MEMBRANE-ASSISTED PROCESSES
The reagents, products, and electrode materials involved in the cathodic
and anodic electrode reactions of an electrochemical cell can sometimes interact
in an undesired manner to foster short-circuits or side reactions that decrease
the figures of merit of the system or can even lead to accidents.
Electrochemists have devised ways to prevent such interactions by using special
separators such as macroseparators, microseparators, and ion selective
membranes. Reviews concerning separators, their technology, and applications
are available; and this body of literature is listed at the end of this chapter.
The main functions of the macroseparators are to prevent physical contact
between electrodes, provide structural support, and promote turbulence to increase
the mass-transfer coefficient (examples include polymeric nets and rigid
plastic open pore sheets). Microseparators must have passage sites large enough
to permit the restricted movement of electrolyte from one side to the other but
small enough as to prevent gross exchange of solution due to convective or
diffusive mass transport. Separator materials include asbestos diaphragms,
porous polymeric materials, porous ceramics, and sintered metals. 287 Unfortunately,
an ohmic drop penalty always must be paid for their use, whose magnitude
depends primarily on the pore size and tortuosity.
Ion selective membranes or ion exchange membranes (IEMs) are polymeric
materials that incorporate within their structure ionic centers capable of binding
ions of opposite charge. Such ions can then selectively and directionally
migrate under the influence of an electric field and be separated from their
counterions. 288 This property is called permselectivity. These membrane separators
are used in a variety of electrochemical processes and shall be explored in
more detail in this section in view of their relevance to the environmental field.
The world market for IEM-based technologies is estimated to be on the order
of $3 x 108/year, which is an important share of the total membrane technology
market estimated to be well over $1 billion. 289,29~ Commercial aspects (i.e., membrane
producers) are identified in Chapter 8.
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