Guiyab, Jim Pauline C.Hugo, Krizelle Ann B.2011-02597-MN-02011-00051-MN-0Anti-corrosion of Metal-based Bipolar Plates on Proton Exchange Membrane Fuel Cells

BS Chemistry 4-1Prof. Dalton R. Fortin

OutlineI. Introductiona. PEM Fuel Celli. Definition and backgroundii. Parts of the PEM fuel cellsb. Bipolar PlatesII. MechanismIII. Environmental and Operational Factors Affecting Metal Bipolar PlatesIV. Anti-corrosion of metal bipolar plates on fuel cellsa. Plate materialb. Plate coating

I. Introduction

Proton Exchange Membrane Fuel CellsFuel cells are power conversion devices that transform the chemical energy stored in form of a fuel and an oxidant directly into electrical energy through clean and efficient electrochemical oxidation processes. One of the most developed type of fuel cell used is the Proton Exchange Membrane or the Solid Polymer Fuel Cells.A PEM fuel cell is an electrochemical cell that is fed hydrogen, which is oxidized at the anode, and oxygen that is reduced at the cathode. These PEMFCs were developed by General Electric in the United States in 1960s for use in the Gemini space flights by NASA.Advances in this technology were stagnant until the late 1980s when the fundamental design underwent significant reconfiguration. Possibly, the most significant barrier that PEM fuel cells had to overcome was the costly amount of platinum required as a catalyst.

Parts of PEMFCAt the heart of the PEM fuel cell is the membrane electrode assembly (MEA). The MEA is pictured in Figure 2. The MEA consists of a proton exchange membrane, catalyst layers, and gas diffusion layers (GDL). Typically, these components are fabricated individually and then pressed to together at high temperatures and pressures. MEA is typically sandwiched by two flow field plates that are often mirrored to make a bipolar plate when cells are stacked in series for greater voltages. The bipolar plate constitutes the structural backbone of the fuel cell assembly normally absorbing the greater cost of a fuel cell.The electrolyte in the PEM is a solid polymer in which protons are mobile. Most common theme used is the use of sulphonated fluoropolymers, usually fluoroethylene (Figure 3).

Figure 1. Schematic diagram of a single typical PEMFC

Figure 2. An example of a membrane electrode assembly (MEA).

Figure 3. Example structure of a sulphonated fluoroethylene.

Bipolar PlatesIn a fuel cell, the voltage produced is quite small, about 0.7 V. To produce a useful voltage many cells have to be stacked in series. One way to do this is to connect the edge of each anode to the cathode of the next cell. But the problem with this method is that the electrons have to flow across the face of the electrode to the edge. If each cell is operating at 0.7 V, even a small voltage drop is important so unless the current flow is very low, or the electrode is a good conductor, or very small, this method is not used. A bipolar plate (Figure 4) is used for a much better cell interconnection. This makes connections all over the surface of one cathode and the anode of the next cell (hencebipolar); at the same time, the bipolar plate serves as a means of feeding oxygen to the cathode and fuel gas to the anode.Bipolar plates serves there purposes: separation of the fuel and oxidant gases, rigidity for the MEA, and electron flow through the fuel cell stack. From a cost perspective, inexpensive metals such as stainless steel and aluminum could easily be processed into bipolar plates. Because bipolar plates take up almost 80% of the mass of the fuel cell, thin nature of the metal substrate allows for smaller stack designs with reduced weight.Figure 5. Two bipolar plates of very simple designFigure 4. Single cell, with end plates for taking current from all over the face of the electrodes.

II. Mechanism

III. Environmental and Operational Factors Affecting Metal Bipolar Plates

ElectrolyteCorrosive EnvironmentOperational Parameters Affecting Metal CorrosionTypical Corrosion Reactions

PEMsolid polymer,proton-conductingelectrolyte sulfuric acid aqueous solutionscontaining fluoride ions pH range 0-3.5 0.1-1M H2SO4 1-5 ppm F- other anions: CO32-, HCO3-,SO42-, HSO4-, SO32- operating at 500

Gold