In Situ Soft XAS Study of Electrocatalysts with an Ultrahigh Vacuum-Compatible Liquid Flow Cell

Synchrotron X-ray absorption spectroscopy offers a unique tool for investigating the element-specific electronic structure of materials in chemistry and quantum physics fields. In particular, in situ/operando X-ray absorption spectroscopy (XAS) has yielded valuable atomic-level insight into electrocatalysts in operation. While most studies have employed "hard" X-rays that are higher than ~5 keV in energy, "soft" X-rays with energies around or below 1-2 keV have the unique capability to probe core-level excitations of light elements such as oxygen or p→d excitations of 1st-row transition metals. The low-energy soft X-rays easily become absorbed and scattered by ambient air, however, requiring ultrahigh vacuum (UHV) environments. To acquire soft XAS spectra of electrocatalysts in action within a UHV chamber, we employ home-fabricated SiN x membranes and a sealed electrochemical liquid flow cell. We demonstrate the use of our technique with canonical water oxidation catalysts RuO 2 and MnO x .