Small fuel cells bipolar plates

Small fuel cells for portable applications require bipolar plates with surface textures (flow-fields) with partial cross-sections <1.0 mm. This requires a tool technology that allows a complete filling of the mold and the transmission of the structures of the tool onto the bipolar plate material. 

With respect to fuel-cell technology itself, the small portable units use commercially available membrane electrode assemblies (MEA) and gas diffusion layers (GDL). As the operating temperature of small fuel-cell stacks usually lies below 50 °C, the requirements with respect to material stability of MEA and GDL, but also of sealing gaskets and bipolar plates are comparable lower than for other applications. For example, it is well known that metallic bipolar plates show significantly lower corrosion below 50 °C than at typical operation temperature of 80 °C [6,7], so that a sufficient lifetime for portable applications can be achieved with stainless steel. 

The traditional structure of a fuel battery is that of a vertical stack of alternating bipolar plates, MEAs, and heat-exchange plates that is compressed with the aid of massive end plates and tie bolts. This structure is poorly adapted for building mini-fuel plants. Particularly for the applications mentioned, special small fuel cells are needed that have design principles different from those of their large cousins. 

Development of small fuel cells for portable power applications has resulted in a myriad of stack configurations. Some stacks are miniaturized replicas of the larger automotive or stationary power fuel cells with the same components, MEAs, gas diffusion layers, bipolar plates, and end plates. Some use planar configuration where the cells are connected with conductive strips. Recently, microfluidic cells manufactured on silicon-based chips are emerging [37,38]. 

Design Principles An individual fuel cell will generate an electrical potential of about 1 V or less, as discussed above, and a current that is proportional to the external load demand. For practical apph-cations, the voltage of an individual fuel cell is obviously too small, and cells are therefore stacked up as shown in Fig. 27-61. Anode/ electrolyte/cathode assemblies are electrically connected in series by inserting a bipolar plate between the cathode of one cell and the anode of the next. The bipolar plate must be impervious to the fuel... 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

From a cross-flow point of view it may be of interest to mention the phosphoric acid fuel cell with the so-called DiGas system (Fig. 9), which is an air-cooled cross-flow configuration for use in utility-power stations [39]. The process air stream is diverted into two types of channels into individual cells with relatively small cross-sectional area, and into cooling plates (approximately one for every five cells) with a lai ge cross-section. Bipolar plates were molded from a mixture of graphite and phenolic resin, with a Pt-on-carbon cathode and a Pt anode combined with colloidal PTFE on a graphite-paper backing. 

Fig. 1. Schematic presentation of a PEFC cross-section. The cell (left) consists of a membrane catalyzed on both sides (referred to as a membrane/electrode (M E) assembly ), gas-diffusion backing layers and current collectors with flow fields for gas distribution. The latter become bipolar plates in a fuel cell stack. The M E assembly described schematically here (right) shows catalyst layers made of Pt/C catalyst intermixed with ionomer and bonded to the membrane (large circles in the scheme correspond to 10 nm dia. carbon particles and small circles to 2 nm dia. platinum particles).

The present market situation is characterized by a small number of units and hand fabrication. Figure 8.18 compares the present situation of the stack production by a research institution with the prediction of ADL by plotting the cost distribution of PE fuel cell stacks. The present cost for the manufacture of a single unit is about a factor of 10-50 more expensive than the values of the ADL analysis. As can be seen, the importance of bipolar plates is presently about equal to that of the MEA and labor-intensive tasks like assembling and quality control contribute significantly to the cost of the stack system. These are the cost drivers that can be... 

The current produced in a fuel cell is proportional to the geometric area of the electrodes. A single fuel cell typically produces a relatively small voltage (about 1 V). In order to produce a higher voltage, several fuel cells are connected in series or in parallel, through bipolar plates separating adjacent fuel cells (i.e., stacked ). 

Individual cells in a stack are coimected in series, i.e. the total current I = J j dS crossing each cell is the same (J is the local current density and S is the cell active area). Due to high conductivity, the potential of the bipolar plate does not vary much over the BP surface. However, below we will show that small variations of BP voltage indicate quite significant inhomogeneities in transversal (through-plane) current in the fuel cell and thus these variations are of great interest. 

FIGURE 6.31 SEM images of a small area of a PP-bonded injection-molded bipolar plate before (left) and after (right) exposure—no changes could be observed—(time of exposure in deionised water 1000 h at 80°C). (Adapted from ZBT. 2010. Original material gained at the Center for Fuel Cell Technology.)... 

The core of a fuel cell power system is the electrodes, the electrolyte, and the bipolar plate that we have already considered. However, other parts frequently make up a large proportion of the engineering of the fuel cell system. These extras are sometimes called the balance of plant (BOP). In the higher-temperature fuel cells used in CHP systems, the fuel cell stack often appears to be quite a small and insignificant part of the whole system, as is shown in Figure 1.19. The extra components required depend greatly on the... 

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