Driving electrochemical membrane processes with coupled ionic diodes

Abstract

Ionic diodes have emerged repeatedly in the literature for gel interfaces, for nanopores and channels, for nano-/micro-fluidic systems, and for asymmetrically ionomer-covered microholes. Concentration polarisation is likely to be the key to under-standing the diode function and the diode time constant tdiode, i.e. the time for approaching steady state following a potential/ polarity switch. For frequencies higher than udiode = 2pf = (tdiode)-1, the polarization mechanism is too slow for ion cur-rent rectification. Below the frequency associated with the diode time constant, irreversibility in ion flow is induced and the diode switches between two resistive states at opposite po-tentials ("open" and "closed"). The irreversible flow of ions allows energy conversion from electrical to electrochemical. For energy conversion, two coupled ionic diodes are neces-sary driven by alternating current (AC) electricity to minimise driver electrode electrolysis and energy losses. Opportunities for AC-desalination and for electroosmotic water harvesting with coupled ionic diodes are discussed.