X-ray absorption spectroscopy (XAS) plays a critical role in the characterization of energy materials, including thinfilm 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.
In electrocatalysis we seldom think about the competing direct reduction reactions that may happen alongside catalytically mediated reduction-with direct redox chemistry often happening slower but in competition with, catalysis. One class of compounds of interest from this perspective are iron sulfides. In addition to being structurally similar to many metalloproteins, iron sulfides are also among nature's strongest chemical reductants and reported to act as catalysts for key chemical reactions including proton, nitrite, and nitrate reduction. It is important that iron sulfides can act as catalysts because they are also strong enough reductants to mediate some of the same reactions directly. This is paradoxical because in order to be a catalyst for reduction, an iron sulfide cannot also be oxidised. To investigate this phenomenon further, we assembled a test set of iron sulfides spanning both amorphous iron sulfide (FeS am ) as well as the crystalline iron sulfides greigite, pyrite, and troilite. These were used to explore the relationship between direct reduction and catalysis of a reduction reaction with a secondary electron source, NO 2 À was chosen as a test substrate. The trends in direct reduction followed the least stable material (FeS am ) to the most stable material (FeS 2 ). Of the phases studied, troilite (FeS) showed the largest difference between direct and catalytic reduction, however amorphous iron sulfide showed the greatest selectivity for NH 3 /NH 4 + production as both a direct reductant and a catalyst.
Iron sulfides are key materials in metalloprotein catalysis. One interesting aspect of iron sulfides in biology is the incorporation of secondary metals, for example, Mo, in nitrogenase. These secondary metals may provide vital clues as to how these enzymes first emerged in nature. In this work, we examined the materials resulting from the coprecipitation of molybdenum with iron sulfides using X-ray absorption spectroscopy (XAS). The materials were tested as catalysts, and direct reductants using nitrite (NO 2 − ) and protons (H + ) as test substrates. It was found that Mo will coprecipitate with iron as sulfides, however, in distinct ways depending on the stoichiometric ratios of Mo, Fe, and HS − . It was observed that the selectivity of reduction products depends on the amount of molybdenum, with the presence of approximately at 10% Mo optimizing ammonium/ammonia (NH 4 + /NH 3 ) production from NO 2 − and minimizing competitive hydrogen (H 2 ) formation from protons (H + ) with a secondary reductant.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.