Figure 1.) Ru K-edge and Ft-EXAFS spectrum for Ru foil, RuO  ₂  and Nd  _0.1  RuO  _x  . Wavelet transform of Ru K-edge EXAFS

X-ray absorption near-edge spectroscopy (XANES) was used to further analyze the valence state of Ru in order to ascertain the impact of the Nd doping. The Ru-O bonds in RuO 2 and Nd 0.1 RuO x cause the  Ru  K- edges  of both materials to move to higher energies than the Ru foil in Nd  _0.1  RuO  _x  . Additionally, Nd  _0.1  RuO  _x  has a greater Ru K-edge energy than RuO  ₂  , indicating that it contains more Ru  ⁴⁺  than RuO  ₂  , which is consistent with the XPS data. This Ru  ⁴⁺ The valence state is advantageous to chemical stability because the low oxidation state Ru will change into the high valence Ru species during the ferocious oxygen evolution reaction and then be dissolved during the OER process, causing the structure to collapse and performance to decline. When compared to Nd powder, the Nd K-edge XANES of Nd  _0.1  RuO  _x  change to a higher energy location, indicating that the Nd in this material is in a substantially higher valence state. Additionally, the coordination structure of Nd  _0.1  RuO  _x  was investigated using the extended X-ray absorption fine structure (EXAFS). Due of the lattice expansion brought on by the difference in ionic radii between Ru and Nd, Nd 0.1 RuO x displays  _a  significantly  _longer Ru- Ru distance (2.87 Å) compared to the Ru-Ru bonds in RuO  ₂  (2.81 Å) and Ru foil (2.67 Å). Additionally, Nd  _0.1  RuO  _x  has a lower relative intensity of Ru-Ru bonds than RuO  ₂  , which would limit structural change during the catalytic process. The long-term stability test showed that the addition of Nd can limit the over-oxidation of the catalyst in its prepared condition, which is advantageous to stability because Ru's valence state barely altered before and after the test . Similar to the outcome of RuO  ₂  , but plainly different from that of Ru foil, the wavelet transform study shows that the intensity of Ru-O coordination in Nd  _0.1  RuO  _x achieves the maximum at ≈2.8  ∼  6.1 Å  ⁻¹  .

Acknowledgments

The authors are thankful to the 21 A X-ray nanodiffraction beamline of Taiwan Photon Source (TPS) for the XAFS tests.  The authors thank the X-ray analysis service via SGService.