Bipolar plates function

Since a typical voltage output from one cell is around 0.4-0.8 V, many cells must be connected together in series to build up a practical voltage (e.g., 200 V). A bipolar plate performs this cell-connecting function and also helps to distribute reactant and product gases to maximize power output. 

The major function of a bipolar plate, or simply called "plate," is to connect each cell electrically and to regulate the reactant gas (typically, hydrogen and air in a hydrogen fuel cell) or reactant liquid (typically, methanol in a DMFC) and liquid or gas coolant supply as well as reaction product removal in desired patterns. This plate must be at least electrically conductive and gas and/or liquid tightened. Considering these important functions and the larger fraction of volume, weight, and cost of the plate in a fuel cell, it is worthwhile to construct this chapter with emphasis on the current status and future trend in bipolar plate research and development, mainly for the plate materials and fabrication process. 

Functions, Structures, and Performance Requirements of Bipolar Plates... 

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. 

According to the structure, location, and role of the plate in fuel cells mentioned earlier, it is clear that the full function of the bipolar plate would be very important for the electrochemical reactions, heat and water management, and electrical current and power transfer in a stack. The specific functions of bipolar plates include ... 

Although it is difficult to determine the quantitative requirements of plate and plate materials appropriately for various fuel cells and different applications in a development phase, such a target would be helpful to direct the development effort and make necessary trade-offs. The cascaded performance requirement targets in 2010 and 2015 for bipolar plates of fuel cells in transportation applications were set by the U.S. DoE (Department of Energy) according to functions of the plate mentioned before and overall requirements of performance, reliability, manufacturability, and cost of a stack, as shown in Table 5.1 [7]. The technical target in the DoE s multiyear research, development, and demonstration plan has been popularly and worldwide... 

Fig. 1.2 Principles, functions and schematic of a flat planar SOFC where the PEN is stacked between two bipolar plates (interconnect with gas flow fields).

Bipolar plates play an important role in fuel cell operation. " - Generally, the functions of bipolar plates can be summarized as (1) supply and separate reactant gases without introducing impurities (2) conduct electrons (3) remove the reaction... 

Each cell in this planar stack is connected in series, that is, the positive electrode of one cell is connected to the negative of the next cell and so on. In a block stack this is accomplished by using bipolar plates, but in the L79 we use circuit board traces with tab wire connects to perform the same function as the bipolar plates. The L79 can also be built in a parallel configuration or a series-parallel configuration depending on the current and voltage desired. 

Phase II focuses upon process development to result in a pilot production line capable of producing 300 bipolar plates per hour. Our goal is a complete functional pilot line, including all relevant quality assurance, failure mode and effects analysis, and statistical manufacturing characterization processes. This will be completed by transferring the most promising mass-production technique to laiger-scale and continuous equipment operation in a dedicated production line. 

Because a fuel cell functions at a low voltage (/.c., well below 1 V), it is customary to build up the voltage to the desired level by electrically connecting cells in series to form a stack . This is achieved by means of a bipolar plate-and-frame arrangement similar to that employed for electrolysers see Section 4.2, Chapter 4. There are a number of different designs of fuel cell, but in each case the unit cell has certain components in common. These are as follows. 

The technological development of electrolyzers started with a mono polar cell consisting of a cathode part and an anode part separated by a diaphragm, hi multi-cell systems, bipolar plates are used carrying the cathode material for one cell and on its backside the anode material for the neighbor cell. The functions of the bipolar plate are the continuous supply of the membrane electrode with H2 on one side and with O2 or air on the other side and the regulation of the water balance by providing moisture for the membrane on the H2 side and remove the product water on the O2 side. 

In addition to the earlier described state of the art, nanotechnology plays a role in the development of micro fuel cells the realisation of special properties of surfaces and the enhancement of functionalities by nanostrnctnres, nanolayers as coatings and nanoparticles raise increasing interest. Nanostructnred electrolytes, carbon snp-ports or coatings for bipolar plates are examples. 

The diffusion model [5, 6] has been developed for mass transfer study within the system bipolar plate - gas diffusion layer (with micro-porous sublayer) - electrocatalytic layer . It was shown that the current density distribution is a complex function and depends mainly on the electrochemical parameters of the MEA (electrocatalytic layer activity) and... 

Polymeric functional materials are of central importance for the polymer electrolyte membrane fuel cell (PEMFC) and DMFC technologies in particular. In addition to the expected cost reduction due to low-cost mass productimi, for example of polymeric bipolar plates (see Sect. 2.1), the polymeric membranes are irreplaceable in the PFMFC and DMFC technologies. 

Bipolar plates have a number of functions in fuel-cell batteries. Mechanically, they are the backbone of the individual fuel cells and of the battery as a whole. They provide the electrical contact between individual fuel cells in the stack and channel the reactant supply to the entire working surface area of the electrodes. The plates must meet a number of requirements in order to fill all these functions They must (i) be sufficiently sturdy, (ii) be electronically conducting, (iii) have a low surface resistance in contact with other conductors, (iv) be impermeable to gases, (v) be corrosion resistant under the operating conditions of the fuel cells. 

Fig. 14.1 Functional layers in a fuel cell. Functional layers can be integrated to a component such as catalyst coated membrane (CCM), catalyst coated substrate (CCS)/gas diffusion electrode (GDE) or bipolar plate (BPP)...

Individual layers can be integrated to components such as the electrolyte membrane and the two catalyst layers to a Catalyst Coated Membrane (CCM). Another option is the integration of the gas diffusion layer and the catalyst layer to a Gas Diffusion Electrode (GDE). The gas functions of distribution, gas separation and coolant distribution are commonly integrated into the BiPolar Plate (BPP). 

Bipolar plates connecting adjacent cells can be considered as combination the functions, current collector, gas distribution (flow field), gas separation and coolant layer into one subunit. Integration of electrolyte membrane, catalytic layers and gas diffusion layers into a Membrane Electrode Assembly (MEA) results into a second major subunit for fuel cell stack integration. Combination of a catalytic layer and a gas diffusion layer to a gas diffusion electrode is yet another possibility of integrating functional layers to subunits. 

Bipolar plates are integrating the functions of reactant distribution, current collection, and thermal management for each cell. For this purpose, the bipolar plates are containing distribution zones (flow fields) for fuel (hydrogen/methanol), oxidant (air) and coolant. The design of the flow fields must ensure spatially homogeneous reactant distribution across the active area as well as reliable and homogeneous removal of product water. 

Carbon constitutes the most abundant element of the different FC components. Setting aside the membrane, which is a polymer with a carbon backbone, all the other components, i.e. the CL, GDL and current collector plates (bipolar plates) are made almost entirely of graphitic carbon. The electrocatalyst support of the CL is commonly carbon black in the form of fine powder. GDLs are thin porous layers formed by carbon fibers interconnected as a web or fabric, while current collector plates are carbon monoliths with low bulk porosity. As explained above each of these components has a particular function within the fuel cell and in particular in the triple phase boundary. The structure and properties of the carbon in each of the different FC components will determine the whole performance of the cell. 

The typical materials for gas diffusion layers are carbon paper and carbon cloth. These are porous materials with typical thickness of 100-300 /um [22], The functions of the gas diffusion layers are to provide structural support for the catalyst layers, passages for reactant gases to reach the catalyst layers and transport of water to or from the catalyst layers, electron transport from the catalyst layer to the bipolar plate in the anode side and from the bipolar plate to the catalyst layer in the cathode side, and heat removal from the catalyst layers. Gas diffusion layers are usually coated with Teflon to reduce flooding which can significantly reduce fuel cell performance due to poor reactant gas transport. 

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