## 3 Results
### OER Efficiency in Saline and Seawater Environments
Figure 5: Comparison of the average OER faradaic efficiencies (3 subsequent measurements) following 30 min at 1.5 V (vs. Ag/AgCl) in 0.5 M NaCl for electrodes doped with (**a**) Co, (**b**) Fe, (**c**) Ru, (**d**) W, and (**e**) Sn. While the OER faradaic efficiency is a crucial factor, the electrodes also need to sustain high catalytic activity at the anode to facilitate efficient HER at the cathode. A summary of the electrode performance both as OER faradaic efficiency and a measure of catalytic activity (total charge passed during the OER runs in seawater) can be seen in Figure 7. This comparison shows the promising performance of the Mn-Mo-Ru system, in addition to that measured in other electrolytes and at lower applied potentials in artificial seawater and phosphate buffer (Figure S4). The Mn-Mo-Sn electrodes had the second highest current; however, these electrodes were less efficient toward OER than the Ru. The Mn-Mo-Ru electrode system was the most promising for suppressing CER and maintaining high catalytic activity. However, the tests performed were relatively short term with a 30 min run length. To realize eventual maturation and deployment in a raw seawater electrolyzer, the stability of the electrodes needs to be assessed and likely improved. Figure S5 shows the measured current during the 30 min runs in raw seawater at 1.5 V (vs. Ag/AgCl) for the Mn-Mo-0.006 M Ru and Mn-Mo-0.009 M Ru electrodes. While the Mn-Mo-0.006 M Ru electrode was initially more catalytically active, the activity decreased over time and, notably, in the second and third runs. In contrast, the initially lower activity of the Mn-Mo-0.009 M Ru was consistent throughout the runs, suggesting that doping level may have a trade-off between catalytical activity and stability. Figure 6: Comparison of the OER faradaic efficiency for replicate electrode samples following 30 min at 1.5 V (vs. Ag/AgCl) in raw filtered seawater for electrodes doped with (**a**) Co, (**b**) Fe, (**c**) W, (**d**) Ru, and (**e**) Sn. Extending testing in a flowing seawater environment was also carried out at PNNL's Marine Sciences Lab. Seawater flowed through a flume at a rate of 75 L/min, with an average temperature of 22.0 \(\pm\) 1.1 \({}^{\circ}\)C, salinity of 31.9 \(\pm\) 0.4 parts per thousand, and a bulk pH of 7.80 \(\pm\) 0.07. The Mn-Mo-0.009 M Ru electrode was placed into the flume along with a Pt counter electrode and a Ag/AgCl reference electrode. A pH probe was placed in close proximity (-5 mm) to the anode and cathode to monitor local pH. Pictures of the test system are shown in Figure 8. While gas or anolyte collection was not feasible in the flow setup to assess OER selectivity at this time, we expect comparable performance intermediate to the tested cases of raw seawater and buffered NaCl (due to the mitigation of pH swings by the flow rate). A comparison was performed on Mn-Mo-Ru electrode stability based on the pH of the electrodeposition solution, through which achieving a more acidic solution is thought to generate a more durable catalyst. Figure 9 compares the chronoamperometric response for a Mn-Mo-0.009 M Ru electrode prepared at either pH 1 or pH 2 in the seawater flume at 1.5 V (vs. Ag/AgCl). The pH 2-deposited electrode had higher catalytic activity than its pH 1 counterpart. The rapid current increase at ~1 day for pH 1 and ~2 days for pH 2 is believed to have occurred due to spalling from the electrode partially exposing the IrO\({}_{2}\) underlayer, expected to decrease the OER faradaic efficiency due to the high CER activity of IrO\({}_{2}\). Coating with a water-permeable polymer, such as Nafion, may improve the mechanical durability of anodes, maintaining or improving OER selectivity albeit at a slight compromise to activity (charge passed, Figure S5). Figure 8: (**a**) The seawater flume electrochemical setup, (**b**) the electrochemical cell region in the flume, and (**c**) the test electrode within the flume showing the pH probe (red) near the surface.