## 3 Conductive Polymer Based 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 \(\sigma\) electrons and \(\pi\) electrons, and \(\sigma\) electrons are fixed and cannot move freely, forming covalent bonds between carbon atoms [91]. The 2 \(\pi\) 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]. [PERSON] 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 \(\Omega\) cm\({}^{2}\) within 50 h in the simulated environment at 70 \({}^{\circ}\)C. [PERSON] et al. [94] applied conductive polymer (2-amino-5-mercapto-1,3,4-thiadiazole)/polypyrrole (PAMT/PPy) single-layer 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]. [PERSON] 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.