## 5 Conclusions and Perspectives  
Among the various water electrolysis hydrogen production technologies, PEMWE has the advantages of high current density, high purity of hydrogen production, and fast response speed, which has attracted wide attention from the scientific community and industry. Electrolysis efficiency, cost, and life are important indicators of PEMWE. As a supporting part and current collector of the PEMWE cell, the BPs require high corrosion resistance and low interfacial resistance. The industries generally believe that metallic BPs are a potential choice for PEMWE in the future, which can be manufactured through an embossing process with a low cost, thin thickness, and high-volume specific power and specific energy density. However, the corrosion and passivation of the metallic surface during the operation of PEMWE could result in high ICR during the long-term operation and, thus, increase the ohmic loss, and reduce the output power of the stack. This puts forward the urgent demand of \"solving the corrosion resistance problem of metallic BPs without affecting the contact resistance\". Currently, metallic BPs with protective coatings, such as precious metals, nitrides, carbides, etc., are effective ways. While the disadvantage of the precious metal coating is high cost, it has high conductivity and remarkable corrosion resistance. While the titanium nitrides/carbide coatings, which have high equipment requirements or thermal treatment under high temperatures and special atmosphere oven, are not suitable for large-scale commercial production.  
At present, the surface modification of metallic BPs based on conductive polymers, such as silane treatment, PANI, PPy, and polymer mixed layers promoted by some nanoparticles, have good corrosion resistance, excellent conductivity, and low production cost, which is one of the development trends. The aim of this review is to summarize and discuss the protective mechanisms, preparation processes, and applications of the above-mentioned polymer-based coatings of BPs for PEMWE in depth. (1) As for the silane treatment, the dense silane layer can not only effectively enhance the corrosion resistance but also improve the adhesion between the substrate and the conductive polymers. (2) As for the conductive polymeric coatings, the poor electrical conductivity of the polymer and serious peeling off during long-term operation greatly limit the application according to the literature. (3) While for the nanoparticle-promoted polymer hybrid coatings, the composition, dispersion of nanoparticles, microstructure, etc., can be controlled to balance the conductivity and mechanical properties. Especially, nanoparticle-promoted polymer hybrid coatings composed of polymers and conductive noble metals or nitrides/carbides, which have the advantages of a simple preparation process, low cost, and large-scale production, are of great significance to the commercial applications of PEMWE and have gradually become a research hotspot.  
Conceptualization, G.L. and F.H.; validation, G.L. and F.H.; formal analysis, G.L. and F.H.; investigation, G.L. and F.H.; resources, G.L.; data curation, G.L. writing--original draft preparation, G.L. and B.F.; writing--review and editing, G.L., B.F., and X.W.; visualization, G.L. and B.F.; supervision, G.L.; project administration, G.L.; funding acquisition, G.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development plan of China (Grant No. 2018 YFB1502403) and the Communication Program for Young Scientist in USTB (Grant No. QNXM20220015).  
Not applicable.  
Data Availability Statement: Data will be available upon request from the corresponding authors.  
The authors declare no conflict of interest.