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Membranes 2021, 11, 575 19 of 29 Test Cation membrane Bipolar membrane Current density (A·m−2 ) LiCl initial concentration (wt%) Number of unit cells LiOH concentration (wt%) Cl− concentration in LiOH solution (wt%) Final purity (wt%) SEC (kWh·kg−1) CE Test 1 CMX Neosepta BP Test 2 CMB Neosepta BP Test 3 CMX Neosepta BP Test 4 CMX Fumasep FBM Test 5 CMX Fumasep FBM Test 6 CMX Fumasep FBM the other hand, this result can be attributed to leakage current associated with a non-ideal selectivity of the membrane [62]. In the case of the Fumasep FBM membrane at higher concentrations (5.0 wt% LiOH and 7.8 wt% HCl), a small plateau is detected in the graph for potential values below 0.8 V. This can be explained by the increase in salt leakage with concentration. For practical purposes of the production of bases and acids at high concentrations, the bipolar membrane Neosepta BP presents better performance with respect to salt leakage reduction. 3.3.3. Linear Sweep Voltammetry in Cation-Exchange Membranes Linear sweep voltammetry was performed on the CMX and CMB membranes at different LiOH concentrations. Figure 9 presents a comparative plot of the different current– voltage curves obtained. Current variation with potential was linear for the measurement range used, and limiting current density was not reached in any case. It can be observed from the graph that current densities close to 5000 A·m−2 for the CMX membrane, and between 2200 and 3500 A·m−2 for the CMB membrane, were reached for a potential range between 0 and 1.8 V. This indicates that the high electrolyte concentrations studied allow high availability of Li+ ions on the membrane surface to migrate across the membrane without the occurrence of concentration polarization. The highest current densities were reached at higher electrolyte concentrations. For theCMX membrane, the current density at 5.0 wt% LiOH was on average 32% higher than the value at 0.5 wt% LiOH. In the case of the CMB membrane, this difference was 60%. On the other hand, when comparing the membranes at the same concentration, the CMX membrane allowed us to obtain current densities 113–159% higher than those obtained with the CMB membrane. Better energy efficiency was observed for the CMX membrane, as it achieved a higher electric current density at a lower potential difference. This can be attributed to the fact that the CMX membrane has 13–21% less thickness than the CMB membrane (see Table 7), and an electrical resistance 22–24% lower (see Table 1). For the application of the membranes in LiOH production, the selected concentra- tions and flux rates (1.0–1.4 cm/s) were adequate for lithium transport through cationic membranes without reaching the limiting current density. 3.4. Long-Running Production Tests of LiOH by BMED Long-running tests of LiOH production were performed using two different bipolar membranes (Neosepta BP and Fumasep FBM) and cation-exchange membranes (CMX and CMB) (Table 5). The current densities used were 500 and 1000 A·m−2. These current densities were chosen according to the LSV results in order to reduce salt leakage through the bipolar membranes (see Figure 7). The initial LiCl feed concentration was between 14 and 34 wt%. Initial LiOH and HCl concentrations equal to 0.5 wt% were used in all tests. The obtained results are summarized in Table 8. Table 8. Summary of the main results obtained in long-running tests. 1000 14 14 25 25 14 34 1.98 3.16 4.05 2.11 3.34 0.10 0.19 0.26 0.05 0.13 94.6 93.5 93.4 97.9 96.0 6.94 7.72 8.71 7.57 8.58 0.72 0.65 0.58 0.77 0.66 4.35 1.93 0.21 0.06 95.4 96.0 9.45 8.23 0.59 0.68 3.25 4.43 0.13 0.46 95.3 88.6 8.58 9.01 0.64 0.60 1.81 2.98 0.07 0.32 92.9 86.5 8.98 9.46 0.63 0.58 3.97 2.05 3.35 4.13 3.10 0.52 0.18 0.40 0.69 0.48 83.8 90.7 87.8 83.6 76.8 10.23 5.97 7.14 9.29 6.81 0.53 0.69 0.58 0.46 0.54 1000 1000 1000 500 500 4 4.80 5.20 1.23 1.24 66.0 54.7 8.92 11.94 0.39 0.31 2 2224PDF Image | Bipolar Membrane Electrodialysis for LiOH Production
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