Membrane cells bipolar design

The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

Aqueous, alkaline fuel cells, as used by NASA for supplemental power in spacecraft, are intolerant to C02 in the oxidant. The strongly alkaline electrolyte acts as an efficient scrubber for any C02, even down to the ppm level, but the resultant carbonate alters the performance unacceptably. This behavior was recognized as early as the mid 1960 s as a way to control space cabin C02 levels and recover and recycle the chemically bound oxygen. While these devices had been built and operated at bench scale before 1970, the first comprehensive analysis of their electrochemistry was put forth in a series of papers in 1974 [27]. The system comprises a bipolar array of fuel cells through whose cathode chamber COz-containing air is passed. The electrolyte, aqueous Cs2C03, is immobilized in a thin (0.25 0.75 mm) membrane. The electrodes are nickel-based fuel cell electrodes, designed to be hydrophobic with PTFE. 

The dimensionally slahle characteristic of the metal anode made the development of the membrane chlorine cell possible. These cells arc typically arranged in ail electrolyzer assembly which docs not allow for anodc-ro-cathode gap adjustment alter assembly. Also, very close tolerances are required. The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

The electrolysis was carried out in stacks of 24 cells in parallel in a filter press (Fig. 6.3). Each cell (Fig. 6.4) consisted of a cathode spacer, lead cathode, catholyte distribution block, membrane, anolyte distribution block and lead dioxide anode and will have its own catholyte and anolyte streams with compositions shown in Fig. 6.2. The catholyte distribution block includes turbulence promoters and the cell was designed to minimize the cathode—membrane gap (0.8—3.2 mm) because of the poor conductivity of the catholyte required to obtain the high selectivity for adiponitrile. The electrical connection was bipolar with 300 V appUed across the stack, i.e. about 12 V per cell, which gives a cathode current density in the range 0.4-0.6 A cm . The total cell current was 2870 A. 

Membrane cells again are more diverse in their design. In some cases, the crane moves entire electrolyzers. In others, notably the large bipolar electrolyzers, components or elements are the items handled. 

Overcoming these problems was an important part of the development of bipolar membrane cells. From bonded bimetallic sheets, electrode design has advanced to more sophisticated assemblies that prevent titanium embrittlement. As designs have become more successful and as die technology has moved toward higher current densities, bipolar designs have increased their market share. Chapter S illustrates some of the specific designs that are now in use. 

The most significant contributions to the ohmic losses are due to the membrane, the bipolar/cooling plates and the electrodes (GDM + catalyst layer), including contact resistance between components. In a H2/air-fed fuel cell, the activation losses are mainly at the cathode. The catalyst layer, the microporous layer (MPL), and the macroporous gas diffusion layer (GDL) of the gas diffusion medium (GDM) as well as the gas channel design (i.e., the bipolar plate) all contribute to transport losses of reactants. 

Polymers are nsed in fnel cells. Those of particular interest are the polymer electrolyte membrane (PEM) and the phosphoric acid fuel cell (PAFC) designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. In March 2005 Ticona reported that it had bnilt the first fnel cell prototype made solely with engineering thermoplastics. They claimed that this approach rednced the cost of the fuel by at least 50% when compared with fuel cells fabricated from other materials. The 17-cell unit contains injection moulded bipolar plates of Vectra liquid crystal polymer and end plates of Fortron polyphenylene sulfide (PPS). These two materials remain dimensionally stable at temperatures up to 200 "C. The Vectra LCP bipolar plates contain 85% powdered carbon and are made in a cycle time of 30 seconds. 

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... 

Electrolyzers for the production of chlorine and sodium hydroxide, including both diaphragm and membrane cells, are classified as either monopolar or bipolar. The designation does not refer to the electrochemical reactions that take place, which of course require two poles or electrodes for all cells, but to the electrolyzer construction or assembly. There are many more chlor-alkali production facilities with monopolar cells than with bipolar cells. 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

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