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Membranes 2021, 11, 575 15 of 29 In the particular case of this work, the transport number decrease can be attributed to the fact that high concentrations of LiCl and LiOH promote higher concentration of Li+ in the membrane as a counterion. This, along with the principle of electroneutrality and Donnan exclusion, causes increase in the concentration of Cl− and OH− co-ions in the membrane [48]. According to the Nernst–Planck flux equation applied to the membrane, the latter would promote more OH− ion leakage across the cation-exchange membrane due to increased concentration and current density [54,55]. In this work, this is manifested by a greater influence of the LiOH solution on the lithium transport number compared to the LiCl solution, which can be attributed to the presence of OH− as a co-ion in the membrane. Regarding the membranes used, the CMX membrane presents a higher transport num- ber for lithium ions than the CMB membrane, which can be attributed to its lower thickness and electrical resistance. On the other hand, at high LiOH concentrations (8.0 wt%), an increase in current density increases the lithium transport number—especially for the CMX membrane. 3.3. Current–Voltage Curves To determine the performance of bipolar and ion-exchange membranes, linear sweep voltammetry was performed as presented in Figures 7–9. 3.3.1. Salt Leakage through Bipolar Membranes In the BMED process for LiOH production, Li+ leakage into the HCl solution and Cl− leakage into the LiOH solution through the bipolar membrane can occur. Figure 7 presents current–voltage curves measured for the determination of salt trans- port through bipolar membranes at two different concentrations: LiCl 14 wt% and 25 wt%. Results are presented for the Fumasep FBM and Neosepta BP membranes. At a concen- tration of LiCl 14 wt%, a plateau in the curve was clearly observed for both membranes, indicating a limiting current density associated with the transport of salts through the membrane. At 25 wt% LiCl concentration, the plateau was less defined and presented a slope, which is probably due to a significant transport of co-ions—which carry electric current—through the membrane [44]. When using 14 wt% LiCl as an electrolyte, the mea- sured limiting current densities were 176 A·m−2 and 77 A·m−2 for the Fumasep FBM and Neosepta BP membranes, respectively. In contrast, for a 25 wt% LiCl electrolyte, the mea- sured limiting current densities were approximately 334 A·m−2 and 79 A·m−2, respectively. Limiting current densities for the Neosepta BP membrane were lower, indicating that it possesses characteristics more suitable for operating at high electrolyte concentrations (see Table 2). Studies have been reported that indicate a decrease in salt leakage through bipolar membranes by increasing the thickness of one of the cationic or anionic layers that compose a bipolar membrane [44]. A similar effect can be expected when comparing the Neosepta BP and Fumasep FBM membranes, with the former being thicker. For production process application, results indicate that for high concentrations (25% LiCl), the use of high current densities could reduce undesired transport of salts (for in- stance, in the case of the bipolar membrane Fumasep FBM, a current density of 1000 A·m−2). Thus, electric current through bipolar membranes is transported mostly by H+ and OH− ions generated by water dissociation in the bipolar membrane, and to a lower extent by leakage of Li+ and Cl− salts. Balster et al. [44] studied salt leakage through a BP-1 membrane using the same characterization method. They found that the first limiting current density associated with salt transport was 0.61 mA·cm−2 (6.1 A·m−2) using a 2 M NaCl solution (approximately 11 wt%). By laminating AMX membranes on the anionic face of the BP-1 membrane (asymmetric bipolar membranes), they achieved a 25–30% decrease in the first current density, at the cost of an increase in electrical resistance by 32% and 84% by the addition of one and two AMX membranes, respectively. As reported in their study, the first limiting current density decreases with the thickness of the anionic layer, meaning reduced saltPDF Image | Bipolar Membrane Electrodialysis for LiOH Production
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