**2.2. Bipolar plate DOE performance targets**

As defined by the US Department of Energy (DOE), an ideal material for bipolar plates must meet the following requirements[18]:


**•** Corrosion rate:

**2.1. Bipolar plate materials**

242 Electrodeposition of Composite Materials

*2.1.1. Aluminum*

bipolar plate performance.

current target for transportation[17].

meet the following requirements[18]:

**•** Hydrogen permeability:

**•** Bulk electrical conductivity (in-plain):

**2.2. Bipolar plate DOE performance targets**

*2.1.2. Stainless steel*

The materials [12, 15] required for bipolar plates fall into two categories:

**Aluminum** 370,000 2.7 1–2 **Stainless Steel** 10,000 8.0 1–2 **Graphite** 110–680 1.8–2 5-6

**Substrate Conductivity (S/cm) Density (g/cm3**

**Table 1.** Showing properties variation of bipolar plates materials

**•** Non-metallic materials, for example, graphite or polymer-based composites.

**•** The metallic materials, for example, stainless steel, aluminum, titanium, nickel, copper, etc.

It has a relatively low density, good strength, and higher thermal conductivity than stainless steel. Aluminum bipolar plates can be produced by casting, machining, and etching methods. This leads to lower production time and costs when compared to graphite. The major setback of aluminum lies in the formation of an electrically insulating oxide layer which impedes

Stainless steel has been shown to have the potential to meet all of the requirements for bipolar plates. It has a relatively low cost with high electrical and thermal conductivity, good me‐ chanical properties, and ease of machining. Stainless steel bipolar plates[16] can be rapidly manufactured in large quantities by stamping. In the acidic environment within the fuel cell, stainless steel passivates forming Cr2O3 which elevates the interfacial contact resistance.

Most conventional coatings for stainless steel have shown to add to overall cost and also leave surface defects which result in local corrosion, damaging the fuel cell. The development of cost-effective coatings is the most significant research area in metallic bipolar plates. Despite the excellent physical properties and high-volume manufacturing processes available for metal bipolar plates, current technology places the estimated cost at \$60–100/kW or 6 to 10 times the

As defined by the US Department of Energy (DOE), an ideal material for bipolar plates must

**) Thickness (mm)**


#### **2.3. Performance control strategies for fuel cell**

#### *2.3.1. Corrosion management*

Corrosion problem is only associated with metallic bipolar plate materials such as stainless steel, aluminum, nickel, and titanium. The bipolar plate works in an acidic environment and is therefore susceptible to corrosion attack[19, 20] through the electrochemical processes. The corrosion products of the metallic ions from the substrate at first will increase the surface contact resistance, then reduce the ionic conduction of the proton via the membrane electrode, and eventually poison it. Metallic coatings through electroplating can be used to address these problems. It will act as a protective barrier between the substrate and the aggressive acidic environment.

#### *2.3.2. Water management*

It has been well established that at the anode interface, the fuel undergoes a splitting process whereby only protons are permitted to pass through the membrane electrode assembly. Fuel cell performance is a function of effectiveness of the Nafion membrane to conduct the protons through it which also depend on the temperature and the level of relative humidity.

The proton conductivity of the Nafion membrane is highly influenced by the quantity of water absorbed in the membrane[21] and the maximum proton conductivity is attained when the membrane is fully saturated with water. Overflooding of water has an adverse effect of blocking the reaction sites of the neighboring electrodes, thereby preventing access of reactant gases into their cell. On the other hand, under low relative humidity, the absorbed water in the membrane vaporizes which remarkably reduces the proton conductivity and drastically increase the ohmic overpotential. To create balance, there is need for incorporation of water retention fillers in the electrolyte membrane such as TiO2, SiO2, ZrO2, and heteropolyacids which are both hygroscopic and proton conductors. Functionalized one-dimensional carbon nanotubes can also be incorporated into the Nafion membrane to improve the membrane performance operated under low relative humidity and dry conditions [22, 23].
