## 4 Discussion  
The results of this study show the effectiveness of doping transition metals into Mn-Mo-X catalysts to suppress the CER and improve overall catalytic activity. The best-performing dopant was Ru in terms of efficiency for OER over CER and catalytic activity. However, despite previous reports of this doping approach being successful for CER suppression, little mechanistic discussion is present in the literature. With the similar response of multiple dopants (Fe, Sn, W, and Co) and the enhanced activity of a known catalytically active species, Ru, dopants may play a role in manipulating the coating density and stability in addition to serving as active catalytic sites. Evidence on both mechanistic roles exists. Ru and Ir are industrial benchmark catalysts for efficient oxygen and chlorine evolution (in the chlor-alkali process), sitting high on the volcano curve for both OER [33] and CER [34]. In the case of IrOx, the OER is not suppressed or influenced by the CER occurring on the same surface, showing scaling between the two reactions on active catalysts [35]. Targeting selective OER over CER can present a challenge but may  
Figure 11: (**a**) SEM micrograph of the Mn-Mo-0.006 M Ru catalyst following chronoamperometric study in a raw seawater flume and the corresponding EDS maps for (**b**) Mn, (**c**) Mo, (**d**) Ru, (**e**) Ir, and (**f**) Mg.  
Figure 10: Chronopotentiometric behavior of Mn-Mo-based electrodes compared with IrO\({}_{2}\) in a flowing seawater flume at 10 A/cm\({}^{2}\) applied current. be realized through a combination of processes. The base catalyst in this study, Mn, is well documented to have a lower overpotential for oxygen evolution than for chlorine evolution [36], while the Mo serves to stabilize the Mn and partially enhance catalytic activity [6]. The work on alkali and alkali-earth intercalated MnO\({}_{2}\) has shown that inducing structural disorder and the presence of oxygen vacancies can create a preference for the OER over CER [37]. The presence of transition metal oxides in the electrodeposited MnO\({}_{2}\) will induce disorder and potentially create bridging mismatches leading to oxygen vacancies. The role of defects within transition and noble metal catalysts to enhance catalytic water splitting ability has also been highlighted recently, especially on Co electrocatalysts [7]. The adsorbed intermediates generated for the CER can also influence selectivity. Surfaces that form a M-Cl intermediate can efficiently sustain the CER, while the generation of the M-OCI intermediate will suppress the CER [38]. The OCI intermediate route further suppresses the CER on RuO\({}_{2}\) with increasing pH due to the change in Gibbs free energy loss. Studies of preferential OER in halide-containing environments on NiB and CoP catalysts suggest that the edge sites on the metalate clusters are crucial to sustaining the OER, as H\({}_{2}\)O will out compete halides for these sites, increasing the likelihood of OER over CER. With the small locations and disordered distribution of the Ru dopants in this study, a higher number of edge sites may dictate an added preference to OER over CER via oxygen-containing intermediates [39]. Ab initio assessment of reaction intermediate binding energies on doped Mn-Mo oxide surfaces and high-index facets would help assess whether Ru plays such roles in the present system. While the partial dissolution and oxidation of RuO\({}_{2}\) cannot currently be ruled out as a contribution to the measured activity, the presence of Ru on the catalyst after extended seawater testing suggests this contribution would be minimal. Many other candidate dopants (and combinations thereof) are available to be tested in a similar approach, and this work shows that dopants that are catalytically active toward CER can also be considered for preferential OER on Mn-Mo-X catalysts.