Frequency-Resolved X-Ray Absorption Spectroscopy: A Synchrotron-Based Technique for Extracting Kinetic and Transport Parameters

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.

Thermoresistive catalytic combustion sensors based on noble metals are very stable stable and highly sensitive devices to monitor potentially explosive atmospheres. We studied and proved the high stability of rhodium oxide-based sensors under working conditions in different CH4/air mixtures (up to 3.5 vol % methane) with the help of operando X-ray-based characterization techniques, DC resistance measurements, and IR thermography using a specially designed in situ cell. Operando X-ray diffraction and X-ray absorption spectroscopy showed that the active Rh species are in the oxidized state and their chemical state is preserved during operation under realistic conditions. The resistance correlated with the surface temperature of the pellistor and is related to the combustion of CH4, confirming the catalytic nature of the observed sensing process. Only under harsh operation conditions such as an oxygen-free atmosphere or enhanced working current, a reduction in the active Rh2O3 phase was observed. Finally, the effect of poisoning causing the lowered activity on the catalytic combustion of methane was investigated. While stable rhodium sulfate might form in a sulfur-poisoned pellistor, silicon dioxide seems to additionally physically block the pores in the alumina ceramics of the pellistor poisoned by hexamethyldisiloxane.

Magnesium rechargeable battery, which uses magnesium metal anode, is one of the candidates for the next generation batteries. The magnesium metal anode has a high theoretical volumetric capacity (3832 mAh/cm3), and relative low reduction potential (-2.356 V vs. standard hydrogen potential). However, conventional electrolytes consisting of inorganic salts and ester solvents, as widely used in lithium ion batteries, do not occur reversible deposition/dissolution of magnesium metal. This is because a passivation layer is formed at the magnesium metal anode surface that blocks both magnesium ions and electron transport. The limitation of available electrolytes prevents the magnesium battery from being used in a practical application.Recently, it has been reported that reversible magnesium deposition/dissolution reactions can occur in inorganic magnesium salts dissolved in glyme-based solvents electrolytes 2, 3. However, the reaction mechanism of magnesium metal anode has not been clearly understood. To clarify the reaction mechanism, it is necessary to investigate the interfacial structure of magnesium ion at anode/electrolyte interface. In this study, we investigated the bulk structure of the glyme-based electrolyte using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and the theoretical calculation. Furthermore, we observed the electronic and local structure of the electrolyte at the interface during magnesium deposition using operando soft x-ray absorption spectroscopy.0.5 M Mg(TFSA)2/triglyme and 0.5 M Mg(TFSA)2/2-MeTHF electrolytes were prepared by stirring Mg(TFSA)2 with triglyme or 2-MeTHF in an Ar-filled glove box. Raman spectroscopic measurements were carried out using LabRAM HR-800 equipped with He-Ne laser (633 nm) at room temperature. operando XAS measurements at Mg K-edge were conducted at the beam line BL27SU at SPring-8 (Japan). XAS spectra were obtained at different potential during magnesium deposition. Magnesium deposition/dissolution occurred in Mg(TFSA)2/triglyme as observed in previous report3. In contrast to Mg(TFSA)2/triglyme, magnesium metal deposition did not observe in Mg(TFSA)2/2-MeTHF. Raman spectroscopy showed that TFSA anion exists as uncoordinated state with magnesium ion in the Mg(TFSA)2/triglyme, while TFSA anion strongly coordinates with magnesium ion in the 2-MeTHF-based electrolyte. operando Mg K-edge XAS observed that the local structure attributed to first coordination shell changed more smaller with decreasing potential in the Mg(TFSA)2/triglyme than in the Mg(TFSA)2/2-MeTHF at anode/electrolyte interface. In the Mg(TFSA)2/triglyme, the solvation structure of magnesium ion without coordination of TFSA anion, may suppress its local structure change during approaching the anode/electrolyte interface, resulting in the reversible magnesium deposition/dissolution. ACKNOWLEDGMENTS This study was partially supported by Synchrotron radiation experiment was performed at BL27SU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). REFERENCES [1] Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich M.; Levi, E., Nature, 2000, 407, 724-727.[2] Ha, S.-Y.; Lee, Y.-W.; Woo, S. W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K. T.; Choi, N.-S., ACS Appl. Mater. Interfaces, 2014, 6, 4063-4073.[3] Orikasa, Y.; Masese, T.; Koyama, Y.; Mori, T.; Hattori, M.; Yamamoto, K.; Okado, T.; Huang, Z. -D.; Minato, T.; Tassel, C.; Kim, J.; Kobayashi, Y.; Abe, T.; Kageyama, H.; Uchimoto, Y., Sci. Rep., 2014, 4, 5622.

Single-atom catalysts (SACs) represent a promising class of low-cost materials with high catalytic activity and 100% atom-utilization efficiency, widely applied in energy conversion fields.[1-2] Benefit from tunable coordination environment, the uniformly dispersed SACs on diverse supports exhibit satisfied catalytic ability. More importantly, different from the multiple crystal facets of nanomaterials can serve as active sites. There is only one active site consisting of the single metal atom and coordinated atoms in SACs, thereby improving the selectivity of products some catalytic reactions. Moreover, atomically dispersed metal catalysts exist in cations, which exhibit better stability than traditional nanoparticulated counterparts.[3] However, SACs are susceptible to undergo restructuring during the reactions because of high surface free energy. Therefore, exploring the active sites of SACs through in-situ characterization techniques plays a critical role in studying the reaction mechanism and guiding the design of optimum SACs.As a synchrotron-based element-specific technique, X-ray absorption spectroscopy (XAS) is influential in studying the probed element's structural information. X-ray absorption near edge structure (XANES) spectra reflect the electronic structure, while extended X-ray absorption fine structure (EXAFS) spectra provide local atomic structural information such as coordination number, bond length, and structural disorder. Thus, in-situ XAS is promising and widely used for monitoring electronic structure and atomic configuration changes of SACs during real-time working conditions. We have prepared Ni, Cu single atoms on nitrogen-doped carbon (NiCu@NC-900), which exhibit improved catalytic ability compared with Ni SACs and Cu SACs. Nuclear magnetic resonance results show only ethanol as a liquid product during CO2 reduction reaction (CO2RR). To explore the origin of improved catalytic performance of SACs toward CO2RR, we are going to monitor changes of SACs under open-circuit voltage (OCV) and working conditions by Operando XAS.[1] Li, Z., Ji, S., Liu, Y., Cao, X., Tian, S., Chen, Y., Niu, Z., Li, Y. Chem. Rev. 120, 2 (2020).[2] Ji, S., Chen, Y., Wang, X., Zhang, Z., Wang, D., Li, Y. Chem. Rev. 120, 21 (2020).[3] Speck, F. D., Paul, M. T. Y., Ruiz-Zepeda, F., Gatalo, M., Kim, H., Kwon, H. C., Mayrhofer, K. J. J., Choi, M., Choi, C. H., Hodnik, N., Cherevko, S. J. Am. Chem. Soc. 142, 36 (2020).

Conversion electrodes, such as magnetite (Fe3O4), offer high theoretical capacities (>900 mAh/g) because of multiple electron transfer per metal center. Capacity retention for conversion electrodes has been a challenge in part because of the formation of an insulating surface electrolyte interphase (SEI). This study provides the first detailed analysis of the lithiation of Fe3O4 using isothermal microcalorimetry (IMC). The measured heat flow was compared with heat contributions predicted from heats of formation for the Faradaic reaction, cell polarization, and entropic contributions. The total measured energy output of the cell (7260 J/g Fe3O4) exceeded the heat of reaction predicted for full lithiation of Fe3O4 (5508 J/g). During initial lithiation (3.0–0.86 V), the heat flow was successfully modeled using polarization and entropic contributions. Heat flow at lower voltage (0.86–0.03 V) exceeded the predicted values for iron oxide reduction, consistent with heat generation attributable to electrolyte decomposition and surface electrolyte interphase (SEI). Operando X-ray absorption spectroscopy (XAS) indicated that the oxidation state of the Fe centers deviated from predicted values beginning at ∼0.86 V, supportive of SEI onset in this voltage range. Thus, these combined results from electrochemistry, IMC, and XAS indicate parasitic reactions consistent with SEI formation at a moderate voltage and illustrate an approach for deconvoluting Faradaic and non-Faradaic contributions to heat, which should be broadly applicable to the study of energy-storage materials and systems.

The reduction of sulfur during discharge in a lithium-sulfur (Li-S) cell is known to occur in a series of reaction steps that involve lithium polysulfide intermediates. We present an operando study of the discharge of a solid-state Li-S cell using X-ray absorption spectroscopy (XAS). In theory, the average chain length of the polysulfides, xavg,cell, at a given depth of discharge is determined by the number of electrons delivered to the sulfur cathode. The dependence of xavg,cell measured by XAS on the depth of discharge is in excellent agreement with theoretical predictions. XAS is also used to track the formation of Li2S, the final discharge product, as a function of depth of discharge. The XAS measurements were used to estimate rate constants of a series of simple reactions commonly accepted in literature.

Lithium-ion batteries using germanium as the anode material are attracting attention because of their high-capacity, higher conductivity, and lithium-ion diffusivity relative to silicon. Despite recent studies on Ge electrode reactions, there is still limited understanding of the reaction mechanisms governing crystalline Ge and the transformations into intermediate amorphous phases that form during the electrochemical charge and discharge process. In this work, we carry out in operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies on Ge anodes during the initial cycles to better understand these processes. These two probes track both crystalline (XRD) and amorphous (XAS) phase transformations with potential, which allows detailed information on the Ge anode to be obtained. We find that crystalline Ge lithiates inhomogeneously, first forming amorphous Li9Ge4 during the beginning stage of lithiation, followed by the conversion of the remaining crystalline Ge to amorphous Ge. The lithiation of amorphous Ge then forms amorphous LixGe, which are then further lithiated to form crystalline Li15Ge4. During delithiation, crystalline Li15Ge4 transforms directly into a heterogeneous mix of amorphous LixGe, which eventually form amorphous Ge, and interestingly, no amorphous Li9Ge4 are detected. Both our in operando XRD and XAS results present new insights into the reaction mechanism of Ge as anodes in LIBs, and demonstrate the importance of correlating electrochemical results with in operando studies.

In-situ/operando X-ray absorption spectroscopic investigation of the electrode/electrolyte interface on the molecular scale

Given their natural abundance and thermodynamic stability, fluoride salts may appear as evolving components of electrochemical interfaces in Li-ion batteries and emergent multivalent ion cells. This is due to the practice of employing electrolytes with fluorine-containing species (salt, solvent, or additives) that electrochemically decompose and deposit on the electrodes. Operando X-ray absorption spectroscopy (XAS) can probe the electrode–electrolyte interface with a single-digit nanometer depth resolution and offers a wealth of insights into the evolution and Coulombic efficiency or degradation of prototype cells, provided that the spectra can be reliably interpreted in terms of local oxidation state, atomic coordination, and electronic structure about the excited atoms. To this end, we explore fluorine K-edge XAS of mono- (Li, Na, and K) and di-valent (Mg, Ca, and Zn) fluoride salts from a theoretical standpoint and discover a surprising level of detailed electronic structure information about these materials despite the relatively predictable oxidation state and ionicity of the fluoride anion and the metal cation. Utilizing a recently developed many-body approach based on the ΔSCF method, we calculate the XAS using density functional theory and experimental spectral profiles are well reproduced despite some experimental discrepancies in energy alignment within the literature, which we can correct for in our simulations. We outline a general methodology to explain shifts in the main XAS peak energies in terms of a simple exciton model and explain line-shape differences resulting from the mixing of core-excited states with metal d character (for K and Ca specifically). Given ultimate applications to evolving interfaces, some understanding of the role of surfaces and their terminations in defining new spectral features is provided to indicate the sensitivity of such measurements to changes in interfacial chemistry.

Operando X-ray absorption spectroscopy (XAS) can be utilized to probe the phase and structural changes of FeNiOx and similar transition metal oxide electrocatalysts during electrocatalytic reactions. However, capturing the temporal changes occurring in the operando chemistry of electrocatalysts has been little studied. In this work, we successfully capture the time-resolved changes at the Ni K-edge for both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions of an aqueous phase synthesized FeNiOx bimetallic nanoparticle electrocatalyst. During a stepped voltage experiment, the temporal change in the Ni K-edge was observed as the applied voltage was stepped from 0.7 to 0.8 V versus the silver/silver chloride reference electrode, and the change observed is associated with the Ni redox transition from the 2+ to 3+/4+ oxidation state. Individual XAS spectra were obtained in 90 s, and a total of 9 scans post voltage step showed a unique transition from a hydroxide to an oxyhydroxide phase. The shift in absorption edge energy position and the changes in spectral shape for individual scans explain the typically observed broadening of the Ni K-edge XANES spectrum during time-averaged operando XAS, and a kinetic analysis revealed a first-order observed rate constant, |kobs|, of 0.00426 s–1 and a half-life (t1/2) of 163 s. Multivariate curve resolution-alternating least squares analysis followed by linear combination fitting analysis for time-averaged versus time-resolved Ni K-edge changes upon voltage step show distinct differences in estimated contributions from Ni2+ oxide/hydroxide phases. Detailed EXAFS modeling shows the phase transition from hydroxide to oxyhydroxide on top of the unchanged metallic core.

We have utilized a comprehensive set of experimental techniques such as transmission electron microscopy (TEM) and synchrotron-based x-ray absorption spectroscopy (XAS) and the respective x-ray linear dichroism and x-ray magnetic circular dichroism to characterize the correlation of structural, chemical and magnetic properties of Co-doped ZnO samples. It can be established on a quantitative basis that the superparamagnetic (SPM) behavior observed by integral superconducting quantum interference device magnetometry is not an intrinsic property of the material but stems from precipitations of metallic Co. Their presence is revealed by TEM as well as XAS. Annealing procedures for these SPM samples were also studied, and the observed changes in the magnetic properties found to be due to a chemical reduction or oxidation of the metallic Co species.

The rechargeable Zn-MnO2 battery system is one of the few systems in development capable of meeting the extreme cost requirements needed for global integration of battery storage in the power grid. Modifying the MnO2 cathode with Bi allows the electrode process to cycle reversibly between layered Mn oxides 𝛿-MnO2 and Mn(OH)2, without detrimental formation of irreversible Mn3O4. The 𝛿-MnO2 and Mn(OH)2 reaction end members represent the Mn(IV) and Mn(II) species and can be easily observed via operando X-ray diffraction (XRD) due to their crystallinity. However, an Mn(III) intermediate is not observed by XRD, indicating that there is a substantial segment of the cycling mechanism that involves only disordered materials. Mn(III) is susceptible to disproportionation, and the Mn(III) intermediate is likely the precursor to Mn3O4. For this reason we hypothesize that the disordered Mn(III) intermediate is likely the species directly impacted by modifying Bi3+ cations. To characterize this intermediate, we turn to extended x-ray absorption fine-structure (EXAFS), which relies on short-range order and does not require crystallinity to be effective. This talk will demonstrate rapidly collected, operando EXAFS data for MnO2 cathodes with and without Bi modifier. Operando techniques are scientifically powerful as they reduce experimental uncertainty resulting from destructive preparation of samples for ex situ analysis. In addition, rapid EXAFS collection allows for analysis of the kinetics of Mn coordination changes.The Bi-modified alkaline MnO2 cathode has been of much interest in literature as only a small amount of Bi2O3 additive (Bi:Mn molar ratio 0.075) incorporated by high-energy ball-milling enables rechargeable behavior. Previously, a disordered phase with no diffraction signal was identified during the first charge of Bi-modified MnO2 cathodes, but no structural characterization was reported.1 In this work, operando X-ray absorption spectroscopy (XAS) was employed to further compare Mn coordination environments in standard and Bi-modified cathodes throughout cycling. Specifically, EXAFS analysis was used to visualize the dynamic first shell (Mn-O) and second shell (Mn-Mn) interatomic distances during the first cycle. Comparison with theoretical EXAFS modeled from CIF data allows for “fingerprinting” of the charge/discharge products, while rapid EXAFS collection time allows for identification of intermediates and precise analysis of the potentials and states of charge at which materials deform. We identify an intermediate that is structurally similar to layered 𝛽-MnOOH, as well as the conversion of this material to Mn3O4 via incorporation of Mn(II) atoms. When Bi modifier is present, this undesired conversion still begins but is quickly extinguished and no Mn3O4 is observed. This work establishes the mechanism through which Bi imparts rechargeability to MnO2, and through this understanding enables further engineering of the system to achieve low-cost rechargeable alkaline Zn-MnO2 batteries. References Bruck, A.M.; Kim, M.A.; Ma, L.; Ehrlich, S.N.; Okasinski, J.S.; and Gallaway, J.W., "Bismuth Enables the Formation of Disordered Birnessite in Rechargeable Alkaline Batteries." Journal of the Electrochemical Society, 2020, 167, 110514. Acknowledgements We acknowledge financial support from the National Science Foundation under Award Number CBET-ES-2044602. This research used resources of beamline 7-BM (QAS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

X-ray absorption spectroscopic studies on solid oxide fuel cells and proton-conducting ceramic fuel cells

Due to the asymmetric structure with multiple materials, SOFCs are subjected to various modes of mechanical stress, resulting in a risk of failure. Of particular importance are thermal and chemical strains, which depend on the micro- and macroscopic distribution of electrochemical reaction sites. Therefore, SOFC modeling must include electrochemistry, mass and charge transport, thermo-fluid dynamics, and structural analysis. In addition, appropriate experiments must be performed to validate the modeling results. This research group is working to develop a comprehensive environment for modeling and validation in order to evaluate the mechanical reliability of SOFCs.Electrochemical modeling of cathode was made based on the defect chemistry and mass transport of (La,Sr)CoO3 (LSC) and (La,Sr)(Co,Fe)O3 (LSCF). An empirical equation for the surface reaction of the porous electrode was determined by electrochemical analysis of a dense film electrode taking into account the difference between the dense film and porous electrodes [1]. Anode reaction rate on a unit length of triple phase boundary of Ni-YSZ-gas was determined using circular Ni model electrodes of various size [2]. Evaluation of 3D oxygen potential distribution inside the constituent oxides was made using an in-house code called SIMUDEL where mixed ionic/electronic conduction and oxygen nonstoichiometry were taken into consideration [3,4]. It was combined with commercial software to make structural analysis. Mechanical properties used for calculation were obtained from measurements by the resonance method and the small punch method or collected from the literature. Non-linear stress-strain behaviors were observed with Ni-YSZ cermet [5] and inelastic behaviors were found in most nonstoichiometric oxide materials. Large ferro-elasticity emerged in the LSC and LSCF [6] and in Sc stabilized ZrO2 electrolyte at elevated temperatures. Chemical strain in vacancy formation was formulated with a linear function of the defect concentration as an independent factor from the thermal expansion. In the structural analysis, it was treated as the additional temperature change of the local material. Volume change of Ni-YSZ cermet on red-ox operation was a difficult factor to formulate because of its complex dependence on composition, temperature, and atmosphere. As expected, Ni-YSZ showed expansion when oxidized, but under certain conditions at an intermediate temperature range, it rather contracted on oxidation [7].In reality, the calculated stress distribution does not always match the actual behavior of the cell since there often are unconsidered factors. Thus, experimental validation is essential. For this purpose, we have developed methods for in-situ or operando measurements of cell shape and the residual stress. Long-focus laser profilometer was used to measure the warping of a planar cell during heating, operating, and cooling operations. For the change in the diameter of a tubular cell, the laser projection type dimension meter was employed. Residual stress measurements were made with a handy X-ray analyzer (μ-X360s, Pulstec Industrial. Co. Ltd.) combined with a newly developed one- or two-chamber cell holder [8,9]. Anode-supported cells from multiple sources were analyzed, showing different stress histories in red-ox operations depending on the microstructure of the anode support layer.Currently, we are developing a protocol for testing robustness of planar SOFCs. Here, temperature distribution is intentionally applied on a cell with segmented heater allays on the top and the bottom of the cell. Severe condition tests will be performed, and the results will be analyzed with the above-mentioned simulation.AcknowledgementThis study was supported by the New Energy and Industrial Technology Development Organization (NEDO) .[1] T. Kawada, Current Opinion in Electrochemistry, 21, 274-282 (2020) DOI: 10.1016/j.coelec.2020.03.016[2] M. Takeda, Master’s thesis, Graduate School of Environmental Studies, Tohoku University (2016) : M. Takeda, et al. Submitted to J. Electrochem. Soc.[3] K. Terada, et. al., ECS Trans. 35(2 part2), 923-933 DOI: 10.1149/1.3570073[4] M. Sato, et. al., Trans. Jpn. Soc. Comp. Eng. Sci. 2017, 14 (2017) DOI: 10.11421/jsces.2017.20170004[5] S. Watanabe et al., J. Mater. Sci. 55, 8679-8693 (2020) DOI: 10.1007/s10853-020-04624-4[6] Y. Kimura et al., J. Electrochem. Soc. 161(11), F3079- F3083 (2014) DOI: 10.1149/2.0131411jes[7] Y. Morishita et al. , ECS Trans., 91(1) 1979-1984 (2019). DOI: 10.1149/09101.1979ecst[8] K. Yashiro et al., submitted to J. Power Sources[9] K. Oshima et al.,ECS Trans. 103(1), 1251-1260 (2021) DOI: 10.1149/10301.1251ecst

For the quantitative determination of the effective reaction area in an SOFC cathode, a new type of model patterned electrode was proposed and fabricated with La0.6Sr0.4CoO3-δ (LSC). In situ micro X-ray absorption spectroscopy (XAS) measurements were performed in 1 bar of p(O2) at 973 K under open circuit and cathodic polarization. Absorption spectra were taken at different positions with the position resolution of less than 1 μm. The increase in the absorbance, i.e. the decrease in the oxygen potential, was observed near the electrod/electrolyte interface, when the electrode was cathodically polarized. The characteristic length of the effective reaction area for the patterned LSC electrode was estimated approximately 20 μm from the interface. Electrochemical impedance spectroscopy (EIS) measurements were also carried out with the patterned LSC electrode. The characteristic length of the effective reaction area estimated by EIS was close to one by XAS.