Electrochemical Looping Green Hydrogen Production by Using Water Electrochemically Treated as a Raw Material for the Electrolyzer

Abstract

In this study, the applicability of an integrated-hybrid process was performed in a divided electrochemical cell for removing organic matter from a polluted effluent with simultaneous production of green H₂. After that, the depolluted water was reused, for the first time, in the cathodic compartment once again, in the same cell to be a viable environmental alternative for converting water into energy (green H₂) with higher efficiency and reasonable cost requirements. The production of green H₂ in the cathodic compartment (Ni-Fe-based steel stainless (SS) mesh as cathode), in concomitance with the electrochemical oxidation (EO) of wastewater in the anodic compartment (boron-doped diamond (BDD) supported in Nb as anode), was studied (by applying different current densities (j = 30, 60 and 90 mA cm⁻²) at 25 °C) in a divided-membrane type electrochemical cell driven by a photovoltaic (PV) energy source. The results clearly showed that, in the first step, the water anodically treated by applying 90 mA cm⁻² for 180 min reached high-quality water parameters. Meanwhile, green H₂ production was greater than 1.3 L, with a Faradaic efficiency of 100%. Then, in a second step, the water anodically treated was reused in the cathodic compartment again for a new integrated-hybrid process with the same electrodes under the same experimental conditions. The results showed that the reuse of water in the cathodic compartment is a sustainable strategy to produce green H₂ when compared to the electrolysis using clean water. Finally, two implied benefits of the proposed process are the production of green H₂ and wastewater cleanup, both of which are equally significant and sustainable. The possible use of H₂ as an energetic carrier in developing nations is a final point about sustainability improvements. This is a win-win solution.

1. Introduction

H₂ production is a subject of great scientific and technological interest because it is one of the most important energy alternatives to the use of fossil fuels. Nevertheless, the cost, technological developments, and environmental impacts during the production of H₂ are still great challenges, and all the possible options should be considered. In the former, the utilization of renewable energies entails the application of commercial H₂ production methods (named green H₂) to effectively meet all energy requirements in a financially efficient manner [1]. Meanwhile, water scarcity is a prominent environmental concern that is closely linked to the production and utilization of this green H₂.

The generation of green H₂ requires a substantial quantity of clean water, which may potentially worsen prevailing water scarcity issues in regions where water resources are already limited [2,3]. However, green H₂ is a product in the EO when this process is applied for the decontamination of effluents [4,5]. Therefore, the development of an integrated hybrid process to remove organic matter in an effluent with simultaneous production of green H₂ is a viable environmental water-to-energy alternative with higher efficiency and reasonable cost requirements, as already proved by our group [6,7,8,9,10,11,12,13]. This strategy brings significant insights and a great opportunity for the electrochemical technologies of wastewater to contribute as a key pillar of decarbonizing the global energy system [14,15].

Hybrid electrolysis processes emerge as a viable substitute for conventional water electrolysis to mitigate the financial and human risks involved. The benefits of replacing oxygen evolution reaction with decontamination of water levels achieved at the anodic process for producing clean water demonstrate the advantages of this strategy for energy applications [7,9]. Additionally, the use of lower/higher j at the anodic material in the hybrid process, which is driven by renewable energies, significantly reduces its cost [16].

In this framework, this win-win alternative implies significant benefits via the production of an energetic carrier and wastewater cleanup, both of which are equally significant and sustainable [11,17]. Also, the possible production and use of green H₂ in developing nations is a final point about sustainability improvements, mainly depolluting and reusing water [17,18]. Therefore, in this work, electrochemical looping was evaluated to produce green H₂ by using water electrochemically treated as a raw material for the electrolyzer. To do that, the electrochemical treatment of a real water matrix was performed in a divided cell by investigating different j (30, 60 and 90 mA cm⁻²), evaluating the water quality obtained at an anodic reservoir and the simultaneous production of green H₂ at the cathodic compartment with an electrolyte solution prepared with distilled water. Afterward, the production of H₂ was newly evaluated by reusing the water previously treated in the same system. Then, the results should establish that it is not necessary to use clean water for the desired production of H₂ to be achieved [11].

2. Results and Discussion

2.1. Electrode Characterizations: BDD as Anode and Ni-Fe-Based SS Mesh as Cathode

Before the EO experiments, the characterization of anodic and cathodic materials was carried out using SEM, EDS, and electrochemical measurements, as shown in Figure 1 and Figure S1 in the Supplementary Material (SM). Up till today, BDD (Figure 1a) remains the best electrocatalytic material for EO-based technologies in wastewater treatment due to its already well-known efficiency and properties, such as excellent corrosion resistance, chemical stability, high wear resistance, excellent electrical conductivity [19,20,21] and effectiveness to produce several oxidants. As can be observed in Figure 1a, the BDD electrode presents a fine polycrystalline diamond surface with sub-micron crystal size. In EO, the nature of BDD plays a determining role in the reaction pathway, selectivity, and efficiency of the oxidation process of organic compounds [22,23,24]; for this reason, polarization profile was recorded for BDD electrode in 0.1 mol L ⁻¹ Na₂SO₄ at a scan rate of 10 mV s ⁻¹ (Figure S1 in the SM) As can be seen in Figure S1a in the SM, the voltammetric BDD profile exhibits significant oxygen evolution potential at about +2.25 V [25]. This response indicates that BDD promotes the production of oxygen evolution after producing hydroxyl radicals (^●OH), which participate efficiently in the degradation of organics [26,27]. This higher potential for oxygen evolution reaction can cause the modification of the BDD surface due to the presence of functional groups generated during oxidation (^●OH, and SO₄²⁻) [27,28], which can promote higher charge transfer at high j. In fact, the diffractogram obtained for the BDD surface (Figure S1b in the SM) clearly identifies the diffraction peak for carbon (diffraction plane (002)), and this peak appears shifted towards higher values of 2θ approximately equal to 36.5°, which promotes an increase in interplanar distance. Also, the corresponding EDS spectrum of the BDD surface (see Figure 1c) clearly confirmed the presence of C, Nb, B and O. Based on these results, the formation of hydroxyl radicals on the electrode surface [20] is effectively promoted under these conditions, indicating that this electrocatalytic material has the capacity to be a very promising tool for treating contaminated effluents.

Conversely, the material chosen as cathode, a Ni-Fe-based SS mesh (Figure 1b), is a low-cost, highly efficient and durable material. It is also a free material of noble metals, even when some dopants or deposits have been used on its surface to increase their effectiveness in producing H₂ [29,30]. As can be seen in Figure 1c, SEM and EDS analyses evidenced the presence of nickel, iron and chromium in its composition, with being Ni the most abundant. In this frame, this cathodic material can be efficiently used to produce H₂ in the integrated-hybrid strategy proposed here. In fact, several studies have shown the effective application of nickel-based electrodes for H₂ production, for example: Ni-Fe alloy nanostructured [31], Ni-W alloy coating [32], and Ni-Cr [32].

2.2. EO of Wastewater with Simultaneous Green H₂ Production

It is evident that the selection of water supply for the electrolysis process determines the sustainable production of green H₂. If the water source has an impact on conflicts over availability with the populace or environmental constraints, sustainability will not exist [17,18,33]. For this reason, using treated water from alternate sources has become a viable choice when it comes to water supply for H₂ production [34].

In this study, untreated water from the mains supply was first electrolyzed in the anodic compartment to obtain clean water parameters needed to feed the electrolysis process in the cathodic compartment. The effluent was anodically treated in the denominated reactor: wastewater (0.1 mol L⁻¹ of Na₂SO₄)||H₂ cell (as illustrated in the SM, Figure S2). The parameters analyzed during the EO of the real water matrix were chemical oxygen demand (COD), total organic carbon (TOC), and others, which are listed in

Table 1. Physicochemical characterization of contaminated water and after anodic treatment by applying different j with an integrated-hybrid electrochemical process.

Parameters	Contaminated Water	After Treatment
		30 mA cm−2	60 mA cm−2	90 mA cm−2
Conductivity (μS cm−2)	2009	31.28	26.51	21.88
pH	7.77	1.73	1.94	1.56
UT Turbidity	16.58	11.75	11.53	11.85
Nitrate (mg/L)	4.69	8.84	13.22	17.88
Total Nitrogen (chemiluminescence) (mg L−1)	49.86	1.34	2.13	2.72
Calcium (mg L−1)	27.09	16.01	6.56	2.56
Sodium (mg L−1)	301.42	2486	3105	3287
Acetic acid (mg L−1)	ND	1.25	1.98	0.46
Formic acid (mg L−1)	ND	2.05	3.74	0.82
Malonic acid (mg L−1)	ND	ND	1.51	0.85
Oxalic acid (mg L−1)	ND	1.1	0.7	0.5

“ND” denotes “not detected”.

and plotted in Figure 2. As can be seen in Figure 2a, COD decay as a function of the electrolysis time during the electrochemical treatment of the real water matrix by using BDD anode, showing the tendency to eliminate organic matter after 180 min. In this frame, COD decay increases as the j increases, reaching 85%, 92% and 99% of organic matter removal by applying 30, 60 and 90 mA cm⁻². The mechanism behind organic matter removal from the water matrix involves the reaction of organic matter with electrochemically generated hydroxyl radicals (^•OH) at the BDD electrode surface (Equation (1)), as well as other oxidizing species produced by electrochemical processes, such as sulfate-based mediators (Equations (2) and (3)) [28]. This finding is supported by the existing literature [26], which demonstrates that organic matter is efficiently degraded through these oxidative processes when higher current densities are applied during electrochemical treatment.

BDD + H₂O → BDD(•OH) + H⁺ + e

(1)

2SO₄²⁻ → S₂O₈²⁻ + 2e⁻

(2)

SO₄²⁻ + ^•OH → 2SO₄^−• → S₂O₈²⁻

(3)

In the case of the water matrix used here, organic matter present, which may be natural organic matter at lower concentrations as evidenced by the initial COD (119 mg L⁻¹) and TOC (638 mg L⁻¹) measurements, is transformed into small organic compounds (like organic acids, as confirmed by results reported in Table 1 [7]) via the reaction with ^•OH, 2SO₄^−• and S₂O₈²⁻. Due to the high concentration production of these oxidants by BDD anode, the degradation of organic matter occurs quickly. This was also confirmed by the COD kinetic analysis (inset Figure 2a), which considers pseudo-first-order behavior. The kinetic profiles show a change in the kinetic constant k, identifying two distinct zones. The first region (0–60 min) represents the effective removal of organic matter, which depends on the electrical charge applied during the process [24,27]. Then, k values, based on a pseudo-first-order, were of about 0.0228 min⁻¹, 0.0316 min⁻¹ and 0.0396 min⁻¹ for 30, 60 and 90 mA cm⁻², respectively. Then, the degradation rate increases when an increase in the j is achieved. Conversely, the degradation rate decreases in the second region as a function of j, indicating a slowdown in removing organic compounds in the effluents [27]. In fact, this behavior is due to the mass transport phenomena, which are related to the lower concentration of organic compounds in the effluent at the end of the electrolysis [35].

The intensity of mineralization of the effluent was accompanied by a decrease in TOC as a function of the j, as shown in Figure 2b. The results showed that TOC significantly decreased from 638 mg L⁻¹ (raw water) to 8.3 mg L⁻¹ by applying 90 mA cm⁻² in 180 min of electrolysis time. This is an important result that indicates the effectiveness of treating real effluent to achieve the quality of clean water (see Table 1). In fact, the quality parameters indicated that lower organic and inorganic compounds are present in the water after EO treatment [36]. For example, no presence of organic acids was detected before the anodic electrochemical treatment of raw water; nevertheless, acetic, formic, malonic and oxalic acids were quantified after the electrolysis at different j by using wastewater (0.1 mol L⁻¹ of Na₂SO₄)||H₂ cell (as illustrated in the SM, Figure S2). In the case of 90 mA cm⁻², lower concentrations of these organic acids were quantified after 180 min of electrolysis time (see Table 1) because the organic matter was efficiently mineralized. Conversely, no complete mineralization of organic matter in the effluent was achieved when 30 and 60 mA cm⁻² were applied under the same experimental conditions, which promoted the production and accumulation of organic acids at the end of the electrolysis (Table 1). Even when the medium contained sulfates (Na₂SO₄), the concentration of secondary oxidizing species, such as SO₄^−• and S₂O₈²⁻, was not enough to promote the complete elimination of intermediates formed. In the case of nitrates, these inorganic species increase their concentration as a function of j, which is associated with the degradation of nitrogen-organic compounds. These compounds release nitrogen-inorganic species, such as nitrate and ammonium, in accordance with the results reported in Table 1.

At this point, the results clearly showed that the hybrid-integrated electrochemical technology can significantly reduce the organic load of the real water matrix, which means that a large part of the compounds present in the contaminated water have been mineralized to CO₂ and H₂O [27]; but some by-products are still recalcitrant, requiring long electrolysis times to complete mineralization. Concomitantly, green H₂ was produced in the cathodic reservoir, and the volume generated was compared with the theoretical values for H₂ production, which were estimated from Equation (4) [5].

$$V_{th}={\displaystyle \frac{k_e\times i\times t}{p}}$$

(4)

where ke = (M/n × F) is the electrochemical equivalent (kgA⁻¹s⁻¹), n is the electron number (2 for H₂), F is the Faraday constant (96,487 C mol⁻¹), M is the molar mass of H₂ (Kg mol⁻¹), i is the intensity of applied total current (A), t is the electrolysis time (s), and ρ is the H₂ gas density (0.0818 kg m⁻³).

As can be seen in Figure 2c, the experimental volumes of green H₂ produced by applying different j are very similar to the H₂ volumes theoretically estimated. This behavior was achieved until approximately 50–60 min; after that, the volume of dry H₂ produced slightly increased until the end of the electrolysis. This increase is more pronounced, reaching about 5%, by applying 90 mA cm⁻². It is associated with an increase in the protons (H⁺) concentration in the anolyte due to the oxidation of organic compounds at the anodic compartment via oxidants produced on the BDD surface [37]. These H⁺ cross the cation exchange membrane from the anodic compartment towards the cathodic reservoir, and subsequently, these H⁺ are reduced to molecular hydrogen at the cathode [37], increasing its volume with respect to the theoretical estimations, as observed in Figure 2c. To confirm these assertions, additional experiments were carried out to produce H₂ in the absence of organic matter (Figure 2d) in the anodic compartment as a traditional water splitting process, only adding supporting electrolyte (0.1 mol L⁻¹ Na₂SO₄ solution) in both compartments.

As can be seen in Figure 2d, the measured volume of green H₂ produced experimentally varied linearly as a function of electrolysis time, showing values very close to those predicted theoretically by Equation (4), with a margin of error of 1% and with a correlation factor of about R² > 0.9998 in all experimental cases. Then, these results clearly indicate that the H₂ production obeys Faday law and depends on the i applied as well as confirm that organic matter in an anodic reservoir when the integrated-hybrid approach is used, plays a key role in the production of green H₂. These assertions agree with a previous report already published by Martínez-Huitle and dos Santos group [11].

On the other hand, the formation of a precipitate was observed on the anode surface during the EO of raw water, which was mainly attained at lower j (30 and 60 mA cm⁻²). The precipitate was then analyzed using thermogravimetry (TG) and FTIR instrumental techniques. The results clearly showed that the composition of the precipitate corresponds to calcium carbonate and calcium oxide. In the case of CaCO₃, it was mainly formed due to the presence of calcium (Ca²⁺) in the real water matrix (see Table 1) as a result of the hardness of the water. Nevertheless, the formation of calcium oxalate (the product of the reaction between Ca²⁺ and oxalic acid (a by-product of the oxidation of organic matter at the end of the electrolysis)) can contribute to the generation of CaCO₃. In fact, insignificant concentrations of oxalic acid are accumulated after the EO of the real water matrix by applying 30 and 60 mA cm⁻² as a consequence of the degradation of organic matter [38]. Meanwhile, the formation of CaO is due to the lower accumulation of oxalic acid in solution, avoiding the production of calcium oxalate and, consequently, promoting the direct formation of CaO from CaCO₃ under specific pH conditions and lower concentration of CaCO₃. Based on the thermogravimetric (TG) curves for the solids formed at three conditions analyzed, dehydration (elimination of H₂O to form anhydrous salt and thermal decomposition of the anhydride salt) and the elimination of CO to form CaCO₃ were observed in the range from 37 to 590 °C for 90 mA cm⁻² and from 37 to 620 °C for 30 and 60 mA cm⁻², approximately (Figure 2e) [39,40]. However, in the case of a solid formed under electrolysis conditions by applying 90 mA cm⁻², two loss mass events were observed, which correspond to CaCO₃ and CaO, respectively (see Figure 2e). Conversely, at 30 and 60 mA cm⁻², only one of these mass loss events occurred, indicating that CaCO₃ was mainly formed. The mass loss observed at the temperatures investigated clearly showed that the thermal decomposition of CaCO₃ towards CO₂ via the formation of CaO in the range of 590 to 712.7 °C at 90 mA cm⁻² and 690 °C to 810 °C for 30 and 60 mA cm⁻², corresponding to a mass loss of 60% for 90 mA cm⁻² and 38% for 30 and 60 mA cm⁻², approximately, according to reaction above [39,40]:

CaCO_{3 (g)} → CO_2(g) + CaO_(g)

(5)

The next stage occurs in a defined manner, starting at 800 °C completing the mass loss process. Through the calculations carried out, a purity percentage of 88%, 84% and 63% of elemental calcium were obtained for the CaCO₃ samples at 30, 60 and 90 mA cm⁻², respectively [39,40]. These results confirm that CaCO₃ is the main solid by-product formed at lower j, while lower concentrations of CaCO₃ were reached at 90 mA cm⁻², achieving a higher depuration level of water with BDD anode.

Alternatively, the FTIR analysis of the precipitate samples collected at different j of electrolysis registered diverse assignments in their spectra (see Figure 2f), as follows: in-plane angular deformations δ_d (OCO) at 712 cm⁻¹; out-of-plane angular deformations γ(CO₃) at 870 cm⁻¹ and antisymmetric stretching ν_as(CO) at 1400 cm⁻¹ [41]. This information confirmed that the main composition of the precipitates was related to CaCO₃, as already revealed by the TG analysis. On the one hand, similar absorption bands obtained at all precipitate samples were registered when a CaCO₃ standard was analyzed, validating their composition. On the other hand, the main differences in the precipitate samples analyzed were associated with the intensity of the bands, which corroborate the lower purity content of the precipitate collected when the integrated-hybrid process was performed at 90 mAcm².

2.3. Reuse of Treated Water in Cathodic Process for H₂ Production

The behavior of the integrated-hybrid electrochemical system, in relation to H₂ production, when the water anodically treated was reused in the cathodic compartment, was investigated. As shown in Figure 3a, the production of green H₂ depended on the intensity of the applied current and the electrolysis time when 0.1 mol L⁻¹ Na₂SO₄ solution as supporting electrolyte was recirculated through the anodic compartment using a peristaltic pump at a constant rate of 125 mL min⁻¹ while a volume of 40 mL of water electrochemically treated was reused in the cathodic compartment, without flow conditions under the similar experimental conditions reported in Section 3.1. It is also important to note that the volume of green H₂ experimentally produced is similar to the theoretical estimations of the volume of H₂ reusing water previously treated electrochemically in Section 3.1. However, there was a slight increase in the production of H₂ by applying 90 mA cm⁻² in the final times of electrolysis, which can be associated with the production of other gases in small volumes, like NH₃ or N₂. These gases increase the final volume of the reservoir.

The electrochemical reduction mechanism of nitrate promotes the production of adsorbed nitrite (NO₂⁻), and afterward, it is converted into N₂ or NH₃ (Equations (6)–(8)). A high overpotential is necessary for this reaction pathway, which is typically preferred in acidic conditions, and this reaction competes with H₂ production [42].

NO₃⁻ + 2H⁺ + 2e⁻ → NO₂⁻ + H₂O

(6)

NO₂⁻ + 4H⁺ + 3e⁻ → NH₃ + H₂O

(7)

NO₂⁻ + 6H⁺ + 5e⁻ → N₂ + 3H₂O

(8)

Therefore, electrode material selection has a key impact on selectivity and efficiency in promoting H₂ or N₂/NH₃ production. For instance, Ni and Pt prefer to absorb hydrogen rather than nitrate ions due to their electrical structure [42].

To determine the composition of the gas phase produced at the cathodic reservoir when the anodically treated water was reused in this compartment, differential electrochemical mass spectroscopy (DEMS) analysis was conducted with a QIC heated flexible capillary inlet ultra-flow. As can be observed in Figure 3b, green H₂ is produced at cathodic material by applying 90 mA cm⁻² as a function of electrolysis time. The production of H₂ follows a similar behavior when compared to the production of the same gas by water splitting process using clean water and supporting electrolytes in the cathodic reservoir. This analysis also indicated that a lower amount of N₂ was produced, which is a by-product of the electroreduction of N-compounds present in the water reused. Conversely, no production of N₂ was detected when distilled water containing 0.1 mol L⁻¹ Na₂SO₄ solution as supporting electrolyte was electrolyzed by applying 90 mA cm⁻². This result clearly demonstrates that H₂ is produced with high purity in concomitance with a lower amount of N₂ gas (Figure 3b) when water previously treated electrochemically was reused in the cathodic reservoir. These experiments confirm three points:

(i)

The increase in the volume of H₂ gas (reported in Figure 3a by applying 90 mA cm⁻²), as a function of time, is explained as the increase of N₂ gas,

(ii)

The lower production of N₂ is due to the electro conversion of NO₃⁻ to NO₂⁻, and after to N₂, and

(iii)

The preference of Ni-Fe mesh cathode to adsorb H₂ is higher than NO₃⁻ [42].

Within this framework, these results establish that it is not necessary to use clean water for the desired production of H₂ to be achieved [13]. Hence, the optimization of a BDD integrated-hybrid process to remove organic matter in an effluent with simultaneous production of green H₂ is a viable environmental water-energy alternative with greater efficiency by ensuring availability and sustainable management of water and sanitation for all (SDG 6) [6].

From an economic perspective, the most crucial factor in electrochemical system assessment is energetic efficiency, which is closely linked to operating costs [6]. The energy efficiency for H₂ production (H₂EE) can vary depending on the actual operating conditions, according to Equation (9). This parameter in electrolysis systems can be defined as the heating value of H₂ versus the rate of H₂ produced (mol h⁻¹) divided by the energy consumed by the electrolyzer, reflecting improved thermodynamic operating conditions [43].

$${\mathrm{H}}_2\mathrm{E}\mathrm{E}(\mathrm{\%})={\displaystyle \frac{39 \mathrm{w}\times \mathrm{h}\times {\mathrm{g}}^{-1}\times H_2 rate\times 2 \mathrm{g} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}}{E_{cell}\times I_{cell}}}\times 100$$

(9)

As observed in Figure 4a, H₂EE was investigated as a function of energy consumption by applying different j. Then, the j increased, the H₂ production, I_cell, and E_cell increased, but the effect on H₂EE decreased. Knowing this, the energy efficiency of the green H₂ produced reaches 44%, 38%, and 37% in 180 min by applying 30, 60, and 90 mA cm⁻², increasing the energy expenditure (Figure 4a). Another important fact is that the energy cost for removing organic pollutants with simultaneous generation of H₂ (180 min of electrolysis) using the PV system is about 0.000034, 0.000091 and 0.00015 US$ for 30, 60 and 90 mA cm⁻² based on costs from the International Renewable Energy Agency ((IRENA): 1 kWh = 0.0057 US$) [9]), which can be considered as a more suitable process and thus allowing energy-saving H₂ production. Figure 4b evidenced the lower energy consumption values when anodically treated water was reused in the cathodic compartment to produce H₂ by applying different j, thus minimizing the costs of the electrochemical system. Therefore, the proposed work also guarantees access to cheap, reliable, sustainable and renewable energy for all (SDG 7).

BDD films are normally formed on a certain substrate, such as Si, Ti, Nb, W, Ta, or Mo. The most common substrate for BDD films is silicon wafers due to their low electrochemical activity and ability to form stable, compact oxide films that prevent film delamination [21,26,44,45]. However, Si substrates are not suitable for severe water treatment conditions because of their brittleness and generally low electrical conductivity, which restricts their usage in large-scale applications. In fact, diamond films supported in Si substrates have only demonstrated a lifetime of around 300 h at lower j (>1 A cm⁻²). Conversely, BDD films supported in Nb have shown a lifespan of over 850 h during electrolysis under acidic conditions (0.5 M H₂SO₄) at higher j like 10 A cm⁻² [21,45,46]. Then, a diamond film in this work can be considered stable material under the experimental conditions used here as well, and this electrocatalytic material can be suitable for large-scale or longer-durability applications.

3. Materials and Methods

The effluent was collected from a municipal duct in Natal-RN and stored at ambient prior to use in EO experiments. The effluent sample was used without any previous treatment, and their initial physical and chemical analysis yielded the information content in Table 1. The highest quality commercially available chemicals were used. All model solutions were prepared with ultrapure water obtained using a Millipore Milli-Q purification (≈18.0 MΩ) system (Merck KGaA, Darmstadt, Germany) at 25 °C. Sulfuric acid (H₂SO₄) and sodium sulfate (Na₂SO₄) were supplied by Dinámica Química LTDA (São Paulo, Brazil).

3.1. Experimental Setup

The experiments were carried out in a two-compartment cell (equipped with a boron-doped diamond (BDD) supported in Nb as the anode and a Ni-Fe-based SS mesh as the cathode, separated by a cation exchange membrane (Nafion type-350), represented by wastewater||H₂ cell, as illustrated in the SM, Figure S2. In general, two experimental stages were considered as follows:

• Firstly, the electrochemical treatment of the 350 mL of raw water (by adding 0.1 mol L⁻¹ Na₂SO₄ as the supporting electrolyte) was carried out for 180 min using a power supply (Minipa, model MLP-3305, Minipa do Brasil Ltd.a, São Paulo, Brazil) under galvanostatic conditions (30, 60, and 90 mA cm⁻²), connected to the solar photovoltaic (PV)-battery system (as reported in SM, Figures S3 and S4). The effluent was recirculated through the anode compartment using a peristaltic pump at a constant rate of 125 mL min⁻¹ while a volume of 40 mL of supporting electrolyte solution (0.1 mol L⁻¹ Na₂SO₄) was used in the cathodic compartment, without flow conditions. The water treatment was reserved for a second stage.
• The treated water in the preceding stage (without additional electrolyte), which was analyzed, was used in the cathodic compartment under similar experimental conditions in (1) to examine the impact of water quality on H₂ generation (

  Table 2. Electrochemical treatment of the wastewater||H₂ cell to produce green H₂. The operating conditions were chosen based on the existing literature.

  Entry	Anodic Compartment	Cathodic Compartment	j, mA cm−2	Flow Rate r(H2), mmol min−1
  1	Wastewater 0.1 mol L−1 Na2SO4	0.1 mol L−1 Na2SO4	30	0.22
  2	Wastewater 0.1 mol L−1 Na2SO4	0.1 mol L−1 Na2SO4	60	0.31
  3	Wastewater 0.1 mol L−1 Na2SO4	0.1 mol L−1 Na2SO4	90	0.45
  4	Wastewater 0.1 mol L−1 Na2SO4	water treated	30	0.28
  5	Wastewater 0.1 mol L−1 Na2SO4	water treated	60	0.44
  6	Wastewater 0.1 mol L−1 Na2SO4	water treated	90	0.66

  ).

3.2. Characterization Methods

Organic matter removal was evaluated by the COD decay during electrolysis by using a smartphone-based protocol [47,48]. The pH conditions were monitored using a Nova Instruments pH-meter HANNA, and the conductivity/salinity/resistivity/total dissolved solids were measured with an Electrical Conductivity meter model HI4321 (HANNA^® instruments Brasil Exp. E Imp. LTDA, São Paulo, Brazil). TOC measurements were determined for the samples collected after 180 min of treatment by using an Analytik Jena MULTIN/C 3100 (Analytik Jena GmbH+Co. KG, Jena, Germany) according to the ASTM D 7573-18 standard methodology. Meanwhile, the intermediates produced during the EO of real water matrix were identified by high-performance liquid chromatography (HPLC) on a dual LC (VQDUO-DUALLC, Thermo Fisher Scientific, Inc., São Paulo, Brazil) with a diode array detector (Ultimate 3000 DAD, Thermo Fisher Scientific, Inc., São Paulo, Brazil). A volume of 10 µL of sample was injected by an Ultimate 3000 autosampler, (Inc., São Paulo, Brazil). The compounds were determined by a column Acclaim OA, 5 mm, 120 Å, 4.0 × 250 mm at 25 °C. The mobile phase consisted of 100 mmol L⁻¹ of Na₂SO₄, pH 2.65 (adjusted with methanesulfonic acid), and was eluted at 600 mL min⁻¹ for 15 min. To reduce experimental errors, each test was run twice, and the average of the replication results was used. For every determination, the differences between the series were always within 3%. Also, differential electrochemical mass spectroscopy (DEMS) analysis was conducted to determine the composition of the gas phase produced at cathodic reservoir by using a HPR-40 DEMS system (3F series HAL/3F RC 301 triple filter quadrupole mass spectrometer, providing 300 amu mass range and enhanced sensitivity for high mass species) with a QIC heated flexible capillary inlet (0.9 m of length, temperature 200 °C) ultra-flow (sample flow rate = 250 L min⁻¹), for evolved gas studies (nitrogen, argon, hydrogen), with a QGA 2 gas analysis calibration and quantification PC software (QGA 2.0 (quantitative gas analyser 2.0) (Hiden Analytical, Warrington, UK). DEMS was interconnected with a potenciostat/galvanostat OrigaFlex-Drive Unit (SAS OrigaLys ElectroChem, Rillieux-la-Pape, France).

The energy consumption (EC, kWh L⁻¹) was calculated as described in Equation (10):

$$Energy consumption={\displaystyle \frac{\left(\mathsf{\Delta }\mathrm{E}\mathrm{c}\mathrm{ }\times \mathrm{ }\mathrm{I}\mathrm{ }\times \mathrm{ }\mathrm{t}\right)}{1000\mathrm{ }\times \mathrm{ }\mathrm{V}}}$$

(10)

where ΔEc (V) and I (A) are the average cell voltage and the electrolysis current, respectively; t is the time of electrolysis (h); and V is the sample volume (L⁻¹). A Fourier transform infrared spectrometer (FTIR) was used to determine the IR spectra of precipitate samples obtained at the end of the electrolysis, using a PerkinElmer Frontier (PerkinElmer Inc., São Paulo, Brazil) with a sampling rate of 400–4000 cm⁻¹.

4. Conclusions

A hybrid-integrated electrochemical system for effluent treatment and simultaneous H₂ production has been proven to improve water quality, so it can be used in the same system to replace ultra-pure water (used in traditional H₂ production technology) to produce green H₂ again. This study showed that water decontamination and the production of green H₂ were always improved when the j was increased. The effectiveness of EO was also proven by the improvement in the physicochemical parameters of the contaminated water. Volumes of H₂ of 0.5, 0.8 and 1.3 L were collected after 180 min of electrolysis at j of 30, 60 and 90 mA cm⁻², respectively. The H₂ production results compared to the anodically treated water added to the cathodic compartment demonstrate the practical feasibility of not needing to use clean water in the system. In addition, the use of the cathodic material proposed here proved to be efficient for H₂ production with the advantage of having a lower cost compared to other commonly used cathodes (e.g.,: Pt). Thus, treated water reuse promotes the optimization of the integrated-hybrid process, bringing economic and environmental advantages (water-energy) and reasonable cost requirements to fulfill SDG 6 and 7.