1. Introduction
In the last decade, the impact of global warming has become more apparent and an environmental crisis. One of the main reasons causing global warming is the wide employment of fossil fuels. Therefore, it is urgent to find clean alternatives to replace traditional fossil fuels [
1,
2,
3]. For the possible clean energy sources, hydrogen has received much attention due to its great potential for practical applications. It possesses higher energy density than other types of fossil fuels, and the cost of developing hydrogen energy is affordable [
4]. Numerous studies have been devoted to the mass production of hydrogen in the literature, in which the electrolysis method is an attractive way to produce hydrogen by splitting water into hydrogen and oxygen in a proton exchange membrane electrolyzer cell (PEMEC) [
5,
6,
7]. In this device, a solid polymer electrolyte membrane (PEM) typically made of a fluorinated polymer (i.e., Nafion) is used to separate the cathode flow channel and the anode flow channel [
8,
9]. The water molecules can be split into hydrogen ions and oxygen gas by passing a current through the cell. The hydrogen ions are transported through the PEM to the cathode side of the cell, and the oxygen gas is released at the anode side simultaneously. The PEM is selective in allowing only protons to pass through while blocking the passage of electrons and other species [
10,
11]. The PEMEC has been used for various purposes, such as hydrogen production in fuel cell vehicles, energy storage, and industrial processes. It has also been considered a promising technology for hydrogen production from renewable energy sources, such as wind [
12] and solar [
13].
The usage of PEMEC for hydrogen production has many advantages. One is its high efficiency in hydrogen generation, which means that most of the supplied electrical energy into the cell could be converted into chemical energy stored in hydrogen gas [
14]. In addition, the produced hydrogen has high purity, which is essential for many applications, such as fuel cells [
15]. It can also operate at relatively low temperatures, typically between 50 °C [
16] and 80 °C [
17]. This characteristic reduces its energy consumption and enables the usage of inexpensive materials in the cell. It also makes it easier to start and stop the cell as needed. However, the PEMEC has one major demerit: low durability, with a short lifespan of around 10,000 h [
18]. After that, the device needs to be replaced or refurbished, which increases the operating cost and reduces the technology’s economic viability.
The pressure drop across the water flow channel is one of the primary causes that may significantly affect the system’s durability. Excessive pressure drop can increase the mechanical stress inside the cell structure, resulting in a short lifespan [
19]. An increase in pressure drop can also produce several adverse effects on the cell; for example, the transport of water from the bulk channel to the membrane depends on the difference in water concentration between these two sites. An increase in pressure drop indicates a lower pressure in the flow channels, which generally implies a lower concentration of water. As a result, the rate of water transport to the membrane would be reduced, which could lead to insufficient hydration and poor cell performance [
20]. In addition, the heat generated during the reaction in the cell needs to be removed to maintain the cell temperature within a suitable range [
21]. The heat transfer rate can be affected significantly by the pressure drop. In the study by Nie et al. [
22], it was noted that due to the relatively high pressure drop within the inlet and outlet channels, the flow velocity tends to increase as the flow approaches the end of the channel, which leads to the occurrence of reverse flow within the channel. Hence, the overall fluid flow rate is reduced, which degrades the performance and durability of the cell. Furthermore, a higher pressure drop represents a greater pressure difference between the inlet and the outlet of the PEMEC. Because the pressure in the flow channel would affect the water transport rate through the porous layer to the membrane, the uniformity of electrolyte distribution would be worse under a higher pressure drop. It may also result in local flooding or drying out of the membrane. This phenomenon could lower the cell performance greatly [
22]. In summary, the pressure drop across the water flow channel influences the water transport, heat transfer, and electrolyte distribution in a PEM electrolyzer cell. Therefore, it is essential to optimize the design of the water flow channel to minimize pressure drop and enhance cell performance and durability [
23].
In the recent experimental study conducted by Kang et al. [
24], they developed a dual-layer structure of a thin/tunable liquid/gas diffusion layer or porous transport layer (TT-LGDL/PTL) to improve the mass transport performance for a PEMEC. Their results revealed that the dual-layer structure presents smaller ohmic resistance and mass transport resistance. Consequently, the PEMEC performance could be enhanced significantly. The dual-layer structure consists of an ~830
pore TT-LGDL/PTL stacking on a ~100
pore TT-LGDL/PTL. They suggested that it is strongly feasible to raise the PEMEC efficiency by stacking the in-plane transport enhancement layer with large pore sizes onto a TT-LGDL/PTL with small pore sizes. However, they did not explore how the dual-layer porous structure affects the flow field and the pressure variation in the flow channel.
The flow field within a PEMEC is important as it governs the processes of mass transport and distribution within the cell. Numerous researchers have utilized computational fluid dynamics (CFD) simulations to offer insights and recommendations for design enhancements and optimization of parameters. Ruiz et al. [
25] employed computational fluid dynamics (CFD) to simulate the pressure distribution across different flow paths and to analyze hydrogen generation. Nafchi et al. [
26] conducted numerical simulations and identified that reducing the film thickness, channel height, and width can effectively decrease electrical pressure while improving overall efficiency. In another study, Tijani et al. [
27] examined the hydrodynamic characteristics of three different flow plate designs, revealing that parallel flow channel configurations exhibited the most promising outcomes.
The optimal flow channel design with dual porous layers has also not been investigated yet. Accordingly, the present study intends to perform exploration to optimize flow channel design for a PEMEC with a dual porous-layer structure. The Taguchi method was employed to perform the analysis for optimization. This method has been widely used to determine the optimal operating parameters for a PEM electrolyzer cell system [
28,
29] or a PEM fuel cell system [
30]. Toghyani et al. [
28] used the Taguchi method to optimize the operating parameters for a PEMEC to decrease the required input voltage. They also employed the analysis of variance (ANOVA) method to evaluate the significance of each of the parameters, and the results showed that the anode exchange current density presents the most significant effect on the input voltage. Saikia et al. [
29] considered the electrolyzer system integrated with a solar photovoltaic device and used the Taguchi method to optimize the hydrogen production rate. Several key operating parameters were selected in the analysis, and they found the hydrogen production rate could be raised dramatically after optimization. Chen et al. [
30] used four methods, including the Taguchi and ANOVA, to optimize the PEM fuel cell stack’s inlet/outlet flow channel geometries. The results indicated the tube diameter is the most impactive factor on the pressure uniformity within the stack.
Because a uniform pressure distribution is quite important for a PEMEC to lengthen its lifespan, this study explores how to minimize the pressure drop across the flow channel by adjusting the geometric design of the flow channel and the geometric properties of the dual-layer porous structure. The flow field in the PEMEC is simulated by computational fluid dynamics (CFD). Then, the Taguchi method is used in the optimization analysis, and the impact of each selected parameter is evaluated by the analysis of variance (ANOVA) method. The results will benefit the enhancement of PEMEC performance.