[PERSON]  
2 Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA; [EMAIL_ADDRESS] (N.D.); [EMAIL_ADDRESS] (D.M.S. ); [EMAIL_ADDRESS] (L.-J.K.); [EMAIL_ADDRESS] (G.A.G. ); [PERSON].A.S. ); [EMAIL_ADDRESS] (J.A.S. ); [EMAIL_ADDRESS] (N.M.A. ); [EMAIL_ADDRESS] (J.V.C. ); [PERSON]] (M.H.E.) [PERSON]  
2 Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA; [EMAIL_ADDRESS] (N.D.); [EMAIL_ADDRESS] (D.M.S. ); [EMAIL_ADDRESS] (L.-J.K.); [EMAIL_ADDRESS] (G.A.G. ); [EMAIL_ADDRESS] (J.A.S. ); [EMAIL_ADDRESS] (N.M.A. ); [EMAIL_ADDRESS] (J.V.C. ); [PERSON]] (M.H.E.) [PERSON]  
2 Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA; nathan.doi@sjsu. A primary limiter of energy efficiency is the sluggishness of the reaction that counters the hydrogen evolution reaction (HER): the oxygen evolution reaction (OER), which in seawater also competes with the hazardous chlorine evolution reaction (CER):  
OER, alkaline: 4[OH]\({}^{-}\)\(\rightarrow\)2 H\({}_{2}\)O + O\({}_{2}\) + 4e\({}^{-}\)  
OER, acidic: 2 H\({}_{2}\)O\(\rightarrow\)4H\({}^{+}\) + O\({}_{2}\) + 4e\({}^{-}\)  
E_OER\({}^{0}\) = +1.23 V - 0.059 pH  
CER, acidic: [2 CI]\({}^{-}\)\(\rightarrow\)[Cl]\({}_{2}\) + [2e]\({}^{-}\)  
E_CER\({}^{0}\) = +1.358 V SHE, pH 0  
The kinetics of the OER are generally slower relative to CER due to the requirement for the transfer of four electrons and protons per oxygen molecule, while the CER is a two-electron process. At pH > 3, chloride can be oxidized to form hypochlorous acid (HOCI, pH 3-7), and at pH > 7, hypochlorite is formed (OCI\({}^{-}\)) [4]. Most research on maximizing oxygen evolution activity to date has focused on highly acidic [5; 6; 7; 8] or alkaline [9; 10; 11; 12] media, and information regarding the selectivity toward chlorine evolution often stems from the chlor-alkali industry in acidic environments where CER is preferred [13]. Although precious metal oxides (e.g., IrO\({}_{2}\)) are highly active for OER in acidic media, such materials are similarly highly active for CER in chloride-containing media [14; 15]. In the neutral conditions of seawater [16; 17; 18], electrolysis studies are scarce and, instead, often incorporate additional buffering [19]. Historically, research in such media has focused on metal oxides primarily composed of Mn [20; 21; 22], Ni [16; 23], and Co [19; 20; 24], often with additional elements, such as Fe or W. Considering abundance and a promising price point [24], we focus here on Mn-based oxides, which are stable and have demonstrated good Faradaic efficiency in a wide range of electrolyte pH. MnOx electrocatalysts (often supported on an IrO\({}_{2}\)/Ti substrate [25]) have demonstrated good selectivity (>90%) toward OER in chlorine-containing electrolyte, with the remainder of the current going toward CER and oxidative dissolution in the form of MnO\({}_{4}\)\({}^{-}\)[21]. However, these electrodes suffered from mechanical instability during cycling at appreciable current densities. The incorporation of Mo\({}^{6+}\) improved durability and Faradaic efficiency [26]. The further incorporation of W\({}^{6+}\) maintained this performance while improving OER activity (lowering overpotential), which has been attributed in part to its enhancement of oxide conductivity [23]. However, numerous other potential metals may serve a similar role, perhaps with better efficacy and stability. In the present work, we consider the OER activity of a Mn-Mo-based oxide electrocatalyst in chlorine-containing neutral electrolytes and raw seawater, comparing the performance upon the gradual incorporation of W, Fe, Co, Sn, and Ru as dopants. It is well documented that the doping of transition and noble metals into electrocatalysts [27] and photocatalysts [28; 29] can enhance their water splitting capacity, and a similar approach was pursued within.