Niklas Köller

Operating electrodialysis (ED) processes beyond the limiting current density offers the potential for reduced capital costs and improved energy efficiency, but is fundamentally constrained by concentration polarization and the associated increase in electrical resistance. Spacer geometries are commonly employed to manipulate hydrodynamics in ED channels and enhance mass transfer; however, their impact on overlimiting ion transport remains insufficiently understood. In this study, we systematically investigate how specifically designed spacer geometries influence limiting and overlimiting current behavior by coupling electrochemical measurements with three-dimensional computational fluid dynamics (CFD) simulations. Custom spacer structures were fabricated by 3D printing and evaluated using current--voltage (I--V) characteristics and chronopotentiometric measurements under buoyancy-stable and buoyancy-unstable configurations. The simulations resolve spacer-induced flow fields, near-membrane shear distributions, ion transport, and current-density patterns, enabling direct correlation with experimental performance. While all spacer geometries enhance average mass transfer compared to a spacerless reference, their electrochemical effectiveness strongly depends on the spatial overlap between regions of increased near-membrane shear and electrically active membrane area. Spacer designs that generate strong local mixing but simultaneously induce electric-field shadowing lead to highly non-uniform current distributions, early local ion depletion, and modest gains in effective limiting and overlimiting currents. In contrast, geometries that promote a more homogeneous alignment of shear enhancement and electric driving force delay concentration polarization and stabilize overlimiting operation, particularly under buoyancy-stable conditions where forced convection dominates. These findings allow us to rationalize why spacer geometries that perform well in terms of classical mass-transfer metrics do not necessarily translate into improved overlimiting electrodialysis performance.