## 3 Results
### OER Efficiency in Saline and Seawater Environments  
The performance of the electrodes was assessed on efficiency toward the OER over the CER, represented as the faradaic efficiency (FE) for OER, and the overall catalytic performance, represented as total charge passed at a given applied potential for 30 min (normalized by electrode geometric area). Electrodes that suppress the CER while facilitating the rate of OER are promising candidates for further development. In a 30 min exposure at 1.4 V vs. Ag/AgCl, an \(\text{IrO}_{\text{x}}\) had a faradaic efficiency of only 18% toward OER (14.7 C/cm\({}^{2}\)), while a Mn-Mo electrode measured 91% faradaic efficiency (31 C/cm\({}^{2}\)). The influence of various dopant levels on the Mn-Mo electrodes on OER faradaic efficiency in neutral chloride (0.5 M NaCl) environments is shown in Figure 5. In Figure 5a, the addition of Co to the Mn-Mo provided efficiencies >95% relatively consistently for the tested compositions, with the exception being 0.009 M Co. Increasing to Mn-Mo-0.1 M Co created a more physically stable electrode. For the inclusion of Fe (Figure 5b), the best performance was measured for electrodes Mn-Mo-0.01 M Fe, Mn-Mo-0.1 M Fe, and Mn-Mo-0.02 M Fe. Below 0.01 M Fe addition, stability and efficiency were compromised. A Mn-Mo-0.003 M W electrode was the best performing of the W set with consistent >94% efficiency for OER (Figure 5d). We observed a drop in pH over time in the unbuffered neutral chloride electrolyte, possibly due to the inability of the Nafion(r) membrane to exchange the protons at the proton generation/consumption rate. The inclusion of these dopants (W, Fe, and Co) also increased the OER faradaic efficiency and/or the charge passed toward OER in circum-neutral phosphate-buffered NaCl (Figure S3). The Ru electrodes with >0.003 M Ru addition all had strong OER efficiency (Figure 5c). The Mn-Mo-0.001 M Ru electrodes had poor physical stability leading to the large deviation between runs. The addition of Sn as a dopant led to a decrease in OER compared with the other dopants, with the Mn-Mo-0.003 M Sn electrode providing \(\sim\)88% efficiency (Figure 5e). Out of the electrodes evaluated, the inclusion of Ru provided the best efficiency toward OER in 0.5 M NaCl (Figure 5) and also showed the highest charge passed toward OER in buffered NaCl (Figure S3). Figure 4: (a) XPS spectra and spectral fits for the (**a**) Ru 3d, (**b**) the Mn 3d, and (**c**) the Mo 3d regions from a Mn-Mo-Ru-0.009 M electrode. The survey scan can be found in the Supplementary Materials, Figure S2. When testing in filtered raw seawater, most of the electrodes saw a slight decrease in OER faradaic efficiency compared with the neutral chloride solution. The differences may originate from the increased ionic concentration in seawater and the application from a higher kinetic overpotential (V vs. RHE) given the increased buffering in seawater and, correspondingly, the higher pH in the anolyte compared to 0.5 M NaCl. A summary of these tests is shown in Figure 6. The inclusion of Co (Figure 6a) generally maintained high OER faradaic efficiency, somewhat greater for the Mn-Mo-0.2 M Co electrode. Surprisingly, for Fe-containing electrodes (Figure 6b), the OER faradaic efficiency in seawater was notably greater than in 0.5 M NaCl for 0.001 M Fe, generally decreasing with Fe content and reaching OER efficiencies notably lower than in 0.5 M NaCl. This switch may be due to buffering occurring in seawater improving the Fe's stability in the electrode. Elucidation of this behavior change at low Fe doping levels will be a focus of future work. The Ru electrodes also performed well, >90% efficiency for the electrodes at 0.003 M Ru and 0.009 M Ru (Figure 6c). For W-containing electrodes, OER faradaic efficiencies were also somewhat higher in seawater, marginally greater for the Mn-Mo-0.003 M W (Figure 6d), which also had similar performance in artificial seawater (Figure S2). The Mn-Mo-0.003 M Sn electrode again had the best performance from the Sn set (Figure 6e).