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The Wikipedia Free Encyclopedia website has a section on dehumidifiers and describes various techniques for reducing humidity.  The ROSAHL dehumidifiers use the technique referred to in the section ionic membrane dehumidifier

Rosahl electrolytic dehumidifier replaces Silica Gel for small remote location

An alternative to traditional dehumidification techniques

Ionic membranes are used in many industrial areas such as fuel cell technology, chemical engineering, and for water improvement. A specialized type of membrane can be used as an “ionic pump” to move humidity into or out of a sealed enclosure, operating at a molecular level without involving visible liquid water.

The solid polymer electrolyte (SPE) membrane is a low power, steady state dehumidifier for enclosed areas where maintenance is difficult. The electrolytic process delivers dehumidifying capacities up to 0.2 grams/day from a 0.2m³ (7 cu ft) space to 58 grams/day from an 8m³ (280 cu ft). SPE systems generally do not have high dehumidification capacities, but because the water vapor is removed through electrolysis, the process is maintenance free. The process also requires very little electrical energy to operate, using no moving parts, making the ionic membranes silent in operation and very reliable over long periods of time. SPE dehumidifiers are typically used to protect sensitive electrical components, medical equipment, museum specimens, or scientific apparatus from humid environments.

The SPE consists of a proton-conductive solid polymer electrolyte and porous electrodes with a catalytic layer composed of noble metal particles. It works when a voltage is applied to the porous electrode attached to the membrane, the moisture on the anode side (dehumidifying side) dissociates into hydrogen ions (H ) and oxygen: the hydrogen ions migrate through membrane to be discharged on the cathode (moisture discharging) side where they react with oxygen in the air, resulting in water molecules (gas), being discharged