A bipolar membrane is a novel type of ion exchange membrane, composed of an anion-selective layer, a cation-selective layer, and an intermediate interfacial layer. When a reverse voltage is applied across the two ends of the bipolar membrane, charged ions migrate from the transition zone of the two ion exchange layers to the bulk solution. Water molecules rapidly dissociate into H⁺ and OH⁻, which then migrate to the bulk solution. The consumed water molecules are replenished from the external solution to the intermediate interfacial layer through diffusion.