Flow-field plates

Ohmic losses AEohmic originate from (i) membrane resistance, (ii) resistance of CLs and diffusion layers, and (iii) contact resistance between the flow field plates. Although it is common practice to split current-voltage characteristics of an MEA into three regions— kinetic (low currents), ohmic (intermediate currents), and mass transport (high currents) [Winter and Brodd, 2004]—implicit separation of vt Afiohmic is not always straightforward, and thus studies of size and... 

Metal foams have been used in the past in the development of FF plates. However, Gamburzev and Appleby [53] used Ni foams as both a DL and a flow field plate with an MPL layer on one of its surfaces. They observed that such a design had high contact resistance between the nickel foam and the MPL and also increased gas diffusion resistance due to the required MPL thickness. Arisetty, Prasad, and Advani [54] were able to demonstrate that these materials can also be used as potential anode diffusion layers in DMFCs (see Figure 4.10). In fact, the nickel foam used in this study performed better than a carbon cloth (Avcarb 1071HCB) and a stainless steel mesh. However, it was recognized that a major drawback for these foams is their susceptibility to corrosion. 

W. Sun, B. A. Peppley, and K. Karan. Modeling the influence of GDL and flow-field plate parameters on the reaction distribution in the PEMFC cathode catalyst layer. Journal of Power Sources 144 (2005) 42-53. 

The bipolar plate with multiple functions, also called a flow field plate or separation plate (separator), is one of fhe core components in fuel cells. In reality, like serially linked batteries, fuel cells are a serial connection or stacking of fuel cell unifs, or so-called unif cells fhis is why fuel cells are normally also called sfacks (Figure 5.1) [2]. The complicated large fuel cells or module can consist of a couple of serially connecfed simple fuel cells or cell rows. Excepf for the special unit cells at two ends of a simple stack or cell row, all the other unit cells have the same structure, shape, and functions. 

GRAFCELL plates (thermoset-based graphite composite plates) made by GrafTech ALT. (GrafTech International. 2008. http //www.graftechaet.com/GRAFCELL/GRAFCELL-Products/ Flow-Field-Plates-(FFP).aspx (accessed Dec. 20, 2008).)... 

The most commonly utilized embedded sensor for temperature distribution mapping is the thermocouple. Wilkinson et al.130 developed a simple, in-situ, and noninvasive method of measuring the temperature distribution of a fuel cell with micro-thermocouples. In this study, thermocouples were located in the landing area of the flow field plates (in contact with the GDL of the MEA) of a fuel cell. The temperature data taken at different locations along the flow channel was then used to find each temperature slope, which in turn were related through mathematical equations to the local current density of each location. Thus, the current density distribution in the fuel cell was determined by simple temperature measurements. The results of this approach are discussed in more detail in Section... 

Subcell Approach Stumper et al.135 presented the subcell approach to measure localized currents and localized electrochemical activity in a fuel cell. In this method a number of subcells were situated in different locations along the cell s active area and each subcell was electrically isolated from each other and from the main cell. Separate load banks controlled each subcell. Figure 8 shows the subcells in both the cathode and anode flow field plates (the MEA also had such subcells). The current-voltage characteristics for the... 

The core of the Ballard fuel cell consists of a membrane electrode assembly (MEA) that is placed between two flow-field plates. The flow-field plates direct H2 to the anode and Oz (from air) to the cathode. To obtain the desired amount of electric power, individual fuel cells are combined to form fuel cell stacks. Increasing the number of cells in a stack increases the voltage, and... 

Flow Field Plate Gas Diffusion Electrode (Anode) Catalvst... 

An elementary PEMFC comprises several elements and components the membrane-electrode assembly (MEA), the flow-field plate (bipolar plate, which also ensures electric contact with the next cell), gaskets to ensure tightness to reactants and end plates (Figure 9.4). 

The bipolar flow-field plate is the main hardware component of the PEMFC and of the DAFC. This has to fulfill several requirements, namely... 

Volume Manufacturing FOR Commercialization OF Flow Field Plates... 

Wilkinson D P etal., 1996a, Embossed Fluid Flow Field Plate for Electrochemical Fuel Cells. US Patent 5,521,018. 

Walkins, D.S. Dircks, K.W. Epp, D.G. Novel Fuel Cell Fluid Flow Field Plate. U.S. Patent 4,988,583, January 29, 1991. 

The L79 is easy to build and uses a few common materials in a unique way (see illustration, next page). It consists of a six layered sandwich composed of one PVC end plate, two PC (printed circuit) board elec-trode/gas flow field plates, one 12 PEM MEA (membrane electrode assembly) layer, and 2 rubber gaskets, one of which also acts as a gas supply line. 

The next step is to tin the tips of the series edge connectors on the oxygen and hydrogen electrode/flow field plates. There are twelve contacts to be tinned on each plate, eleven for the series connections and one for the take off on one end of each plate. 

A bipolar plate that serves to connect individual cells together. Gas channels are usually machined, or moulded, into both sides of the plate to introduce fuel and oxygen/air to the respective electrodes and to remove the reaction products, i.e., pure water in the case of hydrogen fuel. This component is commonly known as a flow-field plate because it serves to smooth the current across the area of the cell stack. 

A prototype of a PEM fuel cell system made of a stack of 15 bipolar plates, was developed to deliver 9 W output power at 8 V [4-6]. The stack of only 30 cm volume (50 mm x 24 nun x 25 mm) was constructed using graphite micromachined bipolar flow-field plates. Adhesive bonding was used for assembly and sealing. The stack is shown in Figure 8-3. 

Flow field plate Proton-conducting Flow field plate... 

The final component of a fuel cell to be discussed in this chapter is the current collector plates (CCP) or more commonly called the bipolar plates (BP) or flow field plate (FFP). As observed in Fig. 7.1 the fuel cell stack is composed by a number of BP, each of which will separate a pair of MEA and with two end plates completing the stack. At either side of the BP, an arrange of channels provide the flow paths for the fuel and oxygen. One of those faces is in contact with the anode of one MEA and the other face with the cathode of the other MEA, hence, the name bipolar plate. The end plates have channels only on one face, and the stack is typically completed by two metal plates with a series of bolts that holds the stack... 

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