Overlimiting current properties at ion echange membranes
(Wetsus/University of Twente)
Joeri de Valença is PhD student in the MESA+ research group Soft Matter, Fluidics and Interfaces. His promoter is Rob Lammertink.
The work presented in this thesis is concerned with the understanding and exploitation of overlimiting current (OLC) phenomena in the vicinity of ion exchange membranes. OLC is a boundary phenomena in which additional ion transport next to ion migration and diffusion starts, thereby increasing the current through the membrane. This work has focused on advection driven OLC at cation exchange membranes (CEM), since at a CEM water dissociation during OLC is negligible. Advection, the transfer of heat or matter by the flow of a fluid, was induced at the membrane in two ways. Firstly, by direct coupling of the electric field and the ions in the charged layer at the membrane interface. Since there is a net charge a net ion flow occurs at the interface, the liquid is set in motion under sufficient electrical forcing. This phenomena is called electroconvection, where convection refers to the sum of advective and diffusive transport. Secondly, the electric field also induces a concentration gradient due to the selectivity of the membrane. This fact induces a density gradient that can become gravitational unstable due to buoyancy forces, giving the second type of convection, called gravitational convection. This occurs only if the lighter (ion depleted) layer lies below the heavier (ion enriched) layer which is referred to as the gravitational unstable orientation.
Both types of convection (electro- and gravitational) are induced in a transparent electrochemical cell in which the fluid is initially stagnant. The time-dependent electrical resistance over the membrane is measured while keeping either the current or voltage drop fixed. Simultaneously the emerging fluid motion is monitored by recording tracer particles suspended in the fluid. The particle images are analysed by following the single particle motion and by using particle image velocimetry (PIV) algorithms to determine the collective particle motion. Particle tracking allows an accurate determination regarding advection onset, growth and magnitude. Using another optical technique called fluorescence lifetime image microscopy (FLIM), the concentration distribution was extracted. The combination of above techniques makes is possible to determine the ion transport via diffusion, migration and advection and compare this to theoretical and numerical predictions (Chapter 1 and 2)
The electroconvective mixing layer near a flat cation exchange membrane starts quickly after the interface gets depleted and an additional threshold voltage is applied. The interface layer becomes unstable which starts a vortical flow which in turn enhances the instability. This positive feedback loop is referred to as the electrokinetic instability (EKI). The PIV data shows a layer of lateral moving vortices with approximately equal height and width. The height of the layer grows, while the number of vortices shrink. Although the mixing layer grows till an approximately fixed saturated thickness, the individual vortices keep moving chaotically along the membrane. Applying a higher current density results in a thicker mixing layer and higher system resistance. The conductivity of the mixing layer was found to be nearly constant in all experiments (Chapter 3). This has led to the hypothesis that the mixing layer has a fixed average concentration, independent of size, while the stagnant layer has a concentration gradient. This hypothesis was supported by measuring the concentration profile within the system with FLIM (Chapter 4).
The development of the concentration profile within the system domain is also predicted with a numerical model based on Fick's laws of diffusion with constant flux boundary conditions in a stagnant liquid. The model predictions agree with the measured concentration profile development and predict the time the interface gets depleted, at which point electroconvection starts. The theoretical depletion time agrees with the electrical transition time and electroconvection onset time observed in the gravitational stable orientation where no gravitational convection can occur (Chapter 3 and 4). This orientation is also called counter-orientation, since the electrical field and the gravitational field point in opposite directions. In co-orientation both fields point downward and the model is also used to predict when the concentration gradient becomes gravitationally unstable and Rayleigh-Bénard (RB) convection occurs. To test this, the cell is operated in two orientations towards the gravitational field and shows that the numerical and experimental transition times are in agreement. The measurements also show that RB convection occurs in the full cell, therefore reducing the resistance. At high current electroconvection via the electrokinetic instability (EKI) and RB coexist, since the additional ion transport via RB is not enough to avoid the interface layer gets fully depleted. The RB motion does reduce the diffusion boundary layer, therefore diminishing the EKI layer thickness drastically (Chapter 4). This is similar to the reduction of the diffusion boundary layer because of pressure driven flow.
The exact coupling between the electric field and the EKI mixing layer is a much debated question. To understand the onset of EKI, membranes with different line undulations are compared under constant potential conditions. The presence of the structures reduces the threshold voltage to start electroconvection up to 60% compared to at a flat membrane (Chapter 5). The structures act as starting points for electroconvection which results in periodic vortices, compared to a flat membrane where the vortices appear at random locations and move lateral along the surface. The structured membrane interface has a part that is parallel to the external electric field, along which an electro-osmotic flow (wall EOF) occurs. Our hypothesis is that wall EOF occurs without a threshold and increases in strength as the concentration depletes further and the space charge layer expands. This leads to periodic heterogeneity in the concentration profile and electric field that allows EKI to start at a lower threshold voltage.
After the onset the vortices grow until saturating in size. When the membrane structures had similar dimension as the saturated mixing layer thickness, the resistance of this layer was 50% lower compared to a flat membrane. The surface structures fix the vortices position allowing steady inflow locations with higher concentration and outflow locations with lower concentration. The PIV and FLIM results show the mixing occurs in vortex rolls along the line structures (Chapter 5). With the same driving force the vortices can grow larger, therefore extend the mixing layer farther from the membrane interface and bring higher concentration solution to the membrane. This results in improved average concentration in the mixing layer and a reduced resistance. From the resistance, concentration and flow data the migration, diffusion and advective transport is estimated. These calculations give the right order of magnitude of ion transport. This hints that in steady state no other major transport processes are significant (e.g. transport by other ions (H-, OH-) or loss of ions due to chemical reactions (copper oxide scaling)).
To summarize, the work presented in this thesis provides the observation and determination of ion concentration polarization (ICP) by applying techniques such as fluorescence lifetime image microscopy (FLIM), particle tracking and particle image velocimetry (PIV). This latter technique was also used for proving that electroconvection is a source of overlimiting current (OLC) regime at cation exchange membranes. A new regime of chaotic motion in the electroconvective mixing layer has been observed in contrast of what was previously described by other researchers. Another new aspect of this work is the observation and classification of the coupling between electroconvection and Rayleigh-Bénard convection. Furthermore, the work presented in this thesis quantifies the behaviour of the electroconvective vortices at geometrically structured membranes. When the mixing layer thickness and the structure periodicity have a similar length the vortices are stabilised which reduces the resistance within the layer.
Location: Kanselarij, Turfmarkt 11 Leeuwarden
Time: 14:30 hrs.
