Designing Fe₂O₃-Ti as Photoanode in H-Type Double-Electrode Coupling Systems for Bidirectional Photocatalytic Production of H₂O₂

Abstract

Developing high-efficiency photoelectrodes plays an important role in the photoelectrocatalytic generation of hydrogen peroxide (H₂O₂) in the photoelectrochemical (PEC) water splitting field. In this work, an innovative strategy was proposed, the synergistic photocatalytic production of H₂O₂ using a bidirectional photoanode–photocathode coupling system under visible-light irradiation. Fe₂O₃-Ti, as the photoanode, which was built by way of Fe₂O₃ loaded on Ti-mesh using the hydrothermal-calcination method, was investigated in terms of the suitability of its properties for PEC H₂O₂ production after optimization of the bias voltage, the type of electrolyte solution, and the concentration of the electrolyte. Afterwards, a H-type double-electrode coupling system with an Fe₂O₃-Ti photoanode and a WO₃@Co₂SnO₄ photocathode was established for the bidirectional synergistic production of H₂O₂ under visible-light irradiation. The yield of H₂O₂ reached 919.56 μmol·L⁻¹·h⁻¹ in 2 h over −0.7 V with 1 mol·L⁻¹ of KHCO₃ as the anolyte and 0.1 mol·L⁻¹ Na₂SO₄ as the catholyte (pH = 3). It was inferred that H₂O₂ production on the WO₃@Co₂SnO₄ photocathode was in line with the 2e^- oxygen reduction reaction (ORR) principle, and on the Fe₂O₃-Ti photoanode was in line with the 2e^- water oxidation reaction (WOR) rule, or it was indirectly promoted by the electrolyte solution KHCO₃. This work provides an innovative idea and a reference for anode–cathode double coupling systems for the bidirectional production of H₂O₂.

1. Introduction

Since Fujishima and Honda developed photoelectrochemical (PEC) water splitting by using a TiO₂ photoanode in 1972, much focus has been placed on PEC as a promising technology [1]. The PEC technique for storing solar energy was deemed to be environmentally sustainable and economical, and is an efficient approach for producing hydrogen fuels by rearranging the chemical bonds of water [2,3,4,5,6]. For the PEC method, the most important challenge is to develop semiconductor photoelectrodes with high photoresponse capability and conversion capability. The photoelectrodes in PEC systems play an essential role in absorbing light to produce active charge carriers (photogenerated electron–hole pairs) for redox applications. To date, most strategies have concerned fabricating semiconductor photoelectrodes for producing and recovering H₂ or O₂, which has always been limited by additional bias [7,8,9]. H₂O₂, as the high-value-added oxidation product produced during water splitting, only generates H₂O or O₂ upon being utilized, and is widely applied in environmental purification, in selective organic conversion, as an energy source of fuel cells, as a cleaning agent, and so on [10,11,12]. However, little focus has been placed on the design of the photoelectrode for the PEC production of hydrogen peroxide (H₂O₂). The PEC technique is a high-efficiency way of preparing H₂O₂, mainly by way of a water oxidation reaction (WOR) and an oxygen reduction reaction (ORR) in the photoanode and photocathode. Therefore, it is valuable for creating a new type of photoelectrode to simultaneously produce H₂O₂ through water splitting.

In recent years, research on photoelectrodes for the production of H₂O₂ through PEC water splitting has been gradually increasing. BiVO₄ photoanodes and carbon cathodes have been found to simultaneously produce electricity and H₂O₂ under light driving [13]. A Cu₃BiS₃ photocathode was used via a PEC method through indirect two-electron ORR for the visible-light-driven synthesis of H₂O₂ [14]. Zemin Zhang et al. [15] employed a CuBi₂O₄-based photocathode to achieve H₂O₂ production of 192.9 μmol·L⁻¹. A core–shell MoSe₂/TiO₂ NRA photocathode was investigated for H₂O₂ production under ultraviolet light illumination, with a yield of 40 μmol·L⁻¹·h⁻¹ [16]. Two morphologically structured In₂O₃-coated BiVO₄ photoanodes were effectively enhanced through selectivity and produced H₂O₂ via PEC water oxidation [17]. Other semiconductor materials have been used for H₂O₂ production through virous kinds of methods, such as noble-metal-free electrocatalysts [18], N-defect-modified g-C₃N₄ [19], a Na-doped BiFeO₃ photocathode [20], a Cu⁺/Cu²⁺-substituted double perovskite Cs₂AgBiBr₆ photoanode [21], and so on. However, no attempts have been made to fabricate a bidirectional photocathode-and-photoanode coupling system for H₂O₂ evolution.

In this work, a H-type double-electrode coupling system, consisting of a WO₃@Co₂SnO₄ photocathode and an Fe₂O₃-Ti photoanode, was designed and investigated for the bidirectional photoelectrocatalytic production of H₂O₂. Firstly, Fe₂O₃-Ti was prepared with the optimization of procedures to discuss the performance of H₂O₂ as the photoanode. Following this, under the best conditions, an H-type bidirectional PEC coupling system was constructed to inspect the effectiveness of the system on the synergistic production of H₂O₂. Simultaneously, the PEC reaction mechanism of the bidirectional coupling system was also deduced.

2. Discussion

2.1. Properties Characterization

The micro morphology and structure of Ti-mesh and Fe₂O₃-Ti can be found in Figure 1a,b. Ti-mesh, as the photoanode substrate, displayed a network structure with a rough surface because of activation treatment. From Figure 1b, it can be seen that many short rod-like structures are attached to the base, which indicates that Fe₂O₃ was successfully loaded on the Ti-mesh.

The crystal structure and composition of Fe₂O₃ were evaluated by using XRD analysis (Figure 1c). The peaks at 2θ = 24.23°, 33.24°, 35.70°, 41.02°, 49.56°, 54.18°, 57.71°, 62.53°, and 64.04° correspond to the (012), (104), (110), (113), (024), (116), (018), (214), and (300) planes of Fe₂O₃ (JCPDS NO. 86-0550) [22], respectively. These peaks are typical characteristic diffraction peaks of α-Fe₂O₃. Moreover, no obvious impurity peak was detected in the XRD analysis.

The Raman spectrum of the Fe₂O₃ nanomaterial is displayed in Figure 1d, which is consistent with the literature [23]. The prominent peaks at 219 cm⁻¹ and 495 cm⁻¹ match with the A1g vibration mode, and those at 287 cm⁻¹, 409 cm⁻¹ and 606 cm⁻¹ belong to the E_g vibration mode. This further proves that the Fe₂O₃ nanomaterial as a photoanode material was successfully prepared.

X-ray photoelectron spectroscopy (XPS) was used to further identify the chemical compositions and electronic structures of the Fe₂O₃ nanomaterial [24,25]. The XPS spectra of Fe-2p in Figure 1e were deconvoluted further into four peaks located at 711.03, 719.28, 724.73, and 733.28 eV, assigned to Fe²⁺ 2p _3/2, Fe²⁺ 2p _1/2, Fe³⁺ 2p _3/2, and Fe³⁺ 2p _1/2, respectively. The O 1s XPS spectrum in Figure 1f demonstrates two peaks located at 530.03 and 531.73 eV, which can be assigned to the bonds of Fe-O and -OH. The XPS results are consistent with those of the XRD and Raman analyses.

The photochemical activity of Fe₂O₃ was estimated using UV-Vis absorption spectroscopy, as shown in Figure 2a. The Fe₂O₃ nanomaterial exhibited outstanding light absorption in the wavelength range of 200–750nm. According to the calculation of the Kubelka–Munk function equation, the band gap of Fe₂O₃ was approximately 2.0 eV (inset of Figure 2a). The flat potential and the semiconductor type of Fe₂O₃ were estimated using a Mott–Schottky plot (Figure 2b). The conduction band potential (ECB) for n-type semiconductors is very close to the flat-band potential (EFB) value. In general, the value of E_FB for the n-type semiconductor was higher than that of E_CB. That is, the flat-band potential was 0.1–0.3 eV higher than the ECB [26,27]. It can be estimated that Fe₂O₃ was an n-type semiconductor with a positive slope, and it can be confirmed that the flat-band potential (E_FB) was 0.15 eV (vs. RHE). Thus, the E_CB value of Fe₂O₃ was +0.05 eV. The approximate band structure of Fe₂O₃ was as depicted in the inset of Figure 2b, from which it can be deduced that the E_VB of Fe₂O₃ met the potential (1.76 eV) for the oxidation of H₂O to generate H₂O₂.

2.2. Photoelectrocatalytic Performance of H₂O₂ Production

When the anode was used for WOR, the formation of O₂ via 4e^- WOR competed with the production of H₂O₂ via 2e^- WOR. The potential for water oxidation to produce H₂O₂ was higher than that of O₂, which made it more difficult to achieve the selective generation of H₂O₂. Therefore, it is important to choose one appropriate semiconductor material as the anode so that E_g can meet the potential of 2e^- WOR. Because of these reasons, the linear scanning voltammetry (LSV) curves of different anode materials (Pt, WO₃, Co₃O₄, and Fe₂O₃) were investigated, and the current changes at different potentials were explored in detail. As shown in Figure 3a, with an increase in voltage, the current via different anodes gradually increased; that is, Fe₂O₃ > WO₃ > Co₃O₄ > Pt. From Figure 3a, Fe₂O₃, when used as the anode material, displayed the lowest onset potential and highest current density. This was the reason for choosing Fe₂O₃ as the photoanode material. Figure 3b shows the LSV curves of Fe₂O₃-C (Fe₂O₃ dripped onto carbon paper) and Fe₂O₃-Ti as the electrode. While the onset potential of Fe₂O₃-Ti, so far, was higher than that of Fe₂O₃-C, Fe₂O₃-Ti only needed 1.68 V, which was lower than the onset potential of Fe₂O₃-C (1.78V) when the current reached 10 mA. In other words, Fe₂O₃-Ti, when used as the electrode, required much lower power consumption, which meant it was more suitable for Fe₂O₃-Ti when used as the photoanode material in the whole application.

The anodic potentials, the type of electrolyte solution, and the concentration of electrolyte solution were all optimized, as shown in Figure 4. The photocurrents of the Fe₂O₃-Ti photoanode at different potentials are shown in Figure 4a; the photocurrent at 2.0V was 2–3 times higher than that at 1.6 V and 1.8 V. Meanwhile, the production of H₂O₂ with the Faraday efficiency at nearly 75.2%, at 2.0 V, was higher than that of the others (Figure 4b). Figure 4c shows that the yield of H₂O₂ using KHCO₃ as the electrolyte solution obviously surpassed that when using KOH as the electrolyte solution. And when the concentration of KHCO₃ was 1 mol·L⁻¹, the yield of H₂O₂ was higher compared to the other concentrations (0.1 mol·L⁻¹ and 2 mol·L⁻¹). Therefore, the optimal conditions for the production of H₂O₂ were 2.0 V as the anode potential and 1 mol·L⁻¹ of KHCO₃ as the electrolyte solution, which applied to the whole photoelectrocatalysis process.

Based on the above optimal conditions, the continuous production of H₂O₂ using the photoanode were investigated. It is clearly presented in Figure 5a that the cumulative concentration of H₂O₂ produced by the Fe₂O₃-Ti photoanode within two hours using 1 mol·L⁻¹ of KHCO₃ as the electrolyte solution was 110.27 μmol·L⁻¹. Figure 5b demonstrates that the cyclic stabilization of the Fe₂O₃-Ti photoanode was performed for 1h three times. After the third time, the yield of H₂O₂ only decreased by 6.8%, indicating the outstanding stability of the Fe₂O₃-Ti electrode as the photoanode material.

2.3. Synergistic Photocatalytic Production of H₂O₂ by Bidirectional Coupling System

On account of the satisfactory performance of WO₃@Co₂SnO₄ as the photocathode (which was discussed in the previous paper [22]) and Fe₂O₃-Ti as the photoanode, a bidirectional coupling system was established for the synergistic photocatalytic production of H₂O₂ within a H-type quartz cell. The cell structure was as follows: a WO₃@Co₂SnO₄ photocathode at one side of the H-type quartz cell with 0.1 mol·L⁻¹ Na₂SO₄ as the catholyte (pH = 3), an Fe₂O₃-Ti photoanode on the other side with 1 mol·L⁻¹ of KHCO₃ as the anolyte, and a Ag/AgCl electrode as the reference electrode on the side of the photocathode. A proton exchange membrane (PEM) was used to divide the anode and cathode. Before the photoelectrocatalytic reaction, O₂ gas was purged on both sides of the H-type reactor to ensure adsorption–desorption equilibrium.

As given in Figure 6a, different potentials, that is, −0.5 V, −0.6 V, −0.7 V, and −0.8 V, were inspected. When the applied voltage across the cell electrodes was at −0.7 V, the yield of H₂O₂ was the highest for both the WO₃@Co₂SnO₄ photocathode and the Fe₂O₃-Ti photoanode. Then, under a potential of −0.7 V, the synergistic photocatalytic production of H₂O₂ using a bidirectional cathode–anode coupling system was carried out for 2 h, as shown in Figure 6b, which shows that the concentration of H₂O₂ reached 1672.7 μmol·L⁻¹ at the WO₃@Co₂SnO₄ photocathode and 98.84 μmol·L⁻¹ at the Fe₂O₃-Ti photoanode. It is obvious that the Fe₂O₃-Ti photoanode and the WO₃@Co₂SnO₄ photocathode can be applied as an effective electrode couple for the production of H₂O₂.

According to the above results, the Synergistic photocatalytic reaction mechanism of the bidirectional coupling system was deduced. As demonstrated in Scheme 1, for the WO₃@Co₂SnO₄ photocathode, the production of H₂O₂ followed the 2e^- ORR process. The conduction band (CB) edge of WO₃@Co₂SnO₄ was around −0.52 eV [22], which was more negative than the potential of O₂/O₂^- (−0.33 eV). And its valance band (VB) edge was more positive than the potential of O₂/H₂O₂ (0.68 eV) and O₂^-/H₂O₂ (1.44 eV). This means the WO₃@Co₂SnO₄ photocathode absorbed the incident light to form photogenerated electron-hole pairs (e^--h⁺). The photogenerated e^- reacted with O₂ to generate H₂O₂ (R3) and also create a superoxide radical (O₂^-); then, O₂^- continued to produce H₂O₂ (R4). On the other hand, H₂O₂ production on the Fe₂O₃-Ti photoanode was inferred to follow the rule of 2e^- WOR, or was indirectly promoted by the electrolyte solution KHCO₃. As shown in Scheme 1, for the Fe₂O₃-Ti photoanode, under the irradiation of visible light, we used only water and photogenerated h⁺ to produce H₂O₂ (R6), because the VB of Fe₂O₃ (2.05 eV) was more positive than the potential of H₂O/H₂O₂ (1.76 eV). Meanwhile, based on previous studies [13,28,29], the HCO₃^- anion can act as an effective catalyst for H₂O₂ production. The HCO₃^- anion was oxidized by h⁺ to HCO₄^- (R7), which further reacted with H₂O to form H₂O₂ and HCO₃^- (R8).

The the conjectural reaction equations were as follows:

The photocathode:

WO₃@Co₂SnO₄ + hv → h⁺ + e^-

(1)

O₂ + e^- → ∙O₂^-

(2)

O₂+ 2H⁺ + 2e^- → H₂O₂

(3)

O₂^- + 2H⁺ + e^- → H₂O₂

(4)

The photoanode:

Fe₂O₃ + hv → h⁺ + e^-

(5)

2H₂O + 2h⁺ → H₂O₂ + 2H⁺

(6)

or

HCO₃^- + H₂O → HCO₄^- + 2H⁺ + 2e^-

(7)

HCO₄^- + H₂O → H₂O₂ + HCO₃^-

(8)

3. Materials and Methods

3.1. Preparation of WO₃@Co₂SnO₄ as Photocathode

The hydrothermal-calcination method was used for the preparation of the WO₃@Co₂SnO₄ heterojunction, and a detailed description of the preparation process of WO₃@Co₂SnO₄ was given in a previous paper [30]. Firstly, NaWO₄·2H₂O was dissolved in deionized water (DI) with 4 mL of HCl under stirring, and then, oxalic acid was introduced into the mixture to solubilize a yellow flocculent precipitate. After a hydrothermal reaction at 180 °C for 8 h, the as-prepared products were calcined under 500 °C for 2 h (2 °C·min⁻¹) to obtain WO₃ nanobricks. Secondly, a mixture of CoCl₂·6H₂O and SnCl₄·5H₂O, with a molar ratio of 2:1, was dissolved in DI, and NaOH (1 mol·min⁻¹) and WO₃ nanobricks were introduced under continuous stirring. Then, the dispersion was transferred to a Teflon-lined steel autoclave for a reaction at 180 °C for 10 h. After calcination at 500 °C for 2 h (2 °C·min⁻¹), the WO₃@Co₂SnO₄ was formed.

3.2. Preparation of Photoanode Substrate

The activation treatment for Ti-mesh was carried out according to the method described in [31]. Ti-mesh with dimensions of 2 × 3 cm² was impregnated with 0.1 mol·L⁻¹ of oxalic acid (C₂H₂O₄) solution at 80 °C for 30 min to remove the surface oxide. Then, the Ti-mesh was dried after rinsing with deionized water.

3.3. Preparation of Photocathode

WO₃@Co₂SnO₄ with a mass of 3 mg was dissolved in 0.5 mL deionized water ultrasonically and then dropped on activated carbon paper (2 × 3 cm²) [32] until it covered over two-thirds of the paper’s area. After it dried, WO₃@Co₂SnO₄ films to be used as as the photocathode were obtained.

3.4. Synthesis of Fe₂O₃-Ti as Photoanode

The pretreated Ti-mesh was added into 50 mL of deionized water that contained Fe (NO₃)₃·9H₂O (0.1 mol·L⁻¹) and 0.007mol·L⁻¹ of fumaric acid (C₄H₄O₄). We performed stirring for 30 min under ambient temperature; then, the mixture was poured into a Teflon-lined steel autoclave and heated constantly at 110 °C for 6h. After the reaction, the autoclave underwent natural cooling, and after centrifugation, the pre-product was thoroughly rinsed with alcohol three times and dried overnight at 70 °C, and then was calcined at 450 °C for 2 h (the heating rate was 2 °C·min⁻¹). Finally, Fe₂O₃-Ti was obtained to be used as the photoanode material, as exhibited in Scheme 2.

3.5. Characterization

The experimental instruments and equipment included a scanning electron microscope (SEM, HITACHI SU-8000, Japan), an X-ray diffractometer (XRD, Bruck D8 Siemens D5000, Germany), a Raman spectrometer (Renishaw, Wotton-under-Edge, UK), a UV-Vis diffuse reflectance spectrometer (DRS, Metash UV-8000S, BaSO₄ as the reflectance material), and a CHI760E electrochemical station (Shanghai Chenhua, China). X-ray photoelectron spectroscopy (XPS) was conducted through PHI5700 (USA).

3.6. Photocatalytic Production of H₂O₂ with Photoanode

The production of H₂O₂ was performed in a quartz reactor under 300 W Xe light (λ ≥ 420 nm) illumination under a 2V (vs. Ag/AgCl) bias voltage with a three-electrode system, that is, Fe₂O₃-Ti as the photoanode, Pt as the cathode, and a Ag/AgCl electrode as the reference electrode. The reaction was carried out with a condensing unit in 1 mol·L⁻¹ of KHCO₃ (20 mL) and 20 °C. Before switching on the power, O₂ was pumped into the electrolyte under dark conditions with stirring for 30 min to ensure adsorption–desorption equilibrium.

3.7. Bidirectional Catalytic Production of H₂O₂ by Coupling System

The bidirectional catalytic production of H2O2, whose coupling system used Fe₂O₃-Ti and WO₃@Co₂SnO₄ films as the photoanode and photocathode and a Ag/AgCl electrode as the reference electrode, was conducted in a H-type quartz reactor with 300 W Xe light (λ ≥ 420 nm) as the visible-light source. A bias voltage of −0.7 V (vs. Ag/AgCl) was set through a CHI760E electrochemical workstation. A total of 20 mL of KHCO₃ (1 mol·L⁻¹) and 20 mL of Na₂SO₄ (0.1 mol·L⁻¹) were used as the electrolyte solution for the anode and cathode, respectively. The pH value of the electrolyte solution was adjusted to 3 by using 0.5 mol·L⁻¹ HClO₄. Then, a condensation unit was installed on the coupling system to keep the temperature at 20 °C. Before switching on the power, magnetic stirring with pumping oxygen was performed for 30 min in the dark to achieve absorption–desorption equilibrium. During the reaction process, the concentration of H₂O₂ was determined using the potassium iodide method for the anode and the potassium titanium oxalate method for the cathode.

During the reaction process, the concentration of H₂O₂ was determined using the titration method; the potassium iodide method was used for the anode and the potassium titanium oxalate method for the cathode.

4. Conclusions

In summary, Fe₂O₃ with a short rod-like morphology was loaded on Ti-mesh by using the hydrothermal-calcination technique. Fe₂O₃-Ti, when used as the photoanode, achieved water oxidation of 110.27 μmol·L⁻¹ in two hours for the generation H₂O₂ under optimized conditions, including the bias voltage, type of the electrolyte solution, and concentration of the electrolyte. Then, the synergistic photocatalytic production of H₂O₂ was investigated using a bidirectional anode–cathode coupling system. A H-type electrolytic tank was established, with a WO₃@Co₂SnO₄ photocathode and an Fe₂O₃-Ti photoanode, to produce H₂O₂ through the bidirectional photoelectrocatalysis method. The synergistic yield of H₂O₂ over a −0.7 V potential reached 919.56 μmol·L⁻¹·h⁻¹. The PEC reaction mechanism of the bidirectional production of H₂O₂ using the double coupling system was discussed and inferred. This work provides an innovative approach for the bidirectional production of H₂O₂.