  Ionic current through a charge-selective interface in a binary electrolyte is
a basic element of many electrochemical engineering and microfluidic processes.
Such current passage is diffusion-limited: it induces a decrease of electrolyte
concentration towards the interface (concentration polarization, CP), expressed
in the saturation of current upon increasing voltage at some value (limiting
current, LC). With further increase of voltage, this saturation breaks down
(overlimiting conductance, OLC). In open systems OLC is mediated by a
microscale vortical flow which develops as a result of electroconvective
instability (ECI) of quiescent CP near LC. Electroconvection (EC) is a flow
driven by the electric force acting either upon the space charge of the
interfacial EDL (electroosmosis, EO) or the residual space charge of the
quasielectroneutral bulk (bulk EC). There are two types of EO, the equilibrium
and the nonequilibrium one. The former relates to the action of the tangential
electric field upon the space charge of the EDL, and the latter pertains to the
similar action upon the extended space charge, forming next to the EDL near LC.
For a perfectly charge-selective interface, CP is stable under the equilibrium
EO or bulk EC, but nonequilibrium EO may cause ECI. For this reason until
recently, ECI in CP was attributed to this latter mechanism. Lately, it was
shown that imperfect charge-selectivity of the interface makes equilibrium ECI
possible, driven by either equilibrium EO or bulk EC, or both. Here we identify
and analyze the major surface and bulk factors affecting the ECI. These factors
(diffusioosmosis, EO, bulk EC, and some newly identified ones) are
manifestations of the electric force and pressure gradient, balanced by the
viscous force acting in various locations in solution. The contribution of
these factors to ECI in CP is analyzed for a varying interface permselectivity.
