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

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^–/{\mathrm{I}}_{\mathrm{3}}^{-} catalytic system, the PEC system toward splitting of H₂S totally converted S^2– into S without any polysulfide ({\mathrm{S}}_x^{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.

Key words： bismuth oxyiodide (BiOI)/ tungsten trioxide (WO₃) nano-flake arrays (NFA); photoelectrocatalytic (PEC); H₂S splitting; H₂; S

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 [1–3]. 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 [6–10]. 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 [13–17]. 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 [18–20]. 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^–/{\mathrm{I}}_{\mathrm{3}}^{-} 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 ({\mathrm{S}}_x^{n-}) since {\mathrm{I}}_{\mathrm{3}}^{-} ions are gentle oxidizers [24–26]. 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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(1){\mathrm{WO}}_{\mathrm{4}}{}^{\mathrm{2}–}+{\mathrm{2H}}^{+}\rightleftharpoons {\mathrm{H}}_{\mathrm{2}}{\mathrm{WO}}_{\mathrm{4}}\downarrow ,

(2){\mathrm{H}}_{\mathrm{2}}{\mathrm{WO}}_{\mathrm{4}}+{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}\rightleftharpoons \mathrm{peroxotungstates}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O},

\mathrm{peroxotungstates}+\mathrm{ethanol}+{\mathrm{H}}^{+}\rightleftharpoons

(3){\mathrm{WO}}_{\mathrm{3}}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}\downarrow +\mathrm{aldehyde},

(4){\mathrm{WO}}_{\mathrm{3}}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to {\mathrm{WO}}_{\mathrm{3}}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}.

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 4f_7/2 and Bi 4f_5/2 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 3d_3/2 and I 3d_5/2 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 4f_7/2 and W 4f_5/2 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 S^2– into {\mathrm{S}}_x^{n–}because of the high oxidation potential. Thus, only a clarifying {\mathrm{S}}_x^{n-} solution with a typical yellow color can be obtained. To further confirm the existence of {\mathrm{S}}_x^{n-}, 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 {\mathrm{S}}_x^{n-}. In this case, a mild oxidant seems to be necessary in order to selectively convert S^2– into S. Herein, I^– ion was introduced into the anodic chamber in the PEC cell where I^– ions can be first oxidized into {\mathrm{I}}_{\mathrm{3}}^{-} ions by BiOI/WO₃ NFA. Since {\mathrm{I}}_{\mathrm{3}}^{-} 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.

(5)\mathrm{Photoanode}+\mathrm{}hv\to {\mathrm{e}}^{–}+{\mathrm{h}}^{+},

(6){\mathrm{3I}}^{–}+{\mathrm{2h}}^{+}\to {\mathrm{I}}_{\mathrm{3}}^{–},

(7){\mathrm{I}}_{\mathrm{3}}^{–}+{\mathrm{H}}_{\mathrm{2}}\mathrm{S}\to {\mathrm{3I}}^{–}+{\mathrm{2H}}^{+}+\mathrm{S}\downarrow .

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 {\mathrm{I}}_{\mathrm{3}}^{-} ions which further quickly reacted with S^2– 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^–/{\mathrm{I}}_{\mathrm{3}}^{-} 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 {\mathrm{I}}_{\mathrm{3}}^{-}. 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^–/{\mathrm{I}}_{\mathrm{3}}^{-} in the operation of the system gradually changed during the conversion of I^– ions to {\mathrm{I}}_{\mathrm{3}}^{-} ions at the initial time (Fig. S6). When H₂S was introduced into the anodic chamber, H₂S reacted with {\mathrm{I}}_{\mathrm{3}}^{-} 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 {\mathrm{I}}_{\mathrm{3}}^{-} 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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