**3.2. Electroless deposition**

Electroless deposition is mainly different from electroplating by not using external electrical power. It is purely chemical or autocatalytic plating that involves a reaction whereby hydrogen is released by reducing agent, normally sodium hypophosphite or thiourea and becomes oxidized, thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating, although silver, gold, and copper layers can also be applied in this manner, as in the technique of Angel gilding.

Electroless nickel plating, according to several studies, is a suitable method of coating metallic bipolar plates. Electroless nickel plating layers are known to provide extreme surface adhesion when plated properly.

#### **3.3. Existing literature review on coating of bipolar plates of fuel cell**

Corrosion resistance has always been the ultimate goal for bipolar plate of fuel cell. Actualizing this through correct application of surface barrier coating will minimize interfacial contact resistance of the fuel cell. Using electrochemical method TiN has been successfully coated on stainless steel 316L bipolar plate for proton-exchange membrane fuel cell [36]. The result of investigation showed that the TiN-coated 316L exhibits promising interfacial contact resistance and improved corrosion resistance in simulated aggressive PEMFC environments. Slight increase in the ICR was also observed after the potentiostatic polarization. This was due to the formation of stable passive film on the surface of the TiN-coated 316L.

Composite coatings are an excellent corrosion inhibitor and can be suitably formed through electrodeposition. To gain high hardness, good thermal stability, and corrosion resistance, multicomponent TiAlSiN coating has been developed by Li and colleagues using different deposition methods [37]. The authors demonstrated the influence of Al and Si on the electro‐ chemical properties of TiN-coated 316L stainless steel bipolar plate in simulated PEMFC environment. The corrosion inhibiting efficiency was improved by incorporation of the Al and Si in TiN coating.

According to research conducted by Nam and colleagues, electroplating Cr-C coatings on SS304 offers an excellent conductivity and corrosion resistance [38]. The deposition was done under different current densities, and it was discovered that carbon content of the composite coating decreased with increasing current density. The surface roughness of Cr-C plated at current density of was observed to be smooth and crack-free. In addition, the lowest contact resistance was recorded at this value. Beyond this current density was formation of cracks and pinholes in the coating network.

A novel epoxy resin (EP)-based system containing polyaniline (PANI) was developed in the research work carried out by Baldissera and Ferreira [39] The PANI serves as an anticorrosive agent to monitor corrosion behavior of mild steel samples. It was found out that the addition of three different forms of PANI-undoped, sulfonated, and fibers to the epoxy resin increased its corrosion protection capacity. Based on the good outcomes, paints prepared with EP and PANI is able to be used as protective coating to metals even when exposed to aggressive marine environment.

In the study conducted by Hung et al. (2009) [40], coated aluminum and graphite composite bipolar plates were installed in separate single PEM fuel cells and tested under normal operating conditions and cyclic loading. After 1000 hours of operation, samples of both the bipolar plates and the membrane electrode assembly (MEA) were collected and characterized. The purpose was to examine the stability and integrity of the plate's coating and evaluate possible changes of the ionic conductivity of the membrane. The SEM/EDX analysis showed very small variation in the surface composition of the coated aluminum bipolar plate after 1000 hours of operation. Chromium was observed in one of the three cathode samples of the MEA. However, it was confirmed that the released Cr did not react with Pt. The microcracks that were observed in the corrosion resistance coating did not seem to completely penetrate through the substrate layer. Aluminum was also detected in the GDLs that were used in both coated aluminum and graphite composite fuel cell which is believed to come from aluminum oxide carried by the reactant gases from the uncoated back plate and gas manifold.

Conducting polymers as a new type of material have high redox potential complimented with properties of both metals and polymers. Polypyrrole coatings have gained outstanding recognition over the years as one of the most important conductive polymers successfully used in fuel cells, chemical sensors, batteries, anti-corrosion coatings, and drug delivery systems. Graphite-polypyrrole has been successfully coated on SS316L substrate as the bipolar plate for polymer electrolyte membrane electrode [41]. Synergy of graphite and polypyrrole as com‐ posite imparts good surface barrier and conductive properties. The polypyrrole enhances good corrosion resistance and electrical conductivity. The graphite further improves the electrical conductivity of the bipolar plate.
