Energy-tunable X-ray Research Articles

(Invited) Synchrotron X-Ray Techniques for Structure-Functionality of Ionomers: Current Status and New Directions

For the desired clean energy transition, hydrogen technologies are sought to play a critical role from electrolyzers to produce clean hydrogen from water to fuel cells to decarbonize heavy-duty transportation. Centerpiece to the successful operation of these technologies is the durable performance of the membrane-electrode assembly systems, which consists of an ion-conducting membrane between the electrodes where electrochemical reactions take place. Ion-conducting polymers, ionomers, play a key function in these systems as the separator solid-electrolyte, which requires typically a well-hydrated, phase-separated morphology that can block reactant species while transporting water and ions (protons). In addition, ionomers are also present in the electrodes as a nanometer thick film with a dual function of conducting medium and catalyst binder. In such heterogeneous structures, ionomer forms many interfaces with catalyst particles (e.g., Platinum in fuel cells) and interactions with the other species thereby making it challenging, yet critically important, to characterize their multi-scale structural features.What emerges from these distinct roles of ionomers in fuel cells and electrolyzers is the need for characterizing their structure in relation to the expected properties and desired performance for the operation. For example, for membrane, while structure-property relationships have been commonly investigated via x-ray scattering techniques, with almost three decade of work on interpretation of the so-called ionomer peak associated with the water nano-domains. However, the need for understanding and mitigating membrane degradation requires an expansion to structure-property-durability relationship. Similarly, for catalyst ionomers, their confined structure interfacing metal substrates have been idealized and examined by grazing-incidence x-ray scattering or reflectometry techniques. Yet, understanding the formation of catalyst layers from the mix of catalyst inks and ionomers require a shift to characterize their structure-processing-property relationships.This presentation will provide an overview of the current state of research on ionomer characterization with synchrotron x-rays and the ongoing an expansion of modalities that aim to image not only ionomer structure, but also their evolution under various applied stimulus via in-situ and operando x-ray techniques. Selected examples will be highlighted as case studies, demonstrating, first, how small/wide-angle x-ray scattering (SAXS/WAXS) can be repurposed with various environmental controls to understand structural changes due to degradation as well as the presence of cationic particles added to mitigate degradation. We will also show how energy-tunable x-rays at targeted absorption energies can be used to enhance the scattering signal and the acquired structural information. Then, we will show how grazing-incidence (GI) SAXS/WAXS can be employed to track the real-time evolution of the ionomer film nanostructure and crystallinity on a substrate during casting from a solution. These examples will be discussed together to provide a broader summary the current status and the future directions of both ionomer research for fuel cells and electrolyzer and the required characterization needs using synchrotron x-rays.

(Invited, Digital Presentation) Fundamental Understanding and Challenges in Cathode Materials for Next Generation Li-Ion Batteries

Introducing high-performance Li-ion batteries in current energy storage technologies has great hope in achieving decarbonization goals for sustainable future. The impact of Li-ion batteries in current energy sector is huge, especially portable electronics to electric vehicles. With this energy demand, recently, researchers have been exploring the so-called “Li-rich anion redox” cathode chemistry, in which electrons stored on oxide anions are reversibly utilized during redox reaction along with conventional transition metal redox to obtain high energy storage capability.1-2 Compared to conventional cathodes, the anion redox cathode chemistry is not straightforward as it goes through a complex redox reaction pathway, leading to fundamental issues such as voltage fade, voltage hysteresis and irreversible oxygen release. However, these anionic redox reactions in transition metal oxide-based cathodes attained by extracting excess lithium ions have resulted in stability issues due to weak metal (M) – oxygen(O) ligand covalency.1 In order to increase the tightness between metal–ligand, an alternative strategy was investigated by replacing M–O framework with M–X (X- S, Se) ligands. Based on the experimental studies, it is found that the metal-ligand tunability is the key to achieve highly reversible chalcogen anion redox reactions in various Li based chalcogen cathode materials.3 Further, the lithiation delithiation reactions of cathodes are investigated using in-depth electrochemical analysis and the electronic as well as structural changes during electrochemical reactions are understood using detailed spectroscopy and microscopy characterization. Findings from this research will inspire Ni and Co free chalcogen cathode design and various functional materials in the pursuit of next-generation cathode materials. In parallel, this presentation will also focus on fundamental understanding and importance of fabricating rechargeable high-temperature Li-ion batteries for various demanding applications such as military applications, sensor applications, oil and gas industry drilling applications. In this work, high temperature compatible electrode materials are identified, and their electrode/electrolyte interfacial stability issues at different depth levels are understood using energy tunable X-ray photon electron spectroscopy and further visualized with advanced microscopy imaging. This proof-of-concept study will lead to a new paradigm for transforming ambient temperature Li-ion battery technology to high-temperature applications.

Chemical and Morphological Origins of Improved Transport in Perfluoro Ionene Chain Extended Ionomers

The performance of proton-conducting ionomer membranes used in electrochemical applications such as fuel cells is complicated by an intricate interplay between chemistry and morphology that is challenging to characterize and control. Here, we report on a class of perfluoro ionene chain extended (PFICE) ionomers that contain either one (PFICE-2) or two (PFICE-3) bis(sulfonyl)imide groups on the side-chain in addition to a terminal sulfonic acid group. PFICE ionomers are promising materials, exhibiting greater water uptake and conductivity over a range of relative humidity values compared to prototypical perfluorinated sulfonic acid (PFSA) ionomers. Advanced in situ synchrotron characterization combined with simulations reveals insights into the connections between molecular structure and morphology that dictate performance. Energy-tunable X-rays with sensitivity to sulfur can decipher the unique bonding environment of different protogenic groups on the polymer side-chain. Guided by simulations, X-ray absorption spectroscopy can be sensitive to hydration level and configuration that dictates proton dissociation. In situ resonant X-ray scattering reveals that PFICE ionomers have a phase-separated morphology with enhanced short-range order that persists in both dry and hydrated state, allowing for improved transport pathways across hydration levels. Furthermore, side-chain chemistry and length can be used as a molecular design parameter to predict phase-separated domain spacing. The enhanced conductivity of PFICE ionomers is attributed to a unique side-chain chemistry and structure promoting hydrogen bonding configurations that facilitate proton dissociation at low water content in combination with a well-ordered phase-separated morphology that forms transport pathways. Overall, these results provide guidelines to design new ionomers with improved transport properties and demonstrate the value of in situ characterization methods such as resonant X-ray scattering and spectroscopy for unraveling the structural features in chemically-heterogeneous materials used in electrochemical systems.

Native-SAD Phasing at Synchrotrons: A Low-dose and High-redundancy Strategy

Native biological macromolecules contain intrinsic light elements such as sulfur in proteins and phosphorus in nucleic acids. Native-SAD phasing utilizes the anomalous signals from light elements for de novo structure determination: first the substructure of anomalous scatterers is determined; phase evaluation for the entire structure then follows. Synchrotron beamlines are expected to be ideal instruments for native-SAD phasing due to their brilliant and energy-tunable x-rays. However, anomalous signals from light elements are typically very weak at x-ray energies accessible to most synchrotron beamlines. Efforts have been made to promote the utility of synchrotrons for routine native-SAD phasing with no requirement for heavy-atom incorporation. Our strategy is to limit the x-ray dose per crystal and to enhance the signal-to-noise ratio by increasing data redundancy through use of multiple crystals. We have devised a robust procedure and applied it for routine native-SAD analyses on real-life membrane proteins, protein-protein complexes, and recalcitrant proteins. Here we use these real-life case studies to illustrate our procedures in sample preparation, x-ray energy selection, data collection, data analysis and phasing.

In-line phase-contrast imaging based on Tsinghua Thomson scattering x-ray source.

Thomson scattering x-ray sources can produce ultrashort, energy tunable x-ray pulses characterized by high brightness, quasi-monochromatic, and high spatial coherence, which make it an ideal source for in-line phase-contrast imaging. We demonstrate the capacity of in-line phase-contrast imaging based on Tsinghua Thomson scattering X-ray source. Clear edge enhancement effect has been observed in the experiment.

Fundamentals of XAFS

The basic physical principles of X-ray Absorption Fine-Structure (XAFS) are presented. XAFS is an element-specific spectroscopy in which measurements are made by tuning the X-ray energy at and above a selected core-level binding energy of a specified element. Although XAFS is a well-established technique providing reliable and useful information about the chemical and physical environment of the probe atom, its requirement of an energy-tunable X-ray source means it is primarily done with synchrotron radiation sources and so is somewhat less common than other spectroscopic analytical methods. XAFS spectra are especially sensitive to the oxidation state and coordination chemistry of the selected element. In addition, the extended oscillations of the XAFS spectra are sensitive to the distances, coordination number and species of the atoms immediately surrounding the selected element. This Extended X-ray Absorption Fine-Structure (EXAFS) is the main focus of this chapter. As it is element-specific, XAFS places few restrictions on the form of the sample, and can be used in a variety of systems and bulk physical environments, including crystals, glasses, liquids, and heterogeneous mixtures. Additionally, XAFS can often be done on low-concentration elements (typically down to a few ppm), and so has applications in a wide range of scientific fields, including chemistry, biology, catalysis research, material science, environmental science, and geology. Special attention in this chapter is given to the basic concepts used in analysis and modeling of EXAFS spectra. X-ray absorption fine structure (XAFS) is the modulation of an atom’s X-ray absorption probability at energies near and above the binding energy of a core-level electron of the atom. The XAFS is due to the chemical and physical state of the absorbing atom. XAFS spectra are especially sensitive to the formal oxidation state, coordination chemistry, and the distances, coordination number and species of the atoms immediately surrounding the selected …

Inverse Compton backscattering source driven by the multi-10 TW laser installed at Daresbury

AbstractInverse Compton scattering is a promising method to implement a high brightness, ultra-short, energy tunable X-ray source at accelerator facilities. We have developed an inverse Compton backscattering X-ray source driven by the multi-10 TW laser installed at Daresbury. Hard X-rays, with spectral peaks ranging from 15 to 30 keV, depending on the scattering geometry, will be generated through the interaction of laser pulses with electron bunches delivered by the energy recovery linac machine, initially known as energy recovery linac prototype and subsequently renamed accelerators and lasers in combined experiments. X-ray pulses containing 9 × 107 photons per pulse will be created from head-on collisions, with a pulse duration comparable to the incoming electron bunch length. For transverse collisions 8 × 106 photons per pulse will be generated, where the laser pulse transit time defines the X-ray pulse duration. The peak spectral brightness is predicted to be ~1021 photons/(s mm2 mrad2 0.1% Δλ/λ).

Production and application of a novel energy-tunable X-ray source at the RPI LINAC

The 60-MeV electron linear accelerator at Rensselaer Polytechnic Institute (RPI) is used to produce parametric X-rays (PXR). PXR is an intense, quasi-monochromatic, energy-tunable, and polarized X-ray source derived from the interaction of relativistic electrons with the periodic structure of crystal materials. Experiments were performed using highly oriented pyrolytic graphite (HOPG), LiF, Si, Ge, Cu, and W target crystal radiators. Smooth X-ray energy tunability is achieved by rotating the crystal with respect to the electron beam direction. Measured energy linewidths consistently agreed with predicted values except in cases using lower quality HOPG. When the predicted energy linewidth was narrower than our Si X-ray detector resolution (350eV at 17.5KeV), a near-absorption edge transmission technique that takes advantage of the PXR energy tunabilty was used to measure the PXR energy linewidth for example, Si(400) FWHM of 134eV at 9.0keV (2%). Per electron, the photon production efficiency of PXR is comparable to synchrotron radiation sources. A theoretical model that considers electron multiple scattering, electron divergence, and crystal mosaicity was used to broaden the PXR photon distribution in order to calculate the predicted PXR photon yield. Comparing measurements and calculations resulted in a typical relative error below 50%. In some cases with LiF, the differences between predicted and measured values were as low as 2% for LiF(400). Finally, this work reports for the first-time PXR imaging. This was achieved using LiF(220) interacting with 56MeV electrons with electron beam currents up to 6μA. The LiF and graphite PXR target crystals were compared for use in soft tissue imaging, e.g. mammography using energies 17–20keV. Low Z materials like graphite and LiF were most suitable for PXR production because of their low Bremsstrahlung production, electron scattering, and photon absorption. Graphite was more efficient at producing PXR photons while the LiF energy line width was narrower.

Energy-tunable x-ray diffraction: A tool for depth profiling in polycrystalline materials

We have developed a new variant of depth-sensitive x-ray diffraction technique to study structural parameters in inhomogeneous polycrystalline materials. In this method, diffraction patterns are measured at different x-ray energies which are varied by small steps, and then the depth-resolved structural characteristics are retrieved from the energy-dependent x-ray diffraction data. In the current articles, this approach is applied to extract preferred orientation with depth resolution. In the case of uniaxial preferred orientation, the analytical algorithm has been developed based on March functions. Application of this technique to seashells allowed us to characterize the microstructure evolution in the nacre layer. Near the inner surface, adjacent to the mollusk mantle, the nacre consists of well-defined lamellas which reveal a high degree of the (001)-preferred orientation. This preferred orientation deteriorates in depth due to the accumulation of cracks and other imperfections. The texture distribution is characterized quantitatively by depth-dependent March parameters, which allows us to compare samples taken from different shells. In a similar way, energy-variable x-ray diffraction can be used for nondestructive characterization of a very broad spectrum of laminated structures and composite materials and systems.

Generation of an X-ray microbeam for spectromicroscopy at SPring-8 BL39XU.

A pair of elliptical mirrors (KB mirror) was designed and fabricated to realize an energy tunable x-ray microbeam for spectromicroscopy at SPring-8 BL39XU. As is commonly recognized, the obtainable beam size with the aspherical total reflection mirrors is strongly affected with the slope error of the mirror. Considering that the extremely high brilliance of the undulator radiation from the SPring-8, the small mirror size and the small mirror-to-focus distance were employed to minimize effects of the slope error. Preliminary evaluation of the KB mirror was carried out using 10 keV monochromatized undulator radiation. Alignment of the mirror was assisted by the beam monitor system composed of a scintillator and a CCD, and the beam size less than 5 microm can be easily achieved even when the source was fully used. The beam size obtained with this experiment was 2 x 4 microm2 with the photon flux of 1 x 10(10) photons/s. Smaller beam size may be expected with the use of intermediate slits. Characterization of trace elements with the spatial resolution will be carried out by using x-ray fluorescence (XRF) analysis and x-ray absorption fine structure (XAFS) measurements with XRF yield method.

Energy-Tunable X-Ray Diffraction in Polycrystalline Materials: a Look at Microstructure in Seashells

AbstractWe developed a depth-sensitive x-ray diffraction technique in which diffraction profiles are measured at x-ray energies that are varied by small steps. The method is intended for synchrotron beam lines and provides non-destructive mapping of structural characteristics in inhomogeneous polycrystalline materials. Depth resolution is achieved due to an energy dependence of the x-ray penetration length. Application of this technique to seashells allowed us to extract spatial distributions of preferred orientation and strain components, which revealed pronounced variations of the shell microstructure in three dimensions. The results shed light on “engineering solutions” by mollusk. The developed technique can be used to characterize various laminated structures and composite materials.

X-ray linear birefringence and linear dichroism in a cobalt crystal measured with a tunable X-ray polarimeter.

X-ray linear birefringence and linear dichroism spectra have been measured simultaneously and quantitatively for the first time. The sample was a hexagonal cobalt single-crystal foil. The apparatus used was an energy-tunable X-ray polarimeter, consisting of a polarizer, a phase retarder and an analyser. The X-ray energy was scanned over a range of 350 eV near the K-absorption edge. The Kramers-Kronig relation between the refraction and absorption anisotropy has been confirmed on an absolute scale. The phase-difference sensitivity of the polarimeter as a polarization inferometer is as small as 2pi/10000, which is about 100 times smaller than that of the Bonse-Hart inferometer.

X-ray triple refraction and triple absorption in a cobalt-complex crystal.

X-ray triple refraction and triple absorption have been quantitatively measured for the first time. The samples were (100)-, (010)- and (001)-oriented plates of a cobalt-complex monoclinic crystal. The apparatus used was an energy-tunable X-ray polarimeter with a phase retarder. The X-ray energy was scanned over a range of 150 eV near the cobalt K-absorption edge. Both the spectra of ellipticity and rotation of polarization were completely different for each crystal plate, which revealed that the biaxial crystal had three refractive indices and three absorption coefficients in the X-ray region. The Kramers-Kronig relation between the real and imaginary parts of the dielectric anisotropy has been quantitatively confirmed for all three different orientations.

移相子付き波長可変Ｘ線ポラリメーターによる結晶の誘電率異方性の研究

A new type of energy-tunable X-ray polarimeter has been developed by introducing a Hirano-Ishikawa-Kikuta's transmission-type X-ray phase retarder into an X-ray polarimeter consisting of Hart-Rodrigues' polarizer and analyzer. By using the new type of polarimeter, simultaneous detections of X-ray linear birefringence and dichroism, and simultaneous detections of X-ray linear triple refraction and trichroism, have been made for the first time. The Kramers-Kronig relation has been confirmed in the absolute scale of dielectric constant between linear birefringence and dichroism (and between linear triple refraction and trichroism), which correspond to the real and imaginary parts of dielectric anisotropy, respectively. Phase shift due to the birefringence (or the triple refraction) has been measured with a precision of 2π 110, 000, which is limited mainly by photon-counting statistics.

X-ray microanalysis with energy tunable synchrotron X-rays

Focused intense monochromatic synchrotron radiation was realized with a combination of a Pt coated ellipsoidal mirror, a double crystal monochromator and a pinhole. Energy tunable X-ray microanalysis using a focused X-ray beam was compared to particle induced X-ray emission analysis. A section of Japanese cedar showed severe damage after proton irradiation, while another section of the tree showed no damage after the measurement of its trace element distribution by synchrotron radiation. Less damage, high sensitivity and energy tunability were demonstrated by utilizing synchrotron radiation.