Carbon-based suspension electrodes are currently intensively investigated for emerging electrochemical systems, such as flow batteries, flow capacitors, and capacitive deionization cells. The main limitation of such electrodes is their low electric conductivity, which is typically orders of magnitude lower than that of traditional static carbon electrodes. Two main categories of suspension electrodes exist: 1) slurry electrodes where particles are not significantly affected by gravity, and 2) fluidized bed electrodes where particles are affected by gravity. We introduce a novel category that we term "combined" suspension electrodes, which combine dilute slurries and dense fluidized beds. We present experimental measurements of the electrochemical impedance and electric conductivity of two combined electrodes. For one set of materials, the measured electric conductivity of the combined electrode is at least an order of magnitude above the fluidized bed and slurry components alone, demonstrating that a synergetic effect can be achieved when adding dilute slurry to dense fluidized bed. For a second set of materials, results show that the combined electrode conductivity is lower than the slurry component alone, a counter-intuitive result, demonstrating that increasing electrode carbon loading does not always enhance the electric conductivity.
In this paper, we investigate electroconvective ion transport at cation exchange membranes with different geometry square-wave structures (line undulations) experimentally and numerically. Electroconvective microvortices are induced by strong concentration polarization once a threshold potential difference is applied. The applied potential required to start and sustain electroconvection is strongly affected by the geometry of the membrane. A reduction in the resistance of approximately 50% can be obtained when the structure size is similar to the mixing layer (ML) thickness, resulting in confined vortices with less lateral motion compared to the case of flat membranes. From electrical, flow, and concentration measurements, ion migration, advection, and diffusion are quantified, respectively. Advection and migration are dominant in the vortex ML, whereas diffusion and migration are dominant in the stagnant diffusion layer. Numerical simulations, based on Poisson–Nernst–Planck and Navier–Stokes equations, show similar ion transport and flow characteristics, highlighting the importance of membrane topology on the resulting electrokinetic and electrohydrodynamic behavior.
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