## 1 Introduction
Among them, BP, also known as a flow field plate, is one of the key materials of PEMWE cell, accounting for about 48% of the total cost of the cell [42; 43; 44]. Reducing the cost of BPs is of importance for the application of the PEMWE. The BP is engraved with flow channels (such as parallel flow field, crossfinger flow field, bionic flow field, etc.). In the process of water electrolysis, the bipolar plate plays the role of conducting electrons, delivering and distributing reactant water to the anode, and collecting and outputting products H\({}_{2}\) and O\({}_{2}\). As the core component of PEMWE, it is also of great significance to the output power, service life, and cost control of PEMWE cell [1]. Therefore, in addition to the optimization of plate flow channel design, the research on BPs also needs to pay attention to BP materials and protective coating as shown in Figure 2. BPs need not only excellent conductivity but also good corrosion resistance and a process convenient for mass production to adapt to the harsh environment, e.g., high potential and acidic environment in PEMWE cell [45; 46]. Metallic materials are very suitable for BP because of their good plasticity, high conductivity, and high strength [50; 51]. Particularly, stainless steel and titanium BPs feature the advantages of high corrosion resistance, high strength, high thermal conductivity, low permeability, and low resistivity. The titanium BPs exhibit good corrosion resistance in the high potential and acidic environment of PEMWE, and the corrosion products of titanium BP are far less toxic to the proton exchange membrane and catalyst than other metals [52; 53]. However, during PEMWE operation, an acidic environment and high potential will cause a dense oxide film on the surface of a titanium BP, increasing the interface contact resistance with the collector, thus increasing the ohmic loss, and reducing the output power of the stack [54]. Moreover, hydrogen embrittlement of titanium also exists during the HER (cathode), which is unacceptable for long-term operation. In long-term operation, the titanium plate will absorb more than 1000 ppm hydrogen during 500 h of operation [55; 56]. Hydrogen embrittlement can damage the mechanical properties of titanium, such as loss of ductility and tensile strength [57; 58]. The modification of the  
Figure 1: Schematic diagram of PEMWE working principle. Figure 2: The photos of bipolar plate with (**a**) polymer coating [47], (**b**) TiN coating [48] and (c) gold coating [49]. titanium BPs with protective coatings can greatly reduce the corrosion rate. The commonly investigated measure of a bipolar plate is to prepare a layer of highly conductive and corrosion-resistant coating on the surface. At present, the surface modification of BP coating mainly includes: (1) precious metal coating, such as Au, Pt, etc., prepared on the substrate surface [59]; (2) metal nitrides/carbide coating [46; 60]; and (3) conductive polymer coatings [61]. The Au, Pt, and other precious metal coatings modified BPs (Figure 2) have excellent performance and can meet the requirements of PEMWE [59; 62]. There is quite a bit of research on mixing these precious metals with other non-noble metal nanomaterials to form a composite coating in order to reduce the usage of precious metals. It is true that both improved corrosion resistance as well as low interfacial contact resistance have been achieved. However, the high cost still limits its commercial development. While for the metal nitrides/carbide coating, the coating process is mainly carried out via _chemical vapor deposition_ (CVD) [63; 64], physical vapor deposition (PVD) [65; 66], and other technologies, which have high equipment requirements or thermal treatment under high temperatures and special atmosphere oven, and they are not suitable for large-scale commercial production. Generally, the research on the surface modification of metallic BPs for PEMFC is commonly used for reference in the PEMWE application, and there is not much research in the area of PEMWE.