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Membranes 2022, 12, 602 4 of 18 1.2. Electrodes Regardless of the process type, the anode is in contact with very reactive oxidants and must therefore meet specific constraints. The first electrodes used were made of graphite, then electrodes, made of platinum, diamond, etc., were developed [9]. Currently, the most common solution is the use of dimensionally stable anodes (DSA) consisting, for example, of titanium covered with a deposit of metal oxides such as ruthenium, iridium, or titanium [4,9]. The cathode in contact with OH− ions poses less of a problem from the point of view of corrosion. While stainless steel electrodes may be suitable, energy criteria lead to the adoption of more sophisticated materials. This is because the choice of electrodes also plays on the optimization of the current/voltage parameters by their influence on the overvoltages necessary for the activation of the reactions and therefore on the cost of energy operation. For this reason, activated nickel-based cathodes coated with a catalyst including nickel and platinum group elements are used [9,11]. 1.3. Cell Design To achieve important reduction of the cell resistance, it is necessary to significantly reduce the thickness of electrolytes. The first idea is to reduce the space between electrodes while avoiding a gas blockage between the electrode and the membrane. The optimal thick- ness is between 0.2 and 1 mm [10]. The distance between the cathode and the membrane is typically set at approx. 1 mm. Another solution is to change the position of the different elements. In the simplest case, porous electrodes are placed directly in contact with the membrane, hence the name zero-gap [6]. A more complex setup can also be used. In this case, the membrane is placed in contact with the cathode by means of a mesh glued to it and connected to a cathode by means of an elastic metal element, thus ensuring the electrical conductivity between the two elements of the cathode [12,13]. 1.4. Monopolar or Bipolar Electrolyzer In the case of a system using several unit cells, it is possible to make two types of electrical connections. These can be powered in parallel (unipolar connection with high current circuit and low voltage) or in series (bipolar connection with a low current and high voltage). Bipolar electrolyzers are preferred due to the reduced investment (simple filter press design leading to easier manufacturing) and operating costs (better energy performance due to smaller voltage drop) as well as to easier maintenance (easier detection of faulty cells by monitoring individual cell voltages, shorter duration of shutdown and start-up phases to replace membranes) [3,14,15]. 1.5. Recycling In the case of the production of a low-concentration disinfectant, it is necessary to optimize the transformation of the produced chlorine. Kim et al. [5] propose reinjecting the anodic chlorine into the cathodic solution containing the formed soda. Production is then significantly improved, whereas an assembly without recirculation yields a production practically equivalent to an assembly without a separator. 1.6. Parameters’ Optimization The chlorine-soda process operates industrially at temperatures above 60 ◦C, most often around 90 ◦C, and the cell is supplied with a saturated NaCl solution. For the formation of disinfectant solutions, it is easier to work at room temperature. On the other hand, due to the disproportionation of chlorine being an exothermic reaction, it is sometimes necessary to provide a method to cool the reaction medium. The activity and the quality change during storage time also depends on the operating parameters.PDF Image | Zero Gap Electrolysis Cell for Producing Bleach
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