RESEARCH ARTICLE

Photoelectrocatalytic generation of H2 and S from toxic H2S by using a novel BiOI/WO3 nanoflake array photoanode

  • Jing BAI 1 ,
  • Bo ZHANG 1 ,
  • Jinhua LI 1 ,
  • Baoxue ZHOU , 2
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  • 1. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
  • 2. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200090, China; Key Laboratory of Thin Film and Microfabrication Technology, Shanghai Jiao Tong University, Shanghai 200240, China

Received date: 03 Apr 2021

Accepted date: 23 Jun 2021

Published date: 15 Sep 2021

Copyright

2021 Higher Education Press

Abstract

In this paper, a photoelectrocatalytic (PEC) recovery of toxic H2S into H2 and S system was proposed using a novel bismuth oxyiodide (BiOI)/ tungsten trioxide (WO3) nano-flake arrays (NFA) photoanode. The BiOI/WO3 NFA with a vertically aligned nanostructure were uniformly prepared on the conductive substrate via transformation of tungstate following an impregnating hydroxylation of BiI3. Compared to pure WO3 NFA, the BiOI/WO3 NFA promotes a significant increase of photocurrent by 200%. Owing to the excellent stability and photoactivity of the BiOI/WO3 NFA photoanode and I/I 3 catalytic system, the PEC system toward splitting of H2S totally converted S2– into S without any polysulfide ( Sx n) under solar-light irradiation. Moreover, H2 was simultaneously generated at a rate of about 0.867 mL/(h·cm). The proposed PEC H2S splitting system provides an efficient and sustainable route to recover H2 and S.

Cite this article

Jing BAI , Bo ZHANG , Jinhua LI , Baoxue ZHOU . Photoelectrocatalytic generation of H2 and S from toxic H2S by using a novel BiOI/WO3 nanoflake array photoanode[J]. Frontiers in Energy, 2021 , 15(3) : 744 -751 . DOI: 10.1007/s11708-021-0775-7

1 Introduction

It has become a global consensus to reduce greenhouse gas emissions to control climate change. However, achieving this goal requires a significant increase in clean energy to replace the use of fossil energy. Hydrogen, as a promising green energy in the past decades, has been widely used, but the production of hydrogen in a green and cheap way still poses certain challenges. Solar hydrogen production is considered to be the most environmentally friendly way to produce hydrogen. The development of various photocatalysts to directly split water to produce hydrogen has always been an important research goal of the majority of researchers. However, direct decomposition of water to produce hydrogen faces the dilemma that few photocatalyst own a proper band structure to meet water oxidation potential and good visible light absorption at the same time. In fact, some hydrogen-rich substances can replace water as proton donors, making photocatalytic hydrogen production more efficient. For example, H2S is an abundant chemical by-product from petroleum, natural gas, and coal gasification industries as well as naturally emanated from the putrefaction of organic substances or wastewater treatment plants, etc [13]. However, H2S is also a potentially valuable chemical because H2 and elemental sulfur S could be extracted [4]. Traditional processing techniques of H2S, such as the Claus process, could only obtain elemental sulfur. Therefore, the hydrogen contained in H2S is wasted. Actually, the thermodynamic decomposition of H2S is only 33 kJ/mol (ΔG0) within the oxidation capacity of most photoanode material [5]. Compared with the energy required to split water (ΔG0 = 273 kJ/mol), the energy required for decomposition of H2S is significantly lower. Therefore, developing an environmentally friendly, sustainable, and economical way of recovering H2 and S from H2S was highly reasonable and desirable.
Over the decade, semiconductor photoelectrocatalysis has attracted a lot of attentions because it facilitates the utilization of sunlight to produce renewable energy and treat pollutants through an eco-friendly route [610]. Owing to the lower oxidation potential of H2S, the photoelectrocatalytic (PEC) technology presents a potential to treat H2S and recover H2 and S simultaneously. Recently, tungsten trioxide (WO3) nanoplates have been reported as promising photoanode materials [11] and a PEC-H2S splitting system has been constructed based on the WO3 phtonaode [12]. Although WO3 has a remarkable catalytic activity, the efficiency of PEC-H2S splitting system is still limited because WO3 has a large band gap (about 3.0 eV) that hinders the absorption of sunlight in a small region (<450 nm). Furthermore, the photoinduced electrons and holes still face the rapid recombination that finally lowers the quantum efficiency. To overcome these limitations, using the semiconductors (BiVO4, g-C3N4, TiO2, CdWO4, Ag3PO4, etc.) coupled with WO3 to form a heterojunction has been approved to improve the electron transportation and thus photoactivity [1317]. However, these semiconductors have a relatively wide band gap (about 2.4–3.2 eV) which cannot remarkably enhance the absorption of visible light. Bismuth oxyiodide (BiOI) is a p-type semiconductor with a narrow band gap of 1.9 eV, which expands the absorption of visible light to 650 nm [1820]. As reported by Odling and Robertson [21], BiOI sensitized TiO2 films has a significant wider light absorption range from UV to visible light. Therefore, by combining the BiOI with WO3, the absorption of visible light could be greatly enhanced and most importantly, a p–n heterojunction is formed at WO3-BiOI interface, which builds an internal electric field for a better separation and lifetimes of photogenerated charge-carrier. Luo et al. [22] and Feng et al. [23] studied the WO3-BiOI nanoparticle photocatalyst for the promoted visible light photocatalytic activity, respectively. However, in a PEC H2S recovery system, a large-scale and economical preparation of BiOI-WO3 photoelectrode material is urgently required.
In this paper, a PEC recovery of toxic H2S into S and H2 system was proposed by using a novel BiOI/WO3 nano-flake arrays (NFA) photoanode combined with a I/I3 electrolyte. The vertically aligned BiOI/WO3 NFA films were prepared via a facile, controllable, and scalable method. In comparison with bare WO3 NFA, the photocurrent of the BiOI/WO3 NFA increased by 200%. In the designed PEC system, H2S was totally oxidized into element sulfur S without any polysulfide (Sxn) since I3 ions are gentle oxidizers [2426]. The evaluation of the system toward splitting of H2S under solar-light irradiation showed a good efficiency and stability during a long-term operation. High generation rates of H2 and S (up to about 0.867 mL/(h·cm) and 1.12 mg/(h·cm)), were obtained respectively. Overall, the PEC system could provide an efficient way to splitting H2S into H2 and S.

2 Experimental section

2.1 Preparation of WO3 NFA

The WO3 NFA photoanode was fabricated using a modified chemical bath method as previously reported [11]. First, 0.4 g of Na2WO4·2H2O and 0.15 g of (NH4)2C2O4 were dissolved in 33 mL of deionized water. Then, HCl (9 mL, w(HCl) = 37%, w is mass fraction), H2O2 (8 mL, w(H2O2) = 37%, w is mass fraction), and ethanol (30 mL) were subsequently added to this solution with a strong agitation for obtaining the precursor solution. Next, the fluorine-doped tin oxide (FTO) glass (13 Ω/cm; 40 mm × 60 mm) was dipped into the solution at a constant temperature and then cooled naturally. After that, the sample was annealed at 500°C in air for 2 h. Finally, the sample was cut into two 20 mm × 60 mm pieces, one for control and the other for the fabrication of BiOI/WO3 NFA.

2.2 Synthesis of BiOI/WO3 NFA photoanode

BiOI was deposited onto the WO3 NFA by using an impregnating hydroxylation method [27]. Typically, 0.94 g of BiI3 was mixed with 2 drops of HCl (w(HCl) = 35%, w is mass fraction) in 20 mL of absolute ethanol solutions. The WO3 NFA photoanode was then immersed into the prepared solution for 30 min and then dried in an oven at 80°C for 1 h. Subsequently, the WO3 NFA photoanode was immersed in distilled water, and BiOI/WO3 NFA photoanode was finally prepared by hydroxylation of BiI3.

2.3 Characterization

The surface morphology of the prepared samples was investigated by scanning electron microscope (SEM) using a microscope equipped with an energy-dispersive X-ray (EDX) spectrometer (Ultra Plus, Zeiss, Germany). An X-ray diffraction (XRD) were performed for the crystalline phases of the samples (Rigaku D-Max B). The composition of BiOI/WO3 NFA sample was analyzed with an ESCALAB250 X-ray photoelectron spectroscopy (XPS) measuring system. The UV-Vis spectra was conducted in a spectrophotometer (TU-1901, Pgeneral, China) using an integrating sphere.

2.4 Photoelectrochemical measurements

The photocurrent of the BiOI/WO3 NFA photoanode was measured in a three electrode system (the NFA as the working electrode, platinum foil as the counter electrode and the NFA as the working electrode and Ag/AgCl electrode as the reference). The electrolyte was a 0.1 mol/L Na2SO4 solution. The current-voltage characteristics of the samples were measured by Linear Sweep Voltammetry (LSV) at a scan rate of 50 mV/s (CHI 660c, CH Instruments Inc. USA). A 300 W xenon lamp (Perfect Light, China) was used as simulated light source (at a calibrated light intensity of 100 mW/cm2).

2.5 PEC splitting of H2S

Customized PEC cell with a proton membrane was applied for the H2S splitting. The nanostructured BiOI/WO3 NFA photoanode was put in the anodic chamber while the Pt photocathode in the cathodic chamber. The anodic compartment contained 100 mL of 0.1 mol/L Na2SO4 and certain amount of KI. The cathodic compartment contained 100 mL of 0.1 mol/L Na2SO4. Before operation, the pH of anodic solution and cathodic solution were adjusted to 1 (Fig. S7). H2S was injected into the anodic chamber following a filtration process for S collection. Meanwhile, H2 was measured by an online gas system (Bofeilai Technology) connected with a gas chromatograph (GC9790, Fuli, China).

3 Results and discussion

3.1 Characterization of BiOI/WO3 NFA

The top-view and cross-section SEM images of pure WO3 NFA and BiOI/WO3 NFA were investigated as shown in Fig. 1. As can be seen in Figs. 1(a) and 1(b), the WO3 film tightly adheres to the FTO substrate, showing a homogeneous flake-like morphology with a thickness of about 850 nm. The formation mechanism of WO3 NFA proposed here can be described as Eqs. (1)–(4): the precipitation of tungstic acid first forms peroxotungstates and then after further reduction uniform and ordered WO3·H2O films was formed. Finally, the WO3 film was obtained via a calcination. This direct growth strategy may benefit the smaller resistance between the WO3 film and the FTO substrate via eliminating their grain boundaries, thus resulting in a higher PEC performance.
Fig.1 SEM images of WO3 NFA and BiOI/WO3 NFA.

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WO4 2 + 2H+H2WO 4,
H2WO 4+ H2O2 peroxotungstates + H2O ,
peroxotungstates+ethanol +H+
WO3H2O+aldehyde,
WO3H2O WO3+ H2O .
After deposition of BiOI NFA, the morphology of the sample revealed that the BiOI NFA homogenously distributes both on the surface and interstice of WO3 NFA (Figs. 1(c) and 1(d)). The heterojunction film has a larger thickness and surface area. It can further load more BiOI on the WO3 surface but the performance is relatively decreased possibly because the thicker BiOI film ineffectively injects charges into the WO3 conduction band (Fig. S1).
Fig.2 XRD patterns of FTO, WO3, BiOI, and BiOI/WO3 electrode.

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The elemental composition of BiOI/WO3 was investigated via an EDX detection spectrometer. As demonstrated in Fig. S2, only W, O, Bi, and I elements appear in the sample. Figure 2 exhibits the XRD patterns of the as-synthesized BiOI, WO3, and BiOI/WO3, respectively. The sharp and intense diffraction peaks of the samples imply that all the photocatalysts are well crystallized. The XRD pattern of pure WO3 shows diffraction peaks at 23.1°, 23.5°, 24.3°, 28.6°, 33.5°, and 34.2°, which are in good agreement with the (002), (020), (200), (–112), (–202), and (202) diffraction planes of the orthorhombic crystalline phase of WO3 (JCPDS Card No.43-1035). It is worth noting that the peak for monoclinic WO3 (002) facet has a relatively low intensity, suggesting that the (002) direction is parallel to the substrate, whereas the (200) direction is vertical to the substrate owing to the intensified (200) diffraction peak. On the other hand, pure BiOI displays four distinct diffraction peaks at 29.6°, 31.6°, and 45.4°, corresponding to (102), (110), and (200) diffraction planes of the tetragonal phase of BiOI (JCPDS Card No.10-0445). Compared with pure the WO3 and BiOI film, the BiOI/WO3 film possesses all these peaks, implying the formation of the heterojunction. In addition, the additional peaks at 26.4°and 37.6° for all samples indicate the FTO substrate.
Fig.3 X-ray photoelectron spectroscopy survey scan for BiOI/WO3 NFA.

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To identify the valence states of the BiOI/WO3 sample, the XPS spectrum are illustrated in Fig. 3. The full spectrum reveals the presence of W, O, Bi, and I and no obvious impurities could be found on the surface of BiOI/WO3 NFA (Fig. 3(a)). The O 1s (Fig. 3(a)) at the peak of 531.0 eV is assigned to the lattice oxygen of the Bi–O or W–O chemical bonding in the BiOI or WO3 [23,28]. Figure 3(b) presents the narrow scans of Bi 4f, where two distinct peaks can be detected. The fitting peaks at 159.9 eV and 165.2 eV are assigned to Bi 4f7/2 and Bi 4f5/2 respectively, indicating the presence of Bi3+. Moreover, I is identified by the other two distinct peaks located at 631.1 eV and 619.8 eV in Fig. 3(c) which are assigned to binding energy of I 3d3/2 and I 3d5/2 respectively. The four peaks indicate the presence of Bi3+ and I in BiOI [29]. Accordingly, Fig. 3(d) shows two strong peaks at 35.9 eV and 37.9 eV, which are assigned to W 4f7/2 and W 4f5/2 in WO3, respectively [30]. The combined results of FE-SEM, XRD, and XPS convincingly prove the formation of BiOI/WO3 NFA heterojunction. Based on the UV-Vis absorption spectra in Fig. 4, the absorption band edge of the pure WO3 lies at below 475 nm owing to the intrinsic band gap of WO3, whereas BiOI/WO3 expands the absorption band edge to around 650 nm, presenting a stronger and broader absorption of the visible light. The band gap energy of WO3 and BiOI is estimated to be about 2.60 eV and 1.88 eV, respectively, which are in good agreement with those reported in Refs. [22,23]. (Fig. 4(b)).
Fig.4 UV-Vis absorption spectra.

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Fig.5 Photo-response and energy bands of BiOI/WO3NFA.

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Figure 5 presents the LSV characteristics of the WO3 and BiOI/WO3 under illumination of chopped AM 1.5 G, respectively. A comparison of the bare WO3 with a photocurrent density of 0.58 mA/cm2 (0.6 V versus Ag/AgCl) clearly reveals that the WO3 modified with BiOI shows a significant enhancement in photocurrent density of approximately 1.19 mA/cm2 (0.6 V versus Ag/AgCl), reaching about 2 times the photocurrent of the bare WO3 film. The semicircle radii (r) of the EIS Nyquist plot follow the order of r(WO3)>r(BiOI/WO3), characterizing the decrease of impedance after the modification of BiOI on WO3 (Fig. S3). The degradation of methylene blue (MB) using the BiOI/WO3 under visible light irradiation also demonstrates the enhanced photocatalytic activities (Fig. S4). Figure 5(b) shows the schematic diagram of energy bands between the BiOI and WO3 under visible-light irradiation. As can be seen, the highly efficient separation of photogenerated electrons and holes should be attributed to the internal electrostatic field in the p–n junction formed between BiOI and WO3, which restrains the recombination of photogenerated electrons and holes. An external electrostatic field can further enhance the hole transfer from the semiconductor to electrolyte as well as the electron diffusion to the back contact. Therefore, the synergetic effects of both the internal and external electric fields realize the highest photocurrent density.

3.2 PEC splitting of H2S

The BiOI/WO3 NFA was then used in the PEC generation of elemental hydrogen and sulfur from H2S. As is known, hydroxyl radical (E0 ≈ 3.2 V versus normal hydrogen electrode (NHE)) or photoholes (E0 = 2.81 V versus NHE) induced by most semiconductor photocatalysts commonly make direct oxidation of S2– into Sxn because of the high oxidation potential. Thus, only a clarifying Sxn solution with a typical yellow color can be obtained. To further confirm the existence of Sxn , a qualitative analysis was conducted through the reaction of the yellow clarifying solution with the hydrochloric acid. As can be seen in Fig. S5, the appearance of milky-white turbid demonstrates the formation of suspending S, indicating the existence of Sxn . In this case, a mild oxidant seems to be necessary in order to selectively convert S2– into S. Herein, I ion was introduced into the anodic chamber in the PEC cell where I ions can be first oxidized into I3 ions by BiOI/WO3 NFA. Since I3 ions only have an oxidation potential of 0.54 V (versus SHE), H2S was favorably oxidized into S and H+ (Eqs. (5)–(7)). Accordingly, I ions were recovered again for the next cycle.
Photoanode + hve+ h+,
3I+ 2h+I3,
I3+H2S 3I+2H++S.
The photocurrent of the system was first investigated in the existence of I ions. As displayed in Fig. 6, without addition of KI, the photocurrent gradually decreases from 1.2 mA/cm2 to 0.4 mA/cm2 during the 1000 s mainly due to the weak stability of BiOI. However, the photocurrent has a tendency to increase with the gradual increase of KI concentration and most importantly, at 200 s reaches a stable value of about 1.8 mA/cm2 at 0.2 mol/L KI. The significant enhancement in photocurrent and stability confirms that I ions promote the separation of electron-hole pairs and stabilize the BiOI in a long-term operation as well.
Fig.6 Photocurrent-time (I-t) curves of BiOI/WO3 NFA with different concentrations of KI. (Condition: bias potential 0.6 V, 0.1 mol/L Na2SO4)

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Figure 7(a) presents the schematic of the PEC H2S splitting system in which the BiOI/WO3 NFA photoanode and Pt cathode are assembled in the anodic chamber and the cathodic chamber respectively and separated with a proton exchange membrane (PEM). The device was illuminated with 100 mW/cm2 intensity and operated at a voltage of 0.6 V. During the operation, I ions was oxidized into I3 ions which further quickly reacted with S2– to transform the latter into elemental S. Meanwhile, H+ was reduced into H2 gas in the cathodic chamber. Due to the continuous transition of I/ I3 pairs, the H2S could be continuously transformed to H2 and S as H2S → H2 ↑ + S ↓.
Fig.7 PEC splitting of H2S for H2 and S recovery.

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Figure 7(b) depicts the photocurrent and the corresponding color change of the electrolyte when H2S is continuously injected into the anodic chamber. In the beginning, the photocurrent reached 1.8 mA/cm2 and the bubbles of H2 was observed in the cathodic chamber. Meanwhile, the color of the solution gradually changed from transparent to light yellow and then turned to bright red due to the formation of I3. Then, the red solution quickly changed to a nepheloid yellowish once H2S was injected into the anodic chamber (Fig. 7(b)). This phenomenon approved the formation of elemental S because of its bigger density and insolubilization in water. After a continuing filtration, the colorless and transparent solution was obtained again. After a long-term operation, much substantial S powder was collected. It is also found that the photocurrent was kept at 1.8 mA/cm2 except a little drop with the injection of H2S into solution. The ion ratio of I/I3 in the operation of the system gradually changed during the conversion of I ions to I3 ions at the initial time (Fig. S6). When H2S was introduced into the anodic chamber, H2S reacted with I3 ions which subsequently turned back to I ions, and S was produced simultaneously. The operation ran into the second cycle immediately (Fig. S5) and the concentration ratio between I and I3 presented a dynamic equilibrium.
Fig.8 Production of hydrogen and sulfur in a long-term operation.

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The H+ originated from H2S was transferred to the cathodic chamber and reduced to the hydrogen gas finally. Figure 8 shows the production rate of H2 and S after 7 h continuous collection. It can be seen that the production rate of H2 and S is kept at about 0.867 mL/(h·cm) and 1.12 mg/(h·cm), respectively. Moreover, the result also indicates that the PEC-H2S splitting system has a good stability and reusability (Fig. S8). Compared to the previous work [12], the production rate for both H2 and S is increased by almost 2.6 times.

4 Conclusions

A novel method to prepare visible-light response BiOI/WO3 NFA photoelectrode were demonstrated and successfully applied to PEC H2S splitting for H2 gas and sulfur generation. The BiOI/WO3 NFA was studied by SEM, XRD, XPS, and EDX to analyze its physical and chemical characterization. The photoelectrochemical measurement also indicated an improved photocurrent of 2.5 mA/cm2 compared to bare WO3 under visible light illumination. Based on the novel photoelectrode, a PEC-H2S splitting system was designed and showed an excellent performance of H2S recovery into H2 and S. The novel PEC-H2S splitting system offers a promising H2S recovery route for green energy generation.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (Nos. 2018YFE0122300 and 2018YFB1502001), Shanghai International Science and Technology Cooperation Fund Project (No. 18520744900), and the SJTU-AMED.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11708-021-0775-7 and is accessible for authorized users.
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