Keywords

1 Introduction

The production of green hydrogen is important for sustainable and eco-friendly energy. It has diverse applications in transportation, electricity generation, and industrial processes [1]. Green hydrogen is produced through water electrolysis using renewable energy sources, resulting in carbon-neutral fuel. Large-scale adoption of green hydrogen can significantly reduce carbon emissions, mitigate climate change, and enhance energy security by decreasing reliance on fossil fuels and increasing renewable energy use. In summary, green hydrogen production promotes a sustainable energy transition and resilient, eco-friendly energy system [2].

Alkaline water electrolysis is the most promising method for large-scale green hydrogen production, but faces challenges due to high costs [3,4,5,6]. The process requires substantial electricity, which can be prohibitively expensive if derived from non-renewable sources. Capital investment and equipment maintenance contribute to the high cost, making it less competitive than alternatives like steam methane reforming. Thus, reducing energy consumption is the main goal for developing alkaline water electrolyzers (AWEs).

To reduce energy consumption, the design of the electrolyzer’s geometry structure is crucial. It determines electrolyte flow behavior and uniformity. Non-uniform flows can decrease reaction rates, cause uneven product distribution, and increase overpotential. The presence of product bubbles creates complex multiphase flow, influencing performance [7,8,9]. Therefore, optimizing the electrolyzer’s geometry is essential to improve efficiency and reduce hydrogen production costs.

Optimizing the geometry structure requires understanding flow behavior, including uniformity and bubble motion. However, commercialized electrolyzers are assembled, making internal flow configurations invisible. Flow visualization setups are essential to investigate flow behavior and test structure designs. Yet, installing an accurate, reliable, and low-cost flow visualization system in large and complex commercial electrolyzers is challenging. Modifications to the electrolyzer design or additional components may be needed, which can be expensive and time-consuming. Flow visualization setups for water electrolysis are occasionally reported in small laboratory-scale electrolyzers [7, 10,11,12]. However, designing a visualization system for commercial-sized alkaline water electrolyzers is challenging due to difficulties in design, sealing, and high cost.

In the present study, a cost-effective, flexible, and convenient electrolysis visualization and testing system were established by using 3D computer-aided design and precise CNC machining. The flow behavior and performance of three structure designs of common alkaline water electrolyzers, namely, concave-convex bipolar plate (CCBP), metal mesh as well as blank electrolyzers were visualized and investigated based on the system. The current platform enables the study of flow behavior and the evaluation of the performance of the component, leading to more effective and sustainable electrolysis technologies. The proposed platform possesses the potential to serve as a comprehensive tool for investigating flow behavior and testing the performance of various components. Its application is anticipated to be of significant assistance in advancing the development of high-performance electrolyzers.

2 Electrolysis Visualization and Test Platform

The transparent electrolyzer module is the main component of the platform. Figure 1 shows the schematic diagram of the assembled transparent electrolysis cell, which provides an inexpensive and convenient experimental platform for studying flow behavior and optimizing novel electrolysis cell structures, including bipolar designs, mesh electrodes, and novel materials. The 3D structure of the transparent electrolysis cell was created using Solidworks software, allowing real-time modification and improvement of structural design parameters. The final design can be directly processed. The main components of the transparent electrolysis module are as follows:

  1. 1.

    Transparent PMMA plate: The default structure is a flat blank, but different electrode structures can be designed and tested. Electrode types can be 3D printed and tightly fitted inside the blank structure for comprehensive testing. The inner circle diameter of the transparent plate is 220 mm, which matches the electrode diameter. This size corresponds to commercial-sized alkaline water electrolyzers with a rated hydrogen production of 2 Nm3/h.

  2. 2.

    Sealing rings: Made of transparent silicone or rubber, with a thickness of 0.6–1 mm, for efficient sealing between the PMMA plate and the Teflon sealing ring.

  3. 3.

    Teflon sealing ring: Made of PTFE material with grooves for placing the current collector ring and achieving effective sealing. The groove thickness is carefully determined for proper sealing.

  4. 4.

    Current collector ring: Connected to the power source to introduce current for hydrogen electrolysis. The outer dimensions must match the inner diameter of the collector ring sealing ring to ensure stable placement. The inside of the collector ring also has grooves for holding the electrode mesh, with a thickness matching that of the mesh.

  5. 5.

    Electrode mesh: Transmits current by contacting the collector ring and facilitates water electrolysis on the anode and cathode electrode meshes. Pure nickel mesh is used in this device.

  6. 6.

    Sealing ring: Made of silicone or rubber material, for efficient sealing between the electrode mesh and the diaphragm.

  7. 7.

    Diaphragm: Isolates hydrogen and oxygen produced during electrolysis to prevent the risk of explosion. PPS material is used as the diaphragm material in this device.

Fig. 1.
figure 1

Structure diagram of the main component of the electrolysis visualization test platform, the transparent PMMA electrolyzer:

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The proposed transparent electrolyzer module can be used to test the performance of various components, including different geometry designs, electrode materials, and diagrams. In the following section, two common plate designs are investigated on the platform, namely concave-convex bipolar plate and metal 3D mesh, as shown in Fig. 2. And the results of the two are compared with one of the ‘blank’ electrolyzers with no geometry units. The geometry designs of CCBP and 3D metal mesh are intended to enhance the flow uniformity inside electrolyzers to improve electrolyzer efficiency. CCBP is a traditional geometry design in pressure filter electrolyzers with several concave and convex round units sculptured in the bipolar plates. This structure is simple and widely used in traditional commercial electrolyzers, but it is of high processing cost. Recently, a novel structure design of a metal 3D mesh support body is developed for alkaline water electrolyzers, in which a 3D structured metal mesh is sandwiched between the flat bipolar plate and the electrodes. This design benefits the precision and speed of assembly. In the experimental section, the flow behavior and performance of the two designs would be investigated and compared.

Fig. 2.
figure 2

Two common configurations of alkaline water electrolyzer: left, concave-convex bipolar plate (CCBP), right: metal 3D metal mesh.

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3 Experiment

3.1 Experiment Setup

The assembled components of the electrolysis process visualization testing platform are secured with bolts to achieve efficient sealing. After passing the sealing test, experimental testing can be conducted. A schematic diagram of the electrolysis process visualization testing platform is shown in Fig. 3. The main equipment includes:

  1. (1)

    Transparent electrolysis module for flow visualization;

  2. (2)

    Peristaltic pump for circulating electrolytes;

  3. (3)

    Constant temperature water bath for maintaining the constant temperature of the electrolysis process;

  4. (4)

    Programmable DC power supply (ITECH, IT6724C, Auto range DC power supply) to generate constant current/voltage through the program to test the voltage of the electrolysis cell under different conditions;

  5. (5)

    Computer for recording and processing experimental data.

Fig. 3.
figure 3

Schematic diagram of experimental apparatus

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3.2 Experiment Procedure

The experimental procedure is as follows: install and connect the equipment according to the schematic diagram, and use deionized water as the working fluid. Check the tightness of the entire system to ensure that there is no leakage. After confirming no leakage, replace the deionized water with 30% KOH as the electrolyte. Turn on the peristaltic pump to circulate the electrolyte inside the platform. The KOH electrolyte enters the transparent PMMA electrolyzer from the reagent bottle through the peristaltic pump, fills the cell, and flows out to the KOH reagent bottle to achieve the circulation of the alkaline solution. At the same time, turn on the water bath to ensure the temperature of the alkaline solution is constant. In this experiment, the temperature of the alkaline solution is set to a constant 80 ℃. Before each group of experiments, set an appropriate flow rate for the alkaline solution and maintain it for more than 10 min to ensure the system reaches a steady state. Then, turn on the DC power supply, and use the programmed power supply strategy to experiment. Record the corresponding volt-ampere characteristic curve of the electrolyzer structure under different alkaline solution flow rates. Observe and record the gas-liquid flow state inside the transparent electrolyzer at the same time.

In this experiment, the output current of the DC power supply increases linearly, and the voltage of the electrolyzer is recorded every 15 s. Finally, the UI performance curve of the electrolyzer under the given conditions can be obtained. The experimental plan is attached in Table 1.

Table 1. Experimental plan in performance test
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4 Result Discussion

4.1 Flow Behavior in the Blank Electrolyzer

Initially, we conducted flow visualization tests on a blank electrolyzer structure and observed the gas-liquid flow inside, as depicted in Fig. 4. At low current (1A), large gas bubbles formed within the electrolytic cell, adhering to the electrode plate walls. This is likely due to the limited upward movement of bubbles and the absence of strong circulation at low current density, resulting in mild gas motion. Consequently, bubbles stick to the electrode mesh and walls due to less drag force, as shown in the figure. As the current density increases, smaller bubbles are generated, exhibiting mist-like movements that are not directly visible to the naked eye. Simultaneously, internal gas bubbles display circulation within the electrolyzer, moving upward along the axial line of the cell. However, due to limited alkaline liquid discharge, a significant portion of bubbles cannot be expelled in time when reaching the outlet. As a result, they reverse direction along the electrolyzer’s edge, creating an internal circulation driven by the density difference between gas and alkaline liquid, facilitated by lift force. This internal circulation disrupts the alkaline liquid flow, preventing bubble adhesion to the cell’s walls and directing them towards the outlet alongside the alkaline liquid. Additionally, near the electrolyzer inlet, a circular region of lower gas-phase fraction (void zone) was observed. This occurs because the flow velocity is higher near the inlet, causing rapid upward movement of gas bubbles generated by the electrolyzer in that position (Fig. 4).

Fig. 4.
figure 4

Diagram of multi-phase flow behavior in the blank electrolyzer, showing a void zone near the entrance and internal convection of product gas bubbles at a higher current (I = 10A)

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4.2 The Influence of Flow Rate for Blank Electrolyzer

We conducted voltammetric measurements on the blank electrolyzer at different flow rates, as depicted in Fig. 5. Increasing the flow rate of the alkaline solution leads to a decrease in cell voltage at the same current density, indicating enhanced electrolyzer performance. This is likely due to the improved removal of bubbles from the cell, reducing gas holdup and decreasing ohmic overpotential. However, our previous findings showed that increasing the flow rate reduces cell uniformity, which should increase the cell voltage and imply reduced performance. This paradox reflects the combined effects of various factors within the electrolyzer. As a highly nonlinear system, the electrolyzer’s performance is influenced by complex factors, resulting in different outcomes under different conditions. In our experiment with a small electrolyzer diameter, increasing the flow rate in the low range improves cell performance more significantly due to faster bubble removal, outweighing the performance reduction caused by decreased uniformity. Thus, an increase in the flow rate improves electrolyzer performance within this range. However, it is anticipated that continuing to increase the flow rate beyond a certain threshold will cause the performance improvement from bubble removal to be insufficient in compensating for the performance reduction due to decreased uniformity. At that point, electrolyzer uniformity becomes the controlling factor for performance. This conclusion requires further experimental verification.

Fig. 5.
figure 5

Polarization curve measurement of blank electrolyzer at different inflow rates.

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4.3 Influence of Different Geometries

Diagrams of multi-phase flow behavior in CCBP and mesh support electrolyzer are shown in Figs. 6 and 7, respectively. As shown in the figure, the void zone of the CCBP electrolyzer is much smaller than that of the blank electrolyzer, indicating an enhanced flow uniformity. From the observation, the flow uniformity in the mesh electrolyzer is the best of the three. The electrolyzer-scale internal convection of product bubbles is not obvious and is confined to the local area. Because of the geometries like CCBP and metal mesh, the internal convection inside of the electrolyzer is subdued, resulting in faster gas product ejectment. Thus, energy consumption is reduced. This is validated by polarization curve measurement, as is shown in Fig. 8. It can be observed that at the same current, the voltage of the blank structureelectrolyzer is significantly higher than that of the other structures, indicating higher resistance and poorer performance. Additionally, as the current gradually increases, the slope of the voltage increase for the blank structure electrolyzer is significantly larger than that of the other structures, indicating poorer performance. Combining the results of the visual experiments, the observed deficiencies in the performance of blank electrolyzer can be attributed to the absence of structural elements, leading to uneven lateral fluid distribution, circulating flow, and non-uniform distribution of fluid within the chamber, which results in the formation of dead zones. The non-uniform flow distribution and resultant dead zones increases the residence time of the production bubbles, which causes considerable larger overpotential. Moreover, at lower currents, the 3D metal mesh structure electrolyzer has the lowest voltage, and as the current increases, the voltage of all the electrolyzer gradually increases. It was found that the voltage of the 3D metal mesh structureelectrolyzer gradually approaches that of the conventional CCBP, and when the current is 20 A, the voltage of the electrolyzer from high to low is blank structure, 3D metal mesh, and CCBP. Among them, the voltage of the 3D mesh structure electrolyzer is higher than that of the traditional CCBP structure, indicating its poorer performance. However, the fluid uniformity of the 3D metal mesh structure is superior to that of the CCBP structure, so why is its electrolysis performance worse than that of the CCBP structure? One possible reason is that through the visual experiment shown in Fig. 7, it was found that when the current density is high, due to the overly dense 3D mesh in the experiment, a large number of bubbles are generated and attached to the 3D metal mesh to form larger bubbles, as shown in Fig. 7, which increases the ohmic overpotential inside the electrolyzer, thereby reducing the performance of the electrolyzer. In summary, the above results confirm that improving the uniformity of fluid does indeed have a positive effect on the performance of the electrolyzer. However, at the same time, the uniformity of fluid flow is not the only factor affecting the performance of the electrolyzer. The structure inside the electrolyzer, such as the obstruction characteristics of product bubbles, also has an important impact on the performance of the electrolyzer. This also demonstrates the complexity of the electrolytic hydrogen production process (Fig. 8).

Fig. 6.
figure 6

Diagram of multi-phase flow behavior in CCBP electrolyzer, showing improved flow uniformity with reduced void zone area.

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Fig. 7.
figure 7

Diagram of multi-phase flow behavior in mesh support electrolyzer, showing bubble impediment.

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Fig. 8.
figure 8

Polarization curve measurement of CCBP, 3D metal mesh, and blank electrolyzers at different inflow rates.

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5 Conclusion

To reduce the energy consumption of water electrolysis, it is important to design electrolyzers with improved flow uniformity, which requires a thorough understanding of flow behavior. Visualizing the flow behavior inside electrolyzers is crucial for this investigation. However, due to difficulties in sealing and fabrication as well as high cost, visualization systems for commercial-sized alkaline water electrolyzers are rare. In this study, an in-situ electrolysis visualization and performance evaluation platform were developed using 3D computer-aided design and precise CNC machining. The platform consists of a transparent bipolar plate and essential electrolyzer components, enabling in-situ observation of multiphase flow configurations during electrolysis and measurement of polarization curves. Two geometries, namely CCBP and 3D mesh electrolyzers, were investigated and tested, and the results were compared with those obtained from a blank electrolyzer. In the blank electrolyzer, internal convection of product bubbles was observed, whereas the CCBP and 3D mesh geometries were effective in achieving improved flow uniformity. The polarization curve measurements indicated that the improved flow uniformity in CCBP and 3D mesh electrolyzers reduced the cell voltage. However, bubble impediment was observed in the 3D mesh electrolyzer, resulting in an inferior performance at higher current densities. The current platform enables the study of flow behavior and component performance, leading to more effective and sustainable electrolysis technologies, which can be applied as a general evaluation platform for the further development of high-performance electrolyzers, including structure design, electrode materials, and other components.