1. Introduction
Hydrogen peroxide (H
2O
2) has been found widespread application in disinfection, bleaching, and chemical synthesis, making it one of the 100 most important chemical compounds in the world [
1,
2]. However, the traditional anthraquinone process for producing H
2O
2 is often associated with environmental challenges, such as high energy consumption and potential generation of hazardous by-products [
3,
4]. Photocatalytic synthesis provides an environmentally friendly and more energetically efficient alternative for producing H
2O
2. During this process, utilizing sunlight as a source of energy, only O
2 and H
2O are required to promote the synthesis of H
2O
2 under normal temperature and pressure conditions, resulting in a reduction of the dependence on fossil fuels and minimizing the environmental impact [
5,
6,
7,
8].
The conventional particle photocatalysts were limited by the rapid recombination of photogenerated carriers, resulting in a slow synthesis rate of H
2O
2 [
9]. To address this issue, photoelectrocatalysis (PEC) systems were developed [
10,
11]. By adding an external applied bias, the photogenerated hole-electron pairs on the photoelectrode were effectively separated, thereby improving the H
2O
2 synthesis efficiency. A typical PEC cell usually contains a photoanode and a photocathode, with the anode and cathode chambers separated by a proton exchange membrane. The oxidation and reduction reactions take place in the photoanode and photocathode, respectively, offering greater selectivity to the reactions. Photocatalytic synthesis of H
2O
2 proceeds mainly through two pathways: water oxidation reaction (WOR) and oxygen reduction reaction (ORR) [
4,
10]. The WOR takes place on the photoanode, while the ORR occurs on the photocathode. For photoanodes, the primary materials used are metal oxides such as BVO [
12,
13,
14,
15], WO
3 [
16,
17], and TiO
2 [
18,
19]. Among these materials, BVO is considered one of the most promising materials for fabricating photoanodes due to its suitable band gap (~2.4 eV) and excellent photo-corrosion stability [
20,
21]. However, its performance for WOR was constrained by limited electrical conductivity and carrier diffusion length, as well as slow water oxidation kinetics [
22]. Therefore, surface modification of BVO, such as the creation of oxygen vacancies, is one of the key strategies to improve its PEC water oxidation activity [
20,
23]. On the other hand, studies on photocathodes are progressing at a slower pace, and a photocathodic material that possesses both high reaction efficiency and selectivity as well as high stability has yet to be identified.
A significant reason for the low rate of H
2O
2 synthesis on photocathodes is that ORR is a proton-coupled electron transfer process [
3]. The solubility of O
2 in aqueous solutions is very low, limiting the mass transfer efficiency and leading to ineffective synthesis of the critical intermediate *OOH (−0.05 V vs. RHE;
eq. 2), which results in a slow H
2O
2 production rate [
24,
25]. However, in the field of electrocatalysis, this problem has been effectively mitigated. A gas diffusion electrode (GDE), as a type of porous membrane electrode, facilitates the transport of a significant amount of gas to the interior of the electrode [
26,
27,
28,
29]. Upon contact with the electrolyte, a unique three-phase boundary system is formed. Utilizing this three-phase interface system can markedly enhance the rate of charge transfer from electrons to oxygen. Among the various GDEs, transition metal phthalocyanine (MPc) has been demonstrated to enhance ORR synthesis of H
2O
2 [
30,
31,
32,
33]. MPc was suitable as a material for the preparation of a GDE due to its low solubility in aqueous media and good thermal and chemical stability [
32,
34]. In addition, the catalytic performance of a GDE can be further optimized by changing the metal center of the phthalocyanine [
30].
In this study, a modified BiVO
4 photoanode was utilized in conjunction with an MPc-modified GDE to overcome mass transfer issues in ORR and enhance the synthesis efficiency of H
2O
2 through a tandem electrode strategy (). Upon irradiation of the BVO photoanode with visible light, the excitation of hole-electron pairs initiated the OER by generating holes. The generated electrons were directed towards the SnPc-GDE for the reduction of O
2 to generate H
2O
2. Under conditions without external bias, the generation rate of H
2O
2 was measured at 952.5 μM·L
−1·h
−1, with a Faradaic efficiency of 48.4%.
Scheme 1. A tandem electrode strategy for the synthesis of H2O2.
2. Experimental
2.1. Chemicals
Unless otherwise stated, all of the chemicals used in the study were of analytical grade and used without further purification. All solutions were prepared by using ultra-pure water. WCl
6, Bi(NO
3)
3·5H
2O, VO(acac)
2, Na
2SO
3, N,N-dimethylformamide (DMF), Triton X-100, and acetic acid of analytical grade were purchased from Wako Pure Chemical Industries, Ltd., Japan. Ni mesh (100 mesh, 99.9% purity, 0.1 mm in thickness) was purchased from Nilaco Corporation, Japan. Hydrophobic carbon (HS-100) was purchased from Denki Kagaku Kogyo K.K, Japan. Hydrophilic carbon (ECP600JD) was purchased from Lion Corp., Japan.
2.2. Fabrication of a WO3 Photoanode
The WO
3 photoelectrode was prepared on an F-doped SnO
2 conductive glass (FTO) substrate via spin coating. Typically, tungsten hexachloride (WCl
6) was dissolved in DMF, adjusted to a concentration of 504 mM, and subsequently applied to the FTO substrate by spin-coating (1000 rpm for 15 s), followed by calcination at 500 °C for 30 mins under an air atmosphere.
2.3. Fabrication of a BVO/WO3 Photoanode
An acetic acid solution of 0.04 M Bi(NO
3)
3·5H
2O and an aqueous solution of 0.04 M VO(acac)
2 were stirred for 24 h, and then the two solutions were mixed and stirred for another 2 h. The mixture solution was spin-coated (1000 rpm, 10 s) onto an WO
3 underlayer and then baked at 500 °C for 30 mins in a muffle furnace under an air atmosphere. By repeating this spin-coating and calcination process 6 times, BVO/WO
3 electrodes were prepared in which BVO thickness was 200 nm.
2.4. Fabrication of an N-Ovac-BVO/WO3 Photoanode
Oxygen vacancy-rich BVO (Ovac-BVO) photoanodes were prepared by the photoetching method. Typically, the BVO/WO
3 electrode was immersed in 0.5 M phosphate buffer solution containing 0.2 M Na
2SO
3 with a xenon lamp (AM 1.5) illumination for 10 mins. In order to further increase the oxygen vacancies on the surface, the BVO/WO
3 photoanode was calcined at 350 °C for 2 h under an N
2 atmosphere with a heating rate of 5 °C/min and an N
2 flow rate of 50 mL/min to obtain an N-Ovac-BVO/WO
3 photoanode.
2.5. GDE Preparation
A GDE was prepared from hydrophobic carbon, hydrophilic carbon and polytetrafluoroethylene (PTFE) by a molding process [
35]. The GDE, consisting of a hydrophobic layer and a hydrophilic layer loaded with an MPc catalyst, was formed on an Ni mesh (Figure S1).
Hydrophobic carbon, surfactant (Triton X-100) and ultra-pure water were mixed in a ratio of 1:1:30 by weight for the preparation of hydrophobic powder. PTFE (67 wt% relative to carbon black) was added and then further mixed for 5 min. The prepared hydrophobic powder was frozen overnight then thawed and filtered. Subsequently, the pellets were dried at 120 °C for 12 h and then heat-treated at 300 °C for 3 h in air.
For preparation of hydrophilic powder, 36 wt% hydrophilic carbon and 32 wt% MPc catalyst were stirred in 100 mL ultra-pure water containing 1 mL butanol for 2 h. Subsequently 32 wt% PTFE was added and further stirred for 1 h. The suspension was filtered and then dried at 120 °C for 12 h.
2.6. Characterization of Catalysts
The crystalline phases were characterized by using a powder X-ray diffraction (XRD) instrument (MiniFlex II, Rigaku Co., Japan) with CuKα (λ = 1.5418 Å) radiation (cathode voltage: 30 kV, current: 15 mA). Scanning electron microscopy (SEM) images of BVO and WO3 electrodes were obtained by a JEOL JSM-6701F field-emission scanning electron microscope. The absorption properties of the electrodes were measured using the diffuse reflection method with a UV-Vis spectrometer (UV-2600, Shimadzu Co., Japan) attached to an integral sphere at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Kratos AXIS Nova spectrometer (Shimadzu Co., Japan) with a monochromatic Al Kα X-ray source. The binding energy was calibrated by taking the C1s peak of contaminant carbon as a reference at 284.6 eV. The amount of H
2O
2 was determined by a colorimetric method using PACKTEST (WAK-H
2O
2, KYORITSU CHEMICAL- CHECK Lab., Corp., Japan) equipped with a digital PACKTEST spectrometer (ED723, GL Sciences Inc., USA).
3. Results and Discussion
3.1. Preparation and Characterization of N-Ovac-BVO/WO3 Photoanode
Both the BVO layer and WO
3 layer were prepared via the spin-coating method [
36,
37]. The surface of the BVO electrode was modified by the photoetching method and subsequently annealed under an N
2 atmosphere to obtain the oxygen vacancy-enriched N-O
vac-BVO electrode. The WO
3 layer was composited with the BVO layer to establish a built-in electric field, thereby enhancing charge separation and improving the efficiency of OER. As shown in the SEM images, the WO
3 layer displayed a porous, loosely packed planar morphology with a thickness of approximately 350 nm (a,c). The pores were filled after the BVO layer was coated with a thickness of ca. 200 nm and the surface of the electrode became smoother (b,c). Generally, a smoother surface of the photoelectrode correlated with improved charge transfer, leading to more satisfactory activity [
38]. XRD spectra was employed to confirm the physical phase components of the prepared materials. Typical diffraction peaks of WO
3 and BVO were observed in the XRD spectra (d) and consistent with the reference patterns of WO
3 (JCPDS No. 43-1035) and BVO (JCPDS No. 14-0688), respectively, indicating the high purity of the prepared thin-film electrodes. The light absorption of BVO and BVO/WO3 films prepared on FTO substrates was characterized by UV-Vis absorption spectroscopy. As illustrated in e, the absorption edge of BVO was observed at approximately 510 nm, which was consistent with reported values [
39]. After the modification of WO
3, the photoanode exhibited similar light absorption curves with a light absorption edge at 510 nm. Furthermore, an appreciable enhancement in light absorption was observed, which may contribute to enhanced photocatalytic activity.
. Top-view SEM images of (a) WO3 and (b) WO3/BVO photoanodes. (c) Cross-sectional SEM photograph of WO3/BVO photoanode. (d) XRD analysis of WO3, BVO, and WO3/BVO photoanodes. (e) UV-Vis spectrum of WO3/BVO photoanode.
The introduction of oxygen vacancies (Ov) has emerged as a highly effective approach to augment photocatalytic performance [
24]. The impact of reported oxygen defects on the photocatalyst includes: 1. an increase in the carrier concentration; 2. promotion of separation and transmission of the carrier flow; 3. adjustment of the structure of energy regulation; 4. regulation of the adsorption and activation of molecular oxygen; 5. enhancement of optical absorption. In this study, inspired by a prior work [
37], the construction of an oxygen-vacancy-enriched BiVO
4 (O
vac-BVO) photoanode was achieved by the photoetching method, and N-O
vac-BVO was obtained by further processing in an N
2 atmosphere for 2 h, which was confirmed by X-ray photoelectron spectroscopy (XPS) ().
. (a) O 1s, (b) Bi 4f, and (c) V 2p XPS spectra of BVO, N-Ovac-BVO and N-Ovac-BVO photoanodes. (d) EPR spectra of BVO and N-Ovac-BVO photoanodes.
In the XPS spectrum of O 1s, the peak corresponding to a lower binding energy (~529.4 eV) was assigned to the lattice oxygen (O
L2−), while the peak with higher binding energy (~531.3 eV) was ascribed to O
vac [
20]. After photoetching treatment, the relative abundance of O
vac increased from 10% to 16.1%. Following heating treatment in an N
2 atmosphere, the O
vac ratio further increased to 20.1%, proving that both photoetching treatment and heating treatment in an N
2 atmosphere could generate O
vac on the BVO electrode surface (a). Compared with BVO, the Bi 4f, V 2p and O 1s peaks of O
vac-BVO and N-O
vac-BVO simultaneously shifted in the direction of high binding energy (), which was consistent with the reported work [
20,
37]. The existence of O
vac was further proved by electron paramagnetic resonance (EPR) spectra. In the EPR spectrum of the N-O
vac-BVO photoanode, the signal corresponding to a g factor of 2.003 was significantly enhanced compared to that in the EPR spectrum of the BVO photoanode, which indicated a notable increase in the defect concentration of N-O
vac-BVO photoanode [
37].
a shows linear sweep voltammograms (LSVs) of WO
3, BVO, O
vac-BVO, N-O
vac-BVO, and N-O
vac-BVO/WO
3 photoanodes in CO
2-bubbling 1.0 M KHCO
3 aqueous solution (1 M, pH = 7.5) under visible light illumination (420–800 nm, 100 mW/cm
2). The LSVs indicated that the abundant oxygen vacancies effectively improve the carrier separation efficiency. Notably, after a WO
3 layer modification, not only was the photocurrent of the N-O
vac-BVO/WO
3 electrode improved, but the onset potential was also reduced from 0.57 V vs. RHE to 0.39 V vs. RHE. This may be attributed to the formation of the built-in electric field that promoted the transfer of electrons between the electrode surface and the electrolyte, thereby reducing the overpotential of the OER process. At a bias of 1.5 VRHE, the current-time (
i-t) curves for the aforementioned photoanodes were measured (Figure S3a). The experiments demonstrated that the N-O
vac-BVO/WO
3 photoanode exhibited the highest photocurrent density, aligning with the trends depicted in the
J-V curves. Notably, all photoanodes displayed low Faradaic efficiencies for H
2O
2 synthesis (Figure S3b), indicating that the photogenerated carriers predominantly do not engage in the two-electron pathway for H
2O
2 formation but rather proceed via the four-electron pathway to effect O
2 evolution. This modality benefits by facilitating an increased electron transfer to the photocathode. b shows the charge separation efficiency on the electrode surface with ηsurface values of 35.6%, 39.7%, 69.4% and 82.2% for BVO, O
vac-BVO, N-O
vac-BVO, and N-O
vac-BVO/WO
3 at 1.23 V vs. RHE, respectively.
Pt foil electrode was used as a reference to demonstrate the advantages of GDE in H
2O
2 synthesis. By adjusting the applied bias within the range from −0.5 to 1.5 V, it was found that the current density generated with SnPC-GDE cathode was significantly higher than that of the Pt foil electrode. Moreover, the generation rate of H
2O
2 was examined under the applied biases of −0.5 V and 1.5 V. After 3 hours testing, 2028.5 µM and 3404.4 µM of H
2O
2 were synthesized when SnPC-GDE cathode was used, which is approximately 13 times that of the Pt electrode under the same condition. Due to the unique three-phase interface of GDE, mass transfer between O
2 and the cathode is more efficient, thereby improving H
2O
2 generation rate [
34].
. (a) J-V curves of WO3, BVO, Ovac-BVO, N-Ovac-BVO, and N-Ovac-BVO/WO3 photoanodes. (b) Charge separation efficiency on the surface (ηsurface) of BVO, Ovac-BVO, N-Ovac-BVO, and N-Ovac-BVO/WO3 photoanodes. (c) Current density generated by SnPc-GDE and Pt foil in an air-bubbling phosphate buffer electrolyte (0.5 M, pH = 6.5) under different potential conditions with visible light irradiation (420–800 nm, 100 mW/cm2). (d) Amount of H2O2 generated by SnPc-GDE and Pt foil.
The GDE system effectively addressed the issue of limited O
2 solubility in aqueous solutions through its three-phase interfacial system, thus improving the efficiency of ORR. In the PEC system, the GDE was used as the working electrode, wherein the electrons transferred from the N-Ovac-BVO/WO
3 photoanode were utilized for the reduction of O
2, thereby facilitating the synthesis of H
2O
2 (Figure S2). Figure 4a illustrated the voltage distribution of the N-Ovac-BVO/WO
3 photoanode and SnPc-GDE under illumination and different applied biases from 0.5 V to −1.5 V (GDE vs N-Ovac-BVO/WO3). At 0.44 V
vs. RHE, the convergence of the two curves indicated that no external bias was applied between the N-O
vac-BVO/WO
3 photoanode and the SnPc-GDE under these conditions.
. (a) Distribution of applied bias on N-Ovac-BVO/WO3 and SnPc-GDE. (b) Amount of H2O2 generated by SnPc-GDE in an air-bubbling phosphate buffer electrolyte (0.5 M, pH = 6.5) under different potential conditions with visible light irradiation (420–800 nm, 100 mW/cm2). (c) i-t curves of CoPc, NiPc, SnPc and without MPc GDE cathodes under a potentiostatic condition at +0.44 VRHE. (d) Amount of H2O2 generated by the CoPc, NiPc, SnPc and without MPc GDE cathodes and corresponding Faraday efficiency values. (e) Five-hour stability test of the SnPc GDE cathode. (f) Amount of H2O2 generated by the SnPc GDE cathode and corresponding Faraday efficiency.
Inexpensive transition metals (Co, Ni, and Sn) are used as coordination centers for phthalocyanine. Transition metal atoms typically have incompletely filled d-orbitals, which enables them to form strong chemical bonds with reaction intermediates such as
∗O
2−,
∗OOH, and
∗OH, thus providing effective active sites [
40]. Different transition metal elements have different electronic configurations, oxidation states and coordination properties, which directly affect the catalytic performance of MPc [
32]. The activity and selectivity of phthalocyanine GDEs modified with Co, Ni, and Sn for ORR were compared under 0.44 V vs. RHE (c). Through the
i-t curves, it was found that the SnPc electrode has good stability, and the generation rate of H
2O
2 reached 952.5 μM·L
−1·h
−1, corresponding to a Faradaic efficiency of 48.4%. The reason Sn acts as a better active center may be due to the fact that SnN4 only requires a dissociation energy of 0.63 eV to break the O-O bond [
41]. The amount of H
2O
2 generation reached ca. 16288.3 μM·L
−1·h
−1 with a corresponding Faradaic efficiency of 97.9% under the potential of 0.2 VRHE, indicating that the current applied by the potentiostat was involved in ORR (b and Figure S4). To further test the stability of the electrodes, a prolonged reaction was performed. After 5 hours of reaction, the photocurrent decreased slightly, while the Faradaic efficiency remained stable (e,f).
4. Conclusions
In this work, a modified BiVO4 photoanode combined with an SnPc-GDE was utilized for the synthesis of H2O2. When visible light was irradiated on the BVO photoanode, the hole-electron pairs were excited and the OER was driven through the holes, and the excited electrons were transferred to the SnPc-GDE to reduce O2 for the synthesis of H2O2. Increase in oxygen vacancy was achieved by photoetching the BVO electrode, followed by annealing in an N2 atmosphere, culminating in the formation of the N-Ovac-BVO electrode. The enriched oxygen vacancies effectively promoted electron-hole pair separation and enhanced the charge separation efficiency on the electrode surface. The N-Ovac-BVO electrode was combined with WO3 to form a built-in electric field, which further promoted charge separation and reduced the onset potential of OER. The SnPc catalyst-modified GDE electrode demonstrated superior selectivity for ORR and maintained its stability throughout prolonged reaction cycles. Under unbiased conditions, the H2O2 production rate reached 952.5 μM·L−1·h−1 accompanied by a Faradaic efficiency of 48.4%.
Supplementary Materials
Summarize the supplementary information with the caption names in this section. The following supporting information can be found at: https://www.sciepublish.com/article/pii/177, Fig. S1: Photos of (a) WO3/BVO photoanode and (b) SnPc-GDE. (c) Schematic diagram of the structure of GDE. The bottom layer is the Ni network, the middle is the hydrophobic layer, and the top layer is the hydrophilic layer; Fig. S2: (a) Photo of the PEC system. (b) Schematic diagram of the PEC system; Fig. S3: (a) CA curves of WO3, BVO, Ovac-BVO, N-Ovac-BVO, and N-Ovac-BVO/WO3 photoanodes under the potential of 1.5 VRHE. (b) Amount of H2O2 generated by WO3, BVO, Ovac-BVO, N-Ovac-BVO, and N-Ovac-BVO/WO3 photoanodes in a CO2-bubbling KHCO3 electrolyte (1 M, pH = 7.6) under the potential of 1.5 VRHE with visible light irradiation (420–800 nm, 100 mW/cm2) and Pt electrode as a counter electrode; Fig. S4: Time courses of the photocurrents of SnPc-GDE in an air-bubbling phosphate buffer electrolyte (0.5 M, pH = 6.5) under different potential conditions with visible light irradiation (420–800 nm, 100 mW/cm2). Potential conditions: (a) 0.2 V vs. RHE, (b) 0.3 V vs. RHE, (c) 0.3 V vs. RHE and (d) 0.5 V vs. RHE; Table S1: Comparison of different photocathodes for H2O2 producing with current study.
Author Contributions
Conceptualization, C.C. and T.O.; Methodology, N.T.; Software, N.T.; Validation, N.T., C.C. and W.C.; Formal Analysis, C.C.; Investigation, N.T.; Resources, N.T.; Data Curation, N.Y.; Writing – Original Draft Preparation, C.C.; Writing – Review & Editing, Q.Z.; Visualization, C.C. and N.T.; Supervision, T.O.; Project Administration, T.O.; Funding Acquisition, T.O. and Q.Z.
Ethics Statement
Not applicable.
Informed Consent Statement
Not applicable.
Funding
This work was funded by the Guangdong Basic and Applied Basic Research Foundation (Grant Nos. 2024A1515010976 and 2020A1515010982) and Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for Scientific Research (B) (20H02847).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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