## 1 Introduction
Ruthenium oxides have excellent electrocatalytic properties, but other transition metal oxides are also used to increase the coating stability during the chlorine evolution reaction, of which iridium oxides present the best mechanical properties [7; 8]. Therefore, nowadays, the composition of the electrodes used in industrial chlor-alkali processes is based on TiO\({}_{2}\)-RuO\({}_{2}\)-IrO\({}_{2}\) applied on a Ti-support [10]. Traditionally, the ruthenium-titanium based oxides exhibit chlorine overpotentials between 40 and 60 mV, for current densities of 0.20-0.25 kA\(\cdot\)m\({}^{-2}\) with 5 M NaCl solutions and temperatures of around 85 \({}^{\circ}\)C. However, after a prolonged operation, the electrocatalytic activity decreases and the anode overpotential can reach 300-400 mV at the final stage with an exponential evolution, a point, at which the anode is considered 'deactivated' [10]. Previous research works showed that the anode deactivation contributes significantly to increasing the electricity consumption. The deactivation mechanisms of the electrodes are dependent on the operating conditions, where pH of the brine and the electrolysis temperature are the most important parameters [11; 12; 13]. One of the best practices in the chlor-alkali industry is an anolyte acidification that helps to reduce the oxygen content in the cell gas products. The pH of the brine is most commonly lowered by adding HCl for neutralizing the back-migrated OH\({}^{-}\) into the anolyte, which leads to the formation of oxygen at the anode [12; 13]. The main anodic reaction is the chlorine evolution reaction, CER, which requires a thermodynamic potential of 1.36 V at 25 \({}^{\circ}\)C vs. SHE (Standard Hydrogen Electrode) [6]. \[2\text{Cl}^{-}\text{ (aq)}\rightarrow\text{Cl}_{2}\text{ (g)}+2\text{e}^{-} \tag{2}\]  
In addition to the chlorine evolution, the undesired oxygen evolution, mainly by water oxidation:\[2\mathrm{H_{2}O}\ (\mathrm{I})\to 4\mathrm{H^{+}}\ (\mathrm{aq})+\mathrm{O_{2}}\ ( \mathrm{g})+2\mathrm{e^{-}} \tag{3}\]  
in which the electrode potential is 1.23 V vs. SHE at 25 \({}^{\circ}\)C [6] also occurs, especially outside the optimum process operational pH \"window\" of 2 to 4 [12]. This reaction is thermodynamically favourable, but the DSA(r) electrode strongly favours the CER. Industrially, the ratio of the chloride ions in the anolyte inlet to the total amount of electric current used by the anode, so-called anodic current efficiency, is monitored to assess the relevance of parasitic reactions as water oxidation. Typically, DSA(r) anodes have an anodic current efficiency between 95% and 97% [14]. Carbon steel electrodes were first used in diaphragm cells for the hydrogen evolution reaction, HER, at the cathode compartment. In the 1990s, after the successful commercialization of ion-exchange membrane-based cells, cathodes with new compositions (such as nickel or nickel with ruthenium oxide coated, so-called Raney nickel cathodes, Ni-Al) and greater surface areas became available and displaying higher corrosion stability and catalytic activity for the HER. In the chlor-alkali typical operating conditions, nickel is a competitive material, because it combines a good electrocatalytic activity with stability under alkaline conditions and supports higher temperatures (up to ca. 80 \({}^{\circ}\)C) [15]. Raney nickel cathodes exhibit an overpotential up to ca. 120 mV in the high current density region [6]. The cathode promotes the half-reaction of water decomposition to produce hydroxide ions and hydrogen and the associated thermodynamic potential is \(-\)0.83 V at 25 \({}^{\circ}\)C vs. SHE [6]. \[2\mathrm{H_{2}O}\ (\mathrm{I})\ +\ 2\mathrm{e^{-}}\to\mathrm{H_{2}}\ ( \mathrm{g})+2\mathrm{OH^{-}}\ (\mathrm{aq}) \tag{4}\]  
Consequently, sodium hydroxide is formed from the hydroxide and sodium ions.