## 1 Introduction  
Chlorine gas is an important chemical commodity in the modern industry, mainly produced via electrolysis of a concentrated aqueous solution of sodium chloride, which demands high-energy consumption. In 2016, the European chlorine production represented nearly 9.7 million tons and the membrane electrolyser technology corresponded to ca. 64% of the chlorine production capacity [1]. It should be stressed that the electrolysis units installed during the last decades are almost exclusively based on membrane technology, which is considered the best available technique in the chlor-alkali industry. Membrane technology presents numerous advantages over older technologies, such as higher energy efficiency, lower environmental impact and higher purity of the produced caustic soda that the mercury-based method. Developments in the membrane and electrodes fields are further driving this process to improved energy efficiency levels [2]. A membrane electrolyser module consists in a set of single cells. In the case study, considered in the present work, the investigated membrane electrolyser was connected in parallel and its membrane cell elements were disposed in series electrolyser in a stack arrangement, where the electrodes have two different coating surfaces corresponding to the anode and to the cathode, respectively. This configuration is the most common in the modern chlor-alkali industry, therefore it is the one considered in this study [3]. Its three major components are: a cation-exchange membrane, a dimensionally stable anode (DSA(r)) and a nickel-based cathode. At the anode, chloride oxidation to chlorine occurs releasing electrons to the cathode half-reaction, where hydrogen and hydroxyl ions are produced from the water reduction; the sodium ions migrate across the membrane to the cathode compartment and combine with the hydroxyl ions produced at the cathode, thus originating sodium hydroxide [4]. The thermodynamic redox potential for the operating conditions applied (temperature and composition) is 2.23 V [5], corresponding to a value of 2.19 V at 25 \({}^{\circ}\)C.  
\[2\text{NaCl (aq) }+2\text{H}_{2}\text{O (l)}\rightarrow\text{Cl}_{2}\text{ (g)}+\text{H}_{2}\text{ (g)}+2\text{NaOH (aq)} \tag{1}\]  
The growth of the electromembrane processes and the availability of the selective and stable ionically conductive materials has opened a very promising field for the chlor-alkali industry [2]. Nowadays, the bilayer cation-exchange membrane is usually made of perfluorinated sulfonated materials with a low ohmic resistance and high sodium selectivity. The membrane configuration allows the movement of Na\({}^{+}\) cations and water molecules while preventing the back migration of Cl\({}^{-}\) and OH\({}^{-}\) anions, even under high current densities and ionic concentration gradients [6]. The membrane is made by a thick layer of tetrafluoroethylene functionalized with sulfonic acid groups at the anode side and a thinner layer of tetrafluoroethylene functionalized with carboxylic groups facing the cathode side. The sulfonic functionalized layer assures proton conductivity, while the carboxylic functionalized layer plays a major role for electrostatic repulsion of OH\({}^{-}\) anions. The electrodes should be in a close contact with the cation-exchange membrane, to minimize the ohmic overpotentials on the electrolyte phase. A reduced spacing between the electrode and the membrane allows also for reducing the mass transport overpotential related to the removal of the gaseous products from the inter electrode phase [2]. As for the electrodes, the development of dimensionally stable anodes, DSA(r), contributed to energy savings in the chlorine industry due to their low activation overpotentials and stability [7; 8; 9]. DSA(r) anodes were developed and commercialized back in the 1970's; they are made of a mixture of ruthenium and titanium oxides applied on a titanium substrate.