## 3 Results  
### 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 IrO\({}_{2}\) layer prevents insulation of the titanium substrate by oxidation [25]. The thermal decomposition process deposited \(\sim\)0.0020 g/cm\({}^{2}\) of IrO\({}_{2}\). To ensure the complete coverage of, first, the IrO\({}_{2}\) layer on the Ti and, then, the Mn-Mo catalyst, important for analysis of Faradaic efficiency toward DER, deposition conditions were tailored to achieve complete attenuation by EDS (information depth on the order of \(\sim\)microns). Three sequential electrodeposition cycles of 20 min each at 80 \({}^{\circ}\)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 \({}^{\circ}\)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 \(\sim\)0.0500 g/cm\({}^{2}\) 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. Figure 1: (**a**) SEM micrograph and the corresponding EDS maps for (**b**) Mn and (**c**) Mo for a 0.2 M Mn-0.003 M Mo catalyst with three 20 min electrodeposition cycles at 80 \({}^{\circ}\)C. There was no signal from the underlying Ti or Ir layers. The XRD pattern of both the Mn-Mo-0.006 M Ru and Mn-Mo-0.009 M Ru electrodes showed the majority of the pattern arising from MnO\({}_{2}\) with a strong signal from the underlying IrO\({}_{2}\) layer (Figure 3). The low concentration of Ru and the overlap in the XRD features of its oxides make it challenging to assess potential phase segregation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Spectrum** & **O (at\%)** & **Mn (at\%)** & **Mo (at\%)** & **Ru (at\%)** \\ \hline
1 & 58.23 & 41.49 & 0.28 & 0.00 \\
2 & 58.17 & 41.29 & 0.40 & 0.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of the regions analyzed by EDS in Figure 2e. Figure 2: (**a**) SEM micrograph and corresponding EDS maps for (**b**) Mn, (**c**) Mo, and (**d**) Ru showing a Mn-rich region. (**e**) SEM micrograph at higher resolution on the Mn-Mo-0.006 M Ru electrode showing the heterogeneous bumps on the sample surface. The red boxes denote the regions quantified in Table 1.  
To probe the oxidation state of the Mn-Mo-0.009 M Ru electrode, characterization with XPS was also performed, and the survey scan can be found in Figure S2a.