_applied sciences_  
_Review_  
**Conductive Polymer and Nanoparticle-Promoted Polymer Hybrid Coatings for Metallic Bipolar Plates in Proton Membrane Exchange Water Electrolysis**  
**[PERSON] 1,2,*(c)**, **[PERSON] 1,2**, [PERSON] 1,2** and [PERSON] 1,2,*(c)  
1 Department of Energy Storage Science and Technology, University of Science and Technology Beijing, 30 College Road, Beijing 100083, China  
2 Department of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 College Road, Beijing 100083, China  
* Correspondence: [EMAIL_ADDRESS] (G.L.); [EMAIL_ADDRESS] (B.F.)  
**Abstract: Proton exchange membrane water electrolysis (PEMWE) is a green hydrogen production technology with great development prospects. As an important part of PEMWE, bipolar plates (BPs) play an important role and put forward special requirements due to the harsh environments on both the anode and cathode. Recently, metal-based BPs, particularly stainless steel and titanium BPs have attracted much attention from researchers all over the world because of their advantages of high corrosion resistance, low resistivity, high thermal conductivity, and low permeability. However, these metallic BPs are still prone to being oxidized and are facing with hydrogen embrittlement problems in the PEMWE working environment, which would result in reduced output power and premature failure of the PEMWE stack. In order to reduce the corrosion rate and maintain low interfacial contact resistance, the surface modification of the metallic BPs with protective coatings, such as precious metals (e.g., Au, Pt, etc.) and metal nitrides/carbides, etc., have been extensively investigated. However, the above-mentioned coating materials are restricted by the high-cost materials, complex equipment, and the complicated operation process. In this review, the surface modification of metallic BPs based on silane treatment, conductive polymers, e.g., polyaniline (PANI) and polypyrrole (PPy) as well as some nanoparticles-promoted polymer hybrid coatings which have been investigated for PEMWE, are summarized and reviewed. As for the silane treatment, the dense silane can not only effectively enhance the corrosion resistance but also improve the adhesion between the substrate and the conductive polymers. As for PANI and PPy, the typical value of corrosion current density of a PANI coating is 5.9 \(\mu\)A cm\({}^{-2}\), which is significantly lower than 25.68 \(\mu\)A cm\({}^{-2}\) of the bare metal plate. The introduction of nanosized conductive particles in PANI can further reduce the corrosion current density to 0.15 \(\mu\)A cm\({}^{-2}\). However, further improvement in the electrical conductivity is still desired to decrease the interface contact resistance (ICR) to be lower than 10 m\(\Omega\) cm\({}^{2}\). In addition, serious peeling off of the coating during long-term operation also needs to be solved. Typically, the conductive polymer reinforced by graphene, noble metals, and their compounds in the form of nanoparticle-promoted polymer hybrid coatings could be a good choice to obtain higher corrosion resistance, durability, and conductivity and to extend the service life of PEMWE. Especially, nanoparticle-promoted polymer hybrid coatings consisting of polymers and conductive noble metals or nitrides/carbides can be controlled to balance the conductivity and mechanical properties. Due to the advantages of a simple preparation process, low cost, and large-scale production, nanoparticle-promoted polymer hybrid coatings have gradually become a research hotspot. This review is believed to enrich the knowledge of the large-scale preparation process and applications of BPs for PEMWE.**  
**Keywords: proton exchange membrane water electrolysis; bipolar plates; conductive polymer; inorganic nanoparticles; coatings; corrosion resistance; contact resistance**