Helical vortex formation in three-dimensional electrochemical systems with ion-selective membranes

The rate of electric-field-driven transport across ion-selective membranes can exceed the limit predicted by Nernst (the limiting current), and encouraging this “overlimiting” phenomenon can improve efficiency in many electrochemical systems. Overlimiting behavior is the result of electroconvectively induced vortex formation near membrane surfaces, a conclusion supported so far by two-dimensional (2D) theory and numerical simulation, as well as experiments. In this paper we show that the third dimension plays a critical role in overlimiting behavior. In particular, the vortex pattern in shear flow through wider channels is helical rather than planar, a surprising result first observed in three-dimensional (3D) simulation and then verified experimentally. We present a complete experimental and numerical characterization of a device exhibiting this recently discovered 3D electrokinetic instability, and show that the number of parallel helical vortices is a jump-discontinuous function of width, as is the overlimiting current and overlimiting conductance. In addition, we show that overlimiting occurs at lower fields in wider channels, because the associated helical vortices are more readily triggered than the planar vortices associated with narrow channels (effective 2D systems). These unexpected width dependencies arise in realistic electrochemical desalination systems, and have important ramifications for design optimization.United States. Advanced Research Projects Agency-Energy (Grant DE-AR0000294)Kuwait-MIT Center for Natural Resources and the EnvironmentNational Research Foundation of Korea (Grant 2012R1A2A2A06047424)Singapore-MIT Alliance for Research and Technolog

A multiscale-pore ion exchange membrane for better energy efficiency

Ion exchange membranes (IEMs) have been adopted in various environmental, chemical, and energy applications. However, the formation of ion-depletion regions, caused by concentration polarization near IEMs, often leads to significant energy and efficiency loss. While much research has been devoted to solving this challenge, complete removal of ion-depletion regions is still difficult, especially when the membrane systems are operating under near- or over-limiting conditions. This paper proposes a novel multiscale-pore (MP) IEM to reduce the effect of the ion-depletion region, by allowing a fluid flow through the MP-IEM, thereby limiting the size (and the resulting resistance) of the ion-depletion region. The electrical resistance and energy consumption in MP and conventional IEM-embedded electrochemical systems were investigated, and their performance during water desalination processes were compared. The current-voltage response suggests a secondary ohmic regime attributed to an internal flow rate through the MP-IEM. Moreover, the electrochemical desalination of seawater with MP-IEMs demonstrated up to 75% reduction of energy consumption, compared with conventional IEMs under comparable operating conditions.United States. Defense Advanced Research Projects Agency (Grant DE-AR0000294