## 1 Introduction  
Hydrogen is one of the most promising clean and sustainable energy carriers. Generally, the current hydrogen production technologies mainly include _steam_ reforming of fossil fuel or biomass gasification, methanol-reforming, an industrial by-product, and water splitting by electrolysis or photocatalysis [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. At present, the process of hydrogen production by water electrolysis has the potential to utilize the fluctuant power generated with renewable energy. Hydrogen and oxygen can be obtained by splitting water with the advantages of environmental friendliness and high purity (99.999%) [11; 12; 13; 14; 15; 16; 17]. Among them, proton exchange membrane water electrolysis (PEMWE) using pure water as a reactant has the advantages of low hydrogen permeability, high purity of hydrogen generated, and only removing water vapor [18; 19; 20; 21]. In addition, PEMWE adopts a zero spacing structure with low ohmic resistance, which can significantly improve the overall efficiency of the electrolysis process. Furthermore, it is compact and flexible in operation, which can overcome the uncertainty and volatility in the process of renewable energy power generation [22]. Additionally, the pressure of PEMWE can be adjusted and controlled in a wide range, and the hydrogen output pressure can reach several MPa, which can adapt to the rapidly changing renewable energy power input [23]. Therefore, PEMWE is a green hydrogen production technology with great development prospects [2; 24]. The PEMWE mainly uses perfluorinated sulfonic acid solid polymer as an electrolyte and Ir, Ru, or Pt-based precious metals as catalysts [25; 26; 27]. Essentially, PEMWE is a reverse device of the PEM fuel cell (PEMFC) [28; 29]. The PEMWE uses an external power supply to make water electrolysis to generate hydrogen and oxygen. During the PEMWE, as shown in Figure 1, pure water is sent to the anode through the anode bipolar plate with the water pump, then penetrates into the porous diffusion layer, and reaches the catalytic layer. Oxygen evolution reaction (OER) will occur at the anode to produce oxygen (O\({}_{2}\)), protons (H\({}^{+}\)), and electrons (e\({}^{-}\)). These protons diffuse to the cathode side through the PEM, and the electrons leave the anode through the external power circuit [30; 31; 32]. After the protons reach the cathode, the electrons transported by the cathode catalyst layer and the external power supply will recombine with them to create a hydrogen evolution reaction (HER) [21]. Figure 1 shows its working principle. The specific equation is as follows:  
\[\text{Anode}:\ H_{2}O\to 2H^{+}+\frac{1}{2}O_{2}+2e^{-} \tag{1}\]  
\[\text{Cathode}:\ 2H^{+}+2e^{-}\to H_{2} \tag{2}\]  
\[\text{Overall}:\ H_{2}O\to H_{2}+\frac{1}{2}O_{2} \tag{3}\]  
In PEMWE, the anode is the main source of the entire overvoltage, and the anode electrocatalys has a very important influence on the entire water electrolysis reaction [33; 34; 35]. At present, the preparation of highly active and stable oxygen evolution electrocatalys has been the key problem in PEMWE, and a lot of research has been carried out around the world [36; 37]. At the same time, due to the high potential, strong oxidation, and acidic solution environment of the anode of PEMWE, general carbon materials and metal materials are very easily corroded and dissolved in PEMWE, which leads to the destruction of the bipolar plate structure [38]. Therefore, titanium with high stability and conductivity has become one of the most promising metal materials for preparing the diffusion layer in PEMWE [39; 40]. However, the high cost of titanium is still the limiting condition for its large-scale application. The bottleneck of PEMWE lies in its cost and service life [41]. The research on reducing the cost of PEMWE focuses on the core components such as the catalyst, membrane electrode, gas diffusion layer, and bipolar plates (BPs).