High-efficiency photocathode for green hydrogen generation from sanitation water using bismuthyl chloride/poly-o-chlorobenzeneamine nanocomposite

2.1 Materials

Dimethylformamide (99.9%) was obtained from Sigma Aldrich, USA. Hydrochloric acid (HCl, 36%) and O-chlorobenzene amine (99.9%) were acquired from Merck Co., Germany. Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) and NH₄)₂S₂O₈ are sourced from Pio-Chem Co., Egypt.

2.2 BiOCl/POCBA core–shell nanocomposite photocathode fabrication

The fabrication of the BiOCl/POCBA core–shell nanocomposite photocathode involves synthesizing the nanocomposite as a thin film, which is then utilized as a photocathode for hydrogen (H₂) gas evolution. This synthesis is performed using a one-pot technique.

In this process, O-chlorobenzene amine monomer (0.06 M) is dissolved in 0.8 M HCl in a separate beaker. Subsequently, 0.06 M bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) is added to the solution. The mixture is vigorously stirred to ensure that the materials dissolve homogeneously. In another beaker, 50 mL of ammonium persulfate ((NH₄)₂S₂O₈) is dissolved in distilled water.

The oxidant solution is then suddenly added to the first beaker containing the dissolved monomer and bismuth nitrate. This rapid addition leads to the formation of the BiOCl/POCBA core–shell nanocomposite via the one-pot synthesis method. For the thin film formation, a glass slide is inserted into the mixture, resulting in the deposition of a highly uniform thin film of the core–shell nanocomposite on the glass slide. Following this deposition, the thin film undergoes standard treatment with distilled water and is then dried at 60°C. Once dried, the nanocomposite material and the thin film are ready for characterization and application in hydrogen gas evolution, respectively. This method effectively creates a promising thin film of BiOCl/POCBA core–shell nanocomposite with additional Bi₂O₃, ideal for use as a photocathode in hydrogen gas evolution applications. The process ensures a homogeneous distribution of materials and a uniform thin film, crucial for efficient photocatalytic activity.

2.3 BiOCl/POCBA core–shell nanocomposite photocathode for H₂ gas generation from wastewater

The generation of H₂ gas from wastewater represents a promising approach due to the abundant availability of wastewater. This technology not only helps in managing contamination but also converts it into a renewable energy source in the form of H₂ gas. For electrochemical testing, the CHI608E device is used to estimate H₂ gas production by measuring the photocurrent density (J _ph) values. In this setup, the BiOCl/POCBA core–shell nanocomposite photocathode serves as the main electrode. The electrochemical cell is completed with a graphite auxiliary electrode and a calomel reference electrode. The electrolyte used in this system is the wastewater itself, without the addition of any external electrolyte, the chemical constituents of this wastewater are illustrated in Table 1.

The frequency of the incident photons is controlled using a series of optical filters, which allow for the precise determination of photon energies. The filters used pass photons with energies of 3.6, 2.8, 2.3, and 1.7 eV.

Through these measurements, the effectiveness of the fabricated BiOCl/POCBA core–shell nanocomposite photocathode (with additional Bi₂O₃ materials inside this composite) in generating H₂ gas from wastewater is assessed. The controlled photon energy provided by the optical filters ensures that the experiment can accurately determine the response of the photocathode under different light conditions, highlighting its efficiency in converting wastewater into a clean energy source. This approach underscores the dual benefits of wastewater treatment and renewable energy generation.

3.1 Analyses

The FTIR spectra of pristine POCBA and the BiOCl/POCBA nanocomposite are displayed in Figure 1. For the unmodified POCBA, the –NH-stretching peak is located at 3,235 cm⁻¹. The bands at 1,570 and 1,492 cm⁻¹ correspond to the aromatic and C═C stretching vibrations of the quinonoid and benzene rings, respectively [13]. Additionally, the C–H stretching band is observed around 1,196 cm⁻¹. The band at 1,250 cm⁻¹ is for the C–N and the in-plane C–H. Moreover, the bands at 700 and 822 cm⁻¹ represent the C–H out-of-plane of a 1,2-disubstituted aromatic ring. Upon the formation of the BiOCl/POCBA nanocomposite, the FTIR spectra still show the same functional groups, indicating that the fundamental structure of POCBA is retained. However, the incorporation of BiOCl with the additional Bi₂O₃ material causes shifts in the positions of these bands, suggesting interactions between POCBA and the BiOCl components. These shifts reflect changes in the local chemical environment around the functional groups due to the presence of the nanocomposite materials.

Figure 1

The estimated chemical analyses: (a) FTIR and (b) XRD patterns of the synthesized POCBA and the BiOCl/POCBA nanocomposite.

The –NH-stretching peak, originally at 3,235 cm⁻¹ in pristine POCBA, shifts in the nanocomposite spectrum to 3,159 cm⁻¹. Similarly, the aromatic and C═C stretching bands at 1,570 and 1,492 cm⁻¹, the C–H stretching band around 1,196 cm⁻¹, and the C–N and in-plane C–H bending band at 1,250 cm⁻¹ all exhibit shifts. The C–H out-of-plane at 700 and 822 cm⁻¹ also show changes in their positions to 871 cm⁻¹, in which this band has a smaller area under the incorporation of the inorganic material [14,15]. These shifts in the FTIR bands, summarized in Table 2, confirm the successful incorporation of BiOCl into the POCBA matrix, altering the chemical environment and interactions within the composite material.

The evaluation of the synthesized BiOCl/POCBA nanocomposite including additional Bi₂O₃ material has shown significant enhancements in crystallinity following the incorporation of BiOCl into the POCBA matrix. This improvement is evident through the detection of peaks corresponding to the Bi₂O₃ material, which are observed at ten distinct positions: 23.5, 25.5, 28.2, 32.8, 34.3, 36.5, 42.1, 44.6, 50.8, 56.4, and 59.6° (2θ). These positions correspond to the crystallographic growth directions of (120), (210), (201), (002), (220), (420), (122), (023), (203), (421), and (402). JCBDS standard 76-1730 [16,17]. Alongside these peaks, additional peaks related to the inorganic BiOCl are observed at nine positions: 15.3, 18.6, 22.6, 24.7, 27.1, 31.1, 35.1, 39.4, 47.0, and 48.8° (2θ). These peaks correspond to the growth directions of (020), (120), (002), (101), (012), (110), (003), (112), (220), and (113), respectively, [18,19]. The pristine POCBA exhibits a broad peak, indicative of lower crystallinity. However, after forming the composite, three distinct peaks emerge, indicating the formation of crystalline materials within the composite. The incorporation of these inorganic materials notably enhances the overall crystallinity of the composite, including the organic polymer matrix. This enhancement is due to the inorganic materials coating the polymer network, which significantly contributes to the improved crystallinity.

These crystallinity enhancements are promising for applications related to charge transfer, particularly in green H₂ generation that operates based on redox reactions. The improved crystallinity of the composite materials enhances their electrochemical properties, making them suitable for high-performance energy transfer devices. The increased crystallinity is also correlated with the nanocrystalline size of the composite materials. The nanocrystalline size of the composite, specifically for the peak observed at 15.3° (2θ), is evaluated using the Scherrer equation. The crystal size (D) is ∼15 nm. This calculation considers the full width at half-maximum (β) of the peak, in the Scherrer equation, (equation (1)) [20,21].

The XRD and XPS analyses complement each other in interpreting the chemical structure of the synthesized material. The XPS analysis for the BiOCl/POCBA nanocomposite (including additional Bi₂O₃ material) is depicted in Figure 2(a), providing a comprehensive survey of the elements present in this nanocomposite. The oxygen (O) element is highlighted in Figure 2(c), appearing through the 1s orbital at a binding energy of 531.5 eV. Alongside oxygen, Figure 2(a) also identifies other elements such as carbon (C), nitrogen (N), and chlorine (Cl). These elements are confirmed by their standard binding energies, specifically 285.5 eV for the C1s orbital, 400 eV for the N1s orbital, and 200 eV for the Cl2p orbital.

Figure 2

The chemical and optical analyses of the synthesized BiOCl/POCBA nanocomposite: (a–c) XPS survey, Bi element, O element, respectively, and (d) optical absorbance.

The significant role of the bismuth (Bi) element in the composite is evident in Figure 2(b), which shows the Bi4f orbital with distinct doublet peaks corresponding to Bi4f_7/2 and Bi4f_5/2. These doublet peaks indicate the formation of two inorganic materials within the composite. The presence of oxygen and chlorine elements further confirms the formation of BiOCl, as evidenced by the Bi4f_7/2 and Bi4f_5/2 peaks at binding energies of 160.3 and 165.6 eV, respectively. Concurrently, the incorporation of oxygen into another inorganic material, Bi₂O₃, is verified by the Bi4f_7/2 and Bi4f_5/2 peaks at 159.6 and 161.1 eV, correspondingly.

Thus, the XPS analysis validates the formation of the BiOCl/POCBA nanocomposite by demonstrating the binding energies and elemental composition. The presence of these characteristic peaks and elements confirms the successful synthesis of this nanocomposite, highlighting the integration of BiOCl within the POCBA matrix with additional Bi₂O₃ material. This synthesis is further substantiated by the complementary information obtained from the XRD analysis, providing a thorough understanding of the chemical structure and confirming the composite’s composition and formation.

The optical spectrum of the synthesized BiOCl/POCBA nanocomposite (including additional Bi₂O₃) is analyzed by evaluating the wavelength in relation to its physical absorbance, as shown in Figure 2(d). This nanocomposite demonstrates significant absorbance extending into the near-infrared region. This behavior confirms the composite’s capability to absorb photons with a wide range of energies up to these regions, indicating its potential for various applications, particularly in the renewable energy sector.

This extensive optical absorbance results from the synergistic effect of combining inorganic material (BiOCl with additional Bi₂O₃) with the organic polymer POCBA. The evaluated bandgap of 2.1 eV reflects the material’s ability to absorb photons with energies exceeding this value, which covers the visible (Vis) and near-Vis regions.

The determination of this bandgap is conducted using the Tauc equation (equation (2)) [22,23], which relies on the absorption coefficient (α) and the absorbance value (A) as key factors. This approach provides a comprehensive understanding of the material’s optical properties and confirms its suitability for applications requiring broad-spectrum photon absorption, such as in renewable energy technologies.

The morphological structure of the synthesized BiOCl/POCBA nanocomposite is depicted in Figure 3(a), showcasing a highly porous architecture. This structure results from the agglomeration of smaller particles into larger ones, with an average size of 210 nm, while the individual small particles ∼25 nm. This structural arrangement is integral to the nanocomposite’s optical properties, as the tiny particles and their porous nature enhance photon absorption and trapping within the granules [24]. Additionally, the theoretical estimation of the composite’s morphological properties is presented in Figure 3(b). The high roughness indicated by this estimation suggests a highly porous structure, with small particles coalescing to form larger ones around 200 nm in size. This theoretical insight complements the observed morphological features, confirming the successful synthesis of the nanocomposite with its intended structural and optical characteristics. The combined data from various analyses underscore the potential of the BiOCl/POCBA nanocomposite in applications requiring efficient photon absorption and trapping due to its innovative porous and fibrous structures. The TEM image in Figure 3(c) further illustrates the nanocomposite, revealing very small particles approximately 15 nm in size. These unique morphologies are pivotal to the composite’s exceptional optical absorbance. The observed color change from dark to faint confirms the integration of the inorganic materials into the POCBA matrix, resulting in the formation of the promising BiOCl/POCBA nanocomposite.

Figure 3

The synthesized -BiOCl/POCBA nanocomposite morphology: (a) SEM, (b) roughness and cross-section theoretical modeling (c) TEM, and (d) SEM of POCBA.

In contrast, the POCBA materials form fibers with diverse morphologies, as shown in Figure 3(d). These fiber structures create large surface areas with gaps that facilitate light trapping, which is crucial for the composite formation.

3.2 The electrochemical green hydrogen generation from the sanitation water using BiOCl/POCBA nanocomposite thin film photocathode

The electrochemical behavior of a fabricated BiOCl/POCBA nanocomposite thin film photocathode for hydrogen gas generation through sanitation water splitting was investigated using an electrochemical workstation. This photocathode was utilized as a working electrode in a three-electrode cell setup, with measurements taken at room temperature using a CHI608E device. The electrolyte was solely sanitation water, whose chemical composition is detailed in Table 1. The other two electrodes in the setup were a calomel reference electrode and a graphite counter electrode.

The BiOCl/POCBA nanocomposite demonstrated excellent performance, attributable to its highly porous morphological structure and small bandgap of 2.1 eV. This small bandgap allows the material to absorb light effectively, facilitating the water-splitting reaction and the providing of OH radicals. These radicals interact with additional water molecules, leading to hydrogen gas production. The superior optical properties of the nanocomposite enhance electron transfer to the conduction band, where hot electron clouds and an electric field are formed. These electrons are subsequently pushed into the sanitation water solution, promoting hydrogen gas production [25].

The efficiency and sensitivity of the fabricated photocathode for hydrogen gas generation were assessed by comparing the current density under illumination (J _ph) and in the dark (J _o), as depicted in Figure 4. The observed current densities were −0.0033 mA cm⁻² under light and −0.0017 mA cm⁻² in the dark, indicating a significant difference that reflects the formation of a photocurrent of −0.0016 mA cm⁻². This difference highlights the photocatalytic properties of the material. The sensitivity to light was further confirmed by the behavior shown in Figure 4(b), where chopped light and dark conditions revealed a rapid and significant change in the produced current. This quick response to light and darkness underscores the effectiveness of the photocathode in hydrogen gas generation under illumination.

Figure 4

(a) Photo-electrochemical testing of the fabricated BiOCl/POCBA nanocomposite thin film photocathode for hydrogen gas generation through sanitation water splitting and (b) chopped light illumination estimates the reproducibility study.

Moreover, the observed electrochemical behavior suggests that the fabricated photocathode is highly reproducible and stable for hydrogen gas production. The use of sanitation water as an electrolyte is particularly advantageous due to the minerals present, which act as self-sacrificing agents providing mobility in the electrochemical reaction and thus aiding the water-splitting process. This makes sanitation water a promising choice for such applications.

The sensitivity of the fabricated BiOCl/POCBA nanocomposite thin film photocathode for hydrogen gas generation from sanitation water was assessed by testing it under different photon energies. These varying energies were achieved by adjusting the incident wavelengths using various optical filters as estimated in Figure 5(a). The wavelengths tested ranged from 730 to 340 nm, corresponding to photon energies of 1.7–3.6 eV, respectively. For the first three photons, with energies of 3.6, 2.8, and 2.3 eV (wavelengths of 340, 440, and 540 nm), the observed J _ph varied from −0.024 to −0.019 mA cm⁻². With further decreases in photon energy to 1.7 eV, the estimated J _ph values decreased, which is attributed to the lower energy of these photons compared to the bandgap of the photocathode materials.

Figure 5

(a) Photo-electrochemical testing of the fabricated BiOCl/POCBA nanocomposite thin film photocathode for hydrogen gas generation from sanitation water splitting using various monochromatic photons and (b) summary column of the current density measured at −0.72 V.

These findings indicate that the fabricated photocathode is a highly effective photocatalyst, exhibiting significant sensitivity to photons, which generate different photocurrents as revealed by electrochemical studies. This sensitivity is linked to the energies of the photons; higher-energy photons (with higher frequencies) can transfer their energy to electrons, exciting them to the conduction band. These excited electrons, referred to as hot electrons, have different energies based on the kinetic energy received from the incident photons.

Figure 5(b) illustrates the estimated current densities obtained from electrochemical measurements, demonstrating the relationship between the incident photon energies and the resulting photocurrents. This relationship underscores the photocathode’s efficiency in utilizing photon energy for the water-splitting reaction, thereby contributing to hydrogen gas generation.

The mechanism of green hydrogen gas generation using the fabricated BiOCl/POCBA nanocomposite (with additional incorporated Bi₂O₃) thin film photocathode involves the photocathode’s response to incident photons as estimated in Figure 6(a). These photons initiate the generation of hot electrons through sequential electron transitions within the photocathode materials [26,27]. The small differences in the energy levels of BiOCl and POCBA facilitate the smooth transition of electrons across these materials, in which the Bi₂O₃ works to facilitate this sequential charge flow. Eventually, the electrons accumulate on the BiOCl material, which acts as an initiator for the water-splitting reaction in the sanitation water solution, leading to hydrogen gas production. The accumulated electrons interact with the sanitation water, generating OH radicals that sequentially attach to water molecules, resulting in hydrogen gas generation [28]. The presence of metals in sanitation water plays a crucial role in the photoelectrochemical reaction. These metals, through their movement, help trigger the water-splitting reaction, facilitating the generation of hydroxyl (OH) radicals. These OH radicals are highly reactive and essential for further reactions that lead to H₂ gas production. The abundance of metals in the wastewater itself eliminates the need for any external materials or metals to act as sacrificial agents. Instead, the inherent metal content is sufficient to drive the reaction effectively. This natural enrichment of metals in the wastewater stream enhances the process efficiency and sustainability by reducing the need for additional chemical inputs. Consequently, the wastewater’s existing composition provides a favorable environment for photoelectrochemical reactions, promoting hydrogen generation without relying on supplementary materials. This approach not only simplifies the process but also lowers costs, making it a more practical and eco-friendly solution for hydrogen production from wastewater. Using sanitation water, which is cost-effective, the elements in this water serve as promising sacrificial agents, promoting the movement of water molecules and enhancing the hydrogen gas reaction. This study highlights a significant conversion of a harmful water source into a valuable renewable energy source for hydrogen gas generation. The great stability of the used materials facilitates the H₂ gas generation that continues to more than 5.0 h with high stability as estimated in Figure 6(b). so this photocathode opens the door for the industrial application of these promising materials to provide H₂ gas from sanitation water.

Figure 6

(a) The mechanism and (b) the stability of the fabricated BiOCl/POCBA photocathode over 5.0 h for green hydrogen gas production under light illumination.

The state of the art of this study compared to existing literature, lies in the use of low-cost sanitation water as an electrolyte. This approach aims to convert wastewater into renewable hydrogen gas, targeting the production of clean energy from harmful sources. A key aspect of our research is the fabrication of a BiOCl/POCBA core–shell nanocomposite, which incorporates Bi₂O₃ and is applied to a cost-effective glass substrate. The thin film technique we employed is highly efficient, using minimal materials to coat the glass slide, thereby reducing costs while maintaining performance.

Additionally, the stability of hydrogen gas generation in our setup is impressive, sustaining for 5.0 h without any noticeable limitations. The J _ph value obtained in this study is also competitive when compared to other systems, such as CuO–C/TiO₂ and TiN–TiO₂ [29]. This demonstrates that our method is not only cost-effective but also efficient in producing hydrogen.

Looking forward, we plan to extend this research by conducting further investigations aimed at generating hydrogen over longer periods, up to 20 h. Additionally, we will scale up the surface area of the system to 100 cm², which will enhance hydrogen production capacity and move our study closer to industrial applications. This combination of cost-efficiency, stability, and scalability makes our approach a promising alternative to other methods for sustainable hydrogen production.