Conductive Polymer and Nanoparticle-Promoted Polymer Hybrid Coatings for Metallic Bipolar Plates in Proton Membrane Exchange Water Electrolysis

: 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


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, the 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: Anode : Cathode : Overall : In PEMWE, the anode is the main source of the entire overvoltage, and the anode electrocatalyst 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 electrocatalysts 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).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, cross finger 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 longterm 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 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.However, there is an urgent demand for "solving the corrosion resistance problem of metallic BPs without affecting the contact resistance" which is of great significance to the commercial applications of PEMWE and has gradually become a research hotspot.Herein, there exist several surface modifications of metallic BPs based on silane treatment, conductive polymers, e.g., polyaniline [67] and PPy [68][69][70], as well as some nanoparticle-promoted polymer hybrid coatings that have both good electrical conductivity and good corrosion resistance and have been investigated for PEMWE.The application of conductive polymers and some nanoparticle-promoted polymer hybrid coatings has a bright future, as the preparation process of polymer coatings is simple, inexpensive, and suitable for large-scale production (Table 1) [71,72].In this review, both the pretreatment of the metal surface with silane treatment technology and the mechanism and application of the conductive polymers and polymer-based hybrid coatings to prevent the oxidization and the hydrogen embrittlement of the metallic surface are summarized for the first time.It is believed that this review could enrich the knowledge of the large-scale preparation process and applications of polymer-based coatings for PEMWE.

Silane Treatment Technology
As a new technology, silane treatment technology can significantly improve the corrosion resistance of metal by preparing a layer of silane coating on the metal surface [73,74].The dense silane layer not only undertakes the charge transfer from the metal surface to the electrolyte solution interface as a conductive layer but also restricts the direct contact between metal and electrolyte solution as an effective isolation layer, which is considered to be the reason why the silane layer can effectively improve the corrosion resistance of metal [75,76].In addition, organic silane coating can be used as a "bridge" for the close connection between metal and other polymer coatings, greatly improving the adhesion between the organic coating and the metal.Silane treatment has the advantages of simple operation, strong adhesion, and environmental safety.Generally, the silane coating has a wide application prospect, and it is affected by the polarity of functional groups of silane itself.Silane coupling agents are generally divided into hydrophobic and hydrophilic.Silanes with non-polar functional groups are hydrophobic and need to use a cosolvent to make them soluble in water for hydrolysis.The structure of silane is generally Y-R-SiX 3 or R'-(CH 2 )n-Si(OR) 3 where R is an aliphatic carbon chain, R' (or Y) is a group that can react with organic compounds (such as epoxy group, amino group, etc.) to improve the compatibility and reactivity of polymer and silane, and OR (or X) is a hydrolytic group (such as acetoxy, alkoxy, etc.), which can bond some metals, SiO 2 , etc.Because of these two functions, silane molecules can connect organic materials and inorganic materials to form an "organic silane chain inorganic" intermediate layer with a certain network structure.Edzatty et al. [77] pointed out that hydroxysilane with a carbon functional group can be used for metal surface treatment.If the silane coupling agent does not have an organic group, it can generally be used to prepare a single protective film.For example, compounds with epoxy silane that can match the coating generally have specific carbon functional groups, which can improve the adhesion between the substrate and the organic coating [78].
There are many causes for silane molecules to adsorb and form films on the surface of metal substrates.At present, most people believe that the chemical bond theory proposed is the closest to the actual situation [79,80].Figure 3 shows the theoretical model of the silane chemical bond, which summarizes the action process of silane on the substrate surface in three steps: (1) Activation of silane molecules.Hydrolysis reaction will occur during the preparation of silane.Under the action of water molecules, the Si-X group will hydrolyze to generate Si-OH, as shown in formula (4).With the gradual hydrolysis of (Si-OR)n, a large amount of Si-OH will be generated in the solution, which will activate the silane molecules for further reaction. Si (2) Adsorption and condensation of silane molecules.After the metal substrate is put into the silane hydrolysate, the activated Si-OH and hydroxylated substrate surface (i.e., Me-OH group) are rapidly combined in the form of hydrogen bond, so as to adsorb on the metal surface, as shown in formula (5).At the same time, there are still a large number of active Si-OH groups in the solution [81,82].Adjacent Si-OH is prone to intermolecular dehydration and condensation reaction, forming an outer silane membrane with a Si-O-Si interpenetrating network structure, as shown in formula (6).
(3) Silane molecules are solidified into films.After adsorption, silane molecules are solidified on the surface of the substrate.In this process, due to the instability of the hydrogen bond, groups lose one molecular water and form stable Si-O-Me covalent bonds, as shown in formula (7).The curing and film forming need to be heated at a certain temperature.Different silane curing processes need different temperatures.Generally, the curing temperature should be greater than 90 • C.During the curing process, a dehydration reaction will occur between hydrogen bonds, forming Si-O-Me bonds at the interface where the surface metal participates in bonding, which can significantly improve the bonding strength between the metal matrix and silane.At the same time, the remaining silanol hydroxyl groups adsorbed on the surface of the matrix that do not participate in bonding at the interface will dehydrate and condense with each other to form Si-O-Si bonds, thus forming a layer of cross-linked mesh three-dimensional silane film on the metal surface.Generally, one end of the silane coupling agent is hydrolyzed with three silanol hydroxyl groups, but the energy required for bonding with the metal substrate surface is different.When one silanol hydroxyl group is bonded, the other two will no longer bond, or dehydrate and condense with the silanol hydroxyl groups in adjacent molecules, or finally exist in the silane film in a free state at one end.
The complete impregnation method and electrodeposition method are the main methods for preparing silane membranes on metal surfaces.Under the action of hydrogen bonding, the silane solution will quickly adsorb to the metal surface after contact with the metal, compared to mature chromate passivation that requires a longer period of dipping, and silanes take much less time to complete adsorption on metal surfaces.
Generally, the surface modification of the metal surface with silane has the following advantages: simple process, non-toxic, non-polluting, a wide range of applications, low cost, and the anticorrosion effect is better than the traditional phosphating, and passivation process.The silane-treated metal surface can also enhance the adhesion between the metal matrix and organic coating.

Polyaniline (PANI)
PANI is one of the earliest found conjugated conductive polymers, and its conductivity is between 5 and 10 S cm −1 .Polyaniline is a typical organic conductive polymer, and although the π electrons in its structure have the ability to delocalize, it is not a free electron.The conjugated structure in the molecule increases the π electron system; the electron delocalization is enhanced.When the conjugated structure reaches a large enough scale, the compound can provide free electrons, so as to be able to conduct electricity.
Many studies on the mechanism and applications of polyaniline in metal corrosion prevention show that the corrosion prevention mechanism of polyaniline on metal mainly has several points (Figure 4): (1) Wessling's [83] research on metal passivation theory found that doped polyaniline can catalyze itself and form a passivation film on the metal surface after oxidation by first reducing.In addition, polyaniline has a self-repairing effect.When the passivation film is damaged, polyaniline can repair the passive film and maintain its stable existence for a long time.(2) Shielding effect: it was found by electrochemical impedance spectroscopy (EIS) that when the thickness of electrochemical polymerized polyaniline coating was greater than 1 µm, the polyaniline coating can completely separate the metal from the corrosion medium, significantly delaying the metal corrosion, which is the physical shielding effect of the polyaniline coating [84].(3) Electric field protection of polyaniline will generate a special electric field at the metal interface.During corrosion, the direction of electron transmission is opposite to that of the electric field.The electric field force becomes an important factor preventing the movement of electrons from the metal to the oxidant.At this time, the metal surface is protected by the electric field [85].In recent years, polyaniline has obtained more and more applications in the field of hydrogen energy due to its protective effect on metals.Jiang et al. [86] deposited a polyaniline layer of 2.5 µm on the surface of 304 stainless steel, showing stable structural characteristics and good corrosion resistance.The adhesion characteristics of the polyaniline coating on the 304 stainless steel did not change significantly within 648 h, and the polarization current density was stable at 0.059 µA cm −2 , which is significantly better than the original 304 stainless steel, revealing the great potential of polyaniline in the field of metal BP coating.

Polypyrrole (PPy)
PPy is a kind of heterocyclic conjugated conductive polymer with pyrrole as a monomer and is made into a conductive film by electrochemical oxidation polymerization or synthesized by the chemical polymerization method [68,88].It is a conductive polymer with good stability and easy electrochemical polymerization into a film, and its conductivity can reach 10 2 -10 3 S cm −1 .The conduction mechanism is: the PPy structure has a conjugated structure composed of carbon-carbon single bonds and carbon-carbon double bonds arranged alternately [89,90].The double bonds are composed of σ electrons and π electrons, and σ electrons are fixed and cannot move freely, forming covalent bonds between carbon atoms [91].The 2 π electrons in a conjugated double bond are not fixed to a certain carbon atom, they can transpose from one carbon atom to another: that is, they have a tendency to extend throughout the molecular chain [92].
Tan et al. [93] (Figure 5) successfully prepared Ti 4 O 7 nanoparticles using titanium powder as a reducing agent, and then prepared a dense Ti 4 O 7 nanoparticles doped polypyrrole (PPy) composite coating on the 316 L surface using constant current deposition technology.Conductive Ti 4 O 7 nanoparticles were modified with methyl sulfonic acid.Ti 4 O 7 nanoparticles were combined with sulfur-doped acid and the PPy chain through electrostatic action as well.In the cathode environment of PEMFC, the composite coating maintained high chemical stability.At the same time, the addition of Ti 4 O 7 nanoparticles also improved the conductivity and hydrophobicity of the PPy coating.In further study, the PPy-Ti 4 O 7 coating showed amazing stability, and the impedance was basically maintained between 5.78 and 6.80 Ω cm 2 within 50 h in the simulated environment at 70 • C. Akula et al. [94] applied conductive polymer (2-amino-5-mercapto-1,3,4-thiadiazole)/polypyrrole (PAMT/PPy) singlelayer and double-layer conductive composite coatings on 316 L stainless steel using the cyclic voltammetry and galvanostatic method.The polarization measurements of the coating by cyclic voltammetry (CV) showed that compared with PPy on the PAMT double-layer coating, the PAMT on the PPy double-layer coating has lower corrosion current density.It was also found the PAMT on PPy double-layer coating showed better chemical stability than the PAMT/PPy single layer and PPy on PAMT double layers, and the corrosion potential was about 250 mV.Under the simulated PEMFC conditions, the performance of the coating was studied in the anode (0.1 V and H 2 purging) and cathode (0.6 V, oxygen purging) environments for 12 h to evaluate the stability of the coating.Under these conditions, all coatings except for the PPy coating were stable in both environments.Although the cost of these conductive polymer coatings is much lower than that of precious metal coatings, in the PEMWE working environment, high temperature will cause partial decomposition, thus losing the protection of the substrate, and the coating may fall off after a long time of operation [87].
Josep et al. [61] found that with the increase of cycles, the coating became thicker, and the adhesion decreased.The pinholes in the polyaniline coating increased with the increase of deposition cycles, while the pinholes in PPy had little or no increase with the increase of deposition cycles.In polyaniline and PPy coatings, the corrosion effect was the best after three deposition cycles.Under the same compaction pressure, the contact resistance of a polymer-coated plate deposited by three cycles was very close to that of the graphite plate.The conductivity of these polymer coatings is expected to improve under the acidic conditions of actual fuel cells.
Compared with PPy, PANI could be a potential conductive polymer due to its excellent performance and relatively simple preparation method.However, the electrical conductivity still requires further improvement and the serious peeling off of the coating during long-term operation also needs to be solved.Currently, conductive polymer combined with graphene, 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.

Nanoparticles-Promoted Polymer Hybrid Coatings 4.1. Epoxy Resin (EP)-Based Hybrid Coatings
As a low molecular weight prepolymer with one or more epoxy groups, epoxy resin is mainly obtained by using different types of curing agents to participate in the curing reaction.Its performance is mainly determined by the type of epoxy resin and the combination mode of the curing agent used.Epoxy resin is widely used in fiber-reinforced materials, general adhesives, packaging materials, high-performance anticorrosion coatings, and other fields due to its high hardness, good chemical and mechanical resistance, high adhesion to many substrates, and good thermal performance [95,96].Luo et al. [97] (Figure 6) investigated the hybrid coating with the graphene nanolayers dispersed in epoxy resin.Its anticorrosion performance was tested by electrochemical tests and salt spray tests.The results showed that only 0.5% graphene content can achieve the best anticorrosion performance, and its protective effect on Q235 steel sheet reached 99.7%.Abdullah et al. [98] used graphene oxide (GO) to modify epoxy resin to prepare GO/EP composites.They found that the tensile strength of the composites first increased and then decreased with the increase of GO volume fraction.When the GO volume fraction was 1.5%, the Lakun strength reached the maximum value of 13 MPa.When the GO volume fraction was >1.5%, due to the large specific surface area of GO, its own agglomeration was strengthened, the stress distribution was uneven, and the tensile strength decreased.Argüelles et al. [99] found that when thermal reduced GO (TRGO) was added to the one-component and twocomponent EP matrix, the reinforcing ability of TRGO to the one-component EP was lower than that of the two-component EP, and the TRGO with a mass fraction of 1% increased the tensile strength of the two-component composites.

Polyimide (PI) Based Hybrid Coatings
Polyimide (PI) is a kind of polymer with an imide ring in the main chain, among which the polymer with phthalimide structure is more important [101,102].According to the structural unit of the molecule, they can generally be divided into aliphatic PIs and aromatic PIs.Because of the poor practicability of aliphatic PIs, polymers with aromatic PIs are particularly important.PI, as a kind of special engineering plastic with excellent performance, is also one of the polymer materials with the highest heat resistance rating so far.Because of its excellent performance, it is widely used in high-tech fields such as microelectronics, separation membranes, aerospace, and so on.PI has strong high-temperature resistance, its glass transition temperature is generally above 200 • C, and the most important feature of all aromatic PI is its good heat resistance, its thermal decomposition temperature can often reach 500 • C, and brittle behavior will not occur at ultra-low temperatures.In addition, it has very high mechanical strength and a low thermal expansion coefficient, which means that PI film can maintain good physical protection performance in a hot and humid environment.Furthermore, PI has excellent dielectric properties, and its dielectric constant is about 3.4.When fluorine is introduced into PI, its dielectric constant can be reduced to ca. 2.5.This makes it widely used in the electrical insulation industry, as the outer coating of insulated wire or direct coating.Due to the self-lubricating property of PI, it can resist aging and electrical breakdown.
Due to the excellent properties of PI, it has a great prospect in the application of high corrosion resistance coatings, especially in corrosion protection under harsh chemical media and mechanical damage conditions.Jia et al. [103,104] (Figure 7) prepared TRGO/PI nanocomposite corrosion-resistant coatings with different mass fractions by adding hightemperature thermally reduced graphene oxide (TRGO) as a two-dimensional nanofiller to the PI polymer matrix and evaluated the electrochemical corrosion behavior of the coating in simulated seawater (3.5 wt% NaCl solution) using alternating current impedance spectroscopy and potentiodynamic polarization curves.The results showed that the addition of TRGO could significantly improve the resistance and corrosion protection efficiency of the coating.When the TRGO content was 0.3 wt%, the corrosion resistance of the coating was best enhanced, and the maximum coating resistance was 1.3176 × 10 6 Ω, the highest corrosion protection efficiency reached 99.65%, and its corrosion protection gain was related to the excellent physical and chemical properties of PI and the physical barrier performance of TRGO.

Other Polymer-Based Hybrid Coatings
Liu et al. [105] electrophoretically deposited a layer of highly conductive and corrosionresistant graphene on metal BPs.In order to avoid the oxidation of metal substrate during the electrophoretic deposition (EPD), p-phenylenediamine (PPD) was first grafted onto GO to obtain modified GO.Then, under a constant voltage, magnesium oxide dispersed in ethanol was coated on a titanium plate through cathodic EPD.Next, the deposited magnesium oxide was reduced with H 2 at 400 • C to obtain a titanium plate coated with reduced magnesium oxide (RMGO@Ti).Under the simulated environment of PEMFC, the RMGO@Ti revealed a corrosion current of less than 10 −6 A cm −2 , about two orders of magnitude lower than that of the pure titanium.In addition, the ICR was studied for the RMGO@Ti, which was as low as 4 mΩ cm 2 , about one-thirtieth of the bare titanium.Lee et al. [106] used EPD technology to coat a layer of titanium nitride (TiN) nanoparticles on the stainless steel surface as the BP of PEMFC.The charging additive was polydimethyldiallylammonium chloride (PDADMAC), and titanium nitride powder was uniformly deposited on stainless steel by EPD.The effects of time, voltage, pH value, and different charging agents on the uniformity and kinetics of TiN nanoparticles were studied.The results showed that the deposition rate and surface morphology of PDADMAC with different chain lengths were different.
Liu et al. [107] prepared a hydrophilic corrosion-resistant polydopamine (PDA) composite protective coating.A layer of PDA film was prepared on the surface of titanium by solution oxidation.The Au nanoparticles mounted on PDA using an in situ reduction method improved the conductivity and corrosion resistance; however, it was found that the Au-PDA coating was not particularly suitable for modifying titanium porous diffusion layers in PEMWE systems.The modification of the coating reduced the polarization current density of the diffusion layer to 0.001 µA cm −2 , which is an order of magnitude reduction compared to the pure titanium.
Nanoparticle-reinforced polymer hybrid coating has the characteristics of easy processing, good corrosion resistance, and excellent mechanical properties.The mechanical properties of polymer coatings are also affected by the feature of nanosized conductive particles, such as the conductive materials, addition amount, dispersion of nanoparticles, and other factors, which need to be carefully controlled to balance the conductive and mechanical properties of the coatings.As for the selection of different kinds of polymers, compared with the PI-based thermoplastic coating, the epoxy resin coating is a thermosetting polymer, which has the advantages of high conductivity and high bending strength, but the disadvantage is that it has a long processing cycle and is not conducive to large-scale production [108,109].

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

Figure 4 .
Figure 4. (a) The concept of ennobling mechanism using PANI.(b) Corrosion protection of iron electrode coated with conductive polymer.Two mechanisms can be identified: (1) barrier protection of the PANI coating suppresses the active metal dissolution, and (2) ennobling effect due to the redox activity of the PANI which shifts the potential into a passive domain at higher potential values.(c) Schematic illustration for the chemical reactions of PANI for corrosion protection of iron through different mechanisms [87].

Figure 6 .
Figure 6.(a) The TEM image of dopamine@BCZT powders, the coating layer is about 15 nm; (b) FTIR spectra of ceramic powders before and after dopamine modification.(c) The XRD patterns of ceramic/epoxy resin composites with different volume fractions of dopamine@BCZT powders.(d)The dielectric constant and dielectric loss tangent of dopamine@BCZT/epoxy resin composites with various concentrations of dopamine@BCZT powders at 1000 Hz and room temperature[100].

Figure 7 .
Figure 7. Photographs of the fractured surfaces of (a) the neat PI and (b) the TRGO0.1/PIcoatings, (c) XRD patterns of GO and TRGO.(d) Tafel plots of the bare steel, pure PI, and TRGO/PI coatings [104].

Table 1 .
Comparison of different coatings.