Ionic diode desalination: Combining cationic Nafion™ and anionic Sustainion™ rectifiers

Microscale ionic rectifier effects are commonly observed in devices based on semipermeable ionomer coated on an array of microholes with potential applications in alternating current (AC) driven desalination and/or electroosmotic pumping. The efficiency of devices is dependent on ionic diode switching speed, the rectification ratio, and the design of materials and the ionic circuit. Here, a new circuit is proposed based on coupling in parallel (i) a cationic diode based on the cation conductor Nafion and (ii) an anionic diode based on the anion conducting Sustainion. With an alternating driving voltage, a net desalination effect is observed without any moving parts and without significant side reactions. Experimentally, a 4-electrode configuration and a 2-electrode configuration are compared. The ionic diode desalination system is shown to work with only two carbon mat driver electrodes, but the performance in particular at higher ionic strengths (>10 mM) still needs to be improved. Based on the experimental prototype, the current/power efficiency are investigated and challenges for future improvements are discussed.