Bipolar flow

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

Fig. 7.2.17 Flow imaging by phase encoding. A bipolar flow-encoding gradient is applied for slice selection to image through-plane flow v (x, y). Both phase and frequency gradients for space encoding are flow compensated.

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. 

After a short historical overview of the history of astrophysical spectroscopy some general properties of star forming regions will be discussed. The bipolar flow region L1228 will be shown as an example and a theoretical model for bipolar flow mechanism is proposed. 

Since the first observational evidence of bipolar flows in LI55I (Snell et al. I98O) was found an Intensive effort has been made both observationally and some extent also theoretically to understand the nature and cause of the bipolar-flows (see e.g. Lada 1985). The existence of bipolar flows is probably the most direct evidence for... 

In an attempt to contribute theoretically towards an understanding of the physics of bipolar-flows, we have developed a model aimed at explaining the major observational features of outflow sources. The initial scenario required by the model is that of a central stellar object embedded in a parental cloud of gas and dust forming a halo whose shape deviates slightly from spherical symmetry, an assumption which should be met in practically all realistic cases. 

FIGURE 4-20 Concept of the two-layer bipolar flow of slurry at an angle of inclination. 

The experiments conducted by Karimi indicate that a larger amount of oxidant stream needs to flow in the bipolar flow fields with increase in stack operating cm-rent density, which resnlts in significantly low pressures. The low pressure improves water transportation and thus reduces flood. 

A possible problem of sealing the electrolyte path is found in the Foreman and Veatch cell. This can be avoided by placing the cells in a vessel. The best known example of this is the Beck and Guthke cell shown in Figure 8 (74). The cell consists of a stack of circular bipolar electrodes in which the electrolyte is fed to the center and flows radially out. Synthesis experience using this cell at BASF has been described (76). This cell exhibits problems of current by-pass at the inner and outer edge of the disk cells. Where this has become a serious problem, insulator edges have been fitted. The cell stack has parallel electrolyte flow however, it is not readily adaptable to divided cell operation. 

The invention of the germanium transistor in 1947 [I, 2] marked the birth of modem microelectronics, a revolution that has profoundly influenced our current way of life. This early device was actually a bipolar transistor, a structure that is mainly used nowadays in amplifiers. However, logical circuits, and particularly microprocessors, preferentially use field-effect transistors (FETs), the concept of which was first proposed by Lilicnficld in 1930 [3], but was not used as a practical application until 1960 [4]. In a FET, the current flowing between two electrodes is controlled by the voltage applied to a third electrode. This operating mode recalls that of the vacuum triode, which was the building block of earlier radio and TV sets, and of the first electronic computers. 

Ion chromatography (see Section 7.4). Conductivity cells can be coupled to ion chromatographic systems to provide a sensitive method for measuring ionic concentrations in the eluate. To achieve this end, special micro-conductivity cells have been developed of a flow-through pattern and placed in a thermostatted enclosure a typical cell may contain a volume of about 1.5 /iL and have a cell constant of approximately 15 cm-1. It is claimed15 that sensitivity is improved by use of a bipolar square-wave pulsed current which reduces polarisation and capacitance effects, and the changes in conductivity caused by the heating effect of the current (see Refs 16, 17). 

Redox flow Positive electrode, negative electrode substrate, electrocatalyst support, current collector, bipolar separator... 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

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