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

Abstract

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.

Tuning the Coordination Structure of CuNC Single Atom Catalysts for Simultaneous Electrochemical Reduction of CO2 and NO3– to Urea

“…Subsequently, we explore the various NO 3 RR reaction pathways that result in the formation of NH 3 (Figure 3c and Figure S26, Supporting Information). Several steps have been proposed as the RDS on single atoms sites for NO 3 RR, including the first step of NO 3 adsorption, *NO reduction to *NHO, and *NHO reduction to *N. [22,41] However, we find that the reaction pathways exhibit similar energies on each site, which is reflected in our experimental results, where a comparable FE NH4…”

“…Subsequently, we explore the various NO 3 RR reaction pathways that result in the formation of NH 3 (Figure 3c and Figure S26, Supporting Information). Several steps have been proposed as the RDS on single atoms sites for NO 3 RR, including the first step of NO 3 – adsorption, *NO reduction to *NHO, and *NHO reduction to *N. [ 22,41 ] However, we find that the reaction pathways exhibit similar energies on each site, which is reflected in our experimental results, where a comparable FE NH4 + is achieved with each catalyst across the potential range tested. We note that in one modeled pathway (Figure 3c), a significant difference is seen in the energy for *NH formation on the Cu‐N 4 sites (Figure 3c), of −4.43 eV (leading to an uphill process) compared to −7.17 eV and −6.51 eV on Cu‐N 3 ‐C 1 and Cu‐N 2 ‐C 2 sites, respectively.…”

“…However, as the Cu‐GS catalysts can contain a combination of structurally different sites, this may have the effect of cancelling out the pre‐edge features. [ 41 ] There is a slight energy difference in the XANES of the materials consistent with a shift to lower effective nuclear charge with higher annealing temperature. The Cu K‐edge extended X‐ray absorption fine edge structure (EXAFS) spectra (Figure 1j) are close to featureless and are well accounted for by four Cu–N interactions.…”

Intrinsic Catalytic Activity for the Alkaline Hydrogen Evolution of Layer-Expanded MoS2 Functionalized with Nanoscale Ni and Co Sulfides

“…Moreover, the data collected at the Mo K-edge notably differ from the Ni, Co, and Fe K-edge EXAFS for the respective bimetallic materials (Figure b), neither of which exhibited a pronounced second peak in the Fourier transform data (Figure c). This indicates that the dominant location of secondary metals is not within an ordered crystalline environment, i.e., provides more strong evidence against substitutional doping of MoS 2 with nickel, cobalt, and iron sulfides.…”

“…Moreover, the data collected at the Mo Kedge notably differ from the Ni, Co, and Fe K-edge EXAFS for the respective bimetallic materials (Figure 3b), neither of which exhibited a pronounced second peak in the Fourier transform data (Figure 3c). This indicates that the dominant location of secondary metals is not within an ordered crystalline environment, 112 i.e., provides more strong evidence against substitutional doping of MoS 2 with nickel, cobalt, and iron sulfides. Simulations of the EXAFS at the secondary metal K-edges for the MS x /MoS 2 LE materials demonstrated that the data could be reasonably well described with a first coordination sphere of sulfur but not oxygen (Table S1 and Figure S10).…”