As the mediators were synthesized in-house, XPS, XRD, and SEM are used to insure the oxidation states and phase purity and reveal the morphology of the mediators. In Figure 2, the Fe 2p XPS spectra for the mediators are similar regardless of the rare-earth element being used. With the Fe 2p3/2binding energy at ~709, Fe is present with an oxidation state of 2+ as one would expect in this type of material. The spectra for the rare-earth elements Y, La, Ce, and Pr (not shown) are consistent with all being present with an oxidation state of 3+. In the case of Ho, we anticipate the oxidization state to be 3+ but it was difficult to confirm due to lack of reference data. The C 1s spectra (not shown) are dominated by a contribution with binding energy near ~285 eV that corresponds to CN and hydrocarbon chemistries and a minor component with binding energy near 289 eV that corresponds to more oxidized carbon species. The XPS spectra of the N1s region are dominated by a major component with binding energy at 398.0 eV that corresponds to the CN chemistry and a minor component with binding energy near ~402 eV which could be attributed to NOx type of species. Additional ex situ and in situ structural characterization after polarization at different states of charge will be used to correlate with performance characteristics in order to explore the critical factors controlling energy, power, and capacity retention.
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