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Membranes 2022, 12, 602 5 of 18 The solution used is more dilute to avoid too much salt transport but must remain suf- ficiently concentrated to limit electric resistance. The concentration of the electrolyte at the anode can vary from 180–240 g·L−1 [5,16] to 1.65–3.5 g·L−1 at pH between 1.0 and 4.5 [10]. Liao et al. [17] studied the influence of the flow rate, NaCl concentration, electrolysis time, and the current density on the solution activity. All obtained curves have a similar shape with an optimal result for a short electrolysis time of 10 s. Although the variation of the parameters is small, the best quality is visibly obtained for the lowest flow rate (10 mL·s−1), the highest salt concentration (0.2 g·L−1), and the highest current density (0.4 A·cm−2). 1.7. Salt Quality When the salt solution is not obtained by redissolving purified NaCl but from recycled solutions, or solutions prepared with non-pure salt, brine, or seawater, the precipitation of bivalent salts will obstruct the membrane and, hence, its resistance. The presence of transition metals can have a negative catalytic effect which leads to the appearance of undesirable secondary products [1,18]. 1.8. Other Cells Another means of reducing energy consumption can be obtained by introducing oxygen-depolarized cathodes (ODC). This method consists of modifying the reaction on the cathode by injecting oxygen by means of a gas diffusion electrode (GDE); a technique well known from alkaline fuel cells. By changing the reaction of hydrogen production by a reduction in oxygen, up to 30% energy can be gained [9,19–22]. A very different approach is proposed by Hou et al. [23], who return to a cell without a separator but based on a different operating principle from conventional electrolysis. The process uses a pair of electrodes based on Na0.44MnO2 operating on a two-phase cycle; the first, in NaOH 1 M, produces the cathode H2 and OH− from water and deintercalates the anode in Na+, the second, in saturated NaCl medium, inserts Na+ in the cathode and produces chlorine on the anode. The process leads to a Faradic efficiency of 100% for hydrogen but it is only 90.2% for chlorine against 97.4% for electrodialysis. For the direct disinfection of water, Isidro et al. [19] compared the use of two electroly- sis cells with boron-doped diamond (BDD) coatings but varying by geometry and the flow conditions. One is a flow-through cell with perforated electrodes; the other is a zero-gap cell in which a Nafion membrane separates the anode and cathode. The performances are quite identical, but the second one minimizes the production of chlorates and perchlorates when operating in a single-pass mode. The main purpose of this study is to develop a new method based on a zero-gap elec- trolysis cell for producing bleach at a relatively high concentration. To generate hypochlorite ions in a more efficient, controllable, and cost-effective way, we have not only used different types of commercial membranes such as ion-exchange membranes (Nafion®, AMX, CMX, Fuji AEM) and a composite membrane (Zirfon®, AGFA, Mortsel, Belgium), but also a homemade BN/PTFE composite membrane, presenting high chemical resistance. This study offers a new possibility to develop an efficient and stable hypochlorite ion generation system in a batch mode. 2. Materials and Methods 2.1. Materials We used the Zirfon membrane (AGFA, Mortsel, Belgium) which is one of the commer- cially available low-cost membranes with very low electrical resistance. This membrane will be used to compare the performance of our BN/PTFE membrane, which was synthesized by laminating a paste based on boron nitride (BN) and polytetrafluoroethylene (PTFE). PTFE provides chemical resistance to the membrane and BN provides thermal resistance and porosity. Zirfon and BN/PTFE membranes are porous and are likely to have significant gas and/or ion leakage. We therefore considered dense perfluorinated cation-exchange mem-PDF Image | Zero Gap Electrolysis Cell for Producing Bleach
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