(Invited) In Situ/Operando X-Ray Characterization of Electronic Structure in Energy Materials Systems

Abstract

Characterizing the evolution in electronic structure of energy materials systems, e.g., (photo)electrochemical, electrocatalytic, etc., during operation is essential for fully understanding their behavior; the structure-property relationships derived from operando studies of functioning energy systems is critical for informing the tailored design of materials with enhanced performance as part of a feedback loop. Synchrotron-based soft x-ray spectroscopies, including both x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES), are ideally suited to the in situ/operando study of energy material structure. As element specific probes of the unoccupied and occupied electronic density of states, XAS and XES can yield insight into the bonding and composition/structure of materials across multiple length scales; moreover, these techniques are equally applicable to ordered and amorphous systems, and have the sensitivity to address structural changes in low concentration additives/dopants and bulk materials alike. Significantly, the advantages of XAS/XES are only fully realized via the close coupling of in situ experimental methods and advanced ab initio modeling, especially for highly complex, multi-component systems, to identify and interpret key spectroscopic signatures. We demonstrate the benefits of a combined experiment/simulation approach via the investigation of nanostructured carbon aerogel (CA) electrodes operating in prototypical electric double-layer capacitors. Simulations are leveraged to interpret spectral changes consistent with complex, reversible, and highly influential structural transformations: profound bias- and time-dependent evolution of the electronic structure under applied bias connotes both mesoscale (distortion of the porous networks) and nanoscale (specific adsorption) phenomena central to device performance. Although XAS/XES have the potential for impact across multiple EMN consortia, further discussion will focus on ongoing work centered around our combined experiment/simulation node within HydroGEN. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.