Cu₂O-Electrodeposited TiO₂ Photoelectrode for Integrated Solar Redox Flow Battery

Abstract

TiO₂ photoelectrode has become an attractive platform due to its excellent photoelectric performance and has been widely used in battery, photocatalysis, and other photoelectric fields. However, when the TiO₂ photoelectrode is used in solar flow batteries, the small photo-charging current is a potential problem, which will extend the charging process and lower the battery utilization efficiency. To address this issue, Cu₂O is introduced to the surface of the TiO₂ photoelectrode, and Cu₂O-TiO₂ forms a heterojunction to improve battery performance in this work. The formation mechanism of Cu₂O-TiO₂ is revealed and utilized to deposit Cu₂O on pre-treated FTO glass covered with TiO₂ films using electrochemical deposition (ECD). The photoelectrochemical properties of Cu₂O-TiO₂ photoelectrodes are characterized using XRD, UV-vis diffuse reflectance spectroscopy, XPS, and electrochemical characterizations. The successful deposition of Cu₂O on the surface of TiO₂ photoelectrode is confirmed, and the UV-vis spectroscopic test results show that the incorporation of Cu₂O enhances and broadens the absorption and utilization of sunlight in the UV range by the TiO₂ photoelectrode. Furthermore, the electrochemical test results manifest that the Cu₂O-TiO₂ photoelectrode possesses a higher carrier concentration under illumination conditions due to the formation of a heterojunction. Finally, a 30 min unbiased photocharging test demonstrates that the Cu₂O-TiO₂ photoelectrode charges in a current density of 425.03 μA·cm⁻², indicating an increased photogenerated carrier concentration and a decreased photogenerated carrier recombination rate, which results from the enlarged doping concentration and improved charge transfer process at the electrolyte/semiconductor interface due to the incorporation of Cu₂O. Compared with the current density of 116.21 μA·cm⁻² for the bare TiO₂ photoelectrode, the performance can be improved by over 365%.

1. Introduction

Currently, due to the emergence of problems such as resource shortages and environmental pollution, people are beginning to pay more and more attention to clean energy sources such as solar and wind power. As one of the solutions to future energy needs, solar energy is expected to replace fossil energy sources while maintaining the current standard of living due to its nearly unlimited energy reserves. However, due to the intermittent nature of solar energy, the use of solar energy needs to be accompanied by the construction of appropriate energy storage facilities. The emergence of redox flow batteries (RFBs) provides new ideas for the construction of energy storage facilities. Due to their adjustable capacity and power design, RFBs are very suitable for large-scale energy storage. Therefore, solar energy and redox flow batteries are combined to form solar redox flow battery, and redox flow batteries are used as energy storage devices to overcome the shortcomings of strong solar intermittency. However, the traditional solar redox flow battery is a split design, i.e., there are two separate systems for solar power generation and redox flow battery energy storage. Its energy conversion process is divided into three steps: solar energy, electrical energy, and chemical energy. Therefore, the traditional solar flow battery needs to be supported by the construction of solar-power-generation-related equipment, such as a maximum power point tracker and DC-DC converter. This undoubtedly increases the cost of system construction and reduces the reliability of the whole system. To address these shortcomings, a one-piece solar liquid flow battery configuration has been proposed [1,2,3,4,5,6,7], and the mechanism and design methodology of solar liquid flow batteries have been systematically investigated [8,9,10]. The Integrated solar flow battery has the advantage of in situ storage over the traditional configuration, i.e., the solar energy is directly converted into chemical energy for storage without the need of conversion into electrical energy in the middle. At the same time, the integrated design reduces many non-essential components, such as maximum power point tracking devices, which reduces the construction and utilization costs.

Among the many components of an integrated solar flow cell, the photoelectrode becomes the core component, and its proper selection can effectively enhance the performance of the cell. Among many photoelectrodes, oxide-based photoelectrodes [11] stand out due to their high abundance, low manufacturing cost, and excellent stability, and they have attracted much attention in various fields of photoelectrochemistry, such as the field of solar–hydrogen energy conversion [12,13,14], the field of lithium batteries [15], and the field of solar water splitting [16,17]. Among the many oxide-based photoelectrodes, the performance of Cu₂O photoelectrodes is comparable to that of many photovoltaic semiconductor-based photoelectrodes [18]. For this reason, the mechanism of heterojunction formation in Cu₂O photoelectrodes [19] has been investigated and various methods [20,21,22] have been used to construct Cu₂O to enhance the electrode performance. In the study of oxide-based photoelectrodes, in addition to Cu₂O, the doping of elements such as Cu²⁺ and Cr³⁺ [23,24,25,26,27] is also a more popular research direction. Low-temperature hydrolysis [28], a microwave-assisted hydrothermal method [29], and sol–gel method [30,31,32,33,34,35] have been used to dope metal ions to enhance battery performance. However, these photoelectrodes are mostly used in dye-sensitized solar cells (DSSCs) and hardly used in SRFBs. In addition to the study of oxide-based photoelectrodes, photoelectrodes of some other materials have also been utilized in the field of solar flow batteries. Hu et al. [36] used III-V materials as photoelectrodes for the cells, and although a good performance was obtained, the photoelectrodes made of this material were extremely susceptible to photocorrosion, which greatly reduced the lifetime of SRFBs. Tian et al. [37] used the MoS₂-doped photoelectrodes which are utilized in SRFBs with good results. Its solar output energy conversion efficiency (SOEE) can reach 4.78% in 1 M electrolyte, but it is still lower at 0.17% in 0.1 M electrolyte.

Taken together as described in the above materials, in this study, Cu₂O was deposited on fluorine-doped tin oxide (FTO) glass using an electrochemical deposition method at a lower cost to form FTO-Cu₂O photoelectrodes, which were characterized using electrochemical methods. Finally, full-cell tests were performed using TEMPO and VCl₃ as redox substances. Compared to similar studies, this study differs in that the FTO-Cu₂O photoelectrode was utilized in a non-aqueous SRFB. Although TEMPO and VCl₃ were used as the redox substances, Deep Eutectic Solvents (DESs) was used as the solvent, which can effectively inhibit the hydrogen precipitation reaction during operation, further reduce the cost of using SRFBs, and improve their performance.

2. Materials and Methods

2.1. Chemicals

Tetrabutyl titanate (C₁₆H₃₆O₄Ti), acetylacetone (CH₃COCH₂COCH₃), absolute ethanol (CH₃CH₂OH), cupric chloride dihydrate (CuCl₂·2H₂O) and polyethylene glycol (HO(CH₂CH₂O)_nH), hydrochloric acid (HCl), 2,2,6,6-tetramethylpiperidinyl-1-oxide(Tempo), choline chloride (C₅H₁₄ClNO), ethylene glycol (C₂H₆O₂), copper(II) sulfate pentahydrate (CuSO₄·5H₂O), vanadium trichloride (VCl₃), sodium hydroxide (NaOH), DL-lactic acid (C₃H₆O₃), deionized water (H₂O), FTO glass. All chemicals were analytical grade and used without further purification.

2.2. Preparation of TiO₂ Sol

The following steps were used to prepare the TiO₂ sol. First, the anhydrous ethanol and deionized water were mixed in ratio, and we used a magnetic stirrer to stir at a constant speed for 30 min to form a homogeneous solution, which was named solution A. Further, the tetrabutyl titanate solute, anhydrous ethanol solution, and acetylacetone were mixed, stirred for 30 min using a magnetic stirrer until a homogeneous solution formed, and it was named solution B. Then, concentrated hydrochloric acid (HCl) was added to a mixed solution of A and B and was stirred for 10 min in a magnetic stirrer. Polyethylene glycol was added to the mixed solution and placed in a magnetic stirrer for 30 min. The resulting mixed solution was sealed and left to age at room temperature for 24 h for use.

2.3. Preparation of Electrodes

The following steps were used to prepare the photoelectrode. First, the treated TiO₂ sol was coated with a homogenizer FTO glass (1 cm × 1.2 cm) and the FTO glass was placed in a drying box for 10 min (70 °C) for each coat for a total of five coatings. Then, the coated FTO glass was put into a muffle furnace (400 °C) to isolate the oxygen from calcination for 1 h. Finally, we prepared photoelectrodes covered with TiO₂ film. The prepared photoelectrode was sealed and stored away from light.

The preparation method of FTO-Cu₂O is described below. First, it was equipped with a solution containing 3 M lactic acid and 0.4 M CuSO₄ and named A. Then, 5 M NaOH was used to adjust the PH of solution A to 10, and this was named solution B. Next, in a three-electrode system, FTO glass (1 cm × 1.2 cm) was electrodeposited in solution B using the galvanostatic method. The current density at the time of electrodeposition was set to −1 mA/cm², the temperature was constant at 50 °C, and stirring occurred for 10 min using a stirrer to ensure that the stirring was uniform. The FTO-Cu₂O electrode was washed with deionized water several times and dried. Finally, the electrodeposited FTO glass was calcined in a muffle furnace (350 °C) for one hour in a nitrogen environment. Figure 1 shows the preparation process for photoelectrodes.

2.4. Preparation of Electrolyte

The following steps were used to prepare the electrolyte. First, choline chloride and ethylene glycol were mixed in a ratio of 1:2 to prepare a DES solution. Next, 0.1 M Tempo was dissolved into DES solution, and the positive electrolyte was obtained by stirring well. The negative electrolyte was obtained by dissolving 0.1 M VCl₃ solution into DES solution.

2.5. Photoelectrochemical Characterisation

Photoelectrochemical studies were conducted in a three-electrode electrochemical cell using FTO-TiO₂ or FTO-Cu₂O as a working electrode, and a saturated calomel electrode (SCE) and platinum sheet electrode as the reference and counter electrodes, respectively. In total, 0.1 M TEMPO in DES was used as the electrolyte. Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 10 mV·s⁻¹ in the voltage range of −0.5 V to 1 V vs. SCE. The illuminated area of the working electrode was 1.0 cm². The photoelectrode was irradiated by simulated solar light using an AM 1.5 G filter. The power density of the incident light was calibrated to 100 mW·cm⁻² using an optical power meter with a silicon photodetector as a certified reference.

3. Results

3.1. Half-Cell Testing

A three-electrode system was constructed using a platinum sheet clip electrode, a platinum sheet electrode, and a SCE, and the prepared FTO-TiO₂ and FTO-Cu₂O electrodes were tested for half-cell performance.

In order to test the photoresponsivity of the two photoelectrodes in a 0.1 M TEMPO electrolyte, a linear sweep voltammetry test was performed. The test results are shown in Figure 2. the FTO-Cu₂O electrode exhibited much higher performance than the FTO-TiO₂ electrode, capable of achieving a photovoltage of about 430 mV vs. SCE, which is about 30 mV higher than that of the FTO-TiO₂ electrode. The photocurrents of the two photoelectrodes increased with the increase in the applied voltage, but the trend of the increase was more obvious for FTO-Cu₂O. Therefore, it can be hypothesized that the FTO-Cu₂O photoelectrode produces significantly more holes per unit time than the FTO-TiO₂ electrode under the same electrolyte and light conditions. From this, it can be hypothesized that FTO-Cu₂O deserves better battery performance, which was also reflected in the subsequent experiments.

The instantaneous current profiles of the two photoelectrodes under different bias voltages are shown in Figure 3. In the test, five different gradients of voltage were applied to the two photoelectrodes; each gradient of voltage was 0 V, 0.2 V, 0.4 V, 0.6 V, and 0.8 V. Each voltage gradient lasted for two minutes. The final experimental data were obtained as shown in Figure 3. As can be seen from the data in the figure, the FTO-Cu₂O photoelectrode exhibits a superior current density compared to the FTO-TiO₂ photoelectrode at all five voltage gradients. This means that the FTO photoelectrode has more excellent electrical conductivity and oxidation ability due to the addition of Cu₂O, which may be attributed to the higher carrier concentration of the FTO photoelectrode due to the addition of Cu₂O, which results in a higher performance. The change in its carrier concentration can be derived from the Mott–Schottky curve (M-S curve) in Figure 4.

The Mott–Schottky technique was applied to examine changes in the electrochemical interfaces between the electrolyte and photoelectrode. Carrier densities and flat band potentials can be accurately measured using the Mott–Schottky equation for p-type and n-type semiconductors. The Mott–Schottky formula is shown below:

$$\frac{1}{C^2}=-\frac{2}{\epsilon {\epsilon }_{0}e{N}_A{A}^2}\left(V-V_{fb}-\frac{k_{B}T}{e}\right)$$

(1)

where C is the interfacial capacitance, ε is the dielectric constant of the semiconductor electrode, ε₀ is the vacuum permittivity (8.854 × 10⁻¹² F·m⁻¹), e is the electron charge (1.603 × 10⁻¹⁹ C), V_fb is the flat band potential, N_A is the carrier density, A is the electrode area, V is the applied voltage, k_B is the Boltzmann constant (1.38 × 10⁻²³ J·K⁻¹), and T is the absolute temperature. The carrier density of p-type Cuprous oxide (N_A) can be determined by plotting the tangent to the Mott–Schottky curve, while the flat band potential (V_fb) can be obtained from the intercept with the x-axis.

The M-S curve test results of FTO-TiO₂ and FTO-Cu₂O electrodes are shown in Figure 4. The flat band potential E_fb = 0.235 V vs. SCE of the FTO-Cu₂O electrode can be derived from Figure 3b. The carrier concentration of the electrode can be calculated by using Equation (2), where $N_D$ is the carrier concentration, $e_0$ is the fundamental charge, ${\epsilon }_{{TiO}_2}$ [38] is the relative dielectric constant of TiO₂, ${\epsilon }_0$ is the dielectric constant, and $m$ is the slope of the curve. According to the data presented in Figure 3b, the Cu₂O film exhibits P-type semiconductor properties with a negative slope and, therefore, the ε of Cu₂O is 7.6. Substituting the slope m (red line) obtained from Figure 3b into the formula gives the carrier concentration of the FTO-Cu₂O electrode in the TEMPO electrolyte, N_D = 5.808 × 10¹⁷ cm⁻². In comparison, the carrier concentration of FTO-TiO₂ is N_D = 1.5 × 10¹⁷ cm⁻². At the same time, the flat band potentials of the FTO-TiO₂ and FTO-Cu₂O electrodes are −0.31 V and 0.235 V, respectively. Since the flat band potential correlates with the driving force for separating electron–hole pairs in the space charge region, the flat band potential can predict the photoelectrochemical performance of the photoelectrode to some extent. For example, a shift of the flat band potential toward a positive potential indicates an improved electron transfer process as well as a reduced complexation rate of electron–hole pairs, which improves the performance of the battery. This further explains that the FTO-Cu₂O electrode has a superior performance compared to the FTO-TiO₂ electrode in the I-t curve test. That is, the FTO-Cu₂O electrode has a higher carrier concentration and a lower electron–hole pair recombination rate.

$$N_D=-\frac{2}{e_0{\epsilon }_{{TiO}_2}{\epsilon }_{0}m}$$

(2)

3.2. Full-Cell Testing

The excellent performance of the FTO-Cu₂O electrode was confirmed after half-cell testing of both photoelectrodes. In order to test its performance in a full-cell test, we tested the photo-charging performance of the FTO-Cu₂O electrode using an electrochemical workstation under standard sunlight (AM 1.5 G). The test results are shown in Figure 5. The temperature of this test was controlled at 35 °C to simulate the long-term operating temperature of a one-piece solar liquid flow battery. Meanwhile, TEMPO was used as the redox substance on the photoelectrode side, and V²⁺-V³⁺ was used as the redox substance on the other side. The experimental results show that the average charging current density of the FTO-Cu₂O electrode can reach 425.03 μA·cm⁻² under 30 min photo-charging conditions; however, the average current density of the FTO-TiO₂ electrode is only 116.21 μA·cm⁻². Electrochemical tests show that the photo-charging performance of the all-in-one solar liquid flow battery has been significantly improved, while the charging current did not undergo significant decay during a longer charging process. This coincides with the test results of the I-t curve and M-S curve in the previous section. That is to say, the carrier concentration of the FTO-Cu₂O electrode is higher than that of the FTO-TiO₂ electrode under the light condition, and due to the flat band potential it possesses, it is able to obtain more open-circuit voltage and an electron driving force, which also makes its matching ability with the redox pair stronger. In addition to this, the enhancement of the photocharging performance may also be attributed to the decrease in the rate of carrier complexation during charge transfer due to the addition of Cu₂O.

3.3. XRD Analysis

In order to preliminarily analyze whether the material deposited on the surface of the FTO-Cu₂O photoelectrode is Cu₂O or not, we performed X-ray diffraction (XRD) of the photoelectrode, as shown in Figure 6, for the surface structure test. The Cu₂O standard card (ICDD#34-1354) shows that the diffraction peaks at 2θ = 29.61°, 36.42°, 42.15°, and 72.9° correspond to the (110), (111), (200), and (311) crystal planes of Cu₂O. Meanwhile, the peaks appeared at 2θ = 26.23°, 33.37°, and 51.22° are the characteristic peaks of FTO glass. The results show that the electrodeposited Cu₂O films are polycrystalline in structure and are predominantly (111) crystalline. Therefore, comparison of the test results in Figure 6 with the standard card confirms that the material deposited on the surface of the FTO-Cu₂O photoelectrode is indeed Cu₂O.

However, the characteristic peak of CuO appeared simultaneously at 2θ = 42°, but with a lower peak. This suggests that some of the Cu₂O was oxidized to CuO during the testing process, which led to the appearance of the characteristic CuO peak. This explains the decrease in the charging current density of the battery after a long period of operation.

3.4. XPS Analysis

After the above half-cell and full-cell tests, the FTO-Cu₂O photoelectrode has a higher photocharging performance than the common TiO₂ electrode. In order to confirm the successful deposition of Cu₂O on the surface of the FTO electrode and to prove that the improved full-cell performance is caused by the presence of Cu₂O, we performed X-ray photoelectron spectroscopy (XPS) analysis on the FTO-Cu₂O photoelectrode. The results of the XPS test of the FTO-Cu₂O photoelectrode are shown in Figure 7. The yellow and blue lines in Figure 7 represent the XPS baselines of each corresponding element, respectively.

We corrected the peaks obtained (BE) for each element with the exotic carbon 1s peak (BE 284.8 eV) and fitted them using a hybrid Gauss–Lorentz curve. As shown in Figure 7b, our fitting of the Cu 2p dual peaks reveals that the binding energies of the Cu₂O film in the deposited state in the FTO-Cu₂O photoelectrode are 932.7 eV and 953.1 eV, which correspond to the binding energy states of Cu 2p_3/2 and Cu 2p_1/2, respectively. Therefore, it can be concluded that the bimodal peaks of the binding energy of Cu 2p on the FTO-Cu₂O photoelectrode correspond to metal Cu and Cu⁺, respectively.

Figure 7c shows the XPS test curve of O 1s, whose binding energy peak at 532.2 eV corresponds to the Ti-O bond in TiO₂.

3.5. UV-Vis Analysis

Figure 8 shows the visible light absorption recorded on the surface of FTO-TiO₂ and FTO-Cu₂O electrodes using ultraviolet–visible spectroscopy (UV-vis). The spectral resolution is 1 nm, and the FTO glass was used as a blank sample. The dotted line in the figure represents the tangential projection of plasma polaritons with absorbance value = 0. As can be seen in Figure 8, the FTO-TiO₂ photoelectrode can only absorb UV light below 350 nm; however, the addition of the Cu₂O heterojunction allows the photoelectrode to extend the absorption wavelength to about 400 nm, and even a small amount of absorption in the visible wavelength band. After tangentially processing the UV spectral data of the two electrodes, Equation (3) can be used to approximate the band gap of the photoelectrode to further explain the excellent performance of the FTO-Cu₂O electrode.

$$E=\frac{hc}{\lambda }$$

(3)

where E corresponds to the absorbed energy, h is the Plank’s constant (6.626 × 10⁻³⁶ J·s), c is the speed of light (3.8 × 10⁸ m·s⁻¹), and λ corresponds to the wavelength, expressed in meters. The band gap of the FTO-TiO₂ photoelectrode is calculated to be about 3.54 eV; however, the band gap of the FTO-Cu₂O photoelectrode is about 2.9 eV. It can be seen that the addition of Cu₂O reduces the band gap of TiO₂, thereby reducing the difficulty of electron transition. Furthermore, the FTO-Cu₂O photoelectrode with a heterojunction structure absorbs wave sunlight not only in the ultraviolet band but also has a certain absorption and utilization capacity in the visible light band. This is consistent with the results obtained in the UV-vis diagram.

However, although the solar light utilization range of the FTO-Cu₂O photoelectrode was extended to 400 nm under the effect of the Cu₂O heterojunction structure, there is still a gap with the performance of the Cu₂O/TiO₂/FTO photoelectrode obtained in the study of Felipe et al. [19], where the solar light absorption range of the photoelectrode could be extended up to 500 nm. This is due to the fact that the Cu₂O electrodeposited on the surface has poor stability and will be oxidized to CuO rapidly during use, thus reducing the performance of the electrode, which can be seen in the previous XRD test. Therefore, how to improve the stability of Cu₂O has become a focus and difficulty in the subsequent research.

4. Conclusions

In this study, Cu₂O was successfully deposited on the surface of FTO glass using an electrochemical deposition method, and the prepared photoelectrode was used in a one-piece solar liquid flow battery. The LSV, instantaneous current plots, and Mott–Schottky test results show that the FTO-Cu₂O photoelectrode possesses P-type semiconductor characteristics. At the same time, the FTO-Cu₂O photoelectrode possesses a more positive flat band potential compared to FTO-TiO₂, which allows it to obtain more holes under the light condition, thus increasing its carrier concentration. In addition, TiO₂, as an n-type semiconductor, forms a heterostructure type with Cu₂O, which reduces the electron–hole pair recombination rate of the photoelectrode and further improves the cell performance. The UV-vis test results showed that the FTO-Cu₂O photoelectrode has a wider absorption range and stronger absorption ability for sunlight in the UV range. The full-cell test results showed that the photo-charging current density of FTO-Cu₂O can reach 425.03 μA·cm⁻² under an unbiased condition, which is 365.74% higher compared with that of the FTO-TiO₂ electrode. Moreover, it can be stabilized at a high current density for a longer time. However, although the electrode can be stabilized for longer, the charging current density of the battery still gradually decreases as the operating time continues to increase, and the SOC value of the battery at this time is far from reaching 100%, which is presumed to be caused by the poor stability of the Cu₂O on the electrode surface, and part of the Cu₂O is oxidized to CuO. This conclusion can be confirmed in the XRD and XPS tests. Overall, this study provides a low-cost option for the selection of photoelectrodes for non-aqueous integrated solar redox flow batteries.