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Bio-based carbon-enhanced tungsten-based bimetal oxides as counter electrodes for dye-sensitized solar cells
Journal of Power Sources 423 (2019) 339–348
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Bio-based carbon-enhanced tungsten-based bimetal oxides as counter electrodes for dye-sensitized solar cells
T
Yangliang Zhang, Sining Yun∗, Chen Wang, Ziqi Wang, Feng Han, Yiming Si Functional Materials Laboratory (FML), School of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
carbon supported W-based • Bio-based bimetal oxides are successfully prepared.
carbon as structure-di• Bio-based recting agent direct self-assembly of nanoflowers.
composite CEs show en• W-based hanced electrocatalytic activity in DSSCs.
high PCE of 7.08% is obtained by • Ausing NiWO /BC catalyst in DSSCs. bio-based carbon is a promising • Using strategy to fabricate efficiently hybrid 4
CEs.
A R T I C LE I N FO
A B S T R A C T
Keywords: Dye-sensitized solar cell Counter electrode Catalysts Tungsten-based bimetal oxide Bio-based carbon
Tungsten-based bimetal oxides are first used as counter electrode catalysts for dye-sensitized solar cells. In this work, we introduce porous bio-based carbon into tungsten-based bimetal oxide by hydrothermal and annealing methods to improve the electrocatalytic properties of tungsten-based bimetal oxides. A structure transformation from nanorod to nanoflower is observed in cobalt tungsten oxide when porous bio-based carbon is used as the structure-directing agent. Bio-based carbon, which has a porous three-dimensional network structure, serves as an electrocatalytic support material in the tungsten-based bimetal oxide/bio-based carbon composites, reducing the agglomeration and grain size of tungsten-based bimetal oxides and providing rapid electron transfer paths. Tungsten-based bimetal oxide/bio-based carbon composite counter electrodes exhibit enhanced electrocatalytic activity and reduced charge-transfer resistance because of the synergistic effects of catalytic tungsten-based bimetal oxides and conductive bio-based carbon. The dye-sensitized solar cell with nickel tungsten oxide/biobased carbon counter electrode shows a high power conversion efficiency of 7.08%, surpassing that of dyesensitized solar cell with Pt counter electrode (6.46%). This work presents a general strategy for designing and fabricating porous bio-based carbon support inorganic compound materials for energy conversion devices.
1. Introduction Dye-sensitized solar cells (DSSCs) have drawn wide attention because of their incomparable advantages including low cost, easy to manufacture, environmental friendly and high theoretical photoelectric
∗
conversion efficiency (PCE) [1–4]. A typical DSSC has a sandwich structure that consists of three parts: a dye-sensitized semiconductor photoanode, counter electrode (CE) and electrolyte which is sandwiched between the photoanode and CE. The CE is an important component of DSSC, which collects electrons from an external circuit
Corresponding author. E-mail addresses: [email protected], [email protected] (S. Yun).
https://doi.org/10.1016/j.jpowsour.2019.03.054 Received 16 January 2019; Received in revised form 5 March 2019; Accepted 14 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
Journal of Power Sources 423 (2019) 339–348
Y. Zhang, et al.
and catalyzes the reduction of I3− ions in the electrolyte solution. However, traditional Pt CE materials are expensive, have a limited source, and are easily corroded by the I3−/I− electrolyte. As these unfavorable features restrict the commercial application of DSSCs [5], exploring Pt-free catalysts as CE material is urgently needed. Transition metal compounds (TMCs) [3], carbon materials [6,7], metals and alloys [8], conductive polymers [9,10], and their corresponding composites [11] have been used as CEs in DSSCs. Of these materials, TMCs, particularly tungsten (W)-based compounds, have excellent catalytic properties owing to their electronic structures, which are similar to that of Pt [12,13]. Lee et al. prepared mesoporous WC through a microwave-assisted method and used it as a CE for DSSC. The DSSC showed an excellent PCE of 7.01% [14]. Ma et al. synthesized W18O49 nanofibers through a template-free solvothermal approach. The DSSC with W18O49 nanofibers CE yielded a PCE of 8.58% [15]. Liu et al. used WB crystals with spontaneously oxidized surface as a CE material in DSSC and yielded a PCE of 7.07% [16]. Qiao et al. created oxygen vacancy to activate WO3 as CE for DSSC and achieved a PCE of 10.5% [17]. Recently, our group used first-principle density functional theory (DFT) calculations to elucidate the intrinsic reason that TMC-based catalysts demonstrate superior electrocatalytic activity as CEs in DSSCs [18,19]. Compared with binary TMCs (oxides, nitrides, carbides, and etc), ternary TMCs (CoMoS4, NiCo2S4, CoFe2O4 and etc) present superior electrocatalytic activities and controllable structures because of the synergistic effects and valence interchanges among their catalytic components [20–22]. Thus, the fabrication of W-based bimetal oxides may be a good means for optimizing the electrocatalytic properties of W-based compounds as CEs. Unfortunately, information on W-based bimetal oxides used as CEs applied in DSSCs is limited. An effective catalytic material for DSSC must possess superior catalytic activity and electrical conductivity. However, most TMCs have inferior electrical conductivities that result in low PCEs. The electrical conductivities of catalysts can be improved by introducing carbon materials, which have excellent electrical conductivities, reasonable chemical durability, and large specific surface areas [23–26]. Recently, composites composed of TMCs and carbon materials have been used as CEs for DSSCs. Yun et al. synthesized HfO2/mesoporous carbon (MC) and Hf7O8N4-HfO2/MC composites through a simple chemical synthesis. DSSCs with the composite CEs rendered PCEs of 6.71% and 7.85%, respectively, owing to the synergetic electrocatalytic effects of various components in the composite [12]. Yuan et al. used CoFe2O4/carbon nanotubes (CNTs) as CE for DSSC which yielded PCE of 6.55%, and this result indicates that the enhanced catalytic performance of CoFe2O4/ CNTs is due to the excellent conductivities of the CNTs and the high catalytic activity of CoFe2O4 [27]. Chen et al. prepared Co3O4-WC-nitrogen-doped carbon (CN)/graphene (rGO), which showed remarkable catalytic activity due to the synergistic effect of Co3O4-WC and electrical conductivity of rGO. The DSSC with the Co3O4-WC-CN/rGO as CE achieved PCE of 7.38% [28]. The carbon materials used as structuredirecting and morphology-controlling agents are conducive to the synthesis of nanomaterials possessing controllable structures and morphologies that enhance CE catalysts performance [29,30]. Although various kinds of carbon materials have been used in DSSCs, complex synthetic approaches and limitations for scale manufacture make the effective utilization of carbon materials difficult [31]. Natural biomass is considered a promising source for carbon material production because of its abundance and facile accessibility [32]. Bio-based carbon (BC) is a remarkable carbon material for CE production in terms of resources cyclic utilization. In this work, waste biomass was used as a carbon source, and a series of porous BC supported W-based bimetal oxides (MWO4, M = Fe, Co, and Ni) composites were prepared through hydrothermal treatment and annealing. In the resulting MWO4/BC composites, porous BC functioned as a structure-directing agent for the synthesis of flower-like CoWO4. As CE catalysts for DSSCs, MWO4/BC showed more preferable electrocatalytic activity for I3− reduction than bare MWO4, thus
resulting in the improved PCEs for DSSCs with MWO4/BC CEs. The DSSC fabricated with NiWO4/BC exhibited excellent PCE of 7.08%, which surpassed that of DSSC with Pt CE (6.46%).
2. Experimental 2.1. Preparation of bio-carbon Bio-carbon (BC) was derived from aloe peel waste. Aloe peels were washed with distilled water for the removal impurities and then dried at 105°C for 24 h. The dried aloe peels were crushed into powder with particle size ∼2 μm. 6 g aloe peel powder was added into 120 mL deionized water and vigorously stirred for 4 h. The obtained mixture was hydrothermally treated at 230°C for 15 h with a stainless-steel autoclave with Teflon lining. The sediment was washed with deionized water and alcohol for several times and then dried at 105°C for 24 h, resulting in hydrothermal carbon. 1 g hydrothermal carbon along with 2 g KOH was added into 20 mL deionized water and vigorously stirred for 4 h. The obtained mixture was dried at 105°C for 24 h, then subjected to activated treatment at 800°C for 2 h in N2 atmosphere. The activated product was washed with hydrochloric acid, alcohol, and deionized water successively until the filtrate was neutral, and dried at 105°C for 24 h, resulting in BC. Further details can be cited in our previous works [33–35].
2.2. Synthesis of MWO4 and MWO4/BC (M = Fe, Co, and Ni) MWO4/BC were synthesized by hydrothermal treatment and annealing (Scheme 1). Fe(NO3)3·9H2O, Co(NO3)2·6H2O, and Ni (NO3)2·6H2O were used as metal (Fe, Co, and Ni) precursors, and ammonium paratungstate was used as a tungsten precursor. In detail, 5 mmol ammonium paratungstate and 20 mmol BC (the mole ratio of BC to MWO4 is 4, based on the previous literature [36]) were added into 50 mL deionized water and then vigorously stirred for 30 min. 5 mmol metal precursor was added, and the resulting solution was vigorously stirred for 4 h and then hydrothermally treated in an autoclave with Teflon lining at 220°C for 12 h. The obtained precipitate was washed with deionized water and alcohol repeatedly and then dried at 80°C for 48 h. Finally, the obtained powder was annealed at 500°C for 2 h in N2, resulting in MWO4/BC. Bare MWO4 was prepared through similar procedures but without BC.
2.3. Fabrication of electrodes and cells In the photoanode fabrication, the obtained TiO2 film electrode with an active area of 0.196 cm2 was heated at 500°C for 30 min in air. The TiO2 electrode was cooled to appropriate temperature and then dipped into a mixture of tert-butyl alcohol, acetonitrile (the volume ratio of tertbutyl alcohol to acetonitrile is 1:1), and 0.5 mM of N719 dye for 24 h. The dye-sensitized TiO2 electrode was rinsed with ethanol and then dried in air. In the fabrication of CEs, 0.2 g MWO4 or MWO4/BC powder was added into 5 mL isopropanol solution and ball-milled for 4 h with 3.5 g ZrO2 pearls as dispersion medium. The prepared mixture was sprayed on the surface of an FTO substrate with an air brush. Then, the FTO substrate loaded with MWO4 or MWO4/BC layer was annealed at 400°C for 30 min in N2. Pt CEs were fabricated by annealing the FTO substrate which was coated with H2PtCl6 isopropanol solution at 500°C under ambient atmosphere for 30 min. The symmetrical cell was assembled by two identical CEs and an I−/I3− electrolyte. The DSSC was assembled by filling the I−/I3− electrolyte between a TiO2 photoanode and a CE. The I−/I3− electrolyte consisted of 0.03 M I2, 0.06 M LiI, 0.1 M guanidinium thiocyanate, 0.5 M 4-tert-butyl pyridine, 0.6 M 1butyl-3-methylimidazolium iodide, and acetonitrile solution [18,37]. 340
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Scheme 1. Schematic of the synthesis of MWO4 and MWO4/BC (M = Fe, Co, and Ni).
(222), (−113), (−231), (320), (−140), and (−312), respectively, for the monoclinic CoWO4 (JCPD: 15–0867). In Fig. 1d, the diffraction peaks at 15.62, 19.28, 23.97, 24.92, 30.93, 31.54, 36.57, 37.25, 39.12, 41.66, 44.74, 46.41, 48.16, 49.04, 51.10, 52.32, 53.18, 54.62, 55.84, 62.72, 64.47, 65.84, 68.95, and 72.62° can be assigned to the crystal planes of (010), (100), (011), (110), (−111), (020), (002), (120), (200), (−102), (−112), (−211), (030), (022), (220), (130), (−122), (−202), (−131), (310), (−222), (−311), (041), and (−302), respectively, for the monoclinic NiWO4 (JCPDS: 15–0775). The characteristic diffraction peaks of synthesized FeWO4, CoWO4, and NiWO4 matched with the data of their respective monoclinic phases. Meanwhile, MWO4/BC displayed similar characteristic peaks with their corresponding MWO4, indicating that the expected MWO4/BC were successfully synthesized. However, the graphitic carbon (002) and (100) diffraction peaks were not clearly observed in the XRD patterns of MWO4/BC. The possible reason is that the highly crystallized diffraction peaks of MWO4 masked the weak diffraction peaks of amorphous carbon [39]. In contrast with the XRD patterns of MWO4, the diffraction peaks of MWO4 in their corresponding MWO4/BC were weak probably because of the presence of BC, which reduced the crystallinity of MWO4. The microstructures and morphologies of BC, MWO4, and MWO4/ BC were investigated through FESEM measurement. BC exhibited a porous three-dimensional (3D) network structure with randomly distributed macropores (size range from 200 nm to 800 nm) over its entire parts (Fig. 2a). This structure is conducive to the reduction of I3− because its interconnected porous structures enable CE to fully contacted with an electrolyte solution, and the interlaced networks of BC provides rapid electron transfer paths [40,41]. Bare CoWO4 showed two types of morphologies: a rod-like shape with broad size distribution of 0.5–2.5 μm in length and 200–600 nm in diameter and an irregular particle shape with a size that ranges from 200 nm to 800 nm (Fig. 2b). By contrast, the CoWO4 showed a hexagonal flower-like nanostructure with a mean diameter of approximately 1.5 μm (Fig. 2c), when BC was used as structure-directing and morphology-controlling agent in CoWO4/BC composites. The magnified structure of hexagonal flower-like CoWO4 could observed in Fig. 2d. The nanoflowers were assembled by six interlaced bunches, which are constructed by many parallel-arranged nanorods. This change of morphology of CoWO4 products might be attributed to the existence of BC. Previous studies synthesized flower-like CoWO4 by
2.4. Characterization X-ray diffraction (XRD) patterns of the BC, MWO4, and MWO4/BC were obtained on an automated X-ray powder diffractometer (D/Max 2200, MDI, America). The morphologies and microstructures of BC, MWO4, and MWO4/BC were observed by field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan). The photocurrent-voltage (J-V) curves of the as-assembled DSSCs were recorded by a Keithley 2400 digital source-meter under the illumination of 100 mW cm−2 provided by a solar simulator (Oriel 94023A, Newport). Cyclic voltammetry (CV) was performed by using a CHI602 electrochemical analyzer (Chenhua, Shanghai, China) at a scan rate of 50 mV s−1 in a three-electrode system. An acetonitrile solution containing 0.1 M LiClO4, 1 mM I2, and 10 mM LiI was used as the electrolyte, the as-prepared CE as the working electrode, Pt as the counter electrode, and Ag/Ag+ as the reference electrode. Tafel polarization tests and electrochemical impedance spectra (EIS) measurements were conducted by using the symmetrical cells on a CHI660E electrochemical analyzer (Chenhua, Shanghai, China). In the EIS measurements, the cells were scanned from 100 mHz to 1 MHz at 0.01 V alternating current amplitude and −0.75 V bias potential. The obtained EIS data were fitted by Z-view software.
3. Results and discussion Fig. 1 shows the XRD patterns of BC, MWO4, and MWO4/BC. In Fig. 1a, two broad diffraction peaks were found at 2θ = 24.3° and 43.3° (Fig. 1a). The presence of peaks corresponding to the (002) and (100) crystal planes of a typical graphitic carbon indicates the formation of the amorphous carbon structure of BC [38]. In Fig. 1b, the diffraction peaks at 15.48, 18.67, 23.71, 24.36, 30.37, 31.27, 36.21, 36.70, 37.95, 41.09, 44.05, 45.23, 47.68, 48.52, 49.96, 51.67, 52.49, 53.49, and 61.38° can be assigned to the crystal planes of (010), (100), (011), (110), (−111), (020), (021), (120), (200), (−121), (−112), (211), (030), (022), (220), (130), (122), (221), and (300), respectively, for the monoclinic FeWO4 (JCPDS: 46–1446). In Fig. 1c, the diffraction peaks at 15.58, 18.98, 23.82, 24.65, 30.63, 31.45, 36.30, 36.99, 38.52, 41.38, 44.35, 47.99, 48.76, 50.58, 52.07, 54.06, 59.35, 61.80, 63.83, 64.88, 65.08, 65.39, 68.69, and 71.98° can be assigned to the crystal planes of (010), (001), (−110), (011), (−111), (020), (200), (021), (002), (−121), (−211), (030), (−220), (022), (031), (−122), (003), (−311), 341
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Fig. 1. XRD patterns of (a) BC, (b) FeWO4 and FeWO4/BC, (c) CoWO4 and CoWO4/BC, and (d) NiWO4 and NiWO4/BC.
reaction of I3−, which will further support pervious viewpoint [40,48–50]. Fig. 3 shows the morphologies and thicknesses of FeWO4/BC, CoWO4/BC, and NiWO4/BC CEs. It could observed that the catalyst powder adhered to the surface of FTO uniformly form the dense film, and no obvious cracks were observed, indicating that the composites have a good bonding property. The thicknesses of the FeWO4/BC, CoWO4/BC, and NiWO4/BC CEs were approximately 5.25, 5.45, and 5.52 μm, respectively, and the thicknesses of different CEs are close, indicating that the effect of thickness of these three CEs on catalytic performance can be omitted. The as-prepared catalysts and Pt were used as CEs in the DSSCs to evaluate their catalytic properties. Fig. 4a shows the characteristic J-V curves of the DSSCs with various CEs, and Table 1 lists the related parameters. The DSSCs assembled by FeWO4, CoWO4, NiWO4, and BC CEs produced PCEs of 4.65%, 4.46%, 5.10%, and 5.20%, respectively; whereas the DSSCs with FeWO4/BC, CoWO4/BC, and NiWO4/BC CEs yielded enhanced PCEs of 5.38%, 6.07%, and 7.08%, respectively. The PCE value of NiWO4/BC based DSSC was higher than that of DSSC with Pt CE (6.46%). Notably, the DSSCs with MWO4/BC CEs exhibited higher short-circuit current density (Jsc) and fill factor (FF) than their corresponding DSSCs with MWO4 CEs. The introduction of BC increased the Jsc values of DSSCs from 13.45 mA cm−2 (FeWO4), 11.96 mA cm−2 (CoWO4), and 12.60 mA cm−2 (NiWO4) to 14.38 mA cm−2 (FeWO4/ BC), 13.96 mA cm−2 (CoWO4/BC), and 14.70 mA cm−2 (NiWO4/BC), respectively, and enhanced the FF values from 0.50 (FeWO4), 0.56 (CoWO4) and 0.59 (NiWO4) to 0.55 (FeWO4/BC), 0.63 (CoWO4/BC) and 0.69 (NiWO4/BC), respectively. However, the open-circuit
an alcohol-thermal process, in which deionized water is substituted with alcohol as solution. Aggregated particles in low polarity system increase the stability of the a colloid and result the formation of flowerlike nanostructures [42]. In this work, the introduced BC has a nonpolar surface and provide a low polarity environment for the hydrothermal system [43]. The amorphous particles and nanorods aggregated on the surface of the BC and gradually evolved to hexagonal flower-like structures through oriented attachment (Scheme 2), similar to the observed in the previous reports [44–46]. The FeWO4 particles (Fig. 2e) had irregular shapes, and their size ranged from 0.3 μm to 1.5 μm. Meanwhile, the NiWO4 particles (Fig. 2g) were spherical, and their sizes ranged from 200 nm to 800 nm, and had a small quantity of nanorod that was 0.5–2 μm in length and 200–500 nm in diameter. In FeWO4/BC (Fig. 2f) and NiWO4/BC (Fig. 2h) composites, FeWO4 and NiWO4 showed a quasi-spherical shape and had smaller grain sizes (size range from 100 nm to 200 nm) than those of corresponding bare FeWO4 and NiWO4. The smaller particle size of MWO4 could increase the contact area of MWO4 with electrolyte, thereby exposing more catalytic sites to accelerate the reduction reaction of I3−. The smaller grain sizes of FeWO4 and NiWO4 may be attributed to the presence of BC, which prevented the growth of FeWO4 and NiWO4 crystals during the synthesis [40,47]. This finding proved the results of the XRD measurement, that is, the weak diffraction peaks of MWO4 in their corresponding MWO4/BC are due to the low crystallinity of MWO4. Fig. 2c, 2f, and 2h show that MWO4 loaded on the surface of BC showed good dispersion, indicating that BC substrate could reduce the agglomeration of MWO4, thereby exposing more catalytic sites. This founding is important for accelerating the reduction 342
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Fig. 2. FESEM images of (a) BC, (b) CoWO4, (c and d) CoWO4/BC, (e) FeWO4, (f) FeWO4/BC, (g) NiWO4, and (h) NiWO4/BC.
I3−+2e−↔3I−
photovoltage (Voc) values for devices had no obvious change whether the BC was added or not, indicating that the Fermi levels of MWO4/BC composite CEs were not affected by BC. We evaluated the electrocatalytic activities of MWO4, MWO4/BC, BC, and Pt CEs by cyclic voltammetry (CV) to further interpret the difference in photovoltaic performance for various CE-based DSSCs. Related data are listed in Table 1. Fig. 4b and 4c show the CV curves of various CEs at a scan rate of 50 mV s−1. The left one pair of oxidationreduction peaks in low potential is related to electrochemical reaction (1), and the right one in high potential corresponded to electrochemical reaction (2) [51].
−
3I2+2e ↔2I3
−
(1) (2)
Among all MWO4 CEs (Fig. 4b), only NiWO4 CE exhibited two typical redox peaks, which indicates the superior electrocatalytic activity of NiWO4, as reported in our previous literature [12,49]. This result is consistent with the photovoltaic performance of MWO4 CE-based DSSCs. As shown in Fig. 4c, two clear oxidation-reduction peaks are present in all MWO4/BC composite CEs, indicating a fluent electrochemical reaction of I3−/I− on the CEs surface and the speedy regeneration of 343
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electrocatalytic capacity of the composites. This enhancement was mainly attributed to the rapid electron transfer paths provided by the porous 3D network structure of the BC and by the catalytic sites from dispersive MWO4 in the composites. Moreover, NiWO4/BC CE showed smaller ΔEp (0.37 mV) and higher Ip (−2.41 mA cm−2) values than Pt CE, demonstrating the remarkable synergetic effect of the catalytic activity of NiWO4 and the excellent electrical conductivity of BC. The synergetic effect of carbon supported binary TMCs have been reported in previous papers [3,12,49,55]. The larger ΔEp and lower Ip values of FeWO4/BC and CoWO4/BC CEs relative to those of NiWO4/BC CE can be attributed to the poor electrocatalytic activity of FeWO4 and CoWO4 in the FeWO4/BC and CoWO4/BC composites. The intrinsic origin is on progress by employing first-principle DFT calculation [18,19,49,56]. Tafel polarization and EIS measurements were conducted by using the symmetrical cells to further expound the electrocatalytic activity for the I3− reduction of various CEs. Tafel polarization curves of MWO4, MWO4/BC, BC and Pt CEs are shown in Fig. 4d. Exchange current density (J0), which represents the intersection of tangent of the cathodic branch with the equilibrium potential line, and limiting diffusion current density (Jlim), which represents the intersection of the cathodic branch with the Y axis, are usually used to investigate the electrocatalytic activity of the catalysts [57,58]. Among all MWO4 CEs, NiWO4 CE had the highest J0 and Jlim values, indicating its superior
Scheme 2. Schematic of the formation of flower-like CoWO4.
the sensitizer [52,53]. Considering that one of the roles of CE in DSSC is to catalyze the reduction of I3−, we mainly discussed the left pair of peaks. Then, we used peak-to-peak separation (ΔEp) and peak current density (Ip) to compare the catalytic activities of the CEs [37,54]. As shown in Fig. 4c, all composite MWO4/BC CEs exhibited lower ΔEp and higher Ip values than their corresponding MWO4 and BC CEs. These results indicate that the introduction of BC enhanced the
Fig. 3. FESEM top (left column) and cross-sectional (right column) images of (a, b) FeWO4/BC, (c, d) CoWO4/BC, and (e, f) NiWO4/BC. 344
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Fig. 4. (a) J-V curves of DSSCs based on FeWO4, FeWO4/BC, CoWO4, CoWO4/BC, NiWO4, NiWO4/BC, BC, and Pt CEs; CV curves of (b) FeWO4, CoWO4, NiWO4, and BC, (c) FeWO4/BC, CoWO4/BC, NiWO4/BC, and Pt CEs at the scan rate of 50 mV s−1; (d) Tafel curves and (e) Nyquist plots for the symmetrical cells based on FeWO4, FeWO4/BC, CoWO4, CoWO4/BC, NiWO4, NiWO4/BC, BC, and Pt CEs; the inset is the relevant equivalent circuit.
Table 1 summarizes the related EIS data. The semicircle at high frequencies reflects charge-transfer resistance (Rct), which represents the electron transfer capability of the catalysts. The intersection of semicircle and the X axis determines series resistance (Rs), which mainly describes the adhesion between the substrate and the electrocatalyst film [61]. Among all CEs, FeWO4 has the lowest Rs value (8.86 Ω cm2), which may be due to the superior adhesive between the CE catalyst film and the FTO substrate. However, its poor electrocatalytic activity results in low PCE of DSSC. In fact, the absolute values of Rs for different CEs are close to that of Pt, and the effect of Rs for photovoltaic performance can be omitted [18]. Rct values of MWO4 CEs increased in the order of NiWO4 < CoWO4 < FeWO4, suggesting that the electron
electrocatalytic activity. This result is consistent with CV and photovoltaic performance. This finding might be attributed to the highly intrinsic catalytic property and the smaller particle size of NiWO4 than that of other MWO4 [59]. J0 values of MWO4/BC CEs were higher than those of their corresponding MWO4 and BC CEs, indicating the enhanced electrocatalytic activity of MWO4/BC CEs, which resulted from the introduction of porous structure of the BC. Compared with Pt, NiWO4/BC CE exhibited higher Jlim, reflecting a faster diffusion velocity of the electrolyte on the surface of NiWO4/BC, which might be a reason why the NiWO4/BC had an excellent electrocatalytic activity [52,60]. Fig. 4e presents the Nyquist curves of various kinds of CEs, and
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Table 1 Photovoltaic and electrochemical parameters of the cells with different CEs.a CEs
Voc (V)
FeWO4 FeWO4/BC CoWO4 CoWO4/BC NiWO4 NiWO4/BC BC Pt
0.69 0.68 0.67 0.69 0.68 0.70 0.68 0.73
± ± ± ± ± ± ± ±
0.02 0.01 0.02 0.02 0.01 0.01 0.02 0.02
Jsc (mA cm−2)
FF
13.45 14.38 11.96 13.96 12.60 14.70 13.25 13.32
0.50 0.55 0.56 0.63 0.59 0.69 0.58 0.66
± ± ± ± ± ± ± ±
0.17 0.32 0.37 0.26 0.24 0.21 0.31 0.35
PCE (%) ± ± ± ± ± ± ± ±
0.02 0.01 0.02 0.01 0.03 0.01 0.03 0.02
4.65 5.38 4.46 6.07 5.10 7.08 5.20 6.46
± ± ± ± ± ± ± ±
0.06 0.02 0.03 0.01 0.03 0.01 0.04 0.06
ΔEp (mV)
Ip (mA cm−2)
Rct (Ω cm2)
Rs (Ω cm2)
996 822 945 454 701 370 580 386
−1.22 −1.58 −1.14 −2.25 −0.78 −2.41 −1.50 −1.98
12.00 8.50 27.41 8.07 11.27 2.17 3.27 1.94
8.86 13.02 14.53 14.44 11.05 11.30 14.67 10.69
a Voc: open-circuit voltage, Jsc: short-circuit current density, FF: fill factor, PCE: power conversion efficiency, ΔEp: peak-to-peak separation, Ip: peak current density, Rct: charge-transfer resistance, Rs: series resistance.
after the continuous CV scanning is not observed (see the inset in Fig. 6). This observation indicated the good electrochemical stability of MWO4/BC. The electrochemical stability of MWO4/BC was further examined. The peak current at various CV scanning cycles for three MWO4/BC CEs are shown in Fig. 6b, 6d, and 6f, respectively. With the increased CV scanning cycles, no obvious change is observed in the peak current of all CEs, indicating the good stability of the three MWO4/BC CEs in the redox reaction of I3− and I−. Notably, a slight but perceptible difference in these three CEs was observed, that is, NiWO4/BC CE demonstrated remarkable repeatability in 50 cycles successive scanning curves, suggesting excellent electrochemical stability for NiWO4/BC in the I3−/I− electrolyte system.
transfer capability of NiWO4 is superior to other oxides, which resulted in the high electrocatalytic activity. In contrast with their corresponding MWO4 and BC, MWO4/BC CEs exhibited smaller Rct, indicating faster electron transfer capability of composites than that of MWO4. This phenomenon could be attributed to the introduction of BC, which provided electron transfer paths and was favorable for catalyzing the reduction of I3−. In addition, Pt and BC CEs exhibited lower Rct (1.94 Ω cm2 and 3.27 Ω cm2) due to their excellent electrical conductivities [62]. The obtained Rct value (2.17 Ω cm2) from NiWO4/BC CE was very close to the Rct value of Pt CE, suggesting a high electrocatalytic activity of NiWO4/BC CE, which is consistent with the results of Tafel polarization. By contrast, the Rct of MWO4/BC showed a decreasing tendency, and the PCE of MWO4/BC-based DSSC displayed an increasing tendency (Fig. 5a). This result indicates that an improved electrical conductivity of MWO4/BC could enhance the PCE of MWO4/BC-based DSSC. The main reasons are as follows. On the one hand, the introduction of BC reduced the agglomeration and grain size of the bimetal oxides, thereby exposing more electrocatalytic sites for the reduction of I3−. On the other hand, the BC with interconnected porous structures provided fast electron transfer paths and accelerated electrons transfer from the FTO substrate to the electrocatalytic sites to promote the reduction of I3− to I− at the CE surface and the regeneration of sensitizer in TiO2 photoanode (Fig. 5b). As a consequence, the synergistic effects of conductive BC and highly catalytically active NiWO4 resulted in the high PCE of DSSC with NiWO4/BC CE [40,63]. One reason for the change in PCE of devices is due to the formation of energy storage at the CE, which has been given by Dao et al. [64]. The electrochemical stability of a CE is a key factor for the evaluation of the potential application of DSSCs [60,65]. The electrochemical stability of each as-prepared composite CEs was determined with continuous CV scanning at a scan rate of 50 mV s−1. Fig. 6a, 6c, and 6e did not show any obvious changes of the CV curves. In addition, the peeling off of CE catalyst films from the surface of the FTO substrate
4. Conclusions Waste biomass-derived porous bio-based carbon (BC) enhanced tungsten-based bimetal oxide composites MWO4/BC (MWO4, M = Fe, Co, and Ni) were synthesized through hydrothermal treatment and annealing process. In the hydrothermal system, BC worked as a structure-directing agent to direct the self-assembly of CoWO4 nanoflowers. In MWO4/BC composites, the BC with porous 3D network structure functioned as the electrocatalytic support material reducing the agglomeration and grain size of MWO4 and providing electron transfer paths. The MWO4/BC used as CEs in DSSCs exhibited excellent electrocatalytic activity for I3− reduction and fast electron transport capability (Rct values of FeWO4/BC, CoWO4/BC, and NiWO4/BC were reduced from 12.00 Ω cm2, 27.41 Ω cm2, and 11.27 Ω cm2 for FeWO4, CoWO4, and NiWO4 to 8.50 Ω cm2, 8.07 Ω cm2, and 2.17 Ω cm2, respectively) owing to the synergistic effects of catalytic MWO4 and conductive BC. The DSSCs fabricated with FeWO4/BC, CoWO4/BC, and NiWO4/BC achieved PCEs of 5.38%, 6.07%, and 7.08%, respectively, and matched to that of DSSC with Pt CE (6.46%). The MWO4/BC CEs illustrated remarkable electrochemical stability in the I3−/I−
Fig. 5. (a) PCE and Rct values from the J-V and EIS measurements and (b) schematic of the synergistic effects of BC and MWO4 for catalyzing the reduction of I3−. 346
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Fig. 6. CV curves for 50 cycles continuous scans and the redox peak currents at various CV scanning cycles at a scan rate of 50 mV s−1. FeWO4/BC (a, b), CoWO4/BC (c, d), and NiWO4/BC (e, f) CEs. The inset show their corresponding digital photographs before and after scanning.
(2010K01-120 and 2015JM5183), and Shaanxi Provincial Department of Education (2013JK0927) is greatly acknowledged. The project was partly sponsored by SRF for ROCS, SEM.
electrolyte system based on successive CV scans. The developed NiWO4/BC composite CE can be a good alternative for traditional Pt electrode. The introduction of porous 3D network structure BC is a promising strategy for fabricating efficient and structure-controllable BC-enhanced tungsten-bimetal oxides.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Bio-based carbon-enhanced tungsten-based bimetal oxides as counter electrodes for dye-sensitized solar cells
T
Yangliang Zhang, Sining Yun∗, Chen Wang, Ziqi Wang, Feng Han, Yiming Si Functional Materials Laboratory (FML), School of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
carbon supported W-based • Bio-based bimetal oxides are successfully prepared.
carbon as structure-di• Bio-based recting agent direct self-assembly of nanoflowers.
composite CEs show en• W-based hanced electrocatalytic activity in DSSCs.
high PCE of 7.08% is obtained by • Ausing NiWO /BC catalyst in DSSCs. bio-based carbon is a promising • Using strategy to fabricate efficiently hybrid 4
CEs.
A R T I C LE I N FO
A B S T R A C T
Keywords: Dye-sensitized solar cell Counter electrode Catalysts Tungsten-based bimetal oxide Bio-based carbon
Tungsten-based bimetal oxides are first used as counter electrode catalysts for dye-sensitized solar cells. In this work, we introduce porous bio-based carbon into tungsten-based bimetal oxide by hydrothermal and annealing methods to improve the electrocatalytic properties of tungsten-based bimetal oxides. A structure transformation from nanorod to nanoflower is observed in cobalt tungsten oxide when porous bio-based carbon is used as the structure-directing agent. Bio-based carbon, which has a porous three-dimensional network structure, serves as an electrocatalytic support material in the tungsten-based bimetal oxide/bio-based carbon composites, reducing the agglomeration and grain size of tungsten-based bimetal oxides and providing rapid electron transfer paths. Tungsten-based bimetal oxide/bio-based carbon composite counter electrodes exhibit enhanced electrocatalytic activity and reduced charge-transfer resistance because of the synergistic effects of catalytic tungsten-based bimetal oxides and conductive bio-based carbon. The dye-sensitized solar cell with nickel tungsten oxide/biobased carbon counter electrode shows a high power conversion efficiency of 7.08%, surpassing that of dyesensitized solar cell with Pt counter electrode (6.46%). This work presents a general strategy for designing and fabricating porous bio-based carbon support inorganic compound materials for energy conversion devices.
1. Introduction Dye-sensitized solar cells (DSSCs) have drawn wide attention because of their incomparable advantages including low cost, easy to manufacture, environmental friendly and high theoretical photoelectric
∗
conversion efficiency (PCE) [1–4]. A typical DSSC has a sandwich structure that consists of three parts: a dye-sensitized semiconductor photoanode, counter electrode (CE) and electrolyte which is sandwiched between the photoanode and CE. The CE is an important component of DSSC, which collects electrons from an external circuit
Corresponding author. E-mail addresses: [email protected], [email protected] (S. Yun).
https://doi.org/10.1016/j.jpowsour.2019.03.054 Received 16 January 2019; Received in revised form 5 March 2019; Accepted 14 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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and catalyzes the reduction of I3− ions in the electrolyte solution. However, traditional Pt CE materials are expensive, have a limited source, and are easily corroded by the I3−/I− electrolyte. As these unfavorable features restrict the commercial application of DSSCs [5], exploring Pt-free catalysts as CE material is urgently needed. Transition metal compounds (TMCs) [3], carbon materials [6,7], metals and alloys [8], conductive polymers [9,10], and their corresponding composites [11] have been used as CEs in DSSCs. Of these materials, TMCs, particularly tungsten (W)-based compounds, have excellent catalytic properties owing to their electronic structures, which are similar to that of Pt [12,13]. Lee et al. prepared mesoporous WC through a microwave-assisted method and used it as a CE for DSSC. The DSSC showed an excellent PCE of 7.01% [14]. Ma et al. synthesized W18O49 nanofibers through a template-free solvothermal approach. The DSSC with W18O49 nanofibers CE yielded a PCE of 8.58% [15]. Liu et al. used WB crystals with spontaneously oxidized surface as a CE material in DSSC and yielded a PCE of 7.07% [16]. Qiao et al. created oxygen vacancy to activate WO3 as CE for DSSC and achieved a PCE of 10.5% [17]. Recently, our group used first-principle density functional theory (DFT) calculations to elucidate the intrinsic reason that TMC-based catalysts demonstrate superior electrocatalytic activity as CEs in DSSCs [18,19]. Compared with binary TMCs (oxides, nitrides, carbides, and etc), ternary TMCs (CoMoS4, NiCo2S4, CoFe2O4 and etc) present superior electrocatalytic activities and controllable structures because of the synergistic effects and valence interchanges among their catalytic components [20–22]. Thus, the fabrication of W-based bimetal oxides may be a good means for optimizing the electrocatalytic properties of W-based compounds as CEs. Unfortunately, information on W-based bimetal oxides used as CEs applied in DSSCs is limited. An effective catalytic material for DSSC must possess superior catalytic activity and electrical conductivity. However, most TMCs have inferior electrical conductivities that result in low PCEs. The electrical conductivities of catalysts can be improved by introducing carbon materials, which have excellent electrical conductivities, reasonable chemical durability, and large specific surface areas [23–26]. Recently, composites composed of TMCs and carbon materials have been used as CEs for DSSCs. Yun et al. synthesized HfO2/mesoporous carbon (MC) and Hf7O8N4-HfO2/MC composites through a simple chemical synthesis. DSSCs with the composite CEs rendered PCEs of 6.71% and 7.85%, respectively, owing to the synergetic electrocatalytic effects of various components in the composite [12]. Yuan et al. used CoFe2O4/carbon nanotubes (CNTs) as CE for DSSC which yielded PCE of 6.55%, and this result indicates that the enhanced catalytic performance of CoFe2O4/ CNTs is due to the excellent conductivities of the CNTs and the high catalytic activity of CoFe2O4 [27]. Chen et al. prepared Co3O4-WC-nitrogen-doped carbon (CN)/graphene (rGO), which showed remarkable catalytic activity due to the synergistic effect of Co3O4-WC and electrical conductivity of rGO. The DSSC with the Co3O4-WC-CN/rGO as CE achieved PCE of 7.38% [28]. The carbon materials used as structuredirecting and morphology-controlling agents are conducive to the synthesis of nanomaterials possessing controllable structures and morphologies that enhance CE catalysts performance [29,30]. Although various kinds of carbon materials have been used in DSSCs, complex synthetic approaches and limitations for scale manufacture make the effective utilization of carbon materials difficult [31]. Natural biomass is considered a promising source for carbon material production because of its abundance and facile accessibility [32]. Bio-based carbon (BC) is a remarkable carbon material for CE production in terms of resources cyclic utilization. In this work, waste biomass was used as a carbon source, and a series of porous BC supported W-based bimetal oxides (MWO4, M = Fe, Co, and Ni) composites were prepared through hydrothermal treatment and annealing. In the resulting MWO4/BC composites, porous BC functioned as a structure-directing agent for the synthesis of flower-like CoWO4. As CE catalysts for DSSCs, MWO4/BC showed more preferable electrocatalytic activity for I3− reduction than bare MWO4, thus
resulting in the improved PCEs for DSSCs with MWO4/BC CEs. The DSSC fabricated with NiWO4/BC exhibited excellent PCE of 7.08%, which surpassed that of DSSC with Pt CE (6.46%).
2. Experimental 2.1. Preparation of bio-carbon Bio-carbon (BC) was derived from aloe peel waste. Aloe peels were washed with distilled water for the removal impurities and then dried at 105°C for 24 h. The dried aloe peels were crushed into powder with particle size ∼2 μm. 6 g aloe peel powder was added into 120 mL deionized water and vigorously stirred for 4 h. The obtained mixture was hydrothermally treated at 230°C for 15 h with a stainless-steel autoclave with Teflon lining. The sediment was washed with deionized water and alcohol for several times and then dried at 105°C for 24 h, resulting in hydrothermal carbon. 1 g hydrothermal carbon along with 2 g KOH was added into 20 mL deionized water and vigorously stirred for 4 h. The obtained mixture was dried at 105°C for 24 h, then subjected to activated treatment at 800°C for 2 h in N2 atmosphere. The activated product was washed with hydrochloric acid, alcohol, and deionized water successively until the filtrate was neutral, and dried at 105°C for 24 h, resulting in BC. Further details can be cited in our previous works [33–35].
2.2. Synthesis of MWO4 and MWO4/BC (M = Fe, Co, and Ni) MWO4/BC were synthesized by hydrothermal treatment and annealing (Scheme 1). Fe(NO3)3·9H2O, Co(NO3)2·6H2O, and Ni (NO3)2·6H2O were used as metal (Fe, Co, and Ni) precursors, and ammonium paratungstate was used as a tungsten precursor. In detail, 5 mmol ammonium paratungstate and 20 mmol BC (the mole ratio of BC to MWO4 is 4, based on the previous literature [36]) were added into 50 mL deionized water and then vigorously stirred for 30 min. 5 mmol metal precursor was added, and the resulting solution was vigorously stirred for 4 h and then hydrothermally treated in an autoclave with Teflon lining at 220°C for 12 h. The obtained precipitate was washed with deionized water and alcohol repeatedly and then dried at 80°C for 48 h. Finally, the obtained powder was annealed at 500°C for 2 h in N2, resulting in MWO4/BC. Bare MWO4 was prepared through similar procedures but without BC.
2.3. Fabrication of electrodes and cells In the photoanode fabrication, the obtained TiO2 film electrode with an active area of 0.196 cm2 was heated at 500°C for 30 min in air. The TiO2 electrode was cooled to appropriate temperature and then dipped into a mixture of tert-butyl alcohol, acetonitrile (the volume ratio of tertbutyl alcohol to acetonitrile is 1:1), and 0.5 mM of N719 dye for 24 h. The dye-sensitized TiO2 electrode was rinsed with ethanol and then dried in air. In the fabrication of CEs, 0.2 g MWO4 or MWO4/BC powder was added into 5 mL isopropanol solution and ball-milled for 4 h with 3.5 g ZrO2 pearls as dispersion medium. The prepared mixture was sprayed on the surface of an FTO substrate with an air brush. Then, the FTO substrate loaded with MWO4 or MWO4/BC layer was annealed at 400°C for 30 min in N2. Pt CEs were fabricated by annealing the FTO substrate which was coated with H2PtCl6 isopropanol solution at 500°C under ambient atmosphere for 30 min. The symmetrical cell was assembled by two identical CEs and an I−/I3− electrolyte. The DSSC was assembled by filling the I−/I3− electrolyte between a TiO2 photoanode and a CE. The I−/I3− electrolyte consisted of 0.03 M I2, 0.06 M LiI, 0.1 M guanidinium thiocyanate, 0.5 M 4-tert-butyl pyridine, 0.6 M 1butyl-3-methylimidazolium iodide, and acetonitrile solution [18,37]. 340
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Scheme 1. Schematic of the synthesis of MWO4 and MWO4/BC (M = Fe, Co, and Ni).
(222), (−113), (−231), (320), (−140), and (−312), respectively, for the monoclinic CoWO4 (JCPD: 15–0867). In Fig. 1d, the diffraction peaks at 15.62, 19.28, 23.97, 24.92, 30.93, 31.54, 36.57, 37.25, 39.12, 41.66, 44.74, 46.41, 48.16, 49.04, 51.10, 52.32, 53.18, 54.62, 55.84, 62.72, 64.47, 65.84, 68.95, and 72.62° can be assigned to the crystal planes of (010), (100), (011), (110), (−111), (020), (002), (120), (200), (−102), (−112), (−211), (030), (022), (220), (130), (−122), (−202), (−131), (310), (−222), (−311), (041), and (−302), respectively, for the monoclinic NiWO4 (JCPDS: 15–0775). The characteristic diffraction peaks of synthesized FeWO4, CoWO4, and NiWO4 matched with the data of their respective monoclinic phases. Meanwhile, MWO4/BC displayed similar characteristic peaks with their corresponding MWO4, indicating that the expected MWO4/BC were successfully synthesized. However, the graphitic carbon (002) and (100) diffraction peaks were not clearly observed in the XRD patterns of MWO4/BC. The possible reason is that the highly crystallized diffraction peaks of MWO4 masked the weak diffraction peaks of amorphous carbon [39]. In contrast with the XRD patterns of MWO4, the diffraction peaks of MWO4 in their corresponding MWO4/BC were weak probably because of the presence of BC, which reduced the crystallinity of MWO4. The microstructures and morphologies of BC, MWO4, and MWO4/ BC were investigated through FESEM measurement. BC exhibited a porous three-dimensional (3D) network structure with randomly distributed macropores (size range from 200 nm to 800 nm) over its entire parts (Fig. 2a). This structure is conducive to the reduction of I3− because its interconnected porous structures enable CE to fully contacted with an electrolyte solution, and the interlaced networks of BC provides rapid electron transfer paths [40,41]. Bare CoWO4 showed two types of morphologies: a rod-like shape with broad size distribution of 0.5–2.5 μm in length and 200–600 nm in diameter and an irregular particle shape with a size that ranges from 200 nm to 800 nm (Fig. 2b). By contrast, the CoWO4 showed a hexagonal flower-like nanostructure with a mean diameter of approximately 1.5 μm (Fig. 2c), when BC was used as structure-directing and morphology-controlling agent in CoWO4/BC composites. The magnified structure of hexagonal flower-like CoWO4 could observed in Fig. 2d. The nanoflowers were assembled by six interlaced bunches, which are constructed by many parallel-arranged nanorods. This change of morphology of CoWO4 products might be attributed to the existence of BC. Previous studies synthesized flower-like CoWO4 by
2.4. Characterization X-ray diffraction (XRD) patterns of the BC, MWO4, and MWO4/BC were obtained on an automated X-ray powder diffractometer (D/Max 2200, MDI, America). The morphologies and microstructures of BC, MWO4, and MWO4/BC were observed by field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan). The photocurrent-voltage (J-V) curves of the as-assembled DSSCs were recorded by a Keithley 2400 digital source-meter under the illumination of 100 mW cm−2 provided by a solar simulator (Oriel 94023A, Newport). Cyclic voltammetry (CV) was performed by using a CHI602 electrochemical analyzer (Chenhua, Shanghai, China) at a scan rate of 50 mV s−1 in a three-electrode system. An acetonitrile solution containing 0.1 M LiClO4, 1 mM I2, and 10 mM LiI was used as the electrolyte, the as-prepared CE as the working electrode, Pt as the counter electrode, and Ag/Ag+ as the reference electrode. Tafel polarization tests and electrochemical impedance spectra (EIS) measurements were conducted by using the symmetrical cells on a CHI660E electrochemical analyzer (Chenhua, Shanghai, China). In the EIS measurements, the cells were scanned from 100 mHz to 1 MHz at 0.01 V alternating current amplitude and −0.75 V bias potential. The obtained EIS data were fitted by Z-view software.
3. Results and discussion Fig. 1 shows the XRD patterns of BC, MWO4, and MWO4/BC. In Fig. 1a, two broad diffraction peaks were found at 2θ = 24.3° and 43.3° (Fig. 1a). The presence of peaks corresponding to the (002) and (100) crystal planes of a typical graphitic carbon indicates the formation of the amorphous carbon structure of BC [38]. In Fig. 1b, the diffraction peaks at 15.48, 18.67, 23.71, 24.36, 30.37, 31.27, 36.21, 36.70, 37.95, 41.09, 44.05, 45.23, 47.68, 48.52, 49.96, 51.67, 52.49, 53.49, and 61.38° can be assigned to the crystal planes of (010), (100), (011), (110), (−111), (020), (021), (120), (200), (−121), (−112), (211), (030), (022), (220), (130), (122), (221), and (300), respectively, for the monoclinic FeWO4 (JCPDS: 46–1446). In Fig. 1c, the diffraction peaks at 15.58, 18.98, 23.82, 24.65, 30.63, 31.45, 36.30, 36.99, 38.52, 41.38, 44.35, 47.99, 48.76, 50.58, 52.07, 54.06, 59.35, 61.80, 63.83, 64.88, 65.08, 65.39, 68.69, and 71.98° can be assigned to the crystal planes of (010), (001), (−110), (011), (−111), (020), (200), (021), (002), (−121), (−211), (030), (−220), (022), (031), (−122), (003), (−311), 341
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Fig. 1. XRD patterns of (a) BC, (b) FeWO4 and FeWO4/BC, (c) CoWO4 and CoWO4/BC, and (d) NiWO4 and NiWO4/BC.
reaction of I3−, which will further support pervious viewpoint [40,48–50]. Fig. 3 shows the morphologies and thicknesses of FeWO4/BC, CoWO4/BC, and NiWO4/BC CEs. It could observed that the catalyst powder adhered to the surface of FTO uniformly form the dense film, and no obvious cracks were observed, indicating that the composites have a good bonding property. The thicknesses of the FeWO4/BC, CoWO4/BC, and NiWO4/BC CEs were approximately 5.25, 5.45, and 5.52 μm, respectively, and the thicknesses of different CEs are close, indicating that the effect of thickness of these three CEs on catalytic performance can be omitted. The as-prepared catalysts and Pt were used as CEs in the DSSCs to evaluate their catalytic properties. Fig. 4a shows the characteristic J-V curves of the DSSCs with various CEs, and Table 1 lists the related parameters. The DSSCs assembled by FeWO4, CoWO4, NiWO4, and BC CEs produced PCEs of 4.65%, 4.46%, 5.10%, and 5.20%, respectively; whereas the DSSCs with FeWO4/BC, CoWO4/BC, and NiWO4/BC CEs yielded enhanced PCEs of 5.38%, 6.07%, and 7.08%, respectively. The PCE value of NiWO4/BC based DSSC was higher than that of DSSC with Pt CE (6.46%). Notably, the DSSCs with MWO4/BC CEs exhibited higher short-circuit current density (Jsc) and fill factor (FF) than their corresponding DSSCs with MWO4 CEs. The introduction of BC increased the Jsc values of DSSCs from 13.45 mA cm−2 (FeWO4), 11.96 mA cm−2 (CoWO4), and 12.60 mA cm−2 (NiWO4) to 14.38 mA cm−2 (FeWO4/ BC), 13.96 mA cm−2 (CoWO4/BC), and 14.70 mA cm−2 (NiWO4/BC), respectively, and enhanced the FF values from 0.50 (FeWO4), 0.56 (CoWO4) and 0.59 (NiWO4) to 0.55 (FeWO4/BC), 0.63 (CoWO4/BC) and 0.69 (NiWO4/BC), respectively. However, the open-circuit
an alcohol-thermal process, in which deionized water is substituted with alcohol as solution. Aggregated particles in low polarity system increase the stability of the a colloid and result the formation of flowerlike nanostructures [42]. In this work, the introduced BC has a nonpolar surface and provide a low polarity environment for the hydrothermal system [43]. The amorphous particles and nanorods aggregated on the surface of the BC and gradually evolved to hexagonal flower-like structures through oriented attachment (Scheme 2), similar to the observed in the previous reports [44–46]. The FeWO4 particles (Fig. 2e) had irregular shapes, and their size ranged from 0.3 μm to 1.5 μm. Meanwhile, the NiWO4 particles (Fig. 2g) were spherical, and their sizes ranged from 200 nm to 800 nm, and had a small quantity of nanorod that was 0.5–2 μm in length and 200–500 nm in diameter. In FeWO4/BC (Fig. 2f) and NiWO4/BC (Fig. 2h) composites, FeWO4 and NiWO4 showed a quasi-spherical shape and had smaller grain sizes (size range from 100 nm to 200 nm) than those of corresponding bare FeWO4 and NiWO4. The smaller particle size of MWO4 could increase the contact area of MWO4 with electrolyte, thereby exposing more catalytic sites to accelerate the reduction reaction of I3−. The smaller grain sizes of FeWO4 and NiWO4 may be attributed to the presence of BC, which prevented the growth of FeWO4 and NiWO4 crystals during the synthesis [40,47]. This finding proved the results of the XRD measurement, that is, the weak diffraction peaks of MWO4 in their corresponding MWO4/BC are due to the low crystallinity of MWO4. Fig. 2c, 2f, and 2h show that MWO4 loaded on the surface of BC showed good dispersion, indicating that BC substrate could reduce the agglomeration of MWO4, thereby exposing more catalytic sites. This founding is important for accelerating the reduction 342
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Fig. 2. FESEM images of (a) BC, (b) CoWO4, (c and d) CoWO4/BC, (e) FeWO4, (f) FeWO4/BC, (g) NiWO4, and (h) NiWO4/BC.
I3−+2e−↔3I−
photovoltage (Voc) values for devices had no obvious change whether the BC was added or not, indicating that the Fermi levels of MWO4/BC composite CEs were not affected by BC. We evaluated the electrocatalytic activities of MWO4, MWO4/BC, BC, and Pt CEs by cyclic voltammetry (CV) to further interpret the difference in photovoltaic performance for various CE-based DSSCs. Related data are listed in Table 1. Fig. 4b and 4c show the CV curves of various CEs at a scan rate of 50 mV s−1. The left one pair of oxidationreduction peaks in low potential is related to electrochemical reaction (1), and the right one in high potential corresponded to electrochemical reaction (2) [51].
−
3I2+2e ↔2I3
−
(1) (2)
Among all MWO4 CEs (Fig. 4b), only NiWO4 CE exhibited two typical redox peaks, which indicates the superior electrocatalytic activity of NiWO4, as reported in our previous literature [12,49]. This result is consistent with the photovoltaic performance of MWO4 CE-based DSSCs. As shown in Fig. 4c, two clear oxidation-reduction peaks are present in all MWO4/BC composite CEs, indicating a fluent electrochemical reaction of I3−/I− on the CEs surface and the speedy regeneration of 343
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electrocatalytic capacity of the composites. This enhancement was mainly attributed to the rapid electron transfer paths provided by the porous 3D network structure of the BC and by the catalytic sites from dispersive MWO4 in the composites. Moreover, NiWO4/BC CE showed smaller ΔEp (0.37 mV) and higher Ip (−2.41 mA cm−2) values than Pt CE, demonstrating the remarkable synergetic effect of the catalytic activity of NiWO4 and the excellent electrical conductivity of BC. The synergetic effect of carbon supported binary TMCs have been reported in previous papers [3,12,49,55]. The larger ΔEp and lower Ip values of FeWO4/BC and CoWO4/BC CEs relative to those of NiWO4/BC CE can be attributed to the poor electrocatalytic activity of FeWO4 and CoWO4 in the FeWO4/BC and CoWO4/BC composites. The intrinsic origin is on progress by employing first-principle DFT calculation [18,19,49,56]. Tafel polarization and EIS measurements were conducted by using the symmetrical cells to further expound the electrocatalytic activity for the I3− reduction of various CEs. Tafel polarization curves of MWO4, MWO4/BC, BC and Pt CEs are shown in Fig. 4d. Exchange current density (J0), which represents the intersection of tangent of the cathodic branch with the equilibrium potential line, and limiting diffusion current density (Jlim), which represents the intersection of the cathodic branch with the Y axis, are usually used to investigate the electrocatalytic activity of the catalysts [57,58]. Among all MWO4 CEs, NiWO4 CE had the highest J0 and Jlim values, indicating its superior
Scheme 2. Schematic of the formation of flower-like CoWO4.
the sensitizer [52,53]. Considering that one of the roles of CE in DSSC is to catalyze the reduction of I3−, we mainly discussed the left pair of peaks. Then, we used peak-to-peak separation (ΔEp) and peak current density (Ip) to compare the catalytic activities of the CEs [37,54]. As shown in Fig. 4c, all composite MWO4/BC CEs exhibited lower ΔEp and higher Ip values than their corresponding MWO4 and BC CEs. These results indicate that the introduction of BC enhanced the
Fig. 3. FESEM top (left column) and cross-sectional (right column) images of (a, b) FeWO4/BC, (c, d) CoWO4/BC, and (e, f) NiWO4/BC. 344
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Fig. 4. (a) J-V curves of DSSCs based on FeWO4, FeWO4/BC, CoWO4, CoWO4/BC, NiWO4, NiWO4/BC, BC, and Pt CEs; CV curves of (b) FeWO4, CoWO4, NiWO4, and BC, (c) FeWO4/BC, CoWO4/BC, NiWO4/BC, and Pt CEs at the scan rate of 50 mV s−1; (d) Tafel curves and (e) Nyquist plots for the symmetrical cells based on FeWO4, FeWO4/BC, CoWO4, CoWO4/BC, NiWO4, NiWO4/BC, BC, and Pt CEs; the inset is the relevant equivalent circuit.
Table 1 summarizes the related EIS data. The semicircle at high frequencies reflects charge-transfer resistance (Rct), which represents the electron transfer capability of the catalysts. The intersection of semicircle and the X axis determines series resistance (Rs), which mainly describes the adhesion between the substrate and the electrocatalyst film [61]. Among all CEs, FeWO4 has the lowest Rs value (8.86 Ω cm2), which may be due to the superior adhesive between the CE catalyst film and the FTO substrate. However, its poor electrocatalytic activity results in low PCE of DSSC. In fact, the absolute values of Rs for different CEs are close to that of Pt, and the effect of Rs for photovoltaic performance can be omitted [18]. Rct values of MWO4 CEs increased in the order of NiWO4 < CoWO4 < FeWO4, suggesting that the electron
electrocatalytic activity. This result is consistent with CV and photovoltaic performance. This finding might be attributed to the highly intrinsic catalytic property and the smaller particle size of NiWO4 than that of other MWO4 [59]. J0 values of MWO4/BC CEs were higher than those of their corresponding MWO4 and BC CEs, indicating the enhanced electrocatalytic activity of MWO4/BC CEs, which resulted from the introduction of porous structure of the BC. Compared with Pt, NiWO4/BC CE exhibited higher Jlim, reflecting a faster diffusion velocity of the electrolyte on the surface of NiWO4/BC, which might be a reason why the NiWO4/BC had an excellent electrocatalytic activity [52,60]. Fig. 4e presents the Nyquist curves of various kinds of CEs, and
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Table 1 Photovoltaic and electrochemical parameters of the cells with different CEs.a CEs
Voc (V)
FeWO4 FeWO4/BC CoWO4 CoWO4/BC NiWO4 NiWO4/BC BC Pt
0.69 0.68 0.67 0.69 0.68 0.70 0.68 0.73
± ± ± ± ± ± ± ±
0.02 0.01 0.02 0.02 0.01 0.01 0.02 0.02
Jsc (mA cm−2)
FF
13.45 14.38 11.96 13.96 12.60 14.70 13.25 13.32
0.50 0.55 0.56 0.63 0.59 0.69 0.58 0.66
± ± ± ± ± ± ± ±
0.17 0.32 0.37 0.26 0.24 0.21 0.31 0.35
PCE (%) ± ± ± ± ± ± ± ±
0.02 0.01 0.02 0.01 0.03 0.01 0.03 0.02
4.65 5.38 4.46 6.07 5.10 7.08 5.20 6.46
± ± ± ± ± ± ± ±
0.06 0.02 0.03 0.01 0.03 0.01 0.04 0.06
ΔEp (mV)
Ip (mA cm−2)
Rct (Ω cm2)
Rs (Ω cm2)
996 822 945 454 701 370 580 386
−1.22 −1.58 −1.14 −2.25 −0.78 −2.41 −1.50 −1.98
12.00 8.50 27.41 8.07 11.27 2.17 3.27 1.94
8.86 13.02 14.53 14.44 11.05 11.30 14.67 10.69
a Voc: open-circuit voltage, Jsc: short-circuit current density, FF: fill factor, PCE: power conversion efficiency, ΔEp: peak-to-peak separation, Ip: peak current density, Rct: charge-transfer resistance, Rs: series resistance.
after the continuous CV scanning is not observed (see the inset in Fig. 6). This observation indicated the good electrochemical stability of MWO4/BC. The electrochemical stability of MWO4/BC was further examined. The peak current at various CV scanning cycles for three MWO4/BC CEs are shown in Fig. 6b, 6d, and 6f, respectively. With the increased CV scanning cycles, no obvious change is observed in the peak current of all CEs, indicating the good stability of the three MWO4/BC CEs in the redox reaction of I3− and I−. Notably, a slight but perceptible difference in these three CEs was observed, that is, NiWO4/BC CE demonstrated remarkable repeatability in 50 cycles successive scanning curves, suggesting excellent electrochemical stability for NiWO4/BC in the I3−/I− electrolyte system.
transfer capability of NiWO4 is superior to other oxides, which resulted in the high electrocatalytic activity. In contrast with their corresponding MWO4 and BC, MWO4/BC CEs exhibited smaller Rct, indicating faster electron transfer capability of composites than that of MWO4. This phenomenon could be attributed to the introduction of BC, which provided electron transfer paths and was favorable for catalyzing the reduction of I3−. In addition, Pt and BC CEs exhibited lower Rct (1.94 Ω cm2 and 3.27 Ω cm2) due to their excellent electrical conductivities [62]. The obtained Rct value (2.17 Ω cm2) from NiWO4/BC CE was very close to the Rct value of Pt CE, suggesting a high electrocatalytic activity of NiWO4/BC CE, which is consistent with the results of Tafel polarization. By contrast, the Rct of MWO4/BC showed a decreasing tendency, and the PCE of MWO4/BC-based DSSC displayed an increasing tendency (Fig. 5a). This result indicates that an improved electrical conductivity of MWO4/BC could enhance the PCE of MWO4/BC-based DSSC. The main reasons are as follows. On the one hand, the introduction of BC reduced the agglomeration and grain size of the bimetal oxides, thereby exposing more electrocatalytic sites for the reduction of I3−. On the other hand, the BC with interconnected porous structures provided fast electron transfer paths and accelerated electrons transfer from the FTO substrate to the electrocatalytic sites to promote the reduction of I3− to I− at the CE surface and the regeneration of sensitizer in TiO2 photoanode (Fig. 5b). As a consequence, the synergistic effects of conductive BC and highly catalytically active NiWO4 resulted in the high PCE of DSSC with NiWO4/BC CE [40,63]. One reason for the change in PCE of devices is due to the formation of energy storage at the CE, which has been given by Dao et al. [64]. The electrochemical stability of a CE is a key factor for the evaluation of the potential application of DSSCs [60,65]. The electrochemical stability of each as-prepared composite CEs was determined with continuous CV scanning at a scan rate of 50 mV s−1. Fig. 6a, 6c, and 6e did not show any obvious changes of the CV curves. In addition, the peeling off of CE catalyst films from the surface of the FTO substrate
4. Conclusions Waste biomass-derived porous bio-based carbon (BC) enhanced tungsten-based bimetal oxide composites MWO4/BC (MWO4, M = Fe, Co, and Ni) were synthesized through hydrothermal treatment and annealing process. In the hydrothermal system, BC worked as a structure-directing agent to direct the self-assembly of CoWO4 nanoflowers. In MWO4/BC composites, the BC with porous 3D network structure functioned as the electrocatalytic support material reducing the agglomeration and grain size of MWO4 and providing electron transfer paths. The MWO4/BC used as CEs in DSSCs exhibited excellent electrocatalytic activity for I3− reduction and fast electron transport capability (Rct values of FeWO4/BC, CoWO4/BC, and NiWO4/BC were reduced from 12.00 Ω cm2, 27.41 Ω cm2, and 11.27 Ω cm2 for FeWO4, CoWO4, and NiWO4 to 8.50 Ω cm2, 8.07 Ω cm2, and 2.17 Ω cm2, respectively) owing to the synergistic effects of catalytic MWO4 and conductive BC. The DSSCs fabricated with FeWO4/BC, CoWO4/BC, and NiWO4/BC achieved PCEs of 5.38%, 6.07%, and 7.08%, respectively, and matched to that of DSSC with Pt CE (6.46%). The MWO4/BC CEs illustrated remarkable electrochemical stability in the I3−/I−
Fig. 5. (a) PCE and Rct values from the J-V and EIS measurements and (b) schematic of the synergistic effects of BC and MWO4 for catalyzing the reduction of I3−. 346
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Fig. 6. CV curves for 50 cycles continuous scans and the redox peak currents at various CV scanning cycles at a scan rate of 50 mV s−1. FeWO4/BC (a, b), CoWO4/BC (c, d), and NiWO4/BC (e, f) CEs. The inset show their corresponding digital photographs before and after scanning.
(2010K01-120 and 2015JM5183), and Shaanxi Provincial Department of Education (2013JK0927) is greatly acknowledged. The project was partly sponsored by SRF for ROCS, SEM.
electrolyte system based on successive CV scans. The developed NiWO4/BC composite CE can be a good alternative for traditional Pt electrode. The introduction of porous 3D network structure BC is a promising strategy for fabricating efficient and structure-controllable BC-enhanced tungsten-bimetal oxides.
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