 0.21

15.2 15.9 15.6 1.99

0.65 0.69 0.67 0.19

6.18 6.75 6.42 0.08

the electrocatalytic activity of different CEs. The larger |Jpc| and smaller Epp imply higher electrocatalytic activity and smaller overpotential for the I3− reduction.4 As shown in Figure 6, the peak current density of Pt, NiSe-hh, and NiSe-sb CEs increased gradually, which indicates that the electrocatalytic activity of these CEs also rises successively. However, the Ni CE did not show obvious redox peaks, which should be related with its very low electrocatalytic activity (i.e., very large Rct).54 Moreover, the as-prepared NiSe CEs have Epp values similar to that of Pt CEs, which means that these counter electrodes possess a similar overpotential for the I3− reduction. In addition, parts a, b, and c of Figure 8 present the CV curves of the Pt, NiSe-sb, and NiSe-hh CEs, respectively, in the I3−/I− electrolyte by increasing the scan rates from 15 to 90 mV/s. As the scanning rate increased, both |Jpc| and the Epp also increased gradually. As shown in Figure 8d−f, the anodic and cathodic peak current densities have a great linear relationship with the square root of the scan rates. It demonstrates that no species interaction occurs between the counter electrode and the I−/I3− redox couple. There is only a diffusion-limited process at the CE/electrolyte interface.55 3.3. The Stability of NiSe CEs. Good stability is very important for counter electrodes. One hundred consecutive CV scans of Pt-, NiSe-sb-, and NiSe-hh-based CEs with a scan rate of 50 mV/s are listed in parts a, b, and c of Figure 9, respectively. Notice that cyclic voltammetric curves still hold steady after 100 consecutive scans, and redox peak current densities do not change significantly upon increasing the number of cyclic scans, as shown in Figure 9d−f. It signifies that the NiSe-based CEs as well as the Pt CE show an excellent electrochemical stability in the iodine-based electrolyte.48 3.4. Photovoltaic Performances of DSSCs Based on Different CEs. Figure 10 shows the J−V curves of DSSCs with Pt-, Ni-, and NiSe-based CEs. The obtained photovoltaic parameters for different DSSCs are listed in Table 1. Impressively, the DSSC constructed with the NiSe-based CEs show better photovoltaic performances than the DSSC based on the Pt and Ni CEs. Furthermore, the device with NiSe-sb CE possesses the best photovoltaic performance among these DSSCs because it has the highest short-circuit current density (Jsc) of 15.9 mA/cm2, a fill factor (FF) of 0.69, an adjacent open-circuit voltage (Voc) of 0.62 V, and then the highest photovoltaic conversion efficiency (η) of 6.75%. However, the DSSC with the Ni CE only shows a 0.08% power conversion efficiency, which further indicates that Ni foil nearly has no catalytic activity for iodine-base electrolyte, and it acts as a supporting substrate and electron transport “speedway”. The improved efficiency of the DSSC constructed from NiSe-based CEs is primarily caused by the increase in Jsc and fill factor. The improvement in Jsc for the NiSe-based CEs results from their superior electrocatalytic activity22,24 and electron transfer properties,56 as displayed in the electrochemical properties tests. The increase in FF can be attributed to the low Rsum (i.e., the sum of Rs, Rct, and Zw),22 as discussed previously.

Figure 6. Tafel curves of the symmetrical dummy cells fabricated with two identical Pt, Ni, and NiSe CEs.

coefficient (D), i.e., a small Zw.52 From Figure 6, the Jlim has the sequence Pt > NiSe-sb > NiSe-hh > Ni, which is consistent with the EIS analysis. Cyclic voltammetry (CV) is also a powerful tool to evaluate the electrocatalytic activity and reversibility of a CE. A single pair of redox peaks tested in the liquid electrolyte is adopted.53 Figure 7 displays the CV curves of Pt-, Ni-, and NiSe-based CEs. There are two important parameters, i.e., peak separation between the anodic and cathodic peaks (Epp) and peak current density (Jpc) in the curves, which can be used to characterize

Figure 7. Cyclic voltammograms of the as-synthesized NiSe, Pt, and Ni CEs, in 0.01 M LiI, 0.001 M I2, and 0.1 M LiClO4 in acetonitrile, at a scan rate of 50 mV·s−1. F


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Figure 8. CV curves of (a) Pt, (b) NiSe-sb, and (c) NiSe-hh CEs at various scan rates, respectively, and the corresponding anodic and cathodic peak current densities as a function of the square root of the scan rate for (d) Pt, (e) NiSe-sb, and (f) NiSe-hh CEs, respectively.

Figure 9. One hundred consecutive cyclic voltammetry scans for (a) Pt, (b) NiSe-sb, and (c) NiSe-hh CEs at a scan rate of 50 mV·s−1 and the relationship between the number of scans and the resultant redox peak current for (d) Pt, (e) NiSe-sb, and (f) NiSe-hh CEs, respectively.

4. CONCLUSION

The DSSC with NiSe-sb CE exhibits better photovoltaic performance, as against the DSSC with NiSe-hh CE. It may be that the NiSe-sb/Ni sample contains a greater amount of electrocatalytic active material (NiSe) per unit area and shows higher crystallinity (as shown in SEM images and XRD patterns). The power conversion efficiency of the device with the NiSe-sb CE is 8.4% higher than that of the device with Pt CE. Moreover, the NiSe-sb CE also shows an excellent electrochemical stability in iodine-based electrolyte (as shown in Figure 9). All these findings suggest that single-crystal NiSesb grown in situ on Ni substrate is a potential candidate to replace Pt/TCO as a cheap and highly efficient counter electrode of DSSCs.

In summary, 1D single-crystal nanostructural NiSe/Ni CEs have been prepared by two simple one-step solvothermal methods and tested as CEs in DSSC for the first time. Extensive electrochemical analyses demonstrated that NiSe/Ni CEs showed low series resistance, high electrocatalytic activity, and good electrochemical stability. As a result, the DSSC with NiSe/Ni CEs exhibited higher fill factor and short-circuit current density than the DSSC with Pt/FTO CE, and then the power conversion efficiency (6.75%) of the DSSC with the NiSe-sb/Ni CE is obviously higher than that (6.18%) of the DSSC with the Pt/FTO CE. These all suggested that singlecrystal NiSe-sb in situ grown on Ni substrate is a potential G


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Figure 10. Photocurrent density−voltage (J−V) characteristics of DSSCs based on Pt-, Ni-, and NiSe-based CEs under AM 1.5G illumination (100 mW·cm−2).

candidate to replace Pt/TCO as a cheap and highly efficient counter electrode of DSSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10198. Nyquist plots of EIS for the symmetrical dummy cells constructed with two identical Ni CEs (Figure s1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-717-6397560. Fax: +86-717-6397559. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51602175 and 51602176), Foundation of Key Laboratory of new building energy and building efficiency, Guangxi Province, China (Grant No. 15-J22-2), and the Foundation of Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education (Grant No. 130026504).

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REFERENCES

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One-Pot Solvothermal in Situ Growth of 1D Single-Crystalline NiSe on

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