Integrated Electrochemical–Electrolytic Conversion of Oilfield-Produced Water into Hydrogen

Abstract

This study tackles the challenge of treating high-oil (≥90 mg/L) and high-salinity (Cl⁻ ≥ 6900 mg/L) oilfield-produced water for green hydrogen production. An integrated technology combining electrochemical cascade purification (EDCF: electro-demulsification–coagulation–flotation) with alkaline water electrolysis is developed. The EDCF process effectively reduces oil, suspended solids, and turbidity to <10 mg/L, <20 mg/L, and <20 NTU, respectively, meeting stringent feedwater criteria for electrolysis. An asymmetric electrolysis strategy employing a nickel felt anode/Raney nickel cathode system achieves a low cell voltage of 1.856 V at 1 A/cm² in 6 M KOH at 85 °C, with 96.58% H₂ purity. Crucially, separate anolyte/catholyte (0.5/6 M KOH) mitigates Cl⁻ corrosion, enabling stable 240 h operation (96.66% ± 0.5% H₂ purity) in a duplex steel electrolyzer. The work establishes comprehensive boundary conditions for scalable hydrogen production from treated produced water.

1. Introduction

The extensive development of hydrocarbon resources within China’s continental sedimentary basins has spurred large-scale oilfield exploitation. Sustained production in major producing regions, such as the Songliao Basin in Northeast China, the Bohai Bay in North China, and the Junggar Basin in Northwest China, has resulted in the majority of oilfields entering a high-water-cut development phase. Geological data indicates that produced fluids from mature fields typically exceed 80% water content, generating millions of cubic meters of produced water daily. Such wastewater exhibits characteristics including multiphase emulsion stability and complex pollutant composition. Beyond residual crude oil and suspended solids, it concentrates recalcitrant substances such as surfactants, polymer flooding agents, and heavy metal ions. Improper disposal can trigger soil permeation toxicity, groundwater system contamination, and heavy metal accumulation effects within ecological chains, severely constraining the environmental carrying capacity of oilfield regions. Concurrently, it leads to production issues such as reduced injection system efficiency and accelerated pipeline corrosion, creating a dual predicament of environmental risk and diminished economic benefits. Consequently, establishing synergistic technologies for the deep purification and resource recovery of produced water has become a core issue in the green transformation of the oil and gas industry [1].

Traditional physicochemical treatment technologies (such as hydrocyclone separation, chemical demulsification, and activated carbon adsorption) exhibit inherent limitations when treating highly emulsified produced water, including substantial chemical dosing requirements, high sludge production rates, and low removal efficiency for insoluble pollutants [2]. In stark contrast, electrochemical water treatment technologies achieve synergistic optimization of pollutant removal and energy utilization by precisely regulating electron transfer processes at electrode interfaces: (1) electric fields disrupt the interfacial film structure of oil droplets via dielectric breakdown, enabling efficient demulsification of emulsified systems; (2) polyhydric hydroxyl complexes generated by soluble anodes (Fe/Al) promote colloidal particle aggregation and sedimentation through electroneutralization and adsorption bridging; and (3) reactive oxygen species (·OH, O₃) generated during electrochemical oxidation deeply mineralize dissolved organic matter while simultaneously achieving disinfection and scale inhibition. This multi-mechanism synergy not only significantly enhances pollutant removal efficiency but also reduces sludge production, demonstrating notable technological superiority [3,4,5].

Notably, produced water subjected to electrochemical pretreatment becomes an ideal feedstock for electrolytic hydrogen production due to optimized electrolyte composition and reduced organic loading. Theoretical studies indicate that residual ions such as Cl⁻ and SO₄²⁻ in produced water effectively lower electrolyte resistance, while the presence of Fe²⁺ further catalyzes hydrogen evolution kinetics. Through proton exchange membrane (PEM) electrolyzers or alkaline electrolysis systems, hydrogen production efficiency from pure water electrolysis can reach 99% [6], simultaneously circumventing the reliance on high-purity deionized water inherent in conventional electrolysis processes. Further research confirms that employing transition metal phosphide/nitrogen-doped carbon composite electrodes effectively suppresses electrode passivation caused by complex produced water constituents, enhancing long-term operational stability [7].

Addressing the high mineralization and oil content inherent in produced water, this research employs electrochemical treatment methods to optimize feedwater quality for alkaline electrolysis. The objectives include identifying the most effective electrochemical treatment approach, determining optimal operating conditions for produced water treatment, and assessing the feasibility of hydrogen production using alkaline electrolysis cells with treated produced water.

2. Experimental System

To address the complex challenge of treating produced water with high levels of oil and salt, this study proposes an innovative three-stage coupled treatment process that integrates electrodemulsification, electrocoagulation, and electroflotation (EDCF), combined with alkaline electrolyzer-based hydrogen production technology. The core treatment systems consist of a produced water purification unit and an electrolytic hydrogen generation unit, as schematically illustrated in Figure 1.

2.1. Produced Water Purification System

The pretreatment stage consists of an integrated treatment system based on electrochemical synergy, which employs the coupled EDCF process to achieve advanced purification of oilfield-produced water. Designed modularly, the system incorporates three core functional units: electrodemulsification (ED), electrocoagulation (EC), and electroflotation (EF). The process flow includes an ED reaction chamber, an EC reaction chamber, inclined plate sedimentation tanks, and an EF purification chamber. Through optimized hydraulic retention times in each functional zone, graded removal of pollutants is accomplished.

As illustrated in Step 1 of Figure 1, the EDCF system operates as follows: first, the oil–water interfacial film in the produced water is efficiently broken under an applied electric field, achieving demulsification. Subsequently, flocs generated from the aluminum electrodes adsorb and remove oil droplets and suspended solids. This is followed by primary solid–liquid separation via inclined plate sedimentation. Finally, residual contaminants are effectively eliminated through the synergistic action of micro- and nano-bubbles catalyzed by the electrodes. This integrated approach not only maximizes the synergistic enhancement of each electrochemical step but also allows the system to be powered by renewable electricity, thereby supporting green energy utilization and contributing to energy savings and carbon reduction. A photograph of the on-site installation is presented in Figure 2.

2.2. Hydrogen Production System

In this hydrogen production system, the pretreated produced water is first conveyed to the pH adjustment tank. At this stage, alkali solution is metered precisely into the produced water to adjust the pH of the water sample to an appropriate range. This step is crucial as it ensures that metal ions such as calcium (Ca) and magnesium (Mg) in the water precipitate as hydroxides, thereby removing these impurities that could impair electrolytic efficiency. Subsequently, the produced water, having undergone pH adjustment and precipitation treatment, is further purified. The supernatant from this process is then conveyed to the electrolytic cell. The hydrogen production experimental procedure is illustrated in Figure 1, Step 2.

The alkaline electrolytic water hydrogen production process comprises an electrolytic water supply and cooling system alongside an electrolytic gas treatment system, as illustrated in Figure 3. The electrolytic water supply and cooling system includes the following: water pump, oxygen separator, radiator, resin tank, and make-up water tank. The electrolytic gas treatment system comprises the following: buffer tank, water separator, filter, drain valve, refrigerated dryer, and purifier. The electrolytic cell assembly employs a square metal tank. Both anodes and cathodes utilize catalysts prepared in-house. Power is supplied via a Gamry 1000 unit, with adjustable output current ranging from 0 to 1 A and adjustable output voltage ranging from 0 to 10 V.

The electrolyzer serves as the core component of the hydrogen production process, in which electrochemical reactions are driven by an applied direct current. At the anode, water molecules undergo oxidation to produce oxygen, while at the cathode, they are reduced to generate hydrogen gas. The produced hydrogen and oxygen, initially carried by the circulating electrolyte, are directed into a gas–liquid separator. Within this unit, the gases are effectively purified and separated through physical partitioning processes. This step ensures high-purity hydrogen output, supplying a clean hydrogen source for subsequent application or storage [8]. The entire system is equipped with a PLC-based automation system, allowing for one-touch startup and automated data acquisition. The on-site installation of the electrolyzer unit is shown in Figure 4.

3. Laboratory Instruments and Water Quality Testing

3.1. Experimental Apparatus and Methods

Using suspended solids, turbidity, and oil content as assessment indicators for produced water treatment, the analytical instruments and measurement methods employed are detailed in

Table 1. Experimental apparatus and methods.

Analytical Indicators	Method of Determination	Principal Instruments
Turbidity	Spectrophotometry	WZS-186 Digital Turbidity Meter/ShangHai/China
Suspended solids content	Photometric method	HM-SS Suspended Solids Detector/ShanDong/China
Oil content	Spectrophotometry	A360 Ultraviolet–Visible Spectrophotometer/ShangHai/China
Hydrogen purity	External standard method for thermal conductivity detector	NK-200A Hydrogen Analyzer/ShanXi/China

.

3.2. Wastewater Quality Testing

This study examined produced water from a facility within China National Petroleum Corporation’s Changqing Oilfield, employing a standardized sampling procedure to obtain representative water samples. Based on an analysis of multiphase flow process characteristics, the effluent outlet of the three-phase separator was systematically selected as a typical sampling point. Water samples collected from the three-phase separator outlet underwent three parallel tests, with the results presented in

Table 2. Water quality at the outlet of the three-phase separator.

Testing Metrics	Water Sample 1	Water Sample 2	Water Sample 3
Oil content (mg/L)	115.1	113.2	115.1
Turbidity (NTU)	350.2	351.2	350.8
Suspended solids (mg/L)	1800.5	1815.3	1785.1
Chloride content (mg/L)	6948.1	6949.6	6943.4
pH	7.6	7.5	7.3
Hardness index	17	16	18

.

Table 2 indicates an oil content of approximately 115.1 mg/L, suspended solids of approximately 1800 mg/L, and turbidity of approximately 350 NTU. The water sample exhibits typical mineralization characteristics of produced water from oil and gas fields: chloride ion concentration (ion chromatography) reaches 6900 mg/L, total hardness exceeds 15.2 GPG (as CaCO₃), and pH (online pH meter) remains stable at 7.5, consistent with the properties of weakly alkaline, highly mineralized electrolyte solutions.

3.3. Produced Water Purification Unit

Through indoor static experimental analysis, a 20 min reaction time for wastewater can achieve a stable treatment state. This apparatus is designed for a flow rate of 10 L per hour. The effective volume of each reaction unit, calculated to be approximately 3 L, results in a calculated residence time of approximately 20 min.

3.3.1. Structural Design of the ED Reaction Zone

The ED reaction zone primarily comprises an inlet, electrode plates, a reaction tank, and an outlet. As the first electrochemical treatment unit within the entire system, ED performs the initial oil–water separation. Through the application of an electric field, it separates oil and water within water-in-oil emulsions, causing small oil droplets to coalesce into larger droplets and separating them from the effluent. Key design parameters are as follows: The electrodispersion reaction zone measures 20 cm × 20 cm × 15 cm with a wall thickness of 5 mm. It incorporates a 2 mm deep recess for embedded electrodes, possesses an effective volume of 3 L, and maintains a residence time of approximately 20 min. The structure of the electrodispersion reaction zone is illustrated in Figure 5.

3.3.2. Structural Design of the EC Reaction Zone

The EC reaction zone represents the core component of the electrocoagulation apparatus. Its primary configuration includes an inlet, electrode plates, a reaction tank, an outlet, an air-blowing orifice, and an external floc collection tank. The aluminum electrode plates are central to the oil removal mechanism: the anode undergoes sacrificial dissolution, releasing aluminum ions that subsequently hydrolyze and polymerize to form aluminum hydroxide flocs. These flocs, along with electrochemically generated bubbles, adsorb and carry dispersed oil droplets and suspended solids to the liquid surface. The accumulated oily sludge is then removed via the integrated air-blowing device.

Key design parameters of the EC zone are as follows: The main reaction tank measures 20 cm × 20 cm × 15 cm with a wall thickness of 5 mm and incorporates a 2 mm embedded electrode groove. An external floc collection tank (20 cm × 5 cm × 5 cm) is connected, providing an effective total volume of 3 L. The hydraulic retention time is approximately 20 min. A schematic diagram of the EC reaction zone is presented in Figure 6.

3.3.3. Structural Design of the EF Reaction Zone

The EF (electroflotation) reaction zone serves as the final stage of the integrated wastewater treatment apparatus. Its core function is to separate residual minute flocs and oil droplets from the polished effluent by attaching them to fine bubbles generated via electrolytic water catalysis, thereby lifting them to the liquid surface for subsequent removal by a skimming mechanism. The primary components include an inlet, electrode plates, a reaction tank, an outlet, an air-blowing port (for skimming), and an attached floc collection trough.

Key design parameters are as follows: the main reaction tank measures 20 cm × 20 cm × 15 cm with a wall thickness of 5 mm and incorporates a 2 mm deep embedded electrode groove. An external floc collection tank (20 cm × 5 cm × 5 cm) is connected, providing an effective system volume of 3 L and a hydraulic retention time of approximately 20 min. A schematic of the EF reaction zone is presented in Figure 7.

4. Experimental Results and Analysis

4.1. Water Quality Analysis of Produced Water Treatment

Based on preliminary water quality analysis, the produced water collected from the outlet of the three-phase separator was characterized as a deeply emulsified, opaque dark liquid with high stability, exhibiting turbidity exceeding 200 NTU. To treat this stable oil–water system, an advanced purification process was developed using an electrochemically synergistic approach integrating EDCF technology.

In this configuration, graphite electrodes were utilized for the ED stage, while aluminum electrodes were employed for EC. The EF unit was equipped with titanium-ruthenium electrodes of identical shape, number, and surface area. Fourteen electrodes were arranged in a cross configuration, providing a total effective surface area of 200 cm². The system was operated under constant current conditions at 3 A (corresponding to a current density of 12 A/m²), with a hydraulic retention time of 20 min (flow rate: 10 L/h) and at an initial pH of 7.2.

Treatment via this electrochemical synergistic process resulted in an 80% reduction in oil content and a 95.23% removal of suspended solids from the produced water. Comparative water quality data are summarized in

Table 3. Water quality before and after the electrochemical synergistic treatment process.

Test Parameters	Water Quality Prior to Treatment	Treated Water Quality
Oil content (mg/L)	115.1	23.1
Turbidity (NTU)	350.2	46.2
Suspended solids (mg/L)	1800.5	85.3
Chloride ion content (mg/L)	6948.1	5232.8
pH	7.6	7.2
Hardness index	17	16

, and the corresponding treatment results are illustrated in Figure 8.

4.2. Optimal Conditions for Hydrogen Production from Produced Water

First, pure alkaline water electrolysis for hydrogen production was conducted to observe equipment performance as a comparative experiment. Using foam nickel as the catalyst, an electrolyzer was assembled with an alkaline AGFA ZIRFON UTP 500 membrane. At room temperature (25 °C) and under 1 M KOH conditions, the effect of varying feed flow rates on hydrogen production performance was investigated by adjusting the pump speed (higher speed yields greater flow; 21 rpm corresponds to 0.8 L/min). Five parallel experiments were conducted, with current–voltage relationship curves recorded.

As shown in Figure 9, the apparent resistance of the electrolytic cell exhibits a pronounced negative correlation with current density, fundamentally attributable to the dynamic double-layer reconstruction effect [9]. When the cell voltage increases from 1.8 V to 2.4 V, the current density grows exponentially. This phenomenon aligns with the Butler-Volmer equation’s description of charge transfer processes, indicating a transition from a concentration-polarization-dominated region to an ohmic-polarization-dominated region [10,11]. Notably, the rotational speed of the circulation pump exerts a significant regulatory effect on the system’s total energy consumption: increased rotational speed elevates the electrolyte flow velocity, thereby reducing the bubble detachment diameter within the gas–liquid two-phase flow. This hydrodynamic optimization shortens bubble residence time, effectively mitigating losses in the electrolyte’s effective conductivity and ultimately achieving reduced energy consumption [12].

Hydrogen purity was measured at 99.25% using the hydrogen analyzer, confirming that both the hydrogen production equipment and testing apparatus are functioning satisfactorily and meet the fundamental requirements for conducting the experiment.

4.2.1. Temperature and Flow Rate Influence Patterns

Using produced water from the purification system as feedstock, an electrolytic cell was assembled employing foam nickel as a catalyst and an alkaline AGFA-500 membrane (Agfa-Gevaert, Mortsel, Belgium). With 1 M KOH as electrolyte, the effects of temperature and feed flow rate on hydrogen production from produced water were investigated.

According to Ohm’s law, R = U/I; at the same cell voltage, a higher current density [13] indicates lower cell resistance and reduced energy consumption. It is evident that as cell voltage increases, current density rises exponentially. Furthermore, as rotational speed increases—that is, as pump flow velocity increases—energy consumption decreases. This indicates that the high flow rates associated with high rotational speeds facilitate the rapid release of entrained bubbles in the electrolyte, thereby contributing to reduced energy consumption.

As shown in Figure 10, at 25 °C, hydrogen purity reached 80.29% following electrochemical treatment of three-phase produced water, as measured by a hydrogen analyzer. This demonstrates the feasibility of hydrogen production from wastewater. However, hydrogen yield was low under low-temperature, low-flow operating conditions, resulting in reduced hydrogen purity.

As the operating temperature was increased incrementally from 45 °C to 55 °C and further to 70 °C alongside elevated flow rates, a clear reduction in cell pressure was observed, accompanied by a significant improvement in electrolyzer performance.

When the temperature was further raised to 85 °C, increasing the flow rate resulted in only a marginal decrease in cell pressure, with the overall cell performance exhibiting a trend toward stabilization. Further analysis indicates that at 85 °C, intensified molecular thermal motion diminishes the influence of bubbles on the cell, rendering the impact of high flow rates on energy consumption negligible.

In summary, elevating the operating temperature can substantially reduce the energy consumption of the cell, while higher flow rates improve performance. This effect is particularly pronounced at lower temperatures, where slower molecular motion allows increased flow rates to effectively promote bubble separation and enhance system efficiency [14].

With the solution temperature fixed at 85 °C, variations in rotational speed and current density were observed, with the corresponding trends in cell voltage depicted in Figure 11. It is evident that a rotational speed of 23 rpm (at 1 L/min) coupled with a current density of 1 A/cm² yielded the lowest cell voltage of 2.15 V. Compared to room-temperature operation (2.51 V), this represents a 14% reduction in energy consumption, indicating that elevated temperatures promote the hydrogen production process. Furthermore, observations of hydrogen purity revealed that as temperature increased, the hydrogen production rate accelerated, air was rapidly expelled, and the produced hydrogen became purer, reaching a maximum purity of 87.12%.

4.2.2. Patterns Influencing Catalyst Performance

In previous experimental investigations, nickel foam (NF) has been utilized as a catalyst for both hydrogen and oxygen evolution reactions. However, its microstructure—characterized by a three-dimensional porous framework with a hollow skeletal morphology—proves susceptible to degradation under the sustained ionic attack present in wastewater. This structural vulnerability significantly compromises the longevity and operational stability of sewage-based hydrogen production. Accordingly, the influence of catalyst performance on hydrogen generation efficiency warrants further examination.

In preliminary studies, nickel foam served as a dual-function catalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Nevertheless, during wastewater electrolysis tests, its three-dimensional foam architecture exhibited notable microstructural shortcomings: the hollow framework was prone to collapse under prolonged ion erosion in wastewater, leading to detachment of active catalytic sites and deterioration of the conductive network [15]. This degradation culminated in a rapid decline in system durability, substantially limiting both the efficiency and stability of hydrogen production. To evaluate catalyst performance under realistic conditions, nickel foam, nickel–aluminum alloy, and nickel fibers were employed as anode and cathode catalysts, respectively, in electrolysis experiments using produced water at varying temperatures.

Through systematic analysis of the experimental data in Figure 12, the following can be observed:

(1)

Upon replacing the conventional nickel mesh with a nickel–aluminum alloy as the cathode catalyst, the cell voltage decreased significantly from 2.15 V to 2.342 V at a current density of 1 A cm⁻² (85 °C), while hydrogen purity increased from 87.12% to 95.13%. Notably, synergistic effects emerged between elevated temperature and increased flow velocity: when the temperature rose from 55 °C to 85 °C, cell voltage decreased by 0.051 V. Concurrently, heightened flow velocity enhanced mass transfer, thereby boosting current density and reducing energy consumption.

(2)

The discovery of an altered anode catalyst reveals that three-dimensional porous nickel fiber felt, owing to its abundant active sites and excellent conductivity, requires only 1.952 V to sustain 1 A cm⁻² under conditions of 85 °C and 23 RPM (1 L/min). This represents a 13% reduction in cell potential compared to nickel mesh anodes, while simultaneously enhancing hydrogen purity to 96.45%.

(3)

The optimal process parameters are as follows: employing a nickel fiber felt anode and a nickel–aluminum alloy cathode, operating at 85 °C and 23 RPM (1 L/min), achieving efficient hydrogen production with a cell voltage of 1.952 V and a current density of 1 A cm⁻², while hydrogen purity exceeds 96%.

4.2.3. Patterns of Electrolyte Concentration Influence

Through systematic analysis of the experimental data in Figure 13, the following conclusions can be drawn:

(1)

As shown in Figure 13a–c, the electrolysis temperature exhibits a significant negative correlation with system energy consumption. When the operating temperature is raised from ambient to 85 °C, the energy consumption per unit of hydrogen produced decreases monotonically, while the hydrogen purity increases to 96.48%. This indicates that elevated temperatures effectively promote the electrolytic reaction kinetics.

(2)

As shown in Figure 13d–f, within the 3M KOH electrolyte system, the regulatory effect of flow rate on system energy consumption diminishes. This is primarily attributable to the high electrolyte concentration (3M) ensuring sufficient OH⁻ ion transport flux, thereby shifting the electrode reaction process from mass transfer control to charge transfer control. Consequently, the influence of rotational speed parameters on performance is significantly reduced. The optimal operating conditions are 85 °C and 23 rpm (1.0 L/min). Under these conditions, hydrogen production using 3M KOH electrolyte reduces the cell potential from 2.15 V to 1.896 V compared to 1M electrolyte, achieving a 12% reduction in energy consumption.

(3)

As shown in Figure 13g–i, the optimum operating conditions are 85 °C and 21 rpm. Under these conditions, hydrogen production using 6M KOH reduces the cell potential from 2.15 V to 1.856 V compared to 1M electrolyte, achieving a 13% reduction in energy consumption.

(4)

The optimum process parameters are as follows: employing a nickel fiber felt anode and a nickel–aluminum alloy cathode, operating at 85 °C, 21 RPM (0.8 L/min), 6 M KOH, and a current density of 1 A cm⁻², yielding hydrogen purity of 96.58%.

Through the above experimental analysis, the optimal conditions for the produced water hydrogen production process were determined, as shown in

Table 4. Optimal boundary conditions for hydrogen production from produced water.

Variables	Optimal Experimental Parameters
Electrolyte pH	6 M KOH
Current intensity	1 A cm−2
Flow rate	0.8 L min−1
Temperature	85 °C

.

4.3. Water Quality Specifications for Hydrogen Production from Produced Water

4.3.1. Problems Encountered in the Reaction

Following 120 min of continuous operation under optimal hydrogen production conditions, online thermal conductivity monitoring revealed a marked decline in product hydrogen purity, plummeting from an initial 87.12% to 55.23%. To investigate the underlying mechanism of this anomaly, the electrolyzer system underwent analytical disassembly. Cell characterization revealed pronounced electrochemical corrosion morphology on the bipolar plate surfaces (Figure 14a), with severe localized corrosion affecting the central region of serpentine flow channels (original width 1.0 mm). Cross-sectional analysis indicated a 40% reduction in effective channel width to 0.6 mm, a structural degradation directly increasing the thickness of the reactant mass transfer boundary layer. Notably, high-density black granular deposits were detected within the flow channels. Their compact accumulation significantly increased fluid transport resistance and disrupted mass transfer equilibrium at the multiphase interface.

Further structural characterization, as illustrated in Figure 14b, reveals that a black heterogeneous deposit layer is similarly adhered to the surface of the alkaline electrolyte membrane. More critically, a 3 × 2 mm rhombohedral characteristic corrosion perforation has emerged in the lower right quadrant of the membrane. This structural defect directly leads to the failure of gas–liquid separation within the anode and cathode chambers. Analysis of the aforementioned phenomena indicates the following:

(1)

Excessively high oil content in produced water leads to the formation of black granular deposits.

(2)

Excessively high chloride ion content in produced water leads to corrosion of the electrolytic membrane.

4.3.2. Solutions

(1)

EC process for removing high crude oil content from produced water.

Following treatment by the original process, the produced water failed to meet the hydrogen production water quality requirements, resulting in excessively high oil content. After operational periods, this caused channel blockages. Consequently, an optimized two-stage EC module was introduced. Through the synergistic effects of electrical neutralization and Fenton reactions, the final effluent oil content was reduced to 6.083 mg/L (as measured by an infrared spectrophotometric oil analyzer). A fine filtration module was further added to decrease the suspended solids content.

(2)

Process improvements for chloride ion suppression.

To address operational challenges such as chloride-induced corrosion and scaling resulting from high concentrations of calcium and magnesium ions, modifications have been made to the process flow of the hydrogen production system to suppress chlorine oxidation and mitigate electrode corrosion. First, the underlying principle of alkaline electrolysis is considered: when direct current is applied across an alkaline electrolyzer, water molecules undergo redox reactions at the electrodes. According to the fundamental mechanism of alkaline water electrolysis, water molecules adsorb onto the catalyst surface where H–OH bonds dissociate, releasing hydroxide ions. Driven by concentration gradients and electrical potential differences, hydroxide ions then permeate through the alkaline membrane toward the anode, where oxygen is generated.

In a conventional symmetric feed configuration, the electrochemical potential gradient established by the difference in OH⁻ concentration between the anode and cathode chambers promotes OH⁻ migration across the membrane. This process, governed by Fick’s second law [16], also entrains Cl⁻ ions, resulting in coupled transport. To suppress chlorine oxidation, an asymmetric feed system was adopted: the KOH concentration in the anode chamber was reduced while maintaining 6 mol/L in the cathode chamber. This adjustment lowers the anode-side electrolyte concentration, effectively inhibiting the oxidation of chloride ions. Moreover, physically separating the anode and cathode feeds helps prevent chloride-induced corrosion of the electrolysis components. The optimized process flow is illustrated in Figure 15.

(3)

Replace the material of the electrolytic cell.

The corrosion resistance characteristics of different alloy materials (304 and 316 austenitic stainless steel and 2205 duplex steel) in high chloride ion concentration media and their impact on electrolytic hydrogen production performance were investigated. Monitoring revealed that 304SS exhibited severe localized corrosion after just 2 h of operation. Upgrading to 316SS extended corrosion initiation to 6 h due to enhanced passivation film stability from molybdenum addition, though continued operation still caused channel cross-sectional area reduction. In contrast, 2205 duplex steel demonstrated outstanding corrosion resistance: after 245 h of continuous electrolysis, channel geometric integrity retention exceeded 98%, with channels remaining clear and free of contaminants, as shown in Figure 16 and Figure 17.

4.3.3. Determination of Hydrogen Production Parameters

Through experimental studies on optimized pretreatment processes, the synergistic enhancement mechanism of electrochemical pretreatment on hydrogen production from oily produced water was elucidated. As shown in

Table 5. Analysis of water quality and hydrogen purity at different reaction times under optimized process conditions.

Processing Time (min)	Effectiveness of Oily Wastewater Treatment			Hydrogen Production Efficiency
	Oil Content (mg/L)	Turbidity (NTU)	Suspended Solids Content (mg/L)	Hydrogen Purity (%)	Voltage of 6M KOH, 21 (0.8 L/min) Cell Voltage (V) at 1 A cm−2 Current Density
0	112.86	260	1639.05	-	2.231
20	-	-	-	-	-
40	-	-	-	-	-
60	7.6263	36.2	65.041	88.13	2.279
80	6.0828	12.9	13.16	96.69	1.856
100	6.5721	15.3	14.136	95.19	1.889
120	6.1245	15.2	17.190	96.12	1.876

, when EC time was incrementally increased from 20 min to 80 min, the oil content in produced water (determined by UV spectrophotometry) exhibited an exponential decay pattern, ultimately decreasing to 6.08 mg/L (94.6% removal rate). Concurrently, the hydrogen production system demonstrated markedly enhanced performance: at a low cell potential of 1.856 V (Gamry constant potential mode), hydrogen purity reached 96.69%.

5. Conclusions

The electrolytic hydrogen production technology, utilizing oilfield-produced water, as proposed in this study, not only reduces wastewater treatment costs but also supplies a novel green energy source for oilfield operations, establishing a fully integrated closed-loop system. This research provides a theoretical foundation for the resource utilization of complex produced water, along with technical support for achieving the “near-zero emissions” target and developing a regional hydrogen energy supply chain in oilfield areas, yielding substantial environmental benefits and strategic value.

By establishing an integrated “water purification–electrolytic hydrogen production” process, this study successfully achieves both purification and hydrogen generation from produced water characterized by high oil content, salinity, and suspended solids. The main conclusions are as follows:

(1)

For the complex produced water from the Changqing Oilfield (suspended solids > 1600 mg/L, turbidity > 350 NTU, oil content 90–120 mg/L), a multi-stage EDCF purification process was developed. After treatment, the oil content was reduced to 6.083 mg/L (removal rate 95%), suspended solids to below 20 mg/L (removal rate 98%), and turbidity to below 20 NTU (removal rate 94%). This provides stable feedwater conditions for subsequent electrolytic hydrogen production.

(2)

Through synergistic optimization of electrochemical kinetics and mass transfer, the optimal hydrogen production performance was achieved using a nickel fiber felt anode and a nickel–aluminum alloy cathode under the following conditions: temperature 85 °C, flow rate 21 revolutions per minute (0.8 L/min), electrolyte concentration 6 M KOH, and current density 1 A/cm². The hydrogen purity attained was 96.58%.

(3)

To address corrosion in flow channels and diaphragm perforation in the electrolyzer, optimization of the feed process and adoption of 2205 duplex stainless steel as the cell material enabled stable continuous operation for 245 h without corrosion while maintaining hydrogen purity at 96.66% ± 0.5%.