Characterization of Energy Materials with X-ray Absorption Spectroscopy-Advantages, Challenges, and Opportunities

X-ray absorption spectroscopy (XAS) plays a critical role in the characterization of energy materials, including thin-film electrocatalysts and battery materials. XAS is well-suited for this purpose because it is element-specific and can target distinct chemical environments within a material, even in a mixed or complicated matrix. Even so, some key energy materials are far from "ideal"XAS samples. This means that both sample preparation and experimental conditions need to be considered when collecting and interpreting data to ensure that conclusions are correct. This review outlines some of the key questions that an XAS experiment is well-suited to answering, including speciation of amorphous materials, understanding how multi-metal systems interact, and the different ways that we may observe single atoms. In addition, we show how XAS can be highly complementary to other analytical techniques in developing a full picture of a material over different scale bars. Importantly, we also examine instances where the sample matrix can distort XAS data, show an example where bond-length disorder can be confused with a change in the coordination number, and discuss some of the advantages and challenges of in situ electrocatalysis. Finally, we examine the future role that XAS will play in innovations in energy materials.