Open access peer-reviewed chapter

# Electrodeposition of Functional Coatings on Bipolar Plates for Fuel Cell Applications – A Review

Written By

Peter Odetola, Patricia Popoola, Olawale Popoola and David Delport

Submitted: September 21st, 2015 Reviewed: December 17th, 2015 Published: March 23rd, 2016

DOI: 10.5772/62169

From the Edited Volume

## Electrodeposition of Composite Materials

Edited by Adel M. A. Mohamed and Teresa D. Golden

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## Abstract

### 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]:

• Bulk electrical conductivity (in-plain):

• Hydrogen permeability:

• Corrosion rate:

• Interfacial contact resistance: at

• Tensile strength:

• Flexural strength:

• Thermal conductivity:

• Thermal stability: up to for PEMFC

• Cost: 2005,

• Chemical and electrochemical stability in acidic environments

• Low thermal expansion

• Acceptable hydrophobicity

• Rapid and inexpensive manufacture

### 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].

### 2.4. Surface modification through coating

By definition, coating is a covering that can be applied to the surface of substrate for enhancement, 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:

• It must have good adhesion to the substrate material without exposing its corrosive media. Proper adhesion of coating to the substrate is achieved by selecting coating materials with thermal expansion coefficients similar to those of the chosen bipolar plate’s material to minimize micro- and macro-crack formation.

• It must be impermeable to the fuel cell reactant gases.

• It must be chemically stable or inert and give low contact resistance.

• It must be conductive so as to enhance electron conduction through it to the external circuit.

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[2629] that have been investigated over the years as suitable candidates for bipolar plates are as follows:

• Carbon-based coatings: includes conductive polymer, graphite, diamond-like carbon, and organic self-assembled monopolymers.

• Metal-based coatings: comprises metal nitrides, metal carbides, metal oxides, and noble metals like gold, platinum, and ruthenium.

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

### 2.5. Stainless steel and their coatings

Bare or uncoated stainless steel cannot satisfy DOE criteria for bipolar plate. There are three types of stainless steel with varying chromium contents: austenitic (AISI SS300)[30] has 18–20% Cr, ferritic (AISI SS400)[31, 32] has 17% Cr, and martensitic Cr quantities ≥ 11.5%. Chromium acts to produce a thin layer oxide of Cr2O3 which gives it self-surface protection and stop progression of corrosion. The passive layer of the thick oxide film however increases interfacial contact resistance between the bipolar plates and the gas diffusion layers which amount to the overall voltage drop. As a result of this, majority of studies[33, 35] on bipolar plates use measurements of interfacial contact resistance as the main criteria for material suitability.

In the stainless steel group, austenitic stainless steel is the most corrosion resistant due to its high Ni composition coupled with substantial level of chromium that gives it a higher formability at all temperatures from the cryogenic region to the melting point of the alloy.

They are the largest produced stainless steel accounting for about 70%. As a result, the grades 316, 310, and 304 SS have been investigated by many researchers as suitable candidates to replace nonporous graphite bipolar plates.

The primary selection criterion for austenitic stainless steel bipolar plates is the Cr, N, and Mo content that comes in different compositions and therefore makes them to behave differently in various environments. Addition of Mo and N is intended to enhance crevice and pitting corrosion resistance. Nickel and chromium addition is to improve strength and high-temperature oxidation resistance.

Over the years, ferritic stainless steel has been considered as bipolar plate material due to its low nickel content that reduces the overall cost of the material and eliminates the potential problem of Ni ion contamination of the membrane.

### 2.6. Titanium and their coating

Titanium[33] has been investigated as a suitable material for bipolar plates because of its properties such as low density, good mechanical strength, and high corrosion resistance. Titanium can form an insulating oxide film such as stainless steel. This surface passive film formed significantly increases ohmic losses with the stack resulting in lower power output compared to uncoated 316 SS. To tackle these challenges, further studies on titanium alloys with niobium and tantalum as viable bipolar plate materials showed that the resistivity of their surface oxides were lower than that of pure Ti. It was also found out that INEOS CHLOR patent PEMcoatTM coated on titanium offered an interfacial contact resistance similar to graphite.

### 2.7. Aluminum and their coating

In terms of cost and density, aluminum offers a better substitute compared to other bipolar plate materials. It also has an inherent problem of developing an oxide film like stainless steel and titanium. This reduces its surface conductivity and rendered it incompetent as bipolar plate material except used in combination with other metals or with suitable coating blend.

### 2.8. Nickel and their coatings

Nickel is comparatively inexpensive, and it exhibits good ductility and ease of manufacturing. Pure nickel does not form protective oxide layer like other known bipolar plate materials but is very susceptible to severe corrosion. Therefore, there is need for alloying it with chromium to be very stable at minimum corrosion rate and low electrical resistivity compared to stainless steel alloys.

### 2.9. Copper and their coatings

Copper emerges as the only bipolar plate material with the highest possible electrical and thermal conductivity. Studies have shown that in a stimulated PEMFC environment, copper beryllium alloy Ce17200 has a corrosion rate of approximately 0.28 µm year at 70°C.

Materials such as Al, Cu, Sn, Ni, and Ni-phosphorous are very susceptible to electrochemical corrosion in acidic solutions that are typical of PEMFC operating conditions. However, gold shows very high resistance to electrochemical corrosion, in comparison to graphite, the traditional bipolar plate material.

In order for its multifunctional roles to be actualized, its material requirement has to be one of excellent electrical and thermal conductivity, good gas permeability, high mechanical strength, high corrosion resistance, and low weight. Having all these required properties locked up in a single material has ever been a challenge facing the research and development community on bipolar plates. As a result, different materials suited for different applications for bipolar plates such as metal, coated metal, graphite, flexible graphite, carbon–carbon composite, and carbon–polymer composites have been adopted over the years. None of these has been able to fulfill at once all the performance requirements and targets set by the US Department of Energy for fuel cell.

## 3. Electrochemical methods of applying coatings on metals

Techniques here include electrophoretic deposition, electrospray, electrodeposition, and electroless deposition.

### 3.1. Electrodeposition

This is a technique of using electrochemical processes to apply metallic coatings on metals or other conducting surfaces. Electrodeposition is done for the following purposes:

• Impartation of special surface properties like harness for wear control, toughness for tribology control, surface roughness for frictional control

• Protection and barrier intermediary between a material and the environments of influence

• Appearance as seen in aesthetic outlook and beautification of materials

• Engineering or mechanical properties

Electrodeposition works on the principle of electrolysis. Electroplating utilizes electrolytic cell setup whereby plating metal (anode) and metal to be plated (cathode) are inserted in plating bath containing the solution of a salt of the metal that is to form the coating. The object to be coated is connected to the negative terminal of an electric battery as cathode while the plating metal is connected to the positive terminal of the electric power source as anode. As the electroplating process continues, the metal salts in the bath are used up. If the anode is a bar of the coating metal, the bar dissolves in the bath at the same rate that the bath gives up its metal to the cathode. If the anode is made of another metal, salts of the coating metal must be added to the bath as metal becomes deposited on the cathode. The longer the process continues, the greater the thickness of the coating on the cathode.

### 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 electrochemical 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 composite 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.

## 4. Summary

It has been shown that metallic bipolar plates are prone to corrosion attack in the fuel cell environment. In addition, the use of precious noble metals is not economical in meeting the future cost projection of fuel cells to successfully compete with combustion engine vehicles. Though noble metals offer excellent conductivity and good corrosion resistance, they are phased out as viable bipolar plate materials over difficulty in forming them into thin strips. Composite-based bipolar plates are known for brittleness which in the long run may easily fail in operation of the fuel cell.

Metallic bipolar plates were proven to surpass the mechanical strength of graphite composite plates as well as giving acceptable electrical conductivity with minimal production cost on commercial scale. As indicated earlier, metallic plates are prone to corrosion in the fuel cell environment. Considerable research work has been conducted to enhance the material’s corrosion resistance and interfacial contact resistance. It is concluded that metallic bipolar plates hold a promising potential as more research and development studies progress on its surface modification through different functional coatings.

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Written By

Peter Odetola, Patricia Popoola, Olawale Popoola and David Delport

Submitted: September 21st, 2015 Reviewed: December 17th, 2015 Published: March 23rd, 2016