Diaphragm Cells

Not like the other fuel cell, diaphragm cells contain a diaphragm, usually made of asbestos fibers, to separate the anode from the cathode. The allows ions to pass through by electrical migration by reduces the diffusion of products. Anodes have usually been made of graphite, cathodes of cast iron. Diaphragms permit the constructions of compact cells of lowered resistance because the electrodes can be placed close together. The diaphragms becoe clogged wit use, as indicated by higher voltage drop and higher hydrostatic pressure on the brine feed. They must be replaced regularly. The diaphragm permits a flow of brine from anode to cathode and thus greatly lessen or prevent side reactions (e.g. sodium hypochlorite formation).

Cells with metal cathodes (titanium coated with rare earth oxides, platinum or noble metals, or oxides) rarely develop clogged diaphragms and operate for 12 to 24 moths without requiring diaphragm replacements. It is expected that diaphragm made of corrosion resistant plastics will increase service life and remove the environmentalist’s objection to any process that may release asbestos fibers into the environment.

A major advantage of the diaphragm cell is that it can dilute (20%), fairly impure brine. Such dilute brines produce dilute sodium hydroxide (typically 11% NaOH with 15% NaCl) contaminated with sodium chloride as a product. Concentration to the usual shipping strength of 50% is required, and this consumes a great deal of energy even when multiple effect evaporators are used. Approximately 2600 kg of water must be evaporated to produce a ton of 50% caustic.