Bipolar Designs

A third approach, not yet fully demonstrated at the limit of dispersed catalysts, is the induction of electrochemical promotion without an intermediate conductive phase (Fig. 12.3). This approach will be discussed in Section 12.3 in relation to the concept of the bipolar design. 

It has been recently found that direct electrical contact, via a metal wire, to the catalyst-electrode is not necessary to induce the effect of electrochemical promotion.8 11 It was found that it suffices to apply the potential, or current, between two terminal electrodes which may, or may not, be catalytically active. The concept appears to be very similar with that of the bipolar design used now routinely in aqueous electrochemistry. 

Electrochemical Promotion of C2H4 Oxidation on Pt Using a Bipolar Design... 

An obvious extension of the bipolar design idea presented in the previous section is the induction of NEMCA using multi-stripe or multi-dot Pt catalysts placed between two terminal Au electrodes, as shown in Figs. 12.8 and 12.9. Both designs have been successfully tested as shown in these figures.10 Larger terminal voltages are applied here between the two Au electrodes, so that the potential difference in each individual cell formed between the Pt stripes or dots is of the order of IV.10... 

A brilliant demonstration of the bipolar design concept came recently from the group of Comninellis atEPFL (Fig. 12.10).9... 

They also dramatically advanced the bipolar design concept, first explored by Marwood and Vayenas to induce NEMCA in monolithic YSZ stmctures, a key step for the practical utilization of NEMCA. 

Two distinct classes of cell design exist the monopolar and the bipolar. Most commercial stacks have the bipolar design, which means that the single cells are connected in series both electrically and geometrically. The bipolar cell design has the advantages of compactness and shorter current paths with lower voltage losses. 

The most widely used brine electrolytic cells are the Hooker and Diamond Shamrock which are both monopolar, but bipolar designs like that of Figure 19.19(a) also are popular. That figure does not indicate the presence of a diaphragm but one must be used. 

In a related study, Lin et al. [42] used a bipolar arrangement of the same reactor and electrode system mentioned above to compare the performance of the two configurations. The bipolar design was shown to yield higher current efficiencies for cyanide destruction at the expense of a higher power consumption. Nonuniform copper deposition on various cathodes showed that the reactions were not occurring uniformly with the bipolar arrangement. 

The major part of water electrolyzers operated today use alkaline electrolytes and follow the bipolar design. Figure 21 represents the principle setup. 

The Hooker MX, Allied PCF, and Ionics Chloromate electrolyzers are all bipolar designs that utilize plastic cell frames The electrolytic area of these cells is typically 1.2-1 5 per cell unit The electrolyzers are typically composed of 10-50 cell units Power consumptions are reported to be 2750 KWH/metric ton of NaOH operating at 3KA/M current density at 95% current efficiency producing 35% caustic soda (65,69,70) ... 

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