The electrocatalytic splitting of water to produce hydrogen can be used to store renewable electricity as either a chemical fuel or as a feedstock. However, this process currently requires an ultra-pure water feed [1], which would thus strain drinking water supplies at hydrogen production levels required for a fleet of fuel cell vehicles [2]. Seawater presents an abundant water resource, but it is yet to be utilized for hydrogen production due to the complexity it presents as an electrolyte and energy-intensive purification processes. If the associated challenges were overcome, seawater electrolysis could generate H₂ fuel from green electricity at coastal sites, on board marine vessels or platforms, or submersed at marine hydrokinetic platforms [3].

The kinetics of the oxygen evolution reaction (OER) are generally slower relative to chlorine evolution reaction (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 (HOCl, pH 3–7), and at pH > 7, hypochlorite is formed (OCl⁻) [4]. Most research on maximizing oxygen evolution activity to date has focused on highly acidic ^[5][6][7][8][5,6,7,8] or alkaline ^{[9][10][11][12]}[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., IrO₂) are highly active for OER in acidic media, such materials are similarly highly active for CER in chloride-containing media ^[14][15][14,15]. In the neutral conditions of seawater ^[16][17][18][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][20,21,22], Ni ^[16][23][16,23], and Co ^[19][20][24][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 are focused, which are stable and have demonstrated good Faradaic efficiency in a wide range of electrolyte pH. MnOx electrocatalysts (often supported on an IrO₂/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 MnO₄⁻ [21]. However, these electrodes suffered from mechanical instability during cycling at appreciable current densities. The incorporation of Mo⁶⁺ improved durability and Faradaic efficiency [26]. The further incorporation of W⁶⁺ 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.

2. Electrode Fabrication and Structure

The electrodeposition process produced electrodes with a cracked mud structure, with the Mn-Mo-X catalyst deposited on top of a sub-layer of thermal decomposition-produced IrO2. The IrO₂ layer prevents insulation of the titanium substrate by oxidation [25]. The thermal decomposition process deposited ~0.0020 g/cm² of IrO₂. To ensure the complete coverage of, first, the IrO₂ layer on the Ti and, then, the Mn-Mo catalyst, important for analysis of Faradaic efficiency toward OER, deposition conditions were tailored to achieve complete attenuation by EDS (information depth on the order of ~microns). Three sequential electrodeposition cycles of 20 min each at 80 °C were used to generate a homogenous catalyst layer. Figure 1 shows the resulting catalyst layer yielding complete coverage of the electrode surface by EDS, similar to prior work [22]. Electrodeposition at room temperature or with a continuous deposition at 80 °C generated incomplete and heterogeneous catalyst layers on the electrode surface. Beyond three cycles, flaking from over-deposition was observed. The deposition cycles resulted in ~0.0500 g/cm² of catalyst deposited. The inclusion of dopants at various levels in the Mn-Mo-X catalyst did not distort the overall cracked mud structure (see SEM images in Figure S1). Some heterogeneous distribution of the catalyst elements was observed as small “bumps” on the catalyst surface. EDS mapping highlighted the variations in element distribution through the catalysts. As an example,  Figure 2a shows the SEM image of a Mn-Mo-0.006 M Ru electrode. While the composition appeared generally uniform across the film, there is also a Mn-rich region (Figure 2b) that is depleted in Mo (Figure 2c) and Ru (Figure 2d). The “cracks” in the microstructure gave a stronger Mn signal, suggesting the Mn distribution may vary with electrode thickness. While the distribution of the Ru in Figure 2d appeared homogenous, magnification of the catalyst surface in Figure 2e showed “bumps” on the catalyst surface. EDS spot analysis showed that the flat region contained only Mn and Mo, whereas the bumps also contained Ru (Table 1). As a result of this heterogenous composition profile, the quantification of the dopant level and Mn/Mo/X ratios was limited with EDS. Thus, electrodes are referred to by the starting electrodeposition solution concentration, where increased solution concentration results in increased electrode dopant concentration.