Composites of Vanadium (III) Oxide (V2O3) Incorporating with Amorphous C as Pt-Free Counter Electrodes for Low-Cost and High-Performance Dye-Sensitized Solar Cells.

To replace precious Pt-based counter electrodes (CEs) with a low-cost Pt-free catalyst of CEs is still a motivating hotspot to decrease the fabrication cost of dye-sensitized solar cells (DSSCs). Herein, four different V2O3@C composite catalysts were synthesized by pyrolysis of a precursor under N2 flow at 1100 °C and further served as catalytic materials of CEs for the encapsulation of DSSCs. The precursors of V2O3@C composites have been prepared via a sol-gel method using different proportions of V2O5 with soluble starch in a H2O2 solution. Power conversion efficiencies (PCEs) of 3.59, 4.79, 5.15, and 5.06% were obtained from different V2O3@C composites, with soluble starch-to-V2O5 mass ratios (S/V) of 1:2, 1:1, 2:1, and 4:1, respectively, as CEs to reduce iodide/triiodide in DSSCs. The improvement of electrode performance is due to the combined effects on the increased specific surface area and the enhanced conductivity of V2O3@C composite catalysts.

Introduction

Solar energy, a kind of renewable energy source, is rich in reserves, clean, and environmentally friendly. It is rarely limited by the geographical scope. However, it is also easily affected by natural factors such as season, climate, day and night, latitude, altitude, and so on and has intermittent and unstable characteristics. To overcome the above problems, dye-sensitized solar cells (DSSCs) had already been applied to store solar energy through the direct conversion of photoelectricity and to further utilize it.[1,2] The optimization of each of the key parts in a DSSC has been widely mentioned corresponding to the porous semiconductor film photoanode,[3−5] sensitized dye,[6] electrolytes for redox mediation,[7] and counter electrodes (CEs).[8] Among these components, CEs, as an indispensable part of DSSCs, have two main functions: connecting the external circuit to transmit electrons and catalyzing redox pairs in electrolytes to recycle and regenerate. At present, the focus of research on CEs should be consistent, that is, to maintain high power conversion efficiencies (PCEs) and to further reduce the cost-effectiveness of CEs at the same time by Pt-free electrocatalytic materials to replace the traditional noble Pt-based catalyst of CEs.[9] Herein, the respective research places particular emphasis on the differences in the composition, preparation, morphology, or optimization of the corresponding catalytic materials, and so on. Hence, a variety of Pt-free materials, for instance, carbonaceous materials,[10−14] organic conductive polymers,[15−18] transition-metal compounds (TMCs),[19−24] alloys,[25,26] metal–organic frameworks (MOFs),[27−29] and multicomponent compounds,[30−33] have been extended as CE materials to replace the high-cost Pt. These materials as CEs in DSSCs have many advantages of abundance, low cost, easy synthesis, and high stability with large electrochemical activity for the regeneration reactions of I3–/I–.

Among the above catalytic materials of CEs, transition-metal oxides (TMOs) of group VB and VIB, as one of the TMCs, are widely applied to the novel Pt-free CEs due to a similar d-band electron density to that of Pt.[34] In VB group oxides of TMOs, V element of V-based oxides could be diverse in the valence state between V2+ and V5+, and it exists in various forms from VO2 to VO2. The extensive interest in V-based oxides as catalysts in various electrochemical devices (batteries, supercapacitors, etc.) is because of their unmatched structural types, multiphase states, and easy modification.[35,36] As shown in our previous work,[37] V2O3-based CEs had been successfully prepared using VOCl3 as metal precursors with the urea–metal route. The DSSCs using V2O3 CEs showed a decent PCE value of 5.40%. Vijayakumar et al. also reported that V2O3 nanofiber CEs yielded an efficiency of 5.0%.[38] To further improve the performance and reduce the costs of DSSCs, researchers have focused on producing single V2O3 catalysts better by incorporating with carbon or carbon-derived materials because of the outstanding conductivity, controllable specific surface, and low-cost fabrication of carbonaceous materials.[39,40] Additional carbonaceous materials as a basic carrier could change the energy band structure and distort the lattice, which result in high-energy activation to improve the electrical conductivity and provide more reaction sites in V-based oxides.[41] Also, in our work, V2O3@AC composite catalysts have been fabricated by the pyrolysis of NH4VO3 with different mass ratios of activated carbon (AC) at high temperatures.[42] Compared with the 4.54% PCE of Pt and 3.33% of pure V2O3, the DSSCs using V2O3@AC composite can reach the highest PCE of 5.55% for the regeneration of the I3–/I– redox couple. Gnanasekar et al. wrapped monoclinic crystal VO2 on one-dimensional carbon as a cost-effective CE and achieved a PCE of 6.53%.[43] As a result, it is significant to explore a stabler, cheaper, and simpler assembly and higher activity to improve the performance of Pt-free CEs in DSSCs.

Herein, different proportions of V2O3@C composite catalysts were synthesized via pyrolysis of a precursor under N2 flow by a solid-state reaction at high temperatures. The precursors were obtained by a sol–gel method with soluble starch (denoted as S) as a carbon source and V2O5 (denoted as V) as a metal source in H2O2. Soluble starch is a kind of common natural biomass material, which has a large specific surface area and shows flammability and network cross-linking; therefore, it is suitable to be used as a carbon source of composite catalysts.[44−46] The introduction of soluble starch into the precursor can reduce the sintering temperature, accelerate the diffusion rate, and even impel grain growth, which makes it easier to control and optimize the composition and structure of V-based oxides.[47] In this work, four proportions of V2O3@C composites with specific composition, structure, morphology, and properties were obtained by guided pyrolysis from different mass ratios (S/V) of 1:2, 1:1, 2:1, and 4:1. The structural characterization of V2O3@C composites (S/V) was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and N2 adsorption/desorption analysis, and the corresponding electrochemical performance and basic parameters were determined by different electrochemical testing techniques.

Results and Discussion

Material Characterization

Figure presents the details of diffraction angles of four V2O3@C (S/V) and the pure V2O3 at 32.9, 36.2, 37.8, 41.1, 43.6, 49.8, 54.0, 58.2, 63.6, 65.1, 71.0, 76.2, 78.3, 80.5, 82.0, and 86.21°, which can be indexed to the crystal facets of the pristine V2O3 in (104), (110), (006), (113), (202), (024), (116), (122), (214), (300), (101), (220), (306), (223), (312), and (134), respectively. The XRD peaks of V2O3@C (S/V = 1:2, 1:1) were similar to those of pure V2O3. With the increase in the proportion of soluble starch in the precursor, the prepared V2O3@C (S/V = 2:1, 4:1) showed an additional wide and weak pattern at around 24.9°, which is caused by the (002) facet diffraction of amorphous C,[48,49] and further demonstrated noncrystalline C incorporating with V2O3 as a carrier by soluble starch carbonized at high temperatures without affecting the diffraction patterns of V2O3. The strong diffraction patterns of V2O3 in the composites almost cover the broad and low diffraction patterns of amorphous C due to the highly ordered crystal structure of V2O3 in the composites.

Figure 1

XRD patterns of the four prepared V2O3@C composites (S/V) and pure V2O3.

The morphologies of four V2O3@C (S/V) and the as-prepared pure V2O3 were observed in Figure . The special characterization of pure V2O3 particles belongs to a solid block material with compact apparent density and less pores in Figure e. The shape of pure V2O3 was an irregular sphere with particle size in the range of 100–550 nm. The nanostructures of V2O3@C (1:2 and 1:1) all presented that the particle shapes were sphere, potato, ellipsoids, etc., with the particle size distribution in a relatively narrow range of 50–150 nm. Also, SEM results show that there is no excess amorphous C on the surface of V2O3@C (1:2, 1:1), as in Figure a,b, indicating that the soluble starch almost completely reacts with V2O5 to form V2O3 in the precursor. With further increase in the soluble starch mass ratio, V2O3@C (2:1, 4:1) obviously composed of two different components, including V2O3 and amorphous C, as observed from Figure c,d. The smaller flocculate C fills the gaps among V2O3 particles with uniform distribution in the analysis of SEM, that is, embodiment of amorphous substances. In particular, the V2O3 morphologies in V2O3@C (2:1) show low apparent density and open structure as nanoscale jars with a diameter of 50–100 nm and a wall thickness of about 15 nm, as shown in Figure c. The surface of V2O3@C (2:1) has several uniformly distributed open nanojars that would improve the effective surface area of the material particles, increase the number of catalytic sites, and greatly promote the electrocatalytic performance of CEs.[41] Simultaneously, there is moderate dual connectivity that consists of uninterrupted nanoparticles and interconnected pore channels in V2O3@C (2:1), which should be an alternative Pt material with high effective catalytic activity as CEs.[50,51] However, the excess carbonization of soluble starch in V2O3@C (4:1) results in that most of the surface of V2O3 was covered by smaller flocculate C particles (Figure d). The higher the amount of soluble starch, the smaller the V2O3 particle size, and the particle morphology becomes more and more spherical, which shows the isotropic growth with a narrower average particle size of <10 nm. This determination of morphology coincides with amorphous carbon in the V2O3@C composites by X-ray diffraction results. Energy-dispersive X-ray (EDX) analysis was further carried out to confirm the elements V, O, and C within V2O3@C (2:1), as in Figure f. EDX patterns of other CE composite materials showed similar results, but only the content of the elements was different.

Figure 2

(a–d) SEM images of V2O3@C with S/V = 1:2, 1:1, 2:1, 4:1 and (e) pure V2O3. (f) EDX pattern (2:1).

The N2 adsorption–desorption isotherms of V2O3@C (S/V) all exhibited narrow loops complying with a type-IV isotherm that indicate that four V2O3@C materials are mesoporous, and the pure V2O3 can be identified by an unapparent hysteresis loop belonging to a type-I characteristic of isotherms, as shown in Figure .[52] With the increase in the soluble starch mass ratio, the hysteresis loop integral areas of V2O3@C composites gradually increase. Table shows that the SBET values of S/V = 4:1 (350.2 m2/g) and 2:1 (181.7 m2/g) are higher than those of 1:1 (85.1 m2/g), 1:2 (45.5 m2/g), and V2O3 (33.5 m2/g); the Vp values follow an increasing order: 0.171 (2:1) > 0.116 (4:1) > 0.115 (1:1) > 0.040 (1:2) > 0.031 cm3/g (V2O3); the dp values of 3.368, 3.782, 3.798, 3.795, and 3.361 nm were obtained from S/V = 1:2, 1:1, 2:1, 4:1, and pure V2O3, respectively. These data indicate that the pure V2O3 was a solid bulk material with a lower Vp and a very small SBET, and the amorphous C incorporated with V2O3 could offer a larger Vp value for I3–/I– diffusion and more active edge sites for the regeneration of I3–/I–.[53] Because too much amorphous C covers the surface of the material, Vp and dp of V2O3@C (4:1) were smaller than those of V2O3@C (2:1). The porous structure of four V2O3@C composites is (S/V) also in good agreement with the SEM (Figure ) results.

Figure 3

(a) N2 adsorption–desorption isotherms and (b) dp of V2O3@C (S/V) and pure V2O3.

Table 1

Textural Properties of V2O3@C (S/V) and Pure V2O3

materials	SBET (m2/g)	Vp (cm3/g)	dp (nm)
S/V = 1:2	45.5	0.040	3.368
S/V = 1:1	85.1	0.115	3.782
S/V = 2:1	181.7	0.171	3.798
S/V = 4:1	350.2	0.116	3.795
V2O3	33.5	0.031	3.361

Electrochemical Properties of Electrodes

The redox reaction of I3– + 2e– → 3I– occurs on the interface between CEs and the electrolyte to recycle and regenerate the I3–/I– pairs of DSSCs at a low potential.[54] Two key parameters ΔEp (peak-to-peak separation) and IP (current density of cathodic peak) can be determined from the CV profile to evaluate the electrocatalytic activity of the electrode. Figure shows ΔEp values of 0.150 (1:2), 0.205 (1:1), 0.202 (2:1), and 0.229V (4:1) and IP values of 1.092 (1:2), 1.238 (1:1), 1.724 (2:1), and 1.636 (4:1) in V2O3@C (S/V) CEs. The smaller the ΔEp value, the larger the electrocatalytic ability and reversibility of the I3–/I– regeneration reactions, which would be beneficial to the enhancement of DSSC performance.[41] In theory, ΔEp ∝ 1/ks, where ks is the rate constant of electrochemical reaction.[55] Thus, smaller ΔEp values indicate that the reaction of I3– + 2e– → 3I– can be carried out at a rapid rate on CEs.[56] The larger IP can also demonstrate the catalytic material on CEs to more effectively catalyze the reduction of I3– to I–. Comparing the ΔEp and IP values of four kinds of V2O3@C (S/V) with Pt, the corresponding values of V2O3@C (2:1 and 4:1) are better than those of Pt CEs (0.151 V, 0.534 mA/cm2). Therein, in the V2O3@C composite (2:1), the open nanojar V2O3 incorporated with amorphous C and activated by high temperature and N-doping provided suitable dual connectivity and more vacancies and defects, which result in lower ΔEp and higher IP values.[57,58] The V2O3@C composite (2:1) as a Pt-free efficient CE would be favorable to enhance the electrocatalytic activity in DSSCs. For the expected catalyst, it is important to evaluate the long-term stability of the catalytic property. Herein, the V2O3@C (2:1) material as a CE catalyst was assessed by 30 consecutive CV cycles. There are minor IP attenuation and little redox peak position shift of I3– + 2e– → 3I–, which demonstrate that V2O3@C CEs possess stability as well as increased catalytic activity against I3–/I– redox couples.

Figure 4

CV profile of V2O3@C (S/V) and Pt CEs.

The characteristic electrochemical impedance spectroscopy (EIS) parameter values of Rct (charge transfer resistance), R (solution resistance), and Zw (Warburg impedance) were collected from the Nyquist plots of the symmetric cells with different V2O3@C (S/V) and Pt CEs in Figure . The increasing order of values of Rct is 28.81 Ω (1:2) > 14.8 Ω (Pt) > 10.72 Ω (1:1) > 7.10 Ω (4:1) > 6.08 Ω (2:1). Rs values of 41.71, 45.82, 46.38, 48.01, and 37.66 Ω and Zw values of 0.064, 0.050, 0.114, 0.103, and 0.126 Ω were obtained from S/V = 1:2, 1:1, 2:1, 4:1, and Pt CEs, respectively. The Rct values of V2O3@C (4:1, 2:1) are much smaller than those of other V2O3@C (S/V) and Pt. In the preparation process of CE composite catalyst materials, the carbonization of soluble starch with cross-linking and natural adhesion can result in the improvement of the dispersion uniformity of flocculate amorphous C in the precursor. Hence, V2O3 incorporating with amorphous C can very meaningfully provide that the continuous conductive layers in between the interface of V2O3@C composites transfer electrons and the interconnective pore channels permeate and diffuse I3–/I– electric pairs in the electrolyte.[50,59] Furthermore, the Warburg impedance (Zw) of V2O3@C (S/V) shows a similar 45° straight line, indicating that the reaction of I3– + 2e– → 3I– indicates the charge transfer process rather than the mass transport process.

Figure 5

Nyquist plots of V2O3@C (S/V) and Pt CEs for symmetrical cells.

The exchange current density (J0) of Tafel polarization was determined as four different V2O3@C (S/V) and Pt CEs in the reduction of I3– + 2e– → 3I–, as shown in Figure . A larger J0 value means that a small applied potential results in an appreciable increase in current.[60] Tafel polarization results show J0 values of four different V2O3@C and Pt CEs in the following order: 2:1 (0.669 mA/cm2) > 4:1 (0.529 mA/cm2) > Pt (0.501 mA/cm2) > 1:1 (0.482 mA/cm2) > 1:2 (0.294 mA/cm2). Particularly, V2O3@C (2:1) CEs show the largest slope of extrapolated straight lines in the Tafel zone of anodic and cathodic branches, which revealed the largest J0 compared to other V2O3@C (S/V) and Pt CEs. The Tafel polarization results indicate that V2O3@C composites (2:1, 4:1) possess a lower polarization potential and a quicker charge exchange rate on the interface between I3– and V2O3@C than that of Pt CEs in DSSCs. In conclusion, the excellent performance of V2O3@C composite (2:1) CEs can be attributed to the moderate dual connectivity with continuous nanoparticles and interconnected pore channels compared to other CEs.

Figure 6

Tafel curves of V2O3@C (S/V) and Pt CEs based on symmetrical cells.

Application of V2O3@C Composites in DSSCs

To make a direct comparison of performance with different V2O3@C (S/V) and Pt as CEs in encapsulated DSSCs, the J–V (photocurrent–photovoltage) curves were characterized under I = 100 mW/cm2 of a solar simulator (Figure ). Four different V2O3@C CEs presented Voc (open-circuit voltage) values of 665, 736, 738, and 747 mV, Jsc (short-circuit current density) values of 7.88, 10.31, 11.31, and 10.56 mA/cm2, fill factor (FF) values of 0.62, 0.63, 0.62, and 0.64, and PCE values of 3.59, 4.79, 5.15, and 5.06%, which were, respectively, obtained from S/V = 1:2, 1:1, 2:1, and 4:1, as shown in Table . Because of the same dye-sensitized commercial TiO2 photoanode used in the process of assembling DSSC devices, the performances of DSSCs only depend on the electrocatalytic reduction of redox couples at CEs, and different CEs also affect the photocurrent generation at the photoanode through dye regeneration. Comparing the parameters (Jsc, FF, Voc, PCE) of four V2O3@C (S/V) under the encapsulation of DSSC in the same way, the Jsc value of V2O3@C (2:1) is the largest and the four FF values are close to 0.63 ± 0.01. The enhanced Jsc value of V2O3@C (2:1) can be attributed to its open nanoscale jar morphology with the larger number of catalytic activity sites, continuity channel, and low Rct.[61]Voc is the difference between the Fermi level of the semiconducting material and the potential energy level of the redox potential in the liquid electrolyte.[62] It should be noted that the Voc values of V2O3@C (S/V) have a little enhancement with the increase in amorphous C. The variation of Voc in the different CEs for DSSCs can be influenced by the surface area of CEs. Despite the exposed geometric area of all V2O3@C composite CEs being 0.5 cm × 0.5 cm, the actual electrochemical active area is different due to the existence of amorphous C in different proportions. The highest value of Voc for V2O3@C composite (4:1) CE may be attributed to its much rougher surface with the maximum SBET compared to other CEs.[63] FF is the ratio of the maximum power of DSSCs to the product of Voc and Jsc. In the J–V curve of DSSCs, the more rectangular the output characteristics representing the level of PCE, the higher the FF. The FF value is closely related to the concentration gradients in DSSCs.[64] It can be seen that the PCEs of V2O3@C (1:1, 2:1, and 4:1) CEs are higher than 4.54% of Pt CEs (Jsc = 11.63 mA/cm2, Voc = 779 mV, and FF = 0.50). Although V2O3@C (2:1) CE has a lower Voc than V2O3@C (4:1), its overall efficiency is higher than that of V2O3@C (4:1) due to various combined factors. Therein, V2O3@C (2:1) CEs yielded the highest PCE (5.15%) in DSSCs. The remarkably enhanced Jsc value had a great contribution to the highest PCE of V2O3@C (2:1) as CEs. With the change of mass ratio (S/V) from 1:2 to 2:1, the PCE of the V2O3@C composites (S/V) gradually increases. However, on further increasing the S/V from 2:1 to 4:1, the PCE of V2O3@C (4:1) was lower than that of V2O3@C (2:1). The possible reason is that too much amorphous C in the V2O3@C (4:1) could lead to the decrease of the electron transfer rate in the puffy materials, and the O in the electrolyte could show a dark current between the photogenerated electrons and dye holes as well as V2O3@C (4:1) CEs with lower Jsc values.[65] The dark J–V curves revealed that DSSCs using V2O3@C (2:1) CEs present much lower dark current than those of DSSCs using other CEs, as in Figure b. That is to say, different degrees of current leakage occurred in DSSCs using V2O3@C composites. This result not only alludes to the suppression of the electron recombination at the CEs/electrolyte interface but also agrees well with the enhanced Jsc values of DSSCs. The J–V experimental results confirmed that the improved PCE and Jsc performances should be mainly attributed to the excellent electrocatalytic activity of V2O3@C composite catalysts by V2O3 incorporating an appropriate amount of amorphous C.

Figure 7

J–V characteristics of DSSC assembled with various CEs: (a) I = 100 mW/cm2 and (b) in the dark.

Table 2

Performances of DSSCs Assembled with V2O3@C (S/V) and Pt CEs

CEs	Voc (mV)	Jsc (mA/cm2)	FF	PCE (%)
S/V = 1:2	665	7.88	0.62	3.59
S/V = 1:1	736	10.31	0.63	4.79
S/V = 2:1	738	11.31	0.62	5.15
S/V = 4:1	747	10.56	0.64	5.06
Pt	779	11.63	0.50	4.54

According to the above-mentioned characterization results of CV, EIS, Tafel, and J–V, V2O3@C composite CEs as Pt-free catalysts can highly and effectively catalyze the reduction of I3– + 2e– → 3I– and further improve the PCE of DSSCs. In addition, it is very important that DSSCs should keep long stability along with maintaining high PCEs as competitive energy storage systems. As shown in Figure , a long-term stability test of the DSSCs using V2O3@C (2:1) CEs was performed after 500 h, and the photovoltaic parameters of Voc, Jsc, FF, and PCE retained 95, 96, 97, and 91% of their initial values, respectively. The high stability of DSSCs can be attributed to the fact that V2O3 incorporating with amorphous C possesses higher electrical conductivity and highest mechanical stability. The excellent stability of V2O3@C composite CEs as their intrinsic features makes them more suitable as Pt-free materials for low-cost and long-term-stable DSSCs.

Figure 8

Stability of V2O3@C composites (2:1) as CEs in DSSCs.

Conclusions

In various syntheses of V-based oxides, the choice of reactants and reaction process will directly affect the difference in the material properties. Thus, in this work, eco-friendly soluble starch was carbonized to amorphous C at high temperatures; the amorphous C incorporating with V2O3 to prepare V2O3@C composites can cause various volume defects, enhance the surface energy and adsorption, shorten the time of electron and hole migration to the particle surface, and improve the conductivity and catalysis. The DSSCs using V2O3@C composite (S/V) CEs with S/V = 1:1, 2:1, and 4:1, respectively, obtained PCEs of 4.79, 5.15, and 5.06% from J–V measurements, which were higher than the PCEs of 4.54% and 3.33% for Pt and pure V2O3 CEs, respectively, for the regeneration of I3–/I– redox couple. The corresponding electrochemical performance and basic parameters were determined by CV, EIS, Tafel, and J–V characterizations revealing that V2O3@C composite (S/V) CEs can catalyze the regeneration of I3–/I– higher than Pt CEs. Pt-free CEs are expected to be highly efficient and stable in the practical application of DSSCs as well as to expand the range of CE catalyst selection in the future.

Experimental Section

Materials

Vanadium pentoxide (V2O5, purity >99%) and hydrogen peroxide (H2O2, 10 wt %) were purchased from Shanghai Reagent Factory. The soluble starch and ammonium vanadate (NH4VO3, purity >99%) were purchased from Aladdin, Shanghai. All chemicals are of analytical reagent (AR) grade and used as received without purification. Deionized water was obtained from an electrothermal distiller and was used in all experiments.

Synthesis of V2O3@C Composites

V2O3 was directly prepared by the pyrolysis of NH4VO3 under high-purity N2 flow (50 sccm) at 1100 °C for 1 h. The precursor of V2O3@C composites (S/V) was prepared via a sol–gel method. First, 0.4 g of V2O5 was slowly dissolved in 30 mL of 10 wt % H2O2 solution and stirred evenly. With the violent reaction in the solution, a large number of bubbles emerge. After standing for 10 h, 0.2, 0.4, 0.8, and 1.6 g of the soluble starch were, respectively, added into the homogeneous mixed solution with vigorous stirring. The same four solutions were heated from ambient temperature to 80 °C until the soluble starch in each solution was completely dissolved. All of the four solutions gradually became a red flocculent sol with the volatilization of the solvent and finally turned into a dark green gel. After drying in a vacuum oven at 50 °C for 4 h, the as-precursor was milled for 0.5 h and further formed an 11.5 mm diameter and 10 mm thick tablet by a hydraulic tablet-press at 4 MPa. Then, the four proportions of V2O3@C composite (S/V) catalysts were synthesized via pyrolysis of the precursor under N2 flow (50 sccm) at 1100 °C for 1 h.

Assembly of DSSCs

The assembly of DSSCs is shown in the Supporting Information.

Characterization and Electrochemical Measurements

The characterization and electrochemical measurements of CEs are shown in the Supporting Information.