Possibility for potentiometric humidity sensor of composite membranes prepared from anion-exchange membranes and conducting polymer

Composite membranes prepared from anion-exchange membranes and polypyrrole have been examined as a potentiometric humidity sensor. The strip prepared from the composite membrane, of which one half is an anion-exchange membrane and the other is the anion-exchange membrane with polypyrrole layers on both surfaces, generates an electrical potential (e.m.f.) which is dependent on the relative humidity. (Both ends of the strip are coated with silver paste.) The value of the potential is dependent on the anion species ion-exchanged with the membrane and doped with polypyrrole: chloride ions >sulfate ions>nitrate ions>perchlorate ions. Although the strip of sulfate-ion form prepared from the anion-exchange membrane with lower water content shows reversible behaviour of the potential with increasing and decreasing relative humidity, the strips of chloride-ion and nitrate-ion forms show an irreversible potential change with relative humidity. The potential of the perchlorate-ion form strip is the lowest and it is not sensitive to humidity change. It is thought that this is due to the difference of hydration force of each anion in the membrane matrix.