## 2 Materials and Methods  
The OER catalyst electrodes were fabricated via electrodeposition. The plate electrodes were fabricated using Ti sheet, 2 mm thickness, spot welded to a 304 stainless steel rod. The plates were first etched in concentrated (18.4 mol/L) H\({}_{2}\)SO\({}_{4}\) at 80-85 \({}^{\circ}\)C for 30 min [30]. The plates were then rinsed in deionized water (DDI, 18.2 M\(\Omega\)-cm) and dried in a hot air stream. Following drying, an IrO\({}_{2}\) base layer was placed onto the plate via thermal decomposition. A solution of iridium (III) chloride (Aldrich) in butanol was manually deposited onto the etched surface and placed into a 450 \({}^{\circ}\)C oven for 10 min and weighed. This process was repeated three additional times until \(\sim\)2 mg of IrO\({}_{2}\) was deposited. The electrodeposition stock solutions were prepared in DDI, 18.2 M\(\Omega\)-cm, and then acidified to pH 1-3 (dependent on test) with concentrated H\({}_{2}\)SO\({}_{4}\). The metal salts were then added in sequence of the dopant, the sodium molybdate dihydrate (Sigma-Aldrich, St. Louis, MO, USA) and then the manganese sulfate monohydrate (Sigma-Aldrich, St. Louis, MO, USA) in the target molar ratios. The dopant salts used were iron (II) sulfate heptahydrate (Sigma-Aldrich, St. Louis, MO, USA), cobalt (II) sulfate heptahydrate (Sigma-Aldrich, St. Louis, MO, USA), ruthenium (III) chloride (Sigma-Aldrich, St. Louis, MO, USA), in (IV) chloride pentahydrate (Sigma-Aldrich, St. Louis, MO, USA), and sodium tungstate dihydrate (Sigma-Aldrich, St. Louis, MO, USA). A 60 mL aliquot of the stock solution was then placed in a glass beaker and heated to 80 \({}^{\circ}\)C. The IrO\({}_{2}\)-coated electrode and a Pt counter electrode were placed into the beaker, and a DC power source attached with 0.06 A at 5.0 V passed for 20 min. Following the deposition, the electrode was removed, rinsed with DDI, dried, weighed, and returned to a fresh stock solution. The deposition was then repeated twice with weight measured in between. The electrodes were named based on their starting concentration of the deposition solution. A ratio of 0.2 M M-0.003 M Mo was used for all catalysts based on the literature and preliminary testing. The dopant concentrations (for W, Ru, Fe, Sn, and Co) were used to define the sample name of the electrodes, while the Mn-Mo values were not included, e.g., Mn-Mo-0.XXX M Y where XXX is the dopant concentration and Y is the dopant element. The composition of the electrodes was measured using quantitative energy-dispersive X-ray spectroscopy (EDS) using the average mapped signal from a 250 \(\times\) 250 \(\upmu\)m region on the sample surface. The measurements were performed on a JSM-7001F microscope (JEOL USA, Inc., Peabody, MA, USA) with an XFlash 6 1 60 EDS Si-drift detector (SDD; Bruker, Kennewick, WA, USA). Electrochemical measurements, both linear sweep voltammetry, chronoamperometry and chronopotentiometry, were made using a Solartron(r) 1287 (Ametek, Berwyn, PA, USA), Biologic VSP-300 potentiostat (BioLogic, Sayisset-Pariset, France), or a CH Instruments 600E (CH Instruments, Austin, TX, USA) using a Ag/AgCl reference electrode (0.230 V vs. the standard hydrogen electrode, SHE) and a Pt wire counter electrode. Laboratory measurements were made using an H-cell configuration with a Nafion(r) 117 membrane separating the compartments, unless otherwise noted. Solution samples were collected using a pipette during and at the end of the experimental run. Between tests, the cell was emptied, acid washed, and rinsed thoroughly in 18.2 M\(\Omega\)-cm water, and a new solution was added before the test electrode was returned to the solution. Electrolytes considered were DDI containing NaCl (0.5 mol/L), MgCl\({}_{2}\) (0.5 mol/L), 0.1 M phosphate-buffered 0.5 M NaCl (all from Sigma-Aldrich, St. Louis, MO, United States), artificial seawater (ASW) [31], or raw seawater (taken from Sequim Bay in Washington State, filtered at 0.45 \(\upmu\)m, and stored in a fridge at 5 \({}^{\circ}\)C).