Bus plate

In Figure 31.24, the experimental stack voltage distribution data are compared with the model prediction for the case of two stainless-steel bus plates. 

Requirements of Endplates Functions and Requirements of Bus Plates Assembly Modes... 

In this chapter, the functions of and current research on seal, end plate, and bus plate are introduced, and their degradation mechanisms and mitigation methods are discussed. 

Typically, a PEMFC stack contains two bus plates, an anode, and a cathode, each of which has one or more current output terminals. Bus plates function as an external electric circuit, exporting the electric current from the fuel cell. 

The current output terminal sends out the electric current generated by the PEMFC single cell or stack. Bus plates for all kinds of fuel cell stacks are made of noble metals such as gold or platinum, or nonnoble metals such as stainless steel, copper, or aluminum. The noble metals not only have good conductivity but also can almost avoid electrochemical corrosion and thus will not produce metallic ions that may poison the fuel cell. However, these noble metals are very expensive. 

If stainless steel, copper, or aluminum is directly used to make a bus plate, electrochemical corrosion will occur if fluids pass through it, resulting in unwanted damage due to the metallic ions produced. In order to avoid this problem, nonnoble metal materials plated with gold or platinum are frequently used (Joh et al., 2008). Unfortunately, gold- or platinum-plated copper or aluminum is still relatively expensive. 

In addition, the bus plate materials may influence fuel cell performance. G.-S. Kim et al. investigated PEMFC stack voltage distribution using a voltage/current distribution model (Kim et al, 2005). The bus plate materials were stainless steel, nickel, and aluminum, and their results showed that the bus plate made of stainless steel might have a better voltage distribution (Figure 7.4). 

FIGURE 7.4 100-Cell stack voltage distribution model computations for one anomalous bus plate (either stainless steel, nickel, or aluminum) and one copper bus plate, at 300 A. Model curves correspond to inlet, middle, and outlet locations. (Reprinted from Journal of Power Sources, 152, Kim, G. S. et al. Electrical coupling in proton exchange membrane fuel cell stacks. 210-217. Copyright (2005), with permission from Elsevier.)... 

As mentioned above, bus plates made of nonnoble materials may corrode during electrochemical reactions. Fortunately, this corrosion or deactivation process is not significant in comparison with what happens to the catalyst, membrane, and MEA. Research on the deactivation process for bus plates is limited, and is mostly focused on fuel cells that use methanol, ethanol, or formic acid (Stumper and Charles, 2008) as the fuel. 

Job et al. studied the methanol solution s corrosion effect on bus plate (Joh et al., 2008). Their results showed that effluent solution with lower pH value was likely to corrode metal bus plates. After running... 

In Figure 7.5, to avoid bus plate corrosion, they used gold-plated brass as bus plates. 

Although investigations into seals, endplates, and bus plates are relatively inadequate, it is obvious that the properties of these components have a significant influence on PEMFC performance, especially durability. 

The bus plate is also important for PEMFCs, but its degradation process is not obvious and bus plate R D is mainly focused on how to retard the corrosion process in fuel cells other than Hj/Oj PEMFCs, such as those using methanol, ethanol, or formic acid. 

FIGURE 7.5 Photographs of corroded bus plates affected by effluent methanol solution (a) anode bus plate and (b) cathode bus plate. (Reprinted from loh, H. et al. 2008. International Journal of Hydrogen Energy 33 7153-7162. With permission from International Association for Hydrogen Energy.)... 

A thorough analysis of these effects has been given by Chang et al. (2006). It shows that not only the fluid dynamics of the reactants and coolant in the manifold can have a distinct influence on the cell homogeneity, but also the resistivity of the bus plates can lead to systematic current density distribution and cell voltage differences of individual cells. 

The main components of a fuel cell stack are the membrane electrode assemblies or MEAs (membranes with electrodes on each side with a catalyst layer between them), gaskets at the perimeter of the MEAs, bipolar plates, bus plates (one at each end of the active part of the stack) with electrical connections, and the end plates (one at each end of the stack), with... 

MEAs with a catalyzed active area of varying fractions of the total flow field area. In the second approach, the authors used a number of "subcells" at various locations along the gas flow channel that were electrically insulated from the main active MEA and controlled by a separate load. In the third approach, a network of passive graphite resistors were placed between the flow field plate and the current collecting bus plate, while the potential drop across these resistors was monitored to establish the current flowing through them. However, none of these methods provided sufficient spatial resolution, and all involved significant errors due to lateral currents and/or contact resistance. 

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