Bipolar flat plate cells

Planar geometry Although there are variations in detail, the design of the planar or flat plate cell, and how cells are interconnected are essentially as illustrated in Fig. 4.27. The construction may involve a self-supporting, coated electrolyte or, if the operating temperature permits, the anode-electrolyte-cathode structure may be supported on a stainless steel bipolar plate. 

Flat plate cells are usually made with electrodes spread on screens or grids like lead-acid batteries or industrial Ni-Cd batteries. In these types of batteries the electrodes itself are good metallic conductors To build manganese dioxide batteries as plate cells good conductive screens would be needed. A well known example for a bipolar Mn02-Zn cell in commercial production is the 6 Volt Polaroid camera film battery, a primary battery with a weakly acidic electrolyte. 

On the other hand, in bipolar cells, only the terminal cells are connected by intercell conductors, and there are typically many unit cells electrically in series between the terminal cells (Fig. 5.2). The two basic types of bipolar cells are the flat plate cell and the finger type cell. A group of bipolar cells that have a common piping system for the fluids, via manifolds, is referred to as an electrolyzer or sometimes a series or a stack. Within a single bipolar electrolyzer, there are sometimes more than one set of terminal cells. Bipolar electrolyzers can be connected via an external bus within a DC circuit in series or in parallel, but usually not both. Furthermore, in the case of mercury-cell plant conversions to membrane cells, the electrolyzers are connected electrically in parallel as shown in Fig. 5.3. 

The most common fuel cell stack design is the so-called planar-bipolar arrangement (Figure 1-2 depicts a PAFC). Individual unit cells are electrically connected with interconnects. Because of the configuration of a flat plate cell, the interconnect becomes a separator plate with two functions ... 

Planar Cell Design (Bipolar/Flat-Plate Design)... 

A special stainless steel was developed in Australia and patented (Jaffray, 1999). Production costs and endurance of the resulting flat plate design (Foger etal, 2000) were improved relative to that of the firm s initial all-ceramic design. The bipolar plates and interconnects were stainless steel. The cell had operational difficulties, and has been discontinued. The firm has just established a UK division, to compete in Europe. 

There are two bipolar filter-press cell designs for manufacturing chlorine, the Dow cell and the Glanor cell. Both use finger-type electrodes, as opposed to flat plates. The Dow cells, developed over the past eighty years [76-79] are simple and rugged. They employ... 

FIGURE 3.21 Cell design typical internal configurations, (a) bobbin construction, (b) Spiral wound construction. (c) Flat-plate construction, (d) Bipolar-plate construction. 

The other example, called the nickd/zinc battery [(—) Zn/KOH/NiOOH (+)], has attracted more attention in two different versions from the viewpoints of application and cell design one is the small cyhndrical consumer cell [148, 149], the other is the flat-plate module for electrotraction [149, 150]. A very interesting review with an extensive collection of references was pubHshed in 1992 [151]. In 1996, an improved bipolar construction of this battery was presented [152]. The most recent version was described by Humble and co-authors [153] a nickel/zinc microbattery developed for direct installation and use in autonomous microsystems. 

This company have constructed a number of cells with energies from 20 to 1300Wh. Tubular and flat plate bipolar electrodes were studied. 

The plate at the two ends of a cell row or stack is called the end plate and has a slightly different structure from that of normal bipolar plates in the stack. The end plate actually is a "single-polar" plate with only the fluid field on the inside surface contacting the anode or the cathode of the unit cell at either end of the stack. The outside surface of the end plate is flat with fluid ports as shown in Figure 5.2. The end plate normally contacts the other cell row or system as electrical and fluid input/output connections. Because the end plate is normally made of the same material through similar processing to that of the bipolar plate in a stack, the bipolar plate and end plate will be called a plate hereafter in this chapter unless their differences are addressed. 

Tolerance and Flatness of Individual Fuel Cell Bipolar Plates To achieve a uniform compression on a stack that can have as many as 300 fuel cells in series, a very tight manufacturing tolerance on the flamess of individual bipolar plates must be achieved. The change in a hundred or so micrometers can have a major effect on the distributed contact resistance. 

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