# The Effects of Acidic, Alkaline, and Neutral Anolytes on Electrochemical Seawater Deoxygenation
## 1 Introduction  
Removal of oxygen from seawater used for water injection for enhanced oil recovery (EOR) is crucial in order to avoid corrosion in pipelines. Amongst the available technologies for oxygen removal, the electrochemical method using highly conductive seawater is very suitable. This method avoids the use of additional chemicals and utilizes only seawater as an electrolyte. The electrochemical system with a silver cathode for oxygen removal works efficiently on both the laboratory scale and the industrial scale, as previously reported by [PERSON] et al. [1; 2]. The slow kinetics for oxygen reduction reactions (ORR) at the cathode is one of the major limiting factors for the development of an electrochemical system involving ORR [3; 4]. Platinum or platinum-based materials have higher activities for oxygen reduction reactions (ORR) [5]; however, higher metal price and slow kinetics for ORR are limiting factors for their commercial use on a large scale. There were several attempts to develop a cheap and efficient catalyst for ORR [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]; however, problems still exist with catalyst degradation during the catalytic reduction of oxygen [19]. The mechanism for the oxygen reduction reaction on silver is similar to the reaction on platinum, but slightly less efficient [20; 21; 22]. In addition, silver has good corrosion resistance properties and a low cost which is an advantage. The development of different oxide-based catalysts for oxygen evolution from water found iridium oxide (IrO\({}_{2}\)) to be a dimensionally stable anode (DSA) and a suitable electrode material for oxygen or chlorine evolution reactions [23; 24; 25; 26]. Thus, a silver cathode and an iridium oxide anode were found to be a good catalyst combination for the deoxygenation cell. The desired reactions and side reactions at the anode and cathode in the deoxygenation cell are shown in Equations (1)-(5). At the anode:  
\[2\mathrm{H_{2}O}\to 4\mathrm{H^{+}}+2\mathrm{O_{2}}+4\mathrm{e^{-}}. \tag{1}\]  
At the cathode:  
\[4\mathrm{H^{+}}+\mathrm{O_{2}}+4\mathrm{e^{-}}\to 2\mathrm{H_{2}O}. \tag{2}\]  
Side reaction at anode:  
\[2\mathrm{Cl^{-}}\rightarrow\mathrm{Cl_{2}}+2\mathrm{e^{-}}. \tag{3}\]  
Side reactions at cathode:  
\[2\mathrm{H^{+}}+2\mathrm{e^{-}}\rightarrow\mathrm{H_{2}}; \tag{4}\]  
\[2\mathrm{H^{+}}+\mathrm{O_{2}}+2\mathrm{e^{-}}\rightarrow\mathrm{H_{2}O_{2}}. \tag{5}\]  
In neutral or alkaline catholytes, the reduction of oxygen to hydroxyl ions is a typical reaction. The hydrogen cation passes from the anode to the cathode compartment through the cation exchange membrane (CEM). The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) take place at the anode and cathode, respectively. Both reactions have higher overpotentials making the reactions sluggish. At the anode, water is oxidized forming oxygen gas as shown in Equation (1). In a deoxygenation cell, operated offshore, the use of seawater as both the anolyte and catholytes could make the process simple. However, the generation of chlorine gas (Equation (3)) prior to the generation of oxygen (Equation (1)) gives more aggressive conditions with respect to corrosion in addition to chlorine being toxic. Thus, the use of a chloride-free electrolyte is preferred at the anode side in the deoxygenation cell. The pH and temperature effects on the OER and ORR at the anode and cathode were extensively studied in the literature [27; 28]. To our knowledge, there is no study on the pH effect of the analytes with respect to the efficiency of the deoxygenation process or their effect on electrode materials. Non-chloride anolytes were studied in a deoxygenation cell in order to understand and utilize suitable anolytes for oxygen removal from seawater. The results from this study provide additional information that is important in order to establish a suitable industrial method for the deoxygenation of injected seawater.