## 3 Results and Discussion
### Phenomena Related to Overlimiting Mass Transfer Mechanisms
#### 3.2.1 Chronopotentiometric Features
In Figure 6c, as the i\({}_{\text{lim}}\) is exceeded, electroconvective vortices take place, thus limiting a continuous increase in U\({}_{\text{m}}\), such as that observed for Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\). The simultaneous evolution of electroconvection and formation of precipitates was also observed in Figure 2c. From the evolution of the curves it can be extracted that electroconvection occurs even at currents lower than the current at which formation of precipitates starts. Accordingly, the mixing of the DBL due to electroconvection does not prevent the formation of precipitates. This could be caused by the slight difference between the electrolyte pH at the beginning of the experiments and the pH where precipitation starts. It can be also observed that the electroconvective vortices do not cease after the formation of precipitates, which would indicate that the extended space charge region still prevails after formation of precipitates. In Figure 2c a decrease of U\({}_{\text{m}}\) associated with electroconvection is observed even after the formation of precipitates. In the case of NiSO\({}_{4}\) solutions, the formation of electroconvective vortices is probably impeding the formation of a more compact and blocking precipitate layer, thus limiting the increase in membrane resistance associated with the deposited precipitates. The advantages of electroconvection are therefore not only related to the mixing of the DBL but also to the limitation of the growth of precipitates, as well as to its contribution to create a reversible and less compact precipitate layer. Finally, Figure 6d shows another example where overlimiting mass transfer is coupled with the formation of precipitates. Here, at the intermediate current density, the typical increase of U\({}_{\rm m}\) related to the precipitate deposition is very defined. As higher current densities are applied, the jump in U\({}_{\rm m}\) due to ion depletion and the jump due to the formation of precipitates are overlapped. Then, a maximum is reached and U\({}_{\rm m}\) decreases afterwards. In contrast with the decrease of U\({}_{\rm m}\) reported in the previous curves, the pH of the bulk solution changed significantly during the chronopotentiometric measurements. The formation of a precipitate layer creates in this case a bipolar structure, which promotes the dissociation of water [21]. It has been demonstrated in previous works, that d-metal hydroxides, like Ni(OH)\({}_{2}\) can catalyze the dissociation of water in electrodialysis systems [22]. Curiously, although the same salt was used in the experiments of Figure 6c,d, the overlimiting mass transfer phenomena arising are different. The higher concentration of Ni\({}^{2+}\) ions in Figure 6d implies that precipitation of Ni(OH)\({}_{2}\) starts at lower pH values, as can be deduced from the solubility plot of  
Figure 6: Examples of the evolution of chronopotentiograms at increasing current densities where overlimiting current transfer mechanisms evolve. (**a**) Curves obtained with C\({}_{\rm 2}\)(SO\({}_{\rm 4}\))\({}_{3}\) solutions where electroconvective vortices evolve; (**b**) curves obtained with Fe\({}_{\rm 2}\)(SO\({}_{\rm 4}\))\({}_{3}\) solutions where the formation of precipitates blocks the membrane surface and hampers overlimiting currents; (**c**) curves obtained with \(5\times 10^{-3}\) M NiSO\({}_{\rm 4}\) solutions where first electroconvection arises and then electroconvective vortices coexist with the formation of precipitates; (**d**) curves obtained with \(2\times 10^{-2}\) M NiSO\({}_{\rm 4}\) solutions where precipitates are formed before activation of electroconvection and then the dissociation of water is promoted. Figure 4. At this stage, it is important to highlight the sensitivity of the predominating mass transfer phenomena during electrodialysis on the concentration and type of electrolyte, as well as on the level of polarization applied to the system.