Synchrotron-based techniques for characterizing STCH water-splitting materials

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 necessitate a multifaceted 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 sensitivity, resolution, and rapid data acquisition times required for the 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 (e.g., oxygen vacancy–induced lattice expansion). 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 for 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. The best practices are discussed for preparing quenched states of reduction and performing synchrotron measurements, which focus on XRD and XAS at soft (e.g., oxygen K-edge, transition metal L-edges, and lanthanide M-edges) and hard (e.g., transition metal K-edges and lanthanide L-edges) X-ray energies. The X-ray absorption spectra of these complex oxides are a convolution of multiple contributions with accurate interpretation being contingent on computational methods. The 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 can elucidate complex structure–property relationships as they pertain to nonstoichiometric 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.

Catalytic enhancement of production of solar thermochemical fuels: opportunities and limitations

“…71 Furthermore, synchrotron techniques are also increasingly applied to determine mechanistic and structural aspects of thermochemical cycles. 72…”

“…Furthermore, in situ XRD, which provides information about phase evolution, can be also of interest for investigating thermochemical processes. 72…”

“…71 Furthermore, synchrotron techniques are also increasingly applied to determine mechanistic and structural aspects of thermochemical cycles. 72 In the following section, we discuss the rationale for using heterogeneous catalysts at high-temperature processes, setting the limits for the feasibility of their use.…”

“…Furthermore, in situ XRD, which provides information about phase evolution, can be also of interest for investigating thermochemical processes. 72 In the case of indirect, two steps transformations to upgrade solar thermochemical fuels improving the design of the solar reactors working in tandem with a catalytic one is essential to manage the flows of heat and mass efficiently. This kind of consecutive reactor has been proposed for coupling sequential catalytic reactions such as for example the production of ethanol from syngas 98 or the recently proposed production of C 3 oxygenates from CO 2 and ethane.…”