Ionic Diodes in Water Desalination & Purification

F. Marken, E. Madrid, B.D.B. Aaronson, N.B. McKeown
University of Bath,
United Kingdom

Keywords: rectification, water purification, polymer with microporosity, ion selectivity, desalination


Ionic rectifiers (or ionic diodes) work like electronic diodes, but allow ion flow to be controlled. We have developed novel ionic diode devices based on ionomer and polymer materials coated onto microholes to typically 5 micron to 40 micron diameter. When a potential is applied across the membrane an open (or “resistive”) state is observed and at the opposite potential a closed (or “limiting”) state occurs. The ratio of current at +1V and at -1V can be employed as rectification ratio to describe the performance of the diode. We have investigated effects of salt concentration on performance and effects of microhole diameter on performance. The material employed for the ionic diode is crucial. Initial work was performed with Polymers with Intrinsic Microporosity (PIMs) which provide a novel class of structurally rigid ion-selective membrane materials with 3D nanofluidic pores of typically 1-2 nm size. The PIM-EA-TB material employed initially was based on a poly-amine with estimated pKA = 4.0 and therefore in the protonated state an anion-conductor [1]. When deposited asymmetrically over a 20 micron diameter hole in poly-ethylene-terephthalate (PET) investigated in a two-compartment electrochemical cell with aqueous electrolyte on both sides, ionic diode effects associated with pKA are observed. These results led us to propose a novel water desalination process based on the ionic rectifier and AC membrane excitation [2]. Other materials such as Metal-Organic Frameworks (MOFs [3]) and more conventional materials, even cellulose, provide similar effects. The AC excitation of the membrane during desalination requires a fast switching rate and therefore work is in progress exploring the parameters that determine the open-to-close switching rate of ionic diodes for various materials. For some materials we observe switching when the external pH is changed which occurs for membranes of only 300 nm thickness [4]. Due to the electrical driving force for the ionic rectifier during desalination there are no high pressures (allowing thinner membranes to be employed) and there is no electrolytic decomposition of the electrolyte (due to the AC excitation at appropriate frequency). In summary, a combination of a novel electrochemical microhole process with ionomer materials allows ionic rectification in salt solution (containing NaCl) and progress can now be made in the development of new desalination methods and in new “depollution” methods with ion selective diodes. References: [1] E. Madrid, Y.Y. Rong, M. Carta, N.B. McKeown, R. Malpass-Evans, G.A. Attard, T.J. Clarke, S.H. Taylor, Y.T. Long, F. Marken, Angew. Chem. Int. Ed. 2014, 53, 10751-10754. [2] E. Madrid, P. Cottis, Y.Y. Rong, A.T. Rogers, J.M. Stone, R. Malpass-Evans, M. Carta, N.B. McKeown, F. Marken, J. Mater. Chem. A 2015, 3, 15849-15853 [3] E. Madrid, M.A. Buckingham, J.M. Stone, A.T. Rogers, W.J. Gee, A.D. Burrows, AP.R. Raithby, V. Celorrio, D.J. Fermin, F. Marken, Chem. Commun. 2016, 52, 2792-2794 [4] Y.Y. Rong, Q.L. Song, K. Mathwig, E. Madrid, D.P. He, R.G. Niemann, P.J. Cameron, S.E.C. Dale, S. Bending, M. Carta, R. Malpass-Evans, N.B. McKeown, F. Marken, Electrochem. Commun. 2016, 69, 41-45