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Membranes 2022, 12, 602 15 of 18 Besides the physicochemical properties of the used membranes, the difference in hypochlorite ion yield by zero-gap electrolysis may be related to the chemical stability of the membranes used and their resistance to aggressive media during operation. Table 2 allows to evaluate the chemical resistance of the employed membranes. It shows the states of the different membranes after 8 h of operation. Table 2. Performance and state of the used membranes after electrolysis. Cathodic Membrane CMX Nafion®911 Nafion®911 Zirfon® BN/PTFE Anodic Chlorometric Membrane Degree (◦Ch) AMX 6.1 BN/PTFE 1.6 AEM Fuji 2.87 Zirfon® 0.85 BN/PTFE 5.8 Membrane State CMX, AMX membranes damaged Nafion®911 degraded after 2 tests Nafion®911 degraded after 2 tests Anodic and cathodic membrane affected from the first test Intact after 10 tests Generally, oxidation degrades the functional groups and some organic material. Ac- cording to Gaudichet-Maurin et al. [44], exposure to NaClO produces the breakage of the macromolecular chain of polysulfone. Similarly, Prulho et al. [45] estimate that the immer- sion of the PES/PVP mixtures in a hypochlorite solution at pH = 8 causes the oxidation of the aromatic rings into phenol groups. This enables us to explain the degradation of Zirfon® which contains 15 wt% of polysulfone [13]; after two tests, even though it has good physicochemical properties, its chemical stability and resistance to aggressive agent re- mains limited. In addition, the degradation of the surface of the Zirfon® membrane, which is located against the central compartment of NaCl, is due to the presence of a quantity of hypochlorite ions in this compartment as a result of the retro-diffusion phenomenon explained previously. The degradation of the IEMs-based poly(styrene-co-divinylbenzene) AMX and CMX can be attributed to the breakage of the polymer chain and the anionic and cationic func- tional sites. Garcia et al. [46] showed that sodium hypochlorite provoked a degradation of the quaternary ammonium sites of anion-exchange membranes and chain scission of the poly(styrene-co-divinylbenzene) backbone from anion and cation-exchange membranes through chain radical oxidation. During operation and under the effect of the electric field, Na+ ions selectively migrate towards the cathode through a cation-exchange membrane. Moreover, due to the formation of hydroxide ions, OH−, following the electrochemical reaction which occurs at the cathode according to Equation (4), an important quantity of NaOH is produced during the operation. The results of the dosage are illustrated in Figure 10. According to this figure, the amount of NaOH produced depends on several factors such as the type of membranes and whether they are exchange or composite membranes. In this comparative study, one can reach about 2.5 M NaOH after 8 h using the CMX/AEM membrane pair. The quantities of NaOH produced in the cathodic compartment differ from one pair to another, and this can be explained by the physicochemical characteristics of the used membranes. More specifically, the difference in exchange capacity, as well as the selectivity and the affinity, of the functional groups of the membranes to Na+ ions are observed.PDF Image | Zero Gap Electrolysis Cell for Producing Bleach
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