Converting Sewage Water into H2 Fuel Gas Using Cu/CuO Nanoporous Photocatalytic Electrodes

This work reports on H2 fuel generation from sewage water using Cu/CuO nanoporous (NP) electrodes. This is a novel concept for converting contaminated water into H2 fuel. The preparation of Cu/CuO NP was achieved using a simple thermal combustion process of Cu metallic foil at 550 °C for 1 h. The Cu/CuO surface consists of island-like structures, with an inter-distance of 100 nm. Each island has a highly porous surface with a pore diameter of about 250 nm. X-ray diffraction (XRD) confirmed the formation of monoclinic Cu/CuO NP material with a crystallite size of 89 nm. The prepared Cu/CuO photoelectrode was applied for H2 generation from sewage water achieving an incident to photon conversion efficiency (IPCE) of 14.6%. Further, the effects of light intensity and wavelength on the photoelectrode performance were assessed. The current density (Jph) value increased from 2.17 to 4.7 mA·cm−2 upon raising the light power density from 50 to 100 mW·cm−2. Moreover, the enthalpy (ΔH*) and entropy (ΔS*) values of Cu/CuO electrode were determined as 9.519 KJ mol−1 and 180.4 JK−1·mol−1, respectively. The results obtained in the present study are very promising for solving the problem of energy in far regions by converting sewage water to H2 fuel.


Introduction
Photocatalytic materials represent an important class of components for potential applications in renewable-energy-related fields such as solar cells, optoelectronic, and photocatalytic H 2 production [1][2][3][4]. On the one hand, the production of H 2 gas from sewage water is a very promising field of renewable energy. This process provides H 2 gas fuel for different uses of normal life such as cooking and warming, especially in remote regions inside deserts or rural areas. Moreover, this H 2 gas is used as a fuel for airplanes and aircrafts, in addition to its normal utilization in industrial factories and companies. On the other hand, this photocatalytic reaction removes contamination (sewage water) through

Characterization
X-ray diffraction pattern (XRD, Malvern Panalytical Ltd., Malvern, UK) analyses were carried out using a Bruker D8 advance diffractometer using Cu Kα radiation (wavelength = 0.15418 nm). Field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800, Schaumburg, IL, USA) was used to assess the morphology. The energy dispersive X-ray analysis (EDAX, Hitachi, Schaumburg, IL, USA) elemental composition and elemental mapping were examined using the EDAX unit attached to the FE-SEM. The optical properties were examined using a double beam spectrophotometer (Perkin Elmer Lamba 950, Shelton, CT, USA).

Electrochemical Measurements
The H 2 generation reaction was carried out from sewage water solution (100 mL, pH 5.5) using a three-electrode cell, in which Cu/CuO nanomaterial (1cm 2 ), a graphite sheet of the same dimensions, and calomel act as working, counter, and reference electrodes, respectively, as shown in Figure 1. All measurements were carried out using a workstation (CHI660E, Tennison Hill Drive, Austin, TX, USA) in a potential range from −1 to 1 V, a xenon lamp acts as a solar simulator. Some parameters were studied such as the light intensity (25 to 100 mW·cm −2 ), light wavelength (390 to 636 nm), on/off chopped current, and temperature on the electrode performance.

Cu/CuO Nanomaterial Preparation
Prior to sample preparation, the Cu foil was cleaned using water, soap, acetone, and ethanol under ultrasonication for 10 min each. The preparation of Cu/CuO was carried out through oxidation combustion of copper foil (99.9%, thickness 0.3 mm) in Nabertherm box furnace (Nitrex Metal Inc., St-Laurent, QC, Canada) at 550 °C for 1 h. Through this combustion process, the Cu metal is oxidized to generate CuO NP material. Then 1 cm 2 of the Cu/CuO was used as electrode for hydrogen generation under wastewater splitting reaction through electrochemical measurements.

Characterization
X-ray diffraction pattern (XRD, Malvern Panalytical Ltd., Malvern, United Kingdom) analyses were carried out using a Bruker D8 advance diffractometer using Cu Kα radiation (wavelength = 0.15418 nm). Field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800, Schaumburg, IL, USA) was used to assess the morphology. The energy dispersive X-ray analysis (EDAX, Hitachi, Schaumburg, IL, USA) elemental composition and elemental mapping were examined using the EDAX unit attached to the FE-SEM. The optical properties were examined using a double beam spectrophotometer (Perkin Elmer Lamba 950, Shelton, CT, USA).

Electrochemical Measurements
The H2 generation reaction was carried out from sewage water solution (100 mL, pH 5.5) using a three-electrode cell, in which Cu/CuO nanomaterial (1cm 2 ), a graphite sheet of the same dimensions, and calomel act as working, counter, and reference electrodes, respectively, as shown in Figure 1. All measurements were carried out using a workstation (CHI660E, Tennison Hill Drive, Austin, TX, USA) in a potential range from −1 to 1 V, a xenon lamp acts as a solar simulator. Some parameters were studied such as the light intensity (25 to 100 mW.cm −2 ), light wavelength (390 to 636 nm), on/off chopped current, and temperature on the electrode performance.
The sewage water was obtained from the drinking water and sanitation of Beni Suef city, Egypt, and the construction was confirmed using gas chromatography-mass spectrometry.

Morphological, Structural, and Optical Properties
The morphology of the CuO NP, prepared from Cu metal using a simple combustion process in air at 550 °C for 1 h, is displayed in Figure 2a  The sewage water was obtained from the drinking water and sanitation of Beni Suef city, Egypt, and the construction was confirmed using gas chromatography-mass spectrometry.

Morphological, Structural, and Optical Properties
The morphology of the CuO NP, prepared from Cu metal using a simple combustion process in air at 550 • C for 1 h, is displayed in Figure 2a,b. The CuO NP substrate consists of island-like structures with an inter-distance of 100 nm. Each island is highly porous with a pore diameter of about 250 nm. The formation of these pores with high homogeneous distribution on the CuO surface enlarges the surface area. This feature is beneficial for the photocatalytic process through high light absorption efficiency, in which the porous surface acts as a cave for light absorption [7,12]. of island-like structures with an inter-distance of 100 nm. Each island is highly porous with a pore diameter of about 250 nm. The formation of these pores with high homogeneous distribution on the CuO surface enlarges the surface area. This feature is beneficial for the photocatalytic process through high light absorption efficiency, in which the porous surface acts as a cave for light absorption [7,12].   [35][36][37]. The other three strong diffraction peaks at 43.5 • , 50.7 • , and 74.5 • correspond to the (111), (200), and (220) reflections, respectively, of the facecentered-cubic Cu (JCPDS #02-1225) [36,38]. The crystal size of the CuO nanomaterials is calculated using the Scherrer equation [39][40][41], Equation (1): where ß is the full width at half maximum (), λ is the X-ray wavelength (CuKα = 0.154 nm), and θ is the Bragg's angle [42]. The optical diffuse reflectance of the CuO NP was determined using a double beam spectrophotometer, as shown in Figure 2f. It is evident that the prepared CuO NP has a high light absorption behavior in a wide optical range (Vis and near IR). This is related to the low reflectance behavior of CuO NP in these regions. The bandgap is calculated using the Kubelka-Munk equation (Equation (2)) [12], as shown in the insert of Figure 2f, where K is the molar absorption coefficient, and S is the scattering factor. From Equation (2), a bandgap of 1.38 eV was determined for the CuO thin film, which is in good agreement with a recent research study [43].
The cross section and roughness is estimated using the modeling program (Image J) as shown in Figure 2g,h. From this figure, the surface roughness appears well with a surface area of 235 µm 2 in 38 µm 2 .
The photoelectrochemical (PEC) activity of the Cu/CuO photoelectrode was assessed in sewage water (chemical composition in Table 1) at room temperature (25 • C) with a sweep rate of 1 mV/s under xenon lamp illumination. The PEC measurements were performed in dark and light without optical filters, as shown in Figure 3a. The broad surface area of the prepared nanotextured electrode generates a high density of electron-hole pairs when exposed to light, leading to the splitting of H 2 O molecules to conduct the hydrogen electro-generation reaction. The effect of light on the Cu/CuO photoelectrode generate J ph values of −0.07 and 4.7 mA·cm −2 at 0 and 1 V, respectively (Figure 3a), although the density of the dark current is very small for the electrode and can be ignored. Therefore, from the J ph values, the Cu/CuO with the lowest photogeneration voltage (0.56 V) is a highly efficient electrode for water splitting and H 2 gas generation.
The substrate Cu metal contributes to a high density of pairs of electrons formed. This will motivate H 2 O molecules to be broken to produce hydrogen when it reaches the CuO surface. The spectral overlap between CuO absorbance oscillator frequencies and the Cu metal oscillator frequency improves the produced photocurrent.
The stability (time-J ph relation) of the prepared Cu/CuO photoelectrode was studied as presented in Figure 3b. At +0.1 V, the produced J ph value under on and off chopped light is mentioned. From the figure, the electrode has high stability and sensitivity to light. From the magnified part in Figure 3c, the change in the J ph values under on and off chopped light appears well. This confirms the high sensitivity of the electrode to light due to the high effect of the light on the electrode. The reproducibility of the electrode for four runs is shown in Figure 3d, in which the voltage-current relation shows the same behavior until four runs. This reproducibility was carried out at 30 • C and in normal light. The standard deviation (SD) value for the Cu/CuO photoelectrode is 0.3%. The effect of light intensity (50 to 100 mW·cm −2 ) on the Cu/CuO photoelectrode is mentioned in Figure 4a,b. This effect appears clearly, in which the J ph values increase from 2.17 to 4.7 mA·cm −2 , directly with the light intensity until 100 mW·cm −2 . This increase is related to the electron-hole pair formation under the increasing light intensity [44], in which many photons activate the active sites on the photocatalytic materials [45]. The J ph represents the electrons collected on the surface of the photoelectrode; this J ph then represented the water splitting and H 2 generation rate [46,47].
The number of photons (N) is directly proportional to the light intensity (P) as shown in Equation (3). This equation depends on other factors such as wavelength (λ), Planck constant (h), and light velocity (c). From this equation, the N per second is changed from 4 × 10 21 to 8 × 10 21 photon/s under light intensity from 50 to 100, mW·cm −2 , respectively.
The effect of the light monowavelength (390 to 636 nm) on the produced J ph for Cu/CuO electrodes is presented in Figure 5a. From this figure, the J ph has varied values under the monochromatic light effect. The optimum J ph value is 4.6 mA·cm −2 at 390 nm, in which these values correspond to the optical analyses data (Figure 3). The inset figure in Figure 6a shows this relation clearly.   The substrate Cu metal contributes to a high density of pairs of electrons formed. This will motivate H2O molecules to be broken to produce hydrogen when it reaches the CuO surface. The spectral overlap between CuO absorbance oscillator frequencies and the Cu metal oscillator frequency improves the produced photocurrent.
The stability (time-Jph relation) of the prepared Cu/CuO photoelectrode was studied as presented in Figure 3b. At +0.1 V, the produced Jph value under on and off chopped light is mentioned. From the figure, the electrode has high stability and sensitivity to light. From the magnified part in Figure 3c, the change in the Jph values under on and off chopped light appears well. This confirms the high sensitivity of the electrode to light due to the high effect of the light on the electrode. The reproducibility of the electrode for four runs is shown in Figure 3d, in which the voltage-current relation shows the same behavior until four runs. This reproducibility was carried out at 30 °C and in normal light. The standard deviation (SD) value for the Cu/CuO photoelectrode is 0.3%.
The effect of light intensity (50 to 100 mW.cm −2 ) on the Cu/CuO photoelectrode is mentioned in Figure 4a,b. This effect appears clearly, in which the Jph values increase from 2.17 to 4.7 mA.cm −2 , directly with the light intensity until 100 mW.cm −2 . This increase is related to the electron-hole pair formation under the increasing light intensity [44], in which many photons activate the active sites on the photocatalytic materials [45]. The Jph represents the electrons collected on the surface of the photoelectrode; this Jph then represented the water splitting and H2 generation rate [46,47].
The effect of the light monowavelength (390 to 636 nm) on the produced Jph for Cu/CuO electrodes is presented in Figure 5a. From this Figure, the Jph has varied values under the monochromatic light effect. The optimum Jph value is 4.6 mA.cm −2 at 390 nm, in which these values correspond to the optical analyses data (Figure 3). The inset figure in Figure 6a shows this relation clearly. The incident photon to current conversion efficiency (IPCE) represents the electrons collected on the surface of the photocatalytic materials under the photon flux (Figure 5b). This IPCE value can be calculated from the wavelength values [48], through Equation (4).
The IPCE is determined at 100 mW·cm −2 for the photoelectrode Cu/CuO and presented in Figure 5b. The IPCE values very much depend on monochromatic light, in which the optimum IPCE value is 14.6% at 390 nm. The IPCE values decrease with increasing wavelengths. Therefore, the photocatalytic electrode has the ability for sewage water splitting and H 2 generation with 14.6% efficiency. This high value was produced without using any additional electrolyte and infers that the electrode converts the sewage water into H 2 and O 2 with high efficiency in comparison with other previous literature [29,[49][50][51][52][53].  The incident photon to current conversion efficiency (IPCE) represents the electrons collected on the surface of the photocatalytic materials under the photon flux (Figure 5b). This IPCE value can be calculated from the wavelength values [48], through Equation (4).

IPCE =
J (mA. cm ). 1240 (V. nm) P(mW. cm ). λ(nm) The IPCE is determined at 100 mW.cm −2 for the photoelectrode Cu/CuO and presented in Figure 5b. The IPCE values very much depend on monochromatic light, in which the optimum IPCE value is 14.6% at 390 nm. The IPCE values decrease with increasing wavelengths. Therefore, the photocatalytic electrode has the ability for sewage water splitting and H2 generation with 14.6% efficiency. This high value was produced without using  The incident photon to current conversion efficiency (IPCE) represents the electrons collected on the surface of the photocatalytic materials under the photon flux (Figure 5b). This IPCE value can be calculated from the wavelength values [48], through Equation (4).

IPCE =
J (mA. cm ). 1240 (V. nm) P(mW. cm ). λ(nm) The IPCE is determined at 100 mW.cm −2 for the photoelectrode Cu/CuO and presented in Figure 5b. The IPCE values very much depend on monochromatic light, in which the optimum IPCE value is 14.6% at 390 nm. The IPCE values decrease with increasing wavelengths. Therefore, the photocatalytic electrode has the ability for sewage water split- There is an inverse relationship between the number of photons and IPCE. The optimum values occur at low wavelengths, in which the light has a high frequency for transferring most electrons to the conducting band, so the J ph value increases, and thereby the H 2 production increases [54].
The water splitting reaction for the H 2 generation process using the Cu/CuO photoelectrode under different temperatures is mentioned in Figure 6a. The J ph values increase from 4.7 to 8.8 mA·cm −2 with increasing the temperature from 30 to 60 • C, respectively. This increasing behavior of the J ph indicates the high mobility of the sewage water ions with increasing temperature, in which the high ionic mobility facilitates the H 2 and O 2 generation in both sides of the electrochemical cell. So increasing the J ph values with the temperature indicates an increase in the H 2 generation rate [55,56].
The activation energy (E a ) for the H 2 generation can be calculated under different temperatures using the Arrhenius equation (Equation (5)) [57]. This E a depends on the rate of collisions (k is the rate constant) and temperature values in Kelvin (T) using the universal gas constant (R) and Arrhenius constant (A) as the standard constant. From the E a value, the initiator temperature for starting the splitting reaction is determined [58][59][60]. E a values are obtained from slope of ln J ph versus 1/T as shown in Figure 6c. The E a value is 11.8 KJ mol −1 for the electrode. The small value of the water splitting reaction indicates the splitting occurs easily for the H 2 and O 2 evolution [61].
For calculating the enthalpy (∆H*) and entropy (∆S*), Equation (6) is applied (Eyring equation) [62,63]. This equation is similar to the Arrhenius equation, but it contains additional parameters and uses additional constants, in which k B is the Boltzman constant and h is the Planck constant. By applying this equation for the sewage splitting reaction of the Cu/CuO photoelectrode, ∆H* and ∆S* values are obtained as 9.519 kJ mol −1 and 180.4 JK −1 ·mol −1 , respectively ( Figure 6c).
Moreover, the H 2 moles are calculated from the Faraday equation, Equation (7) [64,65], under time change (dt), using the Faraday constant (F). The estimated H 2 mole for the Cu/CuO photoelectrode is 60 µmol h −1 cm −2 . In addition, a comparison between the present study and the previously reported literature is added in Table 2.
The mechanism of the CuO photocatalytic materials for the sewage water-splitting reaction is based on the effect of incident light on the motivation of the photoelectrons from the CuO NP material that resulted from the energy band changes (Figure 7). The photoelectrons generated under the effect of light incidence have two steps. First, electronhole generation, in which the generated electrons leave the holes and transfer to the upper level. The second step is the localized surface plasmonic resonance (LSPR); this resonance process causes the energy transfer [66]. These two steps occurred easily due to the small CuO band gap of 1.39 eV in addition to the absence of depletion regions inside the Cu and CuO nanomaterials. Therefore, the results are the transfer of electrons from the Cu to CuO without any restrictions, and the continuous electrons flow is carried out without any restrictions [67]. These hot electrons cause the generation of J ph under the applied potential [68][69][70]. The experimental image of electrons transfer processes is represented in the optical analyses ( Figure 3) and the electrochemical measurements under various effects such as light intensity and light wavelengths. The Cu metal substrate has plasmonic properties that cause the light capture and electron resonance process [52] that motivates the CuO nanomaterials and generates electrons over their surface [71][72][73]. These electrons are represented in J ph values and the H 2 generation reaction rate [74,75].

Conclusions
This work provides promising results in support of H 2 production from sewage water using a CuO NP photoelectrode. All the analyses were carried out for confirming the chemical structure, morphology, and optical properties of the prepared CuO NP. From the SEM, the prepared materials have nanoporous features that look like small islands with diameters of about 300-400 nm, with each island composed of a package of small nanoporous particles. XRD confirmed the monoclinic CuO crystalline phase with crystallite size of 89 nm. The obtained optical band gap value for CuO NP was 1.39 eV. The sewage water was used as a source of H 2 gas produced by the Cu/CuO photoelectrode. The J ph value was changed from 2.17 to 4.7 mA·cm −2 upon increasing the light power density from