AUTHOR=Shulda Sarah , Bell Robert T. , Strange Nicholas A. , Metzroth Lucy , Heinselman Karen N. , Sainio Sami , Roychoudhury Subhayan , Prendergast David , McDaniel Anthony H. , Ginley David S. 

TITLE=Synchrotron-based techniques for characterizing STCH water-splitting materials

JOURNAL=Frontiers in Energy Research

VOLUME=Volume 10 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2022.931364

DOI=10.3389/fenrg.2022.931364

ISSN=2296-598X

ABSTRACT=Understanding the role of oxygen vacancy-induced atomic and electronic structural changes to complex metal oxides during water splitting processes is paramount to advancing the field of solar thermochemical hydrogen production (STCH). The formulation and confirmation of a mechanism for these types of chemical reactions necessitates a multi-faceted experimental approach, featuring advanced structural characterization methods. Synchrotron X-ray techniques are essential to the rapidly advancing field of STCH in part due to properties such as high brilliance, high coherence, and variable energy that provide the sensitivity and resolution required for characterization of complex metal oxides during water splitting cycles. X-ray diffraction (XRD) is commonly used for determining the structures and phase purity of new materials synthesized by solid-state techniques and monitoring the structural integrity of oxides during water splitting processes. X-ray absorption spectroscopy (XAS) is an element specific technique and is sensitive to local atomic and electronic changes encountered around metal coordination centers during redox. While in operando measurements are desirable, the experimental conditions required of such measurements (high temperatures, controlled oxygen partial pressures, and H2O), practically necessitate in- situ measurements that do not meet all operating conditions or ex-situ measurements. Here, we highlight the application of synchrotron X-ray scattering and spectroscopic techniques using both in-situ and ex-situ measurements, emphasizing the advantages and limitations of each method as they relate to water-splitting processes. Best practices are discussed for preparing quenched states of reduction and performing synchrotron measurements, which focus on XRD and XAS at soft and hard X-ray energies. The XAS spectra of these complex oxides are a convolution of multiple contributions with accurate interpretation being contingent on computational methods. State of the art methods are discussed that enable peak positions and intensities to be related to material electronic and structural properties. Through careful experimental design, these studies are able to elucidate complex structure-property relationships as they pertain to non-stoichiometric water splitting. A survey of modern approaches for the evaluation of water splitting materials at synchrotron sources under various experimental conditions is provided and available software for data analysis is discussed.