α-Fe₂O₃/Reduced Graphene Oxide Composites as Cost-Effective Counter Electrode for Dye-Sensitized Solar Cells

Abstract

The counter electrode (CE) is an important and vital part of dye-sensitized solar cells (DSSCs). Pt CEs show high-performance in DSSCs using iodide-based electrolytes. However, the high cost of Pt CEs restricts their large-scale application in DSSCs and the development of Pt-free CE is expected. Here, α-Fe₂O₃/reduced graphene oxide (α-Fe₂O₃/RGO) composites are prepared as the Pt-free CE materials for DSSCs. A simple hydrothermal technique was used to disseminate the α-Fe₂O₃ solid nanoparticles uniformly throughout the RGO surface. The presence of the α-Fe₂O₃ nanoparticles increases the specific surface area of RGO and allows the composites to be porous, which improves the diffusion of liquid electrolyte into the CE material. Then, the electrocatalytic properties of CEs with α-Fe₂O₃/RGO, α-Fe₂O₃, RGO, and Pt materials are compared. The α-Fe₂O₃/RGO CE has a similar electrocatalytic performance to Pt CE, which is superior to those of the pure α-Fe₂O₃ and RGO CEs. After being fabricated as DSSCs, the current–voltage measurements reveal that the DSSC based on α-Fe₂O₃/RGO CE has a power conversion efficiency (PCE) of 6.12%, which is 88% that of Pt CE and much higher than that of pure α-Fe₂O₃ and pure RGO CEs. All the results show that this work describes a promising material for cost-effective, Pt-free CEs for DSSCs.

1. Introduction

Due to their low price, greater energy conversion efficiency, and easy manufacturing technique, dye-sensitized solar cells (DSSCs) have received much interest [1,2]. The choice of counter electrode (CE) material is different for different electrolytes [3]. CE electron transmission from the external circuit to iodine and triiodide (I⁻/I₃⁻) in the redox electrolyte is crucial for developing DSSCs [4]. On the CE of DSSCs, thin films of Pt are often utilized as catalysts. However, large-scale production is not possible because Pt is a precious metal and costly. As a result, various attempts have been made to determine potential alternative materials for replacing Pt CE in DSSCs, such as transition metal oxides, which are cheaper, more conductive, and more chemically stable. Among the transitional metal oxides, iron oxides have the features of low cost, no toxicity, elemental abundance [5,6]. Fe₂O₃ is an important iron oxide found in various forms, such as α-Fe₂O₃, β-Fe₂O₃, γ-Fe₂O₃, and ε-Fe₂O₃ [7]. Each has its own unique characteristics. For example, α-Fe₂O₃ has very good electrochemical activity and is the most stable phase of iron oxide under ambient conditions [8,9], which makes it a promising candidate for cost-effective CE materials for DSSC [10]. Moreover, α-Fe₂O₃ is an n-type indirect semiconductor that is able to utilize about 40% of sunlight. Miao et al. reported that flower-shaped α-Fe₂O₃ could be used as the photoanode of DSSC and achieved 1.24% power conversion efficiency (PCE) [11]. However, the poor conductivity of the α-Fe₂O₃ restricts its further development and application [7].

Graphene is also a suitable material for the CE of DSSCs due to its long-term stability, with atoms organized in close-packed conjugated hexagonal lattices similar to graphite, but a one-atom-thick sheet [12,13]. Graphene has attracted considerable interest for energy convention owing to its superior electroconductibility, chemical stability, large specific area, and broad electrochemical window [14,15]. Graphene CEs possess a large surface area, defects, and oxygen-containing groups (such as reduced graphene oxide, RGO), suggesting the possibility of achieving comparable electrochemical performances to Pt CEs. [16] However, the poor electrocatalytic activity of RGO limits its application in DSSCs as a CE material.

Above all, the synergistic effect arising from interactions between α-Fe₂O₃ and RGO is essential because the presence of RGO can increase the electrical conductivity of α-Fe₂O₃ as well as α-Fe₂O₃ improving the electrocatalytic activity and reducing the cost of the CE materials. Furthermore, previous work found that the open spaces caused by α-Fe₂O₃ between graphene nanosheets may mitigate the effect of RGO volume change [17], which is beneficial to the electron transfer process in catalysis. Therefore, the composites based on α-Fe₂O₃ and RGO (α-Fe₂O₃/RGO) are expected to take advantage of the structural characteristics to yield improved CE performance in DSSCs. Chen et al. reported that a DSSC with 3D α-Fe₂O₃/GFs as the CE material displayed a superior PCE to that of Pt due to the positive synergistic effects of α-Fe₂O₃ and the 3D graphene frameworks (GFs) [18]. However, the synthesis process of the reported α-Fe₂O₃/GF materials not only requires a long time hydrothermal treatment, but also requires a high-temperature annealing process in Ar₂, which limits its large-scale application in DSSCs. Zhao et al. demonstrated 3D α-Fe₂O₃ hollow meso-microspheres on graphene sheets by applying a solvothermal strategy [19]. The synthesis process involves two hydrothermal treatments at 150 °C for hours. This structure delivers decent electrocatalytic performance in a dye-sensitized solar cell that is comparable to that of Pt. Employing α-Fe₂O₃, which has distinct morphology, as well as optimizing the energy and cost can further advance its potential in high-performance catalysts. In previous research, graphene oxide (GO) and Fe(OH)₃ were used to synthesize the α-Fe₂O₃/RGO composites by a facile and cost-effective process [20]. The Fe(OH)₃ sol contributed to the homogeneous size and excellent dispersity of α-Fe₂O₃ solid nanoparticles on RGO. The concentration of the composite was 73% α-Fe₂O₃/RGO, with the best performance exhibited when the volume ratio between the GO solution and the Fe(OH)₃ sol was 2:1.

Here, in this work, α-Fe₂O₃/RGO composites are developed to be used as a cost-effective CE in a DSSC system and the performances of DSSCs with α-Fe₂O₃/RGO, α-Fe₂O₃, RGO, and Pt CEs are compared under the same conditions. A facile synthesis process is used to develop α-Fe₂O₃ solid nanoparticles on RGO with outstanding homogeneity and dispersion by employing Fe(OH)₃ and a GO sol. As a CE material for DSSCs, the α-Fe₂O₃/RGO composites have a larger specific surface area than pure RGO, which induces the composites to have better electrocatalytic activity. The PCE of the DSSC using α-Fe₂O₃/RGO CE is increased by 30.5% compared to the pure α-Fe₂O₃ CE (4.69%), which itself is much higher than pure RGO (98.7% increase). This work presents a promising route for a cost-effective production way for CE materials for DSSCs.

2. Results and Discussion

By comparing the X-ray diffraction (XRD) patterns of α-Fe₂O₃/RGO composites, α-Fe₂O₃, RGO, and GO, it can seen that GO has been transformed to RGO (Figure 1a). Pure and composite α-Fe₂O₃ particles are all highly crystalline, which agrees with the reference (PDF#89-0597). The diffraction peaks at 2θ of 24.1°, 33.1°, 35.6°, 40.8°, 49.4°, 54.0°, 62.4°, and 64.0° correspond to (012), (104), (110), (113), (024), (116), (214), and (300). There are no significant variations in the XRD data of pure α-Fe₂O₃ and α-Fe₂O₃/RGO, except that the α-Fe₂O₃/RGO has a diffraction peak at 2θ = 25.6°, which corresponds to the graphene crystal faces (d-spacing of 3.35 Å) [21].

Figure 1b shows the Raman spectra of the α-Fe₂O₃/RGO, α-Fe₂O₃, RGO, and GO samples. The D and G bands reveal typical peaks at 1350 cm⁻¹ and 1589 cm⁻¹ in all three graphene-related compounds. α-Fe₂O₃/RGO and RGO have higher I_D/I_G values than GO, indicating that GO has been reduced to RGO [22]. GO has an I_D/I_G ratio of 0.95, while α-Fe₂O₃/RGO has the most significant ratio at 1.57. Two peaks are presented for α-Fe₂O₃/RGO and pure α-Fe₂O₃ at 221 cm⁻¹ and 285 cm⁻¹, which correspond to hematite’s standard A_1g and E_g Raman modes, respectively, while the peak for pure α-Fe₂O₃ at 1299 cm⁻¹ is due to two magnetic oscillator scattering of hematite [23]. From the XRD and Raman result, the RGO sheets are probably attached by α-Fe₂O₃ nanoparticles.

FE-SEM was utilized to examine the morphologies of the α-Fe₂O₃/RGO, α-Fe₂O₃, and RGO materials fabricated as films that were used as CEs in DSSCs. Figure 2 presents a top view of films of different materials on FTO at the same magnification. As shown in Figure 2a, the pure α-Fe₂O₃ nanoparticles exhibit uniformly regular solid shape with a size of about 20–50 nm. The pure RGO has a typical wrinkled and folded structure as observed in Figure 2b. Figure 2c illustrates the SEM image of the α-Fe₂O₃/RGO composites film. The graphene sheets are evenly coated with uniform α-Fe₂O₃ particles on both sides and the α-Fe₂O₃/RGO film has a rough surface and many more pores than pure RGO. This variation in shape indicates that the strong force between GO sheets and Fe³⁺ has a significant impact on the crystalline growth of α-Fe₂O₃ nanoparticles [24].

TEM studies of the α-Fe₂O₃/RGO composites were performed to define their microstructure further (Figure 3). Figure 3a shows that α-Fe₂O₃ solid nanoparticles ranging in size from 20 to 50 nm are evenly dispersed over RGO. This result also reveals efficient assembly of the α-Fe₂O₃ particles and graphene sheets during the hydrothermal treatment. The 0.22 nm lattice spacing in the (113) plane identifies the α-Fe₂O₃ particles (Figure 3b) [25], matching with the XRD data. The three images (Figure 3d–f) of the targeted region (Figure 3c) illustrate the TEM elemental mapping findings of the α-Fe₂O₃/RGO composites. Figure 3d demonstrates that carbon (C) atoms are numerous in the composites, but iron (Fe) and oxygen (O) atoms are scarce in Figure 3e,f. However, all three images demonstrate the distribution of C, Fe, and O elements in the α-Fe₂O₃/RGO composites are highly homogeneous.

Figure 4a depicts the N₂ adsorption–desorption isotherms of the α-Fe₂O₃/RGO composites. The sample showed a curve pattern between Type IV (BDDT classification) that displays hysteresis loops predominantly of type H3 in the adsorption isotherms in Figure 4a [26]. The BET specific surface area of the α-Fe₂O₃/RGO samples are measured to be 136.16 m² g⁻¹. According to the previous study [4], the addition of α-Fe₂O₃ provides a larger specific surface area for the composites (the specific area of RGO prepared by the same synthesis method is 32.2 m² g⁻¹). In addition, obvious pressure hysteresis can be observed in the N₂ adsorption–desorption isotherms, revealing the porous nature of the α-Fe₂O₃/RGO nanostructure. This can be further identified by the BJH pore size distribution analysis which is shown in Figure 4b. The total pore volume of α-Fe₂O₃/RGO composites are 0.244 cm³ g⁻¹ according to the BJH method. The α-Fe₂O₃/RGO composites have a narrow pore size distribution centered at 3.7 nm, while the RGO sample was suggested in the before work to be a structure without large number of holes. The incorporation of α-Fe₂O₃ particles enhanced the specific surface area of α-Fe₂O₃/RGO composites with a porous architecture. The abovementioned results are also consistent with the result in Figure 2. Liquid electrolyte may more readily permeate into this porous nanostructure, which significantly improves electrocatalytic performance.

To evaluate the electrocatalytic activities of as-prepared α-Fe₂O₃/RGO, α-Fe₂O₃, RGO, and Pt, three-electrode cyclic voltammetry (CV) was performed (Figure 5). As previously reported, a representative curve with two couples of redox peaks was found for Pt. The I and I’ peaks correspond to the oxidation and reduction peaks of I₃⁻/I₂, whereas the II and II’ peaks correspond to those of I⁻/I₃⁻ [27]. During the electrochemical process in a DSSC, it is crucial that electrons from CE reduce I₃⁻ to I⁻. Therefore, the II and II’ peaks represent the electro-catalytic capabilities of the CEs. By analyzing the reduction peaks of all the CEs, we can see that the α-Fe₂O₃/RGO and Pt have sharper II’ peaks, demonstrating that their catalytic activities are considerably higher than others. It is well known that peak-to-peak separation and peak current density are two essential metrics for assessing the electrocatalytic activities of CE [28]. The magnitude of peak current density is correlated to the capacity of the CE to decrease the I₃⁻ species. Compared with pure RGO and α-Fe₂O₃ CEs, the α-Fe₂O₃/RGO CE exhibits a greater peak current density (Figure 5), indicating that it has stronger electrocatalytic activity and a faster response rate.

To further examine the liquid electrolyte diffusion rate into the material and interfacial charge transfer properties of the triiodide/iodide pair on the electrode surface, Tafel polarization experiments were performed in a virtual device made with two duplicate electrodes. Figure 6 depicts the logarithmic current density (log J₀) versus voltage (U) during the oxidation/reduction of triiodide to iodide. While the I₃⁻ is converted to I⁻ in the electrochemical cell, the impedance to charge transfer is inversely related to the exchange current density (J₀). Using the Equation (1), this may be computed from the intersection of the tangent line of the polarization curve and the prolongation of the linear section to zero bias.

J₀ = RT/nFR_ct

(1)

where R and F are constant, T is the room temperature, n is the amount of electrons participating in the reaction. The limiting diffusion exchange current density (J_lim) can also be computed from the Tafel curve using the Equation (2).

D = l J_lim/2nFC

(2)

where D is the diffusion coefficient of the triiodide, l is the thickness of the spacer, C is the triiodide concentration and n and F retain their defined meanings. J_lim depends on the diffusion rate of the I⁻/I₃⁻ redox couple. An optimal auxiliary counter electrode should have high J₀, J_lim, and lower R_ct values.

Theoretically, the curve with a relatively low potential but higher than 0.1 V corresponds to the Tafel zone, in which the voltage is a linear function of the log of the current density (log J₀) (Equation (1)). It is possible to determine the exchange current density (J₀) in this region using Equation (1). In addition, the steeper the Tafel zone of the curve, the bigger the J₀ and the greater the material’s catalytic activity. Pt has the greatest J₀ in the Tafel zone, followed by the α-Fe₂O₃/RGO, α-Fe₂O₃, and RGO. This reveals the α-Fe₂O₃/RGO composite CE has more electrocatalytic activity when compared to the pure α-Fe₂O₃ and RGO CEs. The Tafel curves of the α-Fe₂O₃/RGO, α-Fe₂O₃, and RGO Ces are asymmetric may due to the different diffusion time of the electrolyte on two electrodes of symmetric dummy cells. To test the Tafel curve, the electrolyte was firstly dripped on one electrode (always the cathode), and then covered with another electrode on the electrolyte. This process causes the electrolyte to diffuse first on the cathode, resulting in asymmetry between the two electrodes in the Tafel curve. For this reason, the Pt electrode is also slightly asymmetrical. However, the asymmetric degree of the Pt CE is lighter because of the better catalytic ability.

To investigate the effects of the α-Fe₂O₃/RGO composites as a CE material for DSSC, the DSSCs were assembled using α-Fe₂O₃/RGO as the CE. For comparison, the properties of DSSCs fabricated with α-Fe₂O₃, RGO, and Pt Ces were also investigated. Five parallel devices for each sample were tested. The photovoltaic properties results of the four different DSSCs are summarized in

Table 1. PCE performances and EIS parameters of the DSSCs with different CEs.

CEs	Rct	jsc	Voc	FF	η
	(Ω)	(mA cm−2)	(mV)		(%)
Pt	1.78	16.33	635	0.67	6.93
		±0.06	±0	±0.00	±0.05
α-Fe2O3	5.42	13.3	635	0.55	4.69
		±0.05	±5	±0.01	±0.03
RGO	11.02	12.47	555	0.45	3.08
		±0.05	±5	±0.01	±0.05
α-Fe2O3/RGO	3.81	15.43	645	0.61	6.12
		±0.00	±0	±0.00	±0.004

. The short-circuit photocurrent (j_sc), open-circuit voltage (V_oc), fill factor (FF), and the power conversion efficiency (PCE, η) calculated photovoltaic parameters are provided. Due to its weak electrocatalytic activity, the DSSC with RGO CE has a poor transfer efficiency, of 3.08%, whereas the DSSC with α-Fe₂O₃/RGO CE has an open-circuit voltage of 645 mV, a short-circuit current of 15.43 mA cm⁻², a fill factor of 0.61, and a cell efficiency of 6.12%, which is 88% of that of the DSSC with Pt CE (6.93%). In comparison to the α-Fe₂O₃ and RGO Ces, the α-Fe₂O₃/RGO CE has a substantially higher FF value. The photocurrent–voltage (I–V) curves of the DSSCs are shown in Figure 7a.

Figure 7b illustrates the resulting Nyquist plots from the EIS measurements performed on the DSSCs using the CEs above to obtain insight into the variation in FF values. The spectra were simulated using the matching circuit shown in the inset of Figure 7b, which contains high-frequency series resistance (Rs). The interfacial charge transfer resistance R_ct and the capacitance of the electrical double layer (CPE1) are related to the RC processes of the CE/electrolyte interface in the intermediate frequency area. In addition, the interfacial charge transfer resistance (R_R) and capacitance of the depletion layer of the TiO₂ electrode (CPE2) are related to the RC processes of the TiO₂ electrode/electrolyte interface in the low frequency range. The electrocatalytic property of the CE for triiodide reduction may be assessed, which is defined from the first hemicycle of the EIS spectrum [29]. According to Table 1, the α-Fe₂O₃ CE exhibits a greater R_ct, suggesting weak electrocatalytic performance (as also proven by CV outcomes). When the RGO and the α-Fe₂O₃ are combined, the R_ct decreases from 5.42 Ω to 3.81 Ω. The R_ct value of the α-Fe₂O₃/RGO composites are much smaller than that of the RGO CE (11.02 Ω). This may be attributable to the integration of α-Fe₂O₃’s intrinsic high electrocatalytic activity on the RGO’s highly active electric transport channel [30]. Nonetheless, the α-Fe₂O₃/RGO CE has a substantially bigger sum of R_s and R_ct than the Pt CE, leading to a lower FF and η values for the photovoltaic properties of the DSSCs [31].

3. Materials and Methods

3.1. Synthesis of Materials

First, a solution was prepared by adding 1.3 g FeCl₃ (Aldrich, 98%, Shanghai, China) in 4 mL distilled water. Second, 50 mL distilled water was boiled. Third, the as-prepared solution (0.5 mL) was added into the boiling distilled water dropwise. Then, the mixture was kept boiling for several minutes to prepare the Fe(OH)₃ sol. GO was manufactured from modified graphite oxide (GO), produced from natural scale graphite using a revised Hummers method technique [32,33]. In a typical synthesis, Fe(OH)₃ sol was added dropwise to the GO solution (1 mg mL⁻¹) at a volume ratio of 1:2 and then stirred for 30 min. Next, the mixture was heated to 85 °C in a water bath and hydrazine hydrate (Aldrich, 85%, Shanghai, China) was added into the mixture. Then, an ultrasonication process was applied for 30 min. After that, the solution was transferred into a Teflon-lined autoclave and heated at 150 °C for 6 h. The comparison mixture was prepared by centrifugation; α-Fe₂O₃ and RGO were prepared using the same technique, but without GO and FeCl₃, respectively.

3.2. Fabrication of CEs and DSSCs

The CEs were fabricated by a doctor blade technique [34]. In one batch, films of a certain thickness were made. Comparatively, platinized CEs were made by a thermal breakdown. The chloro-platinic acid hexahydrate (H₂PtCl₆·6H₂O, Pt ≥ 37.5%, AR, Aladdin, Shanghai, China) was mixed with isopropanol (C₃H₈O, ≥99.5%, AR, Aladdin, Shanghai, China) and the mixture was dropped on the cleaned fluorine-doped tin oxide (FTO) glass sheets (Nippon Sheet Glass Co., Osaka, Japan, surface resistance = 15 Ω/cm², transmittance = 90%) and then heated for 15 min at 390 °C.

TiO₂ was synthesized by using a sol–gel method [35]. Then, the TiO₂ films (ca. 11 μm) were coated on the FTO glass using the doctor blade technique. The FTO glass coated with TiO₂ was heated at 450 °C for 30 min. When the temperature dropped to around 90 °C, the electrodes were submerged for 24 h in a dye solution containing 0.5 mM cis-Ru (H₂dcbpy)₂ (NCS)₂ (H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl) (N3) dissolved in ethanol. After this, the TiO₂ photoanodes are complete. The DSSCs were constructed using the aforementioned TiO₂ photoanode, a CE, and a redox electrolyte comprising 0.5 M LiI (AR, Alfa, Zhengzhou, China), 0.05 M I₂ (AR, Alfa), 0.6 M 4-tert-butylpyridine (TBP, >96%, Tokyo Chemical Industry Co., Ltd. Tokyo, Japan), and 0.6 M 3-hexyl-1-methylimidazolium iodide (HMII, AR, Alfa, Zhengzhou, China) in 3-methoxypropionitrile (MPN, 99%, GC, Alfa, Zhengzhou, China). The tested DSSCs were masked to a working area of 0.2 cm².

3.3. Characterization

XRD (Cu Kαirradiation, D8-ADVANCE, Billerica, MA, USA) was used to analyze the crystalline structure and state of the samples. At room temperature, Raman spectra were acquired using a Raman spectroscope meter (Horiba LabRam HR Evolution, Tokyo, Japan) equipped with a 633 nm laser. Field emission scanning electron microscopy (FE-SEM, Gemini 300, Oberkochen, Germany) was used to examine the sample morphology (FE-SEM, Gemini 300, Oberkochen, Germany). The crystal structure was measured by high-resolution transmission electron microscopy (JEM-F200, Akishima, Japan). The specific sur face area and total pore volume of samples were determined by the Brunauer–Emmett–Teller (BET) method, in which the N₂ adsorption at −195.8 °C was measured using an adsorption instrument (ASAP 2460, Norcross, GA, USA). The pore size distribution was estimated based on the desorption isotherm using the Barrett–Joyner–Halender (BJH) method [26].

Utilizing an electrochemical analyzer, cyclic voltammetry (CV) was performed in a three-electrode setup in an acetonitrile solution containing 0.1 M LiClO₄, 10 mM LiI, and 1 mM I₂ at a scan rate of 100 mV s⁻¹ (Solartron SI 1287, Illinois, IL, USA). The Ag/Ag⁺ combination acted as the comparison electrode, while platinum served as the counter electrode. The spectra were fitted by the Zview software. Using an electrochemical workstation system (Solartron SI 1287, Illinois, IL, USA) in symmetric dummy cells built with two identical CEs and a scan speed of 50 mV S⁻¹, Tafel polarization studies were performed. The photocurrent density–voltage (j_sc-V) performance of the DSSCs was measured with a Keithley digital source meter (Keithley 2410, Cleveland, OH, USA) and simulated under AM 1.5 illumination (100 mW cm⁻², Newport 69907). The incident light was calibrated with a power meter (model 350) and a detector (model 262). Electrochemical impedance spectroscopy (EIS) was performed on the Solartron SI 1260 frequency response analyzer and Solartron SI 1287 electrochemical interface system. The frequency range was 0.1–100 kHz and the AC voltage was 10 mV.

4. Conclusions

In this work, we demonstrated a novel Pt-free CE made of α-Fe₂O₃/RGO composites for DSSCs. The α-Fe₂O₃/RGO composites were proven to be synthesized successfully via a facile method as the α-Fe₂O₃ nanoparticles were found to be dispersed over the RGO surface. Compared with pure α-Fe₂O₃ and RGO, the α-Fe₂O₃/RGO composites have a larger specific surface area and a more porous microstructure, which provides an advantage to catalytic reactions. The α-Fe₂O₃/RGO CE was prepared on an FTO glass-substrate via the doctor blade process. Comprehensive electrochemical investigations revealed that the α-Fe₂O₃/RGO CE exhibited Pt-like electrocatalytic activity for I₃⁻ reduction owing to the synergistic effect between the inherently high catalytic performance of α-Fe₂O₃ and the easy charge transfer properties of RGO. The PCE of the DSSCs constructed with the α-Fe₂O₃/RGO CE was 6.12%, up to 88% of that achieved with the Pt CE. Therefore, the α-Fe₂O₃/RGO CE is a promising candidate for application as a cost-effective CE material in Pt-free transparent DSSCs. This work presents a solution that can easily prepare large-scale DSSCs with low cost, which is expected to improve the possibility of commercialization of DSSCs in the future.