Electrochemical characterization on 182 samples from the 4 composition libraries included 8 Mn-Sb-O, 62 Mn-Sn-Sb-O, 48 Mn-Ti-Sb-O, and 64 Mn-Sn-Ti-Sb-O compositions. For each composition, electrochemical characterization under continuous flow of 1 M H2SO4 electrolyte commenced with a cyclic voltammogram from 1.23 V to 1.79 V vs RHE, and for the present purposes of exploring catalytic activity as a function of cation composition, the geometric current density (using a Fig. : Summary of the 203 XRD measurements from the 4 composition libraries. Each of the 3 columns of composition plots is labelled with the composition system(s) and contains a top and bottom panel corresponding to the 2 rutile lattice parameters. For the Mn-Sb-Sn-Ti-O system, the Mn concentration is indicated by the smaller, right-most composition plot. Each composition within the 6 lattice parameter plots is plotted as a filled or hollow circle for phase-pure and mixed-phase rutile, respectively, with marker colour indicating the lattice parameter value inferred from the XRD pattern. Only the rutile phase is observed for all compositions with Mn concentration below 0.6. Several Mn-rich samples contain some or no rutile phase. Among the phase-pure rutile samples, both lattice parameters vary systematically with cation composition, indicating a broad range of site substitutions within the rutile structure across each composition space. An alternative composition plot of this data is presented in Fig. . representative electrolyte contact area of 0.02 cm 2 ) from the initial cyclic voltammogram experiment at 1.79 V vs RHE is used as the catalytic activity for each composition. This data as well as XRF, XRD, and other electrochemical data are available in an open-source repository. The OER activity measurements are summarized in Fig. where each composition is shown using its concentration of Mn (with respect to all elements except oxygen). The 8 data points from the Mn-Sb-O library illustrate the aforementioned trend that increased concentration of catalytic elements such as Mn results in increased activity, which must be considered in context of the previous demonstration of decreased durability with increasing Mn concentration. 9 Fig. also illustrates that many compositions from each higher-order composition space exhibit higher activity than the Mn-Sb-O baseline, confirming that integration with multiple stabilizing elements enables higher activity at lower Mn concentration, per hypothesis (2).