Transport-limited water splitting at ion-selective interfaces during concentration polarization

Abstract

We present an analytical model of salt-and water-ion transport across an ion-selective interface based on an assumption of local equilibrium of the water-dissociation reaction. The model yields current-voltage characteristics and curves of water-ion current versus salt-ion current, which are in qualitative agreement with experimental results published in the literature. The analytical results are furthermore in agreement with direct numerical simulations. As part of the analysis, we find approximate solutions to the classical problem of pure salt transport across an ion-selective interface. These solutions provide closed-form expressions for the current-voltage characteristics, which include the overlimiting current due to the development of an extended space-charge region. Finally, we discuss how the addition of an acid or a base affects the transport properties of the system and thus provide predictions accessible to further experimental tests of the model.

Concentration polarization, surface currents, and bulk advection in a microchannel

“…Expressions, which are uniformly valid both at underand overlimiting current, are also derived in our previous work Ref. [8], but since these are rather lengthy we will not show them here. We refer to the full model from Ref.…”

“…The development of an extended space-charge region can, however, be captured in an analytical 1D model, and from Ref. [8] we have the limiting expression…”

“…We refer to the full model from Ref. [8] as the analytical bulk conduction (ABLK) model, see Table II.…”

“…A number of different mechanisms have been suggested as explanation for this overlimiting current, most of which are probably important for some system configuration or another. The suggested mechanisms include bulk conduction through the extended space-charge region [4,5], current induced membrane discharge [6], water-splitting effects [7,8], electroosmotic instability [9,10], and most recently, electro-hydrodynamic chaos [11,12].…”

Morphological instability during steady electrodeposition at overlimiting currents

“…For this reason, the electrodeposition rate can be driven to exceed the diffusion limit, at which point the system enters a nonlinear regime not encountered in the purely diffusion-driven systems. One of the features of this nonlinear regime is the development of a nonequilibrium space-charge region, which extends from the cathode into the electrolyte [12,[14][15][16]. This extended space-charge region significantly affects the transport in the system, and it is a central component in the well-known electroosmotic instability [17][18][19].…”