Photoelectrochemical Conversion of Sewage Water into H₂ Fuel over the CuFeO₂/CuO/Cu Composite Electrode

Abstract

This study describes the synthesis of delafossite, CuFeO₂, as a primary photocatalytic material for hydrogen generation. A photoelectrode, CuFeO₂/CuO/Cu, was prepared by combusting a Cu foil dipped in FeCl₃ in ambient air. This photoelectrode showed excellent optical behavior for the hydrogen generation reaction from sewage water, producing 90 µmol/h of H₂. The chemical structure was confirmed through XRD and XPS analyses, and the crystalline rhombohedral shape of CuFeO₂ was confirmed using SEM and TEM analyses. With a bandgap of 1.35 ev, the prepared material displayed excellent optical properties. Electrochemical measurements for H₂ gas generation were carried out using the CuFeO₂/CuO/Cu photoelectrode, comparing the effect of light and dark and monochromatic wavelength light. The electrode exhibited significant enhancement in light compared to dark, with current density (J_ph) values of −0.83 and −0.1 mA·cm⁻², respectively. The monochromatic light also had a noticeable effect, with the J_ph value increasing from −0.45 to −0.79 mA·cm⁻² as the wavelength increased from 640 to 390 nm. This system is cheap and durable, making it a promising solution for hydrogen gas fuel generation in the industry.

1. Introduction

Renewable energy has become a vital energy source for people in today’s society, as non-renewable energy sources are becoming scarce. Additionally, these energy sources are known to release dangerous gases, such as SOx, NOx, and COx, which can have adverse effects on the environment and human health [1,2,3].

Hydrogen gas (H₂) is a promising alternative energy source with numerous potential applications, including fuel for airplanes and factories and everyday household purposes such as cooking and heating. H₂ has a high combustion energy and low cost [4,5,6,7]. It can be produced through different electrolytes, such as strong acids or bases, by relying on the presence of high amounts of H⁺ or OH⁻ ions through an extended mechanism that includes the formation of OH^·. This active radical attacks the H₂O molecule for additional H₂ gas production. However, this reaction has limited economic applications related to the cost of the chemicals, such as NaOH, HCl, or H₂SO₄, and the corrosion problems caused by the implemented electrolytes, affecting the working electrode.

The working electrode must be a semiconductor material, such as an oxide, sulfide, or nitride [8,9]. Oxides have the advantage of being cost-effective and stable [10,11]. Furthermore, the amount of the produced H₂ gas depends on the optical and morphological properties of the semiconductor material, making it crucial to prepare these materials with high surface areas and active sites [12,13,14]. One of the developed methods for increasing H₂ production is using plasmonic materials in the electrode construction; copper (Cu) is a promising and cost-effective plasmonic material [15,16]. In the literature, several studies have used Cu as plasmonic material to increase the activity of ZnO as a photocatalytic material [17]. Plasmonic materials enhance the performance of the neighboring semiconductor material (such as a metal oxide) by transferring electrons to the conducting band and inducing a plasmonic resonance that generates an electric field and increases light absorbance [18].

The physical properties of the prepared photocatalytic material play a crucial role in improving the light absorbance and reducing the bandgap. Copper oxide (CuO), with its black color and semiconducting nature, has a bandgap of 0.7 to 1.6 eV, which falls within the visible light absorbance region [19,20]. This behavior enhances its application as a photocatalytic absorbent and in H₂ production [21,22]. Li et al. synthesized CuO/Cu through annealing Cu(OH)₂ in air conditions at 500 °C [23]. Moreover, Sagadevan et al. synthesized CuO through the combustion process using ascorbic acid as a capping agent at 100–300 °C [24]. Ragupathi et al. synthesized CuO supported with b-C₃N₄ as photocatalytic material for water splitting reaction; the produced J_ph is still very small [25]. Quyen et al. increased the efficiency of the H₂ generation of TiO₂ by using Cu nanomaterials as plasmonic material [26]. In the same manner, Shen et al. slightly increased H₂ generation when g-C₃N₄ was decorated with CuO, but not to the extent desired [27].

Delafossite, CuFeO₂, has a high photocatalytic performance and excellent optical properties, with a bandgap of approximately 1.3 eV [28]. This material can be prepared using various methods, including solvothermal and laser beam techniques. He et al. used CuFeO₂ for carbamazepine degradation with an efficiency of 86% [29]. In addition, GC₃N₄/CuFeO₂ has been used as a catalyst for H₂O₂ degradation through the Fenton reaction [30]. Mao et al. prepared CuFeO₂ using the hydrothermal and solar gel for dye removal and water splitting, and the obtained J_ph was recorded as 4 × 10⁻⁶ µA cm⁻² in 1M NaOH [31]. Baiano et al. investigated the significant photocatalytic reduction of CO₂ utilizing CuFeO₂ through DFT calculations [32]. Furthermore, CuFeO₂ was demonstrated to have photocatalytic antibacterial properties, effectively degrading E. coli, with an improvement in performance when exposed to light [33]. Change et al. used Mg-doped CuFeO₂ for photocatalytic degradation of methylene blue and reduction of CO₂ to form ethylene glycol under the electrolysis conditions [34].

In the previous studies on H₂ generation, the use of additional electrolytes such as HCl or NaOH was required to serve as a source of H⁺ ions [35,36,37]. Additionally, some studies relied on high-cost preparation techniques [38,39].

In this study, a CuFeO₂/CuO delafossite material was synthesized on a Cu metal substrate through a combustion reaction. Subsequently, various analyses were performed to confirm the chemical, optical, and morphological properties of the prepared photoelectrode, CuFeO₂/CuO/Cu. The latter was then evaluated for its ability to produce H₂ gas through photocatalytic decomposition of sewage water, and the effects of light and dark conditions on H₂ production were investigated. The impact of different monochromatic wavelengths on H₂ production was also examined, and the amount of H₂ produced was calculated over time.

2. Results and Discussion

2.1. Characterization of CuFeO₂/CuO/Cu Nanomaterials Photoelectrode

The XRD analysis was employed to investigate and verify the crystalline structure of the prepared CuFeO₂/CuO/Cu nanomaterials. The results showed a well-demonstrated rhombohedral CuFeO₂ phase structure and microstructural parameters of the prepared nanostructured electrode. Figure 1a displays the XRD patterns of the prepared CuFeO₂, which show a poly-oriented nature. Fifteen diffraction peaks were observed; eleven of the peaks correspond to the Miller indices (i.e., (hkl) planes) (006), (101), (012),(104), (009), (018), (110), (1010), (116), (202), and (024) of the rhombohedral CuFeO₂ structure with a secondary CuO impurity phase. The remaining four peaks correspond to the Miller indices (112), (020), (202), and (311) of the CuO monoclinic phase. The peaks are in agreement with the reference data JCPDS 01-075-2146 [40] for the rhombohedral structure of pure CuFeO₂ with $R-3 m$ space group and JCPDS card no. 48-1548 [41] for the monoclinic phase of CuO.

The broadness of the XRD patterns was exploited to estimate the microstructural parameters, such as crystallite size (D) and microstrain (ε), of the prepared CuFeO₂ by using the Williamson–Hall (W–H) approach. The W-H model was applied to the pure peak broadening of the diffraction peaks to calculate the mean values of (D) and (ε). The pure peak broadening (β) was obtained from (Equation (1)) [42,43,44]

$${\mathsf{\beta }}_{ } \cos \mathsf{\theta }=\frac{0.95 \mathsf{\lambda }}{{\mathrm{D}}_{ } }+4{\mathsf{\epsilon }}_{ } \sin \mathsf{\theta }$$

(1)

Where 0.95 is the shape factor value for CuFeO₂ nanoparticles, and λ is the incident XRD wavelength (~0.15418 nm). In Equation (1), it was considered that the investigated samples have an isotropic nature, and the micro-strain is uniform in all (hkl) crystallographic directions. The crystallite size (D) and the micro-strain (ε) values were estimated by plotting the (β_Correct cos θ) versus (4 sin θ) for each XRD peak, yielding a linear regression where the intercept and slope were calculated (Figure 1b). It was found that the crystalline size and micro-strain values for the CuFeO₂ sample are 35 nm and 4.69 × 10⁻³, respectively.

Moreover, the XRD of the CuO nanoparticles is provided in Figure S1; the peaks appearing at 2θ° = 26.55, 35.27, and 41.71° correspond to CuO for the growth direction (110), (111), and (200), respectively. These analyses match the results reported in the literature [45,46,47]. On the other hand, the Cu metal appears at 43.5°, 50.7°, and 74.5° [46,48].

XPS was utilized for further verification of the chemical composition of CuFeO₂. The results are presented in Figure S2a, in which Cu, Fe, and O elements are confirmed through their related peaks. Additionally, the spectra of the Cu_2p spin-orbital components reveal six peaks in the range of 930 to 970 eV, as shown in Figure S2b. The Fe_2P spectra are confirmed through the two peaks at 723 and 726 eV (Figure S2c). The O_1s spin-orbital peaks can be found in the 529–533 eV range (Figure S2d).

In Figure 1c, The morphology of CuO can be clearly seen as protruding appendages that greatly enhance the surface area. The magnified image shows that the appendages are randomly distributed across the surface. After forming CuFeO₂ (Figure 1d), excellent uniform crystalline rhombohedral shapes appear with porous structure. This highly crystalline structure confirms the findings of the XRD analysis. Given the crystalline nature of the prepared materials, it was expected that their optical properties would be superior [49].

The optical reflectance of CuFeO₂ nanoparticles was determined over the wavelength range of 200 to 1200 nm, as shown in Figure 2a. The low reflectance of the CuFeO₂ nanoparticles in the visible region confirms their high absorbance in the UV and IR regions. The bandgap of the material was calculated using the Kubelka-Munk equation (Equation (2)) [12], which takes into account reflectance (R), with K and S values that represent the molar absorption coefficient and scattering factor, respectively. The resulting bandgap of this material is 1.35 eV.

$$\mathrm{K}/\mathrm{S}=\frac{{(1-\mathrm{R})}^2}{2\mathrm{R}}.$$

(2)

The TEM image of CuFeO₂ is displayed in Figure 2b, which confirms the formation of crystalline nanoparticles with an average size of 50 nm. These results are consistent with those obtained from the SEM analysis. The theoretical images of CuO and CuFeO₂, obtained using ImageJ, are presented Figure 2c,d, respectively. Both materials exhibit considerable roughness, but the roughness of the CuFeO₂ is more uniform. This feature indicates the suitability of these materials for photocatalytic applications due to their high ability to capture light through their porous structures, acting like a “cave” for the incident photons [3,12].

2.2. Photoelectrochemical Water Splitting

The photoelectrochemical water splitting reaction was carried out with a three-electrode cell. The prepared CuFeO₂/CuO/Cu photoelectrode served as the working electrode, graphite as the counter electrode, and calomel as the reference electrode. The measurements were conducted using a CHMI608E electrochemical workstation under a 400 W metal halide lamp at a sweep rate of 1 mV/s and a temperature of 25 °C. The measurements were taken both in the dark and under illumination to assess the photocatalytic behavior of the materials.

The impact of light on the performance of CuFeO₂/CuO/Cu is depicted in Figure 3a. When illuminated, the current density (J_ph) value was increased; the J_ph values at 0.88 V in dark and light conditions were −0.10 mA·cm⁻² and −0.83 mA·cm⁻², respectively. The high surface area of the CuFeO₂/CuO/Cu provides a good environment for capturing photons, which leads to the creation of electron-hole pairs. The electrons collected on the surface of CuFeO₂ are then transferred to the adjacent electrolyte (sewage water), leading to further reactions and the generation of OH^. radicals. The results of this study show that using sewage water as an electrolyte (pH 7.2) is a promising approach, as its chemical composition, outlined in

Table 1. The chemical composition of the sewage water used as an electrolyte for H₂ gas production.

Species	Concentration (mg/L)
Phenols	0.015
F−	1.0
Al3+	3.0
NH3	5.0
Hg2+	0.005
Pb2+	0.5
Cd3+	0.05
As3+	0.05
Cr3+	1.0
Cu2+	1.5
Ni3+	0.1
Fe3+	1.5
Mn2+	1.0
Zn2+	5.0
Ag+	0.1
Ba3+	2.0
Co2+	2.0
Other cations	0.1
Pesticides	0.2
CN−1	0.1
Industrial washing	0.5
Coli groups	400

, acts as a catalyst for the water-splitting reaction.

The addition of Cu increased the efficiency of electron transfer. It also acts as a plasmonic material that enhances light capture, causing an electric field to accumulate around the CuFeO₂ materials. This electric field oscillation between Cu and CuFeO₂ increases the number of free electrons available to split water into H₂ gas. The rate of H₂ gas released can be determined from the calculated J_ph values.

The H₂ production was measured and plotted in Figure 3b. It was found that the H₂ production increased with time, reaching 90 µmol/h. This high rate confirms the effectiveness of CuFeO₂ as a photocatalyst in water splitting. The active sites of CuFeO₂ are activated under light, contributing to its high efficiency. Based on Equation (3), the number of photons was estimated to be 8 × 10²¹ photon/s, which leads to a significant number of electrons being generated and collected on the active sites, and then transferred to the neighbor electrolyte for further water splitting and H₂ gas production [50,51]. $\mathsf{\lambda }$, h, P, and c in Equation (3) are wavelength, Planck constant, light intensity, and light velocity, respectively.

The number of generated H₂ moles was determined using Faraday’s law (Equation (4)) [52,53]. This calculation is based on the time change (dt) and the J_ph values, with consideration given to the Faraday constant (F).

$$\mathrm{N}=\mathsf{\lambda }\mathrm{P}/\mathrm{hc}$$

(3)

$${\mathrm{H}}_2 \mathrm{mole}={{\displaystyle \int }}_0^{\mathrm{t}}{\mathrm{J}}_{\mathrm{ph}}·\mathrm{dt}/\mathrm{F}$$

(4)

The performance of the prepared CuFeO₂/CuO/Cu photoelectrode was evaluated under monochromatic light by studying the effect of different light wavelengths (390–640 nm) on the H₂ generation reaction. The results, presented in Figure 4a, show that the J_ph values increased from −0.45 to −0.79 mA·cm⁻² as the wavelength increased from 390 to 640 nm. Figure 4b shows the produced J_ph values at −0.88 V, confirming that the sewage water splitting and H₂ gas evolution reactions were enhanced with increasing incident photons.

The conversion of sewage water into H₂ gas was achieved through the impact of the incident photons on the CuFeO₂/CuO/Cu photoelectrode. The Delafossite material is highly responsive to light illumination, which activates the surface and triggers energy level splitting. Hot electrons are then collected on the surface and transferred to the upper level through resonance energy transfer. The small bandgap, 1.35 eV, is considered promising for electron transfer and confirms the significant optical properties of this material, as seen Figure 2a. The use of Cu as a current collector facilitates this electron transfer process as it can also act as a plasmonic material [52]. The collected electrons are then transferred to the surrounding wastewater solution, generating the current density (J_ph) values shown in Figure 3. These J_ph values determine the rate of H₂ gas produced [51,54,55]. The photocurrent is recorded as −0.11 mA·cm⁻². The generation of H₂ is initiated by the radiation of light that creates electrons and holes, leading to hot electrons being transferred to the photocathode for reduction and, eventually, H₂ production [56].

The IPCE of the photoelectrode, CuFeO₂/CuO/Cu, is determined through Equation (5) and found to be 2.4% at 340 nm, which is a favorable result for producing hydrogen from a solution without using additional electrolytes [57].

$$IPCE=\frac{{\mathrm{J}}_{\mathrm{ph}}\left(\mathrm{mA}·{\mathrm{cm}}^{-2}\right)·1240 \left(\mathrm{V}·\mathrm{nm}\right)}{\mathrm{P}\left(\mathrm{mW}·{\mathrm{cm}}^{-2}\right)· \mathsf{\lambda }\left(\mathrm{nm}\right)}$$

(5)

Finally, a comparison was made between the electrolytes used and the produced J_ph values in the literature and those obtained in this study, where sewage water was used as an electrolyte. The results are summarized in

Table 2. Comparison of the current study (using sewage water as an electrolyte) with the previous studies in the literature [58].

Photoelectrode	Electrolyte	Jph (mA/cm2)
g-C3N4-CuO [25]	NaOH	0.01
CuO-C/TiO2 [59]	Glycerol	0.012
SnO2/TiO2 [60]	Na2S2O3	0.4
TiN-TiO2 [61]	NaOH	3.0 × 10−4
BiFeO3 [62]	NaOH	0.1
Au/Pb(Zr, Ti)O3 [63]	NaOH	0.06
PrFeO [64]	Na2SO4	0.130
Poly(3-aminobenzoic acid) frame [65]	H2SO4	0.08
CuFeO2/CuO/Cu (present study)	Sewage water	0.83

and show that the CuFeO₂/CuO/Cu electrode prepared in this study has one of the highest J_ph values, indicating its high efficiency and sensitivity to light. Furthermore, this study’s results are noteworthy as sewage water was used as the electrolyte without needing additional external electrolytes, making it a significant advantage over other electrodes.

3. Materials and Methods

3.1. CuFeO₂ Delafossite Nanomaterial Preparation

The CuFeO₂ nanomaterial was prepared through a simple combustion reaction. The process began with washing the Cu foil with water, soap, distilled water, and ethanol, followed by immersing it in concentrated H₂SO₄ (99.9%) for 2–3 min. The foil was then rewashed with distilled water and ethanol. Subsequently, it was immersed in 0.05 M FeCl₃ for 15 min on each side and dried at 60 °C for 2 h. The final step involved combusting the substrate at 500 °C for 10 min, forming the CuFeO₂/CuO/Cu photoelectrode.

3.2. Characterization

The prepared CuFeO₂ materials were characterized using various analysis methods. The chemical structure was examined by x-ray diffraction patterns (XRD, Malvern Panalytical Ltd., Malvern, UK) (λ = 0.15418 nm) (advance diffractometer, Bruker D8) and X-ray photoelectron spectroscopy (XPS, K-ALPHA, Waltham, MA, USA). The morphology was observed by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) and transmitted electron microscopy (TEM, JEOL JEM-2100, Oberkochen, Germany). Finally, the optical properties were analyzed using a double-beam spectrophotometer (Elmer Lamba, Waltham, MA, USA).

3.3. Electrochemical Measurements

The electrochemical measurements were performed using a CHI608E electrochemical workstation, as illustrated in Figure 5. The prepared CuFeO₂/CuO/Cu photoelectrode served as the working electrode (1 cm²), while a calomel and graphite electrodes were used as the reference and counter electrodes, respectively. The measurements were carried out using sewage water with a pH of 7.2. A 400 W metal halide lamp (light intensity) with a photon flux of 500 μmol/s was implemented as the light source. The effect of different light wavelengths (390, 440, 540, and 640 nm) and the impact of alternating light and dark conditions were studied. The produced H₂ moles were calculated as a function of time.

4. Conclusions

This study demonstrates the potential of a low-cost photoelectrode made of CuFeO₂/CuO/Cu for the generation of H₂ gas. Using sewage water as an electrolyte, measurements were conducted using an electrochemical workstation in a three-electrode cell setup. The photoelectrode was found to have excellent optical properties, with a small bandgap of 1.35 eV, making it suitable for photocatalytic reactions. The electrochemical testing under a metal halide lamp was applied; the results showed that the J_ph values under light and dark conditions were −0.83 and −0.1 mA·cm⁻², respectively. The effect of monochromatic light on the photoelectrode was studied across the range of 390 to 640 nm, resulting in J_ph values of −0.45 to −0.79 mA·cm⁻². The produced H₂ gas was measured to be 90 µmol/h. Additionally, the cost of a 10 × 10 cm² CuFeO₂/CuO/Cu photoelectrode was found to be 0.5 $, making it a cost-effective option for H₂ gas production and a candidate for industrial applications.