Diaphragm material

The diaphragm separates reaction zones, is impermeable to gases but must be selectively permeable to ions. It is therefore a hydrophilic material that attracts water or other polar components, like OH-, but repels non-polar components like H2 or O2. 

Historically, electrolyser diaphragms were made of 3mm-thick asbestos layers, leading to a total distance between electrodes of about 5mm . Because of asbestos degradation at high temperature, the operating temperature of such electrolysers was restricted to 80°C . 

The ban on asbestos use forced manufacturers to switch to more elaborate materials. Today’s leading material for AEL diaphragm is a porous material sold under the trademarked name of Zirfon Perl  and produced by Agfa. It is composed of an open mesh polyphenylene sulfide fabric, which is symmetrically coated with a mixture of ~15% polysulfone and ~85% of zirconium oxide (ZrO2). It has a thickness of 0.5 mm, a porosity of 55%, the same density as water and a maximum operating temperature of 110°C. 

Some research efforts focus on developing new diaphragm material, notably sulphonated polyether-ether-ketones, capable of operating at higher temperature. Advanced alkaline electrolysers operating up to 150°C have already been demonstrated at the laboratory level. 

Note that the structural component of the separator (polyphenylene sulfide for Zirfon Perl ) is alkali-resistant and can therefore also be used for the cell frame. As it generally has weak mechanical properties, it is reinforced with steel in pressurised electrolysers.