**2.4. Surface modification through coating**

By definition, coating is a covering that can be applied to the surface of substrate for enhance‐ ment, functional, and modification purposes. The major purpose of coating on bipolar plate is to serve as corrosion resistance interface between the substrate and the environment, thereby reducing or eliminating the interfacial contact resistance that affects the overall power output of the fuel cell. They impact special surface properties of hardness, wear control, corrosion, and oxidation resistance without changing the substrate bulk properties. Therefore, improves surface properties.

Coating is mostly needed for application of metallic bipolar plates because of the possible interaction with the stringent acidic fuel cell environments that affects its overall performance. Oxide formation and ion dissolution as a result of metal bipolar plates can be prevented by applications of various coatings. Metallic bipolar plates[24, 25] are often coated with protective coating layers which serve as a barrier between its substrate and the corrosive media thereby preventing corrosion. The coating must be able to satisfy the following important criteria:


In the absence of corrosion, there will be no formation of metallic ions poisoning the membrane assembly electrode and reducing its potency for proton transport. Formation of oxide films in stainless steel as a self-protection against progression of corrosion that eventually result into high surface contact resistance will also be eliminated.

Two types of coatings[26–29] that have been investigated over the years as suitable candidates for bipolar plates are as follows:


The investigation of metallic bipolar plates is divided into two major parts:

**•** Stainless steel and their coatings

**•** Aluminum, nickel, and other non-ferrous alloys and their coatings
