Electrodes bipolar plate construction

Two types of ohmic losses occur in fuel cells. These are potential losses due to electron transport through electrodes, bipolar plates, and collector plates and potential loss due to proton transport through the membrane. The magnitudes of these potential losses depend on the materials used in the construction of the fuel cells and its operating conditions [27]. Membrane conductivity increases with membrane water content. Reduction in the thickness of the membrane between anode and cathode may be thought of as an expedient way to eliminate ohmic overpotential. However, thin membrane may cause the problem of crossover or intermixing of anodic and cathodic reactants [27]. 

The PEMFC is constructed in layers of bipolar plates, electrodes, and... 

The typical stack construction for PEM (and other) fuel cells is a series-connection of cells with bipolar plates interposed between adjacent cells (or membrane-electrode assemblies). The bipolar plate must provide for electronic conduction from one cell to the next isolation of fuel on one side from oxidant (air) on the other side and distribution of fuel and air reactants to the respective adjacent anode and cathode. In an edge-cooled stack of the type described above, the bipolar plate also contributes to heat rejection by conducting heat laterally to its tinned edges, where forced-air convection is employed. 

Bipolar Plate An electrode construction where positive and negative active materials are on opposite sides of an electronically conductive plate. 

A unit fuel cell can be constructed using a membrane electrode assembly (MEA) and two bipolar plates as shown in Fig. 1(a). Multiple cells can be stacked up to obtain more power and such a configuration is shown in Fig. 1(b). The charge carriers and their flow direction are identified in Fig. l(a, b). 

To avoid deterioration of the Au/Cr electrode, C-electrodes have been used. Dual C-fiber electrodes were constructed on PDMS. The separation channel (25 pm wide and 50 pm deep) was fabricated on the top plate. Two C fibers (33 pm dia.) were inserted into the PDMS channels (35 pm wide and 35 pm deep) fabricated on the bottom plate. Consecutive injections (up to 41) could be performed before the electrode was cleaned with a bipolar square wave voltage. The LOD of catechol was found to be 500 nM. With the use of the C electrode, peptides (e.g., Des-Tyr-Leu-enkephalin), which formed stable Cu(II) and Cu(III) complexes, could be detected [762]. Dual-electrode amperometric detection also allowed the positive detection of two peptides [763]. 

In fuel cells, carbon (or graphite) is an acceptable material of construction for electrode substrates, electrocatalyst support, bipolar electrode separators, current collectors, and cooling plates. 

Parallel-plate flow cells Most electrochemical flow cells are based on a parallel-plate electrode design with either horizontal or, more commonly, vertical electrodes in a monopolar or bipolar configuration (see Figure 26.12). With vertical electrodes, the cell is usually constructed in a plate-and-frame arrangement and mounted on a filter press. 

Trickle tower reactors containing a number (typically <60) of vertically stacked, bipolar electrode layers have been studied for metal-ion removal at the University of Southampton [41-44]. Earlier studies [41-43] employed layers of hollow carbon cylinders known as Raschig rings (see figure 10a). The relatively low active cathode area per unit reactor volume together with constructional difficulties associated with these packings led to the examination of alternative materials such as felt, particles, foam and perforated plates [44]. 

The arrangement of plate electrodes in the channel can also be bipolar a typical example is shown in Fig. 6. It is apparent that the critical point is the construction of the bipolar electrodes, since the desired metal must be deposited on one side, while oxygen evolution or oxidation of anions or inorganic compounds proceeds on the other side. The manufacture of such bipolar electrodes is complicated and, of course, their... 

With vertical electrodes the cell is usually constructed in a plate-and-frame arrangement and mounted on a filter press. The electrodes, electrolyte chambers, insulating plates to separate cells electrically and, where used, membranes or separators are constructed individually and mounted with suitable gasketing materials between each component the filter press is then used to seal the cells with up to 100 cells in each unit (see Fig. 2.14). It is difficult to manufacture the cells with an area greater than 1 m. Again the electrical connection may be bipolar instead of monopolar (effectively by removing the intercell insulation) and in a series of cells the electrolyte feeds to the cells may be connected so that the cells are in series or parallel so far as the electrolyte is concerned. 

Basically, the construction of PAFCs differs little from what was said in Section 1.4 about fuel cells with liquid acidic electrolyte. In the development of PAFCs and two decades later in the development of PEMFCs (described in Chapter 3), many similar steps can be distinguished, such as the change from pure platinum catalysts to catalysts consisting of highly disperse platinum deposited on a carbon support with a gradual reduction of platinum content in the catalyst from 4 to 0.4 and then to 0.25 mg/cm, and the change from pure platinum to Pt-Ru catalysts. The bipolar graphite plates that have special channels for reactant snpply and distribution over the entire electrode surface now used widely in PEMFC stacks were first used in PAFCs. 

More than a dozen companies throughout the world oficr membrane cell technology and only two types of cell will be used to illustrate here the important features of the designs. Most membrane cells are based on a filterpress containing a series of plate and frame cells. Both monopolar and bipolar electrical connection is used. The basic structure of this type of cell is illustrated in Fig. 3.12 (note that the relative dimensions are not correct - membrane areas may be up to 2 x 2 m, although more normally I x I m, while the thickness of the electrolyte chambers is usually much less than 1 cm). The electrodes are vertical and constructed with louvres or from expanded metal so that the gases formed at the electrodes are directed to the back of the electrodes and do not stay in the... 

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

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